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Salvitti C, Pepi F, Managò M, Bortolami M, Michenzi C, Chiarotto I, Troiani A, de Petris G. Free N-heterocyclic carbenes from Brønsted acidic ionic liquids: Direct detection by electrospray ionization mass spectrometry. RAPID COMMUNICATIONS IN MASS SPECTROMETRY : RCM 2022; 36:e9338. [PMID: 35729083 PMCID: PMC9542177 DOI: 10.1002/rcm.9338] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 05/07/2022] [Revised: 06/17/2022] [Accepted: 06/17/2022] [Indexed: 06/15/2023]
Abstract
RATIONALE The occurrence of N-heterocyclic carbenes in imidazolium-based ionic liquids has long been discussed, but no spectroscopic evidence has been reported yet due to their transient nature. The insertion of an ionizable acid group into the cation scaffold of an ionic liquid which acts as a charge tag allows for the direct detection of free carbenes by mass spectrometry. METHODS Three different Brønsted acidic ionic liquids were synthesized: 1-methyl-3-carboxymethylimidazolium chloride (MAICl), 1-methyl-3-carboxymethylimidazolium acetate (MAIAc) and the corresponding 2-(3-methyl-1H-imidazol-3-ium-1-yl)acetate zwitterion (MAI - H). The speciation of these compounds was then analysed by electrospray ionization ion-trap mass spectrometry in the negative ion mode. RESULTS The C2-H deprotonation of the imidazolium cation leading to the formation of the corresponding carbene is highly affected by the basic properties of the counter-anion. In the case of MAICl and MAI - H ionic liquids, no charged species corresponding to the free N-heterocyclic carbene was detected. On the contrary, in the presence of a sufficiently basic anion, such as acetate of MAIAc ionic liquid, an intense signal related to the free carbenic species was observed without the addition of an external base. CONCLUSIONS In situ formation of free N-heterocyclic carbenes from Brønsted acidic ionic liquids was demonstrated, highlighting the crucial role of anion basicity in promoting the C2-H proton abstraction from imidazolium cations with a carboxylic side chain.
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Affiliation(s)
- Chiara Salvitti
- Dipartimento di Chimica e Tecnologie del FarmacoSapienza Università di RomaRomeItaly
| | - Federico Pepi
- Dipartimento di Chimica e Tecnologie del FarmacoSapienza Università di RomaRomeItaly
| | - Marta Managò
- Dipartimento di Chimica e Tecnologie del FarmacoSapienza Università di RomaRomeItaly
| | - Martina Bortolami
- Dipartimento di Scienze di Base e Applicate per l'IngegneriaSapienza Università di RomaRomeItaly
| | - Cinzia Michenzi
- Dipartimento di Scienze di Base e Applicate per l'IngegneriaSapienza Università di RomaRomeItaly
| | - Isabella Chiarotto
- Dipartimento di Scienze di Base e Applicate per l'IngegneriaSapienza Università di RomaRomeItaly
| | - Anna Troiani
- Dipartimento di Chimica e Tecnologie del FarmacoSapienza Università di RomaRomeItaly
| | - Giulia de Petris
- Dipartimento di Chimica e Tecnologie del FarmacoSapienza Università di RomaRomeItaly
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Vetica F, Bortolami M, Petrucci R, Rocco D, Feroci M. Electrogenerated NHCs in Organic Synthesis: Ionic Liquids vs Organic Solvents Effects. CHEM REC 2021; 21:2130-2147. [PMID: 33507627 DOI: 10.1002/tcr.202000178] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2020] [Revised: 01/08/2021] [Indexed: 12/14/2022]
Abstract
In the last twenty years, N-heterocyclic carbenes (NHCs) have been extensively studied for their application as organocatalysts in stereoselective synthesis as well as ligands for transition metals-promoted synthetic methodologies. Derived mainly from azolium salts, NHCs have demonstrated exceptional versatility in their generation usually performed by deprotonation or reduction (chemical or electrochemical). In particular, the generation of NHC under electrochemical conditions, starting from azolium-based ionic liquids, has proven to be a successful green approach and demonstrated wide applicability in organic synthesis. In this Personal Account, the application of electrogenerated NHCs in organic synthesis will be discussed, with a particular attention to the different reactivity in ionic liquids compared to classical organic solvents.
