Molecular hydrogen elimination from 2,5-dihydrofuran, 2,3-dihydrofuran, and 2-methyl-2,5-dihydrofuran: Quantum chemical and kinetics calculations

Density Function Theory calculations were carried out to evaluate the potential energy surfaces of H₂ elimination from 2,5-dihydrofuran, 2,3-dihydrofuran, and 2-methyl-2,5-dihydrofuran. The structure and energetics of the reactants, products, and transition states on the surfaces for all the three systems were determined and unimolecular rate constants for the H₂ elimination were calculated from transition state theory. The potential energy surfaces in 2,5-dihydrofuran and 2-methyl-2,5-dihydrofuran have each closed shell, 6-center transition state. The transition state for H₂ elimination from 2,3-dihydrofuran proceeds via two different pathways, both have barriers that are considerably higher than the barriers for H₂ elimination from 2,5-dihydrofuran and 2-methyl-2,5-dihydrofuran. One pathway proceeds via a 4-center transition state and one through a carbene intermediate. In all the cases the transition state leads to the formation of a H₂-furan complex toward a complete separation to H₂ and furan. The calculated rate constants for 2,5-dihydrofuran and 2-methyl-2,5-dihydrofuran are in excellent agreement with the experimental observations. Owing to the very high barriers, the process in 2,3-dihydrofuran cannot compete with other reaction channels, such as isomerizations, and cannot be observed experimentally.