Recent Advances in Catalysis Involving Bidentate N-Heterocyclic Carbene Ligands

Abstract

Since the discovery of persistent carbenes by the isolation of 1,3-di-l-adamantylimidazol-2-ylidene by Arduengo and coworkers, we witnessed a fast growth in the design and applications of this class of ligands and their metal complexes. Modular synthesis and ease of electronic and steric adjustability made this class of sigma donors highly popular among chemists. While the nature of the metal-carbon bond in transition metal complexes bearing N-heterocyclic carbenes (NHCs) is predominantly considered to be neutral sigma or dative bonds, the strength of the bond is highly dependent on the energy match between the highest occupied molecular orbital (HOMO) of the NHC ligand and that of the metal ion. Because of their versatility, the coordination chemistry of NHC ligands with was explored with almost all transition metal ions. Other than the transition metals, NHCs are also capable of establishing a chemical bond with the main group elements. The advances in the catalytic applications of the NHC ligands linked with a second tether are discussed. For clarity, more frequently targeted catalytic reactions are considered first. Carbon–carbon coupling reactions, transfer hydrogenation of alkenes and carbonyl compounds, ketone hydrosilylation, and chiral catalysis are among highly popular reactions. Areas where the efficacy of the NHC based catalytic systems were explored to a lesser extent include CO2 reduction, C-H borylation, alkyl amination, and hydroamination reactions. Furthermore, the synthesis and applications of transition metal complexes are covered.

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

“…These compounds are important because they are the precursors of N -heterocyclic carbenes (NHCs), which are generated by deprotonation [ 106 ]. The NHCs are interesting structures due to the catalytic activity of their metal complexes and are widely used in organic synthesis [ 107 , 108 ].…”

Rh(I) Complexes with Hemilabile Thioether-Functionalized NHC Ligands as Catalysts for [2 + 2 + 2] Cycloaddition of 1,5-Bisallenes and Alkynes

“…Moreover, their simple synthetic procedures and the ability to tune their steric and electronic properties by modifying the nitrogen or backbone substituents make them very attractive for rapid ligand screening. Rhodium­(I) bearing monodentate NHCs have been successfully applied to several metal-catalyzed transformations such as hydrothiolation, hydrophosphination, cross-coupling reaction, and dimerization reactions. , Although most NHCs are monodentate ligands, a strong interest has been shown for the past 10 years and still continues today for the chemistry of functionalized NHC carbenes in which a Lewis base moiety (often rooted on: N, P, or O atom) is attached to the strongly bonded imidazolyl ring . However, although the S-functionalized N-heterocyclic carbenes are less represented, they are also an interesting class of ligands because they can potentially provide an “on and off” dynamic chelating effect for the metal complex during a catalytic cycle.…”

“… 18 , 19 Although most NHCs are monodentate ligands, a strong interest has been shown for the past 10 years and still continues today for the chemistry of functionalized NHC carbenes in which a Lewis base moiety (often rooted on: N, P, or O atom) is attached to the strongly bonded imidazolyl ring. 20 However, although the S-functionalized N-heterocyclic carbenes are less represented, 21 they are also an interesting class of ligands because they can potentially provide an “on and off” dynamic chelating effect for the metal complex during a catalytic cycle. This hemilability has been particularly demonstrated for the thioether function with different metals.…”

Anti-Cancer and Radio-Sensitizing Properties of New Bimetallic (N-Heterocyclic Carbene)-Amine-Pt(II) Complexes

“…Over the last two decades, NHC-transition metal complexes (NHC = N-Heterocyclic Carbene) have attracted significant attention not only for uses in organometallic chemistry and catalysis − but also as bio-organometallics, notably due to the high stability of the NHC-metal bonds under physiological conditions. − …”