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Affiliation(s)
- Fabrizio Vetica
- Department of Chemistry, Sapienza University of Rome, Piazzale Aldo Moro 5, 00185, Rome, Italy
| | - Martina Bortolami
- Department of Basic and Applied Sciences for Engineering (SBAI), Sapienza University of Rome, Via Castro Laurenziano 7, 00161, Rome, Italy
| | - Rita Petrucci
- Department of Basic and Applied Sciences for Engineering (SBAI), Sapienza University of Rome, Via Castro Laurenziano 7, 00161, Rome, Italy
| | - Daniele Rocco
- Department of Basic and Applied Sciences for Engineering (SBAI), Sapienza University of Rome, Via Castro Laurenziano 7, 00161, Rome, Italy
| | - Marta Feroci
- Department of Basic and Applied Sciences for Engineering (SBAI), Sapienza University of Rome, Via Castro Laurenziano 7, 00161, Rome, Italy
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Amit E, Dery L, Dery S, Kim S, Roy A, Hu Q, Gutkin V, Eisenberg H, Stein T, Mandler D, Dean Toste F, Gross E. Electrochemical deposition of N-heterocyclic carbene monolayers on metal surfaces. Nat Commun 2020; 11:5714. [PMID: 33177496 PMCID: PMC7658200 DOI: 10.1038/s41467-020-19500-7] [Citation(s) in RCA: 25] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2020] [Accepted: 10/15/2020] [Indexed: 12/17/2022] Open
Abstract
N-heterocyclic carbenes (NHCs) have been widely utilized for the formation of self-assembled monolayers (SAMs) on various surfaces. The main methodologies for preparation of NHCs-based SAMs either requires inert atmosphere and strong base for deprotonation of imidazolium precursors or the use of specifically-synthesized precursors such as NHC(H)[HCO3] salts or NHC–CO2 adducts. Herein, we demonstrate an electrochemical approach for surface-anchoring of NHCs which overcomes the need for dry environment, addition of exogenous strong base or restricting synthetic steps. In the electrochemical deposition, water reduction reaction is used to generate high concentration of hydroxide ions in proximity to a metal electrode. Imidazolium cations were deprotonated by hydroxide ions, leading to carbenes formation that self-assembled on the electrode’s surface. SAMs of NO2-functionalized NHCs and dimethyl-benzimidazole were electrochemically deposited on Au films. SAMs of NHCs were also electrochemically deposited on Pt, Pd and Ag films, demonstrating the wide metal scope of this deposition technique. N-heterocyclic carbenes (NHCs) have been widely used for the formation of monolayers but self-assembly methods come with drawbacks such as need for dry environment or using specifically-synthesized precursors. Here, the authors demonstrate an approach for surface-anchoring of NHCs which overcomes these limitations by using electrochemically-assisted deprotonation.
