Carbene-trapping agents

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 photolysis of phenyldiazirines [53a]-[53c] in an argon matrix was studied in a similar way. Intense bands of the corresponding carbenes [54a]-[54c] (Table 2) have been observed in all cases. The bands disappeared either when the matrix temperature was raised from 12 to 40-45 K (the corresponding stilbene bands appeared at the same time), or when a trapping agent (HCl, CI2) was doped into the matrix (Mal tsev et al., 1985b, 1987a). 

The yield of trans product (18) is decreased by the presence of a radical scavenger such as 1,1-diphenylethylene and increased by dilution of the reactants with methylene chloride or butane, indicating this product to result from the triplet carbene. A heavy-atom effect on the carbene intermediate was observed by photolysis of a-methylmercuridiazoacetonitrile. With c/s-2-butene as the trapping agent either direct photolysis or triplet benzophenone-sensitized decomposition results in formation of cyclopropanes (19) and (20) in a 1 1 ratio ... 

The initial photochemical step in almost all of the reactions described in this chapter is formation of either trivalent radicals of the type R3E-, or else the divalent analogues of carbenes, R2E . Such species are obviously very reactive, and are only observed as intermediates or in experiments in the presence of trapping agents. The relative stability of the intermediates depends greatly on the nature of the substituents R, and this can influence the type of reaction products ultimately formed. Where appropriate, comparisons with the behaviour of the analogous silicon species are made. 

Miscellaneous. An interesting synthesis of 1,1-difluoro-l-alkenes from ylides and chlorodifluoromethane has been described.47 The ylide acts both as a carbene generator and trapping agent (Scheme 11). 

Thermolysis of aryl chloro diazirine (18) in the presence of acetone and a trapping agent such as A -phenylmaleimide gave rise to cycloadducts such as 41. The unstable adduct hydrolyzed during purification resulting in synthesis of bicyclic hemiacetals 42 and 43 as a mixture of endo and exo adducts in 37 and 8% yield, respectively. The exclusive generation of the singlet carbene was confirmed by low-temperature electron spin resonance (ESR) study of the irradiated diazirine. 

Pyridines are observed in some of these reactions, and the substituent pattern is that expected from nitrene-carbene interconversion Small yields of azepine have been recorded in the photolysis of 158 i ). The bicyclic intermediate 44 [Eq. (64)] has been postulated frequently over the last twenty years. A wavelength dependence of the formation of azepines has been observed i ) the yield of azepine increases with the energy of the light and with the concentration of the trapping agent (diethylamine). At the same time, the yields of competitive processes of the nitrene (carbazole formation from 2-biphenylylnitrene p-cyanophenyl-hydrazine from p-cyanophenylnitrene) decrease. The rate of the azepine-forming... 

Photoexcitation of W(CO)5 CPh2 appears to yield diphenylcarbene as this has been trapped by diethyl fumarate. In the absence of trapping agents PhjC CPhj is formed, probably by reaction of the carbene with W(CO)5CPh2. 

A review has been published in Japanese dealing with low-temperature matrix isolation techniques applied to carbene systems. Low-temperature studies have examined the trapping of carbene with a variety of trapping agents. Thus with oxygen benzophenone O-oxide has been observed spectroscopically when diphenyldiazomethane is irradiated at 515 nm . Interestingly, phenyldiazomethane does not react similarly. However, other carbenes do react in a similar fhshion to diphenylcarbene and the ylide 546 is observed on irradiation of diazocyclopentadiene. This study has shown that the ylide is accompanied by the isomeric dioxirane 547. The related ylides 548 and 549 have also been observed... 

More recently, the reductive activation of halothane (CFaCHBrCl), which is a hepatotoxic anaesthetic molecule, by human hemoglobin results in the modification of the prosthetic heme [57]. The inhibition of the reaction by adding exogeneous CO or the spin trapping agent N-t-butyl-a-phenyl nitrone to the incubation mixture indicated that (i) a reduced and free heme iron is required by Hb to activate the halogenated substrate and (ii) the formation of free radical species is responsible for Hb inactivation. However, no carbene species were detected in these reactions. The mechanism is shown in Scheme 7. 

This method is often utilized for preparation of carbene compounds which do not contain heteroatoms. Trityl salts and Me3P = CH2 are utilized as hydrogen atom trapping agents ... 

The intermediacy of silylenes, equivalent to carbenes, is suggested from hindered dichlorosilanes, e.g., dimesityldichlorosilane.2i6 Their sonochemical dimerization gives tetramesitylsilene in ca, 90% yield instead of 20% electro-chemically. The reaction of di-f-butyldichlorosilane, in the presence of triethyl-silane as a trapping agent, gives mainly the symmetrical disilane with sodium or potassium but also the product of insertion into the Si-H bond with lithium, in 85% yield. In the presence of an olefin, the intermediate is trapped to give silacyclopropanes stereoselectively.2i7 Sonication is essential for success. 

Conversion of C—Cl bonds into C—F requires more strenuous conditions, so conversion of PhHg CClg into PhHg-CFa entails heating the reaction mixture at 90 °C. Phenyl(trifluoromethyl)mercury resists thermal decomposition into phenylmercuric fluoride and difluorocarbene, but serves as an excellent source of the carbene when treated with sodium iodide in benzene or an excess of trapping agent ... 

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