Structural peculiarities of configurational isomers of 1-styrylpyrroles according to 1Н, 13С and 15N NMR spectroscopy and density functional theory calculations: electronic and steric hindrance for planar structure

• Published in: Magnetic resonance in chemistry 2013-06, Vol.51 (6), p.339-349
• Main Authors: Afonin, Andrei V., Ushakov, Igor A., Pavlov, Dmitry V., Schmidt, Elena Yu, Dvorko, Marina Yu
• Format: Article
• Language: English
• Subjects: 13C NMR; 15N NMR; 1H NMR; Carbon Isotopes; configurational isomers; DFT calculations; Electrons; Magnetic Resonance Spectroscopy; Molecular Structure; Nitrogen Isotopes; NMR; Protons; pyrroles; Pyrroles - chemistry; Quantum Theory; Stereoisomerism
• ISSN: 0749-1581; 1097-458X
• DOI: 10.1002/mrc.3951

Comparative analysis of the 1Н and 13С NMR data for a series of the E and Z‐1‐styrylpyrroles, E and Z‐1‐(1‐propenyl)pyrroles, 1‐vinylpyrroles and styrene suggests that the conjugation between the unsaturated fragments in the former compounds is reduced. This is the result of the mutual influence of the donor p–π and π–π conjugation having opposite directions. According to the NMR data combined with the density functional theory calculations, the Z isomer of 1‐styrylpyrrole has essentially a nonplanar structure because of the steric hindrance. However, the E isomer of 1‐styrylpyrrole is also an out‐of‐plane structure despite the absence of a sterical barrier for the planar one. Deviation of the E isomer from the planar structure seems to be caused by an electronic hindrance produced by a mutual influence of the p–π and π–π conjugation. The structure of the E isomer of the 2‐substituted 1‐styrylpyrroles is similar to that of the 2‐substituted 1‐vinylpyrroles. The steric effects in the Z isomer of the 2‐substituted 1‐styrylpyrroles result in the large increase of the dihedral angle between planes of the pyrrole ring and double bond. Copyright © 2013 John Wiley & Sons, Ltd. The E and Z isomers of the 1‐styrylpyrrole have a nonplanar structure because of electronic and steric hindrance. In the E isomer of the 2‐substituted 1‐styrylpyrrole, the double bond has the preferential trans orientation with respect to the substituent. The steric interaction between the substituents in the 2‐position of pyrrole ring and aryl moiety in the Z isomer of the 2‐substituted 1‐styrylpyrrole causes a significant out‐of‐plane turning of the double bond.