Zimmerman Howard E, Novak Tibor
Chemistry Department, University of Wisconsin, Madison, Wisconsin 53706, USA.
J Org Chem. 2003 Jun 27;68(13):5056-66. doi: 10.1021/jo034189j.
The di-pi-methane rearrangement with two pi-groups attached to the central "methane carbon" of the reactant and which leads to a pi-substituted cyclopropane has been studied intensively. Our present research had the goal of elucidating the regioselectivity of the tri-pi-methane counterpart. The reactants with three pi groups attached to the central carbon mechanistically are capable of affording both di-pi-methane and tri-pi-methane photoproducts. In common with the di-pi-methane system, bridging of two of the pi-systems affords a cyclopropyldicarbinyl diradical intermediate that opens to an allyl carbinyl diradical. This diradical has the option of closing to a three-membered or a five-membered ring. It was found that the regioselectivity of the initial pi-pi-bridging step and the three-ring opening of the cyclopropyldicarbinyl diradical exhibit regioselectivity parallel to that of the di-pi counterpart. Both three-ring and five-ring photoproducts were formed with the ratio varying with conversion. Since the three-ring (i.e. di-pi-methane) photoproducts were found to ring expand to the five-ring (i.e. tri-pi-methane) products, kinetics were employed to determine to what extent the reaction proceeds in a two-step versus direct formation of the five-ring product. It was found that the direct route was the major one.
对反应物中心“甲烷碳”连接两个π基团并导致π取代环丙烷的二π-甲烷重排进行了深入研究。我们目前的研究目标是阐明三π-甲烷类似物的区域选择性。中心碳连接三个π基团的反应物从机理上讲能够产生二π-甲烷和三π-甲烷光产物。与二π-甲烷体系一样，两个π体系的桥连会产生一个环丙基二甲基双自由基中间体，该中间体可开环生成烯丙基甲基双自由基。这个双自由基可以选择闭环形成三元环或五元环。研究发现，初始π-π桥连步骤和环丙基二甲基双自由基的三环开环的区域选择性与二π类似物的区域选择性平行。三环和五环光产物均有形成，其比例随转化率而变化。由于发现三环（即二π-甲烷）光产物会扩环生成五环（即三π-甲烷）产物，因此采用动力学方法来确定反应在两步反应与直接形成五环产物的过程中进行的程度。结果发现直接途径是主要途径。
