Polar-group-coupling reactions

The polar pathways are formally equivalent to a discrete electron-transfer step, that is, a pure SET step that is followed by a chemical step. If a hypothetical SET step is followed by coupling of a radical pair that is produced in the SET step, the overall reaction is the equivalent of a polar-group coupling reaction (Scheme 14(b)). If the coupling is accompanied by the elimination of a leaving group, a polar-group transfer reaction results (Scheme 14(a)). 

Polar-group-coupling reactions, a. Nucleophilic-addition reactions... 

Most of the known reactions of HO that produce free radicals probably do not involve a direct single-electron transfer from HO in the primary step because an SET primary step is usually highly endothermic the primary step more often is an approximately thermoneutral polar reaction (polar-group transfer or polar-group coupling), with secondary reactions producing free radicals that are coupled to form stable M-OH bonds (M is a molecule or metal atom with an unpaired electron). 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

The formation of these polar groups contributes increased adhesion. Observation of disappearing vinyl groups in the silane coupling agent and of the formation of polystyrene in the silica by FTIR analysis (Fig. 15) have confirmed the occurrence of a reaction between the polymer and the silane coupling agent [77]. 

Another advantage of the synthesis by mixed Kolbe electrolysis is that polar groups in the carboxylic acid are tolerated in radical coupling. This makes additional protection-deprotection steps unneccessary, which are often needed in polar CC-bond forming reactions and can make these approaches less attractive in such cases. 

Metal oxides, used for manufacturing of ceramic nanofiltration membranes, are intrinsically hydrophilic. This limits the use of these membranes to polar solvents filtration of nonpolar solvents (n-hexane, toluene, cyclohexane) usually yields zero fluxes. Attempts have been made to modify the pore structure by adding hydrophobic groups, for example, in a silane coupling reaction [38, 43]. This approach is similar to modifications of ultrafiltration and microfiltration membranes... 

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