Mechanistic Elucidation of the Arylation of Non-Spectator N-Heterocyclic Carbenes at Copper Using a Combined Experimental and Computational Approach

Transition Metal-Driven Selectivity in Direct C-H Arylation of Imidazo[2,1-b]Thiazole

A selective direct arylation of the different Csp2-H bonds of imidazo[2,1-b]thiazole with (hetero) aryl halides can be achieved simply by switching from a palladium catalyst system to the use of stoichiometric amounts of copper. The observed selectivity, also rationalized by DFT calculations, can be explained by a change in the mechanistic pathways between electrophilic palladation and base-promoted C-H metalation.

Concatenating Structural Constraint Effects at Tin for the Sequential Generation, Stabilization, and Transfer of Acyclic Aminocarbenes

Structural constraint approaches have been employed toward different ends in recent years, from augmenting the nucleophilicity in pyramidalized low-valent p-block compounds to enhancing the Lewis acidities at planarized tetravalent p-block elements. While previous studies exploited these effects separately, this work introduces a strategy to concatenate structural constraint approaches at individual stages of a reaction sequence in a row to unlock a synthetic path unattainable by conventional methodologies. The boosted nucleophilicity resulting from the constrained tetracoordinated calix[4]pyrrolato stannate(II) dianion enables the reductive formation of sterically unprotected acyclic aminocarbenes. These amino carbenes are stabilized at the concomitantly formed square-planar stannane(IV) as air-stable adducts. Transfer of the carbenes onto copper(I) by cooperativity of the calix[4]pyrrole ligand finalizes this protocol to hitherto unreported yet prototypical carbene complexes. Detailed spectroscopic and quantum theoretical analyses establish the synergy of structural constraints and element-ligand cooperation as the linchpin to this reaction path and its selectivity.

Benzimidazolium quaternary ammonium salts: synthesis, single crystal and Hirshfeld surface exploration supported by theoretical analysis

Owing to the broad applications of quaternary ammonium salts (QAS), we present the synthesis of benzimidazolium-based analogues with variation in the alkyl and alkoxy group at N-1 and N-3 positions. All the compounds were characterized by spectroscopic techniques and found stable to air and moisture both in the solid and solution state. Moreover, molecular structures were established through single-crystal X-ray diffraction studies. The crystal packing of the compounds was stabilized by numerous intermolecular interactions explored by Hirshfeld surface analysis. The enrichment ratio was calculated for the pairs of chemical species to acquire the highest propensity to form contacts. Void analysis was carried out to check the mechanical response of the compounds. Furthermore, theoretical investigations were also performed to explore the optoelectronic properties of compounds. Natural population analysis (NPA) has been conducted to evaluate the distribution of charges on the synthesized compounds, whereas high band gaps of the synthesized compounds by frontier molecular orbital (FMO) analysis indicated their stability. Nonlinear optical (NLO) analysis revealed that the synthesized QAS demonstrates significantly improved NLO behaviour than the standard urea.

The Facile Hydrolysis of Imidazolinium Chlorides (N-Heterocyclic Carbene Precursors) Under Basic Aqueous Conditions

The hydrolysis of imidazolinium chlorides takes place readily in a basic water/dichloromethane biphasic mixture at room temperature. Experimental parameters were optimized to afford full conversions and high yields of γ-aminoformamides starting from twelve symmetrical substrates with alkyl or aryl substituents on their nitrogen atoms, and five unsymmetrical 1-alkyl-3-arylimidazolinium chlorides. NMR and XRD analyses showed that the cleavage of unsymmetrical salts led to γ-alkylamino-N-arylformamides with a high regioselectivity and that bulky alkyl or aryl groups on the formamide moiety led to the isolation of the (E)-isomer in high stereoisomeric purity (>95 %), whereas smaller and more flexible alkyl substituents afforded mixtures of (E)- and (Z)-rotamers. Control experiments showed that the hydrolysis of 1,3-dimesitylimidazolinium chloride (SIMes ⋅ HCl) did not occur readily in pure or acidic water and that the presence of bulky aromatic substituents on the nitrogen atoms of 1,3-bis(2,6-diisopropylphenyl)imidazolinium chloride (SIDip ⋅ HCl) efficiently slowed down its hydrolysis under basic aqueous conditions. Most strikingly, this work highlighted the critical influence of the counteranion on the reactivity of imidazolinium cations. Indeed, the chloride salts underwent a facile hydrolysis in the presence of water and Na2 CO3 , whereas various other NHC ⋅ HX derivatives reacted much slower or remained essentially inert under these conditions.

