Reactivity of a Polar Silene toward Terminal Alkynes: Preference for C-H Insertion over Cycloaddition

A variety of terminal alkynes were added to Mes(2)Si=CHCH(2)t-Bu, 4, a naturally polarized silene. Three different modes of reactivity were observed: addition across the acetylenic C-H bond to give silylacetylenes 6a-i and 12, cycloaddition to give silacyclobutenes 7 and 9, and ene-addition to give vinylsilane 8. The reactivity of the naturally polarized silene 4 toward terminal alkynes is compared to that of nonpolar silenes.