Katsuki manganese-salen complex

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... 

Ordinary alkenes (without an allylic OH group) have been enantioselectively epoxidized with sodium hypochlorite (commercial bleach) and an optically active manganese-complex catalyst. Variations of this oxidation use a manganese-salen complex with various oxidizing agents, in what is called the Jacobsen-Katsuki... 

Although the Sharpless catalyst was extremely useful and efficient for allylic alcohols, the results with ordinary alkenes were very poor. Therefore the search for catalysts that would be enantioselective for non-alcoholic substrates continued. In 1990, the groups of Jacobsen and Katsuki reported on the enantioselective epoxidation of simple alkenes both using catalysts based on chiral manganese salen complexes [8,9], Since then the use of chiral salen complexes has been explored in a large number of reactions, which all utilise the Lewis acid character or the capacity of oxene, nitrene, or carbene transfer of the salen complexes (for a review see [10]). 

Manganese salen complex catalyzes C—H oxidation of organic molecules with NaOCl or PhIO, giving alcohols . Larrow and Jacobsen observed kinetic resolution in the benzylic hydroxylation . Katsuki and coworkers used the axis chiral salen manganese complexes for the benzyl hydroxylation and ether hydroxylation, and attained higher ee with the ligand possessing (/f,/f)-diamine and (R)-axis chirality (equation 84). ... 

Although asymmetric aziridination of styrenes was attempted by Burrow and Katsuki and their coworkers using manganese salen complexes in the presence of PhI=NTs, low asymmetric induction was observed ". Nishikori and Katsuki later employed a salen complex synthesized from (/ ,/f)-2,3-diaminobutane and ( i-biphenol, and found that the chirality at the 3,3 -positions is more important for the asymmetric induction (equation 85) . Carreira conducted the stoichiometric amination of enol ethers and alkenes using a manganese nitride salen complex. Komatsu extended the methodology to the catalytic process and attained 94% ee for aziridination of / -isopropylstyrene. ... 

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

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... 

Imido and 0x0 compounds are intermediates in many of the transfers of oxygen atoms and nitrene units to olefins to form epoxides and aziridines, and they are intermediates in many of the insertions of oxygen atoms and nitrene units into the C-H bonds of hydrocarbons to form alcohols and amine derivatives. The enantioselective epoxidation of allylic alcohols (Scheme 13.22) " is the most widely used epoxida-tion process, and the discovery and development of this process was one of the sets of chemistry that led K. Barry Sharpless to receive the Nobel Prize in Chemistry in 2001. The mechanism of this process is not well established, despite the long time since its discovery and development. Nevertheless, most people accept that transfer of the oxygen atom occurs from a titanium-peroxo complex - rather than from an 0x0 complex. Jacobsen s and Katsuki s - manganese-salen catalysts for the enantioselective epoxidations of unfunctionalized olefins, which were based on Kochi s achiral chromium- and manganese-salen complexes, are a second set of... 

Katsuki and coworkers have employed the related chiral manganese salen complexes, in particular the so-called second-generation Mn(salen) 46, in sulfide oidda-tion [148]. This complex was found to serve as an efficient catalyst in asymmetric sulfoxidation, albeit the less atom-efficient iodosylbenzene was required as oxidant (Scheme 11.21). 

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 order to increase the yield and/or the enantioselectivity of the reaction, the reaction temperature and additives were examined. Although aziridination was found to proceed smoothly at 0 °C, the product was not obtained at lower temperatures. Katsuki and co-workers have reported that pyridine /V-oxide is an effective additive for the asymmetric epoxidation catalyzed by salen-manganese(IH) complexes [24], and applied these findings to the asymmetric aziridination of olefins with Phi = NTs [9f]. Thus, the addition of pyridine /V-oxide at 0°C improved the enantioselectivity and allowed the reaction to proceed even at -20 °C (Table 6.1). Other additives, such as 4-phenylpyridine IV-oxide, 4-methylmorphorine N-oxide and 1-methylimidazole were used in the place of pyridine JV-oxide, but positive effects were not observed. 

