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C-H arylation and alkenylation of imidazoles by nickel catalysis: solvent-accelerated imidazole C-H activation - PubMed

  • ️Thu Jan 01 2015

. 2015 Dec 1;6(12):6792-6798.

doi: 10.1039/c5sc02942b. Epub 2015 Sep 8.

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C-H arylation and alkenylation of imidazoles by nickel catalysis: solvent-accelerated imidazole C-H activation

Kei Muto et al. Chem Sci. 2015.

Abstract

The first nickel-catalyzed C-H arylations and alkenylations of imidazoles with phenol and enol derivatives are described. Under the influence of Ni(OTf)2/dcype/K3PO4 (dcype: 1,2-bis(dicyclohexylphosphino)ethane) in t-amyl alcohol, imidazoles can undergo C-H arylation with phenol derivatives. The C-H arylation of imidazoles with chloroarenes as well as that of thiazoles and oxazoles with phenol derivatives can also be achieved with this catalytic system. By changing the ligand to dcypt (3,4-bis(dicyclohexylphosphino)thiophene), enol derivatives could also be employed as coupling partners achieving the C-H alkenylation of imidazoles as well as thiazoles and oxazoles. Thus, a range of C2-arylated and alkenylated azoles can be synthesized using this newly developed nickel-based catalytic system. The key to the success of the C-H coupling of imidazoles is the use of a tertiary alcohol as solvent. This also allows the use of an air-stable nickel(ii) salt as the catalyst precursor.

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Figures

Fig. 1
Fig. 1. C2-arylated and alkenylated imidazoles and benzimidazoles in natural products, pharmaceuticals, and organic materials.
Fig. 2
Fig. 2. Transition metal-catalyzed C–H arylation of imidazoles and benzimidazoles.
Scheme 1
Scheme 1. Substrate scope of imidazole C–H arylation with phenol derivatives. a Unless otherwise noted, the reaction conditions were as follows: 1 (0.40 mmol), 2 (1.5 equiv.), Ni(OTf)2 (10 mol%), dcype (12 mol%), K3PO4 (3.0 equiv.), t-amylOH (1.6 mL), 110 °C, and 12–36 h. b Ni(cod)2 (10 mol%) was used. c Ni(cod)2 (10 mol%) and dcypt (12 mol%) were used. d Starting from ethyl 4-imidazolecarboxylate 1P. (R = Et: 33%; t-amyl: 60%). e Starting from methyl 3-((dimethylcarbamoyl)oxy)benzoate (2f).
Scheme 2
Scheme 2. Dramatic effect of dcypt ligand in the C–H alkenylation of imidazoles.
Scheme 3
Scheme 3. Scope of imidazole C–H alkenylation with Ni(OTf)2/dcypt. a Unless otherwise noted, the reaction conditions were as follows: 1 (1.0 mmol), 2 (1.5 equiv.), Ni(OTf)2 (10 mol%), dcypt (12 mol%), K3PO4 (3.0 equiv.), t-amylOH (4.0 mL), 120 °C, and 36 h.
Scheme 4
Scheme 4. C–H arylation of imidazoles with chloroarenes with Ni(OTf)2/dcype. a Unless otherwise noted, the reaction conditions were as follows: 1 (0.40 mmol), 4 (1.5 equiv.), Ni(OTf)2 (10 mol%), dcype (12 mol%), K3PO4 (3.0 equiv.), t-amylOH (1.6 mL), 110 °C, and 36 h. b Ni(cod)2 (10 mol%) was used.
Scheme 5
Scheme 5. Comparison of the previous and current conditions for the reaction of thiazole 5A.
Scheme 6
Scheme 6. C–H arylation and alkenylation of oxazoles and thiazoles with Ni(OTf)2/dcype/K3PO4 in t-amylOH. a Unless otherwise noted, the reaction conditions were as follows: 5 (0.40 mmol), 2 (1.5 equiv.), Ni(OTf)2 (10 mol%), dcypt (12 mol%), K3PO4 (3.0 equiv.), t-amylOH (1.6 mL), 110 °C, and 36 h. b Ni(OTf)2 (10 mol%) and dcypt (12 mol%) were used. c Ni(OTf)2 (10 mol%) and dcypt (12 mol%) were used.
Fig. 3
Fig. 3. A plausible catalytic cycle.

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