Macrocyclic Salen–Bis-NHC Hybrid Ligands and Their Application to the Synthesis of Enantiopure Bi- and Trimetallic Complexes

Studies on gold(I) and gold(III) alcohol functionalised NHC complexes

Five pairs of novel chiral alcohol functionalised gold(i) and gold(iii) NHC complexes derived from chiral amino alcohols, were synthesized and characterised (NMR, IR, HRMS). Single crystal X-ray diffraction data of gold(i) and gold(iii) complexes are reported and discussed. The chiral imidazolium preligands were readily synthesized via the oxalamides, subsequent reduction and final orthoformate condensation. An improved method was used for generation of gold(i) NHC complexes (up to 92%) and further oxidation afforded the corresponding gold(iii) NHC complexes (up to 99%). All the Au(i) and Au(iii) NHC complexes proved far more catalytically active in a 1,6-enyne alkoxycyclization test reaction than our previously tested N,N- and P,N-ligated Au(iii) complexes. Comparative gold(i) and gold(iii) catalytic studies demonstrated different catalytic ability, depending on the NHC ligand flexibility and bulkiness. Excellent yields (92-99%) of target alkoxycyclization product were obtained with both gold(i) and gold(iii) complexes with the bulky N1-Mes-N2-ethanol based NHC ligand.

Highly Active Cooperative Lewis Acid-Ammonium Salt Catalyst for the Enantioselective Hydroboration of Ketones

Enantiopure secondary alcohols are fundamental high-value synthetic building blocks. One of the most attractive ways to get access to this compound class is the catalytic hydroboration. We describe a new concept for this reaction type that allowed for exceptional catalytic turnover numbers (up to 15 400), which were increased by around 1.5-3 orders of magnitude compared to the most active catalysts previously reported. In our concept an aprotic ammonium halide moiety cooperates with an oxophilic Lewis acid within the same catalyst molecule. Control experiments reveal that both catalytic centers are essential for the observed activity. Kinetic, spectroscopic and computational studies show that the hydride transfer is rate limiting and proceeds via a concerted mechanism, in which hydride at Boron is continuously displaced by iodide, reminiscent to an SN 2 reaction. The catalyst, which is accessible in high yields in few steps, was found to be stable during catalysis, readily recyclable and could be reused 10 times still efficiently working.

Enantiodivergent [4+2] Cycloaddition of Dienolates by Polyfunctional Lewis Acid/Zwitterion Catalysis

Diels-Alder reactions have become established as one of the most effective ways to prepare stereochemically complex six-membered rings. Different catalysis concepts have been reported, including dienophile activation by Lewis acids or H-bond donors and diene activation by bases. Herein we report a new concept, in which an acidic prodiene is acidified by a Lewis acid to facilitate deprotonation by an imidazolium-aryloxide entity within a polyfunctional catalyst. A metal dienolate is thus formed, while an imidazolium-ArOH moiety probably forms hydrogen bonds with the dienophile. The catalyst type, readily prepared in few steps in high overall yield, was applied to 3-hydroxy-2-pyrone and 3-hydroxy-2-pyridone as well as cyclopentenone prodienes. Maleimide, maleic anhydride, and nitroolefin dienophiles were employed. Kinetic, spectroscopic, and control experiments support a cooperative mode of action. High enantioselectivity was observed even with unprecedented TONs of up to 3680.

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.

Synthesis of Double-Bridged Cofacial Nickel Porphyrin Dimers with 2,2'-Bipyridyl Pillars and Their Restricted Coordination Space

Double-bridged cofacial Ni porphyrin dimers 2 with 2,2'-bipyridyl pillars were effectively prepared by a one-step reductive homocoupling reaction of bis(chloropyridyl)-substituted Ni porphyrin derivatives followed by a specific separation of a cyanopropyl-modified silica gel column using pyridine eluent systems. The structural analyses of 2 and its Pd complex were carried out in their solid and solution states by means of X-ray single crystal analysis and NMR, respectively. The complexation of η³-allylpalladium chloride (Pd) with 2 on the spatially restricted 2,2-bipyridine moieties on 2 gave a 2:1 (Pd:2) complex, in which the 2,2'-bipyridine ligands only provided one of the N atoms on a 2,2'-bipyridine ligand to a Pd. Therefore, the 2,2-bipyridine moieties acted as a monodentate ligand.

