Reaction of B 2 ( o ‐tol) 4 with CO and Isocyanides: Cleavage of the C≡O Triple Bond and Direct C−H Borylations

The reaction of highly Lewis acidic tetra( o ‐tolyl)diborane(4) with CO afforded a mixture of boraindane and boroxine by the cleavage of the C≡O triple bond. 13 C labeling experiments confirmed that the carbon atom in the boraindane stems from CO. Simultaneously, formation of boroxine 3 could be considered as borylene transfer to capture the oxygen atom from CO. The reaction of diborane(4) with t Bu−NC afforded an azaallene, while the reaction with Xyl−NC furnished cyclic compounds by direct C−H borylations.