Porphocyanines: Expanded aromatic tetrapyrrolic macrocycles

Article abstract of DOI:10.1021/ja953721k

Porphocyanines are tetrapyrrolic macrocycles containing two dipyrromethene units linked together by two C=N-C bridges. The syntheses of 2,3,7,8,14,15,19,20-octaethylporphocyanine, 5,17-diphenylporphocyanine, 2,3,7,8-tetraethyl-17-phenylporphocyanine, and other meso-aryl and β-alkyl, including a tetra-β-propionate, derivatives and metal complexes are described. The physical and chemical properties of porphocyanines show them to be aromatic. The aromaticity is reflected in the large deshielding of the outer β-protons and shielding of the inner pyrrolic protons in their NMR spectra. The optical spectra of these compounds exhibit Soret and Q bands similar to porphyrins but all significantly bathochromically shifted. The ability of porphocyanines to act as catalysts for the generation of singlet molecular oxygen was investigated using a cholesterol oxidation assay.

Full text of DOI:10.1021/ja953721k

J. Am. Chem. Soc. 1996, 118, 4853-4859
4853
Porphocyanines: Expanded Aromatic Tetrapyrrolic Macrocycles
Lily Y. Xie, Ross W. Boyle, and David Dolphin*
Contribution from the Department of Chemistry, UniVersity of British Columbia,
VancouVer, B.C. V6T 1Z1, Canada
ReceiVed NoVember 6, 1995X
Abstract: Porphocyanines are tetrapyrrolic macrocycles containing two dipyrromethene units linked together by
two CdN-C bridges. The syntheses of 2,3,7,8,14,15,19,20-octaethylporphocyanine, 5,17-diphenylporphocyanine,
2,3,7,8-tetraethyl-17-phenylporphocyanine, and other meso-aryl and â-alkyl, including a tetra-â-propionate, derivatives
and metal complexes are described. The physical and chemical properties of porphocyanines show them to be
aromatic. The aromaticity is reflected in the large deshielding of the outer â-protons and shielding of the inner
pyrrolic protons in their NMR spectra. The optical spectra of these compounds exhibit Soret and Q bands similar
to porphyrins but all significantly bathochromically shifted. The ability of porphocyanines to act as catalysts for the
generation of singlet molecular oxygen was investigated using a cholesterol oxidation assay.
Introduction
Chart 1
Aromatic macrocyclic compounds capable of absorbing in
the far red or near infrared region are now finding applications
in biomedical research, specifically as photosensitizers for use
in photodynamic therapy (PDT) of hyperprolipherative dis-
eases,1,2 e.g. cancer and psoriasis. PHOTOFRIN®, which is
currently the only PDT agent approved for clinical use, consists
of a mixture of oligomeric porphyrins derived from acetylation
and subsequent base treatment of hematoporphyrin.3 It is
important to realize that this “first generation” photosensitizer
can be improved upon. PHOTOFRIN® absorbs weakly above
630 nm and the depth of light penetration through tissue is
attenuated due to the absorption by heme chromophores. The
“second generation” of PDT sensitizers should, therefore, be
compounds which absorb light at wavelengths significantly
longer than 630 nm. Prior to the synthesis of porphocyanine,
we had success in achieving strong absorptions in the far red
region of the spectrum through modification of the porphyrin
periphery.4 As a result a well-characterized chlorin, called
benzoporphyrin derivative (BPD),5 is currently in phase II
clinical trials. Another strategy for achieving strong absorption
in the red region is to extend the conjugate bridges between
the pyrrole groups in the porphyrin to form so-called “expanded
porphyrins”. Examples of such compounds6 are the tetrapyrrolic
vinylogous porphyrins (1), pentapyrrolic sapphyrins (2), pen-
taphyrins (3), hexapyrrolic hexaphyrins (4), and rubyrins (5),
some of which have been shown to be good singlet oxygen
producers7 and promising photosensitizers for PDT and the
photoinactivation of viruses in blood (PDI).8 We adopted a
similar approach in this study; extending the bridges between
the dipyrromethene units in an R,γ-diazaporphyrin, we were
able to synthesize a new class of aromatic expanded tetrapyrrolic
macrocycle, to which we have given the generic name “por-
phocyanine”.9 The ultimate goal of this research is to synthesize
a series of well-characterized compounds having the same “core”
chromophore, but varying in peripheral substituents, to allow
structure-activity relationships to be defined. Many of the new
PDT photosensitizers have shown limited versatility with regard
to altering the pattern of substitution, which has been shown to
affect the biological activity.10,11 We have, therefore, developed
a synthetic scheme that promises a simple preparation and facile
interchange of functional groups on a porphocyanine framework.
Syntheses of 5-aryl- and 5,17-diarylporphocyanines are central
to this strategy. We report here the facile syntheses of a range
of these compounds having different substituents at both meso
and â positions, all of which are shown to be capable of acting
as catalysts for the generation of singlet molecular oxygen.
X Abstract published in AdVance ACS Abstracts, April 15, 1996.
(1) Bonnett, R. Chem. Soc. ReV. 1995, 19-33.
(2) Pass, H. I. J. Natl. Cancer Inst. 1993, 85, 443-456.
(3) Lipson, R. L.; Blades, E. J. Arch. Dermat. 1960, 82, 508-512.
(4) (a) Scherrer Pangka, V.; Morgan, A.; Dolphin, D. J. Org. Chem.
1986, 51, 1094-1100. (b) Yon-Hin, P.; Wijesekera, T.; Dolphin, D.
Tetrahedron Lett. 1991, 32, 2875-2878.
(5) Tang, H.; Xie, L.; Wijesekera, T.; Dolphin, D. Wavelength-Specific
Photosensitive Porphocyanine and Expanded Porphyrin-Like Compounds
and Methods of Use. U.S. Patent No. 5,405,957, 1995.
(6) Sessler, J. L.; Burrell, A. K. Top. Curr. Chem. 1991, 161, 177-273.
(7) Konig, H.; Erickmeier, C.; Moller, M.; Volcker, A.; Rodewald, U.;
Franck, B. Angew. Chem., Int. Ed. Engl. 1990, 29, 1393-1395.
