Silyl enol ethers electrochemical

When furans were tethered to silyl enol ethers at the 2-position, spiroannulation also occurred at the 2-position under electrochemical conditions <06CC194>, as exemplified below. The formation of the kinetic products is the result of the higher nucleophilicity of the furan C2-position. 

Silyl enol ethers are powerful intermediates in organic synthesis. Reactions of silyl enol ethers with various electrophiles provide effective methods for the synthesis of various carbonyl compounds. In this section we will briefly touch on the electrochemical reactions of silyl enol ethers and related compounds. The electrochemical behaviour of silyl enol ethers is expected to be closely related to that of allylsilanes and benzylsilanes because silyl enol ethers also have a silyl group ft to the re-system. 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

Quite recently, stereoselective electrochemical synthesis of silyl enol ethers using a sacrificial magnesium anode was reported, as shown in equation 100126. 

In the electrochemical oxidation, similar reaction was observed (Scheme 12). Cation radical CR26 generated by electrochemical oxidation of a-stannyl sulfides cleaves to give carbocation C26, which react with allyltrimethylsilane or the silyl enol ether of cyclohexanone to give the usual addition products. In this electrochemical reaction, stannyl derivatives also afforded the desired product 27 or 28 in better yield compared with the corresponding silyl derivatives. 

Quite obviously, direct measurement of the oxidation potentials of the intermediate, highly unstable enols poses a severe experimental problem. From work by Kresge, Capon and others [58-60] it is known that under appropriate conditions even enols of aliphatic ketones can be prepared with lifetimes in the order of minutes. However, no electrochemical data on these systems are available so far. Even for arylacetones, where the enol content is in the order of 10 %, direct measurement of the oxidation potential is not known to date. Two approaches have been applied to estimate their oxidation potentials [183]. First, using a linear correlation [184] between the experimental oxidation potentials of benzyl radicals [185,186] and their AMI calculated ionization potentials (IPs) the Fiyj of the four enols 86a-86d were derived from the calculated IPs. Secondly, oxidation potentials of silyl enol ethers were used to... 

A more efficient and economic procedure for the preparation of / -acetamido nitro compounds from alkenes, conjugated dienes and silyl enol ethers is based on the electrochemical generation of nitronium tetrafluoroborate in acetonitrile solution by anodic oxidation of dinitrogen tetrox-ide, using a divided cell (Table 6, Method B)142-143. From 1,3-dienes, mixtures of regio- and diastereomers were obtained. 

Various oxidations of amines can also generate acyliminium ions. The methods most used in synthetic procedures involve electrochemical oxidation to form a-alkoxy amides and lactams, which then generate acyliminium ions. Acyliminium ions are sufficiently electrophilic to react with enolate equivalents such as silyl enol ethers and enol... 

Mannich bases result electrochemically from p-disubstituted dimethylanilines in the presence of silyl enol ether, which form... 

An elegant access to natural product synthesis was realized by Wright and co-workers, who obtained a cis-junction through the cyclization reaction of 91. The silyl enol ether 91 is electrochemically oxidized and subsequently cyclizes to yield compound 92. In three further steps, the cyathin core 93 can be prepared. 

An electrochemical oxidation route to tetrahydrofuran and tetrahydropyran rings has been described, in which a silyl-substituted enol ether reacts with a regioselectivity that is reversed from the normal polarity of enol ethers (Scheme 61) <2000JA5636>. Aldol reactions of a-diazo-/ -ketoesters with aldehydes produce adducts which undergo Rh(ii)-catalyzed 0-H insertion reactions to yield highly substituted tetrahydrofurans <1997TL3837>. 

The success of this transformation depends upon the oxidation potential of the ESE group (Eox 1.5 V), which is lower than that of the alkyl silyl ether group (Eax 2.5 V). Recently, Schmittel et al.35 showed (by product studies) that the enol derivatives of sterically hindered ketones (e.g., 2,2-dimesityl-1-phenyletha-none) can indeed be readily oxidized to the corresponding cation radicals, radicals and a-carbonyl cations either chemically with standard one-electron oxidants (such as tris(/>-bromophenyl)aminium hexachloroantimonate or ceric ammonium nitrate) or electrochemically (equation 10). 

In contrast, allylstannanes act as electrophiles when they were treated with appropriate higher valent metal salts or oxidized by electrochemical process (eq (105)) [100]. In the presence of tin(IV) chloride, allylstannanes react with nucleophiles such as enol silyl ethers or allylsilanes (eq (106)) [101] as well as electrophilic aldehydes. 

Rathore, R. and Kochi, J.K, Quantitative assessment of electron-donor properties of enol silyl ethers charge-transfer complex formation, photoelectron spectra and transient electrochemical oxidation. Tetrahedron Lett., 35, 8577, 1994. 

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