Acid reaction parameter

The product distrihution is influenced hy the catalyst properties as well as the various reaction parameters. The catalyst activity and selectivity are functions of acidity, crystalline size, silica/alumina ratio, and even the synthetic procedure. Since the discovery of the MTG process. 

The enantioselective hydrogenation of a,p-unsaturated acids or esters, using 5wt% Pt/Al203 or Pd/Al203 commercial catalysts doped with cinchonidine (CD), was deeply investigated to evidence the specific activity of Pd or Pt and the role of the reaction parameters and solvent polarity. Finally, the steric and electronic effects of different substituent groups were also studied. 

A simple, efficient, and high-yielding synthesis of quinazolin-4-ylamines and thieno[3,2-d]pyridin-4-ylamines based on the condensation of appropriately functionalized N -(2-cyanophenyl)-N,N-dimethylformamidines and primary amines has been reported by Han and coworkers (Scheme 6.253) [440]. Optimization of the reaction parameters resulted in the use of acetonitrile/acetic acid as a solvent mixture and of 1.2 equivalents of the requisite amine. In general, microwave heating at 160 °C for 10 min provided excellent product yields. 

Aldol reaction of keto-acid 21 with aldehyde 10 and esterification of the resulting acids with alcohol 22 led rapidly to cyclization precursor 23 and its 6S,7R-diastereomer (not shown). RCM using ruthenium initiator 3 (0.1 equiv) in dichloromethane (0.0015 M) at 25 °C afforded macrolactones 24a and 24b in a 1.2 1 ratio. Deprotection and epoxidation of the desired macrolactone, 24a, afforded epothilone A (4) via 25a (epothilone C) (Scheme 5). Varying a number of reaction parameters, such as solvent, temperature and concentration, failed to improve significantly the Z-selectivity of the RCM. However, in the context of the epothilone project, the formation of the E-isomer 24b could actually be viewed as beneficial since it allowed preparation of the epothilone A analog 26 for biological evaluation. 

The application of multiple reaction parameters executed in a parallel array format has been used to expedite the identification of optimal conditions for the synthesis of a collection of almost 600 new interleukin-1/ converting enzyme inhibitors [89]. The reaction in question was the problematic conversion of a / -tert-butyl aspartic acid bromoethylketone to the corresponding acyloxyketone (Scheme 2.63). The study en-... 

As mentioned earlier, in the Ruhrchemie-Rhone Poulenc process for propene hydroformylation the pH of the aqueous phase is kept between 5 and 6. This seems to be an optimum in order to avoid acid- and base-catalyzed side reactions of aldehydes and degradation of TPPTS. Nevertheless, it has been observed in this [93] and in many other cases [38,94-96,104,128,131] that the [RhH(CO)(P)3] (P = water-soluble phosphine) catalysts work more actively at higher pH. This is unusual for a reaction in which (seemingly) no charged species are involved. For example, in 1-octene hydroformylation with [ RhCl(COD) 2] + TPPTS catalyst in a biphasic medium the rates increased by two- to five-fold when the pH was changed from 7 to 10 [93,96]. In the same detailed kinetic studies [93,96] it was also established that the rate of 1-octene hydroformylation was a significantly different function of reaction parameters such as catalyst concentration, CO and hydrogen pressure at pH 7 than at pH 10. 

Phenol methylation to 2,6-xylenol has been widely studied for the past few deeades owing to the room for improvisation from the viewpoint of product selectivity. Generally during phenol methylation to 2,6-xylenol, occurs via sequential methylation of phenol to o-cresol to 2,6-xylenol, various reaction parameters mediate the selectivity between the two. For instance, when the reaetants stoichiometry of methanol to phenol molar ratio > 2, and significant residence time of o-cresol may favor 2,6-xylenol selectivity. However, excess methanol is often used, sinee some amount of methanol tend to undergo oxidation into various reformate produets [71] under vapor phase condition. Similarly, reaction temperature, catalyst acid-base property, and space velocity of the reaetant are the parameters that govern the selectivity to 2,6-xylenol. 

