Department of Chemistry, Wayne State University, 5101 Cass Avenue, Detroit, Michigan 48202, USA.
Department of Chemistry and Biochemistry, Florida International University, Miami, Florida 33199, USA.
Nat Commun. 2014 Jun 6;5:4064. doi: 10.1038/ncomms5064.
Radical addition-elimination reactions are a major pathway for transformation of unsaturated hydrocarbons. In the gas phase, these reactions involve formation of a transient strongly bound intermediate. However, the detailed mechanism and dynamics for these reactions remain unclear. Here we show, for reaction of chlorine atoms with butenes, that the Cl addition-HCl elimination pathway occurs from an abstraction-like Cl-H-C geometry rather than a conventional three-centre or four-centre transition state. Furthermore, access to this geometry is attained by roaming excursions of the Cl atom from the initially formed adduct. In effect, the alkene π cloud serves to capture the Cl atom and hold it, allowing many subsequent opportunities for the energized intermediate to find a suitable approach to the abstraction geometry. These bimolecular roaming reactions are closely related to the roaming radical dynamics recently discovered to play an important role in unimolecular reactions.
自由基加成-消除反应是不饱和烃转化的主要途径。在气相中，这些反应涉及瞬态强束缚中间体的形成。然而，这些反应的详细机制和动力学仍然不清楚。在这里，我们展示了氯原子与丁烯的反应，表明 Cl 加成-HCl 消除途径来自于类似于 Cl-H-C 的几何结构，而不是传统的三中心或四中心过渡态。此外，这种几何结构可以通过 Cl 原子从最初形成的加合物中漫游来实现。实际上，烯烃的π云捕获并保持 Cl 原子，从而为高能中间体提供了许多后续机会，以找到合适的方式接近该离域几何结构。这些双分子漫游反应与最近发现的在单分子反应中起重要作用的漫游自由基动力学密切相关。
