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Easily-preparable N-Heterocyclic Carbene (NHC) Precursors

 NHC-CO2 adducts and NHC hydrogencarbonate salts can be easily prepared for NHCs by heating. Generally, imidazolium halide precursors require pre-treatment with a base followed by salt removal, or to be coexistent in the system during the reactions. In comparison, NHC-CO2 adducts and NHC hydrogencarbonate salts release CO2 and H2O during the formation of the active NHC. As a result, the active NHC can be used as it is or immediately after dehydration.

Advantages

• NHC ligands and catalysts are preparable by heating.

• No salt generated during preparation.

• Applicable under neutral conditions.

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NHC-CO2 Adducts: Transition metal / NHC complex catalyst preparation1)

NHC-CO2 Adducts: Transition metal / NHC complex catalyst preparation

Procedure:
A mixture of [Rh(cod)Cl]2 (54 mg) and NHC-CO2 adduct (2 eq.) is stirred in acetonitrile (3 mL) for 5 min at room temperature in a Schlenk flask, followed by heating at 75 °C for 20 min under argon atmosphere. The reaction mixture is dried in vacuo, and washed three times with diethyl ether. Yellow solid is obtained in 93% yield.

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NHC Hydrogencarbonate Salts: Transition metal / NHC complex catalyst preparation2)

NHC Hydrogencarbonate Salts: Transition metal / NHC complex catalyst preparation

Procedure:
(CH3)2SAuCl (11.4 mg), NHC hydrogencarbonate salt (1.2 eq.), and THF (0.7 mL) are put in a capped vial (air atmosphere). After 1 h of stirring at 50 °C, the solution is filtered over silica gel and dried in vacuo. The Au-NHC complex is obtained as a colorless solid in 95% yield.

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NHC-CO2 Adducts: NHC-Catalyzed Conjugate Cyanations3)

 

NHC-CO2 Adducts: NHC-Catalyzed Conjugate Cyanations

Procedure:
1,4-Dioxane (0.5 mL) is added to a mixture of a p-quinone methide derivative (0.062 mmol), trimethylsilyl cyanide (1.3 eq.) and NHC-CO2 adduct (10 mol%) under argon atmosphere and the resulting mixture is stirred at room temperature. After the reaction is completed, the mixture is quenched with 5 mL of aqueous sodium thiosulfate and extracted with diethyl ether (3 x 5 mL). The combined organic layer is dried over anhydrous sodium sulfate and concentrated under reduced pressure. The residue is then purified through a silica gel column chromatography to give the pure product.

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NHC Hydrogencarbonate Salts: NHC-Catalyzed Benzoin Condensations2)

NHC Hydrogencarbonate Salts: NHC-Catalyzed Benzoin Condensations

Procedure:
NHC hydrogencarbonate salt (10 mol%) and molecular sieves 3A are introduced into a Schlenk tube. The powder is submitted to three Ar/vacuum cycles. Dry THF (2 mL), and then benzaldehyde (0.2 mL, 2 mmol) are added. Conversion to benzoin is calculated as 83% by 1H-NMR in CDCl3 after the reaction for 24 h at 60 °C.

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