Pinacol couplings of a series of aldehydes and ketones with SmI2/Sm/Me3SiCl in DME

Article abstract of DOI:10.1016/j.tet.2015.06.007

The pinacol coupling is one of the most significant methods to synthesize vic-diols. The combination of samarium diiodide (SmI₂) and samarium metal successfully induces the selective pinacol couplings of not only aromatic aldehydes and ketones but also aliphatic ones in the presence of trimethylchlorosilane (Me₃SiCl) in 1,2-dimethoxyethane (DME). DME is the most suitable solvent for the reduction system using SmI₂ and Me₃SiCl. Me₃SiCl, a widely available additive, prevents the decomposition of the formed vic-diols, i.e., meso-isomers, and controls their stereochemistry. In particular, the pinacol couplings of sterically hindered aliphatic aldehydes and ketones proceed with excellent diastereoselectivities to afford dl-isomers in good yields.

Full text of DOI:10.1016/j.tet.2015.06.007

Tetrahedron 71 (2015) 5347e5355
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Tetrahedron
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Pinacol couplings of a series of aldehydes and ketones with
SmI2/Sm/Me3SiCl in DME
*
*
Aya Yoshimura, Tomokazu Saeki, Akihiro Nomoto , Akiya Ogawa
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The pinacol coupling is one of the most significant methods to synthesize vic-diols. The combination of
samarium diiodide (SmI2) and samarium metal successfully induces the selective pinacol couplings of
not only aromatic aldehydes and ketones but also aliphatic ones in the presence of trimethylchlorosilane
(Me3SiCl) in 1,2-dimethoxyethane (DME). DME is the most suitable solvent for the reduction system
using SmI2 and Me3SiCl. Me3SiCl, a widely available additive, prevents the decomposition of the formed
vic-diols, i.e., meso-isomers, and controls their stereochemistry. In particular, the pinacol couplings of
sterically hindered aliphatic aldehydes and ketones proceed with excellent diastereoselectivities to af-
ford dl-isomers in good yields.
Received 9 February 2015
Received in revised form 1 June 2015
Accepted 2 June 2015
Available online 9 June 2015
Keywords:
Pinacol coupling
Samarium diiodide
Carbonyl compound
Trimethylchlorosilane
1,2-Dimethoxyethane
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
from SmI2 and silyl chlorides when silyl chlorides were used as
additives.
The pinacol coupling of carbonyl compounds is an important
method for the construction of functionalized carbonecarbon
bonds.1 Since the pinacol coupling was first developed by Fitting in
1859,2 it has been widely used to synthesize vicinal diols (vic-diols).
A number of vic-diols are found in various important natural
products and medicinal chemicals.3
As to (1), we have revealed that the combination of SmI2 and
samarium metal leads to enhanced reducing ability compared to
those of SmI2 and Samarium metal alone.7 SmI2 has occasionally
been synthesized using excess amounts of Samarium metal, fol-
lowed by the subsequent pinacol coupling reaction performed in
the same vessel. However, most papers do not clearly describe the
details about the presence of Samarium metal. For example, re-
gardless of the use of excess Samarium metal in the preparation of
SmI2, the pinacol coupling reactions are reported as reactions using
only SmI2. Therefore, the presence/absence of Sm metal is an im-
portant issue in the SmI2-induced pinacol coupling reactions of
carbonyl compounds.
As to (2), the presence of H2O is also very important when
discussing the reducing ability of SmI2. Hasegawa and Curran found
that water also dramatically enhances the reducing ability of SmI2,
and noted the difference in yield between acid-quenched and air-
quenched reactions.8 Generally, after a reaction using SmI2, the
resulting reaction mixture is quenched with acids. Even if the de-
sired pinacol coupling did not proceed, the addition of water during
the workup immediately generates SmI2/H2O, which can work
quickly as a reducing agent for the carbonyl compounds. However,
few papers clearly describe the quenching procedures.6f,n
Samarium diiodide (SmI2) is one of the most useful single-
electron reducing agents in synthetic organic chemistry. It can be
prepared conveniently from samarium metal and 1,2-diiodoethane
in THF,4 and a number of reductive transformations involving
single-electron transfer processes have been reported.5 SmI2 itself
can also induce the pinacol couplings of carbonyl compounds, es-
pecially aromatic carbonyl compounds.5qex,6 Furthermore, to scope
substrates such as aliphatic carbonyl compounds and functional-
ized ones, and to improve the diastereoselectivity, SmI2 molar ra-
tios, additives, and solvents were investigated in detail. There have
been many reports on pinacol couplings using SmI2; these reports
show that, in spite of similar reaction conditions, the yields and/or
diastereoselectivities of pinacol couplings often vary. This is most
likely due to (1) the presence/absence of Samarium metal in the
reduction system, (2) the methods for quenching the reaction (acid/
air-quenching), and (3) the influence of in-situ-formed silyl iodides
As to (3), pinacol coupling is promoted with high diaster-
eoselectivity in the presence of chlorosilanes as additives.6 In the
case of SmI2 in THF (a representative solvent for SmI2), however,
undesired reaction of SmI2 with chlorosilane forms in situ silyl
* Corresponding authors. Tel.: þ81 72 254 9290; fax: þ81 72 254 9910; e-mail
addresses: nomoto@chem.osakafu-u.ac.jp (A. Nomoto), ogawa@chem.osakafu-u.ac.jp
(A. Ogawa).
http://dx.doi.org/10.1016/j.tet.2015.06.007
0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.

