N-Heterocyclic carbenes from ylides of indolyl-imidazolium, azaindolyl-imidazolium, and indolyl-triazolium salts, and their borane adducts

Fluorination often confers a range of advantages in modulating the conformation and reactivity of small molecule organocatalysts. By strategically introducing fluorine substituents, as part of a β-fluoroamine motif, in a triazolium pre-catalyst, it was possible to modulate the behaviour of the corresponding N-heterocyclic carbene (NHC) with minimal steric alterations to the catalyst core. In this study, the effect of hydrogen to fluorine substitution was evaluated as part of a molecular editing study. X-ray crystallographic analyses of a number of derivatives are presented and the conformations are discussed. Upon deprotonation, the fluorinated triazolium salts generate catalytically active N-heterocyclic carbenes, which can then participate in the enantioselective Steglich rearrangement of oxazolyl carbonates to C-carboxyazlactones (e.r. up to 87.0:13.0).

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Triazolium Salt Organocatalysis: Mechanistic Evaluation of Unusual Ortho-Substituent Effects on Deprotonation

Catalysts ◽

10.3390/catal11091055 ◽

2021 ◽

Vol 11 (9) ◽

pp. 1055

Author(s):

Peter Quinn ◽

Matthew S. Smith ◽

Jiayun Zhu ◽

David R. W. Hodgson ◽

AnnMarie C. O’Donoghue

Keyword(s):

Kinetic Data ◽

Kinetic Method ◽

Substituent Effects ◽

Catalytic Efficiency ◽

Heterocyclic Carbenes ◽

Pyridyl Nitrogen ◽

Conformational Preferences ◽

Catalytic Mechanisms ◽

Ortho Substituent ◽

Triazolium Salts

Organocatalysis by N-heterocyclic carbenes is normally initiated by the deprotonation of precursor azolium ions to form active nucleophilic species. Substituent effects on deprotonation have an impact on catalytic efficiency and provide insight into general catalytic mechanisms by commonly used azolium systems. Using an NMR kinetic method for the analysis of C(3)-H/D exchange, we determined log kex–pD profiles for three ortho-substituted N-aryl triazolium salts, which enables a detailed analysis of ortho-substituent effects on deprotonation. This includes N-5-methoxypyrid-2-yl triazolium salt 7 and di-ortho-methoxy and di-ortho-isopropoxyphenyl triazolium salts 8 and 9, and we acquired additional kinetic data to supplement our previously published analysis of N-pyrid-2-yl triazolium salt 6. For 2-pyridyl triazoliums 6 and 7, novel acid catalysis of C(3)-H/D exchange is observed under acidic conditions. These kinetic data were supplemented by DFT analyses of the conformational preferences of 6 upon N-protonation. A C(3) deprotonation mechanism involving intramolecular general base deprotonation by the pyridyl nitrogen of the N(1)-deuterated dicationic triazolium salt is most consistent with the data. We also report kDO values (protofugalities) for deuteroxide-catalyzed exchange for 6–9. The protofugalities for 8 and 9 are the lowest values to date in the N-aryl triazolium series.

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An Efficient Synthesis of Achiral and Chiral 1,2,4-Triazolium Salts: Bench Stable Precursors for N-Heterocyclic Carbenes

The Journal of Organic Chemistry ◽

10.1021/jo050645n ◽

2005 ◽

Vol 70 (14) ◽

pp. 5725-5728 ◽

Cited By ~ 199

Author(s):

Mark S. Kerr ◽

Javier Read de Alaniz ◽

Tomislav Rovis

Keyword(s):

Heterocyclic Carbenes ◽

Efficient Synthesis ◽

Triazolium Salts

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Synthetic Approaches to Chiral Non-C 2-symmetric N-Heterocyclic Carbene Precursors

Synthesis ◽

10.1055/s-0037-1611733 ◽

2019 ◽

Vol 51 (08) ◽

pp. 1689-1714 ◽

Cited By ~ 6

Author(s):

Paweł Czerwiński ◽

Michał Michalak

Keyword(s):

