Epoxidation Jacobsen-Katsuki asymmetric

Fig. 4.99 The Jacobsen-Katsuki asymmetric epoxidation of unfunctionalized olefins.

The body of work that constitutes the metallosalen-catalyzed (Jacobsen-Katsuki) asymmetric epoxidation reaction is far too extensive to be detailed here however, it has been comprehensively reviewed <1996JM087, B-1999M1(11)649, B-2000MI287, 2001COR663, 2005CRV1563>. Rather, after some introductory remarks, we will highlight examples in which ring-fused oxiranes are produced. 

Kinetic resolution in the catalytic conversion of racemic chloro propanols to optically active epoxides has been achieved by use of a chiral Co(salen) type complex in combination with K2CO3. Although enantioselectivity was modest (< 35 % ee), this first use in asymmetric epoxide formation of the chiral ligand system that was later brought to fame through the Jacobsen-Katsuki asymmetric epoxidation is noteworthy [56,57]. When applied to the prochiral l,3-dichloro-2-propanol, asymmetric induction of up to ca. 60 % ee was achieved (Sch. 8) [58]. 

Figure 17.32 Use of the Jacobsen-Katsuki asymmetric epoxidation for the preparation of an e.p. intermediate for preparation of structure 92...

The Best results are obtained with cA-alkenes however, the epoxidation of tri-and tetra-substituted double bonds is also possible. Because of its versatility, the Jacobsen-Katsuki epoxidation is an important method in asymmetric synthesis. 

A breakthrough in the area of asymmetric epoxidation came at the beginning of the 1990s, when the groups of Jacobsen and Katsuki more or less simultaneously discovered that chiral Mn-salen complexes (15) catalyzed the enantioselective formation of epoxides [71, 72, 73], The discovery that simple achiral Mn-salen complexes could be used as catalysts for olefin epoxidation had already been made... 

Ten years after Sharpless s discovery of the asymmetric epoxidation of allylic alcohols, Jacobsen and Katsuki independently reported asymmetric epoxidations of unfunctionalized olefins by use of chiral Mn-salen catalysts such as 9 (Scheme 9.3) [14, 15]. The reaction works best on (Z)-disubstituted alkenes, although several tri-and tetrasubstituted olefins have been successfully epoxidized [16]. The reaction often requires ligand optimization for each substrate for high enantioselectivity to be achieved. 

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]. 

The protocol developed by Jacobsen and Katsuki for the salen-Mn catalyzed asymmetric epoxidation of unfunctionalized alkenes continues to dominate the field. The mechanism of the oxygen transfer has not yet been fully elucidated, although recent molecular orbital calculations based on density functional theory suggest a radical intermediate (2), whose stability and lifetime dictate the degree of cis/trans isomerization during the epoxidation <00AG(E)589>. 

The requirement for the presence of an adjacent alcohol group can be regarded as quite a severe limitation to the substrate range undergoing asymmetric epoxidation using the Katsuki-Sharpless method. To overcome this limitation new chiral metal complexes have been discovered which catalyse the epoxidation of nonfunctionalized alkenes. The work of Katsuki and Jacobsen in this area has been extremely important. Their development of chiral manganese (Ill)-salen complexes for asymmetric epoxidation of unfunctionalized olefins has been reviewed1881. 

In 1990, Jacobsen et al. and Katsuki et al. independently reported asymmetric epoxidation of conjugated olefins by using complexes 9 and 10, respectively, as catalysts [29], These Mn-salen complexes were further improved to complexes 11 [30], 12 [31], and 13 [32]. The common features of these first-generation Mn-salen complexes are i) they possess C2-symmetry, ii) two sp3 carbons at the ethylenediamine moiety are replaced with chiral ones, and iii) they have tert-butyl groups or enantiopure 1-phenylpropyl groups at the C3 and C3 positions. 

