Readily Accessible Unsymmetrical Unsaturated 2,6-Diisopropylphenyl N-Heterocyclic Carbene Ligands. Applications in Enantioselective Catalysis

Iridium(I) complexes with bidentate NHC ligands as catalysts for dehydrogenative directed C-H silylation

A series of (NHC)(cod)Ir(I) complexes bearing NHC-carboxylate ligands were efficiently synthesized and fully characterized. Their solid-state structures confirmed the bidentate coordination mode of these LX-type NHC ligands. These unprecedented iridium(I) complexes demonstrated efficient catalytic activities in dehydrogenative directed C-H silylation of arenes, and allowed for excellent ortho-selectivity control with aromatic silylating agents.

Copper-catalyzed asymmetric allylic alkylation of racemic inert cyclic allylic ethers under batch and flow conditions

The Cu-catalyzed AAA reactions employing challenging racemic inert cyclic allylic ethers with sterically hindered Grignard reagents have been disclosed under batch and flow conditions.

Diaza-1,3-butadienes as Useful Intermediate in Heterocycles Synthesis

Many heterocyclic compounds can be synthetized using diaza-1,3-butadienes (DADs) as key structural precursors. Isolated and in situ diaza-1,3-butadienes, produced from their respective precursors (typically imines and hydrazones) under a variety of conditions, can both react with a wide range of substrates in many kinds of reactions. Most of these reactions discussed here include nucleophilic additions, Michael-type reactions, cycloadditions, Diels–Alder, inverse electron demand Diels–Alder, and aza-Diels–Alder reactions. This review focuses on the reports during the last 10 years employing 1,2-diaza-, 1,3-diaza-, 2,3-diaza-, and 1,4-diaza-1,3-butadienes as intermediates to synthesize heterocycles such as indole, pyrazole, 1,2,3-triazole, imidazoline, pyrimidinone, pyrazoline, -lactam, and imidazolidine, among others. Fused heterocycles, such as quinazoline, isoquinoline, and dihydroquinoxaline derivatives, are also included in the review.

Titanium complexes with unsymmetrically substituted imidazolin-2-iminato ligands

The reaction of the unsymmetrical N-heterocyclic carbenes 1-(2,4,6-trimethylphenyl)-3-(adamantyl)imidazolin-2-ylidene (IAdMes, 1a) and 1-(2,6-diisopropylphenyl)-3-(adamantyl)imidazolin-2-ylidene (IAdDipp, 1b) with trimethylsilyl azide furnished the 2-(trimethylsilylimino)imidazolines 2a (Im^AdMesNSiMe₃) and 2b (Im^AdDippNSiMe₃). Desilylation by stirring in methanol gave the corresponding imidazolin-2-imines 3a (Im^AdMesNH) and 3b (Im^AdDippNH). 2a and 2b were treated with [TiCl₄(THF)₂] (THF = tetrahydrofuran) and [CpTiCl₃] (Cp = η⁵-C₅H₅) to form the mono- and bis(imidazolin-2-iminato) titanium(IV) complexes [(Im^AdRN)TiCl₃] (4, R = Mes, Dipp), [Cp(Im^AdRN)TiCl₂] (5, R = Mes, Dipp), and [(Im^AdRN)₂TiCl₂] (6, R = Mes, Dipp). The crystal structures of all compounds except 2b were determined by X-ray diffraction analysis.

Antimicrobial Properties of Amino-Acid-Derived N-Heterocyclic Carbene Silver Complexes

Complexes {Ag[NHC^Mes,R]}_n (R = H, 2a; Me, 2b and 2b’; ⁱPr, 2c; ⁱBu, 2d), were prepared by treatment of imidazolium precursor compounds [Im^Mes,R] (2-(3-mesityl-1H-imidazol-3-ium-1-yl)acetate, 1a, (S)-2-alkyl(3-mesityl-1H-imidazol-3-ium-1-yl)acetate, 1b–d, and (R)-2-methyl(3-mesityl-1H-imidazol-3-ium-1-yl)acetate, 1b’, with Ag₂O under appropriate conditions. They were characterised by analytical, spectroscopic (IR, ¹H, and ¹³C NMR and polarimetry), and X-ray methods (2a). In the solid state, 2a is a one-dimensional coordination polymer, in which the silver(I) cation is bonded to the carbene ligand and to the carboxylate group of a symmetry-related Ag[NHC^Mes,H] moiety. The coordination environment of the silver centre is well described by the DFT study of the dimeric model {Ag[NHC^Mes,H]}₂. The antimicrobial properties of these complexes were evaluated versus Gram-negative bacteria E. coli and P. aeruginosa. From the observed MIC and MBC values (minimal inhibitory concentration and minimal bactericidal concentration, respectively), complex 2b’ showed the best antimicrobial properties (eutomer), which were significantly better than those of its enantiomeric derivative 2b (distomer). Additionally, analysis of MIC and MBC values of 2a–d reveal a clear structure–antimicrobial effect relationship. Antimicrobial activity decreases when the steric properties of the R alkyl group in {Ag[NHC^Mes,R]}_n increase.

