Silyl enol ethers radical attack

We also observed similar phenomena in the reaction of silyl enol ethers with cation radicals derived from allylic sulfides. For example, oxidation of allyl phenyl sulfide (3) with ammonium hexanitratocerate (CAN) in the presence of silyl enol ether 4 gave a-phenylthio-Y,5-un-saturated ketone 5. In this reaction, silyl enol ether 4 reacts with cation radical of allyl phenyl sulfide CR3 to give sulfonium intermediate C3, and successive deprotonation and [2,3]-Wittig rearrangement affords a-phenylthio-Y,6-unsaturated ketone 5 (Scheme 2). Direct carbon-carbon bond formation is so difficult that nucleophiles attack the heteroatom of the cation radicals. 

Since Scheme 4 implies formation of a-carbonyl radicals after deprotonation of enol radical cations, the same oxidation chemistry should potentially be accessible from various enol derivatives as enolates, silyl enol ethers and enol esters (Scheme 5). On the other hand, enol ether radical cations do not fit in this systematization since they are attacked by nucleophiles at the double bond faster than providing a-carbonyl radical intermediates through O-C bond cleavage (Sect. 4.3). 

Aliphatic amines are mainly converted to a-substituted products [99,100], whereby especially the a-methoxylation leads to valuable reagents for synthesis. The intermediate iminium salts can be directly trapped by silyl enol ethers to form Mannich bases [108]. If the a-position is blocked or steric conditions favor it, N,N coupling to hydrazo or azo compounds occurs (Table 5, numbers 17-19). 1,1-Disubstituted hydrazines are dimerized to tetrazenes in fair to excellent yields (Table 5, numbers 20-24). The intermediate diaze-nium ions can attack enolizable carbonyl compounds to form aza-Mannich bases [109]. Arylazonaphthols undergo anodic oxidation, producing radical cations. These couple to biphenylbisazo compounds (up to 34%) or can be trapped by anisidine to form azodiphe-nylamines (up to 74%) [110a]. 

The bromination of ketones is believed to occur via acid-catalyzed enolization, followed by electrophilic attack on the enol form. Unsymmetrical ketones can give rise to mixtures of bromo ketones due to mixtures of enols, and several approaches to overcome this shortcoming have been reported. Radical bromination in the presence of epoxides (as acid scavengers) allows for substitution at the more highly substituted position (eq 17). Silyl enol ethers of aldehydes and ketones react with bromine (or NBS) to give the a-brominated carbonyl compounds (eq 18). This, combined with the ability to regiospecifically prepare silyl enol ethers (kinetic vs. thermodynamic), makes for an extremely useful technique for the preparation of a-bromo carbonyl compounds. 

Radical coupling followed by nucleophilic attack of hydroxyl on a quinone methide intermediate is postulated as the mechanism of the key step in the syntheses of silybin and eusiderin (eq 10). 1,4-Diketones are produced in the reaction of silyl enol ethers with Ag20 in DMSO (eq 11). ... 

Moeller and co-workers published the electron transfer cyclization of silyl enol ethers initiated by anodic oxidation. The in situ generated radical cation of 54 intramolecularly attacks the double bond. The resulting benzylic radical position is further oxidized and attacked by a solvent molecule as well as the remaining cationic center to yield 55. Cleavage of the SiO bond leads to the formation of 56 (Scheme 12). 

Scheme 11.77 shows that the two subunits 372 and 373 were linked by a silyl ketal tether to give 374. An anomeric radical was then produced by treatment with tributyltin hydride. This radical reacted with the enol ether acceptor to give the cyclic derivative 375 as the major product in a yield of 43% together with two of the three possible isomers in yields of 6 and 13%. The combined yield shows that more than 56% of the radical attack occurred from the a face of the gluco residue. However, the intermediate radical, located at C4, is mainly trapped by the a face of the furanose moiety. Although this approach is attractive, further elaboration to the... 

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