161869-72-3Relevant articles and documents
α-Imino and α-oximino carbocations. A comparison with α-carbonyl and α-thiocarbonyl carbocations
Creary, Xavier,Wang, You-Xiong,Jiang, Ziqi
, p. 3044 - 3053 (2007/10/02)
The σ+ 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 α-H. This is interpreted in terms of a cation-stabilizing conjugative effect. The σ+ 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 α-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 α-oximino cations. Despite extensive mesomeric stabilization of α-oximimo cations, the primary cation +CH2CH=NOCH3 cannot be solvolytically generated since nucleophilic solvent displacement processes dominate. Stabilities of a series of cations of type +CH2X, where X is a formal electron-withdrawing group, have been evaluated by ab initia methods. Isodesmic reactions indicate a stability order of α-CHNOCH3 ～ CH=CH2 > CHNHCH3 > CHS > CHO ～ H > CN > NO2.
