Bidentate Donor-Functionalized N-Heterocyclic Carbenes: Valuable Ligands for Ruthenium-Catalyzed Transfer Hydrogenation

Abstract

Ruthenium complexes are by far the most studied compounds that catalyze hydrogen transfer reactions. In this review, we describe the use in this field of ruthenium complexes bearing bidentate donor-functionalized N-heterocyclic carbene ligands. The review specifically covers the application in transfer hydrogenations of (k²-C_NHC,Y)-ruthenacyclic compounds where the Y donor atom is a N, P, O, or S atom, and where the N-heterocyclic carbene ligand is a classical imidazol-2-ylidene, a benzimidazol-2-ylidene, a mesoionic 1,2,3-triazolylidene, or an imidazol-4-ylidene ligand. Tridentate donor-functionalized N-heterocyclic carbene complexes thus fall outside the scope of the review. Applications in (asymmetric) transfer hydrogenation of ketones, aldehydes, imines, alkenes, and nitrobenzene are discussed.

Keywords: bidentate N-heterocyclic carbene; ruthenium; transfer hydrogenation.

Scheme 1

TH of acetophenone and 2-octanone catalyzed by the oxazolinyl-functionalized complex 1a.

Scheme 2

TH of acetophenone and cyclohexanone catalyzed by the pyrimidine-functionalized complex 2.

Scheme 3

TH of acetophenone by the amino-functionalized NHC-Ru(II) complex 3.

Scheme 4

TH of ketones by the amino-functionalized imidazol-2-ylidene complex 4a.

Scheme 5

TH of acetophenone catalyzed by the amino-functionalized NHC-Ru(II) complex 5.

Scheme 6

TH of ketones catalyzed by the picolyl-functionalized NHC-Ru(II) complex 6a.

Figure 1

(η⁶-p-cymene)Ru(II) complexes 7a–e bearing picolyl-NHC ligands.

Scheme 7

Deuterium labeling experiment with 7a establishing a monohydride mechanism.

Scheme 8

TH of acetophenone catalyzed by the imino-functionalized imidazol-2-ylidene-Ru(II) complexes 8a–c.

Scheme 9

TH of benzophenone catalyzed by the pyridyl-functionalized mesoionic triazolylidene complexes 9a,b and 10.

Scheme 10

TH of various ketones catalyzed by the pyridyl- or pyrimidyl-functionalized mesoionic triazolylidene complexes 11a–g and 12.

Scheme 11

TH of benzophenone catalyzed by the picolyl-triazolylidene complexes 13a–c.

Figure 2

Pyridyl-functionalized triazolylidene complexes 14a–c and 16 bearing a pendant carboxylic group and unfunctionalized complex 15.

Figure 3

Pyridyl-functionalized imidazol-2-ylidene complexes 17–20.

Scheme 12

TH of acetophenone in aqueous solution catalyzed by the water soluble complex 17f.

Scheme 13

TH of acetophenone from glycerol catalyzed by the bis-sulfonated complex 21.

Figure 4

Triazole- and pyrazole-functionalized imidazole-2-ylidene complexes 22–27.

Scheme 14

Low-temperature TH of ketones catalyzed by the anion-mixed pyrazole-imidazol-2-ylidene complex 22b.

Scheme 15

Solvent-dependent resonance forms of the pyridylideneamide-imidazol-2-ylidene-Ru(II) complexes 28a,b.

Scheme 16

Solvent dependence of the TH of benzophenone catalyzed by the adaptive pyridylideneamide-triazolylidene complex 29.

Scheme 17

TH of benzophenone catalyzed by the pycolyl-functionalized imidazol-4-ylidene complexes 30 and 31/32.

Figure 5

Pyridyl- or picolyl-functionalized imidazol-, benzimidazol- or triazol-2-ylidene octahedral Ru(II) complexes 33–39.

Scheme 18

Low-temperature TH of aromatic ketones catalyzed by the aqua-Ru(II) complex 40 bearing a 1,8-naphtyrid-2-yl-imidazol-2-ylidene chelate.

Scheme 19

TH of ketones catalyzed by the pyrimidin-2-yl-functionalized imidazol-2-ylidene complexes 41 and 42.

Scheme 20

Synthesis of the hydride complex 44 from the protic NHC complex 43 (A); TH of acetophenone catalyzed by 43 and 44 (B); Proposed TH mechanism (C).

Scheme 21

TH of ketones catalyzed by the ortho-(diphenylphosphino)benzyl-functionalized imidazol-2-ylidene complex 45.

Scheme 22

TH of acetophenone catalyzed by the (diphenylphosphino)ethylene-functionalized imidazol-4-ylidene complexes 46 and 47.

Scheme 23

TH of ketones catalyzed by the (diphenylphosphino)methylene-functionalized imidazol-2-ylidene and imidazol-4-ylidene complex 48.

Scheme 24

TH of ketones catalyzed by the (diphenylphosphino)methylene-functionalized bis-imidazol-4-ylidene complex 49.

Scheme 25

Formation of the dihydride bis-imidazol-2-ylidene complex 50 from 49.

Scheme 26

TH of benzophenone catalyzed by the carboxylate-functionalized imidazol-2-ylidene and 1,2,3-triazolylidene complexes 51a, 52 and 53.

Figure 6

Carboxylate-functionalized NHC-Ru(II) complexes 51b–d.

Scheme 27

TH of ketones catalyzed by the carboxylate-functionalized benzimidazolylidene complex 51d.

Scheme 28

TH of benzophenone catalyzed by the hydroxy-functionalized 1,2,3-triazolylidene complexes 54 and 55 and the 1,4-dibutyl-triazolylidene complex 56.

Scheme 29

ATH attempt of acetophenone with the (S,S)-1,2-diphenylethylamine-functionalized imidazol-2-ylidene complex 57.

Scheme 30

ATH of acetophenone with the hydroxy-amide-functionalized benzimidazol-2-ylidene (η⁶-arene)Ru(II) complexes 58a–c.

Scheme 31

Synthesis of the catalytically inactive dianionic alkoxy-amidate-NHC-Ru complex 59.

Scheme 32

TH of benzaldehyde catalyzed by the pyridine- or pyrimidine-functionalized triazolylidene complexes 11c and 12.

Scheme 33

TH of benzaldehyde from glycerol catalyzed by the bis-sulfonated complex 21.

Scheme 34

TH of N-benzylidineaniline catalyzed by complexes 2, 6a, 7a, 8a, 11c, 12 and 27.

Scheme 35

TH of N-benzylidineaniline from glycerol catalyzed by the bis-sulfonated complex 21.

Scheme 36

TH of N-benzylidineaniline catalyzed by the carboxylate-functionalized benzimidazol-2-ylidene complex 51c.

Scheme 37

TH of alkenes by the pyridyl- and pyrimidyl-functionalized triazolylidene complexes 11c and 12.

Scheme 38

TH of chalcone catalyzed by the carboxylate-functionalized benzimidazol-2-ylidene complex 51c.

Scheme 39

TH of 1-dodecene catalyzed by the thioether-functionalized imidazol-2-ylidene complex 60.

Scheme 40

TH of nitrobenzene to aniline catalyzed by the pyridyl- and pyrimidyl-functionalized triazolylidene complexes 11c and 12.

Scheme 41

Proposed mechanism for the TH of nitrobenzene to aniline catalyzed by 11c and 12.