Oxidation of aryl-substituted allylic alcohols by an optically active Fe(III)(porph*) catalyst:: enantioselectivity, diastereoselectivity and chemoselectivity in the epoxide versus enone formation

A set of aryl-substituted allylic alcohols (+/-)-2 was oxidized using the chiral Fe(porph*) complex I as the catalyst and iodosyl benzene (PhIO) as the oxygen source. Whereas one enantiomer of the allylic alcohol 2 is preferentially epoxidized to give the threo- or cis-epoxy alcohol 3 (up to 43% e.e.) as the main product (d.r. up to > 95:5), the other enantiomer of 2 is enriched (up to 31% e.e.), Some non-stereoselective allylic oxidation to give the enone 4 also takes place. The observed diastereo- and enantioselectivities in the epoxidation reactions are rationalized in terms of a synergistic interplay between the hydroxy-directing effect and the steric interactions of the catalyst 1 and the Substrate 2. (C) 2001 Elsevier Science Ltd. All rights reserved.