(8) Sessler, J. L.; Cyr, M.; Maiya, B. G.; Judy, M. L.; Newman, J. T.;
Skiles, H.; Boriack, R.; Matthews, J. L.; Chanh, T. C. Proc. SPIE Int. Opt.
Eng. 1990, 1203, 233-238.
(9) Dolphin, D.; Rettig, S. J.; Tang, H.; Wijesekera, T.; Xie, L. Y. J.
Am. Chem. Soc. 1993, 115, 9301-9302.
(10) Rousseau, J.; Boyle, R. W.; MacLenan, A. H.; Truscott, T. G.; van
Lier, J. E. Nucl. Med. Biol. 1991, 18, 777-782.
(11) Boyle, R. W.; Leznoff, C. C.; van Lier, J. E. Br. J. Cancer 1993,
67, 1177-1181.
S0002-7863(95)03721-8 CCC: $12.00 © 1996 American Chemical Society

4854 J. Am. Chem. Soc., Vol. 118, No. 20, 1996
Xie et al.
(m, 3H), 7.2 (m, 3H), 8.35 (br s, 2H). HRMS (DCI) for C23H32N4
calcd 364.2627, found 364.2626.
Experimental Section
General. All reagents and solvents were used without purification
unless otherwise stated. Ethanol used in the ammonia reaction was
distilled over magnesium turnings under nitrogen prior to use. THF
used in LiAlH4 reductions was distilled over a sodium benzophenone
complex under nitrogen. The purity of isolated compounds and the
progress of all reactions were monitored by thin layer chromatography
on silica gel plates (Merck kieselgel 60; 0.2 mm) or by observing the
change in the optical absorption spectrum. Purification of porphocya-
nines was carried out on neutral deactivated Alumina (Grade II). In
the case of 2,3,7,8-tetraethyl-17-phenylporphocyanine, reversed-phase
HPLC (Waters µBondapak C18 column) on a Waters 994 instrument
was performed for the separation. Molecular modeling was performed
on a Silicon Graphics IRIS system using the program INSIGHT II by
Biosym (San Diego). The lowest energy van der Waals configurations
were determined using the INSIGHT interactive energy minimization
routine. Microanalyses were performed at the Microanalytic Laboratory
of the University of British Columbia.
Electronic absorption spectra were recorded on a Hewlett-Packard
8452A Diode Array spectrophotometer at room temperature. Fluores-
ence emission spectra were measured on an Aminco Bowman Series 2
Luminescence Spectrometer using a flash lamp excitation source.
Values of pH were recorded on a Fisher Acumet pH meter model 210
with a glass combination electrode. The pH meter was calibrated at
25 °C with standard buffer solutions for pH 4.00 and 7.00 prior to pH
measurements. Nuclear magnetic resonance spectra were recorded on
a Bruker WH-400 spectrometer for 1H and a Varian XL-300 spec-
trometer for 19F. The chemical shift of 19F was referenced to the
chemical shift of CF3CO2H. Mass spectra were recorded on a Kratos/
AEI MS-902 for EI and a Kratos Concept II HQ for both LSIMS and
DCI.
2,3,7,8,14,15,19,20-Octaethylporphocyanine (11). Method 1. A
dry THF solution of 1,9-dicyano-2,3,7,8-tetraethyldipyrromethane (6)
(102 mg, 0.33 mmol) was added dropwise to a suspension of LiAlH4
(102 mg, 3.0 mmol) in THF under N2 at 0 °C. The resulting compound
was not isolated. Instead, after quenching the excess reducing agent
with water, filtering, and drying the solution over anhydrous Na2SO4,
a 5-fold molar excess of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ, Aldrich) was added to the stirred solution. The color of the
solution turned from gold to dark green immediately. The resulting
solution was concentrated and then chromatographed on a neutral
alumina column (Grade II) eluting with 10% ethyl acetate in CH2Cl2.
The bright green eluent containing the macrocycle was collected and
evaporated to dryness. Yield: 46 mg, 48%. 1H NMR (1% TFA in
CDCl3): δ -5.75 (br s, 4H), 2.30 (2t, J ) 7.8 Hz, 24H), 4.78 (2q, J
) 7.8 Hz, 16H), 11.97 (s, 2H), 13.75 (s, 4H). UV/vis λ(ꢀ): 458 (2.4
× 105), 592 (1.7 × 104), 634 (5.8 × 103), 728 (3.2 × 103), 798 (2.7 ×
104) nm in CH2Cl2; 454 (7.6 × 105), 600 (1.0 × 104), 608 (1.2 × 104),
624 (1.1 × 104), 668 (8.4 × 103), 682 (5.0 × 103), 744 (4.4 × 104) nm
in TFA/CH2Cl2. HRMS (EI) for C38H48N6 (M+) calcd 588.3941, found
588.3933. Anal. Calcd for C38H48N6‚HCl: C, 73.00; H, 7.90; N, 13.43;
Cl, 5.66. Found: C, 73.48; H, 7.89; N, 12.97; Cl, 5.67.
Method 2. 2,3,7,8-Tetraethyl-1,9-diformyldipyrromethane15 (25 mg,
0.08 mmol) was suspended in dry ethanol (30 mL). The resulting
ethanol solution was chilled to 0 °C and ammonia gas was bubbled
through it for 30 min. The gas inlet was then removed and the flask,
which was sealed with a septum (suba-seal; Aldrich), was placed in an
oil bath and heated at 60 °C. The reaction was stopped after 80 h and
cooled to 0 °C. Ethanol was removed by rotary evaporation and the
residue was chromatographed on Neutral Alumina (Grade II) with 10%
ethyl acetate in CH2Cl2. Yield: 6 mg (26%).
Preparation of Compounds. 2,3,7,8-Tetraethyl-1,9-diformyldipyr-
romethane and dimethyl 2,8-dimethyl-1,9-diformyldipyrromethane-3,7-
dipropionate were prepared according to literature procedures.12 The
5-phenyl-dipyrromethanes were prepared from pyrrole and aromatic
aldehydes in the presence of trifluoroacetic acid according to a reported
procedure.13 Preparation of 1,9-dicyanodipyrromethane derivatives (6,
8-10) except 1,9-dicyano-5-phenyldipyrromethane (7) has been re-
ported elsewhere.14
Spectroscopic data for this compound were found to be identical
with those for compound 11 prepared by Method 1.
