Ketene-Forming Eliminations from Aryl Phenylacetates Promoted by R<sub>2</sub>NH/R<sub>2</sub>NH<sub>2</sub><sup>+</sup> in Aqueous MeCN. Mechanistic Borderline between E2 and E1cb
作者:Bong Rae Cho、Hyun Cheol Jeong、Yoon Je Seung、Sang Yong Pyun
DOI:10.1021/jo025555m
日期:2002.7.1
Elimination reactions of 2-X-4-NO2C6H3CH2C(O)OC6H3-2-Y-4-NO2 [X = H (1), NO2 (2)] promoted by R2NH/R2NH2+ in 70 mol % MeCN(aq) have been studied kinetically. The base-promoted eliminations from 1 proceeded by the E2 mechanism when Y = Cl, CF3, and NO2. The mechanism changed to the competing E2 and E1cb mechanisms by the poorer leaving groups (Y = H, OMe) and to the E1cb extreme by the strongly electron-withdrawing
在70 mol%MeCN(aq)中由R2NH / R2NH2 +促进的2-X-4-NO2C6H3CH2C(O)OC6H3-2-Y-4-NO2 [X = H(1),NO2(2)]的消除反应已经完成动力学研究。当Y = Cl,CF3和NO2时,通过E2机制从1开始进行碱促消除。该机制由较差的离去基团(Y = H,OMe)变为竞争性的E2和E1cb机制,而由强吸电子的β-芳基(2,X = NO2)转变为E1cb极端。为消除1而计算出的beta = 0.14和beta(lg)= 0.10-0.21的值(Y = NO2)表示类似质子的过渡态,质子转移程度较小,Cα-OAr键断裂。离开的基团较差时,质子转移的程度增加,而离开的基团较弱时,离去基团键的裂解程度增加。也,k(1)和k(-1)/ k(2)值随反应物结构变化的变化与E1cb机制一致。根据这些结果,提出了从E2到E1cb极值机制转变的合理途径。