Reactivities of Stable Rotamers. XIV. Reaction of 9-[2-(Substituted Methyl)-6-methylphenyl]fluorene Rotamers with Butyllithium

The title compounds of which substituents carry atoms with lone-pair electrons have been prepared and their rotamers isolated as stable entities at room temperature. Deprotonation rates of these rotamers with butyllithium in benzene–hexane solution show a striking difference depending upon the substituents and the rotamers. In both sp and ap series, compounds with a methoxyl group exhibit the largest rate constants, while those with a sulfur substituent show the smallest reactivity. The difference is explained in terms of coordination ability of the heteroatoms toward butyllithium and by taking advantage of the entropy effect at the transition state of lithiation. The difference in reactivities between rotamers is further striking in a sense that the reactions differ in the order; sp rotamers show the first-order kinetics while ap rotamers obey the second-order kinetics in the substrate. By assuming that the observed rates are the sum of the first-order and the second-order contributions, we estimate the minimum ratios of the first-order rate constants between sp and ap rotamers, k1sp⁄k1ap, to be >2300 in the methoxyl and dimethylamino derivatives and >240 in the methylthio derivatives.