A facile and reliable method for the synthesis of tetrabenzoporphyrin from 4,7-dihydroisoindole

Article abstract of DOI:10.1002/ejoc.200700014

A new route to tetrabenzoporphyrins from the closest possible precursor of the unstable isoindole was developed. A key feature of this route is a dramatic facilitation of the aromatization of annelated rings, which is the most serious bottleneck in previo

Full text of DOI:10.1002/ejoc.200700014

FULL PAPER
DOI: 10.1002/ejoc.200700014
A Facile and Reliable Method for the Synthesis of Tetrabenzoporphyrin from
4,7-Dihydroisoindole
Mikhail A. Filatov,[a] Andrei V. Cheprakov,*[a] and Irina P. Beletskaya[a]
Keywords: Porphyrinoids / Nitrogen heterocycles / Tetrabenzoporphyrins / π-Extended porphyrins / Dipyrromethanes
A new route to tetrabenzoporphyrins from the closest pos-
sible precursor of the unstable isoindole was developed. A
key feature of this route is a dramatic facilitation of the
aromatization of annelated rings, which is the most serious
bottleneck in previous syntheses of tetrabenzoporphyrins.
The capabilities of the new method are illustrated by the syn-
thesis of meso-tetraaryltetrabenzoporphyrins, as well as the
first unambiguous synthesis and characterization of 5,15-di-
phenyltetrabenzoporphyrin.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Introduction
Of all the so-called π-extended porphyrins,[1] tetrabenzo-
porphyrins (TBP) are probably the most interesting and the
closest relatives of phthalocyanines. Owing to a rare combi-
nation of photophysical,[2] optoelectronic,[3] and physico-
chemical properties,[4] benzoporphyrins show promise in
various applications, for example photodynamic or neutron
capture therapy sensitizers, sensors, and electronic devices,
etc.[5] This potential, however, has been so far largely unex-
plored because of a lack of effective and versatile ap-
proaches to these compounds. For the TBP system, the
straightforward application of standard methods of por-
phyrin chemistry – the condensation of an appropriate
pyrrole with aldehydes – is not possible, as in this case the
progenitor pyrrole, 2H-isoindole, is an unstable transient
molecule (Scheme 1, path A). Therefore, until recently,
the only available approach was through mimicking the
phthalocyanine synthesis by a high-temperature templated
condensation of phthalimide or similar compounds with
various CH-acids,[6] which, as a rule, gives complex mix-
tures of porphyrins requiring HPLC for separation,[7] and
is practically not applicable to anything except the unsubsti-
tuted TBP itself and some simple derivatives.
Scheme 1. General routes to tetrabenzoporphyrins.
proaches used tetrahydroisoindole (Scheme 1, path B), and
the final aromatization was performed by oxidative dehy-
drogenation with DDQ.[8] The other approach used bicyclo-
octadiene-fused pyrrole (Scheme 1, path C), and the aroma-
tization was accomplished by a thermal retro-Diels–Alder
reaction.[9] Both of these approaches enabled fast expansion
of the scope and preparative value of TBP syntheses. How-
ever, as soon became evident, both methods still suffered
from a common serious drawback, which was the harsh
conditions required for the aromatization step (either pro-
longed heating of metallated hexadecahydroTBP precursors
with DDQ leading to partial overoxidation in the tetra-
hydroisoindole method, or heating over 200 °C to effect eth-
ylene extrusion). Thus, the scope of these approaches was
limited, which resulted in losses of valuable target porphy-
rins at the final stage of a long synthesis. Yet, another ele-
gant approach that should be mentioned uses pyrrole annel-
ated to a sulfolene moiety, thus opening the possibility to
Alternatively, the TBP system can be assembled by the
common methods of porphyrin synthesis; however, the use
of the unstable isoindole should be avoided and a stable
precursor of this transient compound, with aromatization
being delayed until after the assembly of the porphyrin
macrocycle, could be used. Indeed, recently two such ap-
proaches were simultaneously elaborated. One of these ap-
[a] Department of Chemistry, Moscow State University
119992 Moscow, Russia
Fax: +7-495-939-1854
E-mail: avchep@elorg.chem.msu.ru
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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Synthesis of Tetrabenzoporphyrin from 4,7-Dihydroisoindole
FULL PAPER
use cycloaddition with appropriate dienophiles either prior pentadienes[19] with only a few examples published on the
or after the porphyrin macrocycle assembly leading to par- cycloaddition to less reactive acyclic dienes with the use of
tially hydrogenated benzoporphyrin precursors.[10] This ap- rather drastic reaction conditions.[20] We could not achieve
proach, however, so far has not been developed into a gene- the cycloaddition of tosylacetylene to 1,3-butadiene when
ral method of tetrabenzoporphyrin synthesis owing to inci- trying to perform the reaction in various solvents by heat-
dental problems, associated with a poor solubility of inter- ing in a thick-walled vessel. However, a smooth and practi-
mediates, which can be overcome only through the use of cally quantitative reaction did take place when a solution
large solubilizing substituents.[11]
In order to remove this bottleneck, the aromatization room temperature for 2 d in a thick-walled tube.
