Conformational analyses of quinolines substituted with a vinyloxy or vinylthio group at the ortho position by means of the NMR selective relaxation method. Experimental verification of the planar conformation in solution

Abstract

Conformations of 2-vinyloxy-4-methylquinoline (1) and 2-vinylthioquinoline (2) have been investigated by means of the NMR selective relaxation method where selective non-inversion T₁, T₁^SNI, has been used to obtain the cross relaxation terms, σ_ij. The interproton distances, r_ij, have been estimated using these σ_ij values. For the rigid part of the molecule, the values of r_ij obtained experimentally give good agreement with the values calculated by the ab initio method.

The conformations defined by the rotations about C(2)–X(O or S) and X(O or S)–C(1′) have been analyzed using the equilibrium model, in which the values of σ_ij obtained experimentally have been compared with the total average of σ_ij, σ_ij^total, and the fractional populations of the pertinent conformers have been determined. The predominance in solution of conformer 1 (see Scheme 2), in which 1′-H is close to the aromatic nitrogen, supporting the planar conformation, has been confirmed for both compounds. Although, for 2, a small proportion of conformer 3′, with a non-planar conformation, is observed as the second-minimum energy one, the conformational equilibria of 1 lie on the side of the conformer 1.