SUBSTITUENT EFFECTS ON THE GAS-PHASE BASICITIES OF 1-ARYLPROPYNES AND1-ARYL-3,3-DIMETHYLBUTYNES - EFFECTS OF THE BETA-ALKYL GROUPS ON THE RESONANCE DEMAND OF VINYL CATIONS

The relative stabilities of 1-aryl-2-methylvinyl cations (I) and 1-ary l-2-t-butylvinyl cations (II) were determined by measuring the proton- transfer equilibria of 1-arylpropynes and 1-aryl-3,3-dimethylbutynes i n 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 substit uent effects on the stability of these vinyl cations could be correlat ed 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 prev ious results, it was found that the r(+) value decreases with the incr ease in stability of the unsubstituted member of the respective vinyli c carbocations while the rho values remain constant for a series of th e phenylvinyl cation system. This indicates that the pi-delocalization of the positive charge into the aryl pi-system decreases with the sta bilization of a vinyl cation by the beta-substituent. This trend is co nsistent with the observation for a series of ordinary sp(2)-hybridize d benzylic carbocations. In addition, it has been shown that both sp- and sp(2)-carbocation systems obey a single linear relationship betwee n the r(+) values and carbocation stabilities of the unsubstituted mem ber of respective series. This result clearly demonstrates a continuou s spectrum of varying resonance demands characteristic of the stabilit ies of carbocations. It was concluded that the a-delocalization mechan ism in the vinyl cation system has no unique feature but it is a part of a continuous spectrum of resonance demands of varying benzylic carb ocations. (C) 1998 Published by Elsevier Science B.V. All rights reser ved.