Acid-catalysed aryl hydroxylation of phenylazopyridines: reaction intermediates, kinetics and mechanism ¹

Abstract

A kinetic and product analysis study of the reactions of the three isomeric phenylazopyridines (PAPys) in aqueous sulfuric acid media (30–97 wt% H₂SO₄) is reported. The final products obtained from the reaction of 4-(phenylazo)pyridine (4-PAPy) are the hydroxylated product 4-(4-hydroxyphenylazo)pyridine, the reduction products 4-aminophenol and 4-aminopyridine, and a small amount of a dimerized product. 3-(Phenylazo)pyridine is unreactive, but 2-(phenylazo)pyridine gives the equivalent 2-(4-hydroxyphenylazo)pyridine, 4-aminophenol and 2-aminopyridine products. This product pattern, an oxidized azo-compound and two reduced amines, is similar to that found in the disproportionation of di-p-substituted hydrazinobenzenes observed in benzidine rearrangement studies. Consequently it has been proposed that the corresponding [N′-(4-hydroxyphenylhydrazino)]pyridines were formed as reaction intermediates in the present system; this is confirmed by showing that [N′-4-(4-hydroxyphenylhydrazino)pyridine synthesized independently gave the same products as 4-PAPy under the same conditions. The kinetic study shows that the 4-isomer reacted faster than the 2-isomer at all the acid concentrations investigated (the 3-isomer being inert). Rate maxima are observed, at ≈72 wt% H₂SO₄ for 4-PAPy and ≈86 wt% H₂SO₄ for 2-PAPy. To facilitate the kinetic analysis, values of pK_BH2²⁺ for the protonation of the substrates and the possible hydroxy products at the azo-group were determined, using the excess acidity method; the first protonation occurs on the pyridine nitrogen in the pH region. An excess acidity analysis of the observed pseudo-first-order rate constants as a function of acidity indicate an A2 mechanism, with the diprotonated substrate and either one HSO₄^– ion or one H₂O molecule in the activated complex. The proposed mechanism thus involves nucleophilic attack of HSO₄^– or H₂O at an aryl carbon of the diprotonated substrate in the slow step, resulting in an intermediate hydrazo species which gives the observed products in a subsequent fast step (cf. benzidine rearrangement).

