Manganese-salen

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... 

An alternative preparation of aziridines reacts an alkene with iodine and chloramine-T (see p. 1056) generating the corresponding A-tosyl aziridine. Bromamine-T (TsNBr Na ) has been used in a similar manner." Diazoalkanes react with imines to give aziridines." Another useful reagent is NsN=IPh, which reacts with alkenes in the presence of rhodium compounds or Cu(OTf)2 to give N—Ns aziridines. Manganese salen catalysts have also been used with this reagent. ... 

Rational Design of Manganese-Salen Epoxidation Catalyts Preliminary Results," Hosoya, N. Hatayama, A. Irie, R. Sasaki, H. Katsuki. T. Tetrahedron, 1994, 50, 4311... 

A typical manganese-salen complex (27)[89] is capable of catalysing the asymmetric epoxidation of (Z)-alkenes (Scheme 18) using sodium hypochlorite (NaOCl) as the principle oxidant. Cyclic alkenes and a, (3-unsaturated esters are also excellent starting materials for example indene may be transformed into the corresponding epoxide (28) with good enantiomeric excess1901. The epoxidation of such alkenes can be improved by the addition of ammonium acetate to the catalyst system 911. 

Epoxidation using manganese salen complexes is very easy to carry out it occurs under aqueous conditions and commercial house bleach can be used as the oxidant. The results are similar to those reported in the literature Table 6.1 gives other examples of alkenes which can be epoxidized using the same procedure. This method gives good results, especially for disubstituted Z-alkenes but trisubstituted alkenes can be epoxidized as well. 

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]). 

The successful application of manganese salen complexes is illustrated in Figure 14.11. In the catalyst used large 2-phenylnaphthyl-l substituents have... 

C. R. Jacob, S. R. Varkey, and R. Ratnasamy, Selective oxidation over copper and manganese salens encapsulated in zeolites, Microporous Mesoporous Mater. 22, 465 74 (1998). 

In Fig. 2.1.6.6, the FTIR spectra of the Jacobsen ligand (a), the Jacobsen catalyst (bj, and the immobilized manganese salen complex in the cages of dealuminated faujasite zeolite (c) are compared. While spectra a and b have been measured using the standard KBr technique, the spectrum c of the ship in a bottle catalyst has been recorded using a self-supported wafer. The bands at wavenumbers 1466 cm, 1434 cm" , 1399 cm" and 1365 cm" in spectrum c can be assigned to the... 

Fig. 2.1.6.6 FTIR spectra of a) the Jacobsen ligand, b) the Jacobsen catalyst, and c) the manganese-salen complex, in zeolite supercages.

The optically active manganese-salen complexes 230 and 231 are effective catalysts for the enantioselective epoxidation of unfunctionalized alkenes357-361. The yield of epoxide... 

In a related study, Jorgensen has examined the regio- and enantioselective catalytic epoxidation of conjugated aliphatic dienes using achiral and chiral manganese salen complexes and sodium hypochlorite or iodosylbenzene as the terminal oxidant. For most substrates, the less substituted diene is epoxidized however, in the case of isoprene, the more highly substituted double bond is the more reactive. Jorgensen proposes an intermediate of type 11, the... 

Ogunwumi, S. B. and Bein, T. Intrazeolite assembly of a chiral manganese salen epoxidation catalyst, Chem. Commun. 1997, 901-902. 

An early model for the active site was found in the manganese salen class of compounds. The active sites of the enzyme and model are shown in Fig. 4.13. The Schiff base model complexes are unusual in that they are able to maintain manganese in the highly reactive +3 oxidation state, ready to catalyse the oxidation of superoxide, yet still be flexible enough to bind the larger manganese(II) ion. 

Many claims have been made for manganese salen derivatives, from the hotly debated report that they extend the lifespan of nematode worms [27, 28] to curative properties in cases of lung damage [29], Unfortunately the compounds suffer from the same problems as other simple enzyme mimics in that they are not shielded from deactivation mechanisms by a protein shell. Furthermore they can also act as prooxidants through mechanisms such as the following ... 

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 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. 

Iron porphyrin catalysts with TBHP have been used for the diastereoselective oxidation of sulfides affording up to 46% d.e.408 A series of manganese(salen) catalysts with hydrogen peroxide has been employed for the oxidation of aralkyl sulfides in 34-70% d.e. and 80-90% yield. The best catalyst was derived from enantiomerically pure trans-1,2-diaminocyclohexane (Figure 3.101).409... 

Figure 3.101 Manganese salen catalyst for diastereoselective oxidation of sulfides with hydrogen peroxide.

---