Chiral Metallocenes. 3.1The Enantioselective Reduction of CX Bonds (X = O or CH2) by the Chiralansa-Metallocenes (R)- or (S)-[TiCl2(η5:η5- C5Me4SiMe2C5H3R*)] (R* = Menthyl or Neomenthyl)

After activating with n -BuLi, the two diastereoisomers of [TiCl 2 (η 5:η 5 -C 5 Me 4 SiMe 2 C 5 H 3 R*)] ( 1a, R* = menthyl), which have different faces of the asymmetric cyclopentadienyl ring coordinated to the titanium, catalyze the hydrosilylation of ketones, with the (1a) R diastereomer being more enantioselective than the (1a) S isomer. For example, with the (1a) R catalyst, hydrosilylation of acetophenone gives, after hydrolysis, S -PhCH(OH)Me in 82% ee, whereas only 16% ee of R -PhCH(OH)Me is obtained with the corresponding (1a) S catalyst. Inspection of the crystal structures of both (1a) R and (1a) S allows this difference in stereoselectivity to be rationalized. Highest ee's and rates of hydrosilylation were observed with aryl ketones containing electron-donating groups in the ring. The diastereoisomer (1a) R catalyzes the hydrogenation of alkenes; thus, 2-phenyl-1-butene, PhC(Et) CH 2, is hydrogenated in 53% ee ( S ). The same reduction carried out with a 1.4:1 mixture of R and S diastereoisomers of the corresponding TiCl 2 (η 5:η 5 -C 5 Me 4 SiMe 2 C 5 H 3 R*) ( 1b, R* = neomenthyl) gives 36% ee of R -PhCH(Me)Et.