Synthesis of 5,15-diaryltetrabenzoporphyrins

Article abstract of DOI:10.1021/jo800509k

(Chemical Equation Presented) A general method of synthesis of 5,15-diaryltetrabenzoporphyrins (Ar₂TBPs) has been developed, based on 2 + 2 condensation of dipyrromethanes followed by oxidative aromatization. Two pathways to Ar₂TBPs were investigated: the tetrahydroisoindole pathway and the dihydroisoindole pathway. In the tetrahydroisoindole pathway, precursor 5,15-diaryltetracyclohexenoporphyrins (5,15-Ar₂TCHPs) were assembled from cyclohexeno-fused meso-unsubstituted dipyrromethanes and aromatic aldehydes or, alternatively, by way of the classical MacDonald synthesis. In the first case, scrambling was observed. Aromatization by tetracyclone was more effective than aromatization by DDQ but failed in the cases of porphyrins with electron-withdrawing substituents in the meso-aryl rings. The dihydroisoindole pathway was found to be much superior to the tetrahydroisoindole pathway, and it was developed into a general preparative method, consisting of (1) the synthesis of 4,7-dihydroisoindole and its transformation into meio-unsubstituted dipyrromethanes, (2) the synthesis of 5,15-diaryloctahydrotetrabenzoporphyrins (5,15-Ar₂OHTBPs), and (3) their subsequent aromatization by DDQ. Ar₂TBP free bases exhibit optical absorption spectra similar to those of meso-unsubstituted tetrabenzoporphyrins and fluoresce with high quantum yields. Pd complex of Ph₂TBP was found to be highly phosphorescent at room temperature.

Full text of DOI:10.1021/jo800509k

Synthesis of 5,15-Diaryltetrabenzoporphyrins
Mikhail A. Filatov,† Artem Y. Lebedev,‡ Sergei A. Vinogradov,*,‡ and
Andrei V. Cheprakov*,†
Department of Chemistry, Moscow State UniVersity, Moscow 119899, Russia, and Department of
Biochemistry and Biophysics, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104
Vinograd@mail.med.upenn.edu; aVchep@elorg.chem.msu.ru
ReceiVed March 4, 2008
A general method of synthesis of 5,15-diaryltetrabenzoporphyrins (Ar2TBPs) has been developed, based
on 2 + 2 condensation of dipyrromethanes followed by oxidative aromatization. Two pathways to Ar2TBPs
were investigated: the tetrahydroisoindole pathway and the dihydroisoindole pathway. In the tetrahy-
droisoindole pathway, precursor 5,15-diaryltetracyclohexenoporphyrins (5,15-Ar2TCHPs) were assembled
from cyclohexeno-fused meso-unsubstituted dipyrromethanes and aromatic aldehydes or, alternatively,
by way of the classical MacDonald synthesis. In the first case, scrambling was observed. Aromatization
by tetracyclone was more effective than aromatization by DDQ but failed in the cases of porphyrins with
electron-withdrawing substituents in the meso-aryl rings. The dihydroisoindole pathway was found to be
much superior to the tetrahydroisoindole pathway, and it was developed into a general preparative method,
consisting of (1) the synthesis of 4,7-dihydroisoindole and its transformation into meso-unsubstituted
dipyrromethanes, (2) the synthesis of 5,15-diaryloctahydrotetrabenzoporphyrins (5,15-Ar2OHTBPs), and
(3) their subsequent aromatization by DDQ. Ar2TBP free bases exhibit optical absorption spectra similar
to those of meso-unsubstituted tetrabenzoporphyrins and fluoresce with high quantum yields. Pd complex
of Ph2TBP was found to be highly phosphorescent at room temperature.
Introduction
Until recently, synthetic approaches to tetrabenzo- and
tetranaphthoporphyrins were limited by the so-called “template
condensation” method and variants thereof, which require
severe reaction conditions and prohibit introduction of many
useful functionalities into the macrocycle. In the search for more
π-Extended porphyrins form a class of porphyrinoids in which
pyrrole rings are fused with external aromatic fragments via
the ꢀ-carbon atoms. The best known representatives of this class
are symmetrically π-extended porphyrins, such as tetraben-
zoporphyrins and tetranaphthoporphyrins, whose optical and
other properties attract interest in materials research,1 biomedical
imaging and sensing,2 up-conversion of noncoherent NIR light,3
and photodynamic and boron neutron-capture therapy.4
5
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† Moscow State University.
‡ University of Pennsylvania.
10.1021/jo800509k CCC: $40.75
Published on Web 05/02/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 4175–4185 4175

Filatov et al.
CHART 1. 5,15-Diaryltetrabenzoporphyrin (Ar2TBP)
practical approaches, efforts were mainly directed toward
tetraarylated tetrabenzo- and tetranaphthoporphyrins (Ar4TBPs
and Ar4TNPs), whose modification can be conveniently ac-
complished via the meso-aryl rings. The method employing
oxidative aromatization6has emerged as a straightforward and
Ar4TNPs.8 and practical approach to Ar4TBPs7 The synthesis
in this route proceeds via the well-established Lindsey
chemistry,9allowing for a variety of functional groups to be
placed at the macrocycle periphery. This strategy was later
extended on the synthesis of meso-unsubstituted tetrabenzo- and
tetranaphthoporphyrins (TBPs and TNPs).10
Recently, a useful extension of the oxidative aromatization
method was developed, which is based on a convenient
precursor, 4,7-dihydroisoindole.11 Using this method, we syn-
thesized and for the first time unambiguously characterized
meso-5,15-diphenyltetrabenzoporphyrin,12 the simplest repre-
sentative of the class of 5,15-diaryltetrabenzoporphyrins
(Ar2TBP, Chart 1).13 It appeared that the optical absorption
spectrum of free base 5,15-Ph2TBP (H2Ph2TBP) strongly
resembled the spectra of meso-unsubstituted H2TBPs,10,14 sug-
gesting that H2Ar2TBPs also possess undistorted planar geom-
etries15 and highly emissive excited states.2a,14,16 The combi-
nation of the red absorption, strong emissivity, and ease of
functionalization suggests that Ar2TBPs can be utilized in a wide
variety of applications, including biomedical imaging, optical
sensing, and PDT.
In addition to forming a distinct group among π-extended
porphyrins, Ar2TBPs belong to a much wider family of meso-
5,15-diarylporphyrins (Ar2Ps). Ar2Ps have been known since
1968,17 and over the years, they have become one of the most
widely used building blocks for construction of porphyrin-based
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4176 J. Org. Chem. Vol. 73, No. 11, 2008

Synthesis of 5,15-Diaryltetrabenzoporphyrins
SCHEME 1. Approaches to Ar2TCHPs
triads in combination with other electro- and/or optically active
25
fragments,
and scaffolds for chiral organopalladium com-
plexes.26 Ar2Ps have also been studied as agents for PDT,27
recognition motifs for aminoacids and other small molecules,28
cores for dendrimers,29 and ligands for DNA intercalation.30
Given such an overwhelming array of applications, it appeared
interesting and relevant to complement the Ar2P family by their
π-extended relatives, which on one hand bear the key structural
features of Ar2P but on the other hand possess distinctly different
optical properties. In this paper, we report a detailed account
on the synthesis of Ar2TBPs with variously substituted meso-
aryl groups, suitable for further functionalization.
Results and Discussion
I. Synthesis of Ar2TBPs via Tetrahydroisoindole Route.
Assembly of Precursor Porphyrins (Ar2TCHPs). Initially, we
examined the approach to Ar2TBPs via aromatization of 5,15-
diarylhexadecahydrotetrabenzoporphyrins or, simpler, 5,15-
diaryltetracyclohexenoporphyrins (Ar2TCHPs), using the meth-
odology developed earlier for Ar4TBPs and TBPs.7,8 Ar2TCHPs
can be assembled using either of the two orthogonal pathways,
A and B/B′, depicted in Scheme 1.
