Radical cation mechanism of aromatic halogenation by halogens or iodine chloride in 1,1,1,3,3,3-hexafluoropropan-2-ol

Abstract

The reaction between aromatic compounds ArH and halogenating agents, viz. iodine chloride, chlorine, bromine, iodine, N-bromosuccinimide and N-chlorosuccinimide, in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) has been investigated. EPR spectroscopy established that these reagents produced persistent radical cations ArH^˙+ from ArH with E_rev(ArH^˙+/ArH) up to 1.6, 1.3, 1.4, 1.1, 1.5 and 1.2 V vs. Ag/AgCl, respectively. Cyclic voltammetry of the halogenating species shows that no effect of complexation with halide ion is observed in HFP, as expected from its capacity to drastically attenuate nucleophilic reactivity, and that the cathodic peak potentials E_pc (referenced to the internal ferricinium/ferrocene redox couple) are significantly or remarkably higher in HFP than in acetonitrile. For N-bromosuccinimide, the difference amounts to almost 1 V.

The persistency of the radical cations in HFP is such that the kinetics of reactions between a halogenating agent, such as iodine chloride or bromine, and ArH, such as 1,4-dimethoxybenzene [E_rev(ArH^˙+/ArH) = 1.50 V vs. Ag/AgCl] or 1,4-dimethoxy-2,3-dimethylbenzene [E_rev(ArH^˙+/ArH) = 1.16 V], have been studied at room temperature over periods of hours. The initial concentration of the radical cation corresponds to yields in the range of 40–100%, depending on the reaction conditions. It is thus possible to establish that the radical cation decays via two pathways, one being the well known oxidative substitution reaction with halide ion. The second mechanism involves halogen atom transfer from the halogenating agent (Cl atom from ICl, Br atom from bromine). In the case of the radical cation of 1,4-dimethoxy-2,3-dimethylbenzene reacting with bromide ion or bromine, the latter reaction is >10² times faster.

