Immobilized salen ligands

Non-covalently Immobilized Catalysts Based on Chiral Salen Ligands. . 152... 

Fig. 8 Symmetrical chiral salen ligands used in solid phase immobilization...

Salen ligands have also been used in the titanium-catalyzed trimethylsilyl-cyanation of benzaldehyde. The complexes were immobilized by substitution of a chloride with a surface silanol from the support. In the first study on this reaction [38], the most efficient ligand was the non-symmetrical salen Im (Fig. 11) (94% ee), whereas the selectivity obtained with the symmetrical ligand la was significantly lower (72% ee). In a recent paper, the immobilization of different titanium species, including monomeric and dimeric systems with... 

The unsymmetrical analog of a Katsuki-type salen ligand was attached to Merri-field s resin (1% cross-Hnked) yielding a catalyst (22) (Fig. 4.3) which showed good efficiency and selectivity in the asymmetric epoxidation of 1,2-dihydronaphtalene with good performance after several recycling steps [81]. Related complexes (23) immobilized on silica were recently disclosed by Seebach and coworkers (Fig. 4.3) [82]. 

In the present work, the Jacobsen s catalyst was immobilized inside highly dealuminated zeolites X and Y, containing mesopores completely surrounded by micropores, and in Al-MCM-41 via ion exchange. Moreover, the complex was immobilized on modified silica MCM-41 via the metal center and through the salen ligand, respectively. cis-Ethyl cinnamate, (-)-a-pinene, styrene, and 1,2-dihydronaphtalene were used as test molecules for asymmetric epoxidation with NaOCl, m-CPBA (m-chloroperoxybenzoic acid), and dimethyldioxirane (DMD) generated in situ as the oxygen sources. 

The immobilization by ionic bonding on Al-MCM-41 was carried out similarly to the immobilization of rhodium-diphosphine complexes as described above. This catalyst was named MCMIHC. For immobihzation via the metal center and covalent bonding of salen, the all-silica MCM-41 was modified with (3-aminopro pyl)triethoxysilane (APTES) [52]. The catalyst obtained by the metal center immobilization was denoted MCM2HC [53], whereas the material obtained by covalent bonding of the salen ligand was named MCM3HC [54]. Detailed procedures are described extensively elsewhere [55]. 

Fig. 6 shows the diffuse reflectance UV-visible spectra of typical salen Co(IH) complexes immobilized on MCM-41, the homogeneous salen complex of the same structure, and a pure Si-MCM-41. The chiral salen ligands of Co(IU) form showed the bands at near 250 and 370 nm on the UV spectra. But the pure siliceous MCM-41 no absorption peak at all. This broad band is probably due to the charge-transfer transitions between metal and ligand. This result indicates that the successful immobilization of chiral salen ligands was achieved. 

Many other projects in the past dealt with the immobilization of the famous chelating salen ligand or salen metal complexes, respectively—the Mnm salen Schiff base complex is also known as Jacobsen s catalyst, and it is used, among others, in asymmetric epoxidation of olefinstttt—mainly onto the surface of MCM-41 and SBA-15. 

Fig. 27 Dendronized salen ligands which were subsequently immobilized by cross-linking of the styryl units...

Smith and Liu <02CC886> have immobilized a Katsuki-type salen ligand by an ester linkage to Merrifield s resin to produce catalyst 7. In a test epoxidation of 1,2-dihydronaphthalene (8)... 

The efficiency of new unsymmetrical chiral salen ligands was examined in the asymmetric trimethylsilylcyanation of benzaldehyde. A very high level of enantioselectivity was attainable over chiral Ti(IV) salen complexes prepared from salicylaldehyde and 3,5-Di-/ert-butylsalicylaldehyde derivative as compared to the conventional salen catalyst. Enantiomeric excess of the corresponding reaction product was generally more than 70% over unsymmetric chiral salen catalysts. The chiral Titanium(IV) salen complexes immobilized on a mesoporous MCM-41 by multi grafting method showed a relatively high enantioselectivity for the addition of trimethylsilyl cyanide to the benzaldehyde. 

First it was considered that as the salen ligand was localized at the crosslink it may induced a steric hindrance and a conformational rigidity responsible of the low ee. Therefore, Laibinis [189-190] and Sherrington [191] synthesized non symmetrieal immobilized salen attached specifically to the polymer in a pendent fashion. The polymer was a styrene base resin. 

Metal-salen complexes immobilized on amorphous or mesostructured SiO were also investigated as acid-base catalysts for the cycHc carbonates synthesis imder continuous operation in gas phase. Despite the high selectivity to the cyclic carbonates the major drawback of these organic-inorganic hybrid catalysts were the catalyst deactivation, likely due to the cleavage of bonds which link the salen ligand and SiO imder the reaction conditions [78,82,106-108,111-113]. 

The freshly prepared catalysts were applied to study the influence of the alkyl spacer length on the catalytic activity of the immobilized salen-type ligands in the presence of the nonpolar polysiloxane backbone. 

Scheme 15.40, Immobilization of salen ligands on dendronized support and their complexation...

Kureshy s group [83] immobilized the Mn(Salen) catalyst axially in the nanopores of MCM-41 via pyridine N-oxide (Scheme 10.15). These immobilized catalysts showed higher enantioselectivity (69% ee) than their homogeneous counterparts (51% ee) for the asymmetric epoxidation of styrene and were also effective for the asymmetric epoxidation of bulkier substrates such as indene and 2,2-dimethylchromene (conversion 82-98% ee 69-92%). The catalysts could be recycled for at least four times without loss in performance. The increase in ee values was attributed to the unique spatial environment constituted by the chiral Salen ligand and the surface of the support. 

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