First pinacol coupling in emulsified water: Key role of surfactant and impact of alternative activation technologies

Article abstract of DOI:10.1002/cssc.201500108

For the first time, the influence of surfactants on the radical pinacol coupling reaction is investigated. The rate and selectivity of this reductive C-C coupling are compared under three different activation technologies: thermal activation, microwave irradiation, and sonication. The use of IgepalCO520, a neutral surfactant, led to the successful conversion of aromatic or α,β-unsaturated aliphatic carbonyl compounds in moderate to excellent yield (55-90 %). An insight on the potential mechanism involved in the reaction is also proposed, based on microscopic observations and particle size measurement.

Full text of DOI:10.1002/cssc.201500108

DOI: 10.1002/cssc.201500108
Full Papers
First Pinacol Coupling in Emulsified Water: Key Role of
Surfactant and Impact of Alternative Activation
Technologies
Muriel Billamboz[a] and Christophe Len*[a, b]
For the first time, the influence of surfactants on the radical pi-
nacol coupling reaction is investigated. The rate and selectivity
of this reductive C-C coupling are compared under three differ-
ent activation technologies: thermal activation, microwave irra-
diation, and sonication. The use of IgepalCO520, a neutral sur-
factant, led to the successful conversion of aromatic or a,b-un-
saturated aliphatic carbonyl compounds in moderate to excel-
lent yield (55-90%). An insight on the potential mechanism in-
volved in the reaction is also proposed, based on microscopic
observations and particle size measurement.
Introduction
The reductive dimerization of carbonyl compounds, the so-
called pinacol coupling, is a useful synthetic method for con-
structing vicinally functionalized carbon–carbon bonds. One-
electron transfer from a metal to a carbonyl function generates
the corresponding ketyl radical, which can dimerize to give
either dl and/or meso isomers of 1,2-diols. These vicinal diols
can serve as structural motif in total synthesis, chiral auxiliaries
or chiral ligands, crosslinker agents in polymer synthesis, pesti-
cides, and substrates useful in structural biology.[1] These diols
can even be converted into a number of high value com-
pounds such as aminoalcohols, epoxides, ketones, and so on.
(Scheme 1).
tween pinacol formation and side reactions (reduction of the
carbonyl group, McMurry reaction, pinacolic rearrangement, or
dehydration)[2] has to be taken into account to design an effi-
cient reaction medium. Pinacol-type couplings were developed
by using a Zn–Cu couple,[3] Mg,[4] Mn,[5] Zn,[6] In,[7] Sm,[8] Al,[9]
Ga,[10] and other metals.[11] Some also reported the use of met-
allocene compounds to promote the reaction such as Ti-, V-, or
Zr- derivatives.[12]
Recently, organic reaction in water or aqueous media has at-
tracted great interest in organic synthesis from the vantage
points of cost, safety, and environmental concerns. Therefore,
the development of an efficient synthetic methodology to
form carbon–carbon bonds in water or aqueous media appears
to be very important. In the case of radical reactions, it was
also demonstrated that solvent plays a crucial role and water
allows control of radical reactions.[13] On the basis of these ob-
servations, pinacol coupling has already been carried out in
aqueous media.[14]
Scheme 1. Pinacol coupling: a versatile reaction.
However, few methods have been reported for pinacol cou-
pling in pure water, and each suffers from drawbacks: use of
an excess of low-valent metal, long time of reaction (from 8 h
to 3 days), limited selectivity, and so on. Moreover, the poten-
tial scope of aqueous pinacol coupling is drastically reduced
when highly hydrophobic substrates are involved in the chemi-
cal transformations.[15] To overcome this problem, some works
deal with the use of additives to enhance the reactivity in pure
water.[16a–d] Among them, micellar catalysis has been intensively
developed during the last two decades.[16] However no suc-
cessful method using surfactant has been already reported for
the pinacol coupling.
A variety of methods have been used to perform this reac-
tion. Among them, the most reliable protocols use low-valent
metals or metal complexes in stoichiometric or catalytic
amounts. In each pinacol coupling reaction, the selectivity be-
[a] Dr. M. Billamboz, Prof. C. Len
Sorbonne Universitꢀs
Universitꢀ de Technologie de Compiꢁgne
Ecole Supꢀrieure de Chimie Organique et Minꢀrale
EA 4297 Transformations Intꢀgrꢀes de la Matiꢁre Renouvelable
Centre de Recherche Royallieu, CS 60 319
F-60203 Compiꢁgne cedex (France)
We herein describe the pinacol coupling reaction in emulsi-
fied water via three different activation methods: thermal acti-
vation, microwave irradiation, and sonication. Comparison of
commercially available surfactants with phase-transfer agents,
kinetics, selectivity, and limitations of the reaction will be dis-
cussed for each activation technology. Moreover, some micro-
E-mail: christophe.len@utc.fr
[b] Prof. C. Len
Department of Chemistry
University of Hull
Hull HU6 7RX (England)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201500108.
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scopic observations and particle size measurements have been
carried out, leading to a mechanistic proposal.
The concentration of ammonium chloride and the tempera-
ture of reaction were then screened for a maximal time of
15 h. At room temperature, the kinetics of reaction improved
with increasing ammonium chloride concentration but was ac-
companied by a decrease in selectivity (Table 1, entries 6–9).
The use of NH4Cl (1m) afforded the same yield and a better se-
lectivity for compound 2A compared with that obtained with
saturated NH4Cl (Table 1, entries 8 and 9). To avoid the incom-
plete conversion of starting material 1A, higher temperatures
were investigated. At 458C, complete conversion was achieved
only after 14 h (Table 1, entry 13) but only 1–2 h were sufficient
to obtain the same results as after 15 h at room temperature
(Table 1, entries 8, 11 and 12).
Results and Discussion
Objectives and methodological study under conventional
heating
In order to design adapted protocol for coupling of highly hy-
drophobic and insoluble substrates in water, 4-bromobenzal-
dehyde (1A) was selected as a model (Table 1). Firstly, 4-bro-
mobenzaldehyde (1A) was subjected to previously reported
For few hours of reaction (1–
Table 1. Initial
study
in
water
with
4-bromobenzaldehyde
1A
as
model
substrate.
2 h), similar results were ob-
served at 458C and 808C
(Table 1, entries 11, 12, 14, and
15). For each temperature, con-
version of 4-bromobenzaldehyde
(1 A) increased with time with-
out affecting the yield of target
compound 2A after 2 h at 458C
(Table 1, entry 12) and 6 h at
808C (Table 1, entry 16). These
results mean that side reactions
are preferentially conducted
when time of reaction was too
long. The main difference ap-
peared after 6 h of reaction at
808C, when 70% yield was ob-
served (Table 1, entry 16). De-
creasing the zinc amount to
1 equivalent led to poorer yield
and selectivity after 2 h (Table 1,
entry 17).
Entry
Metal [equiv]
Acid ([C])
T [8C]
t [h]
Conversion [%]
2A [%]
2A/3A [%]
1[4a]
2[6a]
3[9a]
4
Mg (21 equiv)
Zn (1.5 equiv)
Al (2 equiv)
Mg (10 equiv)
Mg (2 equiv)
NH4Cl (0.1m)
AlCl3 (0.1m)
oxalic acid (0.04m)
NH4Cl (0.1m)
rt
rt
45
rt
rt
15
15
18
15
15
55
33
30
35
<5
45
17
10
90/10
67/23
25/75
89/11
ND[a]
31
5
NH4Cl (0.1m)
traces
6
7
8
9
NH4Cl (0.1m)
NH4Cl (0.5m)
NH4Cl (1m)
NH4Cl saturated
NH4Cl (1m)
NH4Cl (1m)
NH4Cl (1m)
NH4Cl (1m)
NH4Cl (1m)
NH4Cl (1m)
NH4Cl (1m)
NH4Cl (1m)
rt
rt
rt
rt
45
45
45
45
80
80
80
45
15
15
15
15
0.5
1
2
14
1
30
45
60
70
39
57
77
97
56
70
85
58
28
39
52
53
27
43
55
57
46
55
70
45
90/10
89/11
87/13
76/24
82/18
82/18
83/17
82/18
82/18
80/20
82/18
78/22
10
11
12
13
14
15
16
17
Zn (2 equiv)
Zn (1 equiv)
2
6
2
From these results, in order to
achieve short reaction times and
find a compromise between rate
and selectivity, the reaction was
carried out at 458C with 1 M am-
[a] ND=not determined
methods for pinacol coupling in water.[4a,6a,9a] Using a large
excess of Mg in the presence of 0.1m NH4Cl did not allow
complete conversion after 15 h but gave a very good selectivi-
ty (Table 1, entry 1).[4a] Decreasing the quantity of metal led to
lower conversion but still very good selectivity, showing the
potential of NH4Cl as acid source to control the selectivity but
also the limited ability of Mg to activate the reaction (Table 1,
entries 4 and 5). In our study, performed according to the Ha-
zarika and Dutta protocol,[6a] zinc combined with AlCl3 as Lewis
acid at room temperature led to low conversion and selectivity
after 15 h of reaction (Table 1, entry 2).
monium chloride as aqueous medium and 2 equivalents of
zinc as metallic reductor for one hour (Table 1, entry 11). Using
these optimized conditions, phase transfer agents or surfac-
tants were added in order to study their potential effect.
