Silylene with free radicals

Developments in the synthesis and characterization of stable silylenes (RiSi ) open a new route for the generation of silyl radicals. For example, dialkylsilylene 2 is monomeric and stable at 0 °C, whereas N-heterocyclic silylene 3 is stable at room temperature under anaerobic conditions. The reactions of silylene 3 with a variety of free radicals have been studied by product characterization, EPR spectroscopy, and DFT calculations (Reaction 3). EPR studies have shown the formation of several radical adducts 4, which represent a new type of neutral silyl radicals stabilized by delocalization. The products obtained by addition of 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) to silylenes 2 and 3 has been studied in some detail. ... 

The reaction of silylene centers with dihydrogen has been studied in some detail346. It appears to proceed by a free-radical chain mechanism, initiated by radical sites on the silica surface which abstract hydrogen from H2. The H atoms react with the silylene center to give a free radical, and a reaction chain can then ensue (equations 103-105). The hydrogen addition to silylene centers is reversible the hydrogen is completely removed at temperatures near 1000 K. 

Silylene 1 is an unusually versatile catalyst for alkene and alkyne polymerization. The list of compounds polymerized by 1 includes ethene, propene, 1-hexene, styrene, dimethylbutadiene, vinylidene chloride, vinyl ethyl ether, methyl methacrylate, and phenylacetylene. The polymerization does not seem to take place by any of the usual mechanisms, anionic, cationic or free-radical. Instead it somewhat resembles coordination polymerization, as observed for Ziegler-Natta type catalysts. Silylene 2 also catalyzes the polymerization of 1-hexene, but the polymerization is 10 to 100 times slower than with 1. 

Because of their high photosensitivity, polysilanes are suitable sources of free radicals [274]. These silicon-based polymers decompose upon absorption of UV light with quantum yields ranging between 0.2 and 0.97 [251], producing silyl radicals and silylene biradicals ... 

---