Baldwin John E, Leber Phyllis A, Powers David C
Department of Chemistry, Syracuse University, Syracuse, New York 13244, USA.
J Am Chem Soc. 2006 Aug 9;128(31):10020-1. doi: 10.1021/ja0636035.
The gas phase thermal reactions exhibited by bicyclo[4.2.0]oct-2-ene and 7-d and 8-d analogues at 300 degrees C have been followed kinetically through GC and 2H NMR spectroscopic analyses. In contrast to the pattern of transformations exhibited by bicyclo[3.2.0]hept-2-ene and deuterium-labeled analogues, no reactions initiated by C1-C6 bond cleavage are seen, epimerization at C8 is much faster than [1,3] shifts leading to bicyclo[2.2.2]oct-2-ene, and the ratio of rate constants for [1,3] carbon migration with inversion versus migration with retention is approximately 1.4. Homolysis of C1-C8 to give a conformationally flexible diradical intermediate having a relatively long lifetime and multiple options for further reaction (re-formation of C1-C8 with or without net epimerization, fragmentation to 1,3-cyclohexadiene and ethylene, migration to the original C3 with inversion or retention) accords well with the observations. Clearly, orbital symmetry control does not govern stereochemistry for the [1,3] sigmatropic carbon shifts.
双环[4.2.0]辛-2-烯及其7-d和8-d类似物在300℃下呈现的气相热反应，已通过气相色谱（GC）和2H核磁共振（NMR）光谱分析对其动力学进行了跟踪研究。与双环[3.2.0]庚-2-烯及其氘标记类似物所呈现的转化模式不同，未观察到由C1-C6键断裂引发的反应，C8处的差向异构化比导致双环[2.2.2]辛-2-烯的[1,3]迁移要快得多，并且[1,3]碳迁移时构型翻转与构型保持的速率常数之比约为1.4。C1-C8的均裂生成了一个构象灵活的双自由基中间体，该中间体具有相对较长的寿命和多种进一步反应的选择（C1-C8的重新形成，有或没有净差向异构化，裂解为1,3-环己二烯和乙烯，迁移到原来的C3处，构型翻转或保持），这与观察结果非常吻合。显然，轨道对称控制并不决定[1,3]迁移碳的立体化学。
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