Tetraphenyloxybenziporphyrin, a new organometallic ligand for silver(III) and gold(III)

Article abstract of DOI:10.1021/ol062108a

Reaction of 4-hydroxyisophthalaldehyde with excess phenyl magnesium bromide gave a dicarbinol and this condensed with pyrrole and aromatic aldehydes in the presence of BF₃·Et₂O to afford, following oxidation with DDQ, novel tetraaryl

Full text of DOI:10.1021/ol062108a

ORGANIC
LETTERS
2006
Vol. 8, No. 23
5263-5266
Tetraphenyloxybenziporphyrin, a New
Organometallic Ligand for Silver(III) and
Gold(III)†
Jessica A. El-Beck and Timothy D. Lash*
Department of Chemistry, Illinois State UniVersity, Normal, Illinois 61790-4160
tdlash@ilstu.edu
Received August 26, 2006
ABSTRACT
Reaction of 4-hydroxyisophthalaldehyde with excess phenyl magnesium bromide gave a dicarbinol and this condensed with pyrrole and
aromatic aldehydes in the presence of BF3 Et2O to afford, following oxidation with DDQ, novel tetraarylcarbaporphyrinoids in 10 24% yield.
‚
−
Further reaction with silver(I) acetate or gold(III) acetate gave stable organometallic derivatives that retained the aromatic characteristics of
the parent macrocycle.
Oxybenziporphyrin 1 (Scheme 1) was the first reported
example of an aromatic carbaporphyrinoid system.1-3 This
porphyrin-like macrocycle was prepared by a “3 + 1”
MacDonald-type condensation4 of a tripyrrane 2 with 4-hy-
droxyisophthalaldehyde (3) in the presence of TFA in
dichloromethane, followed by oxidation with DDQ. The
benzene unit takes on semiquinoid character in 1, allowing
the macrocycle to have an 18π-electron delocalization
pathway.1,5 Oxybenziporphyrins 1 have been shown to act
both as trianionic ligands, giving silver(III) organometallic
derivatives in reactions with silver(I) acetate,6 and as a
dianionic ligand for palladium(II) derivatives.7 Diverse
carbaporphyrinoid systems have now been synthesized and
many of these form stable organometallic derivatives under
mild conditions.8-11 This chemistry parallels studies on the
better known N-confused porphyrins 4 which share the same
* Author to whom correspondence should be addressed.
† Part 40 in the series Conjugated Macrocycles Related to the Porphyrins.
For Part 39, see: Xu, L.; Ferrence, G. M.; Lash, T. D. Org. Lett. 2006, 8,
5113-5116.
type of CNNN coordination core.12,13 However, tripyrranes
2 must be prepared by multistep procedures14 and this limits
the amounts of these porphyrin analogues that are available
(1) Lash, T. D. Angew. Chem., Int. Ed. Engl. 1995, 34, 2533.
(2) Lash, T. D. Synlett 2000, 279.
(3) Lash, T. D. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 2, pp
125-199.
(7) Stepien, M.; Latos-Grazynski, L.; Lash, T. D.; Szterenberg, L. Inorg.
Chem. 2001, 40, 6892. See also: Venkatraman, S.; Anand, V. G.; Pushpan,
S. K.; Sankar, J.; Chandrashekar, T. K. Chem. Commun. 2002, 462.
(8) (a) Graham, S. R.; Ferrence, G. M.; Lash, T. D. Chem. Commun.
2002, 894. (b) Lash, T. D.; Colby, D. A.; Graham, S. R.; Ferrence, G. M.;
Szczepura, L. F. Inorg. Chem. 2003, 42, 7326.
(4) Lash, T. D. Chem. Eur. J. 1996, 2, 1197.
(5) (a) Lash, T. D.; Chaney, S. T.; Richter, D. T. J. Org. Chem. 1998,
63, 9076. (b) Richter, D. T.; Lash, T. D. Tetrahedron 2001, 57, 3659.
(6) Lash, T. D.; Rasmussen, J. M.; Bergman, K. M.; Colby, D. A. Org.
Lett. 2004, 6, 549.
10.1021/ol062108a CCC: $33.50
© 2006 American Chemical Society
Published on Web 10/13/2006

