CH 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 ^{[1] [2]}. 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”.

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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. In one study ^[3] the alkane pentane is selectively converted to the halocarbon 1-iodopentane with the aid of a tungsten complex.

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 ^[4]:

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