Double-bond saturation during acid-mediated intramolecular electrophilic C–H borylation of 2-(naphthalen-1-yl)vinyl N-heterocyclic carbene-boranes

Three new 2-(naphthalen-1-yl)vinyl N-heterocyclic carbene (NHC)-boranes have been synthesized via radical trans-hydroboration of alkynes. Acid-mediated electrophilic borylation/cyclization reactions of these compounds selectively formed five-membered boracycles via C–H functionalization of naphthalene 2-positions. Unexpectedly, saturation of alkene double bonds was concomitant with this process. So-formed partially saturated five-membered boracyclic NHC-borenium ions were studied by multinuclear NMR spectroscopy, and converted to corresponding isolable boracyclic NHC-borane products by hydride quenching. Mechanistic experiments suggest that the observed double bond saturations may result from rearrangements of electrophilic borylation intermediates. This reactivity presents new considerations for electrophilic C–H borylation synthesis.