Highly efficient and regioselective production of trisubstituted alkenes through heck couplings catalyzed by a palladium phosphinito PCP pincer complex

The PCP ligand precursor 1,3-bis(diisopropylphosphinito)benzene is conveniently prepared from the reaction of chlorodiisopropylphosphine and resorcinol in the presence of 4-dimethylaminopyridine. Refluxing the bis(phosphinito)benzene with Pd(COD)Cl2 in toluene for 5 h yields the palladium complex PdCl{C6H3-2,6-(OPPri 2)2} (1). The molecular structure of the complex was determined through a single-crystal X-ray diffraction study. The complex is a highly efficient catalyst for the olefinic coupling of aryl bromo and iodo compounds under aerobic conditions. Disubstituted alkenes were obtained in good yields from the catalytic coupling of styrene and aryl iodides, whereas styrene reactions with aryl bromides yield exclusively trisubstituted alkenes. Trisubstituted alkenes were also obtained from the reactions of 1,1 disubstituted alkenes (α-methylstyrene, n-butylacrylate) and bromobenzene. Conditions have been found which result in the regioselective product ion of one isomer of the trisubstituted alkene products in≥90% yield.