Cyclic Silylated Onium Ions of Group 15 Elements

Five- and six-membered
cyclic silylated onium ions of group 15 elements I were
synthesized by intramolecular cyclization of transient silylium ions II. Silylium ions II were prepared by the hydride
transfer reaction from silanes III using trityl cation
as hydride acceptor. It was found that smaller ring systems could
not be obtained by this approach. In these cases tritylphosphonium
ions IV were isolated instead. Cations I and IV were isolated in the form of their tetrakispentafluorphenyl
borates and characterized by multinuclear NMR spectroscopy and, in
two cases, by X-ray diffraction analysis. Cyclic onium ions I showed no reactivity similar to that of isoelectronic intramolecular
borane/phosphane frustrated Lewis pairs (FLPs). The results of DFT
computations at the M05-2X level suggest that the strength of the
newly formed Si–E linkage is the major reason for inertness
of I[B­(C6F5)4] versus
molecular hydrogen.