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Abstract
N-Heterocyclic carbene (NHC) catalysis is a by now consolidated organocatalytic platform for a number of synthetic (asymmetric) transformations via diverse reaction modes/intermediates. In addition to the typical umpolung processes involving acyl anion/homoenolate equivalent species, implementation of protocols under oxidative conditions greatly expands the possibilities of this methodology. Oxidative NHC-catalysis allows for oxidative and oxygenative transformations through specific manipulations of Breslow-type species depending upon the oxidant used (external oxidant or O2 /air), the derived NHC-bound intermediates paving the way to non-umpolung processes through activation of carbon atoms and heteroatoms. This review is intended to update the state of the art in oxidative NHC-catalyzed reactions that appeared in the literature from 2014 to present, with a strong focus to crucial intermediates and their mechanistic implications.
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Affiliation(s)
- Carmela De Risi
- Dipartimento di Scienze Chimiche, Farmaceutiche ed Agrarie, Università di Ferrara, Via L. Borsari, 46, 44121, Ferrara, Italy
| | - Arianna Brandolese
- Dipartimento di Scienze dell'Ambiente e della Prevenzione, Università di Ferrara, Via L. Borsari, 46, 44121, Ferrara, Italy
| | - Graziano Di Carmine
- Dipartimento di Scienze Chimiche, Farmaceutiche ed Agrarie, Università di Ferrara, Via L. Borsari, 46, 44121, Ferrara, Italy
| | - Daniele Ragno
- Dipartimento di Scienze Chimiche, Farmaceutiche ed Agrarie, Università di Ferrara, Via L. Borsari, 46, 44121, Ferrara, Italy
| | - Alessandro Massi
- Dipartimento di Scienze Chimiche, Farmaceutiche ed Agrarie, Università di Ferrara, Via L. Borsari, 46, 44121, Ferrara, Italy
| | - Olga Bortolini
- Dipartimento di Scienze dell'Ambiente e della Prevenzione, Università di Ferrara, Via L. Borsari, 46, 44121, Ferrara, Italy
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Li Z, Li Y, Song Z, Zhang Z. NHC-catalyzed [3 + 4] annulation between 2-dromoenal and aryl 1,2-diamine: Insights into mechanisms, chemo and stereoselectivities. Molecular Catalysis 2022; 530:112604. [DOI: 10.1016/j.mcat.2022.112604] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
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Li Y, Song Z, Zhang Z. NHC Catalyzed β-Carbon functionalization of carboxylic esters towards formation of δ-Lactams: A mechanistic study. Molecular Catalysis 2022; 524:112311. [DOI: 10.1016/j.mcat.2022.112311] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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Bołt M, Hanek K, Żak P. Metal-free thioesterification of α,β-unsaturated aldehydes with thiols. Org Chem Front 2022. [DOI: 10.1039/d2qo00678b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
For the first time, the synthesis of thioesters starting from enals and thiols has been performed in the presence of a bulky N-heterocyclic carbene (NHC) as a catalyst.
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Affiliation(s)
- Małgorzata Bołt
- Department of Organometallic Chemistry, Faculty of Chemistry, Adam Mickiewicz University in Poznan, Uniwersytetu Poznańskiego 8, 61-614 Poznan, Poland
| | - Kamil Hanek
- Department of Organometallic Chemistry, Faculty of Chemistry, Adam Mickiewicz University in Poznan, Uniwersytetu Poznańskiego 8, 61-614 Poznan, Poland
| | - Patrycja Żak
- Department of Organometallic Chemistry, Faculty of Chemistry, Adam Mickiewicz University in Poznan, Uniwersytetu Poznańskiego 8, 61-614 Poznan, Poland
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Li Y, Geng L, Song Z, Zhang Z. A DFT study of NHC-catalyzed reactions between 2-bromo-2-enals and acylhydrazones: mechanisms, and chemo- and stereoselectivities. NEW J CHEM 2022. [DOI: 10.1039/d2nj01078j] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
The reaction mechanisms and origins of the chemo- and stereoselectivities of NHC-catalyzed [4 + 2] annulation of 2-bromo-2-enals and acylhydrazones.
