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

Facile synthesis and bonding of 4-ferrocenyl-1,2,4-triazol-5-ylidene complexes

Ferrocene-substituted carbenes have emerged as attractive, redox-active ligands. However, among the compounds studied to date, ferrocenylated 1,2,4-triazol-5-ylidenes, which are closely related to the archetypal imidazol-2-ylidenes, are still unknown. Here, we demonstrate that the triazolium salt [CHN(Me)NCHN(Fc)]I (2; Fc = ferrocenyl), obtained by alkylation of 4-ferrocenyl-4H-1,2,4-triazole (1) with MeI, reacts selectively with metal alkoxide/hydroxide precursors [(cod)Rh(OMe)]2 and [(IPr)Au(OH)] (cod = cycloocta-1,5-diene, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to produce the ferrocene-substituted 1,2,4-triazol-5-ylidene complexes [(cod)RhI{CN(Me)NCHN(Fc)}] and [(IPr)Au{CN(Me)NCHN(Fc)}]I in good yields. The complexes were characterised by NMR and IR spectroscopy, mass spectrometry, cyclic voltammetry, and single-crystal X-ray diffraction analysis. Density function theory (DFT) calculations were used to rationalise the electrochemical behaviour of the carbene complexes and to elucidate the bonding situation in these compounds. An analysis using intrinsic bond orbitals (IBOs) revealed that the 1,2,4-triazol-5-ylidene ligand exerted a strong trans influence and showed a synergistic stabilisation by the negative inductive and positive π-donor effects of the nitrogen atoms adjacent to the carbene carbon atom; these effects were enhanced by conjugation with the CHN bond at the exterior, similar to that in imidazol-2-ylidenes.

Application of first-row transition metal complexes bearing 1,2,3-triazolylidene ligands in catalysis and beyond

Triazole-derived N-heterocyclic carbenes, triazolylidenes (trz) have become an interesting alternative to the ubiquitous Arduengo-type imidazole-derived carbenes, in part because they are stronger donors, and in other parts due to their versatile synthesis through different types of click reactions. While the use of trz ligands has initially focused on their coordination to precious metals for catalytic applications, the recent past has seen a growing interest in their impact on first-row transition metals. Coordination of trz ligands to such 3d metals is more challenging due to the orbital mismatch between the carbene and the 3d metal center, which also affects the stability of such complexes. Here we summarize the strategies that have been employed so far to overcome these challenges and to prepare first-row transition metal complexes containing at least one trz ligand. Both properties and reactivities of these trz complexes are comprehensively compiled, with a focus on photophysical properties and, in particular, on the application of these complexes in homogeneous catalysis. The diversity of catalytic transformations entailed with these trz 3d metal complexes as well as the record-high performance in some of the reactions underpins the benefits imparted by trz ligands.

Advanced Application of Planar Chiral Heterocyclic Ferrocenes

This manuscript is reviewing the superior catalytic activity and selectivity of ferrocene ligands in a wide range of reactions: reduction of ketones, hydrogenation of olefins, hydroboration, cycloaddition, enantioselective synthesis of biaryls, Tsuji–Trost allylation. Moreover, the correlation between a ligand structure and its catalytic activity is discussed in this review.

Copper-Catalyzed Single C-H Amination of 8-Aminoquinoline-Directed Ferrocenes

An unprecedented copper-catalyzed C-H monoamination of ferrocenes directed by an 8-aminoquinoline amide directing group is described. This reaction proceeds in the presence of a catalytic amount of copper catalyst with both cyclic and acyclic amines to afford the various aminoferrocenes. The C-H amination of ortho-substituted ferroceneamides was also achieved, enabling rapid access to multisubstituted ferrocenes that are useful for developing new functional molecules.