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 perfluorohcptaphenylborepin 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.