Chiral oxazolidines acting as transient hydroxyalkyl-functionalized N-heterocyclic carbenes: an efficient route to air stable copper and gold complexes for asymmetric catalysis

Optically pure oxazolidines were synthesized in nearly quantitative yields from chiral hydroxyalkyl-functionalized imidazolinium salts. Acting as transient chiral diamino N-heterocyclic carbenes (NHCs), these oxazolidines allowed the efficient formation of well-defined copper(i) and gold(i) hydroxyalkyl-NHC complexes, which could be isolated, for the first time, as air stable complexes after silica gel chromatography. Interestingly, X-ray analysis of gold complexes revealed that the hydroxyl-function is not chelated to the metal. Computational studies suggested that both cyclisation to produce oxazolidine and O–H bond elimination to form the transient carbene (prior to coordination) occur through a concerted mechanism. The novel chiral copper-catalysts, as well as oxazolidines alone (copper free), demonstrated excellent performances in asymmetric conjugate addition and allylic alkylation with high regio- and enantio-selectivities (up to 99% ee).

Well-defined chiral Cu(i) and Au(i) hydroxyalkyl NHC complexes were synthesized from oxazolidines. The copper-catalysts and oxazolidines alone (copper free), demonstrated excellent performances in asymmetric conjugate addition and allylic alkylation.

Base-free transfer hydrogenation of aldehydes and ketones catalyzed by imidazolin-2-iminato actinide complexes

A series of thorium and uranium complexes decorated with unsymmetrical imidazolin-2-iminato ligands were found to be effective as catalysts in the transfer hydrogenation of aldehydes and ketones with 2-propanol to form the corresponding alcohols.

Continuous Flow Z-Stereoselective Olefin Metathesis: Development and Applications in the Synthesis of Pheromones and Macrocyclic Odorant Molecules*

The first continuous flow Z-selective olefin metathesis process is reported. Key to realizing this process was the adequate choice of stereoselective catalysts combined with the design of an appropriate continuous reactor setup. The designed continuous process permits various self-, cross- and macro-ring-closing-metathesis reactions, delivering products in high selectivity and short residence times. This technique is exemplified by direct application to the preparation of a range of pheromones and macrocyclic odorant molecules and culminates in a telescoped Z-selective cross-metathesis/ Dieckmann cyclisation sequence to access (Z)-Civetone, incorporating a serial array of continually stirred tank reactors.

Multicomponent Synthesis of Unsymmetrical 4,5-Disubstituted Imidazolium Salts as N-Heterocyclic Carbene Precursors: Applications in Palladium-Catalyzed Cross-Coupling Reactions

Various novel (a)chiral 4,5-disubstituted 1-aryl-3-alkyl-imidazolium salts were synthesized via the multicomponent reaction of diketone derivatives, sterically congested arylamines, and alkylamines. Moreover, two novel unsymmetrical bulky cycloalkyl-based NHC-Pd complexes proved highly active as catalysts for Suzuki-Miyaura and Negishi cross-coupling reactions.

Hybrids of cationic [4]helicene and N-heterocyclic carbene as ligands for complexes exhibiting (chir)optical properties in the far red spectral window

Synthesis, electronic and structural properties of a chiral NHC bearing a N-bonded cationic [4]helicene moiety are reported. This ligand is used to construct Au^I, Au^III and Rh^I complexes exhibiting far-red (chir)optical properties regardless of the metal.

Planar-chiral ferrocene-based triazolylidene copper complexes: synthesis, characterization, and catalysis in asymmetric borylation of α,β-unsaturated ester

1,2,3-Triazol-5-ylidenes have recently attracted considerable attention as versatile ligands because of their strong electron-donating properties and structural diversities. While some efforts have been devoted to the development of chiral triazolylidene-metal complexes, there is no example achieving asymmetric induction by base-metal complexes with triazolylidene ligands. Herein, we synthesized planar-chiral ferrocene-based triazolylidene copper complexes, which enabled the asymmetric borylation of methyl cinnamate with bis(pinacolato)diboron with good enantioselectivity.

