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

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.