Tetraethyl 2,8,14,20-Tetramethylporphocyanine-3,7,15,19-tetra-
propionate (16). This compound was prepared and purified according
to Method 2 above using dimethyl 1,9-diformyl-2,8-dimethyldipyr-
romethane-3,7-dipropionate. Yield: 7%. 1H NMR (1% TFA in
CDCl3): δ -5.80 (br s, 4H); 1.05 (t, J ) 7.5Hz, 12H), 3.60 (t, J ) 7.5
Hz, 8H), 4.03 (q, J ) 7.5 Hz, 8H), 4.22 (s, 12H), 5.06 (t, J ) Hz, 8H),
12.28 (s, 2H), 13.67 (s, 4H). UV/vis λ: 458, 592, 632, 724, 800 nm
in CH2Cl2 and 454, 602, 612, 624, 672, 684, 746 nm in TFA/CH2Cl2.
HRMS (EI) for C46H56N6O8 (M+) calcd 820.4159, found 820.4153.
Anal. Calcd for C46H56N6O8‚2H2O: C, 64.47; H, 7.08; N, 9.80;
Found: C, 63.73; H, 7.18; N, 9.64.
5,17-Diphenylporphocyanine (12). A solution of 1,9-dicyano-5-
phenyldipyrromethane (7) (29 mg, 0.1 mmol) in THF (5 mL) was added
dropwise to a stirred suspension of LiAlH4 (44 mg, 1.3 mmol) in THF
(10 mL) under an atmosphere of N2 at 0 °C. The reduction was
followed by thin layer chromatography (silica gel). Excess LiAlH4
was quenched with a few drops of water and the resulting slurry was
filtered. The filtrate was dried over anhydrous Na2SO4 and filtered
and the solvent was evaporated in Vacuo. The residue was redissolved
in dry CH2Cl2 (100 mL) and to the stirred solution was added DDQ in
5-fold molar excess. The oxidation was followed by UV/vis spectros-
copy. When no further porphocyanine formation was detected, the
reaction mixture was filtered through a plug of neutral alumina to
remove excess DDQ. The crude 5,17-diphenylporphocyanine was
purified by chromatography on alumina as described for octaethylpor-
phocyanine. Yield: 15 mg (29%). 1H NMR (CDCl3): δ 7.92 (m,
6H), 8.44 (m, 4H), 9.38 (d, J ) 5.7 Hz, 4H), 9.8 (d, J ) 5.7 Hz, 4H),
12.95 (s, 4H). UV/vis λ: 452, 598, 640, 814 in CH2Cl2. HRMS (EI)
for C34H24N6 (M+) calcd 516.2062, found 516.2058. Anal. Calcd for
C34H24N6: C, 79.07; H, 4.65; N, 16.28. Found: C, 78.94; H, 4.84; N,
15.98.
1,9-Dicyano-5-phenyldipyrromethane (7). The 5-phenyldipyr-
romethane (70 mg, 0.32 mmol) was dissolved in N,N-dimethylforma-
mide (DMF) (5 mL) and the stirred mixture was cooled to -78 °C. To
the cooled mixture was added dropwise a solution of chlorosulfonyl
isocyanate (200 mg, 1.42 mmol) in acetonitrile (1 mL) under 1 atm of
N2. The mixture was stirred at -78 °C for 1 h and then at -40 °C for
1 h before being allowed to rise to room temperature. The reaction
mixture was poured into aqueous KOH (3 N) and ice, then diluted
with brine (200 mL) and extracted with CH2Cl2 (3 × 50 mL). The
organic phase was dried over anhydrous K2CO3, the solvent evaporated
in Vacuo, and the residue chromatographed on silica gel to give the
1,9-dicyano-5-phenyldipyrromethane. Yield: 29 mg, 32%. 1H NMR
(CDCl3): δ 5.5 (s, 1H), 5.95 (d, J ) 1.5 Hz, 2H), 6.7 (d, J ) 1.5 Hz,
2H), 7.05 (m, 2H), 7.4 (m, 3H), 9.65 (br s, 2H). FTIR (CHCl3) νCN
2220 cm-1. HRMS (EI) for C17H12N4 calcd 272.1062, found 272.1060.
1,9-Di(aminomethyl)-2,8-diethyl-3,7-dimethyl-5-phenyldipyr-
romethane (18). A solution of 1,9-dicyano-2,8-diethyl-3,7-dimethyl-
5-phenyldipyrromethane (10)14 (44 mg, 0.12 mmol) in THF (5 mL)
was added dropwise to a stirred suspension of LiAlH4 (44 mg, 1.3
mmol) in THF (10 mL) under an atmosphere of N2 at 0 °C. The
reduction was followed by thin layer chromatography (silica gel), and
when the reaction was complete, excess LiAlH4 was quenched with
water and the slurry was filtered. The filtrate was dried over anhydrous
Na2SO4. The golden colored solution was concentrated and solid 18
was precipitated with hexane and collected by filtration. Yield: 11
mg (24%). 1H NMR (CDCl3): δ 1.0 (t, J ) 7.4 Hz, 6H), 1.85 (s, 6H),
2.35 (q, J ) 7.4 Hz, 4H), 2.7 (br s, 4H), 3.6 (m, 4H), 5.5 (s, 1H), 7.05
5,17-Bis(3′,4′,5′-trimethoxyphenyl)porphocyanine (13). This com-
pound was prepared and purified according to the procedure outlined
in the preparation of 5,17-diphenylporphocyanine using 1,9-dicyano-
5-(3',4',5'-trimethoxyphenyl)dipyrromethane (8).14 Yield: 32%. 1H
(12) Paine, J. B., III; Woodward, R. B.; Dolphin, D. J. Org. Chem. 1976,
41, 2826-2835.
(13) Lee, C. H.; Linsey, J. S. Tetrahedron 1994, 50, 11427-11440.
(14) Boyle, R. W.; Karunaratne, V.; Jasat, A.; Mar, E. K.; Dolphin, D.
Synlett 1994, 939-940.
