Carbanion alkylation

Although most of the reactions of preparative importance involving the a-alkyl carbanions are usually carried out under controlled conditions with NHa /NHs being used as the base, a number of reactions using less severe conditions are known, both in the pyrazine and quinoxaline series. In the case of alkylquinoxalines, where an increased number of resonance possibilities exist, mildly basic conditions are usually employed in condensation reactions. 

The SnI reactions do not proceed at bridgehead carbons in [2.2.1] bicyclic systems (p. 397) because planar carbocations cannot form at these carbons. However, carbanions not stabilized by resonance are probably not planar SeI reactions should readily occur with this type of substrate. This is the case. Indeed, the question of carbanion stracture is intimately tied into the problem of the stereochemistry of the SeI reaction. If a carbanion is planar, racemization should occur. If it is pyramidal and can hold its structure, the result should be retention of configuration. On the other hand, even a pyramidal carbanion will give racemization if it cannot hold its structure, that is, if there is pyramidal inversion as with amines (p. 129). Unfortunately, the only carbanions that can be studied easily are those stabilized by resonance, which makes them planar, as expected (p. 233). For simple alkyl carbanions, the main approach to determining structure has been to study the stereochemistry of SeI reactions rather than the other way around. What is found is almost always racemization. Whether this is caused by planar carbanions or by oscillating pyramidal carbanions is not known. In either case, racemization occurs whenever a carbanion is completely free or is symmetrically solvated. 

Alkyllithium react as though they contain alkanide (R ) ions (or alkyl carbanions), the conjugate base of alkanes. 

Primary [21] and secondary [22] alkyl iodides are reduced in a stepwise fashion at mercury cathodes to form alkyl radicals and alkyl carbanions the alkyl radicals undergo coupling and disproportionation as well as interaction with the electrode to yield diorganomercury compounds, and... 

The initiator required to polymerize a monomer depends on the reactivity of the monomer toward nucleophilic attack. Monomer reactivity increases with increasing ability to stabilize the carbanion charge. Very strong nucleophiles such as amide ion or alkyl carbanion are needed to polymerize monomers, such as styrene and 1,3-butadiene, with relatively weak electron-withdrawing substituents. Weaker nucleophiles, such as alkoxide and hydroxide... 

It has been reported that gem-dihalocyclopropanes, usually undergo electrochemical or metallic reduction with predominant retention of configuration as shown in Scheme 34. 124,126) The stereoselectivity of these reactions may result from the intermediacy of a cyclopropyl carbanion which is unusually stable to racemization compared to other alkyl carbanions. 125>... 

Neutral complexes are represented by XCo(dh)2B, where X is a formally negative ligand (e.g., halide or alkyl carbanion), dh is the monoanion of dimethyl-gly oxime, and B is a neutral two electron-donor base. 

Most alkyl carbanions undergo facile pyramidal inversion. Cyclopropyl anions are an exception, presumably because the transition state, with a planar trigonal carbon, is more strained than the ground state. The configurational stability of cyclopropyl anions is of value in the synthesis of deuterated cyclopropanes by the Haller-Bauer reaction (see Section II.B). An interesting dilemma arises when a cyclopropyl anion is stabilized by a n-electron acceptor substituent such as a nitrile or an ester. Will the anion then retain its pyramidal equilibrium geometry for the strain reasons alluded to above, or will it become planar in order to maximize overlap of the filled orbital on carbon with the n orbital of the substituent Walborsky and coworkers addressed this question in a series of experiments in which rates of H/D exchange and racemization were compared for an optically active cyclopropane exposed to a base in a deuterated hydroxylic solvent. The outcome can be illustrated with the particular example of 1,1-diphenylcyclopropane-2-... 

The first example is a vinyl bromide and vinyl (sp ) carbanions are more stable than saturated (sp ) carbanions because of the greater s character in the C-Li a bond. The second example is saturated like BuLi but, unlike BuLi, it is a tertiary aUcyl bromide. The t-alkyl carbanion would be less stable than the primary BuLi and is not formed. 

Reduction of R4N at a glassy carbon electrode in DMF yields an alkyl carbanion, which deprotonates residual water or R4N" in a Hofmann elimination with formation of an alkene and a trialkylamine. Tetraethylammonium is attacked more easily than tetrabutylammonium [463]. 

