Chiral Metallocenes. 3. The Enantioselective Reduction of CX Bonds (X = O or CH2) by the Chiral ansa-Metallocenes (R)- or (S)-[TiCl2(η5:η5- C5Me4SiMe2C5H3R*)] (R* = Menthyl or Neomenthyl)
journal contributionposted on 20.11.2002 by Paul Beagley, Philip J. Davies, A. John Blacker, Colin White
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After activating with n-BuLi, the two diastereoisomers of [TiCl2(η5:η5-C5Me4SiMe2C5H3R*)] (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)CH2, 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 TiCl2(η5:η5-C5Me4SiMe2C5H3R*) (1b, R* = neomenthyl) gives 36% ee of R-PhCH(Me)Et.