Reaction mechanism and kinetics on the methanolysis–esterification–lactonization of α-phellandrene-derived cyclic acid anhydride

Alkyl bicyclo[2.2.2]octanes (ABCOs) have been designed as high-energy-density hydrocarbons for bio-aviation fuel. In the proposed ABCO synthesis route from α-phellandrene and maleic anhydride, ABCOs yield by hydrotreating depends on the precursor type, and bicyclic dimethyl ester 4 was the most promising candidate, showing the highest yield. However, there has been little discussion about the mechanism and kinetics of dimethyl ester 4 preparation reaction from acid anhydride 3 wherein lactone methyl ester 5 is also produced as a by-product. In this context, herein, we focused on the methanolysis–esterification–lactonization of acid anhydride 3 with methanol in the presence of sulfuric acid. Two different reaction mechanisms (competitive vs. consecutive) were discussed and only consecutive mechanism was favored on the basis of experimental results. In this mechanism, acid anhydride 3 is initially converted to dimethyl ester 4, followed by the formation of lactone methyl ester 5. The direct lactonization of dimethyl ester 4 can be rationalized by stereoelectronic considerations in the cleavage of cyclic hemiorthoester 10. Based on this reaction mechanism, a kinetic model comprising of two consecutive pseudo-first-order-kinetic reactions (methanolysis–esterification (k_1,obs) and lactonization (k_2,obs)) was proposed to elucidate this complex process. Through this kinetic model, the activation parameters were determined; the activation energy (Ea, 59.1 kJ·mol⁻¹), preexponential factor (A, 1.15 × 10⁷ min⁻¹), activation enthalphy (ΔH^‡, 56.5 kJ ·mol⁻¹), and activation entropy (ΔS^‡, −152.5 J·mol⁻¹·K⁻¹) for the methanolysis–esterification; Ea (100.2 kJ·mol⁻¹), A (1.80 × 10¹² min⁻¹), ΔH^‡ (97.6 kJ ·mol⁻¹), and ΔS^‡ (−53.1 J·mol⁻¹·K⁻¹) for the lactonization. Additionally, the reaction orders of apparent rate constants (k_1,obs and k_2,obs) depending on sulfuric acid concentration were determined as 0.54 and 0.78, respectively. Using these parameters, a product prediction model was developed based on reaction temperature and sulfuric acid concentration. The model suggested optimal conditions to achieve a 90 % yield of dimethyl ester 4: a sulfuric acid concentration of 0.3 mol/L at 40°C for 18.8 h. This prediction was experimentally validated.