Annulated bacteriochlorins for near-infrared photophysical studies

Article abstract of DOI:10.1039/c9nj01113g

Molecules with strong absorption in the near-infrared (NIR) spectral region are of interest for a variety of applications in the photosciences. Nature's NIR absorbers are bacteriochlorophylls, which contain a tetrahydroporphyrin chromophore accentuated by

Full text of DOI:10.1039/c9nj01113g

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Annulated bacteriochlorins for near-infrared
photophysical studies†
Cite this:DOI: 10.1039/c9nj01113g
a
a
a
a
Hikaru Fujita,
Haoyu Jing,
Michael Krayer,
Srinivasarao Allu,
a
a
a
b
Gorre Veeraraghavaiah,
Nikki Cecil M. Magdaong,
Dariusz M. Niedzwiedzki,
Zhiyuan Wu,
Amit K. Mandal,
Christine Kirmaier,
Jianbing Jiang,
James R. Diers,
c
c
c
Arpita Roy,
d
c
b
David F. Bocian,
*
Dewey Holten *c and Jonathan S. Lindsey
*
a
Molecules with strong absorption in the near-infrared (NIR) spectral region are of interest for a variety of
applications in the photosciences. Nature’s NIR absorbers are bacteriochlorophylls, which contain
a
tetrahydroporphyrin chromophore accentuated by attached auxochromes, including a b,meso-bridging
isocyclic ring. Modification of the isocyclic ring has provided a convenient means of wavelength tuning. In
other tetrapyrroles, annulation at the b,meso-, b,b-, and b-meso-b-positions also has enabled wavelength
tuning. Here, synthetic bacteriochlorins bearing a gem-dimethyl group in each pyrroline ring have provided
the foundation for studies of annulation. Seven new free base bacteriochlorins have been prepared and seven
others have been elaborated therefrom. Six distinct routes known among porphyrins were explored.
Ultimately, annulation of phenaleno or benzo groups at the b,meso-sites (i.e., spanning the 3,5- and
13,15-positions) of bacteriochlorins was achieved by a new route that entails two successive Pd-mediated
coupling reactions of a dibromobacteriochlorin with a bromoarylboronic acid. The corresponding Phen-BC
or Benz-BC absorbs at 913 or 1033 nm, respectively. Examination by time-resolved spectroscopy revealed
an excited-state lifetime of 150 ps or 7 ps, with commensurably low fluorescence quantum yield (Ff)
values estimated to be B0.004 or 3 ꢀ 10ꢁ5, respectively. The origin of the short excited-state lifetimes
due to b,meso-annulation is unknown, but is not simply a consequence of the energy-gap law for non-
radiative decay.
Received 2nd March 2019,
Accepted 5th April 2019
DOI: 10.1039/c9nj01113g
rsc.li/njc
light absorber in anoxygenic photosynthetic bacteria, contains
the bacteriochlorin chromophore along with an ‘‘isocyclic ring’’
Introduction
Bacteriochlorins are attractive candidates for diverse photo- spanning positions 13–15 (Chart 1).2 The isocyclic ring is a
physical studies such as artificial photosynthesis, photodynamic 5-membered ring containing a keto group conjugated with the
therapy, and optical imaging owing to their strong absorption in bacteriochlorin p-system at the 13-position, but a saturated
the near-infrared (NIR) region.1 Bacteriochlorophyll a, the core methylene unit at the 15-position. Hence, the isocyclic ring
(an annulated oxocyclopentano motif) does not cause conjuga-
tion to be extended across the 13,15-positions. The full comple-
ment of substituents about the perimeter of the macrocycle has
a Department of Chemistry, North Carolina State University, Raleigh,
North Carolina 27695-8204, USA. E-mail: jlindsey@ncsu.edu;
precluded studies to install annulated rings to the native bacter-
iochlorophylls; however, the allomerization of the isocyclic ring
has been exploited to create bacteriochlorin-imides (often referred
to as bacteriopurpurinimides).3,4
Tel: +1-919-515-6406
b Department of Chemistry, University of California, Riverside,
California 92521-0403, USA. E-mail: david.bocian@ucr.edu; Tel: +1-951-827-3660
c Department of Chemistry, Washington University, St. Louis, Missouri, 63130-4889,
USA. E-mail: holten@wustl.edu; Tel: +1-314-935-6502
In previous studies, a synthetic bacteriochlorin was annulated
with imide rings in an effort to extend the position of the long-
wavelength absorption (Qy) band deeper into the NIR spectral
region. For the bacteriochlorin constructed with imides annulated
at the 2,3- and 12,13-positions (i.e., b,b-annulation), the Qy band
appeared at 800 nm.5 On the other hand, the bacteriochlorin
constructed with imides annulated at the 3,5- and 13,15-positions
(i.e., b,meso-annulation) gave the Qy band at 875 nm.5
d Department of Energy, Environmental & Chemical Engineering and Center for
Solar Energy and Energy Storage, Washington University, St. Louis, Missouri
63130-4889, USA
† Electronic supplementary information (ESI) available: Extensive results con-
cerning exploration of alternative routes; spectroscopic data for bacteriochlorin
BC-2; 1H and 13C NMR spectra for all new compounds on the route to Phen-BC
and Benz-BC; and single-crystal X-ray data. CCDC 1900109 (21), 1900117 (24) and
1900118 (18). For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c9nj01113g
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These strategies developed with porphyrins in principle can be
applied to the synthesis of b,meso- or b,b-annulated bacteriochlorins,
albeit with two caveats: (1) in practice, the synthetic chemistry of
bacteriochlorins has significantly lagged that of porphyrins; and (2)
the reactivity of bacteriochlorins appears to be somewhat different
from that of porphyrins.
Representative synthetic approaches to bacteriochlorins1,34
include hydrogenation of porphyrins,35,36 cycloaddition to por-
phyrins,37 breaking and mending of porphyrins,38 semisynthesis
beginning typically with bacteriochlorophyll a,3,4,39 and de novo
synthesis. In de novo synthesis, the saturation characteristic of the
pyrroline rings is built into the acyclic precursors to the bacterio-
chlorin macrocycles. Examples include the synthesis of tolypor-
phins40,41 and a general route that we have been developing,42–47
which has been extended imaginatively by others.48–57
In this paper, we report synthetic approaches for annulation
of de novo synthesized bacteriochlorins. Installing annulated
groups proved more challenging with bacteriochlorins than
appears to be the case with porphyrins, as numerous established
approaches with porphyrins such as oxidative coupling were
unsuccessful. Ultimately, two b,meso-annulated bacteriochlorins
were synthesized via palladium-catalyzed intramolecular arylation.
Photophysical studies were performed to determine the lowest
excited singlet-state lifetimes and fluorescence quantum yields.
These studies were accompanied by density functional theory
(DFT) calculations to examine the forms of the frontier molecular
orbitals (MOs) and by time-dependent DFT (TDDFT) to compare
Chart 1 Structure of bacteriochlorophyll a and bacteriopurpurinimides.
One distinction between the two types of bis(imide)s is that the calculated versus observed absorption spectra.
b,b-annulation affords a 5-membered ring whereas b,meso-
annulation gives a 6-membered ring. The bacteriochlorin-b,b-
bis(imide) MeO-I2-bb-BC gave a singlet excited-state lifetime (ts)
Results
of 3.6 ns and fluorescence quantum yield (Ff) value of 0.17
in toluene.5 In contrast, the bacteriochlorin-b,meso-bis(imide)
Synthetic approach toward b,b-annulated bacteriochlorins
I2-BC gave ts = 1.0 ns and Ff = 0.027 in the same solvent.5 A We first focused on the synthesis of b,b-annulated bacteriochlorins.
synthetic bacteriochlorin bearing a single b,meso-imide (I-BC) Thus, b,b-dibromodihydrodipyrrin-acetal 154 was treated with BF3ꢂ
was prepared and found to exhibit Qy = 818 nm, ts = 1.9 ns and OEt2 in MeCN following the standard procedure for the self-
Ff = 0.036 in toluene.6 The shorter lifetime and diminished condensation leading to synthetic bacteriochlorins42 (Scheme 1).
fluorescence quantum yield with the b,meso-imides were As a result, tetrabromobacteriochlorin BC-1, a potential precursor
surprising and of unknown origin.
of b,b-annulated bacteriochlorins, was obtained in 16% yield.
The annulation of tetrapyrroles with aromatic rings has long Bacteriochlorin BC-1 differs from the tetrabromobacteriochlorin
been pursued as a means to impart a bathochromic shift to the prepared by Oliveira and coworkers54 in lacking a 5-methoxy group.
long-wavelength absorption band.7–12 Issues concerning annu- The Mizoroki–Heck reaction of BC-1 and methyl acrylate afforded
lated tetrapyrroles are ease of synthetic access, solubility, tetrakis(methoxycarbonylvinyl)bacteriochlorin BC-2 in 32% yield.
stability, photophysical characterization, and absorption and However, oxidative cyclization of the adjacent methoxycarbonyl-
fluorescence in the deep NIR region. Annulation with aromatic vinyl groups leading to the corresponding b,b-annulated bacterio-
rings at the meso-positions is a common strategy to synthesize chlorin was unsuccessful, although the corresponding reaction of
b,meso- (and also b,meso,b-) annulated porphyrins. Popular porphyrins affords b,b-benzoporphyrins.58–67
methods for annulation with aromatic hydrocarbons include
Next, phenanthropyrrole 268 was selected as a starting
oxidative coupling13–26 and palladium-catalyzed intramolecular material for the synthesis of b,b-phenanthrobacteriochlorin
direct arylation.27–31 On the other hand, b,b-annulated porphyrins BC-3, similar to the synthesis of phenanthroporphyrins.68,69
have chiefly been prepared by (1) porphyrin formation from Following an established procedure,44 phenanthropyrrole 2
a b,b-annulated pyrrole, or (2) aromatization of a peripheral was converted via the standard intermediates 3–6 to give
non-aromatic ring, which often involves prior cyclization of b dihydrodipyrrin-carboxaldehyde 7 in 5 steps (Scheme 2). The
substituents.7–9,12 Synthesis of the b,b-annulated porphyrins has self-condensation of 7 in neat TFA, conditions used previously
been chiefly pioneered by Lash7,8 whereas b,meso-annulations with for b,b-substituted dihydrodipyrrin-carboxaldehydes,44 gave
porphyrins have been deeply investigated by a number of groups.32,33 bacteriochlorin BC-3 as evidenced by laser-desorption mass
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Scheme 1 Synthesis of tetrakis[(methoxycarbonyl)vinyl]bacteriochlorin
BC-2 via (tetrabromo)bacteriochlorin BC-1.
spectrometry (LD-MS, m/z = 671, [M + H]+) and spectroscopic
analysis (bacteriochlorin-like Qy band absorption and fluores-
cence at 801 and 808 nm in CH2Cl2, respectively, Fig. 1) of a
partially purified sample. However, quality NMR spectra could
not be obtained due to the limited solubility of BC-3 in organic
solvents. Moreover, isolation of BC-3 in pure form was thwarted
by gradual decomposition. Adventitious oxidation in air may
explain the instability of BC-3 because a major contaminant,
which increased gradually upon handling of BC-3, was attributed
to the corresponding oxobacteriochlorin5 given the characteristic
data from LD-MS (m/z = 685, [M + H]+) and absorption spectro-
scopy (Qy band at 758 nm in CH2Cl2).
We envisaged that b,b-annulated bacteriochlorins might be
stabilized by installation of electron-withdrawing groups, which
by adding slight steric protrusions from the plane of the macro-
cycle might also provide improved solubility. However, attempts to
install ester or imide moieties into the macrocycle were unsuccess-
ful. The attempted syntheses are summarized in Chart S1 and
Scheme 2 Attempted synthesis of b,b-phenanthrene-annulated bacterio-
chlorin BC-3.
Synthetic approach toward b,meso-annulated bacteriochlorins
via oxidative annulation
Schemes S1–S10 (ESI†), and the synthesis and characterization As the oxidative annulation of peripheral aromatic hydro-
data are provided for 15 new compounds (see the ESI†). The carbons is well-known for the synthesis of b,meso-annulated
limitations of these routes prompted investigation of methods of porphyrins, di(1-naphthyl)bacteriochlorins BC-4–8 were pre-
oxidative annulation of appended aryl rings.
pared from 870 (Scheme 3) for attempted transformations to
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coupling as described above, halogen-substituted naphthyl
groups are required for such direct arylation processes. Because
avoidance of the steric hindrance of gem-dimethyl groups
still appeared warranted, the Northern–Southern route45 was
adopted to prepare the bacteriochlorin.
The Northern–Southern route relies on the Pd-mediated
coupling of an iodopyrrole and an alkynoic acid; hence, we
planned to install a second halogen (bromide) at the b-pyrrole
position in a subsequent reaction. The reaction of 4-carboethoxy-
2-iodopyrrole45 and 2,2-dimethylpent-4-ynoic acid under slightly
modified conditions afforded known lactone-pyrrole 2045 in
improved yield (80%, Scheme 5).73 Electrophilic bromination of
a pyrrole generally occurs at the a-position. Indeed, as shown in
entry 1 of Table 1, treatment of 20 with N-bromosuccinimide
Fig. 1 Absorption (solid line) and fluorescence (dashed line; using 675 nm
excitation) spectra of partially purified BC-3 in CH2Cl2 at room tempera-
ture, normalized at the Qy band.
give b,meso-phenalenobacteriochlorins. Compound 8 was con- (NBS, 1 equiv.) in DMF gave b-bromopyrrole 21 (minor, 18%) and
verted (via intermediates 9, 10 and 12) into dihydrodipyrrin- a-bromopyrrole 22 (major, 50%). The structure of 21 was con-
acetal 13 following standard procedures42 and use of an improved firmed by X-ray crystallography (Fig. S2 and Table S5, ESI†). Upon
synthesis of Michael acceptor 11.71 The self-condensation of 13 screening of solvents, acetonitrile was found to show better
with BF3ꢂOEt2 in MeCN42 provided 5-unsubstituted bacterio- b-selectivity (24% of 21 versus 17% of 22, entry 2). The use of
chlorin BC-4 in 41% yield, while with TMSOTf in the presence 2,4,4,6-tetrabromo-2,5-cyclohexadienone (TBCO)74 as a brominat-
of 2,6-di-tert-butylpyridine (DTBP) in dichloromethane,43 the reac- ing agent afforded a slightly increased amount of 21 (28%, entry 3).
tion afforded the expected 5-methoxybacteriochlorin BC-5 in 69% Addition of K2CO3 further improved the b-selectivity (31% of 21 and
yield. Furthermore, BC-4 and BC-5 were converted72 into the 19% of 22, entry 4). Remarkably high b-selectivity was observed
corresponding indium(III) and zinc(II) complexes BC-6 and BC-7 (50% of 21 and 6% of 22, entry 5) when the reaction was carried out
in 25% and 80% yield, respectively. Compound 8 was also in the presence of K2CO3 and 2,4,6-tribromophenol (TBP), which is
converted (via intermediates 14 and 16–18) through a recently a coproduct from TBCO. After refinement of the reaction condi-
reported Northern–Southern route45 into dihydrodipyrrin-acetal tions, the yield of 21 reached 61% (entries 6–8). Interestingly,
19 in 5 steps, making use of the known alkynoic acid 15.45
A
bromination by using NBS with TBP and K2CO3 resulted in low
single-crystal X-ray structure of 18 confirmed the integrity of the yields of 21 and 22 (entry 9). To the best of our knowledge, use of
transformations (Fig. S1 and Table S1, ESI†). Compound 19 is a TBCO with TBP/K2CO3 represents a new condition for bromination.
