A new catalytic enantioselective reducing reagent system from (−)-α,α-diphenylpyrrolidinemethanol and 9-borabicyclo[3.3.1]nonane, especially effective for hindered and substituted aralkylketones

New catalytic enantioselective reduction systems were prepared from aminoalcohols and dialkylboranes, for the enantioselective reductions of prochiral aromatic ketones. Among these, the system prepared from (−)-α,α-diphenylpyrrolidinemethanol with 9-borabicyclo[3.3.1]nonane proved especially promising for such reductions. This complex catalyzes the reduction of prochiral aralkyl ketones to the corresponding alcohols with BH3–THF, with enantioselectivities 82–99.2%. Also, this catalyst is particularly effective for the more hindered and substituted aralkyl ketones. Various modifications in this new catalytic reduction system, such as changing reaction conditions, reducing agent and dialkylborane, were also examined.