Stabilized carbanion

HORNER - WADSWORTH - EMMONS Olelination Wittig type reaction of ptwsptionale stabilized carbanions with aldehydes or ketones to form olefins... 

The formation of ethyl cyano(pentafluorophenyl)acetate illustrates the intermolecular nucleophilic displacement of fluoride ion from an aromatic ring by a stabilized carbanion. The reaction proceeds readily as a result of the activation imparted by the electron-withdrawing fluorine atoms. The selective hydrolysis of a cyano ester to a nitrile has been described. (Pentafluorophenyl)acetonitrile has also been prepared by cyanide displacement on (pentafluorophenyl)methyl halides. However, this direct displacement is always aecompanied by an undesirable side reaetion to yield 15-20% of 2,3-bis(pentafluoro-phenyl)propionitrile. 

Allylic conjugation stabilizes carbanions, and pAT values of 43 (in cyclohexylamine) and 47—48 (in THF-HMPA) have been determined for propene. On the basis of exchange rates with cesium cyclohexylamide, cyclohexene and cycloheptene have been found to have pAT values of about 45 in cyclohexylamine. The hydrogens on the sjp-... 

Discuss the relevance of these observations to the structure of sulfur-stabilized carbanions and rationalize your conclusion about the structure of the carbanions in MO terms. 

Enolates of aldehydes, ketones, and esters and the carbanions of nitriles and nitro compounds, as well as phosphorus- and sulfur-stabilized carbanions and ylides, undergo the reaction. The synthetic applications of this group of reactions will be discussed in detail in Chapter 2 of Part B. In this section, we will discuss the fundamental mechanistic aspects of the reaction of ketone enolates with aldehydes md ketones. 

Fluonnation of a sulfoxide-stabilized carbanion provided a route to fluorinated estrones after elimination of the sulfoxide [111] (equation 62). 

The effects of fluonnation on carbanion stability are largely deduced from C-H acidity data (p. 988) [64], a-Halogens stabilize carbanions in the order Br > Cl > F, which IS opposite the inductive electron-withdrawing order and reflects the... 

FIGURE 24.20 The methylmalonyl-CoA epimerase mechanism involves a resonance-stabilized carbanion at the oj-position. 

Several years ago, there was much debate concerning the mechanism of the Darzens condensation.2.3 The debate concerned whether the reaction employed an enolate or a carbene intermediate. In recent years, significant evidence that supports the enolate mechanism has been obtained, wherein the stabilized carbanion (11) of the halide (10) is condensed with the electrophile (12) to give diastereomeric aldolate products (13,14), which subsequently cyclize via an internal Sn2 reaction to give the corresponding oxirane (15 or 16). The intermediate aldolates have been isolated for both a-fluoro- and a-chloroesters 10. 

Michael additions of benzotriazole-stabilized carbanions 98CCC599. Properties and synthetic utility of N-substituted benzotriazoles 98CRV409. 

Aryllead triacetates are also good reagents for aryladon of stabilized carbanions, including le anion of nltro ilkanes fEq 5 70 As a related reaction, ct-vinyladon"° or ct-acetyladon" f nitre compounds is possible using vinyllead triacetates or ilky nyllead tr... 

When a,ct-dinitro compounds are employed, the r stabilized carbanions as shown in Eqs. 7.2-7.5. ... 

Mechanism of nucleophilic aro-malic substitution. The reaction occurs in two steps and involves a resonance-stabilized carbanion intermediate. 

Q Nucleophilic addition of hydroxide ion to the electron-poor aromatic ring takes place, yielding a stabilized carbanion intermediate. 

There are, of course, no Grignard reagents inside living cells, but there are other types of stabilized carbanions that are often carboxylated. One of the... 

CHBr3 or iodoform, CHI3). Note that the second step of the reaction is a nucleophilic acyl substitution of CX3 by OH. That is, a halogen-stabilized carbanion acts as a leaving group. 

Although enolates, their equivalents, and otherwise stabilized carbanions would be interesting candidates for ARO of weso-epoxides, no efficient catalytic method has been developed to date. Crotti reported that 20 mol% of (salen)Cr-Cl complex 2 promoted the addition of the lithium enolate of acetophenone to cyclohexene oxide with moderate ees (Scheme 7.26) [50], However, the very low yields obtained... 

Prior to our work, Helling had reacted a series of hard carbanions with [Fe(mesitylene)2]++ (PFg )2 to obtain an attack on each ring giving the bis(cy-clohexadienyl)Fe" complex [31], Eq. (1). Stabilized carbanions reacted only once [31, 32] first step of Eq. (1) ... 

The nucleophilic substitution of the nitro group in nitro-arene complexes works almost as well as that of Cl" and such substitutions were achieved by Chowdhurry et al. with O, S, and N nucleophiles and with stabilized carbanions [97,98] Eq. (28) and Table 8. 

When the nucleophile is a stabilized carbanion such as the enolate of acetylacetone, 1-benzoylacetophenone, diethylmalonate, or ethyl acetatoacetone, the reaction proceeds similarly. The monosubstituted complex is isolated as long as it contains an acidic hydrogen in the benzylic position. In addition, for the case of diketones CH2(COR)2 (R = Me, Ph, OEt), a deacetylation is observed in an acidic medium [92,93]. These features are the same as described above in the case of the substitution of Cl by stabilized carbanions in monochloroaromatics (the second chlorine being an inert arene substituent [99] Scheme XVII, Eq. (31) and Tables 10 and 11. 

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. 

The Andersen sulphoxide synthesis allows one also to synthesize a variety of a-heteroatom substituted sulphoxides starting from a-heteroatom stabilized carbanions and (—)-(S)-276. The selected examples shown in Scheme 3 are the best illustration of the generality of this approach. The reaction of enolates or enolate like species with (—)-(S)-276 has been used for the synthesis of optically active a-carbalkoxy sulphoxides. For example, treatment of (—)-(S)-276 with the halogenomagnesium enolates of -butyl acetate, t-butyl propionate or t-butyl butyrate resulted in the formation of ( + )-(R)-t-butyl p-toluenesulphinylcarboxylates 298367 (equation 163). 

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