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DOI: 10.1039/A703653A (Paper) Reference Section for: J. Chem. Soc., Perkin Trans. 2, 1997, 2733-2736

Sterically hindered enols of carboxylic acids and esters. The ketonisation reactions of 2,2-bis(2,4,6-trimethylphenyl)ethene-1,1-diol and 2,2-bis(pentamethylphenyl)ethene-1,1-diol

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Barbara M. Allen, Anthony F. Hegarty and Pat O'Neill

Abstract

The ketonisation of two enols of carboxylic acids, 2,2-bis(2,4,6-trimethylphenyl)ethene-1,1-diol 2a and 2,2-bis(pentamethylphenyl)ethene-1,1-diol 2b have been studied at 25 °C in 1∶1 acetonitrile–water. The stability of these compounds arises from the inhibition of the β-carbon site to protonation, and this effect is now quantified. A pKa of 8.18 is reported for ionisation of the first hydroxyl group of (Mes)2C[double bond, length half m-dash]C(OH)2 2a and 8.63 for (PMP)2C[double bond, length half m-dash]C(OH)2 2b. A pH–rate profile has been determined over the full pH range for both species. General acid catalysis was detected, yielding Brønsted α values of 0.35 and 0.34 for 2a and 2b, respectively. Similarly, Brønsted α′ values of 0.48 and 0.45 were calculated for general acid catalysis of both enolate mono-anions. Studies of the ketonisation of the enols of the corresponding methyl esters have permitted comparative studies on the reactivity of enols of carboxylic acids, esters and ketene acetals. The reactivity of Mes2C[double bond, length half m-dash]C(OH)(OCH3) lies closer to the enediol, Mes2C[double bond, length half m-dash]C(OH)2 2a than to the ketene acetal, suggesting that in this case, one hydroxy group is almost as efficient as two such groups in the stabilisation of the positive charge developing at Cβ as protonation occurs.

References

  1. Z. Rappoport, The Chemistry of Enols, John Wiley, 1990 Search PubMed.
  2. P. O'Neill and A. F. Hegarty, J. Chem. Soc., Chem. Commun., 1987, 744 RSC.
  3. B. M. Allen, A. F. Hegarty, P. O'Neill and M. T. Nguyen, J. Chem. Soc., Perkin Trans. 2, 1992, 927 RSC.
  4. B. Urwyler and J. Wirz, Angew. Chem., Int. Ed. Engl., 1990, 29(7), 790 CrossRef.
  5. Y. Chiang, A. J. Kresge, P. Pruszynki, N. P. Schepp and J. Wirz, Angew. Chem., Int. Ed. Engl., 1990, 29, 792 CrossRef.
  6. (a) J. Andraos, Y. Chiang, C. G. Huang, A. J. Kresge and J. C. Scaiano, J. Am. Chem. Soc., 1993, 115, 10 605 CrossRef CAS; (b) J. K. Almstead, B. Urwyler and J. Wirz, J. Am. Chem. Soc., 1993, 116, 954; (c) J. Andraos, A. J. Kresge and V. V. Popik, J. Am. Chem. Soc., 1994, 116, 961 CrossRef CAS.
  7. J. Andraos, Y. Chiang, A. J. Kresge, I. G. Pojarlieff, N. P. Schepp and J. Wirz, J. Am. Chem. Soc., 1994, 116, 73 CrossRef CAS.
  8. J. Frey and Z. Rappoport, J. Am. Chem. Soc., 1996, 118, 5169 CrossRef CAS.
  9. T. L. Amyes and J. P. Richard, J. Am. Chem. Soc., 1996, 118, 3129 CrossRef CAS.
  10. (a) J. Andraos, A. J. Kresge, M. R. Peterson and I. G. Csizmadia, J. Mol. Struct., 1991, 232, 155 CrossRef; (b) J. P. Guthrie, Can. J. Chem., 1993, 71, 2123 CAS; (c) J. P. Guthrie and Z. Liu, Can. J. Chem., 1995, 73, 1395 CAS.
  11. T. T. Tidwell, Ketenes, Wiley, New York, 1995 Search PubMed.
  12. (a) A. D. Allen and T. T. Tidwell, J. Am. Chem. Soc., 1987, 109, 2774 CrossRef CAS; (b) A. D. Allen, A. J. Kresge, N. P. Schepp and T. T. Tidwell, Can. J. Chem., 1987, 65, 1719 CAS.
  13. (a) A. J. Kresge, H. L. Chen, Y. Chiang, E. Murill, M. A. Payne and D. S. Sagatys, J. Am. Chem. Soc., 1971, 93, 413 CrossRef CAS; (b) A. J. Kresge, D. S. Sagatys and H. L. Chen, J. Am. Chem. Soc., 1977, 99, 7228 CrossRef CAS.
  14. (a) Y. Chiang, A. J. Kresge and J. Wirz, J. Am. Chem. Soc., 1984, 106, 6392 CrossRef CAS; (b) Y. Chiang, M. Hojatti, J. R. Keefe, A. J. Kresge, N. P. Schepp and J. Wirz, J. Am. Chem. Soc., 1987, 109, 4000 CrossRef CAS; (c) J. R. Keefe, A. J. Kresge and N. P. Schepp, J. Am. Chem. Soc., 1988, 110, 1993 CrossRef CAS.
  15. F. Kelleher, PhD Thesis, National University of Ireland, 1992.
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