Transition-metal-catalyzed bis-silylation of propyne by [2]chromoarenophanes

A crystal structure determination of the thf adduct of dilithiated bis(benzene)chromium disclosed an unsymmetrical, dimeric composition in the solid state, i.e., a formula of [Cr(eta(6)-C6H5Li)(2)](2)center dot(thf)(7), in which both subunits are connected by three bridging lithium atoms. The [2]silachromoarenophanes [Cr(eta(6)-C6H5)(2)Si2Me4] and [Cr(eta(5)-C5H4)(eta(7)-C7H6)Si2Me4] were prepared in high yields by salt elimination reactions of the corresponding dimetalated sandwich complexes with Cl2Si2Me4. Structural characterization by X-ray diffraction of [Cr(eta(6)-C6H5)(2)Si2Me4] revealed the presence of only weak molecular ring strain demonstrated by a tilt angle of 2.78(12)degrees [cf. [Cr(eta(5)-C5H4)(eta(7)-C7H6)Si2Me4], 2.60(15)degrees]. These species were successfully employed as precursors for the transition-metal-catalyzed bis-silylation of propyne to yield ansa-bis(silyl)alkenes. The reactions were accomplished under homogeneous catalysis conditions employing 5 mol % [Pd(PPh3)(4)] at 125 degrees C. The formation of the bis-silylated products was authenticated by a crystal structure analysis of the trochrocene derivative [Cr(eta(7)-C7H6)SiMe2C(Me)C(H)SiMe2(eta(5)-C5H4)].