should be dramatically facilitated.[12] This could be
Adduct 2 was used for the preparation of 4,7-dihydroiso-
of 1 was treated with a large excess of butadiene and left at
achieved by taking as close a precursor to isoindole as pos- indole 4 by using standard Barton–Zard chemistry.[21] All
sible, as compared to both pyrrolic compounds used in the steps are high-yielding and reproducible on a multigram
published methods. The choice is not wide, as only a single scale. Dihydroisoindole 4 is quite stable and survives during
candidate for this role – 4,7-dihydroisoindole – could be storage in a refrigerator, though it does rapidly darken at
considered (Scheme 1, path D), which is surprisingly a so- room temperature similarly to other electron-rich β-substi-
far-unknown simple pyrrole derivative.[13] Herein we report tuted pyrroles. Therefore, representative NMR spectra
such a precursor, and show that its use allows the synthesis could not be obtained. Surprisingly, though it could be ex-
of TBP to be realized with unprecedented ease thus super- pected that under acidic or basic conditions the unwanted
seding all extant approaches to this system.[14]
migration of double bond would happen, this does not take
place at any of the stages even though at almost each stage
of this synthesis either strong bases (tBuOK in THF, KOH
in boiling ethyleneglycol) or acids (TFA, TsOH, BF3) are
involved. A likely reason for such stability is that the pyrro-
lic residue is more acidic (as NH-acid) as well as more nu-
cleophilic than the respective reaction centers involved in
the anticipated double bond migration; thus, the pyrrolic
ring may serve to protect the double bond from the initia-
tion of both carbocationic and carbanionic shifts.
Results and Discussion
The synthesis of 4,7-dihydroisoindole (Scheme 2) is
straightforward and employs tosylacetylene 1, readily ob-
tained by the published procedure[15] from commercial bis-
(trimethylsilyl)acetylene (BTMSA). Tosylacetylene is re-
garded as a powerful dienophile.[16] Analysis of the pub-
lished data shows that tosylacetylene is largely employed in
very facile reactions with highly reactive, electron-rich cyclic
dienes, such as N-acylpyrroles,[15a,17] furan,[18] and cyclo-
Tetrahydroisoindole 4 was further successfully used for
the preparation of meso-tetraaryloctahydrotetrabenzopor-
phyrins 5 by using the standard Lindsey protocol[22]
(Scheme 3). These porphyrins were formed in good yields
comparable to those obtained with the same benzaldehydes
and 4,5,6,7-tetrahydroisoindole, without the shift of the
double bond or aromatization of the cyclohexadiene rings.
Compounds 5 were isolated as pure diprotonated dication
1
salts[23] and unambiguously characterized by H and 13C
NMR spectroscopy; these compounds possess a highly
symmetrical structure. Otherwise, the properties of these
porphyrins, including the electronic absorption spectra, are
very close to those of hexadecahydrotetrabenzoporphyrins
described earlier.[8c]
Scheme 2. a) TsCl, AlCl3, CH2Cl2, 12 h, 80%; b) NaF, MeOH/
H2O, 0.5 h, 90%; c) 1,3-butadiene (excess), room temp., 48 h, 95%;
d) CNCH2COOtBu, tBuOK, THF, 4 h, 80%; e) TFA, 15 min, 55%
or KOH, HOCH2CH2OH, reflux, 30 min, 85%.
To a certain degree, the method now being described
mimics the dialine version of the synthesis of TNPs, devel-
Scheme 3. a) BF3·Et2O, CH2Cl2, room temp., 2 h, then DDQ, room temp., 1 h, 5a (25%), 5b (40%); b) DDQ, toluene, reflux, 5 min,
95%.
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M. A. Filatov, A. V. Cheprakov, I. P. Beletskaya
FULL PAPER
oped earlier in our laboratory, with 4,7-dihydroisoindole tive. The MALDI spectra of the crude reaction mixtures
serving the role of key synthon in place of benzo[f]-4,7-di- show no significant byproducts. This is in sharp contrast to
hydroisoindole.[24] The synthesis of TNP should lead, after what is observed in the former method of synthesis based
macrocyclization by the Lindsey procedure, to octahydro- on tetrahydroisoindole,[8b–8d] in which the dehydrogenation
tetranaphthalo[2,3]porphyrins (Scheme 4), which are struc- by DDQ was applied only after prior metallation by se-
turally similar to porphyrins 5, possessing exactly the same lected metal salts (Cu, Ni, Pd)[12] and required prolonged
degree and pattern of unsaturation in the inner rings and reflux temperatures in polar solvents. Modest yields of TBP
the same porphyrinic core (both belong to the same products are often obtained[26] owing to both multiple side
meso-tetraarylocta-β-alkyl strained nonplanar porphyrin reactions and the need for metallation–aromatization–de-
class[25]).
