Matrix Isolation Study of the Ozonolysis of 1,3- and 1,4-Cyclohexadiene: Identification of Novel Reaction Pathways

The ozonolysis reactions of 1,3- and 1,4-cyclohexadiene have been studied using a combination of matrix isolation, infrared spectroscopy, and theoretical calculations. Experimental and theoretical results demonstrate that these reactions predominantly do <i>not</i> follow the long-accepted Criegee mechanism. Rather, the reaction of O₃ with 1,4-cyclohexadiene leads to the essentially barrierless formation of benzene, C₆H₆, and H₂O₃. These two species are then trapped in the same argon matrix cage and weakly interact to form a molecular complex. There is also evidence for the formation of a small amount of the primary ozonide as a minor product, formed through a transition state that is slightly higher in energy. The reaction of O₃ with 1,3-cyclohexadiene follows two pathways, one of which is the Criegee mechanism through a low energy transition state leading to formation of the primary ozonide. In addition, with a similar barrier, ozone abstracts a single hydrogen from C5 while adding to C1, forming a hydroperoxy intermediate. <i>This study presents two of the rare cases in which the Criegee mechanism is not the dominant pathway for the ozonolysis of an alkene as well as the first evidence for dehydrogenation of an alkene by ozone.