Base-catalysed mechanisms general

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. ... 

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... 

By way of example, compare the transition state [16] for a typical general base catalysed reaction (E.1.6) with that for the corresponding reaction involving a nucleophilic mechanism [17] (A.2.35). We have already seen that the EM s for these mechanisms are 13 M and 2.6 x 107 M, respectively. In the... 

If this explanation is correct, the looseness of the proton-transfer part of the mechanism is evidently crucial, both because it is part of every general acid-base catalysed reaction, and more specifically because EM s for reactions... 

The extraction mechanism accommodates a large number of anionic reactions and provides a rationale for general absence of catalytic activity by hydrophilic ammonium salts, e.g. tetramethylammonium halides [24, 25], which have negligible solubility in organic solvents. Similarly, owing to the highly hydrophilic nature of the hydroxide anion, the mechanism is also untenable as a rationalization for the majority of base-catalysed reactions. 

The general-base-catalysed formation of dinitramide anion, (N02N ), on reaction of 2-(Af,Af-dinitroamino)propionitrile (16) in aqueous buffer solutions (pH 9.5-11.5), has been ascribed to the E cB mechanism ( 2 > -i[BH+]), for which fcap = oh-[HO ] -I- b[B] -I- h20- The Brpnsted p value is close to unity and the entropy of activation, = 10 1 calmoH K for reaction with hydroxide ion is consistent with the combined effects of bimolecular collision (ca -11 calmoH K ) and near-complete desolvation of HO (ca -1-20 cal mol ... 

A typical role for the histidine imidazole ring is shown below, in the enzyme mechanism for a general base-catalysed hydrolysis of an ester. The imidazole nitrogen acts as a base to remove a proton from water, generating hydroxide that attacks the carbonyl. Subsequently, the alkoxide leaving group is reprotonated by the imidazolium... 

A probable mechanism of these base-catalysed aldol reactions may be written in general terms as follows ... 

In the work concerning the mechanism of solid-catalysed aldol reactions, the analogy between the homogeneous and heterogeneous mechanisms is usually assumed [370,372—375]. The mechanism of base-catalysed condensations, which has received much attention (cf. ref. 371), may be pictured in general as... 

As the concentration of BH increases, the observed catalytic coefficient will decrease until, when 2[BH] > k, the catalytic coefficient equals ,[OH ] and the rate-determining step is the addition of hydroxide ion to the substrate. Choice may be made between a number of unsymmetrical mechanisms depending upon the rate dependence upon hydrogen ion, hydroxide ion or water concentrations at high buffer concentrations or [B] or [BH] at low buffer concentrations. Johnson has tabulated the 18 kinetic possibilities and the 13 different types of kinetic behaviour of general acid-base-catalysed reaction, pointing out that this tabulation uses only one ionic form for the tetrahedral intermediate. 

The mechanism proposed by Johnson300 for the general base-catalysed hydrolysis of acetic anhydride, viz. 

Different rate laws have been observed experimentally, depending on working conditions, i.e. first order in acetaldehyde and general base catalysed, or second order in acetaldehyde and specific base catalysed [ 17]. We shall use the steady-state approximation to deduce rate laws based upon the mechanism of Scheme 4.7, and see how different mechanistic assumptions lead to different rate law predictions. 

The absence of scatter in a Bronsted plot for a general base-catalysed reaction can imply that the reaction mechanism involves a rate-limiting proton transfer step. This is because proton transfer to the base in the reaction is closely similar to the equilibrium proton transfer to the base in the reaction which defines the p Ka of the conjugate acid of that base. The observation of scatter, especially for sterically hindered bases (such as 2,6-dimethylpyridine), is evidence that nucleophilic catalysis is operating as opposed to general base catalysis. 

This is an anion-catalysed mechanism analogous to metal ion or cation-catalysed reactions. But the kinetically equivalent mechanism of general acid hydrolysis by HSO4, analogous to general base hydrolysis, is also possible (Section 8.1.3 and Problem 8.3). 

The catalytic mechanism of cysteine proteinases. The pathway to the acyl-enzyme is shown its hydrolysis probably involves general base catalysed attack of water in the near microscopic reverse of the above mechanism. Only one proton is in flight during deacylation (Szawelski and Wharton, 1981)... 

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