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) & sdot; AlH(3)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 degrees C yielded the NHC alane adduct (SIDipp) & sdot; AlH(3)2. Reaction at elevated temperatures or prolonged reaction at room temperature gave the product of a ring expansion reaction (RER) of the NHC, (NMe3) & sdot; AlH(RER-SIDippH(2)) 3 & sdot; (NMe3). Subsequent reaction of the latter with sterically less hindered NHCs (IMeMe {=1,3,4,5-tetramethyl-imidazolin-2-ylidene}, IiPr(Me) {=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) & sdot; AlH(RER-SIDippH(2)) 3 & sdot; (NHC) (NHC=IMeMe, IiPr(Me), 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 & sdot; (NHC) were also obtained starting from (SIDipp) & sdot; AlH(3)2 and NHC at room temperature. Heating solutions of (SIDipp) & sdot; AlH(3)2 without additional base to 95 degrees 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{mu-N(Dipp)CH2CH2N(Dipp)}](2)4_dimer. Heating solutions of the NHC-ligated aluminum alkyls (SIDipp) & sdot; AlR(3)2 (R) (R=Me, Et) to 145 degrees C instead led to complete scission of the NHC backbone with evolution of ethene and isolation of the dialkylaluminium(III) amidinates {DippNC(R)NDipp}AlR(2)5 (R) (R=Me, Et).