Methoxy-Functionalized N-Heterocyclic Carbenes

Chiral NHC Ligands for Enantioselective Gold(I) Catalysis Under Aerobic Conditions: the Importance of Conformational Flexibility and Steric Hindrance of NHC Ligand on Reactivity

Gold(I) catalysis has been recognized as a valuable tool for the unique transformation of multiple carbon-carbon bonds. Enantioselective π-catalysis based on gold(I) complexes is, however, still underdeveloped due to lack of privileged ligands. Herein, we present an accessible method to a new family of stable yet catalytically active chiral NHC-Au(I)-Cl complexes. The key to preserving a simultaneous fine balance between reactivity and stability in this newly developed family appears to be sterically hindered, but conformationally flexible NHC ligands. These could be easily accessed on a multigram scale by merging sterically hindered anilines with commercially available amino alcohols and amines via a four-steps synthetic sequence without the need for chromatographic purification. Further investigations of the catalytic activity of NHC-Au-Cl complexes identified the OH functionality incorporated into the NHC core as crucial for the level of enantioselectivity as well as the TsO- anion responsible for the activation of NHC-Au(I)-Cl. Finally, NMR studies and X-ray investigations revealed for the first time that the widely accepted ion metathesis (NHC-Au-Cl to NHC-Au-OSO2 R) responsible for the activation of NHC-Au-Cl complexes does not take place (or it is very slow) in commonly used MeNO2 in contrast to DCM.

Nickel-catalyzed direct stereoselective α-allylation of ketones with non-conjugated dienes

The development of efficient and sustainable methods for the construction of carbon-carbon bonds with the simultaneous stereoselective generation of vicinal stereogenic centers is a longstanding goal in organic chemistry. Low-valent nickel(0) complexes which promote α-functionalization of carbonyls leveraging its pro-nucleophilic character in conjunction with suitable olefin acceptors are scarce. We report a Ni(0)NHC catalyst which selectively converts ketones and non-conjugated dienes to synthetically highly valuable α-allylated products. The catalyst directly activates the α-hydrogen atom of the carbonyl substrate transferring it to the olefin acceptor. The transformation creates adjacent quaternary and tertiary stereogenic centers in a highly diastereoselective and enantioselective manner. Computational studies indicate the ability of the Ni(0)NHC catalyst to trigger a ligand-to-ligand hydrogen transfer process from the ketone α-hydrogen atom to the olefin substrate, setting the selectivity of the process. The shown selective functionalization of the α-C-H bond of carbonyl groups by the Ni(0)NHC catalyst opens up new opportunities to exploit sustainable 3d-metal catalysis for a stereoselective access to valuable chiral building blocks.

New class of RSO2-NHC ligands and Pd/RSO2-NHC complexes with tailored electronic properties and high performance in catalytic C-C and C-N bonds formation

Imidazolium salts have found ubiquitous applications as N-heterocyclic carbene precursors and metal nanoparticle stabilizers in catalysis and metallodrug research. Substituents directly attached to the imidazole ring can have a significant influence on the electronic, steric, and other properties of NHC-proligands as well as their metal complexes. In the present study, for the first time, a new type of Pd/NHC complex with the RSO2 group directly attached to the imidazol-2-ylidene ligand core was designed and synthesized. The electronic properties as well as structural features of the new ligands were evaluated by means of experimental and computational methods. Interestingly, the introduction of a 4-aryl(alkyl)sulfonyl group only slightly decreased the electron donation, but it significantly increased the π-acceptance and slightly enhanced the buried volume (%Vbur) of new imidazol-2-ylidenes. New Pd/NHC complexes were obtained through selective C(2)H-palladation of some of the synthesized 4-RSO2-functionalized imidazolium salts under mild conditions. Several complexes demonstrated good activity in the catalysis of model cross-coupling reactions, outperforming the activity of similar complexes with non-substituted NHC ligands.

