Cycloalkylmethyl radicals. Part 4. Electron spin resonance study of conformational equilibria in cyclohexenylmethyl and 4-alkylcyclohexenylmethyl radicals
作者:John C. Walton
DOI:10.1039/p29860001641
日期:——
half-chair conformation of cyclohex-2-enylmethyl radical were determined by line-shape analysis of the exchange-broadened spectra and found to be: log(kf/s–1)= 12.3 –(5.7 kcal mol–1)/2.3RT and log (kb/s–1)= 12.0 –(5.5 kcal mol–1)/2.3RT. The barrier to rotation about the Ċα–Cβ bond in a cyclohexenylmethyl radical is much less than the barrier in a cyclohexylmethyl radical because the former radical has only
对于环己-2-烯基甲基和4-烷基环己-2-烯基甲基,可以通过esr光谱法观察到准轴向和准赤道构象异构体。类似地,环己-3-烯基甲基自由基的轴向和赤道构象异构体可以通过电离能谱法进行区分。该CH的构象的自由能差2在2-位,-Δ˙组ģ ° 300,被发现是0.17±0.03千卡摩尔-1和在3-位-Δ ģ ° 300 = 0.0±0.1大卡摩尔–1。通过交换增宽光谱的线形分析确定了环己-2-烯基甲基自由基半椅构象的阿伦尼乌斯参数,发现为:log(k f / s –1)= 12.3 –(5.7 kcal mol –1)/2.3 RT和log(k b / s –1)= 12.0 –(5.5 kcal mol –1)/2.3 RT。所述屏障以关于C旋转α -C β在游离基的环己烯基键比环己阻挡少得多的自由基,因为前者自由基只有一个顺上C(5)-axial氢阻碍旋转而后者自由基具有两个顺式对C(3)和C(5)-axial氢。