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

Citations Over TimeTop 10% of 2005 papers

Abstract

The synthesis and isolation of 12 alpha-aryl, beta, beta'-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 (delta13C(C(+)) = 178.1-194.5; delta13C (Cbeta) = 83.3-89.9; delta13C (Cipso)) = 113.6-115.2; delta (29)Si = 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 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 (29)Si NMR spectroscopy to be DeltaG++ = (48.9 +/- 4.2) kJ mol(-1). A Hammett-type analysis, which relates the (1)J(SiC(beta)) coupling constant and the low-field shift of the (29)Si NMR signal upon ionization, Deltadelta (29)Si, 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-l, 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.

Related Papers

• → An analysis of substituent effects on 1H and 13C NMR parameters of substituted furans. Linear free energy relationships and PM3 semiempirical calculations(1998)19 cited
• → Linear free energy relationships of 13C NMR chemical shifts in 4-substituted phenyl-4,5-dihydrobenzo[f][1,4]oxazepin-3(2H)-ones(thiones)(2013)5 cited
• → Linear free energy relationships applied to the reactivity and the 13C NMR chemical shifts in 4-[[(substituted phenyl)imino]methyl]benzoic acids(2010)6 cited
• → The molecular structure of allenes and ketenes. Part 16. Phenyl carbon chemical shifts of γ-substituted phenylallenes as a probe for the transmissions of substituent effects across the allenic system(1983)6 cited
• → ChemInform Abstract: An Analysis of Substituent Effects on 1H and 13C NMR Parameters of Substituted Furans. Linear Free Energy Relationships and PM3 Semiempirical Calculations.(1998)