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)] (1) with alkynes led to efficient preparation of 1,1,4,4-tetramethy1-1,4-disilacyclohexadienes (3). The reactions were best catalyzed by Pd(0) species generated from Pd-2(dba)(3)center dot 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 3. A dimethylsilylene (Me2Si:) species, generated by activation of Si-Si bonds in 1 by Pd(0) species, was involved in the reaction mechanism. The DFT calculations suggest that oxidative addition of Si Si bonds in 1 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 3. The extrusion is accompanied by ring contraction of 1 to generate (SiMe2)(5), which also contributes to formation of 3 and (SiMe2)(4) bythePd(0)-catalyzed reaction with an alkyne. Extrusion of Me2Si=Pd(CNAd) and ring contraction generated more than five Me2Si: species from (SiMe2)(6) (1).