BORON-STABILIZED CARBENES .3. [3-CYCLO ADDITIONS TO BORIO NITRILE YLIDES(2])

The borio nitrile ylides Li[Ph3B-C=N-CH-R] (R=CO2Et, Tos) react with v arious dipolarophiles regio- and site-specifically to give boron compl exes of carbenoid five-membered heterocycles which have been alkylated in order to accomplish their isolation. Thus, isocyanates and isothio cyanates/trialkyloxonium salts give rise to the imidazoline-2-ylidene complexes 2-5, CS2/Et3O+ to the thiazoline-2-ylidene complex 6 and ace tone/Et3O+ to the oxazoline-2-ylidene complexes 7 a, b. Aryldiazonium ions are added regiospecifically in the ''wrong'' orientation that lea ds to mesoionic 1,2,4-triazolium-3-ato-triphenylborate complexes (11a- e). The spectroscopic properties of the new complexes are discussed. O f l-4H-5-carbethoxy-triazolium-3-ato)triphenylborate (11a) an X-ray st ructure analysis has been carried out.