References

1. (a) L. Eberson, M. P. Hartshorn and F. Radner, in Advances in Carbocation Chemistry, vol. 2, ed. J. M. Coxon, JAI Press, London, 1995, p. 207; (b) J. K. Kochi, Adv. Phys. Org. Chem., 1994, 29, 185.
2. A. Pross, Adv. Phys. Org. Chem., 1985, 21, 99; A. Pross, Acc. Chem. Res., 1985, 18, 212; S. S. Shaik, Prog. Phys. Org. Chem., 1985, 15, 197; S. S. Shaik, J. Am. Chem. Soc., 1981, 103, 3692.
3. Early proposals and refutation: J. Kenner, Nature, 1945, 156, 369; J. Weiss, Trans. Faraday Soc., 1946, 42, 116; G. A. Benford, C. A. Bunton, E. S. Halberstadt, E. D. Hughes, C. K. Ingold and R. E. Reed, Nature, 1945, 156, 688 Revival: C. L. Rerrin, J. Am. Chem. Soc., 1977, 99, 5516.
4. L. Eberson, M. P. Hartshorn and F. Radner, Acta Chem. Scand., 1994, 48, 937.
5. L. Eberson, M. P. Hartshorn, O. Persson and F. Radner, J. Chem. Soc., Chem. Commun., 1996, 215.
6. See, for example: A. J. Bard, A. Ledwith and H. J. Shine, Adv. Phys. Org. Chem., 1976, 12, 155; F. M. Dean and U. Oyman, Tetrahedron Lett., 1988, 3721; F. M. Dean, S. N. France and U. Oyman, Tetrahedron, 1988, 44, 4857; M. V. Jovanovic and E. R. Biehl, J. Org. Chem., 1984, 49, 1905; H. Chiou, P. C. Reeves and E. R. Biehl, J. Heterocycl. Chem., 1976, 13, 77; F. Effenberger, P. Bäuerle, W. Seufert and W.-D. Stohrer, Chem. Ber., 1990, 123, 193.
7. (a) N. Boden, R. J. Bushby, A. N. Cammidge and G. Headdock, Tetrahedron Lett., 1995, 36, 8685; (b) D. E. Turner, R. F. O'Malley, D. J. Sardella, L. S. Barinelli and P. Kaul, J. Org. Chem., 1994, 59, 7335; (c) S. M. Hubig, W. Jung and J. K. Kochi, J. Org. Chem., 1994, 59, 6233.
8. (a) L. Eberson, M. P. Hartshorn and O. Persson, J. Chem. Soc., Chem. Commun., 1995, 1131; (b) L. Eberson, M. P. Hartshorn and O. Persson, J. Chem. Soc., Perkin Trans. 2, 1995, 1735; (c) L. Eberson, M. P. Hartshorn and O. Persson, Angew. Chem., Int. Ed. Engl., 1995, 34, 2268; (d) L. Eberson, M. P. Hartshorn and O. Persson, Res. Chem. Intermed., 1996, 22, 799; (e) L. Eberson, M. P. Hartshorn, O. Persson and F. Radner, J. Chem. Soc., Chem. Commun., 1996, 2105.
9. L. Eberson, M. P. Hartshorn and O. Persson, J. Chem. Soc., Perkin Trans. 2, 1996, 141.
10. T. M. Bockman and J. K. Kochi, J. Phys. Org. Chem., 1994, 7, 325.
11. A study of the bromination of 4,4′-dimethoxystilbene in 1,2-dichloromethane was recently published: G. Bellucci, C. Chiappe and G. Lo Moro, J. Org. Chem., 1997, 62, 3176.
12. J. K. Kochi, Tetrahedron Lett., 1975, 41.
13. L. Eberson, M. P. Hartshorn, O. Persson, F. Radner and C. J. Rhodes, J. Chem. Soc., Perkin Trans. 2, 1996, 1289.
14. (a) I. M. Kolthoff and J. F. Coetzee, J. Am. Chem. Soc., 1957, 79, 1852; (b) A. I. Popov and D. H. Geske, J. Am. Chem. Soc., 1958, 80, 1340; (c) G. Dryhurst and P. J. Elving, Anal. Chem., 1967, 39, 606; (d) G. Casalbore, G. M. Mastragostino and S. Valcher, J. Electroanal. Chem., 1977, 77, 373; (e) L. G. Feoktistov, G. V. Andreev and A. P. Tomilov, Soviet Electrochem. (Engl. transl.), 1988, 24, 32.
15. J. E. Barry, M. Finkelstein, W. M. Moore, S. D. Ross, L. Eberson and L. Jönsson, J. Org. Chem., 1982, 47, 1292.
16. J. E. Barry, M. Finkelstein, W. M. Moore, S. D. Ross and L. Eberson, J. Org. Chem., 1985, 50, 528.
17. In view of the reaction course shown for 1-ICl in Fig. 5, the kinetics of the reactions of 3 and 4 should be accessible by the stopped-flow method.
18. J. F. Evans and H. N. Blount, J. Am. Chem. Soc., 1978, 100, 4191.
19. J. W. Moore and R. G. Pearson, Kinetics and Mechanism, 3rd edn., Wiley, New York, 1981, p. 20.
20. The previously given value, 0.17 dm³ mol^–1 s^–1(ref. 5), was based on ε= 8500 dm³ mol^–1 cm^–1, valid for 3^●+ in dichloromethane (ref. 7c).
21. F. L. Cozens, R. A. McClelland and S. Steenken, J. Am. Chem. Soc., 1993, 115, 5050 and references therein.
22. L. Eberson, M. P. Hartshorn, O. Persson and F. Radner, Acta Chem. Scand., 1997, 51, 492; L. Eberson, R. González-Luque, M. Merchán, F. Radner, B. O. Roos and S. Shaik, J. Chem. Soc., Perkin Trans. 2, 1997, 463.
23. (a) U. Svanholm and V. D. Parker, Tetrahedron Lett., 1972, 471; (b) W. Lau and J. K. Kochi, J. Org. Chem., 1986, 51, 1801; (c) L. Byrd, L. L. Miller and D. Pletcher, Tetrahedron Lett., 1972, 2419.
24. L. Eberson, M. P. Hartshorn, O. Persson and J. O. Svensson, J. Chem. Soc., Perkin Trans. 2, 1995, 1253.
25. L. Eberson, J. E. Barry, M. Finkelstein, W. M. Moore and S. D. Ross, Acta Chem. Scand., Ser. B, 1986, 40, 283.
26. S. D. Malone and J. F. Endicott, J. Phys. Chem., 1976, 44, 395.
27. It may be argued that chlorination may also result from the action of chlorine, present in equilibrium with ICl and iodine (see note 22 in ref. 7c). However, in HFP chlorination of 4 and compounds of similar reactivity would take place via the ET mechanism.
28. C. W. McCleland, Synthetic Reagents, ed. J. S. Pizey, Harwood, Chichester, 1983, vol. 5, p. 85.
29. H. A. Muathen, J. Chem. Res. (S), 1994, 405.
30. D. Duling, Public EPR Software Tools, National Institute of Environmental Health Sciences, 1996.
31. It is advisable to use the best HFP qualities commercially available, since the cheaper qualities may have hydrogen fluoride as an impurity.
32. L. Eberson, M. P. Hartshorn and O. Persson, Acta Chem. Scand., 1995, 49, 640.
33. This spectrum is also similar to that obtained in HFP or TFA from the oxidation of 1,2-dimethylnapthalene by Tl^III frifluoroacetate: L. Eberson, M. P. Hartshorn and O. Persson, J. Chem. Soc., Perkin Trans. 2, 1995, 409 See also: A. Terahara, H. Ohya-Nishiguchi, N. Hirota and A. Oku, J. Phys. Chem., 1986, 90, 1564.
34. L. Eberson, J. L. Calvert, M. P. Hartshorn and W. T. Robinson, Acta Chem. Scand., 1993, 47, 1025.
35. H. J. Shine and P. D. Sullivan, J. Phys. Chem., 1968, 72, 1390.
36. P. D. Sullivan, J. Phys. Chem., 1972, 76, 3943.
37. K. Scheffler and H. B. Stegmann, Elektronenspinresonanz, Springer-Verlag, Heidelberg, 1970, p. 462.
38. M. J. Drews, P. S. Wong and P. R. Jones, J. Am. Chem. Soc., 1972, 94, 9122.
39. J. T. Cooper and W. F. Forbes, Can. J. Chem., 1968, 46, 1158.
40. L. Eberson, F. Radner and M. Lindgren, Acta Chem. Scand., 1993, 47, 835.
41. P. D. Sullivan and N. A. Brette, J. Phys. Chem., 1975, 79, 474; P. D. Sullivan, J. Am. Chem. Soc., 1975, 97, 3992.
42. L. Eberson and F. Radner, J. Chem. Soc., Chem. Commun., 1991, 1233; Acta Chem. Scand., 1992, 46, 630.
43. P. D. Sullivan and J. R. Bolton, J. Magn. Reson., 1969, 1, 356.
44. L. Eberson, J. Phys. Chem., 1994, 98, 752.
45. B. J. Kennedy and G. A. Heath, Inorg. Chim. Acta, 1991, 187, 149.
46. L. Eberson and J. J. MacCullough, J. Chem. Soc., Perkin Trans. 2, preceding paper.