1
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Uchikura T, Takahashi K, Oishi T, Akiyama T. Visible-light-driven enantioselective intermolecular [2 + 2] photocyclization utilizing bathochromic excitation mediated by a chiral phosphoric acid. Org Biomol Chem 2023; 21:9138-9142. [PMID: 37975203 DOI: 10.1039/d3ob01425h] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2023]
Abstract
We report herein an enantioselective intermolecular [2 + 2] photocyclization of alkenyl 2-pyrrolyl ketones using the bathochromic shift mediated by a chiral phosphoric acid. This synthetic method provides access to cyclobutanes with up to 98% ee. According to the UV-Vis spectra, the bathochromic effect was observed by mixing alkenyl 2-pyrrolyl ketones and a chiral phosphoric acid. A non-linear correlation was observed between the ee of the catalyst and the ee of the cycloadduct, suggesting that both substrates bind to the chiral phosphoric acid and form a dimer complex before photocycloaddition.
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Affiliation(s)
- Tatsuhiro Uchikura
- Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1, Mejiro, Toshima-ku, Tokyo, Japan.
| | - Kazuki Takahashi
- Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1, Mejiro, Toshima-ku, Tokyo, Japan.
| | - Tatsushi Oishi
- Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1, Mejiro, Toshima-ku, Tokyo, Japan.
| | - Takahiko Akiyama
- Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1, Mejiro, Toshima-ku, Tokyo, Japan.
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2
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Gramüller J, Gschwind RM. An NMR Spectroscopy View on London Dispersion in Catalysis: Detection, Quantification, and Application in Ion Pair and Transition Metal Catalysis. Acc Chem Res 2023; 56:2968-2979. [PMID: 37889132 DOI: 10.1021/acs.accounts.3c00431] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/28/2023]
Abstract
ConspectusThe energetic contribution of London dispersion (LD) can cover a broad range from very few to hundreds of kJ mol-1 for extended interaction interfaces due to its pairwise additivity. However, for a designed and successful application of LD in chemical catalysis, there are still many obstacles and questions that remain. In principle, LD can be regarded as the attractive part of the van der Waals potential. Thus, considering the whole van der Waals potential, including the repulsive part (steric repulsion), the ideal solution to the problem in catalysis would be to design compatible interaction interfaces at exactly the correct distance. In the case of a self-assembled, flexible structure arrangement, entropic contributions and solvent interactions might be detrimental. In the case of a rigid catalyst pocket, steric hindrance might not allow for large substituents that are usually applied as dispersion energy donors (DEDs). For a working catalytic system, the following question arises: how is it possible to dissect the complex interaction interfaces in terms of energetic contributions? Usually, the energetic contribution of LD to catalysis is addressed by using calculations. However, adequately computing the correct energetic contributions can be extremely challenging for a vast conformational space with all kinds of intermolecular interactions. Thus, experimental data are essential for comparison or benchmarking.Therefore, in this Account, we describe our quest for detailed experimental data obtained via NMR spectroscopy to experimentally dissect and quantify LD in catalytic systems. In addition, we address the question of whether bulky substituents used as DEDs can be used in confined catalytic pockets. With the example of Pd phosphoramidite complexes, we show how it is possible to experimentally dissect and quantify the contribution of individual interaction areas in complicated transition metal complexes. Furthermore, a correlation between conformational rigidity and heterodimer preference clearly reveals that LD can only unfold its full potential in cases where entropic contributions are minimized. This finding can also explain the small contribution of LD in flexible and solvent-exposed molecular balances. In the field of Brønsted acid catalysis, we demonstrated that LD has a strong influence on the structures, stability, and populations of confined catalytic intermediates. LD is key for populating higher aggregates such as dimers. In addition, offsets between the experimental and computational results were observed and attributed to solvent-solute dispersion interactions. We studied the delicate interplay of attractive and repulsive interactions by adding bulky DED substituents onto a substrate, which can function as a molecular balance system. Intriguingly, the effect of LD on the free substrate was straightforwardly transferred onto the highly confined intermediates. Furthermore, this effect could even be read out in the enantioselectivities of the underlying reaction. This conceptualized a general approach regarding how LD can be used beneficially in catalysis to convert from moderate/good to excellent stereoselectivities. It showcased that bulky groups such as tert-butyl must not only be regarded as occupied volumes.
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Affiliation(s)
- Johannes Gramüller
- Institute of Organic Chemistry, University of Regensburg, Universitätsstraße 31, D-93053 Regensburg, Germany
| | - Ruth M Gschwind
- Institute of Organic Chemistry, University of Regensburg, Universitätsstraße 31, D-93053 Regensburg, Germany
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3
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Harden I, Neese F, Bistoni G. Dimerization of confined Brønsted acids in enantioselective organocatalytic reactions. Chem Sci 2023; 14:10580-10590. [PMID: 37799993 PMCID: PMC10548523 DOI: 10.1039/d3sc03769j] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2023] [Accepted: 09/08/2023] [Indexed: 10/07/2023] Open
Abstract
The formation of Brønsted acid aggregates in the course of asymmetric organocatalytic reactions is often overlooked in mechanistic studies, even though it might have a deep impact on the stereo-controlling factors of the transformations. In this work, we shed light on the influence of the catalyst structure and reaction conditions on the spontaneity of the aggregation process for popular chiral organocatalysts derived from phosphoric acids using high-level quantum mechanical calculations. Our study encompasses small and sterically unhindered chiral phosphoric acids as well as large and "confined" imidodiphosphates and imidodiphosphorimidates. These systems have recently proven particularly effective in promoting a large number of highly relevant asymmetric transformations. While cooperative catalytic effects of sterically less hindered chiral phosphoric acid catalysts are well appreciated in literature, it is found that the formation of catalyst dimers in solution is possible for both standard and confined catalysts. The spontaneity of the aggregation process depends on reaction conditions like solvent polarity, polarizability, temperature, the nature of the interaction with the substrate, as well as the catalyst architecture. Finally, it is shown that, at low temperatures (153 K), the aggregation process can profoundly influence the reaction kinetics and selectivity.
