carbene complex 1 (R ) Ph, M ) W) afforded R-allyl
â-phenyl propionate 6a in 55% yield without detection of
â-allyl â-phenyl propionate. In this way, we obtained only
R-allyl â-phenyl propionate by reaction with allyl alkoxide
in both cases of R,â-unsaturated Fischer carbene complexes
1 and the corresponding tricarbonylchromium-coordinated
R,â-unsaturated Fischer carbene complexes 5.
Scheme 1. [3,4]-Sigmatropic Rearrangement Promoted by
[1,2]-Metal Shift Reported by Barluenga4
We next extended this interesting reaction to the asym-
metric synthesis of R-allyl â-phenyl propionate by using
planar chiral binuclear carbene complexes. When enantio-
merically pure binuclear Fischer carbene complex 5b ([R]24
D
-3400 (c 0.001, CHCl3)), derived from a planar chiral (-)-
(R)-o-methoxybenzaldehyde chromium complex, was reacted
with allyl alkoxide at 25 °C, an R-allyl o-methoxyphenyl
propionate (6b) ([R]23D -25.2 (c 0.09, CHCl3)) was obtained
in 68% yield with 95% ee7 (entry 2).
concerted 1,2-M(CO)5 shift followed by [3,4]-sigmatropic
rearrangement as shown in Scheme 1. The allyl group was
introduced at the â-carbon to the ester carbonyl group.
Therefore, â-allyl arylpropionates 4 would be obtained as
optically active compounds by utilizing planar chiral arene
chromium complexes as a chiral auxiliary.
Expectedly, the R-allyl phenyl propionate 6b was obtained
with high enantiomeric excess despite severe basic condi-
tions. The absolute stereochemistry of 6b was determined
as (R)-configuration by comparison of an optical rotation of
authentic (S)-configurated allyl 2-allyl o-methoxyphenyl-
propionate ([R]23 +27.1 (c 0.38, CHCl3)) prepared from
D
Evans’ oxazolidone derivative.8 When the reaction was
performed at 0 °C, the corresponding R-allyl methyl ester
7b was obtained in 40% yield with 97% ee along with
formation of R-allyl-â-hydroxy ester 8b as an easily sepa-
rable diastereomeric mixture in 40% yield (entry 3).
Binuclear R,â-unsaturated Fischer carbene complexes 5
as starting materials were easily prepared by aldol condensa-
tion of benzaldehyde chromium complexes with methyl-
methoxycarbene complexes in the presence of triethylamine
and trimethylsilyl chloride in reasonable yields.5 We initially
investigated the synthesis of 4 by using the binuclear carbene
complex 5a according to Barluenga’s procedure. The carbene
complex 5a in allyl alcohol was treated with NaH under inert
gas in a balloon and subsequently exposed to sunlight.
Surprisingly, R-allyl â-phenyl propionate 6a was obtained
in 75% yield without formation of â-allyl â-phenyl propi-
onate 4 (Table 1, entry 1). Thus, the allyl group was
regioselectively introduced at the R-position to the ester. The
structure of R-allyl â-phenyl propionate 6a was absolutely
confirmed by comparison with both authentic samples 6a
and 4 (R ) Ph).6 This result is in sharp contrast to
Barluenga’s report, in which the allyl group was introduced
at the â-position. We initially imagined that the distinct
reaction path of R,â-unsaturated Fischer carbene complexes
with allyl alkoxide might be attributed to the strong electron-
withdrawing ability of the tricarbonylchromium fragment.
However, we were surprised that the chromium-uncomplexed
R,â-unsaturated chromium carbene complex 1 (R ) Ph, M
) Cr) gave only R-allyl â-phenyl propionate 6a in 68% yield
without â-allyl â-phenyl propionate 4 (R ) Ph) by the
reaction with allyl alkoxide. Also, the corresponding tungsten
The stereochemistry of the major R-allyl-â-hydroxy ester
8b ([R]D30 -36.0 (c 0.23, CHCl3)) was determined to possess
the anti-(2R,3S)-configuration by comparison with authentic
compound.9 Similarly, with 2-methyl-2-propen-1-ol at 25 °C,
the corresponding product 6c was obtained in 85% yield with
72% ee (entry 4). On reaction of 5b with 2-methyl-2-propen-
1-ol at 0 °C, the corresponding R-allyl-â-hydroxy ester 8c
was only obtained with 83/17 dr (entry 5). Similarly, with
o-methyl-substituted arene chromium complex 5c, the reac-
tion products were controlled by reaction temperature. Thus,
R-allyl-â-hydroxy ester 6d was obtained at 25 °C (entry 6),
while the corresponding R-allyl methyl ester 7d and dia-
stereomeric R-allyl-â-hydroxy ester 8d were obtained at
lower reaction temperature (entries 7 and 8). The optical yield
of 7d increased to 97% ee in the reaction at -30 °C (entry
8). When the ortho substituent of the arene chromium
complex was changed to an isopropoxy group, aldol-type
product 8e was obtained as a major product in 51% yield
with 85/15 dr (entry 9). In this way, binuclear R,â-
unsaturated chromium carbene complexes with planar chiral
arene chromium complexes were reacted with allyl alcohols
in the presence of a base to give 2-allyl o-substituted phenyl
propionate esters with high ee. Furthermore, when the
reaction was performed at lower temperature, methyl R-allyl-
â-hydroxy arylpropionates 8 were obtained with high anti
(3) Some R,â-unsaturated binuclear Fischer carbene complexes: (a)
Sierra, M. A.; del Amo, J. C.; Manchen˜o, M. J.; Go´mez-Gallego, M. J.
Am. Chem. Soc. 2001, 123, 851-861. (b) Barluenga, J.; Lo´pez, S.; Trabanco,
A. A.; Ferna´ndez-Acebes, A.; Flo´rez, J. J. Am. Chem. Soc. 2000, 122, 8145-
8154. (c) Barluenga, J.; Ferna´ndez-Acebes, A.; Trabanco, A. A.; Flo´rez, J.
J. Am. Chem. Soc. 1997, 119, 7591-7592.
(4) Barluenga, J.; Rubio, E.; Lo´pez-Pelegr´ın, J. A.; Toma´s, M. Angew.
Chem., Int. Ed. 1999, 38, 1091-1093.
(5) (a) Aumann, R.; Heinen, H. Chem. Ber. 1987, 120, 537-540 and
ref 3a.
(7) Enantiomeric excess was determined by chiral HPLC with Chiralcel
OD; hexane/2-propanol ) 100/1; flow rate 1.0 mL/min; column temperature
40 °C; UV detector 254 nm, retention time, racemate, 14.3 min, 16.9 min:
6b, 16.9 min.
(8) Evans, D. A.; Ennis, M. D.; Mathre, D. J. J. Am. Chem. Soc. 1982,
104, 1737-1739.
(9) Evans, D. A.; Tedrow, J. S.; Shaw, J. T.; Downey, C. W. J. Am.
Chem. Soc. 2002, 124, 392-393.
(6) Authentic samples 4 (R ) Ph) and 6a were prepared by allyl
esterification of 3-phenyl-hex-5-enoic acid and R-allylation of â-phenyl
propionate, respectively. See Supporting Information.
4308
Org. Lett., Vol. 6, No. 23, 2004
Kamikawa, Ken
