Coupling reactions , mechanism

Gilbert and coworkers18 were able to detect ESR spectra of sulfinyl radicals together with other sulfur-centered radicals during the oxidation of disulfides and thiols with a titanium(III)-peroxide couple reaction mechanisms involving sulfenic acids as intermediates have been discussed. 

Figure 2.1. Cross-coupling reactions - mechanism and name reactions...

Scheme 14. (a) Sonogashira cross-coupling reaction mechanism, (b) Click chemistry [2 + 3]... 

It is also possible to couple reactions mechanically. However, this method succeeds only when conducted as a thought experiment and therefore we will not discuss it any further here. 

Aluminum chloride reacts with an arene to give the arene radical-cation, which further follows the generally accepted oxidative coupling reaction mechanism, by an influence of copper(II) chloride as an oxidant to finally give a well defined polymeric product [65],... 

M. Garcia-Melchor, A. A. C. Braga, A. Lledos, G. Ujaque, F. Maseras. Computational Perspective on Pd-Catalyzed C-C Cross-Coupling Reaction Mechanisms . Acc. Chem. Res. 2013. doi 10.1021/ar400080r. 

Chen P, Solomon EL 2004. O2 activation by binuclear Cu sites noncoupled versus exchange coupled reaction mechanisms. Proc Nat Acad Sci USA 101 13105-13110. 

HCR process models are usually very sophisticated because their complex feedstocks and highly coupled reaction mechanisms make it difficult to build a plantwide HCR process simulation. However, refiners are most concerned about maximizing profit under steady operations, which imply small changes of the process operations and feedstock varieties. Therefore, a good operating model of the refining process only needs to match key product yields, qualities, and process operations under small process changes. 

The system of coupled differential equations that result from a compound reaction mechanism consists of several different (reversible) elementary steps. The kinetics are described by a system of coupled differential equations rather than a single rate law. This system can sometimes be decoupled by assuming that the concentrations of the intennediate species are small and quasi-stationary. The Lindemann mechanism of thermal unimolecular reactions [18,19] affords an instructive example for the application of such approximations. This mechanism is based on the idea that a molecule A has to pick up sufficient energy... 

Lithiation at C2 can also be the starting point for 2-arylatioii or vinylation. The lithiated indoles can be converted to stannanes or zinc reagents which can undergo Pd-catalysed coupling with aryl, vinyl, benzyl and allyl halides or sulfonates. The mechanism of the coupling reaction involves formation of a disubstituted palladium intermediate by a combination of ligand exchange and oxidative addition. Phosphine catalysts and salts are often important reaction components. 

The regioselectivity and syn stereochemistry of hydroboration-oxidation coupled with a knowledge of the chemical properties of alkenes and boranes contribute to our under standing of the reaction mechanism... 

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). 

Mechanisms of Coupling. Because the active coupling species is the ioni2ed coupler (35,52), the rate of the coupling reaction and hence its abihty to compete for oxidi2ed developer is dependent on the pH of the process, the piC or acidity of the coupler or less frequentiy the rate of coupler ioni2ation, and the reactivity of the resulting coupler anion with the QDI (40). 

The lithium oxide-promoted barium oxide also functions as a catalyst for the methane coupling reaction, but the mechanism is not clearly understood at the present time. The only comment that might be offered here is that the presence of ions on the surface of this material might etdrance the formation of methyl radicals drrough the formation of hydroxyl groups thus... 

An alternative reaction mechanism has been suggested for nitroarylation of enolates. An impetus for considering other mechanisms is the fact that the by-products which might be expected from aryl radicals, such as reduction products from hydrogen abstraction from the solvent or biaryls from coupling, are not observed. One alternative is that, rather than being a chain process, the reaction may involve recombination whereby the radicals combine more rapidly than they separate. 

DET calculations on the hyperfine coupling constants of ethyl imidazole as a model for histidine support experimental results that the preferred histidine radical is formed by OH addition at the C5 position [00JPC(A)9144]. The reaction mechanism of compound I formation in heme peroxidases has been investigated at the B3-LYP level [99JA10178]. The reaction starts with a proton transfer from the peroxide to the distal histidine and a subsequent proton back donation from the histidine to the second oxygen of the peroxide (Scheme 8). 

The optimal pH-value for the coupling reaction depends on the reactant. Phenols are predominantly coupled in slightly alkaline solution, in order to first convert an otherwise unreactive phenol into the reactive phenoxide anion. The reaction mechanism can be formulated as electrophilic aromatic substitution taking place at the electron-rich aromatic substrate, with the arenediazonium ion being the electrophile ... 

The mechanism " of the Suzuki reaction is closely related to that of the Stille coupling reaction, and is also best described by a catalytic cycle ... 

Figure 27.9 Mechanism of the coupling reaction of dimethylallyl diphosphate (DMAPP) and isopentenyl diphosphate (IPP), to give geranyl diphosphate (GPP).

Scheme 4. Presumed mechanism for the Ni(n)/Cr(ii)-mediated coupling reaction.

In addition to protons, other electrofugic leaving groups such as SO3 (i. e., anions of sulfonic acids), Cl, Br, I, C02, and others can also be displaced in azo coupling reactions with aromatic substrates. The mechanism of such substitutions is in principle the same as that of dehydrogenation (see Fischer and Zollinger, 1972). 

The coupling reaction of 3-methylindole (Scheme 12-21) is complex, as it involves an initial ipso-addition at the 3-position followed by rearrangement of the arylazo group to the 2-position (Jackson and Lynch, 1987 Jackson et al., 1987). However, under slightly different conditions Spande and Glenner (1973) isolated the unusual triazene 12.41. Based on the change in the UV spectrum of the reaction mixture with time, Sarma and Barooah (1977) proposed a mechanism involving initial formation... 

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