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, (NMe₃) ⋅ AlH(RER-SIDippH₂) 3 ⋅ (NMe₃). Subsequent reaction of the latter with sterically less hindered NHCs (IMe^Me {=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) ⋅ AlH(RER-SIDippH₂) 3 ⋅ (NHC) (NHC=IMe^Me, 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 ⋅ (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{μ-N(Dipp)CH₂CH₂N(Dipp)}]₂ 4_dimer. Heating solutions of the NHC-ligated aluminum alkyls (SIDipp) ⋅ AlR₃ 2^R (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 5^R (R=Me, Et).

Keywords: Alanes; Aluminum; Decarbonization; N-heterocyclic Carbenes; Ring Expansion.