Alane-Centered Ring Expansion of <i>N</i>-Heterocyclic Carbenes

Abstract

The β-diketiminato aluminum dihydrides, [HC{(Me)CNAr} 2 AlH 2 ] [ 4: Ar = 2,6-di- iso propylphenyl (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} 2 AlCH 2 (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- iso propylphenyl)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(SiMe 3 ) 2 in the presence of compound 4 provides facile ring opening of the NHC at room temperature to yield [HC{(Me)CNDipp} 2 AlCH 2 (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} 2 AlCH 2 (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 (AlH 3 ) 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.