Epoxidation with metal complexes

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

Mimoun proposed a mechanism that is general for both stoichiometric epoxidations with peroxo complexes and for catalytic systems employing alkyl hydroperoxides.292-294 It involves an alkylperoxidic species with the alkene complexed through the metal ... 

In contrast with metal-complex catalyzed transformations [52], enantioselective organocatalyzed intermolecular conjugate additions of O-nudeophiles seem to be limited to peroxides such as hydrogen peroxide or tert-butyl hydroperoxide. In these reactions the primary addition product, a / -peroxy enolate, reacts further to yield an epoxide (Scheme 4.31). Consequently, reactions of this type are covered in Section 10.2 Epoxidation of Enones and Enoates . 

Presumably, a PET is involved also in the cationic photopolymerization of epoxides with metal arene complexes [181, 182]. 

The results of quantum chemical DFT and ab initio calculations of the reaction mechanism of the olefin epoxidation with diperoxo complexes and cis-dihydroxylation with oxo complexes show clearly that the organometallacyclic compounds which were previously suggested as intermediates in the reaction are not formed. The epoxidation reactions do not proceed via formation of metalla-2,3-dioxolanes as intermediate but rather via direct attack of the metal peroxo moiety to the olefin. Likewise the reaction of neutral metal oxides in high oxidation states with olefins occurs not as a [2+2] addition via metalla-... 

Whilst the Sharpless epoxidation with titanium catalysts and the Jacobsen-Katsuki epoxidation with manganese(salen) complexes are at the forefront of enantioselec-tive epoxidation with metal catalysts, there are alternative systems available. Ruthenium pyridinebisoxazoline (PYBOX) complexes have been independently reported, using either phenyliodinium diacetate or sodium periodate as... 

S. S. Balula, A. C. Coelho, S. S. Braga, A. HazeU, A. A. Valente, M. Pillinger, J. D. Seixas, C. C. Romao, I. S. Gongalves, Influence of cyclodextrins on catalytic olefin epoxidation with metal-carbonyl componnds, crystal structure of the TRIMEB complex with CpFefCOl Cl, Organometallics, 2007, 26, 6857-6863. 

Other substrates which react with metal complex anions to give metal-carbon bonds are sulfates, epoxides, and onium compounds. Ethylene oxide gives j3-hydroxyethyl compounds 27, 28). 

Several research groups reported that various ILs could be effective cocatalysts in the copolymerization of CO2 and epoxides with metal salen or metal porphyrin complexes [126-130]. In some cases, it was shown that the activities of theses metal complexes were drastically enhanced by the co-presence of IL, although they had no or a very low activity for the coplymerization in the absence of IL. 

Transition metal-catalyzed epoxidations, by peracids or peroxides, are complex and diverse in their reaction mechanisms (Section 5.05.4.2.2) (77MI50300). However, most advantageous conversions are possible using metal complexes. The use of t-butyl hydroperoxide with titanium tetraisopropoxide in the presence of tartrates gave asymmetric epoxides of 90-95% optical purity (80JA5974). 

Although the enantioselective intermolecular addition of aliphatic alcohols to meso-epoxides with (salen)metal systems has not been reported, intramolecular asymmetric ring-opening of meso-epoxy alcohols has been demonstrated. By use of monomeric cobalt acetate catalyst 8, several complex cyclic and bicydic products can be accessed in highly enantioenriched form from the readily available meso-epoxy alcohols (Scheme 7.17) [32]. 

Especially in the early steps of the synthesis of a complex molecule, there are plenty of examples in which epoxides are allowed to react with organometallic reagents. In particular, treatment of enantiomerically pure terminal epoxides with alkyl-, alkenyl-, or aryl-Grignard reagents in the presence of catalytic amounts of a copper salt, corresponding cuprates, or metal acetylides via alanate chemistry, provides a general route to optically active substituted alcohols useful as valuable building blocks in complex syntheses. 

