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

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• 作者：Gilles Schnee ; David Specklin ; Jean-Pierre Djukic ; Samuel Dagorne
• 刊名：Organometallics
• 出版年：2016
• 出版时间：June 13, 2016
• 年：2016
• 卷：35
• 期：11
• 页码：1726-1734
• 全文大小：459K
• 年卷期：0
• ISSN：1520-6041

文摘

The sterically bulky NHCs 1,3-di-tert-butylimidazol-2-ylidene (I^tBu), 1,3-di-tert-butylimidazolin-2-ylidene (S^tBu), 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 I^tBu-H⁺, S^tBu-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 (I^tBu)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 CH₂^2– 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)⁻.