5348
A. Yoshimura et al. / Tetrahedron 71 (2015) 5347e5355
Table 2
iodide, which causes the reductive ring-opening reaction of THF,
resulting in the consumption of SmI2 and the formation of
byproducts (Me3SiOCH2CH2CH2CH2SiMe3) from THF.9 To avoid the
formation of silyl iodide, we previously developed the pinacol
couplings of aromatic carbonyl compounds using Sm metal and
Me3SiCl under sonication in THF in the absence of SmI2.10 Fur-
thermore, this ring-opening problem could be solved through the
use of 1,2-dimethoxyethane (DME) as solvent, because acyclic
ethers do not undergo reductive cleavage of the ethereal CeO bond
even in the presence of silyl iodide.7e Thus, the reductive coupling
of both aromatic and aliphatic carbonyl compounds can be facili-
tated using the SmI2/Sm/Me3SiCl system in DME with high dia-
stereoselectivity. There are a very limited number of systematic
studies on SmI2-induced pinacol couplings of aromatic/aliphatic
aldehydes/ketones using the same reduction system. Herein, we
report an overview of the pinacol couplings of aromatic and ali-
phatic carbonyl compounds using SmI2/Sm/Me3SiCl in DME.
Pinacol couplings of aromatic aldehydes with SmI2/Sm/Me3SiCl in DME
Entry
R1
Isolated yield (%)
dl/meso
1
C6H5 (1a)
85
78
71
84
75
18:82
17:83
24:76
36:64
34:66
2
p-MeC6H4 (1b)
p-MeOC6H4 (1c)
p-BrC6H4 (1d)
p-CF3C6H4 (1e)
3a
4
5
a
DME (10 mL) was used.
DME under the same conditions as those for aromatic aldehydes,
corresponding coupling product 4a was obtained in only 30% yield
(entry 1). When the reductive coupling of 3a was conducted in
refluxing DME (85 ꢀC) for a prolonged reaction time (3 h), the yield
of 4a increased; however, interestingly, the diastereoselectivity
changed and the formation of the dl-isomer was preferred (entry
2). Previously, we reported the reductive dimerization of chlor-
osilanes using SmI2/Sm in DME.7e This dimerization may influence
the pinacol coupling of 3a when a large excess of Me3SiCl is used.
Thus, we examined the reaction using a slight excess of Me3SiCl
(0.55 mmol), and vic-diol 4a was obtained in 58% yield (entry 3).
Prolonging the reaction time further (20 h) decreased the yield of
4a (entry 4). As some side reactions such as benzoin condensation
and the Tishchenko reaction have been reported to occur with
higher concentrations of SmI2,11 the amounts of both SmI2 and Sm
were reduced to 0.55 mmol, resulting in the increase in the yield of
4a (71%) (entry 5). However, when a lower amount of SmI2 than of
3a was used, the pinacol coupling did not occur sufficiently (entry
6). The best results were achieved under diluted conditions (10 mL
of DME), which afforded 4a in 81% yield (entry 7).
Although the pinacol coupling of 3a has been reporte-
d,6a,b,g,m,p,r,11 only a very limited number of pinacol couplings of
other aromatic ketones with SmI2 are known.11a Thus, we examined
the pinacol couplings of several substituted aromatic ketones. The
reductive coupling proceeded successfully to give the correspond-
ing vic-diols in good yields under the optimized reaction conditions
(Table 3, entry 7), as shown in Table 4. In all cases, dl-isomers were
obtained preferentially.
To clarify the behavior of Me3SiCl, we performed some control
experiments (Scheme 1, AeC). In condition A, the pinacol coupling
of benzaldehyde (1.0 mmol) was conducted with 1.0 mmol of both
SmI2 and Sm in DME for 10 min. In condition B, after applying
2. Results and discussion
2.1. Pinacol couplings of aromatic carbonyl compounds with
SmI2/Sm/Me3SiCl in DME
First, we investigated the reductive coupling of benzaldehyde
(1a) with SmI2/Sm/Me3SiCl in DME (Table 1). When 1a (1 mmol) was
treated with SmI2 (1.1 mmol), Sm (1.1 mmol), and Me3SiCl
(1.1 mmol) in DME at room temperature for 10 min, corresponding
vic-diol 2a was obtained in 61% yield with moderate diaster-
eoselectivity (entry 1). In the absence of Me3SiCl, the yield of 2a was
57% (entry 2). When 1.3 equiv of SmI2/Sm/Me3SiCl was used, both
the yield and diastereoselectivity (meso-selectivity) of 2a increased
(entry 3). The best result was obtained when the reaction was
conducted using 0.50 mmol of 1a,1.0 mmol of SmI2,1.0 mmol of Sm,
and 1.5 mmol of Me3SiCl at room temperature for 10 min (entry 4). In
contrast, the absence of Sm metal resulted in a very low yield of 2a
(entry 5). However, the pinacol coupling of benzaldehyde proceeded
well even in the absence of Me3SiCl (entry 6). The results clearly
indicate that, in the case of aromatic aldehydes, Sm metal strongly
induces the pinacol coupling, though Me3SiCl is not effective.
The pinacol couplings of several different aromatic aldehydes
were examined under the optimized reaction conditions (Table 1,
entry 4). As shown in Table 2, various substituted aromatic alde-
hydes (1be1e) can undergo reductive coupling with SmI2/Sm/
Me3SiCl in DME to give corresponding vic-diol 2 in good to excel-
lent yields with preferential formation of the meso-isomers.
