Nanostructured Silicas, a Platform for the Observation of Transient Radicals: Application to Sulfinyl Radicals

Diazenes, precursors of sulfinyl radicals, were used to functionalize nanostructured SBA-15 silicas either in the framework or on the pore by formation of covalent links, or by simple adsorption on the surface. Depending on their design and on the experimental conditions, these materials proved to be effective tools to study the behavior and the reactivity of arylsulfinyl radicals made persistent because of confinement effects. When the covalently linked precursors were irradiated at room temperature, a spectacular increase of the lifetime of the expected sulfinyl radicals was registered (up to 17 h). At higher temperature, upon thermal initiation at 473 K, the decomposition of the adsorbed precursor enabled visualization of the rearrangement of the corresponding arylsulfinyl radicals into sulfonyl radicals via O–S coupling

EPR Investigation of Zinc/Iodine Exchange between Propargyl Iodides and Diethylzinc: Detection of Propargyl Radical by Spin Trapping

The production of propargyl radicals in the reaction of dialkylzincs with propargyl iodides in nondegassed medium was investigated by EPR using tri-<i>tert</i>-butylnitrosobenzene (TTBNB) as a spin trap. The radical mechanism and the nature of the observed species were confirmed by the trapping of propargyl radicals generated by an alternative pathway: i.e., upon irradiation of propargyl iodides in the presence of hexa-<i>n-</i>butyldistannane. In dialkylzinc-mediated experiments a high concentration of adduct was instantaneously observed, whereas no spontaneous production of spin adduct was detected in a blank experiment performed with the propargylic iodide and TTBNB in the absence of diethylzinc. Under irradiation in the presence of distannane, two different species were observed at the very beginning of the irradiation; the nitroxide resulting from the trapping of propargyl radical at the propargyl carbon remained the only species detected after irradiating for several minutes. The absence of adducts resulting from the trapping of allenyl canonical forms was supported by DFT calculations and by the preparation of an authentic sample

Design of Wall-Functionalized Hybrid Silicas Containing Diazene Radical Precursors. EPR Investigation of Their Photolysis and Thermolysis

Bis-silylated diazenes were designed to prepare wall-functionalized hybrid silicas via the sol–gel process. These precursors enabled the straightforward generation of arylsulfanyl radicals by either photolysis or thermolysis. Their location in the framework of the mesoporous materials was evidenced by spin trapping experiments. The incidence of confinement on the radical lifetime was investigated by electron paramagnetic resonance (EPR). An amazing persistence (<i>t</i>_1/2 = 27 min) was recorded at 473 K