Jacobsen complex

Figure 2.1.6.9 and Table 2.1.6.4 present the N2-sorption isotherms and the results of the characterization of the Al-MCM-41 parent material and of the by ion exchange and impregnation onto Al-MCM-41 immobihzed (S,S)-Co( 11)-Jacobsen complexes. The N2-sorption isotherms of the loaded materials clearly illustrate that (S,S)-Co(II)-Jacobsen complex is deposited in the inner surface of the pores... 

Scheme 2.1.6.4 Method for the impregnation of metal-Jacobsen complexes into Al-MCM-41.

Table 2.1.6.4 Nitrogen sorption data of the Co(ll)-Jacobsen complex immobilized on AI-MCM-41 by ion exchange or impregnation.

The (S,S)-Co(II)-Jacobsen complex was immobilized on Al-MCM-41 by both impregnation and ion exchange. For the ion-exchange and impregnation methods different solvents, several metal salts, and different concentrations were tested in order to avoid a structure collapse of the support material. The catalysts were in-... 

The catalytic activity of the (S,S)-Co(ll)-Jacobsen complex immobilized by impregnation was found to be higher in the HKR of meso and terminal epoxides than the activity of the same complex immobilized by ion exchange. 

Several different immobilization methods are currently under investigation in order to immobilize various types of Jacobsen complexes in the mesopores of Al-MCM-48, Al-MCM-41 and Al-SBA-15 types of support materials. The novel chiral heterogeneous catalysts obtained will be characterized and their activity in different test reactions will be investigated. 

A one-sided attachment of a vinyl-functionalized salen monomer to an MCM-41 material was reported by Janssen (95). In the epoxidation of 1-phenylcyclohexene with PhIO in acetonitrile, the Mn-functionalized structure 7g gave an ee of 75%, which is the same as for the soluble Jacobsen complex and considerably higher than that obtained with 7f. Morever, the chemoselectivity, the olefin conversion, and the enantioselectivity remained unchanged over four consecutive cycles. 

Figure 5 shows that the (salen-2) complexes of V and the Co(salen-5) complex retained their catalytic properties upon entrapment in the host materials. In contrast to the Mn(salen-2) complex which loses only some of its epoxide selectivity upon immobilisation, the corresponding Co and Cr complexes show an additional decrease in stereoselectivity as well. Strikingly, the immobilised Co(salen-5) complex achieved with 100 % conversion, 96 % selectivity and 91 % de even better results in the epoxidation of (-)-a-pinene than its homogeneous counterpart. However, it is worth notifying that among the (salen-2) complexes neither the homogeneous nor the occluded Jacobsen complex catalysed the epoxidation of (-)-a-pinene best. 

Figure 8.5 Molecular structure of chiral Mn(III) salen complexes immobilized onto oxidized AC (a) Jacobsen catalyst (b) Katsnki catalyst (c) modified Jacobsen complex.

A catalytic enantio- and diastereoselective dihydroxylation procedure without the assistance of a directing functional group (like the allylic alcohol group in the Sharpless epox-idation) has also been developed by K.B. Sharpless (E.N. Jacobsen, 1988 H.-L. Kwong, 1990 B.M. Kim, 1990 H. Waldmann, 1992). It uses osmium tetroxide as a catalytic oxidant (as little as 20 ppm to date) and two readily available cinchona alkaloid diastereomeis, namely the 4-chlorobenzoate esters or bulky aryl ethers of dihydroquinine and dihydroquinidine (cf. p. 290% as stereosteering reagents (structures of the Os complexes see R.M. Pearlstein, 1990). The transformation lacks the high asymmetric inductions of the Sharpless epoxidation, but it is broadly applicable and insensitive to air and water. Further improvements are to be expected. 

