Triptycene-Based Chiral andmeso-N-Heterocyclic Carbene Ligands and Metal Complexes

C2-Symmetric N-Heterocyclic Carbenes in Asymmetric Transition-Metal Catalysis

The last decades have witnessed a rapid growth of applications of N-heterocyclic carbenes (NHCs) in different chemistry fields. Due to their unique steric and electronic properties, NHCs have become a powerful tool in coordination chemistry, allowing the preparation of stable metal-ligand frameworks with both main group metals and transition metals. An overview on the use of five membered monodentate C2-symmetric N-heterocyclic carbenes (NHCs) as ligands for transition-metal complexes and their most relevant applications in asymmetric catalysis is offered.

Bi- and trimetallic complexes with macrocyclic xanthene-4,5-diNHC ligands

Three different types of bimetallic NHC-metal complexes were synthesized, whose NHC units are attached at the 4,5-positions of xanthene. The NHC units are in close proximity and are designed such that each carbene coordinates one ML unit, while the chelation of one metal by two NHC is not possible. Several xanthene-((NHC)ML)₂ complexes with ML = RhCl(cod), IrCl(cod), RhCl(CO)₂, IrCl(CO)₂, AuCl, AgCl, CuCl and Pd(allyl)Cl were synthesized and investigated.

Simple synthesis of [Ru(CO3)(NHC)(p-cymene)] complexes and their use in transfer hydrogenation catalysis

A novel, efficient and facile protocol for the synthesis of a series of [Ru(NHC)(CO₃)(p-cymene)] complexes is reported. This family of Ru-NHC complexes was obtained from imidazol(in)ium tetrafluoroborate or imidazolium hydrogen carbonate salts in moderate to excellent yields, employing sustainable weak base. The ruthenium complexes were successfully utilized in the transfer hydrogenation of ketones as highly active multifunctional catalysts.

Chiral Triptycenes: Concepts, Progress and Prospects

Triptycenes have been established as unique scaffolds because of their backbone π-structure with a propeller-like shape and saddle-like cavities. They are some of the key organic molecules that have been extensively studied in polymer chemistry, in supramolecular chemistry and in material science. Triptycenes become chiral molecules when substituents are unsymmetrically attached in at least two of their different aromatic rings. This Minireview highlights the chirality of triptycenes from basics to an advanced stage for the development of functional molecules.

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.

Pentiptycene-based concave NHC-metal complexes

The reaction of 1-amino,4-hydroxy-pentiptycene with diacetyl or acenaphthene-1,2-dione gave the respective diimines, followed by alkylation of the hydroxyl groups, and cyclization of the alkylated diimines to the respective bispentiptycene-imidazolium salts NHC·HCl. The azolium salts, being precursors to N-heterocyclic carbenes, were converted into metal complexes [(NHC)MX] (MX = CuI, AgCl, AuCl) and [(NHC)IrCl(cod)] and [(NHC)IrCl(CO)2] in good yields. In the solid state [(NHC)AgCl] displays a bowl-shaped structure of the ligand with the metal center buried within the concave unit.

Alternating ring-opening metathesis polymerization by Grubbs-type catalysts with N-pentiptycenyl, N-alkyl-NHC ligands

A Grubbs–Hoveyda type catalyst with a N-pentiptycenyl, N-cyclohexyl-NHC ligand provides poly(nbe-alt-coe) with an excellent degree of alternation.

Fluorescence resonance energy transfer (FRET) for the verification of dual gold catalysis

Two gold complexes with different bodipy-tagged N-heterocyclic carbene ligands, which are a potential FRET pair, were synthesized. It was shown, that the formation of dinuclear intermediates in alkyne transformations (“dual gold catalysis”) are characterized by a FRET signal.

Iptycenes with an acridinone motif developed through [4+2] cycloaddition of tethered naphthalene and iminoquinone via a radical reaction

A new class of iptycenes was developed by combining 2-(naphthalen-1-yl)anilines and p-benzoquinones through copper(ii)-mediated radical cyclisation.

Methyl and phenyl substituent effects on the catalytic behavior of NHC ruthenium complexes

NHCs with different combinations of methyl and phenyl substituents produce ruthenium second generation catalysts with different RCM behavior.