Reaction of bromopentafluorobenzene and pentafluorophenyllithium with α-lithiated arylacetonitriles

Bromopentafluorobenzene and pentafluorophenyllithium react with α-lithiated arylacetonitriles in ether to give α-aryl-2,3,5,6-tetrafluorophenylacetonitriles and α-aryl-4-bromo-2,3,5,6-tetrafluorophenylacetonitriles, respectively. However, the reaction of bromopentafluorobenzene with α-lithiated arylacetonitriles in ether/THF affords trans-1,2-dicyanostilbenes. Mechanisms for the formation of the various products are proposed.