Rate-Determining Steps in Michael-Type Additions and E1cb Reactions in Aqueous Solution

Article abstract of DOI:10.1021/jo00039a013

Rates of equilibration of a series of 10 substituted pyridines and five Michael acceptors (CH2=CHZ, Z = CHO, COCH3, SO2CH3, CN and CONH2) with the corresponding N(ZCH2CH2) pyridinium cations have been measured in aqueous solution at ionic strength 0.1 and 25 deg C.Analysis of the dependence of the pseudo-first-order rate constants for equilibration as a function of acceptor concentration and of pH allows the evaluation of the second-order rate constants (k_Nu) for the nucleophilic attack of each of these pyridines upon each of these acceptors and also the second-order rate constants (k_OH) for the hydroxide ion catalyzed E1cb elimination reaction which is the microscopic reverse of each of these Michael-type addition reactions.Broensted-type plots for each of these processes as a function of the basicity of the substituted pyridine are concave down for each of Z = CHO, COCH3, and CN and are consistent with a change from rate-determining nucleophilic attack for the more basic pyridines to rate-determining protonation of the carbanionic intermediate by a water molecule for less basic pyridines and the corresponding microscopic reverse processes in the elimination reactions.The "break" in these Broensted-type plots is shown to occur at a pyridine basicity that is a function of the Z-activating substituent.Broensted β_1g and β_nuc are evaluated for each rate-determining step (wherever accessible); these two parameters are shown to pass through minima as a function of reactivity. β_eq is shown to be a simple linear function of reactivity (as log k_Nu) for nucleophilic addition to the acceptor species, although K_eq is relatively insensitive to the nature of the Z-activating substituent.