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.