Effects of additives on yields and selectivity
In a first screening, the amount of commercial additives was
set at 1 wt.% and different families of phase transfer agents
[18-crown-6, tetra-n-butylammonium bromide (TBAB)] and sur-
factants [cetyltrimethylammonium bromide (CTAB), SDS, Brij,
Tween, Triton, IgepalCO] were studied (Table 2). From this
screening it was clear that additives played an essential role
during the pinacol coupling reaction in water. In fact, 70%
yield was achieved using BrijL4 as additive compared with
43% under the same conditions without any promoter
(Table 2, entries 1 and 6). The use of neutral and cationic phase
However, Zn seems more active than Mg at low quantity.
When reaction was carried out according to Yuan et al the
combination of Al/oxalic acid did not allow good conversion
nor selectivity (Table 1, entry 3).[9a] Based on these results, zinc
(2 equiv) as cheaper low valent metal and ammonium chloride
solution as solvent were selected for further studies.
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shorter polar head (Table 3,
entry 1). As a consequence, Ige-
palCO520, exhibiting an HLB
around 10, was selected for fur-
ther optimization.
Table 2. Effects of commercial additives (1 wt.%) when pinacol coupling starting from compound 1A is carried
out 1 hour at 458C.
Entry Additive
Charge Conversion 2A [%] 3A [%] Other [%] Global sel. [%][a] 2A/3A [%]
1
2
3
4
5
6
7
8
none
18-crown-6
TBAB
CTAB
SDS
–
57
43
39
43
44
31
70
66
66
63
60
55
50
50
51
9
4
13
12
4
11
11
8
11
10
27
22
5
13
15
8
14
17
21
16
18
17
75
72
52
56
77
75
72
81
74
69
64
68
66
68
82/18
91/9
neutral 54
cationic 83
cationic 78
anionic 40
neutral 94
neutral 92
neutral 82
neutral 85
neutral 87
neutral 86
77/23
78/22
88/12
86/14
86/14
89/11
89/11
86/14
85/15
86/14
86/14
88/12
Green aspects
BrijL4
Some adjustments were made
to respect as far as possible the
green chemistry concepts. In
these regards the energy saving,
the catalyst loading and the re-
cycling of the medium were
studied with variation of reac-
tion time and surfactant concen-
tration. For IgepalCO520, after
Tween 20
IgepalCO630
Triton X100
Brij98
9
8
10
11
12
13
14
10
10
8
8
7
BrijC10
Monomuls90 L12 neutral 74
Sympatens AC100 neutral 76
Tween 80
neutral 75
[a] Global selectivity is defined as PCP/conversion. PCP=pinacol coupling product.
Table 3. Influence of the PEG length on yield for the IgepalCO family.
transfer agents did not allow any improvement in yield
(Table 2, entries 2 and 3). Cationic phase transfer agent like
Entry
Surfactant
Number of PEG units [n][a]
2A [%]
TBAB and cationic surfactant CTAB seem to boost the kinetics
of reaction. Indeed, cationic additives allowed to achieve high
conversions but limited yields of compound 2A, which dra-
matically decreases the global selectivity of the reaction. In
fact, numerous side products were observed with such cationic
promoters. It could be hypothesized that cationic species such
as quaternary ammonium N+ replace Zn2+ in some intermedi-
ate species of reaction. As Zn2+ is essential for the ketyl radi-
cals to meet, its replacement by a non-chelating species results
in many undesired products. Moreover, using a cationic pro-
moter increases the salinity of the media and acts on kinetics,
for example by increasing the concentration of NH4Cl (Table 2,
entries 3 and 4). Using SDS as promoter led to a decrease in ki-
netics even if global and relative good selectivities were main-
tained (Table 2, entry 5). This could be explained by electrostat-
ic repulsions between the charged intermediates of reaction
(complexes of Zn2+ and ketyl radicals) and the anionic polar
head of SDS.
1
2
3
IgepalCO520
IgepalCO630
IgepalCO720
5
10
12
68
66
57
[a] n=number of PEG units of the hydrophilic head.
1 hour of reaction, the best concentration was confirmed at
1 wt.% (Figure 1). In our hands, higher conversion of aldehyde
1A and selectivity were observed after 2 h in the presence of
IgepalCO520 (1 wt.%) (Table 4, entry 2). Decreasing the quanti-
ty of reductant led to a drop in yield from 80% to 54%
(Table 4, entry 3). As a consequence, the scope of reaction was
investigated with zinc (2 equiv), IgepalCO520 (1wt.%) in NH4Cl
(1m) as medium at 458C over 2 h.
Different neutral surfactants were tested (Table 2, entries 6–
14). In our study, all neutral surfactants led to yields above
50% (compared with 43% without any promoter) and global
and relative selectivities are similar (Table 2, entries 6–14). Par-
ticularly, BrijL4, Tween 20, IgepalCO630, and Triton X-100 all al-
lowed a great improvement in kinetics and good yield (Table 2,
entries 6–9). BrijL4 and IgepalCO630 share lot of structural sim-
ilarities. In fact, they both consist of a hydrophobic tail and
a linear hydrophilic PEG chain. The major difference between
the two surfactants is the presence of an aromatic ring spacer
in IgepalCO630. From these data, IgepalCO630 proved to be
less active on kinetics than BrijL4 but the quality of the reac-
tion seems better as less side-products are obtained. In this re-
gards, the IgepalCO family was further studied to determine
the optimal structure of the hydrophilic part (Table 3).
Figure 1. Yield of 2A versus concentration (wt.%) of IgepalCO520. PCP=pi-
nacol coupling product, RP=reduction product.
In these conditions, recycling of the medium was realized.
The aqueous medium, containing IgepalCO520 and NH4Cl can
be successfully recycled 4 times without significant loss of ac-
tivity or selectivity, although zinc is oxidized and cannot be
reused easily. At the end of the reaction, zinc and zinc salts are
filtered off and the aqueous medium is extracted by ethyl ace-
It seems that a shorter hydrophilic head is better for an im-
proved yield in pinacol coupling product 2A. In fact, for the Ig-
epalCO derivatives, the best candidate is the one with the
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persed in water (Figure 4). The term “oil” refers to the mixture
of bromobenzaldehyde (1A) and surfactant. To prove the for-
mation of droplets, some lipophilic colorant was added in the
pre-formed medium. It is obvious that the lipophilic additive
dissolved gradually in the oil phase (Figure 5).
Table 4. Study of time of pinacol coupling reaction for IgepalCO520
(1 wt.%)
Entry Metal
t [h] 2A [%] 3A [%] Global sel. [%][a] 2A/3A [%]
1
2
3
Zn (2 equiv)
Zn (2 equiv)
Zn (1 equiv)
1
2
2
68
80
54
6
18
10
85
84
84
92/8
82/18
84/16
When combining all results, it could be hypothesized that
the pinacol coupling reaction takes place on the surface of the
droplets formed by the association of the surfactant and alde-
[a] Global selectivity is defined as PCP/conversion.
tate to recover the products. The
organic phase is discarded and
the aqueous phase can be reen-
gaged in reaction with the addi-
tion of zinc (2 equiv) (Figure 2).
