A set of (3,3')-bis(1-Ph-2-R-1H-2,1-benzazaborole) compounds, in which R=tBu (Bab-tBu)2 , R=Dipp (Bab-Dipp)2 or R=tBu and Dipp (Bab-Dipp)(Bab-tBu), was synthesized and fully characterized using 1 H, 11 B, 13 C, and 15 N NMR spectroscopy as well as single-crystal X-ray diffraction analysis. The central HC(sp3 )-C(sp3 )H bond with restricted rotation at the junction of both 1H-2,1-benzazaborole rings displayed an intriguing reactivity. It was demonstrated that this bond is easily mesolytically cleaved using alkali metals to form the respective aromatic 1Ph-2R-1H-2,1-benzazaborolyl anions M+ (THF)n (Bab-tBu)- (M=Li, Na, K) and K+ (THF)n (Bab-Dipp)- . Furthermore, the central HC(sp3 )-C(sp3 )H bond of bis(1H-2,1-benzazaborole)s is also homolytically cleaved either by heating or photochemical means, giving corresponding 1Ph-2R-1H-2,1-benzazaborolyl radicals (Bab-tBu). and (Bab-Dipp). , which rapidly self-terminate. Nevertheless, their formation was unambiguously established by NMR analysis of the reaction mixtures containing products of the self-termination of the radicals after heating or irradiation. (Bab-Dipp). radical was also characterized using EPR spectroscopy. Importantly, it turned out that the essentially non-polarized HC(sp3 )-C(sp3 )H bond in (Bab-tBu)2 is also cleaved heterolytically with 2 equiv of MeLi, giving the mixture of Li+ (SOL)n (Bab-tBu)- (SOL=THF or Et2 O) and lithium methyl-substituted borate complex Li+ (SOL)n (Bab-tBu-Me)- in a diastereoselective fashion.
Keywords: C−C bond cleavage; atropisomerism; boron; heterocycles; hindered rotation; radicals.
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