Deprotonation of Al2Me6 by Sterically Bulky NHCs: Scope, Rationale through DFT Studies, and Application in the Methylenation of Carbonyl Substrates

Abstract

The sterically bulky NHCs 1,3-di-tert-butylimidazol-2-ylidene (ItBu), 1,3-di-tert-butylimidazolin-2-ylidene (StBu), and 1,3-di-tert-butyl-3,4,5,6-tetrahydripyrimidin-2-ylidene (C6-tBu) were found to readily react with excess Al2Me6 at room temperature to form salts 2, 3, and 4, respectively, consisting of the polynuclear Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)- anion associated with either the cation ItBu-H+, StBu-H+, or C6-tBu-H+. Such a reaction involving the deprotonation of an Al2Me6 moiety by a NHC does not proceed with less sterically hindered NHCs such as 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IDipp) and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), for which only the classical Lewis pair adducts (NHC)AlMe3 were isolated. In line with experimental and density functional theory (DFT) calculations data, such reactivity thus appears to be driven by steric frustration, resulting in the destabilization of the corresponding (NHC)AlMe3 adducts, then more prone to dissociate and hence allowing Al2Me6 activation/deprotonation. The DFT-estimated profile of the reaction of model adduct (ItBu)AlMe3 (I) with Al2Me6 agrees with a ready adduct dissociation at low energy cost, with a subsequent deprotonation of Al2Me6 by the NHC fragment to afford model salt II (isostructural to salt 2). Anion Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)- behaves as an efficient CH22- group transfer agent with the methylenation of aldehydes and ketones to afford the corresponding methylene organics in good conversions. Bonding analysis of the latter anion agrees with an enhanced nucleophilicity of the Al-CH2 moiety (compared to the Al-Me groups), in line with the observed reactivity. DFT calculations also allowed a detailed bonding description of the pentacoordinate methylene carbon in the anion Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)-.