Next, we investigated the pinacol couplings of aromatic ketones
using SmI2/Sm/Me3SiCl in DME (Table 3). Although acetophenone
(3a), as a model compound, was treated with SmI2/Sm/Me3SiCl in
Table 1
Pinacol coupling of benzaldehyde with SmI2/Sm/Me3SiCl in DME
Entry
1a (mmol)
SmI2 (mmol)
Sm (mmol)
Me3SiCl (mmol)
Yield of 2aa (%)
dl/mesoa
1
2
3
4
1.0
1.0
1.1
1.1
1.1
1.0
1.0
1.0
1.0
1.1
1.1
1.1
1.0
0
1.1
0
1.1
1.5
1.5
0
61
57
81
88
3
26:74
27:73
19:81
18:82
0:100
18:82
30:70
0.80
0.50
0.50
0.50
0.50
5
6
1.0
1.0
81
81
7b
0
a
Determined by 1H NMR.
Use of THF (3 mL) in place of DME.
b

A. Yoshimura et al. / Tetrahedron 71 (2015) 5347e5355
5349
Table 3
Pinacol coupling of acetophenone with SmI2/Sm/Me3SiCl in DME
Entry
SmI2 (mmol)
Sm (mmol)
Me3SiCl (mmol)
DME (mL)
Conditions
Yielda (%)
dl/meso
1
2
3
4
5
6
7
1
1
1
1
0.55
0.25
0.55
1
1
1
1
0.55
0.55
0.55
1.5
1.7
3
3
3
3
3
rt, 10 min
Reflux, 3 h
Reflux, 3 h
Reflux, 20 h
Reflux, 3 h
Reflux, 3 h
Reflux, 3 h
30
47
58
38
71
56
81
33:67
62:38
78:22
74:26
69:31
66:34
65:35
0.55
0.55
0.58
0.56
0.59
3
10
a
Determined by 1H NMR.
Table 4
Pinacol couplings of aromatic ketones with SmI2/Sm/Me3SiCl in DME
C. The yield of the meso-isomer with condition B was lower than
those with conditions and C. This strongly suggests de-
A
composition of the meso-isomer during the additional 1 h in con-
dition B.12 In contrast, the meso-isomer did not decompose in
condition C. These results suggest that the addition of Me3SiCl can
prevent the decomposition of the meso-isomer, probably through
the bis-silylation of the samarium salt of the pinacol coupling
product.
Entry
R1
Isolated yield (%)
dl/meso
Although several mechanistic pathways of the SmI2-induced
pinacol coupling have been proposed in the literature,5x,6l,13 its
mechanism has not be clarified completely thus far. Mechanistic
consideration is shown in Scheme 2. SmI2-induced pinacol cou-
pling reactions of carbonyl compounds begin with the generation of
samarium-coordinated ketyl radical species A via single-electron
transfer from SmI2. In path A, the generated ketyl radicals A are
coupled in an anti orientation, avoiding the overlap of the sterically
hindered groups, which leads to the meso-diol. In path B, co-
ordination of SmI2 to both the ketyl radical and the substrate car-
bonyl compound is followed by coupling in a syn orientation to give
the dl-diol. Path A likely operates for the reactions of aromatic
1
C6H5 (3a)
74
70
74
73
65:35
67:33
79:21
66:34
2a
3
p-MeC6H4 (3b)
p-CF3C6H4 (3c)
p-ClC6H4 (3d)
4
a
DME (3 mL) was used.
condition A, the reaction continued for another 1 h. In condition C,
after applying condition A, Me3SiCl (1.5 mmol) was added to the
solution and the resulting mixture was stirred for 1 h. Product 2a
was obtained in 55% (dl/meso¼27:73), 40% (dl/meso¼42:58), and
61% (dl/meso¼23:77) yields, respectively, with conditions A, B, and
Scheme 1. Behavior of Me3SiCl.

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A. Yoshimura et al. / Tetrahedron 71 (2015) 5347e5355
Scheme 2. Plausible reaction pathways of aromatic carbonyl compounds.
aldehydes because the concentrations of ketyl radical species are
higher than those of less-reactive aromatic ketones. Accordingly,
the corresponding meso-diols are obtained preferentially (Tables 1
and 2). For aromatic ketones, path B likely operates because the
corresponding ketyl radical species gradually generate in situ and/
or the coupling of ketyl radical species (path A) is inhibited by the
steric hindrance of substituents.
