First catalysis by corrole metal complexes: Epoxidation, hydroxylation, and cyclopropanation

Article abstract of DOI:10.1039/a900571d

The first ever application of corroles shows that their metal complexes are good catalysts, almost as potent as the corresponding metalloporphyrins in the oxygenation of hydrocarbons by iodosylbenzene and superior for the cyclopropanation of olefins by carbenoids.

Full text of DOI:10.1039/a900571d

First catalysis by corrole metal complexes: epoxidation, hydroxylation, and
cyclopropanation
Zeev Gross,* Liliya Simkhovich and Nitsa Galili
Department of Chemistry, Technion - Israel Institute of Technology, Haifa 32000, Israel.
E-mail: chr10zg@tx.technion.ac.il
Received (in Cambridge, UK) 21st January 1999, Accepted 25th February 1999
The first ever application of corroles shows that their metal
complexes are good catalysts, almost as potent as the
corresponding metalloporphyrins in the oxygenation of
hydrocarbons by iodosylbenzene and superior for the
cyclopropanation of olefins by carbenoids.
of an iron(iv) corrole radical4b could be relevant to catalysis by
heme-enzymes and synthetic iron porphyrins, in which the
analogous complexes are key intermediates.5 But, there is still
no information about corroles or their metal complexes in
catalysis or in any other potential application. Presumably, it is
only the lack of obvious procedures for the synthesis of corroles
which prohibits such research. Thus, the first meso-substituted
corrole—meso-aryl substitution is a prerequisite for a reason-
able oxidation catalyst—was reported as late as 1993,6 almost
30 years after the first reported corrole.7 Also, in spite of the
significant recent progress in corrole synthesis, even the most
simple procedures reported to date require the preparation of
non-commercially available starting materials.8
We have very recently contributed to this field by disclosing
the first synthesis of corroles by the direct reaction of pyrrole
with aldehydes.9 In particular, 5,10,15-tris(pentafluorophenyl)-
corrole (H3tpfc, Scheme 1) is now routinely prepared in our
laboratory by this very simple and fast methodology. The
structure of H3tpfc is very similar to that of H2tpfp, whose
iron(iii) complex is among the most active porphyrin-based
catalysts.10 Accordingly, we have now decided to compare the
iron complexes of these two ligands as catalysts for the three
main reactions catalyzed by metalloporphyrins.11 These are the
epoxidation of olefins and the hydroxylation of alkanes by
iodosylbenzene, and the cyclopropanation of alkenes by
carbenoids (Scheme 2).12
Modified porphyrins have received increased attention in recent
years, with emphasis on their syntheses, characterizations, and
potential applications.1 Some of these macrocycles form
complexes with metal ions, which due to the alteration of the
ligand’s structure have quite different properties compared with
the analogous metalloporphyrins.2 The most interesting ligands
in this respect are corroles, the one-carbon atom contracted
porphyrin analogs, which may also be considered as the
aromatic version of corrin—the cobalt’s ligand in Vitamin B12.3
Similar to porphyrins, the corroles provide an equatorial
tetradentate coordination plane, which is however trianionic and
somewhat contracted (see Scheme 1). These last two properties
have some remarkable effects on the coordination chemistry of
corrole metal complexes (metallocorroles),4 among which the
stabilization of exceptionally high metal oxidation states is the
most outstanding. The recent isolation of iron(iv) corroles4a and
Ar
Ar
Cl
The results shown in Table 1 clearly demonstrate that
(tpfc)Fe–Cl is a good catalyst in all three reactions. Still, it is not
as effective as (tpfp)Fe–Cl for the epoxidation and hydroxyla-
tion of hydrocarbons. This is reflected in the higher yields of
oxygenation products with (tpfp)Fe–Cl, as well as from the fact
that at the end of the reactions the corrole complex, but not the
analogous porphyrin complex, is completely bleached. How-
ever, for the cyclopropanation of styrene by 1a (Scheme 2),
(tpfc)Fe–Cl is superior. In addition, the catalyst is stable under
the cyclopropanation conditions.
N
N
NH
N
N
N
FeCl2, DMF
reflux
Ar
Ar
Ar
Ar
FeIII
HN
N
H2tpfp
(tpfp)Fe-Cl
Ar
Ar
Ar
Ar
These results are reasonably accounted for by considering the
proposed key intermediates in these processes, metal–oxo in
oxygenation and metal–carbene in cyclopropanation.5,12 It is
well known that for iron(iii) porphyrins, oxygen atom transfer
from iodosyl benzene results in an oxoiron(iv) porphyrin radical
intermediate, whose protection against self-destruction relies on
the steric crowding of the four meso-aryl groups. By analogy,
the oxygenation of (tpfc)Fe–Cl—an iron(iv) complex—will
lead to an even more reactive intermediate. In addition, the
Cl
N
HN
N
N
FeCl2, DMF
reflux
Ar
FeIV
Ar
Ar
Ar
NH NH
H3tpfc
N
N
Ar = C6F5
(tpfc)Fe-Cl
Scheme 1 Structures of meso-aryl substituted porphyrin and corrole, and of
their iron complexes (note the different oxidation states of the metal).
Ph
O
O
Ph
Ph
Ph
H
Ph
Ph
Ph
Ph
Ph
1a X = OEt
CH3
X
X
+
+
+
O
O
OH
Ph
O
CH3
O
1b X =
Ph
I
Ph
I
Ph
I
I
N
N2
X
CHN2
1a,b
Scheme 2 Three catalytic transformations examined with (tpfc)Fe–Cl and (tpfp)Fe–Cl as catalysts.
O
O
SO2
Chem. Commun., 1999, 599–600
599

