Density functional calculations on alcoholysis and thiolysis of phosphate triesters: Stepwise or concerted?

We have performed density functional theory calculations on the alcoholysis and thiolysis of four triesters with aryl and alkyl leaving groups in the gas phase and in solution. It is found that the alcoholysis of diethyl p-chlorophenyl phosphate proceeds through a concerned mechanism, while an associative mechanism is observed for the thiolysis reaction. The reactivity of diethyl phenyl phosphate and diethyl p-methylphenyl phosphate are similar, they react through an associative mechanism with both −OCH3 and −SCH3, though the second step of alcoholysis reaction has a low energy barrier. For triethyl phosphate, the alcoholysis and thiolysis reactions proceed by an associative mechanism, where the second step is the rate-determining step. The results show that the thiolysis of the four triesters are less favorable than the corresponding alcoholysis both in the gas phase and in solution, the enthalpies of all stationary points and the activation enthalpies for all the processes in solution are higher than those in the gas phase. Our calculations provide a comprehensive data set and allow re-interpretation of previous experimental and theoretical studies, and new experiment is proposed to trace reactions both in the gas phase and in solution.