Substitution reactions carbanions

Substitution reactions (carbanion alkylation, acylation, heteroalkylation)... 

With active methylene compounds, the carbanion substitutes for the hydroxyl group of aHyl alcohol (17,20). Reaction of aHyl alcohol with acetylacetone at 85°C for 3 h yields 70% monoaHyl compound and 26% diaHyl compound. Malonic acid ester in which the hydrogen atom of its active methylene is substituted by A/-acetyl, undergoes the same substitution reaction with aHyl alcohol and subsequendy yields a-amino acid by decarboxylation (21). 

Carbanions are very useful intermediates in the formation of carbon-carbon bonds. This is true both for unstabilized structures found in organometallic reagents and stabilized structures such as enolates. Carbanions can participate as nucleophiles both in addition and in substitution reactions. At this point, we will discuss aspects of the reactions of carbanions as nucleophiles in reactions that proceed by the 8 2 mechanism. Other synthetic aj lications of carbanions will be discussed more completely in Part B. 

The haloform reaction of unsymmetrical perfluoroalkyl and co-hydroper-fluoroalkyl trifluororaethyl ketones gives the alkane corresponding to the longer alkyl chain [54] (equation 53) If the methyl group contains chlorine, the reaction can take different pathways, leading to loss of chlorine (equation 54), because of the variable stability of the chlorine-substituted methyl carbanions in alkali. 

An a-halosulfone 1 reacts with a base by deprotonation at the a -position to give a carbanionic species 3. An intramolecular nucleophilic substitution reaction, with the halogen substituent taking the part of the leaving group, then leads to formation of an intermediate episulfone 4 and the halide anion. This mechanism is supported by the fact that the episulfone 4 could be isolated. Subsequent extrusion of sulfur dioxide from 4 yields the alkene 2 ... 

Mechanistically the reaction can be divided into two steps. Initially the alkyl halide 1 reacts with sodium to give an organometallic species 3, that can be isolated in many cases. In a second step the carbanionic R of the organometallic compound 3 acts as nucleophile in a substitution reaction with alkyl halide 1 to replace the halide ... 

Addition and substitution reactions of nitrile-stabilized carbanions S. Arseniyadis, K. S. Kyler and D. S. Watt, Organic Reactions 31,1 (1984). Note. Includes ArC (OTMS)CN, and HetAr (OTMS)CN. 

Pathway A shows the most common reaction where the nucleophilic substitution reaction occurs at the electron-deficient carbon atom due to the strong electron-attracting character of the sulfonyl group. Nucleophilic displacements at the allylic position (SN2 reaction) are shown in pathway B. Pathway C is the formation of a-sulfonyl carbanion by nucleophilic attack on the carbon atom p to the sulfone moiety. There are relatively few reports on substitution reactions where nucleophiles attack the sulfone functionality and displace a carbanion as illustrated in pathway D3. 

Whereas the reactions of sulfones with nucleophiles via pathways A and B of equation 1 are most frequently observed, the nucleophilic substitution reaction by pathway D has been observed only in the cases where the leaving carbanion can be stabilized, or in the highly strained molecules. Chou and Chang3 has found recently that an organolithium reagent attacks the sulfur atom of the strained four-membered sulfone in 34. When this sulfone is treated with 1 equivalent methyllithium, followed by workup with water or Mel, 38 or 39 are formed in high yield. 

Bordwell and coworkers63 87 have studied the reaction of 9-fluorenyl carbanions (9-RFP) with a series of electron acceptors and in particular a-halosulfones and sulfoxides, in dimethyl sulfoxide solution. The overall reaction is characterized by the formation of the 9,9 -bis-fluorenyl derivative and the reduction of the halogenated acceptor. A family of 9-substituted fluorenyl carbanions covering a basicity range of 9.1 pKa units was used and... 

The carbanion of 2,3-dimethylthiazolidine-4-one reacted with nitroarenes to give either a ting opened product (50) via a VNS (vicarious nucleophilic substitution) reaction or a product resulting from oxidative nucleophilic substitution of hydrogen (51). Ring opening VNS reactions with 5-membered 5-heterocycles are limited to those heterocycles which show some conformational flexibility <96TL983>. 

Organic halides play a fundamental role in organic chemistry. These compounds are important precursors for carbocations, carbanions, radicals, and carbenes and thus serve as an important platform for organic functional group transformations. Many classical reactions involve the reactions of organic halides. Examples of these reactions include the nucleophilic substitution reactions, elimination reactions, Grignard-type reactions, various transition-metal catalyzed coupling reactions, carbene-related cyclopropanations reactions, and radical cyclization reactions. All these reactions can be carried out in aqueous media. 

Ionic dissociation of carbon-carbon a-bonds in hydrocarbons and the formation of authentic hydrocarbon salts, 30, 173 Ionization potentials, 4, 31 Ion-pairing effects in carbanion reactions, 15, 153 Ions, organic, charge density-NMR chemical shift correlations, 11,125 Isomerization, permutational, of pentavalent phosphorus compounds, 9, 25 Isotope effects, hydrogen, in aromatic substitution reactions, 2,163... 

The interfacial mechanism provides an acceptable explanation for the effect of the more lipophilic quaternary ammonium salts, such as tetra-n-butylammonium salts, Aliquat 336 and Adogen 464, on the majority of base-initiated nucleophilic substitution reactions which require the initial deprotonation of the substrate. Subsequent to the interfacial deprotonation of the methylene system, for example the soft quaternary ammonium cation preferentially forms a stable ion-pair with the soft carbanion, rather than with the hard hydroxide anion (Scheme 1.8). Strong evidence for the competing interface mechanism comes from the observation that, even in the absence of a catalyst, phenylacetonitrile is alkylated under two-phase conditions using concentrated sodium hydroxide [51],... 

In the unconventional synthesis of thioethers (Scheme 4.11), cyanide ion is displaced from thiocyanates by carbanions [52, 53], which have been generated under phase-transfer catalytic conditions (cf. 4.1.12). Thiocyanates are readily obtained by a standard catalysed nucleophilic substitution reaction [4, 54-58] (see Table 4.19). Aryl thiocyanates are obtained from activated aryl halides [4, 57] (see Chapter 2). 

In this chapter, C—N couplings, e.g. substitution reactions of carbanions on nitrogen atom of oximes to yield primary amines, have been reviewed. A list of oximes and 0-sulfonyloximes used for electrophilic amination is given in Table 6. These reagents aminate carbanions to A-organylimines as isolable intermediates which are hydrolyzed to primary amines (Scheme 53, path d Scheme 54). Depending on the organometaUic... 

The proposed mechanism of this reaction is composed by an initial S v2-type nucleophilic substitution reaction of 113 with the nucleophilic a-sulfonyl lithium carbanion to give the alkylmagnesium species (114) having a sulfonyl group at the / -position. Then, a -elimination reaction of magnesium sulfinate from the intermediate (114) occurs... 

A summary of the in situ use of the azobenzene probases is given in Table 2. Apart from the generation of ylid, referred to above, the main applications have been for N- and C-alkylation of weak nitrogen and carbon acids, for the promotion of condensation and substitution reactions involving carbanions such as the cyano-methyl anion, for an interesting carboxylation reaction (entries 4 and 17), and for base-promoted cyclisations (entries 5 and 6). 

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