Stereoselectivity chiral rhodium complexes

Chiu s group [193] used this domino process for an entry to pseudolaric acid 6/2-27, starting from 6/2-28, to yield 6/2-29 and 6/2-30 as an almost l l-mixture of di-astereomers (Scheme 6/2.5). Attempts to improve the stereoselectivity by using chiral rhodium complexes did not change the picture very much. The pseudolaric acids A, B and C are diterpenoids, which were isolated from the root bark of Pseudo-larix kaempferi Gordon (Pinaceae), and are components of the traditional Chinese medicine called tujinpi. They reveal antifungal activity and cytotoxicities at submicromolar levels [194]. 

Dehydropeptides (21) were employed for the asymmetric hydrogenation, catalyzed by chiral rhodium complexes of the hydroxyproline derivative (13). It was reported that the stereoselectivity is satisfying (ds = 90-95 %)50). 

Brice Bosnich et al. Chiral rhodium complexes that are highly stereoselective catalysts... 

The chemistry of secondary phosphine oxides, R2P(H)0 and their phosphi-nous acid tautomers, R2POH, has continued to attract attention. The study of the phosphinous acid tautomers has been aided by the development of stereoselective procedures for direct conversion of secondary phosphine oxides to the phosphinous acid-boranes (83). Treatment of the secondary phosphine oxide with either a base-borane complex or boron trifluoride and sodium borohyd-ride provides the phosphinous acid-borane with predominant inversion of configuration at phosphorus. The phosphinous acid tautomers are usually trapped as ligands in metal complexes and further examples of this behaviour have been noted. Discrimination of enantiomeric forms of chiral phosphinous acids, Ph(R)OH, coordinated to a chiral rhodium complex, has been studied by NMR. °° Palladium complexes of di(t-butyl)phosphinous acid have found application as homogeneous catalysts.A lithium salt of the tellurophos-phinite Ph2PTeH has been prepared and structurally characterised. ... 

A similar type of chiral rhodium porphyrin was found to be effective for the carbene-insertion reaction to olefins, where formation of the carbene complex takes place. Chiral rhodium complexes for catalytic stereoselective-carbene addition to olefins were prepared by condensation of a chiral aldehyde and pyrrole. Formation of the metal-carbene complex and substrate access to the catalytic center are crucial to the production of optically active cyclopropane derivatives. Optically active a-methoxy-a-(trifluoro-methyOphenylacetyl groups are linked witfi the amino groups of a,p,0L,p isomers of tetrakis-(2-aminophenyI)por-phyrin through amide bonds. Oxidation reactions of the... 

The effect that homogeneous transition-metal catalysis has had on stereoselective synthesis is especially impressive. Using chiral ligands, it is possible to control hydrogenation of double bonds so that new chirality centers have a particular configuration. The drug L-dopa, used to treat Parkinsonism, is prepared in multiton quantities by enantio-selective hydrogenation catalyzed by an enantiomerically pure chiral rhodium complex. 

It is noteworthy that the immobilized chiral rhodium complex V displays a high catalytic activity as well as stereoselectivity similar to that of the soluble chiral catalyst (6), the results being indicated in parentheses in Table 8. 

The marked effect of hydrosilanes on the stereoselectivity, which is very characteristic of the asymmetric hydrosilylation of ketones as described in the previous Sections, is seen here again. Fairly good optical yields comparable to those obtained in other chiral rhodium complex-catalyzed reactions were attained. For example, the reaction of acetophenone with diphenylsilane catalyzed by (/ )-( S )-MPFA-rhodium complex gave higher optical yield than when (/ )-BMPP or DIOP was used as ligands. 

Ylid trapping was used to establish the tricyclic system 8.529 in a synthesis of pseudolaric acid (Scheme 8.142), using a chiral rhodium complex to achieve better stereoselectivity in favour of the desired isomer. ... 

Hydrogenation with a chiral rhodium complex as catalyst affords stereoselective products as shown in eq. (18.33). When the racemate of the raw material is hydrogenated in 54% conversion, the R isomer, which is less reactive, is obtained in 84% purity, and when the racemate is hydrogenated in 70% conversion, pure R isomer is obtained. HCOOH/NEts is used as the hydrogen source. The reaction shown in eq. (18.33) is the reverse reaction of that shown in Scheme 18.2 which is the hydrogenation of carbon dioxide [67]. 

In terms of stereoselectivity, Muller and coworkers have performed asymmetric C(sp )—H amination with chiral rhodium complexes, though the induction remained modest (31% ee). They have also underscored the stereospecificity of the nitrene C—H insertion that takes place with retention of configuration from (R)-2-phenylbutane. More significantly, they have also addressed the issue of the chemoselectivity in the case of the allylic amination. As observed in carbene chemistry, the product distribution was found to depend on the nature of the substrate and the... 

The most efficient method for the stereoselective formation of amines through rhodium(II)-catalyzed intermolecular C(sp )-H amination, to date, involves a diastereoselective approach based on the synergistic interaction between the stdfonimidamide (S)-7 and the chiral rhodium complex Rh2(S-nta)4 8. Benzylic and allylic amines are isolated from various complex substrates used as the limiting component, in excellent yields of up to 99% and complete stereocontrol (Scheme 34). Of course, the other enantiomer is also efficiendy accessible through the use of the other matched pair of reagents. 