A Chiral Macrocyclic Tetra-N-Heterocyclic Carbene Yields an "All Carbene" Iron Alkylidene Complex

The first chiral macrocyclic tetra-N-heterocyclic carbene (NHC) ligand has been synthesized. The macrocycle, prepared in high yield and large scale, was ligated onto palladium and iron to give divalent C2 -symmetric square planar complexes. Multinuclear NMR and single crystal X-ray diffraction demonstrated that there are two distinct NHCs on each ligand, due to the bridging chiral cyclohexane. Oxidation of the iron(II) complex with trimethylamine N-oxide yielded a bridging oxo complex. Diazodiphenylmethane reacted with the iron(II) complex at room temperature to give a paramagnetic diazoalkane complex; the same reaction yielded the "all carbene" complex at elevated temperature. Electrochemical measurements support the assignment of the "all carbene" complex being an alkylidene. Notably, the diazoalkane complex can be directly transformed into the alkylidene complex, which had not been previously demonstrated on iron. Finally, a test catalytic reaction with a diazoalkane on the iron(II) complex does not yield the expected cyclopropane, but actually the azine compound.

Polyfunctional Bis-Lewis-Acid-/Bis-Triazolium Catalysts for Stereoselective 1,4-Additions of 2-Oxindoles to Maleimides

Achieving enzyme-like catalytic activity and stereoselectivity without the typically high substrate specificity of enzymes is a challenge in the development of artificial catalysts for asymmetric synthesis. Polyfunctional catalysts are considered to be a promising tool for achieving excellent catalytic efficiency. A polyfunctional catalyst system was developed, which incorporates two Lewis acidic/Brønsted basic cobalt centers in combination with triazolium moieties that are crucial for high reactivity and excellent stereoselectivity in the direct 1,4-addition of oxindoles to maleimides. The catalyst is assembled through click chemistry and is readily recyclable through precipitation by making use of its charges. Kinetic studies support a cooperative mode of action. Diastereodivergency is achievable with either Boc-protected or unprotected maleimide.

Recent Advances in Metal-Catalyzed Asymmetric 1,4-Conjugate Addition (ACA) of Nonorganometallic Nucleophiles

The metal-catalyzed asymmetric conjugate addition (ACA) reaction has emerged as a general and powerful approach for the construction of optically active compounds and is among the most significant and useful reactions in synthetic organic chemistry. In recent years, great progress has been made in this area with the use of various chiral metal complexes based on different chiral ligands. This review provides comprehensive and critical information on the enantioselective 1,4-conjugate addition of nonorganometallic (soft) nucleophiles and their importance in synthetic applications. The literature is covered from the last 10 years, and a number of examples from before 2007 are included as background information. The review is divided into multiple parts according to the type of nucleophile involved in the reaction (such as C-, B-, O-, N-, S-, P-, and Si-centered nucleophiles) and metal catalyst systems used.

Linear CuI2 Pd0 , CuI Pd02 , and AgI2 Pd0 Metal Chains Supported by Rigid N,N'-Diphosphanyl N-Heterocyclic Carbene Ligands and Metallophilic Interactions

