Cleavage of C—H bond

Fig. 2. Values of the acidity of ethynylpyrazoles and the energies of heterocyclic cleavage of C—H bond calculated by the CNDO-2 method (75KGS821).

Results discussed above show in several lines a distinct biomimetic-type activity of iron complexes stabilized in the ZSM-S matrix. The most important feature is their unique ability to coordinate a very reactive a-oxygen form which is similar to the active oxygen species of MMO. At room temperature a-oxygen provides various oxidation reactions including selective hydroxylation of methane to methanol. Like in biological oxidation, the rate determining step of this reaction involves the cleavage of C-H bond. 

Cleavage of C—H bonds is also well documented for mononuclear complexes (66-68, 147), and seems likely for dinuclear systems [(84, 386) also Section II,A, Eq. (7)] but apart from some cyclopropane systems (387), cleavage of C—C bonds has not been demonstrated at mononuclear centers. Equation (65) shows such cleavage at a cluster center (388) ... 

The corresponding alkane would then be formed by abstraction of a H atom from the polymer or combination with a H atom formed by radiolytic cleavage of C-H bonds (normally the H atoms form H2). The amounts of... 

Metal cluster complexes containing vinylidene ligands have been considered as models of species present when olefins or alkynes are chemisorbed on metal surfaces (114). Vinylidene has been detected in reactions of ethylene or acetylene with Fe(100), Ni( 111), and Pt(l 11) surfaces (115), and was shown to be an intermediate by theoretical studies on a manganese surface (116). The facile cleavage of C-H bonds which occurs in these systems, together with hydrogen addition or abstraction, also occurs on metal clusters. Typical of the reactions considered is the hydrogen transfer reaction... 

A particularly important synthetic application is in the preparation of complexes with M-C o bonds via cleavage of C-H bonds, by reactions of the type ... 

Haszeldine et al. [267] also performed the photochemical addition of various RjR CHCl (with R1 R2=CH3, F C2H5> H CH3) CH3 n-C3H7, H and C2H5, CH3) telogens to HFP at about 40 °C. They observed, beside the formation of monoadducts mainly produced from the cleavage of C-H bond, few amounts of compounds obtained by the cleavage of C-Cl bond and cyclic or rearranged products. 

In fact, unique cleavage of the C-Si bond from a tertiary SMA is not a general feature as cleavage of C-H bond can also occur. This is illustrated with the reaction of methylvinylketone with tertiary SMAs, leading either to silylated or non-silylated products or both.285... 

The reductive elimination to form C-C and C-H bonds [45] is a crucial step in the cross-coupling processes, as well as many other transition metal-catalyzed reactions. Reductive elimination reactions comprise an early chapter in any organometallic text. Many examples of these reactions have been studied, and a great deal is known about the mechanisms of these processes. Similarly, the cleavage of C-H bonds by oxidative addition, including the C-H bond in methane, is now known [46]. Again, questions remain about how these reactions occur, but a variety of mechanistic studies have revealed key features of these reactions. Even some remarkably mild C-C cleavage reactions have now been observed with soluble transition metal complexes [47,48]. 

Reaction (414) is formally analogous to the oxidative addition of alkyl halides to noble metal complexes described earlier, and both homolytic and heterolytic processes can be envisaged. Heterolytic cleavage of C—H bonds represented in Eq. (415) is analogous to the interaction of the powerful oxidant Co3+ with alkanes in TFA in reaction (229). 

However, this simple model does not explain the (kinetically) enhanced reactivity of arenes relative to alkanes, considering the higher (Dc-h) bond energy of arenes, i.e. Dc-h, benzene — 111 lccal mol-1 vs. Dc-h,alkane 95 lccal mol-1. Many more examples of metal-activated cleavage of C-H bonds are known for aromatic compounds than for alkanes [37]. To account for this difference, arene/metal coordination is proposed [38], although experimental evidence for such intermediate complexes and their structural features is lacking. 

After these pioneering studies, a number of other research groups reported on the cleavage of C-H bonds via the use of a stoichiometric amount of transition-metal complexes [7]. To date, several types of catalytic reactions involving C-H bond cleavage, for example, alkyl, alkenyl, aryl, formyl, and active methylene C-H bonds have been developed [8]. In many cases,for these types of catalytic reactions, ruthenium, rhodium, iridium, platinum, and palladium complexes all show catalytic activity. 

EPR experiments have shown that the redox ability of WZ catalysts is sufficient to initiate a homolytic cleavage of C-H bonds in alkanes. Exposure of a WZ catalyst to n-pentane at 523 K led to the formation of W5+ species and organic radicals on the surface.27 The formation of organic radicals also occurred when WZ catalysts interacted with other hydrocarbons, including benzene.31 We therefore infer that one-electron transfer, although it is not regarded as a step in the catalytic cycle, can initiate catalysis by a process that leads to the formation of the carbenium ion chain carriers,27 as also occurs in acidic solutions.32 We emphasize that a strong redox reactivity is necessary but not sufficient for the catalytic activity of WZ. 

Trimethylphosphine. Trimethylphosphine is very strongly chemisorbed on all the nickel surfaces. On the Ni(lll) surface, thermal decomposition occurs readily and CHi, and H2 are desorbed as decomposition products with desorption maxima at 90 and 98°C respectively. Chemisorption of this phosphine initially must involve a donor-acceptor interaction centered at the phosphorus atom. Models show that the methyl hydrogen atoms can then closely approach the surface metal atoms. Cleavage of C-H bonds probably occurs at or near 25°C, and P-C-Ni bonds are then irreversibly formed. This surface chemistry qualitatively mirrors that of trimethylphosphine in the coordinately unsaturated complex, Fe[P(CH3)3K, which is primarily HFe[r)2-CH2P(CH3)i][P(CH3)3]3 in the solution state (21). 

Cleavage of C—H Bonds Alkane Activation Cyelometallation Reactions 1197... 

The cleavage of C—H bonds represents an oxidative addition reaction in most cases, but is such an important area that we treat it separately. Activation of C—H... 

Bond dissociation energies for the cleavage of C-H bonds are used as a measure of radical stability. For example, two different radicals can be formed by cleavage of the C-H bonds in CH3CH2CH3. 

As with HX, H2O, and ROH (see above), the metals also react with other protic acids such as NH3, PH3, H2S, HCN, mineral acids, and organic acids, with release of H2 The same reaction is observed with a variety of acidic C-H compounds (e.g., R —CH2—R R"CH— ) to give organic derivatives, MR2. In the cases above intermediates, HMR, are not observed. Cleavage of C—H bonds is also common with saturated and unsaturated hydrocarbons, along with C—C fission at high temperatures, metal-carbon bonds being formed . 

A quite obvious solution to this problem is to replace both partners of the crosscoupling reaction with simple arenes by the cleavage of C-H bond during the coupling reaction. To assess this possibility, the catalytic cycle of the Suzuki-Miyaura reaction is depicted in Scheme 2. Palladium-catalyzed Suzuki-Miyaura reaction follows a similar catalytic cycle to those of many other cross-coupling... 

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