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. 

Katsuki T. Catal)4ic asymmetric oxidations using optically active (salen)manganese(in) complexes as catalysts. Coord. Chem. Rev. 1995 140 189-214. 

The mechanism of the J-K epoxidation is not fully understood, but most likely a manganese(V)-specles Is the reactive intermediate, which Is formed upon the oxidation of the Mn(lll)-salen complex. The enantioselectivity Is explained by either a top-on approach (Jacobsen) or by a side-on approach (Katsuki) of the olefin. The three major mechanistic pathways are shown below. The radical intermediate accounts for the formation of mixed epoxides when conjugated olefins are used as substrates. 

Irie, R., Noda, K., Ito, Y., Katsuki, T. Enantioselective epoxidation of unfunctionalized olefins using chiral (salen)manganese(lll) complexes. Tetrahedron Lett. 1991, 32, 1055-1058. 

T. Hashihayata, Y. Ito, T. Katsuki, The first asymmetric epoxidation using a combination of achiral (salen)manganese(III) complex and chiral amine. Tetrahedron 53 (1997) 9541. 

Although it is well known that cyclic ethers are readily oxidized to the corresponding lactols or lactones, their asymmetric desymmetrization was not examined until quite recently. However, desymmetrization of prochiral or meso-cydic ethers is expected to be a useful tool for organic synthesis, since many prochiral or raeso-cyclic ethers are available in bulk. Recently, Miyafuji and Katsuki have reported the desymmetrization of 4-terf-butylcyclotetrahydropyran and meso-tetrahydrofurans with the chiral (salen)manganese(III) complex 13 as catalyst (Scheme 9) [24,25]. The oxidation of the former shows only the modest enantioselectivity, while the reaction of the latter exhibits excellent enantioselectivity. The low enantioselectivity (48% ee) observed in the oxidation of 4-ferf-butyltet-rahydropyran has been attributed to the participation of enantiomeric twist-boat conformers. Although 4-ferf-butyltetrahydropyran exists in an equilibrium mixture of chair and enantiomeric twist-boat conformers and the equilibrium ratio of the latter is very small, the latter is considered to be more reactive than the former for stereo electronic reasons. One of a-C-H bonds in the twist-boat conformer almost eclipses the -orbital while those in the chair conformer are gauche or anti to the -orbital. 

E. N. Jacobsen, in Comprehensive Organometalhc Chemistry II A Review of the Literature 1982-1994 , Eds. E. W. Abel, R G. A. Stone, G. Wilkinson, Pergamon Press, Oxford, U.K., 1995, Vol. 12, 1097-1136 (Transition Metal-Catalyzed Oxidations Asymmetric Epoxidatioriy, T. Katsuki, Coord. Chem. Rev. 1995,140, 189-214 (Catalytic Asymmetric Oxidations Using Optically Active (Salen)Manganese(lll) Complexes as Catalysts), B. M. Trost, C. Heinemann, X. Ariza, S. Weigand, J. Am. Chem. Soc. 1999, 121, 8667. 

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. 

Catalytic asymmetric C-H oxidation has also been achieved using metal porphyrins and metal(salen) complexes. Benzylic oxidations using vaulted binaphthyl iron(III) porphyrins,and manganese(III)(salen) complexes have suffered from moderate yields and/or enantioselectivities. However, Miyafuji and Katsuki have obtained reasonable yield and good enantioselectivity for C-H... 

One of the most elegant methods for the selective formation of C—O bonds is the catalytic Jacobsen-Katsuki epoxidation, the enantioselective synthesis of optically active epoxides by oxygen-transfer reactions with chiral, nonracemic manganese 0x0 salen complexes. These complexes have been suggested as the catalytically active species in epoxidations catalyzed by metal-salen and porphyrin complexes [78]. One of these complexes was for the first time isolated and characterized by Feichtinger and Planner through ESI-MS studies [79]. 

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