How to Decarbonize N-Heterocyclic Carbenes (NHCs): The simple Alane Adducts (NHC)∙AlR3 (R = H, Me, Et).

The reaction of the amine-stabilized alane (NMe3)∙AlH3 1 with the backbone-saturated N-heterocyclic carbene (NHC) SIDipp (SIDipp = 1,3-bis-{2,6-di-iso-propyl-phenyl}-imidazolidin-2-ylidene) at 0°C yielded the NHC alane adduct (SIDipp)∙AlH3 2. Reaction at elevated temperatures or prolonged reaction at room temperature gave the product of a ring expansion reaction (RER) of the NHC, 3∙(NMe3). Subsequent reaction of the latter with sterically less hindered NHCs (IMeMe {= 1,3,4,5-tetramethyl-imidazolin-2-ylidene}, IiPrMe {= 1,3-di-iso-propyl-4,5-dimethyl-imidazolin-2-ylidene}, and IiPr {= 1,3-di-iso-propyl-imidazolin-2-ylidene}) afforded the NHC-stabilized RER-products (NHC)∙AlH(RER-SIDippH2) 3∙(NHC) (NHC = IMeMe, IiPrMe, IiPr), while no reaction was observed with the sterically more demanding NHCs IDipp (= 1,3-bis-{2,6-di-iso-propyl-phenyl}-imidazolin-2-ylidene), SIDipp and ItBu (= 1,3-di-tert-butyl-imidazolin-2-ylidene). The compounds 3∙(NHC) were also obtained starting from (SIDipp)∙AlH3 2 and NHC at room temperature. Heating solutions of (SIDipp)∙AlH3 2 without additional base to 95°C resulted in decarbonization of the NHC and substitution of the carbene carbon atom with aluminum hydride under loss of ethene. Subsequent dimerization afforded cis-[AlH{m-N(Dipp)CH2CH2N(Dipp)}]2 4_dimer. Heating solutions of the NHC-ligated aluminum alkyls (SIDipp)∙AlR3 2R (R = Me, Et) to 145°C instead led to complete scission of the NHC backbone with evolution of ethene and isolation of the dialkylaluminium(III) amidinates {DippNC(R)NDipp}AlR2 5R (R = Me, Et).