Pressure dependence of the thermolysis of tetramethyl-1,2-dioxetane: the volume of activation and its mechanistic implications

Activation and reaction volumes in solution

“…Repetition of the measurement and other mechanistic studies are in order. The dioxetane decomposition of tetramethyldioxetane was studied by Kelm,82 with measurements based on the chemiluminescence of that reaction. It proved difficult to extract V* from the data, and the result of about + 10 cm3/mol could not be interpreted with certainty in terms of the hotly debated question concerning the stepwise or concerted nature of the reaction (the authors favored the concerted mechanism); thus, this case illustrates the experience so often gained with other techniques that no approach is fully reliable if analogs with known mechanism are unavailable.…”

“…The Kyoto group has also reported a comparison of the pressure effects on the thermal and photodissociations of azobisisobutyronitrile, and found only a slight difference in pressure effect. 374 Other work reported in this area includes work by Neuman,375 who has compared the behavior of diradicals generated thermally and photolytically, a study by Kelm,82 who made use of the chemiluminescence generated in the decomposition of oxetanes to follow that reaction, one by Hamann who found that pressure promotes the photodimerization of methyl 3-methoxy-2naphthoate,376 and two studies carried out in Stony Brook to compare cycloadditions carried out photochemically with those done thermally. The well-known pressure-bestowed advantage of [4n + 2] cycloadditions becomes that of the 4n analogs In the photoreactions.…”