Phenol cobalt complex catalyzed

Cobalt-Schifi base complexes catalyze the selective oxidation of phenols by dioxygen into quinols (equation 245561) or quinones (equations 246s62,563 and 247561) under mild conditions. 

Oxidative phenolic coupling.1 This cobalt complex and the related salcomine (2, 160, 3, 245 6, 507), both of which bind oxygen reversibly (102 2Co), catalyze the oxygenation of the phenol 1 to give carpanone 2 in 90-94% yield. Related complexes ol l c(ll) and Mn(II) are less effective. PdCl2 effects this reaction in 46% yield (4, 17(1) singlet oxygen is less efficient (29% yield). 

Bozell, J. J., Hames, B. R., and Dimmel, D. R., Cobalt-Schiff base complex-catalyzed oxidation of parasubsti-tuted phenolics - Preparation of benzoquinones. J Organic Chem 1995, 60 (8), 2398-2404. 

From the viewpoints of reaction mechanism and efficiency in organic synthesis, oxidation of phenols with dioxygen catalyzed by cobalt-, manganese- and related metal-amine complexes has been studied . In particular, much effort has been directed toward constructing new efficient catalysts by a combination of metals with... 

The proposed reaction mechanism of phenols with O2 catalyzed by Co-amine complexes is shown in Scheme 47. On oxidation of 2,6-dimethylphenol (206) to 2,6-dimethyl-p-benzoquinone (223), magnetic field effects in the cobalt(II)-catalyzed oxidations were examined by using two different high- and low-spin cobalt(II) complexes. The former complex, Co bis(3-(salicylideneamino)propyl)methylamine, Co SMDPT (S = 3/2), displays a maximum increase in the initial rate of ca 1000 G, while the low-spin cobalt complex, Co iV,iV -bis(salicylidene)ethylenediamine, Co salen (S = 1/2), in a 1 10 ratio with pyridine displays a maximum decrease in the initial rate at ca 800 G. The difference in the magnetokinetics of both complexes is explained by magnetic field effects on the singlet-triplet (S-T) radical pair and triplet-triplet (T-T) annihilation reactions... 

Bozell JJ, Hames BR, Dimmel DR (1995) Cobalt-Schiff base complex-catalyzed oxidation of ptira-substituted phenolics - preparation of benzoquinones. J Org Chem 60 2398... 

The oxidation of hindered phenols and naphthols catalyzed by Mn(TPP)Cl, cobalt(II) Schiff-base and dimethylglyoximatocobalt(II) (cobaloxime) complexes has been studied by Gaudemer et al. [49]. The following catalysts and substrates were used ... 

As cobalt complex-mediated oxygenation of these hindered phenols (11) occurs 30 times faster than base-catalyzed oxygenation 62), the phenolato Co(III) complex (12) evidently possesses special activation for reaction with molecular oxygen. In keeping with the desirability of a localized, soft anionic center in an oxygenase substrate, we can only assume that the metal is able to localize the n-anionic charge into an orbital with more [sp ] character. 

In benzene solution the rate law for oxidation 2,6-dimethyl-phenol by O2 catalyzed by the bis(3-(salicylideneamino)propyl) methylamine complex of cobalt(II), [Co(SMDPT)], is (35), which suggests participation of a C0-O2 complex in which the oxygen is activated to H abstraction. 

The rate of oxidation of 2,6-ditertiarybutyl,-4-substituted phenols by O2, catalyzed by cobalt(II)-Schiff base complexes, has a similar mechanism with Co(III)02 hydrogen abstraction as an initiating step. The resulting phenoxy radical oxidizes the cobalt(II) in the presence of O2 to give a... 

Competition between metal ion-induced and radical-induced decompositions of alkyl hydroperoxides is affected by several factors. First, the competition is influenced by the relative concentrations of the metal complex and the hydroperoxide. At high concentrations of the hydroperoxide relative to the metal complex, alkoxy radicals will compete effectively with the metal complex for the hydroperoxide. Competition is also influenced by the nature of the solvent (see above). Contribution from the metal-induced reaction is expected to predominate at low hydroperoxide concentrations and in reactive solvents. The contribution from the metal-induced decomposition to the overall reaction is readily determined by carrying out the reaction in the presence of free radical inhibitors, such as phenols, that trap the alkoxy radicals and, hence, prevent radical-induced decomposition.129,1303 Thus, Kamiya etal.129 showed that the initial rate of the cobalt-catalyzed decomposition of tetralin hydroperoxide, when corrected for the contribution from radical-induced decomposition by the... 

The role of Coball-dioxygen complexes in autooxidations other than phenol oxidation is less certain, and ostensibly similar reactions appear to follow radically different pathways. Thus, in the oxidation of thiols to disulfide catalyzed by Co11 species catalysis by the phthalocyanine complex [Con(TSPc)]4 apparently proceeds via a Co1 intermediate and without participation of Co—02 species,680 whereas catalysis by [CoH(TPP)] appears to involve initial formation of an >/ cobalt-dioxygen complex from which Of is displaced by thiolate.681 Several reviews giving extensive coverage to oxidations catalyzed by cobalt(II) complexes are available.649,650,682 683... 

