CHANGES IN TRANSITION-STATE STRUCTURE FOR PROTONATION OF ARYLDIAZOMETHANES IN AQUEOUS-SOLUTIONS

Rate constants are reported for the decay of a series of aryldiazomethanes in aqueous media, 4% acetonitrile, 25 degrees C, ionic strength 0.2 M (NaClO4). Plots of the log of k(0), the buffer independent first-order rate constant for decay, against pH that range from pH 2 to 13 are biphasic. At the low pH end the value of log k(0) decreases with increasing pH with a unit slope, whereas at the high pH end the value of log k(0) is pH independent. The solvent deuterium isotope effect for the pH independent reaction of the 4-methoxyphenyldiazomethane is k(0)(H2O)/k(0)(D2)O = 4.5 +/- 0.1. On the basis of the isotope effect it is concluded that the mechanism of the pH independent reaction of aryldiazomethanes is identical with that deduced previously for the acid-catalyzed reaction and involves rate-limiting protonation by H2O. The value of rho for the hydrogen ion catalyzed reaction is rho(kH) = -1.14 while that for the water-catalyzed reactionis rho(kH2O) = -2.01. Catalysis of decomposition by four oxygen acid buffers was examined for three aryldiazomethanes. The slopes, alpha, of the linear Bronsted plots for catalysis change as a function of the substituent in the benzene ring, yielding the ''cross interaction coefficient'' delta alpha/delta sigma = 0.15. There is a corresponding change in the rho values for sensitivity of the log of k(HA), the rate constant for catalysis by a particular catalyst, to the substituent in the benzene ring, -delta rho/delta pK(HA) = 0.15 These data establish that there are changes in the structure of the transition state for the proton transfer reactions of monoaryldiazomethanes. Examination of a similar range of data obtained by others for the decomposition of diaryldiazomethanes in 80% DMSO/20% H2O indicates two important contrasts: first, cross interaction coefficients in those cases are not significantly different from zero, and second, the Bronsted plots for these latter reactions show strong curvature. The differences between mono- and diaryldiazomethanes are rationalized as being due to a difference in orientation, relative to the orthogonal axes of a reaction coordinate diagram, of the reaction coordinates at the two transition states. The orientation in the case of the monoaryldiazomethanes is more diagonal while the orientation in the case of the diaryldiazomethanes is more parallel to the O-H bond cleavage axis.