Asymmetric Guerbet Reaction to Access Chiral Alcohols.

The first example of an asymmetric Guerbet reaction has been developed. Using commercially available, classic Noyori Ru-II-diamine-diphosphine catalysts, well-known in asymmetric hydrogenation, racemic secondary alcohols are shown to couple with primary alcohols in the presence of a base, affording new chiral alcohols with enantiomeric ratios of up to 99:1. Requiring no reducing agents, the protocol provides an easy, alternative route for the synthesis of chiral alcohols. Mechanistic studies reveal that the reaction proceeds via a Ru-catalyzed asymmetric hydrogen autotransfer process in concert with a base-promoted allylic alcohol isomerization.