N‐Heterocyclic Carbene‐Assisted Reversible Migratory Coupling of Aminoborane at Magnesium

A combined synthetic and theoretical investigation of N-heterocyclic carbene (NHC) adducts of magnesium amidoboranes is presented, which involves a rare example of reversible migratory insertion within a normal valent s-block element. The reaction of (NHC)Mg(N(SiMe3)(2))(2) (1) and dimethylamine borane yields the tris(amide) adduct (NHC-BN)Mg(NMe2BH3)(N(SiMe3)(2)) (2; NHC-BN = NHC-BH2NMe2). In addition to Me2N=BH2 capture at the C-NHC-Mg bond, mechanistic investigations suggest the likelihood of aminoborane migratory insertion from an RMg(NMe2BH2NMe2BH3) intermediate. To elucidate these processes, the carbene complexes (NHC)Mg(NMe2BH3)(2) (8) and (NHC)Mg(NMe2BH2NMe2BH3)(2) (9) were synthesized, and a dynamic migration of Me2N=BH2 between Mg-N and C-NHC-Mg bonds was observed in 9. This unusual reversible migratory insertion is presumably induced by dissimilar charge localization in the (-){NMe2BH2NMe2BH3} anion, as well as the capacity of NHCs to reversibly capture Me2N=BH2 in the presence of Lewis acidic magnesium species.