Divergent Reactivity of Perfluoropentaphenylborole with Alkynes

The reactions of perfluoropentaphenylborole (1) with the symmetrical internal alkynes RCCR (R = C6F5, C6H5, CH2CH3) have been explored. For R = C6F5, a slow reaction occurs via a Diels−Alder pathway established for the non-fluorinated system to give the new borane perfluoroheptaphenylborepin 2. The new perfluoroarylborane 2 and its pyridine adduct 2-py were characterized crystallographically. In contrast, the reaction of 1 with diphenylacetylene is dominated by a reaction path in which the borole electrophilically attacks the alkyne, triggering a 1,2 phenyl migration and borole ring expansion to deliver the boracyclohexadiene 3 as the major product. Minor products in this reaction were identified as diphenyl-substituted borepins resulting from the Diels−Alder reaction channel. Finally, 3-hexyne reacts with 1 via the Diels−Alder pathway, but the kinetic products are not the expected diethyl borepins but rather one of the initial Diels−Alder adducts formed prior to ring-opening to the seven-membered borepin ring. This intermediate was fully characterized and provides concrete evidence for the pericyclic reaction sequence proposed by Eisch for the reaction of pentaphenylborole with diphenylacetylene.

本研究探索了全氟五苯基硼咯（perfluoropentaphenylborole，1）与对称内炔烃RCCR（R分别为C₆F₅、C₆H₅、CH₂CH₃）的反应行为。当R为C₆F₅时，反应以较慢速率进行，遵循非氟化体系已确立的狄尔斯-阿尔德（Diels−Alder）反应路径，生成新型硼烷类产物全氟七苯基硼庚环（perfluoroheptaphenylborepin，2）。对新型全氟芳基硼烷2及其吡啶加合物（pyridine adduct）2-py开展了晶体学表征。与之相反，1与二苯基乙炔的反应主要遵循一条反应路径：硼咯亲电进攻炔烃，引发1,2-苯基迁移与硼咯环扩环，最终以主要产物形式得到硼环己二烯（boracyclohexadiene，3）。该反应的副产物为经狄尔斯-阿尔德反应路径生成的二苯基取代硼庚环。3-己炔（3-hexyne）同样通过狄尔斯-阿尔德路径与1发生反应，但其动力学产物并非预期的二乙基硼庚环，而是开环生成七元硼庚环之前形成的初始狄尔斯-阿尔德加合物之一。该中间体已得到完整表征，为Eisch提出的五苯基硼咯与二苯基乙炔反应的周环反应序列（pericyclic reaction sequence）假说提供了确凿证据。