Cobalt carbaporphyrin-catalyzed cyclopropanation

Article abstract of DOI:10.1039/c0cc03894f

Cobalt complexes of N-confused porphyrins and benziphthalocyanine, which both feature organometallic bonds at the macrocycle cores, catalyze the cyclopropanation of styrene with a higher trans-selectivity than the corresponding porphyrin and phthalocyanine complexes. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0cc03894f

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Cobalt carbaporphyrin-catalyzed cyclopropanationw
Kimberly B. Fields,a James T. Engle,b Saovalak Sripothongnak,b Chungsik Kim,a
X. Peter Zhang*a and Christopher J. Ziegler*b
Received 15th September 2010, Accepted 18th October 2010
DOI: 10.1039/c0cc03894f
Cobalt complexes of N-confused porphyrins and benziphthalo-
cyanine, which both feature organometallic bonds at the macro-
cycle cores, catalyze the cyclopropanation of styrene with a
higher trans-selectivity than the corresponding porphyrin and
phthalocyanine complexes.
the carbaporphyrins have not been extensively studied as
catalysts for organic transformations, although there has
been some work in the catalytic chemistry of N-confused
porphyrin.11
Previously, we have generated several cobalt complexes of
5,10,15,20-tetraphenyl-N-confused porphyrin (NCTPP)12 as
well as benziphthalocyanine (bzpc).10 For benziphthalocyanine,
we used Co2(CO)8 in pyridine solution to insert the metal ion
into the macrocycle, which first resulted in the complex
CoII(bzpc)py and then, upon air oxidation, CoIII(bzpc)py2.
We have reported on the metalation of N-confused porphyrin
with cobalt by Co2(CO)8 to afford the Co(II) adducts.13
We can readily generate both CoII(NCTPP)py and
CoII(N-CH3NCTPP)py via this latter method, and the struc-
tures elucidated from single crystals are shown in Fig. 1 along
with our previously elucidated cobalt carbahemiporphyrazines.
Both N-confused porphyrin complexes resemble each other as
well as the corresponding CoII(bzpc)py complex. The internal
C–H bonds of the N-confused porphyrins have been activated,
resulting in direct Co–C bonds. The compounds are five
coordinate with pyridines occupying the axial positions. In
the case of CoII(NCTPP)py, air oxidation in the presence of
pyridine produces the Co(III) complex Co(NCTPP)py2, which
indicates that the N-confused porphyrin macrocycle stabilizes
higher valent oxidations states than normal porphyrin.12
Directly analogous chemistry is observed in benziphthalocyanine,
and the chemistry of the Co(III) benziphthalocyanine was
investigated. However, the presence of the methyl group on
the external nitrogen position in CoII(N-CH3NCTPP)py
effectively prevents the formation of a trivalent N-confused
The metal complexes of porphyrins and phthalocyanines are
frequently used as catalysts for organic transformations.1
These metallomacrocycles were initially employed as oxygen
transfer catalysts, inspired by heme-based enzymatic catalysis
such as that seen in Cytochrome P-450.2 More recently,
metalloporphyrins and metallophthalocyanines have been
used to catalyze other organic transformations, including
carbene and nitrene transfer reactions.3,4 Cobalt porphyrin
complexes have been reported to be effective catalysts for the
stereoselective cyclopropanation of alkenes with unique
reactivity profiles.5 There are many inherent advantages in
using porphyrin or phthalocyanine-based catalysts, ranging
from their robustness to their facile steric and electronic
modification. Recent development in the synthesis of porphyrin
and phthalocyanine variants has led to new metal complexes
where the skeleton of the macrocycle is perturbed relative to
their parent molecules.6 Not surprisingly, there has been
increasing interest in investigating the catalytic properties of
porphyrin and phthalocyanine analogues, in particular with
regard to oxidations.7 In this report, we present the first
catalytic study on a series of cobalt carbaporphyrinoids and
have observed a high selectivity for trans products, a direct
result of the presence of a metal–carbon bond in the complex.
Carbaporphyrins are porphyrin analogues and isomers that
have one or more carbon atoms in place of nitrogen atoms at
the core of the ring.8 The carbaporphyrin family of macrocyles
includes the porphyrin isomer N-confused porphyrin9 and the
phthalocyanine analogue benziphthalocyanine.10 These
macrocycles are shown along with normal porphyrin and
phthalocyanine in Scheme 1. N-confused porphyrin has the
same skeleton as normal porphyrin, but with one of the four
pyrrole rings inverted. Benziphthalocyanine is a phthalocyanine
analogue with one of the isoindoline rings replaced with a
benzene ring. We have recently developed the synthetic metal
chemistry of both macrocycle types. The metal complexes of
a Department of Chemistry, University of South Florida, Tampa,
FL 33620-5250, USA. E-mail: xpzhang@usf.edu
b Department of Chemistry, University of Akron, Akron,
OH 44325-3601, USA. E-mail: ziegler@uakron.edu
w Electronic supplementary information (ESI) available: Experimental
procedures for the catalytic reactions and crystal data and structure
refinement parameters for CoII(NCTTP)py and CoII(N-MeNCTTP)py.
CCDC 790334 and 790335. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0cc03894f
