Phenols homogeneous catalysis

Kuntz, E., Amgoune, A., Lucas, C. and Godard, G. (2006) Palladium TPPTS catalyst in water C-allylation of phenol and guaiacol with allyl alcohol and novel isomerisation of allyl ethers of phenol and guaiacol./. Mol. Catal. A Chem., 244,124. Kuntz, E.G. (1987) Homogeneous catalysis. .. in water. Chemtech, 17, 570. Cornils, B. (1999) Bulk and fine chemicals via aqueous biphasic catalysis. /. Mol. Catal. A Chem., 143, 1. 

Yang HH, Eckert CA. Homogeneous catalysis in the oxidation of p-chloro-phenol in supercritical water. Ind Eng Chem Res 1988 27 2009-2014. 

As is the case for the Zn-Pro-Phenol aldol reaction described earlier, the development of catalytic reactions that remove the need for stoichiometric reagents is of paramount importance with respect to AE. Trost has been a major contributor in this field, and an overview of some of his labs contributions should provide a reference for the impact of homogeneous catalysis on AE [68, 69], Other modes of catalysis, such as organocatalysis, have also played an important role in the union of two substrates into product with high AE [70],... 

Pal et al. (1994) compared the catalysis of oxidative coupling reactions of various phenolic compounds by the enzymes, laccase and tyrosinase, and mineral catalyst, birnessite. Birnessite acts as a heterogeneous catalyst whereas laccase and tyrosinase function as homogeneous catalysts. Laccase and tyrosinase continue to oxidize catechol after repeated additions of the chemical, while birnessite lost its oxidizing activity after the first addition of catechol (Figure 2.20). In the case of birnessite,... 

Sippola, V. O. and Krause, A. O. I., Oxidation activity and stability of homogeneous cobalt-sulphosalen catalyst -Studies with a phenolic and a non-phenolic lignin model compound in aqueous alkaline medium. J Molecular Catalysis A-Chemical 2003, 194 (1-2), 89-97. 

Like BINOL, salicylaldehyde imines have become very important in asymmetric catalysis and a variety of polydentate ligands prepared from chiral monoamines and diamines are employed in oxidation reactions, carbenoid reactions and Lewis acid catalyzed reactions. As in the previous section, this section emphasizes the effect of the phenol moiety on the asymmetric catalysis. An imine derived from a chiral 1-phenethylamine and salicylaldehyde was employed in the copper catalyzed asymmetric cyclopropanation by Nozaki, Noyori and coworkers in 1966, which is the first example of the asymmetric catalysis in a homogeneous system . Salicylaldehyde imines with ethylenediamine (salen) have been studied extensively by Jacobsen and Katsuki and their coworkers since 1990 in asymmetric catalysis. Jacobsen and coworkers employed the ligands prepared from chiral 1,2-diamines and Katsuki and coworkers sophisticated ligands possess chirality not only at the diamine moiety but also at the 3,3 -positions. 

The interconversion of the various oxidation states of Mn in natural waters is influenced by UVR through its effects on reactions involving ROS [Chapter 8] and natural phenols, photoinduced charge transfer reactions, and microbial processes. The oxidation of Mn + is slow at pH < 8.5 in the absence of a catalyst. The oxidation of Mn(ii) is faster on metal oxide surfaces than in homogeneous solution in the pH range of 8 to 9 [217], and its oxidation also can be biologically mediated in the environment [153]. In comparison to bacteria-free waters, the oxidation rate of Mn(ii) in seawater is increased dramatically by catalysis on bacterial surfaces. However, even with such catalysis, its half-life still is of the order of weeks to months in open ocean waters [153]. 

The fact that Weiss and Downs have been aide to isolate phenol in the products of their reactions with solid catalysts indicates a hydroxylation mechanism similar to that postulated in the case of vapor phase catalysis, in whidi the formation of the monohydroxylated derivative is the first step. The presence of the hydroxyl group as a substituent in the benzene molecule activates the para and ortho positions so that the introduction of a second oxygen molecule would be expected to result in the formation of quinol (C6H4(OH)2l 4) and catechol (C0H4(OH)21 2) with a preponderance of the former. Quinone which would result from the further oxidation of quinol has been found in the oxidation products from benzene for the case of the homogeneous catalytic reaction. 

The present study shows fundamental differences in the hydroxylation kinetics of phenol using strong acid catalysts or titanium silicalites. With the latter, the reaction occurs slowly but regularly, while, with the solid acids, the reaction shows an induction period followed by a very fast autocatalysis. These results cast doubt on the validity of the tests performed by stopping the reaction at a determined time. They also call into question the mechanism of the acid catalysis, the homogeneous as well as the heterogeneous contribution. Finally, taking into account that water is the best solvent for this reaction, solid acids should be considered as valuable catalysts for hydroxylation of phenol. 

It must be noted that the phenol/aldehyde reaction can be catalyzed by Bronsted acids (protonation of the carbonyl oxygen) as well as by Lewis acids (coordination of the carbonyl oxygen). In the latter case one Lewis centre (e.g. Al ) can accommodate and activate both the phenol and the aldehyde (cq. the benzyl alcohol, in the consecutive reaction). As a consequence, ortho-substitution is favoured [14,15]. The high 2,2 -dihydroxydiphenylmethane selectivity we obtained with homogeneous Al " -catalysis and with 7-alumina is consistent with these data. Additionally, the finding that the H - US - Y catalyzed toluene/formaldehyde-condensation gives a low 2,2 -selectivity, 19% [16], compared to the 32% we obtained with phenol, also indicates the hydroxyl-group plays a role. However, transalkylation, reported to lead to ortho-substitution in condensations of phenol with methanol on both zeolite- and non-zeolite Bronsted acid catalysts [17], can t be ruled out. 

Following the development of aerobic conditions, N-heterocychc carbenes (NHCs) were found to be equally efficient under conditions of homogeneous palladium catalysis [33]. Although initially their apphcation had focused on aerobic alcohol oxidation, later examples include Wacker oxidation of styrenes and 2-aUyl phenol cych2ation.[34]... 

Very early experiments revealed that the amine was absolutely critical in the first step of the reaction, hydrolysis. In the absence of any amine catalyst, chloroformate could be recovered virtually unreacted from the interfacial mixture even when pH s approached 14. If base catalysis were the dominant means of chloroformate hydrolysis, then typical phase-transfer catalysts in the presence of sodium hydroxide should at least promote hydrolysis of chloroformate to phenols. However, a variety of phase-transfer catalysts, including n-Bu4N" OH, produced little or no reaction of the bischloroformate during the time frame of a normal cyclization reaction. Under homogeneous conditions or very long reaction times, the chloroformate can be consumed to produce primarily linears and polymer with negligible levels of cyclics. 

It should also be mentioned that SUP catalysis has not only been successfully applied for the immobilisation of homogeneously dissolved transition metal conplexes but also for acidic catalysis. The first example of an immobilised acidic chloroaluminate(III) on a support was reported, as early as 2000, by Holderich and co-workers for the allqrlation of different aromatics (benzene, toluene, naphthalene and phenol) [85]. The acidic ionic liquid... 

Multiphase catalysis occurs in C02/water by adding surfactants, which form micelles that disperse catalysts in pressurised C02/water mixtures. This is known for homogeneous toluene oxidation,toluene oxidation on immobilised micelles, oxybromination of phenol and aniline derivatives, or enzyme catalysis. 

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