C-H bond activation

Carbon-hydrogen bond activation or CH activation may be defined as a facile carbon hydrogen cleavage reaction with an organometallic “MX” species that proceeds by coordination of a hydrocarbon to the inner-sphere of “M” (either via an intermediate “alkane or arene complex” or a transition state) leading to a M-C intermediate. Important to this definition is the requirement that during the CH cleavage event the hydrocarbyl species remains associated in the inner-sphere and under the influence of “M”.

Theoretical studies as well as experimental investigations support this view that classically unreactive CH bonds can be cleaved by such inner-sphere mechanisms. This emphasis on inner-sphere coordination is based on the presumption that cleavage reactions of the CH bond that proceed in this manner, with strong interaction between the CH bond and “M”, can be expected to show unique high selectivity.

Much effort is spent in academic research into the design and synthesis of new catalysts that can bring about CH activation. A big driving force for this type of research is that it enables the conversion of cheap and abundant alkanes into valuable organic compounds with specific desired functional groups. Many time-tested alternative methods exists but they rely on reactive intermediates such as free radicals and carbenes that lack regioselectivity.

Historic overview
The first CH activation reaction is often attributed to Otto Dimroth who in 1902 reacted benzene with Mercury(II) acetate (See: organomercury chemistry) but there is disagreement if this truly is CH activation . In 1955 Jack Halpern demonstrated the electrophilic activation of dihydrogen (in terms of bond strength a H-H bond is comparable to a C-H bond) in water by Copper(II) acetate to the CuH+ salt and a proton. In 1962, Vaska added dihydrogen to what is now known as Vaska's complex in a oxidative addition and in 1965 reactions with Wilkinson's catalyst were found to contain a reductive elimination of a C-H bond. As observed Goldman & Goldberg CH activation just like HH activation can be achieved by electrophilic or oxidative activation. The first true CH activation reaction was reported by Joseph Chatt in 1965 with insertion of a ruthenium atom ligated to dmpe in the C-H bond of naphtalene. Then in 1966 A.E. Shilov discovered that potassium tetrachloroplatinate induced isotope scrambling between methane and heavy water with the reaction of Pt(II) with a C-H bond taken as reversal of reductive elimination. In 1972 he was able to produce methanol and methyl chloride in a similar reaction of stoichiometric potassium tetrachloroplatinate, catalytic potassium hexachloroplatinate, methane and water. As Shilov worked and published in Cold War Soviet Union his work was largely ignored by Western scientists. This so-called Shilov system is today one of the few true catalytic systems for mild alkane functionalizations.

On the other side of the spectrum, oxidative addition, Malcolm Green in 1970 reported on the photochemical insertion of tungsten (as a Cp2WH2 complex) in a benzene C-H bond and George M. Whitesides in 1979 was the first to carry out an intramolecular aliphatic C-H activation

The next breakthrough was reported independently by two research groups 1982, by Robert George Bergman with the first photochemical CH activation of completely saturated hydrocarbons cyclohexane and neopentane forming the hydridoalkylmetal complex Cp*Ir(PMe3)H(C6H5) where Cp* is a pentamethylcyclopentadienyl ligand


 * [[Image:CHactivationBergman1982.svg|600px|CH activation Bergman 1982]]

and by W.A.G. Graham who reacted the same hydrocarbons with Cp*Ir(CO)2 also to the iridiumhydrido complex


 * [[Image:CHactivationGraham1982.svg|600px|CH activation Graham 1982]]

Scope
In one study the alkane pentane is selectively converted to the halocarbon 1-iodopentane with the aid of a tungsten complex.


 * [[Image:PentaneActivation.png|500px|C-H activation of pentane, as seen in Ledgzdins et al., J. Am. Chem. Soc. 2007, 129, 5372-3.]]

The tungsten complex is fitted with a pentamethylcyclopentadienyl, a nitrosyl, a 3η 1-butene and a neopentanyl CH2C(CH3)3 ligand. It is thermally unstable and when dissolved in pentane at room temperature it loses neopentane (gains a proton) and coordinates with a pentane ligand (loses a proton). This proton exchange proceeds via a 16 electron intermediate with a butadiene ligand after beta elimination. In a separate step iodine is added at -60°C and 1-iodopentane is released.

Arene C-H bonds can also be activated by metal complexes despite being fairly unreactive. One manifestation is found in the Murai olefin coupling. In one reaction a ruthenium complex reacts with N,N-dimethylbenzylamine in a cyclometalation also involving CH activation :


 * [[Image:Chactivationbyruthenium.png|400px|Cyclometallation with a Substituted Benzylamine Chetcuti 2007]]

An alkene C-H bond activation with a rhodium catalyst is demonstrated in the synthesis of this strained bicyclic enamine :


 * [[Image:BergMannCHActivation2008.svg|500px|C-H bond activation Yotphan 2008]]