Substituent-Dependent Negative Hyperconjugation in 2-Aryl-1,3-<i>N,N-</i>heterocycles. Fine-Tuned Anomeric Effect?
作者:Anasztázia Hetényi、Tamás A. Martinek、László Lázár、Zita Zalán、Ferenc Fülöp
DOI:10.1021/jo034417+
日期:2003.7.1
different polarization of the acceptor sigma orbitals, caused by the 2-aryl substituent, accounted for the observed propagation of the substituent effect. These results promote a detailed explanation of the useful tautomeric behavior of the 2-aryl-1,3-X,N-heterocycles, and reveal the nature of the connection between the anomeric effect and the Hammett-type linear free energy relationship.
构象上不灵活的2-芳基-1,3-N,N-杂环的差向异构反应被用作模型系统,以研究氮孤对-C2相关的抗键轨道超共轭相互作用在实验观察到的依赖于取代基的广义异头作用中的作用。发现从头算NBO分析得出,测得的无反应焓与C2周围所有邻位供体-受体轨道重叠的超共轭稳定能的总和良好相关,并且这两个量与Hammett-Brown取代基常数线性相关。各个立体电子相互作用(n(N)-sigma(C2)(-)(N),n(N)-sigma(C2)(-)(Ar),n(N)-sigma(C2)(-)( H))还观察到表现出取代基依赖性,尽管它们与2-芳基取代基的距离和它们的非平面排列。2-芳基取代基的吸电子效应越高,对n(N)-sigma(C2)(-)(Ar)的稳定性就越大,而对n(N)-sigma(C2)(- )(N)和n(N)-sigma(C2)(-)(H)的变化方向相反。由2-芳基取代基引起的受体σ轨道的不同