Silyl radical combination

Me3Si)3Si radicals, respectively [14,16]. While the fate of the reaction between two Et3Si radicals is still not known, the termination products of other silyl radicals have been determined. Pentamethyldisilyl radicals, produced by the reaction of Me3SiSi(H)Me2 with photogenerated t-BuO radicals at room temperature, behave similarly to the Me3Si radical [17]. That is, products due to the combination (Reaction 4.4) and disproportionation (Reaction 4.5) of these radicals were detected in a ratio of > 2.1. 

The kinetics data on the reactions of silyl radicals with carbon-centred radicals are also available. The rate constant for the cross-combination of CHs with MesSi was measured to be 6.6 x 10 M s in the gas phase [19]. Studies on the steady-state and the pulse radiolysis of EtsSiH in methanol showed that the cross-combination of Et3Si with CH30 andHOCH2 occurs with rate constants of 1.1 x 10 and 0.7 x 10 M s , respectively [20]. 

A different approach must be used for the photochemical hydrophosphination of electron-poor olefins, and this involves a PET reaction. Silyl phosphites (e.g., 30) were used as electron donors, whereas conjugated ketones have the double role of electron acceptors and absorbing species. Thus, the irradiation of a mixture containing 2-cydohexenone and 30 generated an ion pair. The phosphoniumyl radical cations decomposed to give trimethylsilyl cations (which in turn were trapped by the enone radical anion) and phosphonyl radicals. A radical-radical combination afforded the 4-phosphonylated ketones in yields ranging from 78% to 92% (Scheme 3.20) [49]. This reaction was exploited for the preparation of substituted phosphonates, which serve as key intermediates in the synthesis of a class of biologically active compounds. 

Divalent state stabilization energies are not easy to come by, as they require knowledge of both the first and second BDE, and the reactive intermediates MR2 are not trivially characterized. Quantum mechanical studies are certainly ahead of experiment in this area, and we can combine the results of two separate studies, one by Coolidge and Borden (109) and the other by Luke et al. (88), to assemble a small list of DSSEs for monosubstituted carbenes and silylenes. Specifically, Coolidge and Borden determined the effects of substituents, X, on the stability of methyl and silyl radicals through determination of the heat of reaction... 

Modem acyloin condensations are usually executed in the presence of Me3SiCl, and a bis(silyloxy)alkene is obtained as the immediate product. The bis(silyl-oxy)alkene may then be hydrolyzed to the acyloin upon workup. The yield of the acyloin condensation is greatly improved under these conditions, especially for intramolecular cyclizations. The MesSiCl may improve the yield by reacting with the ketyl to give a neutral radical, which can undergo radical-radical combination more easily with another ketyl radical due to a lack of electrostatic repulsion. 

Since it was only a guess that the rate of addition of silyl radicals to ethylene was rapid, we decided to measure this rate for the first time, by a combination of fiash photolysis and time-resolved ESR spectroscopy... 

Crich and co-worker report an attractive solution to this problem by a combination of efficient iodine abstraction by tributyltin or tris(trimethylsilyl)silyl radicals from an aryl iodide to form an aryl radical and successive intramolecular homolytic attack at sulfur by the aryl radicals to generate acyl radicals (Scheme 8) [60]. 

Photolysis of 1 in the presence of hexa-2,4-diyne gave two compounds in high yield, of which one was easily identified as the propynylsilirene 13 (Scheme 4). Elucidation of the constitution of the 2 1 cycloadduct of the disilene 3 and hexa-2,4-diyne was more difficult finally use of a combination of NMR methods and X-ray crystallography demonstrated the formation of the bicyclic product 12. The mechanism of formation of 12 presumably begins with the addition of two molecules of disilene 3 to the C=C triple bond, followed by a thermally allowed sigmatropic 1,5-hydrogen shift. Silyl radicals could be formed by homolysis of the thus-formed cyclobutane ring and then two consecutive cyclization steps would furnish the isolated bicyclic product 12 [8]. 

In cationic polymerization, the use of decatungstate (Wio032" ) anion in combination with silanes has been fonnd to allow the generation of silyl radicals by irradiation by green flnorescence bulbs or simply by exposing to... 

While the electrografting mechanism is the same for carbon and metals, a different one has been proposed for semiconductors such as hydrogenated Si. In this case, once the aryl radical is formed close to the surface, it reacts with the substrate by homolytic cleavage of Si-H bond to give a stable aromatic compound (aryl-H) and leave a silyl radical (Si ) on the surface. This silyl radical then combines with another aryl radical in the vicinity of the electrode to form the expected Si-C chemical bond. 

It has been reported that (TMS)3SiCl can be used for the protection of primary and secondary alcohols [55]. Tris(trimethylsilyl)silyl ethers are stable to the usual conditions employed in organic synthesis for the deprotection of other silyl groups and can be deprotected using photolysis at 254 nm, in yields ranging from 62 to 95%. Combining this fact with the hydrosilylation of ketones and aldehydes, a radical pathway can be drawn, which is formally equivalent to the ionic reduction of carbonyl moieties to the corresponding alcohols. 

Interestingly, 1,2-adducts 33, 34 and 35 have also been obtained in a 69% yield and a ratio of 1 10 9, when PhSi(SiMe3)3 is photolysed in the presence of Ceo, which clearly indicates the intermediacy of silyl-adduct radical 31 followed by an intramolecular addition to the phenyl ring (32) prior to combining with MesSi radicals to provide these unusual adducts (Scheme 8.7) [30,31]. 

Silyl and polysilyl radicals also combine with nitrogen, arsenic, and other main group 5 elements, as with sulfur and selenium. 

This vanadium method enables the cross-coupling only in combinations of silyl enol ethers having a large difference in reactivity toward radicals and in their reducing ability. To accomplish the crosscoupling reaction of two carbonyl compounds, we tried the reaction of silyl enol ethers and a-stannyl esters based on the following consideration. a-Stannyl esters (keto form) are known to be in equilibrium with the enol form such as stannyl enol ethers, but the equilibrium is mostly shifted toward the keto form. When a mixture of an a-stannyl ester such as 45 and a silyl enol ether is oxidized, it is very likely that the stannyl enol ether will be oxidized preferentially to the silyl enol ether. The cation radical of 45 apparently cleaves immediately giving an a-keto radical, which reacts with the silyl enol ether selectively because of the low concentration of the stannyl enol... 

Hydrosilylation can also be initiated by a free-radical mechanism (227—229). A photochemical route uses photosensitizers such as peresters to generate radicals in the system. Unfortunately, the reaction is quite sluggish. In several apphcations, radiation is used in combination with platinum and an inhibitor to cure via hydro silylation (230—232). The inhibitor is either destroyed or deactivated by uv radiation. 

Scheme 11.77 shows that the two subunits 372 and 373 were linked by a silyl ketal tether to give 374. An anomeric radical was then produced by treatment with tributyltin hydride. This radical reacted with the enol ether acceptor to give the cyclic derivative 375 as the major product in a yield of 43% together with two of the three possible isomers in yields of 6 and 13%. The combined yield shows that more than 56% of the radical attack occurred from the a face of the gluco residue. However, the intermediate radical, located at C4, is mainly trapped by the a face of the furanose moiety. Although this approach is attractive, further elaboration to the... 

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