C-H bonds cleavage

The classification described above is an approximate subdivision of aU reactions known in accordance with their mechanisms. One example was given in eq. (1.6). Such process can proceed with participation of ligands of metal complex. Photochemical reaction between, for example, alkane, RH, and PtC [5], depicted by eq. (1.7) and initiated via mechanism of the third type can lead to the formation of an cr-organyl derivative of the metal and the entire process then belongs to the first type. 

Evidently the unambiguous assignment of a process to a particular type requires a detailed knowledge of the reaction mechanism. However, at the present time the mechanisms of many processes have not been elucidated even in a broad outline. For example, organomagnesium compounds formed by co-condensation of magnesium and aryl halides from the gas phase are capable of catalyzing 

We may note that the mechanisms of reactions included in the last two types are, in general, not the same for paraffins, on the one hand, and aromatic hydrocarbons, on the other hand, even if the products of these reactions are of the same type. For example, alcohols and phenols may be obtained from alkanes and arenes respectively by the reaction in air with hydroxyl radicals generated by the action of a metal complex. However, in the case of alkane, an alcohol can be formed by the reduction of alkyl peroxide, whereas hydroxyl is added to an arene with subsequent oxidation of a radical formed. Hence follows the possibility that arenes and alkanes may exhibit different reactivities in each specific reaction. 

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A number of studies of the acid-catalyzed mechanism of enolization have been done. The case of cyclohexanone is illustrative. The reaction is catalyzed by various carboxylic acids and substituted ammonium ions. The effectiveness of these proton donors as catalysts correlates with their pK values. When plotted according to the Bronsted catalysis law (Section 4.8), the value of the slope a is 0.74. When deuterium or tritium is introduced in the a position, there is a marked decrease in the rate of acid-catalyzed enolization h/ d 5. This kinetic isotope effect indicates that the C—H bond cleavage is part of the rate-determining step. The generally accepted mechanism for acid-catalyzed enolization pictures the rate-determining step as deprotonation of the protonated ketone ... 

A second piece of evidence in support of the E2 mechanism is provided by a phenomenon known as the deuterium isotope effect. For reasons that we won t go into, a carbon-hydrogen bond is weaker by about 5 kj/mol (1.2 kcal/mol) than the corresponding carbon-rfaiiferiwm bond. Thus, a C-H bond is more easily broken than an equivalent C-D bond, and the rate of C-H bond cleavage is faster. For instance, the base-induced elimination of HBv from l-bromo-2-phenylethane proceeds 7.11 times as fast as the corresponding... 

Mikami M, Hatano M, Akiyama K (2005) Active Pd(II) Complexes as Either Lewis Acid Catalysts or Transition Metal Catalysts. 14 279-322 Minatti A, DOtz KH (2004) Chromium-Templated Benzannulation Reactions. 13 123-156 Miura M, Satoh T (2005) Catalytic Processes Involving b-Carbon Elimination. 14 1-20 Miura M, Satoh T (2005) Arylation Reactions via C-H Bond Cleavage. 14 55-84 Mizobe Y, see Hidai M (1999) 3 227-241... 

In aromatic combustion flames, cyclopentadienyl radicals (c-CgHj ) can be precursors for PAH formation. " At high temperatures, benzene is oxidized by reaction with an oxygen molecule to yield phenylperoxy (C6H5O2 ) radical, via the initial formation of the phenyl radical (by C-H bond cleavage) and then the rapid addition of O2 (reaction 6.16). After expulsion of CO from phenylperoxy radical, a resonance-stabilized cyclopentadienyl radical (c-CgHg ) is formed (reaction 6.16). 

The TS for selenoxide elimination has also been examined computationally.334 The C-H bond cleavage runs ahead of the C—Se cleavage. 

Recently, we have demonstrated another sort of homogeneous sonocatalysis in the sonochemical oxidation of alkenes by O2. Upon sonication of alkenes under O2 in the presence of Mo(C0) , 1-enols and epoxides are formed in one to one ratios. Radical trapping and kinetic studies suggest a mechanism involving initial allylic C-H bond cleavage (caused by the cavitational collapse), and subsequent well-known autoxidation and epoxidation steps. The following scheme is consistent with our observations. In the case of alkene isomerization, it is the catalyst which is being sonochemical activated. In the case of alkene oxidation, however, it is the substrate which is activated. 

