σ-Delocalization versus π-Resonance in α-Aryl-Substituted Vinyl Cations

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Abstract
The synthesis and isolation of 12 α-aryl, β, β‘-disilyl-substituted vinyl cations 1b−l, 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 (δ 13C(C+) = 178.1−194.5; δ 13C(Cβ) = 83.3−89.9; δ 13C(Cipso) = 113.6−115.2; δ 29Si = 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−l, 7, and 8, the occurrence of substantial stabilization through π-resonance via the aryl substituents and through σ-delocalization via the β-silyl groups. For vinyl cation 8, the free enthalpy of stabilization via π-resonance by the α-ferrocenyl substituent is determined by temperature-dependent 29Si NMR spectroscopy to be ΔG⧧ = (48.9 ± 4.2) kJ mol-1. A Hammett-type analysis, which relates the 1J(SiCβ) coupling constant and the low-field shift of the 29Si NMR signal upon ionization, Δδ 29Si, with the electron-donating ability of the aryl group, indicates an inverse relation between the extent of Si−C hyperconjugation and π-donation. The computed structures (at B3LYP/6-31G(d)) of the vinyl cations 1a−l, 7, and 8 reveal the consequences of Si−C hyperconjugation and of π-resonance interactions with the aryl groups. The structures, however, fail to express the interplay between σ-delocalization and π-conjugation in that the calculated Si−C bond lengths and the C+-Cipso bond lengths do not vary as a function of the substituent.

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