Add time:07/26/2019         Source:sciencedirect.com

Tetrafluorothiophene-S,S-dioxide reacts with terminal acetylenes by two pathways to give (2 + 2) cycloaddition and (4 + 2) Diels-Alder addition products, the latter being accompanied by extrusion of SO2 and aromatization. The (4 + 2) process is orbital topology-allowed to proceed in concerted fashion, while the former is forbidden to do so, yet the (2 + 2) cycloaddition can dominate the course of reaction. Here the two pathways using phenylacetylene are explored by DFT (B3LYP-D3/6-311G**++), illustrating that the (2 + 2) cycloaddition reaction proceeds via a low energy stepwise pathway involving a singlet diradical intermediate, which competes with the orbital symmetry allowed Diels-Alder path.

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