Asymmetric ligands

The coordination of ligands at the surface of metal nanoparticles has to influence the reactivity of these particles. However, only a few examples of asymmetric heterogeneous catalysis have been reported, the most popular ones using a platinum cinchonidine system [65,66]. In order to demonstrate the directing effect of asymmetric ligands, we have studied their coordination on ruthenium, palladium, and platinum nanoparticles and the influence of their presence on selected catalytic transformations. 

Figure 13 Method of synthesis of a silica-anchored asymmetric ligand according to Pugin.

Other examples from Wilke s group are given in (examples 11-13), leading to highly stereoselective reactions, which have been exploited for asymmetric syntheses in the presence of appropriate asymmetric ligands. This subject requires separate review, however, and will not be treated further here. The reader is referred to the review by Bogdanovic (7).4... 

An important application of an isomerisation is found in the Takasago process for the commercial production of (-)menthol from myreene. The catalyst used is a rhodium complex of BINAP, an asymmetric ligand based on the atropisomerism of substituted dinaphthyl (Figure 5.5). It was introduced by Noyori [1],... 

Fig. 3 a STD curves left panel) for the symmetric ligand-protein complex in Fig. 2a. b STD curves right panel) for the asymmetric ligand-protein complex in Fig. 2b. The P3 and P3 protons are saturated. A spectrometer frequency of 600 MHz and a free ligand correlation time of 2.966 x 10 ° s corresponding to null NOE at 600 MHz were assumed. The protein correlation time was 10 s, Lt/Et =10 1, leakage rate = 0.3 s ...

On the basis of the IR spectra (splitting of the C=S vibration), an asymmetric ligand bonding has been proposed, with one of the As-S or Sb-S... 

Optical yields up to 17% and 25%, respectively, have been reached in the styrene hydroformylation in the presence of cobalt or rhodium catalysts using N-alkylsalicylaldimine or phosphines as asymmetric ligands. Furthermore the hydroformylation of aliphatic and internal olefins have been achieved using rhodium catalysts in the presence of optically active phosphines. With the same catalysts, cis-butene surprisingly undergoes asymmetric hydroformulation with optical yields up to 27%. On the basis of the results obtained for cis-butene and the asymmetric induction phenomena in dichlor(olefin)(amine)platinum( 11) com-... 

The prevailing chirality of the aldehyde produced using ( — )-DIOP as the asymmetric ligand is [ (R) ] for vinyl olefins where the asymmetric carbon atom arises from the attack of carbon monoxide at the carbon atom in position 2. The same chirality [ (R) ] is found also for vinylidene olefins where the asymmetric carbon atom arises from the attack of a hydrogen atom on the same carbon atom. 

To obtain information about the steps in which the asymmetric induction actually takes place, 1-butene, cis-butene, and trans-butene were hydroformylated using asymmetric rhodium catalyst. According to the Wilkinson mechanism, all three olefins yield a common intermediate, the sec-butyl-rhodium complex, which, if the asymmetric ligand contains one asymmetric center, must exist in the two diastereomeric forms, IX(S) and IX(R),... 

The degree of asymmetric induction achieved in asymmetric hydro-formylation may be important in synthetic chemistry as demonstrated by the preparation of optically active hydratropa aldehyde (9). No efforts have been made to date to optimize the process. The most hopeful steps in this direction are a thorough investigation of the reaction variables and the use of new asymmetric ligands. 

The above results were reviewed in 1974 (5). Since then the main advances in the field have been the achievement of asymmetric hydro-carbalkoxylation (see Scheme I, X = -OR) using palladium catalysts in the presence of (-)DIOP (6), the use of other diphosphines as asymmetric ligands in hydroformylation by rhodium (7), and the achievement of the platinum-catalyzed asymmetric hydroformylation (8, 9). Further work in the field of asymmetric hydroformylation with rhodium catalysts has been directed mainly towards improving optical yields using different asymmetric ligands (10), while only very few efforts were devoted to asymmetric hydroformylation catalyzed by cobalt or other metals (11, 12) and it will be discussed in a modified form in this chapter. 

A rather large number of asymmetric ligands have been tested in the hydroformylation of styrene (Table 3), optical yields between 20 and 30% being easily obtained. 

The first asymmetric hydroformylation with platinum catalysts was carried out42 using NMDPP as the asymmetric ligand. An optical yield of 9% was obtained in the hydroformylation of 2-methyl-l-butene to 3-methylpentanal. Subsequently, different types of olefins were asymmetrically hydroformylated using a catalytic system formed from [(—)-DIOP]PtCl2 and SnCl2 2 H20 in situ 42,45) (Table 4). 

Rhodium-Catalyzed Hydroformylation of Styrene with Different Asymmetric Ligands... 

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. 

Table 15. Influence of the structure of the substrate on the prevailing chirality and on the maximum optical yield obtained in asymmetric hydrocarbonylation with different metallic components of the catalyst in the presence of the same asymmetric ligand [(—)-DIOP]... 

Colpas, G.J., B.J. Hamstra, J.W. Kampf, and V.L. Pecoraro. 1994. Preparation of VO(3+) and V02(+) complexes using hydrolytically stable, asymmetric ligands derived from Schiff base precursors. Inorg. Chem. 33 4669 -675. 

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