2- Methylnaphthalene, oxidation

This process competes favorably with benzylic hydrogen abstraction in toluene, less in ethylbenzene, and least in cumene (31). Such reactions do not seem significant in the oxidation of benzene derivatives. However, naphthalene reacts about 20 times as rapidly with phenyl radical as does benzene (16), and radical addition to the naphthalene nucleus may at least partly account for the slow oxidation rate in the methylnapthalenes. Among the minor products from both methylnaphthalene oxidations were compounds of molecular weight 296 ... 

As practiced by Hoffmann-La Roche, the commercial synthesis of vitamin is outlined ia Figure 1. Oxidation of 2-methylnaphthalene (4) yields menadione (3). Catalytic reduction to the naphthohydroquinone (5) is followed by reaction with a ben2oating reagent to yield the bis-benzoate (6). Selective deprotection yields the less hindered ben2oate (7). Condensation of isophytol (8) (see Vitamins, vitamins) with (7) under acid-cataly2ed conditions yields the coupled product (9). Saponification followed by an air oxidation yields vitamin (1) (29). 

Industrially, vitamin is prepared from the chromic acid oxidation of 2-methylnaphthalene (56). Although the yields are low, the process is economical owing to the low cost and availabiUty of the starting material and the oxidizing agent. However, the process is compHcated by the formation of isomeric 6-meth5l-l,4-naphthoquinone. As a result, efforts have been directed to develop process technology to faciUtate the separation of the isomeric naphthoquinone and to improve selectivity of the oxidation. 

In a biotechnology-based approach, Japanese workers have reported on the microbial conversion of 2-methylnaphthalene to both 2-methyl-1-naphthol and menadione by Jiodococcus (64). The intermediate 2-methyl-1-naphthol can readily be converted to menadione by a variety of oxidizing agents such as heteropoly acids (65) and copper chloride (66). A review of reagents for oxidizing 2-methylnaphthalene and naphthol is available (67). 

This procedure illustrates a general method for the preparation of aromatic carboxylic acids by oxidation of the corresponding alkylarenes.2 For example, 2-naphthoic acid (360 g., 93% yield m.p. 184-185°) was obtained from 2-methylnaphthalene (320 g., 2.25 moles), sodium dichromate (975 g., 3.26 moles, 45% excess), and water (1.81.). 

Similar experiments with 1-methyl- and 2-methylnaphthalene involved oxidation of the methyl group to hydroxymethyl and carboxyl groups (Cemiglia et al. 1984a,b). 

An intere.sting example in the context of waste minimization is the manufacture of the vitamin K intermediate, menadione. Traditionally it was produced by stoichiometric oxidation of 2-methylnaphthalene with chromium trioxide (Eqn. (8)), which generates 18 kg of solid, chromium containing waste per kg of menadione. Catalytic alternatives have been reported, but selectivities tend to be rather low owing to competing oxidation of the second aromatic ring (the. selectivity in the classical process is only 50-60%). The best results were obtained with a heteropolyanion as catalyst and O2 as the oxidant (Kozhevnikov, 1993). 

Amoco Amoco Chemicals Company, a subsidiary of Amoco Corporation, formerly Standard Oil Company (IN), is best known in the chemicals industry for its modification of the Mid-Century process for making pure terephthalic acid. /7-Xylene in acetic acid solution is oxidized with air at high temperature and pressure. Small amounts of manganese, cobalt, and bromide are used as catalysts. The modification allows the use of terephthalic acid, rather than dimethyl terephthalate, for making fiber. The process can also be used for oxidizing other methylbenzenes and methylnaphthalenes to aromatic carboxylic acids. See also Maruzen. 

Chemical/Physical. An aqueous solution containing chlorine dioxide in the dark for 3.5 d at room temperature oxidized 2-methylnaphthalene into the following l-chloro-2-methylnaphtha-lene, 3-chloro-2-methylnaphthalene, l,3-dichloro-2-methylnaphthalene, 3-hydroxymethylnaphtha-lene, 2-naphthaldehyde, 2-naphthoic acid, and 2-methyl-l,4-naphthoquinone (Taymaz et al., 1979). 

Methyl methacrylate monomer, see Methyl methacrylate IV-Methylmethanamine, see Dimethylamine Methyl methanoate, see Methyl formate Methyl-a-methylacrylate, see Methyl methacrylate Methyl-2-methyl-2-propenoate, see Methyl methacrylate Methyl 2-methyl-l-propenyl ketone, see Mesityl oxide p-Methylnaphthalene, see 2-Methylnaphthalene IV-Methyl-l-naphthylcarbamate, see Carbaryl IV-Methyl-a-naphthylcarbamate, see Carbaryl IV-Methyl-a-naphthylurethan, see Carbaryl... 

Tanaka et al. (1996,2000) studied the behavior of a series of naphthalene derivatives in AN solution containing NaN02 and CF3SO3H at 0°C in air. Naphthalene showed very low reactivity, and most of the starting material was recovered after the reaction. In case of 1-methylnaphthalene, a coupling reaction took place to produce 4,4 -dimethyl-l,T-binaphthyl in 91% yield alongside mononitro derivatives of the dimer in 1.5% yield. However, when the reaction is carried out on the same conditions but in inert (Nj) atmosphere, the yield of the dimer decreased from 97 to 15%, and no mononitro derivatives were formed. Therefore, the oxidation of NO with O2 to form NO2 (after the electron transfer to NO from 1-methylnaphthalene) is an obvious step of the reaction depicted in Scheme 4.42. 

Aromatic compounds are oxidized to quinones by bis(triorganosilyl) peroxides in the presence of a metal acid catalyst. Thus, 2-methylnaphthalene was oxidized with BTSP in the presence of Re207 and BU3PO in the presence of CHCI3, to a mixture of 59% 2-methyl-1,4-naphthoquinone and 8% 6-methy 1-1,4-naphthoquinone. ... 

Problem 11.20 (a) Why cannot 2-naphthalenecarboxylic acid be formed by oxidation of 2-methylnaphthalene (b) How is 2-naphthalenecarboxylic acid prepared ... 

The methylnaphthalenes were slowest to oxidize of the hydrocarbons studied after the standard 6 hours only a few per cent had been converted to the products shown ... 

Neither the relative number of benzylic hydrogens nor the base strength accounts for the slow oxidation rate of the methylnaphthalenes. Formation of radicals in the presence of aromatic hydrocarbons can lead to radical attack on the aromatic ring. Addition of phenyl or methyl radical to the ring gives a cyclohexadienyl radical that may disproportionate or dimerize, or undergo hydrogen abstraction by another radical (3, 9,13). 

Is it possible that the unexpectedly slow oxidations of molecules such as mesitylene, durene, and the methylnaphthalenes which the author has described are caused by the depletion of the bromide portion of the catalyst In these cases, bromination of the activated nuclei could occur and the bromide would be lost permanently. 

A palladium(II)-exchanged polystyrene sulfonic acid resin (Dowex 50W, H form) catalyzes the oxidation of 2-methylnaphthalene with 60% aqueous H2O2 (reaction 27), affording 2-methyl-l,4-naphthoqu1none (menadione) in 55-60% yield at 90-97% conversion. 3 Menadione is a commercially important vitamin K intermediate and these results compare favourably with those obtained in existing industrial processes that employ stoichiometric quantities of chromium trioxide in sulfuric acid. 

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