Altering the allowed/forbidden gap in cyclobutene electrocyclic reactions: Experimental and theoretical evaluations of the effect of planarity constraints
Altering the allowed/forbidden gap in cyclobutene electrocyclic reactions: Experimental and theoretical evaluations of the effect of planarity constraints
复制标题
DOI：
10.1021/ja028963g
复制
发表时间：
2003-05-14
影响因子：
15
通讯作者：
Houk, KN
中科院分区：
文献类型：
--
作者：
Lee, PS;Sakai, S;Houk, KN
允许的同旋环丁烯开环具有明显的非平面碳骨架。Brauman和Archie以及Freedman等人的经典实验，将允许/禁止间隙设置为大于15 kcal/mol。Wolfgang Roth提出，一个被迫平面化的系统可能比无约束的系统更倾向于对旋模式。这样的系统现在已经在理论和实验上进行了研究，结果证实了罗斯的假设。实验在波鸿进行，计算在大坂和洛杉矶进行。随着环丁烯开环过渡结构接近平面，允许的对旋和禁止的对旋路径之间的能隙减小。对于开环的环丁烯稠合到异戊烯，之间的禁带和允许的过渡态的能隙仅为4.1千卡/摩尔的CASSCF和8.0千卡/摩尔的CAS-MP2相比，分别为13.4和19.2千卡/摩尔，为母体环丁烯。实验研究的3，4-二甲基环丁烯稠合到各种环系统的报告，并发现一种趋势，朝着减少允许/禁止的环丁烯的平面性被强制执行的间隙。
The allowed conrotatory cyclobutene ring-opening has a distinctly nonplanar carbon skeleton. Classic experiments by Brauman and Archie, and by Freedman et al., placed the allowed/forbidden gap at greater than 15 kcal/mol. Wolfgang Roth proposed that a system forced to planarity might have a smaller preference for the conrotatory mode than unconstrained systems. Such systems have now been studied theoretically and experimentally, and results that confirm Roth's postulate are presented here. The experiments were performed in Bochum, and the calculations were carried out in Osaka and Los Angeles. As the cyclobutene ring-opening transition structure approaches planarity, the energy gap between allowed conrotatory and the forbidden disrotatory pathways decreases. For the ring-opening of a cyclobutene fused to norbornene, the energy gap between the forbidden and the allowed transition state is only 4.1 kcal/mol by CASSCF and 8.0 kcal/mol by CAS-MP2 as compared to 13.4 and 19.2 kcal/mol, respectively, for the parent cyclobutene. Experimental studies of 3,4-dimethylcyclobutenes fused to various ring systems are reported, and a trend is found toward a reduced allowed/forbidden gap as the planarity of the cyclobutene is enforced.