The X-ray structure of a vinyl cation.

Vinyl cations, the dicoordinated unsaturated analogues of trivalent carbenium ions, were first detected by Grob and coworkers in the early 1960s in solvolysis reactions of a-aryl vinyl halides. In the 1970s numerous investigations established vinyl cations as reaction intermediates in solvolysis reactions of activated alkenyl halides and in reactions of electrophiles with alkynes. The direct NMR detection of vinyl cations under superacidic reaction conditions at temperatures below 100 8C was achieved in the early 1990s and provided important information about their electronic structure. We report herein the X-ray structure of the b-silyl substituted vinyl cation 1, which provides the first direct experimental structural information about this important class of reaction intermediates. In addition, the molecular structure of cation 1 gives direct structural evidence for the occurrence of b-silyl hyperconjugation in carbocations. Vinyl cation 1 was prepared by reaction of the alkynylsilane 2 with trityl ion as described previously for related cations (see Scheme 1). The counterions were either tetra-