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Affiliation(s)
- Ingolf Harden
- Max-Planck-Institut für Kohlenforschung Kaiser-Wilhelm Platz 1 45470 Mülheim an der Ruhr Germany
| | - Frank Neese
- Max-Planck-Institut für Kohlenforschung Kaiser-Wilhelm Platz 1 45470 Mülheim an der Ruhr Germany
| | - Giovanni Bistoni
- Department of Chemistry, Biology and Biotechnology, University of Perugia Via Elce di Sotto, 8 06123 Perugia Italy
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4
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Liles JP, Rouget-Virbel C, Wahlman JLH, Rahimoff R, Crawford JM, Medlin A, O’Connor V, Li J, Roytman VA, Toste FD, Sigman MS. Data Science Enables the Development of a New Class of Chiral Phosphoric Acid Catalysts. Chem 2023; 9:1518-1537. [PMID: 37519827 PMCID: PMC10373836 DOI: 10.1016/j.chempr.2023.02.020] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 08/01/2023]
Abstract
The widespread success of BINOL-chiral phosphoric acids (CPAs) has led to the development of several high molecular weight, sterically encumbered variants. Herein, we disclose an alternative, minimalistic chiral phosphoric acid backbone incorporating only a single instance of point chirality. Data science techniques were used to select a diverse training set of catalysts, which were benchmarked against the transfer hydrogenation of an 8-aminoquinoline. Using a univariate classification algorithm and multivariate linear regression, key catalyst features necessary for high levels of selectivity were deconvoluted, revealing a simple catalyst model capable of predicting selectivity for out-of-set catalysts. This workflow enabled extrapolation to a catalyst providing higher selectivity than both reported peptide-type and BINOL-type catalysts (up to 95:5 er). These techniques were then successfully applied towards two additional transforms. Taken together, these examples illustrate the power of combining rational design with data science (ab initio) to efficiently explore reactivity during catalyst development.
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Affiliation(s)
- Jordan P. Liles
- Department of Chemistry, University of Utah, 315 S 1400 E, Salt Lake City, UT, 84112, USA
| | | | - Julie L. H. Wahlman
- Department of Chemistry, University of Utah, 315 S 1400 E, Salt Lake City, UT, 84112, USA
| | - Rene Rahimoff
- College of Chemistry, University of California, Berkeley, Berkeley, CA, 94720, USA
| | - Jennifer M. Crawford
- Department of Chemistry, University of Utah, 315 S 1400 E, Salt Lake City, UT, 84112, USA
| | - Abby Medlin
- College of Chemistry, University of California, Berkeley, Berkeley, CA, 94720, USA
| | - Veronica O’Connor
- Department of Chemistry, University of Utah, 315 S 1400 E, Salt Lake City, UT, 84112, USA
| | - Junqi Li
- College of Chemistry, University of California, Berkeley, Berkeley, CA, 94720, USA
| | - Vladislav A. Roytman
- College of Chemistry, University of California, Berkeley, Berkeley, CA, 94720, USA
| | - F. Dean Toste
- College of Chemistry, University of California, Berkeley, Berkeley, CA, 94720, USA
| | - Matthew S. Sigman
- Department of Chemistry, University of Utah, 315 S 1400 E, Salt Lake City, UT, 84112, USA
- Lead contact
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5
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Stockerl WJ, Gschwind RM. Photo enhancement reveals ( E, Z) and ( Z, Z) configurations as additional intermediates in iminium ion catalysis. Chem Commun (Camb) 2023; 59:1325-1328. [PMID: 36644931 DOI: 10.1039/d2cc05976b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Abstract
Imidazolidinone-based α,β-unsaturated iminium ions are the reactive species within countless synthetic protocols in asymmetric organocatalysis. However, (E,Z) and (Z,Z) imidazolidinone iminium ions, i.e. (Z)-CC configurations, have been elusive so far. Herein we describe how in situ photoisomerization enables the observation and assignment of high energetic (Z)-configured intermediates below the detection limit of NMR spectroscopy for (E,Z) and (Z,Z) iminium perchlorate complexes derived from MacMillan's 1st generation catalyst and cinnamaldehyde. Traces of (E,Z) could even be detected under synthetic conditions at 25 °C in MeCN. Using back isomerization studies and diffusion ordered spectroscopy, conditions were found to stabilize the (E,Z) and (Z,Z) isomers for several hours via ion pair aggregation. Thus, at least (E,Z) should be considered for future investigations in asymmetric iminium ion catalysis.