couplings of aromatic ketones (Table 4), the coupling of 5a under
similar conditions did not proceed effectively (entry 5). In the ab-
sence of Me3SiCl, the yield of 6a decreased (28%) and small amounts
of the meso-isomer was also formed (entry 6). Since the reagent
concentrations were important in the pinacol couplings of aromatic
ketones, this coupling reaction was examined using 3,10, and 20 mL
of DME. The best results wereobtained when 10 mL of DMEwas used
(entries 4, 7, 8). When Ph2MeSiCl was used in place of Me3SiCl, the
dl-diol was obtained in 70% yield with excellent diastereoselectivity
(entry 11).14
2.2. Pinacol couplings of aliphatic carbonyl compounds with
SmI2/Sm/Me3SiCl in DME
Table 6 presents the reductive couplings of some other aliphatic
carbonyl compounds under the optimized reaction conditions
(Table 5, entry 7). Aliphatic aldehydes 5ae5d and ketones 5ee5i
successfully underwent reductive coupling with SmI2/Sm/Me3SiCl
in DME to give the corresponding vic-diols 6ae6i in moderate to
high yields. Although the diastereoselectivity was very substrate-
sensitive, complete stereoselectivity (only dl-isomer) was ob-
served with several substrates (5a, 5c, 5e, 5f, 5h, and 5i). In general,
the pinacol couplings of aliphatic carbonyl compounds afford dl-
isomers preferentially, but not exclusively. Notably, the present
SmI2/Sm/Me3SiCl conditions in DME afford only dl-isomers with
many substrates. The diastereoselectivities toward products 6e and
6h have never been discussed.6a,b,i
Compared to aromatic aldehydes and ketones, aliphatic ones are
generally less reactive. As to the diastereoselectivities of the hitherto
known pinacol couplings of aliphatic carbonyl compounds with
SmI2 in the presence of HMPA,6j ferrocene,6k or tetraglym6k as ad-
ditives, dl-diols were obtained preferentially but not completely.
With other additives,6i,j lower diastereoselectivities were observed.
Thus, wenext investigated in detailthe pinacolcouplingsof aliphatic
carbonyl compounds (Table 5). When cyclohexanecarboxaldehyde
(5a) (0.50 mmol), as a model compound, was treated with SmI2
(1.0 mmol), Sm (1.0 mmol), and Me3SiCl (1.5 mmol) in DME at room
temperature for 20 h, corresponding vic-diol 6a was obtained in 39%
yield, surprisingly, with dl-selectivity (entry 1). Elevating the tem-
perature did not facilitate the reaction (entry 2). The yield of 6a did
not increase upon prolonging the reaction time or adding 4 equiv of
Me3SiCl (entry3). Whena smallexcessof Me3SiCl was used, theyield
of 6a increased slightly (52%) (entry 4). Although using small ex-
cesses of SmI2, Sm, and Me3SiCl worked well in the reductive
Since Ph2MeSiCl promoted the reductive coupling of 5a effi-
ciently (Table 5, entry 11), we screened various chlorosilanes using
2-methylbutanal (5b) as a substrate. As shown in Table 7, Me3SiCl
generated the highest yield of corresponding diol 6b. However, the
bulkiness of chlorosilanes did not influence the diastereoselectivity
Table 5
Pinacol coupling of cyclohexanecarboxaldehyde with SmI2/Sm/Me3SiCl
Entry
SmI2 (mmol)
Sm (mmol)
Me3SiCl (mmol)
DME (mL)
Conditions
Yielda (%)
dl/meso
1
2
3
4
5
6
7
8
1
1
1
1
0.55
1
1
1
1
1
1
1
1
0.55
1
1
1
1
1.5
1.5
2.0
0.55
0.56
0
3
3
3
3
3
rt, 20 h
39
40
38
52
44
28
69
63
52
51
70
100:0
100:0
100:0
100:0
100:0
93:7
100:0
100:0
93:7
Reflux, 20 h
Reflux, 40 h
Reflux, 20 h
Reflux, 20 h
Reflux, 20 h
Reflux, 20 h
Reflux, 20 h
rt, 20 h
3
0.55
0.57
0.55
0
10
20
10
10b
10
9
10
11
1
1
1
1
Reflux, 20 h
Reflux, 20 h
100:0
100:0
0.55c
a
Determined by 1H NMR.
Use of THF in place of DME.
Ph2MeSiCl was used.
b
c

A. Yoshimura et al. / Tetrahedron 71 (2015) 5347e5355
5351
Table 6
Pinacol couplings of aliphatic aldehydes and ketones with SmI2/Sm/Me3SiCl in DME.
Entry
1
Substrate
Product
Isolated yield (%)
52
dl/meso
100:0
2
3
61
65
65:35
100:0
4
5
6
65
66
46
62:38
100:0a
100:0
7
8
30
74:26
100:0
43b
9
62
100:0
a
In this case, the dl/meso ratio determined by 1H NMR before isolation was 96:4. As to other substrates, the dl/meso ratios did not change before/after isolation.
b
Determined by 1H NMR.
toward 6b. Since SmI2/Sm in DME may induce reductive coupling of
affected by the quenching method (entries 2e4). These results
suggest that, in the case of aromatic aldehydes, the influence of
water upon quenching with HCl should be considered. To exclude
the influence of water on this pinacol coupling, therefore, air-
quenching is recommended for the pinacol couplings of aromatic
aldehydes.
chlorosilane to give corresponding disilane in situ, the influence of
disilane on this pinacol coupling was also investigated (entry 6). By
comparison with the results of entries 1 and 6, disilane does not
contribute to the yield and diastereo selectivity of the pinacol
coupling.
