Competition between Acid Catalyzed Interconversion and Solvolysis of Adducts of Dimethyl Azodicarboxylate and 2,5-Dimethyl-3,4-diphenylcyclopentadienone

Abstract

Trifluoroacetic acid catalyzes the conversion of each of the isomers, the Diels–Alder adduct 1a, the 1,3,4-oxadiazine 2a, and the 1,2-diazetidine 3a into the others, with the exception of 3a → 1a. The isomerizations compete with solvolysis to the common end products, the 1,3,4-oxidiazinone 4a and methyl trifluoroacetate, directly in the case of 1a and 2a, but perhaps only by way of 2a in the case of 3a. The isomerization and solvolysis rates increase with acid concentration in dilute acid but in concentrated acid there is evidence that all the isomers are converted into a (probably) multi-protonated form of 2a, which is stable to isomerization and whose solvolysis rate decreases with increasing acid concentration.