Synthesis of Electrophiles Derived from Dimeric Aminoboranes and Assessing Their Utility in the Borylation of π Nucleophiles

Dimeric aminoboranes, [H2BNR2]2 (R = Me or CH2CH2) containing B2N2 cores, can be activated by I2, HNTf2 (NTf2 = [N­(SO2CF3)2]), or [Ph3C]­[B­(C6F5)4] to form isolable H2B­(μ-NR2)2BHX (for X = I or NTf2). For X = [B­(C6F5)4]− further reactivity, presumably between [H2B­(μ-NMe2)2BH]­[B­(C6F5)4] and aminoborane, forms a B3N3-based monocation containing a three-center two electron B-(μ-H)-B moiety. The structures of H2B­(μ-NMe2)2BH­(I) and [(μ-NMe2)­BH­(NTf2)]2 indicated a sterically crowded environment around boron, and this leads to the less common O-bound mode of NTf2 binding. While the iodide congener reacted very slowly with alkynes, the NTf2 analogues were more reactive, with hydroboration of internal alkynes forming (vinyl)2BNR2 species and R2NBH­(NTf2) as the major products. Further studies indicated that the B2N2 core is maintained during the first hydroboration, and that it is during subsequent steps that B2N2 dissociation occurs. In the mono-boron systems, for example, iPr2NBH­(NTf2), NTf2 is N-bound; thus, they have less steric crowding around boron relative to the B2N2 systems. Notably, the monoboron systems are much less reactive in alkyne hydroboration than the B2N2-based bis-boranes, despite the former being three coordinate at boron while the latter are four coordinate at boron. Finally, these B2N2 electrophiles are much more prone to dissociate into mono-borane species than pyrazabole [H2B­(μ-N2C3H3)]2 analogues, making them less useful for the directed diborylation of a single substrate.