(15) Xie, L. Y.; Dolphin, D. J. Chem. Soc., Chem. Commun. 1994, 1475-
1476.

Porphocyanines
J. Am. Chem. Soc., Vol. 118, No. 20, 1996 4855
NMR (CDCl3): δ 4.1 (s, 12H), 4.25 (s, 6H), 7.67 (s, 4H), 9.44 (d, J )
4.8 Hz, 4H), 9.8 (d, J ) 4.8 Hz, 4H), 12.95 (s, 4H). UV/vis λ: 460,
602, 644, 818 nm in CH2Cl2. HRMS (EI) for C40H36N6O6 (M+) calcd
696.2696, found 696.2690. Anal. Calcd for C40H36N6O6‚H2O: C,
67.22; H, 5.36; N, 11.75. Found: C, 67.12; H, 5.47; N, 11.67.
5,17-Bis(pentafluorophenyl)porphocyanine (14). This compound
was prepared according to the procedure outlined in the preparation of
5,17-diphenylporphocyanine using 1,9-dicyano-5-(pentafluorophenyl)-
dipyrromethane (9)14 and purified by chromatography on alumina
(Grade II) eluting with acetone. Solvent was evaporated from the eluent
in Vacuo and the residue was triturated with hexane and filtered to
give 14 as lustrous green crystals. Yield: 28%. 1H NMR (acetone-
d6): δ 9.55 (dd, J1 ) 15.22 Hz, J2 ) 4.15 Hz, 4H), 10.1 (d, J ) 4.15
Hz, 4H), 13.24 (s, 4H). 19F NMR (acetone-d6): δ 60.53, 60.92, 61.45.
UV/vis: λ 442, 582, 624, 814 nm in acetone. HRMS (EI) for
C34H14F10N6 (M+) calcd: 696.1120, found: 696.1124. Anal. Calcd
for C34H14F10N6‚1/2H2O: C, 57.87; H, 2.10; N, 11.78. Found: C, 57.96;
H, 2.44; N, 11.57.
2,3,7,8-Tetraethyl-17-phenylporphocyanine (17). 1,9-Dicyano-
2,3,7,8-tetraethyldipyrromethane (6)14 (62 mg, 0.2 mmol) and 1,9-
dicyano-5-phenyldipyrromethane (7) (55 mg, 0.2 mmol) were dissolved
in dry THF (5 mL) separately and the two solutions were mixed. To
a suspension of LiAlH4 (140 mg) in dry THF (10 mL) was added
dropwise the mixed solution under 1 atm of N2 at 0 °C. The reduction
was followed by TLC and, when complete, excess LiAlH4 was
quenched with a few drops of H2O. The resulting slurry was filtered
and the filtrate dried over anhydrous Na2SO4. Evaporation of solvent
in Vacuo followed by redissolution in CH2Cl2 (20 mL) gave a golden
solution. To this solution was added, dropwise, a solution of DDQ
(0.45 g, 2.2 mmol) in toluene (7 mL). The oxidation was followed by
UV/vis spectroscopy, and when no further porphocyanine formation
was observed the solution was applied to an alumina (Grade II) column
and eluted with 10% ethyl acetate in CH2Cl2. The bright green fractions
containing all three porphocyanines were collected, yielding 44 mg of
a dark green solid. 2,3,7,8-Tetraethyl-17-phenylporphocyanine (17)
was separated from octaethylporphocyanine (11) and diphenylporpho-
cyanine (12) on HPLC (reversed phase) with isocratic elution [0.1%
trifluoroacetic acid in water/0.1% trifluoroacetic acid in acetonitrile (1:
4)] yielding 23 mg (21%) of the isolated 2,3,7,8-tetraethyl-17-
phenylporphocyanine (17). 1H NMR (CDCl3): δ 2.05 (t, J ) 7.5 Hz,
3H), 2.07 (t, J ) 7.5 Hz, 3H), 4.21 (q, J ) 7.5 Hz, 2H), 4.32 (q, J )
7.5 Hz, 2H), 7.88 (m, 3H), 8.41 (m, 2H), 9.31 (d, J ) 4.5 Hz, 2H), 9.7
(d, J ) 4.5 Hz, 2H), 10.3 (s, 1H), 12.72 (s, 2H), 12.96 (s, 2H). UV/
vis: λ 456, 592, 632, 806 nm in CH2Cl2 and 454, 602, 608, 670, 748
nm in TFA/CH2Cl2. HRMS (EI) for C36H36N6 (M+) calcd 552.3001,
found 552.2996. Anal. Calcd for C36H36N6‚1/2CF3COOH: C, 72.80;
H, 6.11; N, 13.80. Found: C, 72.48; H, 6.39; N, 13.77.
filtered (Corion #2424) tungsten halogen lamp (250 W) was positioned
at a distance of 1 cm so that the beam passed through the condenser
and tube. The solution was irradiated with red light (>600 nm). The
filter was cooled with a stream of air to prevent cracking. After 60
min the light was turned off and the tube removed. The solution was
reduced in volume by 75% using a stream of nitrogen, and methanol
(Fisher HPLC grade) was added to give a final volume of 5 mL.
Sodium borohydride (Aldrich) (5 mg) was added and agitated to convert
the unstable hydroperoxides to the corresponding alcohols. Samples
(5 µL) were extracted by micropipet and spotted onto thin layer
chromatograpy plates (Merck kieselgel 60; 0.2 mm) along with genuine
samples containing 7R/â-hydroxycholesterol and 5R-hydroxycholesterol
obtained by 2-methyl-1,4-naphthoquinone (Aldrich) and rose bengal
(Aldrich) photosensitized oxidation of cholesterol. Plates were devel-
oped, twice, by elution with ethyl acetate/hexane (1:1) and products
were visualized by immersing the plates in a 5% solution of
concentrated sulfuric acid in ethanol and heating with a heat gun.
Products were then compared against standards by Rf value relative to
cholesterol: 5R-hydroxycholesterol, 0.39; 7â-hydroxycholesterol, 0.29;
7R-hydroxycholesterol, 0.22.
