Eleven model 1-[2-(methoxycarbonyl)phenyl]-3-(2-substituted phenyl)triazenes were synthesized and their cyclization kinetics examined in aqueous-methanolic buffer solutions (51 wt.% methanol) at various pH values. 3(2-Substituted phenyl)benzo[d][1,2,3]triazin-4(3H)-ones were identified as the cyclization products. The log kobs vs pH plot was linear with a slope of unity. Investigation of the steric and electronic effects of substituents in the ortho position revealed that substituents at the ring which is bonded to the N(3) nitrogen affect the cyclization rate through their steric effect only, while their electronic effects are statistically insignificant. This fact was explained in terms of the ring being tilted from the plane in which the remaining part of the conjugate base anion of the model substrate lies. The assumed and confirmed BAc2 mechanism involving specific base catalysis begins by deprotonation of the triazene giving rise to the conjugate base, continues with formation of a tetrahedral intermediate, and ends with elimination of the methanolate ion. Other mechanisms, such as the elimination-addition mechanism via a ketene intermediate or the mechanism involving general base catalysis, are unlikely.
合成了11种模型1-[2-(甲氧羰基)苯基]-3-(2-取代苯基)
三氮唑,并在不同pH值的
水甲醇缓冲溶液中研究了它们的环化动力学。3(2-取代苯基)苯并[
d][1,2,3]
三氮唑-4(3
H)-酮被确定为环化产物。log
kobs与pH的图像呈线性关系,斜率为1。研究取代基在
ortho位置的立体和电子效应表明,与N(3)氮键合的环上的取代基仅通过其立体效应影响环化速率,而其电子效应在统计学上是不显著的。这个事实可以用环从模型底物的剩余共轭碱基阴离子所在的平面倾斜来解释。假定和确认的涉及特定碱催化的B
Ac2机理从去质子化
三氮唑开始,形成四面体中间体,最终消除
甲醇酸根离子。其他机理,如通过酮中间体的消除-加成机理或涉及一般碱催化的机理,则不太可能。