Hydrogen bonds in “push-pull” enamines

Abstract

A comparison of hydrogen bond strengths in various enamines was made by monitoring the differential shifts Δδ(X) as the difference of NMR chemical shifts between E and Z forms of the nuclei directly involved in hydrogen bonding, i.e., X = ¹⁵N, ¹H, ¹⁷O atoms. The interpretation of these differential values was aided by ab initio calculations and X-ray derived geometric parameters for selected compounds. It is shown that Δδ(N¹H) and Δδ(¹⁵N) give conclusive results and their changes are rationally interpreted by invoking established contributing effects that influence the values of chemical shifts. The Δδ(¹⁷O) parameter is sensitive to intramolecular geometry, mainly bond angles around oxygen and co-planarity of the atoms forming the hydrogen bonds. The latter factor is important in determining the strength of hydrogen bonds. Even a weak acceptor such as the lactone function gives a relatively strong hydrogen bond as compared with sp³-hybridized sulfonyl or sulfinyl acceptors. The hybridization of the penultimate atom of an acceptor plays a crucial role in determining the strength of hydrogen bonding.