Theoretical study of neighboring group participation of methyl omega-chloroesters elimination kinetics in the gas phase

摘要

The mechanisms of the homogeneous, unimolecular, gas-phase elimination kinetics of several methyl omega-chloroesters were examined by using the 'ab initio' and DFT level of theories. Theoretical calculations of dehydrochlorination of methyl 3-chloropropionate suggest a planar concerted, non-synchronous, four-membered cyclic transition state to give methyl acrylate. However, the parallel competitive gas-phase elimination of methyl 4-chlorobutyrate and methyl 5-chlrovalerate occurs through neighboring group participation to render methyl chloride and the corresponding lactone through a concerted, semi-polar five- and six-membered cyclic transition state type of mechanism. Calculated thermodynamic and kinetic parameters reasonably agree with the experimental values at DFT B3LYP/6-31G* theory level. Geometrical parameters, NBO charges and bond indexes showed strong polarization at C delta+center dot center dot center dot Cl delta- bond in the transition state suggesting the breaking of C-Cl bond as rate-determining factor for both dehydochlorination and lactone formation reactions. The synchronicity parameters suggest a concerted polar mechanism implying a TS which has ion-pair character for lactone product formation.