Oxidative Coupling mechanisms

Polyheterocycles. Heterocychc monomers such as pyrrole and thiophene form hiUy conjugated polymers (4) with the potential for doped conductivity when polymerization occurs in the 2, 5 positions as shown in equation 6. The heterocycle monomers can be polymerized by an oxidative coupling mechanism, which can be initiated by either chemical or electrochemical means. Similar methods have been used to synthesize poly(p-phenylenes). 

An interesting oxidative coupling mechanism has been proposed for the biotransformation of p-coumaric acid with peroxidase, leading to dimer and trimer structures with stronger antioxidant activities [38]. 

The dibenzo[c,e]-1,2,7-thiadiazepine (43) can be prepared by oxidation of A(,7V -diphenylsulfamide (40) with sodium hypochlorite in the presence of a base, probably via a type of oxidative coupling mechanism involving the intermediates (41) and (42) (Scheme 5) <71JCS(C)993>. 

The preparation of conducting polymers in emulsion is generally via an oxidative coupling mechanism in which the active polymerizing species are free-radicals. 

Oxidative Coupling Mechanism. Poly-2,6-Me2P is a C-0 couplingprod-uct and DPQ is a product via C—C coupling. Control of the C—0 coupling is a most important question (5). Three possible reaction mechanisms for the C—0 coupling... 

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... 

Polymerization Mechanism. The mechanism that accounts for the experimental observations of oxidative coupling of 2,6-disubstituted phenols involves an initial formation of aryloxy radicals from oxidation of the phenol with the oxidized form of the copper—amine complex or other catalytic agent. The aryloxy radicals couple to form cyclohexadienones, which undergo enolization and redistribution steps (32). The initial steps of the polymerization scheme for 2,6-dimethylphenol are as in equation 6. 

Early examples of such branched polysulphides, e.g. Thiokol FA, are believed to possess hydroxyl end groups and are coupled by means of zinc compounds such as the oxide, hydroxide, borate and stearate by a mechanism which is not understood. Later elastomers, e.g. Thiokol ST, have been modified by a restricted reductive cleavage (see below) and this generates thiol (mercaptan) end groups. These may be vulcanised by oxidative coupling as illustrated below with lead peroxide ... 

Detailed kinetic studies in connection with digital simulations do confirm the RR coupling mechanism postulated in older publications as well as the oxidation of the resulting dimer D to the dication D. But the surprising drop in the height of the reduction wave for the redox pair as the concentration... 

Antioxidant capacities of common individual curcuminoids were determined in vitro by phosphomolybdenum and linoleic acid peroxidation methods. Antioxidant capacities expressed as ascorbic acid equivalents (pmol/g) were 3099 for curcumin, 2833 for demethoxycurcumin, and 2677 for bisdemethoxycurcumin at concentrations of 50 ppm. The same order of antioxidant activity (curcumin > demethoxycurcumin > bisdemethoxycurcumin) was observed when compared with BHT (buty-lated hydroxyl toluene) in linoleic peroxidation tests. The antioxidant activity of curcumin in the presence of ethyl linoleate was demonstrated and six reaction products were identified and structurally characterized. The mechanism proposed for this activity consisted of an oxidative coupling reaction at the 3 position of the curcumin with the lipid and a subsequent intramolecular Diels-Alder reaction. ... 

Masuda, T. et ah. Chemical studies on antioxidant mechanism of curcumin analysis of oxidative coupling products from curcumine and linoleate, J. Agric. Food Chem., 49, 2539, 2001. 

Chauhan, M. Dean, F. M. Hindley, K. Robinson, M. Phenoxylium ion from a-tocopherol spirodimer and its significance for the mechanism of oxidative coupling in phenols. Chem. Commun. 1971, 19, 1141-1143. 

For illustration, we consider a simplified treatment of methane oxidative coupling in which ethane (desired product) and CO, (undesired) are produced (Mims et al., 1995). This is an example of the effort (so far not commercially feasible) to convert CH, to products for use in chemical syntheses (so-called Q chemistry ). In this illustration, both C Hg and CO, are stable primary products (Section 5.6.2). Both arise from a common intermediate, CH, which is produced from CH4 by reaction with an oxidative agent, MO. Here, MO is treated as another gas-phase molecule, although in practice it is a solid. The reaction may be represented by parallel steps as in Figure 7.1(a), but a mechanism for it is better represented as in Figure 7.1(b). 

C-C bond formation mediated by silane.6,6a 6f With respect to the development of intramolecular variants, these seminal studies lay fallow until 1990, at which point the palladium- and nickel-catalyzed reductive cyclization of tethered 1,3-dienes mediated by silane was disclosed. As demonstrated by the hydrosilylation-cyclization of 1,3,8,10-tetraene 21a, the /rarcr-divinylcyclopentanes 21b and 21c are produced in excellent yield, but with modest stereoselectivity.46 Bu3SnH was shown to participate in an analogous cyclization.46 Isotopic labeling and crossover experiments provide evidence against a mechanism involving initial diene hydrosilylation. Rather, the collective data corroborate a mechanism involving oxidative coupling of the diene followed by silane activation (Scheme 15). 

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