THE STEREOCHEMICAL DICHOTOMY IN PALLADIUM(0)-CATALYZED AND NICKEL(0)-CATALYZED ALLYLIC SUBSTITUTION

The steric course of the first step of Pd(0)-catalyzed allylic substit ution with stabilized C-nucleophiles can be completely reversed by a s uitably positioned coordinating Ph2P group, resulting in an overall in version (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 resul t of ret.-inv. pathway, whereas 9, lacking the coordinating group, aff ords the ''normal'' inv.-inv. product 23. The intermediate eta(3)-comp lex 32; generated in the former reaction, has been characterized by H- 1 and P-31 NMR spectroscopy. While this stereochemical control is high ly successful with cyclic substrates, it does not operate in acyclic s eries, as documented by the reactivity of the anti-configured 1,4-func tionalized hexenes 14 and 15, which both give the product of inv. -inv . pathway, i.e., 35 and 36, respectively. The syn-configured allylic s ubstrates 21 and 22 exhibit the same pattern, irrespective of the pres ence of the coordinating neighboring group. The lack of overriding con trol in the latter instances has been attributed to a rotation about t he C-C bond connecting the coordinating group to the allylic system, w hich allows the precoordinated Pd(0) to approach the allylic moiety fr om the face opposite to the leaving group (15 --> 41 --> 42). Precoord ination of the catalyst to the Ph2P group is evidenced by substantial acceleration of the reaction in all cases studied. For the Ni(0)-catal yzed reaction of the allylic methoxy derivatives with MeMgBr, precoord ination proved to be the prerequisite for the reaction to occur (50 -- > 51 --> 52); ret.-ret. pathway was observed.