A. N. Vasil'ev

Pyrroles from ketoximes and acetylene 8. Synthesis of 4,5,6,7-tetrahydroindole and its 1-vinyl derivative

Abstract4,5,6,7-Tetrahydroindole or 1-vinyl-4,5,6,7-tetrahydroindole was obtained in 81 and 93% yields, respectively, by reaction of cyclohexanone oxime with acetylene at 90–140‡C in the presence of alkali metal hydroxides or alkoxides in dimethyl sulfoxide (DMSO) or mixtures of DMSO with low-polarity or nonpolar solvents. The reaction is effective both in an autoclave (initial pressure 8–16 gage atm) and at atmospheric pressure.

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. 22. Dihaloethanes in place of acetylene in reactions with cyclohexanone oxime

Abstract4,5,6,7-Tetrahydroindole and its 1-vinyl derivative were obtained in overall yields of 30–60% by the reaction of cyclohexanone oxime with 1,2-dichloro- and 1,2-dibromoethane in the MOH-dimethyl sulfoxide (DMSO) superbase system (M = K, Na, Li). Nucleophilic substitution of the halogen in the dihaloethanes by the oximate anion to give the corresponding ethylene glycol bis (imido ether) is a side reaction. The effect of the reaction conditions on the yields of products and the selectivity of the process was examined.

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.

Free-radical addition of alkanethiols to 1-vinylpyrroles

In the presence of 2,2′-azobisisobutyronitrile or in the absence of an initiator alkanethiols add to 1-vinylpyrroles exclusively counter to Markovnikovs rule to give 1-(2′-alkylthioethyl)pyrroles (in yields up to 94%).

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.