The slow decrease in reactivity
can be attributed to a loss of
surfactant during the extraction
step by ethyl acetate.
Figure 3. Dynamic light scattering: particle distribution in reaction medium.
Figure 2. Recycling of the aqueous medium for 2 h of reaction.
Figure 4. DLS: evolution of the medium when the aqueous medium is soni-
cated. (A) under 10 min of classical stirring; (B) the same solution after 5 min
of sonication at 100 Hz.
Mechanistic proposal
Knowing that size and charge of the surfactant involved in the
reaction is of importance in reactivity and selectivity, some
physical observations and measurements were carried out to
better understand the mechanism of pinacol coupling reaction
in emulsified water under classical activation.
hydes. The droplet could act as a reservoir for bromobenzalde-
hyde (1A). During the course of reaction, apparent orientation
of the different intermediates occurred at the oil–water inter-
face. Indeed, the aromatic ring and the chelate between metal
and oxygen atoms have high affinities with oil and water, re-
spectively. Consequently, a mechanistic proposal for pinacol
coupling reaction could be realized as detailed in Scheme 2.
Aggregation behavior was investigated in reaction condi-
tions. Bromobenzaldehyde (1A), NH4Cl, and zinc were sus-
pended in aqueous IgepalCO520 (1% w/w). The mixture was
stirred at room temperature for 10 min and then aqueous
medium was filtered off. The aqueous phase was analyzed by
dynamic light scattering and by direct ocular microscopy with
and without addition of a lipophilic colorant.
Scope of the reaction
First of all, dynamic light scattering studies (Figure 3) reveal
that compounds spontaneously self-assemble in aqueous solu-
tion and that bromobenzaldehyde (1A) interacts with the ob-
tained micro-structures. The size of the structures observed is
between 3 and 10 micrometers, largely above the classical size
for micelles.
Various carbonyl compounds were submitted to the pinacol
coupling reaction under these optimized conditions. Several
structurally different aromatic or aliphatic aldehydes under-
went reductive coupling to the corresponding 1,2-diols in
good to high yields (Table 5). The reaction rates were faster
than those described in many existing methods. The selectivi-
ties of the pinacol coupling reaction are above 50% for all
Moreover, direct observation by ocular microscopy proved
that the reaction medium is composed of droplets of “oil” dis-
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phobic tail of the surfactant, which gave high stability of the
products in the aggregates and prevented pinacol coupling.
Comparing 2-trans-decenal (1C) with 2,4-decadienal (1D)
having the same skeleton, the global selectivity decreased for
1D due to a higher stability of ketyl radical but the meso
isomer was obtained quantitatively (Table 6, entries 2 and 3).
Due to the absence of chiral inductor, this result was of partic-
ular interest. Some much hindered aliphatic a,b-unsaturated
compounds were then tested, such as trans-2-methylbutenal
(1E), trans-2-methylpentenal (1F), and citral A (1G) (Table 5,
entries 4–6). It is notable that an increase in the steric hin-
drance around the carbonyl group affected distribution of the
ratio of meso and dl isomers. In fact, as proved by products 2E
and 2F, if the position 2 is substituted by a methyl group, the
meso isomer is obtained selectively (Table 5, entries 4 and 5). It
is noteworthy than citral A (1G), a long chain hindered natural
aliphatic a,b-unsaturated aldehyde, reacted well under these
conditions to result in a mixture of meso and dl isomers
(Table 5, entry 6). To our knowledge, it is the first time that
citral A (1G) proved to be a good substrate for pinacol cou-
pling in water. Cinnamaldehyde (1H) (Table 5, entry 7) gave
moderate conversion and yield. It could be attributed to the
particular stabilization of the ketyl radical.
Figure 5. Direct observation of the dissolution of the lipophilic colorant (or-
ganoblue) in the droplets. (A) medium whithout colorant; (B) beginning of
the absorption of colorant in the “oil” phase; (C) progressive dissolution of
the lipophilic additive in the droplet; (D) quasi-totale coloration of the lipo-
philic phase. The length of the scale bars are: (A) 10 mm, (B) 15 mm, (C) 5
mm, (D) 15 mm.
In a second part, some aromatic aldehydes 1 having various
electrostatic effect on the aromatic ring and steric hindrance
near the carbonyl group were subjected to our pinacol cou-
pling system. Products 2 were always obtained with an excess
of the meso-type isomer (Table 5). Compounds 1 J, 1 K, and 1S
reacted slowly under these conditions (Table 5, entries 9, 11,
and 19) probably due to the observed aggregation around the
magnetic stirrer at the end of the reaction. Moreover, 1-naph-
taldehyde (1S) is highly hindered, which could decrease its
own reactivity. To avoid aggregation, much vigorous stirring
was attempted without real success. Aromatic aldehydes sub-
stituted by electron-withdrawing group gave higher yields
than by electron-donating group at the same position. For ex-
ample, 4-bromo- and 4-fluorobenzaldehydes 1A and 1l gave
higher yields than 4-methoxybenzaldehyde (1M) (Table 5, en-
tries 10, 12 and 13). In the same manner, compounds 1N and
1O having a chlorine atom in position 2 and 3 on the aromatic
ring gave better results than compounds 1Q and 1R having
a methoxy group in the same position (Table 5, entries 14, 15,
17 and 18). The 2,3-dichlorobenzaldehyde (1P) gave quantita-
tive conversion and similar yields to those obtained for the
chloro analogues 1N and 1O (Table 5, entry 17).
Scheme 2. Mechanistic proposal for the pinacol coupling assisted by surfac-
tants.
compounds, even if some of them react slowly (Table 5, en-
tries 9, 11, 13, 19, 21 and 22).
First, we were interested in the pinacol coupling of aliphatic
a,b-unsaturated compounds (Table 5, entries 1–7). Indeed,
even if numerous natural a,b-unsaturated carbonyl com-
pounds exist such as citral A (1G), their transformation is rarely
described in the literature. Upon checking the few reported
methods in water, we found that no studies have dealt with
the coupling of such compounds. Moreover, under traditional
conditions (with co-solvents for instance), the yields in pinacol
coupling products are low. For example, pinacol coupling of
trans-2-hexenal was performed using TiI4 in EtCN with a 16%
yield.[17] Geranial was coupled by electrochemical reduction in
EtOH-Ph5 buffer in around 35% yield.[18] Our methodology en-
ables the coupling of aliphatic a,b-unsaturated compounds in
good-to-excellent yields, even for hindered or long chain sub-
strates (Table 5, entries 1–7). Trans-2-hexenal (1B) proved to be
a perfect substrate for pinacol coupling leading 2B in an excel-
lent 90% yield with high selectivity (Table 5, entry 1). Long
chain compounds such as 2-trans-decenal (1C) and 2,4-deca-
dienal (1D) (Table 5, entries 2 and 3) gave moderate yields.
This could be explained by their interaction with the hydro-
Heteroatomic aromatic aldehyde such as furfural 1T reacted
quickly under these conditions giving 2T in a moderate 51%
yield. Numerous side products were observed, which is not
surprising due to the known instability of furfural (Table 5,
entry 20).
Concerning acetophenone derivatives 1U and 1V (Table 6,
entries 21 and 22), their very slow reactivities could be attribut-
ed to the difficulty in generating and stabilizing the ketyl radi-
cal on this aromatic ketone compared with aromatic aldehydes
as previously described.[19] It is noteworthy that total selectivi-
ties and excess in dl isomer were obtained with acetophenone
derivatives.
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Table 5. Scope of the pinacol coupling reaction.
