Abstract
Norbornadiene, cyclohexene, 4-vinylcyclohexene, ¿-limonene, and 2,3-dihydropyran undergo laser-induced retro- Diels-Alder reactions at ambient temperature in the presence of SiF4 when irradiated at 1025 cm-1 by a pulsed megawatt CO2 laser. The reaction cell never gets warm. Norbornadiene affords cyclopentadiene and acetylene along with toluene and, in some experiments, benzene. Lower pressures of sensitizer (SiF4) afford better yields of diene and dienophile at the expense of other products. Cycloheptatriene is not observed under any conditions. Compared with the large number of compounds observed in its thermal reaction, the reaction with cyclohexene is very clean and affords only acetylene and methane along with the usual diene and dienophile. Acetylene and methane are shown to be secondary decomposition products of the diene and dienophile. Vinylcyclohexene, which is stable up to 650 °C, cleanly undergoes the laser-induced decomposition. The reaction with ¿-limo- nene is also clean and more efficient than that previously done under CW laser conditions. In the case of 2,3-dihydropyran, both direct irradiation at 1072 cm-1 and sensitized initiation at 1025 cm-1 with SiF4 afford acrolein and ethylene exclusively. These reactions are examples of the “cold pyrolytic” process which seems typical of laser-induced reactions.