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

Abstract

The partial oxidation of 4-tert-butyltoluene to 4-tert-butylbenzaldehyde by hydrogen peroxide in glacial acetic acid, catalyzed by bromide ions in combination with cobalt(II) acetate or cerium(III) acetate, has been studied in detail. Based on the observed differences in reaction rates and product distributions for the different catalysts, a reaction mechanism involving two independent pathways is proposed. After the initial formation of a benzylic radical species, either oxidation of this intermediate by the metal catalyst or reaction with bromine generated in situ occurs, depending on which catalyst is used. The first pathway leads to the exclusive formation of 4-tert-butylbenzaldehyde, whereas reaction of the radical intermediate with bromine leads to formation of the observed side products 4-tert-butylbenzyl bromide and its hydrolysis and solvolysis products 4-tert-butylbenzyl alcohol and 4-tert-butylbenzyl acetate, respectively. The cobalt(II) catalysts Co(OAc)(2) and Co(acac)(2) are able to quickly oxidize the radical intermediate, thereby largely preventing the bromination reaction (i.e., side-product formation) from occurring, and yield the aldehyde product with 75-80 % selectivity. In contrast, the cerium catalyst studied here exhibits an aldehyde selectivity of around 50 % due to the competing bromination reaction. Addition of extra hydrogen peroxide leads to an increased product yield of 72 % (cerium(III) acetate) or 58 % (cobalt(II) acetate). Product inhibition and the presence of increasing amounts of water in the reaction mixture do not play a role in the observed low incremental yields.

Liquid‐Phase Oxidation Reaction and Mechanism of p‐Diethylbenzene to p‐Ethylacetophenone with H2O2 in a Batch Reactor and a Fixed Bed Reactor

“…In the first step, as already hypothesized by van de Water, bromide radical is probably formed by oxidizing Co 3+ , which is the reaction product of Co 2+ with H 2 O 2 . A weakly bond α‐H atom of PDEB is abstracted by bromide radical to produce alkyl benzene radical and HBr [Step 1 in Scheme ].…”

“…In the pathway I, the reaction of alkyl benzene carbocation with water generates EPEA, which will be converted to EAP by H 2 O 2 [Step 3 and Step 4 in scheme 2]. The carbonyl group on EAP could react with Co 2+ , H 2 O 2 or molecular bromine (formed in situ on reaction of bromide ions with H 2 O 2 ), to produce aldehyde group under outside energy (discussed above) [Step 5 in Scheme ].…”

“…Hence, the oxidants such as molecular oxygen and hydrogen peroxide (H 2 O 2 ) gradually attract more attention by the researchers because of their economy and environmental protection . Due to inefficiency, and safety, of oxygen in the reaction, the utilization of H 2 O 2 as an oxidant draws an increasing interest both in laboratory and industrial scale ,,…”

Homogeneous Liquid-Phase Oxidation of Ethylbenzene to Acetophenone in Continuous Flow Mode

“…In the present study, CoBr 2 was used to catalyze the oxidation, and acetic acid was used as the solvent. Although CoBr 2 in acetic acid is primarily used for C–H oxidations with molecular oxygen (MC system), , the catalytic ability of the Co/Br/acetic acid system has been demonstrated also for oxidations using H 2 O 2 as the oxidant . The bromide ion is thereby essential for the oxidation and almost no reaction was observed in the absence of bromide (see Supporting Information, Table S1).…”

Recyclable Supramolecular Ruthenium Catalyst for the Selective Aerobic Oxidation of Alcohols on Water: Application to Total Synthesis of Brittonin A

“…The use of inexpensive and abundant molecular oxygen results in the formation of water as by product. However, to carry out large-scale aerobic oxidations in flammable solvents, limiting oxygen concentrations are to be maintained to prevent combustion reactions. − Additionally, controlling the overoxidation of aldehydes into acid − and the selective oxidation of alcohols in the presence of oxidizable functional groups − are challenges that need to be addressed. Moreover, very few reports are available in the literature for aerobic oxidation of alcohol in water. − On account of these aforementioned drawbacks, the development of safe and highly selective aerobic oxidation protocols that are usable under ambient pressure conditions using recyclable catalysts in a benign solvent like water is highly desirable.…”