Verdazyl radicals as substrates for organic synthesis: Unique access to tetrahydropyrazolotriazinones, pyrazolotriazinones and dihydrotetrazinylacrylonitriles

摘要

1,5-Dimethyl-3-phenyl-6-oxoverdazyl radical was recently shown to undergo a disproportionation reaction to form an azomethine imine, which reacted with a series of dipolarophiles, to form novel tetrahydropyrazolotetrazinone heterocyclic structures, demonstrating for the first time the use of verdazyl radicals as substrates for organic synthesis. Herein, we report on the chemistry of this verdazyl radical with captodative olefins and show that the reactions that occur, either an addition reaction or a hydrogen abstraction reaction followed by a cycloaddition reaction, are defined by the captodative olefin. Also highlighted is an intramolecular rearrangement of initially formed tetrahydropyrazolotetrazinone cycloadducts to either pyrazolotriazinone or tetrahydropyrazolotriazinone structures. While the rearrangement falls under the very general definition of the Dimroth rearrangement, the mechanism that is proposed is distinct and provides the opportunity to go from one particular structural motif to another under mild conditions, expanding the usefulness of this chemistry. The use of a verdazyl radical as a precursor for organic synthesis is demonstrated. The use of captodative olefins as the dipolarophiles leads to a variety of unexpected products, in a few of the cases as a result of a rearrangement reminiscent of the Dimroth rearrangement but actually proceeding by a unique mechanism. A proposed mechanism for the rearrangement is suggested.