Selective copper(I) to palladium(0) transmetallation of P-donors from the rigid N,N'-diphosphanyl-imidazol-2-ylidene C₃ H₂ [NP(tBu)₂ ]₂ (PC^NHC P) was observed when known [Cu₃ (μ₃ -PC^NHC P,κP,κC^NHC ,κP)₂ ](OTf)₃ was reacted with [Pd(PPh₃ )₄ ]. When 1.2 equivalents of [Pd(PPh₃ )₄ ] was used, the product [Cu₂ Pd(μ₃ -PC^NHC P,κP,κC^NHC ,κP)₂ ](OTf)₂ (2(OTf)₂ ) was obtained, which features a Cu^I -Cu^I -Pd⁰ chain and appears to be the first linear heterotrinuclear complex with d¹⁰ -d¹⁰ interactions between Pd⁰ and Cu^I . When the Cu₃ precursor was reacted with 3.0 equivalents of [Pd(PPh₃ )₄ ], the complex [CuPd₂ (μ₃ -PC^NHC P,κP,κC^NHC ,κP)₂ ](OTf)₂ (3(OTf)₂ ) was obtained, which, on the basis of magnetic measurements, DFT calculations, and computed nuclear shieldings, was formulated as containing a Pd⁰ -Cu^I -Pd⁰ chain with an electron hole delocalized over the whole cation, including the metal chain. Similarly, selective transmetallation of the P-donors in [Ag₃ (μ₃ -PC^NHC P,κP,κC^NHC ,κP)₂ ](OTf)₃ from silver to palladium (originating from [Pd(PPh₃ )₄ ]) gave the linear chain [Ag₂ Pd(μ₃ -PC^NHC P,κP,κC^NHC ,κP)₂ ](OTf)₂ (5(OTf)₂ ), which on the basis of NMR spectroscopy comprises an Ag^I -Ag^I -Pd⁰ metal core. However, X-ray diffraction data collected on various samples of 5(OTf)₂ were modeled with 50:50 metal disorder at the terminal positions, corresponding to a (Ag^I /Pd⁰ )-Ag^I -(Ag^I /Pd⁰ ) formulation. Upon standing in solution, 5(OTf)₂ transformed to 6(OTf)₂ , the regioisomer of 5(OTf)₂ in which the Pd center has migrated to the central position of an Ag^I -Pd⁰ -Ag^I chain. Prolonged standing in CH₂ Cl₂ or by reaction with [PtCl₂ (NCMe)₂ ] converts complex 6(OTf)₂ to the Ag^I /Pd^II complex [Ag₂ PdCl₂ (μ₃ -PC^NHC P,κP,κC^NHC ,κP)₂ ](OTf)₂ (7(OTf)₂ ). The structural data of 2(OTf)₂ , 3(OTf)₂ , and 7(OTf)₂ establish significant heterometallophilic interactions.

Metal complexes with oxygen-functionalized NHC ligands: synthesis and applications

Ligand design has met with considerable success with both categories of hybrid ligands, which are characterized by chemically different donor groups, and of N-heterocyclic carbenes (NHCs). Their spectacular development and diversity are attracting worldwide interest and offers almost unlimited diversity and potential in e.g. coordination/organometallic main group and transition metal chemistry, catalysis, medicinal chemistry and materials science. This review aims at providing a comprehensive update on a specific class of ligands that has enjoyed much attention in the past few years, at the intersection between the two categories mentioned above, that of hybrid NHC ligands in which the functionality associated with the carbene donor is of the oxygen-donor type. For each type of oxygen-donor present in such chelating (Section 1) or bridging (Section 2) hybrid ligands, we will examine the synthesis, structures and reactivity of their metal complexes and their applications, with a special focus on homogeneous catalysis (Section 3). Thus, hydrogenation, C-H bond activation, C-C, C-N, C-O bond formation, hydrolysis of silanes, oligomerization, polymerization, metathesis, hydrosilylation, C-C bond cleavage, acceptorless dehydrogenation, dehalogenation/hydrogen transfer, oxidation and reduction reactions will be successively presented in a tabular manner, to facilitate an overview and a rapid identification of the relevant publications describing which metals associated with a given oxygen functionality are most suitable. The literature coverage includes the year 2015.

Synthesis and structural characterization of metal complexes with macrocyclic tetracarbene ligands

Fourteen Ag(i), Au(i), Ni(ii), Pd(ii) and Pt(ii) complexes were prepared with macrocyclic tetradentate N-heterocyclic carbene (NHC) ligands.