Selector-Induced Dynamic Deracemization of a Selectand-Modified Tropos BIPHEPO-Ligand: Application in the Organocatalyzed Asymmetric Double-Aldol-Reaction

Nonlinear Effects in Asymmetric Catalysis by Design: Concept, Synthesis, and Applications

Asymmetric synthesis constitutes a key technology for the preparation of enantiomerically pure compounds as well as for the selective control of individual stereocenters in the synthesis of complex compounds. It is thus of extraordinary importance for the synthesis of chiral drugs, dietary supplements, flavors, and fragrances, as well as novel materials with tunable and reconfigurable chiroptical properties or the assembly of complex natural products. Typically, enantiomerically pure catalysts are used for this purpose. To prepare enantiomerically pure ligands or organocatalysts, one can make use of the natural chiral pool. Ligands and organocatalysts with an atropisomeric biphenyl and binaphthyl system have become popular, as they are configurationally stable and contain a C2-symmetric skeleton, which has been found to be particularly privileged. For catalysts with opposite configurations, both product enantiomers can be obtained. Configurationally flexible biphenyl systems initially appeared to be unsuitable for this purpose, as they racemize after successful enantiomer separation and thus are neither storable nor afford a reproducible enantioselectivity. However, there are strategies that exploit the dynamics of such ligands to stereoconvergently enrich one of the catalyst enantiomers. This can be achieved, for example, by coordinating an enantiomerically pure additive to a ligand-metal complex, which results in deracemization of the configurationally flexible biphenyl system, thereby enriching the thermodynamically preferred diastereomer. In this Account, we present our strategy to design stereochemically flexible catalysts that combine the properties of supramolecular recognition, stereoconvergent alignment, and catalysis. Such systems are capable to recognize the chirality of the target product, leading to an increase in enantioselectivity during asymmetric catalysis. We have systematically developed and investigated these smart catalyst systems and have found ways to specifically design and synthesize them for various applications. In addition to (i) reaction product-induced chiral amplification, we have developed systems with (ii) intermolecular and (iii) intramolecular recognition, and successfully applied them in asymmetric catalysis. Our results pave the way for new applications such as temperature-controlled enantioselectivity, controlled inversion of enantioselectivity with the same chirality of the recognition unit, generation of positive nonlinear effects, and targeted design of autocatalytic systems through dynamic formation of transient catalysts. Understanding such systems is of enormous importance for catalytic processes leading to symmetry breaking and amplification of small imbalances of enantiomers and offer a possible explanation of homochirality of biological systems. In addition, we are learning how to target supramolecular interactions to enhance enantioselectivities in asymmetric catalysis through secondary double stereocontrol. Configurationally flexible catalysts will enable future resource-efficient development of asymmetric syntheses, as enantioselectivities can be fully switched by stereoselective alignment of the stereochemically flexible ligand core on demand.

Controlling the Enantioselectivity in an Adaptable Ligand by Biomimetic Intramolecular Interlocking

For the preparation of chiral drugs, both stereochemically stable and flexible catalysts in combination with chiral auxiliaries can be used. Here, chiral induction plays an important role in generating an enantiomerically pure catalyst. We demonstrate a successful approach to the spontaneous deracemization of tropos ligands for asymmetric catalysis. Three different constitutional isomers of a bisphosphinite ligand decorated with l-valine moieties (interaction units) linked to the flexible biphenyl system by a phenylene bridge for inducing a chiral switch were prepared. The substitution pattern's influence on the attached intermolecular recognition sites was systematically investigated. We can show that biomimetic intramolecular hydrogen bonding leads to a pronounced diastereoselective enrichment of one of the ligand stereoisomers. As a result, in the asymmetric Rh-catalyzed hydrogenation of prochiral olefins using these ligands, enantiomeric ratios of up to 95.8:4.2 (S) were obtained. Of particular note is the inversion of enantioselectivity relative to the previously reported BIBIPHOS-Rh catalyst due to the altered orientation of the biphenyl moiety from (R_ax) to (S_ax). The enantioselectivities achieved by appropriate intramolecular interlocking are remarkable for a tropos ligand/catalyst. The strategy presented here represents a powerful approach for the spontaneous alignment of tropos ligands, yielding high enantioselectivities in asymmetric catalysis.