As described in the Introduction, the presence of H2O is a very
important issue concerning the reducing ability of SmI2. Thus, we
investigated the effect of H2O by quenching the reaction mixture
with either 1.5 N HCl or air after the pinacol coupling with SmI2/Sm/
Me3SiCl in DME was complete. Carbonyl compounds 1a, 3a, 5a, and
5e were treated under the corresponding optimized conditions,
and then the resulting mixtures were quenched with either 1.5 N
HCl or air (Table 8). In the case of benzaldehyde (1a), which is
highly reactive, a higher yield of product 2a was observed upon
quenching with 1.5 N HCl than by air-quenching (entry 1). On the
other hand, the pinacol couplings of 3a, 5a, and 5e were not
In general, SmI2-induced pinacol couplings of aliphatic carbonyl
compounds are more difficult than those of aromatic carbonyl
compounds. Therefore, the activation of aliphatic carbonyl com-
pounds is essential for pinacol coupling. As shown in path C
(Scheme 3), Me3SiCl coordinates to the oxygen atom of the carbonyl
group, and the carbon atom of the carbonyl group is positively
charged (intermediate B). Depending on this activation by Me3SiCl,
aliphatic carbonyl compounds may receive one electron from SmI2
easily, and a-siloxy radical species C is generated. Although further
reduction of radical anion species A with SmI2 in Scheme 2 is dif-
ficult, radical species C can be reduced easily to corresponding

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A. Yoshimura et al. / Tetrahedron 71 (2015) 5347e5355
Table 7
substrate aliphatic carbonyl compounds mainly contributes to the
diastereoselectivities.
Pinacol coupling of 2-methylbutanal with SmI2/Sm/chlorosilanes
3. Conclusion
We reported an overview of pinacol couplings of aromatic and
aliphatic carbonyl compounds using SmI2/Sm/Me3SiCl in DME. The
pinacol couplings of aliphatic carbonyl compounds, which have low
reactivity, could proceed uneventfully and with diastereoselectivity
by adding Me3SiCl; very interestingly, dl-isomers are obtained
exclusively in many cases. In fact, a wide range of carbonyl com-
pounds could be efficiently coupled by using SmI2/Sm/Me3SiCl in
DME. Moreover, it was found that Me3SiCl prevented the de-
composition of meso-diols and controlled the stereochemistry of
the products.
Entry
R1R2R3SiCl
Yielda (%)
dl/meso
1
2
3
4
5
6
None
34
34
55
40
13
33
63:37
64:36
62:38
62:38
54:46
67:33
Ph2MeSiCl
Me3SiCl
iPr3SiCl
(EtO)3SiCl
(Me3Si)2
a
Determined by 1H NMR.
4. Experimental section
4.1. General information
anion species D. If this process occurs rapidly, the radical coupling
process (path D) leading to the meso-isomer can be inhibited. The
nucleophilic addition of generated anion species D to carbonyl
compounds may be facilitated because of the intermolecular co-
ordination of SmI2 to two oxygen atoms. The steric hindrance of the
1H NMR spectra were recorded on a JEOL JNM-ECS or JNM-ECX
(400 Hz) spectrometer in CDCl3 with tetramethylsilane (TMS) as
Table 8
Effect of H2O in the different quenching methods
Entry
R1C(O)R2
Conditions
Quenched by 1.5 N HCl
Yielda (%)
Quenched by air
Yielda (%)
dl/meso
dl/meso
1
Table 1, entry 4
88
75
18:82
62
70
20:80
2
3
Table 3, entry 7
69:31
70:30
Table 5, entry 7
Table 5, entry 7
70
52
100:0
96:4
65
60
100:0
4
>99:1
a
Determined by 1H NMR.
Scheme 3. Plausible reaction pathways of aliphatic carbonyl compounds.

A. Yoshimura et al. / Tetrahedron 71 (2015) 5347e5355
5353
the internal standard. 13C NMR spectra were obtained on a JEOL
JNM-ECS or JNM-ECX (100 Hz) spectrometer in CDCl3. Chemical
shifts in the 13C NMR spectra were measured relative to CDCl3 and
(1 mmol), and freshly distilled DME (10 mL) under a nitrogen at-
mosphere. The mixture was stirred at ambient temperature for
1.5 h, resulting in a dark blue SmI2 solution. After adding the
substrate (0.5 mmol) and Me3SiCl (0.55 mmol) to the solution, the
flask was set in an oil bath maintained at 97 ꢀC and reaction
mixture was treated at reflux for 20 h. After it was cooled to room
temperature, the reaction mixture was treated with hydrochloric
acid (1.5 N, 20 mL) and extracted with diethyl ether (3ꢁ10 mL). The
combined organic layers were washed with saturated Na2S2O3
solution and brine, dried over anhydrous Na2SO4, and filtered. The
solvent was removed under reduced pressure. Products were
isolated by High Performance Liquid Chromatography (HPLC)
LC-908 (Japan Analytical Industry Co., Ltd.) with chloroform as an
eluent.
converted to dTMS values using
d 77.0 ppm. Mass spectra were ob-
tained on a Shimadzu GCMS-QP5000 gas chromatograph-mass
spectrometer (ionized by electron impact). Melting points were
determined using a Yanagimoto micro melting point apparatus.
DME was freshly distilled from sodiumebenzophenone ketyl prior
to use. Unless otherwise noted, chemical reagents were obtained
from commercial suppliers and used directly. Samarium powder in
oil (99.9%) was purchased from High Purity Chemicals and used
after washing with dry pentane followed by drying for 4 h under
reduced pressure. 1,2-Diiodoethane was purified by washing its
ether solution with saturated sodium thiosulfate aqueous solution
followed by removal of ether under reduced pressure.