metallation sequence in place of the direct aromatization
as in the protocol described here. Additionally, owing to
cleanness of aromatization in the current protocol, target
tetrabenzoporphyrins 6 are readily isolated from the reac-
tion mixtures by crystallization without the need for
chromatography either as free bases or diprotonated dicat-
ion salts.
Thus, tetraaryltetrabenzoporphyrins are made available
by the reported method in only three major preparative
steps (cycloaddition, Barton–Zard condensation, Lindsey’s
porphyrin synthesis) from readily available tosylacetylene.
The aromatization in this protocol is ready and quantitative
and is no longer a bottleneck in the synthesis, as in the
earlier procedures.
It should be noted that when this article was in prepara-
tion, a paper appeared describing the synthesis of mono-,
di-, and tribenzoporphyrins (but not tetrabenzoporphyrins)
by intermediate porphyrins containing cyclohexadiene-an-
nelated rings, the same fragment as in porphyrin 5.[27] These
Scheme 4. meso-Tetraaryloctahydrotetranaphthaloporphyrins are
isostructural (with respect to inner core without peripheral benzene
rings) to porphyrins 5.
However, octahydrotetranaphthalo[2,3]porphyrins are intermediates were accessed from common tetraphenylpor-
readily aromatized in situ to fully aromatic systems during phyrin by electrophilic bromination, followed by Suzuki
DDQ oxidation of the respective porphyrinogen, and could cross-coupling and olefin metathesis performed under high
not be isolated in spite of many efforts to quench DDQ as dilution conditions. The ease of aromatization of cyclohexa-
fast as possible. MALDI spectra of the samples of the reac- diene rings by DDQ in toluene has been noted.
tion mixtures that were obtained by early quenching never
To further illustrate the potential of a new synthon, we
showed any appreciable amounts of these porphyrins, explored the [2+2] McDonald condensation as an easy
though partially dehydrogenated forms with one, two, or route to very interesting 5,15-diaryltetrabenzoporphyrins
three naphthalene residues could be observed and even iso- (Scheme 5). Dipyrromethane-α,αЈ-dicarboxylates 7 can be
lated.
formed by the condensation of 3 with benzaldehyde. We
Bearing this apparent analogy in mind, the observed sta- established that this condensation in our case gives poor
bility of porphyrins 5 towards dehydrogenation is not easily yields under published conditions[28] as a result of incom-
understandable, particularly if we take into account that the plete conversion and tarring. Apparently, such behavior
driving force for aromatization is generally lower for larger may be explained by side reactions at the double bonds,
polycyclic systems as a result of the localization of aroma- effected by the action of acid in the presence of liberated
ticity in subsystems. However, this phenomenon gives us a water. Indeed, the addition of a highly hygroscopic salt,
chance to exploit this new porphyrinic synthon possessing namely, anhydrous tetrabutylammonium chloride, resulted
double bonds in the fixed symmetrical positions ready for in full suppression of tarring and desired dipyrromethane 7
further transformations towards new π-extended porphy- was formed in near-to-quantitative yield. The action of salt,
rins. The examples of such chemistry will be reported sepa- which binds the liberated water, is probably similar to the
rately.
well-known salt effect in the formation of porphyrinogens,
Further aromatization of porphyrins 5 is nevertheless described by Lindsey et al.,[29] though, as far as we know,
straightforward. This can be achieved even at room tem- this effect was never employed in dipyrromethane chemis-
perature, on keeping porphyrins 5 with DDQ for several try. Besides, unlike the salt effect of Lindsey, which is deliv-
days in the dark. Alternatively, a quantitative aromatization ered by a lot of various salts, in our reaction simple salts
to tetraaryltetrabenzoporphyrins 6 takes place on heating such as NaCl or Na2SO4 were helpless, as only the use of
the solution in toluene or another appropriate solvent with anhydrous Bu4NCl led to success. The obtained dipyrrome-
DDQ for a few minutes; the aromatization takes place al- thane was utilized in the [2+2] condensation, with all stages
most instantaneously as the temperature is raised over being selective and free of side reactions arising from ex-
100 °C. In all cases the aromatization is clean and quantita- pectable double bond migration to give expected 5,15-di-
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Synthesis of Tetrabenzoporphyrin from 4,7-Dihydroisoindole
FULL PAPER
Scheme 5. a) TsOH (cat), nBu4NCl, CHCl3, room temp., 24 h, 90%; b) TFA, CH2Cl2, room temp., 30 min (not isolated); c) CH(OMe)3,
TFA, room temp., 30 min, 50% (based on 7); d) TFA, CH2Cl2, room temp., 21 h, then DDQ, 4 h, 15%; e) DDQ, toluene, reflux, 5 min,
Ͼ95%.
aryloctahydrotetrabenzoporphyrin 10, which is cleanly aro-
matized to 5,15-diaryltetrabenzoporphyrin 11.
