N‐Alkenyl amide rotational barriers by 2D EXSY NMR

Abstract

The rotational barriers of some N-alkenylamides were measured by 2D exchange spectroscopy (2D EXSY) NMR techniques. It was found that the conjugated double bond lowers E,Z barriers by 2.6 kJ mol −1 .

The cis‐trans isomerization of N‐methyl‐α,β‐dehydroamino acids

“…Various factors, such us steric and electronic effect of substituents, influence of environment, or presence of hydrogen bonding may perturb the cis‐trans equilibrium 39. It is known that the unsaturated character as well as steric hindrance of the N ‐substituent decrease free‐energy differences between cis and trans isomers and increase the population of the cis isomer in esters and amides 68–70. Although the electronic effect of the double bond cannot be neglected, as the rotational barrier can be lowered due to the potential overlapping of the nitrogen lone pair and C α C β π‐electron, the steric effect seems to play important role in the case of N ‐methyldehydroamino acids.…”

Influence of lithium cations on prolyl peptide bonds

“…This technique allows the investigation of exchange processes that are slow on the NMR time scale. It has been widely used to determine the rotational barriers of amide bonds on the basis of the magnetization transfer during a chemical exchange like a cis / trans isomerization process . For this purpose, NOESY spectra at seven different mixing periods (0.03, 0.5, 0.75, 1.0, 1.5, 2.0, and 3.0 s) were recorded at 300 K. The volume integrals of the cross peaks of the exchanging ortho protons of the para ‐nitro moiety in 1 and the H α protons in peptide 2 , respectively, were determined after normalization of the cross peaks to the diagonal peaks.…”

Synthesis of Z‐N‐Alkenylformamides

“…[10 -13] Using 2D EXSY NMR, we found the E-N-alkenylformamides had a rotational barrier of 17 -19 kcal/ mol. This subsequently led us to study the barriers to rotation in the Z-Nalkenylformamides isomer, which showed similar energy values [14,15] in spite of the fact that the double bond is orthogonal to the benzene ring ( Fig. 1).…”

“…This 1 H NMR spectrum exhibits a doubling of signals due to restricted rotation about the amide (N-CO) bond, leading to two rotational conformers (anti and syn). [14,15] The more abundant rotamer is the syn for both E and Z-N-alkenylformamides (11,12). Anal.…”