Manganese oxidation epoxidation

In subsequent research, it turned out that two-state reactivity can also provide a concept for the understanding of oxidation reactions way beyond the scope of gas-phase ion chemistry and can actually resolve a number of existing mechanistic puzzles. In enzymatic oxidations involving cytochrome P450, for example, changes in spin multiplicity appear to act as a kind of mechanistic distributor for product formation [27-29], and in the case of manganese-catalyzed epoxidation reactions, two-state scenarios have been put forward to account for the experimentally observed stereoselectivities [30-32], Two-state reactivity is not restricted to oxidation reactions, and similar scenarios have been proposed for a number of other experimentally studied reactions of 3d metal compounds [33-37]. Moreover, two-state scenarios have recently also been involved in the chemistry of main group elements [38]. The concept of two-state reactivity developed from the four-atomic system FeO /H2... 

Manganese is the third most abundant transition element [1]. It is present in a number of industrial, hiological, and environmental systems, representative examples of which include manganese oxide batteries [2] the oxygen-evolving center of photosystem II (PSII) [3] manganese catalase, peroxidase, superoxide dismutase (SOD), and other enzymes [4, 5] chiral epoxidation catalysts [6] and deep ocean nodules [7]. Oxidation-reduction chemistry plays a central role in the function of most, if not all, of these examples. 

Cobalt-catalyzed epoxidation of alkenes has been carried out with the cobalt derivative of (174), employing iodosylbenzene as the oxidant. Epoxidation of cfa- -methylstyrene furnishes exclusively the cis-epoxide (equation 62). The reaction proceeds through an active oxo-cobalt(IV) species, and is mote selective than reactions proceeding through oxo-chromium or oxo-manganese species. The catalyst can be recovered unchanged by simple filtration. 

Between 2000 and 2005, several major reviews on asymmetric organic synthesis were published [10-14] which also covered some advances in the dynamic field of polymer-immobilized manganese-saien complexes. In 2000, immobilization of Jacobsen s epoxidation catalyst [26] on polystyrene and polymethacrylate resins was reported [27]. Catalytic performances were evaluated using 1,2-dihydronaphtha-lene, indene, l-phenyl-3,4-dihydronaphthalene and 1-phenylcyclohexene as substrates, and wx-chloroperbenzoic add (m-CPBA) and N-methylmorpholine-N-oxide (NMO) as oxidant/co-oxidant. Epoxide yields up to 61% and ee values up to 91%... 

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. 

Oxidations. Epoxidation of alkenes and hydroxylation (and partial oxidation to ketones) of alkanes have been carried out with Nal04 and manganese(Ill) porphyrin complex on an ion-exchange resin. 

Whereas several catalytic methods are currently available for manganese-catalyzed epoxidation with aqueous H2O2, high turnover numbers for cis-dihydroxylation reactions so far have only been achieved with osmium compounds 161-165). However, manganese-catalyzed oxidation reactions have a few inherent advantages, such as the low price of the manganese salts and complexes and their non-toxic nature. 

Group 7 metal-promoted oxidations epoxidation by salen manganese complexes... 

Topical Procedure for the Epoxidation of Alkenes. The alkene (10 mmol) was dissolved in dichloromethane (3 mL, 2 M concentration in alkene) to which the amine (1.2 mmol) and MTO (0.05 mmol) was added. Then 30% hydrogen peroxide (20 mmol) was added and the reaction vigorously stirred whereupon the solution turns yellow, indicative of the formation of the active catalyst. After Ih a catalytic amount of manganese oxide was added (to destroy any remaining peroxides in solution). When evolution of oxygen had stopped, the layers were separated, the aqueous layer extracted with dichloromethane (3 x 25 mL) and the combined organic extracts dried (Na2S04) and concentrated under reduced pressure. 

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... 

Fueled by the success of the Mn (salen) catalysts, new forays have been launched into the realm of hybrid catalyst systems. For example, the Mn-picolinamide-salicylidene complexes (i.e., 13) represent novel oxidation-resistant catalysts which exhibit higher turnover rates than the corresponding Jacobsen-type catalysts. These hybrids are particularly well-suited to the low-cost-but relatively aggressive-oxidant systems, such as bleach. In fact, the epoxidation of trans-P-methylstyrene (14) in the presence of 5 mol% of catalyst 13 and an excess of sodium hypochlorite proceeds with an ee of 53%. Understanding of the mechanistic aspects of these catalysts is complicated by their lack of C2 symmetry. For example, it is not yet clear whether the 5-membered or 6-membered metallocycle plays the decisive role in enantioselectivity however, in any event, the active form is believed to be a manganese 0x0 complex <96TL2725>. 

Asymmetric epoxidation is another important area of activity, initially pioneered by Sharpless, using catalysts based on titanium tetraisoprop-oxide and either (+) or (—) dialkyl tartrate. The enantiomer formed depends on the tartrate used. Whilst this process has been widely used for the synthesis of complex carbohydrates it is limited to allylic alcohols, the hydroxyl group bonding the substrate to the catalyst. Jacobson catalysts (Formula 4.3) based on manganese complexes with chiral Shiff bases have been shown to be efficient in epoxidation of a wide range of alkenes. 

Iron N,N -bis(2-pyridinecarboxamide) complexes encaged in zeolite Y were used for the partial oxidation of alkanes.99 Epoxidation with manganese N,N -bis(2-pyridinecarboxamide) complexes encapsulated in zeolite Y was also reported.100... 

The vanadium(IV) complex of salen in zeolite was found to be an effective catalyst for the room temperature epoxidation of cyclohexene using t-butyl hydroperoxide as oxidant.88 Well-characterized vanadyl bis-bipyridine complexes encapsulated in Y zeolite were used as oxidation catalysts.101 Ligation of manganese ions in zeolites with 1,4,7-triazacyclononanes gives rise to a binu-clear complex stabilized by the zeolites but allows oxidation with excellent selectivity (Scheme 7.4). 

The reaction of olefin epoxidation by peracids was discovered by Prilezhaev [235]. The first observation concerning catalytic olefin epoxidation was made in 1950 by Hawkins [236]. He discovered oxide formation from cyclohexene and 1-octane during the decomposition of cumyl hydroperoxide in the medium of these hydrocarbons in the presence of vanadium pentaoxide. From 1963 to 1965, the Halcon Co. developed and patented the process of preparation of propylene oxide and styrene from propylene and ethylbenzene in which the key stage is the catalytic epoxidation of propylene by ethylbenzene hydroperoxide [237,238]. In 1965, Indictor and Brill [239] published studies on the epoxidation of several olefins by 1,1-dimethylethyl hydroperoxide catalyzed by acetylacetonates of several metals. They observed the high yield of oxide (close to 100% with respect to hydroperoxide) for catalysis by molybdenum, vanadium, and chromium acetylacetonates. The low yield of oxide (15-28%) was observed in the case of catalysis by manganese, cobalt, iron, and copper acetylacetonates. The further studies showed that molybdenum, vanadium, and... 

---