The elemento-boration of alkynes by boron electrophiles, particularly hydroboration, has been used as a powerful tool to form vinylboranes for decades. Recently, the scope of elemento-boration has increased due to the need for non-toxic, inexpensive routes to vinyl and aryl boronic acid derivatives.1 Notable developments include aminoboration,2 oxoboration,3 trans-hydroboration of alkynes,4 and 1,1-carboboration reactions.5
In contrast to hydroboration, relatively few novel haloboration reactions have been reported, despite this generating a useful ambipolar synthon. 1,2-haloboration was first reported by Lappert et al. in 1964,6 and typically involves the syn-1,2-addition of a B–X bond across an alkyne. We now report that 1-ethynylnaphthalene reagents undergo a range of novel transformations with BBr3. These include alkyne directed C–H borylation of terminal and alkyl ethynylnaphthalenes via a cyclized 1,2-haloboration intermediate, and borylative cyclization and rearrangement sequences of aryl ethynylnaphthalenes to yield boraphenalenes. We also report the synthesis of a well-defined boron-nitrogen doped polyaromatic hydrocarbon using a sequential borylative cyclisation/C-H borylation reaction.
1 R. Barbeyron, E. Benedetti, J. Cossy, J. J. Vasseur, S. Arseniyadis and M. Smietana, Tetrahedron, 2014, 70, 8431–8452.
2 R. L. Melen, M. M. Hansmann, A. J. Lough, A. S. K. Hashmi and D. W. Stephan, Chem. Eur. J., 2013, 19, 11928–11938.
3 A. J. Warner, A. Churn, J. S. McGough and M. J. Ingleson, Angew. Chem. Int. Ed., 2017, 56, 354–358.
4 J. S. McGough, S. M. Butler, I. A. Cade and M. J. Ingleson, Chem. Sci., 2016, 7, 3384–3389.
5 R. L. Melen, Chem. Commun., 2014, 50, 1161–1174.
6 M. F. Lappert and B. Prokai, J. Organomet. Chem., 1964, 1, 384–400.
