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

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