Results and Discussion
Preparation of the Compounds. Originally, 2,3,7,8,14,15,-
19,20-octaethylporphocyanine (11) was isolated in very low
yield by condensing 2,3,7,8-tetraethyl-1,9-diformyldipyrromethane
and 1,9-di(aminomethyl)-2,3,7,8-tetraethyldipyrromethane in
refluxing MeOH in the presence of molecular sieves (3 Å)
followed by air oxidation. Considerable improvements in the
synthesis of porphocyanines have been made in our laboratory
since that time. The number of reaction steps have been reduced
and the yield of 11 has increased significantly, from 6% to 48%,
based on the dicyano derivative via Method 1 (Scheme 1). The
key intermediate (6) was originally prepared by dehydration of
the corresponding oxime in acetic anhydride. This procedure
gave poor yields and involved a difficult separation of product
from impurities.15 A more efficient method for direct cyanation
of dipyrromethanes was developed in our laboratory. The
original lengthy reaction steps (formylation, oxime formation,
and dehydration) were cut to a single step by employing
chlorosulfonyl isocyanate in DMF/acetonitrile at low temper-
ature. This cyanation reaction proceeds under mild conditions
for all the 1,9-unsubstituted dipyrromethanes used in yields
ranging from 30 to 60%.14 The facile preparation of 1,9-
dicyanodipyrromethanes (6-10) from the readily available
dipyrromethanes gave us potential access to a wide range of
differently substituted porphocyanines. Surprisingly, at this
stage it was found that the THF solution of the supposed 1,9-
bis(aminomethyl)-2,3,7,8-tetraethyldipyrromethane from LiAlH4
reduction of 6 turned green after being left standing at room
temperature in air for a few days. The optical spectrum of the
resulting solution had bands characteristic of the previously
isolated porphocyanine. This observation led to a 4-fold
increase in the yield of 11 when the reduction product of 6 was
simply condensed in refluxing MeOH/THF and subsequently
oxidized with air at room temperature.9 The 1,9-diformyldipyr-
romethane component was thus found to be superfluous! The
yield of octaethylporphocyanine was further improved, to 48%,
when the reduction product of 6 was oxidized in situ by DDQ
immediately after quenching the excess LiAlH4.
Preparation of a Zn(II) Derivative of 2,3,7,8,14,15,19,20-Octa-
ethylporphocyanine (24). Octaethylporphocyanine (11) (8 mg, 0.014
mmol) was dissolved in CH2Cl2 (5 mL), to which ZnCl2 (20 mg, 0.15
mmol) in methanol (5 mL) was added. The resulting solution was
heated at reflux for 20 min. The metalation was monitored spectro-
scopically and, when complete, the mixture was evaporated to dryness.
The crude product was redissolved in CH2Cl2, washed three times with
water, and dried over anhydrous MgSO4. Filtration of the solution and
evaporation to dryness gave a brown solid which was in turn dissolved
in chloroform and transferred to a solvent diffusion apparatus. Diethyl
ether was then slowly diffused into the chloroform solution. Several
1
days later, the dark green crystals were collected. Yield: 46%. H
NMR (CDCl3): δ -5.50 (s, 2H), 2.14, 2.20, 2.24, 2.27 (4t, J ) 7.8
Hz, 24H), 4.48, 4.53, 4.60, 4.62 (4q, J ) 7.8 Hz, 16H), 11.2, 11.38
(2s, 2H), 13.30, 13.55 (2s, 4H). UV/vis: λ 464, 476, 656, 666, 736
nm in CH2Cl2; 480, 658, 762 nm in Et3N/CH2Cl2. HRMS (LSIMS)
for C38H48Cl2N6Zn (M+) calcd 722.2603, found 722.2608.
Cholesterol Oxidation Assay for Singlet Molecular Oxygen.
Cholesterol (Sigma) freshly crystallized from methanol was dissolved
in chloroform (Fisher HPLC grade) to a concentration of 5 mM. The
porphocyanine was then dissolved in 10 mL of this solution to attain
a photosensitizer concentration of 5 µM. The mixed solution was
transferred to a stoppered glass tube fitted with gas inlet and outlet,
and this was placed inside a dry-ice condenser with water flowing
through the outer jacket. Air was bubbled through the solution and a
Diphenylporphocyanine derivatives (12-14) were readily
prepared by the 1,9-dicyano route (Method 1) in moderate
yields. Oxidation with DDQ during the formation of 11 was
instantaneous; however, oxidations during the formation of 12-
14 were slower and an excess of oxidant caused decomposition
of the 5,17-diphenylporphocyanines over a prolonged period
of time. Monitoring of porphocyanine formation by UV/vis
was thus essential for compounds 12-14. Excess DDQ must,

4856 J. Am. Chem. Soc., Vol. 118, No. 20, 1996
Xie et al.
Scheme 1
Scheme 2
methylene group and the â-pyrrolic positions. Formation of
11 should thus be more facile than the formation of 12-14.
The isolation and characterization of 1,9-bis(aminomethyl)-2,8-
diethyl-3,7-dimethyl-5-phenyldipyrrromethane (18) might, at
first, seem inconsistent with the instability of R-(aminomethyl)-
pyrrole. However, the extremely low yield (<1%) of 1517 after
DDQ oxidation of the 1,9-dicyano-2,8-diethyl-3,7-dimethyl-
dipyrromethane reduction product mixture implied to us that
both the 1,9-bis(aminomethyl) derivative 18 and the corre-
sponding porphyinogen-like intermediate 21 were present in
solution (Scheme 2), with 18 as the major product. Molecular
modeling showed that steric interaction of the phenyl and methyl
substituents in 18 limited the flexibility of the dipyrromethane,
thus making the attainment of the correct transition state
geometry for macrocycle formation more difficult. Isolation
of 18 may, therefore, simply be the result of increased difficulty
in forming 21. Molecular modeling also indicated that por-
phocyanine 15 was sterically strained by interaction of the 5,-
17-phenyl and 3,7,15,19-methyl groups causing the porphocy-
anine framework to ruffle. A slight twisting in the bis(p-
nitrophenyl)amethyrin framework (a structurally similar non-
aromatic hexapyrrolic version of porphocyanine) was recently
reported by Sessler et al.18 Porphocyanines bearing â-pyrrolic
alkyl groups (11 and 16) are generally more acid resistant than
those which are â-unsubstituted (12-14). Porphocyanines
substituted on both 5,17 and pyrrolic â positions should have
similar stability to â-alkylporphocyanines while retaining the
possibility of placing a wide range of substituents on the phenyl
groups. Knowing that even more strained porphyrins (perha-
logenated porphyrins19 and dodecasubstituted porphyrins20)
therefore, be removed immediately after porphocyanine forma-
tion is complete to limit degradation and ensure a good yield.
