Substrate purity, effect

Optimization of the enantioselective catalytic key steps calls for careful experimental investigation of many reaction parameters. Besides temperature, concentration of substrate, solvent effects, pressure, and conversion rate, a defined robustness of the process towards impurities, for example contained in reagents, as well as its sensitivity towards air (oxygen) or moisture at various temperatures are important aspects. In particular, the purity of prochiral substrates is of utmost importance for the success of asymmetric hydrogenation experiments. As a consequence, considerable attention had to be paid to even the smallest differences in the impurity profile of substrates, which may be due to different preparation and/or purification procedures at lab, pilot, or production scale. 

The simplest vay to measure an isotope effect is the noncompetitive technique, in vhich the rate (kn) with fully protiated substrate ( H labeled), is compared to the rate (kn) at which deuterium labeled substrate ( H labeled) reacts [28]. The label may be in the primary or a secondary position, yielding the primary or secondary KIE, respectively. Steady-state noncompetitive measurements yield the isotope effect on the rate constants or k st/Ku, but suffer from the requirement of both high substrate purity and isotopic enrichment, and from a large uncertainty in the KIE (ca. 5-10%) due to propagated errors. Single-turnover experiments can yield noncompetitive KIEs on the chemical step, but also generally have large uncertainties. Nevertheless, noncompetitive measurements are the only way to obtain KIEs on kcat, which for certain enzymes may be the sole kinetic parameter that reflects the chemical step(s). 

Resolution of Racemic Amines and Amino Acids. Acylases (EC3.5.1.14) are the most commonly used enzymes for the resolution of amino acids. Porcine kidney acylase (PKA) and the fungaly3.spet i//us acylase (AA) are commercially available, inexpensive, and stable. They have broad substrate specificity and hydrolyze a wide spectmm of natural and unnatural A/-acyl amino acids, with exceptionally high enantioselectivity in almost all cases. Moreover, theU enantioselectivity is exceptionally good with most substrates. A general paper on this subject has been pubUshed (106) in which the resolution of over 50 A/-acyl amino acids and analogues is described. Also reported are the stabiUties of the enzymes and the effect of different acyl groups on the rate and selectivity of enzymatic hydrolysis. Some of the substrates that are easily resolved on 10—100 g scale are presented in Figure 4 (106). Lipases are also used for the resolution of A/-acylated amino acids but the rates and optical purities are usually low (107). 

If we consider natural synthetic processes, enzymes are seen to exert complete control over the enantiomeric purity of biomolecules (see Figure 8.2). They are able to achieve this because they are made of single enantiomers of amino adds. The resulting enantiomer of the enzymes functions as a template for the synthesis of only one enantiomer of the product Moreover, the interaction of an enzyme with the two enantiomers of a given substrate molecule will be different. Biologically important molecules often show effective activity as one enantiomer, the other is at best ineffective or at worst detrimental. 

The above brief analysis underlines that the porous structure of the carbon substrate and the presence of an ionomer impose limitations on the application of porous and thin-layer RDEs to studies of the size effect. Unless measurements are carried out at very low currents, corrections for mass transport and ohmic limitations within the CL [Gloaguen et ah, 1998 Antoine et ah, 1998] must be performed, otherwise evaluation of kinetic parameters may be erroneous. This is relevant for the ORR, and even more so for the much faster HOR, especially if the measurements are performed at high overpotentials and with relatively thick CLs. Impurities, which are often present in technical carbons, must also be considered, given the high purity requirements in electrocatalytic measurements in aqueous electrolytes at room temperature and for samples with small surface area. 

The bulk of the ester group has little effect on optical purity of N-acetylphenylalanine ester products formed from the dehydro substrates using the C-4 catalyst, and with this or the DIOP system, variation of the substituent in the phenyl ring of the N-acetylphenylalanine precursors also had little effect (242). 

