Immobilized Schiff-base ligand

A phosgene-free route to aromatic isocyanates, such as M DI and TDI, was reported by Fernandez et al. [42] (Scheme 5.7) According to the patent, the one-pot synthesis involves the use of an immobilized Schiff base type of ligand catalyst that facilitates the oxidative carbonylation of aromatic amines to the corresponding isocyanates. However, 2,2,2-trifluoroethanol (TFE), 1,2-dichlorobenzene, and carbon monoxide were used in this process, so this would not be a totally environmentally friendly process even if these reagents could be recycled and reused. 

Since the affinity of the Schiff base ligand for the Mn is extremely high, Mn immobilization is largely not an issue, provided that a reliable method is used to anchor the ligand irreversibly to a support. The real challenge is to create a catalyst with chemoselectivity and enantioselectivity that matches or even surpasses the results representing the homogeneous catalysts (87, 88). 

Whilst the chiral manganese complexes can epoxidize alkenes with high enantioselectivity (> 90% e.e.), they are not particularly stable. This instability is probably due to the easily oxidizable imine and phenoxide ligands on the complex. Attempts are currently being made to immobilize Schiff-bases in order to increase their stability in a similar manner to the metalloporphyrins discussed earlier. 

Some natural polymers have also been used as support for Schiff base ligands. A starch support has also been used as support for a PyBox. A telomerized starch 96 [91] presenting a final double bound reacted with a thiol-PyBox in the presence of AIBN (Scheme 45) to form the corresponding immobilized PyBox. [92] Although the Ru-97 complex exhibited lower activity (up to 44% after the third reuse) and selectivity (trans/cis 76/24 with 50% ee for the... 

Affinity microparticles (AMPs) were obtained by cross-linking the S-layer lattice on S-layer-carrying cell wall fragments with glutaraldehyde, reducing Schiff bases with sodium borohydride, and immobilizing protein A as an IgG-specific ligand [92]. Thus, AMPs rep-... 

Scheelite, 25 349-350 Scheelite sorting, 16 626 Scheibel column, 10 777-778 Scheibler filter, 11 364 Schiff base(s), 21 203, 204, 25 100-101 chelating agents, 5 712t reaction with amino acids, 2 567 reaction with aniline, 2 786 thermochromic materials, 6 622-623 Schiff base chemistry, 24 42 Schiff base (reductive amination) method, for covalent ligand immobilization, 6 396t... 

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. 

Bi i-naphthaldehyde and salicylaldehyde Schiff s base ligands immobilized on silica Metal ions determination and neconcentration by atomic absorption analysis [21]... 

C-H oxidation (150). The Jacobsen s ligand has been used for the modification of the manganese exchanged Al-MCM-41, that is the immobilized catalyst has been investigated in epoxidation of styrene (151). Chiral manganese-Schiff base complexes were immobilized on a glassy carbon electrode surface (152). Enan-tioselective epoxidation reactions were also catalyzed with Mn(III) complexes of chiral porphirines (97,153). 

Under homogeneous conditions [73], various chiral Schiff base-Cu(II or I) or Ru(II) complexes have been used as chiral catalyst precursors and from all of them the most efficient systems are formed with oxazolines and bis-oxazolines. Thus after their first use by Masamune [74], oxazoline-derived ligand have been extensively employed in homogeneous enantioselective catalytic reaction particularly in cyclopropanation [75]. Thus nitrogenanchoring ligands on organic polymers are mainly formed by immobilized oxazolines. 

Transition metal complexes with Schiff bases as ligands attracted a great interest in this respect. These metal complexes can easily be immobilized on electrode surfaces via electropolymerizatiOTi generating an electrically conductive polymeric film. The reactivity of metal complexes depends on the nature of the metal as well as on the ligand attached. 

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