positional isomer of 13. Treatment of 19 with TMSOTf/DTBP gave The origin of the b-selectivity caused by TBP/K2CO3 is unclear at
10-methoxybacteriochlorin BC-8 in 44% yield.
present. Dihydrodipyrrin 23 was prepared from 21 by the estab-
Oxidation of dinaphthylbacteriochlorins BC-4–7 with DDQ lished procedure45 in 65% yield.
or FeCl3, which are common oxidizing agents in the synthesis
Attempted borylation of b-bromodihydrodipyrrin 23 proved
of annulated porphyrins,7,8 did not afford the expected annu- fruitless (Table S6, ESI†). Compound 23 was converted into the
lated bacteriochlorins (Scheme 4A and Table S2, ESI†). The corresponding difluoroboron- and dibutylboron-complexes73 to
In(III) chelate BC-6 and divalent Zn(II) chelate BC-7 were created thwart possible complexation of 23 with palladium upon direct
to probe whether metalation might facilitate the annulation benzo annulation, but this approach also was to no avail
process, either directly or upon subsequent macrocycle oxida- (Table S7, ESI†). Treatment of 23 with Pd(OAc)2 in CH2Cl2 at
tion. It was straightforward to attribute the failure of annula- room temperature75 gave the bis(dipyrrinato)Pd(II) complex 24,
tion of these cases to steric interactions of the naphthalene which did not survive attempted Suzuki coupling with 2-bromo-
units at the 2,12-positions with the gem-dimethyl groups at the phenylboronic acid but instead reverted to 23. Crystals of 24
8,18-positions (Scheme 4B). Thus, similar oxidative annulation suitable for X-ray analysis were obtained from vapor diffusion
of BC-8 and known bacteriochlorin BC-9,45 wherein the gem- of hexanes into CH2Cl2/ethyl acetate solution at room tempera-
dimethyl groups are positioned away from the expected reaction ture. For 24, steric interaction between the pyrrolinyl methyl
sites, was attempted (Scheme 4C). However, neither b-to-meso nor group of one dihydrodipyrrin and the a-hydrogen on the pyrrole
meso-to-b oxidative annulation was observed with bacteriochlorins ring of the other dihydrodipyrrin was observed in the single-
BC-8 and BC-9 (Tables S2 and S3, ESI†); similar attempts to crystal X-ray structural data (Fig. S3 and Table S8, ESI†).
annulate meso-naphthyldihydrodipyrrinatoboron complexes73
also were unsuccessful (Table S4, ESI†).
At a minimum, the limitation in direct coupling highlights
ongoing challenges associated with the synthetic chemistry
of hydrodipyrrins despite extensive work.45,47,76–79 Thus, the
coupling reaction was carried out after the macrocycle forma-
tion. When oxidation of 23 with SeO2 was performed in the
Synthesis of b,meso-annulated bacteriochlorins via
intramolecular direct arylation
Next, we focused on palladium-catalyzed intramolecular arylation absence of pyridine following the general procedure,45
as a means to gain access to b,meso-annulated bacteriochlorins. dihydrodipyrrin-acetal 25 was obtained in less than 27%
Unlike the unsubstituted naphthyl groups for the oxidative yield following acetalization with trimethyl orthoformate.
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Scheme 3 Synthesis of di(1-naphthyl)bacteriochlorins BC-4–8.
Addition of pyridine to SeO2 for the oxidation80,81 improved the reactions at low concentrations (entries 3–6). Scaled-up reac-
yield of 25 to 37%. Table 2 shows refinement of the conditions tions proceeded smoothly (entries 5 and 6). In prior studies
for the self-condensation of 25. 1,2-Dichloroethane was found with diverse dihydrodipyrrin-acetals,43 the use of TMSOTf/DTBP
to be a better solvent than CH2Cl2 or CHCl3 (Table S9, ESI†). tended to afford exclusively the meso-methoxybacteriochlorin.
When the concentration of 25 was 32 mM, the desired meso- Here, both the meso-unsubstituted and meso-methoxy bacterio-
unsubstituted bacteriochlorin BC-10 was obtained in only 11% chlorin were obtained under these conditions regardless of
yield along with undesired meso-methoxybacteriochlorin BC-11 temperature employed.
in 52% yield (entry 1). Lower concentrations (5.0–18 mM,
Suzuki–Miyaura cross-coupling of BC-10 and excess
entries 2–6) tended to give increased BC-10 (up to 32% in entry 4). (8-bromonaphthalen-1-yl)boronic acid (26) provided annula-
Higher temperature was required for completion of the tion precursor BC-12 in 49% yield (Scheme 6). The molar ratio
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than other ligands screened (PCy3ꢂHBF4/K2CO3 and PtBu3ꢂ
HBF4/K2CO3).
In this synthetic strategy, the aromatic rings for annulation
can be altered by changing the coupling partner of the Suzuki–
Miyaura coupling. Thus, we also aimed for b-to-meso annulation
with benzene rings by reaction of BC-10 and (2-bromophenyl)-
boronic acid (27), affording bis(2-bromophenyl)bacteriochlorin
BC-13 in 85% yield. Similar to BC-12, bacteriochlorin BC-13
was isolated as a mixture of two atropisomers. Subsequent
palladium-mediated arylation gave the desired b,meso-
dibenzobacteriochlorin Benz-BC in 80% yield. In this case,
PCy3 gave a better result than that of SPhos. The NMR chemical
shifts of the inner N–H protons of diverse synthetic bacterio-
chlorins are typically observed around ꢁ2 ppm in CDCl3. In
contrast, those of Phen-BC and Benz-BC were found at 1.97 and
2.89 ppm, respectively, suggesting unusual ring currents in the
annulated macrocycles.
Photophysical properties
Fig. 2 shows absorption spectra for the two new annulated
bacteriochlorins along with several reference compounds, all of
which contain 8,18-geminal dimethyl groups to stabilize the
bacteriochlorin. The Qy band for Benz-BC (1033 nm) lies
120 nm to longer wavelength than that for Phen-BC (913) nm,
which in turn is bathochromic from those for diimide I2-BC
(875 nm),5 monoimide I-BC (818 nm),6 diester Es-BC (753 nm)84
and parent H-BC (713 nm).85 The structures of I2-BC and I-BC
are shown in Chart 2 whereas H-BC and Es-BC are shown in
Chart 3. Fig. 2 also shows fluorescence spectra. The Qy fluores-
cence maximum is bathochromically shifted from the Qy
absorption peak by 4–9 nm (65–84 cmꢁ1) for all constructs
except for Phen-BC, which shows a considerably larger Stokes
shift of 20 nm (235 cmꢁ1). The emission band for Phen-BC is
also broader (49 nm, 586 cmꢁ1) than that of the other bacterio-
chlorins (12–34 nm; 230–346 cmꢁ1). The spectral characteristics
are given in Table 3.
The fluorescence quantum yield (Ff) of Benz-BC is estimated
to be very small (B3 ꢀ 10ꢁ5) and that for Phen-BC is somewhat
larger but also low (0.004). Both annulated bacteriochlorins are
less fluorescent than diimide I2-BC (0.027) and monoimide
I-BC (0.064), which themselves are weak emitters. In turn, all of
Scheme 4 Attempted oxidative annulation from (A) BC-4–7 and (C) BC-8
and BC-9. (B) Possible steric hindrance in BC-4–7 during the oxidative
annulation.
of bromoarylboronic acid 26 to bacteriochlorin BC-10 was 10 : 1, these constructs, which employ exo-macrocycle connectivity
giving 5 equiv. per bromo site on the bacteriochlorin. In this between adjacent meso and b-pyrrole positions, have much
reaction, byproducts less polar than the bacteriochlorins, smaller Ff values than benchmark bacteriochlorins Es-BC and
perhaps oligomerized aromatic rings, were observed by TLC H-BC (0.13–0.14; Table 3). The lifetime (ts) of the lowest singlet
analysis and readily removed by column chromatography. excited state (S1) shows a similar pattern (Table 3): Benz-BC
Compound 26, reported previously but without characteriza- (7 ps) o Phen-BC (150 ps) o I2-BC (0.98 ns) o I-BC (1.9 ns) o
tion data,82,83 was fully characterized here. NMR and TLC Es-BC (3.3 ns) o H-BC (3.8 ns). As will be discussed below, the
analysis indicated that BC-12 was a mixture of two atropi- weak fluorescence and short S1 lifetimes for the annulated
somers (B1 : 1 ratio) attributed to hindered rotation of the bacteriochlorins cannot be attributed solely to a lower excited-
bromonaphthyl groups at the 3- and 13-positions. Since both state energy than the benchmarks.
of the atropisomers should afford the same annulated
The S1 lifetimes noted above were measured by transient
compound, BC-12 was used as the mixture in the next step. absorption (TA) spectroscopy, and in some cases also by time-
Palladium-catalyzed intramolecular direct arylation of BC-12 resolved fluorescence. The TA studies also give insight into
afforded target b,meso-bis(phenaleno)bacteriochlorin Phen-BC yields of the S1 - T1 intersystem crossing and S1 - S0 internal
in 37% yield. The ligand SPhos was found to be better conversion pathways that compete with S1 - S0 fluorescence.
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Scheme 5 Synthesis of bromodihydrodipyrrin 23 en route to dibromobacteriochlorin BC-10.
Table 1 Bromination of lactone-pyrrole 20
Entry
Brominating agent (equiv.)
Solvent
Additive (equiv.)
Recovered 20a (%)
21a (%)
22a (%)
c
1b
2e
3f
4f
5f
6f
7f
8f
9f
NBS (1)
NBS (1)
DMF
None
None
None
K2CO3 (1)
TBP (1), K2CO3 (1)
TBP (1), K2CO3 (2)
TBP (2), K2CO3 (3)
TBP (2), K2CO3 (3.6)
TBP (2), K2CO3 (3)
—
—
18d
24d
28
50d
17d
31
19
6
c
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
TBCO (1)
TBCO (1)
TBCO (1)
TBCO (1)
TBCO (1)
TBCO (1.6)
NBS (1)
16
23
31
23
50
25d
26
53d
49
9
5
Trace
49
61e
7
Trace
3
a
1
b
Calculated from H NMR spectroscopic analysis by using 2-nitromesitylene as an internal standard unless otherwise noted. The reaction was
c
d
e
f
carried out at ꢁ20 1C to r.t. for 10 h. Not determined. Isolated yields. The reaction was carried out at 0 1C to r.t. for 1.5 h. The reactions were
carried out at 0 1C for 1 h.
Representative TA data for the two annulated bacteriochlorins there are minor spectral changes that can be attributed to
is given in Fig. 3. The TA difference spectrum for Phen-BC relaxations (vibrational, solvent, etc.) in the excited state. Sub-
(Fig. 3A) at 0.4 ps after a 0.1 ps excitation flash is dominated by sequently, the prominent 930 nm feature and the rest of the
a feature at B930 nm that is comprised of bleaching of the spectrum diminish uniformly to almost DA = 0 by about 2 ns,
ground state S0 - S1 absorption band along with S1 - S0 reflecting decay of the S1 excited state with a time constant of
stimulated emission. During the next tens of picoseconds, 150 ps (Fig. 3B). Close examination of the data to 7 ns indicates
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Table 2 Yields of BC-10 and BC-11 under different reaction conditions
Entrya Conc. (mM) Temp. Time (h) BC-10b (%) BC-11b (%)
1
2
3
32
18
10
5.6
5.6
5.0
r.t.
r.t.
60 1C
Reflux
Reflux
20
20
4
7
8
11
15
29
32
31
29
52
32
37
38
39
36
4
5c
6d
Reflux 11
a
All reactions were carried out with TMSOTf (7.5 equiv.) and DTBP
(30 equiv.) in 1,2-dichloroethane on a 0.12 mmol scale unless otherwise
b
c
noted. Isolated yields. The reaction was carried out on a 0.71 mmol
scale. The reaction was carried out on a 2.00 mmol scale.
d
Fig. 2 Absorption spectra (blue) and fluorescence spectra (red) of the
annulated bacteriochlorins Phen-BC and Benz-BC and known reference
compounds. The absorption spectra are normalized to the Qy maximum.
The emission spectra were obtained using excitation in the Qx band (490,
522, 544, 557, 675, and 715 nm, respectively); for each compound the
same spectrum was obtained using excitation in the Soret band.
asymptotic signal likely represents the lowest triplet excited
state (T1), which forms from S1 with a yield of Fisc = 0.02. Given
this value and Ff = 0.004 for Phen-BC, the yield of S0 - S1
internal conversion is Fic = 0.98.
The analogous TA data for Benz-BC are dominated by a
spectral feature at B1030 nm (Fig. 3C) that disappears with a
time constant of 7 ps (Fig. 3D), which reflects the S1 lifetime. At
long times, the Qy bleaching is less than 1% of the initial value,
reflecting essentially no formation of the T1 excited state. Given
the extremely small Ff value (o0.0001), this means that decay
of the S1 excited state of Benz-BC occurs 99% by S0 - S1
internal conversion (Table 3).
Molecular orbital characteristics and calculated spectral
properties
The energies and electron density distributions calculated by
DFT for the frontier MOs of Es-BC, Phen-BC, and Benz-BC are
shown in Fig. 4. For benchmark Es-BC (Fig. 4A), the two highest
occupied molecular orbitals (HOMO and HOMOꢁ1) and the
two lowest unoccupied molecular orbitals (LUMO and LUMO+1)
have the familiar electron-density distributions of the frontier
Scheme 6 Synthesis of b,meso-annulated bacteriochlorins Phen-BC and
Benz-BC.
that a very small amount of Qy bleaching remains that corre- MOs of Gouterman’s four-orbital model for tetrapyrrole chromo-
sponds to about 2% of the bleaching at early times. This small phores.87–89 In this model, linear combinations of excited-state
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Chart 3 Bacteriochlorin benchmarks.5,6,84–86 The numbers in parenth-
eses identify the data points in the energy-gap plot (vide infra).
from S0 to S1 (Qy) is along the axis that contains the N–H bonds
(the y-axis) and the transition to S2 (Qx) is essentially perpendi-
cular to that. The S1 transition is also along the y-axis for Phen-BC,
but the S2 transition is notably rotated off the x-axis. In compar-
ison, the S2 transition for Benz-BC is approximately x-polarized,
and the S1 transition is mainly y-polarized but rotated somewhat
off the y-axis and directed toward the annulated rings.
