Unusual formation of 21-oxacorrole from 21-oxaporphyrin induced by phosphoryl chloride

Article abstract of DOI:10.1021/ol303527v

An unusual formation of 21-oxacorrole from 21-oxaporphyrin by concomitant elimination of a meso-aryl group and ring contraction is reported.

Full text of DOI:10.1021/ol303527v

ORGANIC
LETTERS
2013
Vol. 15, No. 5
1040–1043
Unusual Formation of 21-Oxacorrole from
21-Oxaporphyrin Induced by Phosphoryl
Chloride
Avijit Ghosh,† Tamal Chatterjee,† Way-Zen Lee,‡ and Mangalampalli Ravikanth*,†
Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India, and
Instrumentation Center, Department of Chemistry, National Taiwan Normal University,
88 Sec. 4 Ting-Chow Road, Taipei, 11677, Taiwan
ravikanth@chem.iitb.ac.in
Received January 2, 2013
ABSTRACT
An unusual formation of 21-oxacorrole from 21-oxaporphyrin by concomitant elimination of a meso-aryl group and ring contraction is reported.
Corroles are tetrapyrrolic macrocycles containing three
methine bridges and one direct pyrroleꢀpyrrole linkage.1
Corroles contains one carbon atom less than porphyrins
but possesses three inner NH protons instead of two inner
NH protons present in porphyrin. Thus, corroles have a
greater ability to stabilize higher central metal oxidation
states than porphyrins. In addition to their very interesting
metal coordination chemistry, corroles also exhibit novel
spectral, electrochemical, and luminescent properties as
evident from the extensive research on corrole chemistry in
the past decade.2 Although, now the chemistry of corroles
with a regular N4 core is relatively better understood, the
chemistry of core-modified corroles2b largely remains un-
explored. The core-modified corroles resulting from the
replacement of one or two inner nitrogens with other
heteroatoms suchassulfur, oxygen, selenium, phosphorus,
tellurium, etc. are expected to possesses interesting physico-
chemical properties that differ from regular N4 corroles.
However, reports on meso-aryl heterocorroles are very few
because of the lack of proper synthetic protocols and
instability of these macrocycles. Recently we reported3 the
synthesis of stable meso-aryl 22-thiacorroles by condensing
thiophene carbinol with aryl aldehyde and pyrrole under
mild acid catalyzed conditions. Chandrashekar and co-
workers4 isolated the 22-oxacorrole containing a direct
pyrroleꢀpyrrole linkage as a side product during their
attempts toward the synthesis of stable meso-triaryl smarag-
dyrins by 3 þ 2 condensation of 16-oxatripyrrane and meso-
aryl dipyrromethane under acid catalyzed conditions. Lee
and co-workers5 developed a rational approach for the
synthesis of both 21-oxa and 22-oxacorroles by using
appropriate precursors. Thus, the possible two corroles that
† Indian Institute of Technology Bombay.
(3) Shetti, V. S.; Prabhu, U. R.; Ravikanth, M. J. Org. Chem. 2010,
75, 4172–4182.
‡ National Taiwan Normal University.
(1) (a) Paolesse, R. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic: San Diego, CA, 2000; Vol. 2, ch. 11, pp
201ꢀ232. (b) Erben, C.; Will, S.; Kadish, K. M. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic:
San Diego, CA, 2000; Vol. 2, ch. 12, pp 233ꢀ300.
(4) (a) Narayanan, S. J.; Sridevi, B.; Chandrashekar, T. K.; Englich,
U.; Ruhlandt-Senge, K. Org. Lett. 1999, 1, 587–590. (b) Sankar, J.;
Anand, V. G.; Venkatraman, S.; Rath, H.; Chandrashekar, T. K. Org.
Lett. 2002, 4, 4233–4235. (c) Sankar, J.; Rath, H.; PrabhuRaja, V.;
Chandrashekar, T. K.; Vittal, J. J. J. Org. Chem. 2004, 69, 5135–5138.
(5) (a) Cho, W.-S.; Lee, C.-H. Tetrahedron Lett. 2000, 41, 697–701.
(b) Lee, C.-H.; Cho, W.-S.; Ka, J.-W.; Kim, H.-J.; Lee, P.-H. Bull.
Korean Chem. Soc. 2000, 21, 429–433.
(2) (a) Aviv, I.; Gross, Z. Coord. Chem. Rev. 2011, 255, 717–736 and
references cited there in. (b) Gryko, D. T. Eur. J. Org. Chem. 2002, 1735–
1743 and references cited therein.
r
10.1021/ol303527v
Published on Web 02/08/2013
2013 American Chemical Society

