Benzocyclobutenone enolate: an anion with an antiaromatic resonance structure †

Abstract

Benzocyclobutenone enolate (1a) was synthesized by deprotonating its conjugate acid with fluoride ion in the gas phase using a Fourier transform mass spectrometer. Reactions of 1a were explored and C-alkylation was found to be greatly preferred over oxygen attack. Thermodynamic properties (ΔH °_acid(1) = 360.3 ± 2.1 kcal mol^–1, EA(benzocyclobutenyl radical) = 1.90 ± 0.10 eV, and BDE(C–H) = 90.5 ± 3.1 kcal mol^–1) also were measured. The results are contrasted to ab initio calculations on cyclobutanone, benzocyclobutenone, and cyclobutenone enolates. Natural bond orbital and Bader analyses are reported too. Given our gas phase and computational results, we were able to devise conditions for the generation and trapping of 1a in tetrahydrofuran despite previous failings reported in the literature.

References

1. B. M. Trost and I. Fleming, Comprehensive Organic Synthesis: Selectivity, Strategy and Efficiency in Modern Organic Chemistry, Pergamon Press, New York, 1991.
2. B. A. Lorsbach and M. J. Kurth, Chem. Rev., 1999, 99, 5149.
3. M. P. Cava and K. Muth, J. Am. Chem. Soc., 1960, 82, 652.
4. Reported deuterium incorporation into benzocyclobutenone via1a was later retracted (see ref. 5). H. Hart and R. W. Fish, J. Am. Chem. Soc., 1960, 82, 749.
5. H. Hart, J. A. Hartlage, R. W. Fish and R. R. Rafos, J. Org. Chem., 1966, 31, 2244.
6. D. J. Bertelli and P. Crews, J. Am. Chem. Soc., 1968, 90, 3889.
7. T. Matsumoto, T. Hamura, Y. Kuriyama and K. Suzuki, Tetrahedron Lett., 1997, 38, 8985.
8. M. E. Jones, S. R. Kass, J. Filley, R. M. Barkley and G. B. Ellison, J. Am. Chem. Soc., 1985, 107, 109.
9. J. E. Bartmess, R. L. Hays and G. Caldwell, J. Am. Chem. Soc., 1981, 103, 1338.
10. M. D. Brickhouse and R. R. Squires, J. Am. Chem. Soc., 1988, 110, 2706.
11. M. D. Brickhouse and R. R. Squires, J. Phys. Org. Chem., 1989, 2, 389.
12. R. N. Hayes, R. P. Grese and M. L. Gross, J. Am. Chem. Soc., 1989, 111, 8336.
13. I. L. Freriks, L. J. de Koning and N. M. M. Nibbering, J. Am. Chem. Soc., 1991, 113, 9119.
14. I. L. Freriks, L. J. de Koning and N. M. M. Nibbering, J. Phys. Org. Chem., 1992, 5, 776.
15. Trifluoroacetyl chloride has been proposed as a reagent which gives a higher percentage of O-alkylation. However, the percentage of oxygen reactiivity is based on detection of chloride which can, in principle, arise from either C- or O-attack. B. D. Wladkowaski, J. L. Wilbur, M. Zhong and J. I. Brauman, J. Am. Chem. Soc., 1993, 115, 8833.
16. M. Zhong and J. I. Brauman, J. Am. Chem. Soc., 1996, 118, 636.
17. O. Abou-Teim, M. C. Goodland and J. F. W. McOmie, J. Chem. Soc., Perkin Trans. 1, 1983, 2659.
18. S. Takano, K. Inomata, K. Samizu, S. Tomita and M. Yanase, Chem. Lett., 1989, 1283.
19. M. Krumpolc and J. Rocek, Org. Synth., 1990, Coll. Vol. VII, 114.
20. T. C. L. Wang, T. L. Ricca and A. G. Marshall, Anal. Chem., 1986, 58, 2935.
21. M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Peterson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, R. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez and J. A. Pople, GAUSSIAN94 Revisions A, B, C; Gaussian, Inc., Pittsburgh, PA, 1995.
22. J. A. Pople, A. P. Scott, M. W. Wong and L. Radom, Isr. J. Chem., 1993, 33, 345.
23. A. E. Reed, L. A. Curtiss and F. Weinhold, Chem. Rev., 1988, 88, 899.
24. F. Weinhold and J. E. Carpenter, in The Structure of Small Molecules and Ions, Plenum, New York, 1988.
25. R. F. W. Bader, in Atoms in Molecules: A Quantum Theory, Oxford University Press, Oxford, 1990.
26. All thermodynamic data unless otherwise noted comes from S. G. Lias, J. E. Bartmess, J. F. Liebmann, J. L. Holmes, R. D. Levin and W. G. Mallard, J. Phys. Chem. Ref. Data, 1988, 17, Suppl. 1 [or the slightly updated form available on a personal computer, NIST Negative Ion Energetics Database (Version 3.00, 1993); NIST Standard Reference Database 19B] or ref. 27. Note, 4.184 kJ = 1 kcal and 1 eV = 23.06 kcal and 96.48 kJ.