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Affiliation(s)
- Willibald J Stockerl
- Institute of Organic Chemistry, University of Regensburg, 93040, Regensburg, Germany.
| | - Ruth M Gschwind
- Institute of Organic Chemistry, University of Regensburg, 93040, Regensburg, Germany.
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6
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Reyes E, Prieto L, Milelli A. Asymmetric Organocatalysis: A Survival Guide to Medicinal Chemists. Molecules 2022; 28:271. [PMID: 36615465 PMCID: PMC9822454 DOI: 10.3390/molecules28010271] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/22/2022] [Revised: 12/16/2022] [Accepted: 12/21/2022] [Indexed: 12/30/2022] Open
Abstract
Majority of drugs act by interacting with chiral counterparts, e.g., proteins, and we are, unfortunately, well-aware of how chirality can negatively impact the outcome of a therapeutic regime. The number of chiral, non-racemic drugs on the market is increasing, and it is becoming ever more important to prepare these compounds in a safe, economic, and environmentally sustainable fashion. Asymmetric organocatalysis has a long history, but it began its renaissance era only during the first years of the millennium. Since then, this field has reached an extraordinary level, as confirmed by the awarding of the 2021 Chemistry Nobel Prize. In the present review, we wish to highlight the application of organocatalysis in the synthesis of enantio-enriched molecules that may be of interest to the pharmaceutical industry and the medicinal chemistry community. We aim to discuss the different activation modes observed for organocatalysts, examining, for each of them, the generally accepted mechanisms and the most important and developed reactions, that may be useful to medicinal chemists. For each of these types of organocatalytic activations, select examples from academic and industrial applications will be disclosed during the synthesis of drugs and natural products.
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Affiliation(s)
- Efraim Reyes
- Department of Organic and Inorganic Chemistry, University of the Basque Country (UPV/EHU), 48080 Bilbao, Spain
| | - Liher Prieto
- Department of Organic and Inorganic Chemistry, University of the Basque Country (UPV/EHU), 48080 Bilbao, Spain
| | - Andrea Milelli
- Department for Life Quality Studies, Alma Mater Studiorum-University of Bologna, Corso d’Augusto 237, 47921 Rimini, Italy
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7
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Gramüller J, Dullinger P, Horinek D, Gschwind RM. Bidentate substrate binding in Brønsted acid catalysis: structural space, hydrogen bonding and dimerization. Chem Sci 2022; 13:14366-14372. [PMID: 36545144 PMCID: PMC9749107 DOI: 10.1039/d2sc05076e] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/11/2022] [Accepted: 11/24/2022] [Indexed: 11/27/2022] Open
Abstract
BINOL derived chiral phosphoric acids (CPAs) are a prominent class of catalysts in the field of asymmetric organocatalysis, capable of transforming a wide selection of substrates with high stereoselectivities. Exploiting the Brønsted acidic and basic dual functionality of CPAs, substrates with both a hydrogen bond acceptor and donor functionality are frequently used as the resulting bidentate binding via two hydrogen bonds is expected to strongly confine the possible structural space and thus yield high stereoselectivities. Despite the huge success of CPAs and the popularity of a bidentate binding motif, experimental insights into their organization and origin of stereoinduction are scarce. Therefore, in this work the structural space and hydrogen bonding of CPAs and N-(ortho-hydroxyaryl) imines (19 CPA/imine combinations) was elucidated by low temperature NMR studies and corroborated by computations. The postulated bidentate binding of catalyst and substrate by two hydrogen bonds was experimentally validated by detection of trans-hydrogen bond scalar couplings. Counterintuitively, the resulting CPA/imine complexes showed a broad potential structural space and a strong preference towards the formation of [CPA/imine]2 dimers. Molecular dynamics simulations showed that in these dimers, the imines form each one hydrogen bond to two CPA molecules, effectively bridging them. By finetuning steric repulsion and noncovalent interactions, rigid and well-defined CPA/imine monomers could be obtained. NOESY studies corroborated by theoretical calculations revealed the structure of that complex, in which the imine is located in between the 3,3'-substituents of the catalyst and one site of the substrate is shielded by the catalyst, pinpointing the origin or stereoselectivity for downstream transformations.
