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

Abstract

Palladium-catalyzed reactions of dodecamethylcyclohexasilane [(SiMe2)6] (1) with alkynes led to efficient preparation of 1,1,4,4-tetramethyl-1,4-disilacyclohexadienes (3). 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 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 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 (1).

1,2-Palladasilacyclobutene: The Missing Link in the Pd-Catalyzed Annulation of Alkynes for the Silirene-to-Silole Transformation

“…1,2-metallasilacyclobutenes are organometallic complexes with an unsaturated four-membered cyclic structure where a transition metal center is stabilized by two sigma bonds of different nature, σ-C–M and σ-Si–M, respectively. Although often invoked, ,, these complexes are rare and limited in number. A first description was reported by Ishikawa in the case of nickelasilacyclobutene C generated from a silirene and [(PEt 3 ) 4 Ni­(0)] (Figure ).…”

Advances in disilylation reactions to access cis/trans-1,2-disilylated and gem-disilylated alkenes

“…151,152 In 2018, the Nagashima group investigated the palladium-catalyzed reaction of Si 6 Me 12 with alkynes using 1-isocyanoadamantane (AdNC) as the ligand (Scheme 17). 153 They successfully synthesized 1,4disila-2,5-cyclohexadiene 24 from the reaction of the unreactive Si 6 Me 12 with an alkyne.…”

Iron(II)-Catalyzed Activation of Si–N and Si–O Bonds Using Hydroboranes

“…There is a growing focus on developing catalytic approaches to activate Si–heteroatom bonds. While the catalytic activation of Si–Si, , Si–B, − and Si–P has been explored, to the best of our knowledge, there are no examples of catalytic Si–N bond activation, not least by an iron precatalyst. We have previously demonstrated the use of iron β-diketiminate complexes in efficient heterodehydrocoupling reactions, − and during these studies, our mechanistic investigations showed that the Si–N bond forming step was fast and reversible (Scheme e).…”