Sodium trichloroacetate, dichlorocarbene generation

Dichlorocarbene can be generated by heating sodium trichloroacetate. Propose a mechanism for the reaction, and use curved arrows to indicate the movement of electrons in each step. What relationship does your mechanism bear to the base-induced elimination of HC1 from chloroform ... 

Alternative procedures for the generation of dichlorocarbene and dibromocarbene under phase-transfer catalysed conditions are also available. Where the reactive substrate is labile under basic conditions, the thermal decomposition of solid sodium trichloroacetate or bromoacetate under neutral conditions in an organic solvent is a valuable procedure [10-12], The decarboxylation is aided by the addition of a quaternary ammonium salt, which not only promotes dissolution of the trihaloacetate anion in the organic solvent, but also stabilizes the trihalomethyl anion. Under optimum reaction conditions, only a catalytic amount of the quaternary ammonium salt is required, as a large amount of the catalyst causes the rapid generation of the dichlorocarbene with resultant side reactions. 

Phase-transfer catalysed reaction of dichlorocarbene generated from sodium trichloroacetate... 

For example, methyl 3-phenylbicyclofl.l.0]butane-l-carboxylate (11, R1 = Ph R2 = C02Me) was converted to methyl 2,2-dichloro-3-phenylbicyclo[l.l.l]pentane-l-carboxylate (12, R1 = Ph R2 = C02Me) in 46% yield on reaction with dichlorocarbene generated by thermolysis of sodium trichloroacetate. 3,14... 

The dichloromethyl derivative 20 is isolated (10%) from 3,4,5-trimethyI-pyrazole and chloroform in the presence of sodium ethoxide generation of dichlorocarbene by thermolysis of sodium trichloroacetate does not give any 4/Z-pyrazole.30 The same product is formed, in lower yield, using chloroform and base with a phase-transfer catalyst.31... 

The reaction of dihydrobenzothiazepines (1) with dichlorocarbene, generated in situ from chloroform using a phase-transfer catalyst or by thermal decomposition of sodium trichloroacetate, afforded compounds 2 in low yields (18-24%) (Scheme 1). The structure of 2 was postulated on the basis of analytical and spectroscopic data and confirmed by X-ray diffraction (92MI1). 

There are a dozen or so ways by which dichlorocarbene may be generated. In this experiment thermal decomposition of anhydrous sodium trichloroacetate in an aprotic solvent in the presence of cis,cis-l,5-cy-clooctadiene generates dichlorocarbene to give 5,5,10,10-tetrachloro-tricyclo[7.1.0.0 ]decane (1). 

Tetrachloroethylene, a nonflammable solvent widely used in the dry cleaning industry, boils at 121°C and is relatively inert toward electrophilic dichlorocarbene. On generation of dichlorocarbene from either chloroform or sodium trichloroacetate in the presence of tetrachloroethylene, the yield of hexachlorocyclopropane (mp 104°C) is only 0.2-10% (W. R. Moore, 1963 E. K. Fields, 1963). The first ideafor simplifying the procedure was... 

Surprisingly, with dichlorocarbene generated by another method (ther-mocatalytic decomposition of sodium trichloroacetate instead of alkaline hydrolysis of chloroform under phase-transfer catalysis conditions) N-(p-R-benzylidene)-feH-butylamines 55 (R = H, Cl) give 3,3-dichloroazetidi-... 

A comparison of olefin reactivities toward CeHjHgCBrCt in benzene at 80° with the reactivities of the same olefins toward sodium trichloroacetate in 1,2-dimethoxy-ethane at 80° established near identity of the relative reactivities toward both reagents, a result which favors the interpretation that both reactions involve free dichlorocarbene as an intermediate. Of practical significance is the fact that yields are consistently higher by the mercurial route. Thus the latter route proved effective as applied to olefins of low reactivity toward dihalocarbenes generated by other procedures. Examples are formulated ... 

The addition of dichlorocarbene, generated by thermolysis of sodium trichloroacetate, to 7-methylene- and 7-isopropylidenenorbornenes is preferentially syn selective when the norbor-nene contains electron-withdrawing c /o-substituents. ... 

A mixture of the monoadducts 14 and 15, one of which rearranged, was formed from 1-phenyl-buta-1,2-diene and dichlorocarbene generated from ethyl trichloroacetate/sodium methoxide. Under the conditions of the chloroform/base/phase-transfer catalyst or thermolysis of sodium trichloroacetate methods, dichlorocarbene reacts further with the rearranged product to give 16. ... 