can be derived from 21-oxaporphyrin I macrocycle are (1)
the 21-oxacorrole II that has a direct pyrroleꢀpyrrole
link and (2) the 22-oxacorrole III that has a direct pyrroleꢀ
furan link (Scheme 1). Among these two corroles, the
21-oxaporphyrin 2, we carried out the reaction by treating
3 with excess POCl3 in toluene in the presence of triethyl-
amine at reflux for 10h (Scheme 2). TLC analysis showed a
major bright pink spot followed by one clear minor green
spot. The crude compound was subjected to silica gel
column chromatography using petroleum-ether/dichloro-
methane, and the fast moving major pink fraction (8%)
and the subsequent green fraction (6%) were collected
separately. The mass spectral analysis showed a molecular
ion peak at m/z 528.2069 (HRꢀMS) for the pink fraction
and 661.35 (ESꢀMS) for the green fraction (Supporting
Information pp S3ꢀS4). Although we thought that the
pink fraction could be our desired phosphorus derivative
of 21-oxaporphyrin 2, mass spectral analysis indicated that
the minor green fraction is the phosphorus inserted
21-oxaporphyrin 2. Even though we proposed the phosphorus
derivative of 21-oxaporphyrin 2 formed in this reaction as
a phosphorus(III) derivative, we do not rule out the
possibility of formation of a phosphorus(V) derivative
since we did not have any crystal structure evidence at
present.10 The absorption spectrum recorded for the pink
fraction showed Q-type bands at 523, 488, 549, and 604 nm
and a split Soret band at 402 and 416 nm indicating the
formation of a corrole type of macrocycle. A perusal of
literature revealedthat the absorption spectrum of the pink
fraction matched closely with the reported 21- oxacorrole
by Lee et al.5 indicating that the 21-oxaporphyrin underwent
ring contraction by dearylation of the meso-aryl group which
Scheme 1. Possible 21- and 22-Oxacorroles from the Parent
21-Oxaporphyrin
22-oxocorrole was characterized crystallographically and
spectral, electrochemical, photophysical, and metal com-
plexation properties were explored reasonably well,6 whereas
the 21-oxacorrole containing a direct pyrroleꢀfuran linkage
has not been investigated in detail.5 In this paper, we report
the unusual formation of free base 5,10,15-triphenyl-
21-oxacorrole 1 containing a pyrroleꢀfuran direct linkage
during our attempts to prepare a phosphorus derivative
of 21-oxaporphyrin 2 by treating 5,10,15,20-tetraphenyl-
21-oxaporphyrin 3 with POCl3 in the presence of triethyla-
mine in toluene under reflux conditions. Prior to our study,
Chan et al.7 reported the formation of oxorhenium(V)
corrolate during their attempts to metalate the highly electron-
deficient 5,10,15,20-tetrakis(trifluoromethyl)porphyrin in
PhCN at refluxing temperature. Grazynski and co-workers8
showed the unique formation of 3-pyranone dioxacorrole
from 21,23-dioxaporphyrin by solid-supported Achmatowicz
rearrangement in the porphyrin macrocycle. Our serendipi-
tous observation of the formation of free base 21-oxacorrole 1
from free base 21-oxaporphyrin 3by concomitant elimination
of one meso-aryl group and ring contraction is one of the
unusual rare reactions on the porphyrin macrocycle.9
Scheme 2. Formation of 21-Oxacorrole from 21-Oxaporphyrin
along with the Formation of Phosphorous Derivative of
21-Oxaporphyrin10
Since our interest is to insert phosphorus into 21-oxa-
porphyrin 3 to prepare a phosphorus derivative of
(6) Sridevi, B.; Narayanan, S. J.; Chandrashekar, T. K.; Englich, U.;
Ruhlandt-Senge, K. Chem.;Eur. J. 2000, 6, 2554–2563.
(7) Tse, M. K.; Zhang, Z.; Mak, T. C. W.; Chan, K. S. Chem.
Commun. 1998, 1199–1200.
(8) Pawlicki, M.; Bykowski, D.; Szterenberg, L.; Latos-Grazynski, L.
Angew. Chem., Int. Ed. 2012, 51, 2500–2504.
is present between furan and pyrrole to form 21-oxacorrole.
The plausible mechanism for this unusual reaction is pre-
sented in Scheme 3.
(9) (a) Senge, M. O.; Sergeeva, N. N. Angew. Chem., Int. Ed. 2006, 45,
7492–7495. (b) Gros, C. P.; Barbe, J.-M.; Espinosa, E.; Guilard, R. Angew.
Chem., Int. Ed. 2006, 45, 5642–5645. (c) Skonieczny, J.; Latos-Grazynski, L.;
Szterenberg, L. Chem.;Eur. J. 2008, 14, 4861–4874. (d) Jeandon, C.;
Ruppert, R.; Callot, H. J. Chem. Commun. 2004, 1090–1091. (e) Ramdhanie,
B.; Stern, C. L.; Goldberg, D. P. J. Am. Chem. Soc. 2001, 123, 9447–9448. (f)
Jeandon, C.; Ruppert, R.; Callot, H. J. J. Org. Chem. 2006, 71, 3111–3120.
(10) The formation of the phosphorous derivative of 5,10,15,20-
tetraphenyl-21-oxaporphyrin 2 was confirmed by a molecular ion peak
in an ESꢀMS mass spectrum (Supporting Information p S4). The
complete characterization and properties of 2 are under investigation
in our laboratory (Supporting Information pp S11ꢀS12, S14, and S22).
Org. Lett., Vol. 15, No. 5, 2013
1041

