Aqueous acid conditions

The most common synthetic application of mercury-catalyzed addition to alkynes is the conversion of alkynes to ketones. This reaction is carried out under aqueous acidic conditions, where the addition intermediate undergoes protonation to regenerate Hg. ... 

They report that in the reaction between 5-chlorisatin 19 and 5,6-dimethoxindanone 20 under basic conditions at reflux for 16 hours, the desired quinolinic acid 22 is obtained in 38% yield with an unavoidable competing amount of the aldol product 21. However, if the same reaction is carried out using aqueous acid conditions, the quinolinic acid is obtained in a reproducible 86% yield. ... 

Oxime hgations can be significantly accelerated by using aniline as a nucleophilic catalyst . Rate enhancements are achieved by changing the electrophile from a weakly populated protonated carbonyl (Scheme 2a) to a more highly populated protonated aniline Schiff base (Scheme 2b). The transimination of the protonated aniline Schiff base to the oxime proceeds rapidly under aqueous acidic conditions. 

The degradation of several commercial penicillin antibiotics has been reported.110 113 Thus the kinetics and mechanism of decomposition of cefazolin ester (136) in phosphate buffers110 and the effect of hexadecyltrimefhylammonium bromide-based microemulsions on the decomposition of the antibiotic cephalosporin, cephaclor (137)111 have been reported. Reaction of the latter can occur intramolecularly or intermolecularly by hydroxide attack. Degradation in the solid state of cephaclor (137) has also been reported.112 The same group has looked at the decomposition under aqueous acidic conditions.113 The degradation pathways that have been recognized are... 

The Birch reduction of 3,4,5-trimethoxybenzoic add 17a b (cognate preparation in Expt 7.19) is of interest in that the product is 1,4-dihydro-3,5-dimethoxy-benzoic add (46) thus revealing that in this reduction the 4-methoxy group is removed by hydrogenolysis. The product possesses two methyl vinyl ether residues which are labile under aqueous acidic conditions to yield the saturated diketone, 3,5-diketocyclohexanecarboxylic add (47). 

A A /V /V -Tetramethylelhylcncdiaminc (TMEDA) as catalyst of the Morita-Baylis-Hillman reaction has been found to be more efficient than DABCO in aqueous media.146 1-Methylimidazole 3-/V-oxide promotes the Morita-Baylis-Hillman reaction of various activated aldehydes with ,/i-unsaturated ketones and esters CH2= CHCOR (R = Me, OMe) in solvent-free systems.147 In another study, the Morita-Baylis-Hillman reaction has been successfully performed under aqueous acidic conditions at pH 1, using a range of substrates and tertiary amines as catalysts.148... 

More recently, Porta and co-workers have reported a free-radical Mannich type reaction based on the selective a-CH aminomethylation of ethers by a Ti(III)/t-BuOOH system under aqueous acidic conditions (Equation 14.27) [34]. 

Why do TUP ethers, unlike ordinary ethers, cleave under mildly aqueous acidic conditions ... 

The collapse of a polymer gel in response to a change in environment can be scaled down to the single chain level. A layer of polymer chains grafted to a surface forms a polymer brush, for which the collapse transition can be nicely observed using neutron reflectivity. A pore lined with a responsive polymer brush will form a selective valve for example if the grafted polymer is a weak polybase, in aqueous acidic conditions the brush will be charged and will expand to close the pore, while in basic conditions the brush will be neutral. This principle has been used to create a selective membrane, which shows greatly reduced permeability in acidic conditions. 

A convenient method for the synthesis of selenocarboxamides (121) from aromatic and aliphatic nitriles using monoselenophosphate (122) has been elaborated. This method complements the previously reported methods since aqueous acidic conditions are used as opposed to basic conditions or BF3. OEt2 in... 

Treatment of cyclobutanone under aqueous acidic conditions with either hydrogen peroxide/selenium dioxide or thallium(III) salts gives cyclopropanecarboxylic acid in a good yield (equation 176) ... 

The first category includes furan compounds, water, and char—all of which are the expected products of the better understood acid-catalyzed dehydration reactions of carbohydrates under aqueous acid conditions (J). The second category includes glyoxal, acetaldehyde, and other low molecular weight carboxyl compounds, which are sim-... 

The THP group is a widely used protecting group it is readily introduced by reaction of the enol ether dihydropyran with an alcohol in the presence of an acid catalyst, such as TsOH, BF3 OEt2, or POCI3. For sensitive alcohols such as allylic alcohols, PPTS (pyridinium p-toluenesulfonate) is used as a catalyst for tetrahydropy-ranylation. As an acetal, the THP group is readily hydrolyzed under aqueous acidic conditions with AcOH-THF, TsOH, PPTS-EtOH, or Dowex-H (cation exchange resin). 

Hence, the initial major product is a mono-substituted phenol [Eq. (1)]. Because the reaction is done under aqueous acidic conditions, the products shown in Eq. (1) are not isolated. Instead, a methylene bridge is formed between the phenyl rings [Eq. (2)]. In both Eqs. (1) and (2) the mechanism is an electrophilic aromatic substitution. Heating the system so as to promote removal of water and polymerization results in thermoplastic material known as novolac (1). This thermoplastic resin can be mixed with hexamethylenetetramine (formed from ammonia and formaldehyde) and stored until cure. Heating this system produces an excess of formaldehyde and ammonia. A cross-linked polymer results from the cure. The linkages are mostly methylene and amino groups. 

The alcohols 105c and 109c reacted with acetone in the presence of an acidic catalyst to give the oxazolidines 143 and 142a the products were reconverted into the reactants under aqueous acidic conditions. 

The most widely used approach to furans is the cyclising dehydration of 1,4-dicarbonyl compounds, which provide all of the carbon atoms and the oxygen necessary for the nucleus. Usually, non-aqueous acidic conditions are employed to encourage the loss of water. The process involves addition of the enol oxygen of one carbonyl group to the other carbonyl group, then elimination of water. ... 

Each elementary step is reversible. At fist glance, this type of reaction should afford a complex reaction mixture of product, all possible intermediates, and starting materials. However, if the desired adduct is thermodynamically more stable than any other intermediates, the reaction can still be high yielding and become synthetically useful. The Strecker reaction is a typical example. Although the condensation of an aldehyde, an amine leading to iminium intermediate and subsequent nucleophilic addition of cyanide are in principle reversible under aqueous acidic conditions. The a-aminonitrile can be obtained in high yield under appropriate conditions. 

Aqueous acidic conditions favor the formation of the iminothiazinone (227) from the reaction of itaconic acid (228) and thiourea. However, if the reaction is carried out in water alone the product is the amino tautomer (229) (Scheme 43) <91IJC(B)886>. 

The first in-situ PM study was reported by Russell et al. in 1982 [63] and was concerned with the adsorption of CO on Pt. As was discussed above, the first EMIRS paper on this subject [62] had concluded that the dominant poison in the electro-oxidation of MeOH at Pt under aqueous acid conditions was —C = 0, which exists at high coverage, and another CO species thought at that time to be >C = 0. The C = 0 feature appeared as a bipolar band, indicating that its frequency was potential-dependent. Russell and coworkers commented upon the indirect nature of the information derived... 

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