C—H Bonds Methane

A different picture is observed when a polar radical reacts with a C—H bond of a polar molecule. For example, the reaction of an oxygen atom with the methane C—H bond is characterized by the activation energy of thermoneutral reaction /ic0 54.6 kJ mol-1 and parameter bre= 13.11 (kJ mol-1)172 while the reaction with the methanol C—H bond is characterized by Ed) 50 kJ mol-1 and parameter brc 12.55 (kJ mol-1)172 [30]. For these values of bre, the difference between the activation energies is 4.6 kJ mol-1. The decrease in the activation energy can be explained by the fact that the polar O—H group in the O H C—OH transition state interacts with the O H C polar reaction center. 

In contrast to the radical processes described before, high selectivity is characteristic of electrophilic oxidation of methane.61 It reacts, for instance, with H2O2 in superacidic media to give methanol.1,62 The reaction is best explained by electrophilic insertion of the hydrogen peroxonium ion (H3Oj) into the methane C—H bond ... 

The basis of the high selectivity in this system, confirmed by both theoretical and experimental results, is that the active catalyst [XHg]+ reacts at least 1000 times faster with the C-H bonds of methane than with those of CH3OH, which exists primarily as the protonated, or sulfated forms, [CH3OH2]+ or CH30-S03H, respectively, in sulfuric acid. This greater reactivity of the methane C-H bonds compared with those of methanol can be traced to substantially lower reac-... 

For the activation of methane, SO4 /Zr02 was found to be active for the reaction of methane with ethylene to form C3, t-C4, 1-C5 [64] In Table 5, the product distribution is compared with that of the other solid superacid, TaFs/AEC [65] For the chlorination of methane, the selectivity of S04 /Zr02 for the formation of methyl chloride exceeds 90% with 30% of chlorine conversion at 473 K and is above 80% with 70-90% conversion at 439 K The selectivity is enhanced by adding Pt, while Fe203-Mn02-S04 /Zr02 exhibits excellent sclectivities (99-100%) at lower conversions [66] The electrophilic insertion is said to involve electron-deficient metal-coordinated chlorine at the methane C H bond... 

It was also found that insertion into a primary C-H bond was more facile than insertion into a secondary or tertiary C-H bond.This indicated that insertion into a methane C-H bond would be more favorable. Hoyano and Graham have photolyzed [Cp Ir(CO)2] in neopentane to obtain the neopentyl hydride with the loss of CO. Under these conditions, methane was also found to react at 8 atm pressure in perfluorohexane solution [Eq. (6.90)] ... 

Numerous theoretical studies have been performed at various levels of theory for different transition metals. A barrier for C-C insertion was found to be 14-20 kcal mol higher than the barrier for C-H insertion. The lowest barrier for C-H insertion was found to be for the rhodium atom. Studies by Bickelhaupt et al. with Pd(0) favor a 3c-2e transition state in contrast to the SN2-type transformation. - - A recent study by de Almeida and Cesar has predicted that thorium inserts into the methane C-H bond in an essentially barrierless and considerably exergonic reaction (-38 kcal mol ). 

Figiire 6.8. Insertion of HCIC into the methane C-H bond, (a) c Approach and the corresponding transition state 143 (b) ji approach and the corresponding secondary saddle point structure 144. 

The activation parameters (TMP A// +7kcalmoH A5 —39 cal. K-. mor TXP A// +17kcalmor A5 -25cal.K .mor, the rate law (rate = fc[(por)Rh ] [CH4]) and large kinetic isotope effects (fcn/fe >8) derived from kinetic studies of the reactions in Fig. 58 (166, 167), are consistent with the proposed concerted four-centered pathway with a near linear transition state in Fig. 59(a). Similar kinetic smdies suggest that toluene and methane C—H bond cleavage reactions (and also H—H bond reactions of H2) proceed via related mechanisms (166, 168, 169). 

Cyclopentadienyl- and pentamethylcyclopentadienyl-Re complexes are capable of intermolecular C—H activation if appropriate electron-donating ligands are attached to the metal center. Primary, cyclopropyl and methane C—H bonds are attacked but the secondary and tertiary C—H bonds are not. The cleanest results are obtained with the ()7 -Cp)Re(PMe3)3 complex, where an adduct of cyclopropane is isolated as a stable complex by recrystallization. ... 

When an r -C-H coordinated adduct X(PH3)Ir "HCH3 is formed, considerable weakening of the coordinated methane C-H bond occurs [61a]. The calculated enthalpy for the reaction... 

Fig. 3 Activation of the methane C-H bond by metal oxide molecules MO, [43]...

Mechanistically, both reactions are electrophilic insertion reactions into the methane C—H bonds. In the platinum insertion reaction subsequent chlorolysis of the surface... 

We can rationalize the difference between prediction and experiment with VSEPR theory by noting that the electronegativity of chlorine is greater than that of carbon (Table 1.12). In a methane C-H bond, the carbon atom and the hydrogen atom attract the electron pair approximately equally. In CH3CI, however, the electrons in the C-Cl bond will be pulled toward the... 

Now, one must wonder if there is any limit to the ability of metals to bond in a stable way to other o bonds, including those in non-reactive molecules like alkanes. In fact, evidence for an intermediate methane complex has been found at low temperature in the reductive elimination of methane from a cationic rhenium methyl hydride [34]. The ab initio theoretical study of the intermolecular process of oxidative addition of a methane C - H bond has led to the location of transition states where the bond is partially broken [35]. The same results have been fond for intramolecular oxidative additions which are related to agostic interactions. In fact, agostic interaction itself is a kind of non-oxidative coordination [15,36]. For unsaturated substrates like ethylene, the activation of a C - H bond seems to follow an intermolecular path, without any previous coordination of the double bond. A feasible explanation consists here of the fact that metal orbitals suitable for ethylene coordination are the same as those which are responsible for oxidative addition, thus making the processes competitive [37]. 

For aU catalytic complexes the methane C-H bond activation starts from a reactant complex (RC) that is more strongly bound along the series of catalysts, from — 1.9 kcal mol for Pd(PH3)2 to —4.7 kcal mol for Pd(Pl3)2 (Table 2 and... 

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