In spite of these limitations, three examples of (salen)-metal complex adsorption have been described. In the first one, Jacobsen s complex (la-MnCl) was adsorbed on Al-MCM-41 [27] by impregnation with a solution of the complex in dichloromethane, an approach that prevents the possible cationic exchange. The results in the epoxidation of 1,2-dihydronaphthalene with aqueous NaOCl were comparable to those obtained in solution, with only a slight reduction in enantioselectivity (55% ee instead of 60% ee). However, recycling of this catalyst was not described. 

Before turning to epoxide opening with low valent metal complexes, the reduction of epoxides under Birch conditions [10-13] will be discussed very briefly for historical reasons. The initially formed radical is reduced further to give carbanionic species, that do not display the reactivity of radicals. No C - C bond-forming reactions have initially been reported. 

The catalysts which have been tested for the direct epoxidation include (i) supported metal catalysts, (ii) supported metal oxide catalysts (iii) lithium nitrate salt, and (iv) metal complexes (1-5). Rh/Al203 has been identified to be one of the most active supported metal catalysts for epoxidation (2). Although epoxidation over supported metal catalysts provides a desirable and simple approach for PO synthesis, PO selectivity generally decreases with propylene conversion and yield is generally below 50%. Further improvement of supported metal catalysts for propylene epoxidation relies not only on catalyst screening but also fundamental understanding of the epoxidation mechanism. 

A chiral diphosphine ligand was bound to silica via carbamate links and was used for enantioselective hydrogenation.178 The activity of the neutral catalyst decreased when the loading was increased. It clearly indicates the formation of catalytically inactive chlorine-bridged dimers. At the same time, the cationic diphosphine-Rh catalysts had no tendency to interact with each other (site isolation).179 New cross-linked chiral transition-metal-complexing polymers were used for the chemo- and enantioselective epoxidation of olefins.180... 

Complex (17) of Class 3 has no chiral auxiliary, but is endowed with facial chirality by the presence of a bridging strap (Figure 4).65 Treatment of (17) with oxidant generates metal oxo bonds, preferentially on the sterically less hindered (nonbridged) side of the complex, and epoxidation with (17) is low in enantioselectivity (Scheme 10). However, the enantioselectivity is considerably improved by the addition of imidazole. The imidazole has been considered to coordinate the metal center from the nonbridged side and to force the formation of metal oxo bonds on the bridged (chiral) side, thus enhancing enantioselectivity. 

Kureshy developed a polymer-based chiral Mn-salen complex (Figure 21). Copolymerization of styrene, divinylbenzene, and 4-vinylpyridine generated highly cross-linked (50%) porous beads loaded with pyridine ligands at 3.8 mmol g-1. Once the polymer was charged with the metal complex catalyst, enantioselective epoxidation of styrene derivatives was achieved with ee values in the range 16 46%. 79... 

From the energetics point of view, the epoxidation act should occur more easily (with a lower activation energy) in the coordination sphere of the metal when the cleavage of one bond is simultaneously compensated by the formation of another bond. For example, Gould proposed the following (schematic) mechanism for olefin epoxidation on molybdenum complexes [240] ... 

For 1,2-disubstituted epoxides, the regiochemical outcome of nucleophilic attack becomes less predictable. However, in the case of epoxy ethers chelation control can be used to deliver the nucleophile preferentially to the epoxide carbon away from the ether moiety. Thus, treatment of epoxy ether 61 with an imido(halo)metal complex, such as [Cr(N-t-Bu)Cl3(dme)], leads to the clean and high-yielding production of the chlorohydrin 64. The regioselectivity is rationalized in terms of initial formation of a chelated species (62), followed by attack at C-3 to form the more stable 5-membered metallacyclic alkoxide 63 <00SL677>. 

Porphyrin-metal complexes are natural mimetic substances that have attracted much attention during the past decade. The epoxidation of olefins by porphyrin complexes proceeds well, but with only modest enantioselectivity. As this area of research is growing, description of a few selected publications may be useful.96... 

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