Diazene-Catalyzed Oxidative Alkyl Halide-Olefin Metathesis

The first platform for oxidative alkyl halide-olefin metathesis is described. The procedure employs diazenes as catalysts, which effect cyclization of alkenyl alkyl halides to generate cyclic olefins. Mechanistically, the diazene catalyst is converted to a hydrazonium intermediate by alkylation with the alkyl halide component followed by tautomerization. The resulting hydrazonium ion reacts with the olefin component through [3+2] cycloaddition and cycloreversion reactions, leading to new olefin and hydrazonium moieties. Hydrolysis of the newly formed hydrazonium was found to outcompete retro-tautomerization / dealkylation, thus furnishing a carbonyl product instead of an alkyl halide. For this reason, the process is termed oxidative alkyl-halide olefin metathesis. Adventitious O2 and H2O were sufficient for catalyst turnover. The synthesis of phenanthrene, coumarin, and quinolone derivatives is demonstrated, as well as the potential to apply this strategy to other electrophiles.