4.5. Representative procedure for the reductive coupling of
carbonyl compounds mediated by the SmI2/Sm system in DME
(quenching by air)
4.2. Representative procedure for the reductive coupling of
aromatic aldehydes mediated by the SmI2/Sm system in DME
In a 20 mL two-necked flask equipped with a reflux condenser
were placed samarium powder (2 mmol), 1,2-diiodoethane
(1 mmol), and freshly distilled DME (3 mL) under a nitrogen at-
mosphere. The mixture was stirred at ambient temperature for
1.5 h, resulting in a dark blue SmI2 solution. After adding Me3SiCl
(1.5 mmol) and the substrate (0.5 mmol) to the solution, the re-
action mixture was stirred for 10 min at room temperature. After
that, the reaction mixture was treated with hydrochloric acid (1.5 N,
20 mL) and extracted with diethyl ether (3ꢁ10 mL). The combined
organic layers were washed with saturated Na2S2O3 solution and
brine, dried over anhydrous Na2SO4, and filtered. The solvent was
removed under reduced pressure. Products were isolated by pre-
parative thin-layer chromatography (PTLC) with hexane or a mix-
ture of hexane and ethyl acetate (7:3) as eluent. All products
obtained are known compounds, and therefore, the diaster-
eoselectivities of them were unambiguously determined by the
comparison of their chemical shifts in NMR spectra with the cor-
responding literature values.
In a 20 mL two-necked flask equipped with a reflux condenser
were placed samarium powder (2 mmol), 1,2-diiodoethane
(1 mmol), and freshly distilled DME (3 mL) under a nitrogen at-
mosphere. The mixture was stirred at ambient temperature for
1.5 h, resulting in a dark blue SmI2 solution. After adding Me3SiCl
(1.5 mmol) and the substrate (0.5 mmol) to the solution, the re-
action mixture was stirred for 10 min at room temperature. After
that, the reaction mixture was quenched by air until the color of
solution changed to yellow and treated with hydrochloric acid
(1.5 N, 20 mL) and extracted with diethyl ether (3ꢁ10 mL). The
combined organic layers were washed with saturated Na2S2O3 so-
lution and brine, dried over anhydrous Na2SO4, and filtered. The
solvent was removed under reduced pressure. Products were iso-
lated by PTLC with hexane or a mixture of hexane and ethylacetate
(7:3) as eluent. Except 6g, the obtained pinacol coupling products
are all known compounds. Therefore, the determination of di-
astereomers was performed by the comparison of their chemical
shifts in NMR spectra with the corresponding literature values. In
the cases of the pinacol coupling products 6e, 6f, 6g, and 6h, which
have two methyl groups, the chemical shifts of the methyl groups in
dl-isomers appear more up-field compared with those in meso-
isomers. In the case of the pinacol coupling products 6b, which
have two CHeOH groups, the chemical shifts of the underlined
proton in dl-6b appear more down-field compared with those in
meso-6b.
4.3. Representative procedure for the reductive coupling of
aromatic ketones mediated by the SmI2/Sm system in DME
In a 20 mL two-necked flask equipped with a reflux condenser
were placed samarium powder (1.1 mmol), 1,2-diiodoethane
(0.55 mmol), and freshly distilled DME (10 mL) under a nitrogen
atmosphere. The mixture was stirred at ambient temperature for
1.5 h, resulting a dark blue SmI2 solution. After adding the substrate
(0.5 mmol) and Me3SiCl (0.55 mmol) to the solution, the flask was
set in an oil bath maintained at 97 ꢀC and reaction mixture was
treated at reflux for 3 h. After it was cooled to room temperature,
the reaction mixture was treated with hydrochloric acid (1.5 N,
20 mL) and extracted with diethyl ether (3ꢁ10 mL). The combined
organic layers were washed with saturated Na2S2O3 solution and
brine, dried over anhydrous Na2SO4, and filtered. The solvent was
removed under reduced pressure. Products were isolated by PTLC
with hexane or a mixture of hexane and ethylacetate (7:3) as elu-
ent. All products obtained are known compounds, and therefore,
the diastereoselectivities of them were unambiguously determined
by the comparison of their chemical shifts in NMR spectra with the
corresponding literature values.
4.6. Analytical data
4.6.1. 1,2-Diphenyl-1,2-ethanediol (2a, Table 2).15 White solid; mp
133e136 ꢀC; yield 41.1 mg (85%); 1H NMR (400 MHz, CDCl3/TMS,
ppm) (dl and [meso])
d 7.33e7.11 (10H, m), 4.72 [4.84] (2H, s), 2.83
[2.19] (2H, br s); 13C NMR (100 MHz, CDCl3, ppm)
d 139.8, 128.1,
128.1 [128.3], 126.9 [127.1], 78.1; MS (EI) m/z 180 [Mþꢂ2OH].
4.6.2. 1,2-Bis(4-methylphenyl)-1,2-ethanediol (2b, Table 2).16 White
solid; mp 130e132 ꢀC; yield 48.0 mg (78%); 1H NMR (400 MHz,
CDCl3/TMS, ppm) (dl and [meso])
7.15e7.13 (4H, d, J¼7.6 Hz), 4.69 [4.75] (2H, s), 2.30 [2.35] (6H, s),
2.72 [2.04] (2H, br s); 13C NMR (100 MHz, CDCl3, ppm)
137.9,
d
7.21e7.19 (4H, d, J¼8.0 Hz),
d
137.0, 128.4 [129.0], 126.8 [127.0], 78.1, 21.2; MS (EI) m/z 208
[Mþꢂ2OH].
