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Carbon–hydrogen bond activation
Carbon–hydrogen bond activation (C–H activation), or C-H functionalization, is a type of reaction in which a carbon–hydrogen bond is cleaved and replaced with a carbon-X bond (where X is usually carbon, oxygen, or nitrogen).〔“Alkane C–H activation and functionalization with homogeneous transition metal catalysts: a century of progress – a new millennium in prospect”, R. H. Crabtree, J. Chem. Soc., Dalton Trans. 2001, 17, 2437–2450. 〕〔''"Designing Catalysts for Functionalization of Unactivated C–H Bonds Based on the CH Activation Reaction"'' B. G. Hashiguchi, S. M. Bischof, M. M. Konnick, R. A. Periana. ''Acc. Chem. Res.'' 2012, 45, 885–898. 〕〔“Organometallic alkane CH activation”, R. H. Crabtree, J. Organomet. Chem. 2004, 689, 4083–4091. 〕〔“Mechanistic Aspects of C−H Activation by Pt Complexes”, M. Lersch, M.Tilset, Chem. Rev. 2005, 105, 2471–2526. 〕〔“Recent Advances in the Platinum-mediated CH Bond Functionalization”, A. N. Vedernikov, Curr. Org. Chem. 2007, 11, 1401–1416.〕〔“Catalytic C–H functionalization by metalcarbenoid and nitrenoid insertion”, H. M. L. Davies, J. R. Manning, Nature, 2008, 451, 417–424, 〕〔"Mechanisms of C–H bond activation: rich synergy between computation and experiment”, Y. Boutadla, D. L. Davies, S. A. Macgregor, A. I. Poblador-Bahamonde, Dalton Trans. 2009, 5820–5831. 〕〔“C–H Bond Activation in Transition Metal Species from a Computational Perspective”, D. Balcells, E. Clot, O. Eisenstein, Chem. Rev. 2010, 110, 749–823. 〕〔“Palladium-Catalyzed Ligand-Directed C–H Functionalization Reactions”, T. W. Lyons, M. S. Sanford, Chem. Rev. 2010, 110, 1147–1169. 〕〔“Beyond Directing Groups: Transition Metal-Catalyzed C H Activation of Simple Arenes”, N. Kuhl, M. N. Hopkinson, J. Wencel-Delord, F. Glorius, Angew. Chem. Int. Ed. 2012, 51, 10236–10254. 〕〔“Selectivity enhancement in functionalization of C–H bonds: A review”, G. B. Shul’pin, Org. Biomol. Chem. 2010, 8, 4217–4228. 〕 The term is often restricted to reactions that involve organometallic complexes and transformations that proceed by coordination of a hydrocarbon to the inner-sphere of a metal, either via an intermediate “alkane or arene complex” or as a transition state leading to a "M−C" intermediate.〔''Organometallic C–H Bond Activation: An Introduction'' Alan S. Goldman and Karen I. Goldberg ACS Symposium Series 885, Activation and Functionalization of C–H Bonds, 2004, 1–43〕〔Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. “Selective Intermolecular Carbon–Hydrogen Bond Activation by Synthetic Metal Complexes in Homogeneous Solution.” Accounts of Chemical Research, 1995: 28 (3) 154–162.〕〔Periana, R. A.; Bhalla, G.; Tenn, W. J., III, Young, K. J. H.; Liu, X. Y.; Mironov, O.; Jones, C.; Ziatdinov, V. R. “Perspectives on some challenges and approaches for developing the next generation of selective, low temperature, oxidation catalysts for alkane hydroxylation based on the C–H activation reaction.” Journal of Molecular Catalysis A: Chemical, 2004: 220 (1) 7–25. 〕 Important to this definition is the requirement that during the C–H cleavage event, the hydrocarbyl species remains associated in the inner-sphere and under the influence of “M”.
Theoretical studies as well as experimental investigations indicate that C–H bonds, which are traditionally considered unreactive, can be cleaved by coordination. Much research has been devoted to the design and synthesis of new reagents and catalysts that can affect C–H activation. C-H activation chemistry has the potential to transform the chemical world through the development of novel synthetic methods. C-H activation could enable the conversion of cheap and abundant alkanes into valuable functionalized organic compounds and the efficient structural editing of already complex molecules (i.e. natural product synthesis).〔“C–H bond activation enables the rapid construction and late-stage diversification of functional molecules”, J. Wencel-Delord, F. Glorius, Nature Chem. 2013, 5, 369–375. 〕 The abundance of C-H bonds in organic molecules provides precedent for developing methods to convert C-H bonds into useful C-X bonds. Research leads to novel disconnections, drastically shortening synthetic routes.〔Center for Selective C-H functionalization http://www.nsf-cchf.com/index.html〕 There are applications of C-H activation in various fields, including materials science and pharmaceuticals.〔
==Challenges==
Organic compounds often contain many carbon-hydrogen bonds. Hence, selective activation of a specific C-H bond poses a great challenge. The reactions must be regioselective and stereoselective. In addition, the reaction conditions need to be mild enough to tolerate additional functionality within the molecule.
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