Generation from chlorodifluoromethane

In fluorinated agrochemicals, the difluoromethoxy group is less commonly encountered than the trifluoromethoxy group. The difluoromethoxy group is readily available from the reaction of a phenoxy group and difluorocarbene, generated from chlorodifluoromethane and a base (Fig. 27) [6,107-109]. 

Furthermore, 1-alkyl-1-fluorocyclopropanes were prepared by cycloaddition of alkyl-fluorocarbene, generated from an alkyllithium (usually methyllithium) and dichloro-, difluoro-or chlorodifluoromethane, to alkenes. Due to the many competitive side reactions, H-Li, Hal-Li exchange, alkylation etc., this method has no practical value (for details, see Houben-Weyl, Vol. E19b, pp 1464-1466). 

Difluorocarbene is the only dihalocarbene, which, being generated via PT-catalyzed a-elimination from chlorodifluoromethane in a two-phase system does not enter the cycloaddition reaction to alkenes. This is because of the instability of the chlorodifluoro-methyl anion, which due to its very short lifetime cannot be transferred from the interfacial region, where it is formed, into the organic phase. Therefore, difluorocarbene is generated in the interfacial region and undergoes fast hydrolysis. 

Miscellaneous. An interesting synthesis of 1,1-difluoro-l-alkenes from ylides and chlorodifluoromethane has been described.47 The ylide acts both as a carbene generator and trapping agent (Scheme 11). 

The reaction of chlorodifluoromethane with alkoxide ions generated in low concentration from halide ions and epoxides [28, 29] is an interesting, higher temperature method that gives good to excellent yields of gem-difluorocyclo-propanes from just moderately nucleophilic olefins (equation 9)... 

The generation of difluorocarbene has been the subject of intense investigation. Difluorocarbene has been successfully formed from a wide range of fluorinated precursors, including metal salts of chlorodifluoroacetic acid, methyl chlorodifluoroacetate, ethyl chlorodi-fluoroacetate, difluorodiiodomethane, " chlorodifluoromethane." " difiuorotris(trifluoro-methyl)-A -phosphane hexafluoropropylene oxide,tetrafluoroethylene... 

A reaction of haloforms with a base, which generates dihalocarbenes (a-elimination) and their addition to alkenes is an efficient method for the preparation of 1,1-dihalocyclopropanes, with the exception of 1,1-difluoro derivatives (Houben-Weyl, Vol.E19b, pp 1464-1466). When chlorodifluoromethane and an alkene are treated with methyllithium, potassium tcrt-butox-ide, powdered sodium hydroxide in tetraglyme or a concentrated aqueous solution of alkali metal hydroxide and a phase-transfer catalyst, the expected 1,1-difluorocyclopropanes are formed in low yields. Comparable low yields of these products result, if dichlorodi-fluoromethane and an alkene are treated with methyllithium. " The main products formed are those that result from reaction of difluorocarbene (carbenoid), and its precursor, with the base or the solvent present in the system (for examples, see refs 10-12). Therefore, the reaction of chlorodifluoromethane with base and an alkene lacks preparative value. The difficulties mentioned above are circumvented in the method using chlorodifluoromethane, oxirane (or chloromethyloxirane), with tetraalkylammonium halide as a catalyst and an alkene (Houben-eyl, Vol. 4/3, p 380 and Vol. E19b, pp 1468-1469). 

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