Photoisomerizations of 2-methylphenylcyclopropanes. Deuterium labeling
Photoisomerizations of 2-methylphenylcyclopropanes. Deuterium labeling
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2-甲基苯基环丙烷的光致异构化。
DOI：
10.1021/ja00710a081
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发表时间：
1970
影响因子：
15
通讯作者：
G. W. Craig
中科院分区：
文献类型：
--
作者：
P. Mazzocchi;R. Lustig;G. W. Craig
1a的乙烯基和烯丙基亚甲基发生非常容易的交换9，排除了3、11的形成，预计3、11在热力学上是更稳定的异构体。 12 即使在比简并异构化所需的条件更加剧烈的条件下，使 lb 和 3 达到平衡的尝试也会失败。因此，将1b在140℃下加热9.5小时，在该温度下不可逆重排成2-甲基-3-苯基茚(4)具有动力学意义，13不产生3（可以检测到少至5%）。从另一侧接近平衡同样是不成功的。异构体 3 在 190 ℃以下保持稳定，在该温度下它会重新排列成 l-苯基-3, 4-二氢萘 (5), 14，而 1b 和 4 均检测不到。在所述条件下形成不超过 5% 的 3 的观察结果，加上假设的 1b 至 3 的一级异构化的指前因子至少与简并异构化的指数前因子一样大的合理假设，得出前一个过程的活化能至少比后一个反应的活化能大 6.5 kcal/mol 的结论。这种差异代表了对苯基亚甲基环丙烷重排选择性的保守估计。与根据 Chesick 的平衡数据3计算的结果相比，苯基取代基在异构化中的根本重要性也明显体现在 1a 和 2 相互转化的活化参数的大小上 (//52= 22.1 kcal/mol, 5 52==—11.5 eu) 2-甲基亚甲基环丙烷和亚乙基环丙烷（AH*¡ 2= 39.8 kcal/mol, 5 52== 3.2 eu）。异构化活化的负熵可以至少部分地与苯环的特定取向的要求相关，以诱导苯环的裂解。
The remarkably facile exchange9 of the vinylic and allylic methylene groups of la occurs to the exclusion of formation of 3, 11 which is anticipated to be thermodynamically the more stable isomer. 12 Attempts to set lb and 3 into equilibrium fail even under conditions considerably more drastic than those required for the degenerate isomerization. Thus, heating of lb for 9.5 hr at 140, the temperature at which an irreversible re-arrangement to 2-methyl-3-phenylindene (4) assumes kinetic significance, 13 produces no 3 (as little as 5% could have been detected). Approach of the equilib-rium from the other side is equally unsuccessful; isomer 3 is stable up to 190, at which temperature it rear-ranges to l-phenyl-3, 4-dihydronaphthalene (5), 14 with neither lb nor 4 being detectable. The observation that no more than 5% of 3 is formed under the conditions stated, coupled with the reason-able assumption that the preexponential factor for the hypothetical first-order isomerization of lb to 3 would be atleast as great as that for the degenerate isomerization, allows the conclusionthat the energy of activation for the former process is at least 6.5 kcal/mol greater than that of the latter reaction. This difference repre-sents a conservative estimate of the selectivity accorded to the methylenecyclopropane rearrangement by the phenyl groups.The fundamental importance of the phenyl substitu-ents in the isomerization also is evident in the magnitude of the activation parameters for the interconversion of la and 2 (//52= 22.1 kcal/mol, 5 52==—11.5 eu) as compared to those calculated from Chesick’s data3 on the equilibration of 2-methylmethylenecyclopropane and ethylidenecyclopropane (AH*¡ 2= 39.8 kcal/mol, 5 52== 3.2 eu). The negative entropy of activation for the isomerization can be associated, at least in part, with the requirement of a specific orienta-tion of the phenyl ringsin order to induce cleavage of