Thermal stability improvement of exo -tetrahydrodicyclopentadiene by 1,2,3,4-tetrahydroquinoxaline: Mechanism and kinetics

We investigated the thermal stability of exo-tetrahydrodicyclopentadiene (exo-THDCP, C₁₀H₁₆) in the absence and presence of three additives, 3,4-dihydro-2H-1,4-benzoxazine (Benzox), 1,2,3,4-tetrahydroquinoline (THQ), and 1,2,3,4-tetrahydroquinoxaline (THQox), which act as hydrogen donors (H donors). Conversion of exo-THDCP was slowed in the presence of the H donor. The order of the H-donor effects on the decrease in the conversion of exo-THDCP was Benzox ≪ THQ < THQox. The H-donor-induced decrease in the conversion of exo-THDCP was smaller at higher temperature. In addition, the H-donor-induced decrease in the rate of <C₁₀ product formation was smaller than that of ≥C₁₀. We proposed the mechanism for the thermal decomposition of exo-THDCP in the presence of the H donor. The proposed mechanism explains the unusual thermal decomposition kinetics of exo-THDCP and H donors: (i) exo-THDCP does not follow first-order kinetics and (ii) THQ and THQox show the S-shaped concentration-time curves. We also proposed the mechanism for H donations by Benzox, THQ, and THQox. The proposed mechanism elucidates that THQox performs faster H donation than THQ and has higher thermal stability than Benzox, which accounts for the more effective thermal stability improvement of exo-THDCP by THQox compared to THQ and Benzox.