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

Abstract

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 (R_cryst 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.