Structural peculiarities of configurational isomers of 1-styrylpyrroles according to ~1H, ~(13)C and ~(15)N NMR spectroscopy and density functional theory calculations: Electronic and steric hindrance for planar structure

摘要

Comparative analysis of the ~1Cyrillic capital letter en and ~(13)Cyrillic capital letter es 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.