Rhodium N-confused porphyrin-catalyzed alkene cyclopropanation

Article abstract of DOI:10.1039/b608154a

Rhodium N-confused porphyrins constitute unprecedented, highly efficient tools for the cyclopropanation of styrene, affording the expected cyclopropanes in excellent yields with trans: cis ratios reaching 98: 2. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b608154a

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Rhodium N-confused porphyrin-catalyzed alkene cyclopropanation
Teppei Niino,a Motoki Toganoh,a Bruno Andriolettib and Hiroyuki Furuta*ac
Received (in Austin, TX, USA) 9th June 2006, Accepted 13th August 2006
First published as an Advance Article on the web 5th September 2006
DOI: 10.1039/b608154a
efficient, and hence the development of a more simple system is
highly anticipated.
Rhodium N-confused porphyrins constitute unprecedented,
highly efficient tools for the cyclopropanation of styrene,
affording the expected cyclopropanes in excellent yields with
trans : cis ratios reaching 98 : 2.
N-confused porphyrins (NCPs)6 are isomers of porphyrin that
display novel abilities in coordination and supramolecular
chemistries.7 Despite their rich coordination chemistry, to date,
no catalytic reactions involving metallated NCPs have been
reported in the literature. Herein, we examine alkene cyclopropa-
nations catalyzed by rhodium NCPs and find that they display
superior efficiency to other porphyrinoid catalysts in both yield
and regioselectivity.
In the general context of sustainable development, the need for
efficient catalysts able to accelerate reactions and maximize
selectivity becomes more crucial every day. Among the possible
candidates, porphyrins and porphyrin analogues are particularly
promising (Chart 1).1 Indeed, their macrocyclic structure generally
confers high stability on catalysts and their chemical functionaliza-
tion allows spectacular structural variations to be achieved.
Accordingly, remarkable results have been obtained. For instance,
the enantioselective epoxidation of styrene was carried out in
quantitative yield with 97% ee and turnover numbers (TONs)
exceeding 16 000 using a functionalized binaphthyl porphyrin.2
However, the limited synthetic availability of these elaborate
structures often hampers the wide scale use of such catalysts. For
this reason, in the recent years, special emphasis has been placed
on the development of alternative systems that are capable of
performing catalytic reactions with great efficiency by changing the
platform from normal porphyrins to similar macrocycles. While
some expected results were obtained with corrole,3 porphycene4
and N-fused porphyrin5 systems, they are not so accessible or
The preparation of catalysts could be achieved in only 3 steps
from commercially available reagents (Scheme 1). Thus, tetraaryl
NCPs were prepared from pyrrole and the corresponding aryl
aldehyde.8,9 Next, the Rh(I) complexes were synthesized according
to literature procedures.10 Finally, treatment of the Rh(I)
complexes with pyridine or iodine gave the catalyst precursors.11
To elucidate the electronic and steric effects on the cyclopropana-
tion reaction, the rhodium complexes of N-confused tetra(phe-
nyl)porphyrin (NCTPP), N-confused tetrakis(mesityl)porphyrin
(NCTMP)12 and N-confused tetrakis(pentafluorophenyl)por-
phyrin (NCTPFPP) were synthesized as catalysts.
The catalytic experiments were carried out at room temperature
using a solution of 5 mmol catalyst (0.05 mol%) in 5 mL
dichloroethane, 50 mmol styrene and 10 mmol ethyl diazoacetate
(EDA) as the carbene source (Scheme 2). The results of the
Chart 1 Structures of platform macrocycles.
Scheme 1 Preparation of the rhodium NCPs.
aDepartment of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka
819-0395, Japan. E-mail: hfuruta@cstf.kyushu-u.ac.jp;
Fax: +81 92-802-2865; Tel: +81 92-802-2865
bUniversite´ Pierre et Marie Curie-Paris 6, Laboratoire de Chimie
Organique (UMR CNRS 7611), Institut de Chimie Mole´culaire
(FR 2769), case 181, 4 place Jussieu, F-75252 Paris Cedex 05, France
cPRESTO, Japan Science and Technology Agency, 4-1-8 Honcho,
Kawaguchi 332-0012, Japan
Scheme 2 Rhodium-catalyzed cyclopropanation of styrene.
