Carbanions reactions generating

Chapters 1 and 2. Most C—H bonds are very weakly acidic and have no tendency to ionize spontaneously to form carbanions. Reactions that involve carbanion intermediates are therefore usually carried out in the presence of a base which can generate the reactive carbanion intermediate. Base-catalyzed condensation reactions of carbonyl compounds provide many examples of this type of reaction. The reaction between acetophenone and benzaldehyde, which was considered in Section 4.2, for example, requires a basic catalyst to proceed, and the kinetics of the reaction show that the rate is proportional to the catalyst concentration. This is because the neutral acetophenone molecule is not nucleophihc and does not react with benzaldehyde. The much more nucleophilic enolate (carbanion) formed by deprotonation is the reactive nucleophile. 

Olefination Reactions Involving Phosphonate Anions. An important complement to the Wittig reaction involves the reaction of phosphonate carbanions with carbonyl compounds 253 The alkylphosphonic acid esters are made by the reaction of an alkyl halide, preferably primary, with a phosphite ester. Phosphonate carbanions are generated by treating alkylphosphonate esters with a base such as sodium hydride, n-butyllithium, or sodium ethoxide. Alumina coated with KF or KOH has also found use as the base.254... 

Organic cations (carbocations and onium ions) are important reactive intermediates in organic synthesis. From an experimental point of view, it is noteworthy that the manner in which we carry out reactions of organic cations is different from that for carbanions (Scheme 1). Usually, carbanions are generated and accumulated in a solution in the absence of electrophiles. After the generation process is complete, an electrophile is added to the solution of the pre-formed carbanion to achieve a desired transformation. In contrast, organic cations are usually generated in the presence of nucleophiles. This is probably... 

The pioneering work on thia-[2,3]-Wittig rearrangement was reported by Rautenstrauch in 1971 °. The reaction of allyl sulfides 137 with n-BuLi at —30°C for 1.5-4 h gave homoallyl sulfide or thiol 138 ([2,3]-product) exclusively (equation 81). In contrast, a similar reaction of vinyl sulfide 139 gave no [1,2]-rearrangement product 141 albeit a-thio-carbanion was generated as shown by its methylation to 140 (equation 82) °. 

The reaction of arylacetamides with ethyl phenylpropiolate gives 4,5-diaryl-l,5-dihydro-2//,67/-pyridine-2,6-diones (197). - It has been suggested that the pyridinediones in these reactions are formed through the initial addition of either the amide anion (195) or the carbanion (196), generated under basic conditions, to ethyl phenylpropiolate (Scheme 30). 

Along the same line, the reactions of vinyl fluorides with nucleophiles often involve addition-elimination processes. The addition reaction generates a carban-ion, and this latter induces the loss of a fluoride. As the loss of a fluoride ion is irreversible, the equilibrium is displaced toward the formation of the carbanion and, consequently, the reaction is very efficient. These reactions are often concerted ones (Figure 1.11). 

Some interesting synthetic applications have been found by Strauss for the interactions of 3,5-dinitropyridine with carbanions, as generated in situ by base-promoted CH ionization, to yield bicyclic systems by meta bridging,12 under conditions favoring a similar reaction with 1,3,5-trinitrobenzene.9 3,5-Dinitropyridine reacts with 1,3-diphenyl-2-propanone and triethylamine to give the bridged ion 76, according to Scheme 5. The reaction consists... 

Most of the applications of sulfoxides in synthesis make use of the reactions of sulfur-stabilized carbanions with electrophiles [385, 386]. Thus the methylsulfinyl methylene carbanion, conveniently generated through the interaction of sodium hydride with DMSO [387], is a powerful nucleophile. 

The sulfone (18) (see Section VI,P) has been resolved.214,215 Unlike open-chain a-sulfonyl carbanions, whose generation and proton capture proceeds with high retention of configuration, Corey et Z.214, 216 found that the carbanion generated by base-catalyzed decarboxylation of (+ )-18 gave a completely racemic sulfone (19). It was concluded that the lack of stereospecificity of the reaction is evidence for a planar cyclic a-sulfonyl carbanion intermediate. Cram and Whitney216,218 have studied this reaction in some detail their results indicate that symmetrical (planar) a-sulfonyl carbanions in asymmetric environments are involved as discrete reaction intermediates in the decarboxylation reaction. 

These reactions likely proceed vi a the formation of an intermediate carban-ion. Indeed, the carbanion 496 generated by treatment of 491 and 492 with lithium di isopropyl amide gave 81% of 491 (H=D) and 19% of 492 (H=0). The preferential formation of 491 can be explained on the basis of stereoelectronic effects which influence the reactivity of the intermediate carbanion... 

