Phenol-amine complexes

The second statement has to do with the notion that in the Eigen mechanism for proton transfer there must be intermediate ion pairs. The reference to the unpublished work of Kreevoy and Liang (3) reflects the impact of their studies on some of our own recent work surveyed below. In fact, there is an extensive published literature concerning phenol-amine complexes in which the existence of the intermediates in equation 2 has been established in different organic solvents. One of the oldest such papers is that of Bell and Barrow (11) going back to 1959. Others include Hudson and co-workers (12) in 1972, and Baba and co-workers (13) in 1969. 

Further information comes from Sutton s research with additional assumptions. For the phenol-amine complex Sutton computed the relative bond character of the H bond—75 to 85 percent ionic and 25 to 15 percent covalent—on the basis of the calculated dipole moment direction corrected for induced moments. These figures are in general agreement with other estimates based on electrostatic models. The tenuous nature of the argument must be kept in mind. 

Another alternative to obtain a third-order kinetic law would be to consider that the discrete formation of the phenoxide ion is not required. The phenol could just coordinate with the base and form a phenol-amine complex with a partial negative charge on the oxygen. This complex would then react with benzoyl chloride to form the product in the slow step of the reaction (Scheme 10.3). 

By comparison of Eqs. 10.3, 10.5 and 10.6 we conclude that the alternative mechanisms proposed in Schemes 10.2 and 10.3 are kinetically indistinguishable. Provided that in the nucleophile and base catalysis mechanisms the second step would be rate-determining, Eq. 10.3 and Eq. 10.5 are identical, and even a base catalysis by a phenol-amine complex (Eq. 10.6) could not be discarded. Therefore, the discrimination between nucleophile and base catalysis in flie model reaction cannot be exclusively made on kinetic grounds. 

A decrease in reaction rate with phenol-iie relative to phenol would indicate 0-H (0-D) bond cleavage in the rate-determining step. However, neither the base catalysis nor the nucleophile catalysis require the breakage of the 0-H phenolic bond in the slow step of the reaction. Only in the case of the mechanism depicted in Scheme 10.3, a kinetic isotope effect (KIE) could be expected. In consequence, the absence of KIE when perdeuterated phenol is used as substrate is against the mechanism based on the formation of a phenol-amine complex but is not definitive proof to distinguish between the nucleophile and a base catalysis mechanisms postulated in Scheme 10.2. 

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. 

The solubility of most metals is much higher when they exist as organometallic complexes.4445 Naturally occurring chemicals that can partially complex with metal compounds and increase the solubility of the metal include aliphatic acids, aromatic acids, alcohols, aldehydes, ketones, amines, aromatic hydrocarbons, esters, ethers, and phenols. Several complexation processes, including chelation and hydration, can occur in the deep-well environment. 

Similar small experiments, demonstrating the sterilization of potable water, reduction in the hardness of water, degradation of phenol, amines, potassium iodide and indicators, degradation of complexes, formation of complexes may still be added as found in the preceding chapters of this book. 

Tolman (144, 202) and Wieghardt (203, 204) and their co-workers used amine macrocycles with a 1,4,7-triazacyclononane backbone and one, two, or three phenol pendent arms (Table VIII). In all cases, square-base pyramidal (phenolate)copper(II) precursor complexes were isolated and in many instances structurally characterized by X-ray crystallography. Depending on the number of coordinated phenolates, these complexes are reversibly one-electron oxidized yielding the (phenoxyl)copper(II) species that were characterized in solution by UV-vis, EPR, and RR spectroscopy. 

Interactions between tertiary aliphatic amines or N,N-dialkylanilines and substituted phenols are generally reported as models of O—H- N hydrogen bonds affording molecular complexes (equilibrium 4). A number of complexes between primary, secondary and tertiary aliphatic amines and dihydroxy benzenes (or dihydroxynaphthalenes) were isolated55 to investigate the stoichiometry of these complexes. The phenol/amine ratios observed included values of 1 1, 2 1, 3 1 and 3 2. 

Some complications arise from the presence of proton donor-acceptor interactions134 when the donor is a protic amine. The separate evaluation of the two kinds of interactions may be a difficult problem. Similarly, if the electron acceptor is also a proton donor, the overlapping of salification and complexation processes makes the separate investigation of the interactions very difficult. This is the case in the complexes between amines and picric acid or other related phenols. For complexes of 2,4,6-trinitro-3-hydroxypyridine135 and... 

