Conformational preference of the methylsulphinyl group bonded to the furan and thiophene rings: A theoretical approach

Abstract

The ground state conformations and transition states for conformer interconversion in the two isomers of the methylsulphinyl derivatives of furan ( 1 and 2) and thiophene ( 3 and 4) were identified through a theoretical approach based on ab-initio molecular orbital calculations. Two minima and two transition states were found in the potential energy profiles for all the molecules, but the behaviour of 2-methylsulphinyl furan ( 1) differs somewhat from that of the other compounds. The minima correspond to the two orientations of the S0 bond with respect to the heterocyclic heteroatom X (X0 or S), labelled X,O- cis an X,0- trans. The conformer having the S0 bond oriented s-cis with respect to the internal double bond with higher π density (X,0- cis for the 3-methylsulphinyl derivatives and X,0- trans for the 2-methylsulphinyl derivatives) has a twist angle φ (between the S0 bond and the heterocyclic ring) of a few degrees. Larger distortions are present in the conformers with opposite orientation of the S0 bond, and the twist angle has the highest value (φ = 62.8° ) in compound 1. The transition states correspond to the methyl group being nearly coplanar with the heterocyclic ring and this situation seems to be due mostly to a loss of π electron delocalization. More complex is the transition state situation for compound 1, since a barrier in the potential energy for internal rotation is imposed also by the repulsion between the heterocyclic and sulphinyl oxygen atoms. The conformer populations were estimated from the potential energy expressed as a function of φ by employing a Fourier expansion in the hindered rotor approximation; the populations show that for the 3-methylsulphinyl compounds the X,O- cis form is preferred, although the situation is reversed for the 2-substituted derivatives. An approximate prediction of the behaviour of these molecules in solution was attempted in the framework of the reaction field theory. In solvents of increasing polarity the X,0- cis form becomes progressively more dominant for the 2-methylsulphinyl derivatives, in contrast with the behaviour of 3-methylsulphinyl derivatives which shift to the X,O- trans form. For compound 1 an increase in solvent polarity seems to cause stabilization of a third conformer.