The strategy of the catalyst development was to use a rhodium complex similar to those of the Wilkinson hydrogenation but containing bulky chiral ligands in an attempt to direct the stereochemistry of the catalytic reaction to favor the desired L isomer of the product (17). Active and stereoselective catalysts have been found and used in commercial practice, although there is now a more economical route to L-dopa than through hydrogenation of the prochiral precursor. 

Numerous studies have been directed toward expanding the chemistry of the donor/ac-ceptor-substituted carbenoids to reactions that form new carbon-heteroatom bonds. It is well established that traditional carbenoids will react with heteroatoms to form ylide intermediates [5]. Similar reactions are possible in the rhodium-catalyzed reactions of methyl phenyldiazoacetate (Scheme 14.20). Several examples of O-H insertions to form ethers 158 [109, 110] and S-H insertions to form thioethers 159 [111] have been reported, while reactions with aldehydes and imines lead to the stereoselective formation of epoxides 160 [112, 113] and aziridines 161 [113]. The use of chiral catalysts and pantolactone as a chiral auxiliary has been explored in many of these reactions but overall the results have been rather moderate. Presumably after ylide formation, the rhodium complex disengages before product formation, causing degradation of any initial asymmetric induction. 

Stereoselectivity in hydroformylation reactions, as a result of the supporting ligand set (e.g. large bite angle diphosphines or chiral diphosphines) or by stereocontrol of the substrate has also been discussed by Breit. Rhodium complexes supported by large bite angle diphosphines such as bisbi, biphephos and xantphos, shown in Scheme 10, are now well-established... 

So far, while there is a relative abundance of synthetically useful cyclopropana-tion catalysts, all of them provide a mixture of diastereomers with the anti product predominating. Thus, a catalyst able to provide optically active syn cyclopropyl esters would constitute a useful complement to existing methodology. Rhodium complexes of bulky porphyrins ( chiral fortress porphyrins) have been developed for this purpose [27]. The porphyrin ligands bear chiralbinaphthyl groups appended directly to the meso positions. Their rhodium(III) complexes provide predominantly the syn cyclopropane with diazoesters, with very good stereoselectivity in some cases. However, the enantioselectivities observed are modest. 

Hu et al. developed a system that involved cooperative catalysis by a chiral phosphoric acid and an achiral rhodium complex (Scheme 3.37) [81]. They applied the binary catalytic system to a three component coupling reaction among a diazoesters, primary alcohols, and aldimines. The steric bulkiness of the primary alcohol had a significant effect on both diastereo and enantioselectivities. The sterically demanding 9 anthracenemethanol was the best component to give p amino a alkoxy esters with excellent stereoselectivities under the combined and cooperative catalysis by phosphoric acid ll and the rhodium complex. 

Other chirally modified platinum, cobalt and rhodium complexes give lower inductions, although sometimes with higher aldehyde and branched product yields (Table 1). A strong dependance of chemo-, regio- and stereoselectivity on reaction conditions and conversion rates is observed, sometimes with contradictory results. Attempts to optimize the chemo- and re-gioselectivity usually lead to lower asymmetric inductions. Moreover, since double-bond isomerization and racemization take place under hydroformylation conditions, the results reported do not necessarily reflect the primary asymmetric induction of the starting alkene. 

Since pyrrolidinodiphosphines, e.g., Ph-CAPP, and BPPM, gave excellent stereoselectivities, we prepared a series of new chiral pyrrolidinodiphosphines, in which the nitrogen atom of PPM (, 11) is linked up with a variety of a-aminoacyl groups. The rhodium complexes with these ligands may serve as good bio-mimetic models of reductase when they are anchored on polymers especially polyamides. a-Aminoacyl-PPMs ( ) were prepared by the condensation of PPM with an N-CBZ-a-amino acid or an N-CBZ-dipep-tide in the presence of dicyclohexylcarbodiimide (DCC) and 1-hy-droxybenztriazole (HOBT)(eq. 3). 

Photolysis of solutions of C6o(OH)ig at low solute concentration leads to [C6o(OH)i8] by electron transfer from Me2C(OH) radicals or from hydrated electrons, and this has enabled the reduction potential of the C6o(OH)ig/ [C6o(OH)ig] couple to be estimated. The kinetics of the photoreduction of hexanal using RhCl(PMe3)2CO as catalyst have been measured and the feasibility of a photocatalytic synthesis of hexanol from pentane, CO, and H2 in the presence of rhodium complexes has been demonstrated. Irradiation of a chiral bimolecular crystal of acridine and R-(-)- or S-(+)-2-phenylpropionic acid induces photodecarboxylation followed by stereoselective condensation to give a mixture of three optically active products, and the 3-0-S-methyl dithiocarbo-nate derivatives of oleanolic and ursolic methyl esters have been used as models for the photodeoxygenation of alcohols. ... 

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