IMes (IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene) and IPr (IPr=1,3‐ bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) represent by far the most frequently used N‐heterocyclic carbene ligands in homogeneous catalysis, however, despite numerous advantages, these ligands are limited by the lack of steric flexibility of catalytic pockets. We report a new class of unique unsymmetrical N‐heterocyclic carbene ligands that are characterized by freely‐rotatable N‐aromatic wingtips in the imidazol‐2‐ylidene architecture. The combination of rotatable N−CH2Ar bond with conformationally‐fixed N−Ar linkage results in a highly modular ligand topology, entering the range of geometries inaccessible to IMes and IPr. These ligands are highly reactive in Cu(I)‐catalyzed β‐hydroboration, an archetypal borylcupration process that has had a transformative impact on the synthesis of boron‐containing compounds. The most reactive Cu(I)‐NHC in this class has been commercialized in collaboration with MilliporeSigma to enable broad access of the synthetic chemistry community. The ligands gradually cover %Vburgeometries ranging from 37.3 % to 52.7 %, with the latter representing the largest %Vburdescribed for an IPr analogue, while retaining full flexibility of N‐wingtip. Considering the modular access to novel geometrical space in N‐heterocyclic carbene catalysis, we anticipate that this concept will enable new opportunities in organic synthesis, drug discovery and stabilization of reactive metal centers.
- Award ID(s):
- 1904825
- NSF-PAR ID:
- 10230511
- Date Published:
- Journal Name:
- Inorganic Chemistry Frontiers
- Volume:
- 8
- Issue:
- 1
- ISSN:
- 2052-1553
- Page Range / eLocation ID:
- 59 to 71
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract IMes (IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene) and IPr (IPr=1,3‐ bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) represent by far the most frequently used N‐heterocyclic carbene ligands in homogeneous catalysis, however, despite numerous advantages, these ligands are limited by the lack of steric flexibility of catalytic pockets. We report a new class of unique unsymmetrical N‐heterocyclic carbene ligands that are characterized by freely‐rotatable N‐aromatic wingtips in the imidazol‐2‐ylidene architecture. The combination of rotatable N−CH2Ar bond with conformationally‐fixed N−Ar linkage results in a highly modular ligand topology, entering the range of geometries inaccessible to IMes and IPr. These ligands are highly reactive in Cu(I)‐catalyzed β‐hydroboration, an archetypal borylcupration process that has had a transformative impact on the synthesis of boron‐containing compounds. The most reactive Cu(I)‐NHC in this class has been commercialized in collaboration with MilliporeSigma to enable broad access of the synthetic chemistry community. The ligands gradually cover %Vburgeometries ranging from 37.3 % to 52.7 %, with the latter representing the largest %Vburdescribed for an IPr analogue, while retaining full flexibility of N‐wingtip. Considering the modular access to novel geometrical space in N‐heterocyclic carbene catalysis, we anticipate that this concept will enable new opportunities in organic synthesis, drug discovery and stabilization of reactive metal centers.
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Although N-heterocyclic carbenes (NHCs) have been known as ligands for organometallic complexes since the 1960s, these carbenes did not attract considerable attention until Arduengo et al. reported the isolation of a metal-free imidazol-2-ylidene in 1991. In 2001 Crabtree et al. reported a few complexes featuring an NHC isomer, namely an imidazol-5-ylidene, also termed abnormal NHC (aNHCs). In 2009, it was shown that providing to protect the C-2 position of an imidazolium salt, the deprotonation occurred at the C-5 position, affording imidazol-5-ylidenes that could be isolated. Over the last ten years, stable aNHCs have been used for designing a range of catalysts employing Pd( ii ), Cu( i ), Ni( ii ), Fe(0), Zn( ii ), Ag( i ), and Au( i / iii ) metal based precursors. These