Formal (2+2) ring expansion prevails over (4+2) cycloaddition of a kinetically stabilized benzoborirene with reactive cycloaddends

Benzoborirene carrying a bulky Trip 2 C 6 H 3 (Trip = 2,4,6-tri-iso-Pr 3 C 6 H 2 ) group at boron reacts with the dienophile 4-phenyl-1,2,4-triazoline-3,5-dione and the diene 3,6-di(4-pyridyl)-1,2,4,5-tetrazine by opening of the borirene ring rather than undergoing the typical Diels-Alder reactions. The formal insertion results in diazaborole and azaborolo[1,5- b ][1,2,4,5]tetrazine derivatives, respectively. (2+2) prevails in cycloaddition reactions of benzoborirene with PTAD and 4,4′-bipyridyl-1,2,4,5-tetrazine.