O–H stretching frequencies in bicyclo[2,2,1]heptan-2-ols: measurement of repulsion between aromatic π- and oxygen lone pair-type orbitals

Abstract

endo-Bicyclo[2,2,1]heptan-2-ols with a 5,6-endo-benzo- or -naphtho-substituent are found to possess an unusually strong intramolecular hydrogen bond. The alternative conformation, in which the hydroxy-hydrogen is turned away from the aromatic ring with consequent loss of the internal hydrogen bond, is especially unfavourable because of repulsion between the oxygen lone-pair orbitals and the aromatic π-orbital. Measurement of equilibrium constants for association of various bicycloheptanols with di-n-butyl ether yields a value of –1·2 kcal mol^–1 for this orbital–orbital repulsive energy. The interaction is considered to be responsible for other reports of unusually strong OH⋯π bonds.