Base-catalysed reactions

The mecluLnism of this base-catalysed reaction probably involves the intermediate formation of an aldol ... 

Which giyes formaldehyde as one of the starting materials. Base-catalysed reactions with this yery reacfiye aldehyde often giye poor yields because of polymerisation and other side reactions. The Marmich reaction is used instead ... 

Indoles can also be alkylated by lactones[l4]. Base-catalysed reactions have been reported for (3-propiolactone[15], y-butyrolactone[10] and 5-valerolac-tone[10]. These reactions probably reflect the thermodynamic instability of the N -acylindole intermediate which would be formed by attack at the carbonyl group relative to reclosure to the lactone. The reversibility of the JV-acylation would permit the thermodynamically favourable N-alkylation to occur. 

The mechanism of the base-catalysed reaction was proposed as equilibria (251) and (252)... 

A method of converting polycarbonate (PC) to bishydroxyethyl ether of bisphenol A (BHE-BPA) was studied, with a view to recycling PC plastic wastes. Treating PC in ethylene glycol with a catalytic amount of sodium hydroxide produced the monohydroxyethyl ether of bisphenol A (MHE-BPA, 42%), BHE-BPA (11%) and BPA (42%). BHE-BPA was produced quantitatively when 1.6 mol. equiv. ethylene carbonate was added to this reaction system. The reaction of BPA with EC produced both BHE-BPA and MHE-BPA, indicating that ethylene carbonate was formed as an intermediate in the base catalysed reaction of PC with ethylene glycol. A large proportion of this ethylene carbonate formed from PC was, however, lost by decarboxylation so additional ethylene carbonate must be provided for the quantitative preparation of BHE-BPA. 12 refs. 

The base-catalysed reaction of a-bromo-a,P-unsaturated ketones with aliphatic nitro compounds leads to 2-isoxazoline A-oxides by tandem conjugate addition-ring closure (Scheme 5) <95JOC6624>. A -Acyl-3-isoxazolin-5-ones are transformed into oxazoles by photolysis or by flash vacuum pyrolysis (Scheme 6) <96TL675>. 

The base-catalysed reaction of prop-2-ynylamines with carbon dioxide results in 5-methyleneoxazolidin-2-ones (e.g. 80) <96CC1699>. The fused oxazolidinone 82 is formed... 

The formation of the phosphoranes (23) in the preparation of the phosphonites (24) has been shownto be due to two processes firstly, the acid-catalysed disproportionation of the phosphonites to give (23) and cyclopolyphosphines and secondly, the remarkable base-catalysed reaction of the phosphonites with catechol to give (23) and hydrogen. 

In liquid phase reactions the base catalysed reaction may only proceed as far as its self-condensation to DAA with minimal dehydration to MO, however when operating in the gas phase at elevated temperatures, secondary reactions can take place (4). 

Base-catalysed reactions of isocyanates, such as the title compound, with alcohols may be explosively violent in absence of diluting solvents. 

According to Muller (1990) this aldehyde can give ribose-2,4-diphosphate in the presence of formaldehyde via a two-step, base-catalysed reaction. This reaction provides a route to ribose derivatives, and thus to the nucleic acids. 

When p is increased, the concentration of H+ increases, and therefore, rate of acid catalysed reaction will also increase. Similar results are obtained for reactions catalysed by hydroxyl ions. The ionic strength must be kept constant when we examine a general acid/base catalysed reaction because, if ionic strength is not kept constant the rate constant will change in accordance with equation (8.18). 

When H20 is replaced by D20, since D30+ is a stronger acid than H30+, the concentration of SD+ in D20 will be greater than that of SH+ in H20 and, therefore, the reaction will be faster in D20. Similarly, in base catalysed reactions, DO being a stronger base than HO, an increase in the rate in D20 compared to that in water is expected. 

Intramolecular general acid catalysis in reactions of salicylic acid derivatives 196 Why are EM s for general acid-base catalysed reactions so low 198 EM and the nature of the transition state 200 The formation of small rings 205... 

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... 

Intramolecular general base catalysed reactions (Section II, Tables E-G) present less difficulty. A classification similar to that of Table I is used, but since the electrophilic centre of interest is always a proton substantial differences between different general bases are not expected. This section (unlike Section I, which contains exclusively unimolecular reactions) contains mostly bimolecular reactions (e.g. the hydrolysis of aspirin [4]). Where these are hydrolysis reactions, calculation of the EM still involves comparison of a first order with a second order rate constant, because the order with respect to solvent is not measurable. The intermolecular processes involved are in fact termolecular reactions (e.g. [5]), and in those cases where solvent is not involved directly in the reaction, as in the general base catalysed aminolysis of esters, the calculation of the EM requires the comparison of second and third order rate constants. 

The stereochemistry of the seven-membered cyclic phosphites (94), (95), and (96), prepared from the base-catalysed reaction of triphenyl phosphite and the corresponding diol, has been investigated by 1H, 13C, and 31P n.m.r.73 The results suggest that each of the three heterocycles adopts a different conformation in solution. 

Tetraphenylcyclopent-3-enone and dimethyl phosphonate are the major products from the base-catalysed reaction of methyl phosphonate with tetra-cyclone.75 A mechanism involving initial hydride transfer from dimethyl phosphinate anion to the ketone followed by kinetically controlled protonation to give (98) is suggested. 

Reduction of nitroalkanes RNO2 with samarium(II) iodide, obtained from samarium and 1,2-diiodoethane, yields either alkylhydroxylamines RNHOH or alkylamines RNH2, depending on the amount of the reagent434. The base-catalysed reaction of nitroalkanes with phenyl(vinyl) sulphoxide (399) yields the conjugate adducts 400, which fragment to allylic nitro compounds 401 on thermolysis435. 

ForAs [Am] 1 and /ci+ks [Am] k i, a linear response to the nucleophile concentration, such as that depicted in equation 8, is obtained. This behaviour is characteristic of most base-catalysed reactions. On the other hand, whereas all the studied reactions were base-catalysed in n-hexane, only mild acceleration was observed in benzene9. Also, the reactions seem to be inhibited in benzene and other electron-donor solvents, and Sil-ber and coworkers attributed this effect to a preferential solvation exerted through EDA complex formation with the aromatic substrate, as shown in Scheme 49. 

Since morpholine and piperidine are stereochemically similar but exhibit different pKa values, the difference between their rates in the reactions of the fluoro-substrates in acetonitrile could be also due to a change in mechanism, whereby proton transfer from the intermediate 1 in equation 1 becomes rate-limiting when the reagent is morpholine. The change from an uncatalysed to a base-catalysed reaction with decrease in basicity of the nucleophile is well known in ANS for both primary and secondary amines1 200. 

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. 

K. Schwetlick, R. Noack, F. Stebner, Three Fundamental Mechanisms of Base-Catalysed Reactions of Isocyanates with Hydrogen-Acidic Compounds , J. Chem. Soc., Perk. Trans. 2 1994, 599 - 608. 

The observations suggest that the two condensations are direct and distinct. Had the base-catalysed reaction first given the aryl-dihydro-triazine (CXXXIV) which, under the prevailing basic conditions rearranged to the anilino-dihydro-triazine (CXXXV), an equilibrium mixture of the two would have been expected. Since, however, the pure anilino-dihydrotriazine (CXXXV) was isolated, its formation is considered to be direct and independent of that of its isomer (ffS). The formation of (CXXXIII) from 1-aryl-l-alkylbiguanides (see preceding Section), which cannot proceed by way of an aryl-dihydrotriazine, bears out this conclusion. 

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