Additional results of the TDDFT calculations are given in
Fig. 5–7 for Es-BC, Phen-BC and Benz-BC, respectively. Each
Chart 2 Structures of bacteriochlorins bearing b,b- or b,meso-imides5,6 and figure compares the measured absorption spectrum (brown)
bacteriochlorins with b,b- or b,meso-annulated aromatic hydrocarbons.
with one calculated by TDDFT shown as sticks and an asso-
ciated line plot in which calculated transitions are given
Gaussian skirts (dashed blue). The calculations for all three
configurations derived from one-electron promotions among the molecules reproduce the Qy band (S0 - S1) in the NIR, the Qx
four orbitals dominate the spectral properties of the molecules. band (S0 - S2) in the red-green, and nominal Bx (S0 - S3) and
The HOMO is the a1u(p)-like orbital (using D4h nomenclature), the By (S0 - S4) bands in the blue-violet (Soret) region. The
HOMOꢁ1 is a2u(p)-like and LUMO and LUMO+1 are akin to calculations also reproduce the observed bathochromic shift
egx(p*) and egy(p*). Also shown in Fig. 4 are the next two filled of the Qy band in progressing from Es-BC to Phen-BC to Benz-
orbitals (HOMOꢁ2 and HOMOꢁ3) and next two empty orbitals BC, and for the two annulated molecules, place one additional
(LUMO+2 and LUMO+3). The four frontier MOs (HOMOꢁ1 transition in the Soret region that carries significant oscillator
through LUMO+1) of Phen-BC (Fig. 4B) and Benz-BC (Fig. 4C) strength and is located between the nominal Bx and By bands.
are generally similar to those for Es-BC but show significant This latter prediction can be seen by examining the percent
electron density delocalized onto the annulated rings for certain contribution of the various one-electron promotions to the
orbitals. This delocalization is equally or more pronounced for the transitions (e.g. HOMO - LUMO, HOMOꢁ1 - LUMO+1),
HOMOꢁ3, HOMOꢁ2, LUMO+2 and LUMO+3, which show addi- which are given in Tables S10–S12 (ESI†).
tional characteristics that differ from those from the benchmark.
The energies of the various primary, i.e. (0,0), transitions of
At the lower left of each of the three main panels of Fig. 4 are Es-BC, Phen-BC, and Benz-BC are reasonably well reproduced
the transition dipole moment directions for absorption from by the TDDFT calculations; however, the relative intensities of
the ground state to several singlet excited states. The directions certain transitions differ appreciably from those observed. Of
are obtained from TDDFT calculations. For Es-BC, the transition particular note is the disparity in the observed (relatively weak)
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Table 3 Photophysical dataa
Qy abs Qy em FWHM
Compound (nm) (nm)
abs (cmꢁ1
Db abs-em
c
ꢁ1
)
(cmꢁ1
)
S1 energy (eV) ts (ns) Ff
Fisc
Fic
(kf)ꢁ1 (ns) (kisc
)
(ns) (kic)ꢁ1 (ns)
Benz-BC
Phen-BC
I2-BCd
I-BCe
1033
913
875
818
818
753
713
1042 321
84
235
65
74
292
88
1.20
1.34
1.41
1.51
1.50
1.64
1.73
0.007 2 ꢀ 10ꢁ5 r0.01 0.99 35
—
0.01
0.15
1.2
4.4
2.9
11
17
933
880
823
838
758
717
586
278
346
594
286
230
0.15
0.98
1.9
0.004
0.027
0.064
0.14
0.02 0.98 38
0.18 0.79 36
0.51 0.43 30
0.31 0.55 11
0.56 0.31 25
0.63 0.23 27
7.5
5.4
3.7
5.2
8
BC2f
1.6
3.3
Es-BCg
H-BCh
0.13
78
3.8
0.14
6.5
a
b
c
Data were acquired in toluene at room temperature. Stokes shift between the Qy absorption and fluorescence maxima. Average energy of the
d
e
f
Qy absorption and fluorescence maxima. From ref. 5. From ref. 6. Static absorption and fluorescence data as well as TA spectra are available
(Fig. S4 and S5, ESI). From ref. 84. From ref. 85.
g
h
Fig. 3 Transient absorption data for Phen-BC excited at 715 nm (A and B) and Benz-BC excited at 675 nm (C and D) in toluene obtained using 100 fs
flashes.
versus calculated (relatively strong) features in the Soret region picture of the electron density change involved in an optical
for Phen-BC. We also note that TDDFT does not include transition than can be gleaned from the orbitals themselves.
prediction of vibronic transitions, i.e. (1,0), (2,0), etc., which Ideally, the contributions from various configurations should be
are clearly visible to the blue of the Qy(0,0) and/or Qx(0,0) bands distilled down into one pair of NTOs (filled and empty) for a given
for all three molecules. The band of Benz-BC at B913 nm, transition. However, for all three molecules for which TDDFT
which exhibits an energy consistent with a Qy(1,0) vibronic calculations were performed here (Es-BC, Phen-BC and Benz-BC)
satellite, is unusually intense for a tetrapyrrole-related molecule two pairs of NTO promotions are involved, with relative contribu-
and could easily be mistaken for a primary band. The influence tions indicated by the eigenvalues (from analysis of the transition
of the vibronic features of the annulated bacteriochlorins on density matrices) given in Fig. 5–7. This points to the underlying
the photophysical properties will be addressed further below.
complexity and that both pairs of NTOs should be taken into
Fig. 5–7 also give the natural transition orbitals (NTOs) for account in assessing the net electron-density change accompany-
each calculated transition. The construction of these orbitals ing the transition.
takes into account the contributing configurations to give the
The situation is relatively straightforward for benchmark
net electron-density distributions of the occupied orbital and Es-BC (Fig. 5). The S1 (Qy) transition is comprised mainly
the virtual (empty) orbital as if a single one-electron promotion (B90%) of a promotion between an a1u(p)-like filled NTO to
were involved.90 The NTOs provide a more physically meaningful an egx(p*)-like empty NTO and a small (B10%) contribution
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Fig. 4 Structure, transition-dipole directions for absorption from S0 to excited states that carry significant oscillator strength in the near-UV to NIR
regions (S1, S2, etc.), and frontier MOs for Es-BC (A), Phen-BC (B) and Benz-BC (C).
from a2u(p)-like NTO to an egy(p*)-like NTO. On the other hand, modified or supplemented by effects of electron delocalization
the S4 (By) transition has the reverse contributions. The com- onto the annulated rings.
positions of these two y-polarized transitions are consistent
with expectations based on the four-orbital model. Similarly,
the S2 (Qx) and S3 (Bx) transitions are comprised of mixtures of
promotions between an a1u(p)-like filled NTO to an egy(p*)-like
empty NTO and between an a2u(p)-like NTO to an egx(p*)-like
Discussion
Synthesis
NTO. Examination of the NTOs for the Phen-BC (Fig. 6) and While numerous b,b- and b,meso-annulated porphyrins are
Benz-BC (Fig. 7) show some of the same characteristics, known,7,8 hardly any such bacteriochlorins have heretofore
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Fig. 5 Comparison for Es-BC of the absorption spectra in toluene that are measured (brown line) or calculated via TDDFT (dashed blue lines and colored
sticks) normalized at the highest peak. Calculated spectral features were given 10 nm Gaussian skirts. Shown for the lowest four transitions (S0 to S1–S4)
are the natural transition occupied and virtual orbitals (NTOs) along with the wavelength and oscillator strength of the transition obtained from TDDFT.
Two NTO promotions contribute to each transition, with weights (eigenvalues) indicated by the values that flank the arrows.
Fig. 6 Comparison for Phen-BC of the absorption spectra in toluene that are measured (brown line) or calculated via TDDFT (dashed blue lines and
colored sticks) normalized at the highest peak. Calculated spectral features were given 10 nm Gaussian skirts, except for S0 - S1 and S0 - S2, which were
set at 20 nm. Shown for the lowest five transitions that carry significant oscillator strength (S0 to S1, S2, S4, S6, and S8) are the natural transition occupied
and virtual orbitals (NTOs) along with the wavelength and oscillator strength of the transition obtained from TDDFT. Two NTO promotions contribute to
some transitions, with weights (eigenvalues) indicated by the values that flank the arrows.
been synthesized. The finding that the established routes to dihydrodipyrrins45,47,76–79 also differs considerably from that of
porphyrins7–33 were largely unsuited, at least in our hands, was dipyrrins, where annulation has been achieved via several
a surprise, although we were aware of other reactivity differ- means.91 We explored 7 distinct routes to the annulated
ences of bacteriochlorins versus porphyrins. The chemistry of bacteriochlorins. The routes to b,b-annulated bacteriochlorins
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Fig. 7 Comparison for Benz-BC of the absorption spectra in toluene that are measured (brown line) or calculated via TDDFT (dashed blue lines and
colored sticks) normalized at the highest peak. Calculated spectral features were given 10 nm Gaussian skirts. Shown for the lowest five transitions that
carry significant oscillator strength (S0 to S1, S2, S4, S5, and S7) are the natural transition occupied and virtual orbitals (NTOs) along with the wavelength and
oscillator strength of the transition obtained from TDDFT. Two NTO promotions contribute to some transitions, with weights (eigenvalues) indicated by
the values that flank the arrows.
include cyclization of adjacent appended methyl acrylate units (BC-12,13) resembling BC-8,9 in substituent patterns. A present
(Scheme 1), synthesis beginning with annulated pyrroles limitation of the Northern–Southern route is the requirement
(i.e., isoindoles, Scheme 2) and use of a non-conjugated for a Pd-catalyzed coupling of an iodopyrrole and an alkynoic
b,b-annulated pyrrole (i.e., a tetrahydroisoindole). The routes acid to construct the pyrrole-lactone leading to the dihydro-
to b,meso-annulated bacteriochlorins include oxidative cycliza- dipyrrin. The first Pd-coupling in the annulation in Scheme 6
tion of an appended b- or meso-substituent on the bacterio- uses a 5-fold excess of the bromoarylboronic acid versus the
chlorin (Scheme 3), cyclization of an appended b- or meso- bromo sites on the bacteriochlorin with acceptable yields
substituent on a dihydrodipyrrin (Scheme 5), and ultimately the (49%, 85%) on the basis of the limiting bacteriochlorin. Such
successive Pd-catalyzed arylation shown in Scheme 6. Extensive an excess of a b-bromoaryl-substituted iodopyrrole would be far
information, procedures and accompanying literature concerning less practical at the start of a Northern–Southern synthesis. In
attempted routes are provided in the ESI† (Schemes S1–S10). principle, the same substitution pattern could be created via
Some subtleties of the syntheses warrant comment. In particular, the Eastern–Western route, but additional steps would be
the question may arise why two successive Pd-couplings are required to access the requisite 2-formyl-3-ethoxycarbonyl-4-
employed to install the b,meso-annulation as opposed to pre- (bromonaphthyl)pyrrole. In summary, the studies here have
installation of, say, a bromoaryl group in the beginning of the achieved the synthesis of b,meso-annulated bacteriochlorins yet
synthesis at the pyrrole stage.
at the same time have highlighted significant unresolved
b-Aryl groups are carried through the synthesis very success- limitations in bacteriochlorin chemistry.
fully, as exemplified by the naphthyl group in Scheme 3.
Scheme 3 shows two routes to dinaphthyl-diester bacteriochlor-
Photophysical features
ins, which differ (1) in the positions of the substituents relative The trends in the absorption spectral positions among the two
to the position of the gem-dimethyl groups and the unfettered new annulated bacteriochlorins and several benchmark bacterio-
meso-position, (2) the means of creating the dihydrodipyrrin chlorins are reproduced by the TDDFT calculations. Phen-BC
precursors, and (3) the position of the gem-dimethyl groups basically has a naphthalene unit fused with the macrocycle, and
relative to the pyrrole. The bacteriochlorins with the naphthyl the Qy band is at 933 nm. Benz-BC basically has a benzene unit
groups at the 2,12-positions (BC-4–7) were constructed with fused with the macrocycle, and the Qy band is at 1033 nm. The
the longstanding ‘‘Eastern–Western’’ route,42 whereas those two annulated bacteriochlorins thus absorb at the low-energy
at the preferred 3,13-positions (BC-8,9) were constructed with edge of the first NIR region (B700–1000 nm) and the high-
the more recent ‘‘Northern–Southern’’ route.45 The successful energy edge of the second NIR region (1000–1700 nm),92,93
annulations (Scheme 6) were achieved with bacteriochlorins respectively. If the excited-state energies and spectral positions
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simply followed the particle-in-a-box model, one would have
expected the larger Phen-BC to absorb at longer wavelength (lower
energy) than Benz-BC because the S1–S0 energy gap is inversely
proportional to the square of the box size. Examination of the
energy gaps between the relevant orbitals involved in the Qy
transition (HOMO - LUMO and HOMOꢁ1 - LUMO+1) suggests
that the Qy band of Phen-BC would be at lower energy than that for
Benz-BC. However, the measured spectra show the opposite order
and this is supported by the TDDFT calculations.
A factor that the calculations must effectively take into
account, and which is also integral to the four-orbital model,
is the configuration-interaction (CI) energy. This parameter is
sensitive to the electron density distribution in the macrocycle.
We have shown this from analysis of tetrapyrrole-based
panchromatic absorbers, where the CI energy, and not simply
the MO energy gaps, is a dominant factor in determining the
spectral properties.94 Thus, we speculate that annulation of the
bacteriochlorin perturbs the CI energy sufficiently to comple-
ment effects on the MO energies in governing the observed
spectra, and in particular the position of the Qy band for the
annulated bacteriochlorins. Spectral simulations using the
four-orbital model support this conjecture.
The question arose as to whether the observed short excited-
state (S1) lifetime for Phen-BC and Benz-BC was simply a matter
of the long-wavelength of absorption, and commensurate low
energy level of the excited-state in each case. Fig. 8 plots the rate
constant for S1 - S0 internal conversion obtained from analysis of
the TA data for the annulated bacteriochlorins studied here, the
Fig. 8 Energy-gap law plot showing the natural logarithm of the rate
constant for S1 - S0 internal conversion versus the S1 energy for various
types of bacteriochlorins. The blue line is a fit for bacteriochlorins bearing
one or two imide moieties or annulated rings spanning adjacent meso and
b-pyrrole positions. The red line is a fit for the 10 benchmark bacterio-
chlorins from Chart 3 (and one b,b-annulated bis(imide)bacteriochlorin,
from Chart 2). The latter line projects an S1 lifetime of 400 ps for a
‘‘standard’’ bacteriochlorin as opposed to the observed lifetime of 7 ps
for Benz-BC.
benchmark bacteriochlorins noted above (Table 3) and a host of vibronic (Herzberg–Teller) coupling or via improved Franck–
other tetrapyrroles (Chart 3)5,6,84–86 studied previously. The red line Condon factors through better energy matching due to the
in Fig. 8 suggests that if Phen-BC and Benz-BC followed the contribution of multiple vibrational levels.
normal energy-gap law for bacteriochlorins, the molecules would
The reduced excited-state lifetimes caused by b-meso annu-
have much longer S1 lifetimes than is observed. For example, one lation of bacteriochlorins does not occur upon addition of the
would have expected an S1 lifetime of about 400 ps for Benz-BC, 5-membered isocyclic ring that spans the 13–15 (meso–b) posi-
rather than the observed 7 ps. The blue line in Fig. 8 is a fit to the tions of native photosynthetic pigments such as bacteriochloro-
data for Benz-BC, Phen-BC, diimide I2-BC, and monoimides I-BC phyll a. Unlike the annulated bacteriochlorins studied here, the
and MeO-I-BC, all of which have added connectivity (via fused native 5-membered ring does not provide a conjugation path
conjugated motifs; see Chart 2) between nearby meso and b-pyrrole from meso to b-pyrrole positions across the isocyclic unit.
positions.5,6 The correlation suggests that the annulated b-meso Accordingly, the isocyclic ring serves as an auxochrome yet
motif must introduce some factor to enhance nonradiative inter- does not impart enhanced internal conversion.
nal conversion in these constructs over normal bacteriochlorins.
