Hypervalent iodine chemistry: New oxidation reactions using the iodosylbenzene-trimethylsilyl azide reagent combination. Direct alpha- and beta-azido functionalization of triisopropylsilyl enol ethers

Treatment of triisopropylsilyl (TIPS) enol ethers with PhIO/TMSN(3)/at -18 to -15 degrees C rapidly (5 min) gave beta-azido TIPS enol ethers in high yields, with only traces of the alpha-azido adduct. The reaction is very sensitive to temperature changes, with the alpha-pathway being favored at -78 degrees C and the beta-pathway at -15 to -20 degrees C. Addition of catalytic amounts of the stable radical TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) significantly reduced the beta-azidonation and increased the alpha-azidonation reaction. A mechanistic hypothesis is presented that offers a clear distinction between the alpha- and beta-functionalization pathways. The key difference between the two reactions is that the alpha-pathway is an azide radical addition process and the beta-pathway involves ionic dehydrogenation. Efforts to extend the unusual beta-functionalization reaction to other trimethylsilyl derivatives (TMSX) were unsuccessful. The reagent combination PhIO/TMSN(3) is the only system we have found that results in clean, high-yield beta-functionalization. Attempts to substitute iodine with S, Se, P, As, or Te did not result in an active oxidant, although in the case of diphenyltelluroxide we were able to isolate the stable crystalline adduct bis[azidodiphenyltellurium(IV)] oxide.