S. E. Korostova

Vinylation of pyrroles in dimethyl sulfoxide

A number of 1-vinylpyrroles were obtained in up to 97% yields by base-catalyzed addition of substituted pyrroles to acetylene in dimethyl sulfoxide at 80–100°C.

Pyrroles from ketoximes and acetylene. 30. The synthesis of 3-alkyl-2-arylpyrroles

Conditions were found for the synthesis of 3-alkyl-2-arylpyrroles using the Trofimov reaction at 50–70 °C both at elevated and atmospheric pressure. O-Vinyloximes and 3H-5-hydroxy-4,5-dihydropyrroles were isolated as intermediates.

PYRROLES FROM KETOXIMES AND ACETYLENE. 14.* QUANTITATIVE ESTIMATE OF THE EFFECT OF SUBSTITUENTS ON THE NH ACIDITIES OF PYRROLES

The pK values of a series of substituted pyrroles and the ΔvNH values (in dimethylformamide), which characterize their relative acidities were measured by the transmetallation method. It was established that the acidities of pyrroles are determined primarily by the inductive effect of the substituents and are virtually independent of the ability of the latter to enter into conjugation with the ring. This means that the electron pair of the anion that is set free in the ionization of pyrrole retains σ character. The linear dependences of the pK and ΔυNH values on the inductive constants (σI) of the substituents can be used to estimate the pK values of various compounds that contain a pyrrole ring starting from the ΔυNH and σI values.

IR spectra of substituted pyrroles

The IR spectra of 52 substituted pyrroles, including 24 1-vinylpyrroles, were thoroughly analyzed. The principal analytical bands of the pyrrole ring and the vinyl group were isolated. Doublet character of the bands of the stretching vibrations of the double bond (vC=C) and the C-H (CH2=) out-of-plane deformation vibrations, which indicates the presence of rotational isomerism relative to the N-vinyl bond, was detected. The integral intensify of the principal component of the vC=C doublet was measured for 12 of the 1-vinylpyrroles, and it is shown that it is practically independent of the structure of the alkyl substituents in the ring. Proof for the existence of a nonplanar gauche conformation for the 1-vinylpyrroles was obtained.

Pyrroles from ketoximes and acetylene. 23. 2-(1-naphthyl)- and 2-(2-naphthyl)pyrroles and their 1-vinyl derivatives

Abstract2-(1-Naphthyl)- and 2-(2-naphthyl)pyrroles and their 1-vinyl derivatives were synthesized in 22–64% yields by the reaction of 1- and 2-acetylnaphthalene oximes with acetylene at both atmospheric pressure and elevated pressures in an alkali metal hydroxide-dimethyl sulfoxide system.

Relative basicities of n-vinylpyrroles

The relative basicities of a series of N-vinylpyrroles and some of their analogs with saturated substituents attached to the nitrogen atom were investigated (the ΔvOH shifts in the IR spectrum of phenol were measured). The vOH values were measured in the interval 100–150 cm−1. Alkyl substituents in the 2 and 3 positions raise the basicity of the pyrrole ring, while a phenyl substituent in the 2 position lowers it. N-Vinyl-pyrroles are ∼25% less basic than their limiting analogs-N-ethyl- and N-(2′-alkylthioethyl)pyrroles. The inductive effect of the substituent makes the major contribution to the basicities of the investigated pyrroles. The ΔvOH values for N-vinyl-2,3-diphenylpyrrole are not in conformity with the general tendencies and constitute evidence for the appreciable contribution of continuous conjugation through the double bond, the pyrrole ring, and the benzene ring in the 3 position.

Selective hydrogenation of the vinyl group of 1-vinylpyrroles

The hydrogenation of 1-vinylpyrroles over Raney nickel in ethanol at 50–90°C proceeds exclusively at the 1-vinyl group and leads to the formation of 1-ethylpyrroles in 80–90% yields. The IR and PMR spectra are presented.

UV spectra of N-vinylpyrroles

The effect of the structure and the medium on the position and intensity of the absorption bands in the UV spectra that attest to the presence of a single conjugation system in N-vinylpyrroles is thoroughly analyzed. The tendency for a reduction in the intensity of the conjugation (disruption of the coplanarity) is appreciable as branching of the alkyl group in the 2 position increases. A pronounced interaction of the benzene and ethylene fragments through the pyrrole ring is observed.