Salen-metal complex

In spite of these limitations, three examples of (salen)-metal complex adsorption have been described. In the first one, Jacobsen s complex (la-MnCl) was adsorbed on Al-MCM-41 [27] by impregnation with a solution of the complex in dichloromethane, an approach that prevents the possible cationic exchange. The results in the epoxidation of 1,2-dihydronaphthalene with aqueous NaOCl were comparable to those obtained in solution, with only a slight reduction in enantioselectivity (55% ee instead of 60% ee). However, recycling of this catalyst was not described. 

As can be seen, many methods are available for the immobilization of salen-metal complexes, although it is difficult to elucidate which one is the best. Even in the case of epoxidation, the differences in substrates, oxidants, and reaction conditions preclude any direct comparison. 

Larock R (2005) Palladium-Catalyzed Annulation of Alkynes. 14 147-182 Larrow JF, Jacobsen EN (2004) Asymmetric Processes Catalyzed by Chiral (Salen)Metal Complexes 6 123-152... 

C2-symmetric bis(oxazoline) and salen-metal complexes have recently been shown to be very effective ligands for iron(III),28 magnesium(II),29 copper(II),30 and chromium(in)31 complex-catalyzed reactions. 

Larrow, J. F. Jacobsen E. N. (2004) Asymmetric processes catalyzed by chiral (salen) metal complexes.. Topics in Organometallic Chemistry, 123-152. 

Katsuki and coworkers have developed a family of salen-metal complexes capable of effecting a C—H oxidation at activated positions. meso-Tetrahydrofurans may be oxidized to the lactol in good yield and excellent enantioselectivity using iodosylbenzene as the stoichiometric oxidant and a Mn-salen complex as catalyst [Eq. (10.45)]. " Meso acylpyrrolidines behave similarly, providing slightly lower enantioselectivities using a similar catalyst [Eq. (10.46)]d ... 

Salen Metal Complexes as Catalysts for the Synthesis of Polycarbonates from Cyclic Ethers and Carbon Dioxide... 

Abstract This chapter focuses on well-defined metal complexes that serve as homogeneous catalysts for the production of polycarbonates from epoxides or oxetanes and carbon dioxide. Emphasis is placed on the use of salen metal complexes, mainly derived from the transition metals chromium and cobalt, in the presence of onium salts as catalysts for the coupling of carbon dioxide with these cyclic ethers. Special considerations are given to the mechanistic pathways involved in these processes for the production of these important polymeric materials. 

Darensbourg DJ (2007) Making plastics from carbon dioxide salen metal complexes as catalysts for the production of polycarbonates from epoxides and CO2. Chem Rev 107 ... 

Scheme 7.9 Representative chiral salen-metal complexes [30].

The salen-Ni(II) complex 39a derived from (lR,2R)-[N,N -bis(2 -hydroxybenzyl-idene)]-l,2-diaminocyclohexane was also equally effective (Table 7.3, entry 4). In contrast to earlier reports on salen-metal complexes, where the introduction of a bulky tert- butyl substituent increased enantioselectivity [31], the use of complex 39b exhibited a significant decrease in enantioselectivity (entry 5). The presence of a bulky tert-butyl group obstructed the chelation of alkali metal ions by phenolic oxygen atoms. A dramatic increase in selectivity could be achieved when nickel was replaced with copper, and a salen-Cu(II) complex 39c afforded 85% ee (entry 6). Although screening of other bases or 50% NaOH were not advantageous, the use of 3 equiv. NaOH improved the enantiomeric excess to 92% (entry 9) and after recrystallization of a-methylphenylalanine optical purity was increased to 98% ee. 

Many other projects in the past dealt with the immobilization of the famous chelating salen ligand or salen metal complexes, respectively—the Mnm salen Schiff base complex is also known as Jacobsen s catalyst, and it is used, among others, in asymmetric epoxidation of olefinstttt—mainly onto the surface of MCM-41 and SBA-15. 

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