It should be noted that 5,15-diphenylTBP has already
appeared in the literature, but, as far as we know, has never
been unambiguously characterized. All previous attempts
were based on the high temperature templated condensa-
tion reactions, either in trace amounts by tedious HPLC
separation of multiple side products of the reaction of
benzylidenephthalimidine with Zn acetate at 360 °C,[7b,30]
or by fusion of 3-oxoisoindoleninyl-3Ј-oxoisoindolinylid-
ene-1,1Ј-methylidine with zinc acetate and sodium phenyl-
acetate at 350 °C.[31] The templated fusion methods, how-
ever, owing to extreme harshness of reaction conditions fail
to deliver pure tetrabenzoporphyrins and lead instead to
complex mixtures of analogues with a varied degree of sub-
stitution and scrambling of substituents; well-resolved spec-
tra cannot be obtained for such products. In the reported
cases, according to the published descriptions the products
were either inseparable mixtures of 5,10- and 5,15-di-
phenylTBP, or compounds of unidentifiable nature giving
wrong and poorly resolved NMR and UV/Vis spectra.
Therefore, this work is the first to report the unambiguously
characterized 5,15-diphenyltetrabenzoporphyrin system.
Well-resolved NMR spectra (1H, 13C) can be obtained for
the dication salt of this porphyrin (e.g. Figure 1). As in the
spectra of the free base, the signals are dynamically broad-
ened, which is typical behavior for porphyrins with annel-
ated rings, for example tetraaryltetrabenzoporphyrins. A
very large difference in chemical shifts between the α pro-
tons of the benzo rings (695 Hz) is effected by superposition
of ring currents of the porphyrin core plus the benzo rings
(deshielding of H4 protons in the meso unsubstituted bay
of the TBP system) and the ring current of the meso phenyl
groups, which are roughly perpendicular to the effective
plane of the macrocycle is such molecules (shielding of H1
protons in the meso substituted bay).
Figure 1. Aromatic portion (meso protons at 11.01 ppm excluded)
of the 1H NMR (400 MHz, CDCl3) of 5,15-diphenyltetrabenzopor-
phyrin 11 as a dication salt.
The electronic absorption spectrum of porphyrin 11 as a
free base is rather unusual (Figure 2). Unlike tetraaryltetra-
benzoporphyrins, this porphyrin shows very narrow Soret
bands, as well as splitting of the Soret and Q-bands, with all
Figure 2. The electronic absorption spectrum of 5,15-diphenyltet-
rabenzoporphyrin 11 as a free base in CH2Cl2 (spectrum of dicat-
these special features disappearing upon protonation. These ion is shown in the insert).
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122.28, 27.02, 23.77, 21.56 ppm. C13H14O2S (234.31): calcd. C
66.64, H 6.02; found C 66.16, H 5.88.
features almost exactly follow the overall appearance and
pattern of bands in the spectra of meso unsubstituted tetra-
benzoporphyrins.[32] Thus, it is very likely that the structure
of 5,15-diphenyltetrabenzoporphyrin is closer to meso un-
substituted TBPs than to tetraaryl TBPs. The former are
2-tert-Butoxycarbonyl-4,7-dihydro-2H-isoindole (3): A solution of
tert-butyl isocyanacetate (3.28 g, 23.3 mmol) in THF (10 mL) was
added dropwise to
a stirred suspension of tBuOK (2.61 g,
flat with a saddle shaped dication, and the latter are saddle- 23.3 mmol) in THF (20 mL) at 0 °C under an atmosphere of argon.
A solution of 1-tosyl-1,4-cyclohexadiene (5.2 g, 22.2 mmol) in THF
(20 mL) was added dropwise, and the resulting mixture was stirred
at room temp. After 4 h, the volume of the mixture was reduced by
rotary evaporation and CH2Cl2 (100 mL) was added. The resulting
solution was washed with water and brine and dried with Na2SO4.
