Zirconium enolate radical cations in solution—characterisation and kinetics of their mesolytic Zr–O bond cleavage †

Abstract

Six zirconocene enolate radical cations and one hydroquinone biszirconocene radical cation have been generated and characterised in solution by standard and fast scan cyclic voltammetry. The electroanalytical results along with those from product analysis of preparative one-electron oxidations indicate that the sterically congested radical cations undergo a mesolytic Zr–O bond cleavage process. The kinetics of the mesolytic Zr–O bond cleavage yielding zirconocene cations and the lifetime (t_½) of the zirconium enolate radical cations in dichloromethane at room temperature are determined.

References

1. I. Paterson, in Comprehensive Organic Synthesis, C. H. Heathcock, ed., Oxford, UK, 1991, Vol. 2, p. 301.
2. (a) A. Choudhury and E. R. Thornton, Tetrahedron, 1992, 48, 5701; (b) D. A. Evans, F. Urpi, T. C. Somers, J. S. Clark and M. Bilodeau, J. Am. Chem. Soc., 1990, 112, 8215; (c) S. Shirodkar, M. Nerz-Stormes and E. R. Thornton, Tetrahedron Lett., 1990, 31, 4699; (d) M. P. Bonner and E. R. Thornton, J. Am. Chem. Soc., 1991, 113, 1299; (e) D. A. Evans, D. L. Rieger, M. T. Bilodeau and F. Urpi, J. Am. Chem. Soc., 1991, 113, 1047; (f) R. Mahrwald, Chem. Ber., 1995, 128, 919; (g) A. Solladié-Cavallo, J. L. Koessler, J. Fischer and A. DeCian, Gazz. Chim. Ital., 1996, 126, 173.
3. (a) R. O. Duthaler, P. Herold, W. Lottenbach, K. Oertle and M. Riediker, Angew. Chem., 1989, 101, 490; Angew. Chem., Int. Ed. Engl., 1989, 28, 495; (b) R. O. Duthaler and A. Hafner, Chem. Rev., 1992, 92, 807.
4. (a) M. D. Curtis, S. Thanedar and W. M. Butler, Organometallics, 1984, 3, 1855; (b) S. H. Bertz, G. Dabbagh and C. P. Gibson, Organometallics, 1988, 7, 563; (c) C. P. Gibson and D. S. Bem, J. Organomet. Chem., 1991, 414, 23; (d) P. Veya, C. Floriani, A. Chiesi-Villa and C. Rizzoli, Organometallics, 1993, 12, 4892; (e) R. Beckhaus, I. Strau and T. Wagner, J. Organomet. Chem., 1994, 464, 155; (f) W. Adam and F. Prechtl, Chem. Ber., 1994, 127, 667; (g) P. Veya, P. G. Cozzi, C. Floriani, A. Chiesi-Villa and C. Rizzoli, Organometallics, 1995, 14, 4101.
5. M. Schmittel, H. Werner, O. Gevert and R. Söllner, Chem. Ber./Recueil, 1997, 130, 195.
6. (a) J. M. Manriquez, D. R. McAlister, R. D. Sanner and J. E. Bercaw, J. Am. Chem. Soc., 1978, 100, 2716; (b) R. S. Threlkel and J. E. Bercaw, J. Am. Chem. Soc., 1981, 103, 2650; (c) E. J. Moore, D. A. Straus, J. Armantrout, B. D. Santasiero, R. H. Grubbs and J. E. Bercaw, J. Am. Chem. Soc., 1983, 105, 2068; (d) S. Gambarotta, S. Strologo, C. Floriani, A. Chiesi-Villa and C. Guastini, Inorg. Chem., 1985, 24, 654; (e) M. F. Lappert, C. L. Raston, L. M. Engelhardt and A. H. White, J. Chem. Soc., Chem. Commun., 1985, 521; (f) see also ref. 4a and ref. 4e.
