Synthesis and Properties of 1,2,4,3‐Thiadiazaboretidines. Crystal Structure of 2,4‐Di‐tert‐butyl‐3‐phenyl‐1,2,4,3‐thiadiazaboretidineReaction of sulfur diimides with alkyl(aryl)bis(methylthio)boranes leads to reductive 1,3‐cycloaddition of the NSN sequence with formation of the 1,2,4,3‐thiadiazaboretidines2a‐g. NMR (1H,11B,13C,15N,29Si), mass spectra and the results of the X‐ray analysis of21are reported and discussed. The11B chemical shifts have been calculated for 3‐phenyl‐1,2,4,3‐thiadiazaboretidine and some related model compounds by the IGLO method and correlate acceptably