The solvents were removed in vacuo, and the residue was purified
on silica column (CH2Cl2). The product was recrystallized from
hexane to give white crystals (3.89 g, 80%). M.p. 110–111 °C. 1H
NMR (400 MHz, CDCl3, 25°, TMS): δ = 9.59 (br. s, 1 H, position
of this signal is concentration dependent and may vary in a range
9.1–9.6 ppm), 6.72 (d, J = 2.40 Hz, 1 H), 5.94 (AB, J = 10.4 Hz, 1
H), 5.90 (AB, J = 10.4 Hz, 1 H), 3.46 (m, 2 H), 3.25 (m, 2 H), 1.61
(s, 9 H) ppm. 13C NMR (100 MHz, CDCl3, 25°, TMS): δ = 161.57,
124.55, 124.02, 123.90, 118.79, 118.68, 118.19, 80.47, 28.56, 24.30,
22.47 ppm. C13H17NO2 (219.28): calcd. C 71.21, H 7.81, N 6.39;
found C 71.11, H 8.04, N 6.20.
shaped in both the free base and the dication states. We may
therefore at this stage tentatively conclude that the 5,15-
diphenylporphyrin system is very likely a flat system. A de-
tailed study of this important structural phenomenon will
be reported in a forthcoming paper.
Conclusions
Tetraaryltetrabenzoporphyrins are made available in only
three major preparative steps starting from readily available
tosylacetylene in excellent yields and purity. In all the cases
studied so far, the assembly of porphyrin macrocycle is se-
lective and does not involve double bond shifts or sponta-
neous aromatization of annelated rings. Therefore, this pro-
tocol bears a dual advantage: (1) it is the most straightfor-
ward approach to tetrabenzoporphyrins and starts from the
nearest relative of unstable isoindole and (2) it makes octa-
hydrotetrabenzoporphyrins, bearing double bonds in fixed
symmetrical positions, available, and these compounds are
useful as new reaction centers for the preparation of novel
functionalized porphyrins with annelated rings. The pro-
posed method supersedes all extant methods for the synthe-
sis of tetrabenzoporphyrin in terms of preparative value and
the mild reaction conditions of the final aromatization step,
which allows the ready synthesis of previously inaccessibly
structures, as exemplified here by the synthesis and unam-
biguous characterization of 5,15-diphenyltetrabenzopor-
phyrin.
4,7-Dihydro-2H-isoindole (4): Method A: 2-tert-Butoxycarbonyl-
4,7-dihydroisoindole (3; 1.5 g, 6.84 mmol) was dissolved in TFA
(20 mL), and the solution was left for 30 min under an atmosphere
of Ar in the dark at room temp. After the addition of CH2Cl2
(20 mL), the mixture was poured into cold water. The organic layer
was collected, washed with water, then with 10% solution of
Na2CO3, water, and dried with Na2SO4. The solvent was removed
in vacuo, and the residue was passed through a layer of silica using
CH2Cl2 as eluent. The solvent was evaporated to give the product
as a yellow oil (0.45 mg, 55%). Method B: A mixture of 2-tert-
butoxycarbonyl-4,7-dihydroisoindole (2.0 g, 9.13 mmol) and potas-
sium hydroxide (45.6 mmol, 2.55 g) in ethylene glycol (30 mL) was
heated at reflux under an atmosphere of Ar for 30 min. The mix-
ture was cooled to 0 °C and CH2Cl2 (100 mL) was added. The solu-
tion was washed with water and brine, dried with Na2SO4, and the
solvent was evaporated in vacuo. The residue was passed through
a layer of silica using CH2Cl2 as eluent, and the resulting solution
was evaporated to give the product as a yellow oil (0.92 mg, 85%).
The compound was used in further transformations either immedi-
ately, or after storage in the refrigerator at –18 °C. It rapidly dark-
ens if kept open in air at room temp.
Experimental Section
General Procedures: All commercial reagents (Acros, Aldrich) were
used as received, or purified before use by standard procedures.
Ethynyl p-tolyl sulfone (1) was obtained according to the published
procedure.[15a] THF was distilled from LiAlH4 before use; dichloro-
methane and chloroform were washed by standard procedures and
distilled from CaH2 before use. NMR spectra were recorded with
a Bruker Avance-400 spectrometer. MALDI-TOF and LDI-TOF
mass spectra were obtained by using a Bruker Daltonics Alphaflex
II spectrometer with cinnamic acid as matrix. UV/Vis spectra were
recorded with a Hewlett–Packard 8452A instrument.
