Isocyanide and Ylidene Complexes of Boron: Synthesis and Crystal Structures of (2-(Trimethylsiloxy)phenyl isocyanide)−Triphenylborane and (1,2-Dihydrobenzoxazol-2-ylidene)−Triphenylborane

2-(Trimethylsiloxy)phenyl isocyanide ( 1 ) reacts at −60 °C with triphenylborane to form the isocyanide adduct (2-(trimethylsiloxy)phenyl isocyanide)−triphenylborane ( 2 ). Upon desilylation in MeOH with a catalytic amount of KF at −30 °C 2 is converted into the ylidene adduct (1,2-dihydrobenzoxazol-2-ylidene)−triphenylborane ( 3 ). The X-ray crystal structures of 2 and 3 are reported. When it is heated, 2 dimerizes to give 1,4-bis(2-(trimethylsiloxy)phenyl)-2,2,3,5,5,6-hexaphenyl-2,5-dibora-2,5-dihydropyrazine ( 4 ). Si−O bond cleavage leads to 1,4-bis(2-hydroxyphenyl)-2,2,3,5,5,6-hexaphenyl-2,5-dibora-2,5-dihydropyrazine ( 5 ). Attempts to induce formation of a boron-bridged diylidene complex via attack of the hydroxyl oxygens in 5 at the pyrazine carbon atoms failed. Instead, complex 6, containing one bridging N,O-ylidene ligand, was obtained by partial hydrolysis of 5 . The molecular structure of 6, which crystallizes with one molecule of methanol and acetone each per formula unit, is reported.