Adam Waldemar, Stegmann Veit R
Institut für Organische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany.
J Am Chem Soc. 2002 Apr 10;124(14):3600-7. doi: 10.1021/ja017017h.
Toluene-d(8) solutions of cis- and trans-cyclooctene (cis- and trans-1a) as well as (Z)- and (E)-1-methylcyclooctene (cis- and trans-1b) have been irradiated at temperatures between -95 and +110 degrees C in the presence of benzophenone (BP) to afford mixtures of the cis- and trans-configured oxetanes 2a,b and the regioisomeric 2b'. Correspondingly, benzoquinone (BQ) gave with cis- and trans-1a the cycloadducts cis- and trans-3a. The cis/trans diastereomeric ratios of the [2 + 2]-cycloadducts 2 and 3 display a strong temperature dependence; with cis- and trans-1a or cis-1b as starting materials, the diastereoselectivity of the oxetane formation is high at low temperature, under preservation of the initial cyclooctene configuration. With increasing temperature, the cis diastereoselectivity decreases continuously for the cis-cyclooctenes; in the case of the cis-1a, the diastereoselectivity is even switched to trans (cis/trans ca. 20:80) at very high temperatures. For the strained trans-1a, the trans-oxetanes are strongly preferred over the entire temperature range, with only minor leakage (up to 10%) to the cis-oxetanes at very high temperatures. Oxetane formation is accompanied by nonthermal trans-to-cis isomerization of the cyclooctene. The methyl-substituted trans-1b constitutes an exceptional substrate; it displays cis diastereoselectivity in the [2 + 2] photocycloaddition at low temperatures for both regioisomers 2b and 2b', and the trans selectivity increases at moderate temperature (cis/trans = 4:96), to decrease again at high temperature, especially for the minor regioisomer 2b'. This complex temperature behavior of the cis/trans diastereoselectivity may be rationalized in terms of the triplet-diradical mechanism of the Paternò-Büchi reaction. We propose that the cyclooctene may be competitively attacked by the triplet-excited ketone from the higher (syn) or the less (anti) substituted side; such syn and anti trajectories have hitherto not been considered. To account for the unusual temperature behavior in the diastereoselectivity of the present [2 + 2] photocycloaddition, we suggest that temperature-dependent conformational changes of the resulting triplet preoxetane diradicals compete with their cyclization to the cis/trans-oxetane diastereomers and retro cleavage to the cis-cyclooctene.
顺式和反式环辛烯（顺式和反式 - 1a）以及（Z） - 和（E） - 1 - 甲基环辛烯（顺式和反式 - 1b）的甲苯 - d(8)溶液在 - 95至 + 110℃的温度下，于二苯甲酮（BP）存在下进行辐照，得到顺式和反式构型的氧杂环丁烷2a、b以及区域异构体2b'的混合物。相应地，苯醌（BQ）与顺式和反式 - 1a反应生成环加成产物顺式和反式 - 3a。[2 + 2]环加成产物2和3的顺式/反式非对映体比例表现出强烈的温度依赖性；以顺式和反式 - 1a或顺式 - 1b为起始原料，在低温下氧杂环丁烷形成的非对映选择性较高，同时保持初始环辛烯构型。随着温度升高，顺式环辛烯的顺式非对映选择性持续降低；对于顺式 - 1a，在非常高的温度下，非对映选择性甚至转变为反式（顺式/反式约为20:80）。对于具有张力的反式 - 1a，在整个温度范围内反式氧杂环丁烷占绝对优势，在非常高的温度下仅有少量（高达10%）转化为顺式氧杂环丁烷。氧杂环丁烷的形成伴随着环辛烯的非热反式至顺式异构化。甲基取代的反式 - 1b是一个特殊的底物；在低温下，对于两种区域异构体2b和2b'，它在[2 + 2]光环加成反应中表现出顺式非对映选择性，并且在中等温度下反式选择性增加（顺式/反式 = 4:96），在高温下再次降低，特别是对于次要的区域异构体2b'。这种顺式/反式非对映选择性的复杂温度行为可以根据帕特诺 - 布齐反应的三线态 - 双自由基机理来解释。我们提出环辛烯可能受到三线态激发酮从取代程度较高（同侧）或较低（异侧）的一侧进行竞争性攻击；迄今为止尚未考虑过这种同侧和异侧的反应路径。为了解释当前[2 + 2]光环加成反应中非对映选择性的异常温度行为，我们认为所得三线态前氧杂环丁烷双自由基的温度依赖性构象变化与其环化生成顺式/反式氧杂环丁烷非对映异构体以及逆向裂解生成顺式环辛烯相互竞争。