Entry
Substrate (1)
Conversion
2 [%]
3 [%]
dl/meso[a]
Global sel. [%][b]
2/3 [%]
1
2
3
1B
1C
1D
100
65
90
58
65
2B
2C
2D
5
4
3B
3C
3D
40/60
38/62
0/100
90
89
72
95/5
93/7
ND
90
ND
4
5
1E
1F
100
100
90
83
2E
2F
9
7
3E
3F
0/100
0/100
90
83
91/9
92/8
6
1G
90
62
2G
ND
3G
39/61
69
ND
7
8
9
1H
1I
70
100
40
55
78
26
2H
2I
5
20
12
3H
3I
46/54
42/58
48/52
79
78
65
92/8
80/20
68/32
1 J
2J
3 J
10
11
12
13
1 A
1 K
1l
95
40
89
17
80
28
71
12
2A
2K
2l
18
8
3 A
3 K
3l
27/73
26/74
34/66
48/52
84
70
80
71
82/18
78/22
80/20
75/25
18
4
1m
2M
3m
14
15
16
1N
1O
1P
100
100
100
69
77
78
2N
2O
2P
31
22
20
3N
3O
3P
11/89
19/81
16/84
69
77
78
69/31
77/23
80/20
17
18
1Q
1R
90
91
60
73
2Q
2R
29
7
3Q
3R
30/70
24/76
67
80
67/33
91/9
19
1S
10
9
2S
1
3S
12/88
90
90/10
20
21
1T
100
5
51
5
2T
29
0
3T
32/68
54/46
51
64/36
100
1U
2U
3U
100
22
1 V
6
6
2V
0
3 V
59/41
100
100
1
[a] The dl/meso ratio was determined by H NMR of the crude product. [b] Global selectivity is defined as PCP/conversion.
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During sonication, an ice bath
was used to maintain a moderate
temperature in the reaction
medium. The maximal tempera-
ture reached was 808C within
the medium. It is clear that soni-
cation was attempted to help
Table 6. Comparison of activation technologies.
Entry
Substrate
(1)
Activation method
t [h]
T [8C]
2 [%]
1
2
3
4
silent
)))[b]
MW[a]
MW[a]
2
1.5
2
45
rt to 80
45
80
81
25
72
1A
1.5
80
less
reactive
compounds
5
6
7
8
silent
)))[b]
2
1.5
2
45
rt to 80
45
28
72
8
(Table 7). Sonication greatly im-
proved the reactivity of aldehy-
dic substrates 1H, 1J, 1K, 1M,
1S, and 1U allowing total con-
versions for most of them
(Table 7, entries 2, 4, 6 and 10).
As expected, sonication prevents
1K
MW[a]
MW[a]
1.5
80
33
[a] for the reaction under microwave, the temperature was controlled by an external probe. [b] for sonication,
the temperature was followed by an internal probe.
In order to improve the reactivity of slow reactive substrates
or acetophenone derivatives and avoid aggregation issues,
some alternative activation technologies were studied.
aggregation of the substrate and enhances its reactivity. For 4-
methoxybenzaldehyde (1M), the impact of sonication is less
impressive but the conversion and yield are tripled with very
good selectivities (Table 7, entry 8). Acetophenone (1U), even
under sonication, proved to react slowly, but with a total selec-
tivity towards the pinacol coupling product 2U (Table 7,
entry 12). For all compounds except 2K, the dl/meso ratio is
similar under conventional heating and sonication.
Impact of alternative activation technologies
Among all alternative activation technologies, microwave and
ultrasound are the most popular, due to their high ability to in-
crease rate and selectivity of reaction.
The enhancement of reactivity under sonication can be re-
lated to the size of droplets involved in the reaction. In fact, it
is obvious than smaller droplets are obtained when the aque-
ous medium is sonicated (Figure 6). Dynamic light scattering
studies also proved that a new family of smaller droplets is cre-
ated during sonication (Figure 7). As the reaction takes place
at the interface between the oil phase and water, it can be hy-
pothesized that the kinetics of reaction are better if droplets
are smaller. In fact, the interface is larger and the reactivity of
aldehydes (1) increased.
On the one hand, with its high dielectric constant, water is
potentially a very useful solvent for microwave-mediated syn-
thesis.[20] Indeed, microwave heating has been widely recog-
nized as an efficient tool, and its benefits have been well-docu-
mented.[21] Since many reactions are known to result in higher
yield and/or shorter reaction times, this alternative technology
was developed in our group for Suzuki–Miyaura cross-coupling
of uridine analogues or micellar Tsuji–Trost reaction in pure
water.[22]
Nevertheless, to the best of our knowledge, there is no pro-
tocol for pinacol coupling in pure water under microwave
heating. Only two protocols are described using Al/NaOH in
MeOH[23] and SmBr2, SmI2, or YbI2 in THF.[24] On the other hand,
ultrasound has increasingly been used in organic synthesis.
The main advantages of sonication are to increase reaction
rate in the presence of metallic multiphasic medium or gener-
ate radicals.[25] In this regard, Li et al reported two protocols for
pinacol coupling in acidic water with Mg/NH4Br 0,1m and Zn/
H3PO4 3N.[26]
Figure 6. Evolution of the microemulsion when the aqueous medium is soni-
cated. (A) under 10 min of classical stirring; (B) the same solution after
10 min of sonication at 100 Hz.
In order to compare microwave irradiation and sonication,
reaction was attempted on 4-bromobenzaldehyde (1A) and 4-
chlorobenzaldehyde (1K) (Table 6). At 458C, under microwave
irradiation, both aldehydes 1A and 1K reacted slowly giving
lower yield than under thermal activation (Table 6, entries 3
and 7). Increasing temperature to 808C with a 1.5-hour reac-
tion time, compound 1A proved to react smoothly and 1K
was always poorly reactive. Concerning ultrasound activation,
aldehyde 1A afforded the target compound 2A with yields
similar to those obtained by classical activation (Table 6, en-
tries 1–2). However, the slow reactive substrate 1 K furnished
the corresponding pinacol 2K in a good 72% yield (Table 6,
entries 5 and 6).
Conclusion and perspectives
A novel, fast, and efficient protocol for the pinacol coupling of
aromatic and a,b unsaturated aliphatic aldehydes has been re-
alized thanks the use of IgepalCO520, a commercial neutral
surfactant in water. Three activation technologies have been
screened, and show different abilities. For this radical reaction,
microwave irradiation has minimal effect but sonication
proved to highly activate the reaction, avoiding aggregation
issues. A mechanistic proposal has been suggested for the pi-
nacol coupling under thermal activation, explaining all practi-
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Table 7. Scope of the pinacol coupling assisted by IgepalCO520 under sonication and comparison with thermal activation.
Entry
Substrate
(1)
Activation method[a]
Conversion [%]
2 [%]
3 [%]
dl/meso[b]
Global sel. [%][c]
2/3 [%]
1
2
silent
)))
70
100
55
88
5
8
46/54
45/55
79
88
91/9
83/17
2H
2J
3H
3J
1H
3
4
silent
)))
40
100
26
55
12
45
48/52
45/55
65
55
68/32
61/39
1 J
5
6
silent
)))
40
100
28
72
8
18
26/74
45/55
70
72
78/22
80/20
2K
3K
1 K
1M
7
8
silent
)))
17
40
12
37
4
3
48/52
46/54
71
92
75/25
92/8
2M
3M
9
10
silent
)))
10
100
9
88
1
8
12/88
10/90
90
88
90/10
92/8
1S
2S
3S
11
12
silent
)))
5
11
5
11
0
0
54/46
55/45
100
100
100
100
1U
2U
3U
[a] For silent method, the reaction is carried out at 458C during 2 h; for sonication, the reaction is carried out during 1 h 30 at 200 Hz with an ice bath
1
[b] The dl/meso ratio was determined by H NMR of the crude product. [c] Global selectivity is defined as PCP/conversion.
hyde, and so on. As good-to-ex-
cellent
diastereoselectivities
were observed without chiral in-
ductors, further investigation on
the chiral intermediates should
be carried out.
Experimental Section
Materials and methods
All commercially available products
and solvents were used without
further purification. Reactions were
monitored by TLC (Kieselgel
60F254 aluminum sheet) with de-
tection by UV light, or potassium
permanganate acidic solution.
Column chromatography was per-
formed on silica gel 40–60 mm.
Flash column chromatography was
performed on an automatic appa-
Figure 7. Direct observation of reaction medium.
