Hypervalent Iodine Chemistry: New Oxidation Reactions Using the Iodosylbenzene−Trimethylsilyl Azide Reagent Combination. Direct α- and β-Azido Functionalization of Triisopropylsilyl Enol Ethers

Abstract

Treatment of triisopropylsilyl (TIPS) enol ethers with PhIO/TMSN3/at −18 to −15 °C rapidly (5 min) gave β-azido TIPS enol ethers in high yields, with only traces of the α-azido adduct. The reaction is very sensitive to temperature changes, with the α-pathway being favored at −78 °C and the β-pathway at −15 to −20 °C. Addition of catalytic amounts of the stable radical TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) significantly reduced the β-azidonation and increased the α-azidonation reaction. A mechanistic hypothesis is presented that offers a clear distinction between the α- and β-functionalization pathways. The key difference between the two reactions is that the α-pathway is an azide radical addition process and the β-pathway involves ionic dehydrogenation. Efforts to extend the unusual β-functionalization reaction to other trimethylsilyl derivatives (TMSX) were unsuccessful. The reagent combination PhIO/TMSN3 is the only system we have found that results in clean, high-yield β-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.