Supported complexes chiral supports

Supported cationic rhodium(I) phosphine complexes, chiral at a men-thyl moiety, effected hydrogenation of ketones, but the 2-butanol produced from methylethylketone was optically inactive (348). Polystyrene-or silica gel-supported DIOP systems, however, are particularly effective for production of optically active alcohols (up to 60% ee) via asymmetric hydrosilylation of ketones (10, 284, 296, 366, 368 see also Section III, A,4). 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

Epoxidations. Grafting tantalum onto silica to form a useful catalyst for the Sharpless asymmetric epoxidation of allyl alcohols is contrary to the ineffective titanium species on a similar support. Vanadium-complexed chiral hydroxamic... 

Observe that the former presented retrosynthesis and non-stereoselective synthesis of racemic chloramphenicol have no relation to this asymmetric synthesis of —)-chloramphenicol. It is an often-encountered situation in plaiming the synthesis of complex chiral molecules, although it does not detract from the value of ret-rosynthetic consideration. Unavailability of the chiral variant of certain reactions prompts synthetic organic chemists to explore other routes besides the one suggested by retrosynthesis and to discover new, original solutions. Analysis of the sertraline molecule in the next section offers impressive support of this statement. 

The absolute configuration of products obtained in the highly stereoselective cycloaddition reactions with inverse electron-demand catalyzed by the t-Bu-BOX-Cu(II) complex can also be accounted for by a square-planar geometry at the cop-per(II) center. A square-planar intermediate is supported by the X-ray structure of the hydrolyzed enone bound to the chiral BOX-copper(II) catalyst, shown as 29b in Scheme 4.24. 

A chiral separation medium is a complex system. Ideally, interactions that lead to enantioseparation are maximized while nonspecific interactions should be completely suppressed. Typically, a medium for chromatographic separations involves the solid support, the selector, and the linker connecting the two, as shown in scheme 3-1. 

Chiral separation media are quite complex systems. Therefore, neither combinatorial methods nor even the identification of the best selector can ensure that an outstanding chiral separation medium will be prepared. This is because some other variables of the system such as the support, spacer, and the chemistry used for their con-... 

Abstract The immobilization of chiral catalysts through non-covalent methods, as opposed to covalent immobilization, allows an easier preparation of chiral heterogeneous catalysts with, in principle, less influence of the support on the conformational preferences of the catalytic complex. In this review the different possibilities for immobilization without forming a covalent bond between the chiral diazahgand and the support, which can be either solid or liquid, are presented. 

In the last 20 years a great deal of effort has been focused towards the immobilization of chiral catalysts [2] and disparate results have been obtained. In order to ensure the retention of the valuable chiral hgand, the most commonly used immobihzation method has been the creation of a covalent bond between the ligand and the support, which is usually a solid, hi many cases this strategy requires additional functionalization of the chiral hgand, and this change - together with the presence of the very bulky support - may produce unpredictable effects on the conformational preferences of the catalytic complex. This in turn affects the transition-state structures and thus the enantioselectivity of the process. 

In principle this is the method that gives rise to the strongest support-complex interaction. We have considered in this category all the methods in which the support compensates for at least one of the charges of the complex, usually due to the metal, although without considering the exact nature of the metal-support bond, i.e., purely ionic or polarized covalent. In any case, the only possible covalent bond between support and complex would be estabhshed with the metal center, not with the chiral hgand. 

Only a few years after the development of the homogeneous chiral Mn(salen) complexes by Jacobsen and Katsuki, several research groups began to study different immobiUzation methods in both liquid and soUd phases. Fluorinated organic solvents were the first type of Uquid supports studied for this purpose. The main problem in the appUcation of this methodology is the low solubility of the catalytic complex in the fluorous phase. Several papers were pubUshed by Pozzi and coworkers, who prepared a variety of salen ligands with perfluorinated chains in positions 3 and 5 of the saUcyUdene moiety (Fig. 2). 

Electrochemical studies, in combination with EPR measurements, of the analogous non-chiral occluded (salen)Mn complex in Y zeoUte showed that only a small proportion of the complex, i.e., that located on the outer part of the support, is accessible and takes part in the catalytic process [26]. Only this proportion (about 20%) is finally oxidized to Mn and hence the amount of catalyst is much lower than expected. This phenomenon explains the low catalytic activity of this system. We have considered other attempts at this approach using zeolites with larger pore sizes as examples of cationic exchange and these have been included in Sect. 3.2.3. 

Metal complexes of chiral bis(oxazoline) ligands, in most cases Cu(II) complexes, have been supported by cationic exchange on inorganic, organic, and composite anionic solids. 

The solids were used as catalysts in the benchmark cyclopropanation reaction between styrene and ethyl diazoacetate (Scheme 7). As far as the nature of the clay is concerned, laponite was foimd to be the best support for the catalytic complexes. The best enantioselectivity results (Table 7) were obtained with ligand 6b (69% ee in trans cyclopropanes and 64% ee in cis cyclopropanes) but the recovered solid showed a lower activity and enantioselectivity, which was attributed to partial loss of the chiral ligand from the support. In general, the use of the three chiral ligands led to enantioselectivity results that were intermediate between those obtained in homogeneous phase with CuCl2 and Cu(OTf)2 as catalyst precursors. This seemed to indicate that the sohd behaved as a counterion with an intermediate coordinating abihty to the copper centers. 

Probably the first non-covalent immobilization of a chiral complex with diazaligands was the adsorption of a rhodium-diphenylethylenediamine complex on different supports [71]. These solids were used for the hydride-transfer reduction of prochiral ketones (Scheme 2) in a continuous flow reactor. The inorganic support plays a crucial role. The chiral complex was easily... 

An alternative that has received a great deal of attention in recent years is the immobilisation of a chiral catalyst on a nonsoluble support (polystyrene resins, silica gel, zeolites, etc.), thereby creating a chiral heterogeneous catalyst. Unlike homogeneous catalysts, these supported complexes can be recovered from the... 

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