Route A is based on the acid-catalyzed condensation of meso-
unsubstituted dipyrromethanes with aromatic aldehydes.17,31
This route has been used in the past for preparation of various
types of 5,15-diaryloctaalkylporphyrins.21,31b,32 The alternative
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route has two major variants: B, the classical MacDonald’s
method,33 in which the substituent-free methine groups in the target
porphyrin originate from the formyl groups in 1,9-diformyldipyr-
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Chem. 1993, 32, 2072–2077. (b) Ogoshi, H.; Mizutani, T. Acc. Chem. Res. 1998,
31, 81–89. (c) Mamardashvili, G. M.; Storonkina, O. E.; Mamardashvili, N. Z.
Russ. J. Gen. Chem. 2004, 74, 1446–1450. (d) Comte, C.; Gros, C. P.; Koeller,
S.; Guilard, R.; Nurco, D. J.; Smith, K. M. New J. Chem. 1998, 22, 621–626.
(29) (a) Dandliker, P. J.; Diederich, F.; Gross, M.; Knobler, C. B.; Louati,
A.; Sanford, E. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 1739–1742. (b)
Dandliker, P. J.; Diederich, F.; Gisselbrecht, J. P.; Louati, A.; Gross, M. Angew.
Chem., Int. Ed. Engl. 1996, 34, 2725–2728. (c) Collman, J. P.; Fu, L.; Zingg,
A.; Diederich, F. Chem. Commun. 1997, 193–194.
(30) (a) Bejune, S. A.; Shelton, A. H.; McMillin, D. R. Inorg. Chem. 2003,
42, 8465–8475. (b) McMillin, D. R.; Shelton, A. H.; Bejune, S. A.; Fanwick,
P. E.; Wall, R. K. Coord. Chem. ReV. 2005, 249, 1451–1459.
J. Org. Chem. Vol. 73, No. 11, 2008 4177

Filatov et al.
SCHEME 2. Synthesis of Ar2TCHPs 4a-e Following Route
A (Scheme 1)
SCHEME 3. Synthesis of Ar2TCHPs 4a-c from
5-Aryldipyrromethanes Following Route B from Scheme 1
a Inseparable mixture of scrambling products.
Our goal was to establish the simplest, most versatile
approach to Ar2TCHPs and, at the same time, allow alterna-
tive protection-deprotection schemes in the syntheses of
intermediate pyrrole derivatives, permitting different types
of sensitive peripheral functionalities.36 Potentially, approach
A could lead to scrambling since formation of the methylene
bridges between pyrrole residues is reversible under acidic
conditions.37 Nevertheless, this route appeared more simple
and straightforward, as it required only one dipyrromethane
derivative.
In route A, meso-unsubstituted dipyrromethane 3 was
synthesized in high yield using a published procedure38 from
readily available tetrahydroisoindole ester 1a39 and introduced
into the condensation with aromatic aldehydes to afford
Ar2TCHPs 4a-e in good overall yields (Scheme 2).
MALDI-TOF MS analysis revealed that in the majority
of cases scrambling was insignificant, and only small amounts
of “defective” porphyrins (i.e., monoaryl- or meso-unsubsti-
tuted) could be detected in reaction mixtures. However, in
the case of strongly electrophilic pentafluorobenzaldehyde,
extensive scrambling occurred, yielding a mixture of por-
phyrins with one, two, or no meso-aryl groups. As a result,
porphyrin 4e could not be isolated in the pure form.
The classical MacDonald’s synthesis (pathway B in
Scheme 1) requires two dipyrromethane derivatives: dialde-
hyde and diacid. The common precursors of both components
are tert-butyl esters of 5-aryldipyrromethane-1,9-dicarboxylic
acids 5a-c, which were synthesized as shown in Scheme 3.
The yields of esters 5a-c could be improved significantly
in the presence of anhydrous tetrabutylammonium chloride.
Condensation of pyrrole-2-carboxylates with aldehydes usu-
ally results in yields not exceeding 60-70%, which is likely
to be caused by side reactions, promoted by strong acids in
the presence of the liberated water. Addition of anhydrous
Bu4NCl, a highly hygroscopic compound, raised the yields
of dipyrromethanes 5a-c nearly up to quantitative, suggest-
ing the salt effected water scavenging.40 Notably, in the
original MacDonald synthesis, water was sometimes removed
by azeotropic distillation.41
(34) (a) Baldwin, J. E.; Crossley, M. J.; Klose, T.; Orear, E. A.; Peters, M. K.
Tetrahedron 1982, 38, 27–39. (b) Boyle, R. W.; Bruckner, C.; Posakony, J.;
James, B. R.; Dolphin, D. Org. Synth. 1999, 76, 287–293. (c) Boitrel, B.; Lecas,
A.; Renko, Z.; Rose, E. New J. Chem. 1989, 13, 73–99. (d) Boitrel, B.; Lecas,
A.; Renko, Z.; Rose, E. Chem. Commun. 1985, 1820–1821. (e) Lecas-Nawrocka,
A.; Levisalles, J.; Mariacher, C.; Renko, Z.; Rose, E. Can. J. Chem. 1984, 62,
2054–2058. (f) Lecas-Nawrocka, A.; Levisalles, J.; Renko, Z.; Rose, E.
Tetrahedron Lett. 1984, 25, 1563–1566.
(35) Hombrecher, H. K.; Horter, G.; Arp, C. Tetrahedron 1992, 48, 9451–
9460.
(36) Jolicoeur, B.; Chapman, E. E.; Thompson, A.; Lubell, W. D. Tetrahedron
2006, 62, 11531–11563.
(37) Scrambling has been extensively investigated for regular A2B2
porphyrins: (a) Wilson, G. S.; Anderson, H. L. Synlett 1996, 1039–1040. (b)
Littler, B. J.; Ciringh, Y.; Lindsey, J. S. J. Org. Chem. 1999, 64, 2864–2872.
(38) Novak, B. H.; Lash, T. D. J. Org. Chem. 1998, 63, 3998–4010.
(39) Tetrahydroisoindole derivatives could also be obtained via the modified
Knorr chemistry, avoiding expensive malodorous isocyanides required in the
Barton-Zard reaction: (a) Zavyalov, S. I.; Skoblik, T. I. Bull. Acad. Sci. USSR
DiV. Chem. Sci. 1977, 26, 2559–2562. (b) Hombrecher, H. K.; Horter, G.
Synthesis 1990, 389–391. The standard Knorr reaction also can be used: (c)
May, D. A. J. Org. Chem. 1992, 57, 4820–4828.
Diesters 5a-c were further converted into 1,9-unsubstituted
dipyrromethanes 6a-c or, alternatively, ipso-formylated to give
1,9-diformyldipyrromethanes 7a-c for the following porphyrin
4178 J. Org. Chem. Vol. 73, No. 11, 2008

Synthesis of 5,15-Diaryltetrabenzoporphyrins
SCHEME 4. Protonation of Porphodimethenes Occurs
Predominantly at Their Pyrrolenine Nitrogens
(ipso-Protonation, Required for Scrambling, is Improbable)
σ-complex, destabilized by the pyrrolenine nitrogen on the
conjugation path.
For comparison, we have implemented the standard Mac-
Donald’s protocol, for which dipyrromethanes 6 and 7 had to be
isolated and purified. Overall, this method appeared to be less
efficient, as it was more labor intensive and fraught with losses
due to the poor stability of 1,9-unsubstituted dipyrromethanes.