4.4. Representative procedure for the reductive coupling of
aliphatic aldehydes and ketones mediated by the SmI2/Sm
system in DME
4.6.3. 1,2-Bis(4-methoxyphenyl)-1,2-ethanediol (2c, Table
2).15 White solid; mp 163e166 ꢀC; 47.1 mg (71%); 1H NMR
(400 MHz, CDCl3/TMS, ppm) (dl and [meso]) d 7.06e7.04 [7.23e7.20]
In a 20 mL two-necked flask equipped with a reflux condenser
were placed samarium powder (2 mmol), 1,2-diiodoethane
(4H, d, J¼8.8 Hz), 6.78e6.75 [6.87e6.85] (4H, d, J¼8.8 Hz), 4.64
[4.74] (2H, s), 3.77 [3.81] (6H, s), 2.76 [2.06] (2H, br s); 13C NMR

5354
A. Yoshimura et al. / Tetrahedron 71 (2015) 5347e5355
(100 MHz, CDCl3, ppm)
d
132.0 [132.0], 128.1 [128.3], 113.5 [113.7],
J¼6.8 Hz), 0.920 (18H, s); 13C NMR (100 MHz, CDCl3, ppm)
35.2, 25.8; MS (EI) m/z 174 [Mþ].
d 74.9,
78.8 [77.8], 55.3 [55.2]; MS (EI) m/z 256 [MþꢂOH].
4.6.4. 1,2-Bis(4-bromophenyl)-1,2-ethanediol (2d, Table 2).17 White
solid; mp 167e169 ꢀC; 78.0 mg (84%); 1H NMR (400 MHz, CDCl3/
4.6.13. 8,9-Hexadecanediol (6d, Table 6).21 White solid; mp
119e120 ꢀC; 19.1 mg (65%); 1H NMR (400 MHz, CDCl3/TMS, ppm)
TMS, ppm) (dl and [meso])
6.98e6.96 [7.06e7.04] (4H, d, J¼8.8 Hz), 4.60 [4.81] (2H, s), 2.87
[2.31] (2H, br s); 13C NMR (100 MHz, CDCl3, ppm)
138.3 [138.5],
131.3 [131.4], 128.7 [128.7], 122.3 [122.1], 78.5 [77.2]; MS (EI) m/z
338 [Mþꢂ2OH].
d
7.38e7.36 [7.42e7.40] (4H, d, J¼8.8 Hz),
(dl and [meso]) d 3.40 [3.60] (2H, br s), 1.95 [1.78] (2H, br s),
1.50e1.42 (4H, m), 1.30e1.28 (20H, m), 0.90e0.87 (6H, t, J¼7.2 Hz);
d
13C NMR (100 MHz, CDCl3, ppm)
22.6, 14.1.
d 74.5, 33.6, 31.8, 29.6, 29.2, 25.6,
4.6.14. 7,8-Dimethyl-7,8-tetradecanediol (6e, Table 6).22 White
solid; mp 119e120 ꢀC; 43.2 mg (66%); 1H NMR (400 MHz, CDCl3/
4.6.5. 1,2-Bis[4-(trifluoromethyl)phenyl]-1,2-ethanediol (2e, Table
2).15 White solid; mp 117e119 ꢀC; 67.6 mg (75%); 1H NMR
TMS, ppm)
d 1.84e1.83 (2H, d), 1.61e1.51 (4H, m), 1.48e1.24 (16H,
(400 MHz, CDCl3/TMS, ppm) (dl and [meso])
d
7.54e7.51 [7.56e7.54]
m), 1.15e1.14 (6H, d, J¼3.2 Hz); 13C NMR (100 MHz, CDCl3, ppm)
(4H, d, J¼8.0 Hz), 7.25e7.23 [7.31e7.29] (4H, d, J¼8.0 Hz), 4.76 [4.97]
d 77.1 (2C), 36.3, 35.9, 31.9 (2C), 30.2 (2C), 23.7 (2C), 22.7 (2C), 21.1,
(2H, s), 2.93 [2.41] (2H, br s); 13C NMR (100 MHz, CDCl3, ppm)
20.7, 14.1 (2C); MS (EI) m/z 241 [MþꢂOH].
d
143.2, 127.3 [127.3], 125.1 [125.2], 125.2 [125.2], 78.4 [77.2]; MS
(EI) m/z 316 [Mþꢂ2OH].
4.6.15. 2,3-Dicyclohexyl-2,3-butandiol (6f, Table 6).11b Colorless oil;
30.2 mg (46%); 1H NMR (400 MHz, CDCl3/TMS, ppm)
d
2.27 (2H, s),
2.05e1.90 (2H, m), 1.81e1.51 (10H, m), 1.31e0.78 (10H, m); 13C NMR
(100 MHz, CDCl3, ppm) 79.3, 79.2, 44.6, 44.1, 30.0, 29.3, 28.2, 28.0,
4.6.6. 2,3-Diphenyl-2,3-buthanediol (4a, Table 4).18 White solid; mp
87e90 ꢀC; 48.0 mg (74%); 1H NMR (400 MHz, CDCl3/TMS, ppm) (dl
d
and [meso])
d
7.25e7.19 (10H, m), 2.53 [2.37] (2H, br s), 1.51 [1.59]
143.5 [143.9], 127.1
27.1, 27.00, 26.95, 26.9, 26.6, 26.5, 21.5, 19.7; MS (EI) m/z 237
[MþꢂOH].
(6H, s); 13C NMR (100 MHz, CDCl3, ppm)
d
[127.4], 127.2 [127.2], 126.9 [127.0], 78.8 [78.5], 25.0 [25.2]; MS (EI)
m/z 225 [MþꢂOH].