Novel synthetic procedures for C2 substituted imidazoquinolines as ligands for the α/β-interface of the GABAA-receptor

A series of substituted imidazoquinolines, a structurally related chemotype to pyrazoloquinolinones, a well-known class of GABAA ligands, was prepared via two synthetic procedures and the efficiency of these procedures were compared. One method relies on classical heterocyclic synthesis, the other one aims at late-stage decoration of a truncated scaffold via direct C-H functionalization. A pharmacological evaluation disclosed that one of the synthesized derivatives showed interesting activity on a α1β3 containing receptor subtype.

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Supplementary Information

The online version contains supplementary material available at 10.1007/s00706-022-02988-8.

Copper(I) activation of C-X bonds: bimolecular vs. unimolecular reaction mechanism

Experimental kinetic studies and DFT calculations show that the oxidative addition of aryl halides (Ar-X) to complexes [Cu(NHC)R] follow different paths depending on the nature of X. For X = Br a concerted addition leads to cis-[Cu(NHC)XRAr] from which the usual C-C coupled product Ar-R eliminates. However, for X = I trans-[Cu(NHC)IRAr] is formed instead, leading to the elimination of R-I in a metathesis reaction. This behaviour is accounted for by a change in the reaction mechanism for Ar-I, which involves two molecules of copper(I) complex, the second one stabilising the incipient iodide formed in the C-I breaking (oxidative addition) and C-I forming (reductive elimination) processes.

Cu-Catalyzed P-C bond formation/cleavage: straightforward synthesis/ring-expansion of strained cyclic phosphoniums

Upon reaction with copper(i), peri-halo naphthyl phosphines readily form peri-bridged naphthyl phosphonium salts. The reaction works with alkyl, aryl and amino substituents at phosphorus, with iodine, bromine and chlorine as a halogen. It proceeds under mild conditions and is quantitative, despite the strain associated with the resulting 4-membered ring structure and the naphthalene framework. The transformation is amenable to catalysis. Under optimized conditions, the peri-iodo naphthyl phosphine 1-I is converted into the corresponding peri-bridged naphthyl phosphonium salt 2b in only 5 minutes at room temperature using 1 mol% of CuI. Based on DFT calculations, the reaction is proposed to involve a Cu(i)/Cu(iii) cycle made of P-coordination, C-X oxidative addition and P-C reductive elimination. This copper-catalyzed route gives a general and efficient access to peri-bridged naphthyl phosphonium salts for the first time. Reactivity studies could thus be initiated and the possibility to insert gold into the strained P-C bond was demonstrated. It leads to (P,C)-cyclometallated gold(iii) complexes. According to experimental observations and DFT calculations, two mechanistic pathways are operating: (i) direct oxidative addition of the strained P-C bond to gold,(ii) backward-formation of the peri-halo naphthyl phosphine (by C-P oxidative addition to copper followed by C-X reductive elimination), copper to gold exchange and oxidative addition of the C-X bond to gold. Detailed analysis of the reaction profiles computed theoretically gives more insight into the influence of the nature of the solvent and halogen atom, and provides rationale for the very different behaviour of copper and gold in this chemistry.

The key role of R-NHC coupling (R = C, H, heteroatom) and M-NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species

Complexes of metals with N-heterocyclic carbene ligands (M/NHC) are typically considered the systems of choice in homogeneous catalysis due to their stable metal-ligand framework. However, it becomes obvious that even metal species with a strong M-NHC bond can undergo evolution in catalytic systems, and processes of M-NHC bond cleavage are common for different metals and NHC ligands. This review is focused on the main types of the M-NHC bond cleavage reactions and their impact on activity and stability of M/NHC catalytic systems. For the first time, we consider these processes in terms of NHC-connected and NHC-disconnected active species derived from M/NHC precatalysts and classify them as fundamentally different types of catalysts. Problems of rational catalyst design and sustainability issues are discussed in the context of the two different types of M/NHC catalysis mechanisms.

NHC-Copper Mediated Ligand-Directed Radiofluorination of Aryl Halides

[¹⁸F]-labeled aryl fluorides are widely used as radiotracers for positron emission tomography (PET) imaging. Aryl halides (ArX) are particularly attractive precursors to these radiotracers, as they are readily available, inexpensive, and stable. However, to date, the direct preparation of [¹⁸F]-aryl fluorides from aryl halides remains limited to S_NAr reactions between highly activated ArX substrates and K¹⁸F. This report describes an aryl halide radiofluorination reaction in which the C(sp²)-¹⁸F bond is formed via a copper-mediated pathway. Copper N-heterocyclic carbene complexes serve as mediators for this transformation, using aryl halide substrates with directing groups at the ortho position. This reaction is applied to the radiofluorination of electronically diverse aryl halide derivatives, including the bioactive molecules vismodegib and PH089.

