Carbanions olefin forming reactions

The side-chain alkylation reaction of aromatic hydrocarbons has also been studied using unsaturated aromatic olefins, especially styrene. Pines and Wunderlich 43) found that phenylethylated aromatics resulted from the reaction of styrenes with arylalkanes at 80-125° in the presence of sodium with a promoter. The mechanism of reaction is similar to that suggested for monoolefins, but addition does not take place to yield a primary carbanion a resonance stabilized benzylic carbanion is formed [Reaction (23a, b)j. 

An important addition to the Wittig tran -olefination procedure is the introduction of phosphonate-stabilized carbanions as olefin-forming reagents, referred to as the Horner-Wadsworth-Emmons or HWE reaction. The HWE olefination offers several advantages over the Wittig reaction using stabilized ylides ... 

When the carbanion is formed in a rapid pre-equilibrium, isotopic exchange between the substrate and the solvent should be a faster process than formation of the olefin. The elimination reaction of 2-(p-nitrophenyl)ethyltrimethyl-ammonium ion in aqueous solution is accelerated by hydroxide ion but retarded by acid. This observation led to the postulation of the carbanion mechanism acid depressing the rate by reversing the equilibrium step (22). 

In the above cases pure olefin afforded the adducts shown, but recovered olefin proved to be a mixture of isomers. Excess silane suppressed the isomerizations. The fact that neither catalyst component itself isomerized olefins, but reaction mixtures at equilibrium could, suggested that the rearrangement catalyst was being formed during addition. Speier has favored a carbanionic mechanism. 

The success of the reaction depends on the formation of the zwitterionic 7t-allylpalladium complex, which is subsequently trapped with the electron-poor olefins to afford the desired vinylcyclopentane. This is achieved through a nucleophilic addition of Pd onto the vinyl group, which results in an opening of the cyclopropane ring, revealing the zwitterionic 7i-allylpalladium complex. The presence of the ester moieties stabilizes the carbanion, while the carbocation is stabilized by the 7t-allylpalladium complex. This is followed by a Michael addition of the electron poor olefins onto the carbanion to form the second intermediate, which rapidly... 

Olefin synthesis starts usually from carbonyl compounds and carbanions with relatively electropositive, redox-active substituents mostly containing phosphorus, sulfur, or silicon. The carbanions add to the carbonyl group and the oxy anion attacks the oxidizable atom Y in-tramolecularly. The oxide Y—O" is then eliminated and a new C—C bond is formed. Such reactions take place because the formation of a Y—0 bond is thermodynamically favored and because Y is able to expand its coordination sphere and to raise its oxidation number. 

Facile reaction of a carbon nucleophile with an olefinic bond of COD is the first example of carbon-carbon bond formation by means of Pd. COD forms a stable complex with PdCl2. When this complex 192 is treated with malonate or acetoacetate in ether under heterogeneous conditions at room temperature in the presence of Na2C03, a facile carbopalladation takes place to give the new complex 193, formed by the introduction of malonate to COD. The complex has TT-olefin and cr-Pd bonds. By the treatment of the new complex 193 with a base, the malonate carbanion attacks the cr-Pd—C bond, affording the bicy-clo[6.1,0]-nonane 194. The complex also reacts with another molecule of malonate which attacks the rr-olefin bond to give the bicyclo[3.3.0]octane 195 by a transannulation reaction[l2.191]. The formation of 194 involves the novel cyclopropanation reaction of alkenes by nucleophilic attack of two carbanions. 

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

The mechanism of the cyanide- and thioazolium ion-catalyzed conjugate addition reactions is considered to be analogous to the Lapworth mechanism for the cyanide-catalyzed benzoin condensation. Thus the cyano-stabilized carbanion resulting from deprotonation of the cyanohydrin of the aldehyde is presumed to be the actual Michael donor. After conjugate addition to the activated olefin, cyanide is eliminated to form the product and regenerate the catalyst. 

The Julia olefination involves the addition of a sulfonyl-stabilized carbanion to a carbonyl compound, followed by elimination to form an alkene.277 In the initial versions of the reaction, the elimination was done under reductive conditions. More recently, a modified version that avoids this step was developed. The former version is sometimes referred to as the Julia-Lythgoe olefination, whereas the latter is called the Julia-Kocienski olefination. In the reductive variant, the adduct is usually acylated and then treated with a reducing agent, such as sodium amalgam or samarium diiodide.278... 

