Catalysts Jacobsen

To date, a wide variety of structurally different chiral Mn(III)salen complexes have been prepared, of which only a handful have emerged as synthetically useful catalysts. By far the most widely used Mn(III)salen catalyst is the commercially available Jacobsen catalyst wherein R= -C4H8- and R = = i-Bu (Scheme 1.4.1). In... 

Song and Roh investigated the epoxidation of compounds such as 2,2-dimethylchromene with a chiral Mn (salen) complex (Jacobsen catalyst) in a mixture of [BMIM][PFg] and CH2CI2 (1 4 v/v), using NaOCl as the oxidant (Scheme 5.2-12) [62]. 

Jacobsen catalyst 4-Phenylpyridine N-oxide Methylen chloride Hexane... 

Cost evaluation based on mass balancing results in clearly lower cost indices for the biocatalytic process as compared to the chemical catal)d ic process (Figure 5.6). For three of the substances of the chemical procedure (Jacobsen catalyst (catalyst), 4-phenylpyridine... 

A major difference in the evaluation of the two approaches concerns catalyst synthesis. Whereas catalyst production is integrated in the biocatalytic procedure (Scheme 5.4) and thus also contained in the cost index and the environmental factor, it is not considered in the chemical catalytic approach. A more realistic approach is to include the synthesis of the Jacobsen catalyst (Scheme 5.5) in the mass balance. In Figure 5.8, resources used for catalyst production are separately indicated ( Further Syntheses ). For the biocatalytic procedure, water dominates the environmental factor. The environmental factor increases for the chemical procedure, whereas the cost index, when representing only the raw material costs, declines if the (salen)Mn-catalyst is assumed to be synthesized and not bought. 

Figure 5.8 Environmental factors E (top figure) and cost indices Cl (bottom figure) for the biocatalytic (a) and chemical catalytic (b) syntheses of (5)-styrene oxide (Scheme 5.3) including the synthesis of the Jacobsen catalyst and of the bacteria (Scheme 5.4) as further syntheses. Waste produced during biocatalyst synthesis is indicated. However, it has to be considered that biocatalyst and product synthesis cannot be separated.

In the Jacobsen catalyst, the presence of the four tert-butyl groups disfavors all side-on olefin approaches with the exception of the approach from the di-imine bridge (3 in Fig. 7.5).190,199... 

Immobilization of chiral complexes in PDMS membranes offers a method for the generation of new chiral catalytic membranes. The heterogenization of the Jacobsen catalyst is difficult because the catalyst loses its enantioselectivity during immobilization on silica or carbon surfaces whereas the encapsulation in zeolites needs large cages. However, the occlusion of this complex in a PDMS matrix was successful.212 The complex is held sterically within the PDMS chains. The Jacobsen catalyst occluded in the membrane has activity and selectivity for the epoxidation of alkenes similar to that of the homogeneous one, but the immobilized catalyst is recyclable and stable. 

The Schiff bases being derivatives of aldehydes or ketones and various amines have received considerable attention because of their interesting physical and chemical properties, involvement in biologically important reactions and widespread application of their metal complexes. Increasing interest in optically active Schiff bases is connected with the discovery at the beginning of the 1990s of the so-called Jacobsen catalysts used in several asymmetric reactions showing excellent enantioselectivity. 

Soluble polymer-bound catalysts for epoxidation reactions have also been explored, with a complete study into the nature of the polymeric backbone performed by Janda [70]. Chiral (salen)-Mn complexes were appended to MeO-PEG, NCPS, Jan-daJeF and Merrifield resin via a glutarate spacer. It was found that for the Jacobsen epoxidation of ds-/ -mefhylstyrene, the enantioselectivities for each polymer-supported catalyst were comparable (86-90%) to commercially available Jacobsen catalyst (88%). Both soluble polymer-supported catalysts could be used twice before a decline in yield and enantioselectivity was observed. However, neither soluble polymer support proved as suitable as the insoluble JandaJel-supported (salen)-Mn complex for the epoxidation because residual impurities during precipitation and leaching of Mn from the complex, resulted in lowered yields. 

Keeping in mind the recovery of the catalyst issue, Pozzi et al. [59] specifically tailored the salen ligand to suit its application in fluorous biphasic (FB) system (Figure 4). Accordingly, authors made modification at 5 and 5 position of Jacobsen catalyst by replacing fert-butyl group with perfluoroalkyl chain 6 or 3,5-bis heptadecafluorooctylphenyl 7, 8 [60]. 

An interesting reversal of chiral induction in chromium(III)-salen complexes using a tartaric derived alicyclic diamine moiety (i.e., 7) has been observed by Mosset, Saalfrank, and co-workers <99T1063>. Thus, epoxidation of the chromene 8 using catalyst 7 and an oxidant consisting of MCPBA/NMO afforded the 3S,4S epoxide 9, whereas the Jacobsen catalyst (1) provided the corresponding 3R,4R enantiomer. A mechanistic rationalization for this curious crossover has not yet been proposed. 

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.

Epoxidation of Olefins over Immobilized Jacobsen Catalysts... 

In the dendritic [Co(salen)] complexes prepared by Breinbauer and Jacobsen the dendrimer again serves as - covalent - support material for the catalytic entities attached to the periphery [62]. These dendritic Jacobsen catalysts were obtained by reaction of the corresponding PAMAM dendrimers with active ester derivates of chiral ]Co(II)-(salen)] units according to standard peptide coupling methods. In hydrolytic kinetic resolution of vinylcyclohexane oxide the dendrimer 14 (Fig. 6.40) showed a dramatically increased reactivity compared to the commercially available monomeric Jacobsen catalyst [63-67]. Whereas the latter merely gave a conversion of less than 1% with an indeterminable ee, 14 afforded a conversion of 50% with an ee of 98 2. 

Literature concerning the Jacobsen catalyst a) Preparation, see J. F. Larrow,... 

Literature devoted specifically to asymmetric epoxidation with Jacobsen catalysts a) E. N. Jacobsen, Acc. Chem. Res. 2000, 33, 421-431 b) E. N. Jacobsen,... 

Modifications of the Jacobsen catalyst according to Burrows C. J. Burrows, K. J. O Connor, S. J. Wey, Tetrahedron Lett. 

Modifications of the Jacobsen catalyst according to Thornton D. R. Reddy E. R. Thornton, /. Chem. Soc. Chem. Commun. 

The suggested formation of manganaoxetanes receives support from calculations on a theoretical level, and from experiments reported by Katsuki using derivatives of the Jacobsen catalyst. 

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

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