Si prochirality

Sesquiterpenoid. 203, 1071 Sex hormone, 1082-1083 Sharpless, K. Barry. 734 Sharpless epoxidation, 735 Shell (electron), 5 capacity of, 5 Shielding (NMR). 442 Si prochirality, 315-316 Sialic acid. 997 Side chain (amino acid), 1020 Sigma (cr) bond, 11 symmetry of, 11 Sigmatropic rearrangement, 1191-1195... 

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. 

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. 

Note that the hydroxyl-bearing carbon of citrate is a prochirality center and contains two identical "arms." Because the initial aldol reaction of acetyl CoA to oxaloacetate occurs specifically from the Si face of the ketone carbonyl group, the pro-S arm of citrate is derived from acetyl CoA and the pro-R arm is derived from oxaloacetate. 

Complexation of (124) and (125) with [ Rh(COD)Cl 2] in the presence of Si(OEt)4, followed by sol-gel hydrolysis condensation, afforded new catalytic chiral hybrid material. The catalytic activities and selectivities of these solid materials have been studied in the asymmetric hydro-gen-transfer reduction of prochiral ketones and compared to that of the homogeneous rhodium complexes containing the same ligands (124) and (125) 307... 

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. 

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 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). 

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). 

Enzymes are highly selective of the substrates with which they interact and in the reactions that they catalyze. This selective nature of enzymes collectively known as enzyme specificity can be best illustrated with oxidoreductases (dehydrogenases), which display substrate and bond specificities (e.g., acting on —CHOH—, versus —CHO versus —CH—CH— versus —CHNH2, and cis versus trans for unsaturated substrates), coenzyme specificity (e.g., NAD(H) versus NADP(H)), chiral stereospecificity (d- versus l- or R- versus S-stereoisomers), and prochiral stereospecificity (A versus B corresponding to proR- versus proS isomers and re face versus si face, respectively). The table lists some dehydrogenases and their coenzyme, substrate, product and stereospecificities (You, 1982) ... 

A pro-chiral molecule may turn chiral after undergoing a symmetry breaking chemical reaction on a surface. An example where covalent bonds to surface atoms are formed is the reaction of frans-2-butene with Si(100) [30]. As with purely adsorption-induced chirality, the relative alignment of the prochiral reactant with respect to the surface plane determines the handedness of the adsorbate complex (Scheme la). 

Scheme 1 a The [2 + 2] cycloaddition product of prochiral trans 2-butene with Si dimers of the Si(100) surface leads to chiral adsorbate complexes, b Hydrogenation of prochiral a-keto esters over platinum is a heterogeneously catalyzed reaction leading to chiral alcohols. Using cinchonidin as chiral modifier makes this surface reaction enantioselective. In a similar fashion, TA-modified nickel is a highly enantioselective catalyst for /3-keto ester hydrogenation... 

The main elements of chirality possibly present in the intermediates and transition states that can be hypothesised within this framework are as follows [1]. Firstly, a prochiral a-olefin molecule, e.g. propylene, coordinating via its two faces at the catalyst active site gives rise to non-superpo sable re and si diastereoisomeric complexes (Figure 3.24) [362, 363]. According to the considered mechanisms, an isotactic polymer is generated by a long series of... 

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