Acid-catalyzed side

The advantages of m-chloroperbenzoic acid have already been emphasized it is commercially available, stable and highly selective when methylene dichloride is used as solvent, the m-chlorobenzoic acid precipitates from solution, thereby decreasing the danger of acid-catalyzed side reactions." ... 

Epoxidation of olefins with meta-chloroperbenzoic acid, (MCPBA) remains to this day among the most widely used methods for research-scale applications [16], Discovered by Nikolai Prilezahev in 1909 [17], it became popular only decades later, mostly through the works of Daniel Swern in the 1940s [18]. Despite its simplicity, and not unlike most epoxidation methods in use today, it suffers from undesired epoxide opening caused by the slight acidity of the reaction milieu. Although acid-catalyzed side reactions can sometimes be minimized by use of buffered systems... 

In order to destabilize the likely unproductive 6-membered chelate structure of type K (Scheme 17) that might be formed if the catalyst reacts with the 4-pen-tenoate entity, the cyclization was run in the presence of a Lewis acid which competes with the evolving carbene for the Lewis basic ester group. Such an additive has to be compatible with the RCM catalyst, should provoke a minimum of acid-catalyzed side reactions, and must undergo a kinetically labile coordination with the relay substituent. Ti(OiPr)4 was found to meet these stringent requirements... 

As noted earlier, the reaction is initiated by an arylamine cation-radical, and dichloromethane is commonly used as a solvent. Recently, a simple but valuable technique has been elaborated for minimizing acid-catalyzed side reactions under aminiumyl salt conditions (Bauld et al. 2000). This technique consists of using a two-phase system of dichloromethane with water instead of only... 

Superacids were shown to have the ability to effect the protolytic ionization of a bonds to form carbocations even in the presence of benzene.190 The formed car-bocations then alkylate benzene to form alkylbenzenes. The alkylation reaction of benzene with Ci—C5 alkanes (methane, ethane, propane, butane, isobutane, isopentane) are accompanied by the usual acid-catalyzed side reactions (isomerization, disproportionation). Oxidative removal of hydrogen by SbF5 is the driving force of the reaction ... 

It is noted that Mo/DM is the best performing catalyst with the highest steady state activity and lowest deactivation rate. The deactivation rate is the lowest even under the influence of intense acid-catalyzed side reactions known to produce coke, i.e. oligomerization of styrene and cracking of ethylbenzene. Obviously, the high surface area and high connectivity of the support have played a determining role in the catalytic reaction. The effects they exert can be looked at in two ways ... 

In this series, a relatively clean condensation process was demonstrated124-263 by reactions of the silylated carboxylic acids with acetylated phenol moieties. This procedure avoids the presence of acidic protons as well as reduces the effects of acid-catalyzed side reactions.1266 ... 

The Ti,Al-Beta shows both acidic and oxidative properties which is reflected in unwanted side-reaction. The group of Corma used the bifunctionality in the epoxidation/rearrangement of cx-terpineol to cineol alcohol and in the formation of furans from linalool.81,82 Similarly van Klaveren et al. applied Ti,Al-Beta in the one-pot conversion of styrene to phenyl acetaldehyde.83 Sato et alM solved the unwanted acid-catalyzed side reaction by neutralizing the acid site by ion exchange with alkali metals. Nevertheless the bifunctionality restricts the use of this catalyst to a limited number of reactions. 

The scope of the living cationic polymerizations and synthetic applications of these functionalized monomers will be treated in the next chapter on polymer synthesis (see Chapter 5, Section III.B). One should note that the feasibility of living processes for these polar monomers further attests to the formation of controlled and stabilized growing species. Conventional nonliving polymerizations, esters, ethers, and other nucleophiles are known to function as chain transfer agents and sometimes as terminators. In addition, the absence of other acid-catalyzed side reactions of the polar substituents, often sensitive to hydrolysis, acidolysis, etc., demonstrates that these polymerization systems are free from free protons that could arise either from incomplete initiation (via addition of protonic acids to monomer) or from chain transfer reactions (/3-proton elimination from the growing end). 

The importance of the acid-catalyzed side reactions are illustrated in Table 3 by the product distribution obtained using either TBHP or cumene hydroperoxide (CHP) as oxidant. The epoxidation with TBHP is faster and considerably more selective. When using CHP, about 20 mol% of the coproduct 2-phenyl-2-propanol was dehydrated to a-methylstyrene. It is likely that the simultaneously formed water increases the (Brpnsted) acidity of the aerogel and thus accelerates the ring opening and - to a smaller extent - the isomerization reactions. No oxidation products were formed in the absence of peroxide, as expected. Slow isomerization from p- to a-isophorone catalyzed by titania-silica was the only reaction observed. The data in Table 3 indicate that the simultaneous presence of peroxide and catalyst in the reaction mixture markedly accelerates the acid-catalyzed isomerization reaction. 

The epoxide selectivity is considerably lower in the other model reaction, the oxidation of P-isophorone. The acid-catalyzed side reactions could be suppressed by a treatment of the mixed oxide catalyst with a weakly basic salt prior to the reaction. The epoxide selectivity related to the olefin converted could be increased up to 94 % at 90 % peroxide conversion. 

The catalytic activity of Ti-Beta was found to be much lower than that of its aluminum analog, although its tolerance of water was much higher. The latter property, which is related to its hydrophobic character, and especially the absence of acid sites, enables its use in the gas phase at higher reaction temperatures when acid-catalyzed side reactions become significant [32]. In a typical gas-phase... 

As pointed out earlier, the lack of a common solvent, for aqueous and certain organic substrates, may result in a slow reaction rate and poor selectivity. This serious limitation has been circumvented with the aid of phase transfer catalysis, a well known technique in organic synthesis [55]. It consists in the transfer of a water soluble oxidant species into the immiscible organic phase, as a quaternary ammonium or phosphonium salt. Two main results are achieved by this technique. The reaction rate is increased, due to higher concentration of the oxidant species in the organic phase. Acid catalyzed side reactions are decreased, by keeping the products in the organic phase. 

Instead of the 3-(a-aminobenzyl)quinoxahn-2-(lfl)-one 51 hydrochloride in the reaction with acetoacetic ester 138 its free base is used. This has no significant effect on the yield of the product of the rearrangement, which is apparently associated with the occurrence of various acid-catalyzed side reactions involving acetoacetic ether. 

Although the acid-catalyzed side-reaction of EDOT to EDOT-dimers and -trimers,i induced by toluenesulfonic acid, is no dead end in the reaction route, because these intermediates are also leading to doped PEDOT at the end of the reaction (see Chapter 8), detrimental effects cannot be excluded and may be one of the reasons for improvements by the addition of bases. Additionally, catalytic effects on EDOT polymerization by protic acids— reducing pot life—are suppressed, and premature precipitation of doped PEDOT is diminished. 

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