Conformational analysis. Part 29.1 The conformational analysis of 2-substituted fluoro- and trifluoromethyl-benzaldehydes, acetophenones and methyl benzoates by the lanthanide induced shift (LIS) technique

Abstract

An improved LIS technique, using, Yb(fod)3 to obtain the paramagnetic induced shifts of all the spin ½ nuclei in the molecule, together with complexation shifts obtained by the use of Lu(fod)3 has been used to investigate conformational isomerism in 2-fluorobenzaldehyde 1, 2-fluoroacetophenone 2, methyl 2-fluorobenzoate 3 and the corresponding 2-trifluoromethyl compounds 4, 5 and 6. The use of fluorine LIS in these molecules was first established by analysis of the LIS in 4-fluorobenzaldehyde 7 and 4-trifluoromethylbenzaldehyde 8 in which conformational isomerism is not possible and confirmed in subsequent analyses. It is shown that 19F LIS may be used with the same degree of confidence as the corresponding 1H and 13C LIS in theses molecules. Analysis of the LIS data was considered together with ab initio, modelling and solvation calculations to provide a comprehensive account of the conformer geometries and energies for these compounds in a variety of solvents. The 2-fluoro compounds are all planar, with the trans (CO···F) conformer always more stable, in 1 and 2 predominating in all but very polar solvents. In the corresponding 2-trifluoromethyl compounds both the cis and trans conformers of the aldehyde 4 are planar with the trans form predominating, but the ketone 5 is essentially in one orthogonal conformation and the ester 6 interconverting between two nonplanar conformations with the trans conformer predominant.