Unexpected boratranes: BBr3 mediated synthesis and mechanistic insights

Boratranes synthesis are predictably based on the reactions of triethanolamine derivatives with boric acid, or trialkyl borate and require prolonged heating of the reagents under reflux. Herein, the authors report an efficient synthesis of unexpected boratranes featuring on one hand three six-membered chelating ring or two six-membered and one five-membered chelating ring and on the other hand a trans-annular boron atom is bound to one nitrogen, two oxygen and one carbon atom. These novel boratranes have been synthetized by reacting N,N-bis(2-methoxybenzyl) alkynylamines with BBr3 in dichloromethane at low temperature. The reaction mechanism was investigated by using density functional theory (DFT) calculations. A mechanistic pathway proceeding through a key dioxazaborecinium intermediate was proposed. This alternative protocol represents a convenient access to dissymmetric and functionalizable boratranes under mild conditions in good yields.