Effect of solvents and additives

In different ways solvents influence enantioselective hydrogenation of keto esters on chiral modified Pt-supported catalysts. The effect of solvent on enantioselective hydrogenation involves the solubility of hydrogen, the solubility of the reactant, and the interaction between modifier, reactant, and catalyst surface including the carrier (Singh et al. There are no general 

Blaser et al. found that ee s almost linearly decreased with increasing solvent polarity (Table 5.10 ). In apolar solvents with dielectric constants in the range of 2 and 6, ee values obtained in the hydrogenation of MePy were maximal (Baiker Wehrli ). 

The best ee was obtained in solutions of AcOH (dielectric constant 6.15) and with other carboxylic acids with ee s up to 91%. NMR studies showed that in the presence of AcOH the quinuchdine JV-atom of Cnd is protonated, which favors the interaction between modifier and reactant (Baiker 

Solvent Dielectric constant Activity ro (mol kg s ) Conversion (%) Optical yield ee (%) 

The effect of acids and bases as solvents (or as additives) in the hydrogenation of EtPy on Pt-alumina-Cnd was studied by Blaser et al. The addition of AcOH to common applied solvents (toluene, ethanol) or using AcOH as the solvent exhibited unusually strong positive effects on ee s, up to 95%, especially with catalysts modified with MeO-DHCnd (Blaser et al. ). 

---

Effect of solvents and additive (NaOH) on activity and selectivity to benzhydrol over 20%Ni-Fe/TiO2 catalyst. 

Table 5.14. Effect of solvents and additives on the enantioselective hydrogenation of MePy into (5)-(+)-MeLa on 5% Pt-C-Cnd and 5% Pt-alumina-Cnd catalysts (0.5 g catalysts, 70 bar, RT) (according to Orito ).

Within the radical polymer field, two broad areas of application for ab initio kinetic modeling are model development and process optimization. The RAFT case study presented in this chapter is an example of the former type of application we are currently using the same ab initio approach to conventional radical polymerization to develop better models for predicting copolymer composition and microstructure, structural defect formation as a result of side reactions, and the effect of solvent and additives on the stereochemistry of the resulting polymer. 

Ohrai, K., Kondo, K., Sodeoka, M. and Shibasaki, M. (1994) Effects of solvents and additives in the as3mmetric Heck reaction of alkenyl trillates catalytic asymmetric synthesis of decalin derivatives and determination of the absolute stereochemistry of (+)-vernolepin. J. Am. Chem. Soc., 116,11737 8. 

Effect of solvents and additives The role of solvent may be important in such anionic-coordinated" polymerizations. It was shown for example in the case of methyl thiirane that addition of tetrahydrofuran decreased the stereoelectivity, a competition occuring between the monomer and the solvent for the coordination on the metallic atom (30). 

Interest has been shown by several groups on the effect of solvent and of added anions upon the oxidation of alcohols. The oxidation of isopropanol proceeds 2500 times faster in 86.5 % acetic acid than in water at the same hydrogen ion concentration . The kinetics and primary kinetic isotope effect are essentially the same as in water. Addition of chloride ion strongly inhibits the oxidation and the spectrum of chromic acid is modified. The effect of chloride ion was rationalised in terms of the equilibrium,... 

Copolymerizations of nonpolar monomers with polar monomers such as methyl methacrylate and acrylonitrile are especially comphcated. The effects of solvent and counterion may be unimportant compared to the side reactions characteristic of anionic polymerization of polar monomers (Sec. 5-3b-4). In addition, copolymerization is often hindered by the very low tendency of one of the cross-propagation reactions. For example, polystyryl anions easily add methyl methacrylate but there is little tendency for poly(methyl methacrylate) anions to add styrene. Many reports of styrene-methyl methacrylate (and similar comonomer pairs) copolymerizations are not copolymerizations in the sense discussed in this chapter. 

Despite numerous screening studies, the literature contains little evidence that homogeneous catalyst systems based on metals other than Co, Rh, or Ru have significant activity for catalytic CO reduction. As seen for the known active catalytic systems, however, properties of solvents and additives or promoters can have enormous effects on catalytic activities. Solvents and additives can serve many roles in these catalytic systems. One important function of promoters in the Rh and Ru systems appears to be that of stabilizing metal oxidation states involved in catalytic chemistry. Other... 

Effect of solvent. In addition to the solvent effects on certain SeI reactions, mentioned earlier (p. 574), solvents can influence the mechanism that is preferred. As with nucleophilic substitution (p. 356), an increase in solvent polarity increases the possibility of an ionizing mechanism, in this case SeI, in comparison with the second-order mechanisms, which do not involve ions. As previously mentioned (p. 573), the solvent can also exert an influence between the Se2 (front or back) and SEi mechanisms in that the rates of Se2 mechanisms should be increased by an increase in solvent polarity, while SEi mechanisms are much less affected. 

Although pre-annealing the substrate can greatly reduce shrinkage, and careful process design can reduce the swelling effects of solvents and water, additional measures are still required for the dimensional stability. 

Examples of the hydrogenation of various functional groups and reaction pathways are illustrated in numerous equations and schemes in order to help the reader easily understand the reactions. In general, the reactions labeled as equations are described with experimental details to enable the user to choose a pertinent catalyst in a proper ratio to the substrate, a suitable solvent, and suitable reaction conditions for hydrogenation to be completed within a reasonable time. The reactions labeled as schemes will be helpful for better understanding reaction pathways as well as the selectivity of catalysts, although the difference between equations and schemes is not strict. Simple reactions are sometimes described in equations without experimental details. Comparable data are included in more than 100 tables, and will help the user understand the effects of various factors on the rate and/or selectivity, including the structure of compounds, the nature of catalysts and supports, and the nature of solvents and additives. A considerable number of experimental results not yet published by the author and coworkers can be found in this Handbook. 

Transfer processes can be caused by monomer, counterion, and other components of the reaction mixture (additives, solvents, impurities). The latter reactions are sometimes called spontaneous because they are zero order in monomer. However, the spontaneous elimination of /3-protons is very unlikely, and proton elimination must be assisted by some basic reagent. The ratio of the rate constants of /8-proton elimination to that of electrophilic addition depends on several factors. The relative rate of transfer decreases with temperature, and therefore polymers with higher molecular weights are formed at sufficiently low temperatures. The effect of solvent and counterion is not yet sufficiently understood. 

Many experimental results have been reported which show that materials other than the solute and solvent present in the solution may inhibit crystal growth and induce a greater degree of supersolubility than usual. For example, many years ago, Marc (M6) showed that the presence of even small amounts of the dye Ponceau 2R extends the supersaturation limit of potassium chlorate solution. The general effects of impurities and additives on nucleation and growth are discussed further below and in detail by Buckley (B8). 

The mechanism of H-D exchange of H-2 in the thiazolium ions (107) was studied by n.m.r. A tetrahedral intermediate, formed by addition of a nucleophile, is proposed. The effects of solvents and size of the nucleophile on the reaction were studied.However, elsewhere it is claimed that the kinetic data demonstrate that the exchange of H at the 2-position of thiazolium ions cannot occur through a tetrahedral intermediate. ... 

Tab. 10.16. Effect of solvent and pressure in the addition of tert-butylamine to acrylonitrile.

The effect of a particular solvent on the chirality of a lyotropic phase can be altered by the mixing of solvents and additivity of the chiral quantity as the twisting power or the pitch is observed in a first approximation. This may even lead to a compensation of the twist, as represented in Fig. 9. Another example is provided in Fig. 17, where CTC was dissolved in a mixture of ketones and the right-handed twist preserved. 

---