Scheme 3. Proposed Reaction and Ring Contraction
Mechanism from 3 to 1
Figure 1. Single crystal X-ray structure of the compound 1: (a)
perspective view and (b) side view showing the nonplanarity of the
macrocycle and the cross-orientation of the trans meso-aryl groups.
The proof for the formation of 21-oxacorrole 1 by the
ring contraction of 21-oxaporphyrin 3 was unambiguously
elucidated by single-crystal X-ray diffraction analysis.11
Suitable crystals of compound 1 were obtained by slow
diffusion of n-hexane into the dry dichloromethane/
chloroform solution for over a period of one week. The
crystal structure of 1 is shown in Figure 1. The single
crystal X-ray structure of 1 obtained here is compared with
the reported6 analogous 5,10,15-triphenyl-22-oxacorrole4
5 and 5,20-bis(p-tolyl)-10,15-diphenyl-21-oxaporphyrin12
6. The compound 1 was found to be crystallized in the
monoclinic space group P21/c unlike compound 5 which
crystallized in an orthorhombic space group P212121. The
macrocycle 1 adopts a near-planar conformation like
compound 5 as evident from Figure 1. This is in contrast
to21-oxaporphyrin6whichissignificantlydistorted. Thus,
the direct furanꢀpyrrole bond in compound 1 and the
direct pyrroleꢀpyrrole bond in compound 5 assisted these
contracted macrocycles to be more planar compared to the
distorted porphyrin macrocycle 6. The close inspection of
crystal structures of compounds 1, 5, and 6 indicated that
the deviation of the furan ring from the mean plane of the
macrocycle in compounds 1 (5.69°) and 5 (5.50°) are much
less compared to compound 6 (11.35°). Similarly, the
deviation of three pyrrole rings from the mean plane of
the macrocycle are much more pronounced in 6 (16.93°,
11.64°, 11.03°) than in compounds 1 (1.98°, 4.46°, 5.50°)
and 5 (7.40°, 6.08°, 7.72°). Due to the ring contraction in
compounds 1 and 5, the O1ꢀN2 and N2ꢀN3 distances are
We also prepared the other possible 22-oxacorrole such as
5,10,15-tritolyl-22-oxacorrole 4 by following the reported
procedure4 to compare its characteristic properties with
the isolated 21-oxacorrole 1. The absorption spectra of
21-oxacorrole 1 and 22-oxacorrole 4 are significantly differ-
ent from each other, and 21-oxacorrole 1 bands experienced
a blue shift compared to 22-oxacorrole 4 (Supporting
1
Information p S13). The H NMR showed furan signals
as two doublets at 9.55 and 9.06 ppm in 21-oxacorrole 1
which are downfield shifted compared to furan signals in 22-
oxacorrole 4 which appeared at 9.00 and 8.81 ppm
(Supporting Information pp S5ꢀS9). This supports the
direct linkage between furan and pyrrole in 21-oxacorrole 1.
(11) Selected crystal data for compound 1: Formula = [C37H25N3O;
˚
˚
M = 527.60; monoclinic; P21/c; a = 17.131(3) A, b = 10.7442(18) A,
˚
c = 14.687(2) A; R = 90.00°, β = 93.702(6)°, γ = 90.00°; V = 2697.7(8)
3
A ; T = 200(2) K; Z = 4; μ = 0.079 mmꢀ1; F(000) = 1104; reflections
˚
collected = 17 633; independent reflections = 3359 (Rint = 0.047); final
R values [I > 2σ(I)] R1 = 0.0476, wR2 = 0.1261; R values (all data) R1 =
0.0787, wR2 = 0.1548.
(12) Latos-Grazynski, L.; Pacholska, E.; Chmielewski, P. J.; Olm-
stead, M. M.; Balch, A. L. Angew. Chem., Int. Ed. Engl. 1995, 34, 2252–
2254.
1042
Org. Lett., Vol. 15, No. 5, 2013

