Activation of hydrocarbons

Examples of oxidative additions of alkanes to two metal centers are rare but can occur by a process that parallels the addition of Hj to two metal centers that was shown in Equations 6.10a-6.10c. Wayland showed that the Rh(II) complex containing tetramesityl porphyrin cleaves the C-H bond of methane by a termolecular process to generate a 

Addition of H-H and C-H Bonds to Transition Metal Complexes Without Oxidation and Reduction 

Transition metal complexes containing metal centers with d° electron counts cannot undergo oxidative addition, but they can react with dihydrogen and hydrocarbons by alternative reaction pathways. The most common of these alternative pathways include o-bond metatheses of (P metal-alkyl and -aryl complexes and 2+2 reactions of metal carbene and imido compexes with substrates containing H-H, Si-H, and and C-H bonds. 

Like oxidative addition, o-bond metathesis occurs by a multi-step sequence that requires a site of unsaturation and a complex possessing 16 or fewer valence electrons 

The relative reactivities of dihydrogen, arenes, and alkanes toward a-bond metathesis parallels the relative reactivities of these reagents toward oxidative addition. a-Bond metathesis between dihydrogen and zirconocene alkyl complexes occurs (Equation 6.49), - but metathesis between alkanes or arenes and these complexes does not. Examples of a-bond metatheses with arenes are less common than those ivith dihydrogen, but they are more common and occur faster than those with alkanes. For example, Cp ScMe reacts with benzene to form Cp ScPh (Equation 6.50) faster than Cp ScMe reacts with labeled methane. 

The formation of compounds containing crM-C bonds was observed during activation of the C - H bonds by means of transition metal complexes. 

Alkanes may be activated by superacids or complexes of the transition metals. The superacid reactions lead to the formation of cations possessing a five-coordinate carbon atom  

Mlany other carbonium cations such as (CH) + (n = 5,6,1 x=, 2, 3) possessing cluster structures are also known (Chapter 3). Reactions of hydrocarbons with superacids lead to the H-D exchange. 

The C—H bond activation may occur according to various mechanisms. - One of them is a radical mechanism. In the 

As a result of the instability of metal compounds, it is very often impossible to prove which mechanism of the C —H bond activation operates, since R radicals are formed in all cases. 

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The developments which led to the present day concepts of the metabolic activation of hydrocarbons did not arise from the classical approach of identifying metabolites of greater biological potency than the parent compound, but from an approach dependent upon the assumption (or presumption) that the interaction of carcinogens with DNA is a key event in the initiation of the carcinogenic process. Brookes and Lawley (49) found in 1964 that when radioactive hydrocarbons are applied to the skin of mice, they become covalently bound to the DNA of the skin. Moreover, the extents of binding to DNA for various hydrocarbons followed fairly closely their relative carcinogenic activities. 

FIGURE 11.7 Molar 14C activity of hydrocarbons vs. carbon number of co-feeding experiments with ethanol, labeled in 1-position, 14C activity of wax (average carbon number 26) = 2,200. ... 

Walker, J.D., Colwell, R.R. (1976) Measuring the potential activity of hydrocarbon-degrading bacteria. Appl. Environ. Microbiol. 31, 187-197. 

The activation of hydrocarbons on the catalyst surface was also discussed in the literature [255]. There are no clear experimental evidences of this activation with free radical generation [270]. However, examples of dimer (RR) formation as a result of oxidation of RH on the surface of Mn02 are known [270],... 

Walker, J. D. Colwell, R. R. (1976a). Measuring potential activity of hydrocarbon degrading bacteria. Applied and Environmental Microbiology, 31, 189-97. 

Performing plasma processes in a continuous-flow microreactor leads to precise control of residence time and to extreme quenching conditions, therewith enabling control over the composition of the reaction mixture and product selectivity. In a nonequilibrium microplasma reactor, low-temperature activation of hydrocarbons and fuels, which is difficult to obtain in conventional thermochemical processes, can be achieved at ambient conditions. 

This section is concerned with the activation of hydrocarbon molecules by coordination to noble metals, particularly palladium.504-513 An important landmark in the development of homogeneous oxidative catalysis by noble metal complexes was the discovery in 1959 of the Wacker process for the conversion of ethylene to acetaldehyde (see below). The success of the Wacker process provided a great stimulus for further studies of the reactions of noble metal complexes, which were found to be extremely versatile in their ability to catalyze homogeneous liquid phase reaction. The following reactions of olefins, for example, are catalyzed by noble metals hydrogenation, hydroformylation, oligomerization and polymerization, hydration, and oxidation. 

Higher-valent manganese porphyrins have been of interest since they have been shown to be important intermediates in the activation of hydrocarbons under mild conditions (Section 41.4.8). Their use as sensitizers for the photooxidation of water to oxygen has also provided a motivation,479 although the involvement of a manganese porphyrin in the natural system has not been fully documented. 

Direct C-H activation of hydrocarbon by means of transition metals has also been explored. Cyclohexane reacted with Pd(OAc)2 in the presence of potassium persulfate-trifluoroacetic acid under CO pressure and produced the desired cyclo-hexanecarboxylic acid in low yields and TON (eq. (13)). The electrophilic carbox-ylation is explained by the change of Pd(OAc)2 to Pd(OCOCp3)2 in trifluoroacetic acid as solvent. Electrophilic attack on a C-H bond should give an alkyl Pd complex. CO insertion followed by reductive elimination affords a reactive mixed anhydride which was detected before workup. 

Relation Between Ag Cluster and Oxidative Activation of Hydrocarbons... 

The discussion of reactivity focused on the activation of hydrocarbons by zeolitic protons. The deprotonation energy of a proton is weakly dependent on the zeolite crystallographic position but may be strongly zeolite composition dependent, especially at high concentrations of three valent cations (Al, Ga) in the zeolite framework. Nonetheless, the deprotonation energy is a local property of the OH bond, which can be estimated using quantum-chemical calculations by extrapolation from properly terminated cluster calculations. 

Table I. Carbon-Hydrogen Activation of Hydrocarbons Using Compounds 1 and 2 as Catalysts and Iodosylbenzene as the Monooxygen Transfer Agent4...

The involvement of the triplet state of hydrocarbons is a common feature of spin uncoupling in activation of hydrocarbons by transition metal species. This does, however, not mean that the triplet state of the hydrocarbon is involved in the reaction mechanism at the real kinetic stage (this is close to the case of alkene adsorption on cupper surfaces [54]). Instead it represents a way of analysis of the deformation of the wave function during the catalytic reaction in terms of VB structures. 

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