σ-delocalization versus π-resonance in α-aryl-substituted vinyl cations

The synthesis and isolation of 12 alpha-aryl, beta, beta'-disilyl-substituted vinyl cations 1b-I, 7, and 8 with the tetrakis(pentafluorophenyl)borate counteranion is reported. The vinyl cations are characterized by NMR spectroscopy and are identified by their specific NMR chemical shifts (delta C-13(C+) = 178.1-194.5; delta C-13(C-beta) = 83.3-89.9; delta C-13(C-ipso) = 113.6-115.2; delta Si-29 = 25.0-12.0), supported by density functional calculations at the B3LYP/6-311G(2d,p)//B3LYP/6-31G(d) level. All cations are found to be stable at room temperature in solution and in the solid state. The NMR chemical shifts as well as J-coupling data indicate for vinyl cations, 1b-I, 7, and 8, the occurrence of substantial stabilization through pi-resonance via the aryl substituents and through sigma-delocalization via the beta-silyl groups. For vinyl cation 8, the free enthalpy of stabilization via pi-resonance by the alpha-ferrocenyl substituent is determined by temperature-dependent Si-29 NMR spectroscopy to be Delta G(double dagger) = (48.9 +/- 4.2) kJ mol(-1). A Hammett-type analysis, which relates the (1)J(SiCbeta) coupling constant and the low-field shift of the Si-29 NMR signal upon ionization, Delta delta Si-29, with the electron-donating ability of the aryl group, indicates an inverse relation between the extent of Si-C hyperconjugation and pi-donation. The computed structures (at B3LYP/6-31G(d)) of the vinyl cations 1a-I, 7, and 8 reveal the consequences of Si-C hyperconjugation and of pi-resonance interactions with the aryl groups. The structures, however, fail to express the interplay between sigma-delocalization and pi-conjugation in that the calculated Si-C bond lengths and the C+-C-ipso bond lengths do not vary as a function of the substituent.