Can 2‐acylpyrroles form an intramolecular hydrogen bond?

Abstract

The formation of intramolecular hydrogen bonding by certain N-substituted 2-acylpyrroles has been demonstrated by B3LYP/aug-cc-pVDZ calculations, the quantum theory of atoms in molecules, and the natural bond orbital method. Total electron energy densities H BCP at the bond critical point of the H⋯O bond were applied to analyze the strength of these interactions. The relations between quantum theory of atoms in molecules, carbonyl stretching vibrational modes ν C = O , and natural bond orbital parameters associated with the formation of the C-H⋯O interaction have been established. The short contacts were found experimentally in the crystal structure of a new 2-acylpyrrole derivative 5-chloro-2-oxopentyl-1-(5-chloro-2-oxopentyl)pyrrolo-2-carboxylate. The influence of 2-and N-substitution of 2-acylpyrroles on C-H⋯O interaction energy is discussed. It was found that the methylene group may act as a proton donor leading to a red-shift or blue-shift phenomenon of the ν C-H stretching mode.

Does the Intramolecular Hydrogen Bond Affect the Spectroscopic Properties of Bicyclic Diazole Heterocycles?

“…It is in agreement with the properties of the Cl atom as an electronwithdrawing substituent that enhances the proton-donating ability of the adjacent CH group. The latter was suggested in a previous study [45] that the strength of the C-H … B hydrogen bond strongly depends on the nature of the proton donor and increases when hydrogen atoms are replaced by electron-withdrawing substituents [9,46].…”

“…Replacement of the H atom from the group R 2 = CH 3 by Cl results in a reduction in the H … O distance [9]. This is also observed for the systems analyzed here; for example, the H … O distance is equal to 2.17, 2.03 Å for 20 (R 2 = CH 3 ) and 22(R 2 = CHCl 2 ), respectively ( Compound , the H … O distances are within the range acceptable for intramolecular H-bonds.…”

“…The results of optimization correspond to energy minima since no imaginary frequencies were detected. The initial geometry of the pyrrolopyrazinone system was taken from X-ray data, and it was further applied in the geometry optimization [9]. The computed frequencies were multiplied by the uniform factor of 0.97 to obtain a good estimate of the experimental results and to eliminate known systematic errors related to anharmonicity [25].…”

“…The H atom = CH 3 is located at 2.6 Å distance from the hydrogen atom of the R 3 group and has no steric repulsion between the R 2 and R 3 electron clouds. This effect leads to the lack of attractive interaction of HB between the carbonyl group and the neighboring CH 3 group [9].…”

“…We consider here a series of pyrrolopyrazinones (Scheme 1, Table 1) that is an extension of our previous work on Nsubstituted 2-acylpyrroles [9]. For some of the heterocyclic systems analyzed recently, short C=O⋯H-C intramolecular contacts are observed [10].…”