Nucleophilicities and Nucleofugalities of Organic Carbonates

The kinetics of the reactions of the methyl carbonate ion with benzhydrylium ions in acetonitrile have been studied by UV/Vis spectrophotometry Substitution of the resulting second-order rate constants and the electrophilicity parameters E of the benzhydrylium ions into the linear free energy relationship log k = s(N + E) yielded the nucleophilicity parameters N-25 = 16 03 and s(25) = 0.64 for methyl carbonate in acetonitrile. The kinetics of the reverse reactions, i.e., of the solvolyses of ring-substituted benzhydryl alkyl carbonates in different aqueous solvents were followed by conductimetry. The obtained first-order rate constants and the known electro-fugality parameters E-f of benzhydrylium ions were used to determine the nucleofugality parameters N-f and s(f) of the ROCO2- groups by using the linear free energy relationship log k = s(f)(N-f + E-f) The leaving group abilities of carbonates decrease by a factor of about 300 from PhOCO2- over MeOCO2- and and iBuOCO(2)(-) to tBuOCO(2)(-) in various alcoholic and aqueous solvents. tert-Butyl carbonates (tBocO-R) are, thus, considerably more stable with respect to heterolytic cleavage of the O-R bond than other organic carbonates