for study. In addition, meso-tetraaryl-substituted porphy-
rinoids may possess greater stability and superior solubility
characteristics.15 Recently, we demonstrated that azulipor-
phyrins 5 can be synthesized in good yields in a one pot-
procedure by reacting azulene with pyrrole and benzaldehyde
in the presence of BF3‚Et2O followed by oxidation with
DDQ.15 Oxidative ring contractions with alkaline solutions
of tert-butyl hydroperoxide gave the related benzocarbap-
orphyrins.15 Others have shown that benzene dicarbinols 6
react with pyrrole and benzaldehyde under similar conditions
to give tetraphenylbenziporphyrin 7a,16 and we have adapted
this approach to the synthesis of dimethoxybenziporphyrins
7b and 7c.17 However, no direct routes to the aromatic
oxybenziporphyrins have been reported and this has severely
limited studies on the chemistry of this important carbap-
orphyrinoid system.
Scheme 2
romethane or chloroform in the presence of BF3‚Et2O,
followed by oxidation with DDQ. Although 8 was formed
with either solvent, superior results were obtained with
chloroform. The yield was also dependent on the concentra-
tion of catalyst, and better yields were obtained when the
amount of BF3‚Et2O was doubled compared to the amounts
used to prepare 7b and 7c. Catalytic amounts of TFA in place
of BF3‚Et2O also gave some product formation, although the
yields were low compared to using the Lewis acid catalyst.
Under the best conditions, following purification by column
chromatography and recrystallization from hexanes, tetraphe-
nyloxybenziporphyrin was isolated as shiny purple crystals
in 10-13% yield. Reaction of 9 with 4-chlorobenzaldehyde
and pyrrole gave even better results and the related bis(4-
chlorophenyl)diphenyloxybenziporphyrin 8b was isolated in
22-24% yield.
Scheme 1
Scheme 3
We now report a straightforward two-step synthesis of
tetraphenyloxybenziporphyrin 8 (Scheme 3). Dialdehyde 3
was treated with excess phenylmagnesium bromide in
refluxing THF for 2 h, cooled to room temperature, and
treated with aqueous ammonium chloride solution to give
the dicarbinol 9 in virtually quantitative yields. This was
reacted under Lindsey-Rothemund conditions18,19 with 2
equiv of benzaldehyde and 3 equiv of pyrrole in dichlo-
(9) (a) Lash, T. D.; Hayes, M. J.; Spence, J. D.; Muckey, M. A.; Ferrence,
G. M.; Szczepura, L. F. J. Org. Chem. 2002, 67, 4860. (b) Muckey, M. A.;
Szczepura, L. F.; Ferrence, G. M.; Lash, T. D. Inorg. Chem. 2002, 41,
4840. (c) Lash, T. D.; Colby, D. A.; Szczepura, L. F. Inorg. Chem. 2004,
43, 5258. See also: Bru¨ckner, C. J. Chem. Educ. 2004, 81, 1665.
(10) Liu, D.; Ferrence, G. M.; Lash, T. D. J. Org. Chem. 2004, 69, 6079.
(11) (a) Miyake, K.; Lash, T. D. Chem. Commun. 2004, 178. (b)
Bergman, K. M.; Ferrence, G. M.; Lash, T. D. J. Org. Chem. 2004, 69,
7888.
(12) Srinivasan, A.; Furuta, H. Acc. Chem. Res. 2005, 38, 10.
(13) Harvey, J. D.; Ziegler, C. J. Coord. Chem. ReV. 2003, 247, 1.
(14) (a) Sessler, J. L.; Johnson, M. R.; Lynch, V. J. Org. Chem. 1987,
52, 4394. (b) Lash, T. D. J. Porphyrins Phthalocyanines 1997, 1, 29.
(15) (a) Colby, D. A.; Lash, T. D. Chem. Eur. J. 2002, 8, 5397. (b)
Lash, T. D.; Colby, D. A.; Ferrence, G. M. Eur. J. Org. Chem. 2003, 4533.
(16) (a) Stepien, M.; Latos-Grazynski, L. Chem. Eur. J. 2001, 7, 5113.
(b) Stepien, M.; Latos-Grazynski, L. Acc. Chem. Res. 2005, 38, 88.
(17) Szymanski, J. T.; Lash, T. D. Tetrahedron Lett. 2003, 44, 8613.
(18) Lindsey, J. S. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 1,
pp 45-118.
(19) (a) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.;
Marguerettaz, A. M. J. Org. Chem. 1987, 52, 827. (b) Lindsey, J. S.;
Wagner, R.W. J. Org. Chem. 1989, 54, 828. (c) Lindsey, J. S.; MacCrum,
K. A.; Tyhonas, J. S.; Chuang, Y.-Y. J. Org. Chem. 1994, 59, 579. (d)
Geier, G. R., III; Lindsey, J. S. J. Chem. Soc., Perkin Trans. 2 2001, 677.
Oxybenziporphyrin can potentially exist as “keto” or
“enol” tautomers 8 and 10, respectively, but only the former
would possess overall aromatic character. Due to the presence
of a meso-substituent so close to the oxygen atom, this
5264
Org. Lett., Vol. 8, No. 23, 2006

