Catalysts chirality-inducing

Kanemasa et al.63 reported that cationic aqua complexes prepared from the /ram-chelating tridentate ligand (i ,f )-dibenzofuran-4,6-diyl-2,2,-Mv(4-phcnyloxazolinc) (DBFOX/Ph) and various metal(II) perchlorates are effective catalysts that induce absolute chiral control in the Diels-Alder reactions of 3-alkenoyl-2-oxazolidinone dienophiles (Eq. 12.20). The nickel(II), cobalt(II), copper(II), and zinc(II) complexes are effective in the presence of six equivalents of water for cobalt and nickel and three equivalents of water for copper and zinc. 

The mixture of metallocene and co-catalyst is soluble. Its active center, which is chiral, induces with a very low rate of defects only one type of monomer linkage ( single site catalysts )- That is why high activities (some 1,000 kg polymer/g... 

As discussed in Section 3.1.6.1., natural biopolymers are useful chiral selectors, some of which are readily available they are constructed from chiral subunits (monomers), for instance, from L-amino acids or D-glucose. If synthetic chiral polymers of similar type are to be synthesized, appropriate chiral starting materials and subunits, respectively, must be found. Chiral polymers with, for example, a helical structure as the chiral element, are built using a chiral catalyst as chirality inducing agent in the polymerization step. If the chirality is based on a chiral subunit, the chirality of the polymer is inherent, whereas if the polymer is constructed from chiral starting materials, chiral subunits are formed which lead to chirally substituted synthetic polymers that in addition may order or fold themselves to a supramolecular structure (cf. polysaccharides). 

Finally a fourth way to achieve asymmetric induction in the Passerini reaction is by way of a chiral catalyst, such as a Lewis acid. This approach is not trivial since in most cases the Lewis acid replaces the carboxylic acid as third component, leading to a-hydroxyamides or to other kinds of products instead of the classical adducts 7 (vide infra). After a thorough screening of combinations of Lewis acids/ chiral ligands, it was possible to select the couple 13 (Scheme 1.6), which affords clean reaction and a moderate ee with a model set of substrates [17]. Although improvements are needed in order to gain higher ees and to use efficiently sub-stoichiometric quantities of the chiral inducer, this represents the first example of an asymmetric classical Passerini reaction between three achiral components. 

In the phase-transfer processes discussed in Section 11.2 it is assumed that the anionic hydride source, i.e. borohydride or a hypervalent hydrosilicate, forms an ion-pair with the chiral cationic phase-transfer catalyst. As a consequence, hydride transfer becomes enantioselective. An alternative is that the nucleophilic activator needed to effect hydride transfer from a hydrosilane can act as the chiral inducer itself (Scheme 11.6). 

The enantiomers of thiochroman 1-oxide have been obtained by oxidation of thiochroman in the presence of (R,R)-l,2-diphenylethane-l,2-diol (DPED) or L-diethyl tartrate. In the case of the enantioselective oxidation of thiochroman-4-one, (R,R)-DPED and (V,V)-DPED were used as the chiral inducers <2002CH400>. Fligh yields of both (—)-(R)-thiochroman 1-oxide and (—)-(R)-thiochroman-4-one 1-oxide and with enantioselectivities of 98% and 96%, respectively result from the reaction of H202 with the heterocycles when significant amounts of chloroperox-idase are used as catalyst <1998CH246>. 

Corey and Reichard described a more efficient synthesis of the fluoxetine enantiomers.15 This synthesis features a catalytic reduction of prochiral ketone 12 to install the correct absolute stereochemistry at C-3. In this respect this method is very similar to the one previously described by Robertson et al.10 However, the major advantage of the Corey procedure is that the reduction utilizes chiral enzyme-like catalysts to induce the correct stereochemistry and both enantiomers of the catalyst are available (15 and 16). 

The metal component of catalysts, solvent selection, and reaction concentration can influence aggregation and affect reactivity. The titanium catalyst 13 forms monomeric complexes in solution, but the analogous zirconium catalyst 14 forms polynuclear aggregates (Figure 9.7) this may explain why the chirality induced by oxidation of the sulfides is opposite for these two catalysts [23]. 

Soai et al. discovered and developed asymmetric autocatalysis (Figure 9), in which the structures of the chiral catalyst (5)-54 and the chiral product (5)-54 are the same after the addition of diisopropylzinc to aldehyde 53. Consecutive asymmetric autocatalysis starting with (S)-54 of 0.6% ee amplifies its ee, and yields itself as the product with >99.5% ee. Even chiral inorganic crystals, such as quartz or sodium chlorate, act as chiral inducers in this reaction. Soai et alls asymmetric autocatalysis gives us an insight to speculate on the early asymmetric reactions on this planet Earth. However, it can be argued whether such strictly anhydrous organometallic reactions are possible under the nonartificial conditions or not. 

This chapter has presented the current stage in the development of metal-promoted asymmetric C—C and C—X bond forming reactions, in which cinchona alkaloids are utilized as chirality inducers. As shown in many of the examples discussed above, cinchona alkaloids and their derivatives have great potential to serve as chiral ligands or cobase catalysts in diverse metal-promoted asymmetric C—C and C—X bond forming reactions. However, despite the scientific achievements that have been made... 

Consider an ideal gas of 2N molecules, each containing one chiralic center. Initially, we prepare the system in such a way that all the molecules are in one of the enantiomeric forms, say the d enantiomer. We then introduce a catalyst which induces a racemization process in adiabatic conditions. At equilibrium, we obtain N molecules of the d enantiomer and N molecules of the l enantiomer. The entropy change in this spontaneous process is well known ... 

If the interconversion of the Jt-allyl intermediates 34 and 35 is much slower than nucleophihc attack, the product distribution depends on the nature of the substrate. In this case the two enantiomeric chiral substrates 30 and ent-30 are converted to the corresponding product enantiomers 36 and ent-36 with overall retention of configuration. Starting from a racemic mixture of 36 and ent-36, the two product enantiomers 36 and ent-36 are formed in a 1 1 ratio and, therefore, a chiral catalyst cannot induce enantioselectivity (except for kinetic resolution). However, the analogous reaction of the hnear, achiral substrate 31 can be rendered enantioselective if a chiral catalyst is used that adds preferentially to one of the enantiotopic faces of 31 to give either complex 34 or 35. In this case, the enantioselectivity is determined in the oxidative addition of the substrate to the catalyst while nucleophilic addition to the 7i-allyl intermediate is irrelevant for the enantiomeric excess of the overall reaction. The relative rates of k-O-k isomerization and the other processes shown in Scheme 15 strongly depend on... 

Proline r-butylester is the chiral inducer in hydrogenations of dehydrodipeptides and dehydropeptides under various conditions. The resulting amino acids form in 40% to 93% optical yields depending on the catalyst (finely divided Ni, Pd-C, PtOj), the temperature (- 30°C to RT), and the solvent. Experimental data indicate a steric course for this catalytic hydrogenation . 

With IB hgating to Rh2(OAc)4 to form a chiral catalyst for inducing insertion of methyl aryl(diazo)acetate into the O-H bond of a benzy lie alcohol, it also enables further addition of the product into an aldimine. Both reactions are rendered as5munetric. ... 

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