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 H-2 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 unimotecular 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 H-2 elimination from 2,3-dihydrofuran proceeds via two different pathways, both have barriers that are considerably higher than the barriers for H-2 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 H-2-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. (C) 2001 John Wiley & Sons, Inc.