Asymmetric synthesis classification

Basie definitions of terms relating to polymerization reactions [1,2] and stereochemical definitions and notations relating to polymers [3] have been published, but no reference was made explieitly to reaetions involving the asymmetric synthesis of polymers. It is the aim of the present doeument to recommend classification and definitions relating to asymmetrie polymerizations that may produce optically active polymers. 

Reactions that involve asymmetric synthesis are traditionally classified separately from other dia-stereoselective transformations of chiral substrates, even though there is little fundamental differoice between them. The degree of success realized in both categories depends on the ability of the chemist to distinguish between competing, diastereomeric transition states the critical objective is to maximize AAG - This classification system undoubtedly evolved since the chiral auxiliary used in asymmetric reactions, whether it is introduced as part of a catalyst or is covalently bound to the substrate, is not destined to be an integral structural component of subsequent transformation products, while the reverse situation obviously pertains in the more frequently encountered diastereoselective transformations of chiral substrates. Work that has been reported for asymmetric IMDA reactions is summarized in this section." ... 

In this section, after a classification of the different types of membrane reactors, selected examples including some of the most recent developments in asymmetric synthesis, highlight the potential of this approach (cf. Sections 2.9 and 3.3.1). 

There are many reports on the synthetic uses of TMG (1) and its analogues such as Barton s base (2). In this section, their synthetic roles in organic synthesis will be discussed according to their tentative classification into three categories [addition (catalytic reaction), substimtion (stoichiometric reaction) and others] from the view points of a landmark for guanidine mediated asymmetric synthesis. 

The multitude of terms in the literature, describing the outcome of a given chemical transformation, is a result of the need to emphasize a particular characteristic or selective aspect of a given transformation, e.g. stereoselectivity of the process, optical purity of the product(s), the generation or destruction of an asymmetric center during the transformation, etc.. Asymmetric synthesis, chiral synthesis, asymmetric induction, asymmetric destruction, kinetic resolution, asymmetric desymmetrization are such terms - ones that have described well, specific aspects of a wide variety of reactions. To date, there has been no attempt to depict all of these aspects as parts of a "big picture." Indeed, the problem of a systematic universal classification of chemical transformations has remained unsolved. 

The ultimate source of chirality in all asymmetric synthesis is nature. In this chapter we survey the most important naturally occurring chiral compounds and the ways in which they can be used. This is followed by a classification of the known methods of asymmetric synthesis and a general consideration of their mechanisms. 

Chirality element enumeration is essential for the classification of stereoselective reactions 27>. For instance, in order to distinguish an asymmetrically induced synthesis from other reactions whose stereoselectivity is also due to a chiral reference system, one must compare the number of chirality elements in the starting materials and the products. 

Asymmetric bond disconnection is less frequently employed than asymmetric bond formation for the synthesis of chiral, nonracemic compounds. The substrates for these transformations contain either enantiotopic (diastereotopic) hydrogen atoms or enantiotopic (diastereotopic) functional groups. In some cases the classification of a given transformation of such a substrate as asymmetric bond disconnection or bond formation is somewhat arbitrary. Thus, enantiotopic and diastereotopic group differentiation is also described at appropriate places in various sections but more specifically in part B of this volume. 

For the synthesis of stereospecific metallocene catalysts for propylene polymerization, C2 symmetric precursors are necessary to obtain a catalyst for isospecific polymerization, and C5 symmetric precursors to produce a catalyst for syndiospecific polymerization. Asymmetric precursors can be used to synthesize metallocene catalysts that produce hemiisotactic and isotactic-stereoblock PP. Farina et al. [7] have proposed an useful classification of metallocene catalysts based on their symmetry (Figure 3). 

In this chapter, we first give an overview of carbon membrane materials (Section 10.2) and the classification of carbon membranes (Section 10.3). Then, unsupported carbon membranes, based on planar membranes and asymmetric hollow fiber membranes are discussed (Section 10.4). In Section 10.5, the supported CMSMs are reviewed in detail in terms of precursors, supports, fabrications and problems. In Section 10.6, carbon-based membrane reactors are discussed in detail, based on the topics of dehydrogenation reactions, hydration reactions, hydrogen production reactions, H2O2 synthesis, bio-diesel synthesis, and new carbon membranes for carbon membrane reactors (CMRs). In the end, the new concept of using carbon membranes in microscale devices (microcarbon-based membrane reactor) is outlined (Section 10.7). 

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