Chiral stationary phases and applications in gas chromatography

Chiral compounds are ubiquitous in nature and play a pivotal role in biochemical processes, in chiroptical materials and applications, and as chiral drugs. The analysis and determination of the enantiomeric ratio (er) of chiral compounds is of enormous scientific, industrial, and economic importance. Chiral separation techniques and methods have become indispensable tools to separate chiral compounds into their enantiomers on an analytical as well on a preparative level to obtain enantiopure compounds. Chiral gas chromatography and high-performance liquid chromatography have paved the way and fostered several research areas, that is, asymmetric synthesis and catalysis in organic, medicinal, pharmaceutical, and supramolecular chemistry. The development of highly enantioselective chiral stationary phases was essential. In particular, the elucidation and understanding of the underlying enantioselective supramolecular separation mechanisms led to the design of new chiral stationary phases. This review article focuses on the development of chiral stationary phases for gas chromatography. The fundamental mechanisms of the recognition and separation of enantiomers and the selectors and chiral stationary phases used in chiral gas chromatography are presented. An overview over syntheses and applications of these chiral stationary phases is presented as a practical guidance for enantioselective separation of chiral compound classes and substances by gas chromatography.

Diastereoselective synthesis of a cyclic diamide-bridged biphenyl as chiral atropos ligand

Chiral compounds with a 1,2-diamine structure motif and their derivatives are of great interest in organic chemistry and are broadly used in asymmetric transformations, as chiral auxiliaries, (co)ligands, and ligand core structure. Here, we present a straightforward, diastereoselective synthesis for a diamide-bridged biaryl ligand. The ring closing reaction of the racemic atropos biphenyl 6,6'-dimethoxy-[1,1'-biphenyl]-2,2'-dicarboxylic acid with (R,R)-diaminocyclohexane yields the diasteromerically and enantiomerically pure cyclic (S_ax ,R,R)-BIPOL, which can be used as a versatile chiral ligand. By NMR spectroscopy, we observed the formation of intermolecular aggregates of the diamide-bridged BIPOL with anhydrous DMSO-d₆ . DFT calculations at the B3LYP/6-31G* level of theory corroborate the high interconversion barrier for the biaryl axis of ΔG^ǂ  = 148.7 kJ mol^-1 and the favoured formation of (S_ax ,R,R)-BIPOL as single stereoisomer.

Efficient Amplification in Soai's Asymmetric Autocatalysis by a Transient Stereodynamic Catalyst

Mechanisms leading to a molecular evolution and the formation of homochirality in nature are interconnected and a key to the underlying principles that led to the emergence of life. So far proposed mechanisms leading to a non-linear reaction behavior are based mainly on the formation of homochiral and heterochiral dimers. Since homochiral and heterochiral dimers are diastereomers of each other, the minor enantiomer is shifted out of equilibrium with the major enantiomer by dimer formation and thus a reaction or catalysis can be dominated by the remaining molecules of the major enantiomer. In this article a mechanism is shown that leads to homochirality by the formation of a highly catalytically active transient intermediate in a stereodynamically controlled reaction. This is demonstrated by Soai's asymmetric autocatalysis, in which aldehydes are transformed into the corresponding alcohols by addition of dialkylzinc reagents. The mechanism of chirogenesis proposed here shows that an apparently inefficient reaction is the best prerequisite for a selection mechanism. In addition, stereodynamic control offers the advantage that the minor diastereomeric intermediate can be interconverted into the major diastereomer and thus be stereoeconomically efficient. This is supported by computer simulation of reaction kinetics.

