Substrate Scope and Limitations

Since one aldose of any epimeric pair is usually more easily available than the other, either directly isolable from its natural source or in combination with a chain-length modification, the apphcation of the Bilik reaction enables the other, less-available aldose to be obtained. Thus, the following equilibria can be obtained  

D-glycero-L-gluco-HeTptose/D-glycero-L-manno-h.eTptose 4 1 [12] 

D-glycero-D-galacto-Heptose/D-glycero-D-talo-heptose 7-Deoxy-L-glycero-L-galacto-heptosel 4 1 [13] 

The essentiality of the OH group at C-2 for the epimerization of aldoses is obvious. The same is valid for the carbonyl group of aldoses since alditols do not undergo epimerization changes in molybdic acid [20,21]. 

The most stable epimeric pair of aldoses, D-glucose and D-mannose,has been often used as the model system for optimization, kinetic and mechanistic studies of the reaction. 

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Amide substrate scope and limitations were investigated using allyl alcohol 6a and 2-5 mol% of Bi(OTf)3 and KPF6 (Table 9). Using sulfonamides with electron-donating and electron-withdrawing substituents, the reaction completed within... 

The substrate scope is limited, as electron-withdrawing groups (X = p-N02 or p-CF3) on the aromatic substituent are not tolerated. However, this route does provide valuable intermediates to unnatural a-amino phosphonic acid analogues and the sulfimine can readily be oxidized to the corresponding sulfonamide, thereby providing an activated aziridine for further manipulation, or it can easily be removed by treatment with a Grignard reagent. 

The aldehyde structures and the tosylhydrazone salts were varied in an extensive study of scope and limitations, with use of both achiral and chiral sulfur ylides [73]. Aromatic aldehydes were excellent substrates in the reaction with benzaldehyde-derived ylides, whereas aliphatic aldehydes gave moderate yields and transxis ratios. 

Now using the best experimental conditions, it was possible to investigate raughly the scope and limitations of the arylation of alcohols regarding both the substrates the alcohol on one hand and the arylbromide on the other one (Fig. 13). 

Table 3 summarizes the scope and limitation of substrates for this hydrogenation. Complex 5 acts as a highly effective catalyst for functionalized olefins with unprotected amines (the order of activity tertiary > secondary primary), ethers, esters, fluorinated aryl groups, and others [27, 30]. However, in contrast to the reduction of a,p-unsaturated esters decomposition of 5 was observed when a,p-unsaturated ketones (e.g., trans-chalcone, trans-4-hexen-3-one, tra s-4-phenyl-3-buten-2-one, 2-cyclohexanone, carvone) were used (Fig. 3) [30],... 

Recently, the iron-promoted Barbier-type addition of alkyl halides to aromatic aldehydes has been reported (Equation (26)).326 According to the proposed mechanism, the initial step is the formation of an alkyl radical, which can be reduced to the corresponding carbanion. This carbanion nucleophile can react, while coordinated to the iron pentacarbonyl complex, with the corresponding aldehyde. This stoichiometric method is limited with respect to substrate scope and yield. The same authors have also developed the Reformatsky-type addition of cr-halosub-stituted carbonitriles to aldehydes and ketones in the presence of iron pentacarbonyl.3... 

The mechanism of iridium-catalyzed hydrogenation remains unclear. Although several experimental [31, 53, 54] and computational [53, 55, 56] studies have been reported recently, further investigations will be necessary to establish a coherent mechanistic model. Until now, most studies have dealt with simple test substrates hence, it will be important to explore more complex and also industrially important substrates, in order to determine the full scope and limitations of iridium catalysis. 

Organocopper chemistry is of wide applicability, very efhdent and easy to perform. The main problem is to know the most appropriate reagent to use. The reader will find in this book all the details for the reagent of choice, for the scope and limitations, for the type of substrate needed. This book should be helpful not only to advanced research chemists, but also for teaching this chemistry to younger... 

To date, only a few iridium catalysts have been applied to industrially relevant targets, especially on the larger scale. It is likely that several types of Ir catalyst are, in principle, feasible for technical applications in the pharmaceutical and agrochemical industries. At present, the most important problems are the relatively low catalytic activities of many highly selective systems and the fact, that relatively few catalysts have been applied to multifunctional substrates. For this reason, the scope and limitations of most catalysts known today have not yet been explored. For those in academic research, the lesson might be to employ new catalysts not only with monofunctional model compounds but also to test functional group tolerance and-as has already been done in some cases-to apply the catalysts to the total synthesis of relevant target molecules. 

Many of the 60 known reactions catalyzed by monoclonal antibodies involve kinetically favored reactions e.g., ester hydrolysis), but abzymes can also speed up kinetically disfavored reactions. Stewart and Benkovic apphed transition-state theory to analyze the scope and limitations of antibody catalysis quantitatively. They found the observed rate accelerations can be predicted from the ratio of equilibrium binding constants of the reaction substrate and the transition-state analogue used to raise the antibody. This approach permitted them to rationalize product selectivity displayed in antibody catalysis of disfavored reactions, to predict the degree of rate acceleration that catalytic antibodies may ultimately afford, and to highlight some differences between the way that they and enzymes catalyze reactions. 

Cyclic mew-configurated 1,2-dicarboxylic acid dimethyl esters are excellent substrates for pig liver esterase90. Cyclopropanedicarboxylales have been studied not only for synthetic reasons, but also so that an active-site and/or substrate model of pig liver may be developed13 5. The results obtained, compounds 11-17, are a good demonstration of the scope and limitation of PLE in asymmetric synthesis. Enantiomeric excesses of the monoesters can be determined by conversion into the amides with (S)-l-phenylethylamine and analysis either by GC or H-NMR spectroscopy, whereas the absolute configuration rests on chemical correlation. 

Although most transition metal catalyzed processes are built up of similar steps, they are usually divided into categories (sometimes name reactions) by the synthetic chemists. This classification is usually made on the basis of their synthetic utility rather than on mechanistic considerations. This chapter gives an overview of the most commonly used reactions, briefly outlining their mechanism as well as the scope and limitation of substrates in these processes. 

The substrate range - scope and limitations The reaction can be performed efficiently with a broad variety of ketone donors and aldehydes. Enantioselectivity, however, depends on the enolate structure (Scheme 6.11) [60, 61]. In general, eno-lates bearing larger, branched alkyl groups or a phenyl group result in lower enantioselectivity. The best results were obtained with enolates bearing a methyl substituent (product (S)-16, 87% ee) or a siloxymethyl substituent (product (S)-17, 86% ee). 

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