Ab initio theoretical investigation of the mechanism for α-lactone formation from α-halocarboxylates: Leaving group, substituent, solvent and isotope effects

Structures and energetics of geometry optimized species occurring along the reaction pathway for halide elimination from α-halocarboxylates XCHRCO₂ ^- (R = H, X = F, Cl and Br; X = Cl, R = CH₃ and CH=CH₂) have been determined by means of ab initio MO calculations using the HF/6-31++G(d,p) and MP2/6-311++G(d,p) methods. Endothermic heterolysis of the C_α-X bond yields halide and α-lactone by means of a transition structure leading to an ion-molecule complex lying in a shallow well. The ion-molecule complexes are all essentially planar about C_α and possess an almost completely formed C_α-O bond in the α-lactone ring. The transition structures are also essentially planar about C_α, but show only partial ring-closure. The maximum degree of charge separation occurs in the transition structure, which has considerable positive charge about C_α sandwiched between the negatively charged leaving group and internal nucleophile. Aqueous solvation, as treated by the self-consistent reaction field IPCM method, accentuates the charge-separated character of the transition structure but raises the barrier to heterolysis since the localized charge of the reactant is preferentially stabilized; the calculated value of ΔH^‡ = 112 kJ mol^-1 for reaction of α-chloroacetate in water compares favourably with experimental values for hydrolyses of α-bromophenylacetic acids. Calculated secondary α-D kinetic isotope effects suggest an S_N2 transition state for reaction of α-chloroacetate but a more S_N1-like transition state for α-chloropropionate, while the α-¹⁴C effects are typical of S_N2 processes. The calculated secondary β-D₃ kinetic isotope effect for α-chloropropionate is inverse.