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Affiliation(s)
- Yan Li
- School of Chemical Engineering, University of Science and Technology Liaoning, Anshan, 114051, P. R. China
| | - Lina Geng
- School of Chemical Engineering, University of Science and Technology Liaoning, Anshan, 114051, P. R. China
| | - Zhiyi Song
- School of Chemical Engineering, University of Science and Technology Liaoning, Anshan, 114051, P. R. China
| | - Zhiqiang Zhang
- School of Chemical Engineering, University of Science and Technology Liaoning, Anshan, 114051, P. R. China
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Harnying W, Sudkaow P, Biswas A, Berkessel A. N-Heterocyclic Carbene/Carboxylic Acid Co-Catalysis Enables Oxidative Esterification of Demanding Aldehydes/Enals, at Low Catalyst Loading. Angew Chem Int Ed Engl 2021; 60:19631-19636. [PMID: 34010504 PMCID: PMC8457137 DOI: 10.1002/anie.202104712] [Citation(s) in RCA: 16] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/06/2021] [Revised: 05/10/2021] [Indexed: 01/07/2023]
Abstract
We report the discovery that simple carboxylic acids, such as benzoic acid, boost the activity of N-heterocyclic carbene (NHC) catalysts in the oxidative esterification of aldehydes. A simple and efficient protocol for the transformation of a wide range of sterically hindered α- and β-substituted aliphatic aldehydes/enals, catalyzed by a novel and readily accessible N-Mes-/N-2,4,6-trichlorophenyl 1,2,4-triazolium salt, and benzoic acid as co-catalyst, was developed. A whole series of α/β-substituted aliphatic aldehydes/enals hitherto not amenable to NHC-catalyzed esterification could be reacted at typical catalyst loadings of 0.02-1.0 mol %. For benzaldehyde, even 0.005 mol % of NHC catalyst proved sufficient: the lowest value ever achieved in NHC catalysis. Preliminary studies point to carboxylic acid-induced acceleration of acyl transfer from azolium enolate intermediates as the mechanistic basis of the observed effect.
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Affiliation(s)
- Wacharee Harnying
- Department of Chemistry (Organic Chemistry), University of Cologne, Greinstraße 4, 50939, Cologne, Germany
| | - Panyapon Sudkaow
- Department of Chemistry (Organic Chemistry), University of Cologne, Greinstraße 4, 50939, Cologne, Germany
| | - Animesh Biswas
- Department of Chemistry (Organic Chemistry), University of Cologne, Greinstraße 4, 50939, Cologne, Germany
| | - Albrecht Berkessel
- Department of Chemistry (Organic Chemistry), University of Cologne, Greinstraße 4, 50939, Cologne, Germany
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Harnying W, Sudkaow P, Biswas A, Berkessel A. N‐Heterocyclic Carbene/Carboxylic Acid Co‐Catalysis Enables Oxidative Esterification of Demanding Aldehydes/Enals, at Low Catalyst Loading. Angew Chem Int Ed Engl 2021. [DOI: 10.1002/ange.202104712] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023]
Affiliation(s)
- Wacharee Harnying
- Department of Chemistry (Organic Chemistry) University of Cologne Greinstraße 4 50939 Cologne Germany
| | - Panyapon Sudkaow
- Department of Chemistry (Organic Chemistry) University of Cologne Greinstraße 4 50939 Cologne Germany
| | - Animesh Biswas
- Department of Chemistry (Organic Chemistry) University of Cologne Greinstraße 4 50939 Cologne Germany
| | - Albrecht Berkessel
- Department of Chemistry (Organic Chemistry) University of Cologne Greinstraße 4 50939 Cologne Germany
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Pareek M, Reddi Y, Sunoj RB. Tale of the Breslow intermediate, a central player in N-heterocyclic carbene organocatalysis: then and now. Chem Sci 2021; 12:7973-7992. [PMID: 34194690 PMCID: PMC8208132 DOI: 10.1039/d1sc01910d] [Citation(s) in RCA: 54] [Impact Index Per Article: 18.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/05/2021] [Accepted: 04/28/2021] [Indexed: 12/27/2022] Open
Abstract
