Prochiral groups

When the N-1 atom of salts of type (71) bears two prochiral groups an anisochrony of protons analogous to that of diethyl acetals is observed. For instance in compound (74) the benzylic protons show chemical shifts at 8 4.98 and 5.99p.p.m. with Ja.b = 12.2Hz (690MR(1)249). 

Reaction of an achiral (and prochiral) group within a chiral substrate with an achiral (or chiral) reagent. 

From the above observation, we proposed that the reaction rate should be explained by the size of the reaction cavity for the reactive group and that the chirality of the produced group should depend on the shape of the prochiral group. Using the concept of a reaction cavity, more complicated processes such as racemic-to-chiral transformation and chirality inversion will be made clear. 

Chiral derivatives or compounds having a prochiral group in the reactive center are frequently present among substrates, amines, and aldehydes used as reagents in Mannich synthc.sis. When at least two out of the three reagents are chiral or prochiral, the resulting Mannich base will be made up of a diastcrcomeric mixture of products. The main possible combinations of chiral or prochiral couples of reactants leading to diastereo-meric derivatives are reported in Fig. 36. 

A more direct approach to the enantioselective hydrogenation of a prochiral group involves the use of a chiral catalyst for the reaction. " At present most of these reactions are run using chiral homogeneous catalysts but the... 

Many prochiral groups, however, cannot be interchanged by any type of symmetry operation, because of the presence of one or more chiral centers in the molecule. They are diastereotopic and anisochronous, and they constitute AB, AX, etc., systems. For this reason, when encountering prochiral groups, chemists should expect them to be diastereotopic and should be, perhaps, pleasantly surprised when they are not. 

Before further investigating the matter of the chemical-shift nonequivalence of diastereotopic, prochiral groups in the presence of a chiral center, let us examine examples of... 

Let us next consider the methylene protons of the molecule HaHbYC-CRiR2X, shown in Figure A7-3, and focus primarily on the prochiral group comprising Ha and Hb- If R] = R2, these two protons are chemically equivalent and enantiotopic by virtue of a a... 

The R, S system becomes very complex when there are chains of > CHOH groups, which is why the Fischer-Rosanolf system is still used in carbohydrate chemistry. However, the Cahn-Ingold-Prelog system has to be used to distinguish between prochiral groups. 

In this review, we shall concentrate on the stereochemistry of enzymic reactions of amino acids, many of which involve transformations at prochiral centers. We shall use the nomenclature of Hanson (8) to specify the stereochemistry of prochiral atoms and groups as pro-R (Hjj) and pro-S (Hj) and of prochiral faces as Re and Si and the nomenclature of Mislow and Raban (2) to describe prochiral groups as having enantiotopic or diastereotopic relationships. Reviews on the stereochemistry of enzymic reactions of amino acids were published in 1978 (9,10), and since the seminal review by Dunathan in 1971 (11), several reviews comparing the stereochemistry of pyridoxal phosphate-catalyzed enzymic reactions have appeared (12-15). 

Enantiotopic groups are sometimes called prochiral groups. When one or the other of these groups is replaced by a different one, a chiral molecule results. The original molecule is not chiral, but it can produce a chiral molecule. The reaction of prochiral molecules with a chiral reagent, such... 

A special notice is given to the chiral and prochiral groups in molecules. Mathematically, chirality can be endowed with a potential by defining the so called chiral volume Vabcd that is the spade product defined as ... 

Margitfalvi et al. suggested that the catalytic systems Pt- and Pd-cinchona alkaloid-keto esters, are the first examples of a new class of heterogeneous catalytic reactions of supramolecular catalysis, where the prochiral group is part of a conjugated double bond system. 

It is clear that polymerization of MMA via the unimolecular mechanism offers the greatest prospect for both control of polymer MWD, commoner sequence distributions, as well as tacticity. As chiral and prochiral group 4 metallocene complexes are more readily available than their group 3 or lanthanide analogs, we can expect continued effort in this area will eventually lead to a wide range of applications to polymer synthesis. 

Since an asymmetric synthesis is one that preferentially involves only one prochiral face of a planar substrate or reacts with only one prochiral group on a tetrahedral atom, some terminology is needed to unambiguously describe such species. 

Asymmetric synthesis represents a robust field of research, with entire journals and books devoted to the topic. The current literature is full of examples of new and improved chiral reagents, catalysts, and auxiliaries that can be used for enantioselective syntheses. In every case, a chiral environment is produced that favors reaction with one prochiral face of a planar center, or one prochiral group of a tetrahedral center. The ongoing research seeks to not only develop... 

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