Conformational analysis of organic carbonyl compounds. Part 11. Conformational properties of difuryl, dithienyl, and furyl thienyl ketones studied by X-ray crystallography, n.m.r. lanthanide-induced shifts and ab-initio MO calculations

Abstract

The conformational properties of the diheteroaryl ketones of the furan and thiophene series have been studied both experimentally, in the solid state and in solution, and with a theoretical ab-initio approach. X-Ray analysis of the samples which are solid at room temperature was possible only for the compounds which gave suitable crystals, i.e. di(2-thienyl) ketone (2), di(3-thienyl) ketone (5), 2-furyl 3-furyl ketone (7), 2-thienyl 3-thienyl ketone (8) and 3-furyl 2-thienyl ketone (10). The crystal data are: (2) space group F dd2, a= 13.267(2), b= 21.853(4), c= 6.067(1)Å, Z= 8, R= 0.0637 for 388 observed reflections; (5) space group P2₁/n, a= 6.174(1), b= 11.184(3), c= 13.203(2)Å, Z= 4, R= 0.0776 for 1 202 observed reflections; (7) space group P2₁/c, a= 9.900(4), b= 10.934(6), c= 7.485(2)Å, Z= 4, R= 0.0461 for 984 observed reflections; (8) space group P2₁/c, a= 6.183(1), b= 11.109(1), c= 13.379(2)Å, Z= 4, R= 0.0732 for 1 186 observed reflections; (10) space group P, a= 13.799(2), b= 10.039(2), c= 6.084(1)Å, Z= 4, R= 0.0639 for 1 973 observed reflections. The crystal structures of compounds (5) and (8) are disordered: for the former the disorder concerns the position of one ring, whereas both rings are involved in the latter. For compound (10) the structure of the two crystal lographicaIly independent molecules is the same. From the n,m.r. LIS (lanthanide-induced shifts) measurements (on ¹H and ¹³C) indicative information is obtained on the type of conformer and its relative population present in the solution equilibrium mixture. MO ab-initio calculations, performed by employing the 3-21 G or 3-21 G*(with d-orbitals on the sulphur atom) basis sets for the ground-state conformers of all the molecules considered, show that the solid-state structures are very similar to those calculated for the conformers with the lowestenergy content and the calculated conformer populations are in qualitative agreement with measurements in solution. As a general trend, the solid-state structures, the preferred conformations found in solution and those calculated for the free molecules show that the preferred orientation of one heterocyclic ring with respect to the carbonyl group is of the type found in the corresponding formyl and acetyl derivatives and is maintained in all the diheteroaryl ketones. For the molecules having conformers of similar energy content and different dipole moment, i.e. di(2-furyl) ketone (1), it was demonstrated, on the basis of the classical theory of the solvent effect, that the equilibrium mixture of conformers in the vapour phase is different from that in solution and changes in solvents of different polarity.