Alkaline-Earth Metal Mediated Benzene-to-Biphenyl Coupling.

Complex [( BDI)Ca] (C H ), with a C H dianion bridging two Ca ions, reacts with benzene to yield [( BDI)Ca] (biphenyl) with a bridging biphenyl dianion ( BDI=HC[C(Me)N-DIPeP] ; DIPeP=2,6-CH(Et) -phenyl). The biphenyl complex was also prepared by reacting [( BDI)Ca] (C H ) with biphenyl or by reduction of [( BDI)CaI] with KC in presence of biphenyl. Benzene-benzene coupling was also observed when the deep purple product of ball-milling [( BDI)CaI(THF)] with K/KI was extracted with benzene (DIPP=2,6-CH(Me) -phenyl) giving crystalline [( BDI)Ca(THF)] (biphenyl) (52 % yield). Reduction of [( BDI)SrI] with KC gave highly labile [( BDI)Sr] (C H ) as a black powder (61 % yield) which reacts rapidly and selectively with benzene to [( BDI)Sr] (biphenyl). DFT calculations show that the most likely route for biphenyl formation is a pathway in which the C H dianion attacks neutral benzene. This is facilitated by metal-benzene coordination.