Cyclopropanation of alkenes with ethyl diazoacetate catalysed by ruthenium porphyrin complexes

Article abstract of DOI:10.1039/a700098g

Ruthenium porphyrin complexes are active catalysts for the cyclopropanation of styrene derivatives by ethyl diazoacetate with good to very good diastereoselectivity; moderate enantiomeric excesses (34%) are observed using a chiral porphyrin as catalyst.

Full text of DOI:10.1039/a700098g

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Cyclopropanation of alkenes with ethyl diazoacetate catalysed by ruthenium
porphyrin complexes
Erwan Galardon, Paul Le Maux and Ge´rard Simonneaux*
Laboratoire de Chimie Organome´tallique et Biologique, associe´ au CNRS, Universite´ de Rennes 1, 35042 Rennes cedex, France
Ruthenium porphyrin complexes are active catalysts for the
cyclopropanation of styrene derivatives by ethyl diazo-
acetate with good to very good diastereoselectivity; moderate
enantiomeric excesses (34%) are observed using a chiral
porphyrin as catalyst.
the data, the porphyrin structure is important since the use of the
unencumbered TPP instead of the crowded TMP results in an
increase in the anti:syn ratio of the cyclopropyl esters from 7.9
to 13.1. The cyclopropane formation also exhibits a substrate
shape preference that may be useful for selective cyclopropana-
tion of polyalkenes. Only alk-1-enes and 1,1-disubstituted
alkenes react efficiently. Thus styrene and a-methylstyrene are
cyclopropanated efficiently, whereas with b-methylstyrene and
cyclohexene, diethyl maleate and fumarate are formed in very
high yield in a ratio of up 30:1 under the same conditions.8
These coupling products are typically produced when the
carbene transfer to the alkene is not observed.8 The catalyst is
also sensitive to the electronic nature of the alkene since
aromatic alkenes are better substrates, as previously observed
with iron or osmium porphyrins.5,6 For example, only traces of
the cyclopropane products are detected when the reaction is
carried out with 4-vinylcyclohex-1-ene.
The reaction of styrene with ethyl diazoacetate in the
presence of the dioxoruthenium(vi) picket-fence complex
bearing optically active a-methoxy-a-(trifluoromethyl)phenyl-
acetyl residues on both sides of the porphyrin plane (a,b,a,b
isomer)12 (Fig. 1) gave a mixture (anti:syn = 9:1) of optically
active cyclopropane derivatives, e.g. anti and syn ethyl
2-phenylcyclopropane-1-carboxylic esters with 14 and 34%
enantiomeric excess, respectively.§
The development of efficient and selective methods for the
synthesis of cyclopropanes is of great interest in organic
synthesis, in particular because of their role in biological
processes.1 Several years ago, Callot et al. reported that
rhodium porphyrins catalysed the cyclopropanation of alkenes
by ethyl diazoacetate (EDA).2 Recently, efficient asymmetric
cyclopropanation of alkenes with diazoacetates catalysed by
bis(oxazolinyl)pyridine ruthenium complexes has been reported
by Nishiyama and co-workers.3,4 Very recently, osmium5
and iron6 porphyrins were also found to be active catalysts
for the formation of cyclopropanes in good yield. Despite
the periodic relationship of ruthenium to iron and osmium,
and the syntheses of ethoxycarbonyl carbene complexes of
ruthenium 5,10,15,20-tetratolylporphyrin7 and ruthenium
5,10,15,20-tetramesitylporphyrin,8 no cyclopropanation reac-
tions were observed using ruthenium porphyrins as catalysts;
only the coupling products diethyl maleate and fumarate were
found.8 We present herein the first ruthenium porphyrin-
catalysed cyclopropanation of styrene derivatives by ethyl
diazoacetate.
We presume that the active intermediate in the ruthenium
porphyrin-catalysed reactions is a ruthenium carbene species
formed by reaction of ruthenium(ii) with ethyl diazoacetate, by
analogy to previous work in which it was demonstrated that
osmium(ii) porphyrin carbenes are catalytically active.5 Actu-
ally, when the reaction is monitored at room temperature, a RuII
The complexes RuII(por)(CO) (por = TPP,† TMP†)9,10 or
10
RuVI(por)(O)2 (por = TMP†), in catalytic amounts, quickly
react with ethyl diazoacetate in the presence of an excess of
styrene to give quantitatively‡ the corresponding cyclopropyl
esters, with a large excess of the anti isomer (Scheme 1). The
diastereoselectivity (Table 1) is reminiscent of that observed
with osmium and iron catalysts and differs from the syn
selectivity observed with rhodium porphyrins.2,11 As shown in
1
carbene complex is detected by H NMR spectroscopy when
Ru(TMP)(CO) is used as the catalyst. As expected,8 the
a-carbon proton appears in the proton spectrum at d 13.23.¶ The
presence of both the carbene ligand and the carbonyl group
coordinated to the ruthenium has been confirmed, by IR
(nCO = 1941 cm21) after isolation of the complex at the end of
the reaction, when Ru(TMP)(CO) is used as catalyst. Such a
O
CO2Et
CO2Et
Ph
catalyst
+
+
OEt
Ph
Scheme 1
Ph
N2
F3C
CF3
Ph
Ph
Table 1 Cyclopropanation of styrene derivatives by ethyl diazoacetate using
MeO
C
C
OMe
NH
ruthenium porphyrin complexes as catalysts
O
C
C
O
Ratio of
O
Ru
O
HN
anti:syn
productsa
Alkene
yieldb (%)
N
Substrate
Catalyst
N
N
N
Styrene
Sytrene
Styrene
a-Methylstyrene
a-Methylstyrene
a-Methylstyrene
p-Chlorostyrene
p-Chlorostyrene
Ru(TPP)(CO)
Ru(TMP)(CO)
Ru(TMP)(O)2
Ru(TPP)(CO)
Ru(TMP)(CO)
Ru(TMP)(O)2
Ru(TPP)(CO)
Ru(TMP)(CO)
13.1
7.9
7.1
3.1
1.6
1.5
14.0
8.2
7
< 5
< 5
7
< 5
< 5
8
NH
O
C
C
CF3
MeO
OMe
NH
Ph
O
C
C
Ph
< 5
CF3
Fig. 1
a Determined by GC. b Diethyl maleate and fumarate.
Chem. Commun., 1997
927

