Investigations on the dynamic properties of 25,26,27,28-tetraalkoxycalix[4]arenes: para-substituent- and solvent-dependent properties of paco conformers and determination of thermodynamic parameters of the pinched cone/pinched cone conversion

Abstract

The conformational and dynamic behaviour of the 25,26,27,28-tetraalkoxycalix[4]arenes 1–7 (R = methyl, propyl) has been investigated by X-ray crystallography, dynamic 1D and 2D NMR spectroscopy, as well as with a series of force field (PIMM) and semiempirical (PM3, MNDO) calculations. The stereo-electronic effects of the upper rim substituents of tetramethoxycalix[4]arenes characteristically influence the preferred arrangement of the phenyl rings in the paco conformation. The in/out isomerisation (0001≈AAAA ⇌ 0001≈AAAB) taking place in 4 is strongly hindered in 1 due to the smaller diameter of the upper rim. The ease of rotation of the methoxy group attached to the inverted ring in paco-4 can be altered by using a solvent (CD₂Cl₂), which is able to form a host–guest complex with the calixarene cavity. For 1–3 cone/paco and paco/paco equilibria have been observed by 2D EXSY spectroscopy. Gibbs activation energies for the cone(C_2v)/cone(C_2v) conversion were determined for 1, 2, 5 and 6. An NMR line shape analysis provided Arrhenius and Eyring activation parameters for the cone(C_2v)/cone(C_2v) conversion in 5 and 6.