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Affiliation(s)
- Johannes Gramüller
- Institut für Organische Chemie, Universität RegensburgD-93053 RegensburgGermany
| | - Philipp Dullinger
- Institute of Physical and Theoretical Chemistry, University of RegensburgD-93053Germany
| | - Dominik Horinek
- Institute of Physical and Theoretical Chemistry, University of RegensburgD-93053Germany
| | - Ruth M. Gschwind
- Institut für Organische Chemie, Universität RegensburgD-93053 RegensburgGermany
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8
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Gramüller J, Franta M, Gschwind RM. Tilting the Balance: London Dispersion Systematically Enhances Enantioselectivities in Brønsted Acid Catalyzed Transfer Hydrogenation of Imines. J Am Chem Soc 2022; 144:19861-19871. [PMID: 36260790 DOI: 10.1021/jacs.2c07563] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
London dispersion (LD) is attracting more and more attention in catalysis since LD is ubiquitously present and cumulative. Since dispersion is hard to grasp, recent research has concentrated mainly on the effect of LD in individual catalytic complexes or on the impact of dispersion energy donors (DEDs) on balance systems. The systematic transfer of LD effects onto confined and more complex systems in catalysis is still in its infancy, and no general approach for using DED residues in catalysis has emerged so far. Thus, on the example of asymmetric Brønsted acid catalyzed transfer hydrogenation of imines, we translated the findings of previously isolated balance systems onto confined catalytic intermediates, resulting in a systematic enhancement of stereoselectivity when employing DED-substituted substrates. As the imine substrate is present as Z- and E-isomers, which can, respectively, be converted to R- and S-product enantiomers, implementing tert-butyl groups as DED residues led to an additional stabilization of the Z-imine by up to 4.5 kJ/mol. NMR studies revealed that this effect is transferred onto catalyst/imine and catalyst/imine/nucleophile intermediates and that the underlying reaction mechanism is not affected. A clear correlation between ee and LD stabilization was demonstrated for 3 substrates and 10 catalysts, allowing to convert moderate-good to good-excellent enantioselectivities. Our findings conceptualize a general approach on how to beneficially employ DED residues in catalysis: they clearly showcase that bulky alkyl residues such as tert-butyl groups must be considered regarding not only their repulsive steric bulk but also their attractive properties even in catalytic complexes.
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Affiliation(s)
- Johannes Gramüller
- Institute of Organic Chemistry, University of Regensburg, 93040 Regensburg, Germany
| | - Maximilian Franta
- Institute of Organic Chemistry, University of Regensburg, 93040 Regensburg, Germany
| | - Ruth M Gschwind
- Institute of Organic Chemistry, University of Regensburg, 93040 Regensburg, Germany
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9
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Žabka M, Gschwind RM. Ternary complexes of chiral disulfonimides in transfer-hydrogenation of imines: the relevance of late intermediates in ion pair catalysis. Chem Sci 2021; 12:15263-15272. [PMID: 34976346 PMCID: PMC8635212 DOI: 10.1039/d1sc03724b] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2021] [Accepted: 10/22/2021] [Indexed: 01/29/2023] Open
Abstract
In ion pairing catalysis, the structures of late intermediates and transition states are key to understanding and further development of the field. Typically, a plethora of transition states is explored computationally. However, especially for ion pairs the access to energetics via computational chemistry is difficult and experimental data is rare. Here, we present for the first time extensive NMR spectroscopic insights about the ternary complex of a catalyst, substrate, and reagent in ion pair catalysis exemplified by chiral Brønsted acid-catalyzed transfer hydrogenation. Quantum chemistry calculations were validated by a large amount of NMR data for the structural and energetic assessment of binary and ternary complexes. In the ternary complexes, the expected catalyst/imine H-bond switches to an unexpected O-H-N structure, not yet observed in the multiple hydrogen-bond donor-acceptor situation such as disulfonimides (DSIs). This arrangement facilitates the hydride transfer from the Hantzsch ester in the transition states. In these reactions with very high isomerization barriers preventing fast pre-equilibration, the reaction barriers from the ternary complex to the transition states determine the enantioselectivity, which deviates from the relative transition state energies. Overall, the weak hydrogen bonding, the hydrogen bond switching and the special geometrical adaptation of substrates in disulfonimide catalyst complexes explain the robustness towards more challenging substrates and show that DSIs have the potential to combine high flexibility and high stereoselectivity.
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Affiliation(s)
- Matej Žabka
- Institute of Organic Chemistry, University of Regensburg D-93053 Regensburg Germany
| | - Ruth M Gschwind
- Institute of Organic Chemistry, University of Regensburg D-93053 Regensburg Germany
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10
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Sendra J, Reyes E, Prieto L, Fernández E, Vicario JL. Transannular Enantioselective (3 + 2) Cycloaddition of Cycloalkenone Hydrazones under Brønsted Acid Catalysis. Org Lett 2021; 23:8738-8743. [PMID: 34726408 PMCID: PMC8609578 DOI: 10.1021/acs.orglett.1c03190] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
Hydrazones derived from cycloalkenones undergo an enantioselective transannular formal (3 + 2) cycloaddition catalyzed by a chiral phosphoric acid. The reaction provides high yields and excellent stereocontrol in the formation of complex adducts with one or two α-tertiary amine moieties at the ring fusion, and these can be converted into very versatile stereodefined decalin- or octahydro-1H-indene-derived 1,3-diamines through simple reductive N-N cleavage.