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J 7.3 Hz), 2.63 (p-Me, s, 12 H), 1.94 (o-Me, s, 12 H), 1.80 (oA-Me, s, 12 H),
0.30 (CH3, t, 3 H, J 7.3 Hz)
carbene is also observed with ethyl diazoacetate in the presence
of the optically active dioxoruthenium(vi) picket-fence com-
plex and a large excess of the alkene, but is not detectable when
the porphyrin is the TPP.
References
In conclusion, the ruthenium porphyrin compounds described
here, which are more easily handed than their air-sensitive
iron(ii) porphyrin analogues, are promising catalysts for
cyclopropanation reactions.
We gratefully acknowledge the financial support of the
MESR for a grant to E. G.
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Footnotes
* E-mail: gerard.simonneaux@univ-rennes1.fr
† Abbreviations used: TPP = 5,10,15,20-tetraphenylporphyrin dianion;
TMP = 5,10,15,20-tetramesitylporphyrin dianion.
9 J. J. Bonnet, S. S. Eaton, G. R. Eaton, R. H. Holm and J. A. Ibers, J. Am.
Chem. Soc., 1973, 95, 2141.
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‡ Based on ethyl diazoacetate conversion. In a typical experiment, ethyl
diazoacetate (1.14 mmol) is slowly added over approximately 4 h to a
vigorously stirred solution of the ruthenium catalyst (2.6 mmol) and alkene
(1.71 mmol) at room temperature under inert atmosphere. The reaction
mixture is then stirred for 1 h and the cyclopropyl ester formation is
analysed by GC.
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Commun., 1994, 1287.
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§ Enantiomeric excesses for cyclopropyl esters were determined using the
chiral shift reagent tris[3-(heptafluoropropylhydroxymethylene)-(+)-cam-
phorato]europium(iii). For the trans product, measurement of the optical
rotation allowed identification of the absolute configuration as
(+)-(1S,2S).13
¶ Selected spectral data: 1H NMR (CDCl3):d 13.23 (carbene CH, s, 1 H),
8.28 (Hb, s, 8 H), 7.25 (Hm s, 4 H), 7.20 (HmA, s, 4 H), 2.67 (CH2, q, 2 H,
Received in Cambridge, UK, 6th January 1997; Com.
7/00098G
928
Chem. Commun., 1997