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10 µL of liquid samples from the organic phase were withdrawn after each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.