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

Abstract

Comparative analysis of the (1)Н and (13)С 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.

Through‐space 19F–19F spin–spin coupling in ortho‐fluoro Z‐azobenzene

“…Nuclei in the Z isomer are more shielded in the 1 H and 13 C NMR except for the 13 C shifts of the ipso ring. Steric deshielding has been previously reported for ABs and related systems, where there is close proximity of two carbon atoms. Based on the X‐ray analysis of our systems, the ipso carbons are the only atoms having non‐bonded distances less than the sum of the van der Waals radii.…”

Computational1H and13C NMR in structural and stereochemical studies

“…Thus, calculations of carbon‐hydrogen coupling constants in combination with corresponding experimental measurements were utilized for the configurational assignments of silicon and germanium derivatives of acetylenic aldehydes and ketone oximes, [ 244 ] pyrrole‐2‐carbaldehyde oximes, [ 245 ] selenophenyl oximes, [ 249 ] N ‐styrylpyrroles, [ 246 ] bis[( Z )‐cyanomethylidene]diazapentacyclodiene‐dicarboxylates, [ 247 ] and (arylamino)methylidenefuran‐2( 3H )‐ones. [ 248 ]…”

“…Speaking about stereochemical applications of LPE in the values of carbon-hydrogen coupling constants, it should be noted that a major breakthrough in the stereochemistry of heteroatomic compounds was provided with a series of publications by Afonin, et al [244][245][246][247] and partly by Osipov, et al [248] Thus, calculations of carbon-hydrogen coupling constants in combination with corresponding experimental measurements were utilized for the configurational assignments of silicon and germanium derivatives of acetylenic aldehydes and ketone oximes, [244] pyrrole-2-carbaldehyde oximes, [245] selenophenyl oximes, [249] Nstyrylpyrroles, [246] bis[(Z)-cyanomethylidene] diazapentacyclodiene-dicarboxylates, [247] and (arylamino)methylidenefuran-2(3H)-ones. [248] F I G U R E 2 9 13 C NMR chemical shifts (ppm) along the tube axis for (6, 0) and (9, 0) zigzag SWCNTs (10 bamboo units), calculated with the BHandH functional.…”

“…Speaking about stereochemical applications of LPE in the values of carbon‐hydrogen coupling constants, it should be noted that a major breakthrough in the stereochemistry of heteroatomic compounds was provided with a series of publications by Afonin, et al [ 244–247 ] and partly by Osipov, et al [ 248 ]…”