Trisilyl-Substituted Vinyl Cations

A series of beta,beta-disilyl-substituted vinyl cations were prepared by intramolecular addition of transient silylium ions to C C triple bonds. The vinyl cations were isolated from hydrocarbon solutions as tetrakis(pentafluorophenyl) borates at room temperature. The substituents directly attached to the positively charged carbon atom were varied from tert-butyl- to trialkyl-silyl- to trialkylgermyl groups. The cations were identified by their characteristic C-13 and Si-29 NMR data. The NMR investigations revealed for beta,beta-disilyl-alpha-germyl- and trisilyl-substituted vinyl cations a highly dynamic structure in which both vinylic carbon atoms undergo an intramolecular exchange process which is fast on the NMR time scale at room temperature. NMR studies using a doubly C-13-labeled vinyl cation suggest as mechanism for this exchange process a rotation of the dicarbyne C C2- unit within the triangle defined by the three cationic silyl fragments. Therefore the dynamic structure indicated for trisilyl- or disilylgermyl-substituted vinyl cations parallels the situation found for the parent vinyl cation, the protonated acetylene.