Synthesis, structures and reactivity of macrocyclic imidazolylboranes

New macrocyclic imidazolylboranes [imidazolylB(R1)2]n, where selected carbon atoms of imidazolyl rings may bear substituents other than hydrogen, and where n=4 or 5, are obtained from 1-trimethysilylimidazoles and haloboranes XB(R1)2 by boron/silicon exchange using 2-bromoimidazole and benzimidazole. These macrocycles are formally zwitterionic and contain imidazolyl rings linked through their nitrogen atoms by BH2, B(R1)2 or BR1X units. Despite the sterical demand of these derivatives tetrameric macrocycles are formed. A new synthetic strategy to macrocyclic imidazolylboranes includes the preparation and cyclization of bis(imidazolyl)boronium chlorides. In addition dihaloboranes have been tested for cyclization to yield halogen-containing macrocycles. The new compounds are spectroscopically characterized and X-ray structure analyses of tetrameric- (2a) and pentameric 1-imidazolylborane (2a′), tetrameric 1-imidazolyldimethylborane (2b), tetrameric 1-(2-bromo)imidazolylborane (2d) and bis(2-bromoimidazolyl)boronium chloride (3d) are reported.