Carbon–hydrogen bond activation

CH - activation can be defined as a reaction that cleaves a carbon- hydrogen bond. The term is usually used for reactions that take place on organometallic complexes under the coordination of the hydrocarbon to a metal, such as alkane or arene complex.

Both theoretical considerations and experimental studies show that C-H bonds can be cleaved under coordination to a metal. The development of new catalysts for CH activation is at the forefront of research, since it might be possible to selectively convert simple alkanes into valuable functionalized intermediates and also as well as an already complex molecules efficiently to edit structurally. While more recently organocatalysis is used, which can be used more cost-effective without the use of metals, and thus in certain circumstances.

History

The first activation is CH Otto Dimroth attributed the reaction of benzene with mercury (II) - acetate described in 1902. The first true C-H activation has been described by Joseph Chatt in 1965, an insertion of a ruthenium complex in the CH bond of naphthalene. In 1966, Alexander Shilov described that Natriumtetrachloroplatinat induced isotope exchange between methane and heavy water. Pathway as the bonding of the methane to the Pt ( II) complex has been proposed. 1972 succeeded Schilows group to produce a similar reaction of methanol and methylene chloride. The so-called Shilov system is the only one known to today's system, which can be functionalized alkanes under mild condition.

Catalytic C -H Functionalization

For the selective activation of a specific C-H bond, different concepts are used: directing groups coordinate to the metal and thus bring it to the vicinity of the corresponding C-H bond. Heteroaromatics have an inherent reactivity of the various C-H bonds (such as 2- position on the benzofuran ). Furthermore, heteroatoms can increase the acidity of adjacent CH bonds.

In the functionalization are divided into CH oxidations, in which a C-H bond in a C - heteroatom bond is transferred and coupling reactions. In the latter products are obtained analogously to cross-coupling. However, may be replaced by the non-functionalized compound here präfunktionalisierte organometallic substrate. In place of the transmetalation found here according to the CH activation takes. In the dehydrogenative coupling may be unfunctionalized by adding an oxidizing agent in addition to the halogenated coupling Spartans.

The catalysts are typically based on transition metal complexes. As in cross-coupling reactions, palladium is the most widely used metal. Rhodium or ruthenium may also be used frequently.