Supported catalysts manganese-salen complexes

Key Words Polymer-supported catalysts, Manganese(ll) complexes, Manganese-salen complexes, Manganese-porphyrin complexes,... 

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... 

Dendritic and nondendritic polystyrene-boimd manganese-salen complexes were described by Seebach and coworkers [30]. The supported catalysts were prepared by suspension copolymerization of styrene with the vinyl-substituted complexes and employed in the epoxidation of phenyl-substituted alkenes by m-CPBA/NMO. Activities and selectivities were similar to those obtained with the monomeric complexes. High catalyst stabilities were observed and it was demonstrated that the immobilized catalysts can be recycled up to 10 times without loss of performance. Laser ablation inductively coupled plasma mass spectrometry was used to monitor the manganese content in repeatedly used polystyrene beads and a correlation between metal leaching from the support and catalytic activity was disclosed [31]. 

The premiere method for producing chiral oxiranes from non-functionalized olefins is the Jacobsen asymmetric epoxidation, which utilizes a chiral manganese salen complex as a catalyst. Since Jacobsen s first report in 1990, intensive study in this area has generated a plethora of reaction conditions and catalyst type.s, as well as questions regarding the mechanistic parameters. The course of the oxygen transfer itself remains a matter of much debate. Norrby and Ackermark <97AG(E) 1723> maintain support for the intermediacy of a metallaoxetane species... 

Immobilization of a sulfonated chiral manganese-salen catalyst on a fimctio-nalized Merrifield resin yielded a remarkably active epoxidation catalyst [42]. Its activity and enantioselectivity was examined by epoxidation of 6-cyanochromene, indene, styrene, 4-methylstyrene, and trans-stilbene using m-CPBA/NMO and quantitative yields were obtained in less than 5 min. Enantioselectivities were between 33% (4-methylstyrene) and 96% ee (6-cyanochromene). The same complex was also supported on silica and a layered double hydroxide (LDEI) and the catalytic performances of the systems were compared. Recycling experiments were carried out and the silica-based system showed metal leaching combined with a significant decrease in yield and ee. The layered double hydroxide- and resin-catalysts exhibited a slight decrease in activity and constant ee values in five consecutive reactions. 

Polymer-supported salen catalysts were also developed by employing poly (norbornene)-immobihzed salen complexes 139 of manganese and cobalt (Scheme 3.40) [77]. The poly(norbornene) complexes are highly active and selective catalysts for the epoxidation of olefins. The asymmetric epoxidation of cis-P-methylstyrene 132 occurred smoothly at -20 °C to give the chiral epoxide 133 in 100% conversion with 92% ee. Under the same reaction conditions, Jacobsen s catalyst (an unsupported salen complex) afforded the same product with 93% ee. 

Asymmetric epoxidation of unfunctionalized aUcenes catalyzed by chiral Mn(III)(salen) complexes has proven to be a useful solution-phase reaction [88]. To simplify product isolation and to avoid degradation of the Mn(salen) complex through formation of i-oxo-manganese(lV) dimers by spatial redistribution, the polymer-supported catalyst 112 was prepared by co-polymerization of complex 113, styrene 58, and divinylbenzene as a cross-linker (Scheme 20) [89]. As a stoichiometric oxidant, a combination of meta-chlor-operbenzoic acid (mCPBA) and N-methyl-morpholine N-oxide (NMO) in acetonitrile was used. Yields and rates of conversion were satisfactory for the epoxidation of styrene 58 and of methyl styrene, but only low enantioselectivities were obtained. Nevertheless, the catalyst retained its efficiency in terms of yields and enantioselectivities after repetitive use. Similar results have been described by other researchers [90]. 

Asymmetric epoxidation (AE) of unfunctionalized alkenes catalyzed by chiral (salen)Mn(III) complex 38 (Scheme 2.13), developed by Jacobsen et al., is one of the most reliable methods [50]. As shown in Table 2.2, several different strategies have been formulated to immobilize Jacobsen s catalysts on inorganic supports [37-42]. Facilitation of catalyst separation, catalyst reuse, an increase in catalyst stability (e.g. minimization of the possibility of formahon of inachve g-oxo-manganese(lV) species [51a,b]) and sometimes improvement in enanhoselectivity are the main objectives of such research. Heterogenized Mn(salen) systems have recently been reviewed by Salvador et al. [51c] and Garcia et al. [5 Id]. Some selected cases are therefore described herein on the basis of the immobilizahon methods. 

Katsuki and coworkers examined enantiotopic selective hydroxylation of pro-chiral substrates with chiral (salen)manganese complexes as catalysts [13]. This reaction also proceeds via a radical intermediate [13a]. The kinetic isotopic effect (kn/kD = 4.6) observed in the hydroxylation of ethylbenzene with complex lib supports the idea that hydrogen atom abstraction is the rate-determining step [13b]. In the reaction using chiral (salen)manganese complexes which have no chiral cavity, radical decay should occur less selectively and should deteriorate the enan-tioselectivity of hydroxylation. A solvent of intense viscosity constitutes a strong... 

In the first topic - Fine Chemistry - the methodologies used in the het-erogenisation of manganese complexes into solid supports are briefly described. The major part of the section focuses on examples of heterogeneous manganese complexes with salen- and porphyrin-type ligands used as catalysts in the epoxidation of alkenes their catalytic activity is discussed as a... 

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