Activation of an alkyl C-H bond geminal to an agostic interaction: an unusual mode of base-induced C-H activation

L Jonas L Häller; Michael J Page; Stuart A Macgregor; Mary F Mahon; Michael K Whittlesey

doi:10.1021/ja900953d

Activation of an alkyl C-H bond geminal to an agostic interaction: an unusual mode of base-induced C-H activation

J Am Chem Soc. 2009 Apr 8;131(13):4604-5. doi: 10.1021/ja900953d.

Authors

L Jonas L Häller¹, Michael J Page, Stuart A Macgregor, Mary F Mahon, Michael K Whittlesey

Affiliation

¹ School of Engineering and Physical Sciences, Perkin Building, Heriot-Watt University, Edinburgh, EH14 4AS, UK.

PMID: 19281174
DOI: 10.1021/ja900953d

Abstract

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 (i)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 DeltaE (double dagger)(calcd) = +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.