Selective C–C Coupling Reaction of Dimethylphenol to Tetramethyldiphenoquinone Using Molecular Oxygen Catalyzed by Cu Complexes Immobilized in Nanospaces of Structurally-Ordered Materials

Two high-performance Cu catalysts were successfully developed by immobilization of Cu ions in the nanospaces of poly(propylene imine) (PPI) dendrimer and magadiite for the selective C–C coupling of 2,6-dimethylphenol (DMP) to 3,3',5,5'-tetramethyldiphenoquinone (DPQ) with O2 as a green oxidant. The PPI dendrimer encapsulated Cu ions in the internal nanovoids to form adjacent Cu species, which exhibited significantly high catalytic activity for the regioselective coupling reaction of DMP compared to previously reported enzyme and metal complex catalysts. The magadiite-immobilized Cu complex acted as a selective heterogeneous catalyst for the oxidative C–C coupling of DMP to DPQ. This heterogeneous catalyst was recoverable from the reaction mixture by simple filtration, reusable without loss of efficiency, and applicable to a continuous flow reactor system. Detailed characterization using ultraviolet-visible (UV-vis), Fourier transform infrared (FTIR), electronic spin resonance (ESR), and X-ray absorption fine structure (XAFS) spectroscopies and the reaction mechanism investigation revealed that the high catalytic performances of these Cu catalysts were ascribed to the adjacent Cu species generated within the nanospaces of the PPI dendrimer and magadiite.


Preparation of Cu 2+ (mono)-Magadiite
Cu 2+ (mono)-magadiite was prepared in the same manner as reported by Kim et al. [1]. A Cu mononuclear complex Cu(ethylenediamine)2(ClO4)2 was synthesized by addition of ethylenediamine (1.3 mL, 20 mmol) to the MeOH solution containing Cu(ClO4)2·6H2O (10 mmol), and the resulting solution was refluxed for 4 h. After the reaction, the solution was cooled to room temperature to precipitate the purple solid. The precipitate was filtered and washed with ethanol to afford Cu(ethylenediamine)2(ClO4)2. Next, Cu(ethylenediamine)2(ClO4)2 (100 µmol) was dissolved in EtOH (10 mL), and magadiite (0.4 g) was added into the above solution. The resulting mixture was further stirred for 6 h at 313 K. After the reaction, the obtained solid was filtered, washed with EtOH (100 mL), and dried to afford a light purple powder.

Reuse Experiment of G 4 -Cu 2+ 12
1a (0.5 mmol) was reacted using fresh G4-Cu 2+ 12 (Cu: 5 µmol) in 4 mL α,α,α-trifluorotoluene (TFT), after which the synthesized 2a was removed from the reaction mixture by filtration. 1a (0.5 mmol) was added to the filtered TFT solution, still containing the original G4-Cu 2+ 12, and the mixture was vigorously stirred at 323 K for 18 h. By repeating this process, it was determined that the G4-Cu 2+ 12 catalyst could be reused without significant loss of its activity and selectivity; the yield of 2a from the first reaction, as determined by 1 H NMR, was 96%, while the isolated yields were 83% (fresh catalyst), and 93% (first reuse).

S2 ESR Measurement
The spectra were recorded at the Xband using a Bruker EMX-10/12 spectrometer with a 100 kHz magnetic field modulation at a microwave power level of 10.0 mW. G4-Cu 2+ n (n = 2, 8, 12, 16, and 24; Cu: 0.5 µmol) in CHCl3 (0.1 mL) was placed in a quartz ESR tube under an Ar atmosphere, and measured at 298 K. In the case of the heterogeneous Cu catalysts, the Cu catalyst (Cu: 0.5 µmol) was introduced in a quartz ESR tube, evacuated at room temperature, and subjected to analysis at 298 K.

XAFS Measurement
The Cu K-edge XAFS spectrum of G4-Cu 2+ 12 was recorded in transmission mode at room temperature. Fourier transforms of k 3 -weighted EXAFS spectra were performed in the 4 Å < k < 12 Å range to obtain radial structural functions. Curve-fitting analysis was performed with the inverse FT of the 1.2 Å < R < 2.4 Å range. The coordination numbers (CN) and interatomic distances (R) were estimated by curve-fitting analysis using Cu-Cl and Cu-N shell parameters obtained from reference samples of CuCl2 [2] and Cu(ImH)4SO4 [3], respectively. In the case of Cu 2+ -magadiite, the spectrum was recorded at 10 K using a cryostat. Curve-fitting analysis was conducted with the inverse FT of the 1.0 Å < R < 3.3 Å range, and the CN and R were estimated by curve-fitting analysis using Cu-N/O and Cu-Cu shell parameters obtained from a reference sample of [Cu(OH)TMEDA]2Cl2 [4].

Product Identification
The reaction products were identified by 1 H and 13 C NMR and, in each case, the chemical shifts of the products were in agreement with those reported in the literature, as summarized below. 3,3'-Di-tert-butyl-5,5'-dimethyldiphenoquinone (2c) (CAS-RN 2417-00-1) [5] (Likely a mixture of cis and trans isomers [7]).    Figure S4. Simultaneous in situ UV-vis spectra recorded during G4-Cu 2+ 12-catalyzed oxidative coupling of DMP under an Ar atmosphere as shown in Scheme S1.