1 H and 13 C dynamic NMR study of the E , Z isomers of vinylogous diamides. Effect of conjugation and electrostatic and steric repulsion on the C(O)N rotation barriers and low‐temperature conformation

A 1 H dynamic NMR study was carried out on N , N ‐diethylmethacrylamide (1a), N , N , N ′, N ′‐tetraethylitaconamide (2), N , N , N ′, N ′‐tetraethylcitraconamide (3) and N , N , N ′, N ′‐tetraethylmesaconamide (4) over a wide temperature interval. Amide rotational barriers were determined from the coalescence temperatures. In 3 a second dynamic process was observed below 240 K, which made the geminal methylene protons non‐equivalent. The phenomenon is interpreted in terms of a non‐planar conformation caused by electrostatic and/or steric repulsion of the amide carbonyls. Homonuclear NOE experiments revealed an s ‐cis, s ‐ cis ′ conformation for the compounds studied.