STERICALLY HINDERED AZOBENZENES - ISOLATION OF CIS ISOMERS AND KINETICS OF THERMAL CIS-] TRANS ISOMERIZATION
STERICALLY HINDERED AZOBENZENES - ISOLATION OF CIS ISOMERS AND KINETICS OF THERMAL CIS-] TRANS ISOMERIZATION
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DOI：
10.1021/jo00379a015
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发表时间：
1987-02-06
影响因子：
3.6
通讯作者：
STACHNYK, GJ
中科院分区：
文献类型：
--
作者：
BUNCE, NJ;FERGUSON, G;STACHNYK, GJ
首次分离出一系列顺式构型的高位阻邻位烷基化偶氮苯。其中一种化合物顺式-2，2，6，6-四异丙基偶氮苯的晶体结构表明，该分子通过采用歪斜构象和打开N= N桥的角度（C-N= N 126.4（2））来容纳大体积取代基。这一结果证实了基于光谱证据和量子计算对这些化合物的几何结构所做的预测。热顺1反异构化反应的活化参数表明，顺式异构体的存在与不存在之间存在着明显的界限。如果顺式异构体是完全可分离的，它的异构化活化能为ca。90 kj mol-1。没有发现顺式异构体只有边缘稳定性的情况。我们最近报道了一系列在部分或全部邻位被烷基取代的偶氮苯的光谱和量子力学研究。1与本研究相关的是顺式异构体1至7。这些顺式化合物的电子吸收光谱似乎是令人惊讶的不敏感的邻位烷基的大部分。计算表明，光谱可以最好地解释由假设的几何形状
A series of highly hindered ortho-alkylated azobenzenes has been isolated in the cis configuration for the first time. The crystal structure of one of these compounds, cis-2, 2,, 6, 6,-tetraisopropylazobenzene, shows that the molecule accommodates the bulky substituents by adopting a skewed conformation and by opening up the angles of the N= N bridge (C—N= N 126.4 (2)). This result confirms predictions that were made about the geometries of these compounds based on spectroscopic evidence and quantum calculations. The activation parameters for thermal cis—1-trans isomerization in this series show that there is a remarkably sharp demarcationbetween the existence and nonexistence of cis isomers. Provided that the cis isomer is isolable atall, it isomerizes with an activation energy of ca. 90 kj mol'1. No cases were found where the cis isomer is of only marginal stability.We have recently reported spectroscopic and quantum mechanical studies on a series of azobenzenes substituted by alkyl groups in some or all of the ortho positions. 1 Of relevance to the present study were the cis isomers 1 to 7. These cis compounds had electronic absorption spectra that appeared to be surprisingly insensitiveto the bulk of the ortho alkyl groups. Calculations indicated that the spectra could best be explained by postulating a geometry