Chiral Pd-, Cu- and Ni-chelates and their utilities as catalysts in allylic acetoxylation of alkenes

Abstract

A series of chiral ML★ 2 complexes of Pd, Cu and Ni [L★=2-trifluoroacetylcamphorato (tfacam), ( R)- N-α-methylbenzylsalicylaldimine, 2-(2′-phenolato)-4-substituted-oxazoline (phenox)] have been prepared and characterized, including X-ray structures of Pd(tfacam) 2 ( 1) and Pd(4-Ph-phenox) 2 ( 3a). The Pd- and Cu-chelates are active (pre) catalysts for the allylic acetoxylation of alkenes. The yields and chemoselectivities in these reactions are high but little enantioselectivity (0–5%) was found. Several mechanistic probes, including tests of the stability of the product and PdL★ 2 under the reaction conditions, the product distribution from the acetoxylation of D-1-cyclohexene, and the low acetolysis reactivity of L★Pd(η 3-allyl), indicate the intervention of (η 3-allyl)-Pd intermediates in the catalytic process in which either or both L★ groups dissociate or at least dechelate in the product-forming step.