On the Mechanism of Palladium(0)‐Catalyzed Reactions of Allylic Substrates with Nucleophiles. Origin of the Loss of Stereospecificity

Abstract

AbstractThe mechanism of the loss of stereospecificity in palladium‐catalyzed nucleophilic substitution of allylic substrates has been investigated. Eight substrates (cis and trans isomers of 1a‐d) and two nucleophiles (Et2NH and NaCH(SO2Ph)2) were studied. In the animation reactions two pathways are responsible for the formation of anomalous inversion product, viz., isomerization of the starting material (path B, Scheme 2) and isomerization of the π‐allyl intermediate via displacement of palladium by Pd(0) (path C, Scheme 2), the latter of which predominates. In the alkylation the results indicate that loss of stereospecificity is caused only by path C. The use of a more reactive substrate increased the stereospecificity of the reaction and suppressed the isomerization pathway. An analysis of the kinetics is consistent with the hypothesis that path C is the major pathway for the stereochemical loss.