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

1997; Royal Society of Chemistry; Issue: 5 Linguagem: Inglês

10.1039/a606819g

2050-8239

Christopher F. Rodriquez, Ian H. Williams,

Chemical Reactions and Isotopes

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 CHCH2) have been determined by means of ab initio MO calculations using the HF/6-31 and MP2/6-311 Cα–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 Cα and possess an almost completely formed Cα–O bond in the α-lactone ring. The transition structures are also essentially planar about Cα, but show only partial ring-closure. The maximum degree of charge separation occurs in the transition structure, which has considerable positive charge about Cα 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 SN2 transition state for reaction of α-chloroacetate but a more SN1-like transition state for α-chloropropionate, while the α-14C effects are typical of SN2 processes. The calculated secondary β-D3 kinetic isotope effect for α-chloropropionate is inverse.