Repetition of reactions

Successive repetition of reactions (Equation 4.26 or Equation 4.27) lead to formation of Bi°. These redox reactions act as termination steps for polymer degradation. Radical terminations by metal compounds through redox reaction are well known.35-58... 

Thus, chain transfer to polymer does not influence the number average DP , it may however alter the molecular weight distribution. If the reversible chain transfer to polymer described in Eq. (78) occurred frequently, it would lead to statistical distribution, i.e., MJM = 2. The other consequence is that if the two originally present chains are different, the repetition of reaction sequence will lead to segmental exchange (so called scrambling ). Both effects are clearly detectable, for example, in the cationic polymerization of cyclic acetals as it will be discussed in Section III.B. 

Many reaction schemes are discussed and rate equations derived taking into account a variety of possible photophysical processes. To avoid the tiring repetition of reaction schemes, mechanisms, differential equations, and evaluation approaches which are sometimes very similar, the results are summarised in an appendix where the appropriate formalism can be selected. Some of the theoretical derivations and definition are explained in Examples. They are intended to help to set up relationships and to follow the derivation of equations. 

The total process (6-138) imphes double repetition of reaction (6-37) and triple repetition of reaction (6-136). An alternative mechaiusm to (6-134)-(6-137) that also can be... 

The microstructured extraction devices are attractive for process intensification, since they allow precisely controlled and rapid extraction and the small dimensions permit their flexible use in multiple-stage processes including the repetition of reaction and extraction operations. These advantages are effective for reaction... 

The solid-phase synthesis of oligonucleotides consists basically of the sequential repetition of reaction steps similar to those previously described, with the properly protected monomeric nucleotides (for reviews, see Kossel and Seliger, 1975 Amarnath and Broom, 1977). The essential steps are like those used in solid-phase peptide synthesis. 

Finally, the reaction time data and the ERP-data are correlated with each other. It is convenient to take these reaction time data which lie closest to the ERP data chronologically. It is known that the length of the linear pathway varies from series to series.Therefore, the second series of reaction times data was used for the comparison because the ERP task is a repetition of reaction tasks ie. a third series. 

Comparable to peptide synthesis, the liquid-phase synthesis of oligonucleotides is based on the sequential repetition of reaction steps, thus adding one monomeric nucleotide to the other (Scheme 4). 

But)rryl-S-ACP may then condense with malonyl-S-ACP in a reaction analagous with eqn. 60 above when repetition of reactions 61, 62 and 63 will result in the production of propionyl-S-ACP. Thus each complete repetition of the s)rstem described above results in the addition of two carbon atoms to the primer fatty acid until eventually a long chain fatty acid may be built up. 

Constmction of multilayers requires that the monolayer surface be modified to a hydroxylated one. Such surfaces can be prepared by a chemical reaction and the conversion of a nonpolar terminal group to a hydroxyl group. Examples of such reactions are the LiAlH reduction of a surface ester group (165), the hydroboration—oxidation of a terminal vinyl group (127,163), and the conversion of a surface bromide using silver chemistry (200). Once a subsequent monolayer is adsorbed on the "activated" monolayer, multilayer films may be built by repetition of this process (Fig. 8). 

The subject matter of this chapter is arranged largely on the basis of reaction type rather than on specific functionality to be introduced. This arrangement allows a balanced presentation of the various methods and avoids repetition. Reference to the general scope of a given reaction is provided in the text when alternate methods are available for the preparation of a particular system. 

In essence, this series of four reactions has yielded a fatty acid (as a CoA ester) that has been shortened by two carbons, and one molecule of acetyl-CoA. The shortened fatty acyl-CoA can now go through another /3-oxidation cycle, as shown in Figure 24.10. Repetition of this cycle with a fatty acid with an even number of carbons eventually yields two molecules of acetyl-CoA in the final step. As noted in the first reaction in Table 24.2, complete /3-oxidation of palmitic acid yields eight molecules of acetyl-CoA as well as seven molecules of FADHg and seven molecules of NADFI. The acetyl-CoA can be further metabolized in the TCA cycle (as we have already seen). Alternatively, acetyl-CoA can also be used as a substrate in amino acid biosynthesis (Chapter 26). As noted in Chapter 23, however, acetyl-CoA cannot be used as a substrate for gluco-neogenesis. 

The outcomes of intramolecular cyclizations of hydroxy vinylepoxides in more complicated systems can be difficult to predict. In a study of the synthesis of the JKLM ring fragment of dguatoxin, epoxide 44 was prepared and subjected to acid-mediated cydization conditions (Scheme 9.24) [114]. Somewhat surprisingly, the expected oxepane 45 was not formed, but instead a mixture of tetrahydropyran 46 and tetrahydrofuran 47 was obtained, both compounds products of attack of the C6 and C5 benzyl ether oxygens, respectively, on the allylic oxirane position (C3). Repetition of the reaction with dimsylpotassium gave a low yield of the desired 45 along with considerable amounts of tetrahydropyran 48. 

