Electrochemical pinacol coupling of aromatic carbonyl compounds in a [BMIM][BF4]-H2O mixture

Article abstract of DOI:10.1039/c3gc41641k

The electrochemical pinacol coupling reactions of aromatic carbonyl compounds were carried out using an 80% [BMIM][BF₄]-H₂O mixture as the electrolytic medium. The corresponding diols were obtained in good to excellent yields with moderate diastereoselectivity. The stereoselectivity can be explained using the strongly-bound ion-pairs formed between the imidazolium cation and the radical anions of the carbonyl compounds. The ionic liquid replaces both organic solvents and supporting electrolytes generally used in the electrosynthetic method. The electrolytic medium can be recycled and successfully reused at least in five consecutive reactions.

Full text of DOI:10.1039/c3gc41641k

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Electrochemical pinacol coupling of aromatic
carbonyl compounds in a [BMIM][BF4]–H2O
mixture†
Cite this: Green Chem., 2014, 16,
1489
Hannah Kronenwetter, Jakub Husek, Brian Etz, Aaron Jones and
Renuka Manchanayakage*
The electrochemical pinacol coupling reactions of aromatic carbonyl compounds were carried out using
an 80% [BMIM][BF4]–H2O mixture as the electrolytic medium. The corresponding diols were obtained in
good to excellent yields with moderate diastereoselectivity. The stereoselectivity can be explained using
the strongly-bound ion-pairs formed between the imidazolium cation and the radical anions of
the carbonyl compounds. The ionic liquid replaces both organic solvents and supporting electrolytes
generally used in the electrosynthetic method. The electrolytic medium can be recycled and successfully
reused at least in five consecutive reactions.
Received 12th August 2013,
Accepted 5th December 2013
DOI: 10.1039/c3gc41641k
www.rsc.org/greenchem
and supporting electrolytes frequently used in electrosynthesis
increasing its experimental appeal. RTILs display many advan-
Introduction
Electrosynthesis is an economical and environmentally benign tages over common organic solvents, such as nonvolatilities,
technology that possesses enormous potential for the develop- immiscibilities with many organic solvents, and good solvating
ment of eco-friendly transformations.1–3 Electrons are con- properties for both inorganic and organic compounds. RTILs
sidered as redox reactants and are one of the cheapest reagents can be easily recovered and reused in order to eliminate pol-
in chemistry. The reducing or oxidizing power in electrolysis lution risks and reduce costs. They have played an increasingly
is controlled by voltage applied to the electrochemical cells. important role in chemical synthesis and many organic, organo-
The method prevents the use of dangerous and polluting metallic and biocatalyzed reactions have been investigated
external chemical oxidants or reductants (e.g. PCC, OsO4, in ionic liquids in recent years.9–13 RTILs generally exhibit a
LiAlH4 and NaH) and minimizes the waste formation of these wide electrochemical potential window, which is a highly
reactions, making the process environmentally friendly and desirable property for applying them as solvents in electro-
economically feasible. An electrosynthetic reaction typically synthesis.14 Being composed entirely of ions, RTILs are sup-
requires two electrodes (anode and cathode) that can also be posed to be among the most concentrated electrolytic fluids
considered as heterogeneous catalysts and they are in contact with many charge carriers per unit volume. When these charge
with a solution that contains an electrolyte. These electrodes carriers are mobile, very high conductivities are possible. The
can simply be physically removed at the end of the reaction application of RTILs in electrosynthesis has been receiving
without using external reagents. In addition, most electro- more and more attention in recent years. Among the recent
synthetic reactions take place at room temperature, reducing reports, electrochemically promoted nucleophilic aromatic
the energy consumption. The diverse and frequently complex substitution,15 carboxylation,16 the Henry reaction,17 epoxi-
chemistry has been elucidated to date using electrochemical dation of alkenes,18 and reductive coupling19 have been per-
conditions.4–8 However, the electrosynthesis suffers from the formed in RTILs. Notwithstanding the unique advantages of
frequent requirement for organic solvents and expensive sup- RTILs, some distinct ionic liquid properties such as high vis-
porting electrolytes to carry out reactions and the difficulty in cosity and low ion conductivity can be considered as electro-
separating the product from the complex electrolytic media.
chemically disadvantageous.20 Armed with this information,
Room temperature ionic liquids (RTILs), which are comple- we investigated the possibility of using RTILs as a user- and
tely composed of ions, can replace the volatile organic solvents eco-friendly electrolytic medium for one of the reductive coup-
ling reactions: the pinacol coupling of carbonyl compounds.