compound cannot be completely planar and this could reduce
the favorability of tautomer 8. Nevertheless, tetraphenyloxy-
benziporphyrin shows a powerful diamagnetic ring current
in its proton NMR spectrum (Figure 1) and the internal CH
Figure 2. UV-vis spectra of tetraphenyloxybenziporphyrin 8a:
black line, free base in chloroform; red line, dication 11a in 1%
TFA-chloroform.
Figure 1. Proton NMR spectrum (400 MHz) of meso-tetraphe-
nyloxybenziporphyrin 8a in CDCl3. The singlet at -3 ppm
corresponds to the internal CH.
carbon-13 NMR spectra for 11a also showed the disappear-
ance of the carbonyl resonance. The UV-vis spectra for the
diprotonated species also little resembled those of true
porphyrins, showing weakened bands in the Soret region at
398 and 479 nm, and a broad band centered near 900 nm
(Figure 2). Attempts to observe an intermediary monopro-
tonated species 12 by proton NMR or UV-vis spectroscopy
were unsuccessful.
Oxybenziporphyrins 1 have previously been shown to form
silver(III) complexes under mild conditions.6 Tetrapheny-
loxybenziporphyrin (8a) also reacted with silver(I) acetate
in refluxing pyridine to give the silver(III) organometallic
derivative 13a (Scheme 4) in 42% yield. Although silver-
appears upfield near -3 ppm. As expected, the pyrrolic
protons are shifted downfield appearing as a series of
doublets or AB quartets between 7.9 and 8.5 ppm. The NMR
data are fully consistent with the keto form 8a, but the
magnitude of the ring current is diminished compared to the
etio-type system 1, which shows the internal CH at -7.2
ppm.1,5 This is due to steric crowding from the meso-
substituents and similar effects have been noted for tetraary-
lazuliporphyrins and benzocarbaporphyrins. In addition,
alkyl-substituted N-confused porphyrins show a more pro-
nounced diamagnetic ring current in their proton NMR
spectra compared to the corresponding meso-tetrasubstituted
porphyrins.20,21 The presence of a carbonyl resonance at 180
ppm can be discerned in the carbon-13 NMR spectrum of
8a in CDCl3, and this is confirmed in the IR spectrum (KBr),
which shows a carbonyl absorption at 1615 cm-1. Solutions
of oxybenziporphyrins 8 in 1% Et3N-chloroform gave a
porphyrin-like UV-vis spectrum with a Soret band at
466 nm, and Q-type absorptions at 567, 716, and 782 nm
(Figure 2).
Scheme 4
Addition of one drop of TFA to the NMR solution of 8a
or 8b in CDCl3 resulted in the formation of a new species
that is assigned as the phenolic dications 11. The proton
NMR spectrum of 11a in TFA-CDCl3 showed the NH
resonances at 7.53, 8.86, and 9.20 ppm, while the CH was
observed at 4.25 ppm. In addition, the exterior pyrrolic
protons were observed between 7.4 and 8.0 ppm. These data
indicate that very little of the diatropic character associated
with the free base is retained in the dicationic species. The
(III) derivatives are known for many different carbaporphy-
rinoid systems, few examples of the related gold(III)
derivatives have been reported.9c,22 However, 8a reacted with
gold(III) acetate in pyridine at room temperature to give an
excellent yield of the gold(III) porphyrinoid 13b. The
structures of these new metallo-derivatives were confirmed
by mass spectrometry and NMR spectroscopy. The silver
complex 13a gave orange solutions and its UV-vis spectrum
in chloroform showed the presence of a strong Soret band
(20) (a) Lash, T. D.; Richter, D. T.; Shiner, C. M. J. Org. Chem. 1999,
64, 7973. (b) Morimoto, T.; Taniguchi, S.; Osuka, A.; Furuta, H. Eur. J.
Org. Chem. 2005, 3887.
(21) Although the NMR data give valuable insights into the aromatic
character of these porphyrin analogues, it is important to note that proton
chemical shifts are not always reliable indicators for aromaticity. See:
Wannere, C. S.; Corminboeuf, C.; Allen, W. D.; Schaefer, H. F., III;
Schleyer, P. v. R. Org. Lett. 2005, 7, 1457.
(22) Recently, gold(III) organometallic derivatives of hexaphyrins have
also been reported. Mori, S.; Osuka, A. J. Am. Chem. Soc. 2005, 127, 8030.
Org. Lett., Vol. 8, No. 23, 2006
5265

