Murakami Masahiro, Hasegawa Munehiro, Igawa Hideyuki
Department of Synthetic Chemistry and Biological Chemistry, Kyoto University, Katsura, Kyoto 615-8510, Japan.
J Org Chem. 2004 Jan 23;69(2):587-90. doi: 10.1021/jo035433+.
The ring-opening reaction of 3-(trimethylstannyl)cyclobutene gave a mixture of the (Z)- and (E)-1,3-dienes, whereas that of 3-tert-butylcyclobutene exclusively afforded the (E)-1,3-diene due to the steric influences. The contrasting rotational behaviors suggest that there is some effect operating with the 3-stannylcyclobutene to stabilize the inward transition state, counteracting the steric influences. This contrasteric effect is ascribed to negative hyperconjugation. The stannyl group in the inward transition state accommodates electron density from the breaking sigma orbital of the cyclobutene into its low-lying tin-carbon sigma orbital. Theoretical studies supported this interpretation.
3-(三甲基锡基)环丁烯的开环反应生成了(Z)-和(E)-1,3-二烯的混合物，而3-叔丁基环丁烯的开环反应由于空间效应仅生成了(E)-1,3-二烯。这种截然不同的旋转行为表明，3-锡基环丁烯存在某种作用来稳定内向过渡态，抵消空间效应。这种反空间效应归因于负超共轭。内向过渡态中的锡基将环丁烯断裂的σ轨道中的电子密度容纳到其低能的锡-碳σ轨道中。理论研究支持了这一解释。
