Self-supported chiral catalysts

Self-Supported Chiral Catalysts in Asymmetric Reactions... 

Self-supported chiral titanium clusters were prepared by mixing the ligand and titanium alkoxide in anhydrous toluene, followed by addition of a small amount of water (Scheme 7.22). In some cases, the cluster was completely insoluble in the reaction medium, especially in the case of 30. The cyanation works well with this heterogeneous catalyst, which was reused many times with no significant loss of activity. Additional experiments revealed that no appreciable amount of chiral catalyst had leached into the solution and the enantioselectivity came only from the solid catalyst. 

Scheme 5.9 The preparation of self-supported, chiral Mo catalysts 91a-d.

Strecker reaction A robust heterogeneous self-supported chiral titanium cluster (SCTC) catalyst applied both in batch and continuous mode [44] enabled imine cyanation as well as a three-component Strecker reaction using the corresponding aldehydes and amines under continuous flow conditions in a PBR giving aminonitriles enantiomeric excess values of up to 98%. 

Seayad, A.M., Ramalingam, B., Chai, C.LL, Li, C., Garland, M.V., and Yoshinaga, K. (2012) Self-supported chiral titanium cluster (SCTC) as a robust catalyst for the asymmetric cyanation of imines under batch and continuous flow at room temperature. Chem. Eur. J., 18, 5693-5700. doi 10.1002/chem.201200528... 

As a different immobilization strategy for the heterogeneous asymmetric catalyst. Ding et cd. developed a self-supported polymer catalyst for the flow asymmetric reduction [140]. Self-assembly of chiral multitopic ligands and reactive metal ions afforded homochiral metal-organic coordination polymers, which exhibited... 

Scheme 5.16 Generation of self-supported chiral titanium catalysts for carbonyl-ene...

In principle, the self-supporting strategy (type I MOCP) for chiral catalyst immobilization should be applicable whenever the active catalyst species contains at least two ligands coordinated to the metal (M). However, this would be challenging in... 

Self-supported titanium complexes with linked bis-BINOL ligands were used as an alternative approach for the immobilisation of catalysts, as shown in enantioselective sulfide oxidation (see Section 7.2.2). The same ligands were used with success in asymmetric carbonyl ene reactions. The chiral metal-bridged polymer 76, derived from ent-lOa, titanium tetraisopropoxide and water (Scheme 7.45), catalysed the ene reaction between 68b and 71, to give R)-72 in 88% yield and 88% enantiomeric excess. The catalyst can be reused at least five times without affecting its efficiency. 

As such a method for catalyst heterogenization obviates the need of any extraneous supporting materials, the resulting homochiral metal-organic assemblies can be considered as self-supported catalysts [7, 48]. On the basis of this strategy, the chiral multitopic ligand can spontaneously form a chiral environment inside the cavities or on the surface of the soHds for enantioselective control of the reaction, and the metal ions act as the catalytically active centers. 

In principle, in the use of the self-supporting strategy for heterogenization of homogeneous chiral catalysts, the stereochemical characteristics of the multitopic... 

Ding and coworkers [58] have also reported the use of chiral self-supported BINOL-Zn catalysts for the heterogeneous asymmetric catalysis of epoxidation of... 

Ding s group also extended this approach to the generation of self-supported Noyori-type catalysts 45 through hetero-coordination of an achiral bridged diphosphine 44 and a chiral bridged diamine ligand 40 with Ru(II) ions. The... 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

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