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(η6-C6H5Li)2]2·(thf)7, in which
both subunits are connected by three bridging lithium atoms. The [2]silachromoarenophanes [Cr(η6-C6H5)2Si2Me4] and [Cr(η5-C5H4)(η7-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(η6-C6H5)2Si2Me4] revealed the presence of only weak molecular ring strain
demonstrated by a tilt angle of 2.78(12)° [cf. [Cr(η5-C5H4)(η7-C7H6)Si2Me4], 2.60(15)°]. 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 °C. The formation of the bis-silylated products was authenticated
by a crystal structure analysis of the trochrocene derivative [Cr(η7-C7H6)SiMe2C(Me)C(H)SiMe2(η5-C5H4)].