Using bromodichloromethyl(phenyl)mercury, vinyl acetate afforded 2-acetoxy-l,l-dichlo-rocyclopropane (1, 85%), dichlorocyclopropanation of other aldehyde enol esters would also be expected. The cyclopropane 1 ( 10%) together with 2-acetoxy-l,l,l-trichloropropane (2,10%) were formed when the dichlorocarbene was generated from sodium trichloroacetate, the chain product 2 results from the reaction of the trichloromethyl anion (for the mechanism, see ref 197). These reactions are described in Houben-Weyl, Vol. 4/3, pp 177-178. Under phase-transfer catalytic conditions (CHClj/base/PTC), with a typical catalyst such as benzyl-triethylammonium chloride, vinyl acetate gave 2 (65%) only (Houben-Weyl, Vol.E19b, ppl550-1551). 

Esters of cyclohex-1-enecarboxylie acids underwent addition of dichlorocarbene, generated by the ethyl trichloroacetate/sodium methoxide or bromodichloromethyl(phenyl)mercury... 

Esters of, y-unsaturated acids did not isomerize to conjugated isomers when they reacted with the dichlorocarbene generated from sodium trichloroacetate, e.g. cyclopropane 13. ... 

Nogami s group (Tsuda et al., 1993) showed that dichlorocarbene, generated by pyrolysis of sodium trichloroacetate in a mixture of benzene and 1,2-dimethoxy-ethane, reacts with C o to give the [6,6]-closed methanofullerene, as shown by negative ion fast atom bombardment mass spectrometry (FAB MS) and by NMR spectroscopy. 

The reactivity of dichlorocarbene towards carbon-carbon triple bonds was systematically investigated by Dehmlov in the sixties, and this work has been reviewed". Dehmlov generated the carbene e.g. by decomposition of sodium trichloroacetate in 1,2-dime thoxy ethane. In additions to ene-yne substrates, sometimes the triple bond, whereas in other cases the double bond accepted the carbene. He observed, for instance, that reaction of the carbene with (E)-l,4-diphenylbutene-l-yne-3 gave the triple-bond addition product 20 exclusively. In reaction with 2-methylpentene-l-yne-3, however, the carbene preferred the double bond, leading to 21. Dichlorocarbene generated from potassium f-butylate and chloroform does not add to electron-poor alkynes, indicating electrophilic behavour of the carbene. 

Method using Base and a Substituted Halogenomethane. The influence of catalyst anions (as their tetrabutylammonium salts) and cations (as chlorides or bromides) on the generation of dichlorocarbene from chloroform-sodium hydroxide has been studied under standard conditions by determining the yield of dichloronorcarane produced from the addition of the carbene to cyclohexene. The presence of olefin appears to be necessary since in its absence only very slow decomposition of the trichloromethyl anion occurs. Dehmlow has also devised a new procedure for phase-transfer-catalysed cyclopropanation. Treatment of an alkene (or cycloalkene) with sodium trichloroacetate and a tetra-alkylammonium salt in chloroform without... 

The convenience and potential of this method was immediately apparent and its disclosure was followed by numerous examples and extensions. The method was of particular interest because previous methods used to generate dichlorocarbene all required rigorous exclusion of moisture. Among these methods are treatment of chloroform with potassium -butoxide in pentane, pyrolysis of anhydrous sodium trichloroacetate, and the thermal decomposition of phenyl(bromodichloromethyl)-mercury. 

Solid-Liquid Phase Transfer.—Very few examples are known in onium ion chemistry where the anion to be phase transferred is part of a solid phase rather than in aqueous solution. This contrasts with the situation in crown ether chemistry, as will be discussed later in this review. A useful method, which is cheaper than alternatives, for the generation of dichlorocarbene in neutral conditions involves warming solid sodium trichloroacetate in chloroform, in the presence of quaternary salts. Formaldehyde acetals (25) can be synthesized in good yields by the action of solid KOH on alcohols or phenols dissolved in methylene dibromide containing a quaternary ammonium ion, and a phosphonium salt catalyses the alkylation of solid potassium phthalimide in toluene. Solid-liquid phase transfer mediated by polyamines has been reported. ... 

The mechanism of the addition of dichlorocarbene, generated from sodium methoxide and ethyl trichloroacetate, to the olefins (42) has been investigated. ... 

An unusual outcome of addition of dichlorocarbene to olefins has been the direct synthesis of cyclopropanone acetals. When dichlorocarbene is generated from ethyl trichloroacetate by sodium alkoxide in a heptane-alcohol medium, carbene addition and solvolysis apparently proceed to a cyclopropanone acetal subsequent acid hydrolysis leads to the corresponding cyclopropanone. This same report claims the synthesis of the dichlorovinylcyclopropanone (68) without loss of the original chloro-substituents. 

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