Trapping carbanions

A variation at the final stage of the carbanion trapping to a Michael addition provides an entry to the octalone system. A synthon for dihydrocompactin is readily assembled [102]. 

The reaction of trimethylstannyl sodium with primary halides has been studied in detail with emphasis on the effect of solvents and added radical and carbanion traps. By lowering... 

The effect of carbanion traps (Y-BuNH2) was determined for the reaction of 57a and 57b with LiSnMe3 in THF135. Whereas 57b appears to react exclusively by the radical pathway, 57a was proposed to react by the radical (ca 60%) and HME ca 40%) pathways. 

Reich et al. have shown that the equatorial C-Se bond of the 1,3-diselenane derived from ds-3,5-dimethylcyclohexanone can be cleaved kinetically by t-BuLi in THF and the resulting carbanion trapped by chlorotrimethylsilane. This high stereoselectivity was reversed in a sequential experiment. The Seax/Se q isomerization occurred at - 78 °C leading to a 24 1 ratio in favor of the alkylselanyl group in the equatorial position [15] (Scheme 16). The half-life was estimated to be 7 min. 

Burton (34) recently reported that iodotrifluoroethylene (58) couples directly with 1-alkynes (57) in the presence of palladium, cuprous iodide and triethylamine to give excellent yields of trifluoroenynes (59) (Scheme 18). The difluoroenolphosphonate 61, prepared from chlorodifluoromethylaryl ketones (60), can be metalated with Bu2CuLi in the presence of TMEDA and the resulting carbanion trapped by allyl halides to afford difluorostyrenes (62) (Scheme 19) (35). 

Three-Component Coupling Reactions via Aryl Carbanion Trapping by an External Electrophile... 

The stereochemistry observed in hydrogen-exchange reactions of carbanions is very dependent on the conditions under which the anion is formed and trapped by proton... 

In contrast, additions of fluorine and carbon to fluormated olefins are widely investigated The best known processes involve reactions of olefins with fluoride ion to generate carbanionic intermediates [203] that are trapped in situ by carbon-based electrophiles. 

Lithium silylamides react smoothly with tiifluoronitrosomethane to give diazenes Traces ot water initiate the decomposition of the latter with liberation of a trifluoromethyl carbanion, which is trapped by carbonyl compounds [775] (equation 116) Desilylation of trialkyl(trifluoromethyl)silanes by fluoride ion produces also a trifluoromethyl carbanion, which adds to carbonyl carbon atoms [136, 137] (equations 117 and 118)... 

Because of thetr electron deficient nature, fluoroolefms are often nucleophihcally attacked by alcohols and alkoxides Ethers are commonly produced by these addition and addition-elimination reactions The wide availability of alcohols and fliioroolefins has established the generality of the nucleophilic addition reactions The mechanism of the addition reaction is generally believed to proceed by attack at a vinylic carbon to produce an intermediate fluorocarbanion as the rate-determining slow step The intermediate carbanion may react with a proton source to yield the saturated addition product Alternatively, the intermediate carbanion may, by elimination of P-halogen, lead to an unsaturated ether, often an enol or vinylic ether These addition and addition-elimination reactions have been previously reviewed [1, 2] The intermediate carbanions resulting from nucleophilic attack on fluoroolefins have also been trapped in situ with carbon dioxide, carbonates, and esters of fluorinated acids [3, 4, 5] (equations 1 and 2)... 

The nucleophilic reaction of bromotrifluoroethene with alkoxides yields not only the expected addition and addition-elimination products but also a product from a bromophilic reaction of the carbanion intermediate [6] (equation 3) Similar are the reactions of sodium phenoxide with perfluorovinyl ethers in the presence of hexachloroethane or selected vicinal dibromoperfluoroalkanes The intermediate carbanion is trapped in high yield by these sources of Cl or Br, which suggests a... 

When treated with four equivalents of n-BuLi, arylsulphonylhvdrazones give rise to vinyl carbanions/carbanionoids. These species can be trapped (9)... 

Although sulfinate (50) was not actually isolated, its intermediacy was established by trapping as the isolable sulfonyl chloride 51, which suggests the formation of the a-sulfonyl vinyl carbanion 49 as the first species along the reaction route. 

Potassium or lithium derivatives of ethyl acetate, dimethyl acetamide, acetonitrile, acetophenone, pinacolone and (trimethylsilyl)acetylene are known to undergo conjugate addition to 3-(t-butyldimethylsiloxy)-1 -cyclohexenyl t-butyl sulfone 328. The resulting a-sulfonyl carbanions 329 can be trapped stereospecifically by electrophiles such as water and methyl iodide417. When the nucleophile was an sp3-hybridized primary anion (Nu = CH2Y), the resulting product was mainly 330, while in the reaction with (trimethylsilyl)acetylide anion the main product was 331. 