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Affiliation(s)
- Jana Sendra
- Department of Organic and Inorganic Chemistry, University of the Basque Country (UPV/EHU). P.O. Box 644, 48080 Bilbao, Spain.,Departament Química Física i Inorgànica, Universidad Rovira i Virgilli, C/Marcel·lí Domingo s/n, 50009 Tarragona, Spain
| | - Efraim Reyes
- Department of Organic and Inorganic Chemistry, University of the Basque Country (UPV/EHU). P.O. Box 644, 48080 Bilbao, Spain
| | - Liher Prieto
- Department of Organic and Inorganic Chemistry, University of the Basque Country (UPV/EHU). P.O. Box 644, 48080 Bilbao, Spain
| | - Elena Fernández
- Departament Química Física i Inorgànica, Universidad Rovira i Virgilli, C/Marcel·lí Domingo s/n, 50009 Tarragona, Spain
| | - Jose L Vicario
- Department of Organic and Inorganic Chemistry, University of the Basque Country (UPV/EHU). P.O. Box 644, 48080 Bilbao, Spain
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11
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Pecho F, Sempere Y, Gramüller J, Hörmann FM, Gschwind RM, Bach T. Enantioselective [2 + 2] Photocycloaddition via Iminium Ions: Catalysis by a Sensitizing Chiral Brønsted Acid. J Am Chem Soc 2021; 143:9350-9354. [PMID: 34156845 PMCID: PMC8251699 DOI: 10.1021/jacs.1c05240] [Citation(s) in RCA: 34] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
N,O-Acetals derived from α,β-unsaturated β-aryl substituted aldehydes and (1-aminocyclohexyl)methanol were found to undergo a catalytic enantioselective [2 + 2] photocycloaddition to a variety of olefins (19 examples, 54-96% yield, 84-98% ee). The reaction was performed by visible light irradiation (λ = 459 nm). A chiral phosphoric acid (10 mol %) with an (R)-1,1'-bi-2-naphthol (binol) backbone served as the catalyst. The acid displays two thioxanthone groups attached to position 3 and 3' of the binol core via a meta-substituted phenyl linker. NMR studies confirmed the formation of an iminium ion which is attached to the acid counterion in a hydrogen-bond assisted ion pair. The catalytic activity of the acid rests on the presence of the thioxanthone moieties which enable a facile triplet energy transfer and an efficient enantioface differentiation.
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Affiliation(s)
- Franziska Pecho
- Department Chemie and Catalysis Research Center (CRC), Technische Universität München, Lichtenbergstrasse 4, 85747 Garching, Germany
| | - Yeshua Sempere
- Department Chemie and Catalysis Research Center (CRC), Technische Universität München, Lichtenbergstrasse 4, 85747 Garching, Germany
| | - Johannes Gramüller
- Faculty of Chemistry and Pharmacy, Institute of Organic Chemistry, University of Regensburg, Universitätsstraße 31, 93040 Regensburg, Germany
| | - Fabian M Hörmann
- Department Chemie and Catalysis Research Center (CRC), Technische Universität München, Lichtenbergstrasse 4, 85747 Garching, Germany
| | - Ruth M Gschwind
- Faculty of Chemistry and Pharmacy, Institute of Organic Chemistry, University of Regensburg, Universitätsstraße 31, 93040 Regensburg, Germany
| | - Thorsten Bach
- Department Chemie and Catalysis Research Center (CRC), Technische Universität München, Lichtenbergstrasse 4, 85747 Garching, Germany
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12
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Dreier C, Prädel L, Ehrhard AA, Wagner M, Hunger J. Association Equilibria of Organo-Phosphoric Acids with Imines from a Combined Dielectric and Nuclear Magnetic Resonance Spectroscopy Approach. Anal Chem 2021; 93:3914-3921. [PMID: 33600142 PMCID: PMC7931174 DOI: 10.1021/acs.analchem.0c04669] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2020] [Accepted: 02/08/2021] [Indexed: 11/29/2022]
Abstract
Aggregates formed between organo-phosphoric acids and imine bases in aprotic solvents are the reactive intermediates in Brønsted acid organo-catalysis. Due to the strong hydrogen-bonding interaction of the acids in solution, multiple homo- and heteroaggregates are formed with profound effects on catalytic activity. Yet, due to the similar binding motifs-hydrogen-bonds-it is challenging to experimentally quantify the abundance of these aggregates in solution. Here we demonstrate that a combination of nuclear magnetic resonance (NMR) and dielectric relaxation spectroscopy (DRS) allows for accurate speciation of these aggregates in solution. We show that only by using the observables of both experiments heteroaggregates can be discriminated with simultaneously taking homoaggregation into account. Comparison of the association of diphenyl phosphoric acid and quinaldine or phenylquinaline in chloroform, dichloromethane, or tetrahydrofuran suggests that the basicity of the base largely determines the association of one acid and one base molecule to form an ion-pair. We find the ion-pair formation constants to be highest in chloroform, slightly lower in dichloromethane and lowest in tetrahydrofuran, which indicates that the hydrogen-bonding ability of the solvent also alters ion-pairing equilibria. We find evidence for the formation of multimers, consisting of one imine base and multiple diphenyl phosphoric acid molecules for both bases in all three solvents. This subsequent association of an acid to an ion-pair is however little affected by the nature of the base or the solvent. As such our findings provide routes to enhance the overall fraction of these multimers in solution, which have been reported to open new catalytic pathways.
