Oxidation potentials of acylsilanes

Table 6. Oxidation potentials of acylsilane and related compounds...

The activation by cr-n interacation is also applicable for the acyl metal compounds such as acylsilanes. The oxidation potentials of acylsilanes are less positive than those of aldehydes and ketones because the nonbonding p orbital of the carbonyl oxygen atom... 

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. 

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. 

A third possibility of chemical modification is conversion into an acylsilane which reduces the oxidation potential of the corresponding ketone by approximately 1 V. A peak potential of 1.45 V (relative to Ag/AgCl) for the oxidation of undecanoyltrimethylsilane has been reported. Preparative electrochemical oxidations of acylsilanes proceed in methanol to give the corresponding methyl esters. A two-step oxidation process must be assumed because of the reaction stoichiometry —oxidation of the acylsilane results in the carbonyl radical cation which is meso-lytically cleaved to give the silyl cation and the acyl radical, which is subsequently oxidized to give the acyl cation as ultimate electrophile which reacts with the solvent. A variety of other nucleophiles have been used and a series of carboxylic acid derivatives are available via this pathway (Scheme 49) [198]. 

TABLE 9. Oxidation potentials ( p) of acylsilanes and related compounds30,37... 

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. 

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