Acylsilanes, anodic oxidation

Suda and coworkers described the anodic oxidation of 2-silyl-l,3-dithianes which have two sulfur atoms on the carbon adjacent to silicon [42], In this case, however, the C Si bond is not cleaved, but the C-S bonds are cleaved to give the corresponding acylsilanes (Scheme 12). Although the detailed mechanism has not been clarified as yet, the difference in the anode material seems to be responsible for the different pathway of the reaction. In fact, a platinum plate anode is used in this reaction, although a carbon anode is usually used for the oxidative cleavage of the C-Si bond. In the anodic oxidation of 2-silyl-l,3-dithianes the use of a carbon anode results in a significant decrease in the yield of acylsilanes. The effects of the nature of the solvent and the supporting electrolyte may also be important for the fate of the initially formed cation radical intermediate. Since various 2-alkyl-2-silyl-l,3-dithianes can be readily synthesized, this reaction provides a convenient route to acylsilanes. 

Various nucleophiles other than methanol can be introduced onto the carbonyl carbon. Anodic oxidation of acylsilanes in the presence of allyl alcohol, 2-methyl-2-propanol, water, and methyl /V-methylcarbamate in dichlorometh-ane affords the corresponding esters, carboxylic acid, and amide derivatives (Scheme 24) [16]. Therefore, anodic oxidation provides a useful method for the synthesis of esters and amides under neutral conditions. 

Recently, acylsilanes have been utilized as useful intermediates in organic synthesis [57], For example, treatment of acylsilanes with the fluoride ion generates the corresponding acyl anions which react with electrophiles. On the other hand, by using the electrochemical method, acylsilanes serve as acyl cation equivalents because nucleophiles are introduced at the carbonyl carbon. Chemical oxidation of acylsilanes with hydrogen peroxide which affords the corresponding carboxylic acids has been reported [58], However, the anodic oxidation provides a versatile method for the introduction of various nucleophiles... 

It is interesting that anodic oxidation of 2-silyl-l,3-dithianes at a platinum anode provides the corresponding acylsilanes (equation 25)30. In this case, only a C—S bond... 

Electrochemical properties of tosylhydrazones of acylsilanes were also investigated. A decrease in the oxidation potential of tosylhydrazones caused by silylation is much smaller than that for carbonyl compounds (see Tables 7 and Section n.E., Table 9). Anodic oxidation of tosylhydrazones of acylsilanes provides the corresponding nitriles with consumption of a catalytic amount of electricity (equation 29)34. 

Anodic oxidation of acylsilane in the presence of various nucleophiles provides the corresponding ester, acid and carbamate as shown in equation 3641. 

Methoxy(trimethylsilyl)methane and methoxybis(trimethylsilyl)niethane have been proposed as new synthons for the formyl anion and the methoxycarbonyl anion, respectively after alkylation, C-Si cleavage is achieved by anodic oxidation. Similar electrochemical oxidative cleavage of acylsilanes reveals their potential as acyl cation synthons. Anodic oxidation of N-silylmethyl carbamates in methanol produces f -methoxymethyl carbamates in high yield. 

The acylsilane toxylhydrazones exhibit slightly less positive oxidation potentials than the corresponding tosylhydrazones of aldehydes [137]. The preparative anodic oxidation of acylsilane tosylhydrazones in a divided cell gave the corresponding nitriles. This reaction seems to be promoted by the electrogenerated acid. 

The mechanism shown in Scheme 23 has been suggested The first step involves the transfer of an electron from the acylsilane to produce the cation-radical intermediate. Attack of methanol at the silicon cleaves the C-Si bond to give the acyl radical intermediate, although there is no direct evidence for the acyl radical intermediate. The acyl radical is then oxidized anodically to the acyl cation, which reacts with methanol to give the corresponding methyl ester. 

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