Conformational analysis, Part 41. A modelling and LIS/NMR investigation of the conformations of α,β-unsaturated carbonyl compounds

A novel Lanthanide Induced Shift Analysis (LISA) is presented. In this analysis both the paramagnetic and the diamagnetic lanthanide induced chemical shifts are normalised separately in contrast to previous techniques in which only the paramagnetic shifts were normalised. This procedure is used together with molecular mechanics (MMFF94) and ab initio (RHF/6‐31G, RHF/6‐311G** and B3LYP/6‐311G**) calculations to investigate s‐cis/s‐trans isomerism in some α,β‐unsaturated aldehydes, ketones and esters. In tiglic aldehyde 1 and trans‐cinnamaldehyde 4 the s‐trans conformer predominates with energy differences ΔE (s‐cis–s‐trans) of 1.64 and 1.76 kcal/mol. In methyl vinyl ketone 2 and trans‐cinnamyl methyl ketone 5 the populations of the s‐cis and s‐trans isomers are almost equal (ΔE 0.24 and 0.0 kcal/mol) and in methyl crotonate 3 and methyl trans‐cinnamate 6 the s‐cis conformer is more stable (ΔE −0.72 and −0.41 kcal/mol). These results are in agreement with both the MMFF94 and ab initio calculated energies for the compounds except tiglic aldehyde 1 in which all the calculated values are too large and cinnamyl ketone 5. In this compound the ab initio calculations predict the s‐cis form to be more stable than the s‐trans in contrast to both the MM calculations and the observed result which give both forms of equal energy. Also in both the MM and ab initio calculations phenyl substitution in the ketone (2 vs. 5) considerably stabilises the s‐cis form. This is not observed in practise. In phenyl acetate 7 the B3LYP calculations give two equally stable structures, one planar one non‐planar. The MMFF94 and MP2 calculations and the LIS analysis support the existence of only the non‐planar conformer in solution, which is also the conformation of phenyl acetate in the crystal. Copyright © 2006 John Wiley & Sons, Ltd. A novel Lanthanide Induced Shift Analysis (LISA) is devised to allow separate normalization of Yb(fod)3 and Lu(fod)3 induced shifts. This is used to investigate s‐cis/s‐trans conformational equilibria in α,β‐unsaturated aldehydes, ketones and esters. The results confirm the energy calculations of MMFF94, RHF/6‐31G, RHF/6‐311G** and B3LYP/6‐311G** calculations.