Asymmetric epoxidation catalysis

The Sharpless-Katsuki asymmetric epoxidation (AE) procedure for the enantiose-lective formation of epoxides from allylic alcohols is a milestone in asymmetric catalysis [9]. This classical asymmetric transformation uses TBHP as the terminal oxidant, and the reaction has been widely used in various synthetic applications. There are several excellent reviews covering the scope and utility of the AE reaction... 

The asymmetric epoxidation of enones with polyleucine as catalyst is called the Julia-Colonna epoxidation [27]. Although the reaction was originally performed in a triphasic solvent system [27], phase-transfer catalysis [28] or nonaqueous conditions [29] were found to increase the reaction rates considerably. The reaction can be applied to dienones, thus affording vinylepoxides with high regio- and enantio-selectivity (Scheme 9.7a) [29]. 

The past thirty years have witnessed great advances in the selective synthesis of epoxides, and numerous regio-, chemo-, enantio-, and diastereoselective methods have been developed. Discovered in 1980, the Katsuki-Sharpless catalytic asymmetric epoxidation of allylic alcohols, in which a catalyst for the first time demonstrated both high selectivity and substrate promiscuity, was the first practical entry into the world of chiral 2,3-epoxy alcohols [10, 11]. Asymmetric catalysis of the epoxidation of unfunctionalized olefins through the use of Jacobsen s chiral [(sale-i i) Mi iln] [12] or Shi s chiral ketones [13] as oxidants is also well established. Catalytic asymmetric epoxidations have been comprehensively reviewed [14, 15]. 

Another interesting asymmetric epoxidation technique using metal catalysis involves the vanadium complexes of A-hydroxy-[2.2]paracyclophane-4-carboxylic amides (e.g., 19), which serve as catalysts for the epoxidation of allylic alcohols with f-butyl hydroperoxide as... 

A more versatile method to use organic polymers in enantioselective catalysis is to employ these as catalytic supports for chiral ligands. This approach has been primarily applied in reactions as asymmetric hydrogenation of prochiral alkenes, asymmetric reduction of ketone and 1,2-additions to carbonyl groups. Later work has included additional studies dealing with Lewis acid-catalyzed Diels-Alder reactions, asymmetric epoxidation, and asymmetric dihydroxylation reactions. Enantioselective catalysis using polymer-supported catalysts is covered rather recently in a review by Bergbreiter [257],... 

About a decade after the discovery of the asymmetric epoxidation described in Chapter 14.2, another exciting discovery was reported from the laboratories of Sharpless, namely the asymmetric dihydroxylation of alkenes using osmium tetroxide. Osmium tetroxide in water by itself will slowly convert alkenes into 1,2-diols, but as discovered by Criegee [15] and pointed out by Sharpless, an amine ligand accelerates the reaction (Ligand-Accelerated Catalysis [16]), and if the amine is chiral an enantioselectivity may be brought about. 

The attractive (80) features of MOFs and similar materials noted above for catalytic applications have led to a few reports of catalysis by these systems (81-89), but to date the great majority of MOF applications have addressed selective sorption and separation of gases (54-57,59,80,90-94). Most of the MOF catalytic applications have involved hydrolytic processes and several have involved enantioselec-tive processes. Prior to our work, there were only two or three reports of selective oxidation processes catalyzed by MOFs. Nguyen and Hupp reported an MOF with chiral covalently incorporated (salen)Mn units that catalyzes asymmetric epoxidation by iodosylarenes (95), and in a very recent study, Corma and co-workers reported aerobic alcohol oxidation, but no mechanistic studies or discussion was provided (89). 

Since the first two approaches are very well known and exploited, and excellent reviews and books on the topic are available [1], we will deal only with some of the most recent findings in chemical catalysis -excluding the Sharpless asymmetric epoxidation and dihydroxylation, to which the whole of Chapter 10 is devoted. Synthetic catalysts which mimic the catalytic action of enzymes, known as chemzymes, will be also considered. 

An a-substituted system (Entry 28) has also been epoxidised in good yield and with moderate stereoselectivity. Although the absolute stereochemistry of the resulting epoxide has yet to be determined, this is the first example of such an enone undergoing asymmetric epoxidation using polyamino acid catalysis. 

Dioos, B. M. L. Geiutsa W. A. Jacobs, P. A. (2004) Coordination of Crlll(salen) on fimctionalised silica for asymmetric ring opening reactions of epoxides.. Catalysis Lett., 97 125-129. 

The catalytic asymmetric epoxidation of a,p-unsaturated aldehydes has also been an important challenge in iminium catalysis and for chemical synthesis in general. More recently, Jprgensen and coworkers have developed an asymmetric organocatalytic approach to ot, (3-epoxy aldehydes using pyrrolidine catalyst 20 and H2O2 as the stoichiometric oxidant. The reaction appears to be extremely general and will likely receive wide attention from the chemical synthesis community (Scheme 11.6b). 

Lygo, B. and To, D.C.M., Asymmetric Epoxidation via Phase-transfer Catalysis Direct Conversion of Allylic Alcohols into a, -Epoxyketones. Chem. Commun. 2002, 2360-2361. 

