Enantioselective radical cross-coupling reactions of silyl enol ethers using chiral oxovanadium

Article abstract of DOI:10.1246/cl.2001.1324

Asymmetric induction was observed in radical cross-coupling reactions of silyl enol ethers using chiral oxovanadium generated in situ from 8-phenylmenthol and vanadium oxytrichloride in the presence of MS4A (molecular sieves-4A).

Full text of DOI:10.1246/cl.2001.1324

1324
Chemistry Letters 2001
Enantioselective Radical Cross-Coupling Reactions of Silyl Enol Ethers
Using Chiral Oxovanadium
Masaaki Kurihara,* Taeko Hayashi, and Naoki Miyata
Division of Organic Chemistry, National Institute of Health Sciences, Kamiyoga, Setagaya-ku, Tokyo 158-8501
(Received September 3, 2001; CL-010859)
Asymmetric induction was observed in radical cross-cou-
pling reactions of silyl enol ethers using chiral oxovanadium
generated in situ from 8-phenylmenthol and vanadium
oxytrichloride in the presence of MS4A (molecular sieves-4A).
Asymmetric induction in radical-mediated reactions is
presently of considerable interest in synthetic organic chem-
istry,1 and enantioselective radical reactions using organotin
reagents and chiral Lewis acids appear in the literature.2
Generally, most radical-mediated reactions are conducted
through the reductive generation of a radical species, but oxida-
tive radical generation using metal oxidants is now being con-
sidered for this purpose.3 Diastereoselective oxidative radical
coupling reactions using chiral auxiliaries have been reported.4
Oxidative radical cross-coupling reactions of silyl enol
ethers using VO(OEt)Cl2 as a one-electron oxidant to give 1,4-
diketones as reported by Hirao and co-workers are unique radi-
cal-mediated C–C bond forming reactions5 (Figure 1). In these
reactions, when radical intermediates are subjected to a chiral
environment, asymmetric induction may be considered to occur.
This paper presents for the first time an enantioselective
oxidative radical cross-coupling reaction of silyl enol ethers
using the chiral oxovanadium reagents generated in situ from
VOCl3 and chiral alcohols in the presence of MS4A (Figure 2).
Asymmetric induction was observed when using menthol
and 8-phenylmenthol, (runs 2 and 3 in Table 2) and was best
with 8-phenylmenthol as the chiral alcohol (run 3.).
A typical experimental procedure (run 3 in Table 2) is as
follows. To a solution of VOCl3 (1.29 mmol) in CH2Cl2 (5
mL) were added 8-phenylmenthol (1.29 mmol) in CH2Cl2 (1
mL) and powdered MS4A (1 g). The mixture was stirred at r.t.
for 1 h and cooled to –75 °C under Ar. With the temperature
Bases such as K2CO3, NEt3 and collidine were initially
used with VOCl3 and EtOH to form VO(OEt)Cl2 in situ, but no
coupling reactions took place, however, the reaction proceeded
on using MS4A, and the results are summarized in Table 1.
MS4A was found particularly effective for inducing the forma-
tion of oxovanadium reagents in situ and coupling reactions.
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1325
maintained at –75 °C, 1-phenyl-1-(trimethylsilyloxy)ethylene 2
(0.65 mmol) was added to the reaction mixture, and then 1-
trimethylsilyloxycyclohexene 1 (0.43 mmol) dropwise over a
period of 20 min. This was followed by additional stirring at
–75 °C for 3 h and –30 °C for 4 h. At –30 °C, 5% aqueous
Na2S2O3 (0.5 mL) and ether (10 mL) were added to the reaction
mixture followed by extracting with ether and the addition of
concd HCl and extraction again with ether. The organic layer
was washed with saturated aqueous NaHCO3 and brine, dried
and concentrated. The residue was purified by silicagel column
chromatography (hexane:diethyl ether = 10:1) to give 1,4-dike-
tone 3, [α]D25 –9.8 °(c 0.66, CHCl3) in 85% ee. The value of ee
was determined by HPLC analysis using a DAICEL CHIRAL-
CEL OB column.
In treating VOCl3 with EtOH in the presence of MS4A in
CDCl3, VO(OEt)Cl2 formation was confirmed by the observation
of Et peaks of VO(OEt)Cl2 by NMR measurement. When colli-
dine was used instead of MS4A, there was no generation of
VO(OEt)Cl2 at all. MS4A would thus appear effective for induc-
ing formation of chiral oxovanadium reagents. In the case of a
mixture of 8-phenylmenthol and VOCl3 in the presence of
MS4A, it was observed that the formation of chiral oxovanadium
reagent caused the downfield shift of the HA peak (Figure 3.).
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The present reactions represent a new class of enantiose-
lective radical-mediated reactions and the mechanism of the
asymmetric induction is currently being studied.
This study was supported in part by Special Coordination
Funds of the Science and Technology Agency of the Japanese
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