Theoretical study of neighboring group participation of methyl ω‐chloroesters elimination kinetics in the gas phase

Abstract

AbstractThe mechanisms of the homogeneous, unimolecular, gas‐phase elimination kinetics of several methyl ω‐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 Cδ+···Clδ− 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. Copyright © 2008 John Wiley & Sons, Ltd.