Alkyl carbanions are potent bases, so they are protonated by almost any source of hydrogen ions, especially adventitious water in the solvent or supporting electrolyte. In the absence of water and other added proton donors, tetraalkylammonium cations can serve as proton donors toward alkyl carbanions deprotonation of tetraalkylammonium ions leads, via the Hofmann elimination, to the corresponding trialkylamine and olefin. When tetramethylammonium salts are used as supporting electrolytes, there is evidence that proton transfer gives rise to trimethylammonium methyUde [34,35]. 

Another feature of reactions between alkyl halides and electrogenerated aromatic radical anions is that, once a single electron has been transferred to the alkyl halide to form an alkyl radical (R ), the radical can react with another radical anion to form an alkyl carbanion (R ), which is then protonated to form RH, or the radical can bond with the radical anion to yield an alkylated aromatic hydrocarbon ... 

Problem 22.4 Racemization in certain free-radical and carbonium ion reactions has been attributed (Secs. 7.10 and 14.13) to loss of configuration in a )at intermediate. Account for the fact that the formation of alkyl carbanions, R —which are believed to be pyramidal—also lead to racemization. 

Other aliphatic caibanion structures associated with Group Ila cations are known. Some examples of these are dimethylbeiyllium and lithium tri-t-butyl beryllate. Since the beryllium alkyl carbanions have not yet been utilized as common synthetic reagents, diese structures will not be discussed further. 

Simple alkyl carbanions, like CH3Li, are sp hybridized at the carbanionic center and commonly exist as the tetramer, (CH3Li)4. This tetramer exists as a tetrahedron of lithium ions with a carbanion snuggled in between the three lithium atoms of each face. 

Preparation of Aryl Hydrazines. All methods mentioned above for the hydrazination of alkyl carbanions may also be applied to aryl carbanions. Addition of phenyllithium to a cyclic azo compound followed by in situ arylation to give a tetrasubstituted hydrazine was mentioned earlier (Eq. 19). An alternate hydrazination method, not involving aryl anions, is the reaction of electron-rich arenes with azodicarboxylic esters and aroylazocarboxylic esters under the influence of various catalysts.230,377-384... 

Tn 1955 Pines and Schaap (1) discovered that toluene was alkylated by ethylene in the presence of sodium or potassium metal or, more specifically, their organometallic derivatives. This reaction requires a high temperature (about 200°C) and considerable olefin pressure the organometallic catalyst is essentially insoluble in the reaction medium. The catalyst cycle—for example, in the side-chain ethylation of toluene— involves a benzyl carbanion which adds to ethylene to form a primary alkyl carbanion. The latter immediately abstracts a proton from the excess toluene reactant to form n-propylbenzene and to reform the energetically-favored benzylic anion in a catalytic cycle. 

This suggests, as does the (p-d)7r model, that a-silylcarbanions should be stabilized relative to a-alkyl carbanions. 

Yet another way to obtain a 1 -monohalogenated alkylphosphonic diester is based on the application of the Wadsworth-Emmons adaptation of the Wittig reaction (Scheme 10). Here, the anion from tetraisopropyl (fluoromethyl)bisphosphonate reacts with a carbonyl compound to give the (l-fluoroalk-l-enyl)phosphonic esters 126 as an E-Z mixture (80-95 20-5) hydrogenolysis of these mixtures yields (l-fluoroalkyl)phosphonic esters. In a review of the literature the authors pointed out the widely different results experienced by other workers in their attempts to alkylate carbanions derived from fluorinated alkylphosphonic diesters and, as a result of their own work, advocated the use of alkyl triflates, which appear to react with lithiated carbanions very quickly and cleanly. 

Addition of the nucleophile can take place by two sterically different pathways. In the first (path (a)), direct attack on the rj -allyl group occurs trans to palladium. This is by far the commoner route and is followed by stabilized carbanions such as CH(C02R)2, CH(C0R)2, PhCHCN and CgHg , and normally also with amines. Aryl and alkyl carbanions (e.g. R CuLi) or hydride however add initially to the metal centre and then migrate to the allyl group (path (b)). The stereochemistry of the product depends on which mechanism is followed (v.i.). 

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