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Rigid bicyclic amino alcohols such as DIAB catalyse the addition of dimethyl zinc to aldehydes. Initial reaction with one organozinc produces a bimetallic intermediate. This then coordinates with the aldehyde so that the bulkier group (in this case, Ph) occupies the less hindered position. Intramolecular delivery of the methyl leads to the S-enantiomer, which is released on workup.
Animations based on M. Yamakawa and R.Noyori, Organometallics 1999, 18, 128-133