Chem. Commun., 2006, 4335–4337 | 4335
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Table 1 Yield and selectivity data for catalytic cyclopropanation reactions of styrenea
Entry
Catalyst
Yield (%)
Reaction time/h
Temperature
trans : cis ratio
1b
2b,c
3b
Rh(NCTPP)I2
Rh(NCTPP)I2
Rh(NCTPP)(py)2
Rh(NCTMP)I2
Rh(NCTPFPP)(py)2
Rh(TPP)I
Rh(TPP)I
Fe(TPP)Cl
Fe(TPFPP)Cl
Rh(TPFPC)(PPh3)
92
91
60
93
89
71
71
n.r.
43
87
3
3
24
3
3
4
n.r.
10
3
RT
RT
RT
RT
RT
RT
60 uC
40 uC
RT
91 : 9
98 : 2
86 : 14
83 : 17
88 : 12
52 : 48
47 : 53
85 : 15
85 : 15
67 : 33
4b
5b
6b
715
814
93
103
a
1
RT
TPP = tetraphenylporphyrin; TPFPP = tetrakis(pentafluorophenyl)porphyrin; TPFPC = Tetrakis(pentafluorophenyl)corrole; n.r. = not
reported. The yields and trans : cis ratios were determined by GC (n-pentadecane was used as internal standard). c tButyl diazoacetate was
used in place of EDA.
b
catalytic experiments are summarized in Table 1, together with
those of the similar tetrapyrrolic catalysts. It shows that the
rhodium NCPs were actually more active and selective than their
tetrapyrrolic parents. Indeed, the chemical yields measured with
NCPs generally exceeded those obtained with porphyrins or
corroles. The only exception concerns Rh(NCTPP)(py)2 (Table 1,
entry 3), which appeared very stable and relatively inert in the
presence of EDA (24 h, 60% yield). The decoordination of the
poorly-labile pyridine ligand is necessary to advance the reaction,
but may not constitute the limiting step of the process because
Rh(NCTPFPP)(py)2, which maintains a similar pyridine coordina-
tion, catalyzed the reaction efficiently and afforded the expected
Scheme 3 Proposed transition state for the rhodium NCP-catalyzed
cyclopropanes in 89% yield in 3 h (Table 1, entry 5). Apparently,
cyclopropanation of styrene.
the electron-withdrawing groups at the meso-positions influence
the electron density of the p-cloud of the porphyrin ring, and
skeleton. Assuming that the initial step of catalysis by NCP is
consequently the central metal. Thus, this result may infer that
similar to the general pathway proposed for normal porphyrins,16
carbene reactivity is much influenced by the degree of electron
it would involve preliminary reduction of the rhodium NCP by
deficiency at the Rh metal center. More interestingly, the trans : cis
EDA to form the corresponding rhodium carbene complex. In this
ratio is always very favorable in the case of rhodium NCPs. The
complex, the high back-donating effect of the rhodium NCP
best result was obtained with Rh(NCTPP)I2, which afforded
would stabilize the electrophilic carbene moiety and allow the
cyclopropanes in 92% yield (TON = 1840) with a trans : cis ratio
olefin to get close to the carbene in the transition state, which could
reaching 91 : 9 (Table 1, entry 1). In addition, the trans : cis ratio
result in the preferential production of the trans-isomer due to
was even higher when the bulky tert-butyl diazoacetate was used
steric availability (Scheme 3).
instead of EDA. In the latter case, it reached 98 : 2, with the
In conclusion, this first report concerning the use of NCPs in
chemical yield remaining constant at 91% (Table 1, entry 2).
catalytic reactions demonstrates their powerful potential. The
Considering the effect of the diazo esters’ steric bulk on the
catalytic cyclopropanation experiments disclosed here reveal that,
selectivities reported in previous studies,13 the trans-selectivity
in many cases, NCPs can outperform porphyrins and corroles.
