Synthesis of pinacols through electrochemical reduction of carbonyl compounds at platinum cathode in non-aqueous weakly acidic medium

The electrochemical reduction of p-rnethylacetophenone, p-dimclhylaminobcnzaldchydc and p-hydroxyhcn zaldehyde was carried out at controlled potential in weakly acidic medium (phenol, pKa = 9.98) for the 1>urpose of investi~:aling this type of reduction as a possible synthetic procedure for the preparation of 1.2-diol (or pinacol). The producl~ formed during electrolysis at constant cathode potential arc reported here.

In the present communication. we are reporting the result of our studies on the reduction of some carbonyl compounds specially aryl ketone and arylaldehyde viz. pmethy !acetophenone. m-nitro benzaldehyde. p-di methy 1aminobenzaldehyde and p-hydroxybenzaldehyde. Constant potential electrolysis was carried out. At definite potential the electrode surface is covered by dark brownish layer of the reduction product and that very product is continually diffused in the bulk which is visible.

Results and discussion
The electrochemical reduction of carbonyl compounds in aqueous ethanol system is pH dependent and takes place with the intermediacy of corresponding free-radical carbinolate ion in the alkaline region and the radical carbinolate is formed in the acidic medium. The reduction to the carbinol of the resultant free radical and its ion possibly occurs before dimerisation 1 • But as per our experiment in non-aqueous weakly acidic (phenol. pK,. = 9.lJH) medium no such pH-dependency is seen. In this experiment, the dimethylformamide (DMF) and weak acid (phenol) provides the good system 2 , and phenol serves as a proton donor.
It is generally accepted that the behaviour of carbonyl compounds. in aqueous or non-aqueous systems, is determined by the availability of protons. In water and alcohol this can be measured by pH and controlled by buffers. In aprotic solvents, it can be controlled by addition of proton donors 3 . Mechanism of carbonyl reduction involves a reversible one-electron reduction to form an anion radical, which in the absence of proton donors. can diffuse into the solution to dimerise. With sufficient proton-donor, the rate of protonation can be more rapid than the rate of diffusion 4 .
For const<mt-potential electrolysis, the conventional threeelectrode cell with platinum as working as well as counter electrode and saturated calomel electrode (SCE) as reference electrode was used.
In this case 0.1 M aqueous solution of KCI and phenol were used as supporting electrolyte as a proton-donor respectively.
All the electrolysis was carried out at their corresponding reduction potentials and were completed in 3 h (Table 2). After 3 h no reduction product was seen to diffuse in the bulk.
In each case the work involved extracting the non-aqueous solution with double-distilled water (4 x 100 ml). The reddish brown organic liquid obtained in case of pmethylacetophenone dark brownish organic liquid in case of m-nitrobenzaldehyde, blackish semi solid substance in case of p-dimethylaminobenzaldehyde and reddish brown coloured liquid in case of p-hydroxybenzaldehyde are entirely different from those of starting material. Starting materials give positive test of carbonyl functional group with 2,4-dinitrophenylhydrazine; whereas products are not responding the test of carbonyl functional group with the above reagent.
All the products were tested by the usual chemical ways and spectral analyses.
A peak at 1693 em-' (-C=O) was observed in the starting material, which disappeared in the products.
Pinacols are synthetically important compound. Using electrochemical technique pinacol synthesis became more important because this technique is a part of Green Chemistry i.e. it can be prepared without any environmental hazards.

Experimental
Reaction mixture : 50 cc of the non-aqueous solution of the starting material +50 cc of the 0.1 N aqueous solution of KCI + 5 cc phenol (pKa 9.98).
Controlled-potential electrolytic studies were carried out using three-electrode cell assembly having Pt-plate as working as well as counter electrode. and SCE as a reference-electrode; and four-necked reaction bottle (designed by our laboratory). Potential-cum-galvanostat was used for carrying out controlled-potential electrolysis.