1
ratus, using silica gel cartridges. H
and 13C NMR spectra were record-
ed on a 400 MHz/54 mm ultralong hold. Chemical shifts (d) are
quoted in parts per million (ppm) and are referenced to TMS as an
internal standard. Coupling constants (J) are quoted in hertz. Mi-
crowave experiments were conducted in a commercial microwave
reactor especially designed for synthetic chemistry. Monowave300
cal observations. As far as we know, the pinacol coupling as-
sisted by surfactant in water has never been described, espe-
cially for natural high value a,b-unsaturated aliphatic alde-
hydes such as citral A. This should be a method of choice to
valorize other natural compounds such myrtenal, jasminalde-
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Compound 2B:[16,28] (4E,8E)-dodeca-4,8-diene-6,7-diol (dl and
meso)
(Anton Paar, Austria) is a monomode cavity with a microwave
power delivery system ranging from 0 to 850 W. The temperatures
of the reactions were monitored via a contactless infrared pyrome-
ter, which was calibrated in control experiments with a fiber-optic
contact thermometer. Sealed vessels and a magnetic stir bar inside
the vessel were used. Temperature and power profiles were moni-
tored in both cases through the software provided by the manu-
facturer. For sonication, Reactions were performed with a 200 W ul-
trasonic probe (Bioblock Scientific, Vibra cell 75042) and monitored
by HPLC assays. The dl/meso ratios were determined by means of
1H NMR of the crude product for all compounds.
Table 6 entry 1, colorless oil: 1H NMR (400 MHz, CDCl3): d=0.84–
0.80 (m, 6H, 2 CH3), 1.35–1.29 (m, 4H, 2 CH2), 1.97–1.92 (m, 4H, 2
CH2), 2.82 (brs, 2H, OH), 3.81 (d, J=8.4 Hz, 2H, CH-OH dl form),
4.00 (d, J=8.4 Hz, 2H, CH-OH meso form), 5.42–5.31 (m, 2H, CH=
CH), 5.68–5.60 ppm (m, 2H, CH=CH); dl-2B: 13C NMR (100 MHz,
CDCl3): d=13.6 (CH3), 22.2 (CH2), 34.4 (CH2), 76.1 (CH), 128.8 (CH=
CH), 134.2 ppm (CH=CH); meso-2B: 13C NMR (100 MHz, CDCl3):
d=13.6 (CH3), 22.2 (CH2), 34.4 (CH2), 75.7 (CH), 128.0 (CH=CH),
134.5 ppm (CH=CH); HRMS (ESI): found 221.1519; calculated
221.1517 for C12H22O2Na.
Light scattering measurements:
Compound 2C: (8E,12E)-icosa-8,12-diene-10,11-diol (dl and
meso)
Dynamic light scattering measurements were carried out on a Mas-
tersizer 2000 analyzer (Malvern Instruments Ltd, Worchestershire,
UK). Increased sub-micrometer resolution was delivered via a dual-
wavelength detection system. A short wavelength blue light
source (0.3 mW LED, l=470 nm) was used in conjunction with for-
ward and backscatter detection for enhanced sizing performance.
This, combined with red-light measurements (4 mW He–Ne laser,
l=633 nm), provided superior sensitivity across a wide size range.
The Mastersizer 2000 was equipped with a Hydro2000SM wet
sample dispersion unit.
Table 6 entry 2, white solid: 1H NMR (400 MHz, CDCl3): d=0.82–0.79
(m, 6H, 2 CH3), 1.32–1.19 (m, 20H, 8 CH2, 2OH, 1H from non-equiv-
alent CH2), 2.01–1.94 (m, 6H, 2CH2, 1H from non-equivalent CH2),
3.84 (dd, J=8.4, 1.6 Hz, 2H, CH-OH dl form), 3.99 (dd, J=8.4,
1.6 Hz, 2H, CH-OH meso form), 5.42–5.32 (m, 2H, CH=CH), 5.70–
5.64 ppm (m, 2H, CH=CH); dl-2B: 13C NMR (100 MHz, CDCl3): d=
13.1 (CH3), 21.7 (CH2), 28.1 (CH2), 28.1 (CH2), 28.2 (CH2), 31.4 (CH2),
75.0 (CH), 127.4 (CH=CH), 133.8 ppm (CH=CH), meso-2B:
13C NMR (100 MHz, CDCl3): d=13.1 (CH3), 21.7 (CH2), 28.1 (CH2),
28.1 (CH2), 28.2 (CH2), 30.8 (CH2), 74.6 (CH), 126.7 (CH=CH),
134.3 ppm (CH=CH), mp=30–328C; HRMS (ESI): found 333.2756;
calculated 333.2770 for C20H38O2Na.
Synthetic procedures
Conventional heating: A 10 mL flask was charged with the desired
carbonyl compound (2,3 mmol), NH4Cl (2 equiv, 248 mg), Zinc
(2,1 equiv, 320 mg). The solution of the surfactant in water (1 wt.%,
5 mL) was then added. The mixture was heated at 458C for 2 h. At
the end of the reaction, zinc is filtered and the final product is ex-
tracted with EtOAc (3ꢁ5 mL) before purification.
Microwave irradiation: A 30 mL vessel was charged with the de-
sired carbonyl compound (2,3 mmol), NH4Cl (2 equiv, 248 mg), Zinc
(2,1 equiv, 320 mg). The solution of the surfactant in water (1 wt.%,
5 mL) was then added. The mixture was heated at the desired tem-
perature (458C or 808C) for the optimized time (1.5 h or 2 h). At
the end of the reaction, zinc is filtered and the final product is ex-
tracted with EtOAc (3ꢁ5 mL) before purification.
Sonication: In a 20 mL Beaker containing an IgepalCO520 aqueous
solution (10 mg in 10 mL), zinc (451 mg, 6.9 mmol), NH4Cl (500 mg,
4,6 mmol) and carbonyl compound (2.3 mmol) were introduced.
The mixture was sonicated for 90 min in an ice bath (the ultrasonic
probe was placed at 1 cm from the bottom of the beaker to
ensure a homogenous dispersion). At the end of the reaction, zinc
was filtered and the final product was extracted with EtOAc (3ꢁ
5 mL) before purification.
Compound 2D: (6E,8E,12E,14E)-icosa-6,8,12,14-tetraene-
10,11-diol (meso)
Table 6 entry 3, colorless oil: meso-2D: 1H NMR (400 MHz, CDCl3):
d=0.83–0.79 (m, 10H, 2 CH3, 1 CH2, 2 OH), 1.23–1.18 (m, 12H, 6
CH2), 2.00 (dd, 2H, 1 CH2), 4.10 (d, J=8.4 Hz, 2H, CH-OH), 5.49 (dd,
J=15.6, 6.0 Hz, 2H, CH=CH), 5.65 (td, J=14.4, 6.82 Hz, 2H, CH=
CH), 5.97 (dd, J=15.2, 10.4 Hz, 2H, CH=CH), 6.20 ppm (dd, J=
15.2, 10.4 Hz, 2H, CH=CH); 13C NMR (100 MHz, CDCl3): d=13.0
(CH3), 22.5 (CH2), 28.8 (CH2), 31.4 (CH2), 32.6 (CH2), 75.5 (CH), 127.8
(CH=CH), 129.2 (CH=CH), 133.8 (CH=CH), 136.5 ppm (CH=CH);
HRMS (ESI): found 329.2468; calculated 329.2457 for C20H34O2Na.
Compound 2E: (2E,6E)-3,6-dimethylocta-2,6-diene-4,5-diol
(meso)
Table 6 entry 4, colorless oil: meso-2E: 1H NMR (400 MHz, CDCl3):
d=1.54–1.53 (m, 12H, 4 CH3), 2.25–2.14 (bs, 2H, 2 OH), 3.93 (s, 2H,
2 CH-OH), 5.50–5.44 ppm (m, 2H, 2 CH=CH), 13C NMR (100 MHz,
CDCl3): d=11.9 (CH3), 13.1 (CH3), 78.4 (CH), 122.8 (CH=C),
134.5 ppm (CH=C), HRMS (ESI): found 193.1201; calculated
193.1204 for C10H18O2Na.
