Carbanions halogenation

The simplest case is the substitution of a halogen at a saturated carbon atom by an alkyl group. Organocopper reagents exhibit strong carbanionic capacity, and do attack ester groups only slowly (D.E. Bergbreiter, 1975). Ketones, however, should be protected. The relative re-... 

More general procedures for additions of halogen fluorides to highly fluori-nated olefins involve reactions with a source of nucleophilic fluoride ion, such as an alkali metal fluoride, in the presence of aposttive halogen donor [62 107, lOff, 109, 110, 111] (equations 11 and 12) These processes are likely to occur by the generation and capture of perfluorocarbamonic intermediates Tertiary fluormated carbanions can be isolated as cesium [112], silver [113], or tns(dimethylamino)sul-... 

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 effects of fluonnation on carbanion stability are largely deduced from C-H acidity data (p. 988) [64], a-Halogens stabilize carbanions in the order Br > Cl > F, which IS opposite the inductive electron-withdrawing order and reflects the... 

Chloroquinoline (401) reacts well with potassium fluoride in dimethylsulfone while its monocyclic analog 2-chloropyridine does not. Greater reactivity of derivatives of the bicyclic azine is evident also from the kinetic data (Table X, p. 336). 2-Chloroquinoline is alkoxylated by brief heating with methanolic methoxide or ethano-lic potassium hydroxide and is converted in very high yield into the thioether by trituration with thiocresol (20°, few hrs). It also reacts with active methylene carbanions (45-100% yield). The less reactive 3-halogen can be replaced under vigorous conditions (160°, aqueous ammonia-copper sulfate), as used for 3-bromoquino-line or its iV-oxide. 4-Chloroquinoline (406) is substituted by alcoholic hydrazine hydrate (80°, < 8 hr, 20% yield) and by methanolic methoxide (140°, < 3 hr, > 90% yield). This apparent reversal of the relative reactivity does not appear to be reliable in the face of the kinetic data (Tables X and XI, pp. 336 and 338) and the other qualitative comparisons presented here. 

In the initial step " the a-halo ketone 1 is deprotonated by the base at the a -carbon to give the carbanion 4, which then undergoes a ring-closure reaction by an intramolecular substitution to give the cyclopropanone derivative 2. The halogen substituent functions as the leaving group ... 

An a-halosulfone 1 reacts with a base by deprotonation at the a -position to give a carbanionic species 3. An intramolecular nucleophilic substitution reaction, with the halogen substituent taking the part of the leaving group, then leads to formation of an intermediate episulfone 4 and the halide anion. This mechanism is supported by the fact that the episulfone 4 could be isolated. Subsequent extrusion of sulfur dioxide from 4 yields the alkene 2 ... 

Carbanions of ct-chloroalkyl phenyl sulfones react with nitrobenzenes to effect tlirect nucleophilic replacement of hydrogens located on o and para to the nitro group fEq. 9.24. A very important feature is that VNS of hydrogen usually proceeds faster than conventional SnAt of halogen located In equally activated positions fEq. 9.25. The rule that VNS of... 

CHBr3 or iodoform, CHI3). Note that the second step of the reaction is a nucleophilic acyl substitution of CX3 by OH. That is, a halogen-stabilized carbanion acts as a leaving group. 

Bunnett (1991) expresses doubts that the aryldiazene can be formed at all under these strongly basic conditions, after Huang and Kosower (1968) showed that phenyldiazene is destroyed in water at pH 13.8 (25 °C) with a half-life of <10s, doubtless via C6H5-N2. In addition, Broxton s proposal (Scheme 8-53) does not provide a satisfying explanation of why the ortho halogen has such a strong effect in favor of the carbanion mechanism. 

Bordwell and coworkers63 87 have studied the reaction of 9-fluorenyl carbanions (9-RFP) with a series of electron acceptors and in particular a-halosulfones and sulfoxides, in dimethyl sulfoxide solution. The overall reaction is characterized by the formation of the 9,9 -bis-fluorenyl derivative and the reduction of the halogenated acceptor. A family of 9-substituted fluorenyl carbanions covering a basicity range of 9.1 pKa units was used and... 

Sulphonylbenzofurans 638 a-Sulphonyl carbanions 760, 766, 767, 797, 813, 951, 961 acylation of 627-636 addition to unsaturated bonds 636-642 alkylation of 627-636, 781 halogenation of 1058-1060 Michael addition of 642-649 miscellaneous reactions of 653-655 Ramberg-Backlund reaction of 649-653, 1057, 1058... 

Rearrangement of polyhalobenzenes can also be catalyzed by very strong bases for example, 1,2,4-tribromobenzene is converted to 1,3,5-tribromobenzene by treatment with PhNHK." This reaction, which involves aryl carbanion intermediates (Sgl mechanism), has been called the halogen dance. 

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