CH activation

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The CH activation can be defined as a reaction that breaks a carbon - hydrogen bond. The term is mostly used for reactions that take place via organometallic complexes with coordination of the hydrocarbon to a metal, for example as an alkane or arene complex .

Both theoretical considerations and experimental studies show that CH bonds can be cleaved under coordination to a metal. The development of new catalysts for CH activation is in the foreground of the research, since it could be possible to convert simple alkanes into valuable functionalized intermediates in a targeted manner and also to efficiently structurally edit already complex molecules. In recent times, organocatalysis has also been used, which can be used without the use of metals and thus, under certain circumstances, more cost-effectively.

history

The first CH activation is attributed to Otto Dimroth , who in 1902 described the reaction of benzene with mercury (II) acetate . The first true CH activation was described by Joseph Chatt in 1965, an insertion of a ruthenium complex into the CH bond of naphthalene . In 1966, Alexander Schilow described that sodium tetrachloroplatinate induces the isotope exchange between methane and heavy water . The binding of methane to the Pt (II) complex has been suggested as a reaction pathway. In 1972 Schilow's group succeeded in producing methanol and methyl chloride in a similar reaction . The so-called Schilow system is the only known system to date that can functionalize alkanes under mild conditions.

Catalytic CH functionalization

Various concepts are used for the selective activation of a specific CH bond: Directing groups coordinate to the metal and bring it close to the corresponding CH bond. Heteroaromatics have an inherent reactivity of the various CH bonds (e.g. 2-position in benzofuran ). Furthermore, heteroatoms can increase the acidity of neighboring CH bonds.

In functionalization, a distinction is made between CH oxidations, in which a CH bond is converted into a C heteroatom bond, and coupling reactions. In the case of the latter, products are obtained analogously to cross-couplings . However, here the prefunctionalized organometallic substrate can be replaced by the unfunctionalized compound. Instead of the transmetallation step , the CH activation takes place here accordingly. In the dehydrogenating coupling, the halogenated coupling partner can also be unfunctionalized by adding an oxidizing agent.

The catalysts are typically based on transition metal complexes. As in cross-coupling, palladium is the most commonly used metal. Rhodium or ruthenium can also often be used.

Web links

Individual evidence

  1. ^ A b Organometallic CH Bond Activation: An Introduction Alan S. Goldman and Karen I. Goldberg ACS Symposium Series 885, Activation and Functionalization of CH Bonds, 2004 , 1-43.
  2. ↑ Arndtsen, BA; Bergman, RG; Mobley, TA; Peterson, TH "Selective Intermolecular Carbon-Hydrogen Bond Activation by Synthetic Metal Complexes in Homogeneous Solution." Accounts of Chemical Research , 1995: 28 (3) 154-162.
  3. ↑ Periana, RA; Bhalla, G .; Tenn, WJ, III, Young, KJH; Liu, XY; Mironov, O .; Jones, C .; Ziatdinov, VR "Perspectives on some challenges and approaches for developing the next generation of selective, low temperature, oxidation catalysts for alkane hydroxylation based on the CH activation reaction." Journal of Molecular Catalysis A: Chemical , 2004: 220 (1) 7-25, doi : 10.1016 / j.molcata.2004.05.036 .
  4. ↑ “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, doi : 10.1038 / nchem.1607 .
  5. ↑ Pan, SC “Organocatalytic C – H activation reactions” Beilstein Journal of Organic Chemistry 2012 , 8 , 1374–1384, doi : 10.3762 / bjoc.8.159 (Open Access).
  6. ↑ The tautomerism of arene and ditertiary phosphine complexes of ruthenium (0), and the preparation of new types of hydrido-complexes of ruthenium (II) J. Chatt and JM Davidson, J. Chem. Soc. 1965 , 843, doi : 10.1039 / JR9650000843 .