Asymmetric Induction and Amplification in Stereodynamic Catalytic Systems by Noncovalent Interactions

The local transmission of chiral information by noncovalent interactions is one of the most fundamental processes broadly found in nature, i.e. in complex biochemical systems. This review summarizes our accomplishments in investigating chiral induction in stereodynamic ligands and catalysts by weak intermolecular interactions. It includes our efforts to characterize numerous stereodynamic compounds in detail with respect to their thermodynamic and kinetic properties. Furthermore, many stereolabile ligands for enantioselective catalysis are described, where directed stereoinduction afforded highly enantio- or diastereoenriched catalysts for subsequent selective asymmetric transformations. Various approaches for the dynamic enrichment of one of the catalyst’s conformers are presented, such as noncovalent interaction of the ligand with a chiral environment or a chiral solute. Finally, successful chemical systems are presented in which a process of chiral induction can be coupled with an autoinductive mechanism triggered by the chirality of its own reaction product, realizing Nature-inspired feedback loops resulting in self-amplifying, enantioselective catalytic reactions.

1 Introduction

2 Mapping the Stereodynamic Landscape

3 Chiral Induction by Noncovalent Interactions

4 Autoinduction and Chiral Amplification

5 Self-Alignment and Emergence of Chirality

6 Conclusion

Catalytic strategies towards 1,3-polyol synthesis by enantioselective cascades creating multiple alcohol functions

This review highlights the different enantioselective catalyst-controlled cascades creating multiple alcohol functions through the formation of several carbon-carbon bonds. Through subsequent simple derivatization, these strategies ensure the rapid preparation of 1,3-polyols. Thanks to the use of efficient metal- or organo-catalysts, these cascades enable the selective assembly of multiple substrates considerably limiting operations and waste generation. For this purpose, several mono- or bi-directional approaches have been devised allowing successive C-C bond-forming events. The considerable synthetic economies these cascades enable have been demonstrated in the preparation of a wide variety of complex bioactive natural products, notably polyketides.

Design and synthesis of a stereodynamic catalyst with reversal of selectivity by enantioselective self-inhibition

Chirality plays a pivotal role in an uncountable number of biological processes, and nature has developed intriguing mechanisms to maintain this state of enantiopurity. The strive for a deeper understanding of the different elements that constitute such self-sustaining systems on a molecular level has sparked great interest in the studies of autoinductive and amplifying enantioselective reactions. The design of these reactions remains highly challenging; however, the development of generally applicable principles promises to have a considerable impact on research of catalyst design and other adjacent fields in the future. Here, we report the realization of an autoinductive, enantioselective self-inhibiting hydrogenation reaction. Development of a stereodynamic catalyst with chiral sensing abilities allowed for a chiral reaction product to interact with the catalyst and change its selectivity in order to suppress its formation, which caused a reversal of selectivity over time.

Enantioselectivity Induced by Stereoselective Interlocking: A Novel Core Motif for Tropos Ligands

Well-defined supramolecular interactions are a powerful tool to control the stereochemistry of a catalytic reaction. In this paper, we report a novel core motif for fluxional 2,2'-biphenyl ligands carrying (S)-amino acid-derived interaction sites in 5,5'-position that cause spontaneous enrichment of the R_ax rotamer. The process is based on strong non-covalent interlocking between interaction sites, which causes diastereoselective formation of a supramolecular ligand dimer, in which the axial chirality of the two subunits is dictated by the stereochemical information in the amino acid residues. The detailed structure of the dimer was elucidated by NMR spectroscopy and single-crystal X-ray analysis. Three different phosphorus-based ligand types, namely a bisphosphine, a bisphosphinite and a phosphoramidite were synthesized and characterized. Whereas the first one was found to exist in a strongly weighted equilibrium, the two others each exhibited stereoconvergent behavior transforming into the diastereopure R_ax rotamer. Enriched ligands were used in rhodium-mediated asymmetric hydrogenation reactions of prochiral olefins in which very high enantioselectivities of up to 96:4 were achieved.

