Diphenylamine-derived bis-hydroxyamide catalyzed asymmetric borane reduction of prochiral ketones

A series of bis-hydroxyamides were synthesized from diphenylamine-2,2′-dicarboxylic acid and chiral aminoalcohols. Their catalytic activity in asymmetric borane reduction was investigated. After the fine optimization of solvents, temperature, amount of borane complex, and the length of catalyst generating period, good to excellent yields (55–99%) and enantioselectivities (79–97% ee) can be achieved in the reduction of aromatic and alkyl prochiral ketones. A transition state structure was proposed on the basis of absolute configuration and controlled experiment.

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1,1′-(Diphenylamine-2,2′-dicarbonyl)-bis[(2S)-α,α-diphenyl-2-pyrrolidinemethanol]C48H45N3O4Ee = 99.5%[α]D25=-98.0 (c 0.5, CH2Cl2)Absolute configuration: (S,S)Source of chirality: (2S)-α,α-diphenyl-2-pyrrolidinemethanol