How Important is the Inert Matrix of Supported Enantiomeric Catalysts? Reversal of Topicity with Two Polystyrene Backbones

The Role of Binding Constants in the Efficiency of Chiral Catalysts Immobilized by Electrostatic Interactions: The Case of Azabis(oxazoline)–Copper Complexes

“…Although the attachment of a ligand to a support by covalent bond formation is the most widely used method of immobilization,2, 3 on many occasions the synthetic steps required prove to be difficult. Furthermore, binding the ligand covalently to the support may modify the conformational preferences of the catalytic complex, and thereby lead to changes in enantioselectivity 4. Alternatively, immobilization can be achieved by electrostatic interaction, although in spite of its simplicity, this method is not often used 2…”

Polymer-Supported Bis(oxazolines) and Related Systems: Toward New Heterogeneous Enantioselective Catalysts

“…First of all, the appropriate comparisons need to be made. This requires the preparation of the corresponding homogeneous ligands with exactly the same structure as that found at the active site of the functional polymer. ,, In the case of Cu−BOX catalysts, excellent yields and selectivities for the cyclopropanation reaction have been reported with BOX ligands such as 12 or 13 , but those ligands differ significantly from the BOX structures present in our supported BOXes both at the side chain of the original amino acid and at the substituents on the central methylene bridge. Phenylglycine-derived BOX 14 is structurally much closer to our systems, and this is reflected (see Table ) in the observation in solution of lower enantioselectivities when compared to 12 or 13 , but also this homogeneous ligand presents differences in the substitution pattern at the central methylene group.…”

“…Nevertheless, to properly understand the critical influence that the presence of the support can have on the final outcome of a chemical reaction that takes place in the presence of a supported reagent or catalyst, a very careful analysis is needed. In many cases, the support itself can contribute to the modification of the activity, the stability, or the selectivity of the immobilized species relative to that observed for the related homogeneous systems. , In light of the former considerations, much effort is being devoted nowadays to the preparation and study of supported reagents and catalysts, and this is also true for catalysts derived from BOXes and related systems. , Immobilization of a given reagent, ligand, or catalyst can be carried out using either inorganic or organic (polymeric) supports. In general, each support presents its own advantages and drawbacks, but, in general, the use of polymeric supports, in particular polystyrene−divinylbenzene (PS−DVB) resins, allows for a higher flexibility in the design of the final system.…”

Synthesis of Organic Compounds Using Polymer-Supported Reagents, Catalysts, and/or Scavengers in Benchtop Flow Systems

“…It is evident from the foregoing discussion that a considerable range of organic reactions have already been carried out very successfully using PS reactants in flow systems. When asymmetric syntheses are carried out, the stereochemical results are usually comparable with those obtained with LMW systems, but sometimes they were better 27,43 and in one case the stereochemistry was actually reversed . It has been demonstrated that syntheses can be carried out using appropriate columns in sequence provided the reaction solvent remains the same and the flow rate is compatible with that of the slowest component of the sequence.…”

“…Catalysis of the Diels−Alder reaction between cyclopentadiene and 3-crotonyl-1,3-oxazolidin-2-one by TADDOL derivatives has been studied in a flow system by Luis et al: see Scheme . The catalyst was formed by treating a monolith containing residues 14 (Ar = 3,5-dimethylphenyl), prepared using a TADDOL-containing monomer, with a solution of TiCl 2 (OiPr) 2 in toluene.…”