Phenoxide coupling

One postulate which does resolve many of the inconsistencies in the present data base is that there is a strong tendency towards alternation in these polymers. Such alternation could result from a combination of an intrinsically greater reactivity of the ortho-relative to para-positions of coordinated phenoxide, coupled with an incibility of radicals of the type 5 to attack coordinated phenoxide at the ortho-position due to steric blocking. (An examination of models shows this to be a plausible assumption). ... 

The manganese(III)-phenoxide couple is quite unstable in organic chemistry, Mn(III) and Mn(IV) complexes have been demonstrated to be excellent oxidants for phenols and their analogs (145). Indeed, a peroxidase isolated from white rot fungus is dependent on extracellular Mn(II) (146-149). The heme-peroxide moiety of this enzyme is reduced by the Mn, which in turn (as the trivalent species) migrates to the phenolic substrate. The substrate is thus oxidized by the generated... 

It has been shown that reaction of Mn30(02CPh)6(py)2(H20) with [NEt3H]2(biphenoxide) results in the formation of [Mn(III)(biphen)2-(biphenH)]2- (biphen = 2,2 -biphenoxide) (Fig. 5) (143). The complex is a rare example of five-coordinate Mn(III) without softer ligands such as thiolate or chloride. The monodentate biphen-Mn bond represents the first stable example of the Mn phenoxide couple. The monomer displays several intense absorption bands (Table VI). In DMSO (where the compound may be six coordinate), a band at 430 nm, very similar to that of the enzyme, is assigned to a phenoxide - Mn(III) charge transfer band. 

The halogen displacement polymerization proceeds by a combination of the redistribution steps described for oxidative coupling polymerization and a sequence in which a phenoxide ion couples with a phenoxy radical (eq. 11) and then expels a bromide ion. The resultant phenoxy radical can couple with another phenoxide in a manner that is analogous to equation 11 or it can redistribute with other aryloxy radicals in a process analogous to equations 7 and 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 ... 

For many decades intramolecular O-coupling was considered not to take place in the diazotization products of 2-aminophenol and its derivatives (for a contrary opinion see, however, Kazitsyna and Klyueva, 1972). The compounds were assumed to be present as one structure only, which can be represented as a mesomer of a phenoxide diazonium zwitterion 6.63 b and a diazocyclohexadienone 6.63 a (see reviews by Kazitsyna et al., 1966 Meier and Zeller, 1977 Ershov et al., 1981). In IUPAC nomenclature 6.63 is called 1,2-quinone diazide, in Chemical Abstracts 6-diazo-2,4-cyclohexadien-one (see Sec. 1.3). More recently, however, Schulz and Schweig (1979, 1984) were able to identify the intramolecular product of O-coupling, i.e., 1,2,3-benzooxadiazole (6.64) after condensation of 6.63 in vacuo at 15 K in the presence of argon (see Sec. 4.2). 

Fig. 7-2. Potential energy E as a function of the reaction coordinate for reactions of the P-nitrogen of arenediazonium ions with nucleophiles yielding (Z)- and (is)-azo compounds, a) Reactant-like transition states (e. g., reaction with OH) b) product-like transition states (e. g., diazo coupling reaction with phenoxide ions product = cyclohexadienone-type o-complex (see Sec. 12.8).

In addition, Hashida et al. (1971 a) found a linear correlation between log k and the pof fifteen 1,3-dicarbonyl compounds. This indicates that in these cases the nucleophilicity and the basicity of the anions are closely related. The same result was obtained by Hashida et al. (1971b) for the azo coupling reactivity of substituted phenoxide ions. 

Coupling with phenoxide ion could take place either on oxygen or on carbon, and though relative electron-density might be expected to favour the former, the strength of the bond that is formed is also of significance. Thus here, as with other electrophilic attacks on phenols, it is found to be the C-substituted product (31) that is formed ... 

We might well expect the resultant phenoxy radical to attack— through the unpaired electron on its O, or on its o- or p-C, atom—a further molecule of phenol or phenoxide anion. Such homolytic substitution on a non-radical aromatic substrate has been observed where the overall reaction is intramolecular (all within the single molecule of a complex phenol), but it is usually found to involve dimerisation (coupling) through attack on another phenoxy radical ... 

The divergence of oxidative addition and nucleophilic substitution was shown by the reaction of the dichloropyridazinone 19 with phenoxide, which displaced the 6-chlorine, vs. a Suzuki coupling, which showed selectivity for the C-3 halide <06OBC4278>. 

Figure 3.7 Formation of oligolignols by non-enzymatic coupling of phenoxide radicals.

Methylene bromide can function as the coupling reagent if it is used in an excess. This unusual coupling reaction succeeds, presumably, because the intermediate a-bromo ether, 3, reacts much more rapidly with the phenoxide endgroup of another polymer than methylene bromide does to produce the formal linked product, 4. 

Alkoxide or aryloxide anions are also reputed to be inactive in Sr I reactions. There is, however, one example of such a reaction at an sp carbon the nitro-derivative of 4-nitrocumyl reacts with phenoxide and 1-methyl-2-naphthoxide ions yielding the corresponding ethers (Kornblum et al., 1967). A similar reaction has been reported for halobenzenes in t-butyl alcohol upon stimulation by sodium amalgam (Rajan and Sridaran, 1977). This reaction could not, however, be reproduced (Rossi and Pierini, 1980) and other attempts to make phenoxide ions react at sp carbons have been equally unsuccessful (Ciminale et al, 1978 Rossi and Bunnett, 1973 Semmelhack and Bargar, 1980). It has been found, more recently, that phenoxide ions react with a series of aryl halides under electrochemical induction, but that the coupling occurs at the p- or o-phenolic carbon rather than at the phenolic oxygen (Alam et al, 1988 Amatore et al, 1988). This is... 

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