Palladium catalyzed cycloisomerization of 2,2-diallylmalonates in imidazolium ionic liquids

A series of palladium salts and complexes, including PdCl2, PdCl2/AgPF6, PdCl2(PhCN)2, PdBr2 and Pd2(dba)3 were used in imidazolium ionic liquids as catalysts for the cycloisomerization of diethyl 2,2-diallylmalonate. The order of reactivity follows the expected Pd atom hardness and, therefore, “naked” Pd2+ generated from PdCl2 + AgPF6 gives the highest turnover frequencies. The system comprising Pd species dissolved in ionic liquid can be reused with minor decrease in conversion, but with an apparent variation in the product distribution. The latter comes from a diminished catalytic activity and from formation of Brönsted acid sites. Finally, by using 1,10-undecadiene derivative we have observed that this Lewis acid induced cycloisomerization is specific for the formation of five-member rings.