References

1. Studies of azo and azoxy dyestuffs, part 23; part 22 is N. J. Bunce, S. McKinnon, R. J. Schnurr, S.-R. Keum and E. Buncel, Can. J. Chem., 1992, 70, 1028 Also mechanistic studies in strong acids, part 22; part 21 is ref. 27.
2. R. A. Cox and E. Buncel, in The Chemistry of the Hydrazo, Azo and Azoxy Groups, ed. S. Patai, Wiley, London, 1975, vol. 1, pp. 775–859.
3. R. A. Cox, J. Am. Chem. Soc., 1974, 96, 1059.
4. E. Buncel, R. A. Cox and A. J. Dolenko, Tetrahedron Lett., 1973, 215; E. Buncel, Acc. Chem. Res., 1975, 8, 132; A. J. Dolenko and E. Buncel, in The Chemistry of the Hydrazo, Azo and Azoxy Groups, ed. S. Patai, Wiley, London, 1975, vol. 1, pp. 725–773.
5. R. A. Cox, A. J. Dolenko and E. Buncel, J. Chem. Soc., Perkin Trans. 2, 1975, 471; R. A. Cox and E. Buncel, J. Am. Chem. Soc., 1975, 97, 1871.
6. E. Buncel and S.-R. Keum, J. Chem. Soc., Chem. Commun., 1983, 578.
7. E. Buncel and S.-R. Keum, Tetrahedron, 1983, 39, 1091.
8. E. Buncel and S.-R. Keum, Heterocycles, 1983, 20, 1751.
9. E. Buncel, S.-R. Keum, M. Cygler, K. I. Varughese and G. I. Birnbaum, Can. J. Chem., 1984, 62, 1628.
10. E. Buncel and I. Onyido, Can. J. Chem., 1986, 64, 2115.
11. S. Rajagopal and E. Buncel, Dyes and Pigments, 1991, 17, 303.
12. E. Buncel and S. Rajagopal, J. Org. Chem., 1989, 54, 798; E. Buncel and S. Rajagopal, Acc. Chem. Res., 1990, 23, 226.
13. R. A. Cox, I. Onyido and E. Buncel, J. Am. Chem. Soc., 1992, 114, 1358.
14. R. A. Cox and E. Buncel, in The Chemistry of the Hydrazo, Azo and Azoxy Groups, ed. S. Patai, Wiley, London, 1997, vol. 2, pp. 569–602.
15. R. B. Carlin and G. S. Wich, J. Am. Chem. Soc., 1958, 80, 4023; D. V. Banthorpe and M. O'Sullivan, J. Chem. Soc. B, 1968, 627; H. J. Shine and J. P. Stanley, J. Org. Chem., 1967, 32, 905; H. J. Shine, H. Zmuda, K.-H. Park, H. Kwart, A. G. Horgan, C. Collins and B. E. Maxwell, J. Am. Chem. Soc., 1981, 103, 955; H. J. Shine, J. Habdas, H. Kwart, M. Brechbiel, A. G. Horgan and J. San Filippo, J. Am. Chem. Soc., 1983, 105, 2823; E. S. Rhee and H. J. Shine, J. Am. Chem. Soc., 1986, 108, 1000; J. Org. Chem., 1987, 52, 5633.
16. D. V. Banthorpe, E. D. Hughes and C. K. Ingold, J. Chem. Soc., 1964, 2864; D. V. Banthorpe, Top. Carbocyclic Chem., 1969, 1, 1; H. J. Shine, Aromatic Rearrangements, Elsevier, Amsterdam, 1967, pp. 126–179; in Mechanisms of Molecular Migrations, ed. B. S. Thyagarajan, Wiley, New York, 1969, vol. 2, pp. 191–247; J. Phys. Org. Chem., 1989, 2, 491; in Isotopes in Organic Chemistry, ed. E. Buncel and W. H. Saunders, Elsevier, Amsterdam, 1992, vol. 8, pp. 1–40.
17. E. Buncel and K.-S. Cheon, J. Chem. Soc., Perkin Trans. 2, 1998, following paper.
18. R. A. Cox, Acc. Chem. Res., 1987, 20, 27; R. A. Cox and K. Yates, Can. J. Chem., 1979, 57, 2944.
19. R. A. Cox, J. Phys. Org. Chem., 1991, 4, 233.
20. R. A. Cox, D. B. Moore and R. S. McDonald, Can. J. Chem., 1994, 72, 1910.
21. J.-P. Bégué, F. Benayoud, D. Bonnet-Delpon, A. D. Allen, R. A. Cox and T. T. Tidwell, Gazz. Chim. Ital., 1995, 125, 399; Y. Chiang, A. J. Kresge, P. A. Obraztsov and J. B. Tobin, Croat. Chem. Acta, 1992, 65, 615 and earlier papers; M. Lajunen, M. Virta and O. Kylläinen, Acta Chem. Scand., 1994, 48, 122 and earlier papers.
22. M. Lajunen, M. Himottu and K. Tanskanen-Lehti, Acta Chem. Scand., 1997, 51, 515; M. Lajunen and J. Setälä, Acta Chem. Scand., 1997, 51, 334 and earlier papers.
23. M. Ali and D. P. N. Satchell, J. Chem. Soc., Perkin Trans. 2, 1995, 167; 1993, 1825.
24. R. A. Cox, Can. J. Chem., 1996, 74, 1774.
25. R. A. Cox, Can. J. Chem., 1996, 74, 1779.
26. R. A. Cox, J. Chem. Soc., Perkin Trans. 2, 1997, 1743.
27. R. A. Cox, Can. J. Chem., 1997, 75, 1093.
28. R. A. Cox and K. Yates, J. Am. Chem. Soc., 1978, 100, 3861.
29. R. A. Cox and K. Yates, Can. J. Chem., 1981, 59, 1560.
30. R. A. Cox, L. M. Druet, A. E. Klausner, T. A. Modro, P. Wan and K. Yates, Can. J. Chem., 1981, 59, 1568; R. A. Cox and K. Yates, Can. J. Chem., 1981, 59, 2116; 1984, 62, 2155.
31. R. A. Cox and K. Yates, Can. J. Chem., 1983, 61, 2225.
32. E. V. Brown and G. R. Granneman, J. Am. Chem. Soc., 1975, 97, 621.
33. E. Hayashi, T. Shimura, H. Yamazoe and Y. Tamura, Nippon Yakurigaku Zasshi, 1967, 63, 95; Chem. Abstr., 1968, 69, 42 567v.
34. D. Betteridge and D. John, Analyst, 1973, 98, 377; 390.
35. L. F. Fieser and K. L. Williamson, Organic Experiments, D. C. Heath, Lexington, 4th edn., 1979, ch. 39.
36. K.-S. Cheon and E. Buncel, unpublished results.
37. C. T. Davis and T. A. Geissman, J. Am. Chem. Soc., 1954, 76, 3507.
38. W. F. Giauque, E. W. Hornung, J. E. Kunzler and T. R. Rubin, J. Am. Chem. Soc., 1960, 82, 62.
39. M. Kerker, J. Am. Chem. Soc., 1957, 79, 3664; G. C. Hood and C. A. Reilly, J. Chem. Phys., 1957, 27, 1126; T. F. Young, L. F. Maranville and H. M. Smith, in The Structure of Electrolytic Solutions, ed. W. J. Hamer, Wiley, New York, 1959, p. 35; R. M. Wallace, J. Phys. Chem., 1966, 70, 3922; R. E. Lindstrom and H. E. Wirth, J. Phys. Chem., 1969, 73, 218; H. Chen and D. E. Irish, J. Phys. Chem., 1971, 75, 2672; D. J. Turner, J. Chem. Soc, Faraday Trans. 1, 1974, 70, 1346.
40. R. J. Gillespie, E. A. Robinson and C. Solomons, J. Chem. Soc., 1960, 4320.
41. R. A. Cox and K. Yates, Can. J. Chem., 1981, 59, 2853.
42. A. Bagno, G. Scorrano and R. A. More O'Ferrall, Rev. Chem. Intermed., 1987, 7, 313.
43. E. Buncel and W. M. J. Strachan, Can. J. Chem., 1969, 47, 911.