However the formation of thin polymer film on the electrode, i.e. passivation of the electrode, resulted in cessation of the polymerization, which restricted the electro-oxidation as a polymerization procedure. The electro-oxidative polymerization as a method of producing poly(phenyleneoxide)s had not been reported except in one old patent, in which a copper-amine complex was added as an electron-mediator during the electrolysis (4). The authors recently found that phenols are electro-oxidatively polymerized to yield poly-(2,6-disubstituted phenyleneoxide)s, by selecting the electrolysis conditions This electro-oxidative polymerization is described in the present paper. 

We have examined the proton transfer reaction AH-B A -H+B in liquid methyl chloride, where the AH-B complex corresponds to phenol-amine. The intermolecular and the complex-solvent potentials have a Lennard-Jones and a Coulomb component as described in detail in the original papers. There have been other quantum studies of this system. Azzouz and Borgis performed two calculations one based on centroid theory and another on the Landau-Zener theory. The two methods gave similar results. Hammes-Schiffer and Tully used a mixed quantum-classical method and predicted a rate that is one order of magnitude larger and a kinetic isotope effect that is one order of magnitude smaller than the Azzouz-Borgis results. 

In 1959 Hay, et al., (36) reported that certain 2.6-disubstituted phenols reacted with oxygen in the presence of an amine complex of a... 

Polymerization also takes place when 4-halo-2.6-disubstituted phenols are oxidized with copper-amine catalysts and oxygen (5,35). In this case, stoichiometric amounts of copper salt or some other chloride acceptor (inorganic bases or strongly basic amines) are necessary since the amine complexes of copper (II) halides are not catalysts for the polymerization. Blanchard (5) has also described the polymerization of these 4-halo-phenols under conditions similar to those used by Price using certain copper (II) complexes as initiators. 

Phenols such as 2.6-dimethyIphenol are converted rapidly and in high yield to high molecular weight polymers at room temperature with oxygen in the presence of amine complexes of copper salts as catalyst. Much of the work described in the literature has been performed with copper (I) chloride as catalyst and pyridine as ligand and solvent. Other amines, primary, secondary or tertiary can be used as ligands for the catalyst. Autoxidation of copper (I) chloride in pyridine results in the... 

Another particularly interesting class of amine complex that has had relatively little recent attentionistheclathrateseriesM(X)2(A) Y,whereM = Ni11 orCdu,X = CN orNCS, A = NH3 or KNHjfCHj NHj) and Y is the host, e.g. benzene, thiophene, furan, pyrrole, aniline or phenol.44,45... 

T isubstituted phenols react with oxygen in the presence of amine complexes of copper to yield linear poly(arylene oxides) the molding resin marketed under the trade name PPO is produced in this way by the oxidative polymerization of 2,6-dimethylphenol (14) ... 

Organic compounds having labile hydrogens, such as phenols [41,42], phenylene-diamines [43], and acetylenes [44], can be oxidatively coupled in the presence of specific metal complexes to form polymeric compounds. The oxidative polymerization of 2,6-disubstituted phenols with a copper-amine complex produces poly(2,6-disubstituted phenylene ether) [45-51], Poly(2,6-dimethylphenylene ether) and poly(2,6-diphenylphenylene ether) are commercially produced from 2,6-dimethyl phenol and 2,6-diphenylphenol, respectively (Figure 5). These polymers exhibit excellent performance as engineering plastics. 

Dimethylphenol is oxidatively polymerized to poly(2,6-dimethyl-1,4-phenyl-ene ether) with a copper-amine complex by a laccaselike reaction. The activated phenols are coupled to form a dimer. The dimer is activated by a mechanism similar to that by which the polymerization proceeds. The effects of the amine ligands are to improve the solubility and the stability of the copper complex as well as the phenol-coordinated complex and to control the redox potential of the copper complex. 

When phenols are oxidized by molecular oxygen in the presence of copper-amine complexes as catalysts, oxidative polymerization to polyphenylene ethers results,344-349 e.g.,... 

Fig. 10 Chemical structure of the amine-phenol gallium complex...

When compared to aliphatic amines, aromatic amines generally have reduced exotherm and reactivity. Elevated temperatures are required to achieve optimum properties. In certain cases aromatic amines can be cured at room temperature with catalysts such as phenols, BF3 complexes, and anhydrides. 

Many amine-copper complexes, as well as a few amine complexes of other metals, and certain metal oxides have since been shown to induce similar reactions (17, 18, 22, 23, 30). This chapter is concerned largely with the mechanism of oxidative polymerization of phenols to linear polyarylene ethers most of the work reported has dealt with the copper-amine catalyzed oxidation of 2,6-xylenol, which is the basis for the commercial production of the polymer marketed under the trade name PPO, but the principal features of the reaction are common to the oxidative polymerization of other 2,6-disubstituted phenols. 

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