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. 

In general, the reaction of carbanions with carbenes takes place smoothly to form primary product carbanions. However, they react further with electrophiles and this secondary reaction is difficult to control.36 To overcome this disadvantage, carbanions bearing an appropriate leaving group were designed and utilized for olefin synthesis (Scheme 23). 

Once formed, the radical intermediate (R ) can couple to afford a dimer (R2), can disproportionate to give an alkane (RH) and an olefin (R(—H)), or can accept a hydrogen atom from a donor (such as the solvent, SH) to give an alkane. A carbanion (R ) can be protonated by the solvent (or a deliberately added acid, HB) to yield an alkane. In addition, RX can undergo E2 and Sn2 reactions with B , and R can attack RX to form a dimer. 

Part of the discrepancies can be removed by considering a reaction which becomes important only in water. It was found that in acidic aqueous solutions water soluble phosphines react with activated olefins yielding alkylphosphonium salts [83-85] (Scheme 3.5). The drive for this reaction is in the fast and practically irreversible protonation of the intermediate carbanion formed in the addition of TPPMS across the olefinic bond. Under... 

Since the basic or carbanion intermediate can continue to go to product by Steps 2 and 3, we have a chain reaction which is consistent with the rapid isomerizations which may be obtained using these catalysts. This mechanistic interpretation was proposed in one of the first papers published on this subject (5) it and similar interpretations have been very helpful in bringing about an understanding of base-catalyzed reactions. The chain-reaction sequence may be terminated by reaction with a formation of a material which is not basic enough to metallate the olefin. Such compounds may be polyunsaturated hydrocarbons which may be formed by elimination of hydride ions from a carbanion. 

Thus ethyl groups may be added to a-carbons as long as benzylic hydrogens are available for replacement. The mechanism which has been proposed by Pines et al. 19) for the reaction consists of the addition of the benzylic carbanion formed by reaction of the aromatic and the catalyst with the olefin followed by a transmetalation reaction with more of the aromatic [Reaction (4)]. 

The production of n-butylbenzene may be attributed to an inherent lack of complete selectivity in carbanion reactions, because the greater stability of an intermediate does not exclude the formation of the less stable product. This stability is only important when the step in forming intermediates is slow or when energy differences are large. On the other hand, the formation of n-butylbenzene from toluene and propylene may be due to a partial radical character of benzyl alkali metals. The latter would not seem to be the case because the potassium compounds should have greater ionic character, but they yield more n-butylbenzene. This agrees with the idea that lack of selectivity may be due to greater rate of reaction of potassium compounds with olefins. 

Tosylhydrazones of a,/ -unsaturated ketones can give dienes on treatment with lithium aluminum hydride, but normally rearranged mono-olefins are formed.318 Again, a vinyl carbanion intermediate is suggested by deuterium incorporation studies.290 Reaction with alkyl lithium gives high yields of dienes 325 tosylhydrazones of A4-3-ketones give A2,4-dienes ... 

The ylid character of X5-phosphorins and their Cr(C0)3 complexes again is evident when one or both groups on phosphorus are CHR2 as one can abstract a+ protpn giving a carbanion. Reaction with electrophiles (e.g. D, CH3, and RCHO) causes side chain addition. No Wittig olefination is found with aldehydes. Instead a 1(2 -hydroxy) product 9 is formed which can be dehydrated to the X5-phosphorin derivative 10. 

Various mechanistic routes, such as addition, cyclization, etc., are available for the carbanions formed in the reactions of nucleophiles with activated olefins (Patai and Rappoport, 1962). Their competition with substitution can give information regarding the life-time of the carbani-onic intermediate. The retention of configuration of both isomers of... 

From this, it can be seen that the amount of KOH within the hydroxide mixture would probably be critical in removing organo-sulfur from coal. While the particular role of KOH has not been determined, evidence from the literature has shown that the size of the cation may be important in stabilizing intermediate carbanions. Wallace et al. (J ) conducted a series of base- catalyzed, beta-elimination reactions with isopropyl sulfide and measured the amount of olefin production. The proposed mechanism involved initial abstraction of a proton by the t-butoxide base, and formation of a carbanion, with subsequent elimination of the sulfur moiety (which can be considered a good leaving group) to form the olefin (Equation 5). 

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