Compound 2A:[6a,7,27] 1,2-Bis(4-bromophenyl)-1,2-ethanediol
(dl and meso)
1
Table 6 entry 10, white solid: dl-2A: H NMR (400 MHz, CDCl3): d=
Compound 2F: (3E,7E)-4,7-dimethyldeca-3,7-diene-5,6-diol
(meso)
4.53 (s, 2H, CH-OH), 6.89 (d, J=8.4 Hz, 4H, CHAr), 7.30 ppm (d, J=
8.4 Hz, 4H, CHAr); 13C NMR (100 MHz, CDCl3): d=78.5 (CH), 122.2
(CIV), 128.7 (2 CHAr), 131.4 (2 CHAr), 138.4 ppm (CIV); meso-2A:
1H NMR (400 MHz, CDCl3): d=4.74 (s, 2H, CH-OH), 6.98 (dd, J=8.4,
1.6 Hz, 4H, CHAr), 7.34 ppm (d, J=8.4, 1.6 Hz, 4H, CHAr); 13C NMR
(100 MHz, CDCl3): d=77.2 (CH), 122.1 (CIV), 128.7 (2 CHAr), 131.3 (2
CHAr), 138.3 ppm (CIV), MS (ESI): 370.92 (51%) [M+H]+, 372.92
(100%), 374.92 (49%); mp=155–1578C.
Table 6 entry 5, colorless oil: meso-2F: 1H NMR (400 MHz, CDCl3):
d=0.86 (t, 6H, 2 CH3), 1.53 (s, 6H, 2 CH3), 1.94 (quint, J=7.6 Hz,
4H, 2 CH2), 2.30 (bs, 2H, 2 OH), 3.91 (s, 2H, 2 CH-OH), 5.35 ppm
(td, 2H, 2 CH=C); 13C NMR (100 MHz, CDCl3): d=11.8 (CH3), 13.8
(CH3), 20.8 (CH2), 78.8 (CH), 130.7 (CH=C), 133.0 ppm (CH=C);
HRMS (ESI): found 221.1521; calculated 221.1517 for C12H22O2Na.
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dl form), 128.4 (CHAr), 128.4 (CHAr), 128.4 (CHAr), 128.9 (CHAr), 131.6
(CIV), 133.9 (CIV), 137.8 (CIV), 137.9 ppm (CIV), MS (ESI): 283.02
(100%), 285.02 (64%), 284.02 (15%) [M+H]+ mp=147–1498C.
Compound 2G: (6E,10E)-2,6,11,15-tetramethylhexadeca-
2,6,10,14-tetraene-8,9-diol (dl and meso)
1
Table 6 entry 6, yellow oil: H NMR (400 MHz, CDCl3): d=1.70–1.54
(m, 18H, 6 CH3), 2.06–2.10 (m, 8H, 4 CH2), 4.13 (m, 1H, CH-OH dl
form), 4.23 (m, 1H, CH-OH meso form), 5.19–4.99 ppm (m, 4H, 4
CH=C); 13C NMR (100 MHz, CDCl3): d=16.9 (CH3), 17.1 (CH3), 17.7
(CH3), 17.7 (CH3), 23.5 (2 CH3), 23.6 (2 CH3), 25.7 (2 CH3), 25.7 (2
CH3), 26.3 (CH2), 26.4 (CH2), 26.6 (CH2), 26.7 (CH2), 32.6 (CH2), 32.7
(CH2), 39.7 (CH2), 39.8 (CH2), 70.9 (CH), 71.3 (CH), 71.8 (CH), 72.1
(CH), 122.8 (CH=C), 123.1 (CH=C), 123.6 (CH=C), 123.8 (CH=C),
123.9 (CH=C), 123.9 (CH=C), 123.9 (CH=C), 124.0 (CH=C), 131.7
(CH=C), 131.9 (CH=C), 132.4 (CH=C), 132.5 (CH=C), 141.5 (CH=
C), 141.6 (CH=C), 141.7 (CH=C), 142.1 ppm (CH=C), HRMS (ESI):
found 329.2461; calculated 329.2457 for C20H34O2Na.
Compound 2l:[4a,9b,31,33] 1,2-Bis(4-fluorophenyl)-1,2-ethane-
diol (dl and meso)
1
Table 6 entry 13, white solid: dl-2l: H NMR (400 MHz, CDCl3): d=
2.49 (bs, 2H, OH), 4.56 (s, 2H, CH-OH), 6.84 (t, J=8.4 Hz, 4H,
1
2CHAr), 6.96 ppm (dd, J=8.4, 5.6 Hz, 4H, 2 CHAr); meso-2l: H NMR
(400 MHz, CDCl3): d=2.49 (bs, 2H, OH), 4.76 (s, 2H, CH-OH), 6.89 (t,
J=8.4 Hz, 4H, 2 CHAr), 7.07 ppm (dd, J=8.4, 5.6 Hz, 4H, CHAr);
13C NMR (100 MHz, CDCl3): d=77.3 (CH meso form), 78.8 (CH dl
form), 115.0 (CHAr), 115.2 (CHAr), 128.6 (CHAr), 128.7 (CHAr), 128.7
(CHAr), 135.1 (CIV)„ 135.2 (CIV), 161.3 (CIV), 163.7 ppm (CIV), MS (ESI):
251.08 [M+H]+ 274.07 [M+Na]+ mp=151–1538C [litt. 143–
Compound 2H:[29] (1E,5E)-1,6-diphenylhexa-1,5-diene-3,4-
diol (dl and meso)
,
,
1778C].
1
Table 6 entry 7, white solid: H NMR (400 MHz, CDCl3): d=4.21 (dd,
J=11.4, 5.4 Hz, 2H, CH-OH dl form), 4.36 (dd, J=9.4, 3.8 Hz, 2H,
CH-OH meso form), 6.25–6.17 (m, 2H, CH=CH), 6.68–6.61 (m, 2H,
CH=CH), 7.33–7.22 ppm (m, 10H, CHAr); 13C NMR (100 MHz, CDCl3):
d=78.5 (CH), 122.2 (CIV), 128.7 (2 CHAr), 131.4 (2 CHAr), 138.4 ppm
(CIV); 13C NMR (100 MHz, CDCl3): d=75.8 (CH meso form), 75.9 (CH
dl form), 126.6 (CH), 126.7 (CH), 127.1 (CH), 127.8 (CH), 127.9 (CH),
128.0 (CH), 128.6 (2 CH), 132.7 (CH), 133.0 (CH), 136.4 (CIV),
136.5 ppm (CIV), MS (ESI): 267.13 [M+H]+, 289.12 [M+Na]+ mp=
110–1128C [litt. 106–1558C].
Compound 2M:[4a,5a,9a,c,14a,29a–c,31,32,34] 1,2-Bis(4-methoxyphen-
yl)-1,2-ethanediol (dl and meso)
1
Table 6 entry 14, white solid: dl-2M: H NMR (400 MHz, CDCl3): d=
3.69 (s, 6H, CH3), 4.56 (s, 2H, CH-OH), 6.68 (dd, J=6.8, 4.2 Hz, 4H,
2CHAr), 6.96 ppm (dd, J=8.4, 5.6 Hz, 4H, 2 CHAr); meso-2M:
1H NMR (400 MHz, CDCl3): d=3.73 (s, 6H, CH3), 4.66 (s, 2H, CH-OH),
6.78 (dd, J=8.4, 2.0 Hz, 4H, 2CHAr), 7.14 ppm (dd, J=6.8, 2.0 Hz,
4H, CHAr); 13C NMR (100 MHz, CDCl3): d=55.2 (OCH3), 55.3 (OCH3),
77.8 (CH meso form), 78.8 (CH dl form), 113.5 (CHAr), 113.7 (CHAr),
128.2 (CHAr), 128.3 (CHAr), 132.0 (CIV), 132.1 (CIV), 159.0 (CIV),
159.2 ppm (CIV), MS (ESI): 275.12 [M+H]+ 298.11 [M+Na]+ mp=
Compound 2I:[4a,b,5a,6a,9a,c,11a,12a,14a,28,29,30] 1,2-Diphenyl-1,2-
ethanediol (dl and meso)
,
,
165–1668C [litt. 164–1688C].