Aromatization of Ar2TCHPs Using DDQ. Conversion of
tetracyclohexenoporphyrins into tetrabenzoporphyrins requires
removal of 16 hydrogens by way of oxidative aromatization.6
For this reaction, Ar4TCHPs need to be first transformed into
their metal complexes,7a whereas meso-unsubstituted TCHPs
can be converted into TBPs with or without metalation.10
Our attempts to synthesize Ar2TBPs by oxidizing metalated
Ar2TCHPs surprisingly turned out unsuccessful. First of all, the
metalation itself appeared to be difficult because of the very
poor solubility of 4a-c free bases (H2Ar2TCHP). For example,
prolonged refluxing of H2Ar2TCHPs with Zn or Cu salts in
common solvents (CHCl3, CHCl3/MeOH, MeCN, or DMF) gave
only traces of the corresponding metal complexes, while the
attempts to insert Pd in either DMF or PhCN simply led to
decomposition of the starting porphyrins. Using porphyrin
dications (H4Ar2TCHP2+), which are much more soluble than
free bases, improved the metalation yields. In a typical
procedure, a mixture containing the dication chloride (or
trifluoroacetate) and, for example, Cu or Zn acetate was brought
to reflux in CHCl3/MeOH and treated with triethylamine (Et3N).
The free base, formed instantaneously upon the addition of Et3N,
was metalated prior to aggregation and precipitation. To insert
Pd, a similar procedure in benzonitrile was adopted. Metalated
Ar2TCHPs (MAr2TCHP: M-4, M ) Zn, Ni, Cu, Pd) appeared
to be much more soluble than the corresponding free bases,
especially in CHCl3 and CH2Cl2.
The second, more serious, problem was encountered during
the aromatization step. The protocol developed for the synthesis
of MAr4TBPs,7 that is, treatment of MAr2TCHPs with DDQ in
boiling solvents, failed to give MAr2TBPs. Analysis of the
reaction mixtures revealed that aromatization of MAr2TCHPs
occurred very slowly and never came to completion.
MAr2TCHPs remained largely unaffected even when DDQ was
activated by Lewis acids (e.g., Sc(OTf)3). Initially, we speculated
that meso-unsubstituted porphyrins undergo oxidative oligo-
merization,44 which could account for the disappearance of both
DDQ and the starting metalloporphyrins. However, MALDI MS
spectra showed no presence of dimers or trimers. Lower
reactivity of MAr2TCHPs compared to that of either
MAr4TCHPs or MTCHPs remains to be understood.
We further attempted to carry out aromatization of Ar2TCHPs
without premetalation6a—a strategy employed successfully in
the synthesis of meso-unsubstituted TBPs.10 Refluxing free bases
4 in toluene with DDQ indeed effected some oxidation, although
complete conversions could not be achieved. Therefore, in terms
of reactivity, H2Ar2TCHPs occupy an intermediate position
between H2Ar4TCHPs, which are altogether inactive in
aromatization,7a and meso-unsubstituted H2TCHPs, which can
be fully aromatized by DDQ.10 Such trend in the reactivity could
be a consequence of different basicities of the respective free
bases;45 that is, more basic H2Ar4TCHPs form dications easier
synthesis. Since the synthesis of 6a-c and the Lindsey reaction9
required practically the same conditions, 6a-c could be used
without isolation and purification. The acid-catalyzed ester
cleavage/decarboxylation of 5a-c was followed by the im-
mediate addition of diformyldipyrromethanes 7a-c and oxida-
tion by DDQ to afford target porphyrins 4a-c in good yields.
MALDI MS analysis of the crude reaction mixtures contain-
ing 4a-c did not reveal any traces of scrambling. Notably,
condensation of diformyldipyrromethane with dipyrromethane
gives porphodimethene,42 but not porphyrinogen—the key
intermediate in the type A condensations (Scheme 1). Por-
phodimethenes are expected to be much more stable toward
the acid-catalyzed cleavage. Unlike porphyrinogens, whose
fission is initiated by protonation of their R-carbons, por-
phodimethenes contain more basic nitrogen atoms in the
azafulvene rings (Scheme 4).43 Besides, even if the ipso-
protonation was to occur, it would result in an unfavorable
(40) Bu4NCl has been occasionally used in the porphyrin syntheses as a water
scavenging agent: Zaidi, S. H. H.; Fico, R. M., Jr.; Lindsey, J. S. Org. Process
Res. DeV. 2006, 10, 118–134.
(41) (a) Archibald, J. L.; Walker, D. M.; Shaw, K. B.; Markovac, A.; F.,
M. S. Can. J. Chem. 1966, 44, 345. (b) Senge, M. O.; Medforth, C. J.; Forsyth,
T. P.; Lee, D. A.; Olmstead, M. M.; Jentzen, W.; Pandey, R. K.; Shelnutt, J. A.;
Smith, K. M. Inorg. Chem. 1997, 36, 1149–1163.
(42) (a) Dolphin, D. J. Heterocycl. Chem. 1970, 7, 275. (b) Harmjanz, M.;
Gill, H. S.; Scott, M. J. J. Org. Chem. 2001, 66, 5374–5383. (c) Bonomo, L.;
Solari, E.; Scopelliti, R.; Latronico, M.; Floriani, C. Chem. Commun. 1999, 2227–
2228. (d) Botulinski, A.; Buchler, J. W.; Abbes, N. E.; Scheidt, W. R. Justus
Liebigs Ann. Chem. 1987, 305–309. (e) Senge, M. O.; Runge, S.; Speck, M.;
Ruhlandt-Senge, K. Tetrahedron 2000, 56, 8927–8932.
(44) Osuka, A.; Shimidzu, H. Angew. Chem., Int. Ed. Engl. 1997, 36, 135–
137.
(43) Wood, T. E.; Thompson, A. Chem. ReV. 2007, 107, 1831–1861, and
references therein.
(45) Finikova, O. S.; Cheprakov, A. V.; Carroll, P. J.; Dalosto, S.;
Vinogradov, S. A. Inorg. Chem. 2002, 41, 6944–6946.
J. Org. Chem. Vol. 73, No. 11, 2008 4179

Filatov et al.
SCHEME 5. Aromatization of Ar2TCHPs 4a-d by
Tetracyclone
than H2TCHPs, and dications are inactive in aromatization.
Another possible reason for lower reactivity of H2Ar2TCHPs
is their poor solubility.
The yields of aromatization were greatly improved by using
molten naphthalene as a solvent. We have found that molten
naphthalene provides an excellent solubilizing medium for
poorly soluble porphyrins. However, in this particular reaction,
the role of naphthalene could not be fully explained by its
solubilizing capacity. No other high-boiling solvent, such as di-
or trichlorobenzenes, benzonitrile, nitrobenzole, or durene, was
as effective as naphthalene, but at the same time, aromatization
in mixtures containing small amounts of naphthalene was in
some cases as effective as that in neat naphthalene.
The reaction of H2Ar2TCHPs 4a-d with DDQ in molten
naphthalene at 180-190 °C resulted in the full conversion after
about 10 h. Unfortunately, MALDI MS analysis revealed the
presence of chlorinated Ar2TBPs as satellite products, and
similar results were obtained when PhCN was used as a solvent
for aromatization of Pd complexes (Pd-4). Preparative separation
of Ar2TBPs from chlorinated Ar2TBPs is not feasible since
introduction of the chlorine atom(s) into a large porphyrin
molecule produces only negligible effect on its chromatographic
mobility.
In summary, all attempts to use DDQ for aromatization of
Ar2TCHPs failed to produce pure Ar2TBPs. The reactions were
either incomplete or suffered from inseparable byproduct.
Aromatization of Ar2TCHPs Using Tetracyclone. Recently,
we have found that 2,3,4,5-tetraphenylcyclopentadienone (tetra-
cyclone) provides an effective alternative to DDQ in aromatization
of porphyrins.46 Tetracyclone is not a single electron-transfer agent,
and therefore, it is incapable of inducing oxidative oligomerization.