The development of chromophores with fluorescence at wave-
lengths 41000 nm is an active area of research.95
Conclusions
A potential source of enhanced internal conversion in the
annulated (and imide) bacteriochlorins that employ b-meso Numerous routes established for annulation of porphyrins
connectivity can be gleaned from the optical spectra in Fig. 3. were not productive on application to bacteriochlorins. Instead,
In particular, note the extraordinarily intense and broad Qy(1,0) successive Suzuki–Miyaura coupling enabled construction of
band at B913 nm for Benz-BC. The breadth suggests that there the b,meso-annulated bacteriochlorins. The photophysical stu-
are likely a number of unresolved vibronic components, and dies of the b,meso-annulated bacteriochlorins indicate that this
the intensity indicates that there are modes coupling Qy with structural motif provides an effective means of bathochromi-
higher energy states, such as By or perhaps Qx. For the imides, cally shifting the lowest energy electronic transition far into the
particularly I2-BC, the peak intensity in the nominal Qy(1,0) NIR region of the spectrum. The magnitude of the shift is
region for that molecule (780–820 nm) is normal, but there is a significantly larger than has been achieved via addition of
ladder of small overlapping features that extend up toward the typical substituents (e.g., acetyl, formyl, cyano) at the b and/or
Qy(0,0) band. Thus, internal conversion may be enhanced in meso positions and also larger than that achieved to date via
constructs that utilize conjugated b-meso connectivity either via b,b-annulation. A striking photophysical characteristic of the
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b,meso-annulated bacteriochlorins is an extremely short lowest- the mixture was extracted with CH2Cl2. The combined organic
excited singlet-state lifetime and accompanying vanishingly small extract was washed with saturated aqueous Na2CO3 solution,
fluorescence quantum yield. The photophysical studies trace dried (Na2SO4) and concentrated to afford a pale brown solid
1
these features to extremely rapid internal conversion to the (1.14 g, 83%): mp 184–186 1C; H NMR d 1.73 (s, 9H), 7.45–7.62
ground electronic state. The origin of the rapid internal conversion (m, 4H), 8.16–8.19 (m, 1H), 8.43–8.47 (m, 2H), 9.49–9.53 (m, 1H),
is not certain; however, one unusual spectral feature of the two 10.32 (s, 1H), 10.35 (s, 1H); 13C NMR d 28.4, 83.5, 122.7, 123.0,
b,meso-annulated bacteriochlorins studied herein is exceptionally 123.7, 124.2, 125.7, 125.9, 126.30, 126.33, 126.5, 127.1, 127.2,
strong vibronic activity. The strong vibronic activity accompanied 127.3, 127.4, 127.9, 129.7, 130.8, 159.4, 179.6; ESI-MS obsd
by the relatively low energy of the electronic transition possibly 346.1431, calcd 346.1438 [(M + H)+, M = C22H19NO3].
open more decay channels to the ground electronic state. The new
tert-Butyl 3-(2-nitroethyl)-2H-dibenzo[e,g]isoindole-1-
molecules seem well suited for applications such as photoacoustic carboxylate (4). Following a general procedure,43 a stirred
imaging96 or photothermal therapy,97 where an exquisitely short mixture of 3 (1.04 g, 3.00 mmol), potassium acetate (0.236 g,
excited-state lifetime and the absence of fluorescence are desired, 2.40 mmol), and methylamine hydrochloride (0.162 g,
but not as NIR fluorescent markers or long-lived energy/electron 2.40 mmol) in absolute ethanol (15 mL) was treated with
donors in solar energy conversion.
nitromethane (0.81 mL, 15 mmol). The reaction mixture was
heated at 70 1C for 3 h, and then CH2Cl2 and water were added.
The organic layer was dried (Na2SO4) and concentrated. The
resulting yellow solid was dried under high vacuum and used
directly in the next step. The crude solid material was dissolved
in CHCl3/2-propanol (3 : 1, 120 mL). Silica (6 g) and NaBH4
(0.227 g, 6.00 mmol) were added, and the mixture was stirred at
room temperature under argon for 30 min. The reaction
mixture was filtered, and the filtrate was concentrated. The
resulting crude material was dissolved in CH2Cl2. The organic
solution was washed (water, brine), dried (Na2SO4), and
concentrated. Column chromatography [silica, hexanes/ethyl
acetate (10 : 1)] afforded a white solid (0.562 g, 48%): mp 168–
170 1C; 1H NMR (400 MHz) d 1.73 (s, 9H), 3.79 (t, J = 7.2 Hz, 2H),
4.72 (t, J = 7.2 Hz, 2H), 7.47–7.63 (m, 4H), 7.82–7.85 (m, 1H),
8.49–8.54 (m, 2H), 9.73–9.75 (m, 1H), 9.79 (br s, 1H); 13C NMR
(100 MHz) d 27.1, 28.6, 72.9, 82.1, 115.8, 118.3, 122.4, 122.7,
123.0, 123.9, 124.7, 125.5, 126.9, 127.0, 127.3, 127.4, 127.8,
128.4, 129.5, 130.1, 160.0; ESI-MS obsd 391.1653, calcd
391.1652 [(M + H)+, M = C23H22N2O4].
Experimental section
General methods
1H NMR and 13C NMR spectra were collected at room temperature
in CDCl3 unless noted otherwise. Silica (40 mm average particle
size) was used for column chromatography. THF was freshly
distilled from sodium/benzophenone ketyl. All other solvents
(anhydrous or reagent-grade) were employed as received from
commercial suppliers. Electrospray ionization mass spectrometry
(ESI-MS) data generally enable accurate mass measurements, were
obtained in the positive-ion mode (unless noted otherwise), and
are reported for the molecular ion or protonated molecular ion.
Laser-desorption ionization mass spectrometry without a matrix
(LD-MS) or with a matrix (MALDI-MS) affords low resolution data,
was carried out in the positive-ion mode with the matrix POPOP,
and provided quick screening of reactions. Deactivated silica for
purification of acid-labile compounds was prepared by treatment
of silica with 5% triethylamine in hexanes (1 g/2 mL) and washing
with CH2Cl2 followed by hexanes. Commercial compounds were
used as received unless noted otherwise. NBS was recrystallized
from water. The known compounds 1,54 2,68 8,70 11,71 15,45 20,45,73
and BC-945 were prepared as described in the literature.
tert-Butyl 3-(3,3-dimethyl-2-nitro-5-oxohexyl)-2H-dibenzo[e,g]-
isoindole-1-carboxylate (5). Following an established procedure44
with modification, samples of 4 (156 mg, 0.400 mmol), mesityl
oxide (196 mg, 2.00 mmol), TBAF (1.0 M in THF, 0.6 mL,
0.6 mmol), and 4 Å molecular sieves (400 mg) were stirred together
in DMF (8 mL) for 4 h. The mixture was then diluted with water
(30 mL) and extracted with ethyl acetate (2 ꢀ 30 mL). The
Synthesis of bacteriochlorin precursors
tert-Butyl 3-formyl-2H-dibenzo[e,g]isoindole-1-carboxylate (3). combined organic extract was washed [HCl solution (2 M,
Following a reported procedure,44 DMF (1.0 mL, 13 mmol) was 10 mL), saturated aqueous NaHCO3 solution and brine], dried
cooled in an ice bath, and POCl3 (1.2 mL, 13 mmol) was added (Na2SO4) and concentrated. The crude solid was triturated with
dropwise. An exothermic reaction occurred with the formation of diethyl ether (20 mL) followed by filtration to afford a white solid
the Vilsmeier complex. The ice bath was removed, and the (0.132 g, 68%): mp 150–152 1C; 1H NMR (400 MHz) d 1.22 (s, 3H),
mixture was stirred at room temperature for 15 min. The mixture 1.45 (s, 3H), 1.71 (s, 9H), 2.15 (s, 3H), 2.57 (ABq, DdAB = 0.16, J =
was treated with CH2Cl2 (6 mL), and the reaction temperature was 18.0 Hz, 2H), 3.71 (d, J = 6.0 Hz, 1H), 3.73 (s, 1H), 5.45–5.49 (m, 1H),
lowered to 0–2 1C with the aid of a salt-ice bath. A solution of 2 7.51–7.62 (m, 4H), 7.93–7.95 (m, 1H), 8.53–8.59 (m, 2H), 9.67 (br s,
(1.27 g, 4.00 mmol) in CH2Cl2 (20 mL) was added to the stirred 1H), 9.73–9.76 (m, 1H); 13C NMR (100 MHz) d 23.9, 24.6, 28.0, 28.6,
mixture over a period of 15 min. After the addition was completed, 31.7, 37.2, 51.2, 81.9, 92.5, 116.1, 122.8, 122.96, 123.01, 124.0, 125.5,
the ice bath was replaced with an oil bath and the mixture was 126.8, 127.0, 127.1, 127.6, 128.1, 128.5, 129.7, 130.2, 159.9, 206.5;
refluxed for 15 min. The mixture was then cooled to 25–30 1C, a ESI-MS obsd 489.2387, calcd 489.2384 [(M + H)+, M = C29H32N2O5].
solution of sodium acetate (10 g) in water (40 mL) was added
dropwise, and the reaction mixture was refluxed for an additional methyl)-2H-dibenzo[e,g]isoindole-1-carboxylate (6). Following
15 min. The two layers were separated, and the aqueous portion of general procedure,43 in a first flask a solution of 5 (120 mg,
tert-Butyl (Z)-3-((3,3,5-trimethyl-3,4-dihydro-2H-pyrrol-2-ylidene)-
a
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0.246 mmol) in freshly distilled THF (5.0 mL) and anhydrous 4.99; N, 4.81. ESI-MS obsd 316.0937, calcd 316.0944 [(M + Na)+,
MeOH (100 mL) at 0 1C was treated with NaOMe (66 mg, 1.2 mmol). M = C18H15NO3].
The mixture was stirred and degassed by bubbling argon through
4-(Ethoxycarbonyl)-3-(naphthalen-1-yl)-2-(2-nitroethyl)pyrrole
the solution for 45 min. In a second flask purged with argon, TiCl3 (10). Following a general procedure,43 a stirred mixture of 9
(1.54 mL, 20% w/v in 2 N HCl solution, 2.0 mmol), 8 mL of THF, (12.6 g, 43.0 mmol), potassium acetate (3.3 g, 34 mmol), and
and NH4OAc (1.04 g, 13.5 mmol) were combined under argon, and methylamine hydrochloride (2.3 g, 34 mmol) in absolute etha-
the mixture was degassed by bubbling argon for 45 min. Then the nol (16 mL) was treated with nitromethane (6.0 mL, 0.11 mol).
first flask mixture was transferred via cannula to the buffered TiCl3 The mixture was stirred for 2 h, whereupon water was added.
mixture. The resulting mixture was stirred at room temperature for The reaction mixture was filtered, and the filtered material was
16 h under argon. Saturated aqueous NaHCO3 was added, and the washed with water and a small amount of cold water/ethanol
resulting mixture was then extracted with ethyl acetate. The organic (1 : 1). The filtered material was dried under high vacuum to
extract was washed with brine, dried (Na2SO4), and concentrated. afford a yellow solid, which was used directly in the next step.
Column chromatography [silica, hexanes/ethyl acetate (10 : 1)] The crude solid material was dissolved in CHCl3/2-propanol
1
afforded a yellow solid (45 mg, 41%): mp 164–166 1C; H NMR d (3 : 1, 390 mL). Silica (34 g) and NaBH4 (3.0 g, 78 mmol) were
1.37 (s, 6H), 1.74 (s, 9H), 2.34 (s, 3H), 2.64 (s, 2H), 6.68 (s, 1H), 7.47– added, and the mixture was stirred at room temperature under
7.66 (m, 4H), 8.40 (d, J = 7.5 Hz, 1H), 8.54–8.60 (m, 2H), 9.93 (dd, J = argon for 2 h. The reaction mixture was filtered, and the filtrate
8.4 Hz and 1.2 Hz, 1H), 12.82 (br s, 1H); 13C NMR d 21.0, 28.8, 29.3, was washed with saturated aqueous NaHCO3. The organic layer
42.0, 54.0, 80.7, 103.1, 116.3, 118.5, 122.9, 123.77, 123.83, 124.9, was separated and concentrated to afford a pale brown solid
126.4, 126.8, 127.0, 127.7, 128.2, 128.5, 129.4, 130.0, 130.2, 160.5, (10 g, 69%): mp 48–52 1C; 1H NMR (300 MHz, CDCl3) d 0.73
165.9, 180.8; ESI-MS obsd 439.2376, calcd 439.2380 [(M + H)+, (t, J = 7.0 Hz, 3H), 2.98 (dt, J = 17.6, 6.5 Hz, 2H), 3.88 (dq, J = 18.7,
M = C29H30N2O2].
3.6 Hz, 2H), 4.20 (t, J = 6.5 Hz, 2H), 7.28–7.60 (m, 5H), 7.85 (t, J =
tert-Butyl (Z)-3-((5-formyl-3,3-dimethyl-3,4-dihydro-2H-pyrrol-2- 7.8 Hz, 3H), 8.96 (br s, 1H); 13C NMR (75 MHz, CDCl3) d 13.8,
ylidene)methyl)-2H-dibenzo[e,g]isoindole-1-carboxylate (7). 23.7, 59.6, 75.0, 116.8, 122.0, 124.2, 125.5, 125.9, 126.0, 126.1,
Following a general procedure,44 a solution of 6 (45 mg, 126.2, 127.88, 127.91, 128.4, 132.9, 133.7, 164.9; anal. calcd for
0.10 mmol) in 1,4-dioxane (2.0 mL) was treated with SeO2 (33 mg, C19H18N2O4: C, 67.44; H, 5.36; N, 8.28. Found: C, 66.95; H, 5.27;
0.30 mmol) under argon. The mixture was stirred for 1.5 h at N, 7.86. ESI-MS obsd 361.1267, calcd 361.1154 [(M + Na)+,
room temperature. The reaction mixture was treated with satu- M = C19H18N2O4].
rated aqueous NaHCO3 solution and extracted with ethyl acetate.
