Abstract
Dimeric aminoboranes, [H(2)BNR(2)](2) (R = Me or CH(2)CH(2)) containing B(2)N(2) cores, can be activated by I(2), HNTf(2) (NTf(2) = [N(SO(2)CF(3))(2)]), or [Ph(3)C][B(C(6)F(5))(4)] to form isolable H(2)B(μ-NR(2))(2)BHX (for X = I or NTf(2)). For X = [B(C(6)F(5))(4)](-) further reactivity, presumably between [H(2)B(μ-NMe(2))(2)BH][B(C(6)F(5))(4)] and aminoborane, forms a B(3)N(3)-based monocation containing a three-center two electron B-(μ-H)-B moiety. The structures of H(2)B(μ-NMe(2))(2)BH(I) and [(μ-NMe(2))BH(NTf(2))](2) indicated a sterically crowded environment around boron, and this leads to the less common O-bound mode of NTf(2) binding. While the iodide congener reacted very slowly with alkynes, the NTf(2) analogues were more reactive, with hydroboration of internal alkynes forming (vinyl)(2)BNR(2) species and R(2)NBH(NTf(2)) as the major products. Further studies indicated that the B(2)N(2) core is maintained during the first hydroboration, and that it is during subsequent steps that B(2)N(2) dissociation occurs. In the mono-boron systems, for example, (i) Pr(2)NBH(NTf(2)), NTf(2) is N-bound; thus, they have less steric crowding around boron relative to the B(2)N(2) systems. Notably, the monoboron systems are much less reactive in alkyne hydroboration than the B(2)N(2)-based bis-boranes, despite the former being three coordinate at boron while the latter are four coordinate at boron. Finally, these B(2)N(2) electrophiles are much more prone to dissociate into mono-borane species than pyrazabole [H(2)B(μ-N(2)C(3)H(3))](2) analogues, making them less useful for the directed diborylation of a single substrate.