Table 6 entry 8, white solid: dl-2I: 1H NMR (400 MHz, CDCl3): d=
2.25–2.10 (bs, 2H, OH), 4.61 (s, 2H, CH-OH), 7.23–7.02 ppm (m,
10H, CHAr); 13C NMR (100 MHz, CDCl3): d=79.1 (CH-OH), 127.0 (2
CHAr), 128.0 (CHAr), 128.1 (2 CHAr), 139.8 ppm (CIV); meso-2I:
1H NMR (400 MHz, CDCl3): d=2.25–2.10 (bs, 2H, OH), 4.74 (s, 2H,
CH-OH), 7.23–7.02 ppm (m, 10H, CHAr); 13C NMR (100 MHz, CDCl3):
d=78.1 (CH-OH), 127.1 (2CHAr), 128.1 (CHAr), 128.2 (2CHAr),
139.7 ppm (CIV), MS (ESI): 215.1 [M+H]+, 237.1 [M+Na]+, mp=
120–1248C [litt. 119–1598C].
Compound 2N:[4a,6a,9a,c,14a,29a,b,31,32,33b] 1,2-Bis(2-chlorophenyl)-
1,2-ethanediol (dl and meso)
1
Table 6 entry 15, white solid: dl-2N: H NMR (400 MHz, CDCl3): d=
2.87 (bs, 2H, OH), 5.24 (s, 2H, CH-OH), 7.18–7.01 (m, 6H, CHAr),
7.54 ppm (dd, J=7.6, 1.6 Hz, 2H, CHAr); 13C NMR (100 MHz, CDCl3):
d=73.1 (CH), 126.9 (CHAr), 129.2 (CHAr), 129.2 (CHAr), 129.2 (CHAr),
129.5 (CHAr), 132.6 (CIV), 137.2 ppm (CIV); meso-2N: 1H NMR
(400 MHz, CDCl3): d=2.87 (bs, 2H, OH), 5.49 (s, 2H, CH-OH), 7.18–
7.01 ppm (m, 8H, CHAr); 13C NMR (100 MHz, CDCl3): d=72.1 (CH),
126.5 (CHAr), 128.8 (CHAr), 128.8 (CHAr), 128.9 (CHAr), 133.4 (CIV)„
136.4 ppm (CIV), MS (ESI): 283.02 (100%), 285.02 (64%), 284.02
(15%) [M+H]+ mp=137–1398C [litt. 132–1468C].
Compound 2J:[4a,5a,9a–c,14a,29a–c,27e,31] 1,2-Bis(4-methylphenyl)-
1,2-ethanediol (dl and meso)
Table 6 entry 9, beige solid: 1H NMR (400 MHz, CDCl3): d=2.22 (s,
6H, 2 CH3), 2.26 (s, 6H, 2 CH3), 4.57 (s, 2H, CH-OH dl form), 4.65 (s,
2H, CH-OH meso form), 7.10–6.93 ppm (m, 8H, CHAr); 13C NMR
(100 MHz, CDCl3): d=21.2 (4 CH3), 78.1 (CH-OH meso form), 78.8
(CH-OH dl form), 126.9 (2CHAr), 127.1 (2CHAr), 128.8 (2CHAr), 129.0
(2CHAr), 137.0 (2CIV), 137.5 (CIV), 137.8 ppm (CIV), MS (ESI): 243.13
[M+H]+, 267.12 [M+Na]+, mp=161–1638C [litt 161–1808C].
Compound 2O:[4a,29a–c,31,32] 1,2-Bis(3-chlorophenyl)-1,2-etha-
nediol (dl and meso)
1
Table 6 entry 16, white solid: dl-2O: H NMR (400 MHz, CDCl3): d=
Compound 2K:[4a,5a,9a–c,14a,27e,29a–c,31,32] 1,2-Bis(4-chlorophen-
yl)-1,2-ethanediol (dl and meso)
4.50 (s, 2H, CH-OH), 6.79 (d, J=8.0 Hz, 2H, CHAr), 7.18–7.05 ppm
(m, 6H, CHAr); 13C NMR (100 MHz, CDCl3): d=78.3 (CH), 125.2 (CHAr),
127.0 (CHAr), 128.3 (CHAr), 129.4 (CHAr), 134.2 (CIV), 141.6 ppm (CIV);
1
1
Table 6 entry 11, white solid: dl-2K: H NMR (400 MHz, CDCl3): d=
meso-2O: H NMR (400 MHz, CDCl3): d=4.68 (s, 2H, CH-OH), 6.91
4.55 (s, 2H, CH-OH), 6.95 (d, J=8.4 Hz, 4H, 2CHAr), 7.14 ppm (d, J=
8.4 Hz, 4H, 2CHAr); meso-2K: H NMR (400 MHz, CDCl3): d=4.77 (s,
2H, CH-OH), 7.03 (dd, J=8.4, 1.6 Hz, 4H, 2CHAr), 7.18 ppm (m, 4H,
CHAr); 13C NMR (100 MHz, CDCl3): d=77.2 (CH meso form), 78.6 (CH
(dd, J=8.0, 1.2, 2H, CHAr), 7.18–7.05 ppm (m, 6H, CHAr); 13C NMR
(100 MHz, CDCl3): d=77.1 (CH), 125.3 (CHAr), 127.2 (CHAr), 128.3
(CHAr), 129.4 (CHAr), 134.2 (CIV), 141.5 ppm (CIV), MS (ESI): 283.02
(100%), 285.02 (64%), 284.02 (15%) [M+H]+ mp=140–1428C.
1
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Full Papers
Compound 2P:[5a] 1,2-Bis(2,3-dichlorophenyl)-1,2-ethanediol
(dl and meso)
128.8 (CHAr), 131.4 (CIV), 133.5 (CIV), 135.8 ppm (CIV), MS (ESI): 315.13
[M+H]+, 337.12 [M+Na]+, mp=179–1818C.
1
Table 6 entry 17, white solid: dl-2P: H NMR (400 MHz, CDCl3): d=
Compound 2T:[4a,5a,9b,14a,31,32] 1,2-di(furan-2-yl)ethane-1,2-diol
(dl and meso)
2.12 (bs, 2H, OH), 5.31 (s, 2H, CH-OH), 7.20 (t, J=8.0 Hz, 2H, CHAr),
7.36 (dd, J=8.0, 1.6 Hz, 2H, CHAr), 7.55 ppm (dd, J=8.0, 1.6 Hz, 2H,
CHAr); 13C NMR (100 MHz, CDCl3): d=73.0 (CH), 129.8 (CIV), 127.2
(CHAr), 127.3 (CHAr), 130.1 (CHAr), 131.5 (CIV), 139.7 ppm (CIV); meso-
2P: 1H NMR (400 MHz, CDCl3): d=2.12 (bs, 2H, OH), 5.57 (s, 2H,
CH-OH), 7.03 (t, J=8.0 Hz, 2H, CHAr), 7.11 (dd, J=7.9, 1.7 Hz, 2H,
CHAr), 7.55 ppm (dd, J=8.0, 1.6 Hz, 2H, CHAr),
13C NMR (100 MHz, CDCl3): d=77.1 (CH), 126.9 (2 CHAr), 129.8
(CHAr), 129.8 (CIV), 132.5 (CIV), 138.5 ppm (CIV), MS (ESI): 352.94
(100%), 350.94 (78%), 354.94 (48%) [M+H]+ mp=177–1798C.
1
Table 6 entry 21, yellow oil: H NMR (400 MHz, CDCl3): d=2.75 (bs,
2H, OH dl and meso forms), 4.98 (s, 2H, CH-OH dl form), 5.01 (s,
2H, CH-OH meso form), 6.33–6.23 (m, 4H, CHAr dl and meso forms),
7.38–7.34 ppm (m, 2H, CHAr dl and meso forms); 13C NMR (100 MHz,
CDCl3): d=69.9 (CH-OH dl form), 70.1 (CH-OH meso form), 108.0
(CHAr), 108.2 (CHAr), 110.3 (CHAr), 110.4 (CHAr), 142.4 (CHAr), 142.4
(CHAr), 152.6 (CIV), 152.8 ppm (CIV), MS (ESI): 195.06 [M+H]+ 218.06
,
[M+Na]+
.
