Silyl radical double bond

The silyl radicals formed in the initial scission appear to undergo further reactions, which may be complex. A possible secondary reaction is hydrogen transfer from an alpha carbon atom to give Si-H and a silicon carbon double bond (21)... 

The addition of silyl radicals to double bonds in benzene or substituted benzenes (Reaction 5.2) is the key step in the mechanism of homolytic aromatic substitution with silanes [8,9]. The intermediate cyclohexadienyl radical 2 has been detected by both EPR and optical techniques [21,22]. Similar cyclohex-adienyl-type intermediates have also been detected with heteroaromatics like furan and thiophene [23]. 

The addition of silyl radicals to various 2,6-disubstituted quinones takes place at two different sites, i.e., at the less hindered C=0 and at the C=C double bond, the former being ca 4 times slower (Scheme 5.5) [48]. However, kinetic studies showed that radical adducts 27 are prone to rearrange to the thermodynamically more stable isomers 28 via a four-membered transition... 

Scheme 5.10 Reaction paths for the addition of silyl radicals to C=N double bonds...

The addition of silyl radicals to nitrogen-nitrogen and nitrogen-heteroatom double bonds has mainly been investigated by EPR spectroscopy. 

A convenient route to triquinanes is based on a strategy of silyl radical addition to conjugated dienes to form allylic type radicals and their subsequent intramolecular addition to C=C double bonds. By exposure of 10 to (TMS)3SiH and AIBN at 80 °C (Reaction 7.16) the triquinane 11 is obtained with an unoptimized 51 %> yield [26]. 

The chemoselective addition of silyl radical to the double bond of the (3-alkenyloxyenone derivative 24 was instead planned in Reaction (7.29) and accompanied by a 5-exo-trig radical cyclization leading to the diastereomeric cyclic ether products [40]. 

Photoinitiation. Since photolysis of polysilanes generates silyl radicals, which can add to carbon—carbon double bonds, these polymers have been used for the free-radical polymerization of unsaturated oiganic monomers (135,136). Though about one-tenth as efficient as other oiganic photoinitiators, polysilanes are nevertheless quite insensitive to oxygen effects, which somewhat compensates for their lower efficiency. 

Recently, several studies have been made of the photolysis of disilanes or polysilanes in the presence of an electron-deficient alkene using a photosensitizer (such as phenanthrene) and acetonitrile as solvent. These conditions result in the addition of silyl groups to one end of the alkene double bond and hydrogen to the other end (equation 18) and evidently involve the reaction of the radical anions of the electron-deficient silene with silyl radicals67 (see also Section VIII.A). 

Lycoramine. Lycoramine (300)[79] synthesised by Parker et al [80] utilised an intramolecular 5-exo radical addition to an appropriately located double bond to generate the quaternary carbon centre (Scheme 46). The starting material, the silyloxybromo ketone 301, was secured initially as a mixture of isomers from 4-methoxy-3-cyclohexen-l-ol (302) by silylation to 303 followed by bromination. On standing or in contact with silica, the mixture was converted essentially into 304. The overwhelming preference for the substance 301 to exist... 

Photolysis of polysilanes produces silyl radicals, which can be used to induce free-radical reactions. Because the silyl radicals can add to carbon-carbon double bonds and begin the formation of polymer chains, polysilanes can be used as radical photoinitiators. In early studies, (PhSiMe) and its copolymers (PhQIUSiMe), and (cyclo-HexSiMe) were shown to photoinitiate the polymerization of styrene and several acrylate... 

A simple example involves the reaction of the silyl ether 213, made from the corresponding 4-hydroxy-alkene by treatment with (bromomethyl)chloro-dimethylsilane, with tributyltin hydride and a radical initiator. Bromine abstraction and intramolecular cyclization with the double bond leads to the bicyclic 214, which upon oxidation with hydrogen peroxide gives the branched-chain 215 in an overall yield of 73% from the alcohol precursor of 213 (Scheme 21). When the sequence is conducted with the C-4 epimeric starting alcohol, the final product again has the hydroxymethyl group cis-related to the hydroxy group.217... 

Similarly, for the purpose of studying silyl radicals in the gas phase in the presence of a spin trap, it has been established that the trimethylsilyl radical added to the double bond of a nitrone to yield the a-trimethylsilylated hydroxylamine.204 Other studies have been... 

Substrates containing an electron-rich double bond, such as enol ethers and enol acetates, are easily oxidized by means of PET to electron-deficient aromatic compounds, such as dicyanoanthracene (DCA) or dicyanonaphthalene (DCN), which act as photosensitizers. Cyclization reactions of the initially formed silyloxy radical cation in cyclic silyl enol ethers tethered to an olefinic or an electron-rich aromatic ring, can produce bicyclic and tricyclic ketones with definite stereochemistry (Scheme 9.14) [20, 21]. 

Addition of the silyl radical to carbon-carbon double bonds is an elementary reaction of radical hydrosilation (Scheme 1). Homolytic aromatic silationalso occurs involving silyl radicals. Silyl radicals are nucleophilic owing to the high SOMO energy, as evidenced by the directive effects in the hemolytic aromatic substitution. The intermediate cyclohexadienyl radicals have been observed by ESR. 

Since Scheme 4 implies formation of a-carbonyl radicals after deprotonation of enol radical cations, the same oxidation chemistry should potentially be accessible from various enol derivatives as enolates, silyl enol ethers and enol esters (Scheme 5). On the other hand, enol ether radical cations do not fit in this systematization since they are attacked by nucleophiles at the double bond faster than providing a-carbonyl radical intermediates through O-C bond cleavage (Sect. 4.3). 

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