Racemic l-methyl-2-butenylboronates (E)- and (Z)-3 may be prepared selectively via reactions of the l-methyl-2-butenyl Grignard reagent with the appropriate borate ester. Use of triisopropyl borate provides a 96 4 mixture of (E)-3l(Z)-3 on a 0.36 mol scale15. Use of a bulkier borylating agent, such as 2-isopropyloxy-4,4,5,5-tetramethyl-l,3,2-dioxaborolane, reverses the selectivity, enabling a 91 9 mixture of (Z)-3/( )-3 to be obtained on a 0.5 mol scale. The diastereomeric purity of this mixture may be enhanced to 95 5 by treatment with 0.15 equivalents of benzaldehyde, since ( )-l-mcthyl-2-butenylboronatc ( )-3 is more reactive than (Z)-3. Repetition of this process provides (Z)-3 that is 98% isomerically pure. 

Infrared (IR) investigations can be made on a sample of reactant previously heated to a known extent of reaction (a) and studied in the form of a mull or in an alkali halide disc. An alternative approach is to incorporate the reactant substance in a compact alkali halide disc [287] which is intermittently withdrawn from the reaction vessel for infrared measurements at appropriate intervals. Heated sample holders [288,289] permit repetitive scanning of the spectrum or continuous monitoring of a peak of interest during decomposition. 

Under nitrogen, anhydrous DMF (10 mL) was added to a mixture of 83 (0.240 g, 0.5 mmol), 84 (0.063 g, 0.5 mmol), and Et3N (1 mL). Pd(PPh3)4 (0.027 g, 0.025 mmol) and Cul (0.005 g, 0.025 mmol) were then added and the reaction mixture was stirred at 100°C for 48 h. After being cooled to room temperature, die reaction mixture was poured into MeOH and filtered. The solid was washed with MeOH and dried under vacuum. The repetition of the precipitation procedure gave polymer 85 as orange powder in 96% yield (0.212 g). GPC (polystyrene standards) Mn — 15.100. 

The mechanism of the orf/to-dibromination of phenol with NBS in the presence of amines is considered as follows. The hydrogen bonding between phenol and N-bromoamine which are generated from the reaction of NBS and amines (ref. 14), is the driving force, and causes the bromination at one o/t/io-position of phenol and regeneration of the amines. A catalytic amount of the amines is enough because of the regeneration of the amines. The repetition of the above process causes one more substitution at the other orf/io-position of 2-bromophenol. In the cases of 2-substituted phenols the orf/io-bromination can occur only once (Scheme 5). 

The second analytical method uses a combustion system (O Neil et al. 1994) in place of reaction with BrF,. This method was used for the crocodiles because they were represented by very thin caps of enamel. The enamel was powdered and sieved (20 mg), pretreated in NaOCl to oxidize organic material and then precipitated as silver phosphate. Approximately 10-20 mg of silver phosphate were mixed with powdered graphite in quartz tubes, evacuated and sealed. Combustion at 1,200°C was followed by rapid cooling in water which prevents isotopic fractionation between the CO2 produced and the residual solid in the tube. Analyses of separate aliquots from the same sample typically showed precisions of 0.1%o to 0.4%o with 2 to 4 repetitive analyses even though yields are on the order of 25%. 

We succeeded in showing that recycling of the enzyme was indeed possible in our IL solvent system, though the reaction rate gradually dropped with repetition of the reaction process. Since vinyl acetate was used as acyl donor, acetaldehyde was produced hy the hpase-catalyzed transesterification. It is well known that acetaldehyde acts as an inhibitor of enzymes because it forms a Schiff base with amino residue in the enzyme. However, due to the very volatile nature of acetaldehyde, it easily escapes from the reaction mixture and therefore has no inhibitory action on the lipase. However, this drop in reactivity was assumed to be caused by the inhibitory action of acetaldehyde oligomer which had accumulated in the [bmim][PFg] solvent system. In fact, it was confirmed that the reaction was inhibited by addition of acetaldehyde trimer. =... 

One of the most important characteristics of IL is its wide temperature range for the liquid phase with no vapor pressure, so next we tested the lipase-catalyzed reaction under reduced pressure. It is known that usual methyl esters are not suitable for lipase-catalyzed transesterification as acyl donors because reverse reaction with produced methanol takes place. However, we can avoid such difficulty when the reaction is carried out under reduced pressure even if methyl esters are used as the acyl donor, because the produced methanol is removed immediately from the reaction mixture and thus the reaction equilibrium goes through to produce the desired product. To realize this idea, proper choice of the acyl donor ester was very important. The desired reaction was accomplished using methyl phenylth-ioacetate as acyl donor. Various methyl esters can also be used as acyl donor for these reactions methyl nonanoate was also recommended and efficient optical resolution was accomplished. Using our system, we demonstrated the completely recyclable use of lipase. The transesterification took place smoothly under reduced pressure at 10 Torr at 40°C when 0.5 equivalent of methyl phenylthioacetate was used as acyl donor, and we were able to obtain this compound in optically pure form. Five repetitions of this process showed no drop in the reaction rate (Fig. 4). Recently Kato reported nice additional examples of lipase-catalyzed reaction based on the same idea that CAL-B-catalyzed esterification or amidation of carboxylic acid was accomplished under reduced pressure conditions. ... 

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