Enantioselective self-assembly

Enantioselective self-assembling of amino acids 209 Host-guest inclusion complexes 213 Reactivity of chiral ion-dipole complexes 233... 

While the above studies are representative of the area, it should be noted that a considerable number of other [2]-pseudorotaxanes have been investigated. These include a system exhibiting enantioselective self-assembly during its formation as well as systems incorporating a new structural motif involving a l,2-bis(pyridini-um)ethane axle and a 24-crown-8 ether wheel .A number of poly-pseudoro-taxanes (and related poly-rotaxanes) as well as constitutionally asymmetric and chiral [2]-pseudorotaxanes have been reported. New, multiply-stranded and multiply-encircled systems have also been prepared. The solid state structure of... 

Abstract Recent advances in synthetic aspects of the rhodium-catalyzed hydroformylation of alkenes are reviewed. Emphasis is given to practical improvements, efficient new catalysts for regioselective and enantioselective hydroformylation, and to applications of the reaction in organic synthesis. Furthermore, new developments in directed hydroformylation are covered as well as new approaches toward efficient hydroformylation catalysts employing the concept of self-assembly. 

According to R. M. Hazen, these results are in agreement with the postulate that some self-assembly processes of chiral molecules are highly enantioselective (DiGregorio, 2006). 

Scheme 36.19 Use of self-assembled ligands in enantioselective hydrogenation.

The self-assembly of a chiral Ti catalyst can be achieved by using the achiral precursor Ti(OPr )4 and two different chiral diol components, (R)-BINOL and (R,R)-TADDOL, in a molar ratio of 1 1 1. The components of less basic (R)-BINOL and the relatively more basic (R,R)-TADDOL assemble with Ti(OPr )4 in a molar ratio of 1 1 1, yielding chiral titanium catalyst 118 in the reaction system. In the asymmetric catalysis of the carbonyl-ene reaction, 118 is not only the most enantioselective catalyst but also the most stable and the exclusively formed species in the reaction system. 

Self-Assembly into the Most Enantioselective Catalyst... 

Crusats, J., Claret, J., Di ez-Perez, I., et al. (2003). Chiral shape and enantioselective growth of colloidal particles of self-assembled meso-tetra(phenyl and 4-sulfonatophenyl)porphyrins. Chem. Commun., 13, 1588-9. 

Birkholz, M.-N., Dubrovina, N.V.. fiao. H.. Michalik, D., Holz, f., Paciello, R., Breit. B. and Boerner, A. (2007) Enantioselective hydrogenation with self-assembling rhodiumphosphane catalysts Influence of ligand structure and solvent. Chem.—Eur. J., 13, 5896-5907. 

K., Matsukawa, S., Volk, T, and Terada, M. (1998) Self-assembly of several components into a highly enantioselective Ti catalyst for carbonyl-ene reactions. Angew. Chem., Int. Ed., 36, 2768-2771. 

Significant improvement in the catalytic activity of ALB was realized without any loss of enantioselectivity by using the second-generation ALB [27] generated by the self-assembled complex formation of ALB with alkali metal-malonate or alkoxide. This protocol allowed the catalyst loading to be reduced to 0.3 mol %, for example, the Michael addition of methyl malonate to cyclohexenone catalyzed by the self-assembled complex of (ff)-ALB (0.3 mol %) and KO Bu (0.27 mol %) in the presence of MS 4A gave the adduct in 94% yield and 99% ee [28]. This reaction has been successfully carried out on a 100-g scale wherein the product was purified by recrystallization. The kinetic studies of the reactions catalyzed by ALB and ALB/Na-malonate have revealed that the reactions are second-order to these catalysts (the rate constant ALB = 0.273 M 1h 1 ALB/Na-maionate = 1-66 M 1h 1) [27]. This reaction was used as the first key step for the catalytic asymmetric total synthesis of tubifolidine (Scheme 8D. 11) [28]. 

MIP films, applied to a QCM transducer, have been employed for chiral recognition of the R- and 5-propranolol enantiomers [107]. MIP films were prepared for that purpose by surface grafted photo-radical polymerization. First, a monolayer of 11-mercaptoundecanoic acid was self-assembled on a gold electrode of the quartz resonator. Then, a 2,2 -azobis(2-amidinopropane) hydrochloride initiator (AAPH), was attached to this monolayer. Subsequently, this surface-modified resonator was immersed in an ACN solution containing the MAA functional monomer, enantiomer template and trimethylolpropane trimethacrylate (TRIM) cross-linker. Next, the solution was irradiated with UV light for photopolymerization. The resulting MIP-coated resonator was used for enantioselective determination of the propranolol enantiomers under the batch [107] conditions and the FIA [107] conditions with an aqueous-ACN mixed solvent solution as the carrier. The MIP-QCM chemosensor was enantioselective to 5-propranolol at concentrations exceeding 0.38 mM [107]. 

Notably, even if the V monomer precursors (1) are racemic at the V center in solution, only a particular arrangement of two V monomers among possible structural arrangements is allowed as a stable structure at the Si02 surface, creating a new chiral V center (2) to provide a chiral reaction space for the enantioselective catalysis. Chiral self-assembly is a common phenomenon that is applied to other catalytic materials on surfaces. 

New ligands 49-52 were prepared based on these encouraging results. Unfortunately the enantioselectivity was not improved. For example, compound 52 was found to be unsuitable due to self-assembled structures formed in... 

Coordination of flexible amidic structures to the palladium ion resulted in the formation of a rigid hydrophobic cavity of 61 [82]. This self-assembly takes place in water, and aromatic carboxylates are recognized in it. It was found using NMR titration in D20 (pD=8.5, borate buffer). The structure of the complex formed is shown schematically in Fig. 4. It is apparent that cooperative binding of both carboxylates is essential for the stability of the complex formed nevertheless, the enantioselection of carboxylates is only very weak (Table 5). 

Enantioselectivity control in a [2 + 2]-photocydoaddition reaction was achieved in a chiral, self-assembled host. Fluoranthenes and N-cyclohexylmaleimide underwent an intramolecular reaction in a cage made of M6L4, with the metal M being palladium (II) coordinated to a chiral diamine, and the ligand Lbeing2,4,6-tris(4 -pyridyl)-l,3,5-triazine. Up to 50% ee was observed [117]. 

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