2.4- Trifluoromethyl -1,4-diene formation

Bromination of the enol ether product with two equivalents of bromine followed by dehydrobromination afforded the Z-bromoenol ether (Eq. 79) which could be converted to the zinc reagent and cross-coupled with aryl halides [242]. Dehydrobromination in the presence of thiophenol followed by bromination/dehydrobromination affords an enol thioether [243]. Oxidation to the sulfone, followed by exposure to triethylamine in ether, resulted in dehydrobromination to the unstable alkynyl sulfone which could be trapped with dienes in situ. Alternatively, dehydrobromination of the sulfide in the presence of allylic alcohols results in the formation of allyl vinyl ethers which undergo Claisen rearrangements [244]. Further oxidation followed by sulfoxide elimination results in highly unsaturated trifluoromethyl ketonic products (Eq. 80). 

A particularly useful reaction of this type involves the direct formation of hexakis(trifluoromethyl)cyclopentadiene (71) (Scheme 31), or the corresponding cyclopentadienide (72), from the diene (38) by a fluoride ion induced reaction with pentafluoropropene [67-69]. Recent work [54] has shown that very active sources of fluoride ion can be generated by direct reaction of amines, especially TDAE (43), with perfluorinated alkenes or perfluorinated aromatic compounds and these essentially solventless systems promote both oligomerisations (see above) and polyfluoroalkylations. The absence of solvent makes recovery of product very easy, e.g. in high-yielding formation of (73), (74) or (75) (Scheme 32). 

Five-membered heterocycles with two vicinal chalcogen atoms in the ring system can be used as stable precursors for sulfur as well as for selenium-containing hetero-1,3-dienes in cycloaddition reactions. Consequently, 3//-1,2,4-thiaselenazoles have been used for the in situ formation of 4,4-bis(trifluoromethyl)-l-thia-3-azabuta-1,3-dienes, which exist at room temperature only as 4,4-bis(trifluoromethyl)-2//-l,3-thiazetes. This strategy was applied to the synthesis of the first stable selenophosphorane from bis(trifluoromethyl)-substituted 3//-diselenazol and 2-methoxy-1,3,2-dioxaphospholan [78AG(E)774] (Scheme 83). 

Trifluoromethyl-substituted sulfides, sulfoxides, and sulfones can undergo [4-F 2]-cycloaddition reactions with various dienes under thermal conditions (Table 2) sec also formation of 15 and 17. 5... 

Other examples of favored formation of cyclobutanes include photocycloadditions of tri-fluoromethyl-substituted coumarins with dienes,and of 5-(trifluoromethyl)-l,3-dioxin-4-ones, e.g. 3, with alkenes, " - - " as well as intramolecular photocycloadditions, such as those of bicyclo[2.2.2]octadiene and tricyclononadiene derivatives. 

In the case of dienes, amounts of mono- and diadducts formed depend on the ratio of phenyl(trifluoromethyl)mercury to diene, e.g. formation of 7, 8, and 9. ... 

When a mixture of benzene and perfluorobutyne was irradiated in a Vycor tube with 253.7 nm light, slow formation of at least eight compounds was detected by gas chromatography. The major compound was l,2-bis(trifluoromethyl)cycloocta-l,3,5,7-tetraene (10, 40%), accompanied by three isomeric bis(trifluoromethyl)tricyclo[3.3.0.0 ]octa-3,6-dienes (semibullvalenes) 11-13 in 25, 12 and 5% yield, respectively. ... 

Transition metal compounds can be replaced by other substances, for example, by phosphites. Thus, 2-substituted 4-trifluoromethyl-5-fluoro-l,3-oxazole-2,4-dienes 9 were obtained by the reaction of 4,4-bis(trifluoromethyl)-N-acylimine 10 with trimethylphosphite (71CB1826, 74CB1448). The reaction occurs via the intermediate formation of 5-substituted 3,3-bis(trifluoromethyl)-2,2-dihydro-1,4,2-oxazaphos-pharol-4-ene 11 (Scheme 15). 

The photochemically-induced cleavage (high pressure Hg lamp) of the trifluoromethyl-substituted cyclopenta- and cyclotetra-arsanes in the presence of 1,3-dienes yields the (4 + 2) cycloadducts (141) and (142) <95ZN(B)189). These results underline the intermediate formation of the bistrifluoromethyl diarsene. 

The template reaction leading to the formation of a 1 2 fiuorinated Schiff base complex, bis[l,l,l,2,2,2-hexafluoro-2,ll-bis(trifluoromethyl)-4,9-dimethyl-2,ll-diolato-5,8-diazadodeca-4,8-diene(2-)]-cerium(IV), was investigated by Timmons et al. (1980). The pale yellow adduct crystallizes in space group P2j/c the Ce(IV) is 8-coordinate with mean Ce-O and Ce-N bond lengths equal to 2.21(2) and 2.62(1) A, respectively. This is the first crystallographic evidence for Ce-N bonding. 

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