Carbenes difluorocarbene

Difluoroaminofluorodiazirine (225) extrudes nitrogen at only 75 °C. Intramolecular stabilization gives trifluoromethylenimine (226) added tetrafluoroethylene is cyclopropa-nated. This type of dichotomy is not often found in carbene chemistry alkylcarbenes undergo intramolecular stabilization as a rule, whereas intermolecular stabilization is observed exclusively with alkoxycarbonylcarbenes and with difluorocarbene. In the latter case CF2 attacks its precursor when no other reaction partner is present. 

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)... 

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). 

Difluorocarbene cannot be generated (<1%) under liquiddiquid phase-transfer catalytic conditions [29] owing to the rapid hydrolysis of the carbene at the interface [30], although it has been indicated that it is possible to obtain low yields of 1,1-difluorocyclopropanes under soliddiquid conditions [1]. More successful is the reaction of dibromomethane and dibromodifluoromethane under basic conditions. It is assumed that the initially formed dibromomethyl anion is transported into the organic phase where an equilibrium reaction with dibromodifluoromethane produces the bromodifluoromethyl anion and, subsequently, the difluorocarbene [31]. 

The additions of difluorocarbene onto the bicyclopropylidene derivative 121 (Scheme 24) [116a], of dibromocarbene onto oligospirocyclopropanated bicy-clopropylidenes 56 [33,65] (66% yield) and 62 [117] (94% yield), of dichloro-carbene onto 56 [33] (90% yield), 28a [118] (38% yield) and 121 (99% yield) 1116a] as well as of chloro(phenylthio)carbene (90%yield) [33,1191,of bromo-... 

In the photolysis of difluorodiazirine (218) a singlet carbene was also observed (65JA758). Reactions of the difluorocarbene were especially studied with partners which are too reactive to be used in the presence of conventional carbene precursors, such as molecular chlorine and iodine, dinitrogen tetroxide, nitryl chloride, carboxylic acids and sulfonic acids. Thus chlorine, trifluoroacetic acid and trifluoromethanesulfonic acid reacted with difluorodiazirine under the conditions of its photolysis to form compounds (237)-(239) (64JHC233). 

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... 

Difluorocarbene.1 This carbene can be generated from CF2Br2 by reaction with zinc in THF catalyzed by I2. This reagent should prove useful for the generation of difluorocyclopropanes. 

There is now substantial evidence that hydrogenolysis of CC12F2 on supported noble metal catalysts, which leads to mixtures of CH2F2 and CH4, occurs via a difluorocarbene intermediate [106,107,108], a pleasing parallel with one of the mechanisms proposed for CC14 fluorination [104], A carbene intermediate is pro-... 

In the presence of methyl vioiogen as a catalyst, yields of up to 100% can be obtained. Difluorocarbene can be added to benzenethiols in moderate to good yields. The carbene generation proceeds via a solid-liquid phase-transfer technique."... 

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... 

Pyrolysis of various phosphorane, e.g. (CF3)2PF2 [73], and fluoroalkylated tin compounds [74] has been used to generate difluorocarbene. Diazirines decompose to give carbenes by either photolysis or pyrolysis [75] (Figure 6.49). 

On the basis of spectroscopic and thermodynamic data, it has been concluded that TT-bonding is significant in difluorocarbene [86] and to a degree which accounts for the fact that it is surprisingly stable and relatively unreactive compared with CH2 and other carbenes [86]. That the balance between the two effects is clearly dominated by tt-bonding is illustrated by the relative reactivities of various carbenes with different alkenes it is concluded that electrophilicity decreases in the series CH2 > CBr2 > CCI2 > CFCl > CF2 and CH2 > CHF > CF2. Indeed, CF2 is considered to be amphiphilic [87]. 

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 ... 

Dihalocarbenes generated from haloforms and strong bases e.g. alkoxides) or from haloforms and alkali metal hydroxides under phase transfer conditions " or organomeicuiytrihalomethanes, add to azomethines to yield 2,2-dihaloaziridines (49 equation 30). The following carbenes have been used for such purposes dichlorocarbene, " dibromocarbene, " difluorocarbene, bromochlorocar-bene and bromofluorocarbene. The addition of dihalocarbenes to azoxybenzene or azobenzene affords dihaloaziridines e.g. 50 equation 31) among other products. 

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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