27. J. E. Bartmess, Negative Ion Energetics Data in Secondary Negative Ion Energetic Data, November 1998, National Institute of Standards and Technology, Gaithersburg, MD 20899 (http://webbook.nist.gov).
28. J. E. Bartmess and R. M. Georgiadis, Vacuum, 1983, 33, 149.
29. K. J. Miller, J. Am. Chem. Soc., 1990, 112, 8533.
30. The error in k_–1 is related to the magnitude of the rate and the gauge reading for the neutral pressure. A minimal pressure of the ketone was required to slow the reaction and enable it to be followed over two half-lives. The resulting pressure (ca. 1 – 1.5 × 10^–8 Torr) was not much greater than the background pressure (0.3 × 10^–8 Torr), so the error in the ion gauge reading was larger than usual.
31. G. H. Herzberg, Molecular Spectra and Molecular Structure: Infrared and Raman Spectra of Polyatomic Molecules, D. Van Nostrand, Princeton, NJ, 1945; Vol. 2.
32. J. W. Gauthier, T. R. Trautman and D. B. Jacobson, Anal. Chim. Acta., 1991, 246, 211.
33. J. J. Grabowski, J. M. Van Doren, C. H. DePuy and V. M. Bierbaum, J. Chem. Phys., 1984, 80, 575.
34. Aside from the ions at m/z 247 (adduct –HF) and m/z 147 (perfluoroacetone enolate), two additional products were detected at m/z 169 and 219 (6% and 8%, respectively, relative to the base peak at m/z 247). The m/z 219 species could arise from decarbonylation of the adduct –HF ion and subsequent loss of CF₂ would give the anion at m/z 169.
35. A. G. Marshall and D. C. Roe, J. Chem. Phys., 1980, 73, 1581.
36. T. B. McMahon and P. Kebarle, Can. J. Chem., 1985, 63, 3160.
37. T. B. McMahon, T. Heinis, G. Nicol, J. K. Hovey and P. Kebarle, J. Am. Chem. Soc., 1988, 110, 7591.
38. We have used the electron anity of the enolate's corresponding radical as a measure of the ion's HOMO energy. The computed MP2 HOMO energy gives a similar value.
39. G. J. Heiszwolf and H. Kloosterziel, Recl. Trav. Chim. Pays-Bas, 1970, 89, 1153.
40. G. K. King, M. M. Maricq, V. M. Bierbaum and C. H. DePuy, J. Am. Chem. Soc., 1981, 103, 7133.
41. J. J. Grabowski and L. Zhang, J. Am. Chem. Soc., 1989, 111, 1193.
42. R. W. Taft and F. G. Bordwell, Acc. Chem. Res., 1988, 21, 463.
43. F. G. Bordwell, J. E. Bartmess, G. E. Drucker, Z. Margolin and W. S. Matthews, J. Am. Chem. Soc., 1975, 97, 3226.
44. Ketone 9 was found to be stable to the acidic workup conditions.
45. B. L. Chenard, C. Slapak, D. K. Anderson and J. S. Swenton, J. Chem. Soc., Chem. Commun., 1981, 179.
46. D. N. Hickman, K. J. Hodgetts, P. S. Mackman, T. W. Wallace and J. M. Wardleworth, Tetrahedron, 1996, 52, 2235.
47. Z. Y. Wang, L. Kuang, X. S. Meng and J. P. Gao, Macromolecules, 1998, 31, 2935.
48. Bond order is calculated as a function of the electron density at the critical point, ρ_c Bond order = exp{6.458*(ρ_c– 0.2520)}. See: K. B. Wiberg, R. F. Bader and C. D. H. Lau, J. Am. Chem. Soc., 1987, 109, 985.
49. Ellipticity is a measure of the anisotropy of the charge distribution. A larger number suggests more p character. For example, ethane = 0.0, benzene = 0.23 and ethylene = 0.45.
50. The extended C1–C4 bond might suggest a ring-opened α-ketene phenide isomer; however, this structure is 23.6 kcal mol^–1 higher in energy than 1a at the MP2/6-31 + G(d)//HF/6-31 + G(d) level of theory.
51. D. Kovacek, Z. B. Maksic and I. Novak, J. Phys. Chem., 1997, 107, 1147.
52. M. C. Baschky, PhD Thesis, Unversity of Minnesota, 1994.
53. We estimate that 11 is 1 kcal mol^–1 more acidic than phenyl-acetaldehyde.
54. M. Hare, PhD Thesis, University of Minnesota, 1998.
55. L. Moore, R. Lubinski, M. C. Baschky, G. D. Dahlke, M. Hare, T. Arrowood, Z. Glasovac, M. Eckert-Maksic and S. R. Kass, J. Org. Chem., 1997, 62, 7390.