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 B 4 N 4 bicyclo[4.2.0]octa‐2,4,7‐triene intermediate . An alternative mechanism, slightly higher in energy, proceeds through a B 4 N 4 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 © 2014 John Wiley & Sons, Ltd.