Pathways for the cyclotetramerization of dibenz[c,e][1,2]azaborine, a BN-phenanthryne

The BN-phenanthryne dibenz[c,e][1,2]azaborine (9) was previously inferred as a reactive intermediate in the solution phase thermolysis of 9-azido-9-borafluorene by isolation of its cyclic tetramer. The mechanism of the cyclotetramerization of BN-phenanthryne (9) is investigated using a meta-generalized gradient approximation density functional (TPSS-D3) in conjunction with a polarized split valence basis set and single energy points using a double-hybrid functional (B2PYLP-D3) with a polarized triple-zeta basis set. The most favorable mechanism involves the dimerization of 9 to diazadiboretidine derivative 10, followed by dimerization of 10 for which two different pathways were identified computationally. The more favorable one involves a B4N4 bicyclo[4.2.0]octa-2,4,7-triene intermediate. An alternative mechanism, slightly higher in energy, proceeds through a B4N4 cube isomer that lies in an unexpectedly deep potential energy minimum. The low barriers of dimerization of 10 make it a short lived reactive intermediate that likely is too reactive to be trapped by the still more reactive cyclic iminoborane 9. Copyright (c) 2014 John Wiley & Sons, Ltd.