Synthesis, structural studies and computational evaluation of cyclophanes incorporating imidazole-2-selones

We report new cyclophanes containing imidazole-2-selone groups linked by xylylene rings. A set of imidazole-2-selone cyclophanes is synthesized by reaction corresponding to imidazolium cyclophanes with selenium in the presence of K₂CO₃. The structural behavior of the new imidazole-2-selone cyclophanes was determined by ¹H and ¹³C NMR spectra and X-ray diffraction studies. Cyclophanes incorporating o-xylylene or mesitylene-m-cyclophane linked by selone groups were mutually syn in both the solid state and solution, and the cyclophanes showed a conformation similar to the cone conformation of calix[4]arenes. Cyclophanes incorporating p-xylylene or m-xylylene linked by selone groups showed two conformations in the solution: one mutually syn and the other mutually anti. There was no interconversion for both conformations observed on the NMR timescale. In the solid state, three conformations were detected for the p-xylylene-linked cyclophane: one is mutually syn and the other two are mutually anti and partial cone conformations. In the m-xylylene-linked case, only anti-conformation was characterized in the solid state. A density functional analysis was conducted to interpret the stability of the studied compounds and shed light on their origin. The energy preference analysis is in consistent agreement with the observed geometries and their co-existence.

(NHC)Pd(II) hydride-catalyzed dehydroaromatization by olefin chain-walking isomerization and transfer-dehydrogenation

Transition-metal-catalyzed homogeneous dehydrogenation and isomerization are common organic molecular activation reactions. Palladium hydrides are good olefin isomerization catalysts but are usually short-lived species under redox-active dehydrogenation conditions. Here, we show that Pd-H in the presence of an N-heterocyclic carbene ligand and an alkene regulator enables transfer-dehydroaromatization, avoiding the homo-disproportionation pathway. The desired product is obtained with up to 99:1 selectivity, and the exo-to-endo olefin isomerization can be carried out in one pot. In contrast to previously reported methods that rely on the efficient removal of Pd-H, the approach reported herein benefits from the steric effects of the N-heterocyclic carbene and the choice of alkene to regulate the competing reactivity of allylic C‒H activation and hydropalladation. This method circumvents the challenges associated with tedious olefin separation and a low exo-to-endo olefin isomerization ratio and expands the scope to include challenging endo- and exo-cyclic olefins under mild, neutral, and oxidant-free conditions. Overall, herein, we provide a strategy to synthesize (hetero)aromatic compounds via chemoselective dehydrogenation of cyclic alkenes over ketones and the dehydrogenative Diels-Alder reaction of a cyclic enamine.

Aromatic compounds can be prepared via dehydrogenation of cyclic compounds. Here the authors report the dehydroaromatization of endocyclic and exocyclic olefins via chain-walking isomerization and transfer-dehydrogenation catalyzed by palladium N-heterocyclic carbene complexes in the presence of alkenes as sacrificial reagents.

Reactions of N-heterocyclic carbene-based chalcogenoureas with halogens: a diverse range of outcomes

We have investigated the reactions of chalcogenoureas derived from N-heterocyclic carbenes, referred to here as [E(NHC)], with halogens. Depending on the structure of the chalcogenourea and the identity of the halogen, a diverse range of reactivity was observed and a corresponding range of structures was obtained. Cyclic voltammetry was carried out to characterise the oxidation and reduction potentials of these [E(NHC)] species; selenoureas were found to be easier to oxidise than the corresponding thioureas. In some cases, a correlation was found between the oxidation potential of these compounds and the electronic properties of the corresponding NHC. The reactivity of these chalcogenoureas with different halogenating reagents (Br₂, SO₂Cl₂, I₂) was then investigated, and products were characterised using NMR spectroscopy and single-crystal X-ray diffraction. X-ray analyses elucidated the solid-state coordination types of the obtained products, showing that a variety of possible adducts can be obtained. In some cases, we were able to extrapolate a structure/activity correlation to explain the observed trends in reactivity and oxidation potentials.

Rhodium and Iridium Complexes of Anionic Thione and Selone Ligands Derived from Anionic N-Heterocyclic Carbenes