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Affiliation(s)
- Christian Dreier
- Max
Planck Institute for Polymer Research, Department for Molecular Spectroscopy, Ackermannweg 10, 55128 Mainz, Germany
| | - Leon Prädel
- Max
Planck Institute for Polymer Research, Department for Molecular Spectroscopy, Ackermannweg 10, 55128 Mainz, Germany
| | - Amelie A. Ehrhard
- Max
Planck Institute for Polymer Research, Department for Molecular Spectroscopy, Ackermannweg 10, 55128 Mainz, Germany
| | - Manfred Wagner
- Max
Planck Institute for Polymer Research, Department for Molecular Spectroscopy, Ackermannweg 10, 55128 Mainz, Germany
| | - Johannes Hunger
- Max
Planck Institute for Polymer Research, Department for Molecular Spectroscopy, Ackermannweg 10, 55128 Mainz, Germany
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13
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Pecho F, Zou YQ, Gramüller J, Mori T, Huber SM, Bauer A, Gschwind RM, Bach T. A Thioxanthone Sensitizer with a Chiral Phosphoric Acid Binding Site: Properties and Applications in Visible Light-Mediated Cycloadditions. Chemistry 2020; 26:5190-5194. [PMID: 32065432 PMCID: PMC7216904 DOI: 10.1002/chem.202000720] [Citation(s) in RCA: 25] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/10/2020] [Indexed: 11/06/2022]
Abstract
A chiral phosphoric acid with a 2,2’‐binaphthol core was prepared that displays two thioxanthone moieties at the 3,3’‐position as light‐harvesting antennas. Despite its relatively low triplet energy, the phosphoric acid was found to be an efficient catalyst for the enantioselective intermolecular [2+2] photocycloaddition of β‐carboxyl‐substituted cyclic enones (e.r. up to 93:7). Binding of the carboxylic acid to the sensitizer is suggested by NMR studies and by DFT calculations to occur by means of two hydrogen bonds. The binding event not only enables an enantioface differentiation but also modulates the triplet energy of the substrates.
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Affiliation(s)
- Franziska Pecho
- Department of Chemistry and Catalysis Research Center (CRC), Technical University Munich, Lichtenbergstr. 4, 85747, Garching, Germany
| | - You-Quan Zou
- Department of Chemistry and Catalysis Research Center (CRC), Technical University Munich, Lichtenbergstr. 4, 85747, Garching, Germany
| | - Johannes Gramüller
- Faculty of Chemistry and Pharmacy, Institute of Organic Chemistry, University of Regensburg, Universitätsstraße 31, 93040, Regensburg, Germany
| | - Tadashi Mori
- Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-871, Japan
| | - Stefan M Huber
- Faculty for Chemistry and Biochemistry, Organic Chemistry I, Ruhr-Universität Bochum, Universitätsstraße 150, 44801, Bochum, Germany
| | - Andreas Bauer
- Department of Chemistry and Catalysis Research Center (CRC), Technical University Munich, Lichtenbergstr. 4, 85747, Garching, Germany
| | - Ruth M Gschwind
- Faculty of Chemistry and Pharmacy, Institute of Organic Chemistry, University of Regensburg, Universitätsstraße 31, 93040, Regensburg, Germany
| | - Thorsten Bach
- Department of Chemistry and Catalysis Research Center (CRC), Technical University Munich, Lichtenbergstr. 4, 85747, Garching, Germany
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14
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Jansen D, Gramüller J, Niemeyer F, Schaller T, Letzel MC, Grimme S, Zhu H, Gschwind RM, Niemeyer J. What is the role of acid-acid interactions in asymmetric phosphoric acid organocatalysis? A detailed mechanistic study using interlocked and non-interlocked catalysts. Chem Sci 2020; 11:4381-4390. [PMID: 34122895 PMCID: PMC8159434 DOI: 10.1039/d0sc01026j] [Citation(s) in RCA: 20] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/20/2020] [Accepted: 04/01/2020] [Indexed: 11/21/2022] Open
Abstract
Organocatalysis has revolutionized asymmetric synthesis. However, the supramolecular interactions of organocatalysts in solution are often neglected, although the formation of catalyst aggregates can have a strong impact on the catalytic reaction. For phosphoric acid based organocatalysts, we have now established that catalyst-catalyst interactions can be suppressed by using macrocyclic catalysts, which react predominantly in a monomeric fashion, while they can be favored by integration into a bifunctional catenane, which reacts mainly as phosphoric acid dimers. For acyclic phosphoric acids, we found a strongly concentration dependent behavior, involving both monomeric and dimeric catalytic pathways. Based on a detailed experimental analysis, DFT-calculations and direct NMR-based observation of the catalyst aggregates, we could demonstrate that intermolecular acid-acid interactions have a drastic influence on the reaction rate and stereoselectivity of asymmetric transfer-hydrogenation catalyzed by chiral phosphoric acids.
