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Alane-Centered Ring Expansion of N‑Heterocyclic Carbenes

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journal contribution
posted on 03.03.2017, 20:04 by Mathew D. Anker, Annie L. Colebatch, Kalon J. Iversen, David J. D. Wilson, Jason L. Dutton, Lucía García, Michael S. Hill, David J. Liptrot, Mary F. Mahon
The β-diketiminato aluminum dihydrides, [HC­{(Me)­CNAr}2AlH2] [4: Ar = 2,6-di-isopropylphenyl (Dipp), 5: 2,4,6-trimethyl­phenyl (Mes)] react directly with N-aryl-substituted N-heterocyclic carbenes (NHCs) by C–N bond activation to afford aluminum amido-alkyl derivatives of the form [HC­{(Me)­CNAr}2AlCH2(N­(Ar′)­CH)2]. The more sterically congested alane (4), bearing N-Dipp substitution, does not react with either 1,3-bis­(2,6-di-isopropylphenyl)­imidazol-2-ylidene (IPr) or 1,3-bis­(2,4,6-trimethyl­phenyl)­imidazol-2-ylidene (IMes), even under forcing conditions. In contrast, in situ generation of 1,3-bis­(phenyl)­imidazol-2-ylidene through deprotonation of the corresponding imidazolium tetrafluoroborate by KN­(SiMe3)2 in the presence of compound 4 provides facile ring opening of the NHC at room temperature to yield [HC­{(Me)­CNDipp}2AlCH2(N­(Ph)­CH)2]. Although compound 5 also does not react with IPr, relaxation of the steric demands of the supporting β-diketiminate ligand to N-mesityl substitution enables analogous ring opening of IMes, with the formation of [HC­{(Me)­CNMes}2AlCH2(N­(Mes)­CH)2] (7), when the reaction is heated to 80 °C. DFT calculations performed on model systems suggest that in comparison to the parent alane (AlH3) the enhanced propensity of these systems to induce NHC ring cleavage is a consequence of the relative stability of the initially formed five- and four-coordinate adducts as well as the augmented hydridic character of the Al–H bonds within the β-diketiminate-supported molecules.