Divergent Reactivity of Perfluoropentaphenylborole with Alkynes

The reactions of perfluoropentaphenylborole ( 1 ) with the symmetrical internal alkynes RCCR (R = C 6 F 5, C 6 H 5, CH 2 CH 3 ) have been explored. For R = C 6 F 5, 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.