Copper aromatic hydroxylation

Encapsulated Cu—chlorophthalocyanines oxidize hexane at C-l using 02 and at C-2 using H202 as oxidants. The dimeric structure of copper acetate is intact when it is incorporated into the zeolite. This is a regioselective aromatic hydroxylation catalyst, which mimics the specificity of the monooxygenase enzyme tyrosinase.82,89 Zeolite NaY catalysts made with a tetranuclear Cu(II) complex were synthesized and characterized.90... 

There has been enormous activity in the field of copper(I)-dioxygen chemistry in the last 25 years, with our information coming from both biochemical-biophysical studies and to a very important extent from coordination chemistry. This has resulted in the structural and spectroscopic characterization of a large number of copper dioxygen complexes, some of which are represented in Figure 14.2. The complex F, first characterized in a synthetic system was subsequently established to be present in oxy-haemocyanin, and is found in derivatives of tyrosinase and catechol oxidase, implying its involvement in aromatic hydroxylations in both enzymes and chemical systems. 

There has been extensive work aimed at elucidating the mechanism of aromatic hydroxylation. Most of that research was carried out with the aim of modeling tyrosinase, a copper-containing monooxygenase, and other aromatic systems of biological relevance. This subject is discussed in connection with the hydroxylation of phenols. 

Extensive work has been carried out by Karlin et al. on aromatic hydroxylation by tyrosinase models designed to mimic the coordination environment of copper in this enzyme. Copper(I) complexes of m-xylyl type dinucleating ligands bind 0 to form a p-peroxodicopper... 

Tolman and co-workers have also synthesized a model system which undergoes endogenous aromatic hydroxylation using an amine and a pyridine ligand tethered to an arene. (Scheme 6) Unlike our XYL-H system, oxygenation of copper(I)-complex forms a bis-M.-oxo-dicopperClll) complex instead of a peroxodicopper(II) complex. Upon decomposition, the... 

In contrast to phenolic hydroxyl, benzylic hydroxyl is replaced by hydrogen very easily. In catalytic hydrogenation of aromatic aldehydes, ketones, acids and esters it is sometimes difficult to prevent the easy hydrogenolysis of the benzylic alcohols which result from the reduction of the above functions. A catalyst suitable for preventing hydrogenolysis of benzylic hydroxyl is platinized charcoal [28], Other catalysts, especially palladium on charcoal [619], palladium hydride [619], nickel [43], Raney nickel [619] and copper chromite [620], promote hydrogenolysis. In the case of chiral alcohols such as 2-phenyl-2-butanol hydrogenolysis took place with inversion over platinum and palladium, and with retention over Raney nickel (optical purities 59-66%) [619]. 

Stack and co-workers recently reported a related jx-rf / -peroxodi-copper(II) complex 28 with a bulky bidentate amine ligand capable of hydroxylating phenolates at - 80 °C. At - 120 °C, a bis(yu,-oxo)dicopper(III) phenolate complex 29 with a fully cleaved 0-0 bond was spectroscopically detected (Scheme 13) [190]. These observations imply an alternative mechanism for the catalytic hydroxylation of phenols, as carried out by the tyrosinase metalloenzyme, in which 0-0 bond scission precedes C - 0 bond formation. Hence, the hydroxylation of 2,4-di-tert-butylphenolate would proceed via an electrophilic aromatic substitution reaction. 

These copper-mediated reactions very often involve dinuclear intermediates, but detailed mechanistic studies on stoichiometric systems are relatively few. The key features are the formation of p-peroxo or p-superoxo complexes by electron transfer from cop-per(i) to dioxygen. The co-ordinated oxygen may then act as an electrophile to the aromatic ring. A possible mechanism for the ortho-hydroxylation of phenol by dioxygen in the presence of copper catalysts is shown in Fig. 9-29. 

The mechanism proposed in Figure 3 can easily be applied to the oxidative depolymerization of the lignin macromolecule by nitrobenzene or copper(II) in aqueous alkali. A recent model of softwood lignin (24) contains 27% free benzylic hydroxyl groups and 12% aromatic ring-conjugated double bonds (stilbene... 

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