In order to exploit the hydrophobic cavity of the calixarene platform in aldol reactions, two novel p-tert -butylcalixarene-based chiral organocatalysts bearing L-proline units on the lower rim have been reported by Yilmaz et al. Scheme Synthesis of p-tert -butylcalixarene-based chiral organocatalysts 75 and 78 derived from L-proline. Scheme Synthesis of p-tert -butylcalixarene-based chiral organocatalysts 75 and 78 derived from L-prolin In , an upper rim functionalized calixarene-based L-proline derivative has been reported .
Finally, deprotection of Boc groups under standard conditions afforded the targeted organocatalyst Scheme Synthesis of upper rim-functionalized calixarene-based L-proline derivative Magnetic nanoparticles MNPs due to their high surface area have been widely used as catalytic supports. Surface modification of MNPs with chiral organocatalysts for asymmetric catalysis provides sustainable materials which could perform chiral transformations robustly and readily [62,63]. In this context, the use of chiral calixarenes is very limited.
In , three new lower rim functionalized calixarene-based L-proline catalysts 87—89 containing ester, amide and acid units respectively have been designed by Yilmaz et al. Figure 9: Calixarene-based L-proline catalysts containing ester, amide and acid units. Scheme Synthesis of calixarene-based prolinamide Aldol reaction between 21 and 70 in water showed that catalyst 75 provided faster reaction times and higher isolated yields compared to those of catalyst A general organocatalyzed aldol reaction mechanism in water was suggested by the authors.
Accordingly, the observed catalytic activity and stereoselectivity was explained by the formation of a hydrophobic and hydrophilic region via hydrogen bonds between 75 and interfacial water molecules. Scheme Calixarene-based catalysts for the aldol reaction of 21 with The catalytic activity of compound 83 as an organocatalyst was evaluated in the enantioselective aldol reaction of 21 and 70 in different amounts of water. Under the optimized reaction conditions, the substrate scope for 83 -catalyzed aldol reactions were also probed in DMF.
Various aromatic aldehydes reacted well with 70 to give the aldol products in excellent yields with good ees. In addition, when DMF was used as solvent, a positive effect on the enantioselectivity but negative effect on the diastereoselectivity were observed.
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Scheme Asymmetric aldol reactions of 72 with cyclic ketones catalyzed by calixarene-based chiral organocatalysts derived from L-proline. Scheme Asymmetric aldol reactions of 72 with cyclic ketones catalyzed by calixarene-based chiral organo The catalytic activity of Calix-Pro-MN 86 was also tested on the asymmetric aldol reactions in water without organic solvents and almost full conversion of substrates into products with high enantioselectivity and diastreoselectivity were achieved. The catalyst could readily be isolated by applying an external magnetic field and recycled five times without significant loss of activity.
Under the optimum reaction media, a series of diverse benzaldehydes bearing electron withdrawing groups were reacted with Aldol reaction of 21 and 71 in the presence of catalyst 86 resulted in the formation of syn -aldol product as the major diastereomer with moderate yield but the selectivities were low. In order to create a more compact transition state and increase the selectivity by this way in direct stereoselective aldol reactions, optically active calixarene-imine derivative 92 containing an L-prolinamide functionality was also tested as a catalyst for enantioselective aldol reactions between 70 and a variety of aromatic aldehydes Based on the results, a plausible reaction mechanism similar to that previously suggested for the L-proline-catalyzed aldol reactions in water which proceeds via an enamine intermediate was proposed.
This in situ-formed system enhanced the activity and selectivity of catalyst The Houk—List transition state model was suggested to account for the observed high selectivity. Figure A proposed structure for catalyst 92 in H 2 O. Just recently, design and synthesis of a series of upper-rim-functionalized calixarene-based L-proline derivatives have been reported by Sun et al. The high catalytic efficieny of this proline-functionalized calixarene organocatalyst was attributed to the presence of the amide group and its synergistic effect through hydrogen-bonding interaction with the substrate.
Generally, the aldol adducts were provided in good to excellent yields, diastereoselectivities and enantioselectivities. The aromatic aldehydes bearing strong electron-withdrawing groups mainly gave anti -aldol products with high ee.
Ferrocene-derived P,N ligands : synthesis and application in enantioselective catalysis
In addition, the present method demonstrated selectivity towards cyclic ketones. According to the results observed, cyclic ketones of five- and six-membered sizes evolved into intermediates which show ideal affinity to the calixarene cavity. Scheme Synthetic route for organocatalyst Scheme Asymmetric aldol reactions catalyzed by A mechanism based on general organo-mediated aldol reactions in H 2 O was proposed for the present methodology.
Accordingly, a hydrophobic region and a hydrophilic region may be created through the hydrogen bonds between functionalized calixarene and water molecules.
