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

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.