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The final ionic liquids were dried in a vacuum line (∼0.1 132.2, 159.2; IR (ν/cm−1) 3320, 3012, 2959, 1681, 1601, 1513,
Torr) for 48 h and kept under a nitrogen atmosphere. The 1463, 1249, 1035.
amount of residual water was determined to be 500–700 ppm
using Karl-Fisher coulometric titration (Karl Fisher 652 White solid (0.028 g, 56%); H NMR (CDCl3, 400 MHz) δ 4.64
1,2-Di(4-chlorophenyl)ethane-1,2-diol [4d] (meso/dl mixture).
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Metrohm).
(s, 2H, dl), 4.86 (s, 2H, meso), 7.03–7.28 (m, 8H); 13C NMR
(CDCl3, 100 MHz) δ 76.8, 78.6, 128.4, 128.5, 128.8, 133.9,
138.1; IR (ν/cm−1) 3390, 3034, 2895, 1697, 1604, 1495, 1213.
1,2-Di(4-bromophenyl)ethane-1,2-diol [4e] (meso/dl mixture).
General procedure for the electrochemical pinacol coupling
reaction
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White solid (0.040 g, 62%); H NMR (CDCl3, 400 MHz) δ 4.59
The carbonyl compound (0.35 mmol) and a mixture of 80%
[BMIM][BF4]–H2O (4.0 mL of [BMIM][BF4] and 1.0 mL of H2O)
were placed in an electrochemical cell fitted with a sacrificial
tin foil anode (1.0 cm2) and a platinum plate cathode
(1.0 cm2). The mixture was stirred and degassed by bubbling
nitrogen for 30 minutes. A controlled potential of −2.0 V vs.
the Ag/AgCl reference electrode was applied under a nitrogen
atmosphere for 5 hours using a BASi PWR-3 potentiostat. The
resulting solution was filtered to remove any insoluble tin salts
and extracted with diethyl ether (2 × 5 mL). The ether extract
was dried with anhydrous sodium sulfate, filtered and concen-
trated in vacuo. When necessary, the residue was purified via
silica gel flash column chromatography (10% ethyl acetate in
a hexane eluent) to afford the final product. The product
structures and diastereomeric ratios were confirmed by NMR
spectroscopy. The NMR spectra were recorded on a JOEL
eclipse 400 MHz spectrometer in chloroform-d.
(s, 2H, dl), 4.82 (s, 2H, meso), 6.98–7.24 (m, 8H); 13C NMR
(CDCl3, 100 MHz) δ 78.4, 128.4, 128.7, 129.1, 133.8, 138.2; IR
(ν/cm−1) 3393, 3020, 3010, 2908, 1620, 1494, 1199, 1047, 912.
2,3-Diphenylbutane-2,3-diol [6a] (meso/dl mixture). White
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solid (0.039 g, 91%); H NMR (CDCl3, 400 MHz) δ 1.50 (s, 6H,
dl), 1.58 (s, 6H, meso), 2.64 (s, 2H, OH), 7.23–7.25 (m, 10H);
13C NMR (CDCl3, 100 MHz) δ 25.0, 78.7, 78.9, 127.1, 127.2,
127.5, 143.5; IR (ν/cm−1) 3480, 3070, 2980, 1640, 1446, 1062.
2,3-Di(4-methylphenyl)butane-2,3-diol [6b] (meso/dl mixture).
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White solid (0.044 g, 86%); H NMR (CDCl3, 400 MHz) δ 1.45
(s, 6H, dl), 1.53 (s, 6H, meso), 2.34 (s, 6H), 2.64 (s, 2H, OH),
7.04–7.14 (m, 8H); 13C NMR (CDCl3, 100 MHz) δ 21.1, 25.1,
25.3, 78.7, 79.0, 126.9, 127.4, 127.9, 128.1, 136.2, 136.5, 136.6,
140.7, 141.0; IR (ν/cm−1) 3606, 2984, 2925, 1512, 1372, 909.
2,3-Di(4-methoxylphenyl)butane-2,3-diol [6c] (meso/dl mixture).
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White solid (0.027 g, 50%); H NMR (CDCl3, 400 MHz) δ 1.45
(s, 6H, dl), 1.54 (s, 6H, meso), 3.79 (s, 6H), 6.76 (d, J = 8 Hz,
4H), 7.09 (d, J = 8 Hz, 4H); 13C NMR (CDCl3, 100 MHz) δ 25.1,
55.3, 78.7, 78.9, 112.5, 112.6, 128.2, 128.6, 135.8, 158.6; IR
(ν/cm−1) 3528, 2959, 2954, 1655, 1611, 1511, 1249.
Recycling of RTIL
After extraction of the pinacol product, 5.0 mL of methylene
chloride was added to the separatory funnel containing a
[BMIM][BF4]–aqueous layer. The organic layer was separated
and the aqueous layer was extracted with another portion of
methylene chloride (5.0 mL). The combined organic layers
were dried with anhydrous sodium sulfate, filtered and con-
centrated using a rotary evaporator. The RTIL was then dried
in a vacuum line for 24 h prior to use in the next experiment.
