Zimmerman H E, Církva V
Chemistry Department, University of Wisconsin, Madison, WI 53706, USA.
J Org Chem. 2001 Mar 9;66(5):1839-51. doi: 10.1021/jo001601b.
The di-pi-methane rearrangement with two pi-groups bonded to a single carbon leading to pi-substituted cyclopropanes is now well established. The present research had as its goal the exploration of molecular systems having three pi-moieties attached to an sp(3)-hybridized atom in a search for a tri-pi-methane rearrangement. Indeed, it was found that such systems do rearrange photochemically to afford cyclopentenes. However, it was also established that vinylcyclopropanes ring-expand to cyclopentenes on direct irradiation. Since both three-ring and five-ring photoproducts often are found to be produced, it was important to establish that the observed photochemistry was really the result of a true single-step tri-pi-methane rearrangement and not the consequence of two sequential rearrangements, first to form a vinyl cyclopropane which subsequently ring expanded to the cyclopentene. The general situation has three species-A, B, and C-corresponding to tri-pi-methane reactant A, vinylcyclopropane photoproduct B, and cyclopentene photoproduct C. Three rate constants are involved, k(1) for A --> B, k(2) for A --> C, and k(3) for B --> C. The kinetics were applied to two examples with provision to avoid differential light absorption; this utilized singlet sensitization. It was determined that direct formation of the cyclopentene photoproduct proceeds more rapidly than the ring-expansion route. In contrast to the di-pi-methane rearrangement, the tri-pi-methane reaction was found to be preferred by the singlet, while in these sterically congested systems, the triplet led to di-pi-methane reactivity. Finally, a ground-state counterpart of the reaction was obtained.
与单个碳原子相连的两个π-基团发生的二π-甲烷重排反应生成π-取代环丙烷，现已得到充分证实。本研究的目标是探索分子体系，其中三个π-部分连接到一个sp(3)杂化原子上，以寻找三π-甲烷重排反应。实际上，已发现此类体系确实会发生光化学反应生成环戊烯。然而，也已确定乙烯基环丙烷在直接照射下会扩环生成环戊烯。由于经常发现三环和五环光产物都会生成，因此确定观察到的光化学过程确实是真正的单步三π-甲烷重排反应的结果，而不是两个连续重排反应的结果，即首先形成乙烯基环丙烷，随后其扩环生成环戊烯，这一点很重要。一般情况有三种物种——A、B和C，分别对应三π-甲烷反应物A、乙烯基环丙烷光产物B和环戊烯光产物C。涉及三个速率常数，A→B的k(1)、A→C的k(2)和B→C的k(3)。将动力学应用于两个示例，以避免光吸收差异；这利用了单线态敏化。已确定环戊烯光产物的直接形成比扩环途径进行得更快。与二π-甲烷重排反应不同，发现三π-甲烷反应更倾向于单线态，而在这些空间拥挤的体系中，三线态导致二π-甲烷反应性。最后，获得了该反应的基态对应物。