Pinacol coupling is one of the most important and widely
Department of Chemistry, Susquehanna University, 514 University Avenue,
studied carbon–carbon bond forming reactions in organic
Selinsgrove, PA 17870, USA. E-mail: manchanayakage@susqu.edu
chemistry. The popularity of this reaction stems from its
intrinsic ability to furnish 1,2-diols which can serve as a
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c3gc41641k
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structural motif in total synthesis and as chiral ligands or
auxiliaries.21,22 Pinacol coupling has been playing an impor-
tant role in the synthesis of pharmacologically important
agents such as taxol, cotylenol, and HIV-I protease.23,24 The
reaction also enables controlling both diastereoselectivity and
enantioselectivity in a single step and remains a challenging
target for organic chemists. Various low-valent metal reducing
agents have been used to promote the pinacol coupling
reaction.25–28 However, some of these methods require an
absolutely anhydrous system under an inert atmosphere, and
some reagents and solvents are costly, moisture sensitive, and
toxic. During the past few decades, great efforts have been
made to explore environmentally benign systems for the
pinacol reaction. Different catalysts/co-catalysts in aqueous
media including InCl3/Al, TiCl3, VCl3/Al, Mn/AcOH, etc. have
been reported with promising results.29–31 Electrochemical
pinacol coupling in RTILs can provide convenient and environ-
mentally friendly conditions for this reaction.
Table 1 Comparison of solvents for electrochemical pinacol coupling
reactiona
Percent yield
Percent yield
Entry
Solvent system
of 1b (%)
of 2b (%)
1
2
3
4
5
6
7
8
9
[BMIM][BF4]
[BMIM][PF6]
28
12
95
89
65
62
60
32
46
26
20
5
80% [BMIM][BF4]–H2O
70% [BMIM][BF4]–H2O
50% [BMIM][BF4]–H2O
80% [BMIM][PF6]–H2O
70% [BMIM][PF6]–H2O
50% [BMIM][PF6]–H2O
80% [BMIM][BF4]–CH3CN
80% [BMIM][PF6]–CH3CN
91
80
61
54
48
24
33
18
10
a Electrolysis was performed using the carbonyl compound
(0.035 mmol) in 5.0 mL of the solvent, and applying a constant voltage
of −2.0 V vs. Ag/AgCl for 5 h. b Isolated yields.
Results and discussion
Initially, two of the most commonly used RTILs, 1-butyl-
3-methylimidazolium tetrafluoroborate, [BMIM][BF4], and
1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6],
were chosen as the solvents for electrochemical pinacol coup-
ling reactions. Carbonyl compounds, benzaldehyde and aceto-
phenone were used to develop the electrolysis conditions.
However, after five hours of electrolysis in these RTILs, the
reactions did not proceed to completion, thus providing lower
yields (Table 1, entries 1–2).
Table 2 Physical property comparison for RTILs
Molecular
Viscosity Conductivity weight
Density
(g cm−3
)
Ionic liquid
(mPa s) (S m−1
)
(g mol−1
)
[BMIM][BF4]32
154
308
7.5
0.35
0.146
3.35
226.01
284.18
—
1.26
1.35
—
[BMIM][PF6]32
80% [BMIM][BF4]–H2O33
Even though ionic liquids are comprised entirely of ions,
RTIL itself was not efficient enough as an electric conductor.