On the other hand, it does not contain chlorine atoms. Heating of
porphyrins 4a-d with tetracyclone in the absence of solvent at
180-230° (Scheme 5) led to their complete aromatization into
Ar2TBPs. MALDI MS analysis revealed only two products in
the reaction mixtures (i.e., Ar2TBPs 8a-d) and the Diels-Alder
adducts of the intermediate porphyrins and tetracyclone, which
were aromatized by way of CO extrusion.47 The byproduct did
not form in large quantities, and Ar2TBPs could be purified by
chromatography.
Porphyrins 8a (Ar ) Ph) and 8c (Ar ) 4-BrC6H4) were
obtained in moderate yields; however, yields of 8b and 8d were
quite low. Even prolonged heating of the corresponding precur-
sors with tetracyclone resulted in very low conversions. The
reason for such dramatic difference in reactivity of Ar2TCHPs
with different substituents in the meso-aryl rings remains
unclear.
In conclusion, because of the problems arising at the stage
of aromatization, tetrahydroisondole approach as a whole was
not regarded as general, although in certain cases, it was a
relatively straightforward and cost-effective way to Ar2TBPs.
II. Synthesis of Ar2TBPs via Dihydroisoindole Route.
Recently, we have demonstrated that problems associated with
oxidative aromatization in the synthesis of tetrabenzoporphyrins
can be effectively overcome by employing 4,7-dihydroisoin-
doles.11 The latter compounds lead to precursor porphyrins with
fewer hydrogen atoms in the pendant rings, facilitating aroma-
tization. Earlier, we have shown that reduction in the number
of hydrogens greatly improved the efficiency of aromatization
in a similar synthesis of Ar4TNPs.8
4,7-Dihydroisoindoles are studied very scarcely, although the
first representative of this class was synthesized over 60 years
ago.48 An early attempt to approach π-extended porphyrins via
dihydroisoindole—the closest relative of thermodynamically
unstable 2H-isoindole—was performed by Fuhrhop et al. in
198549 but was unsuccessful. 4,7-Dihydroisoindole was prepared
using the classical Paal-Knorr chemistry (Scheme 6A) under
conditions which probably harmed the electron-rich pyrrole ring.
The apparent instability of 4,7-dihydroisoindole led researchers
to conclude that it was not a useful intermediate in the porphyrin
chemistry. It turned out that changing the conditions made it
possible to isolate 4,7-dihydroisoindole, which proved to be an
extremely practical synthon on the way to tetrabenzoporphyrins.
4,7-Dihydroisoindole is accessible in three major steps from
readily available tosylacetylene (Scheme 6B).11 A modified
variant of the Barton-Zard reaction50 (Scheme 6B) allows
synthesis of 2-alkoxycarbonyl derivatives of 4,7-dihydroisoin-
dole, which are perfectly stable and, in fact, do not differ much
(48) Schenck, G. O. Chem. Ber. 1947, 80, 226–231.
(49) Fuhrhop, J. H.; Hosseinpour, D. Justus Liebigs Ann. Chem. 1985, 689–
695.
(46) Presented in part at ICPP-4 (2006), Rome, Italy: Filatov, M. A.;
Cheprakov, A. V. J. Porphyrins Phthalocyanines 2006, 10, 629.
(47) Ogliaruso, M. A.; Romanell, M. G.; Becker, E. I. Chem. ReV. 1965, 65,
261–367.
(50) (a) Barton, D.; Zard, S. Chem. Commun. 1985, 1098–1100. (b) Barton,
D.; Kervagoret, J.; Zard, S. Tetrahedron 1990, 46, 7587–7598. (c) Abel, Y.;
Haake, E.; Haake, G.; Schmidt, W.; Struve, D.; Walter, A.; Montforts, F. P.
HelV. Chim. Acta 1998, 81, 1978–1996.
4180 J. Org. Chem. Vol. 73, No. 11, 2008

Synthesis of 5,15-Diaryltetrabenzoporphyrins
SCHEME 6. Approaches to 4,7-Dihydroisoindole
SCHEME 7. Synthesis of 5,15-Diphenyloctahydro-
tetrabenzoporphyrin
in their chemical behavior from well-known and much studied
ꢀ-disubstituted 2-alkoxycarbonylpyrroles. Removal of the alkox-
ycarbonyl group can be accomplished either under acidic
(trifluoroacetic acid in CH2Cl2) or basic (reflux with KOH in
ethylene glycol) conditions. In either case, the double bond
remains intact.
Similarity in the behavior of 4,7-dihydroisoindole and tet-
rahydroisoindole enabled us to directly apply methods developed
for the tetrahydroisondole pathway to the syntheses originating
in 4,7-dihydroisoindole. Both routes shown in Scheme 1 (A and
B) were realized in this pathway, leading to precursor 5,15-
diaryloctahydrotetrabenzoporphyrins (Ar2OHTBPs).
Route B (Scheme 7) almost entirely matched its tetrahy-
droisoindole analogue (see Scheme 3).
Similar to the synthesis depicted in Scheme 3, addition of
Bu4NCl improved the yield of dipyrromethane 10a. In this case,
water scavenging was especially beneficial, as in the absence
of Bu4NCl, tarring occurred, and 10a could be isolated only in
a modest yield. Apparently, in the presence of water, the double
bond of the cyclohexadiene ring interferes with acid-catalyzed
condensations.
The formylation reaction, leading to dipyrromethane 12a,
gave a somewhat lower yield than the similar reaction in the
synthesis of Ar2TCHPs (Scheme 3). This also could be caused
by side reactions, affecting the isolated double bonds in the
presence of the strong electrophilic reagent.
In general, route A (Scheme 1) when applied to the dihy-
droisoindole pathway appeared to be more reliable than route
B, and therefore, it was developed into a general preparative
protocol (Scheme 8).
meso-Unsubstituted dipyrromethane 15 was obtained in a
good yield from 2-ethoxycarbonyl-4,7-dihydroisoindole 9a.
Remarkably, the double bond tolerated harsh conditions of the
ester cleavage and decarboxylation. Dipyrromethane 15 was
introduced into the condensation with aromatic aldehydes under
Lindsey conditions to yield the respective Ar2OHTBPs 13a-e.
These porphyrins were identified in the reaction mixtures by
MALDI MS analysis, although they were not isolated and/or
characterized. The MALDI spectra revealed only an insignificant
degree of scrambling, that is, porphyrins with different numbers
of aryl groups. The aromatization was effected directly by
changing the solvent to higher-boiling toluene, followed by a
short heating with an extra portion of DDQ. Like in the synthesis
of Ar4TNPs,8 no metalation was required for aromatization.
Interestingly, in spite of the overall facile reaction, electron-
withdrawing groups in meso-aryls, especially the nitro group,
decreased the rate of aromatization. Porphyrins 13a and 13d
were oxidized almost instantaneously upon bringing their
solutions with DDQ to reflux, porphyrin 13c had to be refluxed
for 30 min, but 4-nitrophenyl-substituted porphyrin 13b could
not be completely aromatized even after prolonged heating. For
quantitative aromatization, 13b had to be treated with DDQ in
J. Org. Chem. Vol. 73, No. 11, 2008 4181

Filatov et al.
SCHEME 8. Synthesis of Ar2TBPs via Dihydroisoindole Pathway (Route A from Scheme 1)
benzonitrile. It is possible that this effect is associated with
solubility rather than with electronic factors. In general, contrary
to the tetrahydroisoindole pathway, aromatization of the precur-
sors was smooth and no traces of chlorination byproduct were
observed.
Overall, the dihydroisoindole pathway is by far the method
of choice for the synthesis of Ar2TBPs, permitting introduction
of various functionalities and giving pure target compounds in
good yields.
