The subtle electronic effects of alkyl groups on the conformational equilibria and intramolecular hydrogen-bond strength in cis-3-alkoxycyclohexanols

Abstract

1H NMR data for cis-3- n-propoxycyclohexanol ( cis-3-PCH) and cis-3-isopropoxy-cyclohexanol ( cis-3-ICH) show that a concentration increase shifts the conformational equilibrium from the diaxial (aa) conformer, stabilized by an intramolecular hydrogen bond (IAHB), to the diequatorial (ee) conformer [ X ee = 42% and 21% (at 0.01 mol L −1) to 58% and 56% (at 0.40 mol L −1), in CCl 4, respectively] due to intermolecular hydrogen bonds (IEHB), as confirmed by IR data. The Δ ν values, obtained by IR spectra, indicated that increasing the size of the OR group [R = CH 3, CH 2CH 2CH 3 and CH(CH 3) 2], increases the IAHB strength, due to an increase in the inductive effect of R group, which makes the oxygen lone pairs more available for an IAHB with OH group, in opposition to the steric effect. The percentage of ee conformer increases with the solvent basicity for cis-3-PCH and cis-3-ICH, from 48% and 36% in CCl 4 to 97% and 96% in DMSO, respectively. Values of 4.58, 6.06 and 6.33 kcal mol −1 for the IAHB strength in cis-3-PCH, cis-3-ICH and cis-3-TCH ( cis-3- tert-butoxycyclohexanol), respectively, were obtained, from the theoretical data through the CBS-4M method, confirming the experimental results and indicating that the IAHB strength increases with the increasing bulk of OR substituent in this series of compounds.