Exchange experiments (H/D) carried out under the conditions of catalysis but without ethylene showed that the C-H bond cleavage is facile and unselective (Equation (52)). However, such processes must be rapid and reversible.52,54... 

The alkylation of the sp3 C-H bonds adjacent to a heteroatom becomes more practical when the chelation assistance exists in the reaction system. The ruthenium-catalyzed alkylation of the sp3, C-H bond occurs in the reaction of benzyl(3-methylpyridin-2-yl)amine with 1-hexene (Equation (30)).35 The coordination of the pyridine nitrogen to the ruthenium complex assists the C-H bond cleavage. The ruthenium-catalyzed alkylation is much improved by use of 2-propanol as a solvent 36 The reaction of 2-(2-pyrrolidyl)pyridine with ethene affords the double alkylation product (Equation (31)). 

Palladium-catalyzed reaction of 2-hydroxy-2-methylpropiophenone with aryl bromides shows a unique multiple arylation via successive C-C and C-H bond cleavages, giving tetraarylethanes.96 For example, the reaction of 2-hydroxy-2-methylpropiophenone with bromobenzene in the presence of Pd(OAc)2, P(/-Bu)3, and CS2CO3 gives 1,1,2,2-tetraphenylethane quantitatively, together with l,4,4-triphenyl-7-methylisochroman-3-one (13% yield) (Equation (74)). 

For the oxidative addition pathway, however, it is not obvious why the C-H bond cleavage reaction should be more facile if the hydrocarbon first binds in the coordination sphere of the metal (Scheme 5, c). One argument could be that the equilibrium between the Pt(II) alkane complex and the five-coordinate Pt(IV) alkyl hydride has an intrinsically low activation barrier. Insight into this question together with detailed information about the mechanisms of these Pt(II) a-complex/Pt(IV) alkyl hydride interconversions has been gained via detailed studies of reductive elimination reactions from Pt(IV), as discussed below. 

The experimental data available to date consistently indicate that ligand dissociation precedes reductive elimination from six-coordinate platinum(IV). In the reverse direction (oxidative addition), it seems necessary that the hydrocarbon molecule coordinates in the square plane of platinum(II). C-H bond cleavage then forms a five-coordinate Pt(IV) species consistent with the principle of microscopic reversibility. 

Let us consider that a C—H bond cleavages in the mechanism of reaction. Now, if deuterium is substituted for hydrogen, the rate is reduced. 

Activated water expulsion in cyclodextrin, 32 422-426, 432 Activation barrier C—H bond cleavage, 37 143... 

Alkanes (continued) aromatic, 41 312-313 aromatization, 30 36, 39 195-201 C-H bond cleavage, 37 133-134 chemical mechanism for isotopic exchange of, 20 84 conversion... 

Calculations performed for cyclopropanation with Fischer-type carbene complexes [28] indicate that the electrophilic attack of the carbene complex at the alkene and the final ring closure are concerted. Extrapolation from this result to the C-H insertion reaction (in which a a-bond instead of a 7i-bond is cleaved) suggests that C-H bond cleavage and the formation of the new C-C and C-H bonds might also be concerted (Figure 3.38). 

Ackermann L (2007) Chelation-Assisted Arylation via C-H Bond Cleavage. 24-. 35-60 Akiyama K, see Mikami M (2005) 14-. 279-322... 

Scheme 10 Proposed mechanism for C-C bond coupling via sp C-H bond cleavage...

Scheme 11 Cyclization of alkene-amide via sp C-H Bond cleavage...

Whilst C—H bonds represent the most ubiquitous chemical linkage in Nature, they are at the same time some of the most difficult bonds to cleave, although they are not completely inert [5b,c]. For example, in 1963 Kleinman and Dubeck reported the possibility of C—H bond cleavage in azobenzene by the Cp2Ni complex (Scheme 13.1) [1]. The structure originally proposed by Kleinman and Dubeck considered the nickel center to be coordinated T 2 to the N=N n-bond (2) (Scheme 13.2). 

A mechanism in which reductive activation of dioxygen precedes the C—H bond cleavage cannot explain such a tight coupling of oxygen and substrate consumption. (From Crespo et ah, 2006)... 

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