Pd tetrabenzoporphyrins.7,10 These characteristics suggest suit-
ability of PdAr2TBPs for oxygen sensing and imaging2 as well
as for PDT.4
Conjugated porphyrin-based molecules are currently under
intensive scrutiny as nonlinear optical materials.19a,53 Tetraben-
zoporphyrins in particular have been investigated as potential
two-photon probes for biomedical imaging.2g We have deter-
mined that, upon irradiation with 110 fs pulses from a
Ti:sapphire laser (76 MHz rep. rate), solutions of 8a exhibit
strong fluorescence, whose intensity was proportional to the
square of the excitation flux, confirming that the two-photon
absorption was the main process underlying the emission from
the singlet excited state (Figure 2).
III. Optical Properties of Ar2TBPs. In this section, we only
briefly outline the optical properties of Ar2TBPs, leaving the
detailed discussion for a separate account.
The optical absorption and fluorescence spectra of porphyrin
8a are shown in Figure 1A. The optical absorption spectra of
free base 8a (A) are strikingly similar to those of meso-
unsubstituted H2TBPs,14 showing a well-resolved vibronic
The two-photon absorption cross-section (σ2) of 8a, deter-
mined by the relative fluorescence method versus Rhodamine
B in methanol (σ2 ) 210 GM,54 λex ) 840 nm, ꢁfl ) 0.555),
was found to be 28 GM (1 GM ) 10-50 cm4 s photon-1), which
is several times higher than the values reported for regular
nonextended porphyrins.56 This value is consistent with other
measurements performed on tetrabenzoporphyrins,57 confirming
that π-extension has a profound effect on nonlinear absorption
of tetrapyrroles.
structure in the Q-band region and a significant (750 cm-1
)
splitting of the B (Soret) band. This feature is typically attributed
to a strong mixing of the Q and B states and to an increase in
the oscillator strengths of the Q-bands in TBPs compared to
regular nonextended porphyrins.14a,51 The fluorescence of 8a
exhibits a sharp band (λmax ) 670 nm), characterized by a very
small Stokes shift (only 2 nm) and a very high for porphyrins
quantum yield (ꢁfl ) 0.38), pointing toward rigid planar
structure, similar to that of meso-unsubstituted H2TBP.15
The Pd complex of 8a, prepared by refluxing the free base
with PdCl2 in benzonitrile, exhibits spectral properties (Figure
1B) close to those of meso-unsubstituted PdTBPs.2a,10,16b,52 An
unusual feature of Pd-8a is the slight splitting of the Q-band
(λmax ) 615 nm) and a shoulder of the main band in the
phosphorescence spectrum (λmax ) 796 nm). The latter property
is unique for Pd-8a and probably for other PdAr2TBPs since
meso-unsubstituted PdTBPs and Pd meso-tetraaryl-TBPs exhibit
quite symmetrical phosphorescent bands, accompanied by small
vibrational satellites. The phosphorescence lifetime of Pd-8a,
measured in Ar-purged dimethylacetamide solution, was found
to be 490 µs, and on air, the phosphorescence was completely
quenched. The phosphorescence quantum yield in the absence
of oxygen was 0.19, which is quite high compared to the other
In conclusion, Ar2TBPs possess an array of useful optical
properties. They combine strong emissivity of the planar TBP
system and enhanced two-photon absorption cross-sections with
the possibility of derivatization via functional substituents in
the meso-aryl rings. These characteristics suggest a wide array
of utilities, including biomedical imaging, sensing, and nonlinear
optical applications. The developed method of synthesis provides
a general route to Ar2TBPs with various substituents, facilitating
engagement of these molecules into applied research.
(53) For examples see: (a) Karotki, A.; Drobizhev, M.; Dzenis, Y.; Taylor,
P. N.; Anderson, H. L.; Rebane, A. Phys. Chem. Chem. Phys. 2004, 6, 7–10. (b)
Drobizhev, M.; Stepanenko, Y.; Dzenis, Y.; Karotki, A.; Rebane, A.; Taylor,
P. N.; Anderson, H. L. J. Am. Chem. Soc. 2004, 126, 15352–15353. (c) Ahn,
T. K.; Kim, K. S.; Kim, D. Y.; Noh, S. B.; Aratani, N.; Ikeda, C.; Osuka, A.;
Kim, D. J. Am. Chem. Soc. 2006, 128, 1700–1704.
(54) Xu, C.; Webb, W. W. J. Opt. Soc. Am. B 1996, 13, 481–491.
(55) Karstens, T.; Kobs, K. J. Phys. Chem. 1980, 84, 1871–1872.
(56) (a) Goyan, R. L.; Cramb, D. T. Photochem. Photobiol. 2000, 72, 821–
827. (b) Karotki, A.; Khurana, M.; Lepock, J. R.; Wilson, B. C. Photochem.
Photobiol. 2006, 82, 443–452.
(51) (a) Nguyen, K. A.; Pachter, R. J. Chem. Phys. 2001, 114, 10757–10767.
(b) Rosa, A.; Ricciardi, G.; Baerends, E. J.; van Gisbergen, S. J. A. J. Phys.
Chem. A 2001, 105, 3311–3327.
(52) Vogler, A.; Kunkely, H.; Rethwisch, B. Inorg. Chim. Acta 1980, 46,
101–105.
(57) Karotki, A.; Drobizhev, M.; Kruk, M.; Spangler, C.; Nickel, E.;
Mamardashvili, N.; Rebane, A. J. Opt. Soc. Am. B 2003, 20, 321–325.
4182 J. Org. Chem. Vol. 73, No. 11, 2008

Synthesis of 5,15-Diaryltetrabenzoporphyrins
isoindole 1a (1.93 g, 10 mmol), dimethoxymethane (0.38 g, 5
mmol), and p-toluenesulfonic acid (0.095 g, 0.5 mmol) was
dissolved in acetic acid (50 mL). The mixture was stirred at room
temperature for 24 h and poured into cold water (200 mL). The
precipitate formed, and it was collected by filtration, dried in
vacuum over P2O5, and recrystallized from ethanol: yield 1.79 g
(90%), white powder, mp 210-211 °C; 1H NMR (CDCl3-d6-
DMSO) δ 10.86 (br s, 2H), 4.15 (q, J ) 7.11 Hz, 4H), 3.60 (s,
2H), 2.62 (m, 4H), 2.38 (m, 4H), 1.60 (m, 8H), 1.23 (t, J ) 7.11
Hz, 6H); 13C NMR (CDCl3-d6-DMSO) δ 161.2, 129.7, 127.5, 117.3,
115.7, 58.7, 22.9, 22.8, 22.8, 21.2, 20.9, 14.2. Anal. Calcd for
C23H30N2O4 (398.49): C, 69.32; H, 7.59; N, 7.03. Found: C, 69.40;
H, 7.50; N, 7.31.
Bis(4,5,6,7-tetrahydro-2H-isoindolyl)methane (3) and Bis(4,7-
dihydro-2H-isoindolyl)methane (15). A mixture of the correspond-
ing dipyrromethane 2 or 14 (3 mmol), KOH (18 mmol, 1.00 g)
and ethylene glycol (30 mL) was refluxed under argon for 1 h.
The mixture was cooled to room temperature, diluted with CH2Cl2
(50 mL), and washed with water (5 × 100 mL) and saturated brine
(20 mL). The mixture was concentrated by evaporation in vacuum,
passed through a short silica gel column (5 cm, eluent: CH2Cl2),
and evaporated in vacuum to dryness. The obtained dipyrromethane
was either immediately used in the porphyrin synthesis without
further purification or stored at -18 °C.
Ar2TCHPs (4a-d) via Route A (Scheme 2). Dipyrromethane 3
(1 mmol, 0.25 g) and aromatic aldehyde (1 mmol) were dissolved
in CH2Cl2 (60 mL) and freshly distilled over CaH2. The mixture
was stirred for 10 min under Ar, and trifluoroacetic acid (TFA)
(0.135 mmol, 0.015 g) was added and the mixture stirred for 6 h.
