The terminal alkynyl-substituted phenol, 1a, was utilized in initial studies as it will further disfavor five membered boracycle formation as the interaction of the boron centre in the Ar-ylOBX2 intermediate with the internal alkyne carbon will lead to a high in energy vinyl cation intermediate (inset, Scheme 1). Initial reactions combined 1a, excess BCl3 and a hindered base to sequester the HCl by-product from the formation of Ar-ylOBCl2. This led to rapid (< 1 h at ambient temperature) formation of a compound that by in-situ multinuclear NMR spectroscopy was consistent with 2-Cl. Compound 2-Cl contains a diagnostic 1H singlet at 6.65 for the vinylic C-H and a 11B at 35.1 consistent with a (Ar)B(Cl)(OR) species. Scheme 1: The synthesis of C4-halogenated benzoxaborinines by o-alkynyl phenol trans-haloboration. Inset, the putative vinyl cation from reaction of boron at the internal alkyne carbon.