C=O and C=S bond activation by an annulated 1,4,2-diazaborole

The reaction of an ambiphilic 1,4,2-diazaborole with C=O and C=S bonds results in formal (3+2) cycloaddition and has allowed the synthesis of a family of 1,3,2-oxazaborole and 1,3,2-thiazaborole derivatives. Computational calculations have indicated a dipolar mechanism where the π bond is concertedly activated via the Lewis acidic boron centre and the nucleophilic C5 position of the 1,4,2-diazaborole. In the case of methylisothiocyanate, preference for C=S over C=N addition is observed, and has been rationalized according to mechanistic calculations. A spirocyclic bis(1,3,2-thiazaborole) has been observed from the double activation of CS2.