Phenols dehydrogenative coupling

A single enzyme is sometimes capable of many various oxidations. In the presence of NADH (reduced nicotinamide adenine dinucleotide), cyclohexanone oxygenase from Acinetobacter NCIB9871 converts aldehydes into acids, formates of alcohols, and alcohols ketones into esters (Baeyer-Villiger reaction), phenylboronic acids into phenols sulfides into optically active sulfoxides and selenides into selenoxides [1034], Horse liver alcohol dehydrogenase oxidizes primary alcohols to acids (esters) [1035] and secondary alcohols to ketones [1036]. Horseradish peroxidase accomplishes the dehydrogenative coupling [1037] and oxidation of phenols to quinones [1038]. Mushroom polyphenol oxidase hydroxylates phenols and oxidizes them to quinones [1039]. 

Soluble lignins and biphenyls have shallower minima at 250-270 nm than monomeric phenols. Pew and Connors [51,52] prepared a number of o-o -dihydroxybiphenyl derivatives by dehydrogenative coupling of simple guaiacyl phenols. They concluded that much of the difference between the spectra of lignins and monomeric phenols... 

The total biosynthesis of L. represents the interplay of enzymatic phenol dehydrogenation and the non-enzymatic coupling of radicals generated by loss of an electron from a phenolate ion, a process referred to as reductive polymerization. 

SCHEME 7. 26 Copper-catalyzed oxidative cross-dehydrogenative-coupling of 3-ketoesters or 2-carbonyl-substituted phenols with ethers. 

In 2012, Pappo and coworkers also reported a chemo-, regio-, and stereoselective FeCl3/l,10-phenanthroline-catalyzed cross-dehydrogenative-coupling reaction between phenols and a-substituted P-ketoesters (Scheme 9.24) [29]. The reaction creates a new quaternary carbon center within a polycyclic hemiacetal or polycyclic spirolactone architecture. 

Scheme 9.24 Cross-Dehydrogenative-Coupling between Phenols and a / -Ketoesters.

A number of compounds react rapidly with DDQ at room temperature. They include allylic and benzylic alcohols, which can thus be selectively oxidized, and enols and phenols, which undergo coupling reactions or dehydrogenation, depending on their structure. Rapid reaction with DDQ is also often observed in compounds containing activated tertiary hydrogen atoms. The workup described here can be used in all these cases. 

Organic compounds having labile hydrogen atoms, such as phenols, anilines, and acetylenes, are also oxidatively polymerized by metal-complex catalysts (Eqs. 1-3). The oxidative coupling is a dehydrogenation reaction the polymer chain produced contains the dehydrogenated monomer structure as a repeating unit. As a remarkable example, poly(phenylene ether), one of the... 

The polymerization of compounds having active methyne groups has also been reported [81] (Eq. 8). The oxidative coupling polymerization of these monomers follows a mechanism similar to that of phenols. The catalytic cycle observed in the polymerization of / -phcnylcncdiaminc with Fe(edta) as the catalyst in an aqueous solution differs from that in the polymerization of phenols as follows The activation of monomers usually involves either electron transfer from the anion or elimination of a hydrogen atom from the monomer. The oxidative polymerization of phenols uses the former mechanism of the electron transfer. In contrast, in the case of the polymerization of aromatic diamines as monomers, the neutral amines are coordinated to the catalyst, followed by the subsequent electron transfer and dehydronation. The dehydronation proceeds by the reaction with 02. Another mechanism has also been proposed where dehydrogenation... 

These results, summarized in Scheme 2, indicate that the methylation pattern can be important in deciding the metabolic fate of precursors at this particular stage of biosynthesis (c/. ref. 75) as well as during oxidative phenol coupling. In addition in vivo O- and 7V-methylation in P. somniferum was found to be dependent, and variably so, on the configuration of the substrate. Further, dehydrogenation of (59) depends on the stereochemistry at C-1 since only the (-)-isomer affords papaverine (62). Moreover it is (-)-norreticuline [as (55)] rather than the ( + )-isomer which is implicated. Finally it is to be noted that norlaudanidine (66) is as efficient a precursor for papaverine as is (59). ... 

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