Stereochemistry of the palladium-catalyzed allylic substitution: the syn-anti dichotomy in the formation of (π-allyl)palladium complexes and their equilibration

Abstract

The mechanism of palladium(0)-catalyzed allylic substitution has been investigated with the aim of finding whether or not the intermediate (π-allyl)palladium complexes can arise in a syn fashion as an alternative to the well known anti-mechanism. Using (diphenylphosphino)acetate as a leaving group and stereochemically biased substrates 30b and 35b evidence for the syn stereochemistry has been acquired ( 30b → 31 and 35b → 36). This reversal of stereochemistry is facilitated by severe steric congestion in the starting allylic esters (which impairs the ordinary anti-mechanism) and is boosted by the pre-coordination of the Pd(0) reagent to the leaving group. The latter effect apparently lowers the activation entropy. With cyclohexene derivatives 10b, 18b, and 19b and acyclic substrate 25b, where steric hindrance does not operate, the anti-mechanism producing η 3-complexes dominates even for (diphenylphosphino)acetates. At elevated temperature, rapid equilibration of η 3-complexes ( 13 ⇄ 14 and 20 ⇄ 21) has been observed prior to the reaction with a nucleophile. This effect has been attributed to the presence of (diphenylphosphino)acetate ion acting as a ligand for palladium.