Chloroacetophenone borane

Related catalysts for asymmetric borane reduction of ketones are open chain and cyclic phosphoric amides, in the oxidation state +3 or +5 (Scheme 11.3) [10, 11]. Early examples are the phosphonamides and phosphinamides 5a and 5b of Wills et al. [12] and the oxazaphospholidine-borane complex 6a of Buono et al. [13]. In the presence of 2-10 mol% catalysts 5a,b, co-chloroacetophenone was reduced by BH3 SMe2 with 35-46% ee [12]. For catalyst 6a a remarkable 92% ee was reported for the catalytic reduction of methyl iso-butyl ketone and 75% ee for acetophenone... 

Reduction of a-chloroacetophenone using the catalyst prepared from the related (5)-diphenylisoleucinol (4) and borane gives (5)-chlorohydrin (5), which is readily transformed to (S)-... 

On the basis of such hypothesis, the synthesis and use of new phosphine oxides from (S)-prolinol has been realized. Thus, enantioselective borane reduction of chloroacetophenone at 60 °C in THF in presence of 1 mol% of 24 led to the expected alcohols in up to 94% ee [34]. 

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