Synthesis, Structure, and Reactivity of (1,2,3-η3-Butadien-3-yl)palladium Complexes

The cationic exo-isopropylidene-π-allylpalladium complex [Pd(η3-CH2CMeCCMe2)(dppb)]BArF4 (1; ArF = C6H3-3,5-(CF3)2), was prepared, and its solid-state structure was determined by X-ray crystallography. The CMe2 plane of the exo-isopropylidene moiety and the η3-CH2CMeC plane are not coplanar, with a dihedral angle of 66.06°. A reaction between 1 and a stabilized nucleophile, Na[CMe(CO2Me)2], took place at the terminal methylene carbon of η3-CH2CMeC to give the corresponding allene exclusively. On the other hand, a reaction of 1 with a Grignard reagent proceeded via an initial nucleophilic attack at the Pd center to give a conjugated diene. Complex 1 showed good catalytic activity in the reactions between 2-bromo-1,3-dienes and stabilized nucleophiles, and a variety of allenes were obtained in good yields. The dynamic process of forming the alkylidene-π-allylpalladium complexes from 2-bromo-1,3-dienes and Pd(0) was also examined.