Electronic properties of the nitrone substituent. Stabilization of benzylic carbocations

The nitrone substituent CH=N(O) t-Bu is electron-withdrawing with a Taft si gma (1) value of 0.20. It also retards the solvolysis rate of a cumyl chlor ide when placed in the meta-position (sigma (+) = 0.20). However, CH=N(O) t -Bu becomes weakly cation stabilizing when placed in the para-position of a cumyl cation (sigma (+) = -0.04). This weak cation stabilization is a resu lt of a conjugative interaction which delocalizes charge and offsets the in ductive effect of the nitrone. When the nitrone is placed in the para-posit ion, but then twisted out of conjugation with the aromatic ring by incorpor ation of flanking 3,5-dimethyl groups, it again retards solvolysis rates. C omputational studies (B3LYP/6-31G*) show that the nitrone substituent stabi lizes a para-substituted benzyl cation relative to the meta-substituted ana log by a conjugative interaction. However, the calculated stabilization gre atly overestimates the cation stabilization seen in solvolytic reactions. C opyright (C) 2001 John Wiley & Sons, Ltd.