Selective reductions. 56. Exploration of the B-haloisopinocampheylboranes for asymmetric reduction of ketones

A series of diisopinocampheylhaloboranes and monoisopinocampheyldihaloboranes were synthesized by the reaction of the corresponding boranes with the respective HX or X2 (X = halogen) or by the hydroboration of α‐pinene with the corresponding haloboranes. Stabilities of these haloboranes in various solvents were studied. Most of these haloboranes proved capable of reducing prochiral ketones to the alcohols with significant optical induction. When tested against a representative aromatic and aliphatic prochiral ketone, acetophenone and 3‐methyl‐2‐butanone, respectively, α‐phenethyl alcohol in 65–98% ee and 3‐methyl‐2‐butanol in 28–59% ee were obtained. Some of them exhibited anomalous behavior.