Lanthanide‐induced shifts in proton NMR spectra of cyclohexanones

Abstract

AbstractThe lanthanide‐induced shifts (LIS) of the lanthanide shift reagent (LSR) Eu(FOD)3 are reported for a large number of cyclohexanones, especially those which are highly substituted. The following compounds were synthesized and characterized: 3‐(aryl)‐3,5,5‐trimethylcyclohexanones, in which aryl = 1‐naphthyl, phenyl, o‐anisyl, m‐anisyl, p‐anisyl and p‐chlorophenyl. Some analogous compounds were also studied: 3,5,5‐trimethylcyclohexanone, 4‐tert‐butyl‐cyclohexanone, 3,3,5,5‐tetramethylcyclohexanone 3‐(1‐naphthyl)5,5‐dimethylcyclohexanone and para‐tert‐butyl‐anisole. A method for the regression analysis of the concentration dependence of the LIS of these substrates is developed and reported, and used to derive the limiting incremental LIS (Δ2) for the LS2 complex and to evaluate the proton chemical shifts (δ0) in the absence of LSR. An ‘incremental dilution’ technique was found to be most appropriate to insure constant substrate concentrations, needed to extract both Δ2 and δ0. The conformations of the 3‐(aryl)‐type systems and analogous compounds were studied via LIS and found to conform to:—(i) an axially disposed aryl substituent in the 3,3,5,5‐tetra‐substituted cyclohexanones and (ii) a flattened chair form of the cyclohexanone ring with distortions in this chair form being an increase in the syn‐diaxial (C‐3, C‐5) substituent distance (C‐3 and C‐5 substituents still eclipsed). The LIS were fully compatible with these structural assumptions.