The IPA system does not require a co-solvent, but one can be used if this proves advantageous. In the TEAF system a solvent is normally used, though neat TEAF or formic acid can be used if required. The solvent can have a large effect on the reaction rate and optical purity of the product this may in part be because the substrate seems to bind by weak electrostatic interactions with the catalyst, and is also partly due to the pH of the system. Solvents have a dramatic effect on the ionization of formic acid for example, in water the piCa is 3.7, but in DMF it is 11.5. This is because formation of the formate anion becomes less favorable with less polar solvents (see Table 35.2). The piCa of triethy-lamine is far less sensitive. As a consequence, formic acid and triethylamine may remain unreacted and not form a salt. The variation in formic acid piCa can also have a significant impact on the catalyst and substrate, particularly when this is an imine. 

As the representative data in Table 6.4 indicate, the Zr-catalyzed resolution technology may be applied to medium-ring heterocycles as well in certain instances (e. g. entries 1 and 2), the starting material can be recovered with outstanding enantiomeric purity. Comparison of the data shown in entries 1 and 3 of Table 6.4 indicates that the presence of an aromatic substituent can have an adverse influence on the outcome of the catalytic resolution. The fact that the eight-membered ring substrate in Table 6.4 (entry 4) is resolved more efficiently may imply that the origin of this unfavorable effect is more due to the... 

Feuer and co-workers ° conducted extensive studies into alkaline nitration with nitrate esters, exploring the effect of base, time, stoichiometry, concentration, solvent, and temperature on yields and purity. Reactions are generally successful when the substrate a-proton acidity is in the 18-25 p A a range. Alkoxide bases derived from simple primary and secondary aliphatic alcohols are generally not considered compatible in reactions using alkyl nitrates. Optimum conditions for many of these reactions use potassium tert-butoxide and amyl nitrate in THF at —30 °C, although in many cases potassium amide in liquid ammonia at —33 °C works equally well. 

Similarly, CALB has been used in combination with a palladium/alkaline earth metal-based racemization catalyst to effect a DKR on the benzylic amine 56e (Scheme 2.27). The (R)-amide 57e was obtained in very good yield and excellent optical purity. Several other substrates also underwent the reaction [29],... 

Inhibitors of lactic dehydrogenase have been reported in commercial preparations of NAD+ and NADH (B4, M6, S28). The concentration of inhibitory substances varied from lot to lot. In a serum lactic dehydrogenase study with NAD+ from 8 sources, activities were found to vary from 145 to 75 units (B4). Inhibitors of lactic dehydrogenase activity have also been observed in dialyzates in uremic patients (W8) and in human urine (G8). The purity of available substrate can also effect enzyme activity. Schwartz and Bodansky observed that, in 6 batches of fructose 6-phosphate, all weighed to a 0.5 mM concentration, the actual concentration varied from 0.13 mAf to 0.55 mM (S14). 

The chiral enolate-imine addition methodology was examined in detail (Thiruvengadam et al., 1999). Enolate formation proceeds to completion within an hour at temperatures from — 30 to 0°C with either 1 equiv. TiCl4 or TiClaO-iPr (preformed or prepared in the presence of substrate by addition of TiCl4 and followed by a third of an equivalent Ti(0-iPr)4 and two equivalents of a tertiary amine base). Unlike the aldol process with the same titanium enolate, the nature of the tertiary amine base had no effect on the diaster-eoselectivity. The diastereoselectivity is maximized by careful control of the internal temperature to below — 20°C during the imine addition (2 equiv.) as well as during the acetic acid quench. The purity of the crude 2-amino carboxamide derivatives (17a or... 

The electrowinning process is connected with higher power consumption but, on the other hand, the electrolytically produced zinc has higher purity. Therefore, further investigations are in progress. The main factors that must be considered in electrowinning process are (1) the electrochemical properties of the cathode materials, (2) the effect of ionic impurities in the electrolyte, and (3) the cohesion strength between the deposited metal and its substrate. 

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