Nucleophilicity toward ketenes: Rate constants for addition of amines to aryl ketenes in acetonitrile solution

Second-order rate constants (k(Nu)) have been measured for the addition of amines to ketenes 4-6 in acetonitrile solution by the laser flash photolysi s technique. These ketenes are formed from a photochemical Wolff rearrangem ent of diazoketones 1-3, respectively. For all diazoketones studied, the pr esence of amines as nucleophiles in the reaction medium results in the form ation of an intermediate that later converts to the amide. The rate of form ation of these intermediates is linearly dependent on amine concentration. Various classes of amines, such as primary, secondary, and tertiary, aromat ic, and aliphatic, have been used to investigate the ketene reactivity, and rate constants in the range 10(4)-10(9) M-1 s(-1) have been measured. Reac tion rates are dependent upon steric effects in both the ketene and the nuc leophile, which is consistent with a reaction mechanism involving nucleophi lic attack at C alpha in the molecular plane of the ketene. On the basis of these data, a set of N+ parameters for the reaction of amines with ketenes was determined.