The stereochemical dichotomy in palladium(0)- and nickel(0)-catalyzed allylic substitution

The steric course of the first step of Pd(0)-catalyzed allylic substitution with stabilized C-nucleophiles can be completely reversed by a suitably positioned coordinating Ph2P group, resulting in an overall inversion (1 --> 4 --> 5), as opposed to the normally observed retention (1 --> 2 --> 3). Thus, on reaction with NaCH-(CO2Me)(2), the allylic acetate 10, containing a phosphinous amide moiety, gives 24 as a result of ret.-inv. pathway, whereas 9, lacking the coordinating group, affords the "normal" inv.-inv. product 23. The intermediate eta(3)-complex 32; generated in the former reaction, has been characterized by H-1 and P-31 NMR spectroscopy. While this stereochemical control is highly successful with cyclic substrates, it does not operate in acyclic series, as documented by the reactivity of the anti-configured 1,4-functionalized hexenes 14 and 15, which both give the product of inv. -inv. pathway, i.e., 35 and 36, respectively. The syn-configured allylic substrates 21 and 22 exhibit the same pattern, irrespective of the presence of the coordinating neighboring group. The lack of overriding control in the latter instances has been attributed to a rotation about the C-C bond connecting the coordinating group to the allylic system, which allows the precoordinated Pd(0) to approach the allylic moiety from the face opposite to the leaving group (15 --> 41 --> 42). Precoordination of the catalyst to the Ph2P group is evidenced by substantial acceleration of the reaction in all cases studied. For the Ni(0)-catalyzed reaction of the allylic methoxy derivatives with MeMgBr, precoordination proved to be the prerequisite for the reaction to occur (50 --> 51 --> 52); ret.-ret. pathway was observed.