Carbenes from phase-transfer catalysis

Treatment of the title compound with an aqueous base under phase-transfer catalysis conditions generates, by a-elimination, chloro(trifluoromethyl)carbene that can be trapped by cycloaddition to allylsilane and allylstannane derivatives, e.g. formation of 1 and 2. This method represents a fast and mild alternative to the Seyferth method that starts from the same alkyl halide and generates the carbene by thermal decomposition of l-bromo-l,2,2,2-tetra-fluoroethyl(phenyl)mercury (see Section 1.2.1.2.4.1.1). 

Unsaturated compounds possessing an allylic C-H bond(s) are prone to insertion of dichlorocarbene, aside from the addition of dichlorocarbene to the double bond. This reaction occurs, in particular, more frequently if the dichlorocarbene is generated from chloroform by phase-transfer catalysis or from dichlorohalomethyl(phenyl)mercury. Thus, careful investigation reveals that, in contrast to earlier findings, 9,10-octalin forms both carbene addition 1 and insertion 2 products in comparable yields." ... 

The synthesis of 1,1-diiodocyclopropane is carried out via addition of diiodocarbene (car-benoid) to alkenes the carbene, in turn, is generated from iodoform and a base. The process is realized using phase-transfer catalysis,(33% or 50%aqueous sodium hydroxide and a quaternary ammonium salt, typically benzyltriethylammonium chloride, as a catalyst) or in the presence of potassium ter/-butoxide in /ert-butyl alcohol 25,127,128 g... 

Phenylthiocarbene, generated by phase-transfer catalysis from (phenylthio)chloromethane adds stereospecifically to ( )-1,2-diphenylethene to provide, 2-trans-2,3-cis-, 2-diphenyl-3-(phenylthio)cyclopropane1. The simple diastereoselectivity of reactions with olefins such as cyclohexene and phenylethene is moderate to good, favoring the endo- or cis-products. For further examples of sulfur-substituted carbenes, see Houben-Weyl Vol. E 19b, pp 1682-1745. 

If a protected fluoromalonaldehyde is used as the dicarbonyl component, direct access to unsubstituted 4-fluoropyrazole is realized. This approach was made more efficient by the development of a new synthesis of protected fluoromalonaldehyde using chlorofluorocarbene chemistry. Reaction of the carbene, generated from dichlorofiuoromethane by phase-transfer catalysis (PTC), with dihydropyran followed by acid-catalyzed ethanolysis gave a mixture of acetals that could be directly used for condensation with hydrazine to give 4-fluoropyrazole 119 in good yield (Fig. 3.67). ... 

Nowadays, phase-transfer catalysis has proved to be particularly effective in the generation of carbenes by a-ehmination. The halomethyl anion is transported as an ion-pair with a tetraalkylammonium ion, or a crown ether-complexed alkah-metal ion, from a strongly basic aqueous solution into an organic phase, where ehmination takes place and the resulting carbene reacts with the substrate in organic phase to give halocyclopropanes (Scheme 5.11). 

Reactions of carbenes other than cyclopropanation can also be performed, and recent examples include the ring expansion of five-ring heterocycles, such as the indoles (18), to their six-membered counterparts (19), and the production of formamides from secondary amines (Scheme 7), both with dichlorocarbene. The latter method is of interest because of its relation to the catalysis of dichlorocarbene generation by tertiary amines in two-phase systems. Recent work indicates that such catalysis is possible because the carbene, after generation at the phase boundary, is transferred into the organic phase (to undergo reaction) in the form of the N-ylid adduct (20). 

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