A solution of DDQ (1.5 mmol, 0.34 g) in toluene (20 mL) was
added, and the mixture was stirred for an additional 2 h, evaporated
in vacuum, and chromatographed on silica gel (eluent: CH2Cl2/
Et3N, 100:1 v/v). Fractions containing the desired porphyrins
(selected by UV-vis spectroscopy) were combined, evaporated in
vacuum to dryness, treated with TFA (0.02 mL), dried in vacuum,
and precipitated from CH2Cl2 solution as free bases upon addition
of diethyl ether.
4a: Yield 0.16 g (48%), dark red crystalline powder, mp >300
°C; UV/vis (CH2Cl2/TFA) λmax (log ꢀ) 426 (5.62), 566 (4.28), 610
(3.93); 1H NMR (CDCl3/TFA) δ 10.12 (s, 2H), 8.22 (m, 4H), 7.88
(m, 6H), 3.78 (m, 8H), 2.66 (m, 8H), 2.31 (m, 8H), 1.73 (m, 8H),
-2.14 (br s, 4H); 13C NMR (CDCl3/TFA) δ 142.6, 140.6, 139.1,
138.5, 138.2, 134.9, 130.0, 128.7, 119.3, 96.7, 25.2, 23.6, 22.9,
22.1; ESI HRMS m/z 679.3823; calcd for C48H47N4 (MH+)
679.3801.
FIGURE 1. Absorption and emission (insets) spectra of 8a (A) in
toluene and of Pd-8a (B) in Ar-purged dimethylacetamide at 23 °C. A
(inset): fluorescence, λex ) 571 nm, ꢁfluo ) 0.38. B (inset): phospho-
rescence, λex ) 610 nm, ꢁphos ) 0.19.
4b: Yield 0.049 g (13%), dark red crystalline powder, mp >300
°C; UV/vis (CH2Cl2/TFA) λmax (log ꢀ) 432 (5.47), 568 (4.28), 614
(3.91); 1H NMR (CDCl3/TFA) δ 10.18 (s, 2H), 8.77 (m, 4H), 8.49
(m, 4H), 3.79 (m, 8H), 2.63 (m, 8H), 2.33 (m, 8H), 1.76 (m, 8H),
-1.94 (br s, 4H); 13C NMR (CDCl3/TFA) δ 148.9, 143.6, 141.4,
139.8, 137.7, 135.8, 123.7, 116.5, 97.7, 25.7, 23.6, 22.8, 21.9;
MALDI-TOF MS m/z 768.96; calcd for C48H45N6O4 (MH+) 769.35.
4c: Yield 0.11 g (27%), dark red crystalline powder, mp >300
°C; UV/vis (CH2Cl2/TFA) λmax (log ꢀ) 428 (5.61), 562 (4.26), 612
(3.83); 1H NMR (CDCl3/TFA) δ 10.14 (s, 2H), 8.12 (m, 4H), 8.05
(m, 4H), 3.78 (m, 8H), 2.70 (m, 8H), 2.33 (m, 8H), 1.77 (m, 8H),
-2.24 (br s, 4H); 13C NMR (CDCl3/TFA) δ 142.3, 140.9, 139.3,
138.1, 137.0, 136.3, 132.1, 125.3, 117.7, 97.0, 25.5, 23.6, 22.9,
22.0; MALDI-TOF MS m/z 836.77; calcd for C48H45Br2N4 (MH+)
837.20.
4d: Yield 0.24 g (60%), dark red crystalline powder, mp >300
°C; UV/vis (CH2Cl2/TFA) λmax (log ꢀ) 428 (5.59), 568 (4.23), 612
(3.82); 1H NMR (CDCl3/TFA) δ 10.18 (s, 2H), 8.58 (m, 2H), 8.35
(m, 2H), 4.15 (s, 3H), 3.79 (m, 8H), 2.65 (m, 8H), 2.32 (m, 8H),
1.74 (m, 8H), -2.37 (br s, 4H); 13C NMR (CDCl3-TFA) δ 167.1,
142.0, 141.7, 140.9, 139.6, 138.3, 135.0, 131.3, 129.8, 118.0, 97.2,
52.8, 25.4, 23.5, 22.8, 21.9; ESI HRMS m/z 795.3878; calcd for
C52H51N4O4 (MH+) 795.3905; MALDI-TOF MS m/z 795.33; calcd
for C52H51N4O4 (MH+) 795.39.
FIGURE 2. Fluorescence of 8a induced by two-photon absorption at
840 nm (110 fs, 76 MHz rep. rate) and its dependence on the incident
laser power (inset). The data points (integrated fluorescence intensity
(I) versus excitation power (P)) were fitted to a quadratic function I(P)
) aP2 (solid line).
Experimental Part
For general description of methods, see Supporting Information.
Tetrahydroisoindole esters 1a and 1b39 and dihydroisoindole 9b11
were synthesized as described previously.
Bis(3-ethoxycarbonyl-4,5,6,7-tetrahydro-2H-isoindolyl)meth-
ane (2). A mixture of 2-ethoxycarbonyl-4,5,6,7-tetrahydro-2H-
J. Org. Chem. Vol. 73, No. 11, 2008 4183

Filatov et al.
Arylbis(3-tert-butoxycarbonyl-4,5,6,7-tetrahydro-2H-isoindolyl)-
methanes (5a-c). p-Toluenesulfonic acid (0.25 mmol, 0.048 g) and
n-tetrabutylammonium chloride (0.1 mmol, 0.028 g) were added
to a solution of 2-tert-butoxycarbonyl-4,5,6,7-tetrahydro-2H-isoin-
dole 1b (5 mmol, 1.105 g) and the corresponding aldehyde (2.5
mmol) in CHCl3 (50 mL). The reaction mixture was stirred at room
temperature under argon for 12 h, washed by NaHCO3 (10%
aqueous solution, 20 mL) and saturated brine (20 mL), and dried
over Na2SO4. The solution was evaporated to dryness, and the
residue was purified by column chromatography on silica gel
(eluent: petroleum ether/ethyl acetate, 5:1). The fractions containing
the target dipyrromethane were identified by TLC (silica gel plates,
eluent: petroleum ether/ethyl acetate, 5:1). Upon heating to 150
°C the spots with Rf 0.85-0.9 turned red. These fractions were
combined and evaporated to dryness.
stirred for an additional 2 h. The workup and the isolation of the
target compounds were performed as in route A. Yields: 4a 0.07 g
(20%), 4b 0.10 g (26%), 4c 0.07 g (17%).
Metal Complexes of Porphyrins 4a-c. Porphyrins 4a-c (0.1
mmol) were dissolved in TFA (0.2 mL), and the solution was
evaporated to dryness. The remaining solid was dissolved in CH2Cl2
(50 mL), and a solution of Cu or Zn acetate in MeOH (5 mL) was
added. The mixture was stirred for 5 min, and Et3N (0.05 mL) was
added in one portion by a pipet. The mixture was stirred for an
additional 5 min, transferred to a separatory funnel, washed with
water (20 mL) and brine (20 mL), and dried over Na2SO4. The
solution was concentrated in vacuum to about 10 mL, and the
resulting metal complex was precipitated by ether. The crystalline
solid was filtered off, washed with ether, and dried. The purity of
the metal complex was assessed by MALDI MS analysis and
UV-vis spectroscopy.
Aromatization of Porphyrins 4a and 4c by 2,3,4,5-Tetraphe-
nylcyclopentadienone (Tetracyclone) (Scheme 5). A mixture of the
porphyrin (4a or 4c) (0.1 mmol) and tetracyclone (2.6 mmol, 1 g)
was placed into a high-pressure glass tube, which was purged with
Ar and closed with a PTFE screw cap. The mixture was heated to
at 230 °C and kept under stirring for 3 h. After cooling to the room
temperature, the mixture was dissolved in CH2Cl2 and chromato-
graphed on silica gel (eluent: CH2Cl2). Further purification of
porphyrins 8a and 8c was achieved by recrystallization from
CH2Cl2/ether. Yields: 8a 0.036 g (55%), 8c 0.037 g (45%). See
the analytical data for porphyrins 8a and 8c below.
