Kinetic metal Salen complexes

The third investigation track demonstrated the immobilization of metal-salen complexes in mesoporous materials and their use in the hydrolytic kinetic resolution of meso and terminal epoxides. The best results were obtained over cobalt-Ja-cobsen catalysts. The catalytic activity of the (S,S)-Co(II)-Jacobsen complex immobilized on Al-MCM-41 was comparable with that of the homogeneous counterpart. Several other immobilization methods are still under investigation. 

To illustrate the utility of the metal salen complexes, several reactions are outlined in Scheme 1. They include the asymmetric epoxidation of unfimctionalized cw-disubstituted and trisubstituted olefins, which are promoted by (salen)Mn complexes." In the case of trani-disubstituted olefins, the simple (salen)Mn complexes do not exhibit the same levels of enantioselectivity as they do with the cis- and trisubstituted derivatives. Promising alternatives include more elaborate (salen)Mn complexes based on the binaphthyl imit, (salen)Cr complexes,and (salen)Ru-based catalysts. Catalysts based on (salen)Co moiety have exhibited amazing levels of selectivity in the hydrolytic kinetic resolution (HKR) of terminal epoxides. The HKR allows access to terminal epoxides and diols with very high enantioselectivities. 

In the first method the metal complex is assembled in the zeolite cavities by allowing the metal-exchanged zeolite to react with ligands that are small enough to access the micropores. The metal complex, once formed, is too large to diffuse out. For example, bis- or tris-bipyridyl complexes of Fe", Ru", Mn", Co" and Cu" have been encapsulated in zeolite Y (FAU) [12-15, 36], Metal-Salen and related SchifFs base complexes have been similarly encapsulated in faujasites [12-15, 37, 38]. However, in this case there is virtually no difference in kinetic diameter between the complex and the free ligand and metal-Salen complexes are readily leached by protic solvents, such as ethanol [12]. 

The kinetic resolution (KR) of racemic mixtures of terminal epoxide catalyzed by chiral metal Salen complexes, such as Cr(Salen) and Co(Salen) (Salen = N,N-bis(3,5-di-tert-butyl-salicylidene)-l,2-cyclohexene diamine), is of great interest in many total syntheses of natural products and drugs. Both Co(Salen) and Cr(Salen)... 

Heterogenization of homogeneous metal complex catalysts represents one way to improve the total turnover number for expensive or toxic catalysts. Two case studies in catalyst immobilization are presented here. Immobilization of Pd(II) SCS and PCP pincer complexes for use in Heck coupling reactions does not lead to stable, recyclable catalysts, as all catalysis is shown to be associated with leached palladium species. In contrast, when immobilizing Co(II) salen complexes for kinetic resolutions of epoxides, immobilization can lead to enhanced catalytic properties, including improved reaction rates while still obtaining excellent enantioselectivity and catalyst recyclability. 

A so far still unsolved problem is the direct enantioselective epoxidation of simple terminal olefins. For example the epoxidation of propylene that was achieved with a 41% ee almost twenty years ago by Strukul and his coworkers using Pt/diphosphine complexes is still unsurpassed. Unfortunately such low ee s are of no practical interest. The problem was circumvented by Jacobsen using hydrolytic kinetic resolution of racemic epoxides (Equation 26) and is practised on a multi 100 kg scale at Chirex. The strategy used is to stereose-lectively open the oxirane ring of a racemic chiral epoxide leaving the other enantiomer intact. Reactions are carried out to a 50% maximum conversion. The catalyst belongs to the metal-salen class described above and can be recycled. The products are separated by fractional distillation. 

Following these results, Darensbourg et al. have continued the research and used other bifunctional Cr(salen) complexes as catalysts for polycarbonate synthesis. They observed that when a monofunctional Cr(salen) complex (5) was used to catalyze the reaction between epoxide and CO2, the product formed was cyclic carbonate. However, when a bifunctional Cr(salen) catalyst (6) was used, 79% selectivity towards the polycarbonate was obtained at 70 °C. The reason for this difference lies in the structure of the bifunctional catalyst, which provides steric hindrance in the epoxide ring-opening process to form the cyclic carbonate. Therefore, it can be inferred that spatial requirements in the active site of the metal catalyst determine the selectivity for the kinetic polymer product over the thermodynamically more stable cyclic carbonate product. 

Lin, M.-H. RajanBabu, T. V. Ligand-assisted rate acceleration in transacylation by a yttrium-salen complex. Demonstration of a conceptually new strategy for metal-catalyzed kinetic resolution of alcohols. Org. Lett. 2002, 4, 1607-1610. 

This is also the case for the kinetically inert octahedrally coordinated chromium(III) centre (d ) found in a salene chromium(III) complex with an organoazide in close proximity to the metal site (Figure 12.10). Salene chromium(II) complexes are known to catalyze the enantioselective opening of prochiral epoxides with TMS-N3. The organoazide derivative was obtained by stoichiometric azide transfer from an azido chromium(III) salene complex to epoxycyclopentane. 

A detailed kinetic study on the disproportionation of (49b) in neutral aqueous solutions has been performed recently in relation to the mechanisms of oxidative DNA cleavage promoted by this complex.208 Such Crv oxo complexes are among the few known types of metal complexes that cause oxidative DNA cleavage in the absence of reactive oxygen species.209 The interactions of (49b) or [CrO(salen)]+ with DNA have been studied in detail and several possible mechanisms of these reactions have been proposed (reviewed in 2000-2003).11,13,210 Several mechanistic studies on the reactions of [CrO(salen)]+ and related complexes with organic reductants have been performed in relation to the roles of these complexes in Crm-salen-catalyzed oxo-transfer reactions (Section 4.6.5.8.4).211-215... 

Chiral Catalysts Containing Group 7 Metals (Mn, Tc, and Re). Most of the chiral manganese complexes belong to the Mn(III)-salen-type complexes (Fig. 17), which are effective catalysts in asymmetric epoxidation (147). (The most widely used one is the Jacobsen s catalyst, iV,Ar -bis(3,5-di-terf-butylsalicylidene)-l,2-cyclohexanediamino-manganese(III) chloride.) These types of catalysts are also efficient for enantioselective aziridination (148), kinetic resolution of racemic allene via enantiomer differentiating oxidation (149), and enantiotopic selective... 

The Salen motif has been widely utilized as a ligand for transition metals. Jacobsen et al. reported that chiral salen-cobalt complex (Co-salen) could be utilized as a Lewis acid catalyst for hydrolytic kinetic optical resolution of racemic... 

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