J . Org. Chem. 1995,60, 1965-1970
1965
Structure and Dynamics of 3,5-Di-tert-butylpyrazole Probed by Combined X-ray Crystallography and 15N Solid State NMR Francisco Aguilar-Parrilla,+Hans-Heinrich Limbach,*,?Concepcih Foces-Foces,*,$ FBlix Hernandez Cane,* Nadine Jagerovic,n and Jose Elguero*,s Institut fur Organische Chemie, Freie Universitat Berlin, Takustr. 3, 0 - 1 4 1 9 5 Berlin, F.R.G., Departamento de Cristalografia, Instituto de Quimica-Fisica “Rocasolano”,CSIC, Serrano 119, E-28006Madrid, Spain, and Instituto de Quimica Midica, CSZC, J u a n de la Cierva 3, E-28006Madrid, Spain Received December 17, 1993 (Revised Manuscript Received January 27, 1995@)
The crystal structure of the title compound was determined by X-ray crystallography: CIIHZON~, Pbca, 2 = 8, a = 11.4779(3),b = 21.1004(1),c = 9.9801(2) A, D , = 0.99 g ~ m - ~The . results show that crystals of 3,5-di-tert-butylpyrazole are formed by three types of dimers AA, AB or BA, and BB differing in the conformation of the tert-butyl groups. Variable temperature 15N-CPMAS-NMR experiments which were carried out on a solid I5N-1abeled sample indicate a conformational interconversion of the different dimers via tert-butyl group rotation as well as a nondegenerate intradimer double proton transfer. The rate and equilibrium constants of the tautomerism were obtained by l5N-CPMAS-NMR line-shape analysis. These constants represent averages of the different conformational states. The analysis of the data suggests that the tautomerism takes place only in the symmetric conformer AA where the tautomerism is degenerate. The observed nondegeneracy is then not the consequence of solid state effects, but of the intrinsic conformational exchange.
1. Introduction Pyrazoles constitute a very interesting class of heterocyclic compound despite their simple structure in the gas phase.’ A large variety of hydrogen bonded complexes was found to be present in the solid state, depending on the substituents of the pyrazole r i r ~ g . ~Whereas -~ the parent pyrazole compound forms linear hydrogen bonded chains in the solid state where proton tautomerism is suppressed8-10(See Figure 11, unexpected and remarkable changes in the crystal structure occur when substituents are introduced at the carbon atoms C3, C4, and C5 of the pyrazole ring. In addition, intermolecular proton transfer processes in these pyrazole complexes were detected for the first time by 13C-and in particular, by 15N-CPMAS-NMRspectroscopy (CP: cross polarization, MAS: magic angle ~ p i n n i n g ) . ~For - ~ example, in the case of the symmetric substituted 3,5-dimethylpyra-
’ Freie Universitat Berlin * Instituto “Rocasolano”.