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Affiliation(s)
- Einav Amit
- Institute of Chemistry, The Hebrew University, Jerusalem, 91904, Israel.,The Center for Nanoscience and Nanotechnology, The Hebrew University, Jerusalem, 91904, Israel
| | - Linoy Dery
- Institute of Chemistry, The Hebrew University, Jerusalem, 91904, Israel.,The Center for Nanoscience and Nanotechnology, The Hebrew University, Jerusalem, 91904, Israel
| | - Shahar Dery
- Institute of Chemistry, The Hebrew University, Jerusalem, 91904, Israel.,The Center for Nanoscience and Nanotechnology, The Hebrew University, Jerusalem, 91904, Israel
| | - Suhong Kim
- Department of Chemistry, University of California, Berkeley, CA, 94720, USA
| | - Anirban Roy
- Bruker Nano Surfaces Division, 112 Robin Hill Road, Santa Barbara, CA, 93117, USA
| | - Qichi Hu
- Bruker Nano Surfaces Division, 112 Robin Hill Road, Santa Barbara, CA, 93117, USA
| | - Vitaly Gutkin
- The Center for Nanoscience and Nanotechnology, The Hebrew University, Jerusalem, 91904, Israel
| | - Helen Eisenberg
- Institute of Chemistry, The Hebrew University, Jerusalem, 91904, Israel.,The Fritz Haber Center for Molecular Dynamics Research, The Hebrew University, Jerusalem, 91904, Israel
| | - Tamar Stein
- Institute of Chemistry, The Hebrew University, Jerusalem, 91904, Israel.,The Fritz Haber Center for Molecular Dynamics Research, The Hebrew University, Jerusalem, 91904, Israel
| | - Daniel Mandler
- Institute of Chemistry, The Hebrew University, Jerusalem, 91904, Israel.,The Center for Nanoscience and Nanotechnology, The Hebrew University, Jerusalem, 91904, Israel
| | - F Dean Toste
- Department of Chemistry, University of California, Berkeley, CA, 94720, USA
| | - Elad Gross
- Institute of Chemistry, The Hebrew University, Jerusalem, 91904, Israel. .,The Center for Nanoscience and Nanotechnology, The Hebrew University, Jerusalem, 91904, Israel.
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Chiarotto I, Mattiello L, Pandolfi F, Rocco D, Feroci M. NHC in Imidazolium Acetate Ionic Liquids: Actual or Potential Presence? Front Chem 2018; 6:355. [PMID: 30211149 PMCID: PMC6121013 DOI: 10.3389/fchem.2018.00355] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/19/2018] [Accepted: 07/30/2018] [Indexed: 11/13/2022] Open
Abstract
Ionic liquids (ILs) are considered in the majority of cases green solvents, due to their virtually null vapor pressure and to the easiness in recycling them. In particular, imidazolium ILs are widely used in many fields of Chemistry, as solvents or precursors of N-heterocyclic carbenes (NHCs). The latter are easily obtained by deprotonation of the C2-H, usually using strong bases or cathodic reduction. Nevertheless, it is known that weaker bases (e.g., triethylamine) are able to promote C2-H/D exchange. From this perspective, the possibility of deprotonating C2-H group of an imidazolium cation by means of a basic counter-ion was seriously considered and led to the synthesis of imidazolium ILs spontaneously containing NHCs. The most famous of this class of ILs are N,N'-disubstituted imidazolium acetates. Due to the particular reactivity of this kind of ILs, they were appointed as “organocatalytic ionic liquids” or “proto-carbenes.” Many papers report the use of these imidazolium acetates in organocatalytic reactions (i. e., catalyzed by NHC) or in stoichiometric NHC reactions (e.g., with elemental sulfur to yield the corresponding imidazole-2-thiones). Nevertheless, the actual presence of NHC in N,N'-disubstituted imidazolium acetate is still controversial. Moreover, theoretical studies seem to rule out the presence of NHC in such a polar environment as an IL. Aim of this Mini Review is to give the reader an up-to-date overview on the actual or potential presence of NHC in such an “organocatalytic ionic liquid,” both from the experimental and theoretical point of view, without the intent to be exhaustive on N,N'-disubstituted imidazolium acetate applications.
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Affiliation(s)
- Isabella Chiarotto
- Dipartimento di Scienze di Base e Applicate per l'Ingegneria, Sapienza University of Rome, Rome, Italy
| | - Leonardo Mattiello
- Dipartimento di Scienze di Base e Applicate per l'Ingegneria, Sapienza University of Rome, Rome, Italy
| | - Fabiana Pandolfi
- Dipartimento di Scienze di Base e Applicate per l'Ingegneria, Sapienza University of Rome, Rome, Italy
| | - Daniele Rocco
- Dipartimento di Scienze di Base e Applicate per l'Ingegneria, Sapienza University of Rome, Rome, Italy
| | - Marta Feroci
- Dipartimento di Scienze di Base e Applicate per l'Ingegneria, Sapienza University of Rome, Rome, Italy
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