Scheme 1 Structures of porphine (1, top left), N-confused porphines
(2, 3, top right), phthalocyanine (4, bottom left) and benziphthalo-
cyanine (5, bottom right).
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Fig. 1 The structures of CoII(NCTTP)py (a)w, CoII(N-MeNCTTP)py (b)w, CoII(bzpc)py (c)10 and CoIII(bzpc)py2 (d)10 with 35% thermal
ellipsoids. Hydrogen atoms have been omitted for clarity with the exception of those on the external nitrogen positions.
porphyrin ligand, and thus is more similar to normal
porphyrin.
the result from catalytic cyclopropanation by cobalt N-methylated
N-confused porphyrin, which can less readily form the
Co(III) oxidation state. As can be seen in the data in Fig. 2,
the N-methylated N-confused porphyrin complex exhibits an
appreciably higher cyclopropanation yield of 51%. The yield
was further increased to 85% when the reaction was carried
out at room temperature. Since the amounts of dimerization
side products were practically the same at the two tempera-
tures, an additional unknown factor must be responsible for
the higher yield obtained at lower temperature. Another
notable aspect of the observed catalysis can be seen in the
benziphthalocyanine complexes. Both the Co(II) and Co(III)
complexes produced the cyclopropane product in essentially
identical yields and selectivity. This implies that the active
intermediates for both compounds are identical and that
therefore the Co(III) complex is reduced to the Co(II) complex
during the reaction.
We investigated the catalytic cyclopropanation of styrene
with ethyl diazoacetate (EDA) using the above cobalt
complexes and compared them to normal cobalt porphyrin
and phthalocyanine. To probe the effect of ligand environment
on the Co-catalyzed cyclopropanation, these catalytic reactions
were carried out under same conditions in toluene at 80 1C
with styrene as the limiting reagent using only 1 mol% catalyst
loading. The results of this initial study are shown in Fig. 2
and are remarkably consistent across the series. Both the
normal porphyrin and phthalocyanine complexes exhibited
higher product yields than the corresponding carbamacrocycle
complexes, and the differences can be as much as a factor of
two. The reduction in yield most likely results from the stabili-
zation of higher valent oxidation states in the carbamacro-
cycles versus the divalent preference seen in normal porphyrin
and phthalocyanine. Consequently, the putative cobalt carbene
intermediates of carbaporphyrinoids are more electrophilic
in nature compared to the radical carbenes associated with
cobalt(II) porphyrins.14 The observed high percentages of
dimerization side products from EDA in the reactions
catalyzed by cobalt carbaporphyrinoids are consistent with
this hypothesis. This hypothesis is further substantiated with
All of the cobalt carbamacrocycle complexes exhibit a
higher diastereoselectivity for the trans isomer over the
cis isomer in product formation. In the case of the cobalt
N-methylated N-confused porphyrin, the difference is striking,
with a 98% selectivity for the trans isomer. Since the normal
cobalt porphyrinoid and the cobalt carbaporphyrinoid have
minimal or no differences in steric bulk, the current effects are
Fig. 2 Carbaporphyrinoid mediated cyclopropanation of styrene with EDA. The table shows reactions using Co(N-Me-NCTPP)py as the
catalyst. The dimerization products were (Z) maleate ethyl esters.
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entirely electronic in nature. In rhodium-based cyclopropanation
reactions, more covalently bound ligands tend to favor the
formation of trans products.15 Similarly, in the cobalt
complexes of the carbaporphyrins, there is a greater degree
of covalency than in normal metalloporphyrins or phthalo-
cyanines in the metal ligand interaction. This increase in
covalency results from the substitution of a carbon for a
nitrogen, which produces a less ionic interaction at the core
of the ring. Previously, this change in electronic structure
at the metal center in carbaporphyrin complexes has been
observed spectroscopically,16 but the present observations are
rare examples of how this change in bonding affects reactivity
and selectivity, specifically via the stabilization of specific
transition state pathways.
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In conclusion, we have carried out a preliminary investi-
gation into the catalysis of cyclopropanation reactions using a
series of cobalt carbaporphyrinoids. While the overall yields
of cyclopropanation are lower than observed in normal
porphyrin and phthalocyanine, we observed a notable increase
in selectivity for the trans isomer across the carbaporphyrin
series. Since the sterics for these carbaporphyrinoids are
identical to their normal porphyrin and phthalocyanine parents,
the difference in product selectivty results from electronic
factors. We are continuing our work in this area on the catalytic
chemistry of the metal complexes of the porphyrinoids.
XPZ is grateful for financial support by the National
Science Foundation (CAREER Award: CHE-0711024). CJZ
would like to thank the National Foundation for support of
this work (CHE-0616416).
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Chem. Commun., 2011, 47, 749–751 751