2. Experimental section
2.1 Materials
All chemicals used in this work were purchased from commercial reagent suppliers and used without further purification: acetophenone (Sigma-Aldrich, 99%), sodium formate (Sigma-Aldrich, 99.0%), RuCl(p-cymene)[(S,S)-Ts-DPEN] (Sigma-Aldrich), 1-butanol (Sigma-Aldrich, 99.8%), 1-pentanol (Sigma-Aldrich, ≥ 99%), 1-hexanol (Alfa Aesar, 99%), 1-heptanol (Sigma-Aldrich, 98%), 1-octanol (Sigma-Aldrich, ≥ 99%).
2.2 Experimental apparatus and procedure
The phase transfer hydrogenation of acetophenone to produce 1-phenylethanol was studied in a specially made hollow cylindrical glass reactor with two ports for electrodes on the ends. The inner diameter of the reactor is about 20 mm. Details of the experimental unit may be found elsewhere.25 Briefly, the biphasic system was set up with a sodium formate/water solution as the more dense aqueous phase and acetophenone/organic solvents as the less dense organic phase in which the Ru complex (RuCl(p-cymene)[(S,S)-Ts-DPEN]) was dissolved. The organic solvents used in conjunction with acetophenone were 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, and 1-octanol. An external electric field (DC voltage) from a high voltage power supply (PASCO SF-9585A) was applied across the two phases without agitation. Stainless steel or titanium electrodes were used. In a single experimental study, 0.34 g of sodium formate was fully dissolved in 1.5 mL of deionized water, while 117 µL of acetophenone was mixed with 1.5 mL of organic solvent to dissolve 0.0064 g Ru catalyst. The reactions were conducted at room temperature and pressure with positive or negative electric field applied during reaction. The detailed orientations of the electric field have been described previously.25 A positive orientation in this work is defined as the direction of the electric field when the anode is placed in the organic phase with the cathode in the aqueous phase. The conversion of acetophenone, total yield of 1-phenylethanol, and enantioselectivity of (S)-1-phenylethanol were determined by gas chromatography analysis.
2.3 Gas chromatography (GC) analysis
10 µL of liquid samples from the organic phase were withdrawn after each reaction and diluted to 1.5 mL with methanol for gas chromatography analysis. An Agilent 6890N gas chromatograph equipped with a CP-Chirasil-Dex CB column (25m×0.25 mm) and a flame ionization detector was used to identify reaction products and determine enantioselectivity under the following conditions: helium as carrier gas (3 mL/min), inlet and detector temperature 250 °C, oven temperature 115°C.
2.4 Nuclear Magnetic Resonance (NMR) spectroscopy analysis
Four samples were prepared for NMR analysis. Sample 1 was prepared by dissolving 5.8 mg of RuCl(p-cymene)[(S,S)-Ts-DPEN] in 500 µL of MeOH-d4 (Cambridge Isotopes). Sample 2 was obtained by dissolving 6.4 mg of RuCl(p-cymene)[(S,S)-Ts-DPEN] in a mixture of 1.5 mL 1-butanol and 117 µL acetophenone. Samples 3 and 4 were collected from reactions under positive and negative 15 V of electric potential respectively after 24 h with titanium electrodes and 1-butanol as solvent. 500 µL of each liquid sample were pipetted into a 5 mm NMR tube (Wilmad Lab-Glass, Vineland, NJ). All NMR spectra were acquired on a 500 MHz Bruker AVIII spectrometer equipped with a cryogenically cooled X-channel observe probe. All data was analyzed using MestreNova NMR software (Santiago De Compostela, Spain).
2.5 X-ray florescence (XRF) analysis
A handheld XRF analyzer (Olympus, Delta Professional) with Ru anode and silicon drift detector was used for fast determination of the metal contents in the degradation products of stainless-steel electrode. These solid samples were collected from the interface after reaction and dried prior for analysis. Precious metal mode with a targeted collimator was employed during analysis. All samples were scanned three times and analyzed with the workstation setup.