catalysts were utilized for different organic transformations such as the Suzuki–Miyaura cross-coupling reaction, C–H bond activation, dehydrogenative coupling, Huisgen 1,3-dipolar cycloaddition (click reaction), hydroheteroarylation, hydrosilylation reaction and migratory insertion of carbenes. Main-group metal complexes were also synthesized, including K( i ), Al( iii ), Zn( ii ), Sn( ii ), Ge( ii ), and Si( ii / iv ). Among them, K( i ), Al( iii ), and Zn( ii ) complexes were used for the polymerization of caprolactone and rac -lactide at room temperature. In addition, based on the superior nucleophilicity of aNHCs, relative to that of their nNHCs isomers, they were used for small molecules activation, such as carbon dioxide (CO 2 ), nitrous oxide (N 2 O), tetrahydrofuran (THF), tetrahydrothiophene and 9-borabicyclo[3.3.1]nonane (9BBN). aNHCs have also been shown to be efficient metal-free catalysts for ring opening polymerization of different cyclic esters at room temperature; they are among the most active metal-free catalysts for ε-caprolactone polymerization. Recently, aNHCs successfully accomplished the metal-free catalytic formylation of amides using CO 2 and the catalytic reduction of carbon dioxide, including atmospheric CO 2 , into methanol, under ambient conditions. Although other transition metal complexes featuring aNHCs as ligand have been prepared and used in catalysis, this review article summarize the results obtained with the isolated aNHCs.more » « less
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Abstract The role of ligands in rhodium‐ and iridium‐catalyzed
Para hydrogen Induced Polarization (PHIP) and SABRE (signal amplification by reversible exchange) chemistry has been studied in the benchmark systems, [Rh(diene)(diphos)]+and [Ir(NHC)(sub)3(H)2]+, and shown to have a great impact on the degree of hyperpolarization observed. Here, we examine the role of the flanking moieties in the electron‐rich monoanionic bis(carbene) aryl pincer ligand,ArCCC (Ar=Dipp, 2,6‐diisopropyl or Mes, 2,4,6‐trimethylphenyl) on the cobalt‐catalyzed PHIP and PHIP‐IE (PHIP via Insertion and Elimination) chemistry that we have previously reported. The mesityl groups were exchanged for diisopropylphenyl groups to generate the (DippCCC)Co(N2) catalyst, which resulted in faster hydrogenation and up to 390‐fold1H signal enhancements, larger than that of the (MesCCC)Co‐py (py=pyridine) catalyst. Additionally, the synthesis of the (DippCCC)Rh(N2) complex is reported and applied towards the hydrogenation of ethyl acrylate withpara hydrogen to generate modest signal enhancements of both1H and13C nuclei. Lastly, the generation of two (MesCCC)Ir complexes is presented and applied towards SABRE and PHIP‐IE chemistry to only yield small1H signal enhancements of the partially hydrogenated product (PHIP) with no SABRE hyperpolarization. -
The crystal structures of ligand precursor bis(imidazolium) salts 1,1′-methylenebis(3- tert -butylimidazolium) dibromide monohydrate, C 15 H 26 N 4 + ·2Br − ·H 2 O or [ t Bu NHC 2 Me][Br] 2 ·H 2 O, 1,1′-(ethane-1,2-diyl)bis(3- tert -butylimidazolium) dibromide dihydrate, C 16 H 28 N 4 + ·2Br − ·2H 2 O or [ t Bu NHC 2 Et][Br] 2 ·2H 2 O, 1,1′-methylenebis[3-(2,4,6-trimethylphenyl)imidazolium] dibromide dihydrate, C 25 H 30 N 4 2+ ·2Br − ·2H 2 O or [ Mes NHC 2 Me][Br] 2 ·2H 2 O, and 1,1′-(ethane-1,2-diyl)bis[3-(2,4,6-trimethylphenyl)imidazolium] dibromide tetrahydrate, C 26 H 32 N 4 2+ ·2Br − ·4H 2 O or [ Mes NHC 2 Et][Br] 2 ·4H 2 O, are reported. At 293 K, [ t Bu NHC 2 Me][Br] 2 ·H 2 O crystallizes in the P 2 1 / c space group, while [ t Bu NHC 2 Et][Br] 2 ·2H 2 O crystallizes in the P 2 1 / n space group at 100 K. At 112 K, [ Mes NHC 2 Me][Br] 2 ·2H 2 O crystallizes in the orthorhombic space group Pccn while [ Mes NHC 2 Et][Br] 2 ·4H 2 O crystallizes in the P 2 1 / c space group at 100 K. Bond distances and angles within the imidazolium rings are generally comparable among the four structures. All four bis(imidazolium) salts co-crystallize with one to four molecules of water.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0QI00638F
