Activation of an Alkyl C-H Bond Geminal to an Agostic Interaction: An Unusual Mode of Base-Induced C-H Activation
Activation of an Alkyl C-H Bond Geminal to an Agostic Interaction: An Unusual Mode of Base-Induced C-H Activation
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DOI：
10.1021/ja900953d
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发表时间：
2009-04-08
影响因子：
15
通讯作者：
Whittlesey, Michael K.
中科院分区：
文献类型：
--
作者：
Haller, L. Jonas L.;Page, Michael J.;Whittlesey, Michael K.
氘标记的研究表明，碱诱导的分子内C-H激活的agostic配合物2-D的收益与排他性的质子从一个'Pr取代基的N-杂环卡宾（NHC）配体的甲基臂。计算研究表明，这种烷基C-H键活化反应涉及的C-H键，是geminal的agostic相互作用之一，而不是agostic C-H键本身的去质子化。该反应在室温下容易进行，当NHC 1，3-二异丙基-4，5-二甲基咪唑-2-亚基用作外碱时，计算活化能垒Δ E-计算双匕首=+11.8 kcal/mol。电荷分析表明，孪位氢实际上比正质子酸性更强，这与它们更容易去质子化一致。
Deuterium labeling studies indicate that base-induced intramolecular C-H activation in the agostic complex 2-D proceeds with exclusive removal of a proton from the methyl arm of an 'Pr substituent on the N-heterocyclic carbene (NHC) ligand. Computational studies show that this alkyl C-H bond activation reaction involves deprotonation of one of the C-H bonds that is geminal to the agostic interaction, rather than the agostic C-H bond itself. The reaction is readily accessible at room temperature, and a computed activation barrier of Delta E-calcd double dagger = + 11.8 kcal/mol is found when the NHC 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene is employed as the external base. Charge analysis reveals that the geminal hydrogens are in fact more acidic than the agostic proton, consistent with their more facile deprotonation.