Structural requirements in chiral diphosphinerhodium complexes. Part 9. ¹H Nuclear magnetic resonance determination of E,Z-configuration and cis,trans-amide conformations in prochiral substrates used in asymmetric hydrogenation reactions: Methyl and ethyl α-formamido-β-substituted acrylates

By ¹H n.m.r. spectroscopy, E-methyl or -ethyl α-formamido-β-substituted acrylates may be characterized by the relatively large difference in chemical shift values (Δδ = 0.75-1.25 p.p.m. in CDCl₃) of the two H_B (vinylic) proton signals exhibited by the cis- and trans-amide conformers. Change of solvent from CDCl₃ to CF₃CO₂H causes this difference in H_B chemical shift values to be considerably reduced, and also results in an increase in the quantity of the minor cis-amide conformer. Z-Methyl or -ethyl α-formamido-β-alkylacrylates may be characterized by a much smaller difference in chemical shift values (Δδ = 0.05-0.10 p.p.m. in CDCl₃) for the corresponding H_B signals. Change of solvent from CDCl₃ to CF₃CO₂H causes the Hp proton signals (for the cis- and trans-amide conformers) to both undergo similar downfield shifts, but did not result in an increase in the quantity of the minor cis-amide conformer [in the series of compounds studied]. E-Methyl or -ethyl α-formamido-β-arylacrylates may be further characterized by the anisotropy of the aromatic ring causing an upfield shifting of the proton signals within the ester moiety as compared to the corresponding signals in olefins having the Z-configuration or to non-aromatic substituted α,β-unsaturated esters in general.