Oxidation of 4-methylphenol

JC Farrand and DC Johnson. Peroxy acetic acid oxidation of 4-methylphenols and their methyl ethers./. Org. Chem. 36 3606-3612, 1971. 

OXIDATION OF 4-METHYLPHENOL IN ACETIC MEDIA IN THE PRESENCE OF PALLADIUM-BASED CATALYSTS... 

As expected, we found that the oxidation of 4-methylphenol in acetic acid medium in the presence of Pd-Sn / C catalyst leads to 4-hydroxybenzylic acetate with a good selectivity (Table 2, entry 2). The study of the reactionnal intermediates in such a medium shows the difficulty to oxidize the acetate into 4-hydroxy-benzaldehyde under these conditions (Table 2, entry 5) while the esterification of alcohol by acetic acid is complete (Table 2, entry 3). 

Scheme 1 Oxidation of 4-methylphenol in the presence of Pd-based catalysts in water-acetic acid media.

Various Pd-based catalysts used in acetic acid / water media allow the oxidation of 4-methylphenol into 4-hydroxybenzaldehyde with good selectivity. The best results are obtained with two kinds of catalysts Pd-Sn / C and prereduced Pd/C (Table 2, entries 1 and 8). With total conversion of cresol, 4-hydroxybenzaldehyde can be obtained with an average selectivity of 65 %. 4-hydroxybenzoic acid is the main by-product (Table 1, entry 9). 

FIGURE 6.18 Oxidation of ortAo-methylphenols to the corresponding ortho-quinone methide via transient zwitterionic intermediates that are stabilized by forming a complex 19 with the 2,5-dihydroxy[l,4]benzoquinone-derived bis(sulfonium ylide). 

Oxidative coupling was used for the synthesis of some stabilizers having properties of AO or FR. Products of oxidation of 4,4 -isopropylidenebis(2-methyl-6-tert-butylphenol with pota ium ferricyanide [151] or of 4,4 -thiobis(2-rcr/-butyl-5-methylphenol) with oxygen in the presence of copper salts [152] (118) were tested as AO. Thermostable fireproofing additives containing 1 to 4 bromine atoms on a phenolic moiety and designed for the stabilization of thermoplastics, e.g. 119, were prepared by oxidative coupling of brominated phenols [153]. 

SCHEME 20. Anodic oxidation of 4,5-dimethoxy-2-methylphenol in the presence of ethyl vinyl ether... 

More recently, the direct formation of hydroxybenzaldehydes by the oxidation of the corresponding alkylphenols was reported. However, the oxidation of 4- and 2-methylphenol respectively into p- and o-hydroxybenzaldehydes remains difficult, leading very often to heavies. For instance, the catalytic systems used for the... 

Formation of these compounds, not or poorly oxidable under the reactionnal conditions, is due to the nucleophilic reaction of 4-methylphenol on 4-hydroxybenzylalcohol or on the corresponding acetate. 

Unfortunatly, oxidation of 2-methylphenol with the previous catalysts (Pd-Sn/C and Pd/C) only gives small amounts of 2-hydroxybenzoic acid and heavies (table 2, entry 10). These heavies are polyethers probably obtained by reaction of o-cresol itself with 2-hydroxybenzylacetate or 2-hydroxybenzylalcohol. Apparently, palladium catalysts activate the benzylic C-H bond of o-cresol, but the oxidation of the intermediates seems less rapid than side reactions. On the other hand, we have check that platinum catalysts, which are known to be excellent catalysts for the oxidation of 2-hydroxybenzylalcohol into 2-hydroxybenzaldehyde in basic aqueous medium (ref. 3), is unable to activate efficiently the benzylic C-H bond of cresols. We synthesized bimetallic catalysts, Pd-Pt / C, with the hope that palladium would activate benzylic C-H bond and platinum would accelerate the oxidation of intermediate alcohols. Effectively, this new catalyst allows to recover 2-hydroxybenzaldehyde with 14 % selectivity at 70% conversion (Table 2, entries 11-12). Addition of bismuth salts are known to improve the aldehyde yield in the saligenin process. With such additives, the selectivity of the aldehyde can reached 60% for a total cresol conversion. Of course Pd-Pt / C can also oxidize 4-methylphenol but it does not bring significant improvement compared to initial catalysts. 

Oxidative cross-coupling has been effected moderately selectively. For example, by the addition of aluminium chloride in nitromethane to a nitromethane solution of 4-methylphenol and 4-methoxyphenol under nitrogen followed by gradual treatment with with dichlorodicyano-1,4-benzoquinone and reaction at ambient... 

Di-/ f2 -butyl-4-methylphenol, which is commonly known as BHT (butylated hydroxy toluene), is a widely used phenoHc antioxidant in the stabilization of oils, mbber, and polyolefins (44). BHT is also one of the few phenolic antioxidants approved by the FDA as a direct food additive where it is used to retard the oxidation of naturally occurring oils in food. 

In contrast with the Schiff base salen, salicylaldehyde oxime (79) (salox) complexes of Co have received comparatively little attention, but a series of bis-bidentate divalent complexes of the form iraiis-Co(sa 1 ox)2( D M SO)2 have been reported.343 The heterocyclic bidentate oxime violurate (lH,3H-pyrimidine-2,4,5,6-tetrone 5-oximate, Hvi) (80) and its /V-methyl (mvi) and /V,/V -dimethyl (dmvi) derivatives form high-spin divalent [Co(vi)]+ and Co(vi)2 complexes, whereas [Co(vi)3] is low spin.344 The mixed-ligand Co(dmvi)2(phen) complex is also low spin. The crystal structure of m-Co(pxo)2Br2 (pxo = 2-acetylpyridine-l-oxide oxime) is isostructural with the Ni11 relative.345 The dichloro complex also adopts a cis configuration. The tridentate dioximes 2,6-diformyl-4-methylphenol dioxime and 2,6-diacetyl-4-methylphenol dioxime (Hdampo) form binuclear complexes of the type (81a) and (81b) respectively.346 Cobalt oxide nanoparticles were prepared by... 

The reaction is retarded by the addition of the radical chain scavengers 2,6-di-/-butyl-4-methylphenol and hydroquinone. The oxidation rate is not affected by the oxygen pressure (Table 2), indicating that re-oxidation of Ce(III) in 1 is not rate limiting. The oxidation rate is first order in formaldehyde and goes through a maximum with increasing concentration of 1. All these phenomena are consistent with a chain radical mechanism of oxidation (11,12). 

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