Substituent effects on the gas-phase basicities of 1-arylpropynes and 1-aryl-3,3-dimethylbutynes -: Effects of the β-alkyl groups on the resonance demand of vinyl cations

The relative stabilities of 1-aryl-2-methylvinyl cations (I) and 1-aryl-2-t-butylvinyl cations (II) were determined by measuring the proton-transfer equilibria of 1-arylpropynes and 1-aryl-3,3-dimethylbutynes in the gas phase. The stabilities of 1-phenyl-2-methylvinyl cation and 1-phenyl-2-t-butylvinyl cation were found to be 1.8 and 5.5 kcal mol(-1) higher than that of 1-phenylvinyl cation, respectively. The substituent effects on the stability of these vinyl cations could be correlated in terms of the Yukawa-Tsuno equation, giving an r(+) of 1.13 and a rho of - 9.5 for I, and 1.03 and - 9.4 for II. Combined with the previous results, it was found that the r(+) value decreases with the increase in stability of the unsubstituted member of the respective vinylic carbocations while the rho values remain constant for a series of the phenylvinyl cation system. This indicates that the pi-delocalization of the positive charge into the aryl pi-system decreases with the stabilization of a vinyl cation by the beta-substituent. This trend is consistent with the observation for a series of ordinary sp(2)-hybridized benzylic carbocations. In addition, it has been shown that both sp- and sp(2)-carbocation systems obey a single linear relationship between the r(+) values and carbocation stabilities of the unsubstituted member of respective series. This result clearly demonstrates a continuous spectrum of varying resonance demands characteristic of the stabilities of carbocations. It was concluded that the a-delocalization mechanism in the vinyl cation system has no unique feature but it is a part of a continuous spectrum of resonance demands of varying benzylic carbocations. (C) 1998 Published by Elsevier Science B.V. All rights reserved.