The lithium salts of anionic N‐heterocyclic thiones and selones [{(WCA‐IDipp)E}Li(toluene)] (1: E=S; 2: E=Se; WCA=B(C₆F₅)₃, IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene), which contain a weakly coordinating anionic (WCA) borate moiety in the imidazole backbone were reacted with Me₃SiCl, to furnish the silylated adducts (WCA‐IDipp)ESiMe₃ (3: E=S; 4: E=Se). The reaction of the latter with [(η⁵‐C₅Me₅)MCl₂]₂ (M=Rh, Ir) afforded the rhodium(III) and iridium(III) half‐sandwich complexes [{(WCA‐IDipp)E}MCl(η⁵‐C₅Me₅)] (5–8). The direct reaction of the lithium salts 1 and 2 with a half or a full equivalent of [M(COD)Cl]₂ (M=Rh, Ir) afforded the monometallic complexes [{(WCA‐IDipp)E}M(COD)] (9–12) or the bimetallic complexes [μ₂‐{(WCA‐IDipp)E}M₂(COD)₂(μ₂‐Cl)] (13–16), respectively. The bonding situation in these complexes has been investigated by means of density functional theory (DFT) calculations, revealing thiolate or selenolate ligand character with negligible metal‐chalcogen π‐interaction.

Recent advances in the synthesis and derivatization of N-heterocyclic carbene metal complexes

N-heterocyclic carbene (NHC) metal complexes have gained an incredible amount of attention in the course of the last two decades and have become indispensable as an intricate part of a plethora of applications. The areas of their synthesis and derivatization are constantly evolving and bring new, more sustainable, cost-effective and simpler approaches to the design of existing and next generation catalysts and materials. This article provides an overview of the latest developments, focusing on those which have appeared during the last two years.

Accelerating the insertion reactions of (NHC)Cu-H via remote ligand functionalization

Most ligand designs for reactions catalyzed by (NHC)Cu-H (NHC = N-heterocyclic carbene ligand) have focused on introducing steric bulk near the Cu center. Here, we evaluate the effect of remote ligand modification in a series of [(NHC)CuH]2 in which the para substituent (R) on the N-aryl groups of the NHC is Me, Et, t Bu, OMe or Cl. Although the R group is distant (6 bonds away) from the reactive Cu center, the complexes have different spectroscopic signatures. Kinetics studies of the insertion of ketone, aldimine, alkyne, and unactivated α-olefin substrates reveal that Cu-H complexes with bulky or electron-rich R groups undergo faster substrate insertion. The predominant cause of this phenomenon is destabilization of the [(NHC)CuH]2 dimer relative to the (NHC)Cu-H monomer, resulting in faster formation of Cu-H monomer. These findings indicate that remote functionalization of NHCs is a compelling strategy for accelerating the rate of substrate insertion with Cu-H species.

Recent advances in computational liquid-phase 77Se NMR

This review aims to highlight significant progress in the calculation of 77Se NMR chemical shifts and spin – spin coupling constants involving selenium substantiated with a vast amount of experimental data. The material is arranged in two basic sections: the first one dealing with the calculation of 77Se NMR chemical shifts and the second one dealing with the computation of spin – spin coupling constants involving 77Se nucleus, namely 77Se–1H, 77Se–13C and 77Se–77Se together with some more exotic types of couplings, 77Se – 15N, 77Se–19F, 77Se–29Si and 77Se–31P. A special attention is focused on the stereoelectronic effects involving selenium atom and their manifestation in the 77Se NMR spectra of organoselenium compounds studied with the aid of the modern calculation of 77Se NMR parametres in combination with experimental results.

The bibliography includes 114 references.

A tropylium annulated N-heterocyclic carbene

Derivatives of the cationic tropylium annulated imidazolylidene ITrop⁺ are obtained by hydride abstraction from related cycloheptatriene compounds. Spectroscopic, structural and theoretical data indicate that, as a cationic relative of benzimidazolylidenes, ITrop⁺ has highly reduced σ-donor and strong π-acceptor character.

A Mechanistically and Operationally Simple Route to Metal-N-Heterocyclic Carbene (NHC) Complexes

We have been puzzled by the involvement of weak organic and inorganic bases in the synthesis of metal-N-heterocyclic carbene (NHC) complexes. Such bases are insufficiently strong to permit the presumed required deprotonation of the azolium salt (the carbene precursor) prior to metal binding. Experimental and computational studies provide support for a base-assisted concerted process that does not require free NHC formation. The synthetic protocol was found applicable to a number of transition-metal- and main-group-centered NHC compounds and could become the synthetic route of choice to form M-NHC bonds.

C. A. Bayrakdar T, Nolan SP, Nahra F, Van Hecke K. Straightforward access to chalcogenoureas derived from N-heterocyclic carbenes and their coordination chemistry

Herein, we describe a straightforward access to chalcogenoureas derived from N-heterocyclic carbenes, and we investigate the coordination chemistry of selenoureas with coinage metals.

