SELECTIVE REDUCTIONS .56. EXPLORATION OF THE B-HALOISOPINOCAMPHEYLBORANES FOR ASYMMETRIC REDUCTION OF KETONES

A series of diisopinocampheylhaloboranes and monoisopinocampheyldihalo boranes were synthesized by the reaction of the corresponding boranes with the respective HX or X(2) (X = halogen) or by the hydroboration o f alpha-pinene with the corresponding haloboranes. Stabilities of thes e haloboranes in various solvents were studied. Most of these halobora nes proved capable of reducing prochiral ketones to the alcohols with significant optical induction. When tested against a representative ar omatic and aliphatic prochiral ketone, acetophenone and 3-methyl-2-but anone, respectively, alpha-phenethyl alcohol in 65-98% ee and 3-methyl -2-butanol in 28-59% ee were obtained Some of them exhibited anomalous behavior.