Prochiral atoms

We can view this reaction as the replacement of one or the other of the two methylene protons at C 2 of butane These protons are prochiral atoms and as the red and blue protons m the Newman projection indicate occupy mirror image environments... 

Tactic macromolecule, essentially comprising only one species of configurational base unit, which has chiral or prochiral atoms in the main chain in a unique arrangement with respect to its adjacent constitutional units. 

Isotactic polymer that contains two chiral or prochiral atoms with defined stereochemistry in the main chain of the configurational base unit. 

Note The absence of one or more of the dashes from a chiral or prochiral atom, or of dashes from atoms linked by a double bond, signifies lack of knowledge about the configuration of the corresponding site of stereoisomerism or lack of intention to specify it [1, 3]. 

The re face of the prochiral atom is preferably formylated at temperatures below 100 °C... 

The results of the hydroformylation of internal olefins are reported in Table 9. In the case of (Z)- and (E)-2-butene, the same fare of the unsaturated carbon atom is formylated with either a rhodium- or platinum (—)-DIOP-containing catalytic system. With the rhodium catalyst, when an acyclic olefin is used as the substrate, the same fare is always attacked, and it is only the notation but not the geometric requirement that is different for (E)-l-phenyl-1-propene. The only exception is represented by bicyclo[2,2,l]heptene. However, using (—)-CHIRAPHOS instead of (—)-DIOP, also bieyelo[2,2,l]heptene behaves like internal butenes. No regularity is observed for the cobalt or ruthenium (—)-DIOP catalytic systems. With the same system, only in 3 cases out of 15 the face of the prochiral atom preferentially formylated has different geometric requirements. 

Any reaction that forms a bond between two prochiral atoms in a stereoselective manner is a valuable synthetic method. Some of the natural products that have been made in nonracemic form using the [2,3]-Wittig rearrangement as the key step are illustrated in Figure 6.7. The stereocenters formed in the Wittig rearrangement are indicated ( ). 

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). 

As noted above, synthetically important prochiral centers are the carbonyl of a ketone or aldehyde and the double bond of an alkene. These functional groups do not contain a pro-R or pro-S group but it is clear that delivery of a fourth point ligand from one face or the other will lead to an (R) or (5) chiral center, as in conversion of 108 to 109 and/or 110. If the carbonyl is oriented as in ketone lllA, priorities can be assigned to the three atoms connected to the prochiral atom, based on the CIP rules. For lllA, the a b c priority is... 

Determine the correct re/si label for each prochiral atom in the following molecules. 

The different chemical shifts that arise from enantiotopic atoms and groups in chiral solvents can be explained by assuming that the chiral solvent or additive must be part of the solvation shell of the solute molecule. Thus, the presence of chiral molecules nearby will result in different intermolecular interactions depending, on the right- or left-handedness of the prochiral atoms or groups. 

Chirality kI-rol adj [chir- -h -a/] (1894) The property of an organic molecule of not being identical with its mirror image a compound whose molecules are chiral can exist as enantiomers, but non-chiral compounds cannot be enantiomers. All asymmetric molecules are chiral however, not all chiral molecules are asymmetric since some having axes of rotational symmetry are chiral. Chiral and prochiral atoms are sites or potential sites, respectively, of Stereoisomerism. 

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