Instituto de Quimica Medica. Abstract published in Advance ACS Abstracts, March 15, 1995. (1)Elguero, J. In ComprehensiveHeterocyclic Chemistry;Katritzky, A. R., Rees, C. W., Eds.; Pergamon Press: Oxford, 1984; pp 167-303. (2) Baldy, A.; Elguero, J.; Faure, R.; Pierrot, M.; Vincent, E. J. J . Am. Chem. SOC.1985,107, 5290-5291. (3) Smith, J. A. S.; Wehrle, B.; Aguilar-Parrilla, F.; Limbach, H.H.; Foces-Foces, C.; Cano, F. H.; Elguero, J.; Baldy, A,; Pierrot, M.; Khurshid, M. M. T.; Larcombe-McDouall, J. B. J . Am. Chem. SOC.1989, 111, 7304-7312. (4) Aguilar-Parrilla, F.; Scherer, G.; Limbach, H.-H.; Foces-Foces, C.: Cano, F. H.: Smith. J. A. S.: Toiron, C.: Elmero, J. J . Am. Chem. Soc. 1993, 114,’9657-9659. (5)Elguero, J.; Cano, F. H.; Foces-Foces, C.; Llamas-Saiz, A. L.; Limbach, H.-H.; Aguilar-Parrilla, F.; Claramunt, R. M.; L6pez, C. J . Heterocycl. Chem. 1994,31, 695-700. (6) Toda, F.; Tanaka, K.; Foces-Foces, C.; Llamas-Saiz, A. L.; Limbach, H.-H.; Aguilar-Parrilla, F.; Claramunt, R. M.; L6pez, C.; Elguero, J. J . Chem. SOC.Chem. Commun. 1993,1139-1142. (7)Aguilar-Parrilla, F.; Claramunt, R. M.; Lopez, C.; Sanz, D.; Limbach, H.-H.; Elguero, J. J . Phys. Chem. 1994,98,8752-8760. ( 8 ) Ehrlich, H. W. W. Acta Crystallogr. 1960,13, 946. (9) Berthou, J.;Elguero, J.; Rerat, C. Acta Crystallogr.Sect. B 1970, 26, 1881-1882. (10)Larsen, F. K.; Lehmann, M. S.; Sotofte, I.; Rasmussen, S. E. Acta Chem. Scand. 1970,24,3248-3258. 4
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zole (DMP), cyclic hydrogen bonded trimers are formed in the solid state where a correlated degenerate triple proton transfer takes place as shown in Figure l.2,3 By combining X-ray crystallography with 15N-CPMAS-NMR spectroscopy, the crystal structure and proton tautomerism were fully c h a r a c t e r i ~ e d . ~ - ~ Further X-ray crystallographic investigations showed that a cyclic tetramer formed when the methyl groups in the DMP were replaced by phenyl rings leading to the 3,5-diphenylpyrazolecompound (DPP). When, in addition, a bromine atom is finally introduced a t position 4 of the DPP, a dimer results (3,5-diphenyl-4-bromopyrazole = DPBrP). In both compounds degenerate proton transfers were again detected by 15N-CPMAS-NMRspectroscopy (see Figure l 1 4 z 5 In contrast, asymmetric substitution of the pyrazole ring with nitro,ll hydroxymethyl,12and carboxy groups,13 or other pyrazole rings14 leads to a suppression of the solid state proton tautomerism. The question then arises as to how chemically identical substituents which can also adopt different conformations affect the tautomerism. In order to study this problem we chose 3,5-di-tert-butylpyrazole(DTBUP) as a model compound because of the variable dihedral angles between the pyrazole ring and the methyl groups (see Figure 2). If this angle is different for both tert-butyl groups, one could expect a perturbation of the gas phase degeneracy of the proton tautomerism. Since tert-butyl groups may rotate in solid organic an influence of this rotation on the proton tautomerism could be the consequence. (11)Foces-Foces, C.; Cano, F. H.; Elguero, J. Guzz. Chim. Ztal. 1993, 123,477-479. (12) Llamas-Saiz, A. L.; Foces-Foces, C.; Elguero, J.; Meutermans, W. Acta Crystallogr. 1992,C48,714-717. (13) Llamas-Saiz, A. L.; Foces-Foces, C.; Elguero, J.; Meutermans, W. Supramolec. Chem. 1994,4, 53-62. (14)Monge, M. A.; Gutierrez-Puebla, E.; Elguero, J.; Toiron, C.; Meutermans, W.; Sobrados, I. Spectrochim.Acta 1994,50A,727-734. (15) Riddell, F. G.; Arumugam, S.; Harris, K. D. M.; Rogerson, M.; Strange, J. H. J . Am. Chem. SOC.l99S,115, 1881-1885.
0022-326319511960-1965$09.00/00 1995 American Chemical Society
1966 J. Org. Chem., Vol. 60, No. 7, 1995
Aguilar-Parrilla et al.
Pyr a zo le
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DPBrP
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