The uniqueness of this self-condensation reaction had attracted
our attention from the outset. Spectroscopic analysis of the
crude reduction product indicated a mixture of several species.
Attempts to trap this supposed 1,9-bis(aminomethyl)-2,3,7,8-
tetraethyldipyrromethane via acetylation resulted in decomposi-
tion. Purification via column chromatography also failed to
provide cleaner compounds. It is known in the literature that
R-(aminomethyl)pyrrole is unstable and can eliminate ammonia
to form a resonance-stabilized azafulvene which is extremely
electrophilic.16 Nucleophilic attack of a primary amine on such
an azafulvene could lead to the formation of a secondary amine.
In the present case, attack of a 1,9-bis(aminomethyl)dipyr-
romethane molecule (e.g. 18) on an azafulvene intermediate (e.g.
19) could result in a cyclic, porphyrinogen-like structure (21,
Scheme 2), which upon oxidation gives porphocyanine 11. The
differences in yields for octaethyl- and diphenylporphocyanines
may be rationalized on the basis of the azafulvene intermediate
since azafulvene 19, with ethyl groups at all four â-pyrrolic
positions, will be both electronically stabilized by induction and
sterically protected from nucleophilic attack at these positions;
conversely, 20 has no electronically stabilizing groups on the
pyrrolic rings and is open to attack at both the exocyclic
(17) This green compound was partially characterized by UV/vis and
mass spectroscopy. UV/vis: λ 462, 604, 642, 802 nm in CH2Cl2. HRMS-
(EI) for C46H48N6 (M+) calcd 684.3940, found 684.3932.
(18) Sessler, J. L.; Weghorn, S. J.; Hiseada, Y.; Lynch, V. Chem. Eur.
J. 1995, 1, 56-67.
(19) Mandon, D.; Fischer, J.; Weiss, R.; Jayaraj, K.; Austin, R. N.; Gold,
A.; White, P. S.; Brigaud, O.; Battioni, P.; Mansuy, D. Inorg. Chem. 1992,
31, 2044-2049.
(20) Shelnutt, J. A.; Medford, C. J.; Berber, M. O.; Barkigia, K. M.;
Smith, K. M. J. Am. Chem. Soc. 1991, 113, 4077-4087.
(16) Paine, J. B., III The Porphyrins; Dolphin, D., Ed.; Academic Press:
New York, 1978; Vol. 1, pp 101-234.

Porphocyanines
J. Am. Chem. Soc., Vol. 118, No. 20, 1996 4857
Scheme 3
Figure 1. Spectral changes of octaethylporphocyanine (11) in CH2-
Cl2 equilibrated with 0.1 M NaCl solutions from pH 3.15 to 7.20 at
22 °C.
Scheme 4
ammonia solution, decomposing over the course of a few hours
without the formation of porphocyanine.
Chemical Properties. The porphocyanine nucleus contains
six nitrogens. The relative basicity of the pyrroleneic nitrogens
verses the bridging imino nitrogens and the pKa values of these
nitrogens are of particular interest to us, as we know that
protonation of porphocyanine not only affects the light absorp-
tion properties but also plays an important role in the biodis-
tribution of photosensitizers in ViVo.21 We have found in our
study that when the pyrrolic â-positions are free of substituents
porphocyanines become sensitive toward acid, particularly
strong acids such as TFA, while â-alkylated porphocyanines
are stable under such conditions. Octaethylporphocyanine (11)
was therefore chosen, and titrated spectrophotometrically in a
biphasic system (CH2Cl2/aqueous NaCl 1:1). Three species
(free base, monocation and dication) were present in solution
throughout the titration (eqs 1 and 2).22 Two equivalents of
acid were added to 11 to give the spectrum of the dication,
which remained unchanged upon addition of more acid. The
bridging nitrogen atom in an etioazaporphyrin is about 7 pKa
units less basic than the pyrroleneic nitrogen.23 However , the
protonation of more than two nitrogen atoms was never observed
with porphocyanine even in concentrated H2SO4.
have been synthesized and are stable, new routes for synthesiz-
ing porphocyanines of this type are being explored.
By analogy with porphyrinogen formation, it is reasonable
to assume that an equilibrium exists between the 1,9-bis-
(aminomethyl)dipyrromethanes and the corresponding non-
aromatic tetrapyrrolic macrocycle. The presence of such an
equilibrium offers the possibility of an efficient mixed conden-
sation to obtain an asymmetric porphocyanine. When the 1,9-
dicyano-5-phenyldipyrromethane (7) and 1,9-dicyano-2,3,7,8-
tetraethyldipyrromethane (6) were reduced together and the
products subsequently oxidized with DDQ (Scheme 3), three
porphocyanines (11, 12, 17) were formed with 11 and 17 being
detected by 1H NMR spectroscopy in a 1.3:1 ratio and 12 only
being detected by reversed-phase HPLC (<3%). The low yield
of 5,17-diphenylporphocyanine in this case would seem to
suggest an order of thermodynamic stability for the three
products of 11 > 17 >> 12.
We have also found that 2,3,7,8-tetraethyl-1,9-diformyldipyr-
romethane reacts with ammonia in dry ethanol (Scheme 4)
(Method 2) leading to the formation of 11 in 26% yield.15 To
our knowledge, there has been no literature precedent where
two aldehydes are directly condensed by the reaction with
ammonia to form an aromatic macrocyclic structure. An
obvious advantage of this method over the cyano route is that
no reduction is involved in the synthesis. Consequently,
porphocyanines with reduction-sensitive functional groups, such
as 16, can be readily prepared by this method. Under these
conditions, transesterification takes place and small amounts of
amide are also formed during the reaction. The amides thus
formed were insoluble and easily removed. Attempts to
condense 1,9-diformyl-5-phenyldipyrromethane with ammonia
in dry ethanol were unsuccessful. The 1,9-diformyl-5-phenyl-
dipyrromethane appeared to be unstable in the ethanolic
K3
H2P + H+ T H3P+
(1)
(2)
K4
H3P+ + H+ T H4P2+
The pKa values of octaethylporphocyanine were estimated
to be 6.0 and 4.4 from the spectral changes recorded at specific
wavelengths for the monocation and dication, respectively22
(Figure 1). Compared to that of sapphyrin (2) whose pKa’s were
also estimated by the same method to be 9.5 and 3.5,
(21) (a) Pottier, R. H.; Kennedy, J. C.; Chow, Y. F. A.; Cheung, F. Can.
