Silyl radicals generation

Silyl radicals generated from both phenyl and n-hexyl substituted poly-(hydrosilane)s add readily to a variety of compounds containing C = C and 0 = C moieties to give the corresponding radical adducts for which EPR spectra have been recorded. ... 

When a thin film prepared from poly[(tetraethyldisilanylene)bis(2,5-thienylene)] was irradiated in air with a 6-W low pressure mercury lamp bearing a Vycor filter, the absorption maximum near 340 nm disappeared within 40 min. Poly[(tetraethyldisilanylene)(2,5-thienylene)] also exhibited a rapid UV change when its thin film was irradiated. IR spectra of the resulting films reveal strong absorption bands due to Si-O-H and Si-O-Si bonds at 3300 and 1100 cm [. The formation of the Si-O-H and Si-O-Si bonds can be best explained by the reaction of the silyl radicals generated by homolytic scission of the silicon-silicon bonds in the polymer backbone with oxygen in air. The other polymers are also photoactive,... 

Silyl radicals have also been observed during /-irradiation of solid polysilanes. Tagawa and coworkers examined the EPR spectrum observed upon irradiation of solid poly-(dimethylsilane) and concluded that the spectrum corresponded to silyl radicals generated by homolysis of the silicon skeleton in the polysilane (equation 5)18. Indeed, the EPR spectrum of the poly(dimethylsilane) radical (13), with hyperfine splitting constants H) and a(y-1 H) of 0.813 and 0.046 mT respectively, corresponded remarkably well to that published for the dimethyl(trimethylsilyl)silyl radical [a( -1H) = 0.821 mT a(/-1H) = 0.047 mT]1. Radical (13) appears to be very stable in solid poly(dimethyl-silane), since the EPR signal was strong and clearly observable at room temperature. 

Similar to pyridinium salts, iodonium salts can be reduced by silyl radicals generated by photolysis of polysilanes. However, due to the tail absorption bands of iodonium salts at about 300 nm, polysilanes with relatively longer... 

Hyperfine Coupling Constants (gauss) of Some silyl Radicals Generated by Photolysis3... 

Encounters between silyl radicals in solution or in the gas phase usually result in recombination and disproportionation (45, 46). Disproportionation results in the production of silanes and highly reactive silenes. The disproportionation reaction is thermodynamically favorable because of the formation of a silicon-carbon double bond, which, although subsequently chemically reactive, is worth —39 kcal/mol (44). For pentamethyldisilanyl radicals, disproportionation is kinetically competitive with radical dimerization (46). In an earlier study, Boudjouk and co-workers (47) demonstrated conclusively by isotopic substitution and trapping that the silyl radicals generated by photolysis undergo disproportionation, as well as, presumably, dimerization (Scheme I). In deuterated methanol, the silanes produced were predominantly undeuterated, whereas methoxymethyldiphenylsilane was extensively deuterated in the a position. The results of these experiments strongly implicated the substituted silene produced by disproportionation. 

When the silicon atom in the acyisilane was chiral, the chiral silyl radical generated had considerable optical stability compared to a carbon radical, since the chlorosilane isolated showed retention of configuration at silicon with an enantiomeric excess of about 80%" (equation 63). 

Mizuno and his co-workers reported the photosilylation of electron-deficient alkenes by use of hexamethyldisilane and unsymmetrical disilanes. The photoreaction of l,l-dicyano-2-phenylethene and disilanes in acetonitrile in the presence of phenanthrene affords sily-lated dicyanoethanes in good yields. A key intermediate in this process is the silyl radical, generated by solvent (nucleophile)-assisted cleavage of the radical cation of disilanes. Silyl radical attack at the radical... 

The mode of cyclization of a-silyl radicals generated from homoallylic (bromomethyl)dimethylsilyl ethers is highly dependent upon the substitution pattern on the distal alkenic carbon. Thus while those with unsubstituted distal alkene carbons provide seven-membered siloxanes (J-endo) (eq 1) those having substituted carbons undergo a 6-exo-mode of radical cyclization in a highly regio- and stereoselective manner (eq 8). These six-membered siloxanes provide, upon Tamao oxidation, branched chain 1,4-diols (eq 8). ... 

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 reactions are radical chain processes (Scheme 3) and, therefore, the initial silyl radicals are generated by some initiation. The most popular thermal initiator is azobisisobutyronitrile (AIBN), with a half-life of 1 h at 81 °C. Other azocompounds are used from time to time depending on the reaction conditions. EtsB in the presence of very small amounts of oxygen is an excellent initiator for lower temperature reactions (down to —78°C). The procedures and examples for reductive removal of functional groups by (TMSlsSiH are numerous and have recently been summarized in the book Organosilanes in Radical Chemistry. ... 

The effects of silyl groups on the chemical behavior of the anion radicals generated by cathodic reduction is also noteworthy. It is well known that silyl groups stabilize a negative charge at the a position. Therefore, it seems to be reasonable to consider that the anion radicals of re-systems are stabilized by a-silyl substitution. The interaction of the half-filled re orbital of the anion radical with the empty low-lying orbital of the silicon (such as dx-pK interaction) results in partial electron donation from the re-system to the silicon atom which eventually stabilizes the anion radical. 

It should be recognized that the stability of cation radicals generated by anodic oxidation is also affected by jS-silyl substitution. Stabilization of car-bocations by a silyl group situated at the -position is well known as the / effect . The interaction of the C Si a orbital with the empty p orbital of the carbon stabilizes the carbocation. Therefore, we can expect similar effects of silicon for cation radical species. The interaction of the filled C-Si a orbital with the half-filled orbital of the carbon may stabilize the cation radical. 

Regarding this proposal, it should be noted that while 1,1-eliminations on Si-Si-C units to generate silylenes are well known thermal processes (54) the photochemical variant seems not to have been described. The rearrangement of silylsilylenes (4) to disilenes is known to be rapid (55), and silyl radical addition at the least hindered site would produce the observed persistent radical. Preliminary evidence for the operation of 1,1-photoelimination processes in the polysilane high polymers has been obtained, in that the exhaustive irradiation at 248 nm of poly(cyclohexylmethylsilane) (PCHMS) produces —10-15% volatile products which contain trialkylsilyl terminal groups. For example, the following products were produced and identified by GC— MS (R=cyclohexyl,R = methyl) H(RR Si)2H (49%), H(RR Si)3H (19%), R2R SiH (2%), R 2RSiRR SiH (5%) and R2R SiRR SiH (7%). 

The reaction of thermally and photochemically generated tert-butoxyl radicals with trisubstituted silanes [Eqs. (6) and (7)] has been used extensively for the generation of silyl radicals in ESR studies, in time-resolved optical techniques, and in organic synthesis. Absolute rate constants for reaction (7) were measured directly by LFP techniques,56,62,63 whereas the gas phase kinetic values for reactions of Me3SiH were obtained by competition with decomposition of the tert-butoxyl radical.64,65... 

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