Jacobsen manganese asymmetric epoxidation

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. 

As a solvent for the asymmetric epoxidation of 2,2-dimethylchromene mediated by Jacobsen s chiral (salen)-manganese catalyst.49... 

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... 

The applicability of the Sharpless asymmetric epoxidation is however limited to functionalized alcohols, i.e. allylic alcohols (see Table 4.11). The best method for non-functionalized olefins is the Jacobsen-Kaksuki method. Only a few years after the key publication of Kochi and coworkers on salen-manganese complexes as catalysts for epoxidations, Jacobsen and Kaksuki independently described, in 1990, the use of chiral salen manganese (111) catalysts for the synthesis of optically active epoxides [276, 277] (Fig. 4.99). Epoxidations can be carried out using commercial bleach (NaOCl) or iodosylbenzene as terminal oxidants and as little as 0.5 mol% of catalyst. The active oxidant is an oxomanganese(V) species. 

Asymmetric epoxidation The catalytic asymmetric epoxidation of alkenes has been the focus of many research efforts over the past two decades. The non-racemic epoxides are prepared either by enantioselective oxidation of a prochiral carbon-carbon double bond or by enantioselective alkylidenation of a prochiral C=0 bond (e.g. via a ylide, carbene or the Darzen reaction). The Sharpless asymmetric epoxidation (SAE) requires allylic alcohols. The Jacobsen epoxidation (using manganese-salen complex and NaOCl) works well with ds-alkenes and dioxirane method is good for some trans-alkenes (see Chapter 1, section 1.5.3). 

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. 

In this context it is worth noting that neither the titanium(IV) tartrate catalyst nor other metal catalyst-alkyl hydroperoxide reagents are effective for the asymmetric epoxidation of unfunctionalized olefins. The only system that affords high enantioselectivities with unfunctionalized olefins is the manganese(III) chiral Schiff s base complex/NaOCl combination developed by Jacobsen [42]. There is still a definite need, therefore, for the development of an efficient chiral catalyst for asymmetric epoxidation of unfunctionalized olefins with alkyl hydroperoxides or hydrogen peroxide. 

Asymmetric epoxidation (AE) of unfunctionalized alkenes catalyzed by chiral (salen)Mn(III) complex 38 (Scheme 2.13), developed by Jacobsen et al., is one of the most reliable methods [50]. As shown in Table 2.2, several different strategies have been formulated to immobilize Jacobsen s catalysts on inorganic supports [37-42]. Facilitation of catalyst separation, catalyst reuse, an increase in catalyst stability (e.g. minimization of the possibility of formahon of inachve g-oxo-manganese(lV) species [51a,b]) and sometimes improvement in enanhoselectivity are the main objectives of such research. Heterogenized Mn(salen) systems have recently been reviewed by Salvador et al. [51c] and Garcia et al. [5 Id]. Some selected cases are therefore described herein on the basis of the immobilizahon methods. 

Polymer-supported salen catalysts were also developed by employing poly (norbornene)-immobihzed salen complexes 139 of manganese and cobalt (Scheme 3.40) [77]. The poly(norbornene) complexes are highly active and selective catalysts for the epoxidation of olefins. The asymmetric epoxidation of cis-P-methylstyrene 132 occurred smoothly at -20 °C to give the chiral epoxide 133 in 100% conversion with 92% ee. Under the same reaction conditions, Jacobsen s catalyst (an unsupported salen complex) afforded the same product with 93% ee. 

The catalyhc asymmetric epoxidation of alkenes offers a powerful strategy for the synthesis of enantiomerically enriched epoxides. Among the several existing catalyhc methods, the asymmetric epoxidahon of unfunctionalized alkenes catalyzed by chiral Mn(lll)(salen) complexes such as homochiral [( N.N )-bis(3,5-di-tert-butylsalicylidene)-l,2-cyclohexanediamine]manganese(lll) chloride (22) (Figure 7.7), as developed by Jacobsen and coworkers, represents one ofthe most reliable methods [39]. 

From the work of Jacobsen and Katsuki, it is known that chiral manganese salen complexes are excellent catalysts for the asymmetric epoxidation of alkenes... 

