ELECTROCATALYTIC COUPLING OF ARYL HALIDES WITH (1,2-BIS(DI-2-PROPYLPHOSPHINO)BENZENE)NICKEL(0)

Dibromo- and dichloro(1,2-bis(di-2-propylphosphino)benzene)nickel(II) is compared with tetrakis(triphenylphosphino)nickel(II) as an electrocatalyst for the reductive coupling of aryl halides. In many of the reactions examined, dehalogenation of the substrate predominated over coupling; however, preparative yields of biphenyls as high as 96% can be obtained with aryl chlorides and 2 mol % of the title catalyst in polar, coordinating solvents. Experimental factors governing the efficiency of these reactions are discussed, and possible mechanisms for coupling and catalyst deactivation are considered. Much better selectivity for aryl chlorides is attained in electroreductive couplings catalyzed by the title compound, whereas electrocatalysis with (Ph3P)4NiCl2 allows for selective intra- and intermolecular coupling at aryl bromide and vinyl chloride sites. Modest yields of cyclization products can be attained with either electrocatalyst in the presence of appropriately functionalized aryl or vinyl halides.