CONCERTED AND NONCONCERTED PATHWAYS FOR THERMAL-CONVERSIONS OF DEUTERIUM-LABELED CYCLOHEXENES TO BUTADIENES AND ETHYLENES

The retro Diels-Alder reactions of cyclohexene-3,3,6,6-d4 and the 4,5-cis isomers of cyclohexene-1,2,3,4,5,6-d6 have been followed using single-pulse shock tube kinetic and tunable diode laser infrared spectroscopic techniques. At temperatures above 1000 K, both labeled cyclohexenes give some products indicative of nonconcerted reaction paths: ethylene-1,1-d2 from the first and trans-ethylene-1,2-d2 from the second. Kinetic modeling calculations indicate that the most important pathway for formation of these products is a sequence of steps: cyclohexene --> vinylcyclobutane --> ethylene plus butadiene. Some relatively minor direct conversion of cyclohexene to ethylene plus butadiene through a stereochemically nonconservative pathway involving a diradical intermediate may also occur.