Competing intermolecular and intramolecular hydride transfers in the ionic hydrogenation of (2-alkoxyphenyl)di(1-adamantyl)methanols†

Abstract

ortho-Lithiation of anisole followed by reaction with di(1-adamantyl) ketone gives syn-(2-anisyl)di(1-adamantyl)methanol with the C–OH proton intramolecularly hydrogen-bonded to the methoxy group. Reaction of the alcohol with trifluoroacetic acid (TFA) in dichloromethane leads to a trifluoroacetate and a substituted phenol. These are formed via a carboxonium ion, resulting from an intramolecular 1,5-hydride shift of the initially formed carbocation. Ionic hydrogenation of the alcohol with TFA and a hydrosilane in dichloromethane results in the expected anti and syn deoxygenation products as well as the trifluoroacetate and the phenol. anti-(2-Anisyl)diadamantylmethane (benzylic hydrogen remote from methoxy, confirmed by a single crystal X-ray diffraction study) is formed directly from the carbocation while the syn isomer results from reduction of the carboxonium ion. Reduction of the carbonium ion is more hydrosilane-selective than that of the carboxonium ion. Kinetic isotope effects (k_H/k_D) on the reaction of the carbocation at room temperature average 1.50 for triethylsilane and dimethylphenylsilane. Analogous reaction of the (2-ethoxyphenyl) derivative gives only syn-(2-ethoxyphenyl)diadamantylmethane and the substituted phenol. Kinetic isotope effects on the reduction of the corresponding carboxonium ion average 1.34 for the same hydrosilanes.