Theoretical study on the aminolysis of p-substituted phenyl acetates with dimeric ammonia in vacuo and acetonitrile

The mechanism and substituent effects on the aminolysis reaction of p-substituted phenyl acetates (CH3C(O)OC6H4X, X = Cl, H, and NH2) with dimeric ammonia molecules are studied using the B3LYP/6-31+G(d,p) method in the gas phase. Single-point computations at the MP2/6-311++G(d,p) level are performed for more precise energy predictions. Two pathways are considered: the concerted and the neutral stepwise mechanisms. The results show that the stepwise mechanism is slightly energetically preferred to the concerted one for the three reaction systems in the gas phase. The solvent effect of acetonitrile is also assessed through single-point calculations by use of the conductor-like polarizable continuum model (CPCM) at the MP2/6-311++G(d,p) level on the gas-phase optimized geometries. Our computational results indicate that the reaction of CH3C(O)OC6H4X with ammonia dimer is more favorable for X = Cl than for X = H and NH2 in the gas phase and in acetonitrile. The solvation effect decreases the barrier height. When X = NH2, the stepwise process is also favored in acetonitrile, while the concerted mechanism is favorable in solution when X = H and Cl.