Carbanions olefination reactions

The phosphorus ylides of the Wittig reaction can be replaced by trimethylsilylmethyl-carbanions (Peterson reaction). These silylated carbanions add to carbonyl groups and can easily be eliminated with base to give olefins. The only by-products are volatile silanols. They are more easily removed than the phosphine oxides or phosphates of the more conventional Wittig or Homer reactions (D.J. Peterson, 1968). 

The main aim of this review is to survey the reactions by which the Co—C bond is made, broken, or modified,.and which may be used for preparative purposes or be involved in catalytic reactions. Sufficient evidence is now available to show that there exists a general pattern of reactions by which the Co—C bond can be made or broken and in which the transition state may correspond to Co(III) and a carbanion (R ), Co(II) and a radical (R-), Co(I) and a carbonium ion (R ), or a cobalt hydride (Co—H) and an olefin. Reactions are also known in which the organo ligand (R) may be reversibly or irreversibly modified (to R ) without cleavage of the Co—C bond, or in which insertion occurs into the Co—C bond (to give Co—X—R). These reactions can be shown schematically as follows ... 

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... 

Aldol addition and related reactions of enolates and enolate equivalents are the subject of the first part of Chapter 2. These reactions provide powerful methods for controlling the stereochemistry in reactions that form hydroxyl- and methyl-substituted structures, such as those found in many antibiotics. We will see how the choice of the nucleophile, the other reagents (such as Lewis acids), and adjustment of reaction conditions can be used to control stereochemistry. We discuss the role of open, cyclic, and chelated transition structures in determining stereochemistry, and will also see how chiral auxiliaries and chiral catalysts can control the enantiose-lectivity of these reactions. Intramolecular aldol reactions, including the Robinson annulation are discussed. Other reactions included in Chapter 2 include Mannich, carbon acylation, and olefination reactions. The reactivity of other carbon nucleophiles including phosphonium ylides, phosphonate carbanions, sulfone anions, sulfonium ylides, and sulfoxonium ylides are also considered. 

These P elimination reactions have been used in an olefine synthesis called the Peterson olefination reaction which is analogous (and sometimes superior) to the Wittig reaction. The Peterson olefination reaction involves the addition of an a-silyl carbanion to an aldehyde or ketone to give P-hydroxysilane, followed by P-elimination to give the olefine. 

A hydrogen-transfer reaction involving the olefin and the addition product of the benzylic carbanion and olefin may accompany the side-chain alkylation reaction. The result is that alkanes and arylalkenes are produced 19). This hydride-tiansfer reaction may take place by elimination of a hydride ion from the carbanion adduct followed by addition of the hydride ion to the olefin [Reaction (6)]. The amount of this side reaction probably depends largely on the severity of reaction conditions used. 

Ethylene is the template for olefin reactions, but ethylene itself is rather unreactive, undergoing electrophilic attack by moderately strong Lewis acids. Nucleophilic attack on the bond even by the strongest Lewis bases has not been reported. The following sequence involves intramolecular addition of a carbanion to an unactivated olefin [111, 112]. The reaction is undoubtedly facilitated by active participation of the lithium cation as a Lewis acid [113]. 

Reviews have featured epoxidation, cyclopropanation, aziridination, olefination, and rearrangement reactions of asymmetric ylides 66 non-phosphorus stabilized carbanions in alkene synthesis 67 phosphorus ylides and related compounds 68 the Wittig reaction 69,70 and [2,3]-Wittig rearrangement of a-phosphonylated sulfonium and ammonium ylides.71 Reactions of carbanions with electrophilic reagents, including alkylation and Wittig-Homer olefination reactions, have been discussed with reference to Hammett per correlations.72... 

Maryanoff, B.E. and Reitz, A.B. (1989) The Wittig olefination reaction and modifications involving phosphoryl-stabilized carbanions. Stereochemistry, mechanism, and selected synthetic aspects. Chemical Reviews, 89(4), 863-927. 

B. E. Maryanoff, A. B. Reitz, The Wittig Olefination Reaction and Modifications Involving Phospho-rylstabilized Carbanions. Stereochemistry, Mechanism, and Selected Synthetic Aspects, Chem. Rev. 1989, 89, 863-927. 

Peterson reaction (olefination). Reaction of a-silylated carbanions with carbonyl compounds, yielding (3-hydroxyl-alkyl silanes, which undergo instantaneous elimination to afford olefins. 

Similarly, the reaction of diethyl 1-formyletliylphosphonate witli diethyl 1-methoxy-l-(meth-oxycarbonyl)methylphosphonate in the presence of NaHMDS in THF at room temperature affords the expected Y-methoxy-P,Y-unsaturated phosphonate in 39% yield. Diethyl 1,1-difluoro-l-formylmethylphosphonate hydrate undergoes an olefination reaction with stabilized methylphosphonate carbanions under a variety of conditions to give 3-substituted diethyl l,l-difluoro-2-propenylphosphonates in 45-78% yields. ... 

The stabilized a-substituted cyanomethylphosphonate carbanion resulting from the addition of potassium diethyl cyanomethylphosphonate to one equivalent of acrylonitrile in THF/HMPA appears to be a useful reagent in Homer-Wadsworth-Emmons olefination reaction. Thus, the reaction with aromatic aldehydes is completely stereoselective and produces ( )-l-aryl-2,4-dicyano-1-butenes in high yields (71-83%, Scheme 6.34).- ... 

Two convenient approaches to (diphenylphosphono)acetic esters (258) have been reported. One approach involves the reaction of chloroformate esters with the carbanion of diphenyl methylphosphonate and the other a Michaelis-Becker reaction of bromoacetic esters (Scheme 41) the former reaction gives better yields than the latter. Olefination reactions of (258) with benzaldehyde and... 

The most obvious explanation for the smaller imbalances in the olefin reactions is that the lag in resonance development and solvation of the carbanion is less extreme than in proton transfers because the sp2 hybridization makes it difficult to localize the negative charge on carbon, unless the carbon assumes substantial sp3 character in the transition state. Possibly, the negative charge does not accumulate at all on carbon and the imbalances may be entirely due to late solvation. 

An extensive study of olefination reactions of phosphonate carbanions in heterogeneous media using weak bases, e.g., carbonate and bicarbonate, has been carried out. (For phosphine oxide-based olefin synthesis see Chapter 4.)... 

Intramolecular olefination reactions are possible using phosphonate carbanions because they are relatively stable. This type of cyclization is also possible with Wittig reagents, but it is more facile with phosphonate carbanions. One example is Weinreb s treatment of phosphonate 571 with potassium carbonate and a crown ether to give a 84% yield of 572 in a synthesis of phyllanthine.50... 

The reaction between a carbanion derived from alkyl 3,5-bis(trifluoromethyl)phenyl sulfones and aldehydes affords, with good yields and stereoselectivities, the corresponding 1,2-disubstituted alkene through the Julia-Kocienski olefination reaction. This one-pot protocol can be performed using the phosphazene base at —78 °C and has been successfully used in a high yielding and stereoselective synthesis of various stilbenes such as resveratrol [47] (Scheme 5.28). 

Condensation of ot-silylalkyl compounds with carbonyl compounds to form alkenes has been known as the Peterson olefination reaction. Usually an a-silanyl carbanion generated in situ... 

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