Reactions catalyzed by transition-metal complexes allow the synthesis of a variety of esters ruthenium(II) promotes the addition of acids to alkynes,379 380 e.g. 2,6-difluorobenzoic acid (9) undergoes addition to but-l-en-3-yne to furnish the enol ester 10.380 Aryl bromides381 and aryl or vinyl triflates,382-384 but also aryl chlorides when their tricarbonylchromium(O) complexes are used,385 react with palladium382- 385 or cobalt complexes38 to form a C —M bond. Insertion of carbon monoxide into the carbon-metal bond followed by trapping with an alcohol or phenol leads to ester formation, e.g. triflate 11 gives ester 12.382... 

Oxidation of 2,6-di(tert-butyl)phenol (23) provides a useful test for comparing the activity of various catalysts 23 is oxidized with O2 catalyzed by metal-amine complexes to give only two products, 2,6-di(terr-butyl)-p-benzoquinone (74) and 3,3, 5,5 -tetra(ferf-butyl)diphenoquinone (24) (Scheme 46). Of the cobalt catalysts 230, 231, 232 and 237, the use of Co(salN-Medpt) in MeCN (room temp., 1 h) provided the most effective results, in which 74 was obtained in 100% yield. The oxidation rate and yield were dependent on... 

For example, the cobalt(II) complex for phthalocyanine tetrasodium sulfonate (PcTs) catalyzes the autoxidation of thiols, such as 2-mercaptoethanol (Eq. 1) [4] and 2,6-di(t-butyl)phenol (Eq. 2) [5]. In the first example the substrate and product were water-soluble whereas the second reaction involved an aqueous suspension. In both cases the activity of the Co(PcTs) was enhanced by binding it to an insoluble polymer, e.g., polyvinylamine [4] or a styrene - divinylbenzene copolymer substituted with quaternary ammonium ions [5]. This enhancement of activity was attributed to inhibition of aggregation of the Co(PcTs) which is known to occur in water, by the polymer network. Hence, in the polymeric form more of the Co(PcTs) will exist in an active monomeric form. In Eq. (2) the polymer-bound Co(PcTs) gave the diphenoquinone (1) with 100% selectivity whereas with soluble Co(PcTs) small amounts of the benzoquinone (2) were also formed. Both reactions involve one-electron oxidations by Co(III) followed by dimerization of the intermediate radical (RS or ArO ). 

Analogously, in the presence of silica-supported palladium catalysts, benzene is oxidized under ambient conditions to give phenol, benzoquinone, hydroquinone and catechol [37b]. Palladium chloride, used for the catalyst preparation, is believed to be converted into metallic palladium. The synthesis of phenol from benzene and molecular oxygen via direct activation of a C-H bond by the catalytic system Pd(OAc)2-phenanthroline in the presence of carbon monoxide has been described [38]. The proposed mechanism includes the electrophilic attack of benzene by an active palladium-containing species to to produce a a-phenyl complex of palladium(ll). Subsequent activation of dioxygen by the Pd-phen-CO complex to form a Pd-OPh complex and its reaction with acetic acid yields phenol. The oxidation of propenoidic phenols by molecular oxygen is catalyzed by [A,A"-bis(salicylidene)ethane-l,2-diaminato]cobalt(ll)[Co(salen)] [39]. 

Copper, titanium, cobalt and iron substituted mesoporous silicas (Cu-, Ti-, Co-, and Fe-HMS) were synthesized with dodecylamine surfactant as templating reagent. Three assembled pathways were used to bond Ti tartrate complex over mesoporous silicas (HMS). The above described catalysts were characterized by XRD and FT-IR, their metal loadings were measured by chemical analysis method. In catalytic testing, Cu-HMS and especially Fe-HMS show the best catalytic activity for hydroxylation of phenol with H2O2 in the presence of water. Ti-HMS and especially Ti tartrate complex assembled HMS catalysts exhibit the best epoxidative activity for catalyzing epoxidation of styrene with rcrt-butyl hydroperoxide. 

The cobalt(II) Schiff-base complexes Co(salen) and Co(salpr), as well as their 5-coordinate derivatives LCo(salen) and LCo(salpr) catalyze the oxidation of substituted phenols by dioxygen. This propensity is due to the formation of dioxygen complexes upon contact with 0, which is usually reversible, especially in nonprotic solvents. 

Phthalocyanine complexes of cobalt(II), copper(II), manganese(11), and iron(II) catalyze the oxidation of substituted phenols to the corresponding benzoquinones and diphenoquinones. Typical selectivity data are listed in Table III. 

A variety of transition metal complexes, especially those of cobalt and copper, catalyze the oxidation of phenols. Autoxidations of 2,6-disubstituted phenols in organic solvents produce mainly the 2,6-disubstituted-1,4-benzoquinone and the 3,5,3, 5 -tetrasubstituted-4,4 -diphenoquinone with Co catalysts and certain copper catalysts (eq. Poly(2,6-... 

An example of a kinetic study of a coordinated ligand reaction at ruthen-ium(III) is provided by that of hydrolysis of p-nitrophenylacetate catalyzed by [Ru(NH3)5(im)] . This complex is a very effective catalyst, being 10,000 times more effective than its cobalt(III) equivalent.Acetylacetone exchange at [Ru(acac)3] will be discussed under cobalt(III) (see Section 5.7.5.2). The rate law for ruthenium trichloride catalysis of oxidation of (substituted) phenols by periodate in alkaline aqueous solution is claimed to indicate preequilibrium formation of a ruthenium(III)-phenol complex. This is assumed to arise from re action between the phenol and RuOHaq present in aqueous solutions of ruthenium trichloride.The substitution reaction... 

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