Transmission of substituent effect in ethyl 2-aroyl- and ethyl 2-arylcarbamoyl-4, 5-dimethyl-1, 2, 3, 6-tetrahydropyridazine-1-carboxylates: An infrared and theoretical study

Nineteen ethyl 2-aroyl- (1a-1h) and ethyl 2-arylcarbamoyl-4, 5-dimethyl-1, 2, 3, 6-tetrahydropyridazine-1-carboxylates (2a - 2k) were synthesized and their FTIR spectra in CCl4 and CHCl3 were measured in the regions of C=O, C =C and N-H stretching vibrations. The wave numbers of the C=O and N-H stret ching vibrations as well as the integrated intensities of the N-H stretchin g absorption bands exhibit significant linear correlations with Hammett sig ma substituent constants. In the series 2 the substituent effects are trans mitted to the esteric C=O group comparably or even more readily as to the n earest carbamoyl C=O groups. The analysis of the effects of the substituent s, solvents and temperature as well as the results of PM3 calculations show that such an efficient transmission is possible when the molecules exist i n a conformation possessing a quasiplanar arrangement of the (O)C-N-N-C(O)- N(H)-C6H4X system of live sigma-bonds.