Conformations of 2-formyl-, 2-acetyl-, and 2 benzoyl-pyrroles

Abstract

Experimental dipole moments (1 03*p/C m) and molar Kerr constants [l 027703(mK2)/m5 V-2 mol-l] are reported for the following as solutes in cyclohexane at 25" : 2-formylpyrrole, 4.33, +76 ; 2-acetylpyrrole, 3.98, +50 ; and 2benzoylpyrrole, 4.06, +42. The results are analysed to provide information concerning the preferred conformations of these molecules.THE conformational preferences of heteroaromatic aldehydes and ketones are continuing to attract much attention. Various physical methods have been applied to the %substituted pyrroles (dipole m ~m e n t s , l -~ i.r.4-7 and n.m.r.8-14 spectra, dielectric relaxation 2, but some problems remain. It might reasonably be expected that

Infrared, Density-Functional Theory, and Atoms in Molecules Method Studies on Conformers of Some 2-Substituted 1H-Pyrroles

“…This confirms earlier experimental results. The conformational preferences of some heteroaromatic aldehydes and their N -methyl derivatives were recognized previously on the basis of various physical methods: 1 H, 13 C NMR, − measurements of dipole moments, molar Kerr constants, ab initio studies, and IR spectral investigations. , These studies showed a very strong preference for the s-cis arrangement of the N−H and CO groups. Furthermore, it was demonstrated that for PC-NMe the s-cis conformer is a dominant form (95%). , The IR spectroscopic investigations 1,4 revealed the presence of one absorption band for the free carbonyl group.…”

“…The calculated dipole moments for eight analyzed structures are also presented in Table 1. In all cases, the dipole moments are lower for the s-cis form, in which the component dipoles of the pyrrole ring and the carbonyl group 12 are in the antiparallel arrangement.…”

“…As we proposed previously, on the basis of ab initio calculations, the absorption band at higher frequency (1719 cm -1 ) was ascribed to the less stable s-trans conformer and the band at 1701 cm -1 to the s-cis form. According to the IR data concerning conformational analysis, , the population of the more polar form increases with the polarity of the solvent. Thus, the ratio / of integrated intensities of the carbonyl bands of MPC conformers in CCl 4 is 1.59, whereas in the more polar acetonitrile it is 1.43 and is close to unity for 10 -3 M ethyl ether solution.…”

“…It was recognized previously 12 that the position of equilibrium between alternative rotational isomers of a particular molecule is medium-dependent. So, solvent effects play an important role in determining equilibrium constants.…”

“…13 Polar solvents stabilize the more polar form causing its higher proportion; in less interactive solvents its lower proportion was observed. Cheng et al 12 explained such behavior as resulting from the reduction of the free energy difference between the s-cis and the s-trans forms. Furthermore, the frequency shifts of the carbonyl group depend on the solvent dielectric constants, solvent-solute interactions such as hydrogen bonds, and steric effects.…”