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

Abstract 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 3 ) 2 ) 2 ( 1 ) and dimethylamine borane yields the tris(amide) adduct (NHC−BN)Mg(NMe 2 BH 3 )(N(SiMe 3 ) 2 ) ( 2 ; NHC−BN = NHC−BH 2 NMe 2 ). In addition to Me 2 N=BH 2 capture at the NHC C−Mg bond, mechanistic investigations suggest the likelihood of aminoborane migratory insertion from an RMg(NMe 2 BH 2 NMe 2 BH 3 ) intermediate. To elucidate these processes, the carbene complexes (NHC)Mg(NMe 2 BH 3 ) 2 ( 8 ) and (NHC)Mg(NMe 2 BH 2 NMe 2 BH 3 ) 2 ( 9 ) were synthesized, and a dynamic migration of Me 2 N=BH 2 between Mg−N and NHC C−Mg bonds was observed in 9 . This unusual reversible migratory insertion is presumably induced by dissimilar charge localization in the − {NMe 2 BH 2 NMe 2 BH 3 } anion, as well as the capacity of NHCs to reversibly capture Me 2 N=BH 2 in the presence of Lewis acidic magnesium species.