Different mechanisms for the reaction of disubstituted aromatic esters and thioic S-esters with electrochemically generated superoxide

Abstract

Dialkyl pyridine and benzene dicarboxylates and dicarbothioates (dithioic S,S′-diesters) react with electrochemically generated superoxide to form the monocarboxylate anions in almost 100% yield (step 1) before being converted to the dicarboxylate dianions (step 2). This result indicates that the rate of reaction with superoxide to form the anion (rate constant k₁) is considerably faster than the rate to form the dianion (rate constant k₂) (k₁ ⪢ k₂). However, the number of moles/equivalent of superoxide needed to bring about the conversion of the diesters to the carboxylate anions is 2.0 ± 0.1 for each step, while for the dithioic S-esters only 1.7 ± 0.2 moles/equivalent are needed for each step. The reason for this difference between the diesters and dithioic S-esters is most likely due, in the case of the dithioic S,S′-diesters, to homolytic rather than heterolytic bond cleavage occurring, where the C(O)–S bond is cleaved to form the thiyl radical (˙SPr), which either dimerises to form PrSSPr or further reacts with O₂^˙- to form ^-SPr. Previously postulated mechanisms do not account for the difference between esters and thioic S-esters. From a synthetic perspective, this work provides a useful route for the preparation of mixed carboxylates–thioic S-esters from dithioic S-esters.

References

1. D. L. Maricle and W. G. Hodgson, Anal. Chem., 1965, 37, 1562.
2. E. L. Johnson, K. H. Pool and R. E. Hamm, Anal. Chem., 1966, 38, 183.
3. D. Vasudevan and H. Wendt, J. Electroanal. Chem., 1995, 392, 69.
4. F. Magno and G. Bontempelli, J. Electroanal. Chem., 1976, 68, 337.
5. J. San Filippo, Jr., L. J. Romano, C.-I. Chern and J. S. Valentine, J. Org. Chem., 1976, 41, 586.
6. M. J. Gibian, D. T. Sawyer, T. Ungermann, R. Tangpoonpholvivat and M. M. Morrison, J. Am. Chem. Soc., 1979, 101, 640.
7. D. T. Sawyer and J. S. Valentine, Acc. Chem. Res., 1981, 14, 393.
8. A. I. Vogel, A Text-Book of Practical Organic Chemistry, 3rd edn., Longmans, New York, 1956.
9. M. M. Bates, T. J. Cardwell, R. W. Cattrall, L. W. Deady and C. G. Gregorio, Talanta, 1995, 42, 999.
10. A. J. Fry and W. E. Britton, in Laboratory Techniques in Electroanalytical Chemistry, ed. P. T. Kissinger and W. R. Heineman, Marcel Dekker, New York, 1984, ch. 13.
11. R. D. Webster, A. M. Bond and T. Schmidt, J. Chem. Soc., Perkin Trans. 2, 1995, 1365.
12. R. D. Webster, A. M. Bond and R. G. Compton, J. Phys. Chem., 1996, 100, 10288.
13. T. Awata, M. M. Baizer, T. Nonaka and T. Fuchigami, Chem. Lett., 1985, 371.
14. R. D. Webster and A. M. Bond, J. Org. Chem., 1997, 62, 1779.
15. K. Boujlel and J. Simonet, Tetrahedron Lett., 1979, 1063.
16. F. Magno, G. Bontempelli and G. Pilloni, J. Electroanal. Chem., 1971, 30, 375.
17. J. K. Howie, J. J. Houts and D. T. Sawyer, J. Am. Chem. Soc., 1977, 99, 6323.