Solvent effects on the barrier to rotation in carbamates

Journal of Organic Chemistry

Volumn 63, Issue 8, 1998, Pages 2426-2427

  Cox, Christopher ^a   Lectka, Thomas ^a  

^a JOHNS HOPKINS UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

CARBAMIC ACID DERIVATIVE; SOLVENT;

ARTICLE; CHEMICAL BINDING; DRUG BINDING; ISOMERISM; MOLECULAR INTERACTION; NUCLEAR MAGNETIC RESONANCE; REACTION ANALYSIS; SOLVATION; STRUCTURE ANALYSIS; THEORY;

EID: 0032540472     PISSN: 00223263     EISSN: None     Source Type: Journal    
DOI: 10.1021/jo9800863     Document Type: Article

References (30)

1. For leading reviews, see: (a) Stewart, W. E.; Siddall, T. H., III. Chem. Rev. 1970, 70, 517. (b) Oki, M. Applications of Dynamic NMR Spectroscopy to Organic Chemistry; VCH: Deerfield Beach, 1985; Chapter 2.
2. For leading reviews, see: (a) Stewart, W. E.; Siddall, T. H., III. Chem. Rev. 1970, 70, 517. (b) Oki, M. Applications of Dynamic NMR Spectroscopy to Organic Chemistry; VCH: Deerfield Beach, 1985; Chapter 2.
3. Souza, W. F.; Kambe, N.; Sonoda, N. J. Phys. Org. Chem. 1996, 9, 179 and references therein.
4. Takayama, H.; Shirakawa, S.; Kitajima, M.; Aimi, N.; Yamaguchi, K.; Hanasaki, Y.; Ide, T.; Katsuura, K.; Fujiwara, M.; Ijichi, K.; Konno, K.; Sigeta, S.; Yokota, T.; Baba, M. Bioorg. Med. Chem. Lett. 1996, 6, 1993.
5. Shi, Z.; Griffin, J. H. J. Am. Chem. Soc. 1993, 115, 6482.
6. (a) Drakenberg, T.; Dahlqvist, K.-I.; Forsen, S. J. Phys. Chem. 1972, 76, 2178.
7. (b) Eberhardt, E. S.; Loh, S. N.; Hinck, A. P.; Raines, R. T. J. Am. Chem. Soc. 1992, 114, 5437.
8. For applications of saturation transfer to amide isomerization, see: (a) Perrin, C. L.; Thouburn, J. D.; Kresge, J. J. Am. Chem. Soc. 1992, 114, 8800. We have used this technique in previous related studies; see: (b) Cox, C.; Ferraris, D.; Murthy, N. N.; Lectka, T. J. Am. Chem. Soc. 1996, 118, 5332. (c) Cox, C.; Young, V. G., Jr.; Lectka, T. J. Am. Chem. Soc. 1997, 119, 2307.
9. For applications of saturation transfer to amide isomerization, see: (a) Perrin, C. L.; Thouburn, J. D.; Kresge, J. J. Am. Chem. Soc. 1992, 114, 8800. We have used this technique in previous related studies; see: (b) Cox, C.; Ferraris, D.; Murthy, N. N.; Lectka, T. J. Am. Chem. Soc. 1996, 118, 5332. (c) Cox, C.; Young, V. G., Jr.; Lectka, T. J. Am. Chem. Soc. 1997, 119, 2307.
10. For applications of saturation transfer to amide isomerization, see: (a) Perrin, C. L.; Thouburn, J. D.; Kresge, J. J. Am. Chem. Soc. 1992, 114, 8800. We have used this technique in previous related studies; see: (b) Cox, C.; Ferraris, D.; Murthy, N. N.; Lectka, T. J. Am. Chem. Soc. 1996, 118, 5332. (c) Cox, C.; Young, V. G., Jr.; Lectka, T. J. Am. Chem. Soc. 1997, 119, 2307.
11. To our knowledge, only two cursory reports on the isomerization of simple carbamates in a hydrogen-bond-donating solvent (both in MeOH) have appeared. See: (a) De Leer, E. W. B.; Toorn, J. M. Recl. Trav. Chim. Pays-Bas 1975, 94, 119. (b) Yamagami, C.; Takao, N.; Takeuchi, Y. Aust. J. Chem. 1986, 39, 457.
12. To our knowledge, only two cursory reports on the isomerization of simple carbamates in a hydrogen-bond-donating solvent (both in MeOH) have appeared. See: (a) De Leer, E. W. B.; Toorn, J. M. Recl. Trav. Chim. Pays-Bas 1975, 94, 119. (b) Yamagami, C.; Takao, N.; Takeuchi, Y. Aust. J. Chem. 1986, 39, 457.
13. Because the relatively hydrophobic carbamates 6 and 7 have limited solubility in aqueous solutions, we chose to perform the experiments in Table 3 at 1 mg/mL so all substrates could be analyzed under identical conditions.
