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

Abstract

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).