The high viscosity of RTILs would limit the ion mobility,
mixture is closely similar to that of the calculated value. The
decrease in the viscosity of ionic liquids by the addition of co-
resulting in an important ohmic drop, and therefore a high solvents varies due to the differences in polarities which lead
to different interactions with the ions in the ionic liquids.35
The conductivity for the 80% [BMIM][BF4]–H2O mixture was
cell voltage when the current intensity is increased.14 The
mass transfer of the substrate in the solution to the electrode
surface is also slower in high viscous ionic liquids. The visco- measured to be about ten times higher than the conductivity
of pure ionic liquid (Table 2). The co-solvent can increase the
conductivity by solvating the constituent ions of the ionic
sities for [BMIM][BF4] and [BMIM][PF6] are given in Table 2.
The ionic liquid, [BMIM][PF6], displays higher viscosity and
lower conductivity than [BMIM][BF4] and the pinacol coupling liquid. This solvation reduces the ion pairing or ion aggrega-
tion in the ionic liquid, resulting in an increase in the number
of available charge carriers and an increase in the mobility of
conducted in [BMIM][PF6] resulted in a lower yield than the
reaction conducted in [BMIM][BF4] (Table 1, entries 1–2).
Therefore, both water and acetonitrile were tested as co- these charge carriers. However, it is worth noting that the
electrochemical potential window was decreased with the
addition of water as the co-solvent.36
After the initial studies, the 80% [BMIM][BF4]–H2O mixture
was used as the electrolyte for subsequent pinacol coupling
reactions. The reduction potentials for aromatic aldehydes and
solvents for this electrosynthesis. The addition of a co-solvent
substantially increased the yields of pinacol coupling in both
RTILs (Table 1, entries 3–10). However, the combination of the
ionic liquid, [BMIM][BF4], with the co-solvent, H2O, resulted in
the best yields (Table 1, entries 3–5).
The conductivity and viscosity of RTILs can be changed by ketones ranged between −1.8 V and −1.2 V vs. the Ag/AgCl
reference in 80% [BMIM][BF4]–H2O. A controlled potential of
adding a co-solvent. The viscosity for the 80% [BMIM][BF4]–
−2 V was applied against Ag/AgCl at a platinum cathode for
H2O mixture which gave the best yields was determined to be
substantially lower than that of the pure ionic liquid (Table 2). 5 hours. The resulting pinacol products were separated and
characterized using spectroscopic techniques. The results of
the electrochemical pinacol coupling of aromatic aldehydes
Seddon et al. reported an equation that can be used to predict
the viscosity for ionic liquid–cosolvent mixtures.34 The experi-
mentally determined viscosity for the 80% [BMIM][BF4]–H2O are shown in Table 3.
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Table 3 Results of the pinacol coupling reactions of aldehydesa
Entry
Aldehyde
Product R-group
Yieldb (%)
meso/dlc
1
2
3
4
5
Benzaldehyde
4-Tolualdehyde
4-Anisaldehyde
4-Chlorobenzaldehyde
4-Bromobenzaldehyde
(4a) H
95
97
93
56
62
31/69
56/44
18/82
40/60
45/55
(4b) CH3
(4c) OCH3
(4d) Cl
(4e) Br
a Electrolysis was performed using the aldehyde (0.35 mmol) in 5.0 mL of the 80% [BMIM][BF4]–H2O mixture and applying a constant voltage of
−2.0 V vs. Ag/AgCl for 5 h. b Isolated yields. c Determined by NMR spectroscopy.
