CHIRAL PD-CHELATES, CU-CHELATES AND NI-CHELATES AND THEIR UTILITIES AS CATALYSTS IN ALLYLIC ACETOXYLATION OF ALKENES

A series of chiral ML2* complexes of Pd, Cu and Ni [L* = 2-trifluoroacetylcamphorato (tfacam), (R)-N-alpha-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 PdL2* under the reaction conditions, the product distribution from the acetoxylation of D-1-cyclohexene, and the low acetolysis reactivity of L*Pd (eta3-allyl), indicate the intervention of (eta3-allyl)-Pd intermediates in the catalytic process in which either or both L* groups dissociate or at least dechelate in the product-forming step.