Prochiral face

In this example addition to the double bond of an alkene converted an achiral mol ecule to a chiral one The general term for a structural feature the alteration of which introduces a chirality center m a molecule is prochiral A chirality center is introduced when the double bond of propene reacts with a peroxy acid The double bond is a prochi ral structural unit and we speak of the top and bottom faces of the double bond as prochiral faces Because attack at one prochiral face gives the enantiomer of the com pound formed by attack at the other face we classify the relationship between the two faces as enantiotopic... 

The double bond m 2 methyl(methylene)cyclohexane is prochiral The two faces however are not enantiotopic as they were for the alkenes we discussed m Section 7 9 In those earlier examples when addition to the double bond created a new chirality cen ter attack at one face gave one enantiomer attack at the other gave the other enantiomer In the case of 2 methyl(methylene)cyclohexane which already has one chirality center attack at opposite faces of the double bond gives two products that are diastereomers of each other Prochiral faces of this type are called diastereotopic... 

FIGURE 17 14 (a) Binding sites of enzyme discriminate between prochiral faces of substrate One prochiral face can bind to the enzyme better than the other (b) Reaction attaches fourth group to substrate producing only one enantiomer of chiral product... 

Reaction of an achiral reagent with a molecule exhibiting enantiotopic faces will produce equal quantities of enantiomers, and a racemic mixture will result. The achiral reagent sodium borodeuteride, for example, will produce racemic l-deM/eno-ethanol. Chiral reagent can discriminate between the prochiral faces, and the reaction will be enantioselective. Enzymatic reduction of acetaldehyde- -[Pg.106]

Chiral chemical reagents can react with prochiral centers in achiral substances to give partially or completely enantiomerically pure product. An example of such processes is the preparation of enantiomerically enriched sulfoxides from achiral sulfides with the use of chiral oxidant. The reagent must preferential react with one of the two prochiral faces of the sulfide, that is, the enantiotopic electron pairs. 

Enzymatic oxidation of naphthalene by bacteria proceeds by way of the intermediate ciy-diol shown. Which prochiral faces of C-1 and C-2 of naphthalene are hydroxylated in this process ... 

When a chiral ansa-type zirconocene/MAO system was used as the catalyst precursor for polymerization of 1,5-hexadiene, an main-chain optically active polymer (68% trans rings) was obtained84-86. The enantioselectivity for this cyclopolymerization can be explained by the fact that the same prochiral face of the olefins was selected by the chiral zirconium center (Eq. 12) [209-211]. Asymmetric hydrogenation, as well as C-C bond formation catalyzed by chiral ansa-metallocene 144, has recently been developed to achieve high enantioselectivity88-90. This parallels to the high stereoselectivity in the polymerization. 

Hence, this analysis indicates that the stereoselectivity of these models is due, not to direct interactions of the tt-ligands with the monomer, but to interactions of the n-ligands with the growing chain, determining its chiral orientation (0i —60° preferred to 0i +60°), which in turn discriminates between the two prochiral faces of the propene monomer.15,37... 

Figure 1.10 Preinsertion intermediates for secondary propene insertion into primary polypropylene chain for (a) isospecific model complex based on (R, R)-coordinatedisopropyl-bis(l-indenyl) ligand and (b) syndiospecific model complex based on isopropyl(cyclopentadienyl-9-fluorenyl) ligand for R chirality at metal atom. Stereoselectivity of isospecific model site is in favor of opposite monomer prochiral faces for primary and secondary insertions (cf. Figures 1.4 and 1.10a). Stereoselectivity of syndiospecific model site is in favor of same monomer prochiral face for primary and secondary insertions (cf. Figures 1.6a and 1.1 Ob).

In summary, there is substantial stereoselectivity of this isospecific C2 symmetric catalytic model for the lower energy (and experimentally observed) primary monomer insertion, and the stereoselectivity would also be higher for the higher energy (minor but experimentally detected) secondary monomer insertion. It is worth noting that the stereoselectivity of the isospecific model site is in favor of opposite monomer prochiral faces for primary and secondary insertions,37d... 

The preferred primary and secondary insertions of opposite monomer prochiral faces into isospecific C2-symmetric catalysts, and of a same prochiral face into syndiospecific Cs-symmetric catalysts, have been confirmed by recent characterization studies on propene-ethene-styrene terpolymers.79... 

If the catalyst is chiral, it can transfer hydride selectively to one prochiral face of an acceptor to provide an optically active product (Fig. 35.1). 

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... 