6-(4-(Ethoxycarbonyl)-3-(naphthalen-1-yl)pyrrol-2-yl)-1,1-
The organic extract was washed with brine, dried (Na2SO4), and dimethoxy-4,4-dimethyl-5-nitrohexan-2-one (12). Following
concentrated. Column chromatography [silica, hexanes/ethyl a general procedure,43 a mixture of 10 (9.3 g, 28 mmol) and
acetate (10 : 1)] afforded a dark orange solid (25 mg, 55%): mp 11 (8.8 g, 55 mmol) was treated with DBU (12.3 mL,
1
216–218 1C; H NMR (400 MHz) d 1.42 (s, 6H), 1.74 (s, 9H), 2.85 82.5 mmol). The reaction mixture was stirred at room
(s, 2H), 7.13 (s, 1H), 7.54–7.67 (m, 4H), 8.36 (d, J = 8.0 Hz, 1H), temperature for 16 h under argon. A saturated solution of
8.58–8.62 (m, 2H), 9.85–9.87 (m, 1H), 10.08 (s, 1H), 12.38 (br s, cold aqueous NH4Cl was added. The mixture was extracted
1H); 13C NMR d 28.6, 29.3, 41.6, 46.2, 81.6, 112.4, 119.4, 121.2, with ethyl acetate, and the organic layer was washed with brine,
122.9, 123.9, 124.0, 124.7, 125.8, 126.2, 126.7, 127.0, 127.2, 127.5, dried (Na2SO4), and concentrated. Column chromatography [silica,
128.3, 128.4, 130.1, 130.4, 164.3, 171.5, 189.6; ESI-MS obsd CH2Cl2/ethyl acetate (9 : 1)] afforded an amorphous brown solid
453.2169 calcd 453.2173 [(M + H)+, M = C29H28N2O3].
(7.9 g, 47%): ESI-MS obsd 497.2277, calcd 497.2282 [(M + H)+, M =
4-(Ethoxycarbonyl)-2-formyl-3-(naphthalen-1-yl)pyrrole (9). A C27H32N2O7]; NMR data were collected from pure fractions of the
solution of 8 (16.9 g, 63.7 mmol) in DMF (300 mL) was treated two existing rotamers. Rotamer A: 1H NMR (300 MHz, CDCl3) d 0.78
2
with POCl3 (7.1 mL, 76 mmol) at 0 1C. The resulting mixture (s, 3H), 0.82 (s, 3H), 1.24 (t, J = 7.2 Hz, 3H), 2.28, 2.45 (AB, J =
3
2
was stirred under argon at 0 1C for 1 h, and then 16 h at room 18.2 Hz, 2H), 2.56 (ABX, JBX = 2.2 Hz, JAB = 15.1 Hz, 1H), 3.05
temperature. The reaction mixture was treated with a mixture (ABX, 3JAX = 11.6 Hz, 2JAB = 15.1 Hz, 1H), 3.28 (s, 3H) 3.32 (s, 3H),
3
3
of saturated aqueous sodium acetate/CH2Cl2 [400 mL, 3 : 1 (v/v)] 3.82–4.01 (m, 2H), 4.22 (s, 1H), 4.92 (ABX, JBX = 2.2 Hz, JAX
=
and stirred for 1 h. The water phase was separated and 11.6 Hz, 1H), 7.30–7.65 (m, 5H), 7.75–7.91 (m, 3H), 9.29 (br s,
extracted with CH2Cl2. The combined organic phase was 1H); 13C NMR (75 MHz, CDCl3) d 13.9, 23.6, 23.9, 25.5, 36.4,
washed with a saturated LiCl solution, dried (Na2SO4) and 44.7, 55.0, 55.1, 59.5, 77.1, 95.1, 104.4, 116.3, 121.8, 124.4,
concentrated. The resulting solid was chromatographed [silica, 125.5, 125.8, 126.1, 126.4, 127.7, 128.3, 128.5, 133.0, 133.5,
hexanes/ethyl acetate (2 : 1)] to give a light-brown solid (12.9 g, 133.7, 165.1, 203.4. Rotamer B: 1H NMR (300 MHz, CDCl3)
1
69%): mp 147–150 1C; H NMR (300 MHz, CDCl3) d 0.73 (t, J = d 0.74 (s, 3H), 0.76 (s, 3H), 1.24 (t, J = 7.2 Hz, 3H), 2.20, 2.43 (AB,
3
2
7.2 Hz, 3H), 3.82–4.04 (m, 2H), 7.32–7.58 (m, 4H), 7.65 (d, J = 2J = 18.2 Hz, 2H), 2.80 (ABX, JBX = 2.2 Hz, JAB = 15.1 Hz, 1H),
7.4 Hz, 1H), 7.80 (s, 1H), 7.90 (t, J = 6.9 Hz, 2H), 9.18 (d, J = 1.1 2.99 (ABX, JAX = 11.6 Hz, 2JAB = 15.1 Hz, 1H), 3.29 (s, 3H), 3.30
3
Hz, 1H), 11.10 (br s, 1H); 13C NMR (75 MHz, CDCl3) d 13.7, 60.2, (s, 3H), 3.77–4.04 (m, 2H), 4.16 (s, 1H), 4.55 (ABX, 3JBX = 2.2 Hz,
118.7, 125.1, 126.1, 126.3, 126.5, 128.4, 128.7, 128.9, 129.9, 3JAX = 11.6 Hz, 1H), 7.28–7.67 (m, 5H), 7.77–7.90 (m, 3H), 9.34
130.9, 131.7, 133.5, 135.9, 163.8, 181.1; anal. calcd for (br s, 1H); 13C NMR (75 MHz, CDCl3) d 13.8, 23.5, 23.7, 25.3,
C
18H15NO3: C, 73.71; H, 5.15; N, 4.78. Found: C, 73.41; H, 36.4, 44.2, 55.07, 55.14, 59.6, 77.0, 94.8, 104.4, 116.5, 121.9,
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124.4, 125.4, 125.8, 126.1, 126.4, 127.70, 127.71, 128.2, 133.1, layer was separated, dried (Na2SO4) and filtered. The filtrate was
133.7, 133.9, 165.1, 203.3. concentrated and chromatographed [silica, hexanes/ethyl acetate
8-(Ethoxycarbonyl)-2,3-dihydro-1-(1,1-dimethoxymethyl)- (3 : 1)] to afford a pale yellow crystalline solid (1.83 g, 76%): mp
7-(naphthalen-1-yl)-3,3-dimethyldipyrrin (13). Following a gen- 202–203 1C; 1H NMR (400 MHz, CDCl3) d 0.69 (t, J = 7.2 Hz, 3H),
eral procedure,43 in a first flask, a solution of 12 (6.65 g, 1.07 (s, 3H), 1.14 (s, 3H), 2.50 (dd, J = 2.0, 16.0 Hz, 1H), 2.40 (dd, J =
13.4 mmol) in freshly distilled THF (30 mL) and anhydrous 2.0, 16.0 Hz, 1H), 3.78–3.98 (m, 2H), 5.93 (dd, J = 2.0, 2.0 Hz, 1H),
MeOH (1.0 mL) at 0 1C was treated with NaOMe (2.2 g, 7.30–7.40 (m, 2H), 7.40–7.52 (m, 2H), 7.60 (d, J = 3.2 Hz, 1H), 7.63–
40 mmol). The mixture was stirred and degassed by bubbling 7.67 (m, 1H), 7.79–7.87 (m, 2H), 8.50 (br s, 1H); 13C NMR
argon through the solution for 45 min. In a second flask purged (100 MHz, CDCl3) d 13.6, 24.6, 24.9, 39.6, 39.9, 59.5, 97.5, 116.5,
with argon, TiCl3 (50 mL, 20% wt/v in 3% HCl solution, 122.2, 125.07, 125.14, 125.5, 125.6, 126.2, 127.5, 128.17, 128.21,
80 mmol), 125 mL THF, NH4OAc (50 g, 650 mmol) and water 132.8, 133.3, 133.5, 147.6, 165.0, 179.6; ESI-MS obsd 390.1701,
(20 mL) were combined under argon, and the mixture was calcd 390.1700 [(M + H)+, M = C24H23NO4].
degassed by bubbling argon for 45 min. Then, the first flask
mixture was transferred via cannula to the buffered TiCl3 furan-2(3H)-ylidene)methyl)-3-(naphthalen-1-yl)pyrrole
4-(Ethoxycarbonyl)-(E)-2-(1-(4,4-dimethyl-5-methylenedihydro-
(17).
mixture. The resulting mixture was stirred at room temperature Following a standard procedure,45 a solution of TiCp2Cl2
for 16 h under argon. The reaction mixture was then poured (3.045 g, 12.37 mmol) in toluene (33 mL) was treated dropwise
over a pad of Celite and eluted with ethyl acetate. The eluant with MeLi solution (17 mL in Et2O, 27 mmol, 1.6 M) at 0 1C
was washed with a saturated aqueous solution of NaHCO3. The under an argon atmosphere. After 1 h at 0 1C, saturated
organic layer was dried (Na2SO4) and concentrated. Column aqueous NH4Cl solution was added. The organic layer was
chromatography [silica, CH2Cl2/ethyl acetate (98 : 2)] afforded washed with water and brine, dried (Na2SO4), and filtered.
1
an amorphous brown solid (3.34 g, 56%): H NMR (400 MHz, The filtrate (now B30–40 mL owing to solvent losses in hand-
CDCl3) d 0.65 (t, J = 7.2 Hz, 3H), 0.98 (s, 3H), 1.08 (s, 3H), 2.59 ling) contained the Petasis reagent. The filtrate in its entirety
(s, 2H), 3.48 (s, 6H), 3.75–3.96 (m, 2H), 5.07 (s, 1H), 5.52 (s, 1H), was treated with 16 (1.02 g, 2.61 mmol) and additional TiCp2Cl2
7.30–7.47 (m, 3H), 7.48–7.55 (m, 1H), 7.67 (d, J = 2.9 Hz, 1H), (39 mg). The solution was heated to 80 1C in the dark for 6 h
7.70–7.76 (m, 1H), 7.81–7.90 (m, 2 H), 11.31 (br s, 1H); 13C NMR under argon. Afterward, the resulting mixture was allowed to
(100 MHz, CDCl3) d 13.7, 29.1, 29.2, 40.5, 48.6, 54.9, 59.4, 102.8, cool to room temperature whereupon MeOH (3.1 mL), NaHCO3
105.6, 116.6, 125.3, 125.5, 125.6, 127.1, 127.4, 128.1, 128.5, (130 mg), and water (31 mL) were added. The mixture was kept
130.5, 133.4, 133.7, 134.1, 161.6, 165.2, 175.7; ESI-MS obsd at 40 1C for 12 h with stirring and then filtered through Celite.
447.2258, calcd 447.2278 [(M + H)+, M = C27H30N2O4].
The filtrate was concentrated under reduced pressure. Column
4-(Ethoxycarbonyl)-2-iodo-3-(naphthalen-1-yl)pyrrole (14). chromatography [silica, hexanes/ethyl acetate (3 : 1)] afforded a
Following a literature procedure45 with slight modification, a yellow crystalline solid (733 mg, 72%): mp 158–159 1C; 1H NMR
stirred solution of 15 (3.98 g, 15.0 mmol) in anhydrous DMF (400 MHz, CDCl3) d 0.68 (t, J = 7.2 Hz, 3H), 1.07 (s, 3H), 1.13
(75.0 mL) was treated with NIS (3.38 g, 15.0 mmol) in batches at (s, 3H), 2.26–2.50 (m, 2H), 3.76–3.98 (m, 3H), 4.24 (d, J = 2.4 Hz,
0 1C. After 2 h, the reaction mixture was diluted with ethyl acetate 1H), 5.64 (s, 1H), 7.28–7.38 (m, 2H), 7.38–7.50 (m, 2H), 7.54
(150 mL) and washed with water and brine. The organic layer was (d, J = 3.6 Hz, 1H), 7.67 (d, J = 8.4 Hz, 1H), 7.74–7.84 (m, 2H),
dried (Na2SO4) and filtered. The filtrate was concentrated and 8.45 (br s, 1H); 13C NMR (100 MHz, CDCl3) d 13.6, 27.3, 27.7,
chromatographed [silica, hexanes/ethyl acetate (4 : 1)] to give a 40.2, 41.8, 59.4, 80.3, 92.3, 116.6, 120.6, 124.0, 125.2, 125.3,
pale yellow crystalline solid (2.85 g, 48%): mp 199–200 1C; 125.4, 126.4, 127.2, 127.4, 128.0, 128.3, 133.3, 133.51, 133.54,
1H NMR (400 MHz, CDCl3) d 0.67 (t, J = 7.6 Hz, 3H), 3.80–3.98 153.3, 165.1, 168.9; ESI-MS obsd 388.1913, calcd 388.1907
(m, 2H), 7.32–7.40 (m, 2H), 7.42–7.48 (m, 1H), 7.48–7.54 (m, 1H), ((M + H)+, M = C25H25NO3).
7.57 (d, J = 8.4 Hz, 1H), 7.65 (d, J = 2.8 Hz, 1H), 7.82–7.94 (m, 2H),
8-(Ethoxycarbonyl)-1-methyl-2,2-dimethyl-7-(naphthalen-1-yl)-
8.78 (br s, 1H); 13C NMR (100 MHz, CDCl3) d 13.3, 59.6, 117.2, 2,3-dihydrodipyrrin (18). Following a literature procedure45 with
125.1, 125.4, 125.5, 126.2, 127.6, 128.0, 128.1, 128.2, 128.4, 129.1, slight modification, a solution of 17 (660 mg, 1.7 mmol) in DMF
133.1, 133.3, 133.7, 164.1; ESI-MS obsd 392.0143, calcd 392.0142 (17 mL) was treated with 1 M HCl (1.0 mL). After 30 min,
[(M + H)+, M = C17H14INO2].
NH4OAc (2.63 g, 34.1 mmol) and triethylamine (4.8 mL, 35 mmol)
4-(Ethoxycarbonyl)-(E)-2-(1-(4,4-dimethyl-5-oxodihydrofuran- were added. The resulting mixture was stirred at 55 1C for 16 h.
2(3H)-ylidene)methyl)-3-(naphthalen-1-yl)pyrrole (16). Follow- The reaction was quenched by the addition of saturated aqueous
ing a literature procedure45 with slight modification, a mixture KH2PO4 solution. The resulting mixture was extracted with ethyl
of 14 (2.4 g, 6.1 mmol), 15 (1.55 g, 12.3 mmol), and BnNEt3Cl acetate (100 mL). The organic layer was washed with water, dried
(1.4 g, 6.1 mmol) in anhydrous acetonitrile (25.0 mL) and (Na2SO4), and filtered. The filtrate was concentrated and chroma-
triethylamine (6.1 mL) in a Schlenk flask was deaerated by tographed [silica, hexanes/ethyl acetate (3 : 1)] to afford a light-
three freeze–pump–thaw cycles. Then, a sample of Pd(PPh3)4 yellow crystalline solid (520 mg, 79%): mp 73–75 1C; 1H NMR (400
(354 mg, 0.307 mol, 5 mol%) was added, and the resulting MHz, CDCl3) d 0.65 (t, J = 7.2 Hz, 3H), 1.12 (s, 3H), 1.14 (s, 3H),
reaction mixture was further deaerated. The reaction mixture 2.15 (s, 3H), 2.39 (q, J = 16.8 Hz, 2H), 3.74–3.96 (m, 2H), 5.47
was stirred and heated at 80 1C for 24 h. After allowing to cool to (s, 1H), 7.30–7.46 (m, 3H), 7.50 (t, J = 7.6 Hz, 1H), 7.64 (d, J =
room temperature, CH2Cl2 and water were added. The organic 3.2 Hz, 1H), 7.71 (d, J = 8.4 Hz, 1H), 7.82 (d, J = 8.0 Hz, 1H), 7.85
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(d, J = 8.0 Hz, 1H), 11.44 (br s, 1H); 13C NMR (100 MHz, CDCl3) d hexanes/ethyl acetate (4 : 1)] to afford the title compound (25.9 mg,
13.5, 15.7, 25.6, 25.7, 43.9, 48.4, 59.1, 104.2, 116.2, 120.7, 124.5, 50%) as a white solid and 21 (9.2 mg, 18%) as a white solid. Data
125.1, 125.3, 125.4, 126.9, 127.1, 127.9, 128.3, 130.8, 133.4, 133.6, for the title compound: mp 75 1C (dec.); 1H NMR (400 MHz,
134.0, 150.5, 165.1, 187.5; ESI-MS obsd 387.2076, calcd 387.2067 CDCl3) d 1.37 (s, 6H), 1.38 (t, J = 7.2 Hz, 3H), 2.94 (d, J = 1.8 Hz,
[(M + H)+, M = C25H26N2O2].