J. Spectrosc. 1988, 33, 57-63. (b) Pottier, R. H.; Laplante, J. P.; Chow, Y.
F. A.; Kennedy, J. C. Can. J. Chem. 1985, 63, 1463-1467.
(22) Xie, L. Y.; Dolphin, D. Can. J. Chem. In press.
(23) Grigg, R.; Hamilton, R. J.; Jozefowicz, M. L.; Rochester, C. H.;
Terrell, R. J.; Wickwar, H. J. Chem. Soc., Perkin Trans. 2 1973, 2, 407-
413.

4858 J. Am. Chem. Soc., Vol. 118, No. 20, 1996
Xie et al.
porphocyanine are retained. Addition of acid caused a normal
hypsochromic shift of the Q-type absorption, whereas in 22 Q
bands shift to longer wavelength upon protonation. The
absorption spectra of the three forms of 11 in methanol are
presented in Figure 3.
The 1H NMR spectrum of 11 in 1% TFA/CDCl3 showed well-
resolved peaks at 13.75 (s, 4H) and 11.95 (s, 2H) ppm. The
meso proton signals observed for this compound are more than
1 ppm downfield compared to the corresponding signals for
the dihydrotrifluoroacetate salt of octaethylporphyrin (Table 1),
but are strikingly similar to that of 22.24 The methine protons
of the C-NdC bridge are shifted about 3 ppm downfield from
the meso protons of octaethylporphyrin (23) and 1 ppm from
that of 22, presumably due to the deshielding effect of nitrogen.
Figure 2. Electronic absorption spectra of porphocyanine (11),
sapphyrin (2), and vinylogous porphyrin (1).
1
Attempts to measure the H NMR spectra in 1% TFA/CDCl3
led invariably to the decomposition of the â-unsubstituted
porphocyanines 12-14. The 1H NMR of 12 in CDCl3,
however, also showed well-resolved peaks at 12.95 (s, 4H), 9.8
1
(d, 4H), and 9.38 (d, 4H) ppm. For comparison, the H NMR
spectrum of 11 in CDCl3 was also recorded. Resonance of the
C-NdC methine protons appeared at 12.92 (s, 4H) ppm, while
the meso protons were observed at 10.50 (s, 2H) ppm. The
chemical shift of the four protons on the C-NdC bridges for
12 appears at 12.95 ppm, very similar to that of 11. This signal
stayed relatively constant in 12-14, the substituents on the
phenyl groups do not seem to influence the chemical shifts of
these protons. The â-pyrrolic hydrogen resonances observed
in 12-14, however, are about 1 and 0.5 ppm downfield
compared to the corresponding signals for the tetraphenylpor-
phyrin in CDCl3.25 The shifts reflect the larger ring current in
22π-electron porphocyanines relative to the 18π-electron por-
phyrins. The spectrum recorded for 11 in 1% TFA/CDCl3 also
shows a singlet upfield of TMS, located at -5.75 ppm,
comparable to the signal of NH protons in 22.24
Electrophilic substitution of the meso protons by deuterium
is common in polypyrrolic aromatic macrocycles. The exchange
is rapid for chlorins,26 corroles,27 and sapphyrins,28 but slow
for porphyrins, even at elevated temperature.29 The outer meso
protons of vinylogous octaethylporphyrin (22) readily undergo
electrophilic substitution while the inner (CH) protons are
completely inert. The ease of electrophilic attack on the
porphocyanine nucleus was also demonstrated: when the CF3-
COOD solution of octaethylporphocyanine 11 was left standing
at room temperature overnight complete exchange of all six
protons attached directly to the macrocycle for deuterium
resulted, as shown by mass spectral analysis.
Figure 3. Electronic absorption spectra of octaethylporphocyanine (11)
in methanol.
Table 1. 1H NMR Data for Octaethyl Derivatives of Porphyrin,
[22]Porphyrin, and Porphocyanine in CDCl3/1% TFA
octaethylporphyrin [22]octaethylporphyrin octaethylporphocyanine
(23)
(22)
(11)
12.78 (d)
11.96 (s)
-6.61 (s)
-9.54 (t)
13.75 (s)
11.97 (s)
-5.75 (s)
10.66 (s)
-3.85 (s)
porphocyanine 11 appeared to be less basic than sapphyrin. The
electronic absorption spectra of all the porphocyanines 11-17
exhibit an intense Soret-like band at 440-460 nm and a weaker
band at 798-818 nm, consistent with a fully conjugated
aromatic porphyrin-like structure. The position of the Soret-
like band seems quite constant within the 22π-electron systems,
porphocyanine, vinylogous porphyrin (1) sapphyrin (2), and
pentaphyrin (3) (Figure 2). However, the Q-type band of
porphocyanine 11 exhibited the largest bathochromic shift
among the 22π-electron “expanded porphyrins”. Most striking
is the observation that replacement of the two C-CdC bridges
of vinylogous octaethylporphyrin (22) with two C-NdC
bridges in 11 shifts the longest wavelength absorption band by
more than 200 nm in the free base form! Substituents on the
porphocyanine perimeter cause shifts in the wavelength of the
absorption maxima; however, the basic characteristics of the
(24) Konig, H.; Eickmeier, C.; Moller, M.; Rodewald, U.; Franck, B.
Angew. Chem., Int. Ed. Engl. 1990, 29, 1393-1395.
(25) Storm, C. B.; Teklu, Y.; Sokolshi, E. A. Ann. N.Y. Acad. Sci. 1973,
206, 631-638.