Table 4 Results of the pinacol coupling reactions of ketonesa
Product structure
R, R1, X
Yield of the pinacol
productb (%)
meso/dl ratio for the
Yield of the reduction
productb (%)
Entry
Ketone
pinacol productc
1
2
3
4
5
6
7
8
Acetophenone
(6a) H, CH3, C
91
86
50
48
88
83
—
—
21/79
11/89
14/86
17/83
42/58
50/50
NA
—
—
—
—
—
—
85
96
4-Methylacetophenone
4-Methoxyacetophenone
4-Bromoacetophenone
2-Acetylpyridine
Propiophenone
Isobutyrophenone
Benzophenone
(6b) CH3, CH3, C
(6c) OCH3, CH3, C
(6d) Br, CH3, C
(6e) H, CH3, N
(6f) H, CH2CH3, C
(7a) H, CH(CH3)2, C
(7b) H, Ph, C
NA
a Electrolysis was performed using the ketone (0.35 mmol) in 5.0 mL of the 80% [BMIM][BF4]–H2O mixture and applying a constant voltage of
−2.0 V vs. Ag/AgCl for 5 h. b Isolated yields. c Determined by NMR spectroscopy.
The pinacol coupling of substituted benzaldehydes under spectroscopic methods. According to NMR spectroscopy, a
developed conditions afforded excellent to good yields. For the mixture of dl and meso isomers was formed for each synthesis
electroreductive coupling of 4-chloro- and 4-bromobenzalde- with very low diastereoselectivity.
hydes, the starting materials were remaining after five hours of
The electrochemical pinacol coupling of aromatic ketones
electrolysis and the pinacol products were separated by flash was also carried out using 80% [BMIM][BF4]–H2O as the electro-
column chromatography (Table 3, entries 4–5). The Faradic lytic medium. The coupling products were obtained in high
efficiency for these reactions varied in the range of 72–87%. In yields with low to moderate diastereoselectivity, confirmed by
the presence of water, it is a probability that some side reac- NMR spectroscopy (Table 4). After five hours of electrolysis,
tions such as hydrogen generation can take place at the Pt unreacted starting ketone was still remaining for 4-methoxya-
cathode. This may have contributed to a somewhat low faradic cetophenone and 4-bromoacetophenone, and the pinacol
efficiency of these reactions. However, a Pt cathode–Sn anode product was separated by flash column chromatography
combination has been successfully used previously in the pres- (Table 4, entries 3–4). Interesting results were obtained when
ence of water (acetonitrile–H2O) for reductive coupling reac- the methyl group (R1) of acetophenone was changed with
tions.5,37 As a comparison, a pinacol coupling of benzaldehyde different alkyl and aryl groups. Both acetophenone and propio-
in 80% [BMIM][BF4]–H2O was conducted using the RVC phenone resulted in the corresponding pinacol product as
cathode.38 The Faradic efficiency and the percent yield were expected in 91% and 83% yields respectively (Table 4, entries
found to be equal to those from the experiment that used a 1, 6). However, when isobutyrophenone and benzophenone
Pt cathode. The resulting 1,2-diols were characterized by were subjected to electrolysis under these conditions, the
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pinacol coupling products were not formed, but the corres-
ponding alcohols, 2-methyl-1-phenylpropanol (7a) and diphenyl-
methanol (7b), were produced in good yields (Table 4,
entries 7–8). The steric hindrance from large substituents (iso-
butyl and phenyl) prevents the pinacol coupling and promotes
the reduction at the carbonyl group under these conditions.
The developed method uses a Sn sacrificial anode in the
electrochemical cell. It has been previously reported that the
cations formed by sacrificial anodes can stabilize the anionic
intermediates by ion-bridge formation.37,39 Such ion-bridge
formation between Sn cations and two radical anions should
increase the diastereoselectivity of the reaction favoring the dl
isomers. However, the pinacol coupling of both aldehydes and
ketones underwent very low diastereoselectivity. In this situ-
ation, it is possible that ion-bridging with Sn cations is not
favored and an ion-pair is more efficiently formed between one
radical anion and the imidazolium cation. It has been reported
that the 1-butyl-3-methylimidazolium cation demonstrates an
unusual ion pairing tendency when compared to tetraalkyl-
ammonium ions.40 The moderate diastereoselectivity for the
pinacol product of acetophenone in [BMIM][NTf2] was also
reported by Lagrost et al.41 The observed moderate diastereo-
selectivity is a result of the strongly-bound ion-pairs formed
between the imidazolium cation and the radical anions of the
carbonyl compounds formed after the first electron transfer
(Scheme 1). The negatively charged carbonyl oxygen can
closely face the imidazolium cation due to the planar geometry
of a cationic ring. This minimizes the cation–anion distance in
the ion pair leading to strong interactions. Another reason for
the strong interactions between the imidazolium cation
and the radical anion is the hydrogen bond formation between
the hydrogen at the C-2 position of the imidazolium cation
and the carbonyl oxygen in the radical anion.42 These tightly-
Fig. 1 Reuse of [BMIM][BF4] in electrochemical pinacol coupling reac-
tions. Reaction conditions as Table 1, entry 3.
