Conformational analysis. Part 24. A lanthanide-induced-shift (LIS) NMR investigation of aromatic ketones. Lutetium versus lanthanum reagents in probing diamagnetic complexation shifts

The possible use of Lu(fod)3vs. La(fod)3 in probing diamagnetic complexation contributions to the shifts induced Yb(fod)3 on the 1H and 13C NMR signals of aromatic ketones has been investigated. For all the compounds examined the patterns of the shifts caused by the two diamagnetic reagents are qualitatively similar, reflecting predominantly π-electron-density transmission. For the less activated substrates fluoren-9-one 1 and 2,6-dimethyl-4 nitroacetophenone 3 the use of either La(fod)3 or Lu(fod)3 yielded very similar sets of data, indicating in 3 a ca. 90° torsion angle between the acetyl and the ring. More interstingly, in the case of acetophenone 2, 2-acetylnaphthalene 4,3-methoxyacetophenone 5 and 2-methylacetophenone 6 the use of Lu(fod)3 gave a markedly improved definition of essentially the same solutions. These are a planer conformation for 2 a prevalence(ca. 68%) of the E conformer for 4 and of the Z conformer for 5 and 6(ca. 63% and 92%, respectively). In the case of 6 the geometries of the two conformers to be submitted to LIS analysis have been obtained through an ab initio RHF 6-13G* full-optimization process, indicating a planar structure for the Z and a 35°-rotated structure for the E conformer. In conclusion the analysis using Lu(fod)3, while supporting the results obtained with its lanthanum analogue, appears to reproduce better the diamagnetic contribution to induced shifts in strongly coordinating substrates. With the unhindred substrates 2, 4 and 5, where very reliable starting geometries could be adopted, solutions obtained through the use of Lu(fod)3 have been particularly good (Rcryst 0.2–0.4%), as pseudoconatact shifts ranging up to 130–150 ppm could be reproduced with an average absolute error of ±0.1 ppm. This shows how accurate the LIS method can be in the absence of additional problems and leaves little doubt about the accuracy of its theoretical background.