Epoxides are key chiral synthetic intermediates and their enantioselective preparation by oxidation of achiral alkenes is a key reaction in many synthetic strategies. Sharpless asymmetric epoxidation is suitable for most allylic alcohols [26, 27], but few general procedures exist for unfunctionalized olefins. Jacobsen s manganese salen-mediated epoxidation is suitable for and gives good selectivities with Z-olefins (85 to 90% ee) [28]. The enzyme chloroperoxidase... 

The premiere method for producing chiral oxiranes from non-functionalized olefins is the Jacobsen asymmetric epoxidation, which utilizes a chiral manganese salen complex as a catalyst. Since Jacobsen s first report in 1990, intensive study in this area has generated a plethora of reaction conditions and catalyst type.s, as well as questions regarding the mechanistic parameters. The course of the oxygen transfer itself remains a matter of much debate. Norrby and Ackermark <97AG(E) 1723> maintain support for the intermediacy of a metallaoxetane species... 

Metal complexes of enantiomericaUy pure N,N -ethylenebis(salicylideneaminato) (salen) complexes in combination with stoichiometric oxidants currently provide the most selective method for the catalytic asymmetric epoxidation of unfunctionalised alkenes. The use of C2-symmetric salen complexes of manganese(lll) were reported independently in 1990 by Jacobsen and coworkers and Katsuki and coworkers. The first generation catalysts are represented by the general structure (4.33). The complex with R = Bu is known as Jacobsen s catalyst. All of the first generation catalysts are composed of a enantiopure diamine core and possess large substituents at the 3/3 and 5/5 positions. Subsequently Katsuki and coworkers developed second generation catalysts such as (4.34) with axially chiral groups at the 3/3 positions. 

Related epoxidations of olefins with PhIO in the presence of Salen and related complexes of chromium(III), manganese(III) and cobalt(III) have been reported by Kochi and coworkers [58]. The use of nickel(II) Salen in conjunction with NaOCl was also described [59]. More recently, these systems formed the basis for the development, by Jacobsen and coworkers [60], of chiral manganese(III) Salen complexes for the enantioselective epoxidation of prochiral olefins by ArlO or NaOCl. Similarly, asymmetric epoxidations with moderate to good... 

The Sharpless asymmetric epoxidation is reliable, but it works only for allylic alcohols. There is an alternative, however, which works with simple alkenes. The method was developed by Eric Jacobsen and employs a manganese catalyst with a chiral ligand built from a simple diamine. The diamine is not a natural compound and has to be made in enantiomeric form by resolution, but at least that means that both enantiomers are readily available. The diamine is condensed with a derivative of salicylaldehyde to make a bis-imine known as a salen. ... 

Chiral Catalysts Containing Group 7 Metals (Mn, Tc, and Re). Most of the chiral manganese complexes belong to the Mn(III)-salen-type complexes (Fig. 17), which are effective catalysts in asymmetric epoxidation (147). (The most widely used one is the Jacobsen s catalyst, iV,Ar -bis(3,5-di-terf-butylsalicylidene)-l,2-cyclohexanediamino-manganese(III) chloride.) These types of catalysts are also efficient for enantioselective aziridination (148), kinetic resolution of racemic allene via enantiomer differentiating oxidation (149), and enantiotopic selective... 

Figure 11.4 Chiral manganese complexes introduced by Jacobsen (13) and Katsuki (14, IS) for asymmetric epoxidation of nonfunctionalized alkenes [71].

Preparation of (1S,2/7)-1,2-epoxy-1,2,3,4 tetrahycb onaphthaiene (Structure 16). Asymmetric epoxidation using NaOCl catalysed by Jacobsen s chiral manganese salen complex (Structure 17),... 

Unlike the Sharpless epoxidation, which gives ehiral epoxides fix)m allylic alcohols, asymmetric epoxidation of unfimctionalized alkenes achieved by Jacobsen et al. [74],by using chiral salen-metal catalysts. Salen-Mn catalysts are preferred since manganese itself is relatively a low eost and nontoxic metal, primarily because of fewer side reaetions over other metal eomplexes. Variety of simple oxidants, such as PhlO, NaClO, and oxone are employed as reoxidants and best possible enantioselectivity for a given substrate could be achieved by choosing the proper metal-salen catalyst and reaction conditions [81], The catalyst can be Irne-tuned for required steric and electronic properties by making a variety of chiral salen ligands fi om various chiral diamines with salicylaldehyde derivates [82]. 

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