Table 1 A comparison of (tpfc)Fe–Cl and (tpfp)Fe–Cl as catalysts for the processes shown in Scheme 1 with 1a
Epoxidation of styrenea Hydroxylation of ethylbenzeneb
Cyclopropanation of styrenec
Products
Epoxide
Aldehyde
Alcohol
Ketone
trans
cis
With (tpfc)Fe–Cl as catalystd
With (tpfp)Fe–Cl as catalystd
66%
90%
21%
10%
6.6%
15.7%
4.2%
8.9%
46%
40%
20%
7%
a
0.36 mmol catalyst, 36 mmol iodosylbenzene, 360 mmol styrene, and 36 mmol nitrobenzene (internal standard), in 1 mL benzene, at RT for 3.5 h and
b
45 min, respectively. 0.45 mmol catalyst, 50 mmol iodosylbenzene, 500 mmol ethylbenzene, and 50 mmol nitrobenzene (internal standard), in 1 mL
benzene, overnight at RT. c 0.3 mM catalyst, 0.15 M 1a, 1.5 M styrene, in 4 mL CH2Cl2, for 2.75 h at RT. 34 and 18% of olefins (diethyl maleate and traces
of fumarate) were obtained in the reactions catalyzed by (tpfc)Fe–Cl and (tpfp)Fe–Cl, respectively. Yields with regard to the limiting reagent were
d
determined by GC relative to the internal standard.
Table 2 The results for cyclopropanation of styrene by 1a and the unichiral
carbenoid 1b, catalyzed by the iron porphyrin complex (tpfp)Fe–Cl and the
iron, cobalt, and rhodium complexes of H3tpfca
In conclusion, this is the first report of catalysis by corrole
metal complexes, and actually the first ever application of
corroles. We have demonstrated that metallocorroles are good
catalysts for the reactions which are traditionally investigated
With 1a
With 1b
with metalloporphyrins. We trust that these promising prelimi-
nary results will encourage further exploration of the chemistry
of corroles and their metal complexes.
trans:
% Yieldb cis
trans:
% Yieldc cis
Catalyst
% de trans % de cis
This research was partially supported by ‘The Technion
V.P.R. Fund - R. and M. Rochlin Research Fund’.
(tpfp)Fe–Cl
(tpfc)Fe–Cl
(tpfc)Co–PPh3
24
71
8
5.5
1.8
2.1
2.0
10
2.6
0.9
67 (1S,2S) 63 (1S,2R)
25 (1S,2S) 66 (1S,2R)
41
d
Notes and references
(tpfc)Rh–PPh3 87
56
1.0
60 (1R,2R) 10 (1S,2R)
1 For some excellent recent reviews, see: J. L. Sessler and S. J. Weghorn,
Expanded, Contracted, & Isomeric Porphyrins, Pergamon, Oxford,
1997; A. Jasat and D. Dolphin, Chem. Rev., 1997, 97, 2267; E. Vogel,
J. Heterocycl. Chem., 1996, 33, 1461.
2 For some recent publications, see: K. M. Kadish, P. L. Boulas, M.
Kisters, E. Vogel, A. M. Aukauloo, F. D’Souza and R. Guilard, Inorg.
Chem., 1998, 37, 2693; Z. Gross, I. Saltsman, R. P. Pandian and C. M.
Barzilay, Tetrahedron Lett., 1997, 38, 2383; P. J. Chmielewski, L.
LatosGrazynski, M. M. Olmstead and A. L. Balch, Chem. Eur. J., 1997,
3, 268.
a
0.25–0.28 mM catalyst, catalyst:1:styrene = 1:100:1000, in 2 mL
b
CH2Cl2, at RT under Ar.
Combined yields of the trans- and cis-
cyclopropyl esters after 1 h, except for (tpfc)Co–PPh3 (24 h). With (tpfc)Fe–
Cl and (tpfc)Rh–P(Ph)3, the reactions are complete after 1 h (the rest of 1a
is transferred into its dimerization products), while with (tpfp)Fe–Cl the
reaction continues (43% yield after 3 h). c Combined yields of the trans- and
cis-isomers after 24 h, except for (tpfp)Fe–C1 (48 h). d Not determined.
absence of the fourth aryl ring in (tpfc)Fe–Cl leads to reduced
steric protection. Accordingly, the lower efficiency and the
greater bleaching of (tpfc)Fe–Cl during catalysis may be
rationalized on both electronic and steric grounds, whose
relative importance still needs to be resolved. On the other hand,
both factors seem to be beneficial for the cyclopropanation
process. It is known that the active oxidation state in catalysis by
iron(iii) porphyrins is iron(ii), which for (tpfp)Fe–Cl is formed
via reduction by 1a.13 Thus, the results suggest that the
