ALPHA-IMINO AND ALPHA-OXIMINO CARBOCATIONS - A COMPARISON WITH ALPHA-CARBONYL AND ALPHA-THIOCARBONYL CARBOCATIONS

The sigma(+) values for the groups m-CH=N-t-Bu and p-CH=N-t-Bu have been determined to be 0.16 and 0.078, respectively The CH=N-t-Bu group is therefore cation-destabilizing relative to hydrogen when placed in the meta position of a cumyl cation. Although the p-CH=N-t-Bu group is still cation-destabilizing, the effect is reduced by a competing resonance effect when the imino group is placed in the para position. However, when attached directly to a cationic center, this imino group enhances rates relative to alpha-H. This is interpreted in terms of a cation-stabilizing conjugative effect. The of value of the p-oximino group, p-CH=NOCH3, is -0.03 and indicates that a cation-stabilizing conjugative effect can essentially offset the inductive effect of this electron-withdrawing substituent in a cumyl cation, However, when attached directly to a developing cationic center, this oximino group greatly enhances cation formation rates relative to hydrogen. This group is even more cation-stabilizing than the methyl group. Studies on systems of type ArCH(OMs)C(NOCH3)Ph, where the oximino group can be syn or anti to the developing cationic center, indicate the existence of isomeric alpha-oximino cations, with the anti cations forming faster than the syn cations, Computational studies at the MP2/6-31G** level support the idea of extensive conjugative stabilization of alpha-oximino cations. Despite extensive mesomeric stabilization of alpha-oximimo cations, the primary cation (CH2CH)-C-+=NOCH3 cannot be solvolytically generated since nucleophilic solvent displacement processes dominate, Stabilities of a series of cations of type (+)CH(2)X, where X is a formal electron-withdrawing group, have been evaluated by ab initio methods. Isodesmic reactions indicate a stability order of alpha-CHNOCH3 similar to CH=CH2 > CHNHCH3 > CHS > CHO similar to H > CN > NO2.