HINDERED ROTATION AROUND THE ARYL-TO-NITROGEN BOND OF N-ACETYL, N-ETHOXYCARBONYLMETHYL-2-AMINO, 5-NITROTHIOPHEN

Article abstract of DOI:10.1016/0040-4020(80)80170-0

Two stereoisomers of the title compound are observed by H NMR at 10 deg C.Their spectra coalesce at higher temperature (10 deg C-90 deg C).The equilibrium and rate constants K and k, strongly dependent on the solvent used (1,4-dioxane, tetrahydrofuran, acetone, chloroform); typical values for these parameters and the related thermodinamic functions are: K(25 deg C)=0.170; k(25 deg C)=23.2s^-1; ΔH_R and ΔH(excit.)=4.94 and 17.9kcal.mol.^-1; ΔS_R and ΔS(excit.)=13.1 and 7.7 e.u. in a 0.2 molar solution in 1,4-dioxane.The two isomers are shown to result from a hindered rotation around the aryl-to-nitrogen bond, presumably due to a direct resonance effect between the amide and nitro groups.The more abundant isomer was assigned a planar molecular structure in which the O atom of the amide group is close to the S atom of the thiophen ring, presumably on account of an electrostatic interaction between these two atoms which bear partial electrical charges of opposite sign.