Chloro carbene generation

The rearrangement of metal carbenes can also provide halodienes in a very elegant manner. For instance, Fischer chromium chloro carbenes, generated from CrCl2 and a trihalomethyl group borne by the substrate, were shown to evolve under microwave irradiation to provide stereoselectively (Z, )-l-halo-l,3-dienol esters in... 

The first synthesis of a cyclopropenone was reported in 1959 by Breslowls who achieved the preparation of diphenyl cyclopropenone (11) by reacting phenyl ketene dimethylacetal with benzal chloride/K-tert.-butoxide. The phenyl chloro carbene primarily generated adds to the electron-rich ketene acetal double bond to form the chlorocyclopropanone ketal 9, which undergoes 0-elimination of HC1 to diphenyl cyclopropenone ketal 10. Final hydrolysis yields 11 as a well-defined compound which is stable up to the melting point (120—121 °C). 

An alternative method for in situ carbene generation starts from methanol, chloro-form or pentafluorobenzene adducts that allow the release of the free carbenes at elevated... 

CARBENES, GENERATION Benzyltrieth-ylammonium chloride. Dehydro-N,N, N -tricyclohexylguanidinohexacarbonyldi-iron(O). Ethylene oxide. Ethyl trichloro-acetate. Phenyl(l-bromo-l-chloro-2,2,2-trifluoroethyI)mercury. Phenyl(trihalo-methyl)mercury. Trimethyl-silyldiazo-methane. 

Whether a cyclopropanation reaction is stereospecific or not, is often tested with (Z)- and ( )-but-2-ene, but of course, this problem also holds for other diastereomeric alkenes. The following carbenes, generated by direct irradiation of the corresponding a-diazocarbonyl compound, undergo stereospecific or highly stereoselective cycloaddition to (Z)- and ( )-but-2-ene methoxycarbonylcarbene, chloro-, bromo- and iodo(ethoxycarbonyl)carbene, ° eth-oxycarbonyl(trimethylsilyl)carbene, ethoxycarbonyl(trimethylgermyl)carbene, ethoxy-... 

Chloro(methyl)carbene, and chloro(trifluoromethyl)carbene, generated from the corresponding diazirines, add stereospecifically to (Z)- and ( )-but-2-ene, e.g. formation of 17. 

Chloro(cyclopentyl)carbene, generated photolytically from the corresponding diazirine, isomer-izes via hydride rather than carbon shift. ... 

Enamines derived from cyclohexanone and and a-tetralones react with chloro-(phenyl)carbene [generated from dichloro(phenyl)methane and potassium tert-butoxide] to give either 1-chloro-l-phenyl-substituted cyclopropanes, or other products.The type of products formed depends on the structure of the amine moiety in the starting enamines (Table 7). 

Addition of chloro(phenyl)carbene, generated from diazirine, to alkenes is stereospecific, yet diethyl (Z)-but-2-enedioate isomerized partially to give a mixture of stereoisomeric cyclopropanes(for a more detailed discussion see Houben-Weyl, Vol. E19b, p995). 

A sequence of carbene additions to methylenetriangulanes can be used to build branched triangulanes. i - 3 3 When chloro(methyl)carbene generated from 1,1 -dichloroethane with butyl-lithium is used, new exocyclic double bonds for further cyclopropanation reactions can be generated by HCl elimination. For the same purpose l,l-dichloro-3-methoxypropane... 

Carbene generation Alumina—Potassium hydroxide. Bis(tribromomethyl)mercury. Bis (trimethylsilyldichloromethyl)mercury. Bromotrifluoromethane. n-Butyllithium. Copper (II) acetylacetonate. Diazoacetaldehyde. Di-n-chloro-x-allyldipalladium. Dichloromethyl 2-chloroethyl ether. Dicyanodiazomethane. sym-Difluorotetrachloroacetone. Ethylene oxide. Methyl dichlorofluoroacetate. Methyllithium. Phenyl (trichloromethyl)mercury. Sodium chlorodifluoroacetate. Sodium trichloroacetate. Thallous ethoxide. / -Toluene-sulfonyl-hydrazine. 

