Excited State Selectivity in the Thermolysis of a 3,4-Diaryl-3,4-dimethyl-1,2-dioxetane

Article abstract of DOI:10.1021/jo00021a033

Total efficiencies for the production of triplet ketones from cis- and trans-3-(3,4-dimethylphenyl)-4-phenyl-1,2-dioxetane (11) are 14.1 +/- 0.2 percent and 21.3 +/- 0.5percent, respectively.The specific efficiency for the production of triplet acetophenone from trans-11 was determined to be 19 +/- 4 percent by trapping the triplet with 2-methyl-2-butene to give the oxetane.These results show that the production of triplets from the dioxetane is state selective, since the n,?* acetophenone triplet is higher in energy than the alternative ?,?* triplet of 3,4-dimethylacetophenone.This state-selective production of triplet ketone is most reasonably dictated by orbital symmetry control in the thermolysis of the dioxetane.With this assumption, implications on the mechanism of dioxetane decomposition in terms of biradical intermediates or a concerted biradicaloid process are considered.Activation parameters for thermolysis of cis- and trans-11 are consistent with a biradical or biradicaloid mechanism.From these activation parameters and molecular mechanics calculations, it was concluded that the transition state was not quite half-way between the dioxetane reactant and a biradical intermediate, if the reaction proceeded through a biradical intermediate.In conjunction with earlier reported triplet efficiencies of 3,4-diaryl-3,4-dimethyl-1,2-dioxetanes and the efficiencies of cis- and trans-11, the participation of a triplet exciplex was suggested.