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

Abstract

The kinetics and yields for the formation of benzaldehydes from benzylic alcohols via aerobic oxidation using homogeneous catalysts in acetic acid are reported. For compounds of the type X-C 6 H 4 CH 2 OH where X ¼ H, 4-CH 3 , 4-OCH 3 , 4-Cl, 4-COOH the observed yields were 84, 78, 57, 88 and 52 mol %, respectively. These yields are comparable to those reported for the stoichiometric Co(III)/Br oxidation of these same benzylic alcohols. These results are limited to benzylic alcohols. The high yields are possible for two reasons: 1) the rate of reaction exhibits two maxima in which the benzaldehyde is predominately being generated in the first phase and the carboxylic acid in the second and 2) the competing esterification of the benzylic alcohol with the acetic acid solvent is an order of magnitude slower. A variety of other catalysts were also evaluated which give lower yields mainly because they are less active and the competing esterification reaction becomes more important. Water addition to the acetic acid increased the rate of acetylation of the alcohol, contrary to expectation. The oxidation of the benzaldehyde appears to be inhibited by the presence of the benzylic alcohol. Even though the rate of oxidation of the benzaldehyde, by itself, is equally reactive as the benzylic alcohol, the benzaldehyde does not significantly react until the benzylic alcohol concentration is very low. Two possible reasons, using a free-radical chain mechanism, are suggested to explain this behavior. A mechanism demonstrating why the stoichiometric and catalytic reactions give similar yields is given.

The benzaldehyde oxidation paradox explained by the interception of peroxy radical by benzyl alcohol

“…1; the analysis revealed that no other products were formed. These simple studies confirm Partenheimer's observations that in benzyl alcohol oxidation catalysed by Co (III), benzoic acid production only begins to accelerate when the benzyl alcohol level in the reaction mixture falls below B10% 25 . That a very small amount of benzyl alcohol, present in benzaldehyde, is evidently involved in preventing the oxidation of benzaldehyde to benzoic acid forms the premise for the more detailed studies presented below.…”

“…The question has been posed previously and it has long been recognized that benzyl alcohol is one of a diverse group of organic molecules that can act as an inhibitor of benzaldehyde oxidation 25 . Investigations on the reactivity and stability of benzaldehyde, under oxidation conditions date back to the late 1920s; McNesby and Heller gave a comprehensive critical review of the early work in 1954 (ref.…”

“…Authors have suggested a variety of explanations for the high selectivity to benzaldehyde when benzyl alcohol is oxidized, including: (a) preferential bonding of benzyl alcohol over benzaldehyde to the catalytic sites 27 , (b) solvent molecules (e.g. acetic acid) forming hydrogen bonds with intermediate peroxy radicals, reducing their reactivity and hence preventing further oxidation 25 and (c) component(s) of the catalyst selectively inhibiting the oxidation of aldehyde 28 . All these proposals have been made with less than adequate experimental proof while most are speculative in nature.…”

Aerobic Oxidation of Benzylic Alcohols in Water by 2,2,6,6‐Tetramethylpiperidine‐1‐oxyl (TEMPO)/Copper(II) 2‐N‐Arylpyrrolecarbaldimino Complexes

“…[1,3] Oxidation of alcohols to their corresponding carbonyl products is one of the fundamental transformations in organic chemistry; [4] benzylic aldehydes, for example, are versatile intermediates for perfumery and other syntheses of fine chemicals. [5] In large-scale applications the replacement of classical methodologies, based on stoichiometric oxidants such as chromates, selenium dioxide or manganese dioxide, by green sources of oxygen (dioxygen, air, H 2 O 2 ) is highly desirable. [6] The price, atomic efficiency and non-toxic by-products of these end oxidants make them attractive candidates for sustainable alcohol oxidation.…”

Partial Oxidation of 4‐tert‐Butyltoluene Catalyzed by Homogeneous Cobalt and Cerium Acetate Catalysts in the Br−/H2O2/Acetic Acid System: Insights into Selectivity and Mechanism

“…Apart from this, side reactions can be suppressed to some extent, as the relative reactivity of a hydrocarbon substrate and that of its partial oxidation products is known to change considerably in the presence of a catalyst. [3] Several transition metal catalyzed autoxidation reactions have been applied to industrial processes, for example, the production of terephthalic acid, used in the production of polyesters, from p-xylene, which is catalyzed by cobalt(II) acetate bromide, with air as oxidant. [4] The cobalt(II) acetate catalyzed oxidation of hydrocarbons with hydrogen peroxide as oxidant, bromide ions as cocatalyst, and acetic acid as solvent has been reported for the partial oxidation of (substituted) toluenes to the corresponding benzaldehydes.…”