Synthesis of N-Aryl- and N-alkyl-Substituted Imidazolium Silver Complexes: Cytotoxic Screening by Using Human Cell Lines Modelling Acute Myeloid Leukaemia

A series of N-aryl- and N-alkyl substituted imidazoles has been synthesised and complexed with Ag⁺ to obtain silver-NHC complexes of the form [Ag(NHC)₂ ]X. These silver-NHC complexes were tested in vitro against the human cell lines HL-60 and MOLM-13, which both model acute myeloid leukaemia (AML). A substantial difference in cytotoxicity was revealed varying in the range 13-4 μM and 22-9 μM for HL-60 and MOLM-13, respectively. Furthermore, this study revealed that when an alkyl group is installed on the imidazole scaffold, its position substantially influences the cytotoxicity of the corresponding silver NHC complex.

Chiral N-heterocyclic Carbene Gold Complexes: Synthesis and Applications in Catalysis

N-Heterocyclic carbenes have found many applications in modern metal catalysis, due to the formation of stable metal complexes, and organocatalysis. Among a myriad of N-heterocyclic carbene metal complexes, gold complexes have gained a lot of attention due to their unique propensity for the activation of carbon-carbon multiple bonds, allowing many useful transformations of alkynes, allenes, and alkenes, inaccessible by other metal complexes. The present review summarizes synthetic efforts towards the preparation of chiral N-heterocyclic gold(I) complexes exhibiting C2 and C1 symmetry, as well as their applications in enantioselective catalysis. Finally, the emerging area of rare gold(III) complexes and their preliminary usage in asymmetric catalysis is also presented.

Copper-Catalyzed Asymmetric Conjugate Additions of Bis(pinacolato)diboron and Dimethylzinc to Acyl- N-methylimidazole Michael Acceptors: A Highly Stereoselective Unified Strategy for 1,3,5,... n (OH, Me) Motif Synthesis

A unified strategy for the construction of prevalent 1,3,5,... n (OH, Me) motifs based on consecutive copper-catalyzed asymmetric conjugate borylation (ACB) and methylation (ACA) reactions involving α,β-unsaturated 2-acyl- N-methylimidazoles is described. Good yields and high diastereoselectivities have been obtained in ACA and ACB reactions for both matched and mismatched pairs as illustrated in the synthesis of syn/ anti and anti/ anti (Me, OTBS, Me) and (OH, OTBS, Me) motifs.

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.

Highly selective macrocyclic ring-closing metathesis of terminal olefins in non-chlorinated solvents at low dilution

New ruthenium complexes featuring two unsymmetrical NHCs proved to be highly selective in macrocyclic RCM performed in green solvents at low dilution.

Palladium complexes of N-heterocyclic carbenes displaying an unsymmetricalN-alkylfluorenyl/N′-aryl substitution pattern and their behaviour in Suzuki–Miyaura cross coupling

Pd–PEPPSI complexes with a mixedN-alkylfluorenyl/N′-aryl substitution pattern were found to be considerably more active in Suzuki–Miyaura cross coupling than the standard complexes [PdCl₂(IMes)(Py)] and [PdCl₂(IPr)(Py)].

A kinetic resolution strategy for the synthesis of chiral octahedral NHC–iridium(iii) catalysts

The transmetalation reaction of a chiral-bidentate NHC–silver complex to racemic [lr(μ-Cl)(ppy)₂]₂ operates with kinetic resolution leading to chiral octahedral NHC–iridium(iii) complexes and enantio-enriched bis-cyclometalated iridium(iii) complexes.

Well-Defined Chiral Copper NHC Complex in the Asymmetric Conjugated β-Borylation and One-Pot Metathesis-Asymmetric β-Borylation Reactions

Highly stereoselective conjugate β-borylation, using a new chiral NHC-based copper catalyst, has been achieved. The chiral NHC copper complex was prepared in gram scale and showed high enantioselectivity and activity (up to 10 000 turnovers at 100 ppm of catalyst loading). This method was employed in the synthesis of a chiral β-borylated ester from simple unconjugated alkenes though an unprecedented one-pot cross metathesis-asymmetric borylation sequence.