V. Strehmel et al. / Tetrahedron Letters 49 (2008) 586–588
587
16.1 G; 3 (ionic liquid): s = 5.4 ns, Aiso (14N) = 16.0 G; 4
(ionic liquid): s = 6.9 ns, Aiso (14N) = 16.1 G) determined
by the method of Budil et al. show a lower mobility in
the ionic liquid than in dimethylsulfoxide and a distinct
micropolarity.23 This is caused by the strong interactions
between the 4-sulfonatooxy-2,2,6,6-tetramethylpiperidine-
1-yloxyl derivatives and 1-butyl-3-methylimidazolium tetra-
fluoroborate and by the higher viscosity of this ionic liquid
with respect to the solvent dimethylsulfoxide.24,25
In conclusion, the one step synthesis of 4-sulfonatooxy-
2,2,6,6-tetramethylpiperidine-1-yloxyl results in a high
yield on the basis of commercially available 4-hydroxy-
2,2,6,6-tetramethylpiperidine-1-yloxyl using chlorosulfuric
acid trimethylsilylester. This opens new possibilities for
synthesis of 4-sulfonatooxy-2,2,6,6-tetramethylpiperidine-
1-yloxyl salts with various cations that are useful for inves-
tigation of ionic liquids. Moreover, selection of counter
ions belonging to the ionic liquid should give more detailed
information about the influence of structural effects of
ionic liquids on micropolarity and microviscosity.
Fig. 1. Formation of 4-sulfonatooxy-2,2,6,6-tetramethyl (piperidine-1-
yloxyl) (2) and its potassium (3) and sodium (4) salts.
ESR spectra of 4-sulfonatooxy-2,2,6,6-tetramethylpiper-
idine-1-yloxyl and its salts are similar if they are dissolved
in the same solvent, and they are characteristic for 2,2,6,6-
tetramethylpiperidine-1-yloxyl derivatives. However, the
solvent influences the habitus of the spectra as can be seen
in Figure 2. The spectrum of 3 measured in DMSO is typ-
ical for a low viscous solvent. On the other side, the spec-
trum of
3 dissolved in 1-butyl-3-methylimidazolium
tetrafluoroborate exhibits a broader linewidth.
The average rotational correlation times (s) and the
hyperfine coupling constants (Aiso (14N)) of the spin probes
(2 (DMSO): s = 0.4 ns, Aiso (14N) = 15.8 G; 3 (DMSO):
s = 0.4 ns, Aiso (14N) = 15.7 G; 4 (DMSO): s = 0.3 ns, Aiso
(14N) = 15.8 G; 2 (ionic liquid): s = 7.4 ns, Aiso (14N) =
Acknowledgments
The authors gratefully acknowledge A. Laschewsky for
the use of laboratory equipment, the group of E. Kleinpet-
er, especially I. Starke and S. Furstenberg for mass spectro-
¨
metric investigation, and the DFG for financial support
within the priority program SPP 1191.
References and notes
1. Wasserscheid, P.; Keim, W. Angew. Chem. 2000, 112, 3926.
2. Rogers, R. D.; Seddon, K. R. In Ionic Liquids Industrial Applications
to Green Chemistry; ACS Symp. Ser. 818; American Chemical Society:
Washington DC, 2002.
3. Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley-VCH:
Weinheim, 2003.
4. Rogers, R. D.; Seddon, K. R.; Kubisa, P. In Ionic Liquids as Green
Solvents, ACS Symp. Ser. 856; American Chemical Society: Wash-
ington, DC; 2003.
5. Rogers, R. D.; Brazel, C. S. In Ionic Liquids in Polymer Systems; ACS
Symp. Ser. 913; American Chemical Society: Washington, DC; 2005.
6. Ohno, H. Electrochemical Aspects of Ionic Liquids; John Wiley &
Sons: Hoboken, New Jersey, 2005.
7. Wang, J.; Tian, Y.; Zhao, Y.; Zhuo, K. Green Chem. 2003, 5, 618.
8. Noel, M. A. M.; Allendoerfer, R. D.; Osteryoung, R. A. J. Phys.
Chem. 1992, 96, 2391.
9. Kawai, A.; Hidemori, T.; Shibuya, K. Chem. Lett. 2004, 11, 1464.
10. Kawai, A.; Hidemori, T.; Shibuya, K. Chem. Phys. Lett. 2005, 414,
378.
11. Evans, R. G.; Wain, A. J.; Hardacre, C.; Compton, R. G.
ChemPhysChem 2005, 6, 1035.
12. Stoesser, R.; Herrmann, W.; Zehl, A.; Strehmel, V.; Laschewsky, A.
ChemPhysChem 2006, 7, 1106.
13. Strehmel, V.; Laschewsky, A.; Stoesser, R.; Zehl, A.; Herrmann, W.
J. Phys. Org. Chem. 2006, 19, 318.
14. Grampp, G.; Kattnig, D.; Mladenova, B. Spectrochim. Acta A 2006,
63, 821.
15. Stoesser, R.; Herrmann, W.; Zehl, A.; Laschewsky, A.; Strehmel, V.
Z. Phys. Chem. 2006, 220, 1309.
16. Keith, A. D.; Snipes, W.; Mehlhorn, R.; Gunter, T. Biophys. J. 1977,
19, 205.
Fig. 2. ESR spectra of 3 dissolved in (a) 1-butyl-3-methylimidazolium
tetrafluoroborate (Aiso
sulfoxide (Aiso
14N) = 15.7 G; s = 0.4 ns) at room temperature.
(
14N) = 16.0 G; s = 5.4 ns) and in (b) dimethyl-
(