Solving the challenging synthesis of highly cytotoxic silver complexes bearing sterically hindered NHC ligands with mechanochemistry

Challenging silver(i) NHC complexes displaying cytotoxicity 256 times higher than cisplatin were synthesized by ball-milling.

Chalcogen complexes of anionic N-heterocyclic carbenes

Chalcogen complexes of anionic N-heterocyclic carbenes were isolated as lithium salts and their protonation and oxidation were studied.

Evaluation of benzthiazolidine-based formamidinium salts for the synthesis of penem-type β-lactams by uncatalysed carbonylation of acyclic diaminocarbenes

The reaction of [iPr2N=CHCl]Cl with 2-phenylbenzthiazolidine (Ph-BtzH) and subsequent anion metathesis afforded the formamidinium salt [(Ph-Btz)CH(NiPr2)][PF6] ([1aH][PF6]). The reaction of [1aH][PF6] with (Me3Si)2NNa in the presence of grey selenium furnished the selenourea derivative 1aSe as trapping product of the acyclic diaminocarbene 1a. In the absence of selenium, a 2H-1,4-benzothiazine derivative (2) was obtained, which is an isomer of 1a, whose formation is plausibly initiated by deprotonation of [1aH]+ not at the cationic N2CH unit, but at the neighbouring PhCH position. CO was ineffective as trapping reagent for 1a. [1aH][PF6], 1aSe and 2 have been structurally characterised by X-ray diffraction.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Stable and Persistent Acyclic Diaminocarbenes with Cycloalkyl Substituents and Their Transformation to β-Lactams by Uncatalysed Carbonylation with CO

Four new acyclic diaminocarbenes (ADACs), viz. [(cyclo-C_n H_2n-1 )₂ N]₂ C (n=5-7) and iPr₂ N-C-N(cyclo-C₆ H₁₁ )₂ , were synthesised by reacting the corresponding formamidinium hexafluorophosphates with NaN(SiMe₃ )₂ . Their nucleophilicities and electrophilicities were respectively judged from the ¹ J_CH values determined for the N₂ CH unit of the corresponding formamidinium cations and from the ⁷⁷ Se NMR chemical shifts of the selenourea derivatives obtained from the reaction of elemental selenium with the corresponding ADACs. An ambiphilic profile essentially identical to that of the "Alder carbene" (iPr₂ N)₂ C was found in each case. Similar to the latter carbene, the new ADACs undergo a well-defined thermal decomposition by β-fragmentation, affording an alkene and a formamidine. The stabilities of [(cyclo-C_n H_2n-1 )₂ N]₂ C depend strongly on the value of n, following the order 6>5>7, with the latter congener being too unstable for isolation. [(cyclo-C₆ H₁₁ )₂ N]₂ C shows no thermal decomposition at room temperature in solution and is thus significantly more stable than (iPr₂ N)₂ C. The stability of iPr₂ N-C-N(cyclo-C₆ H₁₁ )₂ is intermediate between that of (iPr₂ N)₂ C and [(cyclo-C₆ H₁₁ )₂ N]₂ C, its β-fragmentation selectively affording propene and iPrN=CH-N(cyclo-C₆ H₁₁ )₂ . [(cyclo-C_n H_2n-1 )₂ N]₂ C (n=5-7) react readily with CO under mild conditions, selectively affording trisubstituted spirocyclic β-lactam derivatives with an antimicrobial activity spectrum similar to that of penicillin G.

Quantifying and understanding the steric properties of N-heterocyclic carbenes

This Feature Article presents and discusses the use of different methods to quantify and explore the steric impact of N-heterocyclic carbene (NHC) ligands. These include (a) the percent buried volume (%V_bur), which provides a convenient single number to measure steric impact and (b) steric maps, which provide a graphical representation of the steric profile of a ligand using colour-coded contour maps. A critical discussion of the scope and limitations of these tools is presented, along with some examples of their use in organometallic chemistry and catalysis. This Article should provide all users of NHCs, from organic, organometallic, and inorganic chemistry backgrounds, with an appreciation of how these tools can be used to quantify and compare their steric properties.

