C-H cleavage

These results do not prove that the ester is an essential intermediate in aqueous solution even though it is present, but the result with the hindred triterpene is convincing In this case the esterification step, which is normally fast, has become rate-determining and the disappearance of the isotope effect must mean that C-H cleavage occurs after the formation of the ester and not independently of it. The generality of this result is apparent from the stopped-flow investigation of isopropanol oxidation ... 

The available data do not permit a conclusive mechanism to be formulated. C-H cleavage is clearly rate-determining but no choice between a one- or two-... 

Rosenblatt etal have examined the effect of structure and isotopic substitution upon the permanganate oxidation of some alky famines (Table 4). The isotope effect of 1.84 is considered to be sufficiently low to be compatible with aminium radical-cation formation, and it is felt that, while C-H cleavage is significant for oxidation of primary amines, the dominant mode of oxidation of tertiary amines is electron-transfer, e.g. 

The oxidation of formaldehyde by V(V) has an isotope effect of ca 4.5 which accords with the depicted C-H cleavage. ... 

The dehydrogenation studies with labeled cyclohexane suggest that C-H cleavage is involved in the rate... 

There are also reactions in which hydride is transferred from carbon. The carbon-hydrogen bond has little intrinsic tendency to act as a hydride donor, so especially favorable circumstances are required to promote this reactivity. Frequently these reactions proceed through a cyclic TS in which a new C—H bond is formed simultaneously with the C-H cleavage. Hydride transfer is facilitated by high electron density at the carbon atom. Aluminum alkoxides catalyze transfer of hydride from an alcohol to a ketone. This is generally an equilibrium process and the reaction can be driven to completion if the ketone is removed from the system, by, e.g., distillation, in a process known as the Meerwein-Pondorff-Verley reduction,189 The reverse reaction in which the ketone is used in excess is called the Oppenauer oxidation. 

The short-wavelength radiation necessary for this decomposition is absent in the lower layers of the atmosphere it is likely that the photolysis of ethyne occurs via C-H cleavage to give radicals, which react with methane to give methyl radicals, the recombination of which affords ethane ... 

The Pr+ ion has been reacted with a series of fluorocarbon molecules and in all cases the ions [PrF]+ and [PrF2]+ were formed (120). Very few other metal ions have been studied with fluorocarbon molecules but in the case of Au+ reacting with C6F5H neither C-F nor C-H cleavage occurred. The only product was the addition complex ion [AuC6F5H]+ (121). 

Ru" (0)(N40)]"+ oxidizes a variety of organic substrates such as alcohols, alkenes, THE, and saturated hydrocarbons. " In all cases [Ru (0)(N40)] " is reduced to [Ru (N40)(0H2)] ". The C— H deuterium isotope effects for the oxidation of cyclohexane, tetrahydrofuran, 2-propanol, and benzyl alcohol are 5.3, 6.0, 5.3, and 5.9 respectively, indicating the importance of C— H cleavage in the transitions state. For the oxidation of alcohols, a linear correlation is observed between log(rate constant) and the ionization potential of the alcohols. [Ru (0)(N40)] is also able to function as an electrocatalyst for the oxidation of alcohols. Using rotating disk voltammetry, the rate constant for the oxidation of benzyl alcohol by [Ru (0)(N40)] is found to be The Ru electrocatalyst remains active when immobilized inside Nafion films. 

The number of backbone radicals produced in Equation 1 is not simply the C—H cleavage value since an appreciable amount of hydrogen removal can proceed by a molecular process to give an unsaturated group. Ng and Freeman s work indicates that approximately half of the hydrogen atoms are scavengeable. If these atoms react by abstraction (Reaction 4), more polymer backbone radicals would be formed ... 

Phenols A conspicuous molecular ion peak facilitates identification of phenols. In phenol itself, the molecular ion peak is the base peak, and the M — 1 peak is small. In cresols, the M - 1 peak is larger than the molecular ion as a result of a facile benzylic C—H cleavage. A rearrangement peak at m/z 77 and peaks resulting from loss of CO (M - 28) and CHO (M -29) are usually found in phenols. 

A comparison of C—C and C—H bond reactivities in protolytic reactions indicated that under the conditions of alkylation C—H cleavage is much more pronounced (in the reaction of ethane, the relative ratio of C—C C—H cleavages is 9 1 in the absence of benzene and 1 10 in the presence of benzene). This indicates that overall alkylation can also involve the attack of protonated benzene (benzenium ion) on the C—H bond of the alkane [Eq. (5.48)] or interaction of benzene with the five-coordinate alkonium ion prior to its cleavage to the trivalent cation ... 

Evidence for the Fe2+/Fe3+ redox cycle was provided later by ESR measurements [205], while recent experiments with deuterium-labelled butene indicate that C—H cleavage is involved in the rate-controlling step [138]. In agreement with the views of Schuit [281], chemisorption of the olefin on an anion vacancy is assumed, but O- is postulated as the active oxygen species rather than O2-. An argument in favour of O" is that otherwise much more, and rather complicated, elementary reaction steps are required to account for the transfer of charge. 

The favorable bonding situation between hydrogen and cluster metal atoms also provides some driving force for the many observed HX oxidative additions to clusters ranging from C- H cleavage to cluster expansion with... 

A second-order dependence on both the reductant and acidity was observed in the oxidation of alcohols with butyltriphenylphosphonium dichromate study of MeCD2OH and Me2CDOH indicated the presence of a substantial kinetic isotope effect. The reaction was studied in 19 organic solvents and the rates were correlated with mul-tiparametric equations. The reaction is susceptible to both electronic and steric effects of the substituents. A mechanism involving the formation of a dichromate ester and an a-C-H cleavage has been proposed.4... 

It should be noted that several authors have indicated the increase of the yield of hydrogen with the molecular weight of the extractant (18, 84), which is consistent with a higher probability of C-H cleavages. 

Various other telogens have also been successfully attempted undergoing S-Cl, S-S, P-H, C-H cleavages or from boranes (Table 14). 

Thus, the historical context reveals that the term C-H bond activation was introduced with a clear purpose to distinguish metal-mediated C-H cleavage from traditional radical and ionic substitution, and as such was essentially a mechanistic term [8], As a result we may formulate the organometallic definition the term C-H bond activation refers to the formation of a complex wherein the C-H bond... 

---