Nucleophilic Anions

MOCR (a metal carboxylate a source of the nucleophilic anion RC—O )... 

These compounds are sources of the nucleophilic anion RC=C and their reaction with primary alkyl halides provides an effective synthesis of alkynes (Section 9 6) The nucleophilicity of acetylide anions is also evident m their reactions with aldehydes and ketones which are entirely analogous to those of Grignard and organolithium reagents... 

Because of the rapid ring opening by the nucleophile, ayiridinium salts cannot usually be isolated. However, in a few cases it is possible to isolate such compounds (54), eg, at low temperatures, when the ayiridinium salts ate sparingly soluble or where there is steric hindrance to substitution. Stable ethyleneiminium salts can be prepared by reaction of ethyleneimine with acids not containing nucleophilic anions, for example HBF (55). 

For continuing polymerization to occur, the ion pair must display reasonable stabiUty. Strongly nucleophilic anions, such as C/ , are not suitable, because the ion pair is unstable with respect to THE and the alkyl haUde. A counterion of relatively low nucleophilicity is required to achieve a controlled and continuing polymerization. Examples of anions of suitably low nucleophilicity are complex ions such as SbE , AsF , PF , SbCf, BE 4, or other anions that can reversibly coUapse to a covalent ester species CF SO, FSO, and CIO . In order to achieve reproducible and predictable results in the cationic polymerization of THE, it is necessary to use pure, dry reagents and dry conditions. High vacuum techniques are required for theoretical studies. Careful work in an inert atmosphere, such as dry nitrogen, is satisfactory for many purposes, including commercial synthesis. 

Two mechanisms are among those that have been postulated for decomposition of aryl diazonium salts in aqueous solution containing nucleophilic anions, A ... 

The fluoraza reagents consist of two types of compounds one in which a fluorine atom is bound to the nitrogen atom of an amide or, more often, a sulfonamide and one in which a fluorine atom is bound to the nitrogen atom of a tertiary amine such as pyridine, quinuclidine, or triethylenediamine 1,4-diaza-bicyclo[2 2.2]octane. The positive charge on the nitrogen is counterbalanced by a non-nucleophilic anion such as triflate or tetrafluoroborate. 

Stable heterocyclic compounds having the intermediate-complex structure are well known. Where these compounds result from addition of a strongly nucleophilic anion to an A-alkylazinium cation or to a very activated substrate or must pass through a high-energy second... 

Finally, intermediate cationic allyl complexes of palladium15,16 and ruthenium17, produced from allylic esters by the action of substoichiometric amounts of the metal catalyst, have been electronically inverted by reduction to become nucleophilic anion equivalents, which are capable of carbonyl addition. 

Acidity has an important influence on diazotization. Correlations of rates of diazotization with acidity and their implications regarding the reaction mechanism were first evaluated by Ridd (reviews Ridd, 1959, 1961, 1965, 1978 Williams, 1983, 1988). In this section we will concentrate mainly on aqueous solutions of sulfuric acid and perchloric acid, as the weakly nucleophilic anions of these acids do not interact with the nitrosating species. The mechanism of diazotization in the presence... 

Y = Activating or electron-withdrawing group X = Leaving group Nu = Attacking nucleophilic anion... 

If the nucleophilicity of the anion is decreased, then an increase of its stability proceeds the excessive olefine can compete with the anion as a donor for the carbenium ion, and therefore the formation of chain molecules can be induced. The increase of stability named above is made possible by specific interactions with the solvent as well as complex formations with a suitable acceptor 112). Especially suitable acceptors are Lewis acids. These acids have a double function during cationic polymerizations in an environment which is not entirely water-free. They react with the remaining water to build a complex acid, which due to its increased acidity can form the important first monomer cation by protonation of the monomer. The Lewis acids stabilize the strong nucleophilic anion OH by forming the complex anion (MtXn(OH))- so that the chain propagation dominates rather than the chain termination. 

In the synthesis of DMC fiom the transesterification of EC and methanol, quaternary ammonium salt catalysts showed good catalytic activity. The main byproduct was ethylene glycol. The quaternary salt with the cation of bulkier alkyl chain laigth and witii more nucleophilic anion showed better reactivity. Hi temperature and large amount of catalyst increased the conversion of EC. The EC conversion and DMC selectivity increased as the pressure of CO2 increased from 250 to 350 psig. 

Then, substitution of the sulfur atom of Cys with an oxygen would greatly decrease the rate of reaction, because nucleophilicity, anion-stabilizing effect and proton-donating ability of OH group are far smaller than that of an SH group. 

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