Quantum chemical calculations of the homogeneous, unimolecular, gas-phase elimination kinetics of primary alkyl acetates and (dimethylamino)alkyl acetates: Neighboring group participation in 4-(dimethylamino)-1-butyl acetate

The kinetics and mechanisms of the gas-phase elimination reaction of several alkyl and (dimethylamino)alkyl acetates were studied by means of electronic structure calculations using MP2/6-31G(d,p) and DFT B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), MPW1PW91/6-31G(d,p), MPW1PW91/6-31++G(d,p), PBEPBE/6-31G(d,p), PBEPBE/6-31++G(d,p) level of theory. Theoretical calculations demonstrated that ethyl acetate, 2-(dimethylamino)ethyl acetate, propyl acetate, 3-(dimethylamino)propyl acetate, and butyl acetate decompositions proceed through a concerted six-membered cyclic transition state to give acetic acid and the corresponding olefin. Conversely, an alternative path occurs for 4-(dimethylamino)butyl acetate, where a late transition state structure resembles the products N-methylpyrrolidine and methyl acetate. The observed products and the nature of the TS suggest that the nitrogen atom assists the elimination of the acetate. An intimate ion-pair intermediate has been considered, followed by decomposition to the final products methyl acetate and N-methyl pyrrolidine. The high reaction rate observed in 4-(dimethylamino)butyl acetate when compared to the parent compound is discussed. The nature of these reactions is examined in terms of geometrical parameters, electron distribution, and bond order analysis.