Tetrahedron p. 1599 - 1606 (1980)
Update date:2022-07-30
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Cherest
Prudent
Reduction of ketones L-CHMe-CO-R [L=Ph and Cy(Cy = cyclohexyl) and R=Me, Et, iPr and tBu] affords pairs of diastereoisomeric alcohols L-CHMe-CHOH-R. The predominant diastereoisomer is always that predicted by Cram's rule, and the stereoselectivity of the reaction generally increases as R is made more bulky. Thus, with LiAlH4 in ether at 35°, the diastereoisomer ratios are respectively 2.8:1, 3.2:1, 5.0:1, and 49:1 when L=Ph and 1.6:1, 2.0:1, 4.1:1, and 1.6:1 when L=Cy. With NaBH4 in isopropanol at 50°, these ratios are respectively 1.6:1, 2.0:1, 2.7:1, and 7.3:1 when L=Ph, and 1.2:1, 1.6:1, 3.2:1, and 3.5:1 when L=Cy. It is suggested that an important factor determining the steric course of the reduction of both open-chain ketones and cyclohexanones is torsional strain in the transition state, and that torsional strain involving partial bonds can represent a substantial fraction of the strain between fully-formed bonds, even when the degree of bonding is quite low. This postulate has been made the basis of an internally consistent interpretation of the steric course of the reaction between carbonyl compounds and nucleophilic reagents such as hydrides and Grignard reagents.
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