A Comparative Study of Dibenzylideneacetone Palladium Complexes in Catalysis

Commercial Pd x (dba) y from various suppliers was found to vary considerably in appearance, homogeneity, purity, and catalytic activity. The Buchwald–Hartwig amination of 4-bromoanisole (5) with aniline (6) was established as a sensitive test reaction to probe the efficiency of Pd x (dba) y batches in catalytic transformations. The yields obtained with 17 different Pd x (dba) y batches ranged from 10% to nearly quantitative and could not be predicted reliably on the basis of any physical or spectroscopic descriptor alone. The best results in the catalytic test reaction were consistently achieved with a self-made slowly crystallized Pd2(dba)3·toluene adduct. A protocol is disclosed that allows batches of Pd x (dba) y with unsatisfactory or inconsistent performance to be converted into this reliable precatalyst.