Inducing Enantioselectivity in a Dynamic Catalyst by Supramolecular Interlocking

The design of a new class of fluxional biphenyl bisphosphinite (BIBIPHOS) ligands decorated with amino acid-based diamide interaction sites is reported that undergo spontaneous desymmetrization. Hydrogenation of prochiral alkenes using Rh-BIBIPHOS results in enantiomeric ratios of up to 96:4 (R/S). This stereoconvergent behavior of the fluxional BIBIPHOS ligand is triggered by pronounced intermolecular interlocking of the recognition sites, leading to the formation of a supramolecular assembly, where the axial orientation of the biphenyl ligand backbone is governed by the chirality of the amino acid moieties. Stereoinduction during catalysis is decoupled from this process and occurs as an immediate consequence of the emergent behavior of the ligands. This supramolecular system is very robust and has the potential to be adopted for other ligand designs in enantioselective catalysis.

Copper Complexes in the Promotion of Aldol Addition to Pyridine-2-carboxaldehyde: Synthesis of Homo- and Heteroleptic Complexes and Stereoselective Double Aldol Addition

CuCl₂·2H₂O and Cu(ClO₄)₂·6H₂O are able to promote aldol addition of pyridine-2-carboxaldehyde (pyca) with acetone, acetophenone, or cyclohexenone under neutral and mild conditions. The general and simple one-pot procedure for the aldol addition to Cu(II) complexes accesses novel Cu complexes with a large variety of different structural motifs, from which the aldol-addition ligand can be liberated by treatment with NH₃. Neutral heteroleptic complexes in which the ligand acts as bidentate, or homoleptic cationic complexes in which the ligand acts as tridentate can be obtained depending on the copper salt used. The key step in these reactions is the coordination of pyca to copper, which increases the electrophilic character of the aldehyde, with Cu(ClO₄)₂ leading to a higher degree of activation than CuCl₂, as predicted by DFT calculations. A regio- and stereoselective double aldol addition of pyca in the reaction of Cu(ClO₄)₂·6H₂O with acetone leads to the formation of a dimer copper complex in which the novel double aldol addition product acts as a pentadentate ligand. A possible mechanism is discussed. The work is supported by extensive crystallographic studies.

Stereodynamic Quinone-Hydroquinone Molecules That Enantiomerize at sp3-Carbon via Redox-Interconversion

Since the discovery of molecular chirality, nonsuperimposable mirror-image organic molecules have been found to be essential across biological and chemical processes and increasingly in materials science. Generally, carbon centers containing four different substituents are configurationally stable, unless bonds to the stereogenic carbon atom are broken and re-formed. Herein, we describe sp³-stereogenic carbon-bearing molecules that dynamically isomerize, interconverting between enantiomers without cleavage of a constituent bond, nor through remote functional group migration. The stereodynamic molecules were designed to contain a pair of redox-active substituents, quinone and hydroquinone groups, which allow the enantiomerization to occur via redox-interconversion. In the presence of an enantiopure host, these molecules undergo a deracemization process that allows observation of enantiomerically enriched compounds. This work reveals a fundamentally distinct enantiomerization pathway available to chiral compounds, coupling redox-interconversion to chirality.

Sequential Catalysis of Phosphine Oxide for Stereoselective Synthesis of Stereopentads

An efficient method for accessing enantiomerically pure stereopentads via a catalytic asymmetric sequential aldol reaction has been developed for the first time. The enantioselective sequential aldol reaction produces a wide range of chiral stereopentad precursors in good yields with excellent enantioselectivities. The key to success is the use of the sequential catalytic system involving a chiral phosphine oxide catalyst and trichlorosilyl triflate.

