Cyclic Ketiminoboranes

The cyclocondensation reaction of benzil-bis(trimethylsilylimin) (3) and Cl2B-NiPr2 via silicon-boron exchange results in the solvent-dependent formation of the 1,3,2-diazaborole 1a or the 1,3,6,8,2,7-tetraazadiborecin 2a. Using the more rigid 9,10-bis (trimethylsilylimino)phenanthrene instead of 3 only the five-membered heterocycle 4,5-biphenylene-2-diisopropylamino-1,3,2diazaborole (1b) is formed. The cyclic ketiminoboranes are characterized by NMR and X-ray diffraction data. The 1,3,2diazaborole ring is almost planar, while the conformation of the 1,3,6,8,2,7-tetraazadiborecin has the shape of a figure-eight loop.