1833 with Z > 3u(O were retained for solution and refinement of the structure. The large number of unmeasured reflections stems from the small crystal size of 0.020 X 0.175 X 0.325 mm. The structure was solved by direct methods and refined to a final R value of 0.069. In the final refinement Fe and C1 atoms were assigned anisotropic thermal parameters and hydrogen atoms were included a t calculated positions using a riding model and U(H) = 1.2U(bonded C). The largest feature in a final difference map was 0.5, the size of a hydrogen and of no possible chemical significance. H(45) is the largest peak on a difference map computed with it omitted. Tables of anisotropic thermal parameters, hydrogen atom coordinates, and complete tables of bond distances and angles are available as supplementary material.Refluxing 3,4,5,-tribromo-l-nitro-1H-pyrazole (la) in benzene results in the evolution of bromine and NO2 and gives the 4-nitro-1H-pyrazole 2 and the 1-phenyl-1H-pyrazoles 4 and 5, while heating la in toluene gives 2 and benzyl bromide. Thermolysis of la in refluxing acetonitrile affords both 2 and the isomeric 5-nitro-1H-pyrazole 6a. Refluxing la mixed with the electron-rich 3,5-dimethyl-lH-pyrazole (7) in all three solvents gives 6a and 4-bromo-3,5-dimethyl-lH-pyrazole (8), whereas refluxing la mixed with anisole in benzene solution gives 2 and bromoanisoles. 3,5-Dibromo-l-nitro-lH-pyrazole (lb) in refluxing acetonitrile gives mainly 3,4-dibromo-5-nitro-1H-pyrazole (6a) and 3,5-dibromo-IH-pyrazole (3b), but refluxing l b mixed with 7 affords 3-bromo-5-nitro-1H-pyrazole (6b). Possible mechanisms are discussed involving intramolecular rearrangements to intermediates 3-bromo-3-nitro-3H-pyrazoles 9a,b and 4-bromo-4-nitro-4H-pyrazole 10 responsible both for the loss of bromine and NOP as well as for the electrophilic bromination of 7 and anisole. (1) Pyrazoles Studies. Part 19. Part 18: Berbee, R. P. M.; Habraken, C. L. J. Heterocycl. Chem. 1981, 18, 559-560. (2) Janssen, J. W. A. M.; Habraken, C. L.; Louw, R. J. Org. Chem. 1976, 41, 1758-1762 and earlier publications cited therein. (3) (a) Habraken, C. L.; Cohen-Fernandes, P. J. Chem. SOC., Chem. Commun. 1972, 38. (b) Pevzner, M. S.; Kulibabina, T . N.; Loffe, S. L.; Maslina, I. A,; Gidaspov, B. V.; Tartakovskii, V. A. Khim. Geterotsikl. Soedzn. 1979, 15, 550-554. Fernandes, P.; Habraken C. L. J . Org. Chem. 1984, 49, 3310-3314. (4) Zibuck, R.; Stahl, M. A.; Barchiesi, B.; Waalwijk, P. S.; Cohen-0 1986 American Chemical Society Selective Thermolysis Reactions Scheme I1
