Leaving ability

This variation from the ester hydrolysis mechanism also reflects the poorer leaving ability of amide ions as compared to alkoxide ions. The evidence for the involvement of the dianion comes from kinetic studies and from solvent isotope effects, which suggest that a rate-limiting proton transfer is involved. The reaction is also higher than first-order in hydroxide ion under these circumstances, which is consistent with the dianion mechanism. 

The extent to which intramolecular nucleophilic catalysis of the type depicted in mechanism I is important is a function of the leaving ability of the alkoxy group. This has been demonstrated by the study of the hydrolysis of a series of monoesters of phthalic acid ... 

An opportunity to investigate the relative leaving ability of fluorine in the same molecule was presented by the intramolecular cyclization of (25) [89JFC(43)393]. It was found that there was a greater distinction between the two possible sites than when S (i.e., side chain CH=C(C02")S [90JFC(50)229]) was the attacking nucleophile. 

Kinetic studies showed that the leaving ability is increased in the order C(CN)2N02 < C(CN)3 < C(CN)2N=NC6H4N02-p. 

As part of a study on the synthesis of the anticancer natural product camptothecin, alcohol 96, was synthesized (Scheme 11) <1997T10953>. Conversion of the hydroxyl group under standard conditions resulted in 97 which subsequently underwent intramolecular cyclization due to the better leaving ability of the halide. The quaternary spirocyclic product 2 was formed on standing. 

On the other hand, cleavage of the S-C bond is facile in nucleophilic substitutions involving sulfonium salts92 and the leaving ability of the dial-kylsulfide group compares with that of a bromide ion. As a result, there are several examples of type B substitutions in the literature. 

Vinyl iodonium salts 1 are reactive due to the high leaving ability of the positive iodine group, and the stability is inversely dependent on the stability of the carbocation 2 formed upon departure of the iodonio group (eq 1). ... 

The reactions involved are unimolecular, and the cyclohexenyl derivative 3 undergoes solely the spontaneous heterolysis while both spontaneous heterolysis and ligand coupling occur with the iodane 14. The relative contributions of the two reactions of 14 depend on the solvent polarity. The results summarized in Table I show that the iodonium ion and the counteranion are in equilibrium with the hypervalent adduct, X3-iodane. The equilibrium constants depend on the identity of the anion and the solvent employed, and the iodane is less reactive than the free iodonium ion as the k /k2 raios demonstrate. Spontaneous heterolysis of 3 occurs more than 100 times as fast as th t of the adduct 14 as observed in methanol the leaving ability of the iodonid group is lowered by association by more than 100 times. 

A bicyclic urea (123) was an unexpected product of the reaction between pyrrolidine and the phenyl ester of 2-cyano-l,4,5,6-tetrahydro-l-pyridinecarboxylic acid (124 R = Ph) the corresponding methyl ester (124 R = Me) reacted, as expected, to give the product of Michael addition (125). ° The better leaving ability of phenoxide vs methoxide presumably tilted the reaction towards the substitution rather than the addition product, although thiols (e.g. PhSH) underwent only the addition reaction. 

Tseng and Ullman studied leaving groups such as tosyl, trityl, piperidyl, and substituted carboxylic acids. They found that the quantum yields for the elimination from 18 correlated directly with leaving ability of the p-substituent from the ground state (Table 1). 

The parent 2(3/i/)-oxazolone moiety functions as a bifunctional leaving group when carboxyl groups are activated for acylations and condensations, similar to other five- and six-membered heterocycles such as imidazole, triazole, and 2-pyridinethiol. The excellent leaving ability of a 2(3//)-oxazolone moiety has led to the development of versatile reagents. Thus, 3-acyl- and 3-alkoxycarbonyl-2(3//)-oxazolones serve as ready-to-use -type agents for the regioselective and chemoselective N-protection of amino alcohols, amino phenols and polyamines. 

The 2-benzoxazolinone moiety is also effective for carboxy-activation as a comparable leaving group. In contrast, the saturated 2-oxazolidinone skeleton fails to show such a high leaving ability. Regioselective acylation of the primary... 

The ability of the Y group to be eliminated results apparently from the superposition of two factors on the one hand, the specific leaving ability of the Y group, and on the other, the possibility of intramolecular interactions between the Y group and the phosphorus atom. Further, the EHa mechanism is favoured by low hydroxide ion concentration680,719, since it is first order with respect to hydroxide ion, whereas the competitive mechanisms or p] are second order. 

The reduced leaving ability of electron-poor silicon is demonstrated by the reactions of styrene derivatives with trifluoromethanesulphonic acid, as shown in Scheme 3. When the silicon has one or zero electronegative substituents, protiodesilylation takes place. However, when two chloro substituents are present, the loss of the silyl group is disfavoured such that an intramolecular cyclization of the intermediate /J-carbocation 93 takes place116,117. 

The well-known action of silver(i) salts on nucleophilic substitution in alkyl halides is another commonplace example of this effect. The silver ion interacts with the halide, thus weakening the carbon-halogen bond and enhancing the leaving ability of the halide... 

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