Activation of pentafluorophenylsilanes by weak Lewis bases in reaction with iminium cations

The Lewis base mediated carbon-carbon bond forming reactions between pentafluorophenylsilanes and iminium cations were studied theoretically and experimentally. The complexation of silanes with anionic Lewis bases was analyzed computationally using DFT methods at the B3LYP/6-31+G(d) level. The pentafluorophenyl group was found to exhibit a significant stabilizing effect on the formation of pentacoordinate silicon species, with (C6F5)3SiF and C6F5SiF3 being the strongest Lewis acids. Comparison of geometrical isomers of trigonal bipyramidal siliconates R2(C6F5)SiXY- (R = Me, F, Cl; X, Y = F, Cl, ClO4) revealed that the heteroatoms and the C6F5 group prefer to occupy apical and equatorial positions, respectively. Calculations of C6F5 group transfer processes from silanes to N,N-dimethyliminium cation lead to the following conclusions: (1) when employing weak Lewis bases X- = Cl-, ClO4- as activators, R2(C6F5)SiF are more reactive than R2(C6F5)SiX (X = Cl, ClO4); (2) the C6F5 group is much more reactive in an apical position than in an equatorial position of the siliconate; (3) a solvent greatly increases the reaction barrier by stabilizing the reactants; and (4) the efficiency of C-C bond formation can be improved by rendering the process intramolecular. It was shown experimentally that reactions of (C6F5)3SiF with PhCH=NMe2+TfO- can be promoted by a series of weak Lewis bases, from which the chloride anion was the most effective. Even more facile was the reaction of N-(silyloxyethyl)iminium ion (C6F5)3SiOCH2CH2N(Me)=CHPh+TfO-, likely proceeding via intramolecular transfer of a C6F5 group. The optimal conditions for pentafluorophenylation of iminium ions involved the use of benzyltriethylammonium chloride in refluxing dichloroethane.