Fused Polycyclic NHC Ligands in Gold Catalysis: Recent Advances

Abstract

Thanks to their appealing properties, Nheterocyclic carbene (NHC) ligands gained considerable significance as highly powerful ancillary ligands in gold catalysis. While NHC-gold catalysis is still dominated by the classical imidazole-derived NHC ligands, annelated NHC systems, which feature fused aromatic rings to the carbenic heterocycle, have drawn wider attention over the last few years thanks to their unique structural and electronic properties.Fused polyaromatic NHCs are indeed versatile platforms that offer unique stereoelectronic tuning possibilities to exhibit superior activity and/or selectivity in NHC-gold catalysis. But their polycyclic scaffolds also provide further scope for functionalization and the introduction of chirality, stimuliresponsiveness, or cooperativity within the NHC-gold systems. In this review, the recent advances in this area are discussed and a particular attention is drawn on the unique role of the annelation.

Azobenzene-Integrated NHC Ligands: A Versatile Platform for Visible-Light-Switchable Metal Catalysis

“…This twisted conformation would place the arylazo group further away from the metal atom, diminishing its steric effect. Hence, we began our exploration using the rigid imidazo­[1,5- a ]-pyridin-3-ylidene (ImPy) skeleton, an established ring-annellated NHC class with diverse catalytic applications (Figure b). − Despite the successful synthesis of 5-arylazo attached ImPy (azImPy) precursor salts and azImPy gold chloride complexes, this compound class did not disclose satisfactory photoswitch performance (see the Supporting Information (SI) Section 2.1). Guided by density functional theory (DFT), a novel azobenzene-integrated tricyclic framework grounded on imidazo­[5,1- b ]­benzoxazol-1-ylidene (ImBA) was designed (Figure c).…”

A laterally-fused N-heterocyclic carbene framework from polysubstituted aminoimidazo[5,1-b]oxazol-6-ium salts

“…With the novel 3-aminoimidazo[5,1- b ]oxazol-6-ium salt in hand, we examined its use as an NHC precursor for the preparation of late transition metal complexes. Treating compound 9a with triethylamine and either dimethyl sulfide gold(I) chloride or copper(I) chloride in acetone led to the formation of the desired AImOx AuCl and AImOx CuCl metal chloride complexes 13 and 14 , respectively ( Scheme 2 ) [ 7 ].…”

“…The imidazo [1,5-a]pyridin-3-ylidene motif (ImPy), independently introduced by the groups of Lassaletta [4] and Glorius [5], is the most widely explored framework for L-shaped ligands (Figure 1a). Even when only considering gold catalysis [6], the ImPy framework has been used to great effect [7]. The motif has been used to introduce sterically demanding NHCs with secondary gold-ligand interactions [8][9][10], chiral environments [11][12][13] including those enabling secondary interactions with substrates for asymmetric catalysis [14], cooperative and bimetallic catalysis [7,15], and redox-enabling function for Au(I)/(III) cycles [16,17].…”

“…Even when only considering gold catalysis [6], the ImPy framework has been used to great effect [7]. The motif has been used to introduce sterically demanding NHCs with secondary gold-ligand interactions [8][9][10], chiral environments [11][12][13] including those enabling secondary interactions with substrates for asymmetric catalysis [14], cooperative and bimetallic catalysis [7,15], and redox-enabling function for Au(I)/(III) cycles [16,17]. Such L-shaped ligands provide scope to influence the reactivity profile of their resulting metal complexes through steric shielding, direct stabilising interactions with the metal, or by proximal effects to reactive species.…”

[Au(Np#)Cl]: highly reactive and broadly applicable Au(i)–NHC catalysts for alkyne π-activation reactions

“…Furthermore, effects of non-symmetrical wingtip substitution and steric flexibility of NHC ligands remain unexplored, despite the potential of wingtip modification to improve the efficiency of this class of processes using Au( i )–NHC catalysis. 15,16…”