achieved here is remarkable. On the contrary, the presence of the
Accordingly, the high yields and selectivities obtained in this study
too-distant bulky meso-mesityl substituent did not significantly
are particularly significant. We believe that these promising
influence the selectivity of the reaction (Table 1, entry 4). It is
preliminary results will open up new routes to further applications
worth noting that slight modification of the trans-selectivity was
of NCPs in catalytic reactions.
observed with bulky meso-substituents in the case of the iron
porphyrins,14 and even preferential production of the cis-isomer
was observed in the case of the rhodium porphyrin.15 The
regioselectivity trend of NCPs is apparently different to that of
normal porphyrins, implying the unique role of confusion in the
catalytic reactions.
Notes and references
1 F. Montanari and L. Casella, Metalloporphyrins Catalyzed Oxidations,
Kluwer, Drodrecht, 1994.
2 E. Rose, Q.-Z. Ren and B. Andrioletti, Chem.–Eur. J., 2004, 10, 224.
3 Z. Gross, L. Simkhovich and N. Galili, Chem. Commun., 1999, 599.
4 W.-C. Lo, C.-M. Che, K.-F. Cheng and T. C. W. Mak, Chem.
Commun., 1997, 1205.
While no direct evidence explaining the trans preference in the
rhodium NCP-catalyzed reactions has been obtained as yet, we
postulate that the trans-selectivity is caused by a late transition
state. Since the trans : cis ratios were little affected by the meso-
substituents and the structures of the rhodium NCPs should
resemble those of normal rhodium porphyrins, the regioselectivity
would originate from the intrinsic electronic nature of the NCP
5 M. Toganoh, S. Ikeda and H. Furuta, Chem. Commun., 2005, 4589.
6 (a) H. Furuta, T. Asano and T. Ogawa, J. Am. Chem. Soc., 1994, 116,
´
767; (b) P. J. Chmielewski, L. Latos-Graz˙ynski, K. Rachlewicz and
T. Glowiak, Angew. Chem., Int. Ed. Engl., 1994, 33, 779.
/
7 (a) H. Furuta, H. Maeda and A. Osuka, Chem. Commun., 2002, 1795;
(b) J. D. Harvey and C. J. Ziegler, Coord. Chem. Rev., 2003, 247, 1; (c)
4336 | Chem. Commun., 2006, 4335–4337
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A. Srinivasan and H. Furuta, Acc. Chem. Res., 2005, 38, 10; (d)
´
P. J. Chmielewski and L. Latos-Graz˙ynski, Coord. Chem. Rev., 2005,
249, 2510.
8 G. R. Geier, III, D. M. Haynes and J. S. Lindsey, Org. Lett., 1999, 1,
1455.
9 Practical synthesis of NCTPFPP was achieved in a stepwise manner,
see: H. Maeda, A. Osuka, Y. Ishikawa, I. Aritome, Y. Hisaeda and
H. Furuta, Org. Lett., 2003, 5, 1293.
11 A. Srinivasan, M. Toganoh, T. Niino, A. Osuka and H. Furuta, Inorg.
Chem., submitted.
12 G. R. Geier, III and J. S. Lindsey, J. Org. Chem., 1999, 64, 1596.
13 M. P. Doyle, V. Bagheri, T. J. Wandless, N. K. Harn, D. A. Brinker,
C. T. Eagle and K.-L. Loh, J. Am. Chem. Soc., 1990, 112, 1906.
14 J. R. Wolf, C. G. Hamaker, J.-P. Djukic, T. Kodadek and L. K. Woo,
J. Am. Chem. Soc., 1995, 117, 9194.
15 H. J. Callot and C. Piechocki, Tetrahedron Lett., 1980, 21, 3489.
16 J. L. Maxwell, K. C. Brown, D. Bartley and T. Kodadek, Science, 1992,
256, 1544.
10 A. Srinivasan, H. Furuta and A. Osuka, Chem. Commun., 2001,
1666.
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