Electrochemical Reaction of Diphenylacetaldehyde in the Presence of Alcohols: Successive Reactions at the Anode and Cathode

We recently described the anodic oxidation of 1,1-diphenylacetaldehyde (1) to benzophenone in the presence of water.¹ The reaction was postulated to involve nucleophilic attack by water upon a cationic intermediate to afford alpha-hydroxydiphenylacetaldehyde, which then undergoes anodic decarbonylation to afford benzophenone. This hypothesis was tested by carrying out the electrolysis in ¹⁸O-labelled water. As expected, the benzophenone product was found to have incorporated the ¹⁸O label. We then anticipated that substitution of an alcohol (2) for water in the electrolysis of 1 would produce an a-alkoxyaldehyde (3) that would be more resistant to subsequent oxidation. We describe here the results of a study confirming this expectation but revealing further unsuspected mechanistic complexity, Electrochemical reaction in acetonitrile of diphenylacetaldehyde (1) in a undivided cell containing two carbon electrodes in the presence of cyclohexanol under constant current conditions affords alpha-cyclohexyloxydiphenylacetaldehyde cyclohexanol hemiacetal (2, R = cyclohexyl), which then ejects cyclohexanol to afford the corresponding aldehyde (3). Upon continued electrolysis, 3 is converted to a benzhydryl alkyl ether 4(Scheme 1).

 

Scheme 1. (insert image here)

 

The most surprising and indeed unprecedented feature of the electrochemistry of 1 under these conditions is the fact that both electrodes are involved. Since 1 is converted to 3 at the anode via 2, we initially assumed that the ethers 4 are produced by a succeeding anodic oxidation of 3. However, two control experiments disproved this hypothesis. First, 3 (independently synthesized by reaction of alpha-bromo-diphenylacetaldehyde² with cyclohexanol in the presence of silver tetrafluoroborate³) was placed in the anode compartment of a divided cell. It was found to be stable to anodic oxidation. On the other hand, when it was placed in the cathode compartment, it was quickly decarbonylated to 4a.The mechanism of this unusual mild cathodic cleavage reaction is under investigation.

Baizer introduced the term ‘paired electrolyses” for processes in which both electrodes participate.⁴ Examples include (a) parallel electrolyses, in which two components of a solution undergo simultaneous independent and unrelated transformation;⁵ (b) generation of two reagents, one at each electrode, that can react with a third component of the medium, as in the conversion of ethylene to propylene oxide by the dual action of electrogenerated hydroxide and bromine;⁶ or the simultaneous oxidation of glucose to gluconic acid at the anode and reduction to sorbitol at the cathode.⁷ Conversion of 1a to ethers 4 is, however, the first example of an organic electrode reaction involving significant structural transformations of a substance at both electrodes, that is, not merely changes in oxidation state, such as in the anodic oxidation of t-butylphenol to t-butylquinone followed by cathodic reduction of the latter to the hydroquinone.⁸

REFERENCES

(1) Merzel, R. L.; Fry, A. J. J. Electrochem. Soc. 2012, 159.

(2)  Padwa, A.; Dehm, D. J. Org. Chem. 1975, 40, 3139.

(3)  Fry, A. J.; Migron, Y. Tetrahedron Lett. 1979, 20, 3357.

(4) Baizer, M. M. in Lund, H. Baizer, M. M., eds., Organic Electrochemistry, 3^rd ed., Dekker: New York, N.Y., p. 1421.

(5) Baizer, M. M.; Hallcher, R. C. J. Electrochem. Soc. 1976, 123, 809.

(6) Manji, A.; Oloman, C. W. J. Appl. Electrochem. 1987, 17, 532.

(7) Pintauro, P. N.; Johnson, D. K.; Park, K.; Baizer, M. M.; Nobe, K. J. Appl. Electrochem. 1984, 14, 209.

(8) Ref. 4, Table 1 and references therein.