By the addn. of iminoboranes RB.tplbond.NR' (I; R = Me, Et, Pr, CHMe2, R' = CHMe2, CMe3) to the corresponding diazadiboretidines II (same R, R'), a ring expansion is obsd., borazines III being formed. The particular iminoborane Me2CHB.tplbond.NCHMe2 can be added to the diazadiboretidines II to give borazines B3N3R2R'2(CHMe2)2 with an unsym. set of ligands. From PhB.tplbond.NCMe3 and (PhBNCMe3)2, the Dewar borazine IV is formed, which crystallizes in the space group P2l/c. The diazadiboretidines II, the borazines III and the Dewar borazine are split into the corresponding iminoboranes RB.tplbond.NCMe3 at 500-600 Deg in the gas phase. The molten borazines III and Dewar borazines are contracted to the corresponding diazadiboretidines II at 160-320 Deg.