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This reduction works best when the two carbonyl substituents are well-differentiated sterically - ie Ph and Me. When the carbonyl oxygen is complexed to the boron it becomes electrophilic enough to be reduced by the weak hydride source. The hydride itself is delivered via a six-membered cyclic transition state. The enantioselectivity arises from the preference of the larger carbonyl substitutent for the pseudoequatorial position of the ring.