Syntheses of Substituted 1,4-Disila-2,5-cyclohexadienes from Cyclic Hexasilane Si₆Me₁₂ and Alkynes via Successive Si–Si Bond Activation by Pd/Isocyanide Catalysts

Palladium-catalyzed reactions of dodecamethylcyclohexasilane [(SiMe₂)₆] (<b>1</b>) with alkynes led to efficient preparation of 1,1,4,4-tetramethyl-1,4-disilacyclohexadienes (<b>3</b>). The reactions were best catalyzed by Pd(0) species generated from Pd₂(dba)₃·CHCl₃ and 1-isocyanoadamantane (AdNC). Terminal and internal alkynes bearing aryl and alkyl substituents could be used as substrates, and the reaction allowed gram-scale preparation of <b>3</b>. A dimethylsilylene (Me₂Si:) species, generated by activation of Si–Si bonds in <b>1</b> by Pd(0) species, was involved in the reaction mechanism. The DFT calculations suggest that oxidative addition of Si–Si bonds in <b>1</b> to Pd­(CNAd)₂ species is followed by extrusion of a Me₂SiPd­(CNAd) intermediate. Reaction of the resulting palladium-coordinated silylene with an alkyne forms a silacyclopropene, which dimerizes to give <b>3</b>. The extrusion is accompanied by ring contraction of <b>1</b> to generate (SiMe₂)₅, which also contributes to formation of <b>3</b> and (SiMe₂)₄ by the Pd(0)-catalyzed reaction with an alkyne. Extrusion of Me₂SiPd­(CNAd) and ring contraction generated more than five Me₂Si: species from (SiMe₂)₆ (<b>1</b>)