the data reported in Table 3 show that 1dЈ is much more stable
than 1b.
For pyrazole itself, our computed NICS value is Ϫ14.95
ppm, which is very close to pyrrole. The OH tautomers have
NICS values of Ϫ14.55 (1c) and Ϫ14.45 ppm (1dЈ), indicating
that the OH substituent does not alter the aromaticity of pyr-
azole, i.e. that dipolar charged forms are not important. Finally,
the NH tautomer (1b) has a NICS of Ϫ6.75 ppm, intermediate
between those of the aromatic pyrazoles and that of the non-
aromatic CH tautomer (1a, NICS = Ϫ0.25 ppm).
Concerning the NMR experiments, there is an apparent con-
tradiction between the conclusions of the calculations (only 1a
at 0 K) and those reported for 5 (only 5a in the gas phase)8 on
one hand and those of the NMR experiments (only 5b or 5c
at 0 K in methanol or DMSO) on the other. Obviously, the
explanation should lie on solvent effects. Three publications
have reported theoretical studies of solvent effects on these equi-
libria. According to Hillier et al.28 water shifts the equilibrium
towards the NH (1b) or the OH (1c) tautomers, depending on
the model used. Cao, Schäffer et al.29 conclude that the NH
tautomer (1b) is the most stable in water (Luque et al.30 have
only examined the solvent effects on the equilibrium 1b/1d/1dЈ
which is not relevant for the present discussion). Ono et al.31
conclude, from B3LYP/6-31G* calculations, that the CH
tautomer a of 1,3-dimethylpyrazolin-5-one is the most stable in
both the aqueous and vapour phases. This last conclusion is
probably wrong since we found, also in the gas phase and
B3LYP level, that 1a was more stable than 1dЈ and this is in
contradiction with our MP2 results and those of Shäfer et al.29
In summary, in solution, the effect of solvents like methanol
and DMSO, dominates over temperature effects.
Supplementary information available
All the calculated geometries and ring critical points coordin-
ates are available on simple request from one of us (I.R.:
rozas@pinar1.csic.es).
Acknowledgements
This work was supported by the Consejo Nacional de Investi-
gaciones Científicas y Tecnológicas (CONICET-Argentina),
the Consejo de Investigaciones de la Provincia de Córdoba
(CONICOR) and Fundación Antorchas. Lic. Walter Vera’s
help with the FVP experiments is greatly acknowledged. The
Spanish group acknowledged the financial support obtained
through the project “SAF 97-0044-C02”. One of us (C. D.) is
a holder of a Marie Curie Research Training Grant for which
he thanks the European Commission, Directorate General XII
(Science, Research and Development).
A last comment concerning the equilibrium between 1c and
1dЈ. The difference in stability between 1c and 1dЈ (3.5 kcal
molϪ1) is very unusual for annular tautomerism of azoles where
the 3-R and 5-R tautomers have similar stabilities.36–38 The only
explanation is the presence of an intramolecular hydrogen bond
(IMHB) in 1dЈ which stabilizes this tautomer. This HB is not
present in 3(5)-methoxypyrazole which is a “bad” model for
3-hydroxypyrazole.
References
1 J. D. Pérez and G. I. Yranzo, J. Org. Chem., 1982, 47, 2221;
J. D. Pérez and G. I. Yranzo, Bull. Soc. Chim. Fr., 1985, 473.
2 J. D. Pérez, G. I. Yranzo and L. M. Phagouapé, Bull. Soc. Chim.
Fr., 1986, 129.
3 J. D. Pérez and L. M. Phagouapé, Int. J. Chem. Kinet., 1987, 19, 571;
Ibid., 1988, 20, 217; Ibid., 1988, 20, 603; Ibid, 1989, 21, 227.
4 J. D. Pérez, G. I. Yranzo, M. A. Ferraris, R. M. Claramunt and
J. Elguero, Tetrahedron, 1988, 44, 6429.
5 J. D. Pérez and G. I. Yranzo, J. Anal. Appl. Pyrol., 1989, 16, 165.
6 J. D. Pérez, G. I. Yranzo and J. Elguero, J. Fluorine Chem., 1993,
63, 271.
7 E. L. Moyano, G. I. Yranzo and J. Elguero, J. Org. Chem., 1998, 63,
8188.
8 J. Elguero, C. Marzin, A. R. Katritzky and P. Linda, The
Tautomerism of Heterocycles, Academic Press, New York, 1976,
p. 266. New edition: Advances in Heterocyclic Chemistry, 1998, in
the press.
9 Vogel’s Textbook of Practical Organic Chemistry, Fifth Edition,
1989, (a) p. 1050; (b) p. 433.
10 V. F. Matinov and I. B. Belov, J. Gen. Chem. USSR (Engl. Transl.),
1963, 33, 1079.
11 K. von Auwers and W. Mauss, J. Prakt. Chem., 1925, 110, 204.
12 Aldrich NMR FT Catalog 1, 1993, 2, 808 C.
13 K. Sonogashira, Y. Toda and N. Hagihara, Tetrahedron Lett., 1975,
4467.
NICS and the aromaticity of the four tautomers
The problem of the stability of pyrazolinone tautomers is
related to their aromaticity.8,39 It was always clear that the
CH tautomer is not aromatic and that the OH tautomers are
aromatic, but more or less aromatic than pyrazole? The NH
tautomer was considered aromatic although less than the OH
ones. These qualitative reasonings were based on our ability to
write down charged resonance structures, that in the case of the
NH tautomer would justify its aromaticity and in the case of
the OH tautomers would decrease if compared with pyrazole.
We decided to employ Schleyer’s approach40 using the NICS
(Nuclear Independent Chemical Shifts) values computed at
the ring critical points calculated using Bader’s theory22 (see
Fig. 3). To be consistent with Schleyer’s data, we used HF/6-
31ϩG* calculations to evaluate the NICS with geometries
optimized at the MP2/6-31G** level and ring critical points
calculated in these geometries. As a general rule negative NICS
values correspond to aromatic compounds40 and some refer-
ence NICS values, at HF/6-31ϩG* level, for pyrrole, thiophene
and furan are Ϫ15.1, Ϫ13.6 and Ϫ12.3 ppm respectively.40
14 Aldrich NMR FT Catalog 1, 1993, 2, 926 C.
15 A. I. Meyers and E. W. Collington, J. Am. Chem. Soc., 1970, 92,
6676.
16 Aldrich NMR FT Catalog 1, 1993, 2, 508 C.
17 Varian-NMR Spectra Catalog, 1966, 1, 60.
18 S. Danishefsky and M. Bednarski, Tetrahedron Lett., 1983, 24,
3451.
19 M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G.
Johnson, M. A. Robb, J. R. Cheeseman, T. A. Keith, G. A.
Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham,
V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, C. Y. Peng, P. Y. Ayala,
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, Gaussian, Inc.,
Pittsburgh, PA, 1995.
20 P. C. Hariharan and J. A. Pople, Theor. Chim. Acta, 1973, 28,
213.
21 C. Møller and M. S. Plesset, Phys. Rev., 1934, 46, 618.
22 R. F. W. Bader, Atoms in Molecules. A Quantum Theory, Oxford
University, New York, 1990.
Scheme 4
J. Chem. Soc., Perkin Trans. 2, 1999, 211–216
215