Nuclear magnetic resonance spectra of adamantyl-substituted phenols and solvent-induced shifts of sterically hindered protons

Abstract

The n.m.r. spectra of 2,6-di-1-adamantyl-4-alkylphenols (alkyl = Me, Et, Prⁱ, Bu^t, or 1-adamantyl) have been recorded for dilute solutions in [²H]chloroform, carbon disulphide, carbon tetrachloride, hexane, and [²H₆]benzene. The internally referenced chemical shifts of the ring and phenolic protons are presented. The resonance of the sterically hindered 3- and 5-protons is shifted, relative to solutions in carbon tetrachloride or hexane, downfield in [²H₆]benzene and in [²H]chloroform, and upfield in carbon disulphide. These internally referenced solvent shifts become larger as steric hindrance of the protons increases, except in the case of solutions in [²H]chloroform, for which the shift, relative to hexane solution, becomes smaller as the steric hindrance of the protons increases. Comparison with data for the corresponding 2,6-di-t-butylphenols reveals that the adamantyl group causes shielding of neighbouring protons. The chemical shifts of the sterically hindered phenolic protons of 2,4,6-tri-t-butylphenol and 2,6-di-1-adamantyl-4-t-butylphenol have been obtained for solutions in carbon tetrachloride–benzene over the whole range of mixture concentrations. The factors causing solvent shifts for the sterically hindered protons in these substituted phenols are discussed.