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.