Triazole Ring ◽

Imidazole Ring ◽

Heterocyclic Carbenes ◽

Thiazole Ring ◽

Organic Transformations ◽

Synthetic Approaches ◽

First Time ◽

Thiazolium Ring ◽

The Ideal ◽

Triazolium Salts

N-Heterocyclic carbenes and their metal complexes have found applications in many organic transformations. Apart from the privileged C 2-symmetry present in modern enantioselective catalysis, ligands bearing C 1-symmetry have witnessed growing attention due to the better control of process stereoselectivity in many cases. The present review summarizes, for the first time, the seminal synthetic efforts for the preparation of N-heterocyclic carbene precursors exhibiting C 1-symmetry. The well-established methods will be discussed in the light of recent achievements, giving a direct opportunity for comparison of the existing methods, and simultaneously a chance to find the best synthetic pathway for the ideal chiral ligand.1 Introduction2 Five-Membered Rings2.1 Five-Membered Saturated (Imidazolinium) Ring2.1.1 Amino Alcohol Derivatives2.1.2 Amino Acid Derivatives2.1.3 Amine and Diamine Derivatives2.2 Five-Membered Unsaturated Ring2.2.1 Cyclization Strategy2.2.2 Functionalization of the Existing N-Imidazole Ring3 Triazolium Salts3.1 Substitution in Oxadiazolium Salts3.2 Lactam-Derived Triazolium NHC (The Leeper and Knight Methodology)3.3 Monoterpenoid-Derived NHC Triazolium Salts3.4 Modifications of the Existing Triazole Ring4 Thiazole-Derived Salts4.1 Cyclization of a Thiazolium Ring4.2 Modifications of the Existing Thiazole Ring5 Summary and Outlook

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Aryl-substituted Triarsiranes: Synthesis and Reactivity

10.26434/chemrxiv.13176842 ◽

2020 ◽

Author(s):

André Schumann ◽

Jonas Bresien ◽

Malte Fischer ◽

Christian Hering-Junghans

Keyword(s):

Organic Chemistry ◽

Electronic Structure ◽

Heterocyclic Carbenes ◽

Synthetic Approaches ◽

Inorganic And Organic

Cyclotriarsanes are rare and limited synthetic approaches have hampered reactivity studies on these systems. Described in here is a scalable synthetic protocol towards (AsAr)3 (Ar = Dip, 2,6-iPr2-C6H3; Tip, 2,4,6-iPr3-C6H2), which allowed to study their reactivity towards [Cp2Ti(C2(SiMe3)2], affording titanocene diarsene complexes and towards N-heterocyclic carbenes (NHCs) to give straightforward access to a variety of NHC-arsinidene adducts. The electronic structure of the titanium diarsene complxes has been studied and they are best described as Ti(IV) species with a doubly reduced As2Ar2 ligand. These findings will make (AsAr)3 valuable precursors in the synthetic inorganic and organic chemistry.

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Application of Nitrogen Heterocyclic Carbenes in Organocatalysis

Current Catalysis ◽

10.2174/2211544709999201201120617 ◽

2020 ◽

Vol 09 ◽

Author(s):

Minita Ojha ◽

R. K. Bansal

Keyword(s):

Asymmetric Catalysis ◽

Building Blocks ◽

Structural Features ◽

Heterocyclic Carbenes ◽

Stetter Reaction ◽

Metal Pollutants ◽

Synthetic Strategy ◽

Wide Range ◽

Benzoin Condensation ◽

Nitrogen Heterocyclic

Background: During the last two decades, horizon of research in the field of Nitrogen Heterocyclic Carbenes (NHC) has widened remarkably. NHCs have emerged as ubiquitous species having applications in a broad range of fields, including organocatalysis and organometallic chemistry. The NHC-induced non-asymmetric catalysis has turned out to be a really fruitful area of research in recent years. Methods: By manipulating structural features and selecting appropriate substituent groups, it has been possible to control the kinetic and thermodynamic stability of a wide range of NHCs, which can be tolerant to a variety of functional groups and can be used under mild conditions. NHCs are produced by different methods, such as deprotonation of Nalkylhetrocyclic salt, transmetallation, decarboxylation and electrochemical reduction. Results: The NHCs have been used successfully as catalysts for a wide range of reactions making a large number of building blocks and other useful compounds accessible. Some of these reactions are: benzoin condensation, Stetter reaction, Michael reaction, esterification, activation of esters, activation of isocyanides, polymerization, different cycloaddition reactions, isomerization, etc. The present review includes all these examples published during the last 10 years, i.e. from 2010 till date. Conclusion: The NHCs have emerged as versatile and powerful organocatalysts in synthetic organic chemistry. They provide the synthetic strategy which does not burden the environment with metal pollutants and thus fit in the Green Chemistry.

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