Carbamates anodic oxidation

Nitrogen compounds are also effective as nucleophiles in the anodic oxidation of silyl-substituted ethers. The electrochemical oxidation in the presence of a carbamate or a sulfonamide in dry THF or dichloromethane results in the selective cleavage of the C-Si bond and the introduction of the nitrogen nucleophile at the carbon (Scheme 21) [55]. Since a-methoxycarbamates are useful intermediates in the synthesis of nitrogen-containing compounds [44], this reaction provides useful access to such compounds. Cyclic silyl-substkuted ethers such as 2-silyltetrahydrofurans are also effective for the introduction of nitrogen nucleophiles. The anodic oxidation in the presence of a carbamate or a... 

Anodic amide oxidations have also proven useful for selectively converting amides into enamides that have in turn been used to functionalize the carbon beta to nitrogen. For example, the four-electron oxidation of carbamates was used to introduce a carbonyl beta to a nitrogen (Scheme 21) [50]. In this example, the starting carbamate was oxidized at a carbon... 

The anodic oxidation of amides and carbamates is one of the most general and efficient electrochemical reactions known [115]. The primary intermediate... 

Anodic oxidation of the carbamates 17 and 23 in methanol, followed by reaction with chlorodiphenylphosphine affords the a-diphenylphosphinylcarbamates 20 and 25, from which the readily generated carbanions react with aldehydes to give the 4-phosphinyl-2-oxazolidinones 21 and 26. The removal of the diphenylphosphinyl group by a mild thermal treatment provides a route to the 2(3//)-oxazolones 22 and 27 (Fig. 5.6). ... 

Ethers can be converted to acetals, and acetals to ortho esters, by anodic oxidation in an alcohol as solvent.194 Yields are moderate. In a similar reaction, certain amides, carbamates, and sulfonamides can be alkoxylated a to the nitrogen, e.g., MeS02NMe2 — Me-S02N(Me)CH20CH3.195 OS VII, 307. 

The anodic oxidation of methyl N,/V-dialkylcarbamate in methanol, containing Et4NOTs, yielded three types of products a-methoxylated compounds, enamine-type products, and dealkylated carbamates.197 Methyl N-pyrrolidinylcarbamate (137), for example, gave on constant current oxidation... 

On the basis of oxidation potentials, current-potential relationships, and isotope effects, an electron-transfer mechanism is suggested for the anodic oxidation of methyl N,N-dialkyl substituted carbamates, which can reasonably explain the formation of all three types of products. Also, N-acylazacycloalkanes are converted anodically at a platinum electrode in R0H-Et4NBF4 into a-monoalkoxy or a,a -dialkoxy derivatives depending on the electrolysis conditions employed.198... 

Anodic oxidation of allylsilanes in the presence of nucleophiles results in replacement of a trimethylsilyl group by a nucleophile which is introduced into the allylic carbon. Various oxygen and nitrogen nucleophiles such as alcohols, water, caboxylic acids, p-toluenesulfonic acid, carbamates or a sulfonamide can be employed in this reaction (equations 9—11)11 —13. 

Anodic oxidation of A-benzyl-Ar-(a-silylmethyl)carbamate provides a-methoxylated product as a single regioisomer (equation 26), while the unsilylated parent carbamate gives a mixture of regioisomeric products (equation 27). Thus, the introduction of a silyl group can control completely the regiochemistry of the anodic methoxylation and can also activate the nitrogen atom toward anodic oxidation. 

Anodic oxidation of w-silyl ethers in the presence of nitrogen nucleophiles such as sulfonamides and carbamates provides N,0-acatals (equations 34 and 35)2. 

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

The anodic oxidation of carbamates of pyrrolidine and piperidine in a nucleophilic solvent such as acetic acid or methanol yields a,/ -disubstituted compounds in which the nature of the a- and -substituents depends on the reaction conditions employed (equation 9)10. This a,/ -disubstitution has been explained by the intermediary formation of the enecarbamate and its further anodic oxidation. 

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. 

As described in the previous section, the anodic oxidation of aliphatic amines is utilized only rarely in organic synthesis due to the instability of the generated intermediates, whereas amides tuid carbamates of aliphatic amines yield relatively stable intermediates which are sufficiently promising as starting materials in organic synthesis (equations 47 and... 