(26) Woodward, R. B.; Skaric, V. J. Am. Chem. Soc. 1961, 83, 4676-
4678.
(27) Johnson, A. W.; Kay, I. T. Proc. Chem. Soc., London 1964, 89-
90.
(28) Bauer, V. J.; Clive, D. L. J.; Dolphin, D.; Paine, J. B., III; Harris,
F. L.; King, M. M.; Loder, J.; Wang, S-W. C.; Woodward, R. B. J. Am.
Chem. Soc. 1983, 105, 6429-6436.
(29) Paine, J. B., III.; Dolphin, D. J. Am. Chem. Soc. 1971, 93, 4080-
4081.

Porphocyanines
J. Am. Chem. Soc., Vol. 118, No. 20, 1996 4859
Figure 4. Observed mass spectrum of Zn(octaethylporphocyanine)-
Cl2 by electron impact ionization (insert: theoretical mass spectrum
of Zn2(octaethylporphocyanine)Cl2 [C38H46N6Zn2Cl2]).
We had originially expected that the large cavity in the center
of the porphocyanine ring and the planar multidentate array of
nitrogens might make this macrocycle a good ligand for
lanthanides30 and actinides.31-33 However, we have been unable
to form a complex between UO2+ and octaethylporphocyanine.
Conversely, when methanolic solutions of the acetate salts of
Cd2+, Mn2+, Co2+, and Zn2+ were added to CH2Cl2 or CHCl3
solutions of octaethylporphocyanine at elevated temperatures,
spectral changes indicated that metal complexes were formed
and the Zn2+ complex (24) was isolated as a crystalline solid
in low yield. The visible spectrum of the complex which
exhibits a split Soret (464, 476 nm) and a weaker Q band (736
nm) changes to a single Soret (480 nm) and a red-shifted Q
band (762 nm) upon addition of base, e.g. Et3N, and can be
restored by neutralization with acetic acid. This suggested the
presence of acidic pyrrolic protons (NH), which was consistent
with the observation of a sharp singlet at δ -5.50 (2H) in the
1H NMR spectrum. The 1H NMR spectrum of 24 displays two
singlets for the protons of the C-NdC bridges (13.3 and 13.55
ppm), and also shows a spliting of the meso signal into two
singlets. Coordination of zinc, therefore, causes the chemical
environments of the two “dipyrromethene” halves of the
porphocyanine to become nonequivalent. These spectroscopic
data suggested the formation of a mono zinc complex. The
mass spectrum, however, displayed an intense cluster of peaks
centered at m/e 788.1717 (relative intensity 100%), indicative
of the presence of two zinc and two chlorine atoms by isotope
analysis (Figure 4). The mass spectrum finding was surprising
in that chloride, which was present only in trace amount as
impurity in the CH2Cl2 solvent, was strongly prefered over
acetate anion which was present in large excess, when forming
the zinc complex. The yield of zinc complex was dramatically
increased by replacing zinc acetate with zinc chloride at the
chelation step. The X-ray structure of the zinc complex (24)
(Figure 5)9 revealed that the metal was tetrahedrally coordinated
to two pyrrolic nitrogens and two chlorine atoms, leaving the
bridging nitrogens uncoordinated and the other two pyrrolic
nitrogens protonated with hydrogen-bonding interactions to the
chlorines. The crystal structure of the zinc complex compli-
Figure 5. ORTEP presentation of molecular structure of Zn(octa-
ethylporphocyanine)Cl2 (24). View of the Zn complex showing the
tetrahedral coordination around zinc and hydrogen-bonding interactions
of NH and chlorine ligands. Mean derivation from the plane is 0.0095
Å. Some relevant bond lengths (Å) are Zn-N1 2.050, Zn-N2 2.063,
Zn-Cl1 2.247, and Zn-Cl2 2.240. The molecule is situated at a
crystallographic center of symmetry, and an asterisk implies 50%
occupancy of the atom at these sites and 50% occupancy at the
corresponding nonasterisked site.
ments the 1H NMR and electronic absorption spectral findings.
The molecular mass, as revealed by liquid secondary ion mass
spectrometry, indeed confirmed the structure revealed by X-ray.
Therefore, it is likely that a disproportionation reaction occurs
on the probe prior to evaporation. A dizinc unit can indeed be
accommodated in the central cavity of amethyrin which has
essentially the same core size as porphocyanine.18
As we were particularly interested in the porphocyanines as
possible new, long-wavelength-absorbing photosensitizers for
photodynamic therapy, we believed it would be prudent to assay
these compounds for their ability to catalyze the singlet oxygen
mediated oxidation of a biological substrate. The photosensi-
tized oxidation of cholesterol can proceed by two different path-
ways: type I (radical mediated) and type II (singlet oxygen
mediated).34 Each pathway leads to discrete products: 7-R/â-
hydroperoxycholesterol for type I and 5R-hydroperoxycholes-
terol for type II. Thin layer chromatographic analysis of the
products of these photosensitized reactions against genuine
samples of 7-R/â- and 5R-hydroxycholesterol indicated that
compounds 11-15 all catalyzed the type II photooxidation of
cholesterol under these experimental conditions.
Concluding Remarks
The ease of preparation of the porphocyanines, their long-
wavelength absorptions in the visible-near infrared region, and
their ability to generate singlet oxygen makes them potential
candidates for photodynamic therapy.
(30) Sessler, J. L.; Murai, T.; Hemmi, G. Inorg. Chem. 1989, 28, 3390-
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada for financial support.
3393.
(31) Sessler, J. L.; Mody, T. D.; Lynch, V. Inorg. Chem. 1992, 31, 529-
531.
(32) Burrell, A. K.; Hemmi, G.; Lynch, V.; Sessler, J. L. J. Am. Chem.
Soc. 1991, 113, 4690-4692.
(33) Day, V. W.; Marks, T. J.; Wachter, W. A. J. Am. Chem. Soc. 1975,
97, 4519-4527.
JA953721K
(34) van Lier, J. E. Photobiological Techniques; Valenzeno, D. P.; Pottier,
R. H.; Mathis, P.; Douglas, R. H., Eds.; Plenum Press: New York, 1991;
pp 85-98.