Difluorocarbene, formation

Tnmethyl(trifluororaethyl)tin can also be prepared via in situ formation and capture of tnfluorometbide by trimethyltin chlonde [13, 14] (equation 9) This tin analogue has been used as a precursor for difluorocarbene either by thermal decomposition or by reaction with sodium iodide m 1,2-dimethoxyethane This carbene generation procedure has been used to study difluorocarbene selectivity with steroidal olefins [75] (equation 10). 

Similarly, fluorosulfonyldifluoroacetic acid can be decarboxylated easily to give difluoromethanesulfonic acid by using catalytic amounts of sodium chloride, but the reaction can proceed readily to the next step, resulting in loss of sulfur dioxide and formation of difluorocarbene [101, 102] (equations 68 and 69)... 

UV irradiation (A>300nm) of an argon matrix containing tetra-fluoromethane led to the formation of difluorocarbene CF2 (Milligan and Jacox, 1968a). It was shown that the IR spectrum of this species contains three bands at 1222 (i i), 1102 (v ), and 668 (i 2)cm . Some time later difluorocarbene was stabilized in a neon matrix at 4.2 K from the gas phase after vacuum flash pyrolysis (1300°C) of perfluoroethene (Snelson, 1970b). In this case the IR bands of CF2 differed from those in an argon matrix by less than 2 cm . ... 

The metathesis reaction of fluorinated alkyl iodides with reagents such as CdMe2 is a well-established procedure for the synthesis of fluoroalkyl cadmium derivatives. A recent application of this method has allowed for the synthesis of compound 186, as shown in Scheme 22.248 This reaction is accompanied by formation of 187, which results from the insertion of the difluorocarbene in the methyl-cadmium bond. Presumably, the difluorocarbene is generated by a radicalar process involving HCF2I and CdMe2. 

Dipolar cycloaddition is another route to benzopyrrolo[l,2-a]azepines by pyrrole ring formation. The azomethine ylide derived from imine 88 and difluorocarbene adds to DMAD to produce dimethyl 3-fluoro-9H-dibenzo[c,/]-pyrrolo[l,2-fl]azepine-l,2-dicarboxylate 89 in 20% yield (Equation (12)... 

It was shown later that this reaction, and also pyrolyses of tetrafluoroethene (3) and of PTFE, can be adapted36 to give hexafluoropropene and also perfluoro(2-methylpropene), the key stage being formation of difluorocarbene. 

A similar method involves the reaction of difluorocarbene on enones462 or on Mannich bases (205) with formation of furosteroids (206).463... 

One of the best known reactions of dialkyl cadmium compounds is that with acyl halides which results in the formation of ketones. The reactions of (CF3)2Cd glyme with CH, COBr and C6HhCOC1 were the third class of reactions of the trifluoromethylating reagent examined. However, the mixed trifluoromethylalkyl or -aryl ketones were not observed. The organic products obtained were the acyl fluorides (95% yield) along with difluorocarbene. The latter could be stereospecifically trapped at temperatures as low as -78°C. The addition of either lithium or magnesium halides to the reaction mixture did not affect the outcome (106). 

Another possible mechanism is formation of pentafluorophenoxide anion that combines with difluorocarbene, and the intermediate anion is rapidly protonated. 

Sonoyama, N., Ezaki, K., Fujii, H. and Sakata, T. (2002) Electrochemical conversion of CFC-12 to tetrafluoroethylene Electrochemical formation of difluorocarbene. Electrochim. Acta, 47, 3847-3851. 

Difluorocarbene converts aldehydes and ketones into ge/n-difluoroalkanes 1 (Table 5). The proposed mechanism involves formation of a carbonyl ylide, which gives the difluoro compound by loss of carbon monoxide. 

The extremely electrophilic nature of bis(trifluoromethyl)carbene has already been illustrated by the formation of addition products, even with hexafluorobenzene [83] (Section lllB), and the high reactivity results in more side-reactions, such as insertion into C—H bonds, than with difluorocarbene (Figure 6.62). 

Reaction with difluorocarbene leads to the formation of novel cyclopropene and bicyclobutane systems [323] (Figure 7.93), and similar reactions are observed using polyfluoroalkyne derivatives of some metals [324]. 

Addition of difluorocarbene to perfluorobenzene has been proposed to account for the formation of perfluorotoluene and other perfluoromethylbenzenes in the pyrolysis of perfluorobenzene with potassium fluoride, or with polytetrafluoroethene as a difluorocarbene source [184, 185]. Indeed, Russian workers have studied the pyrolysis and... 

Elimination of difluorocarbene from a o-complex 9.79A has been proposed, although the fate of the rest of the molecule is not clear, and the addition could involve either an insertion reaction (a) or formation of a tropylidene (b) (Figure 9.79). 

More direct evidence for the formation of difluorocarbene as an intermediate comes from the photolysis of difluorodiazirine in the presence of chlorine, iodine, dinitrogen tetroxide, and nitryl chloride when the corresponding difluoromethane derivatives were formed, and in the presence of isobutene, buta-l,3-diene, and cis- and frans-but-2-ene when the corresponding 1,1-difluorocyclopropane derivatives were formed. Even with considerable excesses of olefins, appreciable tetra-... 

Trifluoroacetic acid at 300-390 °C produces mainly carbon dioxide, difluoro-methyl trifluoroacetate, carbon monoxide and trifluoroacetyl fluoride. Blake and Pritchard propose that the decomposition proceeds through the elimination of hydrogen fluoride, followed by the formation of difluorocarbene which largely adds to trifluoroacetic acid to form the difluoromethyl ester. The kinetic order is about 0.5 and the overall activation energy for the formation of carbon dioxide and the difluoromethyl ester is about 45 kcal.mole" ... 

The imine reaction with carbenes is also efficient when the carbonyl group participating in oxazoline ring formation is incorporated in an imine, rather than a carbene, component. This is exemplified by a reaction of dimethoxycarbene with acylimines, resulting in good yields of substituted oxazoles 545, as well as by a reaction of difluorocarbene with imine 13, affording oxazoline 14 in 62% yield (79IZV1826). 

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