Re-engineering potential energy surfaces: trapezoidally distorted _π2_s + _π2_s thermal cycloaddition/elimination reactions

Abstract

Calculations at the PM3 or B3LYP/6-31G(d) SCF-MO levels of theory indicate that electron-withdrawing substituents can modify the potential energy surface for a simple thermal _π2_s + _π2_s pericyclic cycloaddition–elimination via a trapezoidal geometric distortion. We propose this thermally allowed synchronous reaction mode as an alternative to the Woodward–Hoffmann antarafacial mode or the stepwise biradical pathways normally invoked for such thermally activated cycloadditions. Reaction of the substituted syn tricyclic system 1 via the trapezoidal and synchronous transition state 6 is predicted as a potential experimental test of this new reaction mode.

References

1. C. Conesa and H. S. Rzepa, J. Chem. Soc., Perkin Trans. 2, 1998, 857.
2. J. J. P. Stewart, MOPAC, Quantum Chemistry Program Exchange, University of Indiana, Bloomington, USA, Program 455.
3. Gaussian program systems available from Gaussian Inc., 4415 Fifth Avenue, Pittsburgh, PA, USA.
4. H. B. Schlegel, J. Comput. Chem., 1982, 3, 214.
5. C. Peng and H. B. Schlegel, Isr. J. Chem., 1993, 33, 449.
6. M. J. S. Dewar, S. Olivella and H. S. Rzepa, J. Am. Chem. Soc., 1978, 100, 5650.
7. B. A. Horn, J. L. Herek and A. H. Zewail, J. Am. Chem. Soc., 1996, 118, 8755.
8. F. Bernardi, A. Bottoni, M. A. Robb, H. B. Schlegel and G. Tonachini, J. Am. Chem. Soc., 1985, 107, 2260; F. Bernardi, A. Bottoni, M. A. Robb and A. Venturini, ibid., 1990, 112, 2106.
9. J. M. Bofill, J. Gomez and S. Olivella, J. Mol. Struct. (THEOCHEM), 1988, 63, 285.
10. H. S. Rzepa and W. A. Wylie, J. Chem. Soc., Perkin Trans. 2, 1991, 939; H. S. Rzepa, ibid., 1989, 2115.
11. W. R. Dolbier, H. Koroniak, D. J. Burton, A. R. Bailey, G. S. Shaw and S. W. Hansen, J. Am. Chem. Soc., 1984, 106, 1871; N. D. Epiotis, J. Mol. Struct. (THEOCHEM), 1990, 66, 225; S. Niwayama and K. N. Houk, Tetrahedron Lett., 1992, 33, 883.