2,3-Di(4-bromophenyl)butane-2,3-diol [6d] (meso/dl mixture).
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White solid (0.025 g, 48%); H NMR (CDCl3, 400 MHz) δ 1.46
(s, 6H, dl), 1.57 (s, 6H, meso), 7.16–7.19 (m, 8H); 13C NMR
(CDCl3, 100 MHz) δ 24.9, 79.5, 126.9, 127.0, 127.1, 127.5,
129.9, 131.9, 143.8; IR (ν/cm−1) 3191, 2977, 2933, 1594, 1570,
1465, 909.
2,3-Di(2-pyridinyl)butane-2,3-diol [6e] (meso/dl mixture).
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White solid (0.049 g, 88%); H NMR (CDCl3, 400 MHz) δ 1.24
Product characterization
(s, 6H, dl), 1.65 (s, 6H, meso), 6.90–8.47 (m, 8H); 13C NMR
1,2-Diphenylethane-1,2-diol [4a] (meso/dl mixture). White (CDCl3, 100 MHz) δ 24.3, 25.8, 78.3, 78.3, 120.5, 121.4, 122.1,
solid (0.035 g, 95%); 1H NMR (CDCl3, 400 MHz) δ 2.75 (br 137.0, 137.4, 146.2, 146.4, 165.1, 168.1; IR (ν/cm−1) 3419, 2986,
s, 2H, OH), 4.63 (s, 2H, dl), 4.77 (s, 2H, meso), 7.05–7.26 2937, 1621, 1468, 1357, 1067.
(m, 10H); 13C NMR (CDCl3, 100 MHz) δ 78.1, 79.2, 127.1,
3,4-Diphenylhexane-3,4-diol [6f] (meso/dl mixture). White
solid (0.043 g, 83%); H NMR (CDCl3, 400 MHz) δ 0.60 (t, J =
1
127.2, 127.9, 128.1, 128.2, 128.3, 139.9, 139.9; IR (ν/cm−1
)
3368, 3040, 2920, 1648, 1510, 764.
6 Hz, 6H), 1.71 (q, J = 6 Hz, 4H, dl), 2.07 (q, J = 6 Hz, 4H,
1,2-Di(4-methylphenyl)ethane-1,2-diol [4b] (meso/dl mixture). meso), 2.67 (s, 2H, OH), 7.24–7.26 (m, 10H); 13C NMR (CDCl3,
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White solid (0.041 g, 97%); H NMR (CDCl3, 400 MHz) δ 2.30 100 MHz) δ7.7, 27.8, 82.0, 16.9, 127.2, 128.4, 140.4; IR (ν/cm−1
)
(s, 6H, dl), 2.92 (s, 6H, meso), 2.41 (s, 2H, OH), 4.57 (s, 2H, dl), 3560, 3060, 2974, 1669, 1491, 970.
4.68 (s, 2H, meso), 6.98–7.08 (m, 8H); 13C NMR (CDCl3,
100 MHz) δ 21.3, 78.1, 79.1, 127.0, 127.2, 128.9, 128.2, 129.0, 85%); H NMR (CDCl3, 400 MHz) δ 0.79 (d, J = 7 Hz, 3H), 0.95
130.0, 130.1, 137.1, 137.8; IR (ν/cm−1) 3360, 3028, 2923, 1650, (d, J = 7 Hz, 3H), 1.94–1.97 (m, 1H), 4.35 (d, J = 7 Hz, 2H),
2-Methyl-1-phenylpropan-1-ol [7a]. Clear liquid (0.042 g,
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1606, 1450, 909.
7.43–7.53 (m, 5H); 13C NMR (CDCl3, 100 MHz) δ 18.4, 19.7,
1,2-Di(4-methoxylphenyl)ethane-1,2-diol [4c] (meso/dl mixture). 35.0, 80.0, 126.7, 127.2, 127.4, 144.2; IR (ν/cm−1) 3609, 2966,
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White solid (0.045 g, 93%); H NMR (CDCl3, 400 MHz) δ 3.01 2932, 1620, 1445, 1369, 1166, 1011.
(br s, 2H, OH), 3.77 (s, 6H, dl), 3.82 (s, 6H, meso), 4.58 (s, 2H,
Diphenylmethanol [7b]. White solid (0.047 g, 96%); 1H
dl), 4.71 (s, 2H, meso), 6.73 (d, J = 8 Hz, 4H), 7.01 (d, J = 8 Hz, NMR (CDCl3, 400 MHz) δ 2.05 (br s, 1H, OH), 5.83 (s, 1H),
4H); 13C NMR (CDCl3, 100 MHz) δ 55.3, 79.0, 113.6, 128.3, 7.31–7.36 (m, 10H); 13C NMR (CDCl3, 100 MHz) δ 76.8, 126.6,
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 1489–1495 | 1493