Carbanions displacement reactions

A number of carbanion displacement reactions are discussed in Chapter... 

Almost all alkylations carried out in basic media are carbanion displacement reactions. The following syntheses are examples. 

The halogenation process may be regarded as a carbanion displacement reaction on a halogen molecule in equilibrium with the hypo-halite 18... 

Groups other than a carbonyl can stabilize a carbanion. Meyers described the preparation of a-lithio derivatives of dihydro-1,3-oxazines (282) and 2-oxazolines (284) and their use in alkylation and condensation reactions via a-lithio derivatives such as (283). Meyers treated the commercially available 285 with n-butyl-lithium to form the a-lithio derivative (286), stabilized by chelation with the nitrogen. When iodomethane was added, a carbanion displacement reaction occurred to give 287.The imine unit in 287 was reduced with borohydride (sec. 4.4.A) to give amine 288, and hydrolysis produced the aldehyde (in this case propanal). 

Solvent for Displacement Reactions. As the most polar of the common aprotic solvents, DMSO is a favored solvent for displacement reactions because of its high dielectric constant and because anions are less solvated in it (87). Rates for these reactions are sometimes a thousand times faster in DMSO than in alcohols. Suitable nucleophiles include acetyUde ion, alkoxide ion, hydroxide ion, azide ion, carbanions, carboxylate ions, cyanide ion, hahde ions, mercaptide ions, phenoxide ions, nitrite ions, and thiocyanate ions (31). Rates of displacement by amides or amines are also greater in DMSO than in alcohol or aqueous solutions. Dimethyl sulfoxide is used as the reaction solvent in the manufacture of high performance, polyaryl ether polymers by reaction of bis(4,4 -chlorophenyl) sulfone with the disodium salts of dihydroxyphenols, eg, bisphenol A or 4,4 -sulfonylbisphenol (88). These and related reactions are made more economical by efficient recycling of DMSO (89). Nucleophilic displacement of activated aromatic nitro groups with aryloxy anion in DMSO is a versatile and useful reaction for the synthesis of aromatic ethers and polyethers (90). 

The formation of ethyl cyano(pentafluorophenyl)acetate illustrates the intermolecular nucleophilic displacement of fluoride ion from an aromatic ring by a stabilized carbanion. The reaction proceeds readily as a result of the activation imparted by the electron-withdrawing fluorine atoms. The selective hydrolysis of a cyano ester to a nitrile has been described. (Pentafluorophenyl)acetonitrile has also been prepared by cyanide displacement on (pentafluorophenyl)methyl halides. However, this direct displacement is always aecompanied by an undesirable side reaetion to yield 15-20% of 2,3-bis(pentafluoro-phenyl)propionitrile. 

There are also nucleophilic displacement reactions, of considerable synthetic importance, in which the attacking atom in the nucleophile is carbon in either a carbanion (p. 288) or a source of negatively polarised carbon (cf. p. 221) new carbon-carbon bonds are thus formed ... 

Carbanions, or similar species, are involved in a variety of displacement reactions, either as intermediates or as attacking nucleophiles. 

Both overt carbanions and organometallic compounds, such as Grignard reagents, are powerful nucleophiles as we have seen in their addition reactions with C=0 (p. 221 et seq.) they tend therefore to promote an SN2 pathway in their displacement reactions. Particularly useful carbanions, in preparative terms, are those derived from CH2(C02Et)2, (3-ketoesters, l,3-( 3-)diketones, e.g. (55), a-cyanoesters, nitroalkanes, etc.—the so-called reactive methylenes ... 

Organometallic compounds or carbanions undergo a number of reactions in which the carbanion or carbanion-like moiety of the organometallic compound acts as a nucleophilic displacing agent. Examples are the formation of hydrocarbons from alkyl halides, alkyl halides from halogens, and ketones from acid chlorides or esters. The latter two reactions are closely related to the base-catalyzed condensations and are perhaps additions as well as displacement reactions. Related addition reactions are the carbonation of organometallic compounds and the addition to ketones or aldehydes. 

In the absence of a proton donor, the alkoxide ion generated by carbanion addition to the carbonyl function can interact with a carbon-halogen bond in the 8 2 displacement reaction. Reactions of this type have led to some novel carbon chain forming processes. Ketones are converted to homologated enones in good yield by... 

Reaction of the mesylated lignin prepared in step 1 (Fig. 1) with the polystyryl carbanion (living polystyrene) from step 2 (Fig. 2). The carbanion displaces the mesylate groups on the lignin in a nucleophilic displacement reaction with the formation of the polystyrene-lignin graft copolymer (Fig. 3). 

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