Reaction of an N-heterocyclic carbene-stabilized silicon(II) monohydride with alkynes: [2+2+1] cycloaddition versus hydrogen abstraction.

An in depth study of the reactivity of an N-heterocyclic carbene (NHC)-stabilized silylene monohydride with alkynes is reported. The reaction of silylene monohydride 1, tBu3 Si(H)Si←NHC, with diphenylacetylene afforded silole 2, tBu3 Si(H)Si(C4 Ph4 ). The density functional theory (DFT) calculations for the reaction mechanism of the [2+2+1] cycloaddition revealed that the NHC played a major part stabilizing zwitterionic transition states and intermediates to assist the cyclization pathway. A significantly different outcome was observed, when silylene monohydride 1 was treated with phenylacetylene, which gave rise to supersilyl substituted 1-alkenyl-1-alkynylsilane 3, tBu3 Si(H)Si(CHCHPh)(CCPh). Mechanistic investigations using an isotope labelling technique and DFT calculations suggest that this reaction occurs through a similar zwitterionic intermediate and subsequent hydrogen abstraction from a second molecule of phenylacetylene.