Sources of difluorocarbenes

Pubhcations have described the use of HFPO to prepare acyl fluorides (53), fluoroketones (54), fluorinated heterocycles (55), as well as serving as a source of difluorocarbene for the synthesis of numerous cycHc and acycHc compounds (56). The isomerization of HFPO to hexafluoroacetone by hydrogen fluoride has been used as part of a one-pot synthesis of bisphenol AF (57). HFPO has been used as the starting material for the preparation of optically active perfluorinated acids (58). The nmr spectmm of HFPO is given in Reference 59. The molecular stmcture of HFPO has been deterrnined by gas-phase electron diffraction (13). 

Dialkylcadmium reagents are often useful alternatives to the more reactive Gngnard reagents in the preparation of ketones from acyl halides However, bis(trifluotomethyl)cadmium glyme is decomposed by acyl halides and does not give trifluoromethyl ketones [, 124] Nevertheless, this reaction can be used as a low-temperature source of difluorocarbene [S, 124] (equation 102)... 

Among the many methods of generating difluorocarbene, the treatment of bromodifluoromethylphosphonium bromides with potassium or cesium fluoride is particularly useful at room temperature or below [II, 12 13] The sodium iodide promoted decomposition of phenyl(trifluoromethyl)mercury is very effective at moderate temperatures [S, 14] Hexafluoropropylene oxide [/5] and chlorodifluo-roacetate salts [7] are excellent higher temperature sources of difluorocarbene... 

In general, halodifluoroniethanessuch as dibromodifluoromethane, difluorodiiodomethane and dichlorodifluoromethane are very poor sources of difluorocarbene for the synthesis of cyclopropanes.Cyclic systems 5-8 contain a difluoromclhyl group which can be added to multiple bonds. 

When heated above 170 C hcxafluoropropylene oxide (5) is a source of difluorocarbene thus, heating 5 in the presence of an alkene allows the synthesis of fluorinated cyclopropancs under neutral conditions.The less readily available tetrafluoroethylcnc oxide (6) is also a precursor of difluorocarbene. ... 

Hexafluorocyclopropane (7) is also a source of difluorocarbene under conditions similar to those used with hexafluoropropylene oxide. It is also possible to generate difluorocarbene from the explosive difluoro-3//-diazirine (8) thermally or by photolysis with UV light. Difluorocarbene can be generated from trimethyl(trifluoromethyl)tin (McjSnCFj) by heating at a temperature of 150 or at a lower temperature of 80 C in the presence of sodium iodide. ° ... 

Fluorinated epoxides are very important synthetically. Perfluoroalkenes can be oxidized by numerous means to the corresponding epoxides. The most commonly encormtered epoxide is hexafluoropropene oxide (HFPO), which is made from hexafluoropropene and is used in the synthesis of oligomers and as a source of difluorocarbene. HFPO is a colorless, nonflammable gas, boiling at -27.4 °C it can be ring-opened easily by nncleophiles or electrophiles imder a variety of conditions. HFPO can be rearranged by flnoride ion to perfluoropropionyl flnoride and by Lewis acids snch as UO2 to hexafluoroacetone (HFA). Nncleophiles typically attack the most substituted carbon of the epoxide, freeing the oxyanion to lose fluoride or act as a nucleophile itself Some examples of HFPO reactivity are shown in Scheme 3. 

Remarkably, tetrafluoroethene [76] and even PTFE [77] (Figure 6.50) may be used as sources of difluorocarbene, if sufficiently high temperatures are used. 

Epoxyhcxafluoropropane has been used as a source of difluorocarbene by thermal decomposition " more complicated oxiranes, however, do not decompose to simple fluorocar-benes.117118... 

Above 375 °C perfluorocyclobutane is also formed in considerable quantities. Haszeldine et al. have demonstrated that other ew-difluorohalogenated cyclopropanes can provide useful low temperature sources of difluorocarbene. 

In addition to phenyl(trifluoromethyl)mercury, iodo(trifluoromethyl)mercury and bis(tri-fluoromethyl)mercury may serve as a source of difluorocarbene and, hence, difluorocyclo-propanes. Bis(trifluoromethyl)mercury is particularly useful as it is fairly readily available and generates two molecules of carbene. However, it has been seldom used so far, e.g. formation of 12. ... 

In a search for a photochemical source of difluorocarbene, 4,4-difluoro-pyra-zolidine-3,4-dione (4) was synthesized as a precursor for the corresponding pyrazolinedione (5). The carbene CF2 was expected to be produced by photoelimination of N2 and CO from (5). Upon oxidation of (4), however, the azo compound (5) turned out to be too unstable to be isolated, although it could be trapped as a cycloadduct with cyclopentadiene. 

As reported in Section 2 above, an attempt to use the pyrazolinedione (5) as a photochemical source of difluorocarbene failed on account of the instability of... 

---