This leads to in situ formation of a microreactor that improves the activity and stereoselectivity of catalyst Figure Proposed catalytic environment for catalyst 99 in the presence of water. Genc et al. Although the authors did not propose a plausible mechanism, it was clear in this tertiary amine-thiourea-catalyzed aldol reactions formation of an enamine intermediate was not involved. Whether the reaction works through the enolate mechanism and complete noncovalent catalysis still needs further investigation.
Scheme Asymmetric aldol reactions between 94 and 72 catalyzed by 55a. The results show that all five chiral calixarene catalysts promote the formation of the dihydropyrimidine product with different extents in terms of chemical yield and enantioselectivity. This result could be ascribed to the effect of steric hindrance on the upper rim of calixarene.
In order to confirm the role of the calixarene skeleton, an L-prolinamide analogue was prepared as a model catalyst from p -butoxybenzenamine. It was also proposed that the observed enantioselectivity resulted from the approach of the enamine double bond to the Re face of the imine via a stable six-membered-ring transition state similar to that described earlier by Feng et al. Scheme Enantioselective Biginelli reactions catalyzed by 69f. In Sciotto et al. While Mn III complexes a , b have been used as catalysts for asymmetric epoxidation of styrene and substituted styrenes in the presence of NaClO as an oxygen donor and 4-phenylpyridine N -oxide 4-PPNO as a coligand, their uranyl- salen derivatives a , b have been employed as models of the oxo-Mn V — salen oxidant species.
It was worth to note that for all the studied alkenes, catalyst a afforded higher ee values than catalyst b and this was ascribed to the length of the methylene spacers and more rigid structure of a. Scheme Synthesis of calixarene— salen complexes. In , Katz et al. The results clearly showed the effect of both denticity of ketone reactant and cooperativity between normal and inherent chirality on the selectivity.
Scheme Synthesis of inherently chiral calixarene catalysts and Scheme Enantioselective MPV reduction. More recently, Al III —calixarene complexes a—c have been prepared from enantiopure chiral hemispherical calixarene ligands a—c in which the asymmetric carbon is directly attached to the calixarene lower rim as shown by Katz et al.
Chiral sec -butylalcohol was used as a hydride donor. Scheme Synthesis of chiral calixarene ligands a—c. Scheme Asymmetric MPV reduction with chiral calixarene ligands. Scheme Asymmetric MPV reduction in the presence of chiral calixarene diphosphites. The corresponding tetrahydropyridine derivatives were obtained in good to excellent yields but the enantioselectivity remained moderate. Scheme Synthesis of enantiomerically pure inherently chiral calixarene phosphonic acid.
Scheme Asymmetric ring opening of epoxides catalyzed by cR,pR - This review demonstrates that considerable progress in the field of catalytic applications of calixarenes has been achieved in the last ten years. Since the first use of optically active diphosphines based on calixarene skeleton, a remarkable number of new chiral calixarenes have been developed able to effectively catalyze several important transformations: phase-transfer catalysis, Henry reaction, Suzuki—Miyaura cross-coupling and Tsuji—Trost allylic substitution, hydrogenation, Michael addition, aldol and multicomponent Biginelli reactions, epoxidation, Meerwein—Ponndorf—Verley reduction, aza-Diels—Alder and epoxide ring-opening reaction.
In these reactions, calixarenes are used either as ligands in metal-catalyzed reactions or as organocatalysts. The application of inherently chiral calixarenes as chiral catalysts is still quite scarce, except in aldol, Michael and Henry reactions.
Asymmetric catalysis with chiral calixarenes in which the chirality is induced by attaching chiral groups to the calixarene backbone is more common and the obtained results are more promising. In addition, the calixarene backbone is much more preferred beyond other ring sizes both in transition metal catalysis and organocatalysis.
It can be argued that this is due to the difficulty of regiochemical control of calixarene and calixarene functionalization. In calixarene-catalyzed systems, the substrate scope is generally limited to the previously reported ones with small organic molecules. From the summarized studies, although significant progress has been achieved with chiral calixarenes, there are still several opportunities for further enhancements such as catalyst design, loading and substrate scope in the reaction systems.
Immobilization of these calixarene derivatives onto polymers or magnetic nanoparticles would also lead to achieve reusable catalysts for batch and continuous-flow studies. Geib, Dennis P.
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US6258979B1 - Chiral ferrocene phosphines active in asymmetric catalysis - Google Patents
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Jump to Figure 9. Wong of the Chinese University of Hong Kong. He was awarded an Alexander von Humboldt research fellowship and completed two years of postdoctoral research with Professor Emanuel Vogel at Cologne University, Germany. He is currently Professor at the SIOC and his research interests include OMCOS, the design of chiral ligands and their applications in asymmetric catalysis and the transformation of organic molecules under non-metal conditions.
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