5,10,15,20-Tetraaryloctahydrotetrabenzoporphyrins (5a,b): 4,7-Di-
hydroisoindole (2.5 mmol, 0.30 g) was dissolved in CH2Cl2
(250 mL) and an aromatic aldehyde (2.5 mmol) was added. The
mixture was stirred under an atmosphere of Ar for 10 min in the
dark at room temp. BF3·Et2O (0.5 mmol, 0.071 g) was added in
one portion, and the mixture was stirred at room temp. for an
additional 2 h. DDQ (2.8 mmol, 0.63 g) was added, and the mix-
ture was stirred for an hour. The resulting solution was washed
with 10% aqueous Na2SO3 and dried with Na2SO4. The solvent
was evaporated in vacuo, and the residue was purified on a silica
1-Tosyl-1,4-cyclohexadiene (2): Ethynyl p-tolyl sulfone (5.0 g,
27.7 mmol) was added to 1,3-butadiene (20 mL), and the mixture
was stirred for 48 h at room temp. in a heavy-walled pressure tube. column (CH2Cl2, then CH2Cl2/AcOH, 50:1; green band collected),
The excess amount of butadiene was evaporated, and the residue
was purified by flash chromatography on a silica column (hexane/
EtOAc). The product was recrystallized from petroleum ether to
give a white crystalline powder (6.2 g, 95%). M.p. 68–69 °C. 1H
and then by recrystallization from CH2Cl2/MeOH. Zn complex was
obtained by reaction of porphyrins 5 with Zn(OAc)2 in CH2Cl2/
MeOH, controlled by UV/Vis spectroscopy. 5a: green crystals
(0.13 g, 25%). M.p. Ͼ300 °C. 1H NMR (400 MHz, CDCl3, 25°,
NMR (400 MHz, CDCl3, 25°, TMS): δ = 7.75 (m, 2 H), 7.33 (m, TMS, as dication salt): δ = 8.41 (m, 8 H), 7.88 (m, 12 H), 5.48 (s,
2 H), 7.01 (m, 1 H), 5.72–5.60 (m, 2 H), 2.98–2.90 (m, 2 H), 2.85– 8 H), 3.20 (m, 8 H), 2.69 (m, 8 H), 1.50 (br. s., 4 H) ppm. 13C
2.77 (m, 2 H), 2.43 (s, 3 H) ppm. 13C NMR (100 MHz, CDCl3, NMR (100 MHz, CDCl3, 25°, TMS): δ = 143.41, 138.45, 136.38,
25°, TMS): δ = 144.24, 137.83, 143.59, 129.76, 128.15, 122.82,
132.23, 130.03, 128.72, 123.02, 118.62, 25.63 ppm. UV/Vis
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Synthesis of Tetrabenzoporphyrin from 4,7-Dihydroisoindole
(CH2Cl2): λ (logε) = 438 (5.23), 532 (4.03), 568 (3.66), 576 (3.67), give the product as a grey powder. Yield: 0.382 g (50%). M.p.
FULL PAPER
1
614 (3.31) nm. UV/Vis (CH2Cl2/TFA, as dication): λ (logε) = 460
Ͼ200 °C (dec.). H NMR (400 MHz, [D8]DMSO, 25°, TMS): δ =
(5.28), 610 (3.89), 670 (4.17) nm. UV/Vis (CH2Cl2, as Zn complex): 11.58 (s, 2 H), 9.50 (s, 2 H), 7.40–7.25 (m, 3 H), 7.14–7.08 (m, 2
λ (logε) = 436, 574, 624 (sh) nm. MALDI-TOF: calcd. for
C60H47N4 [M + H]+ 823.38; found 823.39. MALDI-TOF (as Zn
complex): calcd. for C60H44N4Zn 884.28; found 884.38. 5b: green
crystals (0.264 g, 40%). M.p. Ͼ300 °C. 1H NMR (400 MHz,
CDCl3, 25°, TMS, as dication salt): δ = 8.57 (m, 16 H), 5.50 (m, 8
H), 4.15 (s, 12 H), 1.56 (br.s.) ppm. 13C NMR (100 MHz, CDCl3,
25°, TMS): δ = 166.78, 143.01, 141.73, 136.29, 132.63, 131.51,
129.88, 122.77, 118.13, 52.72, 25.85 ppm. UV/Vis (CH2Cl2): λ
(logε) = 446 (5.29), 540 (4.14), 574 (3.84), 616 (3.80), 682 (3.41)
nm. UV/Vis (CH2Cl2/TFA, as dication): λ (logε) = 468 (5.34), 614
(4.05), 672 (4.27) nm. UV/Vis (CH2Cl2, as Zn complex): λ (logε) =
448, 578, 625 (sh) nm. MALDI-TOF: calcd. for C68H55N4O8 [M +
H]+ 1055.40; found 1055.46. MALDI-TOF (As Zn complex): calcd.
for C68H52N4O8Zn 1116.31; found 1116.40.
H), 5.85 (br. d, J = 10.23 Hz, 2 H), 5.80 (br. d, J = 10.23 Hz, 2 H),
5.70 (s, 1 H), 3.44 (m, 4 H), 2.83 (m, 4 H) ppm. 13C NMR not
obtained because of very poor solubility. C25H22N2O2 (382.45):
calcd. C 78.51, H 5.80, N 7.32; found C 78.45, H 6.02, N 7.04.
