A one-pot electro oxidative nuclear and side chain acetoxylation of aromatic compounds at platinum electrode

Manish K. Srivastav, Apoorv Saraswat and R. K. P. Singh*

Department of Chemistry, University of Allahabad, Allahabad - 211 002 (India)

A direct, one-pot electrooxidative acetoxylation of aromatic compounds is reported here. Preparative electrolysis of various aromatic compounds was studied in the presence of acetic acid using DMF or acetonitrle as a non-aqueous solvent and suitable supporting electrolytes. The controlled potential electrolysis of aromatic compounds afforded nuclear and side chain acetoxylation depending upon the nature of the solvent and supporting electrolytes. A plausible mechanism for the acetoxylation process is discussed.

Pt-electrode; supporting electrolyte; electrochemical oxidation; green chemistry

Introduction

Electrochemical routes for organic synthesis must compete with more traditional chemical methods, as well as with emerging methods (e.g., enzymatic catalysis, photocatalysis). In view of environmental mandate, there is a global effort to replace the conventional catalysts by eco-friendly catalysts. Recently, extensive research work has been carried out on the procedure of esterification of acid with alcohol using catalysts such as zeolitic molecular sieves¹, ion-exchange resin², zirconium sulfate³, hafnium or zirconium salt⁴, polymer supported titanium tetrachloride⁴, ZrO₂ and Mo-ZrO₂ solid acid catalysts⁵ and heteropoly acids⁶ etc. Most of the reported procedures for the synthesis of esters require the use of sulfuric acid, hydrochloric acid and toxic chemicals like dimethyl sulfate, methyl iodide or unsafe reagents such as diazomethanes which are environmentally hazardous and unacceptable.

The electroorganic synthesis has certain merits because these reactions do not require oxidizing chemicals. It involves application of a potential in the presence of active electrode surfaces and the resulting flow of current drives the oxidation or reduction and subsequent recombination of reactants⁷. Electrochemistry can be considered a green methodology in organic synthesis, due to its non-polluting reagent, the electron⁸. At constant potential, a variety of electrochemical reactions takes place when a solution containing a reacting material is oxidized or reduced which depend on the structure of reacting molecules and the environment viz. nature of solvent, value of electrode potential and supporting electrolytes. After some preliminary discussion it is clear that one or other electrochemical processes can be making favorable by adjusting experimental parameters⁹. In the present communication we are reporting the result of our comparative study on the electrochemical nuclear and side chain acetoxylation of some aromatics viz. benzene, toluene, ethyl benzene, anisole, naphthalene, anthracene etc.

Results and Discussion

In any electrochemical study, the first question which must be answered is which component of the solution is actually undergoing electron transfer because the direct electrochemical oxidations or reductions of substrates utilize practically mass-free electrons as the only reagent¹⁰. During electrochemical acetoxylation, voltametric data shows that aromatic rings are more easily oxidized than acetate ion¹¹. Acetoxylation of aromatic substrates has often been cited as evidence for the existence of the acetoxy radicals in the Kolbe reaction^12,13. Recent studies have demonstrated that acetoxy radicals are intermediates in the Kolbe synthesis, but not in the formation of ring-substituted aromatic acetates¹⁴. Aromatic compounds are generally discharged below the critical potential for oxidation of acetate ion (1.90- 2.10 V vs SCE) and undergo what appears to be an electrophilic substitution reaction. In homologous solution, the lifetime of the acetoxy radical (10-9 – 10-10 sec) is estimated to be too short for reaction outside of the solvent cage in which it is formed ¹⁵.

In this paper, we discussed in details a selective one-pot electrochemical acetoxylation of our model aromatic compounds, naphthalene (A) and toluene (B), at room temperature in acetic acid and acetonitrile under various experimental conditions, affording 1-acetoxynaphthalene (C) and benzyl acetate (D) as the exclusive product. At room temperature, the electrooxidative acetoxylation of aromatic systems was carried out with sodium acetate or lithium perchlorate in acetic acid via an EC type mechanism at potential range from 1.25V to 1.95V. Most anodic reactions of aromatic compounds are initiated by oxidation of hydrocarbon, it is then appropriate to inquire into the nature of the oxidation step. It is generally accepted that most oxidations of aromatic hydrocarbons involve removal of one electron from the hydrocarbon to afford a radical cation. The structure of the radical cation and the environment in which it is created both influence its subsequent reactions. When the nucleophile is another reactant, it can attack the ring to afford a cationic intermediate after a second electron transfer, which most commonly will lose a proton to regenerate the aromatic ring and afford a nuclear substitution product.

Figure Electrochemical oxidation of aromatic ring at Pt-electrode

Scheme I Click here to View Scheme

Indeed the electrochemical oxidation of alkyl aromatics in acetic acid affords clean side-chain substitution when a non-nucleophilic salt is used as the supporting electrolyte, whereas a mixture of side-chain and ring substitution products is obtained when sodium acetate is used as the supporting electrolyte. Nuclear substitution is favoured in the presence of good nucleophile and where the ring is not alkylated. The electrooxidative acetoxylation of aromatic nuclear ring was done by electrogenerated intermediate viz. radical cation. The proposed mechanism of this electrochemical reaction can be explained on the basis of attachment of acetoxy group on aromatic ring through the electrochemically generated radical cation of aromatic ring (Scheme II). The possible mechanism of electrooxidative nuclear ring acetoxylation of aromatic substrates is as follows:

Scheme II Click here to View Scheme

Electrochemical oxidation of toluene and ethyl benzene gives side-chain substituted acetoxylated products (6, 7 and 8) when lithium perchlorate or potassium nitrate is used as the supporting electrolyte in acetic acid as a solvent. The possible mechanism of electrooxidative side-chain acetoxylation of aromatic substrates is as follows:

Scheme III Click here to View Scheme

Single nuclear ring viz. benzene, naphthalene, anthracene in acetic acid affords the formation of nuclear substituted products when sodium acetate is used as a supporting electrolyte. In case of substituted aromatic ring, when lithium perchlorate and other supporting electrolytes are used, only side chain substituted products are obtained. In case of acetoxylation of polynuclear aromatic ring, the main products are nuclear substituted acetoxy aromatic ring. G. Bookmain and H. P. Frintz¹⁶ reported nuclear methoxylation of substrates possessing rather high oxidation potential is achieved with difficulty whereas naphthalene and anthracene are readily methoxylated. Electrochemically nuclear substitution in aromatic compounds is almost always preferred, which is reasonable since loss of a proton restored the aromatic stability and therefore has a low activation energy¹⁷. These results suggest that the substrate undergoes electron transfer to form a cationic species which reacts with acetate ion. Both cation radical and dication mechanisms in addition to a concerted two electron transfer to the anode followed by C-O bond formation have been proposed¹⁸.

Table 1: Current-potential data as recorded by potential cum galvenostat

P*= Potential in Volt, C*= Current in mA, Y*= Yield in percentage and AN= Acetonitrile