Chiral catalyst sites

For stereospecific polymerization of a-olefms such as propene, a chiral active center is needed, giving rise to diastereotopic transition states when combined with the prochiral monomer and thereby different activation energies for the insertion (see Figure 2). Stereospecificity may arise form the chiral /0-carbon atom at the terminal monomer unit of the growing chain - chain end control - or from a chiral catalyst site - enantiomorphic site control . The microstructure of the polymer produced depends on the mechanism of stereocontrol as well as on the metallocene used [42-44]. 

While the detailed structures of most catalyst sites are still unknown, it was established that stereoselectivity does not come from the chirality of the growing chain end. Rather, it is built into the catalyst site itself. Normal preparations of the catalysts give equal numbers of (/ ) and (5) chiral catalyst sites. These coordinate selectively with (R) and (5) monomers, respectively, in the process of catalytic-site control. ... 

The Diels-Alder reaction was performed with cyclopentadiene and methacroleine (Scheme 56) in presence of 15 mol% of the oxazaborolidine and oxazaborolidinone catalysts derived respectively from supported aminoalcohols and from V-sulfonylamino acid polymers. The oxazaborolidine and oxazaborolidinone catalysts were formed in situ by action of BH3, BH2Br, BHBr2 or BBrs. The diastereoselectivity was excellent in favour of the exo adduct and yields from 65 to 99%. It is noteworthy that higher loading of chiral catalyst site in the polymer, lower exo selectivity and enantioselectivity. The diastereoselection depended not only on the nature of the supported ligand, the crosslinker but also on the borane and the solvent. Results are summarized in Table 6. 

Abstract The unique and readily tunable electronic and spatial characteristics of ferrocenes have been widely exploited in the field of asymmetric catalysis. The ferrocene moiety is not just an innocent steric element to create a three-dimensional chiral catalyst enviromnent. Instead, the Fe center can influence the catalytic process by electronic interaction with the catalytic site, if the latter is directly coimected to the sandwich core. Of increasing importance are also half sandwich complexes in which Fe is acting as a mild Lewis acid. Like ferrocene, half sandwich complexes are often relatively robust and readily accessible. This chapter highlights recent applications of ferrocene and half sandwich complexes in which the Fe center is essential for catalytic applications. 

The most conventional investigations on the adsorption of both modifier and substrate looked for the effect of pH on the amount of adsorbed tartrate and MAA [200], The combined use of different techniques such as IR, UV, x-ray photoelectron spectroscopy (XPS), electron microscopy (EM), and electron diffraction allowed an in-depth study of adsorbed tartrate in the case of Ni catalysts [101], Using these techniques, the general consensus was that under optimized conditions a corrosive modification of the nickel surface occurs and that the tartrate molecule is chemically bonded to Ni via the two carbonyl groups. There were two suggestions as to the exact nature of the modified catalyst Sachtler [195] proposed adsorbed nickel tartrate as chiral active site, whereas Japanese [101] and Russian [201] groups preferred a direct adsorption of the tartrate on modified sites of the Ni surface. 

A number of groups have reported the preparation and in situ application of several types of dendrimers with chiral auxiliaries at their periphery in asymmetric catalysis. These chiral dendrimer ligands can be subdivided into three different classes based on the specific position of the chiral auxiliary in the dendrimer structure. The chiral positions may be located at, (1) the periphery, (2) the dendritic core (in the case of a dendron), or (3) throughout the structure. An example of the first class was reported by Meijer et al. [22] who prepared different generations of polypropylene imine) dendrimers which were substituted at the periphery of the dendrimer with chiral aminoalcohols. These surface functionalities act as chiral ligand sites from which chiral alkylzinc aminoalcoholate catalysts can be generated in situ at the dendrimer periphery. These dendrimer systems were tested as catalyst precursors in the catalytic 1,2-addition of diethylzinc to benzaldehyde (see e.g. 13, Scheme 14). 

