Department of Chemistry and the Research Center for Smart Molecules, Rikkyo University, Nishi-Ikebukuro, Toshima-ku 171-8501 Tokyo, Japan.
J Am Chem Soc. 2010 Mar 17;132(10):3413-22. doi: 10.1021/ja908899u.
The N(2) liberation from iminodiazonium ion (2-X) is a key step of the Schmidt rearrangement of ketones. Molecular orbital calculations showed that two concurrent reaction channels, syn-benzyl fragmentation and anti-Me rearrangement, exist for syn-2, whereas anti-2-X proceeds via a single TS. Substituent effect analyses of the reactions of syn-2-X gave concave-upward plots, typical for a concurrent reaction mechanism. On the other hand, the reactions of anti-2-X gave linear Hammett plots, indicative of a single reaction mechanism for all anti-2-X. IRC calculations, however, revealed that the TS led to either an anti-benzyl rearrangement or an anti-benzyl fragmentation product depending on the substituent. Thus, the change of the mechanism (identity of the product) could not be detected by the Hammett plots. Ab initio dynamics simulations for anti-2-X were found to follow the IRC path for X = p-NO(2), giving the rearrangement product, and almost so for X = p-MeO, giving the fragmentation products. However, in borderline cases where X is less donating than p-MeO and less withdrawing than p-NO(2), the trajectories did not follow the minimum energy path on the potential energy surface but gave both rearrangement and fragmentation products directly from the single TS. This is a novel example of path bifurcation for a closed shell anionic reaction. It was concluded that a reactivity-selectivity argument based on the traditional TS theory might not always be applicable even to a well-known textbook organic reaction.
亚氨基二氮烯离子（2-X）的 N(2) 释放是酮的施密特重排的关键步骤。分子轨道计算表明，对于 syn-2 存在两个并发反应通道，顺式苄基断裂和反式-Me 重排，而 anti-2-X 则通过单个过渡态进行。syn-2-X 反应的取代基效应分析给出了向上凹的图，典型的协同反应机制。另一方面，anti-2-X 的反应给出了线性哈米特图，表明对于所有 anti-2-X，反应机制都是单一的。IRC 计算表明，过渡态取决于取代基，导致反式苄基重排或反式苄基断裂产物。因此，哈米特图无法检测到反应机制的变化（产物的身份）。对于 anti-2-X 的从头动力学模拟发现，对于 X = p-NO(2)，遵循 IRC 路径，得到重排产物，而对于 X = p-MeO，则几乎如此，得到断裂产物。然而，在 X 比 p-MeO 的供电子能力弱且比 p-NO(2) 的吸电子能力弱的边界情况下，轨迹不会沿着势能面上的最小能量路径进行，而是直接从单个过渡态得到重排和断裂产物。这是一个封闭壳层阴离子反应的路径分叉的新例子。得出的结论是，即使对于一个众所周知的教科书有机反应，基于传统过渡态理论的反应性选择性论点也不一定总是适用。
