Xenon difluoride silyl enol ethers

Silyl enol ethers are fluonnated in high yields with xenon difluoride [62 93, 94 95] Applications of this reaction to the preparation of fluonnated... 

Iodotoluene difluoride reacts with steroidal silyl enol ethers to give a-fluoro-ketones. Whilst the reaction is slower than with xenon difluoride, the rea nt may find use since the stereochemical outcomes of the reactions differ, suggesting an electrophilic mechanism for the latter reagent, but a nucleophilic mechanism (via an iodonium intermediate) for the iodo-arene derivative. 

SCHEME 10 Mechanism of the reaction between silyl enol ethers and xenon difluoride. 

Ramsden et al. have obtained similar results for tetralone silyl enol ether radical cations. They investigated the reaction of various silyl enol ethers with xenon difluoride in acetonitrile and found a new method for the selective preparation of a-fluoroketones (Scheme 9). 

The reaction of the a-tetralone silyl enol ether 3 with xenon difluoride gave fluoroketone 40 in a high yield of 90%. Ketone 41 was obtained as a byproduct in 4% yield. In contrast, the (i-tetralone based silyl enol ether 4 formed fluoroketone 42 in 14% yield and ketone 43 as main product in 52% yield. The reaction mechanism proposed is shown in Scheme 10. 

One electron oxidation of silyl enol ether 44 leads to the formation of a radical ion pair of 45 and the xenon difluoride radical anion. Subsequent transfer of a fluoride radical yields cation 46, which reacts by loss of the trimethylsilyl cation to yield the fluoroketone 47. The formation of ketone 49 is explained by a [1,5] -hydrogen migration from the trimethylsilyl group to the radical cationic moiety of 45, leading to the formation of 48. 

Ramsden, C.A. and Smith, R., Reaction of silyl enol ethers with xenon difluoride in MeCN evidence for a nonclassical radical cation intermediate, Org. Lett., 1,1591, 1999. 

Five research groups have investigated fluorination of enol acetates and silyl ethers with xenon difluoride, and mainly a-fluoroketones were formed22"24, 78-81. HF-catalyzed fluorination of cyclic enol acetates with xenon difluoride at room temperature gave a-fluorocycloalkanones, while the yield and contamination with cycloalkanones depend on... 

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