N-Heterocyclic carbenes (NHCs) belong to the popular family of organocatalysts used in a wide range of reactions, including that for the synthesis of complex natural products and biologically active compounds. In their organocatalytic manifestation, NHCs are known to impart umpolung reactivity to aldehydes and ketones, which are then exploited in the generation of homoenolate, acyl anion, and enolate equivalents suitable for a plethora of reactions such as annulation, benzoin, Stetter, Claisen rearrangement, cycloaddition, and C-C and C-H bond functionalization reactions and so on. A common thread that runs through these NHC catalyzed reactions is the proposed involvement of an enaminol, also known as the Breslow intermediate, formed by the nucleophilic addition of an NHC to a carbonyl group of a suitable electrophile. In the emerging years of NHC catalysis, enaminol remained elusive and was largely considered a putative intermediate owing to the difficulties encountered in its isolation and characterization. However, in the last decade, synergistic efforts utilizing an array of computational and experimental techniques have helped in gaining important insights into the formation and characterization of Breslow intermediates. Computational studies have suggested that a direct 1,2-proton transfer within the initial zwitterionic intermediate, generated by the action of an NHC on the carbonyl carbon, is energetically prohibitive and hence the participation of other species capable of promoting an assisted proton transfer is more likely. The proton transfer assisted by additives (such as acids, bases, other species, or even a solvent) was found to ease the kinetics of formation of Breslow intermediates. These important details on the formation, in situ detection, isolation, and characterization of the Breslow intermediate are scattered over a series of reports spanning well over a decade, and we intend to consolidate them in this review and provide a critical assessment of these developments. Given the central role of the Breslow intermediate in organocatalytic reactions, this treatise is expected to serve as a valuable source of knowledge on the same.
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Affiliation(s)
- Monika Pareek
- Department of Chemistry, Indian Institute of Technology Bombay Powai Mumbai 400076 India
| | - Yernaidu Reddi
- Department of Chemistry, Indian Institute of Technology Bombay Powai Mumbai 400076 India
| | - Raghavan B Sunoj
- Department of Chemistry, Indian Institute of Technology Bombay Powai Mumbai 400076 India
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Biswas A, Neudörfl JM, Schlörer NE, Berkessel A. Acyl Donor Intermediates in N-Heterocyclic Carbene Catalysis: Acyl Azolium or Azolium Enolate? Angew Chem Int Ed Engl 2021; 60:4507-4511. [PMID: 33140529 PMCID: PMC7986403 DOI: 10.1002/anie.202010348] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2020] [Revised: 10/20/2020] [Indexed: 11/23/2022]
Abstract
Azolium enolates and acyl azolium cations have been proposed as intermediates in numerous N‐heterocyclic carbene (NHC) catalyzed transformations. Acetyl azolium enolates were generated from the reaction of 2‐propenyl acetate with both saturated (SIPr) and aromatic (IPr) NHCs, isolated, and characterized (NMR, XRD). Protonation with triflic acid gave the corresponding acetyl azolium triflates which were isolated and characterized (NMR, XRD). Acyl azolium cations have been proposed as immediate precursors of the ester product, for example, in the redox esterification of α,β‐enals. Studies with d3‐acetyl azolium triflate suggest that ester formation originates instead from an azolium enolate intermediate. Furthermore, the acetyl azolium enolate selectively reacted with alcohol nucleophiles in the presence of amines. While the acetyl azolium cation did not react with alcohols, an ester‐selective reaction was induced by addition of base, by intermediate formation of the acetyl azolium enolate.
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Affiliation(s)
- Animesh Biswas
- Department of Chemistry (Organic Chemistry), University of Cologne, Greinstraße 4, 50939, Cologne, Germany
| | - Jörg-M Neudörfl
- Department of Chemistry (Organic Chemistry), University of Cologne, Greinstraße 4, 50939, Cologne, Germany
| | - Nils E Schlörer
- Department of Chemistry (Organic Chemistry), University of Cologne, Greinstraße 4, 50939, Cologne, Germany
| | - Albrecht Berkessel
- Department of Chemistry (Organic Chemistry), University of Cologne, Greinstraße 4, 50939, Cologne, Germany
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