An Adaptable N-Heterocyclic Carbene Macrocycle Hosting Copper in Three Oxidation States

A neutral hybrid macrocycle with two trans-positioned N-heterocyclic carbenes (NHCs) and two pyridine donors hosts copper in three oxidation states (+I-+III) in a series of structurally characterized complexes (1-3). Redox interconversion of [LCu]+/2+/3+ is electrochemically (quasi)reversible and occurs at moderate potentials (E1/2 =-0.45 V and +0.82 V (vs. Fc/Fc+ )). A linear CNHC -Cu-CNHC arrangement and hemilability of the two pyridine donors allows the ligand to adapt to the different stereoelectronic and coordination requirements of CuI versus CuII /CuIII . Analytical methods such as NMR, UV/Vis, IR, electron paramagnetic resonance, and Cu Kβ high-energy-resolution fluorescence detection X-ray absorption spectroscopies, as well as DFT calculations, give insight into the geometric and electronic structures of the complexes. The XAS signatures of 1-3 are textbook examples for CuI , CuII , and CuIII species. Facile 2-electron interconversion combined with the exposure of two basic pyridine N sites in the reduced CuI form suggest that [LCu]+/2+/3+ may operate in catalysis via coupled 2 e- /2 H+ transfer.

Transition-metal-catalyzed reactions involving reductive elimination between dative ligands and covalent ligands

This review summarizes transition-metal catalyzed reactions with reductive elimination between covalent ligands and dative ligands as the key elementary step.

The C-H functionalization of organic cations: an interesting and fresh journey

Organic ionic compounds, especially those with organic cations, are commonly applied in ionic liquids (ILs), organocatalysts, (a)NHC ligands, ion recognition, and optoelectronic materials. The direct C-H functionalization of organic cations offers valuable opportunities for the rapid assembly of diverse functionalized cations and for their further exploitation in material science applications. This review summarizes the substantial progress that has been made in the C-H functionalization of organic cations from the 1960s to May 2020, including transition metal-mediated/catalyzed C-H alkylation, arylation, and annulation, and photo-induced C-H functionalization. Substrate scopes, limitations, regio-/chemoselectivity, and reaction mechanisms are discussed. In addition, the applications of some new organic functional materials are briefly exemplified. This review also aims to serve as a reminder that much care should be taken when using organic ionic compounds as solvents, because they can behave as reactants that can break up desired coupling reactions.

Domino N-/C- or N-/N-/C-arylation of imidazoles to yield polyaryl imidazolium salts via atom-economical use of diaryliodonium salts

Herein, we disclose a Cu-mediated domino di-/triarylation reaction of imidazoles to efficiently access polyaryl imidazolium salts in a single step by using two aryls as well as an anion of a diaryliodonium salt. The diarylation shows high atom economy and excellent selectivity with unsymmetrical iodonium salts.

High-Oxidation-State 3d Metal (Ti-Cu) Complexes with N-Heterocyclic Carbene Ligation

High-oxidation-state 3d metal species have found a wide range of applications in modern synthetic chemistry and materials science. They are also implicated as key reactive species in biological reactions. These applications have thus prompted explorations of their formation, structure, and properties. While the traditional wisdom regarding these species was gained mainly from complexes supported by nitrogen- and oxygen-donor ligands, recent studies with N-heterocyclic carbenes (NHCs), which are widely used for the preparation of low-oxidation-state transition metal complexes in organometallic chemistry, have led to the preparation of a large variety of isolable high-oxidation-state 3d metal complexes with NHC ligation. Since the first report in this area in the 1990s, isolable complexes of this type have been reported for titanium(IV), vanadium(IV,V), chromium(IV,V), manganese(IV,V), iron(III,IV,V), cobalt(III,IV,V), nickel(IV), and copper(II). With the aim of providing an overview of this intriguing field, this Review summarizes our current understanding of the synthetic methods, structure and spectroscopic features, reactivity, and catalytic applications of high-oxidation-state 3d metal NHC complexes of titanium to copper. In addition to this progress, factors affecting the stability and reactivity of high-oxidation-state 3d metal NHC species are also presented, as well as perspectives on future efforts.

Cu-catalyzed controllable C–H mono-/di-/triarylations of imidazolium salts for ionic functional materials

The first Cu-catalyzed direct C–H mono-, di- and triarylations of imidazolium salts with aryl iodides/bromides are reported.

Theoretical rationalisation for the mechanism of N-heterocyclic carbene-halide reductive elimination at CuIII, AgIII and AuIII

A dichotomy in carbon versus bromide reductive elimination in Cu^III–NHC complexes is rationalized by computational methods (DFT).