Palladium, iridium, and rhodium complexes bearing chiral N‐heterocyclic carbene ligands applied in asymmetric catalysis

Abstract

The field of asymmetric catalysis is rapidly developing and the chiral ligands play a key role in enantioselective transition‐metal catalysis. The electron‐rich chiral N‐heterocyclic carbenes (NHCs) have established themselves as a popular class of stereodirecting ancillary ligands to catalyze enantioselective organic transformations in more efficient ways. Several novel transition‐metal complexes in combination with tailored ligand design have emerged during last few decades in asymmetric catalysis. The tailor‐made NHCs can easily be accessed due to the modular synthesis of their parent azolium salt precursors. Their donor capability and the molecular shape can easily be tuned by changing substituent at N‐atom or by changing the cyclic backbone framework. This review article aims to describe the recent advances in this rapidly evolving research area of enantioselective catalysis using well‐defined transition‐metal complexes possessing chiral NHC donor ligands.

Trinuclear PdII Complexes with a C3‐Symmetric Triethynylbenzene‐Based Tris‐NHC Ligand: Catalytic Benefits

“…The palladium‐nitrogen bond distances in the palladium(II) coordinated pyridine moieties are measured as 2.028(13), 2.106(15) and 2.04(2) Å. These bond distances along with C NHC −Pd−N Py bond angles (177.2(7)°, 176.3(10)° and 173.1(10)°) fall in the range that has been previously described for the Pd II NHC complexes [7a,8c] . Interestingly, the central benzene ring, three alkynyl moieties, and three 5‐membered NHC rings are almost coplanar in the complex [ 5 ].…”

“…These resonances appear in the range previously reported for cis‐ (24.18 ppm) and trans‐ (16.27 ppm) type complexes. We take this as indication for the presence of both cis ‐ and trans ‐ coordinated palladium atoms [7a,8c,16,17] . The 13 C{ 1 H} NMR spectrum of the complex also showed the characteristic signal for the carbene carbon atoms for both isomeric complexes at δ =165.3 and 159.3 ( 2 J =190.1 Hz) ppm.…”

Palladium(II) complexes bearing mesoionic carbene/phosphine donor ligands: Catalytic applications in various C–C coupling reactions and redox activities

“…The chemistry of carbenes featuring N-heterocycles has succeeded in drawing the attention of the researchers for their pervasive applications in various research areas such as metal N-heterocyclic carbene (NHC) complexes with biological activity, 1 ligands in organometallic chemistry, 2 spectator ligands in homogeneous catalysis, 3 NHC-based self-assembly, 4 and also in asymmetric catalysis. 5 Besides the traditionally dominated research on classical NHCs such as 1,2,4-triazolylidenes or imidazol-2-ylidenes, the mesoionic 1,2,3-triazol-5-ylidenes (MICs or αNHCs) counterparts have also gained immense popularity as a branch of NHCs showing better sigma donating ability than the corresponding classical counterparts. 6 Subsequently, the complexes with mesoionic carbene ligands have been extensively employed as catalysts in reduction of aromatic nitro compounds, 7 Suzuki-Miyaura cross-coupling reactions, 8 hydroarylation of alkynes, 9 "click" reactions, 10 cyclization reactions, 11 and oxidation reactions.…”

Enantioselective Hydroboration of Styrenes with Markovnikov Selectivity Catalyzed by a Rhodium(I)-NHC Complex

“…Since then, chiral NHCs have found a large application in asymmetric catalysis (Figure ). However, compared to their phosphine counterparts, their incorporation in asymmetric catalysis is still underrepresented . One such underrepresented field is the enantioselective hydroboration of styrenes, which is highly dependent on phosphine-ligated rhodium catalysts .…”