Katsuki’s catalysts

Conjugated dienes can be epoxidized to provide vinylepoxides. Cyclic substrates react with Katsuki s catalyst to give vinylepoxides with high ees and moderate yields [17], whereas Jacobsen s catalyst gives good yields but moderate enantiose-lectivities [18]. Acyclic substrates were found to isomerize upon epoxidation (Z, )-conjugated dienes reacted selectively at the (Z)-alkene to give trans-vinylepoxides (Scheme 9.4a) [19]. This feature was utilized in the formal synthesis of leuko-triene A4 methyl ester (Scheme 9.4b) [19]. 

Various other chromene derivatives 176a-d could be epoxidized with Katsuki s Mn-salen catalyst 173d using either H2O2 or TMS2O2 as oxidant. With this catalytic system several axial ligands (none, 7V-methylimidazole, pyridine TV-oxide) and additives (none. 

Most of the oxidants used with these catalysts tend to be water soluble. However, the use of tetrabutylammonium monopersulfate (5) has been reported to provide relatively smooth oxidation with good ee s e.g, 4 -> 6) under monophasic conditions in organic solvents. The enantioselectivity is both substrate and catalyst dependent, with best results being obtained from electron-rich alkenes and Katsuki-type catalysts (c/. 2) <99TL1001>. Che and co-workers <99CC1789> have also reported on the use of an immobilized chromium binaphthyl catalyst, which offers the advantage of simple work-up and catalyst recovery. 

Lipkowitz, K.B. and Schefzick, S. (2002) Ligand distortion modes leading to increased chirality content of Katsuki—Jacobsen catalysts. Chirality, 14, 677. 

Fig. 7. Katsuki s pre-catalyst for the enantioselective oxidative dimerization of 2-naphthol (68a).

Many interesting reports on the epoxidation of alkcnes have appeared in the literature, particularly involving asymmetric methods. In this regard, the chiral salen catalysts (1 and 2) developed by Jacobsen and Katsuki <01C()C663> find frequent use, and convenient methods for their industrial preparation continue to be reported. For example, Jacobsen s catalyst (1, Rl = R2 = <-Bu X = Q) is now available in 75% overall yield from commercially available starting materials <01SC2913>. 

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. 

Catalyst ( Bu-4.33) is effective in the epoxidation of many ds-alkenes, such as the acyclic alkenes (4.42) and also cychc alkenes, especially 2,2-dimethylchromenes such as (4.43). Enhanced enantioselectivities are often observed using Katsuki s second generation catalysts (4.34) where the phenyl substituents of the BINAP moiety undergo greater steric interactions with the alkene substrate. Dihydronaphthalene (4.44) has been epoxidised with record ees using this catalyst system. ... 

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... 

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 unsymmetrical analog of a Katsuki-type salen ligand was attached to Merri-field s resin (1% cross-Hnked) yielding a catalyst (22) (Fig. 4.3) which showed good efficiency and selectivity in the asymmetric epoxidation of 1,2-dihydronaphtalene with good performance after several recycling steps [81]. Related complexes (23) immobilized on silica were recently disclosed by Seebach and coworkers (Fig. 4.3) [82]. 

Katsuki has extended his earlier work on asymmetric induction using achiral catalysts such as 13. In these systems, the stereochemical bias is imbued by a chiral non-racemic axial ligand, such as (+)-3,3 -dimethyl-2,2 -bipyridine A2,A -dioxide (14), which was purified by crystallization with (5)-binaphthol. Epoxidation using these conditions resulted in good ee s and fair yields, as exemplified by the preparation of chromene epoxide 16 <99SL783>. 

Jonsson, S., Odille Fabrice, G.J., Norrby, P.-O. and Warnmark, K. (2006) Modulation of the reactivity, stability and substrate- and enantioselectivity of an epoxidation catalyst by noncovalent dynamic attachment of a receptor functionality - aspects on the mechanism of the Jacobsen-Katsuki epoxidation applied to a supramolecular system. Org. Biomol. Chem., 4, 1927-1948 Jonsson, S., Odille Fabrice, G.J., Norrby, P.-O. and Warnmark, K. (2005) A dynamic supramolecular system exhibiting substrate selectivity in the catalytic epoxidation of olefins. Chem. Commun., 549-551. 

The Jacobsen-Katsuki Schiff base Mn complexes (6a and 6b) are the most advanced catalysts for enantioselective epoxidation of double bonds. With the typical reactants, cis disubstituted and trisubstituted aromatic olefins, ee values up to 98% are achieved, even if the total number of turnovers is quite limited. In Jacobsen s complex 6a, particularly the bulky /-butyl substituents at positions 3 and 5 of the aromatic ring are crucial in directing the reactant and obtaining high ee values (86). 

Smith and Liu <02CC886> have immobilized a Katsuki-type salen ligand by an ester linkage to Merrifield s resin to produce catalyst 7. In a test epoxidation of 1,2-dihydronaphthalene (8)... 

The Jacobsen-Katsuki-catalysts (Fig. 13) have recently received much attention as the most widely used alkene epoxidation catalysts. An example of Jacobsen s manganese-salen catalyst is shown in Fig. 13. They promote the stereoselective conversion of prochiral olefins to chiral epoxides with enantiomeric excesses regularly better than 90% and sometimes exceeding 98%.82,89,92,93,128 The oxidation state of the metal changes during the catalytic cycle as shown in Scheme 8. 

The epoxidation of simple olefins which cannot benefit from secondary interactions brings some formidable problems that were solved by sophisticated catalyst design, mainly by the groups of Jacobsen and Katsuki in the 1990 s. A class of square planar salen complexes was chosen (Figure 19, for example) capable of giving a metal-oxo derivative by reaction with monooxygen donors such as iodosobenzene or sodium hypochlorite (the preferred oxidant). A series... 

This past year s literature has shown extraordinary activity in this realm. Perhaps the most firmly entrenched methodology for the preparation of chiral epoxides is the metallosalen mediated epoxidation of unfunctionalized alkenes (the Jacobsen-Katsuki epoxidation), which has been recently reviewed <03SL281 >. It is widely accepted that this reaction proceeds through an 0X0 intermediate, and that the observed enantioselectivities depend upon the electronic stability of this species. For example, Jacobsen found empirically that electron-donating substituents in the 5 and 5 positions of catalyst 1 gave better enantioselectivities <91JA6703>. More recent... 

Potvin, P. G., Bianchet, S. The nature of the Katsuki-Sharpless asymmetric epoxidation catalyst. J. Org. Chem. 1992, 57, 6629-6635. 

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