College of Engineering & Science, Louisiana Tech University , Ruston, Louisiana 71272, United States.
J Org Chem. 2013 Nov 1;78(21):10776-83. doi: 10.1021/jo401763v. Epub 2013 Oct 14.
The first computational study of the rearrangement reactions of oxiranes initiated by lithium dialkylamides is presented. Aside from the well-known carbenoid insertion pathways, both β-elimination and α-lithiation have been suggested as the exclusive mechanism by which oxiranes react in the presence of organolithium bases. The products of the former are allyl alcohols (and, in some cases, dienes) and are ketones in the case of the latter. The computational studies reported in this work indicate that both mechanisms could be simultaneously operational. In particular, our work shows that the allyl alcohols from β-elimination are unlikely to undergo 1,3-hydrogen transfer to the vinyl alcohols and thus to the ketones, suggesting that ketones are formed through the opening of the oxirane ring after α-substitution. Elimination of LiOH from the lithiated allyl alcohol is found to result in the diene product. Low activation barriers for β-elimination are offered as the explanation for the few special cases where the allyl alcohol is the dominant or exclusive product. These findings are consistent with the product distributions observed in several experiments.
本文首次对锂二烷基酰胺引发的环氧化合物重排反应进行了计算研究。除了众所周知的碳烯插入途径外，β-消除和α-锂化都被认为是在有机锂碱存在下环氧化合物反应的唯一机制。前者的产物是烯丙醇（在某些情况下是二烯），而后者的产物是酮。本工作中的计算研究表明，这两种机制可能同时起作用。特别是，我们的工作表明，β-消除产生的烯丙醇不太可能经历 1,3-氢转移到乙烯醇，从而转移到酮，这表明酮是通过α-取代后开环形成的。从锂化的烯丙醇中消除 LiOH 被发现会导致二烯产物。β-消除的低活化能垒为烯丙醇是主要或唯一产物的少数特殊情况提供了解释。这些发现与几个实验中观察到的产物分布一致。