References

1. C. D. Gutsche, Calixarenes, The Royal Society of Chemistry, Cambridge, UK, 1989.
2. Calixarenes, A Versatile Class of Macrocyclic Compounds, ed. J. Vicens and V. Böhmer, Kluwer, Dordrecht, 1991.
3. V. Böhmer, Angew. Chem., 1995, 107, 785.
4. S. Shinkai, Tetrahedron, 1993, 49, 8933.
5. K. Iwamoto, K. Araki and S. Shinkai, J. Org. Chem., 1991, 56, 4955.
6. E. Ghidini, F. Ugozzoli, R. Ungaro, S. Harkema, A. A. El-Fadl and D. N. Reinhoudt, J. Am. Chem. Soc., 1990, 112, 6979.
7. C. D. Gutsche, B. Dhawan, A. J. Levine, K. H. No and L. J. Bauer, Tetrahedron, 1983, 39, 409.
8. T. Harada, J. M. Rudzinski and S. Shinkai, J. Chem. Soc., Perkin Trans. 2, 1992, 2109.
9. L. C. Groenen, J. D. van Loon, W. Verboom, S. Harkema, A. Casnati, R. Ungaro, A. Pochini, F. Ugozzoli and D. N. Reinhoudt, J. Am. Chem. Soc., 1991, 113, 2385.
10. S. Fischer, P. D. J. Grootenhuis, L. C. Groenen, W. P. van Hoorn, F. C. J. M. van Veggel, D. N. Reinhoudt and M. Karplus, J. Am. Chem. Soc., 1995, 117, 1611.
11. W. P. van Hoorn, Conformational and Dynamical Properties of Calixarenes, Proefschrift, University of Twente, 1997, and references cited therein.
12. J. C. van Loon, L. C. Groenen, S. S. Wijmenga, W. Verboom and D. N. Reinhoudt, J. Am. Chem. Soc., 1991, 113, 2378.
13. J. Blixt and C. Detellier, J. Am. Chem. Soc., 1994, 116, 11 957.
14. P. D. J. Grootenhuis, P. A. Kollman, L. C. Groenen, D. N. Reinhoudt, G. J. van Hummel, F. Ugozzoli and G. Andreetti, J. Am. Chem. Soc., 1990, 112, 4165.
15. A. Arduini, M. Fabbi, M. Mantovani, L. Mirone, A. Pochini, A. Secchi and R. Ungaro, J. Org. Chem., 1995, 60, 1454.
16. M. Conner, V. Janout and S. L. Regen, J. Am. Chem. Soc., 1991, 113, 9670.
17. A. Ikeda, H. Tsuzuki and S. Shinkai, J. Chem. Soc., Perkin Trans. 2, 1994, 2073.
18. For our semiempirical calculations we used the VAMP program package: G. Rauhut, A. Alex, J. Chandrasekhar, T. Steinke, W. Sauer, B. Beck, M. Hutter, P. Gedeck and T. Clark, VAMP7.0, Oxford Molecular Ltd., Madawar Centre, Oxford Science Park, Standford-on-Thames, Oxford, OX4 4GA, UK, 1996 (a) MNDO: J. S. Dewar and W. Thiel, J. Am. Chem. Soc., 1977, 99, 4899; (b) PM3: J. J. Stewart, J. Comput. Chem., 1989, 10, 209;221.
19. PIMM: H. J. Lindner, Tetrahedron, 1972, 30, 1127; A. E. Smith and H. J. Lindner, J. Comput.-Aided Mol. Des., 1991, 5, 235; M. Kroeker, PhD Thesis, Technische Hochschule, Darmstadt, 1994.
20. Notation introduced by Reinhoudt and co-workers.¹⁰ The numbers denote the anisole units whereby 0 accounts for a non inverted anisole ring and 1 accounts for an inverted one. The letters A and B mark the position of the methoxy group attached to the corresponding benzene ring. A denotes a methoxy group pointing outward and B denotes a methoxy group pointing nward the cavity.
21. F. A. Bovey, Nuclear Magnetic Resonance Spectroscopy, Academic Press, New York and London, 1969, pp. 61–71, 264–274.
22. We used the PM3 structure as a model because its higher symmetry facilitates such qualitative geometrical considerations.
23. Values taken from ref. 11.
24. For the equilibrium (2)ΔH=–9.8 kcal mol^–1was calculated.¹¹.
25. I. Thondorf, J. Brenn, W. Brandt and V. Böhmer, Tetrahedron Lett., 1995, 36, 6665.
26. G. D. Andreetti, V. Böhmer, J. G. Jordan, M. Tabatabai, F. Ugozzoli, W. Vogt and A. Wolff, J. Org. Chem., 1993, 58, 4023.
27. J. Scheerder, R. H. Vreekamp, J. F. J. Engbersen, W. Verboom, J. P. M. van Duynhoven and D. N. Reinhoudt, J. Org. Chem., 1996, 61, 3476.
28. For the 25,26,27,28-tetraoctyloxycalix[4]arene Arduini et al.¹⁵ found ΔG^‡= 10.2 kcal mol^–1. The introduction of four COOH or four CONH₂ groups in the upper rim increases ΔG^‡ due to intramolecular H-bonding to 14.1 and 13.3 kcal mol^–1 respectively.¹⁶ Böhmer and co-workers²⁶ reported a ΔG^‡ of 13.3 kcal mol^–1 for the C₂/C₂ interconversion in a dissymetric calix[4]arene.
29. This value is similar to the ΔS^‡ of –3.8 cal mol^–1 K^–1 for the paco-4/cone-4 conversion determined by Blixt and Detellier,¹³ and should be compared with ΔS^‡=–R ln 4 =–2.8 cal mol^–1 K^–1 accounting for the changes in the symmetry numbers of those species.
30. Obviously the PM3 method is inappropriate to quantitatively reproduce the experimental results. However it has to be considered that 0 K structures in the gas phase neglecting all solvent interactions are considered. Better agreement with experimental results was achieved with force field calculations.^10,11.
31. This value is comparable with ΔS^‡=–16.7 cal mol^–1 K^–1 for the 4-paco/4-1,2-alt conversion.¹³.
32. C. D. Gutsche and P. F. Pagoria, J. Org. Chem., 1985, 50, 5795.
33. W. Verboom, A. Durie, R. J. M. Egberink, Z. Asfari and D. N. Reinhoudt, J. Org. Chem., 1992, 57, 1313.
34. R. A. Jakobi, V. Böhmer, C. Grüttner, D. Kraft and W. Vogt, New J. Chem., 1996, 20, 493.
35. I. Bitter, A. Grün, B. Ágai and L. Tôke, Tetrahedron, 1995, 51, 7835.
36. H. Günther, NMR-Spektroskopie, Georg Thieme Verlag, Stuttgart, New York, ch. 9, 1992.
37. J. Baker, J. Comput. Chem., 1986, 7, 385.
38. G. Rauhut and T. Clark, J. Comput. Chem., 1993, 14, 503.
39. TRAMP 1.1b, H. Lanig, R. Koenig and T. Clark, Erlangen, 1995.
40. B. A. Murtagh and R. W. H. Sargent, Comput. J., 1970, 13, 185.