Efficient epoxidation catalyst

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. 

E. Pizzo, P. Sgarbossa, A. Scarso, R. A. Michelin, G. Strukul, Second-generation electron-poor platinum(II) complexes as efficient epoxidation catalysts for terminal alkenes with hydrogen peroxide, Organometallics 25 (2006) 3056. 

A remarkable approach was reported in 2004 by Simormeaux and coworkers [53]. Manganese complexes of spirobifluorenyl-substituted porphyrins were elec-tropolymerized by anodic oxidation and the resulting poly(9,9 -spirobifluorene manganese porphyrin) films were shown to be efficient epoxidation catalysts in the presence of imidazole. The polymers were tested in the epoxidation of cyclooctene and styrene using PhIO or PhI(OAc)2 as oxidants. Epoxide yield reached 95% in the case of cyclooctene and 77% in the case of styrene. The electrosynthesized polymers could be recovered by filtration and reused up to eight times without loss of activity and selectivity. 

The major breakthrough was the discovery that titanium silicate could used as an efficient epoxidation catalyst.39 The reaction with TS-1 may be performed under mild conditions, for example at room temperature in dilute aqueous or methanolic solutions. 

Efficient epoxidation catalysts, have been discovered over the past decade. Re... 

The Ni(II) complex of salen 35 was found to be a highly efficient epoxidation catalyst when used in conjunction with NaOCl. Enhanced selectivity and high turnover rate were observed (96). By adjusting pH of the aqueous phase, significant increase in turnover rate as high as >600 min for frans-jS-methylstyrene can be achieved, which is comparable to that of the Mn(III) tetraarylporphyrin (97). From the pH dependence of the rate constants, it was proposed that transfer of HOCl to the organic phase was required to become an efficient catalyst, and its reaction with the Ni(II) salen complex led to formation of the active species LNi -0. ... 

Chiral Homo-Quadridentate Ligands. Chiral porphirines (PORPH) com-plexed to manganese(III) have been published recently as efficient epoxidation catalysts (97). The synthesis of the ligand is based on the condensation of enan-tiomerically pure [2.2]-p-cyclophane-4-carbaldehyde and pyrrol (Fig. 11). 

The reason for the efficient epoxidation of explicitly allylic alcohols with this system can be found in the strong associative interactions occurring between the substrate and the catalyst. The [Ti(tartrate)(OR)2]2 dimer 1, which is considered to be the active catalyst in the reaction, will generate structure 2 after the addition of... 

The introduction of chlorinated porphyrins (10) allowed for hydrogen peroxide to be used as terminal oxidant [62], These catalysts, discovered by Mansuy and coworkers, were demonstrated to resist decomposition, and efficient epoxidations of olefins were achieved when they were used together with imidazole or imidazo-lium carboxylates as additives, (Table 6.6, Entries 1 and 2). 

The second major discovery regarding the use of MTO as an epoxidation catalyst came in 1996, when Sharpless and coworkers reported on the use of substoichio-metric amounts of pyridine as a co-catalyst in the system [103]. A change of solvent from tert-butanol to dichloromethane and the introduction of 12 mol% of pyridine even allowed the synthesis of very sensitive epoxides with aqueous hydrogen peroxide as the terminal oxidant. A significant rate acceleration was also observed for the epoxidation reaction performed in the presence of pyridine. This discovery was the first example of an efficient MTO-based system for epoxidation under neutral to basic conditions. Under these conditions the detrimental acid-induced decomposition of the epoxide is effectively avoided. With this novel system, a variety of... 

Begue and coworkers recently achieved an improvement in this method by performing the epoxidation reaction in hexafluoro-2-propanol [120]. They found that the activity of hydrogen peroxide was significantly increased in this fluorous alcohol, in relation to trifluoroethanol, which allowed for the use of 30% aqueous H202. Interestingly, the nature of the substrate and the choice of additive turned out to have important consequences for the lifetime of the catalyst. Cyclic dis-ubstituted olefins were efficiently epoxidized with 0.1 mol% of MTO and 10 mol%... 

Dimethylchromene has also proven to be a useful substrate for the assessment of various transition metal complexes as epoxidation catalysts. Chiral Mn(III)-salen complexes are efficient <00CC615 00T417> and can be recycled when used in an ionic liquid <00CC837>. The enantioselective aziridination of a chromene has been achieved using a chiral biaryldiamine-derived catalyst (Scheme 22) <00JA7132>. 

Run no. Catalyst Reactant Initial/ before H202 addition pH After H202 addition At die end of die reaction TOF Olefin conversion (mol%) h2o2 efficiency Epoxide selectivity (mol%)... 

Conditions were found under which simple inorganic oxorhenium species act as efficient olefin epoxidation catalysts. This has been achieved by simply replacing aqueous H2O2 with BTSP as an oxygen atom source in the epoxidation. Representative substrates include fairly unreactive olefins and/or progenitors of sensitive epoxides . For example, terminal olefins can be efficiently converted to epoxides (equation 40, Table 9). 

Previously, some fluorocyclohexanones were used in a catalytic amount with Oxone for asymmetric epoxidation reaction, but they gave a poor ee . It was found later that chiral ketones derived from fructose work well as asymmetric epoxidation catalysts and show high enantioselectivity in reactions of /rani-disubstituted and trisubsti-tuted olefins ". Cis and terminal olefins show low ee under these reaction conditions. Interestingly, the catalytic efficiency was enhanced dramatically upon raising the pH. Another asymmetric epoxidation was also reported using Oxone with keto bile acids. ... 

---