5,15-Diphenyloctahydrotetrabenzoporphyrin (10): Dipyrromethane
7 (1.05 g, 2 mmol) was stirred in trifluoroacetic acid (5 mL) for
30 min at room temp. under an atmosphere of argon. The mixture
was diluted with CH2Cl2 (60 mL), and a solution of diformyldipyr-
romethane (9; 0.76 g, 2 mmol) in CH2Cl2 (90 mL) was added. The
reaction mixture was stirred for 21 h at room temp. under an atmo-
sphere of argon. DDQ (0.68 g, 3 mmol) was added, and the mixture
was stirred for an additional 4 h. The resulting solution was washed
with 10% aqueous Na2SO3 and brine and then dried with Na2SO4.
The solvent was evaporated in vacuo, and the residue was purified
on a silica column (CH2Cl2, then CH2Cl2/THF) and by crystalli-
zation from CH2Cl2/Et2O to afford dark-purple powder (0.10 g;
15%). M.p. Ͼ300 °C. 1H NMR (400 MHz, CDCl3/TFA): δ = 10.26
(c, 2 H), 8.33–8.22 (m, 4 H), 8.03–7.88 (m, 6 H), 6.22 (br. d, J =
10.10 Hz, 4 H), 5.97 (br. d, J = 9.98 Hz, 4 H), 4.58 (m, 8 H), 3.35
(m, 8 H), 3.71 (m, 8 H), –2.04 (br. s, 4 H) ppm. UV/Vis (as free
base, CH2Cl2): λ (logε) = 406 (5.04), 504 (3.91), 538 (3.41), 572
(3.47), 624 (2.58) nm. UV/Vis (as dication, CH2Cl2/TFA): λ (logε)
= 422(5.49), 526 (3.28), 562 (4.15), 607 (sh, 3.54) nm. MALDI-
TOF: calcd. for C48H39N4 [M+ H]+ 671.32; found 671.28.
5,10,15,20-Tetraaryltetrabenzoporphyrins (6a,b): Porphyrin 5a
(50 mg) was dissolved in toluene (10 mL). To this solution was
added DDQ (69 mg, 5 equiv.), and the solution was heated at re-
flux. The reaction was controlled by removing samples and measur-
ing the electronic absorption spectra. After 5 min of heating at re-
flux, the conversion was complete as evidenced by the full disap-
pearance of the Q-bands of the starting porphyrin. After cooling,
the mixture was washed by Na2SO3 solution, water, brine, dried
with Na2SO4, and the solvents were evaporated to dryness. The
residue was recrystallized from CH2Cl2/MeOH to afford green
crystals of tetraphenyltetrabenzoporphyrin 6a (47 mg, 96%), iden-
tical to the material obtained by the tetrahydroisoindole method.[8c]
Aromatization of porphyrin 5b took place analogously to afford
tetrakis(p-methoxycarbonylphenyl)tetrabenzoporphyrin 6b in 98%
yield, identical to the material obtained by the tetrahydroisoindole
method.[8c]
5,15-Diphenyltetrabenzoporphyrin (11): Porphyrin 10 (35 mg) was
dissolved in toluene (10 mL). To this solution was added DDQ
(60 mg, 5 equiv.), and the solution was heated at reflux for 5 min
(monitored by UV/Vis spectra). After cooling, the mixture was
washed with a Na2SO3 solution, water, and brine, dried with
Na2SO4, and the solvents were evaporated to dryness. The residue
was eluted through a short silica column (CH2Cl2) collecting the
first green band. After evaporation of the solvent, a green crystal-
line powder was obtained (33 mg, 95%). 1H NMR (400 MHz,
CDCl3/TFA): δ = 11.01 (s, 2 H), 9.35 (d, J = 7.96 Hz, 4 H), 8.43
(m, 4 H), 8.20 (ddd, J1 = J2 = 7.52 Hz, J3 = 0.65 Hz, 4 H), 8.12
(m, 2 H), 8.03 (m, 4 H), 7.86 (ddd, J1 = J2 = 7.76 Hz, J3 = 0.78 Hz,
4 H), 7.62 (d, J = 8.35 Hz, 4 H), –0.25 (br. s., 4 H) ppm. 13C NMR
(100 MHz, CDCl3, 25°, TMS): δ = 140.20, 138.78, 138.60, 134.49,
132.94, 132.29, 130.82, 130.39, 130.28, 129.94, 125.43, 123.37,
116.32, 92.24 ppm. UV/Vis (CH2Cl2, as free base): λ (logε) = 396
(3.78), 422 (5.64), 436 (5.71), 450 (3.47), 570 (3.35), 610 (3.88), 666
(3.59) nm. UV/Vis (CH2Cl2/TFA, as dication): λ (logε) = 454
(5.67), 616 (3.30), 672 (3.84) nm. LDI-TOF: calcd. for C48H30N4
662.25; found 662.29.
