Structural Effects in Solvolytic Reactions. 45. Carbon-13 NMR Studies of Carbocations. 9. Variation of the Cationic Carbon Chemical Shifts with Increasing Electron Demand in 1,1-Diaryl-1-ethyl Carbocations. Importance of the Inductive Localized 7r-Polarization Effect in Causing Deviations from Linearity

Abstract

A series of 1-aryl-1-[3’,4,-(ethyleneoxy)phenyl]-1-ethyl (or 1-aryl-1-(5′-coumaranyl)ethyl) (9), l-aryl-l-(4′-methoxyphenyl)-1-ethyl (10), 1-aryl-1-(4′-methylphenyl)-1-ethyl (11), 1-aryl-1-[4′-(trifluoromethyl)phenyl]-l-ethyl (12), and 1-aryl-1-[3’,5′-bis(trifluoromethyl)phenyl]-1-ethyl (13) carbocations were prepared in either SbF5/FSO3H/SO2ClF or FSO3H/SO2ClF solutions at -78 °C and their 13C NMR spectra recorded at -70 °C. The plots of the substituent chemical shifts ΔδC+ against ∼c are linear for the electron-donating substituents Z = 3,4-CH2CH2O to Z = H of 13, 12, 11, and the previously reported 1-aryl-1-ethyl cations (1), with deshielding of the cationic carbon with increasing electron demand. However, from Z = H to Z = 3,5-(CF3)2 the cationic carbons are shielded in comparison with those predicted by the correlation line for the electron donors. The plots for the highly resonance stabilized cations 9 and 10 are in the form of shallow curves with minima at Z = m-CH3 after which increasing electron demand results in shielding of the cationic carbon. This behavior is similar to that observed for the a carbon atom of substituted acetophenones and styrenes. The similarity indicates that the anomalous 13C shifts that give rise to the deviations in the plots are due to substantial double-bond character between the cationic carbon and the ipso carbon (C1') of the phenyl group bearing the fixed substituent. Inductive 7r-polarization of this double bond by the Z substituent results in shielding of the cationic (C1) carbon atom. Thus the cationic carbon chemical shifts of 1,1-diaryl-1-ethyl carbocations are determined by two competing effects, the normal resonance effect of Z (deshielding with increasing electron demand) and an inductive π-polarization effect of Z (shielding with increasing electron demand). © 1983, American Chemical Society. All rights reserved.