Compound 2Q:[9b,c,35] 1,2-Bis(2-methoxyphenyl)-1,2-ethane-
diol (dl and meso)
Compound 2U:[4a,9a,c,14a,28a,c,31,32,35,37] 2,3-diphenylbutane-2,3-
diol (dl and meso)
1
Table 6 entry 18, white solid: dl-2Q: H NMR (400 MHz, CDCl3): d=
1
Table 6 entry 22, white solid: H NMR (400 MHz, CDCl3): d=1.41 (s,
2.92 (bs, 2H, OH), 3.56 (s, 6H, CH3), 4.96 (s, 2H, CH-OH), 6.67 (d, J=
8.0 Hz, 2H, CHAr), 6.76 (td, J=7.5, 1.2 Hz, 2H, CHAr), 7.10–7.07 ppm
(m, 2H, 2 CHAr); 13C NMR (100 MHz, CDCl3): d=55.2 (OCH3), 74.5
(CH), 110.2 (CHAr), 120.5 (CHAr), 128.3 (CHAr), 128.4 (CIV), 128.6 (CHAr),
6H, CH3 dl form), 1.49 (s, 6H, CH3 meso form), 2.39 (bs, 2H, OH dl
and meso forms), 7.16–7.10 ppm (m, 10H, CHAr dl and meso forms);
13C NMR (100 MHz, CDCl3): d=25.0 (CH3 dl form), 25.1 (CH3 meso
form), 78.7 (CIV meso form), 78.9 (CIV dl form), 126.9 (CHAr), 127.0
(2CHAr), 127.1 (CHAr), 127.2 (2CHAr), 127.3 (2CHAr), 127.4 (2CHAr),
1
157.0 ppm (CIV); meso-2Q: H NMR (400 MHz, CDCl3): d=2.92 (bs,
2H, OH), 3.60 (s, 6H, CH3), 5.17 (s, 2H, CH-OH), 6.72 (dd, J=8.2,
1.2 Hz, 2H, CHAr), 6.81 (td, J=7.5, 1.2 Hz, 2H, CHAr), 7.07 (dd, J=7.5,
1.2 Hz, H, CHAr), 7.12 ppm (td, J=7.5, 1.2 Hz, 2H, CHAr); 13C NMR
(100 MHz, CDCl3): d=55.3 (OCH3), 73.7 (CH), 110.2 (CHAr), 120.5
(CHAr), 128.3 (CHAr), 128.5 (CHAr), 128.5 (CIV), 156.9 ppm (CIV); MS
143.4 (CIV), 143.8 ppm (CIV), MS (ESI): 243.13 [M+H]+ 267.12 [M+
,
Na]+, mp=127–1308C.
Compound 2V:[9c,29c,31,38] 2,3-bis(4-chlorophenyl)butane-2,3-
(ESI): 275.12 [M+H]+ 298.11 [M+Na]+, mp=156–1588C.
,
diol (dl and meso)
1
Table 6 entry 23, white solid: H NMR (400 MHz, CDCl3): d=1.39 (s,
Compound 2R:[4a,9b,14a,27b,32,35] 1,2-Bis(3-methoxyphenyl)-1,2-
6H, CH3 dl form), 1.46 (s, 6H, CH3 meso form), 2.22 (bs, 2H, OH dl
and meso forms), 7.23–7.00 ppm (m, 10H, CHAr dl and meso forms);
13C NMR (100 MHz, CDCl3): d=24.8 (CH3 dl form), 25.1 (CH3 meso
form), 78.3 (CIV meso form), 78.5 (CIV dl form), 126.8 (CHAr), 127.3 (2
CHAr), 127.4 (CHAr), 128.5 (2 CHAr), 128.6 (2 CHAr), 128.8 (2 CHAr),
133.0 (CIV), 133.2 (CIV), 141.7 (CIV), 142.2 ppm (CIV), MS (ESI): 243.13
ethanediol (dl and meso)
Table 6 entry 19, pale yellow solid: dl-2R: 1H NMR (400 MHz,
CDCl3): d=2.39 (bs, 2H, OH), 3.62 (s, 6H, CH3), 4.56 (s, 2H, CH-OH),
6.77–6.61 (m, 6H, CHAr), 7.06 ppm (bt, J=7.8 Hz, 2H, CHAr);
13C NMR (100 MHz, CDCl3): d=55.2 (OCH3), 78.9 (CH), 112.2 (CHAr),
113.7 (CHAr), 119.2 (CHAr), 129.2 (CHAr), 141.5 (CIV), 159.4 ppm (CIV);
[M+H]+, 267.12 [M+Na]+ mp=141–1428C.
,
1
meso-2R: H NMR (400 MHz, CDCl3): d=2.39 (bs, 2H, OH), 3.65 (s,
6H, CH3), 4.69 (s, 2H, CH-OH), 6.77–6.61 (m, 6H, CHAr), 7.14 ppm
(bt, J=7.9 Hz, 2H, CHAr); 13C NMR (100 MHz, CDCl3): d=55.2
(OCH3), 78.0 (CH), 112.3 (CHAr), 114.0 (CHAr), 119.5 (CHAr), 129.2
Acknowledgements
(CHAr), 141.5 (CIV), 159.5 ppm (CIV), MS (ESI): 275.12 [M+H]+ 298.11
,
M.B. would really like to thank Prof. I. Pezron, from the IMID
team for its precious help and discussion on emulsions and
during microscopic observations. M.B. also thanks M. Blanchot,
from the ESCOM, for his practical help. And special thanks to the
4A ESCOM students for their collaboration.
[M+Na]+, mp=161–1638C.
Compound 2S:[4a,5a,27b,32,33c,36] 1,2-Bis(1-naphtyl)-1,2-ethane-
diol (dl and meso)
1
Table 6 entry 20, white solid: dl-2S: H NMR (400 MHz, CDCl3): d=
Keywords: alternative technologies · C–C coupling reaction ·
heterogeneous catalysis · phase-transfer catalysis · surfactants
2.25–2.00 (bs, 2H, OH), 5.78 (s, 2H, CH-OH), 7.27 (m, 2H, CHAr),
7.41–7.34 (m, 4H, CHAr), 7.75–7.67 (m, 6H, CHAr), 7.86 ppm (d, J=
8.4 Hz, 2H, CHAr); 13C NMR (100 MHz, CDCl3): d=74.4 (CH-OH),
123.0 (CHAr), 124.9 (CHAr), 125.1 (CHAr), 125.4 (CHAr), 125.8 (CHAr),
128.6 (CHAr), 128.7 (CHAr), 130.9 (CIV), 133.7 (CIV), 136.1 ppm (CIV), MS
(ESI): 315.13 [M+H]+, 337.12 [M+Na]+, mp=178–1808C; meso-
2S: 1H NMR (400 MHz, CDCl3): d=2.25–2.00 (bs, 2H, OH), 5.95 (s,
2H, CH-OH), 7.39–7.33 (m, 4H, CHAr), 7.44 (td, J=8.0, 1.4 Hz, 2H,
CHAr), 7.57 (dd, J=7.2, 1.2 Hz, 2H, CHAr), 7.76 (d, J=8.2 Hz, 2H,
CHAr), 7.83 (d, J=8.0 Hz, 2H, CHAr), 7.93 ppm (d, J=8.0 Hz, 2H,
CHAr); 13C NMR (100 MHz, CDCl3): d=74.2 (CH-OH), 123.1 (CHAr),
124.9 (CHAr), 125.2 (CHAr), 125.5 (CHAr), 126.0 (CHAr), 128.6 (CHAr),
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Received: February 13, 2015
Published online on && &&, 0000
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ChemSusChem 0000, 00, 0 – 0
www.chemsuschem.org
12
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

FULL PAPERS
M. Billamboz, C. Len*
&& – &&
First Pinacol Coupling in Emulsified
Water: Key Role of Surfactant and
Impact of Alternative Activation
Technologies
Emulsion as the solution: Combining
surfactant effects with acoustic cavita-
tion allowed the design of a novel, fast,
and efficient protocol for pinacol cou-
pling of aromatic and a,b-unsaturated
aliphatic aldehydes in water. This proto-
col could be applied to natural high-
value compounds such as citral A. The
use of IgepalCO520, a neutral surfactant,
led to the successful conversion of aro-
matic or a,b-unsaturated aliphatic car-
bonyl compounds in moderate-to-excel-
lent yield.
ChemSusChem 0000, 00, 0 – 0
www.chemsuschem.org
13
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