5-Hydroxy-3-phenyl-5-trifluoromethyl-4,5-dihydro-1H-pyrazole

The five-membered dihydropyrazole ring in the title compound, C10H9F3N2O, is approximately planar (r.m.s. deviation 0.111 Å for all non-H atoms) and its phenyl substituent is aligned at an angle of 14.7 (2)°. Adjacent molecules are linked by N—H⋯O and O—H⋯N hydrogen bonds, generating ribbons running along the b axis of the monoclinic unit cell.

The five-membered dihydropyrazole ring in the title compound, C 10 H 9 F 3 N 2 O, is approximately planar (r.m.s. deviation 0.111 Å for all non-H atoms) and its phenyl substituent is aligned at an angle of 14.7 (2) . Adjacent molecules are linked by N-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds, generating ribbons running along the b axis of the monoclinic unit cell.

Comment
We had intended to synthesize 5-phenyl-3-(trifluoromethyl)pyrazole, whose crystal structure has been reported (Dias & Goh, 2004). However, the strongly electron-withdrawing nature of the α,β-diketone used in the synthesis led to the isolation of a stable intermediate, a dihydropyrazole (Scheme I), that when dehydrated, should furnish the pyrazole. The synthesis of the dihydropyrazole has previously been reported (Yakimovich et al., 2002;Zelenin et al., 1995). The five-membered dihydropyrazole ring of C 10 H 9 F 3 N 2 O is approximately planar, the ring being buckled at the methylene carbon, and its phenyl substituent is aligned at 14.7 (2)° ( Fig. 1). Adjacent molecules are linked by N-H···O and O-H···N hydrogen bonds (Table 1) to generate a helical chain running along the b axis of the monoclinic unit cell (Fig. 2).
The crystal structure of the 2-naphthyl substituted analog has been reported (Yang & Raptis, 2003); both compounds should similar hydrogen-bonding features.

Refinement
Carbon-bound H-atoms were placed in calculated positions [C-H 0.95 Å, U iso (H) 1.2U eq (C)] and were included in the refinement in the riding model approximation.
The amino and hydroxy H-atoms were located in a difference Fourier map, and were refined isotropically with distance restraints of N-H 0.88 (1) Å and O-H 0.84 (1) Å.
In the absence of anomalous scatterers, 727 Friedel pairs were merged. Fig. 1. Anisotropic displacement ellipsoid plot (Barbour, 2001) of C 10 H 9 F 3 N 2 O at the 70% probability level; H atoms are drawn as spheres of arbitrary radius.