stabilization by ion pairing also allows a fast dimerization step
between two radical anions. The coupling arrangement [A]
leads to the dl isomer while the arrangement [B] leads to the
meso isomer (Scheme 1). Both approaches of the radical
anions could be identically involved thereby decreasing the
stereoselectivity.
Room temperature ionic liquids are known for their recycl-
ability.43,44 In this study, the possibility of recycling the ionic
liquid electrolytic medium was also investigated. The RTIL was
recycled using the method reported in the Experimental
section. The NMR spectroscopy of the recovered ionic liquid
indicated no change in the structure. The electrochemical
pinacol coupling reactions of benzaldehyde and acetophenone
conducted in the recovered ionic liquids showed no significant
decrease in yields or selectivity for up to five cycles (Fig. 1).
This indicates that the ionic liquid as an electrolyte for pinacol
coupling of aromatic aldehydes and ketones is recyclable.45
bound complexes promote coupling with
orientation of the radical anion, in order to avoid strong
interactions from large imidazolium cations. The charge
a head-to-tail
Conclusion
We have demonstrated that the electrochemical pinacol coup-
ling of aromatic aldehydes and ketones can effectively be per-
formed using 80% [BMIM][BF4]–H2O as the electrolytic
medium. The resulting products were obtained in high yields
with moderate diastereoselectivity. The developed electro-
chemical method avoids the use of a catalyst–co-catalyst systems
for reductive coupling, thereby preventing the formation of
any metallic byproducts. Owing to the inherent ionic nature of
the RTILs, the use of a supporting electrolyte for the electro-
lysis was also not needed which avoided the salt byproducts
and simplified the separation and purification steps. With the
ease of product recovery, this method provides a simple
synthetic route to 1,2-diols. The electrolyte can be recycled and
reused at least up to five cycles. Efforts are currently underway
to utilize chiral RTILs in stereoselective electrosynthesis.
Experimental
The RTILs [BMIM][BF4] and [BMIM][PF6] were synthesized
using a previously reported method.46
Scheme 1 Mechanism for the electrochemical pinacol coupling of
carbonyl compounds in [BMIM][BF4].
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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).
1
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
1
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
1
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).
1
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).
1
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).
1
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).
1
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,
1
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,
1
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,
1
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,
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3.5 mmol of the carbonyl compound (benzaldehyde, 0.37 g or
acetophenone, 0.42 g) and 80% [BMIM][BF4]–H2O (30.0 mL)
were placed in an electrochemical cell fitted with a sacrificial
tin foil anode (2.0 cm2) and a platinum plate cathode
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nitrogen for 30 minutes. A controlled potential of −2.0 V and a
Ag/AgCl reference electrode was applied under a nitrogen
atmosphere for 5 hours. The resulting solution was filtered to
remove any insoluble tin salts and extracted with diethyl ether
(2 × 15 mL). The ether extract was dried with anhydrous
sodium sulfate, filtered and concentrated in vacuo. The residue
was purified via silica gel flash column chromatography (10%
ethyl acetate in a hexane eluent) to afford the final product.
The pinacol product of benzaldehyde was obtained in 76%
yields while the pinacol product of acetophenone was obtained
in 68% yields.
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Acknowledgements
This work was supported by a Susquehanna University Faculty
Research grant. The authors thank Dr L. Viyannalage from the
University at Buffalo for some electrochemical characteriz-
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