Activating Unactivated Carbon-Hydrogen Bonds

Alkanes are abundant components of petroleum but normally react only under tbe most vigorous conditions (combustion free radicals sucb as Cl and HO). If tbeir C-H bonds could be activated under mild conditions, especially if this could be done catalytically, then it would be feasible to convert alkanes from mere fuel to fine chemicals that might be used and recycled. This is particularly desirable for methane (CH4), a by-product of anaerobic bacteria and a potential source of methanol (CH3OH). Nature routinely activates C-H bonds on saturated 4-coordinate carbon when enzymes called oxygenases employ cytochrome-P450 to catalyze conversions to alcohols  

Just as the stable Vaska compound cannot function as a catalyst, so too did the stability of the Bergman complex described here preclude its use in a catalytic sense. However, Wilkinson s catalyst (see chapter 7) is labile and discoveries of various catalytic activators of C-H bonds followed in short order. 

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C. L. Hill, in C. L. Hill (Ed.), Activation and Functionalization of Alkanes, Catalytic Oxygenation of unactivated carbon-hydrogen bonds-. Superior oxo transfer catalysts and the inorganic metallopor-phyrin, Wiley, New York, 1989, p. 243. 

A combination of spectroscopies including Mossbauer and resonance Raman were used to identify the presence of a diiron-oxo cluster with properties similar to those identified in ribonucleotide reductase (RB2) and methane monooxygenase (MMO). These enzymes all share the ability to break unactivated carbon-hydrogen bonds with a nonheme diiron cluster cofactor. Fatty acid desaturation and methane oxidation require a two-electron reduction of the diiron cluster to initiate the oxygen activation reaction. Identification of a diiron cluster in the desaturase allowed us to propose a consensus diiron-oxo binding motif consisting of two repeats of (D/E)EXXH. 

As shown in the manganese- and ruthenium-catalyzed intermolecular nitrene insertions, most of these results supposed the transfer of a nitrene group from iminoiodanes of formula PhI=NR to substrates that contain a somewhat activated carbon-hydrogen bond (Scheme 14). Allylic or benzylic C-H bonds, C-H bonds a to oxygen, and very recently, Q spz)-Y bonds of heterocycles have been the preferred reaction sites for the above catalytic systems, whereas very few examples of the tosylamidation of unactivated C-H bonds have been reported to date. 

The C-H activation reaction is a reaction that cleaves a caihon-hydrogen bond. Here the carbon-hydrogen bond is mostly referred to unactivated caibon-hydrogen bonds. 

On the other hand, the direct arylation of carbanionic species generated from substrates having relatively acidic hydrogens such as active methylene compounds and ketones can occur (mechanism B) [5,6]. Aryl halides are also capable of coupling directly with appropriately functionalized aromatic substrates and five-membered heteroaromatic compounds as formal carbon nucleophiles via cleavage of their unactivated C-H bonds [5,7-9]. The Fujiwra-Moritani reaction, which is the arylation of alkenes with arenes, is also useful for preparing arylalkenes without employing any halides (mechanism D) [10,11]. 

Other Reductions. The (porphinato)irons could realize the reduction of alkenes and alkynes with NaBILj. Various unsaturated carbon-carbon bonds were saturated by meso-tetraphenylporphinatoiron chloride (TPPFe Cl) derivatives (up to 81% yield). Ruthenium(III) complexes also pair with NaBH in the reduction of unsaturated carbon-carbon bonds (as does cobalt boride). In the presence of a catalytic amount of Ru(PPh3)4H2 (0.5-1 mol %) and NaBHj, unsaturated carbon-carbon bonds in a wide variety of alkenes and alkynes were saturated in toluene at 100 Addition of water was required to provide a proton source. Similar systems with RUCI3 in aqueous solution reduce unsaturated bonds under milder conditions. Various unactivated mono- or disubstituted olefins and activated trisubstituted olefins were reduced with RUCI3 (10 mol %) and NaBH4 in THF-H2O at 0 °C to room temperature (eq 36). When the RuCl3-catalyzed reductions of olefins were carried out in aqueous amide solution, unactivated trisubstituted olefins were also hydrogenated. ... 

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