Newer halo-and hydroxy-aryl pyrazolines and pyrazoles and their infrared spectra

ABSTRACT

It is found that [la] reacts with one mole of phenylhydrazine in boiling ethanol to give the expected l,3,-diphenyl-5 [3.4-dichlorophenyl]-delta[2]-pyrazoline [2]. The latter is readily dehydrogenated with tetrachloro-o-benzo-quinone at room temperature within a few minutes reaction period to give l,3-diphenyl-5 [3,4-dichlorophenyl] pyrazole[3] in good yield, along with tetra-chlorocatechol in a manner similar to that described by Latif et al. [1], as shown in scheme [A]. Structure of [2] is based on analogy [2-3], analytical data, yellow colour, strong fluorescence characteristic of pyrazolines, as well as spectral features. Its I.R. spectrum lacks nu c=o, nu nh; meanwhile its U.V. spectrum includes two maxima [lambda[max] 245, E 13630; lambda[max]. 350 nm E, 14155 ethanol], and thus supporting the delta[2]- pyrazoline structure [4] and excluding a possible hydrazone formation. The pyrazole[3], in contrast to the parent pyrazoline lacks colour and the absorption in its U.V. spectrum; meanwhile its U.V. spectrum exhibits the expected wave band [lambda[max] 290 nm E, 7390 ethanol] supporting its structure; its I.R. spectrum lacks VG=O and shows VC=N at 1600 cm[-1]. In contrast to the propenone [la], the hydroxyphenyl analogue [Ib] reacts with phenylhydrazine in equimolar amounts under the same conditions to give the expected yellow l,3-diphenyl-5 [3,5-dichloro-2-hydroxyphenyl]-delta[2]-pyrazoline [4a], in addition to the colourless dihydropyrazole [9]. However, when an excess of phenylhydrazine is used, [9] is exclusively produced without possible isolation of [4a]. Structure of [9] is inferred from analytical data, lack of colour and spectral features. Its I.R. spectrum is devoid of nu c=o and includes a broad band in the 3400-3100 cm[-1] region for NH and OH functions. Meanwhile, its U.V. spectrum exhibits a fairly strong styrene like band [[lambda[max]255 nm, E,7580 ethanol] due to the PHC= CH chromophore and lacks long -wave absorption, and thus excluding a delta[2] pyrazoline structure. The production of [9] apparently proceeds through an initial enhanced conjugate addition of the hydrazine to the beta-unsaturated ketone, followed by intramolecular cyclization as shown in scheme [B], which is rather unusual and in contrast to the usual delta[2]-pyrazoline formation which proceeds through the initial 1,2 addition to the carbonyl function