Addition versus rearrangement of chloro(methyl)-, chloro(ethyl)-, benzyl(chloro)-, chlo-ro(methoxymethyl)- and chloro(isopropyl)carbene generated by photolytic or thermal decomposition of 3-alkyl-3-chloro-3//-diazirines has also been investigated.81 However, it is known that chloro(isopropyl)- or ie/7-butylchlorocarbene does not undergo addition to 2,3-dimethyl-but-2-ene, which is used as a carbene trap. Photolysis or thermolysis of 3-arylmethyl-3-chloro-3//-diazirines in the presence of an alkene has also been studied extensively.84 Addition of the ambiphilic benzylchlorocarbene82 to an alkene to give 1-benzyl-1-chlorocyclopropanes is usually accompanied by its rearrangement to co-chlorostyrene (1,2-hydride shift, for details see ref 83). This isomerization was seriously impeded when the decomposition of 3-benzyl-3-chloro-3//-diazirines was performed in the presence of an alkene,85 with 2,3-dimethylbut-2-ene only traces of co-chlorostyrene were observed.81,86 The results of reactions of benzylchlorocar-benes with some alkenes are collected in Table 6.85... 

Methoxy (4-mcthylphenylsulfonyl)carbene, generated by phase-transfer catalyzed a-elimina-tion from the chloro precursor, adds with excellent diastereoselectivity lo enol ethers to afford the corresponding cyclopropanes4. 

Dimethylvinylidene carbene generated from 3-chloro-3-methylbut-l-yne using aqueous KOH and a phase-transfer catalyst, has been added to cis,cis-cyclo-octa-l,5-diene to give the mono-adduct. ... 

Dimethylvinylidine carbene (52) has been shown to add to indole-3-carbaldehyde (53) to mainly yield /3-(dihydroprenyl)indole (54). Aryl- and heteroaryl-chloro-carbenes (55) have been shown to add to 1-azabuta-1,3-dienes via a formal [4- -1]-cycloaddition, involving a transient azomethine ylide (56) giving rise to the corresponding pyrrole (57). A formal [4 -1- l]-cycloaddition is also observed during the addition of a triplet carbene (dicarboethoxycarbene generated with a triplet sensitizer) to cyclooctatetraene which leads to bicyclo[4.2.1]nonatriene (58). ... 

Methylthio(chloro)carbene has been generated under phase transfer conditions from dichloromethyl methyl sulfide. This carbene cyclopropanates tetramethyl-ethylene in 43%yield, accompanied by a 24%yield of l,2-dichloro-l,2-Z /5-methyl-thioethylene [24]. 

Transition metal-catalysed carbene abstraction has been reported once, whereby a-oxo Au carbenes generated on oxidation of terminal alkynes are prone to abstract halogen from solvents such as 1,2-dichloroethane and 1,2-dibromoethane. This method constitutes an attractive approach for preparing chloro- or bromo-methyl ketones in one step starting from terminal alkynes. 

Due to the complication of alternate pathways involved in the photochemistry of diazo compounds or diazirines, new strategies for carbene generation from non-nitrogenous precursors have been studied. Phenyliodonium ylides" (27) andmethyl-8-chloro-3a,7a-methanoindan-8-carboxylates" (28) are alternative precursors to dicarboethoxycarbene and carbomethoxychlorocarbene, respectively. Photoreactions from these precursors presumably will not be plagued by the excited-state chemistry observed for the diazo compounds and diazirines. 

Chloro(phenyl)carbene or carbenoid generated from benzal chloride reacts with potassium salts of benzylic, allylic, and other alkoxides to produce phenyl-substituted oxiranes 9 in high yields, as an approximately 1 1 mixture of cis... 

Chloro(methylthio)carbene is generated from dichloro(methylthio)methane... 

Generation and reaction of chloro(phenylthio)carbene and phenylthiocarbene... 

Generation and reaction of chloro(methylthio)carbene Procedure 7.1.15 is used replacing the PhSCHCl2 with MeSCHCl2 (5.24 g, 40 mmol). 

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. 

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