Carbenes fluorocarbenes

Substituted fluorocarbenes have been generated by several methods, but organometallic transfer reagents again are most useful synthetieally [64, 65, 66, 67, 68] (equations 25-27) Diazirines cleanly produce carbenes thermally [69, 71] or... 

In the cyclobutylfluoro system, the excited diazirine, 21-F, is considerably less involved. A parallel analysis indicates that only about 12% of 22-F and 23-F arise from excited diazirine, while 88% of those products descend from carbene 17-F.28 The increased carbene involvement in the photolysis of 21-F presumably reflects the greater stability of fluorocarbene 17-F over its chloro analogue 17-C1 and, consequently, a more favorable partition (ki/k3) of excited diazirine 21-F to the carbene. 

Attempted insertions of fluorocarbenes into Sn—Mn and Sn—Fe bonds were unsuccessful thus, the thermal reaction of (CO)sMnSnMe3 with Me3SnCF3 (a good source of carbene CF2) led instead to a disproportionation reaction involving formation of (CO)5MnSnMe2(CF3) and SnMe4344. 

Substituted fluorocarbenes, for example, arylfluorocarbenes 1, (alkoxycarbonyl)fluorocar-benes 2. fluoro(trichlorovinyl)carbene (3). fluoro(trifluoromethyl)carbenc (4). and (difluoromethyl)fluorocarbene (5), have been reported. Their addition to alkenes to give fluo-rinated cyclopropanes has been studied (see Table 2). 

Thermolysis of both F3CSiH3 and F2HCSiH3 in the presence of efficient carbene trapping agents have shown that they decompose predominantly by a-fluorine shift to give fluorosilane and the fluorocarbenes CF2 and CHF, respectively [24]. Secondary reactions at elevated temperature in the absence of any... 

The stereochemistry of reactions between carbenes and alkenes is determined by the states of the carbenes (when generated), whereby singlet carbenes react in a stereospecific one-step concerted process whilst triplet carbenes lead to a mixture of products via a diradical intermediate (Figure 6.58). Consequently, since fluorocarbenes are singlets in the ground state (Table 6.2), cyclopropanation of alkenes is often stereospecific [91] (Figure 6.59) (for more examples, see Sections A and B). 

The selectivity of carbenes has been qualitatively estimated by a series of competition reactions between various carbenes and mixtures of different alkenes it is found that electrophilic carbenes react preferentially with the most electron-rich alkene present [87, 92]. Fluorocarbenes, being less reactive, give rise to fewer products from C—H insertion reactions than CCI2 [91] (Figure 6.60). However, selectivity may be temperature-dependent [93, 94]. 

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 ... 

J-Fluorocarbenes like bis(trifluoromethyl)carbene and tnfluoromethylcar-bene by contrast are highly electrophilic, ground state tnplet species that display little selectivity m their reactions [159, 162]... 

The photolysis and pyrolysis of difluorodiazirine has been shown to involve difluoromethylene and stereospecific addition to 2-butene indicates that it is probably in a singlet state. In argon or nitrogen matrices, Cp2 has been detected by spectroscopic measurements . Products arising from the thermolysis or photolysis of fluoromethoxydiazirine, cyanofluorodiazirine, difluoroaminodiazirine and chlorofluorodiazirine have been reported the main feature of these reactions is that the intermediate carbene in all cases can be trapped by olefin reagents, whereas fluorocarbenes from other sources are quite unreactive. 

Stereospecific cycloaddition of fluorocarbene to (Z)- and ( )-4-methylpent-2-ene supports the carbene rather than the radical mechanism for this step. An isomer ratio (cisjtrans) close to unity suggests that photolysis of fluorodihalomethanes generates a free carbene. Photolysis of fluorodibromomethane in cyclohexene gave 7-fluorobicyclo[4.1.0]heptane in 9% yield. ... 

The product 4 results from the addition of (difluoromethyl)fluorocarbene to 5, which in turn is the rearrangement product of this carbene. ... 

The stereospecific addition to alkenes is consistent with (difluoromethyl)fluorocarbene being generated in the singlet state, Adducts of this carbene with alkenes were observed during the pyrolysis of trimethyl(l,l,2,2-tetrafluoroethyl)silane. ... 

Syntheses of these fluorodiazirines were published only after publication on their decomposition reactions. " As expected, thermal as well as photochemical decomposition led to the corresponding fluorocarbenes. Typical carbene reactions, such as cyclopropanation of added olefins and formation of substituted ethylenes in the absence of reaction partners, were observed. 

The Fischer carbene-complex 386 undergoes tricyclization to 387 in boiling benzene. Chlorofluorocarbene adds to alkynes much more readily than dichlorocarbene thus the sterically shielded acetylene 388, which does not react with dichlorocarbene, forms an adduct with fluorocarbene which was converted into the cyclopropenone 389 by hydrolysis in situ. ... 

Miyazaki and Yamada report that irradiation of cyclopentadiene in an argon matrix with a super-high-pressure Hg lamp results in the formation of bicyclo[2.1.0]pent-2-ene. By irradiation at shorter wavelengths the initial product is transformed into allylacetylene and vinylallene. The photochemical ring opening of the cyclohexadiene moiety within the triterpenoids (106) yields the corresponding trienes. The tricyclo[4.3.1.0]dienes (107) have been used as a source of some fluorocarbenes irradiation affords the carbenes with indane produced as the by-product. 

---