New insights in Au-NHCs complexes as anticancer agents

Within the research field of antitumor metal-based agents alternative to platinum drugs, gold(I/III) coordination complexes have always been in the forefront due mainly to the familiarity of medicinal chemists with gold compounds, whose application in medicine goes back in the ancient times, and to the rich chemistry shown by this metal. In the last decade, N-heterocyclic carbene ligands (NHC), a class of ligands that largely resembles the chemical properties of phosphines, became of interest for gold(I) medicinal applications, and since then, the research on NHC-gold(I/III) coordination complexes as potential antiproliferative agents boosted dramatically. Different classes of gold(I/III)-NHC complexes often showed an outstanding in vitro antiproliferative activity, however up to now very few in vivo data have been reported to corroborate the in vitro results. This review summarizes all achievements in the field of gold (I/III) complexes comprising NHC ligands proposed as potential antiproliferative agents in the period 2004-2016, and critically analyses biological data (mainly IC₅₀ values) in relation to the chemical structures of Au compounds. The state of art of the in vivo studies so far described is also reported.

Role of C, S, Se and P donor ligands in copper(i) mediated C–N and C–Si bond formation reactions

The first comparative study of C, S, Se and P donor ligands-supported copper(i) complexes for C–N and C–Si bond formation reactions are described. The syntheses and characterization of eight mononuclear copper(i) chalcogenone complexes, two polynuclear copper(i) chalcogenone complexes and one tetranuclear copper(i) phosphine complex are reported. All these new complexes were characterized by CHN analysis, FT-IR, UV-vis, multinuclear NMR and single crystal X-ray diffraction techniques. The single crystal X-ray structures of these complexes depict the existence of a wide range of coordination environments for the copper(i) center. This is the first comparative study of metal–phosphine, metal–NHC and metal–imidazolin-2-chalcogenones in C–N and C–Si bond formation reactions. Among all the catalysts, mononuclear copper(i) thione, mononuclear copper(i) N-heterocyclic carbene and tetranuclear copper(i) phosphine are exceedingly active towards the synthesis of 1,2,3-triazoles as well as for the cross-dehydrogenative coupling of alkynes with silanes. The cross-dehydrogenative coupling of terminal alkynes with silanes represents the first report of a catalytic process mediated by metal–imidazolin-2-chalcogenones.

The first comparative study of C, S, Se and P donor ligands-supported copper(i) complexes for C–N and C–Si bond formation reactions.

Coinage metal complexes of selenoureas derived from N-heterocyclic carbenes

The coordination chemistry of selenoureas derived from N-heterocyclic carbenes with copper and silver is explored, and compared to previous work with gold.

Synthesis and complexation of superbulky imidazolium-2-dithiocarboxylate ligands

The superbulky N-heterocyclic carbenes (NHCs) 2,6-bis(diphenylmethyl)-4-methylimidazol-2-ylidene (IDip*Me) and its 4-methoxy analogue (IDip*OMe) reacted instantaneously with carbon disulfide to afford the corresponding imidazolium-2-dithiocarboxylate zwitterions in high yields. These new dithiolate ligands were fully characterized and their coordination chemistry toward common Re(i) and Ru(ii) metal sources was thoroughly investigated. Neutral [ReBr(CO)₃(S₂C·NHC)] chelates featured three facially-arranged carbonyl groups on a distorted octahedron, whereas cationic [RuCl(p-cymene)(S₂C·NHC)]PF₆ complexes displayed a piano-stool geometry. The molecular structures of the six new compounds revealed that the NHC·CS₂ inner salts were highly flexible. Indeed, the torsion angle between their anionic and cationic moieties varied between ca. 63° in the free ligands and 3° in the ruthenium complexes. Concomitantly, the S-C-S bite angle underwent a contraction from 131° to 110-113° upon chelation. Computation of the %V_Bur parameter showed that the dithiocarboxylate unit of the NHC·CS₂ betaines chiefly determined the steric requirements of the imidazolium moieties, irrespective of the metal center involved in the complexation. The replacement of the p-methyl substituents of IDip*Me with p-methoxy groups in IDip*OMe did not significantly affect the ligand bulkiness. The more electron-donating methoxy group led, however, to small changes in various IR wavenumbers used to probe the electron donor properties of the carbene moiety.