Application of the Variable Oxygen Probe to Derivatives of 2,6-Dimethyltetrahydropyran-4-ol: Evidence for Through-Bond nO–σCC–σ*CO Interactions

Abstract

The variable oxygen probe has been applied to axial and equatorial 4-pyranols 4 and 5 and their ester and ether derivatives. Plots of C–OR bond distance versus pKa (ROH) provided evidence for slightly stronger donation into the σ*C–OR antibonding orbital in the equatorial derivatives 5 than in the axial derivatives 4, which is consistent with the presence of a through-bond nO–σCC–σ*CO interaction in 5. Evidence in support of this interpretation was also provided by density functional theory (DFT) calculations and natural bond orbital (NBO) analyses of the various orbital interactions in the 4-pyranols 4 and 5, their protonated analogues 4·H2O+ and 5·H2O+, and the corresponding cyclohexane derivatives 6, 7, 6·H2O+, and 7·H2O+.