Poor synthesis efficiency

The other situation supposes the formation of stable rings that contain the maximum number of A1 atoms, from a Si-rich gel. This can be a slow process, kinetically limited by the slow A1 incorporation stemming from A1 poor gel phases. Such a system will yield a limited number of Al-rich zeolite nuclei with a fairly low rate. In such a case, a poor "synthesis efficiency", i.e. poor correlation between the Si/Al atomic ratio in the gel and that of the final zeolite, is observed (52) "Figure 5". 

Fig 5 Relationship between Si/Al ratio in the gel and in a series of faujasite zeolite samples crystallized from it example of poor synthesis efficiency (adapted from ref. 53) ... 

The polymerization of 2-(diethylamino)ethyl methacrylate, DEAEMA, was studied under different conditions. It was shown that the best system providing narrow molecular weight distribution polymers involved the use of p-toluenesulfonyl chloride/CuCl/HMTETA as the initiator/catalyst/ligand at 60 °C in methanol [72]. Taking advantage of these results, well-defined PDEAEMA-fr-PfBuMA block copolymers were obtained. The synthesis was successful when either fBuMA or DEAEMA was polymerized first. Poor results with bimodal distributions were obtained when CuBr was used as the catalyst. This behavior was attributed to the poor blocking efficiency of PDEAEMA-Br and the incomplete functionalization of the macroinitiator. 

When mixed with Et2AlCl, the vanadium(III) complex (87) polymerizes propylene at —78 °C in a living manner.241,242 Poor initiator efficiency ( 4%) and low activities were improved by employing complex (88) activities of lOOgmmol h bar 1 were reported and the polymerization of propylene remained living (Mw/Mn= 1.2-1.4) up to 40 °C.243 244 The synthesis of end-functionalized PP and PP copolymers has also been achieved using these initiators. 

The field of solid-phase synthesis instrumentation is continually advancing. Improvements in synthesis reagents, reaction monitoring, and instrument hardware and software will extend the limits of the instrumentation. As synthesizer capabilities improve, there is the potential that more and more control will be taken from the user until the instrument becomes a black box. It is important, however, to maintain an understanding of the principles of instrument operation and the chemistry that is being performed. The instrument is secondary to the chemistry but is an essential tool to help carry out the synthesis efficiently. The best instrument cannot improve ineffective chemistry and, conversely, a poorly designed instrument can compromise a very efficient chemical process. As long as the basic principles of reaction kinetics, fluid mechanics, and instrument safety are sustained, a solid-phase synthesizer can be used to its maximum potential and benefits. 

The synthesis of Arens and van Dorp also has a poor atom efficiency, though not quite as bad as that of Barbier, Bouveault and Tiemann. The use of sodium in liquid ammonia would require refrigeration and the ability to distil and recover ammonia and this would lead to high... 

Flogel et al. [97] described a silicon microreactor (the same as in Figure 1.11) for peptide synthesis, which also allows a quick screening of reaction conditions. Using peptide couplings with Boc- and Fmoc-protected amino acids, significant amounts of peptides could be made in 1-5 min at temperatures as high as 120 °C. Synthesis efficiency was further enhanced by the use of a fiuorous benzyl tag for the assembly of P peptides this method is particularly useful for the purification of poorly soluble products. 

The principal drawback to using alkaloids as catalysts in asymmetric synthesis is the separation of the product from the catalyst. This has been overcome by radical copolymerization of cinchona alkaloids with vinyl monomers in a way that retains the stereoselectivity of the alkaloid. The reaction of dodecanethiol with isopropenyl methyl ketone in toluene in the presence of quinidine acrylonitrile copolymer (21) gives the (+) enantiomer, in excess, 57 % ee, which is claimed to be the highest value ever achieved in the asymmetric reactions catalysed by synthetic organic copolymers. Previous studies with this type of polymeric catalyst had shown poor asymmetric efficiencies. ... 

Wichiansee et al. demonstrated the synthesis of HgS quantum dots following the well established TOP/TOPO route which involved the injection of freshly prepared hot tri- -octylphosphine sulfide (TOPS) solution to mercury(II) acetate in TOPO at room temperature. Then the reaction temperature was raised to 120 °C to produce spherical HgS QDs of ca. 4nm size. The band gap of the QDs was estimated to be 1.2eV with poor quantum efficiency. ... 

Early attempts to apply the Sheehan penicillin synthetic strategy to the total synthesis of cephalosporins were not particularly successful. Although the key step, formation of the /3-lactam CO—N bond, could be carried out efficiently (46->47), subsequent conversion of the lactone to a free C-4 carboxyl could only be accomplished in poor yield (B-72MI51007). 

A problem with employment of ASON in a larger clinical setting is their poor uptake and inappropriate intracellular compartmentalization, e.g., sequestration in endosomal or lysosomal complexes. In addition, there is a need for a very careful selection of the ASON-mRNA pair sequences that would most efficiently hybridize. To date, several computer programs are used to predict the secondary and tertiary structures of the target mRNA and, in turn, which of the mRNA sequences are most accessible to the ASON. However, even with this sophisticated techniques, the choice of base-pairing partners still usually includes a component of empiricism. Despite these principal limitations, it has become clear that ASON can penetrate into cells and mediate their specific inhibitory effect of the protein synthesis in various circumstances. 

The reactions of polymeric anions with appropriate azo-compounds or peroxides to form polymeric initiators provide other examples of anion-radical transformation (e.g. Scheme 7. 6). ""7i However, the polymeric azo and peroxy compounds have limited utility in block copolymer synthesis because of the poor efficiency of radical generation from the polymeric initiators (7.5.1). 

Even though synthesis of organic chemicals in the body occurs very efficiently in water, chemists have traditionally been taught that water is not generally a good solvent for carrying out synthetic reactions, because of either its poor solvent properties or the hydrolytic instability of reagents... 

One reaction that is quite efficient for lithium reagents but poor for Grignard reagents is the synthesis of ketones from carboxylic acids.112 The success of the... 

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