Aldol products do not have to come from an aldol condensation. In another example of catalysis by a small organic molecule, Jeffrey Bode of UC Santa Barbara reports (J- Am. Chem. Soc. 2004,126, 8126) that the thioazolium salt 7 effects the rearrangement of an epoxy aldehyde such as 6 to the aldol product 8. This is a net oxidation of the aldehyde, and reduction of the epoxide. As epoxy aldehydes such as 6 are readily available by Sharpless asymmetric epoxidation, this should be a general route to enantiomerically-aldol products. The rearrangement also works with an aziridine aldehyde such as 9, to give the ff-amino ester 10. 

In an effort to introduce C2 symmetry into nickel complexes for their application in catalysis of asymmetric epoxidation, a series of oxocyclam analogues derived from amino acids 34 were synthesized. They did not react in the presence of PhIO as oxidant. However, they showed enhanced reactivity with NaOCl as the terminal oxidant under phase-... 

Finn, F M, Hofmann, K 1976, in Neurath, H, Hill, R L (eds), The Proteins, 3rd edn, Vol II, chapter 2(p 106—237), Academic Press New York London Finn, M G, Sharpless, fC. B 1985, On the Mechanism of Asymmetric Epoxidation with Titanium-Tartrate Catalysts, in Momson, J D (ed), Asymmetric Synthesis, Vol 5 Chiral Catalysis chapter 8, p 247, Academic Press New York Fischer, E 1914, Chem Ber 47,196 Fischer, H, Slangier G 1927, Liebigs, Ann Chem 459, 53 Fischer, H Neber, M 1932. Liebigs Ann Chem 496,1... 

Rossiter, B E 1985, Synthetic Aspects and Applications of Asymmetric Epoxidation, in Morrison, J D (ed), Asymmetric Synthesis, Vol 5 Chiral Catalysis, chapter 7, p 193, Academic Press New York Rotermund, G W 1975, m Houben-Weyl, Methoden der Organtschen Chemte, Vol IV/] b Oxidation If Thieme Stuttgart... 

The catalytic asymmetric epoxidation of electron-deficient olefins, particularly a,P-unsaturated ketones, has been the subject of numerous investigations, and as a result a number of useful methodologies have been elaborated [44], Among these, the method utilizing chiral phase-transfer catalysis occupies a unique position in terms of its practical advantages. Moreover, it also allows the highly enantioselective epoxidation of trans-a,P-unsaturated ketones, particularly chalcone. 

Asymmetric epoxidation catalyzed by chiral phase-transfer catalysts is another reaction which has been extensively studied following an initial report by Wynberg [2,44]. Shioiri et al. further improved the enantioselective epoxidation of naphthoquinones under cinchona alkaloid-derived chiral phase-transfer catalysis [45],... 

H. Q. Tian, X. G. She, and Y. Shi, Electronic probing of ketone catalysts for asymmetric epoxidation. Search for more robust catalysis, Org. Lett. 2001, 3, 715-718. 

Wu in Comprehensive Asymmetric Catalysis I-III (Eds. Jacobsen, E. N. Pfaltz, A. Yamamoto H.), Springer, Berlin, 1999, p. 649f. (c) For organocatalytic asymmetric epoxidations, see chapter 10. 

Phase-transfer catalysis has been widely been used for asymmetric epoxidation of enones [100]. This catalytic reaction was pioneered by Wynberg et al., who used mainly the chiral and pseudo-enantiomeric quaternary ammonium salts 66 and 67, derived from the cinchona alkaloids quinine and quinidine, respectively [101-105],... 

In the metal-free epoxidation of enones and enoates, practically useful yields and enantioselectivity have been achieved by using catalysts based on chiral electrophilic ketones, peptides, and chiral phase-transfer agents. (E)-configured acyclic enones are comparatively easy substrates that can be converted to enantiomeri-cally highly enriched epoxides by all three methods. Currently, chiral ketones/ dioxiranes constitute the only catalyst system that enables asymmetric and metal-free epoxidation of (E)-enoates. There seems to be no metal-free method for efficient asymmetric epoxidation of achiral (Z)-enones. Exocyclic (E)-enones have been epoxidized with excellent ee using either phase-transfer catalysis or polyamino acids. In contrast, generation of enantiopure epoxides from normal endocyclic... 

Asymmetric Phase Transfer Catalysis Asymmetric Epoxidation Reactions... 

Chiral asymmetric epoxidations have been intensively investigated due to the fundamental importance of epoxides in organic chemistry [69, 70], Nevertheless, catalytic asymmetric Lewis acid epoxidation of a,/i-unsaturated aldehydes remains a challenge to chemists. Recently, Jorgensen and co-workers developed the first asymmetric approach to epoxides of enals, in which chiral pyrrolidine 11 was used as catalyst and H2O2 as oxidant, thus following the concept of iminium catalysis (Scheme 3.9) [71-73]. Importantly, reaction conditions are tolerant to a variety of functionalities and this chemical transformation proceeds in different solvents, with no loss of enantioselectivity. (For experimental details see Chapter 14.13.1). 

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