The stereochemistry of the π,R(3s) excited state ring opening of a series of bicyclic alkylcyclobutenes has been studied in hydrocarbon solution with 228 nm excitation. In these compounds, the C=C bond is shared between the cyclobutene ring and a five-, six-, or seven-membered ancillary ring, which has the effect of restricting the torsional mobility about the central CC bond in the isomeric diene products. It has previously been shown that monocyclic alkylcyclobutenes undergo stereospecific conrotatory ring opening upon excitation at the long wavelength edge of the π,R(3s) absorption band (228 nm), and nonstereospecific ring opening upon irradiation at shorter wavelengths (within the π,π* absorption band). Different behaviour is observed for the bicyclic systems studied in the present work. The bicyclo[3.2.0]hept-1-ene, bicyclo[4.2.0]oct-1-ene, and one of the bicyclo[5.2.0]non-1-ene derivatives yield nearly the same mixtures of E,E- and E,Z-diene isomers upon irradiation at 214 and 228 nm, with the product mixtures being heavily weighted in favor of the isomer(s) corresponding to disrotatory ring opening. The results may indicate that the stereochemical characteristics of the Rydberg-derived ring opening of alkylcyclobutenes depends on the ability of the molecule to twist about the "central" bond (i.e., the C=C bond in the cyclobutene) as ring opening proceeds. It is proposed that restricting the torsional mobility about the central bond activates internal conversion from the π,R(3s) to the π,π* potential energy surface, from which predominant disrotatory ring opening ensues.Key words: cyclobutene, Rydberg, ring opening, photopericyclic, electrocyclic.
一系列双环烷基环丁烯的π,R(3s)激发态环开启的立体化学已在烃溶液中通过228纳米激发进行了研究。在这些化合物中,C=C键在环丁烯环和一个五、六或七元辅助环之间共享,这限制了异构双烯产物中心C-C键周围的扭转活性。先前已经表明,单环烷基环丁烯在π,R(3s)吸收带(228纳米)的长波长边激发时发生立体特异的同向环开启,而在较短波长(在π,π*吸收带内)照射时发生非立体特异的环开启。在本研究中研究的双环系统表现出不同的行为。双环[3.2.0]庚-1-烯,双环[4.2.0]辛-1-烯和双环[5.2.0]壬-1-烯衍生物之一在214和228纳米照射时产生几乎相同的E,E-和E,Z-双烯异构体混合物,产物混合物在向有利于相反方向环开启的异构体方向严重倾斜。结果可能表明,烷基环丁烯的Rydberg导出的环开启的立体化学特性取决于分子围绕“中心”键(即环丁烯中的C=C键)扭转的能力,随着环开启的进行,扭转活性受到限制。提出限制中心键周围的扭转活性会激活从π,R(3s)到π,π*势能表面的内部转化,从而导致主要的相反方向环开启。关键词:环丁烯,Rydberg,环开启,光周环,电环。