Conformational analysis of organic carbonyl compounds. Part 10. Ab-initio MO calculations of the conformational properties of 3-formyl-furan, -thio-phene, and -pyrrole

Abstract

The relative stability of the conformers of 3-formyl-furan, -thiophene, and -pyrrole, and the changes in conformational composition in solutions of different polarity, have been studied using theoretical methods. The total electronic energy of the ground states and that of the transition state for conformer interconversion were calculated with ab-initio MO methods (GAUSSIAN-82) in different AO basis sets. The energy difference between the conformers changes remarkably as a function of the AO basis sets and, as a general rule, the X,O-trans form is increasingly preferred in the more extended basis sets. Geometry relaxation shows that the most significant changes occur on passing from the minimal STO-3G to the 3-21G basis while further implementation fails to produce appreciable changes. The molecular parameter most affected is the exocyclic C–C bond distance which become shorter (from 1.50 to 1.46 Å) and approaches experimental values determined for structurally similar compounds. The calculated electronic structures of these molecules indicate that the relative stability of their conformers is a balance of conjugative interactions, polar character of the conformers, and local electrostatic interactions between the atoms of the formyl group and those of the ring; these effects give a different result in the different calculated schemes. Conformer populations were obtained by solving the Schrödinger equation for the hindered rotor and compared with those available from experimental studies. For the 3-formyl-furan and -thiophene derivatives the X,O-trans conformer predominates, in agreement with experimental results; for the furan derivative this conformer population increases on employing energy values from extended basis sets. For 3-formylpyrrole the N(H),O-cis form is energetically preferred; only to a small extent when energies from large basis sets (6-31G) are employed this conformer is favoured. Reaction field theory was applied in order to study the behaviour of the conformational equilibrium in media of different polarity and the results are compared with those of experimental studies, showing that the trends are qualitatively predicted for the three molecules examined.