3,3Ј-Bis(tert-butoxycarbonyl)-1,1Ј-phenylmethylenebis-4,7-dihydro-
2H-isoindole (7): A mixture of 2-tert-butoxycarbonyl-4,7-dihydro-
isoindole (1.095 g, 5 mmol), benzaldehyde (0.265 g, 2.5 mmol), p-
toluenesulfonic acid (0.094 g, 0.5 mmol), and tetrabutylammonium
chloride (0.1 g, 0.35 mmol) in chloroform (50 mL) that was de-
gassed by purging with argon for 20 min was stirred at room temp.
under an atmosphere of argon for 24 h. The reaction mixture was
washed with 10% aqueous NaHCO3, brine, and dried with
Na2SO4. The solution was evaporated in vacuo, and the residue
was purified on a silica column (hexane/EtOAc, 4:1). The solvents
were evaporated to give the product as a yellow semisolid froth.
1
Yield: 1.18 g (90%). H NMR (400 MHz, CDCl3, 25°, TMS): δ =
8.43 (br. s, 2 H), 7.38–7.27 (m, 3 H), 7.17–7.12 (m, 2 H), 5.91–5.84
(m, 2 H), 5.80–5.73 (m, 2 H), 5.46 (s, 1 H), 3.42 (m, 4 H), 2.83 (m,
4 H), 1.56 (s, 18 H) ppm. 13C NMR (100 MHz, CDCl3, 25°, TMS):
δ = 161.15, 138.42, 129.56, 129.19, 128.31, 127.62, 125.00, 124.44,
123.38, 117.92, 116.58, 80.41, 41.46, 28.51, 24.43, 21.97 ppm.
C33H38N2O4 (526.67): calcd. C 75.26, H 7.27, N 5.32; found C
74.98, H 7.40, N 5.02.
Supporting Information (see footnote on the first page of this arti-
cle): The selected spectra of the newly obtained compounds.
Acknowledgments
3,3Ј-Diformyl-1,1Ј-phenylmethylenebis-4,7-dihydro-2H-isoindole (9):
Dipyrromethane 7 (1.052 g, 2 mmol) was dissolved in trifluoro-
acetic (20 mL) acid under an atmosphere of argon, and the solution
was cooled to 0–5 °C. After 30 min, trimethylorthoformate (2.12 g,
20 mmol) was added. The mixture was stirred for an additional
30 min at room temp. and then poured into cold water. The product
was extracted with CH2Cl2, then washed with 10% aqueous
NaHCO3 and brine, and dried with Na2SO4. The solution was
evaporated in vacuo, and the remaining material was purified on a
silica column (CH2Cl2/MeOH). The solvents were evaporated to
The authors thank the Russian Foundation of Basic Research for
grants RFBR-04-03-32650-a and RFBR-07-03-01121-a. The au-
thors are also indebted to Prof. S. Vinogradov, who was the first
to point out the likely flatness of the 5,15-diphenyl TBP structure.
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www.eurjoc.org
3473

M. A. Filatov, A. V. Cheprakov, I. P. Beletskaya
FULL PAPER
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 3468–3475

Synthesis of Tetrabenzoporphyrin from 4,7-Dihydroisoindole
FULL PAPER
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[26] For example, for direct comparison with the results of this
study, in tetrahydroisoindole method[8c] Ni complex of porphy-
rin 6a is obtained by aromatization of the respective Ni com-
plex of hexadecahydro TBP in 25–30% yield, and Ni cannot
be withdrawn from this complex to afford free base 6a by any
means. Alternatively, for the respective Cu complexes aromati-
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[32] See, for example spectra, discussion, and references in ref.[8d]
Interestingly, very similar narrow and split Soret bands were
recently observed in 5,15-dialkynyl and 5-alkynyl-15-alkenylte-
trabenzoporphyrins reported in: H. Yamada, K. Kushibe, T.
Okujima, H. Uno, N. Ono, Chem. Commun. 2006, 383–385.
Moreover, there are sparse literature data on electronic absorp-
tion spectra of 5,10-disubstituted as well as 5-monosubstituted
tetrabenzoporphyrins, which betray that all these systems may
also belong to the same structural class:a) M. Yasuike, T. Ya-
maoka, O. Ohno, K. Ichimura, H. Morii, M. Sakuragi, Inorg.
Chim. Acta 1991, 185, 39–47; b) M. O. Senge, I. Bischoff, Het-
erocycles 2005, 65, 879–886.
Received: January 8, 2007
Published Online: June 5, 2007
Eur. J. Org. Chem. 2007, 3468–3475
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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