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

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A rare case of dynamic migratory insertion of unsaturated species in normal valent s-block chemistry has been observed at magnesium bis(amidoboranes). The process is facilitated by N-heterocyclic carbenes, and is fundamentally a dynamic acid/base exchange between aminoborane (Me2N=BH2) and Lewis acidic magnesium species.


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(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 NHCC−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 NHCC−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.

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