It is well recognized that chiral tridentate ligands generally form a deeper chiral cavity around the metal center than a bidentate ligand. For example, as mentioned in previous chapters, the chiral tridentate ligand Pybox 120 has been used in asymmetric aldol reactions (see Section 3.4.3) and asymmetric Diels-Alder reactions (see Section 5.7). The two substituents on the oxazoline rings of 120 form a highly enantioselective chiral environment that can effectively differentiate the prochiral faces of many substrates. 

All the new chiral centers are controlled by the rigid structure of the dithia-decalin system. Taking the NaBH4 reduction of 14 as an example, the orientation of the fused rings forces the H- to attack from the Sl -prochiral face of the carbonyl group (Fig. 7-1). 

The formation of an isotactic polymer requires that insertion always occur at the same prochiral face of the propylene molecule. Theoretically, both a chiral catalytic site (enantiomorphic site control) and the newly formed asymmetric center of the last monomeric unit in the growing polymer chain (chain end control) may... 

Occasional defects in the polymer chain remain isolated. When propylene reacts from the opposite prochiral face bringing about a syndiotactic error (Scheme 13.7, a), the original chirality center preceding the error continues to be formed. The same is true for a secondary defect or type 2-1 insertion (Scheme 13.7, b) which is a head-to-head addition and creates a CH2-CH2 sequence. If the chirality were controlled by the growing chain end, an error would be perpetuated, giving rise to a polymer block with altered chirality. Instead, isotacticity is maintained in both cases. 

For the three internal trans-olefins used, because of the Hanson nomenclature, prochiral faces with different names are attacked (Table VI). However, as shown in Scheme 7, the difference in this case is purely formal. In fact, by orienting equally the olefin in the plane of the sheet, with the bulkiest substituent on the left side up, the face of the olefin attacked is in all three cases the same and corresponds to the face attacked in the vinyl olefins. 

Contrary to the trans olefins, the only cis olefin having different prochiral faces which has been examined, the cw-2-hexene, yields two aldehydes arising from the attack at the two opposite prochiral faces (Scheme 8). 

From a stereochemical point of view, the results obtained in the hydrocarbalkoxylation of 2-phenyl-1-butene using [(S -phenyl-butyl] diphenylphosphine as the chiral ligand are particularly interesting. In this case two chiral products are obtained the prevailing antipodes arise from the same enantioface but have different optical purities (see Scheme VI), showing that, at least formally, regioselection is different on the two prochiral faces of the olefin (28). 

The addition criterion may similarly be applied to recognize diastereotopic faces. Methyl a-phenethyl ketone, 58 in Fig. 19 has a chiral center addition clearly gives rise to diastereomers (59a, 59b) the faces of the carbonyl carbon are diastereotopic and the C = 0 group is prochiral. This case is of importance in conjunction with Cram s rule 10). Compounds 60, 62 and 64 also display diastereotopic faces even though the products 61, 63 and 65 are not chiral 60, 62 and 64 have prostereogenic rather than prochiral faces. The C=0 group in 60 is propseudoasymmetric, since C(3) in 61 is a pseudoasymmetric center. a-Phenethyl methyl sulfide (66) displays diastereotopic sides of a molecular plane not due to a double bond 5,24> and may alternatively be considered a case of diastereotopic phantom ligands (unshared pairs on sulfur). This case does involve chirality and the sulfur atom is prochiral. 

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... 

An isotactic stereospecific polymerization arises essentially from the favored complexation of one prochiral face of the a-olefin, followed by a stereospecific process. The stereospecific insertion process and the stereospecific polymerization of racemic a-olefins giving isotactic polymers may be expected to be stereoselective whenever the asymmetric carbon atom is in an a- or /3-position relative to the double bond, and when the interaction between the chirality center of the olefin and the chiral catalytic site is negligible. 

The experimental observation was that C2-symmetric metallocene complexes of zirconium (Fig. 6) produced isotactic polymers, while Cs-symmet-ric metallocene complexes (Fig. 6) produced syndiotactic polymers. Pure MM calculations with frozen core showed that the stereoselectivity is not related to direct interactions of the -ligands of the chiral metallocene with the entering monomer, but to interactions of the -ligands with the growing chain. It is therefore the chirally oriented growing chain which discriminates between the prochiral faces of the propene monomer. For C2-symmetric complexes, identical enantiofacial orientation in all insertion steps results in isotactic polymer formation for Cs-symmetric complexes the enantiofacial orientation alternates between insertion steps and leads to syndiotactic polymers. 

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