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(SiMe ) ) (1) and dimethylamine borane yields the tris(amide) adduct (NHC-BN)Mg(NMe BH )(N(SiMe ) ) (2; NHC-BN = NHC-BH NMe ). In addition to Me N=BH capture at the C-Mg bond, mechanistic investigations suggest the likelihood of aminoborane migratory insertion from an RMg(NMe BH NMe BH ) intermediate. To elucidate these processes, the carbene complexes (NHC)Mg(NMe BH ) (8) and (NHC)Mg(NMe BH NMe BH ) (9) were synthesized, and a dynamic migration of Me N=BH between Mg-N and C-Mg bonds was observed in 9. This unusual reversible migratory insertion is presumably induced by dissimilar charge localization in the {NMe BH NMe BH } anion, as well as the capacity of NHCs to reversibly capture Me N=BH in the presence of Lewis acidic magnesium species.