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Affiliation(s)
- Dennis Jansen
- Faculty of Chemistry (Organic Chemistry) and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Universitätsstrasse 7 45141 Essen Germany
| | | | - Felix Niemeyer
- Faculty of Chemistry (Organic Chemistry) and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Universitätsstrasse 7 45141 Essen Germany
| | - Torsten Schaller
- Faculty of Chemistry (Organic Chemistry) and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Universitätsstrasse 7 45141 Essen Germany
| | - Matthias C Letzel
- Institute of Organic Chemistry, University of Münster Corrensstrasse 40 48149 Münster Germany
| | - Stefan Grimme
- Mulliken Center for Theoretical Chemistry, Rheinische Friedrich-Wilhelms Universität Bonn Beringstrasse 4 53115 Bonn Germany
| | - Hui Zhu
- Mulliken Center for Theoretical Chemistry, Rheinische Friedrich-Wilhelms Universität Bonn Beringstrasse 4 53115 Bonn Germany
| | - Ruth M Gschwind
- Organic Chemistry, University of Regensburg 93040 Regensburg Germany
| | - Jochen Niemeyer
- Faculty of Chemistry (Organic Chemistry) and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Universitätsstrasse 7 45141 Essen Germany
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15
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Rothermel K, Žabka M, Hioe J, Gschwind RM. Disulfonimides versus Phosphoric Acids in Brønsted Acid Catalysis: The Effect of Weak Hydrogen Bonds and Multiple Acceptors on Complex Structures and Reactivity. J Org Chem 2019; 84:13221-13231. [PMID: 31550152 PMCID: PMC6863592 DOI: 10.1021/acs.joc.9b01811] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/05/2019] [Indexed: 12/20/2022]
Abstract
In Brønsted acid catalysis, hydrogen bonds play a crucial role for reactivity and selectivity. However, the contribution of weak hydrogen bonds or multiple acceptors has been unclear so far since it is extremely difficult to collect experimental evidence for weak hydrogen bonds. Here, our hydrogen bond and structural access to Brønsted acid/imine complexes was used to analyze BINOL-derived chiral disulfonimide (DSI)/imine complexes. 1H and 15N chemical shifts as well as 1JNH coupling constants revealed for DSI/imine complexes ion pairs with very weak hydrogen bonds. The high acidity of the DSIs leads to a significant weakening of the hydrogen bond as structural anchor. In addition, the five hydrogen bond acceptors of DSI allow an enormous mobility of the imine in the binary DSI complexes. Theoretical calculations predict the hydrogen bonds to oxygen to be energetically less favored; however, their considerable population is corroborated experimentally by NOE and exchange data. Furthermore, an N-alkylimine, which shows excellent reactivity and selectivity in reactions with DSI, reveals an enlarged structural space in complexes with the chiral phosphoric acid TRIP as potential explanation of its reduced reactivity and selectivity. Thus, considering factors such as flexibility and possible hydrogen bond sites is essential for catalyst development in Brønsted acid catalysis.
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Affiliation(s)
| | | | - Johnny Hioe
- Institute of Organic Chemistry, University of Regensburg, D-93053 Regensburg, Germany
| | - Ruth M. Gschwind
- Institute of Organic Chemistry, University of Regensburg, D-93053 Regensburg, Germany
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16
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Lokesh N, Hioe J, Gramüller J, Gschwind RM. Relaxation Dispersion NMR to Reveal Fast Dynamics in Brønsted Acid Catalysis: Influence of Sterics and H-Bond Strength on Conformations and Substrate Hopping. J Am Chem Soc 2019; 141:16398-16407. [PMID: 31545037 PMCID: PMC6863621 DOI: 10.1021/jacs.9b07841] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/23/2019] [Indexed: 12/25/2022]
Abstract
NMR provides both structural and dynamic information, which is key to connecting intermediates and to understanding reaction pathways. However, fast exchanging catalytic intermediates are often inaccessible by conventional NMR due its limited time resolution. Here, we show the combined application of the 1H off-resonance R1ρ NMR method and low temperature (185-175 K) to resolve intermediates exchanging on a μs time scale (ns at room temperature). The potential of the approach is demonstrated on chiral phosphoric acid (CPA) catalysts in their complexes with imines. The otherwise inaccessible exchange kinetics of the E-I ⇌ E-II imine conformations and thermodynamic E-I:E-II imine ratios inside the catalyst pocket are experimentally determined and corroborated by calculations. The E-I ⇌ E-II exchange rate constants (kex185 K) for different catalyst-substrate binary complexes varied between 2500 and 19 000 s-1 (τex = 500-50 μs). Theoretical analysis of these exchange rate constants revealed the involvement of an intermediary tilted conformation E-III, which structurally resembles the hydride transfer transition state. The main E-I and E-II exchange pathway is a hydrogen bond strength dependent tilting-switching-tilting mechanism via a bifurcated hydrogen bond as a transition state. The reduction in the sterics of the catalyst showed an accelerated switching process by at least an order of magnitude and enabled an additional rotational pathway. Hence, the exchange process is mainly a function of the intrinsic properties of the 3,3'-substituents of the catalyst. Overall, we believe that the present study opens a new dimension in catalysis via experimental access to structures, populations, and kinetics of catalyst-substrate complexes on the μs time scale by the 1H off-resonance R1ρ method.
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Affiliation(s)
- N. Lokesh
- Institute of Organic Chemistry, University of Regensburg, D-93053 Regensburg, Germany
| | - Johnny Hioe
- Institute of Organic Chemistry, University of Regensburg, D-93053 Regensburg, Germany
| | - Johannes Gramüller
- Institute of Organic Chemistry, University of Regensburg, D-93053 Regensburg, Germany
| | - Ruth M. Gschwind
- Institute of Organic Chemistry, University of Regensburg, D-93053 Regensburg, Germany
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17
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Retini M, Bartolucci S, Bartoccini F, Mari M, Piersanti G. Concise and Convergent Enantioselective Total Syntheses of (+)- and (-)-Fumimycin. J Org Chem 2019; 84:12221-12227. [PMID: 31476858 DOI: 10.1021/acs.joc.9b02020] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
The concise and convergent total syntheses of (+)- and (-)-Fumimycin have been achieved by taking advantage of strategies for the asymmetric aza-Friedel-Crafts reaction of a highly substituted hydroquinone and N-fumaryl ketimine generated from the corresponding dehydroalanine. The enantiomerically pure natural product and its enantiomer were prepared in seven steps and 22% overall yield by employing both enantiomers of a BINOL-derived chiral phosphoric acid (CPA) catalyst.
