Pushing the limits of concertedness. A waltz of wandering carbocations
Pushing the limits of concertedness. A waltz of wandering carbocations
复制标题
DOI：
10.1039/c8sc03567a
复制
发表时间：
2019-02-21
期刊：
影响因子：
8.4
通讯作者：
Tantillo, Dean J.
中科院分区：
文献类型：
--
作者：
Castineira Reis, Marta;Silva Lopez, Carlos;Tantillo, Dean J.
在一系列复杂的萜烯形成碳正离子环化/重排反应中，所谓的“三移位”反应是最意想不到的。这种反应包括三个1，n移位的异步组合成一个协调的过程，例如，一个1,2-烷基移位接着一个1,3-氢化物移位接着第二个1,2-烷基移位。到目前为止，这类反应被认为发生在二萜和固醇侧链的生物合成过程中。在这里，我们描述了通过设计和描述量子化学计算，可以将额外的1，n移位事件耦合到三移位的系统来推动这种类型的碳正离子反应的协同性极限的努力，原则上可以导致四，五等移位。虽然我们的设计并没有导致四倍等变化的明确例子，但它们确实导致了具有令人惊讶的平坦能量表面的反应，其中超过五个化学事件将反应物和合理的产物连接起来。从头算分子动力学模拟表明，这些表面上的形式极小值在短时间尺度上相互交换，并与其他意想不到的结构交换，使我们能够瞥见一个非常复杂的流形，从而可以随时访问巨大的结构多样性。
Among the array of complex terpene-forming carbocation cyclization/rearrangement reactions, the socalled "triple shift" reactions are among the most unexpected. Such reactions involve the asynchronous combination of three 1, n-shifts into a concerted process, e.g., a 1,2-alkyl shift followed by a 1,3-hydride shift followed by a second 1,2-alkyl shift. This type of reaction so far has been proposed to occur during the biosynthesis of diterpenes and the sidechains of sterols. Here we describe efforts to push the limits of concertedness in this type of carbocation reaction by designing, and characterizing with quantum chemical computations, systems that could couple additional 1, n-shift events to a triple shift leading, in principle to quadruple, pentuple, etc. shifts. While our designs did not lead to clear-cut examples of quadruple, etc. shifts, they did lead to reactions with surprisingly flat energy surfaces where more than five chemical events connect reactants and plausible products. Ab initio molecular dynamics simulations demonstrate that the formal minima on these surfaces interchange on short timescales, both with each other and with additional unexpected structures, allowing us a glimpse into a very complex manifold that allows ready access to great structural diversity.