Zeolites chiral complex

Zeolite-anchored complexes may exhibit a greater activity and selectivity in chiral catalysis. This has been demonstrated by Corma and colleagues.181-184 They prepared a chiral ligand from L-proline (1 in Fig. 7.3). 

Immobilization of chiral complexes in PDMS membranes offers a method for the generation of new chiral catalytic membranes. The heterogenization of the Jacobsen catalyst is difficult because the catalyst loses its enantioselectivity during immobilization on silica or carbon surfaces whereas the encapsulation in zeolites needs large cages. However, the occlusion of this complex in a PDMS matrix was successful.212 The complex is held sterically within the PDMS chains. The Jacobsen catalyst occluded in the membrane has activity and selectivity for the epoxidation of alkenes similar to that of the homogeneous one, but the immobilized catalyst is recyclable and stable. 

ZEOLITE ENCAPSULATED CHIRAL OXIDATION CATALYSTS The issue of encapsulation of chiral complexes in zeolites and the retention of their... 

Corma et al have anchored Rh(I), Ru (II), Co(II) and Ni(II) chiral complexes based on p-aminoalcohols such as (L) prolinol onto silica and modified USY-zeolites (scheme 3) to perform enantioselective hydrogenation of the same prochiral alkenes than shown in scheme 2.20,34... 

The discovery, in the mid-eighties, of the remarkable activity of TS-1 as a catalyst for selective oxidations with aqueous H2O2 fostered the expectation that this is merely the progenitor of a whole family of redox molecular sieve catalysts with unique activities. However, the initial euphoria has slowly been tempered by the realization that framework substitution/attachment of redox metal ions in molecular sieves does not, in many cases, lead to a stable heterogeneous catalyst. Nevertheless, we expect that the considerable research effort in this area, and the related zeolite-encapsulated complexes, will lead to the development of synthetically usefril systems. In this context the development of chiral ship-in-a-bottle type catalysts for intrazeolitic asymmetric oxidation is an important goal. Such an achievement would certainly justify the appellation mineral enzyme . 

The catalytic chemistry of chiral complexes entrapped in zeolite cages has been the subject of recent reviews. [75-78]. Much attention has been devoted to the encapsulation of the Jacobsen... 

Although the main applications of zeohtic sohds in catalysis will continue to be as solid acids in the synthesis and transformations of petrochemicals and commodity chemicals they continue to be considered as catalysts and catalyst supports for a range of reactions of synthetic and industrial relevance. The most important of these are of titanium- and tin-containing solids in selective oxidations. Other well-studied reactions over zeohtes include light hydrocar-bons-to-aromatics (Ga-zeolites) selective catalytic reduction of NO (transition metal exchanged zeolites) C C bond formation (Pd zeohtes) selective alkane oxyfunctionalisation with air (MAPOs, M Mn, Fe, Co) and chiral catalysis over encapsulated chiral complexes. 

Other zeolite host-guest systems include Rh(CO)Cl(PPh3)2 [281, 282]. Chiral complexes of rhodium as zeolite guests are good enantioseleetive hydrogenation catalysts [283]. 

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. 

The mesoporous character of MCM-41 overcomes the size limitations imposed by the use of zeolites and it is possible to prepare the complex by refluxing the chiral ligand in the presence of Mn +-exchanged Al-MCM-41 [34-36]. However, this method only gives 10% of Mn in the form of the complex, as shown by elemental analysis, and good results are only possible due to the very low catalytic activity of the uncomplexed Mn sites. The immobihzed catalyst was used in the epoxidation of (Z)-stilbene with iodosylbenzene and this led to a mixture of cis (meso) and trans (chiral) epoxides. Enantioselectivity in the trans epoxides was up to 70%, which is close to the value obtained in solution (78% ee). However, this value was much lower when (E)-stilbene was used (25% ee). As occurred with other immobilized catalysts, reuse of the catalyst led to a significant loss in activity and, to a greater extent, in enantioselectivity. 

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... 

Chiral dioxomolybdenum complexes were synthesized from (25,4R)-4-hydroxyproline and connected to the surface of USY zeolite by covalent bonding (2 in Fig. 7.4). 

Scheme 27. Enantioselective ring opening of cyclic oxides G and H by TMSN3 catalyzed by chiral Cr(III) Schiff base complexes within the cavities of zeolites Y 53, EMT 54 and into the interlamellar region of K-10 montmorillonite 55.

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). 

Chiral Mo02(acac)(L ) complexes, where L is a chiral bidentate 0,0-ligand derived from (L)-frans-4-hydroxyproline bearing a Si(OEt)3 moiety, have been successfully heterogenised on zeolite-Y and tested for the epoxidation of allylic alcohols (geraniol and nerol) with TBHP (Scheme 4) [18]. 

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