Barriers of syn-anti-transitions of phenyl-?,?-dichlorovinyl ketone 2,4-dinitrophenylhydrazone

5(4)-Chloro-1-(2,4-dinitrophenyl)pyrazoles from 2,4-Dinitrophenylhydrazones of Chlorovinyl Ketones and β,β-Dichloroacrolein

“…The DNPH of aliphatic ketones exist preferably in the s-cis-anti form, but the DNPH of aromatic ketones exist in the s-trans-syn form, capable of achieving intramolecular heterocyclization. The energy barrier for the transition of one form into the other is fairly high [17], but as was established in [18] the process of syn-anti conversion of geometric isomers of DNPH is catalyzed by acids.…”

“…DNPH 1b-d and pyrazole 2d have been described previously and their physicochemical properties corresponded to the literature [17,18]. The structures of hydrazones 1a,e-g, 3 and pyrazoles 2a-c,e-g obtained for the first time were demonstrated by IR and NMR spectroscopic methods (Table 2), and the compositions were confirmed by elemental analysis (Table 1).…”

“…It was shown simultaneously that 2,4-dinitrophenylhydrazones (DNPH) of aliphatic 2,2-dichlorovinyl ketones are not cyclized on thermolysis into the corresponding 5-chloropyrazoles. In a systematic study of the structure of DNPH of dichlorovinyl ketones [17,18] the determining role of the configuration of the hydrazone was established unequivocally. The DNPH of aliphatic ketones exist preferably in the s-cis-anti form, but the DNPH of aromatic ketones exist in the s-trans-syn form, capable of achieving intramolecular heterocyclization.…”