Chiral (salen)Mn(III)Cl complexes are useful catalysts for the asymmetric epoxidation of isolated bonds. Jacobsen et al. used these catalysts for the asymmetric oxidation of aryl alkyl sulfides with unbuffered 30% hydrogen peroxide in acetonitrile [74]. The catalytic activity of these complexes was high (2-3 mol %), but the maximum enantioselectivity achieved was rather modest (68% ee for methyl o-bromophenyl sulfoxide). The chiral salen ligands used for the catalysts were based on 23 (Scheme 6C.9) bearing substituents at the ortho and meta positions of the phenol moiety. Because the structures of these ligands can easily be modified, substantia] improvements may well be made by changing the steric and electronic properties of the substituents. Katsuki et al. reported that cationic chiral (salen)Mn(III) complexes 24 and 25 were excellent catalysts (1 mol %) for the oxidation of sulfides with iodosylbenzene, which achieved excellent enantioselectivity [75,76]. The best result in this catalyst system was given by complex 24 in the formation of orthonitrophenyl methyl sulfoxide that was isolated in 94% yield and 94% ee [76]. 

The potential of a catalytic process for use on a large scale can be a good indication of its efficiency. During recent decades there has been an increasing tendency to apply asymmetric catalytic processes in industry [1], The asymmetric Noyori hydrogenation [2] and the Sharpless and Jacobsen-Katsuki epoxidation [3] are representative examples of impressive developments in this field [1]. 

In practice in the literature of the past 20 years the important results with ruthenium in epoxidation are those where ruthenium was demonstrated to afford epoxides with molecular oxygen as the terminal oxidant. Some examples are presented (see later). Also ruthenium complexes, because of their rich chemistry, are promising candidates for the asymmetric epoxidation of alkenes. The state of the art in the epoxidation of nonfunctionalized alkenes is namely still governed by the Jacobsen-Katsuki Mn-based system, which requires oxidants such as NaOCl and PhIO [43,44]. Most examples in ruthenium-catalysed asymmetric epoxidation known until now still require the use of expensive oxidants, such as bulky amine oxides (see later). 

In the same year (1990) that Jacobsen reported his asymmetric epoxidation, a group led by Tsutomu Katsuki at the University of Kyushu in Japan reported a closely related asymmetric epoxidation. The chiral catalyst is also a salen and the metal manganese. The oxidant is iodosobenzene (Phl=0) but this method works best for E-alkenes. It is no coincidence that Katsuki and Jacobsen both worked for Sharpless. It is not unusual for similar discoveries to be made independently in different parts of the world, the Katsuki manganese salen complex... 

Using the clear homology of epoxidation of olefin and the oxidation of sulfide, Jacobsen and co-workers65 and Katsuki and co-workers66,67 applied their system developed for the asymmetric epoxidation of simple olefin to the asymmetric oxidation of prochiral sulfides. 

Both chemical and enzymatic synthetic methods for the asymmetric oxidation of the carbon-carbon double bond have been developed [46], but the area of carbon-carbon double bond oxidations has been shaped by the breakthrough discovery of asymmetric epoxidation of allylic alcohols with the Katsuki-Sharpless method [47]. Catalytic asymmetric synthesis of epoxides from alkenes by Jacobsen... 

Initial successes with these ligands came independently from the groups of Jacobsen and Katsuki in the asymmetric epoxidation of unfimctionalized olefins. Since these seminal works in 1990, metal salen complexes have become workhorse in asymmetric catalysis, finding applications in a wide variety of reactions. In Figure 1 is illustrated a variety of metal salen complexes. Scheme 1 lists some of the transformations in which they have been used, demonstrating the broad utility of these complexes. 

The report by Kochi and co-workers in 1986 that a (salen)manganese(lll) complex (Mn(salen) complex) was an efficient epoxidation catalyst for simple olefins <1986JA2309> quickly led to independent reports from the groups of Jacobsen <1990JA2801> and Katsuki <1990TL7345> that chiral Mn(salen) complexes could catalyze asymmetric epoxidation reactions. The reaction requires the use of a stoichiometric oxidant initially iodosylarenes were utilized, but it was quickly found that NaOCl was also successful. 

Related reactions Jacobsen-Katsuki epoxidation, Prilezhaev oxidation, Rubottom oxidation, Sharpless asymmetric epoxidation, Shi... 

Related reactions Davis oxaziridine oxidation, Jacobsen-Katsuki epoxidation, Priiezhaev reaction, Shi asymmetric epoxidation ... 

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