Stereodynamic tetrahydrobiisoindole "NU-BIPHEP(O)"s: functionalization, rotational barriers and non-covalent interactions

Stereodynamic ligands offer intriguing possibilities in enantioselective catalysis. “NU-BIPHEPs” are a class of stereodynamic diphosphine ligands which are easily accessible via rhodium-catalyzed double [2 + 2 + 2] cycloadditions. This study explores the preparation of differently functionalized “NU-BIPHEP(O)” compounds, the characterization of non-covalent adduct formation and the quantification of enantiomerization barriers. In order to explore the possibilities of functionalization, we studied modifications of the ligand backbone, e.g., with 3,5-dichlorobenzoyl chloride. Diastereomeric adducts with Okamoto-type cellulose derivatives and on-column deracemization were realized on the basis of non-covalent interactions. Enantioselective dynamic HPLC (DHPLC) allowed for the determination of rotational barriers of ΔG^‡_298K = 92.2 ± 0.3 kJ mol⁻¹ and 99.5 ± 0.1 kJ mol⁻¹ underlining the stereodynamic properties of “NU-BIPHEPs” and “NU-BIPHEP(O)s”, respectively. These results make the preparation of tailor-made functionalized stereodynamic ligands possible and give an outline for possible applications in enantioselective catalysis.

Concise Asymmetric Construction of C2 -symmetric 1,9-Diarylnonanoids Using a Hypervalent Silicon Complex: Total Synthesis of (-)-Ericanone

By using a phosphine oxide-catalyzed enantioselective double aldol reaction, we achieved the concise construction of C2 -symmetric 1,9-diarylnonanoids, enabling the synthesis of (-)-ericanone from p-hydroxybenzaldehyde in 6 steps with 65 % overall yield. The enantioselective double aldol reaction is useful for establishing C2 -symmetric 1,9-diaryl-3,7-dihydroxy-5-nonanones with a single operation. Furthermore, the use of o-nosyl-protected p-hydroxybenzaldehyde and a 4,4'-disubstituted BINAP dioxide catalyst dramatically improved the reactivity and selectivity in the double aldol reaction, enabling the total synthesis of (-)-ericanone with high yield and with excellent enantiopurity.

Enantiodivergent asymmetric catalysis with the tropos BIPHEP ligand and a proline derivative as chiral selector

A catalytic system based on the tropos ligand BIPHEP and (S)-proline methyl ester as chiral selector was studied for Rh-catalysed asymmetric catalysis. By careful control of the catalyst preformation conditions, the enantioselectivity could be completely reversed in asymmetric hydrogenation of prochiral olefins maintaining the same absolute level in favorable cases. The enantiodivergent asymmetric catalysis could be rationalised by the interplay of the dynamic chirality (tropos) of the phosphine ligand and the coordination of the proline selector. Treating a suitable Rh-BIPHEP precursor with the (Sc)-proline-based ionic liquid led to an equimolar mixture of (RaSc)- and (SaSc)-diastereomers that is kinetically stable at 0 °C. At higher temperature, an irreversible diastereomerisation process was observed resulting in the diastereomerically pure (RaSc)-complex [Rh{(Ra)-BIPHEP}{(Sc)-ProlOMe}]. Whereas the use of the pure (RaSc)-complex led to 51% ee (R) in the hydrogenation of methyl 2-acetamidoacrylate, the S-product was formed with almost identical enantioselectivity when the (RaSc)/(SaSc)-mixture was applied under identical conditions. This inversion was associated with the relative stability of the diastereomers in the equilibria forming the catalytically active substrate complex. The possibility to use this different reactivity to control the direction of enantioselectivity was demonstrated for the hydrogenation of different substrates whereby ee's of up to 80% could be achieved. Moreover, the (RaSc)-complex led to high enantioselectivities of up 86% ee in the asymmetric hydroboration of styrene, approaching the performance of the atropos BINAP ligand for this reaction.

5,5′-Diamino-BIPHEP ligands bearing small selector units for non-covalent binding of chiral analytes in solution

A dynamic axially chiral BIPHEP-ligand with 3,5-dichlorobenzoyl amide selector units for non-covalent binding of phenylalanine derivatives has been developed.

From stereodynamics to high-throughput screening of catalysed reactions

In this review we summarised recent developments in high-throughput kinetic monitoring of reactions including the dynamics of interconverting stereoisomers and the simultaneous combination of (catalysed) reactions with chemical analysis in on-column reaction chromatographic devices.