7. (a) M. Schmittel and R. Söllner, Angew. Chem., 1996, 108, 2248; Angew. Chem., Int. Ed. Engl., 1996, 35, 2107; (b) M. Schmittel and R. Söllner, Chem. Ber./Recueil, 1997, 130, 771; (c) M. Schmittel, A. Burghart, W. Malisch, J. Reising and R. Söllner, J. Org. Chem., 1998, 63, 396.
8. For a preliminary communication, see M. Schmittel and R. Sollner, Chem. Commun., 1998, 565.
9. E. Samuel and M. D. Rausch, J. Am. Chem. Soc., 1973, 95, 6263.
10. All potentials are referenced to the ferrocene/ferrocenium (Fc) redox couple unless otherwise noted. To obtain values vs. SCE, simply add +0.39 V.
11. (a) P. G. Gassman, D. W. Macomber and J. W. Hershberger, Organometallics, 1983, 2, 1470; (b) M. J. Burk, W. Tumas, M. D. Ward and R. D. Wheeler, J. Am. Chem. Soc., 1990, 112, 6133.
12. The expression mesolytic was coined by Maslak to describe bond cleavage of radical ions to yield radical and ionic products: P. Maslak and J. N. Narvaez, Angew. Chem., 1990, 102, 302; Angew. Chem., Int. Ed. Engl., 1990, 29, 283.
13. R. S. Nicholson and I. Shain, Anal. Chem., 1964, 36, 706.
14. H. Trenkle, PhD Thesis, Würzburg, 1997.
15. (a) M. Röck and M. Schmittel, J. Chem. Soc., Chem. Commun., 1993, 1739; (b) M. Schmittel, G. Gescheidt and M. Röck, Angew. Chem., 1994, 106, 2056; Angew. Chem., Int. Ed. Engl., 1994, 33, 1961.
16. (a) M. Schmittel, J. Heinze and H. Trenkle, J. Org. Chem., 1995, 60, 2726; (b) M. Schmittel, M. Keller and A. Burghart, J. Chem. Soc., Perkin Trans. 2, 1995, 2327; (c) M. Schmittel, J.-P. Steffen and A. Burghart, Chem. Commun., 1996, 2349; (d) M. Schmittel and H. Trenkle, Chem. Lett., 1997, 299.
17. R. F. Jordan, C. S. Bajgur, W. E. Dasher and A. L. Rheingold, Organometallics, 1987, 6, 1041.
18. S. L. Borkowsky, R. F. Jordan and G. D. Hinch, Organometallics, 1991, 10, 1268.
19. One methyl group is replaced by a chlorine in Cp₂Zr(IV)Y⁺ but two methyl groups by two chlorines in Cp₂Zr(IV)Y₂(with Y = Me, Cl).
20. (a) A. Lasia, J. Electroanal. Chem., Interfacial Electrochem., 1983, 146, 397; (b) J. Heinze, M. Störzbach and M. Mortensen, J. Electroanal. Chem., Interfacial Electrochem., 1984, 165, 61; (c) J. Heinze and M. Störzbach, J. Electroanal. Chem., 1993, 346, 1.
21. W. Kaminsky, Macromol. Chem. Phys., 1996, 109, 3907 and literature cited therein.
22. E. Samuel, Bull. Soc. Chim. Fr., 1966, 3548.
23. S. E. Biali and Z. Rappoport, J. Am. Chem. Soc., 1984, 106, 5641.
24. E. B. Nadler and Z. Rappoport, J. Am. Chem. Soc., 1987, 109, 2112.
25. S. E. Biali and Z. Rappoport, J. Am. Chem. Soc., 1985, 107, 3669.
26. (a) M. Schmittel and U. Baumann, Angew. Chem., 1990, 102, 571; Angew. Chem., Int. Ed. Engl., 1990, 29, 541; (b) M. Röck and M. Schmittel, J. Prakt. Chem., 1994, 336, 325.
27. C. Amatore, C. Lefrou and F. Pflügler, J. Electroanal. Chem., Interfacial Electrochem., 1989, 270, 43.
28. R. S. Nicholson, Anal. Chem., 1966, 38, 1406.
29. (a) M. Rudolph, J. Electroanal. Chem., Interfacial Electrochem., 1992, 338, 85; (b) M. Rudolph, D. P. Reddy and S. W. Feldberg, Anal. Chem., 1994, 66, A589.