Base-catalysed mechanisms

A kinetic smdy of the acylation of ethylenediamine with benzoyl chloride (110) in water-dioxane mixtures at pH 5-7 showed that the reaction involves mainly benzoylation of the monoprotonated form of ethylenediamine. Stopped-flow FT-IR spectroscopy has been used to study the amine-catalysed reactions of benzoyl chloride (110) with either butanol or phenol in dichloromethane at 0 °C. A large isotope effect was observed for butanol versus butanol-O-d, which is consistent with a general-base-catalysed mechanism. An overall reaction order of three and a negligible isotope effect for phenol versus phenol- /6 were observed and are consistent with either a base- or nucleophilic-catalysed mechanism. Mechanistic studies of the aminolysis of substituted phenylacetyl chlorides (111) in acetonitrile at —15 °C have revealed that reactions with anilines point to an associative iSN2 pathway. ... 

Two mechanisms are shown above. The base-catalysed mechanism proceeds through the enolate anion. The acid-catalysed process would be formulated as involving an enol intermediate. Note that the terminal hydrogens in pentan-3-one are not exchanged, since they do not participate in the... 

Fig. Base-catalysed mechanism for keto-enol tautomerism.

And you get the hydroxide ion back again at the end of the base-catalysed mechanism. 

Base catalysis. Mechanism 2 is base-catalysed mechanism 1 is not. 

The synthesis and properties of cyclic hydrazine derivatives with N—N and N—S bonds have been investigated.119 The reaction of hydrazine-1,2-di-carboxylic esters with suiphanyl chlorides by a base-catalysed mechanism leads to the formation of six-, seven-, and eight-membered heterocycles. 

Base catalysis mechanism 2 is base catalysed, mechanism 1 isn t. 

Notice that both of these reactions are genuinely catalytic. You get the proton back again (in the form of HjO ) at the end of the acid-catalysed mechanism, and you get the hydroxide ion back again at the end of the base-catalysed mechanism. 

Although intramolecular pyramidal inversion is the most important and general pathway, some compounds can racemise following other mechanisms. A typical example is secondary phosphines, which are relatively stable towards pyramidal inversion (Table 1.2, entry 12) but can racemise following acid- and base-catalysed mechanisms (Scheme 1.5). ... 

Other studies of rate and equilibrium constants of the formation and breakdown of hemithioacetals (MeCHO+PhSH, or AcSH, or p-NOaQ-H4SH) reveal a diffusion-controlled rate-determining step, with proton transfer in some sense concerted with cleavage and formation of the C—S bond. A general base-catalysed mechanism involves attack of the RS anion on the carbonyl group. ... 

A computational study of the two possible mechanisms for the asymmetric epoxidation of conjugated aldehydes with H2O2, catalysed by chiral pyrrolidine derivatives lacking proton donor groups, indicates that the more probable route is via formation of an iminium intermediate rather than the general base-catalysed mechanism. The oxidant H2O2 acts as a co-catalyst in the initial formation of the iminium species. The epoxide... 

The hydrolysis of triamido-, diamido-, and monoamido-phosphoric acid has been studied. The triamido-compound reacts by both acid- and base-catalysed mechanisms, and also exhibits spontaneous hydrolysis in the pH range 7—8. The diamido-and monoamido-compounds react by an acid-catalysed pathway, though the dependence of rate upon pH is complicated by zwitterion formation. The reaction of A-(4-chlorophenyl)phosphoramidate anion, [ArNHPOsH]-, in hydroxylamine buffers involves a common intermediate for hydrolysis and hydroxylaminolysis. This is described as a solvated dipole pair, derived from a zwitterion ... 

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