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Carbanions hydrogen abstraction

To summarize under favorable conditions the acidity of a-hydrogens facilitates the generation of a-sulfoxy and a-sulfonyl carbanions in thiirane and thiirene oxides and dioxides as in acyclic sulfoxides and sulfones. However, the particular structural constraints of three-membered ring systems may lead not only to different chemical consequences following the formation of the carbanions, but may also provide alternative pathways not available in the case of the acyclic counterparts for hydrogen abstraction in the reaction of bases. [Pg.405]

The four hitherto known routes of the C-H insertion are shown in Scheme 1. In general, the insertion by singlet carbenes proceeds via route a in one step, whereas the reaction by triplet carbenes proceeds sequentially via route b, i.e., hydrogen abstraction followed by recombination of the radical pairs.4 Other stepwise mechanisms are hydride abstraction (route c) and proton abstraction (route d), both being followed by the recombination of ion pairs. However, extended study on routes c and d for synthetic purposes had not been done before we started, except for a few earlier studies on carbanion-promoted P C-H insertion reactions.5,6 Recent advances in transition metal-catalyzed... [Pg.288]

The sodium hydride eliminated in Reaction 5-73 may also participate in hydrogen abstraction from XXI. The order of stability of polystyryl carbanions depends on the alkali metal in the order K > Na > Li. [Pg.417]

The autoxidation mechanism by which 9,10-dihydroanthra-cene is converted to anthraquinone and anthracene in a basic medium was studied. Pyridine was the solvent, and benzyl-trimethylammonium hydroxide was the catalyst. The effects of temperature, base concentration, solvent system, and oxygen concentration were determined. A carbanion-initi-ated free-radical chain mechanism that involves a singleelectron transfer from the carbanion to oxygen is outlined. An intramolecular hydrogen abstraction step is proposed that appears to be more consistent with experimental observations than previously reported mechanisms that had postulated anthrone as an intermediate in the oxidation. Oxidations of several other compounds that are structurally related to 9,10-dihydroanthracene are also reported. [Pg.214]

The products of reaction of e aq with olefins are carbanions which react with the solvent within less than 10-n sec. (47) to give the corresponding hydrogen adducts >C = C< + e aq(> C = C <)- + H + - > C — CH <. In the case of acetone, the adduct (CH8)2CHO was observed spectrophotometrically by pulse radiolysis and was found to be identical to the radical obtained from isopropyl alcohol by hydrogen abstraction (69). (CH8)2CO- was also detected by electron spin reso-... [Pg.80]

The addition of hydrogen fluoride to electron-deficient aUcenes and aUcynes can be achieved in certain cases via an indirect process in which reaction of the aUcene or alkyne with fluoride ion leads to a carbanion that abstracts a proton from the solvent. This method, however, is limited to cases where other reactions of the carbanion are hindered or less favourable [254, 255] (Figure 3.54). [Pg.77]

Carbanion studies (96-98), infrared and mass spectrometry (97,99), hydrogen abstraction (13,28,349,572,843). [Pg.179]

Bicyclic systems with a vinylcyclopropane fragment reacted in a similar way. The mechanism postulated for these reactions involves the formation of a zwitterionic palladium complex in which the three-membered ring has already been opened. Hydrogen abstraction from the C-H acidic agent gives a carbanion which adds to the -aIlyl ligand of the palladium with removal of the metal. ° ... [Pg.2106]

The aldol condensation involves the reaction of two molecules of an aldehyde or ketone that has alpha hydrogens. Abstraction of an alpha hydrogen by base produces a carbanion which attacks the carbonyl carbon of the other molecule by base-initiated nucleophilic addition an alcohol group is formed. Often the alcohol dehydrates to form the final product, an unsaturated aldehyde or ketone. In a crossed aldol condensation, a carbonyl compound with alpha hydrogens reacts with one without alpha hydrogens. [Pg.259]

The Claisen condensation is a carbanion reaction in which the carbanion produced by alpha hydrogen abstraction on an ester displaces the alkoxy group of another ester molecule the reaction produces keto esters. [Pg.295]

This can only be done by strong bases, and is made easier if the carbanion to be formed is stabilized, for example by conjugation with an adjacent C=0 group. In such cases (e.g. reaction 4.4a), the equilibrium concentration of the carbanion produced is often very small, but since carbanions of this type are very reactive, as the carbanion is removed it will be replaced by further hydrogen abstraction by the base, and nearly 100% reaction can often be achieved. [Pg.78]

Carbanions can exist as metal salts, and are also important intermediates in reactions carried out under basic conditions. They react as strong bases and nucleophiles, undergoing hydrogen abstraction reactions, nucleophilic substitution and addition, fragmentation, and loss of halide ions to give carbenes, They can be identified spectroscopically or by trapping. [Pg.92]

This may be followed by other reactions (Odian, 1991), such as abstraction of an aUylic hydrogen from (XIII) by a carbanion center (to yield an unreactive anion) or hydrogen abstraction by the sodium hydride (XIV) formed in reaction (8.20). [Pg.485]

The effects of structure on reactivity found in oxidation are very similar to some reported in the literature for hydrogen abstraction by free radicals. These include abstraction by methyl radicals in the gas phase (159), abstraction by methyl radicals in solution (140), and abstraction by e/ <-butoxy radicals in liquid phase (141). The oxidation reactivities are widely different from those for carbanion intermediates. [Pg.195]

After an initial a-hydrogen abstraction, transfonnalions at P-carbon or y-carbon of a-amino acids may occur. When a-amino acid substrates possess a substituent at either P- or y-position, they can function as good leaving groups such as COO , OH(CH3) or SH. Either elimination or replacement of the substituent yields corresponding oxo acids. In the case of reactions, labilization of P-hydrogen to form a P-carbanion intermediate is necessary. [Pg.372]

The initial formation of a ketyl radical C=0 by an one-electron transfer was proposed. In absence of methanol, the duplication leads to the pinacol. Protonation by methanol gives the hydroxy-radical C-OH which is reduced by Sml to the carbanion C-OH and then protonated. A minor pathway is the hydrogen abstraction from THF by the hydroxyalkyl radical This was established by the use of MeOD which gives from RCHO, 90% RCHDOH and 10% RCH2OH. [Pg.54]

The base-catalyzed dehydration, hke the aldol condensation reaction, involves a surface acid-base pair site and proceeds through formation of a carbanion intermediate by a-hydrogen abstraction, as presented in Scheme 5. [Pg.63]


See other pages where Carbanions hydrogen abstraction is mentioned: [Pg.342]    [Pg.883]    [Pg.127]    [Pg.576]    [Pg.116]    [Pg.883]    [Pg.69]    [Pg.60]    [Pg.331]    [Pg.157]    [Pg.305]    [Pg.177]    [Pg.36]    [Pg.911]    [Pg.860]    [Pg.883]    [Pg.191]    [Pg.331]    [Pg.73]    [Pg.73]    [Pg.290]    [Pg.883]    [Pg.677]    [Pg.1354]    [Pg.254]    [Pg.331]    [Pg.576]    [Pg.151]   
See also in sourсe #XX -- [ Pg.65 ]




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Hydrogen abstraction

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