at 452 nm, followed by three smaller bands at 539, 593, and
641 nm. The spectrum for the gold complex 13b was very
similar, showing a Soret band at 462 nm and smaller
absorptions at 533, 587, and 641 nm (Figure 3). Both of the
Figure 4. Proton NMR spectra (400 MHz) of metallotetrapheny-
loxybenziporphyrins 13 in CDCl3: lower spectrum, silver(III)
derivative 13a; upper spectrum, gold(III) complex 13b. Both spectra
show increased diatropic character compared to 8a, although the
gold complex shows slightly larger downfield shifts for the pyrrolic
protons.
Figure 3. UV-vis spectrum of gold(III) complex 13b in chloro-
form showing a porphyrin-like Soret band and three Q-type
absorptions.
the macrocyclic cavity and can hold the system in a more
planar arrangement that facilitates π-conjugation.
The direct synthesis of oxybenziporphyrins with Lindsey-
Rothemund conditions makes this system far more accessible
and will allow the chemistry of this aromatic macrocycle to
be further explored. In addition, the ability of this framework
to stabilize organometallic derivatives for metals in higher
oxidation states may also allow the development of new
catalytic systems.
spectra for the metallo-species are far more porphyrin-like
than the free base 8a, suggesting increased aromatic char-
acter, and this was confirmed by the NMR data. The proton
NMR spectra for 13a and 13b showed the presence of strong
diatropic ring currents where the pyrrolic hydrogens showed
up as a series of resonances between 8.3 and 8.7 ppm,
although the downfield shifts were slightly larger for the
gold(III) complex (Figure 4). Carbon-13 NMR data showed
the presence of a carbonyl resonance for 13a and 13b near
200 ppm, while the IR spectra showed peaks at 1653 cm-1
confirming the presence of a cross-conjugated carbonyl
moiety. These data indicate that these metallo-derivatives
are substantially more aromatic than 8a, although the effect
is slightly larger for the gold(III) complex. The free base
form 8a has three internal hydrogens that produce additional
steric interactions in addition to those due to the crowded
macrocyclic periphery. However, as we have demonstrated
previously,9 the silver(III) and gold(III) cations fit nicely into
Acknowledgment. This material is based upon work
supported by the National Science Foundation under Grant
Nos. CHE-0134472 and CHE-0616555, and the Donors of
the Petroleum Research Fund, administered by the American
Chemical Society.
Supporting Information Available: Experimental pro-
1
cedures and UV-vis, H NMR and 13C NMR spectra for
selected compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
OL062108A
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Org. Lett., Vol. 8, No. 23, 2006