1,2-Dihydro[1,3,2]diazaborolo[1,5-a]pyridines: a novel class of heterocycles.
Reaction of a pyridine carbaldimine 2-tBuN:CHC5H4N (1a) with a molar equivalent of BBr3 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 equiv. of BF3 gave nonionic yellow adducts 3a and 3b. The redn. of compds. 2a, 3a and 3b with an excess of Na amalgam in a hexane slurry gave 1-X-2-R-1,2-dihydro[1,3,2]diazaborolo[1,5-a]pyridines (shown as I) (4; X = Br; R = tBu), 5a (F; tBu) and 5b (F; 2,6-Me2C6H3) as yellow oils (4a, 5a) or a yellow wax (5b), resp. Treatment of 4 with an excess of chlorotrimethylsilane caused a Br/Cl exchange to afford chloro deriv. The addn. of a Me group to the B atom was effected by reaction of heterocycle 4 with MeLi. The BCN deriv. (8) resulted from treatment of 4 with Ag cyanide. Redn. of 4 with Li Al hydride gave the 1-hydro-deriv., whereas the BStBu compd. was obtained from the reaction of 4 with KStBu. Compd. 8 was subjected to an x-ray diffraction anal.
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