Allylic substitution mediated by water and palladium:: Unusual role of a palladium(II) catalyst and ESI-MS analysis

Allylic substitution of PhCH(OAc)CH=CHPh by CH2(COMe)(2) in a basic MeOH/H2O mixture could be achieved in the absence of a palladium catalyst and lead to a mixture of PhCH(CH(COMe)(2))CH=CHPh and PhCH(OMe)CH=CHPh in 40% and 55% yields, respectively. The process is induced by water, and nucleophilic attack addition occurred on a stabilized carbocation as the intermediate. Addition of a catalytic amount of PdCl2(CH3CN)(2) did not accelerate the reaction but improved the selectivity, and PhCH(CH(COMe)(2))CH=CHPh was then obtained in 92% yield while PhCH(OMe)CH=CHPh was observed in trace amounts. An ESI-MS analysis of the reaction mixture led us to assume that a palladium acetylacetonate complex is involved in the formation of PhCH(CH(COMe)(2))CH=CHPh.