Encapsulated Chiral Complexes

It was a major improvement when Jacobsen and associates subsequently Kat-suki and colleagues discovered the chiral Mn(III) complexes for the enantiose- 

In the Jacobsen catalyst, the presence of the four tert-butyl groups disfavors all side-on olefin approaches with the exception of the approach from the di-imine bridge (3 in Fig. 7.5).190,199 

Ogunwumi and Bein synthesized the (R,R)-N,N -bis(3-tert-butyl-5-methyl-salicylidene)-l,2-cyclohexanediaminato Mn(III) chloride (5 in Fig. 7.6) (with 

The encapsulated complex with bulky alkyl groups was more active than the complex without alkyl groups. The catalytic activity increases on the addition of axial ligands as pyridine N-oxide, and the highest enantiomeric excess, 88%, was also achieved in the presence of the pyridine N-oxide. 

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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. 

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... 

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... 

An octahedral coordination capsule 560 with triangular faces covered by threefold chelating synthones has been prepared in [57], One-pot coordination-driven self-assembly of tris(2-hydroxybenzylidene)triaminoguanidinium chloride with palladium(II) chloride and sodium 5,5-diethylbarbiturate by Scheme 4.59 in the presence of triethylamine and tetraeth-ylammonium chloride gave the crystals of a cage complex of 560. This T-symmetric chiral complex contains four encapsulated sodium cations and caged solvent water molecules as well [57]. 

Very few examples have been described for the non-covalent immobilization of chiral porphyrin complexes (Fig. 26). In the first case, the porphyrin-dichlororutheninm complex was encapsulated in silica, which was prepared around the complex by a sol-gel method [78], in an attempt to prevent deactivation observed in solution in the epoxidation of different alkenes with 2,6-dichloropyridine N-oxide. In fact, the heterogeneous catalyst is much more active, with TON up to 10 800 in the case of styrene compared to a maximum of 2190 in solution. Enantioselectivities were about the same imder both sets of conditions, with values aroimd 70% ee. 

Expanding on the encapsulation of organometallic guests, half-sandwich complexes of the form CpRu(q -diene)(H2O) were encapsulated in 1 [32]. When the diene portion of the half sandwich complex is unsymmetrically substituted, the ruthenium atom becomes a chiral center. Addition of CpRu(2-ethylbutadiene) (H2O) (4) to 1 revealed the existence of two diastereomers. Encapsulation of these racemic ruthenium complexes in racemic 1 leads to diastereomeric pairs of enantiomeric host-guest complexes (A/R, A/S, A/R, A/S) (Figure 7.3). However, chiral discrimination was not observed with the diastereomeric ratio (d.r.) being 50 50. 

Fiedler, D., Pagliero, D., Brumaghim, J.L., Bergman, R.G. and Raymond, K.N. (2004) Encapsulation of cationic ruthenium complexes into a chiral self-assembled cage. Inorg. Chem., 43 (3), 846-848. 

Chiral (salen) Mn(III) complexes have been found to be highly enantioselective for the asymmetric epoxidation of conjugated cis-disubstituted and trisubstituted oleftns[10]. The increasing interest towards this reaction brought some authors to develop the heterogeneous chiral salen catalysts. However, to date three kinds of approach have been adopted for the immobilization of chiral salens (1) Chiral Mn salen complexes were supported on polymers[l 1], (2) The encapsulation of salen complex using ship-in-bottle method was... 

Fig. 12 b Preparation of chiral molecular nanocapsules 15 and 16 with Bis(2,3-di-hydroxypropyl)-10-undecenylamine and trimethylolpropane (TMP), Respectively, and a schematic encapsulation of sulfonated platinum pincer complexes of type 3 in the hydrophilic compartment of the nanocapsules... 

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