2H), 4.33 (q, J = 7.2 Hz, 2H), 6.10 (t, J = 1.8 Hz, 1H), 6.40 (d, J =
2.4 Hz, 1H), 8.74 (br s, 1H); 13C NMR (100 MHz, CDCl3) d 14.6,
8-(Ethoxycarbonyl)-1-(1,1-dimethoxymethyl)-2,2-dimethyl-7-
(naphthalen-1-yl)-2,3-dihydrodipyrrin (19). Following a litera- 25.5, 40.1, 40.7, 60.5, 96.9, 104.8, 109.0, 116.1, 128.0, 149.0, 163.6,
ture procedure45 with slight modification, a solution of 18 179.7; ESI-MS obsd 342.0327, calcd 342.0336 [(M
+
H)+,
(193 mg, 0.499 mmol) in 1,4-dioxane (9 mL) was treated with M = C14H16BrNO4].
SeO2 (166 mg, 1.50 mmol) at room temperature. After 30 min,
7-Bromo-8-ethoxycarbonyl-1,2,2-trimethyl-2,3-dihydrodipyrrin
ethyl acetate and water were added. The organic layer was (23). Following a reported procedure45 with modification, MeLi
washed with brine, dried (Na2SO4), and concentrated under (6.52 mL, 10.4 mmol, 1.6 M in Et2O) was added dropwise to a
reduced pressure. The residue was dissolved in HC(OMe)3 solution of TiCp2Cl2 (1.180 g, 4.740 mmol) in toluene (12.6 mL) at
(4.0 mL) and treated with p-TsOHꢂH2O (29 mg, 0.15 mmol) at 0 1C. After 1 h at 0 1C, the reaction mixture was washed with
room temperature. After 12 h, the reaction mixture was aqueous NH4Cl (6%, 11 mL), H2O (11 mL), and brine (11 mL).
quenched by the addition of saturated aqueous NaHCO3 The organic layer was dried (Na2SO4) and filtered. Samples of 21
solution and then extracted with ethyl acetate. The organic (342.2 mg, 1.000 mmol) and additional TiCp2Cl2 (14.9 mg,
layer was dried (Na2SO4) and filtered. The filtrate was concen- 0.0598 mmol) were added to the filtrate containing the Petasis
trated and chromatographed [silica, hexanes/ethyl acetate reagent at room temperature. The mixture was heated to 80 1C in
1
(4 : 1)] to afford a brown viscous liquid (93 mg, 42%): H NMR the dark for 2.5 h and then allowed to cool to room temperature.
(400 MHz, CDCl3) d 0.66 (t, J = 7.6 Hz, 3H), 1.20 (s, 3H), 1.23 NaHCO3 (50 mg), MeOH (1.2 mL), and H2O (12 mL, 0.1% v/v
(s, 3H), 2.30–2.54 (m, 2H), 3.46 (s, 3H), 3.47 (s, 3H), 3.70–4.00 relative to MeOH) were added. The mixture was heated to 40 1C
(m, 2H), 5.13 (s, 1H), 5.62 (t, J = 2.0 Hz, 1H), 7.30–7.60 (m, 4H), for 14 h and filtered through Celite. The filtrate was concentrated
7.60–7.80 (m, 2H), 7.80–8.00 (m, 2H), 11.27 (br s, 1H); 13C NMR under reduced pressure. DMF (9.6 mL) and aqueous HCl
(100 MHz, CDCl3) d 13.5, 25.9, 26.0, 45.6, 48.2, 54.46, 54.48, (6 M, 84 mL, 0.50 mmol) were added to the residue at room
59.1, 102.0, 107.4, 116.3, 121.8, 125.1, 125.2, 125.4, 125.5, 126.8, temperature. Aqueous HCl (6 M, 3 ꢀ 84 mL, 3 ꢀ 0.50 mmol) was
127.2, 128.0, 128.3, 130.4, 133.3, 133.4, 133.9, 149.5, 165.0, added after 100, 110, and 240 min. After 3 h at room tempera-
182.0; ESI-MS obsd 447.2288, calcd 447.2278 [(M + H)+, ture, NH4OAc (1.574 g, 20.42 mmol) and NEt3 (2.84 mL,
M = C27H30N2O4].
20.4 mmol) were added. After 12 h at room temperature, the
3-Bromo-4-ethoxycarbonyl-(E)-2-[(4,4-dimethyl-5-oxodihydro- reaction mixture was quenched with aqueous KH2PO4 (10%,
furan-2(3H)-ylidene)methyl]pyrrole (21). A suspension of K2CO3 10 mL) and extracted with CH2Cl2 (10 mL). The organic layer
(497.6 mg, 3.600 mmol) in MeCN was treated with 2,4,6- was washed with H2O (40 mL) and brine (40 mL). The organic
tribromophenol (661.6 mg, 2.000 mmol) at room temperature. layer was dried (Na2SO4) and filtered. The filtrate was concentrated
After 30 min, the mixture was cooled to 0 1C. Compound 20 under reduced pressure. Column chromatography [silica, hexanes/
(263.3 mg, 1.000 mmol) followed by 2,4,4,6-tetrabromo-2,5- ethyl acetate (4 : 1)] afforded a yellow solid (219.0 mg, 65%):
cyclohexadienone (655.5 mg, 1.600 mmol) were added at 0 1C. 1H NMR (300 MHz, CDCl3) d 1.37 (s, 6H), 1.36 (t, J = 7.1 Hz,
After 1 h at 0 1C, the reaction mixture was diluted with ethyl 3H), 2.14 (s, 3H), 2.60 (d, J = 1.8 Hz, 2H), 4.30 (q, J = 7.1 Hz, 2H),
acetate (10 mL). The mixture was washed with aqueous NaHSO3 5.98 (s, 1H), 7.45 (d, J = 2.7 Hz, 1H), 11.40 (br s, 1H); 13C NMR
(10%, 10 mL), saturated aqueous NaHCO3 (10 mL), and brine (75 MHz, CDCl3) d 14.6, 15.9, 25.6, 44.2, 48.6, 59.9, 95.5, 103.2,
(10 mL). The organic layer was dried (Na2SO4) and concentrated 114.4, 124.6, 131.1, 151.9, 163.9, 188.7; ESI-MS obsd 339.0703,
under reduced pressure. Column chromatography [silica, hexanes/ calcd 339.0703 [(M + H)+, M = C15H19BrN2O2].
ethyl acetate (7 : 3)] afforded a pale orange solid (210.3 mg,
Bis(7-bromo-8-carbethoxy-2,3-dihydro-1,2,2-trimethyldipyr-
61%): mp 81 1C (dec.); 1H NMR (400 MHz, CDCl3) d 1.35 (s, 6H), rinato)palladium(II) (24). Following a reported method75 with
1.36 (t, J = 7.2 Hz, 3H), 2.91 (d, J = 2.0 Hz, 2H), 4.31 (q, J = 7.2 Hz, some modification, a solution of 23 (15 mg, 43 mmol) in CH2Cl2
2H), 6.21 (t, J = 2.0 Hz, 1H), 7.46 (d, J = 3.2 Hz, 1H), 8.59 (br s, (0.50 mL) was treated with Pd(OAc)2 (19 mg, 86 mmol). The
1H); 13C NMR (100 MHz, CDCl3) d 14.4, 25.1, 40.3, 60.3, 96.7, reaction mixture was stirred at r.t. for 2 h and then chromato-
97.8, 115.2, 125.4, 126.1, 149.7, 163.8, 179.7; ESI-MS obsd graphed [silica, hexanes/ethyl acetate (2 : 1 to 1 : 1)] to afford a
1
342.0328, calcd 342.0336 [(M + H)+, M = C14H16BrNO4].
yellow-green solid (6.0 mg, 36%): mp 4230 1C (dec.); H NMR
5-Bromo-4-ethoxycarbonyl-(E)-2-[(4,4-dimethyl-5-oxodihydro- (CDCl3, 400 MHz) d 6.80 (s, 2H), 6.32 (s, 2H), 4.30–4.15 (m, 4H),
furan-2(3H)-ylidene)methyl]pyrrole (22). A solution of 20 (39.5 2.85 (d, J = 14.8 Hz, 2H), 2.65 (d, J = 14.8 Hz, 2H), 1.71 (s, 6H), 1.30–
mg, 0.150 mmol) in DMF (1.50 mL) at ꢁ20 1C was treated with 1.26 (m, 12H), 1.04 (s, 6H); 13C NMR (CDCl3, 100 MHz) d 195.6,
NBS (26.7 mg, 0.150 mmol). After 30 min, the reaction mixture 163.6, 139.4, 136.2, 131.7, 115.5, 110.5, 97.3, 59.1, 49.5, 43.8,
was allowed to warm to room temperature. After 9.5 h, saturated 25.5, 23.3, 17.0, 14.6; ESI-MS obsd 781.0204, calcd 781.0211
aqueous Na2S2O4 (1 mL) and ethyl acetate (1 mL) was added. The [(M + H)+, M = C30H36Br2N2O4Pd]; labs (toluene) 364 nm.
mixture was washed with H2O (5 mL) and brine (2 mL). The
7-Bromo-8-ethoxycarbonyl-1-(1,1-dimethoxymethyl)-2,2-dimethyl-
organic layer was dried (Na2SO4) and chromatographed [silica, 2,3-dihydrodipyrrin (25). Following a literature procedure45 with
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slight modification, a solution of 23 (111.9 mg, 0.3299 mmol)
2,3,12,13-Tetrakis(methoxycarbonylvinyl)-8,8,18,18-tetramethyl-
and pyridine (53 mL) in 1,4-dioxane (6.60 mL) was treated with bacteriochlorin (BC-2). Following a reported procedure59 with
SeO2 (73.2 mg, 0.660 mmol) at room temperature. After 5 h, the slight modification, bacteriochlorin BC-1 (12.0 mg, 17.5 mmol),
reaction mixture was diluted with ethyl acetate (10 mL) and Pd(OAc)2 (11.9 mg, 53.0 mmol), PPh3 (32.8 mg, 0.125 mmol) and
filtered through Celite. The filtrate was washed with H2O K2CO3 (21.0 mg, 0.152 mmol) were added to a Schlenk tube and
(10 mL) and brine (10 mL). The organic layer was dried dried under high vacuum. The vacuum was released under argon
(Na2SO4), filtered, and concentrated under reduced pressure. to allow the addition of a solution of DMF (3 mL), toluene (3 mL)
The residue was treated with trimethyl orthoformate (3.30 mL) and methyl acrylate (175 mL, 1.93 mmol). The mixture was then
and p-TsOHꢂH2O (12.6 mg, 0.066 mmol) at room temperature. deaerated via four freeze–pump–thaw cycles before the vessel was
After 1.5 h, the reaction mixture was quenched with saturated purged again with argon. The Schlenk flask was sealed and heated
aqueous NaHCO3 (5 mL) and extracted with ethyl acetate at 105 1C for 10 h. The resulting mixture was washed with water
(5 mL). The organic layer was washed with brine (5 mL), dried (20 mL) and extracted with CH2Cl2 (2 ꢀ 20 mL). The combined
(Na2SO4), and filtered. The filtrate was concentrated and twice organic extract was dried (Na2SO4) and concentrated. The residue
chromatographed [silica, hexanes/ethyl acetate (4 : 1) for the was first purified by chromatography [deactivated silica, hexanes/
first chromatography, CH2Cl2/ethyl acetate (20 : 1) for the sec- CH2Cl2 (2 : 1 to 1 : 1)], then on preparative TLC to afford a reddish
1
ond chromatography] to afford a pale orange solid (48.3 mg, solid (4.0 mg, 32%): H NMR (600 MHz, CDCl3) d ꢁ0.93 (s, 2H),
37%): 1H NMR (300 MHz, CDCl3) d 1.29 (s, 6H), 1.35 (t, J = 1.94 (s, 12H), 4.01 (s, 6H), 4.02 (s, 6H), 4.37 (s, 4H), 6.92 (d, J =
7.2 Hz, 3H), 2.63 (d, J = 1.8 Hz, 2H), 3.45 (s, 6H), 4.30 (q, J = 16.0 Hz, 4H), 8.69 (s, 2H), 8.83 (s, 2H), 8.97 (t, J = 16.4 Hz, 4H);
7.2 Hz, 2H), 5.11 (s, 1H), 6.13 (t, J = 1.8 Hz, 1H), 7.48 (d, J = 13C NMR (150 MHz, CDCl3) d 30.8, 46.2, 51.7, 52.1, 52.1, 95.1,
3.6 Hz, 1H), 11.19 (br s, 1H); 13C NMR (100 MHz, CDCl3) d 14.5, 97.4, 124.5, 124.6, 126.8, 126.9, 134.0, 134.9, 137.1, 137.2, 161.3,
26.0, 45.8, 48.5, 54.5, 59.9, 96.8, 101.9, 106.4, 114.6, 125.3, 167.2, 167.3, 173.2; ESI-MS obsd 706.2990, calcd 706.2997
130.7, 150.9, 163.8, 183.1; ESI-MS obsd 399.0907, calcd [M+, M = C40H42N4O8]; labs (CH2Cl2) 319, 359, 393, 543, 815 nm.
399.0914 [(M + H)+, M = C17H23BrN2O4].
