Porphyrin epoxidation catalyst

The use of inorganic supramolecular compounds in catalysis has also been successful in recent years. Hupp etal. incorporated a Mn(IIl)-porphyrm (see Porphyrin) epoxidation catalyst inside a molecular square, a system that shows enhanced catalyst stabihty and substrate selectivity as compared to the free catalyst. In another example, chiral metaUocyclophanes were constructed from Pt(PEt3)2 units and enantiopme atropoisomeric t,t -binapthyl-6,6 -bis-(acetylenes) and used in enantioselective diethyl zinc addition to aldehydes to afford chiral secondary alcohols. The first organometaUic triangle based on Pt(II) and alkyne-di-substituted-binaphfhyl system was reported and found to effect asymmetric catalysis reactions of aldehydes to alcohols with excellent conversion rates and enantiomeric excess/ ... 

M. L. Merlau, W. J. Grande, S. T. Nguyen, J. T. Hupp, Enhanced activity of manganese(III) porphyrin epoxidation catalysts through supramolecular complexation, /. Mol. Catal. A. Chem. 156 (2000) 79. 

This actual complex as shown has been utilized in aziridination reactions (discussed later), which it proved effective and catalytic. This same porphyin with a Mnni-Cl core, instead of Mnin(OH) (CH3OH), has been applied to epoxidation catalysis and has been shown to be one of the more efficient porphyrin epoxidation catalysts... 

The immobilization of homogeneous Mn-porphyrin epoxidation catalysts on silica to achieve easy catalyst recovery has been realized through anchoring of the porphyrin ligand A [54] or the axial imidazole ligand B (Figure 10.3) [55]. The advan-... 

These reports sparked off an extensive study of metalloporphyrin-catalyzed asymmetric epoxidation, and various optically active porphyrin ligands have been synthesized. Although porphyrin ligands can make complexes with many metal ions, mainly iron, manganese, and ruthenium complexes have been examined as the epoxidation catalysts. These chiral metallopor-phyrins are classified into four groups, on the basis of the shape and the location of the chiral auxiliary. Class 1 are C2-symmetric metalloporphyrins bearing the chiral auxiliary at the... 

There has recently been much work in this area using Ru-based catalysts, particularly with porphyrin-based catalysts, following the work by Sharpless et al. on asynunetric epoxidation of allylic alcohols by a titanium-based tartrate system. There are reviews on asymmetric epoxidations catalysed by chiral Ru porphyrins [5, 18]. 

Figure 6.5 (a) Structure of porphyrin clips 11, Znll and Mnll. (b) Two approaches in which Mnll is used as an epoxidation catalyst in combination with pyridine or 4-tert-butylpyridine as the axial ligand. 

Chiral porphyrins, prepared in different ways84,85 (chiral units attached to preformed porphyrins84, chiral substituents introduced during the synthesis of porphyrins86 or chiral porphyrins synthesized without the introduction of chiral groups69,87-90), proved to be effective as asymmetric epoxidation catalysts. 

Cydooctene has been epoxidized with oxygen using a manganese porphyrin as catalyst (equation... 

Ru-porphyrin complexes covalently bound to MCM-41 were used as catalysts for the oxidation of alkenes, giving turnover numbers 20-40 times higher than the free complex [190]. Porphyrin complexes have also been attached to Nb dopants in the MCM-41 matrix, and behaved as cyclohexene epoxidation catalysts and were stable towards ligand degradation [44b]. Grafting of ethylenediamine ligands on... 

A remarkable approach was reported in 2004 by Simormeaux and coworkers [53]. Manganese complexes of spirobifluorenyl-substituted porphyrins were elec-tropolymerized by anodic oxidation and the resulting poly(9,9 -spirobifluorene manganese porphyrin) films were shown to be efficient epoxidation catalysts in the presence of imidazole. The polymers were tested in the epoxidation of cyclooctene and styrene using PhIO or PhI(OAc)2 as oxidants. Epoxide yield reached 95% in the case of cyclooctene and 77% in the case of styrene. The electrosynthesized polymers could be recovered by filtration and reused up to eight times without loss of activity and selectivity. 

With respect to the widely investigated metalloporphyrins for catalytic epoxidation, progress was made in the area of polymer-supported ruthenium porphyrins for asymmetric epoxidation. Manganese-porphyrin complexes attached via peptide linkers to organic polymers showed enhanced selectivity and catalyst stability due to donor atoms in the linker that could coordinate to the metal center. This shows that improvement can be achieved not only by optimization of the polymer or metal complex but also by appropriate choice of the linker. Furthermore, electropolymerization by anodic oxidation of suitable manganese-porphyrin complexes proved to be a promising technique for the preparation of efficient immobilized epoxidation catalysts. 

The design of viable, highly enantioselective epoxidation catalysts based on porphyrin ligands is confronted by the inherent difficulty associated with inducing dissymmetry from remote parts of an sp -hybridized coordination sphere, and the relative difficulties in constructing the chiral porphyrin rings. As research in this field has been the subject of an insightful review [90], only more recent developments will be covered here. 

Gross, Z., S. Ini, M. Kapon, and S. Cohen (1996). First utilization of a homochiral ruthenium porphyrin as enantioselective epoxidation catalyst. Tetrahedron Lett. 37, 7325-7328. 

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