Scheme 2. Cyclopropanation of o-Bromostyrene (1f) and
p-Bromostyrene (1d) with 2b
84% ee). The optical rotation of (1R,2S)-5, according to the
literature is [R]24 -104 (c ) 1.12, PhH, for >99% ee),7
D
Figure 2. Ligands L*H for Rh(II)-catalyzed cyclopropanations.
which allows assignment of the (1R,2S)-configuration to the
cyclopropanes 3a and 3b, except the ones resulting from
reaction with [Rh2{(R)-ntv}4], where it is (1S,2R)-. The
cyclopropane 3b, resulting from reaction with [Rh2{(S)-
nttl}4], was subjected to X-ray analysis to confirm both
relative and absolute configurations.8,9
Introduction of para substituents in the styrene had only a
minor effect on the enantioselectivity. Thus, the p-chloro and
the p-bromostyrenes were converted to the cyclopropanes
3c and 3d with 94 and 92% ee, respectively, in the presence
of [Rh2{(S)-nttl}4], while the p-methoxy substituent led to a
slightly lower ee of 89% (Table 3). Surprisingly, however,
instead of the double bond, with formation of an intermediate
ylide 6 that may dissociate from the metal.10 The formation
of ylides between carbenes and organohalides is well
documented.11 If direct intramolecular transfer of the carbene
from ylide 6 to the double bond occurs without intervention
of the chiral catalyst, the resulting cyclopropane 3f will be
racemic. The analogous reaction of p-bromostyrene (1d) may
afford the ylide 7. Since the cyclopropanation of 1d is
enantioselective, this suggests that either the pathway in-
volving 7 is not significant or carbene transfer of 7 must
involve the catalyst. Asymmetric carbene transfer from
isolable phenyliodonium ylides, although not from bro-
monium ylides, has been reported in the past with Rh(II)
and Cu(I) catalysts.12
Table 3. Cyclopropanation of Substituted Styrenes (1c-g)
with 2b (y ) Tips) in the Presence of [Rh2{(S)-nttl}4] as the
Catalysta
The performance of the vinyldiazoacetate 2 in conjunction
with [Rh2{(S)-nttl}4] may be appreciated in the light of
compd
X
catalyst
yield (%)
ee (%)b
(8) Crystal data for 3b: C22H34O3Si, Mr ) 374.6; µ ) 0.12 mm-1, dx )
1.132 g/cm3, trigonal, P32, Z ) 3, a ) 15.3674(9), c ) 8.0593(4) Å, V )
1648.3(2) Å3; F(000) ) 612, θmax ) 26.02°, crystal size ) 0.17 × 0.21 ×
0.28 mm. Cell dimensions and intensities were measured at 200 K on a
Stoe IPDS diffractometer with graphite-monochromated Mo KR radiation
(λ ) 0.71073 Å). Data were corrected for Lorentz and polarization effects
and for absorption (Tmin, Tmax ) 0.9690, 0.9854). The structure was solved
by direct methods (SIR97). A total of 20 679 reflections collected and 16 215
reflections with I > 2u(I ), not merged (on account of twinning), were used
in the full-matrix least-squares (|F|2) refinement (SHELXL97), S ) 0.862,
R1 ) 0.0348 (all data ) 0.0498), wR2 ) 0.0694 (all data) for 242 parameters.
Largest residual peak (hole) 0.169 (-0.127) e/Å3. The crystal shows
twinning by a 180° rotation about [1h10] and displaying a volume ratio of
0.564(1)/0.436(1) of the two domains. The Flack parameter (x ) 0.05(6))
was estimated from a refinement taking into account the eight possible twin
laws associated to the space group P32 with respect to its holohedry as
described by H. D. Flack and G. Bernardinelli.9 Full details are given in
the CIF file (CCDC 230695) containing the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
lographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK.; fax:
(+ 44) 1223-336-033; or deposit@ccdc.cam.ac.uk).
1c
1d
1e
1f
p-Cl
p-Br
p-MeO
o-Br
o-Me
[Rh2{(S)-nttl}4]
[Rh2{(S)-nttl}4]
[Rh2{(S)-nttl}4]
[Rh2{(S)-nttl}4]
[Rh2{(S)-nttl}4]
3c 77
3d 70
3e 80
3f 71
3g 84
94
92
89
11
91
1g
a Conditions: 8.7 mmol of 1a-g, 0.6 mmol of 2b in PhCH3 (5.0 mL),
2 mol % catalyst, 0 °C. b Cis isomer not identified.
a bromo substituent in ortho position of the styrene resulted
in an almost racemic cyclopropane 3f. Initially, we attributed
this loss of selectivity to a steric effect. However, this
hypothesis had to be abandoned when it was found that the
cyclopropane 3g resulting from reaction with o-methylstyrene
(2g) had an ee of 91%. Thus, the low selectivity observed
in the case of 3f should not have a steric origin. Conceivably,
an unselective competitive pathway could involve reaction
of the metal-complexed carbene with the bromo substituent
(9) Flack, H. D.; Bernardinelli, G. Acta Crystallogr. A 1999, 55, 908-
915.
(10) Hodgson, D. M. Chem. Soc. ReV. 2001, 30, 50-61.
(11) (a) Doyle, M. P.; Tamblyn, W. H.; Bagheri, V. J. Org. Chem. 1981,
46, 5094-5102. (b) Doyle, M. P.; Acc. Chem. Res. 1986, 19, 348-356.
(7) Corey, E. J.; Gant, T. G. Tetrahedron Lett. 1994, 35, 5373-5376.
Org. Lett., Vol. 6, No. 11, 2004
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Muller, Paul
