Kageyama Yoshiyuki, Murata Shigeru
Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan.
J Org Chem. 2005 Apr 15;70(8):3140-7. doi: 10.1021/jo047737h.
In the presence of a catalytic amount of 3-butyl-4-methylthiazolium bromide, the reaction of benzaldehydes with azobenzene in dichloromethane containing octanethiol and Et(3)N gave the corresponding S-octyl thiobenzoates in good yields. The thiolesters were produced by trapping of the 2-benzoylthiazolium salts with the thiol, which were generated through the azobenzene oxidation of the active aldehydes. This is the first example for the thiolester formation mimicking the function of the pyruvate dehydrogenase complex. An electron-withdrawing substituent at the 4-position of benzaldehyde enhanced the reaction rate. The effect of benzaldehyde substituents on the reaction rate was examined quantitatively on the basis of kinetic measurements, leading to a nonlinear correlation of log(k(obs)) with Hammett's substituent constants (sigma). The origin of the nonlinear Hammett plot was interpreted in terms of a shift in the rate-determining step of the multistep reaction with change of the electronic nature of substituent. Further support for this assumption was given by the observation that the reaction constant (rho) of the Hammett plot for the azobenzene substituent effect on the oxidation rate of 4-bromobenzaldehyde was much smaller than that of 4-cyanobenzaldehyde.
在催化量的3-丁基-4-甲基噻唑溴化物存在下，苯甲醛与偶氮苯在含有正辛硫醇和三乙胺的二氯甲烷中反应，以良好的产率得到相应的S-辛基硫代苯甲酸酯。硫酯是通过硫醇捕获2-苯甲酰基噻唑盐生成的，而这些盐是由活性醛的偶氮苯氧化产生的。这是模拟丙酮酸脱氢酶复合体功能形成硫酯的首个实例。苯甲醛4-位上的吸电子取代基提高了反应速率。基于动力学测量定量研究了苯甲醛取代基对反应速率的影响，结果表明观测速率常数的对数（log(k(obs))）与哈米特取代基常数（sigma）呈非线性相关。非线性哈米特图的起源可通过多步反应速率决定步骤随取代基电子性质变化的移动来解释。4-溴苯甲醛氧化速率的偶氮苯取代基效应的哈米特图的反应常数（rho）远小于4-氰基苯甲醛的反应常数，这一观察结果进一步支持了该假设。