BC-3. Following an established procedure,44 a sample of 7
(8-Bromonaphthalen-1-yl)boronic acid (26)82,83. A solution (21 mg, 0.046 mmol) was dissolved in TFA (2.6 mL) under argon
of 1,8-dibromonaphthalene (1.430 g, 5.00 mmol) in THF at and stirred for 2 h at room temperature. Then the reaction
ꢁ60 1C was treated dropwise with n-butyllithium (3.34 mL, mixture was poured into saturated aqueous NaHCO3 solution
1.6 M in hexane) over 5 min. After 1 h at ꢁ60 1C, trimethyl and extracted with CH2Cl2. The organic extract was washed
borate (668 mL, 6.00 mmol) was added dropwise at ꢁ60 1C. with brine and water, dried (Na2SO4), and concentrated. The
Then, the reaction mixture was allowed to warm to room residue was washed with Et2O followed by MeOH. The title
temperature. After 2.5 h, saturated aqueous NH4Cl (50 mL) compound could not be fully characterized because of instability;
was added at room temperature. After 2.5 h, the mixture was with only limited data obtained the assignment remains provi-
extracted with ethyl acetate (50 mL). The organic layer was sional: LD-MS obsd 670.8 [M+, M = C28H38N4]; labs (CH2Cl2)
washed with brine (50 mL), dried (Na2SO4), and filtered. The 801 nm; lflu (CH2Cl2) 808 nm.
filtrate was concentrated and chromatographed [silica, hexanes/
3,13-Bis(ethoxycarbonyl)-2,12-di(naphthalen-1-yl)-8,8,18,18-
1
acetone (7 : 3)] to afford a white solid (823.9 mg, 59%): H NMR tetramethylbacteriochlorin (BC-4). Following an established
(400 MHz, acetone-d6) d 7.20 (s, 2H), 7.38 (dd, J = 8.0, 7.6 Hz, 1H), procedure,42 a solution of 13 (2.31 g, 5.17 mmol, 18 mM) in
7.52 (dd, J = 8.0, 6.8 Hz, 1H), 7.63 (dd, J = 6.8, 1.2 Hz, 1H), 7.83 (dd, anhydrous CH3CN (290 mL) was treated with BF3ꢂOEt2 (5.1 mL,
J = 7.6, 1.2 Hz, 1H), 7.90 (dd, J = 8.0, 1.2 Hz, 1H), 7.93 (dd, J = 41 mmol, 140 mM). The reaction mixture was stirred at room
8.0, 1.2 Hz, 1H); 13C NMR (125 MHz, acetone-d6) d 124.2, 126.75, temperature for 16 h. The reaction was quenched by the
126.83, 129.5, 129.6, 131.3, 132.4, 134.5, 136.0; ESI-MS obsd addition of solid NaHCO3 to the reaction mixture. The mixture
272.6963, calcd 272.6963 [(M + Na)+, M = C10H9BBrO2].
was filtered. The filtrate was concentrated, and the residue was
chromatographed [silica, CH2Cl2/hexanes (2 : 1)] to afford a
purple/green solid (825 mg, 41%). The 1H NMR spectrum indicated
Synthesis of bacteriochlorins
2,3,12,13-Tetrabromo-8,8,18,18-tetramethylbacteriochlorin a mixture of atropisomers (B1:1 ratio): 1H NMR (500 MHz, CDCl3)
(BC-1). Following a standard procedure,42 a solution of 1 (348 mg, d ꢁ0.98 (br s, 4H), 0.66 (t, J = 7.0 Hz, 12H), 1.67 (m, 12H), 1.86 (m,
0.857 mmol, 18 mM) in anhydrous CH3CN (48 mL) was treated 12H), 4.12–4.19 (m, 8H), 4.34–4.43 (m, 8H), 7.30 (t, J = 7.5 Hz, 4H),
with BF3ꢂOEt2 (0.83 mL, 6.7 mmol, 140 mM). The reaction mixture 7.54 (t, J = 7.5 Hz, 4H), 7.68 (d, J = 8.5 Hz, 4H), 7.81 (t, J = 7.5 Hz,
was stirred at room temperature for 15 h. Excess triethylamine 4H), 7.93 (d, J = 7.0 Hz, 4H), 8.07 (d, J = 8.0 Hz, 4H), 8.15 (d, J =
(2.0 mL) was added to the reaction mixture. The reaction mixture 8.0 Hz, 4H), 8.37 (s, 4H), 9.86 (s, 4H); 13C NMR (100 MHz, CDCl3) d
was concentrated, and the residue was chromatographed [silica, 13.5, 30.8, 30.95, 31.04, 46.1, 52.1, 60.7, 97.7, 99.4, 125.3, 126.1,
hexanes/CH2Cl2 (19 : 1)] to afford a green solid (46 mg, 16%): 126.3, 127.0, 128.4, 128.6, 130.0, 133.8, 134.1, 134.6, 135.0, 135.1,
1H NMR (400 MHz, CDCl3) d ꢁ1.86 (br s, 2H), 1.96 (s, 12H), 4.43 136.9, 161.8, 166.3, 172.5; ESI-MS obsd 767.3578, calcd 767.3592
(s, 4H), 8.75 (s, 2H), 8.88 (s, 2H); 13C NMR (150 MHz, CDCl3) d [(M + H)+, M = C50H46N4O4]; labs (CH2Cl2) 356, 381, 526, 768 nm.
30.97, 46.18, 51.38, 95.64, 97.85, 113.4, 113.5, 132.0, 132.9, 159.9,
3,13-Bis(ethoxycarbonyl)-2,12-di(naphthalen-1-yl)-5-methoxy-
8,8,18,18-tetramethylbacteriochlorin (BC-5). Following an estab-
lished procedure,43 a solution of 13 (1.00 g, 2.24 mmol, 18 mM)
171.9; ESI-MS obsd 681.8571, calcd 681.8573 [(M)+,
M =
C24H22Br4N4]; labs (CH2Cl2) 347, 373, 496, 743 nm.
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in HPLC-grade CH2Cl2 (124 mL) was treated first with DTBP (B1:1 ratio): 1H NMR (500 MHz, CDCl3) d 0.570.61 (m, 6H), 1.02–
(9.9 mL, 45 mmol, 360 mM) and second with TMSOTf (2.0 mL, 1.07 (m, 6H), 1.56–1.79 (m, 24H), 3.97–4.08 (m, 4H), 4.18 (s, 6H),
11 mmol, 90 mM). The reaction mixture was stirred at room 4.27–4.35 (m, 12H), 7.27–7.31 (m, 4H), 7.49–7.55 (m, 4H), 7.70–
temperature for 16 h. The reaction mixture was quenched by the 7.87 (m, 10H), 7.92–7.99 (m, 2H), 8.03–8.12 (m, 10H), 8.19 (s, 2H),
addition of saturated aqueous NaHCO3. The organic phase was 9.61 (s, 2H); 13C NMR (100 MHz, CDCl3) d 13.3, 14.1, 30.3, 30.7,
separated, concentrated, and chromatographed (silica, CH2Cl2). 30.8, 44.7, 45.0, 47.2, 50.6, 60.0, 61.4, 64.3, 97.6, 99.3, 123.8, 123.9,
A single green band was isolated and concentrated to afford a 125.2, 125.5, 125.8, 125.9, 126.1, 127.4, 127.9, 128.0, 128.2, 128.3,
purple/green solid (620 mg, 69%). The 1H NMR spectrum 128.6, 129.5, 129.7, 130.0, 130.1, 132.9, 133.6, 134.0, 134.6, 135.4,
indicated a mixture of atropisomers (B1 : 1 ratio): 1H NMR 136.9, 139.7, 140.3, 144.5, 145.7, 146.5, 151.6, 156.2, 164.8, 167.1,
(500 MHz, CDCl3) d ꢁ1.37 (br s, 2H), –1.05 (br s, 2H), 0.65– 167.8, 169.5; MALDI-MS obsd 859.0533; ESI-MS obsd 858.2753,
0.68 (m, 6H), 1.04–1.07 (m, 6H), 1.58–1.79 (m, 24H), 4.12–4.18 calcd 858.2754 (M+, M = C51H46N4O5Zn); labs (CH2Cl2) 352, 384,
(m, 4H), 4.27 (s, 6H), 4.31–4.37 (m, 12H), 7.28–7.32 (m, 4H), 7.55 550, 762 nm.
(m, 4H), 7.69 (t, J = 8.8 Hz, 2H), 7.77–7.83 (m, 6H), 7.91–7.93
2,12-Bis(ethoxycarbonyl)-10-methoxy-8,8,18,18-tetramethyl-
(m, 2H), 8.04–8.08 (m, 6H), 8.15 (d, J = 8.5 Hz, 4H), 8.18 (s, 2H), 3,13-di(naphthalen-1-yl)bacteriochlorin (BC-8). Following a lit-
8.29 (s, 2H), 9.72 (s, 2H); 13C NMR (100 MHz, CDCl3) d 13.4, 14.2, erature procedure45 with slight modification, a solution of 19
30.8, 30.9, 31.0, 31.1, 45.7, 46.2, 48.2, 51.8, 60.5, 61.7, 64.7, 96.7, (39 mg, 0.087 mmol) in CH2Cl2 (4.4 mL) was treated with DTBP
98.3, 98.5, 120.7, 125.2, 125.5, 126.0, 126.2, 126.3, 127.02, 127.04, (400 mL, 1.78 mmol) followed by TMSOTf (80 mL, 0.44 mmol).
127.7, 128.27, 128.31, 128.48, 128.53, 129.2, 130.0, 130.7, 130.8, The reaction mixture was stirred under a nitrogen atmosphere
131.8, 133.0, 133.4, 133.7, 133.96, 134.01, 134.2, 134.6, 135.3, at room temperature for 15 h, and then diluted with CH2Cl2
135.5, 135.7, 136.5, 157.1, 161.7, 166.3, 168.1, 169.2, 172.9; ESI- and washed with saturated aqueous NaHCO3. The organic
MS obsd 797.3689, calcd 797.3697 [(M + H)+, M = C51H48N4O5]; layer was dried (Na2SO4) and filtered. The filtrate was concen-
labs (CH2Cl2) 371, 528, 748 nm.
trated under reduced pressure. Column chromatography
In(III)X-3,13-Bis(ethoxycarbonyl)-2,12-di(naphthalen-1-yl)- [deactivated silica, hexanes/CH2Cl2 (3 : 1 to 2 : 1)] afforded a
8,8,18,18-tetramethylbacteriochlorin (BC-6). Following an dark purple solid (16.0 mg, 44%): the 1H NMR spectrum
established procedure,72 a solution of BC-4 (25 mg, 0.033 mmol) indicated a mixture of atropisomers (B1 : 1 ratio): 1H NMR
in anhydrous THF (1.6 mL) was treated with NaH (13 mg, (600 MHz, CDCl3) d ꢁ1.20 (br s, 2H), ꢁ0.88 (br s, 2H), 0.67–0.72
0.33 mmol, 60% in mineral oil). The reaction mixture was stirred (m, 6H), 1.00–1.05 (m, 6H), 1.87–2.06 (m, 24H), 4.07–4.19 (m,
at room temperature for 5 min, whereupon the solution turned 12H), 4.20–4.23 (m, 6H), 4.25–4.33 (m, 4H), 7.26–7.32 (m, 4H),
from purple/green to bright green. The mixture was then treated 7.49–7.55 (m, 4H), 7.58–7.68 (m, 4H), 7.77–7.82 (m, 4H), 7.84–
with InBr3 (584 mg, 1.65 mmol), affording immediately a bright 7.89 (m, 2H), 7.98–8.08 (m, 6H), 8.15 (d, J = 7.8 Hz, 4H), 8.21
pink solution. The mixture was then stirred at 50 1C for 16 h. The (s, 2H), 8.27 (s, 2H), 9.60 (s, 2H); 13C NMR (150 MHz, CDCl3) d
reaction mixture was quenched by the addition of saturated 13.23, 13.77, 26.60, 29.22, 29.33, 29.71, 30.14, 30.70, 30.74,
aqueous NaHCO3. The organic phase was separated, concen- 30.90, 30.95, 46.10, 47.30, 53.83, 60.20, 61.32, 68.54, 95.99,
trated, and chromatographed [silica, CH2Cl2/ethyl acetate (9 : 1)]. 98.68, 100.1, 100.5, 115.2, 120.9, 125.0, 125.2, 125.6, 125.8,
The first band (green) was the free-base starting material (10 mg), 126.0, 126.05, 126.06, 126.10, 126.9, 127.3, 128.03, 128.05,
followed by a pink band (7.5 mg, 25%): 1H NMR (400 MHz, CDCl3) 128.2, 128.8, 129.3, 129.41, 129.44, 130.4, 130.5, 131.84,
d 0.63–0.68 (m, 6H), 1.50–1.52 (m, 3H), 1.72–1.79 (m, 6H), 1.87– 131.86, 132.5, 132.78, 132.80, 133.4, 133.6, 133.9, 134.02,
1.90 (m, 3H), 3.87–4.16 (m, 4H), 4.33–4.42 (m, 2H), 4.51–4.60 134.05, 134.4, 136.4, 137.1, 137.2, 158.2, 160.0, 162.5, 165.7,
(m, 2H), 7.17–7.22 (m, 2H), 7.34–7.39 (m, 1H), 7.42–7.51 (m, 2H), 166.7, 167.5, 173.7; ESI-MS obsd 797.3697, calcd 797.3698 [(M + H)+,
7.57–7.61 (m, 1H), 7.64–7.67 (m, 1H), 7.73–7.77 (m, 1H), 7.84–7.88 M = C51H48N4O5]; labs (toluene) 362, 375, 531, 752 nm.
(m, 2H), 7.96–7.98 (m, 1H), 8.04–8.16 (m, 4H), 8.27 (s, 1H), 8.30
3,13-Dibromo-2,12-bis(ethoxycarbonyl)-8,8,18,18-tetramethyl-
(s, 1H), 9.82 (s, 1H); ESI-MS obsd 879.2387, calcd 879.2401 bacteriochlorin (BC-10). Following a literature procedure45 with
[(M ꢁ X)+, M = C50H44InXN4O4]; labs (CH2Cl2) 355, 392, 565, modification, TMSOTf (2.71 mL, 15.0 mmol) was added to a
793 nm. The identity of the indium(III) counterion, X, is not solution of 25 (798.6 mg, 2.000 mmol) and DTBP (13.2 mL,
known with certainty; the yield is calculated assuming X = Cl.
60.0 mmol) in 1,2-dichloroethane (400 mL) at room temperature.