Quinine and quinidine, as well as cinchonidine and cinchonine, are diastereo-meric pairs. However, at the critical sites—the P-hydroxyamine portions of the molecules—they are enantiomeric. Thus if quinine is used as the chiral catalyst in an asymmetric transformation (i.e., with one enantiomer being formed in excess), the other enantiomer is formed in excess when quinidine is used. Table 2 gives a representative example, the thiol addition reaction (19). 

Hence, a reaction of Type I will involve a racemic or achiral/me,t(9 nncleophile which will react enantioselectively with an achiral acyl donor in the presence of a chiral catalyst, while on the other hand, a reaction of Type II will associate an achiral nncleophile and a racemic or udm lmeso acyl donor in the presence of a chiral catalyst. In both cases, when a racemic component is implicated the process constitntes a KR and the maximum theoretical yield of enantiomerically pure product, given perfect enantioselectivity, is 50%. When an achiral/mera component is involved, then the process constitutes either a site-selective asymmetric desymmetrisation (ASD) or, in the case of tt-nucleophiles and reactions involving ketenes, a face-selective addition process, and the maximum theoretical yield of enantiomerically pure product, given perfect enantioselectivity, is 100%. 

Once this process is explored with the model system to assess the level of enantioselectivity, we will then prepare alkyl zinc reagent 48 from 44 using standard methods - - and cross couple 48 to aryl bromide 18 using the appropriate chiral catalysts (Scheme 7). Although the acetonide stereocenter in 48 is somewhat remote from the coupling site, the stereocenter may serve to enhance the stereoselectivity of the cross-coupling process because the two possible products are diastereomers, not simply enantiomers. This reaction will produce 49 from (S)-48 and 30 from (R)-48 that can then be converted to epoxides 31 and 32 using standard methods. Epoxide 31 leads to heliannuol D 4 after base-promoted epoxide cyclization and deprotonation. Similarly, epoxide 32 leads to heliannuol A 1 after acid-promoted cyclization. 

The reactions with nG-105 dendrimers show a different trend. Enhanced catalytic performance was observed with increasing dendrimer generation (77.3% yield, 97.2% selectivity, 15.5% ee for Gl 85% yield, 98% selectivity, 37% ee for G4). This comparison indicates that the introduction of an alkyl spacer not only facilitates the access of reactant to the catalytic active sites but also prevents the formation of frozen-in conformations and thus different chiral active sites. However, when the fifth-generation dendrimer is reached, the multiple interactions between end groups become more pronounced, which leads to a decrease in the catalyst performance (68% yield, 95.3% selectivity, 17.6% ee). 

Some chiral initiators have structures such that alternate monomer placements occur with opposite faces of the monomer to yield the syndiotactic polymer. This is syndioselective polymerization proceeding with catalyst site control and is usually observed only with some homogeneous initiators, both traditional Ziegler-Natta and metallocene. 

The high level of stereocontrol in the formation of complexes 25 and 27 suggests that compounds of this type may be useful as chiral catalysts. Indeed, several examples of enantioselective catalytic reactions carried out with half-sandwich complexes have been published recently [23, 25]. However, it seemed desirable to have access to complexes of the [21 Ru(solv)2] type, which have two easily removable solvent molecules coordinated to the central metal, in order to provide coordination sites for a substrate to be transformed. Although the chloride ligand could be easily removed from 23 and 25 all attempts to strip off the PPhs were unsuccessful. Therefore a new reaction scheme was developed which precluded the use of phosphine ligands, and the bis (acetonitrile) complex 28 could be obtained in a multi-step protocol via the T1 salt T1 21 (Scheme 1.5.12) [26]. 

Recently, dendrimers, which are hyperbranched macromolecules, were found to be an appropriate support for polymer catalysts, because chiral sites can be designed at the peripheral region of the dendrimers (Scheme 5). Seebach synthesized chiral dendrimer 14, which has TADDOLs on its periphery and used an efficient chiral ligand in the Ti(IV)-promoted enantioselective alkylation [21]. We developed chiral hyperbranched hydrocarbon chain 15 which has six p-ami-no alcohols [22], It catalyzes the enantioselective addition of diethylzinc to aldehydes. We also reported dendritic chiral catalysts with flexible carbosilane backbones [23]. 

---