Xenon difluoride enols

Xenon difluoride [55], xenon difluoride complexed with dialkyl sulfides [59], and xenon difluoride intercalated with graphite [90] are all effective reagents for the fluonnalion of acids, enolates, or enols (Table 2)... 

Xenon difluoride reaction with nitro enolates provides a useful entry into a wide variety of fluonnated synthetic intermediates [91 92] (equation 52)... 

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

Xenon difluoride is characteristic in reacting with polarized double bonds (glycals and enol acetates ) in the presence of acid or Lewis acid catalyst,... 

The reaction of a citronellic enolate—a potential precursor to a 5-hexenyl-type radical clock—with various fluorinating agents leads exclusively to the corresponding a-fluorinated ester derivatives. The complete absence of cyclic fluorinated products is proof that the fluorination does not occur via radical intermediates, while the formation of a cyclic product in the reaction with xenon difluoride is an indication that electron transfer is a competitive process which does not give a fluorinated product31 (Scheme 9). 

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... 

Reaction of the r-butyldimethylsilyl enol ether of a protected dipeptide with xenon difluoride in a mixture of acetonitrile and 1,1,2-trichlorotrifluoroethane gave the fluoro ketone in 71 % yield as a 1 1 mixture of diasteroisomers, while further transformation to enol ether and fluorination affords the difluoro ketone81 (Scheme 23). 

In general, examples of substitution of sodium and potassium in organometallic derivatives by fluorine are limited to the salts of active methylene compounds. Their fluorination is achieved with perchloryl fluoride.fluorine, N-F reagents. " and xenon difluoride " (Table 10). The fluorination of metal enolates is discussed under metal-assisted substitution ot... 

Xenon difluoride has also been used for the fluorination of enol derivatives [5] and 1,3-dicarbonyl compounds [140], and fluorination of activated aromatic substrates is possible in the presence of a Lewis acid [141]. 

Chemical oxidation of silyl enolates has been performed with a variety of inorganic and organic oxidants such as ozone, copper(II) salts, Pb(OAc)4, Ag20, hypervalent iodine compounds such as iodosobenzene in methanol, (NH4)2[Ce(N03)6], xenon difluoride, tetranitromethane, halogens, nitronium-, diazonium- and triphenylmethyl salts, chloranil and ddq. 

Silyl enolates are useful starting materials for the smooth a-halogenation of ketones, which can be carried out by oxidation with A-halosuccinimides , halogens and iodoben-zenedifluoride or xenon difluorides. As shown in equation 32 , the reaction of 45 with Xep2 gives a 1 1 mixture of a-fluoroketone diastereoisomers 46a and 46b. 

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. 

---