Trihalomethyl anions

Dihalocarbenes are formed when trihalomethanes are treated with a strong base such as potassium tert butoxide The trihalomethyl anion produced on proton abstraction dissociates to a dihalocarbene and a halide anion... 

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. 

Not surprisingly, electron-poor systems react with chloroform, bromoform and other haloalkanes under basic conditions by Michael-type addition of the trihalomethyl anion to the C=C bond. However, a subsequent base-catalysed ring closure to give the cyclopropane derivatives frequently occurs [e.g., 6,7,26,31,39,72, 84,93,111, 113, 115, 118]. 

Not surprisingly, when the aromaticity of thiophene is destroyed by the formation of the, S,S-dioxidc, the ring reacts readily with chloroform and bromoform under basic catalytic conditions. However, instead of forming the insertion adduct, the major product isolated in each case is the 3-(dihalomethylene)-2,3-dihydrothio-phcnc-,S S-dioxidc via initial reaction with the trihalomethyl anions [15]. 

One-pot synthesis of thiophene S,S-dioxide and reaction with trihalomethyl anions... 

This result is due to specific properties of the phase-transfer catalytic system. Both trihalomethyl anions as well as halide ions form, with the quaternary ammonium cation, lipophilic ion pairs which are readily soluble in the organic phase. The generation of dihalocarbenes is in fact a reversible process and, if the starting haloform contains different halogen atoms, exchange reactions afford all possible dihalocarbenes (Houben-Weyl, Vol. E19b, pp 1591-1592). 

Such equilibrations take place in spite of the relative order of leaving trihalomethyl anions by halogen Br > I > Cl. ... 

It should be pointed out that the carbon tetrahalides are difficult to rationalize via this mechanistic proposal. A conceivable but unprecedented possibility would be the reaction of the haloform-produced trihalomethyl anion an enzymatically produced halonium ion (X ) or its equivalent. 

The loss of halide ion appears to add driving force to the proton removal. Thus it is possible to go directly to what is probably the most stable of the di-halocarbenes and by-pass what would be a relatively unstable trihalomethyl anion . The kinetic isotope effect observed with CDBrF2 k jki, = 2.Ql ... 

M Fedoryhski. Generation and some reactions of trihalomethyl anions—dihalocarbene precursors. PhD dissertation, Warsaw University of Technology, Poland, 1974. 

An exception to this generalization is the elimination of trihalomethyl anion in the haloform reaction (Figure 7.10). 

The haloform reaction (Section 18.3C) is one of the rare instances in which an alkanide anion can act as a leaving group, but then only, as we shall see, because the leaving group is a weakly basic trihalomethyl anion.]... 

The EGB derived from 2-pyrrolidone promotes (Eq. 27) [18, 19]. The trifluoromethylation of the formation of trihalomethyl anions from trihalo- carbonyl compounds is remarkably useful for the methane and their addition to carbonyl compounds synthesis of fluorine containing compounds. 

Haloform reaction (Section 19.4) The conversion of a methyl ketone into a molecule of a carboxylic acid and a molecule of haloform.The trihalo carbonyl compound is formed, base adds to the carbonyl group, and the trihalomethyl anion is... 

Nitrothiophene readily undergoes dihalomethylation using chloroform or bromoform in basic medium via the VNS mechanism [170]. The actual nucleophile in this reaction is the trihalomethyl anion. Either 3-dichloromethyl- and 3-dibromomethyl-2-nitrothiophenes are obtained [170] (hydrolysis of 3-dichloromethyl-2-nitrothiophene with boiling formic acid produces the corresponding aldehyde) (Scheme 110) [170]. 

Despite the many advantages of the phase transfer method for generating dichlorocarbene, it should be noted that the reactive species arises from the trichloromethide anion as it does in most other methods. Because of this, attempted reactions with electron poor olefins will yield products arising from Michael addition of ClaC" to the olefin instead of, or in addition to, cyclopropanation products. The thermal decomposition of trihalomethyl metal compounds remains the unique method for generating dihalocarbenes without prior formation of a trihalomethyl anion [8]. 

In analogy to the previously reported addition of trihalomethyl anions to enol acetates (see Sect. 2.11 and Eq. 2.19), it is shown that a number of carbanions will likewise add to the double bond of such systems. Several substituted phenylacetoni-triles add to vinyl acetate under phase transfer catalytic conditions to give the adducts shown in the equation below in 15-70% yields. 

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