The Unusual Characteristics of the Aerobic Oxidation of 3,4‐Dimethoxytoluene with Metal/Bromide Catalysts

Abstract

DMtoluene (3,4-dimethoxytoluene; DM ¼ 3,4-dimethoxy), a model compound for lignin oxidation, can be autoxidized in acetic acid using a Co/Mn/ Br catalyst to its benzaldehyde in 51 mol % yield, and to its acid in 85 -92 mol % yield. This synthesis is unusual because a number of unprecedented phenomena occur. 1) The rate of molecular oxygen reacting with the substrate is bi-phasic, i.e., two maximum in the rate of reaction are observed. 2) In the first phase, all of the 3,4-dimethoxytoluene is converted to intermediates, but very little to the carboxylic acid. 3) During the second maximum of activity, virtually all the intermediates are converted to the carboxylic acid with 95 -100% mass accountability. 4) The rate of carbon monoxide and carbon dioxide formation is considerably higher during the second phase during which 5) 9 -12% of methyl 3,4-dimethoxybenzoate (the methyl ester of the benzoic acid) is formed. Mechanisms are suggested for these unusual phenomena.

The Aerobic Oxidative Cleavage of Lignin to Produce Hydroxyaromatic Benzaldehydes and Carboxylic Acids via Metal/Bromide Catalysts in Acetic Acid/Water Mixtures

“…[1,2] Products observed during the oxidation of hydrocarbons in acetic acid form from both the oxidation of the hydrocarbon and the solvent. [7,9,20] The same by-products associated with the oxidation of the acetic acid solvent (methyl acetate, methyl bromide, glycolic acid, maleic acid) have been detected during lignin autoxidation, see Table 3. The amount of ethyl acetate varied widely from the same sample which may suggest the the samples were non-uniform in character, i.e., truly representative samples were not always obtained.…”

“…[2] Indeed permanganate oxidation of lignin, after methylation, does produce the acids (as their methyated derivatives) given in Figure 1. 6) We have already shown that 3,4-dimethoxytoluene, [9] a lignin model compound, can be successfully oxidized to its corresponding acid although it did have many unusual characteristics. The maximum yield was 51% to the benzaldehyde and 92% to the carboxylic acid [Eq.…”

“…The rate constant for the disappearance of 3,4-dimethoxytoluene in acetic acid, as reported by us, is 1.6 10 À4 s À1 (t 1/2 = 1.2 h). [9] The reactivity of 4-methoxytoluene is about 16% higher than that of 3,4-dimethoxytoluene. [9] Thus, in an acetic anhydride/acetic acid mixture it is at least as active, if not more so, than in acetic acid alone.…”

“…The 3-methoxy group only slightly deactivates the oxidation of benzaldehyde so the comparison of vanillin acetate with 4-hydroxybenzoic acid is valid. [9] The oxidation of vanillin acetate is not autocatalytic as the reaction immediately initiated. Also the rate of reaction of vanillin acetate is much faster than that of 4-hydroxybenzaldehyde.…”

The High Yield Synthesis of Benzaldehydes from Benzylic Alcohols using Homogeneously Catalyzed Aerobic Oxidation in Acetic Acid

“…Such biphasic behavior has not been observed in our laboratory during the last nine years during which time numerous different alkylaromatic substrates have been similarly evaluated -except for the case of 3,4-dimethoxytoluene -which has been previously reported in detail. [13] Other non-equilibrium behavior has been reported for the oscillating metal/bromide catalyzed aerobic oxidation of benzaldehyde [14] in acetic acid.…”