MECHANISM OF SEO2 PROMOTED OXIDATIVE REARRANGEMENT OF 2-SUBSTITUTED OXAZOLINES TO DIHYDROOXAZINONES - ISOTOPIC LABELING AND KINETIC-STUDIES

The mechanism of the recently reported(1) SeO2 promoted oxidative rear rangement of 2-alkyl oxazolines was investigated. C-13 labeling studie s with 1a-2-C-13 showed that rearrangement proceeds by fission and mig ration of the C=N bond, not the C-C bond as first proposed. Oxidation- rearrangement of 1 proceeds by rapid SeO2 promoted oxidation to 2-acyl oxazoline 3 followed by slower rearrangement of 3 to oxazinone 2. The second order rate constant for the oxidation step for 2,4-diphenyloxaz oline 1b in 1,4-dioxane at 40 degrees C was k(1) = 2.85 +/- 0.62 x 10( -2) M(-1). s(-1). The second and rate limiting rearrangement step (3b- ->2b) had a first order rate constant of k(2) = 1.15 +/- 0.09 x 10(-3) s(-1). while the relatively low apparent energy of activation E(A) of 4.2 +/- 1.6 kcal . mol(-1) in the presence of 2.0 equiv of SeO2 sugge sts heterogeneous catalysis. The reaction is catalyzed by Lewis acids, including SeO2 or small amounts of H2O. A plausible mechanism is prop osed involving Lewis add activation and nucleophilic catalysis with H2 O. Copyright (C) 1996 Elsevier Science Ltd