4.6.16. 3,4,5,6-Tetaramethyl-4,5-octandiol (6g, Table 6). Colorless
oil; 16.6 mg (34%); 1H NMR (400 MHz, CDCl3/TMS, ppm) (dl and
4.6.7. 2,3-Bis(4-methylphenyl)-2,3-buthanediol (4b, Table
4).18 White solid; mp 64e67 ꢀC; 44.7 mg (70%); 1H NMR
[meso])
m), 1.15e1.14, 1.14e1.13 [1.17e1.16] (6H, d, J¼3.6 Hz), 1.03e0.73
(14H, m); 13C NMR (100 MHz, CDCl3, ppm)
79.6, {41.1, 41.0, 40.8},
{40.7, 40.3, 40.2}, {26.2, 26.0}, {25.9, 25.6}, {24.6, 24.5, 24.5}, {21.4,
21.3}, {20.7, 20.4}, {19.3, 19.0, 18.9}, {16.1, 15.8, 15.5}, {14.6, 14.4},
{14.3,14.2}, {12.9,12.9,12.8}; MS (EI) m/z 185 [MþꢂOH]. HRMS (ESI)
calcd for C15H25O2Na [MꢂHþNa]þ: 224.1747, found: 224.1741.
d
2.25e2.23 [2.29e2.27] (2H, d, J¼7.2 Hz), 2.02e1.55 (4H,
(400 MHz, CDCl3/TMS, ppm) (dl and [meso])
d
7.17e7.05 (8H, m),
d
2.34 [2.33] (6H, s), 2.52 [2.17] (2H, br s), 1.47 [1.54] (6H, s); 13C NMR
(100 MHz, CDCl3, ppm)
d 140.5, 136.6, 127.8 [128.0], 127.3 [126.8],
78.8 [78.5], 25.1, 21.0; MS (EI) m/z 236 [MþꢂOH].
4.6.8. 2,3-Bis(4-trifluoromethylphenyl)-2,3-buthanediol (4c, Table
4).18 Yellow oil; 76.4 mg (74%); 1H NMR (400 MHz, CDCl3/TMS,
4.6.17. 4,5-Dimethyl-4,5-octandiol (6h, Table 6).23 1H NMR
ppm) (dl and [meso])
d
7.52e7.50 (4H, d, J¼8.0Hz), 7.30e7.28
(400 MHz, CDCl3/TMS, ppm) d 2.61e1.33 (8H, m), 1.88 (2H, s),
1.15e1.14 (6H, d, J¼1.6 Hz), 0.97e0.93 (6H, t).
[7.46e7.44] (4H, d, J¼8.4 Hz), 2.57 [2.18] (2H, br s), 1.54 [1.57] (6H,
s); 13C NMR (100 MHz, CDCl3, ppm)
d 147.1, 127.7 [127.7], 127.4,
124.2 [124.2], 78.5, 24.8; MS (EI) m/z 361 [MþꢂOH].
4.6.18. 3,4-Dimethyl-1,6-diphenyl-3,4-hexanediol (6i, Table
6).24 Colorless oil; 44.9 mg (62%); 1H NMR (400 MHz, CDCl3/TMS,
4.6.9. 2,3-Bis(4-chlorophenyl)-2,3-buthanediol (4d, Table
4).18 White solid; mp 114e116 ꢀC; 58.9 mg (73%); 1H NMR
ppm) d 7.31e7.15 (10H, m), 2.86e2.75 (2H, m), 2.73e2.64 (2H, m),
1.98 (2H, s), 1.97e1.87 (2H, m), 1.75e1.64 (2H, m), 1.28e1.26 (6H, d,
(400 MHz, CDCl3/TMS, ppm) (dl and [meso])
d
7.21e7.19
J¼4.8 Hz); 13C NMR (100 MHz, CDCl3, ppm)
d 142.8, 142.8, 128.5,
125.9, 77.3, 38.7, 38.4, 30.4, 30.4, 21.2, 20.9; MS (EI) m/z 281
[7.23e7.21] (4H, d, J¼8.8 Hz), 7.11e7.09 (4H, d, J¼8.8 Hz), 2.49
[2.20] (2H, br s), 1.48 [1.55] (6H, s); 13C NMR (100 MHz, CDCl3,
[MþꢂOH].
ppm) d 133.2, 128.8 [128.8], 128.4, 127.3 [127.3], 78.5, 24.9; MS (EI)
m/z 294 [MþꢂOH].
Acknowledgements
4.6.10. 1,2-Dicyclohexyl-1,2-ethanediol (6a, Table 6).19 White solid;
mp 145e147 ꢀC; 29.1 mg (52%); 1H NMR (400 MHz, CDCl3/TMS,
This research was supported by a Grant-in-Aid for Challenging
Exploratory Research (26620149) and Adaptable and Seamless
Technology Transfer Program (JST: A-STEP) from the Ministry of
Education, Culture, Sports, Science, and Technology, Japan.
ppm)
d
3.35e3.34 [3.88e3.86] (2H, d, J¼6.0 Hz), 1.90e1.43 (11H, m),
1.33e1.10 (11H, m); 13C NMR (100 MHz, CDCl3, ppm)
29.7, 28.3, 26.5, 26.3, 26.1; MS (EI) m/z 209 [MþꢂOH].
d 75.2, 40.4,
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