Benzyl silane radical

Mariano s group [61] have extensively investigated the synthetic potentials of the cleavage of the —C-Si— bond from allyl and benzyl silane radical cations produced by the photoreaction of electron-deficient iminium salts. Mechanistically, this result is interpreted by considering one electron oxidation... 

Mariano and coworkers have exploited this PET bond cleavage chemistry in intramolecular as well as intermolecular cyclization reactions [38]. The combined iminium-benzylsilane functionalities undergo intramolecular PET to provide an intramolecular amine radical/benzyl silane cation-radical pair by exciting either the iminium salt or the arene chromophore [38]. Cleavage of the benzyl C—Si bond presumably takes place with assistance of moderately weak n ldeophiles such as methanol. The diradical couples intramolecularly to provide an indolizidine derivative [38] in 90% yield. 

Allyl and benzyl silanes may be anodically oxidized in the presence of an alcohol, a carboxylic acid, or water to the corresponding ether, ester, or alcohol [208]. The initially formed radical cation is believed to lose the silicon moiety by attack by the nucleophile on silicon ... 

A second example is found in the photoaddition of allylic or benzylic silanes to iminium salts reported by Mariano and his co-woikers. - PET reactions of allylic or benzylic silanes with iminium salts generate two kinds of neutral radicals, allyl or benzyl radicals and a-amino radicals. The C-C bond-forming reaction between two different radicals occurs by cross coupling (Scheme 4). 

PhotochemicaUy induced electron-transfer reactions involving electron-donating organo-siUcon compounds such as oligosilanes, polysilanes, allylic and benzylic silanes have attracted much attention in recent years -. While these reactions are synthetically interesting, we have not dealt with them in this chapter, because most of the reactions can be understood as due to the behavior of the corresponding radical cations generated by the electron transfer in the environment. 

The role of the nucleophile was confirmed for a benzyldimethylsilane (141) bearing a nucleophile (OH, OMe) on an oligomethylene tether n = 3-5). The radical cations of these silanes have very short lifetimes even in nonpolar solvents, and the resulting benzyl radicals couple with the radical anion. ... 

Functionally substituted benzylic, allylic, and vinylic compounds containing alkoxides, esters, ethers, nitriles, or amides can be reacted with halosilanes under Barbier conditions using HMPT to yield C- and O-silylated products, 1,2- or 1,4-addition products, as well as reductive dimers. Radical and anionic intermediates are postulated, based on SET reactions from the metal, and multiple silated species can be obtained. The use of the TMSCl-Mg-HMPT system has been extensively investigated by Galas group [85] at the University of Bordeaux, and their work has greatly advanced the science of the Barbier reaction with silanes. 

If, in silane, one hydrogen is replaced by a phenyl group, the silicon-hydrogen bond dissociation enthalpy is only reduced by about 9 7 kJ/mol (19). This is in sharp contrast with the decrease observed for the carbon analogues, where the large stabilization energy of benzyl radical makes D(Me-H)-D(PhCH -H) = 71 4 kJ/mol. The effect of a second phenyl group in the hydrocarbon family is also known, D(Ph CH-H) = 340 8 kJ/mol (27,28) and there is a recent estimate for D(Ph C-H), 338 13 kJ/mol (28). 

Two examples belonging to this class of reaction are shown in Scheme 14.8. In the first transformation, o-dicyanobenzene was benzylated to form o-benzylbenzo-nitriles when irradiated in the presence of either benzyltrimethylsilane (75% yield) [43] or benzyltributylstannane (81%) [44] via the benzyl radical formed by fragmentation of the organometallic radical cations. The reaction led to a clean monoben-zylation and the formation of disubstituted products could be prevented, because the reduction potential of the monobenzylated benzonitrile in the excited state is not sufficient to allow any further oxidation of either the silane or the stannane (Scheme 14.8a). 

Pagliaro and coworkers [87] mixed APTMS with 4-oxo-2,2,6,6-tetramethylpi-peridine-l-oxyl (4-oxo-TEMPO) radical in a methanol solution of NaBHsCN forming TEMPO-fimctionalized silane. The latter was co-electrodeposited with MTMS on ITO. The obtained films were observed to be efficient in electrochemical oxidation of aromatic alcohols in water. Figure 12.32 shows the degradation of benzyl alcohol by electrolysis with the TEMPO-functionalized silane-modified ITO electrode at 1.4 V versus Ag/AgQ. It is seen that after 200 h of electrolysis the concentration of benzyl alcohol decreases to about 10% of its initial value. 

Similar reactions can occur using tetraalkylsilanes including benzyl- and allyl-silanes and, for example, photo-induced electron transfer in the presence of aromatic nitriles can afford alkylated products. These reactions can lead to the regioselective replacement of a cyano substituent with an alkyl group as shown in Scheme 6. Mechanistic studies support the unimolecular fragmentation of the intermediate radical cation [Bu4Si],+ followed by radical-radical combination. ... 

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