With this end in view, phenyldimcthylsilyl tri-n-butylstannane was added under the influence of zero-valent palladium compound with high regioselectivity and in excellent yield to the acetylene 386 to give the metallated olefin 387 (Scheme 56). The vinyl lithium carbanion 388 generated therefrom, was then converted by reaction with cerium(lll) chloride into an equilibrium mixture (1 1) of the cerium salts 389 and 390 respectively. However, the 1,2-addition of 389 to the caibonyl of 391, which in principle would have eventually led to ( )-pretazettine, did not occur due to steric reasons — instead, only deprotonation of 391 was observed. On the other hand, 390 did function as a suitable nucleophile to provide the olefinic product 392. Exposure of 392 to copper(II) triflate induced its transformation via the nine membered enol (Scheme 55) to the requisite C-silyl hydroindole 393. On treatment with tetrafluoroboric acid diethyl ether complex in dichloromethane, compound 393 suffered... 

Transketolase removes a two-carbon fragment from ketols such as fructose 6-phosphate (alternatively xylulose 5-phosphate or sedoheptu-lose 7-phosphate) through the participation of thiamine diphosphate. Nucleophilic attack of the thiamine diphosphate anion on to the carbonyl results in an addition product which then fragments by a reverse aldol reaction, generating the chain-shortened aldose erythrose 4-phosphate, and the two-carbon carbanion unit attached to TPP (Figure 8.5) (compare the role of TPP in the decarboxylation of a-keto... 

Carbopalladation occurs with soft carbon nucleophiles. The PdCl2 complex of COD (100) is difficult to dissolve in organic solvents. However, when a heterogeneous mixture of the complex, malonate and Na2C03 in ether is stirred at room temperature, the new complex 101 is formed. This reaction is the first example of C—C bond formation and carbopalladation in the history of organopalladium chemistry. The double bond becomes electron deficient by the coordination of Pd(II), and attack of the carbon nucleophile becomes possible. The Pd-carbon n-bond in complex 101 is stabilized by coordination of the remaining alkene. The carbanion is generated by treatment of 101 with a base, and the cyclopropane 102 is formed by intramolecular nucleophilic attack. Overall, the cyclopropanation occurs by attack of the carbanion twice on the alkenic bond activated by Pd(II). The bicyclo[3.3.0]octane 103 was obtained by intermolecular attack of malonate on the complex 101 [11]. 

This reaction generates a carbanion in a o orbital in the rate-determining step (Chapter 2, Section 2.A) and so does not directly involve the ir cloud. Inductive effects are therefore of primary, and conjugative effects only of secondary, importance. The 2- 3-position rate ratio for deuteria-tion of thiophene in DMSO at 25°C is 2.5 x 105 (64MI3, 64MI4 66MI2), but no direct comparison with benzene is available thiophene is likely to... 

Dihydrothiophene 1,1-dioxide in the presence of l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) reacts with CO2 to give the carboxylic acid (Equation 69), which is a stable precursor to l,3-butadiene-2-carboxylic acid <2003SC3643>. The reaction proceeds through initial deprotonation at the 2a-position the resonance-stabilized carbanion thus generated reacts with CO2 to form the carboxylate. Abstraction of a proton from the 3-position by another molecule of the base generates a dianion, which isomerizes to the stable dianion as shown in Scheme 39. Final protonation produces 3-sulfolene-3-carboxylic acid. 

Reduction of carbon-halogen bond by metal yields carbanion. Reaction of alkyl halide with Mg in the presence of anhydrous ether as solvent generates Grignard reagent. The Grignard reagent behaves like a carbanion. Alkyllithiums are also obtained from alkyl halides and behave as carbanions. 

Studies of proton transfers from carbon with f1/2 less than one microsecond are quite rare. The ionization of hydrogen cyanide in aqueous alkaline solution has been studied [20] using the ultrasonic stationary method which is applicable to reactions with tl/2 in the range 10 5 to 10 9 sec. Several reactions of benzyl carbanion having f 1/2 in the range 10-6—5 x 10-8 sec have been studied in tetrahydrofuran. The carbanion was generated by pulse radiolysis of solutions of dibenzyl mercury and its subsequent reaction with water and alcohols was followed spectrophotometrically [21]. 

In the Perkin reaction, the carbanion is generated by abstraction of an a-hydrogen from an acid anhydride, with the anion of the corresponding acid acting as the base. For example, reaction of benzaldehyde with acetic anhydride in the presence of sodium acetate at high temperature yields 3-phenylpropenoic acid (Scheme 6.22). Although the mech-... 

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