Carbanions lithium

A good deal of the preparative utility of dihalocyclopropanes arises from the easy halogen-metal exchange. The corresponding carbanion (lithium carbenoid , see Section VLE.l) can be trapped at low temperature by a variety of electrophiles, resulting in substituted or functionalized halocyclopropanes (equation 128). 

Unusual carbanions. Lithium homoenolates are formed from P-aryl-a,P-unsaturated ketones and esters. Their reaction with carbonyl compounds leads to y-lactols and lactones. Reductive dechlorination of a-chloroimines provides a-amino carbanions. Access to 1,2-amino alcohols is assured. 

The Birch reductions of C C double bonds with alkali metals in liquid ammonia or amines obey other rules than do the catalytic hydrogenations (D. Caine, 1976). In these reactions regio- and stereoselectivities are mainly determined by the stabilities of the intermediate carbanions. If one reduces, for example, the a, -unsaturated decalone below with lithium, a dianion is formed, whereof three different conformations (A), (B), and (C) are conceivable. Conformation (A) is the most stable, because repulsion disfavors the cis-decalin system (B) and in (C) the conjugation of the dianion is interrupted. Thus, protonation yields the trans-decalone system (G. Stork, 1964B). 

Sodium borohydride and lithium aluminum hydride react with carbonyl compounds in much the same way that Grignard reagents do except that they function as hydride donors rather than as carbanion sources Figure 15 2 outlines the general mechanism for the sodium borohydride reduction of an aldehyde or ketone (R2C=0) Two points are especially important about this process... 

A variety of conjugated dienones are reduced by lithium-ammonia, presumably via dienyl carbanions analogous to the allyl carbanions encountered in enone reductions. Cross-conjugated l,4-dien-3-ones afford 4-en-3-ones as the major reduction products, indicating that the cyclohexadienyl carbanion (55) protonates largely at C-1. Some protonation at C-5 does occur as shown by examination of the NMR spectrum of the crude reduction product derived from the 17-ethylene ketal of androsta-l,4-diene-3,17-dione. The 17-ethylene ketal of androst-4-ene-3,17-dione is formed in 75%... 

Reduction of linearly conjugated 4,6-dien-3-ones with lithium-ammonia yields either 5-en-3-ones or 4-en-3-ones depending upon the work-up procedure. Protonation of the dienyl carbanion intermediate (58) occurs at C-7 to give ultimately the enolate ion (59) kinetic protonation of (59) occurs largely at C-4 to give the 5-en-3-one (60). ... 

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)... 

All that has been said in this section applies with equal force to the use of organo-lithium reagents in the synthesis of alcohols. Grignard reagents are one source of nucleophilic carbon organolithium reagents are another. Both have substantial carbanionic char acter in their- car bon-metal bonds and undergo the same kind of reaction with aldehydes and ketones. 

Lithium aryls react as typical carbanions in nonpolar solvents giving carboxylic acids with CO2... 

One way to generate carbanions is to combine an acidic molecule with one equivalent of a very strong base, such as n-butyl lithium (n-BuLi). For example, reaction of the alkyne shown below with n-BuLi leads to a carbanion of formula CsH, 02 , which then undergoes an Sn2 reaction with n-propyl bromide (n-PrBr),... 

Enantioselective synthesis with lithium/(-)-sparteine carbanion pairs 97AG(E)2282. 

Intramolecular cyclization of 2-phenysulfonylmethyl lactam 3 took place upon reaction with lithium hexamethyldisilazan via generating its a-sulfonyl carbanion to give a cyclized postulated intermediate that can be quenched with trimethylchlorosilane to afford the stable silyl ketal 4. The later ketal was desulfonylated by Raney-Ni and desilylated through treatment with tetrabutyl ammonium fluoride (BU4NF) to afford the carbacephem 5 (94M71) (Scheme 1). 

The rearrangement of an ether 1 when treated with a strong base, e.g. an organo-lithium compound RLi, to give an alcohol 3 via the intermediate a-metallated ether 2, is called the Wittig rearrangement. The product obtained is a secondary or tertiary alcohol. R R can be alkyl, aryl and vinyl. Especially suitable substrates are ethers where the intermediate carbanion can be stabilized by one of the substituents R R e.g. benzyl or allyl ethers. 

I he diaryImethanes (105). The protons on the methylene group of lliese compounds are sufficiently acidic to be removed by strong b.ises such as sodium amide or butyl lithium. Alkylation of the resulting carbanion with w-C2-chloroethyl)dimethylamine affords,... 

Treatment of dimethylsulfoxide (DMSO) with sodium hydride generates methylsulfinyl carbanion (dimsyl ion), which acts as an efficient base in the production of ylides. The Wittig reaction appears to proceed more readily in the DMSO solvent, and yields are generally improved over the reaction with -butyl lithium (i). Examples of this modification are given. 

Many initiators, such as alkyl and aryllithium and sodium and lithium suspensions in liquid ammonia, effect the polymerization. For example, acrylonitrile combined with n-butyllithium forms a carbanion intermediate ... 

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