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Affiliation(s)
- Michele Retini
- Department of Biomolecular Sciences , University of Urbino "Carlo Bo" , P.zza Rinascimento 6 , 61029 Urbino , PU , Italy
| | - Silvia Bartolucci
- Department of Biomolecular Sciences , University of Urbino "Carlo Bo" , P.zza Rinascimento 6 , 61029 Urbino , PU , Italy
| | - Francesca Bartoccini
- Department of Biomolecular Sciences , University of Urbino "Carlo Bo" , P.zza Rinascimento 6 , 61029 Urbino , PU , Italy
| | - Michele Mari
- Department of Biomolecular Sciences , University of Urbino "Carlo Bo" , P.zza Rinascimento 6 , 61029 Urbino , PU , Italy
| | - Giovanni Piersanti
- Department of Biomolecular Sciences , University of Urbino "Carlo Bo" , P.zza Rinascimento 6 , 61029 Urbino , PU , Italy
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18
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Rothermel K, Melikian M, Hioe J, Greindl J, Gramüller J, Žabka M, Sorgenfrei N, Hausler T, Morana F, Gschwind RM. Internal acidity scale and reactivity evaluation of chiral phosphoric acids with different 3,3'-substituents in Brønsted acid catalysis. Chem Sci 2019; 10:10025-10034. [PMID: 32015815 PMCID: PMC6977555 DOI: 10.1039/c9sc02342a] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2019] [Accepted: 09/02/2019] [Indexed: 12/16/2022] Open
Abstract
NMR H-bond analysis reveals an offset of internal and external acidities of catalysts and allows for a detailed reactivity analysis.
The concept of hydrogen bonding for enhancing substrate binding and controlling selectivity and reactivity is central in catalysis. However, the properties of these key hydrogen bonds and their catalyst-dependent variations are extremely difficult to determine directly by experiments. Here, for the first time the hydrogen bond properties of a whole series of BINOL-derived chiral phosphoric acid (CPA) catalysts in their substrate complexes with various imines were investigated to derive the influence of different 3,3′-substituents on the acidity and reactivity. NMR 1H and 15N chemical shifts and 1JNH coupling constants of these hydrogen bonds were used to establish an internal acidity scale corroborated by calculations. Deviations from calculated external acidities reveal the importance of intermolecular interactions for this key feature of CPAs. For CPAs with similarly sized binding pockets, a correlation of reactivity and hydrogen bond strengths of the catalyst was found. A catalyst with a very small binding pocket showed significantly reduced reactivities. Therefore, NMR isomerization kinetics, population and chemical shift analyses of binary and ternary complexes as well as reaction kinetics were performed to address the steps of the transfer hydrogenation influencing the overall reaction rate. The results of CPAs with different 3,3′-substituents show a delicate balance between the isomerization and the ternary complex formation to be rate-determining. For CPAs with an identical acidic motif and similar sterics, reactivity and internal acidity correlated inversely. In cases where higher sterical demand within the binary complex hinders the binding of the second substrate, the correlation between acidity and reactivity breaks down.
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Affiliation(s)
- Kerstin Rothermel
- Institut für Organische Chemie , Universität Regensburg , Universitätsstraße 31 , D-93053 Regensburg , Germany .
| | - Maxime Melikian
- Institut für Organische Chemie , Universität Regensburg , Universitätsstraße 31 , D-93053 Regensburg , Germany .
| | - Johnny Hioe
- Institut für Organische Chemie , Universität Regensburg , Universitätsstraße 31 , D-93053 Regensburg , Germany .
| | - Julian Greindl
- Institut für Organische Chemie , Universität Regensburg , Universitätsstraße 31 , D-93053 Regensburg , Germany .
| | - Johannes Gramüller
- Institut für Organische Chemie , Universität Regensburg , Universitätsstraße 31 , D-93053 Regensburg , Germany .
| | - Matej Žabka
- Institut für Organische Chemie , Universität Regensburg , Universitätsstraße 31 , D-93053 Regensburg , Germany .
| | - Nils Sorgenfrei
- Institut für Organische Chemie , Universität Regensburg , Universitätsstraße 31 , D-93053 Regensburg , Germany .
| | - Thomas Hausler
- Institut für Organische Chemie , Universität Regensburg , Universitätsstraße 31 , D-93053 Regensburg , Germany .
| | - Fabio Morana
- Institut für Organische Chemie , Universität Regensburg , Universitätsstraße 31 , D-93053 Regensburg , Germany .
| | - Ruth M Gschwind
- Institut für Organische Chemie , Universität Regensburg , Universitätsstraße 31 , D-93053 Regensburg , Germany .
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19
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Singh S, Sunoj RB. Computational asymmetric catalysis: On the origin of stereoselectivity in catalytic reactions. ADVANCES IN PHYSICAL ORGANIC CHEMISTRY 2019. [DOI: 10.1016/bs.apoc.2019.07.002] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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