Re prochirality

In a similar manner, butadienyl phenylacetate 71, an achiral diene, is expected to approach the chiral dienophile (R)-10 from its Re-prochiral face. The two faces of the chelate ring are differentiated by the small hydrogen and large benzyl groups attached to the chiral center of (R)-10 (Scheme 1-18) the ratio of the Si attack product to the Re attack product is 1 8.88... 

The enzyme-catalyzed interconversion of acetaldehyde and ethanol serves to illustrate a second important feature of prochiral relationships, that ofprochiral faces. Addition of a fourth ligand, different from the three already present, to the carbonyl carbon of acetaldehyde will produce a chiral molecule. The original molecule presents to the approaching reagent two faces which bear a mirror-image relationship to one another and are therefore enantiotopic. The two faces may be classified as re (from rectus) or si (from sinister), according to the sequence rule. If the substituents viewed from a particular face appear clockwise in order of decreasing priority, then that face is re if coimter-clockwise, then si. The re and si faces of acetaldehyde are shown below. 

The hand-in-glove fit of a chiral substrate into a chiral receptor is relatively straightforward, but it s less obvious how a prochiral substrate can undergo a selective reaction. Take the reaction of ethanol with NAD+ catalyzed by yeast alcohol dehydrogenase. As we saw at the end of Section 9.13, the reaction occurs with exclusive removal of the pro-R hydrogen from ethanol and with addition only to the Re face of the NAD+ carbon. 

A molecule is prochiral if can be converted from achiral to chiral in a single chemical step. A prochiral sp2-hybridized atom has two faces, described as either Re or Si. An sp3-hybridized atom is a prochirality center if, by changing one of its attached atoms, a chirality center results. The atom whose replacement leads to an R chirality center is pro-R, and the atom whose replacement leads to an S chirality center is pro-S. 

Complex 7-AI2O3/PTA/ (/< ./< )-(Mc-DuPHOS)Rh(COD) 1 (1) was prepared and tested in the hydrogenation of the prochiral substrate methyl-2-acetamidoacrylate (MAA). After full conversion, the products were separated from the catalyst and analyzed for Rh and W content and product selectivity. The catalyst was re-used three times. Analytical results show no rhodium leaching is observed. Complex 1 maintains its activity and selectivity in each successive run. The first three runs show tungsten (W) leaching but after that no more W is detectable. The leached W comes from the excess of PTA on alumina. The selectivity of both tethered and non-tethered forms gave the product in 94% ee. 

Possible elements of chirality in stereospecific polymerizations will be briefly recalled in order to indicate the used terminology. First of all, upon coordination, a prochiral olefin such as propene gives rise to not superpos-able si and re coordinations.22 According to the mechanism described, the isotactic polymer is generated by a large series of insertions of all si- or all re-coordinated monomers, while the syndiotactic polymer would be generated by alternate insertions of si - and re-coordinated monomers. 

Due to re- and si-coordination of prochir-al substrates at a catalyst with C2-sym-metric chiral ligands two diastereomeric catalyst-substrate complexes emerge. In the case of C,-symmetric ligands already four stereoisomer intermediates result. 

When an appropriate chiral phosphine ligand and proper reaction conditions are chosen, high enantioselectivity is achievable. If a diphosphine ligand with C2 symmetry is used, two diastereomers for the enamide-coordinated complex can be formed because the olefin can interact with the metal from either the Re- or Sf-face. Therefore, enantioselectivity is determined by the relative concentrations and reactivities of the diastereomeric substrate-Rh complexes. It should be mentioned that in most cases it is not the preferred mode of initial binding of the prochiral olefinic substrate to the catalyst that dictates the final stereoselectivity of these catalyst systems. The determining factor is the differ-... 

The results of the study of the last-mentioned reaction, DCA-PhCOCH3 —> 167, provided a surprise. X-Ray analyses of the structure of the clathrate before and after partial reaction, and of the final product, 167, showed that the prochiral Re face of the ketone, initially the face more distant from the to-be-attacked host, and not the close-lying Si face, is the one that adds to the steroid. A rationalization of this extraordinary sterochemical effect, which results in formation of a new chiral center with quantitative asymmetric induction, has been proposed (241). 

Site control versus chain-end control. Over the years two mechanisms have been put forward as being responsible for the stereo-control of the growing polymer chain firstly the site-control mechanism and secondly the chain-end control mechanism. In the site control mechanism the structure of the catalytic site determines the way the molecule of 1-alkene will insert (enantiomorphic site control). Obviously, the Cossee mechanism belongs to this class. As we have seen previously, propene is prochiral and a catalyst may attack either the re-face or the, v/-facc. If the catalyst itself is chiral as the one drawn in Figure 10.2, a diastereomeric complex forms and there may be a preference for the... 

The reactions of l,3-thiazolium-4-olates with aliphatic aldehydes carried out in refluxing benzene or dichloromethane, have been reported to produce a series of highly functionalized (3-lactams and thiiranes at the same time [230]. The critical issue of the stereoselection was discussed in terms of the endo and exo approaches (respective to the aldehyde substituent) to any enantiotopic face of the heterocyclic dipole. Such orientations involved either the Re or the Si faces of the prochiral aldehydes (Scheme 105). 

In this Section we shall use the ideas of prochirality in assignment of stereochemical configuration S8) (usually relative — especially meso vs. dl — rather than absolute configuration) and we shall also discuss assignment of prochirality symbol (i.e. recognition of which group is pro-R and which pro-S at a prochiral center). (Recognition of prochiral faces as Re or Si is usually obvious from the stereochemistry of the addition products thereto and will not be discussed here examples are found in Section 5.2). 

Furthermore, in the case of the asymmetric catalytic system containing rhodium and (—)-DIOP always the same, prochiral face (re) is preferentially formylated for six other monosubstituted olefins (Table 7, column 1). Similar results are obtained with rhodium catalysts when monophosphines are used instead of DIOP. The only... 

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

The investigation of platinum(II)-chiral olefin complexes has shown that, when the diastereomeric equilibrium is reached, which diastereoface of the olefin is preferentially bound to the metal depends on the type of chirality of the olefin used61-63. When an optically active asymmetric ligand is present in the complex and a racemic olefin, is used, one diastereoface will be preferred for complexation and correspondingly one of the antipodes is preferentially complexed61 63). Let us suppose that with a certain catalytic system (e.g., Rh/(—)-DIOP), the re-re enantioface of a prochiral a-olefin reacts preferentially. With the same catalytic system the same face of all a-olefins, including the racemic a-olefins, is expected to react preferentially. However, when a racemic olefin is used, two diastereomeric transition states (e.g. a and b in Fig. 11) can form for each of the transition states shown in Fig. 7, depending on which one of the antipodes of the racemic monomer approaches the catalyst. 

As we mentioned earlier (Sect. 2.1.5.), a further complication arises from the fact that, with the exception of the C2v or C2h olefinic substrates, two isomeric reaction products could be formed by cis attack of the metal hydride to one face of the prochiral substrate. In principle, if rc-olefin complexes are intermediates, the isomeric ratio could be determined in the re-complex formation, two non-interconvertible conformers of each of the two diastereomeric rc-complexes being formed. Each conformer then gives rise to a different structural isomer of the reaction products (Fig. 14, paths a, c and a, c ). 

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