14. Early reports on carbamates in organic solvents claimed relatively large, negative entropies of activation but have subsequently been proven unreliable owing to systematic errors in the methods of kinetic analysis; see: Martin, M. L.; Mabon, F.; Trierweiler, M. J. Phys. Chem. 1981, 85, 76 and references therein.
15. (a) Lemire, A. E.; Thompson, J. C. Can. J. Chem. 1970, 48, 824.
16. (b) Hobson, R. F.; Reeves, L. W.; Shaw, K. N. J. Phys. Chem. 1973, 77, 1228.
17. (c) Reference 1a.
18. These simple carbamates are not amenable to analysis in pure water due to lack of solubility.
19. Questions have been raised about the reliability of ΔS‡ values from data obtained over a limited temperature range; see ref 9. We conservatively report our error as ±4 cal/mol·K, even though least-squares analysis of the Eyring data indicated ±2 cal/mol·K. We have performed in excess of 25 Eyring analyses on amides and carbamates and in no case other than the carbamates discussed here have we found ΔS‡ to deviate from zero by more than 3.3 cal/mol·K.
20. There is precedent for cases such as this where the ΔG‡ of a process does not vary as expected with changes in solvent, most notably in decompositions involving radicals; see: (a) Leffler, J. E. J. Org. Chem. 1955, 20, 1202. In those cases and in our present ones, the ΔH‡ and ΔS‡ do change with solvent, but vary in such a way as to cancel and cause no net effect on ΔG‡. This compensating effect has also been observed in enzymes; see: (b) Stein, R. Adv. Protein Chem. 1993, 44, 1 and references therein.
21. There is precedent for cases such as this where the ΔG‡ of a process does not vary as expected with changes in solvent, most notably in decompositions involving radicals; see: (a) Leffler, J. E. J. Org. Chem. 1955, 20, 1202. In those cases and in our present ones, the ΔH‡ and ΔS‡ do change with solvent, but vary in such a way as to cancel and cause no net effect on ΔG‡. This compensating effect has also been observed in enzymes; see: (b) Stein, R. Adv. Protein Chem. 1993, 44, 1 and references therein.
22. For leading references on solution-phase calculations, see: (a) Wiberg, K. B.; Rablen, P. R.; Rush, D. J.; Keith, T. A. J. Am. Chem. Soc. 1995, 117, 4261. (b) Gao, J. J. Am. Chem. Soc. 1993, 115, 2930. (c) Duffy, E. M.; Severance, D. L.; Jorgensen, W. L. J. Am. Chem. Soc. 1992, 114, 7535.
23. For leading references on solution-phase calculations, see: (a) Wiberg, K. B.; Rablen, P. R.; Rush, D. J.; Keith, T. A. J. Am. Chem. Soc. 1995, 117, 4261. (b) Gao, J. J. Am. Chem. Soc. 1993, 115, 2930. (c) Duffy, E. M.; Severance, D. L.; Jorgensen, W. L. J. Am. Chem. Soc. 1992, 114, 7535.
24. For leading references on solution-phase calculations, see: (a) Wiberg, K. B.; Rablen, P. R.; Rush, D. J.; Keith, T. A. J. Am. Chem. Soc. 1995, 117, 4261. (b) Gao, J. J. Am. Chem. Soc. 1993, 115, 2930. (c) Duffy, E. M.; Severance, D. L.; Jorgensen, W. L. J. Am. Chem. Soc. 1992, 114, 7535.
25. The authors cited in ref 2 found that in contrast to amides, TS2 is favored for simple carbamates in the gas phase at the MP2//3-21G* level. We believe 3-21G* is an inadequate basis set for this case, and correlation makes little difference in energy orderings. At all higher levels of theory, we found that TS1 is favored.
26. We used the Spartan 5.0.3 program (Wavefunction, Inc.) and Gaussian 94 for molecular orbital calculations.
27. rel = 15.3 kcal/mol).
28. Bandekar, J.; Okuzumi, Y. THEOCHEM 1993, 281, 113.
29. Reichardt, C. Solvents and Solvent Effects in Organic Chemistry; VCH: New York, 1988; Chapter 2.
30. Winstein, S.; Fainberg, A. H. J. Am. Chem. Soc. 1957, 79, 5937.