5a: Yield 1.26 g (95%), orange amorphous solid; 1H NMR
(CDCl3) δ 8.27 (br s, 2H), 7.36-7.26 (m, 3H), 7.12 (m, 2H), 5.39
(s, 1H), 2.78 (m, 4H), 2.17 (m, 4H), 1.69 (m, 8H), 1.54 (s, 18H);
13C NMR (CDCl3) δ 161.2, 138.8, 129.9, 129.0, 128.2, 127.4, 119.4,
117.8, 80.1, 41. 4, 28.5, 23.3, 23.2, 23.1, 21.2. Anal. Calcd for
C33H42N2O4 (530.70): C, 74.69; H, 7.98; N, 5.28. Found: C, 74.93;
H, 8.16; N, 4.94.
5b: Yield 1.22 g (85%), yellow amorphous solid; 1H NMR
(CDCl3) δ 8.52 (br s, 2H), 8.09 (m, 2H), 7.22 (m, 2H), 5.47 (s,
1H), 2.74 (m, 4H), 2.13 (m, 4H), 1.65 (m, 8H), 1.50 (s, 18H); 13
C
NMR (CDCl3) δ 161.2, 147.1, 146.7, 129.2, 128.3, 128.1, 124.1,
119.9, 118.7, 80.6, 40.9, 28.5, 23.3, 23.1, 21.4. Anal. Calcd for
C33H41N3O6 (575.70): C, 68.85; H, 7.18; N, 7.30. Found: C, 68.41;
H, 7.47; N, 7.02.
2-Ethoxycarbonyl-4,7-dihydro-2H-isoindole (9a). 9a was ob-
tained by the method used previously for the synthesis of tert-butyl
ester 9b:11 yield 73%, white powder, mp 87-88 °C; H NMR
1
5c: Yield 1.44 g (95%), orange amorphous solid; 1H NMR
(CDCl3) δ 8.30 (s, 2H), 7.41 (m, 2H), 6.95 (m, 2H), 5.32 (s, 1H),
2.74 (m, 4H), 2.13 (m, 4H), 1.66 (m, 8H), 1.51 (s, 18H); 13C NMR
(CDCl3) δ 161.2, 138.0, 132.1, 130.0, 129.2, 128.2, 121.3, 119.5,
118.1, 80.2, 40.5, 28.5, 23.3, 23.1, 23.1, 21.3. Anal. Calcd for
C33H41BrN2O4 (609.59): C, 65.02; H, 6.78; Br, 13.11; N, 4.60.
Found: C, 65.42; H, 6.77; N, 4.21.
(CDCl3) δ 8.97 (br s, 1H), 6.70 (m, 1H), 5.87 (m, 2H), 4.30 (q, J
) 7.12 Hz, 2H), 3.43 (m, 2H), 3.22 (m, 2H), 1.35 (t, J ) 7.12 Hz,
3H); 13C NMR (CDCl3) δ 161.6, 124.9, 124.4, 123.8, 118.9, 117.5,
59.8, 24.0, 22.3, 14.5. Anal. Calcd for C11H13NO2 (191.23): C,
69.09; H, 6.85; N, 7.32. Found: C, 68.77; H, 7.00; N, 7.03.
Bis(3-ethoxycarbonyl-4,7-dihydro-2H-isoindolyl)methane (14).
Compound 14 was obtained from 9a by the same procedure as used
in the synthesis of dipyrromethane 2: yield 1.81 g (92%), white
Arylbis(3-formyl-4,5,6,7-tetrahydro-2H-isoindolyl)methanes
(7a-c). Dipyrromethanes 5a-c (2 mmol) were dissolved in TFA
(20 mL) under argon. The solution was stirred for 5 min and cooled
to 0 °C on an ice bath. Trimethoxymethane (20 mmol, 2.1 g) was
added dropwise to the mixture. The mixture was stirred for 15 min
at room temperature, diluted with CH2Cl2 (50 mL), washed with
saturated aqueous NaHCO3 (2 × 20 mL) and saturated brine (20
mL), and dried over Na2SO4. The resulting solution was evaporated
to dryness, and the remaining solid was chromatographed on silica
gel (eluent: CH2Cl2/MeOH). Compounds 7a-c could not be
recrystallized due to their rather poor solubility, and therefore, they
did not give satisfactory elemental analyses.
1
powder, mp 269-270 °C; H NMR (CDCl3-d6-DMSO) δ 11.21
(br s, 2H), 5.76 (m, 4H), 4.18 (q, J ) 7.08 Hz, 4H), 3.71 (s, 2H),
3.25 (m, 4H, overlapped with solvent), 3.00 (m, 4H), 1.26 (t, J )
7.08 Hz, 4H); 13C NMR (CDCl3-d6-DMSO) δ 160.7, 129.3, 124.0,
123.9, 123.1, 115.3, 114.2, 58.6, 23.8, 21.6, 21.4, 14.2. Anal. Calcd
for C23H26N2O4 (394.46): C, 70.03; H, 6.64; N, 7.10. Found: C,
69.88; H, 6.34; N, 6.85.
Ar2TBPs 8a-d via Dihydroisoindole Pathway (Scheme 8).
Freshly prepared dipyrromethane 15 (1 mmol, 0.25 g) was dissolved
in CH2Cl2 (60 mL), distilled over CaH2, and the aromatic aldehyde
(1 mmol) was added to this solution. The mixture was stirred under
Ar at room temperature for 10 min and treated with TFA (0.135
mmol, 0.015 g). The reaction mixture was stirred at room
temperature for 4 h, treated with DDQ (1.5 mmol, 0.34 g) in toluene
(20 mL), and stirred for an additional 2 h. The solution was
evaporated to dryness, and the residue was dissolved in either
toluene (30 mL, 8a,c,d) or in benzonitrile (30 mL, 8b). DDQ (2
mmol, 0.45 g) was added, and the mixture was refluxed for 5-30
min under Ar. The reaction was monitored by UV-vis spectros-
copy. The solvent was removed in vacuum, and the residue was
purified by chromatography on silica gel (eluent: CH2Cl2). Emerald
green fractions were collected, combined, and evaporated to dryness.
The residue was treated with TFA (0.01 mL) and dried in vacuum.
The porphyrins were additionally purified by reprecipitation from
CH2Cl2 solutions by ether. Porphyrins 8a-e were isolated as free
bases.
1
7a: Yield 0.46 g (60%), gray powder, mp >300 °C; H NMR
(CDCl3) δ 9.85 (br s, 2H), 9.35 (s, 2H), 7.30 (m, 3H), 7.11 (m,
2H), 5.50 (s, 1H), 2.83 (m, 4H), 2.30 (m, 4H), 1.77 (m, 8H); 13C
NMR (CDCl3) δ 176.2, 138.1, 129.0, 128.6, 128.2, 127.5, 127.5,
120.9, 40.4, 23.1, 22.7, 21.0, 20.9.
1
7b: Yield 0.69 g (80%), gray powder, mp >300 °C; H NMR
(CDCl3-d6-DMSO) δ 11.31 (s, 2H), 9.38 (s, 2H), 8.02 (m, 2H),
7.16 (m, 2H), 5.50 (s, 1H), 2.74 (m, 4H), 2.41 (m, 4H), 1.70 (m,
8H); 13C NMR (d6-DMSO) δ 176.8, 147.3, 146.4, 133.7, 129.4,
127.7, 123.6, 120.3, 30.3, 22.8, 22.3, 20.5.
