Enantioselective Isomerization of Primary Allylic Alcohols into Chiral Aldehydes with the tol-binap/dbapen/Ruthenium(II) Catalyst

摘要

Asymmetric isomerization of primary allylic alcohols is a direct and simple transformation to afford optically active aldehydes (Scheme 1). In the presence of an appropriate chiral catalyst, one of the a hydrogens of a prochiral allylic alcohol 1 is formally transposed to the y position (1,3-hydrogen shift) with migration of the C-C double bond to give the chiral enol intermediate. The following keto-enol tautomerism produces the desired aldehyde 2 with a stereo-genie center at the |3 position. Undesired reduction of 2 to afford the saturated alcohol 3 is a significant problem under reductive conditions. High reactivity (resulting in low catalyst loading), enantioselectivity, and chemoselectivity are major requirements to achieve an efficient isomerization, leading to highly productive environmentally benign reaction.