“…Furthermore, 4-HBOAc was inclined to undergo side reactions, therefore forming unwanted dimers and having a direct influence on the selectivity of the reaction. It has been reported that dimer formation in such solutions may be suppressed by ensuring that the substrate loading is kept below 5% (mass/vol), and this was confirmed by the current authors.…”
Mechanistic proposals and predictions made in a preceding
paper (Part A) were evaluated by carrying out the catalytic
air oxidation of p-cresol in an alternative solvent system,
comprising either a mixture of ethylene glycol and acetic acid
(for oxidations under acidic conditions) or ethylene glycol and
water (for oxidations under basic conditions). The results
obtained in these experiments confirmed that ethylene glycol
acts as a nucleophile in these solvent systems, thereby stabilizing
the quinomethide intermediate and resulting in highly efficient
oxidations in both alkaline and acidic media. 4-Hydroxybenzaldehyde, the desired product, was thus obtained in isolated
yields of up to 98% and purities >99%. The inherent drawbacks associated with alkaline methanol and aqueous acetic acid
solutions were thus circumvented, and the result is a highly
efficient process for the production of 4-hydroxybenzaldehyde.
“…Furthermore, 4-HBOAc was inclined to undergo side reactions, therefore forming unwanted dimers and having a direct influence on the selectivity of the reaction. It has been reported that dimer formation in such solutions may be suppressed by ensuring that the substrate loading is kept below 5% (mass/vol), and this was confirmed by the current authors.…”
Mechanistic proposals and predictions made in a preceding
paper (Part A) were evaluated by carrying out the catalytic
air oxidation of p-cresol in an alternative solvent system,
comprising either a mixture of ethylene glycol and acetic acid
(for oxidations under acidic conditions) or ethylene glycol and
water (for oxidations under basic conditions). The results
obtained in these experiments confirmed that ethylene glycol
acts as a nucleophile in these solvent systems, thereby stabilizing
the quinomethide intermediate and resulting in highly efficient
oxidations in both alkaline and acidic media. 4-Hydroxybenzaldehyde, the desired product, was thus obtained in isolated
yields of up to 98% and purities >99%. The inherent drawbacks associated with alkaline methanol and aqueous acetic acid
solutions were thus circumvented, and the result is a highly
efficient process for the production of 4-hydroxybenzaldehyde.
“…Oxidations of cresols through acetoxylation of the cresolic methyl group using acetic acid media in the presence of palladium catalysts have also been investigated extensively. , Fache et al have proposed the following reaction pathway to explain their observations under these reaction conditions (Scheme ): 13 According to their proposal, p -cresol 10 is oxidized primarily to 4-hydroxybenzyl acetate 11 when the solvent is glacial acetic acid. This acetate is then only slowly oxidized to the aldehyde 13 .…”
Section: Resultsmentioning
confidence: 99%
“…The main disadvantage of their system is the requirement that low substrate loadings (2−5 mass%) be used, since dimers and oligomers result at higher loadings. 2 Fache et al reaction pathway for cresol oxidation in acetic acid medium …”
Section: Resultsmentioning
confidence: 99%
“…Recently, it was shown that certain supported platinum group metal (PGM) catalysts could be used to afford benzylic oxidation of a cresol without protection of the phenolic group with reasonable success . These procedures are supposed to be flexible enough, particularly with hindered cresols, to allow for the selective preparation of various oxidation products, including nucleophilic addition products and a range of coupled compounds .…”
Section: Introductionmentioning
confidence: 99%
“…Deactivation of the very expensive PMG catalysts is partially relieved by the use of transition metal promoters. Regeneration of such catalysts is, however, problematic and, therefore, expensive. , …”
The catalytic air oxidation of p-cresol and 2,6-di-tert-butyl-4-methylphenol to the corresponding benzaldehydes was investigated to determine the mechanism at work in these oxidation
reactions. A number of intermediates and byproducts, mainly
in the form of dimers, were observed during the course of the
reactions, and their structures were elucidated by spectroscopic
and chromatographic methods. The existence of these compounds in the reaction mixtures, and their proposed methods
of formation, provided further insight into the mechanism
involved in these oxidations.
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