Intermolecular Vicinal Diaminative Assembly of Tetrahydroquinoxalines via Metal-free Oxidative [4 + 2] Cycloaddition Strategy.

Reported herein is the first metal-free oxidative [4 + 2] coupling of o-phenylenediamines with various alkenes. Differing from the known strategy that hinged on reactive π-allyl Pd intermediates from restrained allylic alcohol/acetate and diene substrates, this metal-free method features easy accessibility of starting materials, step economy, benign reaction conditions, and more importantly broad C-C double bonds (styrenes, vinyl (thio)ethers, benzofurans, indoles) with diastereospecificities. Mechanistic studies suggest the intermediacy of the benzoquinone diimides, a class of useful but yet underexploited synthons. Of note, they efficiently furnished functionalized tetrahydroquinoxalines and complement the well-studied alkene vicinal diamination typically toward acyclic diamine derivatives.