1
7c: Yield 0.56 g (60%), gray powder, mp >300 °C; H NMR
(d6-DMSO) δ 11.39 (br s, 2H), 9.43 (s, 2H), 7.51 (m, 2H), 6.99
(m, 2H), 5.56 (s, 1H), 2.75 (m, 4H), 2.40-2.18 (m, 4H), 1.64 (m,
8H); satisfactory 13C NMR spectrum could not be obtained because
of the very low solubility.
Ar2TCHPs (4a-d) via Route B (Scheme 3). Aryldipyrromethanes
5a-c (1 mmol) were dissolved in TFA (20 mL) under argon. The
solution was stirred for 5 min, diluted with CH2Cl2 (50 mL), and
added to a solution of the respective diformyldipyrromethane 7a-c
in CH2Cl2 (45 mL). The resulting mixture was stirred at room
temperature for 4 h, treated with DDQ (1.5 mmol, 0.34 g), and
8a: Yield 0.073 g (22%), the material was characterized in ref
11.
8b: Yield 0.056 g (15%), green powder, mp >300 °C; UV/vis
(CH2Cl2/TFA) λmax (log ꢀ) (CH2Cl2) as free base 394 (4.92), 420
1
(5.70), 436 (5.62), 570 (4.64), 612 (4.99), 666 (4.75); H NMR
4184 J. Org. Chem. Vol. 73, No. 11, 2008

Synthesis of 5,15-Diaryltetrabenzoporphyrins
(CDCl3/TFA) δ 11.01 (s, 2H), 9.36 (d, J ) 7.96 Hz, 4H), 8.32 (m,
4H), 8.24 (m, 4H), 8.20 (m, 4H), 7.94 (m, 4H), 7.69 (d, J ) 8.34
Hz, 4H), -0.35 (br s, 4H); 13C NMR (CDCl3/TFA) δ 139.9, 139.0,
137.3, 136.0, 133.4, 132.9, 131.9, 130.8, 130.6, 125.9, 125.2, 123.6,
114.9, 92.4; LDI-TOF MS m/z 752.17; calcd for C48H28N6O4 (M+)
752.22. Anal. Calcd for C48H28N6O4 ·0.33Et2O: C, 76.21; H, 4.06;
N, 10.81. Found: C, 76.07; H, 4.20; N, 10.73.
4H), 8.12 (m + d overlapped, 6H), 7.77 (m, 4H), 7.51 (m, 4H),
1.57 (s, 36H), -1.33 (br s, 2H); 13C NMR (CH2Cl2) δ 151.7, 141.8,
139.0, 138.9, 137.2, 127.2, 127.0, 126.7, 125.3, 122.0, 120.8, 117.8,
92.8, 35.4, 31.7; ESI HRMS m/z 887.5054; calcd for C64H63N4
(MH+) 887.5053.
Pd-8a. Porphyrin 8a (11 mg, 0.017 mmol) was dissolved in
PhCN (5 mL) and heated to about 100 °C. PdCl2 (10 mg, 0.056
mmol) was added, and the mixture was refluxed under Ar. The
reaction was stopped when the absorption bands of the free base
porphyrin disappeared. Complete conversion required refluxing for
about 2 h. The solvent was evaporated in vacuum. The remaining
emerald green solid was dissolved in a small volume of CH2Cl2
(∼3 mL) and chromatographed a silica gel column (L2 × 20 cm,
eluent: CH2Cl2). The emerald green band was collected, and
evaporation of the solvent gave Pd-8a as a dark green powder:
8c: Yield 0.041 g (10%), green powder, mp >300 °C; UV/vis
(CH2Cl2/TFA) λmax (log ꢀ) as dication 454 (5.67), 616 (4.30), 672
(4.84), as free base 394 (4.78), 422 (5.64), 436 (5.71), 570 (4.35),
1
610 (4.88), 666 (4.59); H NMR (CH2Cl2/TFA) δ 11.00 (s, 2H),
9.36 (d, J ) 7.96 Hz, 4H), 8.36 (m, 4H), 8.22 (m, 8H), 7.93 (m,
4H), 7.70 (d, J ) 8.21 Hz, 4H), 0.02 (br s, 4H); 13C NMR (CDCl3/
TFA) δ 139.9, 139.0, 137.4, 136.0, 133.3, 132.9, 131.9, 130.6,
130.4, 125.8, 125.2, 123.5, 114.9, 92.4; ESI HRMS m/z 819.0765;
calcd for C48H29N4Br2 (MH+) 819.0759.
1
yield 11 mg (86%); H NMR (DMF-d7, 100 °C) δ 11.11 (s, 2H),
9.75 (s, 4H), 8.64 (aa′bb′, J1 ) 6.83 Hz, J2 ) 1.46 Hz, 4H), 8.30
(t, J ) 6.8 Hz, 4H), 8.06 (overlapped with solvent), 7.60 (m, 4H),
7.23 (d, J ) 7.71 Hz, 4H); MALDI-TOF (m/z) calcd for
C48H28N4O12Pd 767.2, found 770.9 [M + H+].
8d: Yield 0.105 g (27%), green powder, mp >300 °C; UV/vis
(CH2Cl2/TFA) λmax (log ꢀ) as dication salt 454 (5.64), 618 (4.40),
672 (4.92), (CH2Cl2) as free base 394 (4.90), 422 (5.59), 436 (5.62),
570 (4.52), 610 (4.93), 666 (4.72); 1H NMR (CH2Cl2/TFA) δ 11.04
(s, 2H), 9.37 (d, J ) 7.96 Hz, 4H), 8.73 (m, 4H), 8.60 (m, 4H),
8.22 (m, 4H), 7.88 (m, 4H), 7.60 (d, J ) 8.21 Hz, 4H), 4.25 (s,
6H), -0.24 (br s, 4H); 13C NMR (CH2Cl2/TFA) δ 168.1, 142.7,
139.4, 139.2, 134.8, 133.0, 131.9, 131.8, 131.2, 130.8, 130.7, 125.2,
123.7, 115.0, 92.7, 53.4; LDI-TOF MS m/z 778.11; calcd for
C52H34N4O4 (M+) 778.26. Anal. Calcd for C52H34N4O4 ·0.5Et2O:
C, 79.49; H, 4.82; N, 6.87. Found: C, 79.30; H, 4.72; N, 6.74.
8e: Yield 0.14 g (32%), green crystals, mp > 300 °C; UV/vis
(CH2Cl2/TFA) λmax (log ꢀ) (CH2Cl2) as free base 394 (4.85), 420
(5.51), 438 (5.55), 570 (4.44), 612 (4.86), 666 (4.70); 1H NMR as
dication (CH2Cl2/TFA) δ 10.97 (s, 2H), 9.32 (d, J ) 7.83 Hz, 4H),
8.33 (d, J ) 1.01 Hz, 4H), 8.15 (dd, Japp ) 7.33 Hz, 4H), 8.13 (d,
2H, overlapped), 7.80 (dd, Japp )7.83 Hz, 4H), 7.60 (d, J ) 8.08
Acknowledgment. Support of the Grants RFBR-04-03-32650
and RFBR-07-03-01121 from Russian Foundation of Basic
Research and the Grants EB007279 and HL081273 from the
NIH USA is gratefully acknowledged. We thank Dr. Alexei
Averin (MSU) for help with NMR and MALDI analysis, and
Dr. Thomas Troxler (Penn RLBL) for assistance with the
femtosecond laser experiments.
Supporting Information Available: Additional procedures,
details of photophysical experiments, and characterization data
for the new compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
Hz, 4H), 1.55 (s, 36H), 0.49 (br s, 4H); H NMR as free base
(CH2Cl2) δ 11.13 (s, 2H), 9.70 (d, J ) 6.94 Hz, 4H), 8.19 (m,
JO800509K
J. Org. Chem. Vol. 73, No. 11, 2008 4185