Proton chemical shifts in NMR. Part 12.1 Steric, electric field and conformational effects in acyclic and cyclic ethers
The proton resonance spectra of tetrahydropyran, at room temperature and –85 °C where the ring inversion is slow on the NMR timescale, 2-methoxy-, 3-methyl- and several 4-substituted tetrahydropyrans, 2-methyl-1,3-dioxolane and the rigid cyclic ethers 7-oxabicyclo[2.2.1]heptane and 1,8-cineole have been recorded and completely analysed.
These results together with literature data on acyclic and cyclic ethers (1,3- and 1,4-dioxane, dioxolane, 4-oxa-5α-androstane etc.) have allowed the determination of the oxygen substituent chemical shifts (SCS) in these systems. This data set consisting of 78 proton chemical shifts in 17 compounds has been used to test the application of a previous theoretical model of proton chemical shifts to these compounds.
It is shown that the model gives a very good account of the proton chemical shifts in these systems. The ether oxygen SCS are due to both steric and electrostatic terms, the steric term predominating at short distances (e.g. in the 1,3-diaxial interactions in methoxycyclohexanes).
Conformational isomerism in these compounds has also been investigated. Low temperature NMR gave ΔG (eq–ax) +1.0 kcal mol–1 for 4-hydroxy-THP. Analysis of the couplings in the CHCH2OH side chain of 2-(hydroxymethyl)-THP has shown that the preferred conformer is gt in both chloroform and acetone solvents.