Structural effects on valence tautomerism : VII. An NMR and molecular mechanics study of the steric effects of t-butyl substituents on the cycloheptatriene-norcaradiene equilibrium of 7-cyano-7-m |
| |
Authors: | Ken ichi Takeuchi,Toshikazu Kitagawa,Akihiko Ueda,Yoshihide Senzaki,Kunio Okamoto |
| |
Affiliation: | Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606, Japan |
| |
Abstract: | 7-Cyano-7-methylcycloheptatrienes containing one t-butyl group on the 1-position () or two t-butyl groups on the 1- and 3-, 1- and 4-, or 1- and 5-positions (, , or , respectively) were synthesized and their cyclo- heptatriene (CHT)-norcaradiene (NCD) equilibria measured by variable-temperature 1H NMR for CS2-CD2Cl2 solutions. The 1H NMR chemical shifts of the 7-methyl group indicate that these compounds are composed of essentially one CHT and one NCD tautomer with an endo geometry of the methyl group. The introduction of a t-butyl group at the 1-position of 7-cyano-7-methylcycloheptatriene () markedly shifts the equilibrium to the NCD side and the addition of the second t-butyl group further favors the NCD form, with the NCD populations for , , , and at 25 °C 70.9, 96.5, 92.3, and 99.3%, respectively. An application of molecular mechanics (MMPI) calculations to various t-butylated CHT-NCD systems suggests that the t-butyl groups sterically destabilize the CHT form more than the NCD form, bringing about increased NCD populations. |
| |
Keywords: | |
本文献已被 ScienceDirect 等数据库收录! |
|