The a-sulfonyl carbanions can be trapped with a variety of electrophiles19. The method provides a synthetically useful synthon for a propylene 1,3-dipole. Reductive cleavage of the sulfone 28 thus prepared, with lithium phenanthrenide in THF, furnishes bicyclooctane 29 (equation 19)16. 

Finally, an ingenious synthetic sequence by Trost, Cossy and Burks201 includes a unique desulphonylation reaction that involves an electron-transfer process. The synthetic sequence uses 1, l-bis(phenylsulphonyl)cyclopropane as a source of three carbon atoms, since this species is readily alkylated even by weakly nucleophilic species. Given an appropriate structure for the nucleophile, Trost found that desulphonylation with lithium phenanthrenide in an aprotic solvent allowed for an efficient intramolecular trapping of the resultant carbanion (equation 88). This desulphonylation process occurs under very mild conditions and in high yields it will undoubtedly attract further interest. 

In the case of carbanion and radical intermediates the solvent is less important but the products are partially determined by the resistance of the medium to proton or hydrogen atom abstraction respectively. The increased stability of these intermediates compared with carbonium ions allows the reaction mechanism to be more readily modified by the addition of trapping agents. For example, carbanions are trapped in high yields by the presence of carbon dioxide in the electrolysis medium (Wawzonek and Wearring, 1959 Wawzonek et al., 1955). 

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radicals trapped in this manner. Even under these conditions, the methyl radical decomposes with a half-life of 10-15 min in a methanol lattice at 77 K. Since the lifetime of a radical depends not only on its inherent stabihty, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

Deprotonation of allylic aryl sulfoxides leads to allylic carbanions which react with aldehyde electrophiles at the carbon atom a and also y to sulfur . With benzaldehyde at — 10 °C y-alkylation predominates , whereas with aliphatic aldehydes at — 78 °C in the presence of HMPA a-alkylation predominates . When the a-alkylated products, which themselves are allylic sulfoxides, undergo 2,3-sigmatropic rearrangement, the rearranged compounds (i.e., allylic sulfenate esters) can be trapped with thiophiles to produce overall ( )-l,4-dihydroxyalkenes (equation 24). When a-substituted aldehydes are used as electrophiles, formation of syn-diols 27 occurs in 40-67% yields with diastereoselectivities ranging from 2-28 1 (equation 24) . ... 

Several examples of conjugate addition of carbanions carried out under aprotic conditions are given in Scheme 2.24. The reactions are typically quenched by addition of a proton source to neutralize the enolate. It is also possible to trap the adduct by silylation or, as we will see in Section 2.6.2, to carry out a tandem alkylation. Lithium enolates preformed by reaction with LDA in THF react with enones to give 1,4-diketones (Entries 1 and 2). Entries 3 and 4 involve addition of ester enolates to enones. The reaction in Entry 3 gives the 1,2-addition product at —78°C but isomerizes to the 1,4-product at 25° C. Esters of 1,5-dicarboxylic acids are obtained by addition of ester enolates to a,(3-unsaturated esters (Entry 5). Entries 6 to 8 show cases of... 

Evidence that carbanion intermediates, e.g. (44), are involved is provided by carrying out the decarboxylation in the presence of bromine. This is without effect on the overall rate of the reaction but the end-product is now Me2CBrN02 rather than Me2CHN02— under conditions where neither Me2C(NO2)CO20 nor Me2CHN02 undergoes bromination. The bromo product (45) arises from rapid attack of Br2 on the carbanion intermediate (44), which is thereby trapped (cf. the base-catalysed bromination of ketones, p. 295) ... 

The ylid intermediate (50) can be trapped by carrying out the decarboxylation in the presence of carbonyl compounds, e.g. PhCOMe, to yield the carbanion addition product, e.g. (52) this process can indeed be used preparatively. The reason for the much easier decarboxylation of (51), than of its 3-, and 4-isomers, is the stabilisation that the N can effect on the adjacent carbanion carbon atom in the intermediate ylid (50). 

Nucleophilic addition to an acylsilane followed by a 1,2-Brook rearrangement and final trapping of the resulting carbanion in either an acylation or intramolecular... 

Carbonylation at the 3-position of a pyridine is illustrated in Scheme 130.197 In these processes the intermediate carbanion from arylation of pyridine can be trapped by iron pentacarbonyl and the ensuing acyliron... 

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