Isomerisation Reactions of α-Methyl Allyl [Acetate,Trifluoroacetate]: Theoretical Study

Abstract

The [3,3]-sigmatropic rearrangement reactions of α-methyl allyl acetate (1), and α-methyl allyl trifluoroacetate (2) have been studied using ab initio molecular orbital (MO) methods at the HF, B3LYP, B3PW91, and BHandHLYP levels with a 6-31++G^** basis set. Among the methods used in this study, the values for activation parameters obtained with the BHandHLYP/6-31++G^** method are in good agreement with experimental values. The calculated data demonstrate that in the rearrangement reactions of the compounds studied, the polarisation of the O₃–C₄ bond is rate determining. Optimised transition states at the BHandHLYP/6-31++G^** level were used for calculating the nucleus-independent chemical shift (NICS) and also a natural bond orbital (NBO) analysis was carried out at the same level.

Based on the optimised ground state geometries using the BHandHLYP/6-31++G^** method, the NBO analysis of donor–acceptor (bonding–antibonding) interactions revealed that the stabilisation energies associated with electronic delocalisation from the σ_O(3)–C(4) bonding orbital to the π^* _C(5)=C(6) antibonding orbital, increase from compounds 1 to 2. The σ_O(3)–C(4) →π^* _C(5)=C(6) resonance energies for compounds 1 and 2 are 1.59 and 1.62 kcal mol⁻¹, respectively. Also, analysis of the donor–acceptor interactions σ_O(3)–C(4) →π^* _C(5)=C(6) suggests that in these rearrangements, the TS structure in compound 2 has more aromatic character than for compound 1. The NBO results revealed that in these rearrangements, activation energies are not controlled by the σ_O(3)–C(4) →π^* _C(5)=C(6) resonance energy. NICS results indicate that the aromaticities of the transition states are controlled by the out-of-plane component.

The predicted high-pressure-limit rate constants for the rearrangement reactions of compounds 1 and 2 are represented as 6.12 × 10¹² exp(–21604/T), and 1.04 × 10¹³ exp(–17846/T) s⁻¹, respectively. The fall-off pressures for the rearrangement reactions 1 and 2 are found to be 9.56 × 10⁻⁵ and 1.09 × 10⁻³ mmHg, respectively. The fall-off pressures of compounds 1 and 2 in both reactions are in the following order: P_1/2(2) > P_1/2(1) and the rates follow as: rate (2) > rate (1). Analysis of bond order, NBO charges, bond indices and synchronicity parameters suggest the Claisen rearrangement reactions of 1 and 2 occur through a concerted and slightly asynchronous six-membered cyclic transition state type of mechanism.

Keywords

rearrangement aromaticity NBO NICS fall-off pressure

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