Zn(II)-3,13-Bis(ethoxycarbonyl)-2,12-di(naphthalen-1-yl)-5- The reaction mixture was heated at reflux for 11 h and then
methoxy-8,8,18,18-tetramethylbacteriochlorin (BC-7). Following allowed to cool to room temperature. The reaction mixture was
an established procedure,72 a mixture of BC-5 (48 mg, 0.060 mmol) quenched with pyridine (1.61 mL, 200 mmol) and concentrated
and NaH (43 mg, 1.8 mmol) was added to DMSO (6.0 mL) under under reduced pressure. Column chromatography [deactivated
argon and stirred for 5 min. Zn(OAc)2ꢂ2H2O (658 mg, 3.0 mmol, silica, hexanes/CH2Cl2/MeCN (500: 50 : 1 to 100 : 10: 1)] afforded
50 eq.) was then added and the suspension was stirred for 16 h in the title compound as a dark purple solid (196.5 mg, 29% and
an oil bath at 60 1C. The crude mixture was washed with brine and the 10-methoxy derivative thereof, namely 3,13-dibromo-2,12-
extracted by ethyl acetate. The combined ethyl acetate extract was bis(ethoxycarbonyl)-10-methoxy-8,8,18,18-tetramethylbacteriochlorin
dried (Na2SO4), concentrated and chromatographed [silica, (BC-11)) as a dark purple solid (250.2 mg, 36%). Data for the title
hexanes/ethyl acetate (3 : 2)] to afford a reddish solid (41 mg, compound: 1H NMR (400 MHz, CDCl3) d 1.77 (t, J = 7.2 Hz, 6H), 1.94
80%). The 1H NMR spectrum indicated a mixture of atropisomers (s, 12H), 4.38 (s, 4H), 4.82 (q, J = 7.2 Hz, 4H), 8.95 (s, 2H), 9.56 (s, 2H);
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13C NMR (100 MHz, CDCl3) d 14.8, 31.0, 46.4, 51.4, 61.6, 97.7, 99.1, 762.3191, calcd 762.3201 [(M)+, M = C50H42N4O4]; labs (toluene)
115.6, 121.2, 132.4, 135.7, 161.1, 164.7, 173.8; ESI-MS obsd 671.0858, 306, 336, 383, 425, 622, 675, 829, 913 nm.
calcd 671.0863 [(M + H)+, M = C30H32Br2N4O4]; labs (toluene) 357,
3,13-Bis(2-bromophenyl)-2,12-bis(ethoxycarbonyl)-8,8,18,18-
385, 528, 731, 772 nm.
tetramethylbacteriochlorin (BC-13). A suspension of BC-10
Data for BC-11: 1H NMR (400 MHz, CDCl3) d 1.58 (t, J = (6.7 mg, 0.010 mmol), (2-bromophenyl)boronic acid (20.1 mg,
7.0 Hz, 3H), 1.74 (t, J = 7.0 Hz, 3H), 1.91 (s, 6H), 2.02 (s, 6H), 4.15 0.100 mmol), K2CO3 (27.6 mg, 0.200 mmol), and Pd(PPh3)4
(s, 3H), 4.31 (s, 2H), 4.34 (s, 2H), 4.76 (q, J = 7.0 Hz, 2H), 4.80 (3.5 mg, 0.0030 mmol) in toluene (333 mL) and DMF (167 mL)
(q, J = 7.0 Hz, 2H), 8.72 (s, 1H), 8.87 (s, 1H) 9.41 (s, 1H); 13C was deaerated by two freeze–pump–thaw cycles. The resulting
NMR (100 MHz, CDCl3) d 14.5, 14.8, 29.3, 30.9, 46.4, 47.4, 51.1, mixture was heated to 100 1C for 5 h and then allowed to cool to
54.0, 61.6, 62.4, 68.8, 96.5, 98.1, 99.5, 109.9, 116.2, 120.8, 125.7, room temperature. The mixture was passed through a pad of
129.6, 131.3, 132.0, 136.3, 137.0, 159.1, 160.7, 164.6, 166.8, basic alumina (CH2Cl2 as an eluent). The eluent was concen-
167.1, 174.1; ESI-MS obsd 701.0974, calcd 701.0969 [(M + H)+, trated under reduced pressure. Column chromatography [deac-
M = C31H34Br2N4O5]; labs (toluene) 360, 372, 532, 719, 756 nm. tivated silica, hexanes/CH2Cl2 (9 : 1 to 8 : 1)] afforded a dark
3,13-Bis(8-bromonaphthalen-1-yl)-2,12-bis(ethoxycarbonyl)-
purple solid (7.0 mg, 85%). 1H and 13C NMR spectra indicated a
8,8,18,18-tetramethylbacteriochlorin (BC-12). A suspension of mixture of atropisomers (B1 : 1 ratio). Two distinguishable
BC-10 (6.7 mg, 0.010 mmol), (8-bromonaphthalen-1-yl)boronic spots were found on TLC analysis with close Rf values
acid (25.1 mg, 0.100 mmol), K2CO3 (27.6 mg, 0.200 mmol), and [0.33 and 0.37, deactivated silica, 3 ꢀ hexanes/ethyl acetate
Pd(PPh3)4 (1.2 mg, 0.0010 mmol) in toluene (333 mL) and DMF (9 : 1)]. This sample was used as a mixture of atropisomers in
1
(167 mL) was deaerated by two freeze–pump–thaw cycles. The the next step. Data for the title compound: H NMR (400 MHz,
resulting mixture was heated to 100 1C for 14 h and then CDCl3) d ꢁ1.14 (s, 4H), 1.24 (t, J = 7.2 Hz, 12H), 1.94 (s, 12H),
allowed to cool to room temperature. The mixture was passed 1.96 (s, 6H), 1.97 (s, 6H), 4.27 (s, 8H), 4.44 (dq, J = 7.2, 10.7 Hz,
through a basic alumina pad (CH2Cl2 as an eluent). The eluent 4H), 4.53 (dq, J = 7.2, 10.7 Hz, 4H), 7.47–7.58 (m, 4H), 7.58–7.69
was concentrated under reduced pressure. Column chromato- (m, 4H), 7.69–7.83 (m, 4H), 7.87–7.98 (m, 4H), 9.73 (s, 4H), 8.29
graphy [deactivated silica, hexanes/CH2Cl2 (9 : 2)] afforded a (s, 4H); 13C NMR (100 MHz, CDCl3) d 14.0, 31.0, 31.1, 31.2, 31.3,
dark purple solid (4.5 mg, 49%). 1H and 13C NMR spectra 46.4, 51.4, 60.8, 97.7, 99.3, 120.57, 120.58, 125.91, 125.94,
indicated a mixture of atropisomers (B1 : 1 ratio). Two distin- 126.80, 126.83, 129.6, 132.4, 132.5, 132.9, 133.0, 133.1, 135.6,
guishable spots were found on TLC analysis with close Rf values 137.4, 137.5, 137.6, 160.6, 165.7, 173.6; ESI-MS obsd 822.1400,
[0.23 and 0.26, deactivated silica, 3 ꢀ hexanes/ethyl acetate calcd 822.1411 [(M)+, M = C42H40Br2N4O4].
(9 : 1)]. This sample was used as a mixture of atropisomers in
Benz-BC. A suspension of BC-13 (52.0 mg, 0.0631 mmol),
1
the next step. Data for the title compound: H NMR (400 MHz, K2CO3 (43.6 mg, 0.315 mmol), tricyclohexylphosphine tetra-
CDCl3) d ꢁ1.0 (s, 4H), 0.72 (t, J = 7.2 Hz, 12H), 1.90 (s, 6H), 1.91 fluoroborate (69.7 mg, 0.189 mmol), and Pd(OAc)2 (14.2 mg,
(s, 6H), 1.95 (s, 6H), 1.96 (s, 6H), 4.32–4.06 (m, 16H), 7.38 (t, J = 0.0632 mmol) in DMF (6.3 mL) was deaerated by three freeze–
7.8 Hz, 4H), 7.84–7.66 (m, 12H), 8.08 (d, J = 8.4 Hz, 4H), 8.16 pump–thaw cycles. The resulting mixture was heated to 110 1C
(d, J = 8.4 Hz, 4H), 8.28 (s, 4H), 9.76 (s, 4H); 13C NMR (100 MHz, for 2 h and then allowed to cool to room temperature. The
CDCl3) d 13.4, 30.9, 31.6, 31.2, 31.3, 46.4, 51.4, 60.2, 97.4, 100.2, mixture was diluted with CH2Cl2 (40 mL) and washed with H2O
121.2, 121.3, 122.0, 125.1, 126.2, 129.3, 129.9, 132.0, 132.8, (50 mL) and brine (20 mL). The organic layer was dried
132.9, 133.6, 133.8, 134.9, 135.80, 135.83, 135.9, 139.34, (Na2SO4) and passed through deactivated silica. The eluent
139.36, 160.3, 165.9, 173.4; ESI-MS obsd 922.1715, calcd was concentrated under reduced pressure. The residue was
922.1724 [(M)+, M = C50H44Br2N4O4].
washed with MeOH, MeCN, and hexanes to give a dark green
1
Phen-BC. A suspension of BC-12 (8.6 mg, 0.0093 mmol), solid (33.3 mg, 80%): H NMR (400 MHz, CDCl3) d 1.69 (t, J =
K2CO3 (12.9 mg, 0.0933 mmol), 2-dicyclohexylphosphino-20,60- 7.2 Hz, 6H), 1.78 (s, 12H), 2.89 (s, 2H), 4.00 (s, 4H), 4.71 (q, J =
dimethoxybiphenyl (11.5 mg, 0.0280 mmol), and Pd(OAc)2 7.2, 4H), 6.97–7.04 (m, 2H), 7.04–7.10 (m, 2H), 7.49–7.54
(2.1 mg, 0.0094 mmol) in DMF (1.00 mL) was deaerated by (m, 2H), 8.02–8.06 (m, 2H), 8.71 (s, 2H); 13C NMR (175 MHz,
two freeze–pump–thaw cycles. The resulting mixture was CDCl3) d 14.9, 30.7, 46.5, 49.1, 61.1, 101.0, 113.1, 113.4, 123.6,
heated to 80 1C for 5.5 h, 100 1C for 0.5 h, and then allowed 126.5, 127.4, 129.3, 136.5, 141.3, 142.1, 143.9, 149.1, 159.0,
to cool to room temperature. The mixture was diluted with 165.3, 174.7; ESI-MS obsd 662.2881, calcd 662.2899 [(M)+,
CH2Cl2 and passed through deactivated silica. The eluent was M = C42H38N4O4]; labs (toluene) 311, 403, 425, 442, 488, 658,
concentrated under reduced pressure. Column chromatogra- 719, 821, 914, 1033 nm.
phy [deactivated silica, hexanes/CH2Cl2 (11 : 9)] afforded a dark
green solid (2.6 mg, 37%): 1H NMR (300 MHz, CDCl3) d 1.57
Photophysical measurements
(s, 12H), 1.72 (t, J = 7.0 Hz, 6H), 1.97 (s, 2H), 4.31 (s, 4H), 4.88 Photophysical studies were carried out on samples in dilute (mM)
(q, J = 7.0 Hz, 4H), 7.58–7.80 (m, 4H), 7.76–8.02 (m, 4H), 8.30– argon-purged solutions of toluene. Excitation and detection band-
8.48 (m, 2H), 8.35 (s, 2H), 8.84–9.06 (m, 2H); 13C NMR widths of 5 nm were employed for fluorescence spectra. Fluores-
(125 MHz, CDCl3) d 14.8, 29.3, 46.1, 55.9, 61.7, 97.5, 110.1, cence quantum yields were determined for argon-purged samples
115.1, 126.4, 126.7, 126.8, 127.4, 127.5, 128.2, 128.3, 129.8, relative to standards meso-tetraphenylporphyrin (Ff = 0.070 in
133.5, 133.6, 134.0, 135.9, 161.6, 166.8, 168.2; ESI-MS obsd toluene with ambient O2),98 H-BC (Ff = 0.14 in argon-purged
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toluene)85 and the zinc chelate of I2-BC (Ff = 0.031 in argon-
purged toluene).5 Each emission spectrum was integrated to
include both the origin and vibronic satellite features. For each
Ff determination, the emission spectrum was acquired using
at least three excitation wavelengths spanning the Soret and
Qx (and in some cases Qy) regions with good agreement, and the
results were averaged. S1 lifetimes were determined by time-
correlated single photon counting (TCSPC) fluorescence
spectroscopy and by transient absorption (TA) spectroscopy,
both employing B100 fs excitation flashes from an ultrafast
laser system.73
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K. M. Smith and R. Guilard, Academic Press, San Diego,
2000, vol. 2, pp. 125–199.
Molar absorption coefficients
8 T. D. Lash, J. Porphyrins Phthalocyanines, 2001, 5, 267–288.
9 S. Fox and R. W. Boyle, Tetrahedron, 2006, 62, 10039–10054.
10 J. P. Lewtak and D. T. Gryko, Chem. Commun., 2012, 48,
10069–10086.
11 A. M. V. M. Pereira, S. Richeter, C. Jeandon, J.-P. Gisselbrecht,
J. Wytko and R. Ruppert, J. Porphyrins Phthalocyanines, 2012,
16, 464–478.
The molar absorption coefficients were determined by dissol-
ving a known quantity of each bacteriochlorin (1.01–2.66 mg) in
toluene (5.00 mL). Then a known amount (40.0–100.0 mL) of
this solution was diluted to 5.00 mL with toluene. The absorp-
tion spectrum was recorded at room temperature. The averages
of 5 runs were calculated: BC-10 (e756nm = 36 800 Mꢁ1 cmꢁ1
,
,
e
772nm = 135 000 Mꢁ1 cmꢁ1); BC-11 (e756nm = 117 000 Mꢁ1 cmꢁ1
e772nm = 20 800 Mꢁ1 cmꢁ1).
12 H. Mori, T. Tanaka and A. Osuka, J. Mater. Chem. C, 2013, 1,
2500–2519.
13 O. Yamane, K.-I. Sugiura, H. Miyasaka, K. Nakamura,
T. Fujimoto, K. Nakamura, T. Kaneda, Y. Sakata and
M. Yamashita, Chem. Lett., 2004, 33, 40–41.
14 K. Kurotobi, K. S. Kim, S. B. Noh, D. Kim and A. Osuka,
Angew. Chem., Int. Ed., 2006, 45, 3944–3947.
15 M. Tanaka, S. Hayashi, S. Eu, T. Umeyama, Y. Matano and
H. Imahori, Chem. Commun., 2007, 2069–2071.
16 N. K. S. Davis, A. L. Thompson and H. L. Anderson, Org.
Lett., 2010, 12, 2124–2127.
17 C. Jiao, K.-W. Huang, C. Chi and J. Wu, J. Org. Chem., 2011,
76, 661–664.
Density functional theory calculations
DFT calculations were performed with Gaussian 09 version
D.01.99 Calculations used the polarization continuum model
for the arrays in toluene. Molecular geometries were fully
optimized using the hybrid B3LYP functional and the basis
set 6-31G*. These calculations used Gaussian defaults with the
exception of keyword Int = (Grid = Ultrafine, Acc2E = 14).
TDDFT calculations used the long-range corrected oB97XD
functional and the basis set 6-31++G**. These calculations used
Gaussian defaults with the exception of keywords TD (nStates =
16), Int = (Grid = Ultrafine, Acc2E = 14), and Pop = Full.
18 N. K. S. Davis, A. L. Thompson and H. L. Anderson, J. Am.
Chem. Soc., 2011, 133, 30–31.
19 L. Jiang, J. T. Engle, L. Sirk, C. S. Hartley, C. J. Ziegler and
H. Wang, Org. Lett., 2011, 13, 3020–3023.
Conflicts of interest
The authors declare no competing financial interests.
20 J. P. Lewtak, D. Gryko, D. Bao, E. Sebai, O. Vakuliuk,
´
M. Scigaj and D. T. Gryko, Org. Biomol. Chem., 2011, 9,
8178–8181.
Acknowledgements
21 V. V. Diev, C. W. Schlenker, K. Hanson, Q. Zhong, J. D.
Zimmerman, S. R. Forrest and M. E. Thompson, J. Org.
Chem., 2012, 77, 143–159.
22 N. Fukui, W. Y. Cha, S. Lee, S. Tokuji, D. Kim, H. Yorimitsu
and A. Osuka, Angew. Chem., Int. Ed., 2013, 52, 9728–9732.
23 P. Chen, Y. Fang, K. M. Kadish, J. P. Lewtak, D. Koszelewski,
A. Janiga and D. T. Gryko, Inorg. Chem., 2013, 52, 9532–9538.
24 K. Ota, T. Tanaka and A. Osuka, Org. Lett., 2014, 16,
2974–2977.
This work was supported by a grant from the Chemical Sciences,
Geosciences and Biosciences Division, Office of Basic Energy
Sciences, of the U. S. Department of Energy (DE-FG02-05ER15661).
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