reduction of the iron(iv) corrole is easier and that the absence of
the fourth aryl group is favorable for formation of the relatively
large metal–carbene intermediate. The last effect is also
reflected in the relatively small trans: cis ratio of the cyclopro-
pyl esters, 2.3 with (tpfc)Fe–Cl vs. 5.7 with (tpfp)Fe–Cl.
Because of the superiority of (tpfc)Fe–Cl in cyclopropanation
catalysis, we turned our attention to other metal complexes of
H3tpfc, as well as to the unichiral carbenoid 1b.14,15 Table 2
summarizes the results of the reaction of styrene with 1a and 1b,
catalyzed by (tpfp)Fe–Cl, (tpfc)Fe–Cl, (tpfc)Co–P(Ph)3, and
(tpfc)Rh–P(Ph)3. Several aspects are clearly evident. First, the
larger activity of the iron corrole relative to the analogous
porphyrin in cyclopropanation by 1a is further amplified in the
reaction with the much bigger 1b. Secondly, within the series of
the corrole metal complexes the catalytic efficiency increases in
the order of Co < < Fe < Rh, similar to what is obtained with
metalloporphyrins.13 Finally, quite low trans: cis ratios are
obtained for all metallocorroles, together with modest diastereo-
meric excesses (% de) in the reactions with 1b. By analogy to
our recent studies with the related metalloporphyrins,14a we
anticipate that a significant increase in diastereoselectivity
might be achieved by utilizing metal complexes of corroles with
larger o-phenyl substituents. We have one such derivative,9 but
we still have to improve the synthetic methodology for its
preparation.
3 S. Licoccia, and R. Paolesse, Struct. Bond., 1995, 84, 71.
4 (a) E. Vogel, S. Will, A. S. Tilling, L. Neumann, J. Lex, E. Bill, A. X.
Trautwein, and K. Wieghardt, Angew. Chem., Int. Ed. Engl., 1994, 33,
731; (b) E. VanCaemelbecke, S. Will, M. Autret, V. A. Adamian, J. Lex,
J. P. Gisselbrecht, M. Gross, E. Vogel and K. M. Kadish, Inorg. Chem.,
1996, 35, 184; (c) S. Will, J. Lex, E. Vogel, V. A. Adamian, E. Van
Caemelbecke and K. M. Kadish, Inorg. Chem., 1996, 35, 5577; (d) S.
Will, J. Lex, E. Vogel, H. Schmickler, J. P. Gisselbrecht, C. Haubtmann,
M. Bernard and M. Gross, Angew. Chem., Int. Ed. Engl., 1997, 36,
357.
5 J. Groves and Y. Han, Cytochrome P450, Structure, Mechanism, and
Biochemistry, ed. P.R. Ortiz de Montellano, Plenum Press, New York,
1995, pp. 1–48.
6 R. Paolesse, S. Licoccia, M. Fanciullo, E. Morgante and T. Boschi,
Inorg. Chim. Acta, 1993, 203, 107.
7 A. W. Johnson and I. T. Kay, Proc. Chem. Soc., 1964, 89.
8 S. Neya, K. Ohyama and N. Funasaki, Tetrahedron Lett., 1997, 38,
4113; R. Paolesse, S. Licoccia, G. Bandoli, A. Dolmella and T. Boschi,
Inorg. Chem., 1994, 33, 1171.
9 Pending Israel Patent application, No.126426; Z. Gross, N. Galili and I.
Saltsman, Angew. Chem., Int. Ed., 1999, in the press.
10 C. K. Chang and F. Ebina, J. Chem. Soc., Chem. Commun., 1981,
778.
11 (tpfp)Fe–Cl was available from our previous studies and the full
characterization (including by X-ray crystallography) of (tpfc)Fe–Cl
and (tpfc)Rh–P(Ph)3 will be reported in the near future.
12 F. Montanari and L. Casella, Metalloporphyrins Catalyzed Oxidations,
Kluwer, Dordrecht, 1994.
13 J. R. Wolf, C. G. Hamaker, J.-P. Djukic, T. Kodadek and L. K. Woo,
J. Am. Chem. Soc., 1995, 117, 9194.
14 For the preparation of 1 and its utilization in cyclopropanation, see:
(a) Z. Gross, N. Galili and L. Simkhovich, Tetrahedron Lett., 1999, 40,
1571; (b) N. Haddad and N. Galili, Tetrahedron: Asymmetry, 1997, 8,
3367.
15 For the term unichiral and its advantages as compared to non-racemic,
homochiral, and enantiopure, see: J. Gal, Enantiomer, 1998, 3, 263.
Communication 9/00571D
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Chem. Commun., 1999, 599–600