Chlorosilanes, reduction potential

This result is consistent with the anionic mechansim, outlined in Scheme 16, since the lower reduction potential of 28 should lead to the formation of the triphenylsilyl anion, which would then react with the chlorosilanes 27 and 28... 

Concerning the chlorosilanes, the methyl substituted ones showed in each case higher reduction potentials than the corresponding phenylsilanes. This is in accordance with the results of the galvanostatic... 

In this reaction, only one half of the engaged chlorosilane is reduced, the second half acting as the electrophile, so the theoretical total current is 1 F mol . In spite of its very negative reduction potential, Me3SiCl could also be reduced to Me6Si2 in the above conditions. 

Assuming an anionic intermediate due to a two-electron transition, there are three possible reaction products (Eq. 1). Competition between C-C, Si-C and Si-Si bond formation is dependent on the reduction potentials of the starting materials on the one hand, and their electrophilicity on the other. Thus, given that a chlorosilane is a better electrophile than an organic halide [2], crosscoupling can occur only if cathodic cleavage of the C-Cl bond is the initial reaction step. Comparing cyclic voltammetric data (Table 2, below) on the basis of this assumption, /-butyl chloride should be even better suited as the substrate than chlorobenzene. 

Electrolyses of several chlorosilanes with PhCl indicate that competition between Si-Si and Si-C bond formation is dependent not only on the reduction potentials of the starting silanes, but also on steric or kinetic effects on the electrode surface. Although the reduction potentials of phenylated chlorosilanes lie very close to those of methylated ones [3], Si-C bond coupling was successful with MesSiCl and Me2SiCl2 (Table 1) only. 

Hengge et al. reported the formation of Si-Si bond in the electrochemical reduction of chlorosilanes in DME without control of the applied potential. For the formation of the Si-Si bond the choice of electrode materials seems to be of great importance [81]. Lead and mercury have proved suitable as cathode materials. 

Perhaps the most useful type of alkene substrates for these reactions are enol ethers, enol esters and vinyl sulfides. Silyl enol ethers have excellent electron-donor properties, with an ionization potential of about 8 eV and an oxidation potential in various solvents of approximately 1.0-1.5 V vs SCE161. These compounds are easily synthesized by reaction of an enolate with a chlorosilane. (A very recent report synthesized a variety of silyl enol ethers with extremely high stereochemical yield, using the electrogenerated amidate of 2-pyrolidinone as the base.)162 An interesting point is that the use of oxidative or reductive cyclization reactions allows carbonyl functionalities to be ambivalent, either oxidizable or reducible (Scheme 65)163. 

Since Me3SiCI does not show any reduction wave at more positive potentials than -3.0 V vs SCE [6] in contrast with phenylchlorosilanes, it acts as the electrophile in the cross-coupling of phenyl(methyl)-chlorosilanes with Me3SiCl. Eq. 7 and Table 1 summarize the results ... 

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