Hydrogen bonding and steric effects on rotamerization in 3,4-alkylenedioxy-, 3-alkoxy- and 3,4-dialkoxy-2-thienyldi(tert-butyl)methanols: an NMR, IR and X-ray crystallographic study

Abstract

The equilibrium constant for the anti ↔ syn rotamerization (anti: intramolecularly hydrogen-bonded hydroxy group; syn: “free” hydroxy group) of 3,4-alkylenedioxy-, 3-alkoxy- and 3,4-dialkoxy-2-thienyldi(tert-butyl)methanols depends on the 3,4-alkylenedioxy or alkoxy group(s) and the solvent, hydrogen-bonding solvents such as DMSO and pyridine favouring the syn isomer. Equilibrium constants ([syn]/[anti]) in chloroform and benzene decrease in the order: 3,4-OCH₂O-, 3,4-O(CH₂)₂O-, 3-OMe, 3-OEt, 3,4-(OMe)₂ ≈ 3-Oi-Pr, 3,4-(OEt)₂, ranging over about 2.5 orders of magnitude. Variations in the IR OH stretching frequencies and the NMR OH proton shifts for the anti isomer indicate that intramolecular hydrogen bonding increases in roughly the same order. The syn → anti rotation barrier in DMSO increases with substituent size and number. The 3,4-methylenedioxythienyl derivative has a rather lower barrier (17.5 kcal mol⁻¹) than all the others (21.0–22.3 kcal mol⁻¹). The syn → anti rotation barrier is largely determined by steric effects but intramolecular hydrogen bonding in the anti isomer contributes to the variation of the anti → syn rotation barrier. A single crystal X-ray diffraction study of the anti-3,4-diethoxy derivative shows that the orientation of the 3-alkoxy group is very different from that in anti-3-methoxy-2-thienyldi(1-adamantyl)methanol. Molecular mechanics and quantum mechanical calculations are used in an attempt to rationalize the equilibrium data.