Difluorocarbene chlorodifluoromethane

Copyrolysis of 1,1-diehloroperfluoroindane and chlorodifluoromethane or tetrafluoroethylene gives 1-perfluoromethyleneindane as the major product and three minor products [3] (equation 2) Insertion of difluorocatbene into the benzylic carbon-chlorine bond and subsequent loss of a chlonne molecule is observed in the copyrolysis of chlorodifluoromethane and pentafluorobenzotnchlonde to give a-chloroperfluorostyrene as the major product. Aromatic carbon-chlorine bonds are unreactive to the difluorocarbene in this reaction [4] (equation 3). 

Regioselective N-difluoromethylation of 3-phenyl 1,2,4-triazole 39 has been achieved using chlorodifluoromethane in the presence of a base. The reaction yielded a mixture of the three possible products 40-42 and proceeds by the insertion of a difluorocarbene into an N-H bond (Equation 10) <1998JFC(92)141>. 

All fluorocarbenes are ground state singlets. For laboratory use there are some precursors which thermally generate difluorocarbene.42 Its identification is usually made by a subsequent chemical insertion reaction. A few industrially important processes proceed via difluorocarbene. The thermal pyrolysis of chlorodifluoromethane (CHF2C1) for the production of tetrafluoroethene and hexafluoropropene gives the intermediate CF2 which dimerizes to the alkene. 

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]. 

At 620°C, chlorodifluoromethane eliminates hydrogen chloride and generates difluorocarbene which is inserted between chlorine and carbon in the trichloromethyl group. Subsequent elimination of a molecule of chlorine gives the final product C, a-chloroperfluorostyrene [85]. 

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... 

Difluorocarbene can be generated by high-temperature reaction of chlorodifluoromethane in the production of tetrafluoroethylene (TFE) (equation 48). Another method of in situ generation of the carbene is thermal decomposition of... 

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). 

The most important reason why chlorodifluoromethane does not react with alkenes to give 1,1-difluorocyclopropanes under phase-transfer conditions is due to its synchronous, rather than stepwise, cleavage to difluorocarbene. The difluorocarbene thus generated at the interface of the two-phase system (without formation of the intermediate chlorodifluoromethyl anion) reacts quickly with the hydroxy anion and/or water, instead of the alkene. 

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. 

Pyrolysis of hexafluoropropylene oxide generates difluorocarbene for cyclo-propanation reactions, as does the action of heat on chlorodifluoromethane. Halogenocarbenes formed in the thermal decomposition of polyhalogenomethanes at 500—650°C can be trapped by cyclopentene and cyclohexene. The primary adducts are unstable under these conditions, suffering dehydrohalogenation to halogeno-benzenes and cycloheptadienes, respectively. ... 

The synthesis of dichloronorcarane from cyclohexene by the chloroform-base-PTC method has been improved further as has the preparation of a-halogeno-aP-unsaturated ketones via em-dihalogenocyclopropanes by employing trimethylsilyl vinyl ethers rather than ethyl vinyl ethers. The formation of gem-difluorocyclo-propanes proceeds in high yield (60— 90%) when chlorodifluoromethane is treated with halide ion and an epoxide in the presence of an olefin. The epoxide-halide ion combination is employed to produce a base of sufficient strength, and in sufficient concentration, to maximize the production of difluorocarbene oxiran and chloro-methyloxiran afford the most suitable bases when treated with chloride ion (Scheme 4). 

Kinetic studies on the static gas-phase thermal decomposition of the halogenomethanes CHFgCI (temperature range 434—S23 °C, initial reactant pressure 100—500 Torr) and CHF (437—515 C, 20—250 Torr) have established that the initial mechanistic step is a-eliraination of HX (X = Cl or Br) leading to difluorocarbene release, a reaction that has been put to preparative use in the synthesis of gem-difiuorocyclopropylsilanes [via flow copyrolysis of vinylchlorosilanes with chlorodifluoromethane at 485— 505 C (see p. 197)]. Fluoro-carbenes, -carbenoids, or -carbenoid complexes have also featured in several other pieces of work on the synthesis of halogenocyclopropanes [for example, see Schemes 29, 30 (olefins used... 

Fluorodibromo- [19] and fluorodiiodomethane [20] each react to form a fluorohalocarbene. Chlorodiiodomethane and triiodomethane (iodoform) have likewise been subjected to phase transfer conditions and have yielded the expected car-benes [21 ]. On the other hand, chlorodifluoromethane (Freon 22) did not yield the desired difluorocarbene and it appears that no attempt to prepare either chlorobromo-... 

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