˚
much shorter (by ∼0.2ꢀ0.3 A) compared to 21-oxaporphyrin
one reversible and one irreversible reduction (Supporting
Information p S16). Compared to 21-oxaporphyrin 3, the
21-oxacorrole 1was found to be difficult to oxidize and reduce
(Supporting Information pp S16ꢀS18). Both 21-oxacorrole
1 and 22-oxacorrole 4 are brightly fluorescent with emission
maxima at 611 and 643 nm respectively (Supporting Infor-
mation p S15), and the quantum yield is in the range of
0.3ꢀ0.4.
6. Furthermore, in compound 1, the O1ꢀN2 distance
˚
˚
(3.81 A) is little longer than the N2ꢀN3 (3.77 A) distance.
But these distances are in reverse order in compound 5
˚
where the O1ꢀN2 distance (3.76 A) is a little shorter than the
˚
N2ꢀN3 (3.86 A) distance. Interestingly, the direct distance
˚
between the pyrrole and furan rings in compound 1(1.40A) is
little shorter than the direct pyrroleꢀpyrrole distance in
˚
compound 5 (1.43 A). In compound 1, the meso-phenyl
In conclusion, we report the unusual formation of
21-oxacorrole in 8% yield by treating 21-oxaporphyrin
with POCl3 in toluene in the presence of base under reflux
conditions. The corrole formation was not observed when
we treated 21-oxaporphyrin with different phosphorylat-
ing reagents such as PCl3, PCl5, and POBr3. Furthermore,
the corrole formation was not observed with other por-
phyrin macrocycles containing N4, N3S, N2SO cores under
similar reaction conditions.
groups which are trans to each other are oriented in opposite
directions with respect to each other by crossing the mean
macrocycle plane (Figure 1b), where, as in compound 5, the
trans meso-phenyl groups are oriented in the same direction.
The more coplanar nature of the meso-phenyl groups with the
corrole rings in compounds 1 and 5 compared to porphyrin 6
is revealed from the average dihedral angles between the meso-
phenyl groups and the mean macrocycle plane which are quite
less in compounds 1and 5. In crystal packing, the two adjacent
molecules are arranged in a parallel direction by the πꢀπ
stacking between the directly linked pyrroleꢀfuran moieties of
both macrocycles with an average plane-to-plane separation
Acknowledgment. M.R. thanks BRNS for financial sup-
port, and A.G. thanks CSIR, Govt. of India for a fellowship.
Authors thank Prof. I. N. N. Namboothiri, Department of
Chemistry, IIT-Bombay for helpful discussions.
˚
of 3.328 A (Supporting Information p S19). Thus, the crystal
structure analysis revealed that the oxacorrole macrocycles 1
and 5 are more planar compared to 21-oxaporphyrin macro-
cycle 6.
The electrochemical and photophysical properties of
corroles 1 and 4 were investigated and compared with
21-oxaporphyrin 3. The21-oxacorrole1 isredoxactiveand
exhibited one reversible and one irreversible oxidation and
Supporting Information Available. General experimen-
tal procedures and characterization data and crystal
packing diagram. This information is available free of
charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 5, 2013
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