1,2-Dihydro[1,3,2]diazaborolo[1,5-a]pyridines: a novel class of heterocycles

Abstract

Reaction of a pyridine carbaldimine 2-^tBuNCHC₅H₄N 1a with a molar equivalent of boron tribromide afforded a bicyclic 1,3,2-diazaborolium bromide 2a as an orange solid, whereas 1a and the corresponding 2-(2,6-Me₂C₆H₃)N CHC₅H₄N 1b with two equivalents of boron trifluoride gave non-ionic yellow adducts 3a and 3b. The reduction of compounds 2a, 3a and 3b with an excess of sodium amalgam in a hexane slurry led to the formation of 1-X-2-R-1,2-dihydro[1,3,2]diazaborolo[1,5-a]pyridines 4 (X = Br; R = ^tBu), 5a (F; ^tBu) and 5b (F; 2,6-Me₂C₆H₃) as yellow oils (4a, 5a) or a yellow wax (5b), respectively. Treatment of 4 with an excess of chlorotrimethylsilane caused a Br/Cl exchange to afford chloro derivative 6. The addition of a methyl group to the boron atom was effected by reaction of heterocycle 4 with methyllithium. The BCN derivative 8 resulted from treatment of 4 with silver cyanide. Reduction of 4 with lithium aluminium hydride gave the 1-hydro-derivative 9, whereas the BS^tBu compound was obtained from the reaction of 4 with KS^tBu. Compound 8 was subjected to an X-ray diffraction analysis.