Insertion of difluorocarbene

Copyrolysis of 1,1-dichloroperfluoroindane and chlorodifluoromethane or tetrafluoroethylene gives 1-perfluoromethyleneindane as the major product and three minor products [3] (equation 2) Insertion of difluorocarbene into the benzylic carbon-chlorine bond and subsequent loss of a chlorine 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). 

A remarkable double insertion of difluorocarbene, derived from trifluoromethyl copper, has been reported [227] (Figure 9.87). 

Diethyl 1-bromo-l.l-difluoromethylphosphonate has also been obtained in 92% yield by treating (bromodillnoromethyl)triphenylphosphonium bromide with excess triethyl phosphite in CI bC b at room temperatnre for 10 min. The proposed mechanism involves an insertion of difluorocarbene into bromotriethoxyphosphonium bromide. ""... 

In order to test this effect, difluoromethyl perfluoroalkanesulfonates have been synthesized and investigated. Difluoromethyl triflate was synthesized earlier from the insertion of difluorocarbene, generated by photolysis of difluorodiazirine into the O—H bond of triflic acid114 (equation 94). By using HCF2S03H as a difluorocarbene precursor, difluoromethyl perfluoroalkanesulfonate can be obtained in 30-50% yields115 (equation 95). 

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. 

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>. 

The reaction proceeds via difluorocarbene elimination from the CF3C group and insertion of the formed CF2 into a C — F bond. 

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). 

Caulton and Eisenstein have examined the interconversion of metal-alkyl, metal-alkylidene and metal-alkylidyne species using combined experimental and DFT theoretical approaches. This useful strategy has yielded examples of such interconversions. For instance, in the first reaction below, McsSiCFs is a source of nucleophilic CF3 ligand. It is proposed that the transient Ru-CFs bond does form and further rearranges by a-F elimination producing the difluorocarbene-metal-fluoride product. The latter reacts with CO which provokes the reverse reaction, insertion of the carbene into the Ru-F bond leading to the Ru-CFs product. ... 

Difluorocarbene generated by the thermolysis of trimethyltnfluoromethylsilane reacts with disilanes by insertion into the silicon-silicon bond [S] (equation 9) Thermolysis of pentafluoroethyltnfluorosilane at 200 °C gives tetrafluoro ethylidene carbene, which reacts with phosphorus trifluonde to give trifluoro vinyltetrafluorophosphorane [9] (equation 10) and with perfluorotnmethylphos-phine to give perfluorodimethyhsopropylphosphine and perfluoro-2-butene [9] (equation 10)... 

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. 

Difluorocarbene generated by the thermolysis of trimethyltnfluoromethylsilane reacts with disilanes by insertion into the silicon-silicon bond [S] (equation 9)... 

The attempted trifluoromethylation of pentamethylcyclopentadienyllithium by reaction with F3 Cl in the presence of A-phenylmaleimide gave rise to the unexpected Diels-Alder adduct (25).37 The authors proposed that the cyclopentadienyl anion captures the iodine of CF3I to give (26) and trifhjoromethyllithium which then produces difluorocarbene (F2C ) which inserts into the C—I bond of (26) to form the difluoroiodomethylated product. The observed regioselectivity of addition of thiazyl chloride (N=S—Cl) to 2,5-diarylftirans provided support for a mechanism involving carbene and nitrene intermediates (27).38... 

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 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). 

IS the most popular, one-step method for m ag fluorinated cyclopropanes and cyclopropenes a-Fluorocarbenes are particularly well behaved, because they all have singlet ground states [/, 2] and therefore usually add stereospecifically to alkenes and do not insert into C-H bonds competitively with addition Moreover, quantitative competition studies of carbene additions to alkenes near room temperature show that a-fluorocarbenes are more selective than other a-halocarbenes, with difluorocarbene being the most selective electrophihc carbene known [3, 4] The relative selectivities, however, can be quite temperature dependent [5, d] The numerous preparations and cycloaddmons of fluorocarbenes have been reviewed thoroughly [7, 8 9,10 ... 

Scheme 4.2 Technical synthesis of tetrafluoroethylene by thermal fragmentation of CHCIF2. In a side reaction difluorocarbene is inserted...

Carbenes,—Attempts to insert Cp2 into several metal-metal bonds failed, because the difluorocarbene precursor MeaSnCCFg) underwent exchange reactions at one of the metals, e.g. 

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