Fluorinated heterocyclic compounds. The first example of an irreversible ring-degenerate rearrangement on five-membered heterocycles by attack of an external bidentate nucleophile

The reactions of 5-perfluoroalkyl-1,2,4-oxadiazoles 3 with hydroxylamine in DMF give the regioisomeric 3-perfluoroalkyl-1,2,4-oxadiazoles 4 in excellent yields. This process is thefirst example of ring-degenerate rearrangement (RDR) occurring on five-membered heterocycles by attack of an external bidentate nucleophile, which replaces two heteroatoms of the ring. We suggest that an ANRORC-like mechanism occurs in which the addition of the nucleophilic nitrogen atom (NH2OH) on the C(5) atom of 3 is followed by ring opening and irreversible ring-degenerate closure by attack of the nucleophilic oxygen atom (=NOH) on the C(3) atom of the originalring, realizing an elegant and efficient synthesis of 4 by a C(3)−C(5) annular switch. Ab initio computations on the starting materials and final products, as well as on the proposedintermediates, support the mechanism and shed light on the features of the reaction.