The products obtained by the anodic oxidation of amides or carbamates in methanol have the same structures as the compounds which can be synthesized from amides (carbamates), aldehydes and methanol (equation 50). l e regeneration of iminium cations from these a-methoxyamides and subsequent reactions of the iminium cations with nucleophiles such as active methylene compounds or nucleophilic aromatic nuclei is well known under the term amidoalkylation (equation 50). ... 

Anodic oxidation of amides and carbamates on the other hand are far more useful than those of amines. Scheme 46 shows the synthesis of a pharaoh ant trail pheromone in which the anodic oxidation of 2-pyrrolidone is an important step [270]. 

Addition to the double bond of enecarbamates is observed during anodic oxidation in methanol in presence of an ammonium halide, leading to the product of halomethox-ylation [Eq. (28)] [155]. The enecarbamates are available through electrochemical a-meth-oxylation of the carbamate followed by methanol elimination (see Sec. VIII.B). 

In Scheme 8, five possible electrochemical pathways for the formation of V-acyli-minium ions are represented. Pathway a (see Sec. VIII.A) describes the direct anodic oxidation of amides and carbamates to the intermediated V-acylium ions via removal of one electron from the nitrogen lone pair followed by deprotonation in a-position of the nitrogen atom and further one-electron oxidation. In pathway b (see Sec. VIII.C), a decarboxylative methoxylation of an V-acylated amino acid (Hofer-Moset reaction) leads to the same intermediate. The radical that is formed after anodic decarboxylation is immediately further oxidized to the cation due to the electron donation of the nitrogen. Pathway c (see Sec. VIII.B) describes the anodic oxidation of an V-acylated amino... 

Alkoxylation may be achieved by anodic oxidation in an alcohol, often methanol (MeOH), containing a suitable electrolyte, such as KOH, NaOMe, NaCN, NaBp4, or NH4NO3 [9-12,31,32]. Substrates that have been alkoxylated in this manner include aromatic compounds (both nuclear and side-chain alkoxylation has been observed), alkenes, ethers including vinyl ethers, enamines, N, AA-dialkylamines, AA-alkylsubstituted amides, and A-alkylsubstituted carbamates. In many cases alkoxylation by substitution is a side reaction only to concurrent alkoxylation by addition [7,9,11,33]. 

In analogy with what has been observed for carbamates (described earlier), the trimethylsilyl substituent in, for example, 1-phenylthio-l-trimethylsilylalkanes is easily replaced by methoxy during anodic oxidation in MeOH [111,112]. Similarly, the anodic oxidation of a. a-bis(trimethylsilyl)xylenes in MeOH results in replacement of one trimethylsilyl group by methoxy [113]. 

For example, the anodic oxidation of a silyl-substituted carbamate to generate a solution of N-acyliminium ion and the cathodic reduction of cinnamyl chloride in the presence of chlorotrimethylsilane to generate the corresponding allylsilane can be carried out simultaneously in a single electrochemical microflow cell under continuous flow conditions (Figure 5.10). The N-acyliminium ion, the anodic product, is allowed to react with the allylsilane, the cathodic product, to give the coupling product. 

The reaction proceeds via electrogenerated cationic intermediates as is seen in the anodic oxidation of nonfluorinated amines, carbamates, and amides (Scheme 19). However, the regiochemistry in these processes is not governed by the stability of the cationic intermediates 1 and r because the main products are formed via the less stable intermediates I. Indeed, the promotion effect and unique re-gioselectivity observed in reactions of the fluoroalkyl amines can be explained mainly in terms of the a-CH kinetic acidities of the cation... 

It is noteworthy that both anodic and cathodic reactions can be used for desired transformations in some cases. For example, the anodic oxidation of silyl-substituted carbamates can be combined with the cathodic reduction of allyUc halides in the presence of chlorotrimethylsilane (paired electrolysis) [51]. The products of both reactions, i.e. N-acyliminium ion and the allylic silane, are then allowed to react with each other to obtain a final coupling product (Table 7.2). 

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