The highly ordered cyclic transition state of the Diels-Alder reaction permits design of reaction parameters which lead to a preference between the transition states leading to diastereomeric or enantiomeric adducts. (See Part A, Section 2.3, to review the principles of diastereoselectivity and enantioselectivity.) One way to achieve this is to install a chiral auxiliary.56 The cycloaddition proceeds to give two diastereomeric products which can be separated and purified. Because of the lower temperature required and the greater stereoselectivity observed in Lewis acid-catalyzed reactions, the best enantioselectivity is often observed in catalyzed reactions. Chiral esters and amides of acrylic acid are particularly useftd because the chiral auxiliary can be easily recovered upon hydrolysis of the adduct to give the enantiomerically pure carboxylic acid. 

Stronger Lewis acids such as SnCLi, TiCLt, and CH3AICI2 yield fast but uncontrolled polymerization with broad PDI. LCP of vinyl ethers can be achieved if the other components and reaction parameters are appropriately adjusted by various combinations of lower reaction temperature, added nucleophile, added common salt, and solvent prolarity. For example, polymerization of isobutyl vinyl ether using HC1 as the initiator (or one can use the preformed adduct of monomer and HC1) with SnCLt or TiCLj in CH2CI2 is non-LCP... 

A detailed study on the reaction parameters has shownthat the reaction temperature and the pressure of CO as well as the amount of LiBr and sulfuric acid are critical for the reaction. High pressures (80-120 atm) of CO guarantee the efficient insertion of CO in the presence of triphenylphosphine. Thus, CO and triphenylphosphine (PPh3) are competing ligands for Pd metal, and PPh3 serves as the stabilizer for Pd(0) catalyst species although the excess use of it inhibits the reaction. " ... 

When the above-mentioned approach was used, a variety of quaternary ammonium ions were synthesized in high yields under both batch and flow conditions on calcined acidic zeolites HY and HZSM-5 (257). The reaction parameters were optimized by investigation of the conversion with MAS NMR spectroscopy. The reaction temperatures and the NMR shifts of the cationic species formed are summarized in Table 4. 

The relation between the acid strength of the catalysts and the mechanism has also been demonstrated by correlations [55,123] of the reaction parameter, p, of the Taft equation for the dehydration of secondary alcohols on A1203 + NaOH, Zr02, Ti02 and Si02 (see Table 4) with the sensitivity to pyridine poisoning, the heat of adsorption of water and diethylether and the kinetic isotope deuterium effects (Table 3) on the same catalysts (Fig. 5). The parameter p reflects the mechanism being... 

Oxidation results from the interactions between atmospheric oxygen and the double bonds of unsaturated fatty acids. Several parameters can catalyze lipid oxidation, while others can prevent or slow down the reactions. Metals, light, moisture and heat can all enhance oxidation, while antioxidant compounds (e.g., BHT and vitamin E) can be utilized to retard oxidation. Oxidation of double bonds leads to intermediate peroxides that eventually break down into a variety of stable compounds. 

In general, the reasons for rate-enhancements in microwave-assisted transformations in comparison to conventional heating are not always fully understood. Some authors have postulated a specific non-thermal microwave effect for those effects that could not be rationalised as a simple consequence of superheated solvents and higher reaction temperatures. Stadler and Kappe therefore carried out a kinetic comparison of the thermal coupling of benzoic acid to chloro-Wang resin at 80° C, with the microwave-assisted coupling at the identical temperature of 80°C and otherwise identical reaction parameters. However, the reaction rates for the two runs were quite similar and the small observed differences could not be attributed to non-thermal effects. In order to confirm this hypothesis, the authors also carried out coupling experiments with... 

The oxidation of a trialkylborane may be effected by per-benzoic acid or by aqueous hydrogen peroxide in the presence of alkali.20 A detailed systematic study of the reaction parameters (oxidation temperature, base concentration, hydrogen peroxide concentration) of the latter method has led to the development3 of a standard and common procedure for oxidizing organoboranes, and is illustrated in the present procedure. 

HA compounds is not necessary for the formation of a polyester. Nevertheless, an acceleration effect of HA compounds on the rate of copolymerization was detected later 36 57 74), even for systems in which proton donors are directly bound to monomers 67). This effect is not the sum of the contributions from the tertiary amine and the proton donor but even stronger. Hence, proton donors display a cocatalytic effect. Concerning the effect of HA compounds Tanaka and Kakiuchi 36) established a linear correlation between Hammett s ct constants and the logarithm of the gelation time for various substituted derivatives of benzoic acid, benzyl alcohol and phenol, and positive reaction parameters q were found in all cases. This means that electron-withdrawing substituents increase the effect of HA compounds, or their effect becomes more pronounced with increasing hydrogen atom acidity. 

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