Divergent Reactivity of TEMPO with MBr3 (M = B, Al)

Addition of TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) to a toluene slurry of AlBr3 results in rapid formation of AlBr3(η1-TEMPO) (1), which can be isolated in 65 % yield. In contrast, addition of TEMPO to a hexanes solution of BBr3 results in formation of [TEMPO][BBr4] (2) and (TEMPO)2BBr (3), the products of TEMPO disproportionation. Complexes 1–3 have been fully characterized, including analysis by X-ray crystallography. The divergent reactivity is likely dictated by the Lewis acidity of the group 13 halide, and in the case of the stronger Lewis acid BBr3, coordination of TEMPO to the boron center generates an adduct that is capable of oxidizing free TEMPO.