The Jacobsen-Katsuki epoxidation reaction is an efficient and highly selective method for the preparation of a wide variety of structurally and electronically diverse chiral epoxides from olefins. The reaction involves the use of a catalytic amount of a chiral Mn(III)salen complex 1 (salen refers to ligands composed of the N,N -ethylenebis(salicylideneaminato) core), a stoichiometric amount of a terminal oxidant, and the substrate olefin 2 in the appropriate solvent (Scheme 1.4.1). The reaction protocol is straightforward and does not require any special handling techniques. 

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... 

In 1990, Jacobsen and subsequently Katsuki independently communicated that chiral Mn(III)salen complexes are effective catalysts for the enantioselective epoxidation of unfunctionalized olefins. For the first time, high enantioselectivities were attainable for the epoxidation of unfunctionalized olefins using a readily available and inexpensive chiral catalyst. In addition, the reaction was one of the first transition metal-catalyzed... 

The Jacobsen-Katsuki epoxidation reaction has been widely used for the preparation of a variety of structurally diverse complex molecules by both academia and the pharmaceutical industry. Summarized below are a few examples. 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

Jacobsen et al. took an important step towards the development of a more general catalytic enantioselective cycloaddition reaction of carbonyl compounds by introducing chiral tridentate Schiff base chromium(III) complexes 15 (Scheme 4.15)... 

Non-functionalized alkenes 6, with an isolated carbon-carbon double bond lacking an additional coordination site, can be epoxidized with high enantiomeric excess by applying the Jacobsen-Katsuki epoxidation procedure using optically active manganese(iii) complexes ... 

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 et al. have made a convincing argument that these types of reaction proceed by way of discrete copper-nitrene complexes, rather than by some sort of single-electron process (Scheme 4.16) [13c]. 

A breakthrough in the area of asymmetric epoxidation came at the beginning of the 1990s, when the groups of Jacobsen and Katsuki more or less simultaneously discovered that chiral Mn-salen complexes (15) catalyzed the enantioselective formation of epoxides [71, 72, 73], The discovery that simple achiral Mn-salen complexes could be used as catalysts for olefin epoxidation had already been made... 

Subsequent to the development of the (salen)Cr-catalyzed desymmetrization of meso-epoxides with azide (Scheme 7.3), Jacobsen discovered that the analogous (salen)Co(n) complex 6 promoted the enantioselective addition of benzoic acids to meso-epoxides to afford valuable monoprotected C2-symmetric diols (Scheme 7.15) [26], Under the reaction conditions, complex 6 served as a precatalyst for the (salen) Co(iii)-OBz complex, which was fonned in situ by aerobic oxidation. While the enantioselectivity was moderate for certain substrates, the high crystallinity of the products allowed access to enantiopure materials by simple recrystallization. 

Jacobsen also showed that 2,2-disubstituted epoxides underwent kinetic resolution catalyzed by (salen)Cr-N3 complex 3 under conditions virtually identical to those employed with monosubstituted epoxides (Scheme 7.34) [64]. Several epoxides in this difficult substrate class were obtained with high ees and in good yields, as were the associated ring-opened products. The kinetic resolution of TBS-... 

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>. 

Only a few years after the development of the homogeneous chiral Mn(salen) complexes by Jacobsen and Katsuki, several research groups began to study different immobiUzation methods in both liquid and soUd phases. Fluorinated organic solvents were the first type of Uquid supports studied for this purpose. The main problem in the appUcation of this methodology is the low solubility of the catalytic complex in the fluorous phase. Several papers were pubUshed by Pozzi and coworkers, who prepared a variety of salen ligands with perfluorinated chains in positions 3 and 5 of the saUcyUdene moiety (Fig. 2). 

The first application of ionic hquids for salen complexes dealt with the epoxidation of alkenes [14]. Jacobsen s Mn complex was immobilized in [bmimjlPFe] and different alkenes were epoxidized with aqueous NaOCl solution at 0 °C. As the ionic solvent sohdified at this temperature, dichloromethane was used as a cosolvent. The recychng procedure consisted of washing with water, evaporation of dichloromethane, and product extraction with hexane. The results (Table 3) were excellent and only a slow decay in activity and enantioselectivity was detected after several cycles. 

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