Heterogeneous Catalytic Racemization of 4,4'-Disubstituted 1,1'-Binaphthyls by Active Carbons and by Modified Carbon Catalysts

Article abstract of DOI:10.1021/jo01300a030

The racemization of four 4,4'-disubstituted 1,1'-binaphthyls is heterogeneously catalyzed by active carbon or carbon black suspended in chloroform solutions.In the presence of 1 mg/mL of Norit SG1 in chloroform the observed first-order rate constants for racemization of 0.025 M substrate were increased over the uncatalyzed rate constants by factors of 2.9, 6.7, 8.3, 8.1, and 14 for disubstituents NH2, CH3, Br, NO2, and H, respectively.The rates of uncatalyzed racemization of these binaphthyls are electronically influenced by the para substituents (giving a Hammett ρof -0.88) and the catalyzed racemization rates show a similar but slightly decreased substituent effect (ρ = -0.57).However, a small steric effect is indicated for the catalyzed reaction since all substituted compounds are less sensitive to catalysis than is 1,1'-binaphthyl itself.Bromination or chlorination of a carbon black results in increased catalytic activity, and potassium-graphite intercalate is an effective, but irratic, racemization agent.An electron-accepting binaphthyl molecule, loosely bound on electron-donor sites of the graphitic basal planes of carbon catalysts, is suggested as an intermediate complex in the catalyzed racemization.