Unexpected transformation of aldehydes into benzoins with Copper(I)/Samarium

Article abstract of DOI:10.1080/00397911.2013.804572

The reductive coupling of aldehydes to afford pinacolic alcohols using all kinds of reducing agents involving samarium is well known. In this report, however, treatment of aromatic aldehydes with Cu(I)/Sm system produces benoins and/or benzils in good yields. A possible mechanism is proposed where Cu(I) not only activates the Sm metal but also promotes the dehydrogenation of the intermediates, during which a Cu(III) species may be involved. [Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications for the following free supplemental resource(s): Full experimental and spectral details.]

Full text of DOI:10.1080/00397911.2013.804572

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An Unexpected Transformation of Aldehydes into
Benzoins with Copper(I)/Samarium
Yongjun Liu , Hui Wang , Yulong Fu , Yan Qi & Kuiwei Yang
Accepted author version posted online: 27 Aug 2013.
To cite this article: Synthetic Communications (2013): An Unexpected Transformation of Aldehydes into Benzoins with
Copper(I)/Samarium, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, DOI: 10.1080/00397911.2013.804572
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ACCEPTED MANUSCRIPT
An Unexpected Transformation of Aldehydes into Benzoins with
Copper(I)/Samarium
Yongjun Liu, Hui Wang, Yulong Fu, Yan Qi, Kuiwei Yang
Corresponding author. State Key Laboratory Base of Eco-chemical Engineering, College
of Chemistry and Molecular Engineering, Qingdao University of Science and Technology,
Qingdao 266042, P. R. China Tel.: +86-532-84022681. Fax: +86-532-84023927 E-mail:
pubsliu@163.com
Abstract
The reductive coupling of aldehydes to afford pinacolic alcohols is well-known by using
all kinds of reducing agents involving samarium. In this report, however, treatment of
aromatic aldehydes with Cu(I)/Sm system produces benoins and/or benzils in good yields.
A possible mechanism is proposed where Cu(I) not only activates the Sm metal but also
promotes the dehydrogenation of the intermediates, during which a Cu(III) species is
proposed to be involved.
[
Supplementary materials are available for this article. Go to the publisher’s online
edition of Synthetic Communications® for the following free supplemental resource(s):
Full experimental and spectral details.]
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KEYWORDS: Samarium; copper; iodides; aldehydes; benzoins
Copper mediated coupling reactions have played important roles for constructing
[
1]
organic molecules with a large variety of structures. Besides, the incorporation of
copper salts into other metals for the achievement of specific purposes is also
impressive. With the incorporation into sec-alkyl Grignard reagent, copper salt was
recenlty reported to achieve the challenging cross-coupling[2b] of nonactivated
sec-alkyl halides and pseudohalides. By using copper salt as a catalyst, zinc mediated
conjugate addition reaction was able to proceed in water smoothly,[2c] while in the
presence of CuI a cyclopropylzinc species can be afforded by the diastereoselective
addition of diorganozinc reagents to cyclopropene derivatives.[2d] The combination of
copper and titanium catalyzes the three-component coupling of unactivated ketones
with amines and terminal alkynes to form tetrasubstituted propargylamines under
solvent-free conditions.[2e] In addition to the classic Sonogashira reaction, palladium
has been explored with the corporation of copper salts for many exclusive coupling
reactions in most recent reports.[2f,i] In these typical reactivity patterns of
organocuprates, the formation of the key carbon-carbon or carbon-heteroatom bond
was often mediated by a Cu(III) intermediate, which has long been proposed to be
involved during the catalytic process of copper salts.[3]
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Samarium reagents have drawn much attention due to their particular importance in
[
4]
reductive coupling reactions. Although the applications of samarium diiodide are
obviously efficient due to its advantages in organic transformations, the direct use of
metallic samarium as a reducing agent showed certain advantages as of being more
[
5]
practical and electron-economical than the use of samarium diiodide. The reactions
promoted by samarium metal generally requires the presence of an auxiliary[5,6] such
as iodine, hydrochloric acid, alkyl halides and TMSCl, etc. so as to activate the
surface of the samarium metal and thus improve the reactivity.[6]
[
7]
Our studies on the direct use of samarium have directed our efforts into exploiting a
copper / samarium system. Inspired by the extraordinary effect resulting from the
[
3]
combination of copper salt with other metal and organometallic reagents, we
assume that the incorporation of copper salts into samarium metal may possess unique
reactivity. Herein we report the coupling of aldehydes into benzoins and/or benzils
with Cu(I)/Sm system. It should be noted that although aldehydes underwent
Pinacolic coupling in the presence of samarium reagents,[6e,8] no piancols were
detected with the Cu(I)/Sm system.
Benzaldehyde was chosen as the model substrate (Scheme 1) and the reaction
conditions for the coupling was optimized.
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Table 1 summarized the results of the investigation on screening a variety of copper salts
for this condensation. No reaction occurred by using copper(I) chloride, while copper(I)
iodide gave good yield of benzoin (Table 1, entry 2). Improvement of the yield as well as
acceleration of the reaction rate was observed when KI or NaI was added as an auxiliary
catalyst to CuI or CuBr (Table 1, entries 4 and 5). Divalent Cu(II) salts, such as
(AcO)
2
Cu or CuSO
4
, were also examined but found unsuccessful (Table 1, entries 7 and
8). Although CuCl
2
can promote the self-condensation of benzaldehyde into benzoin in
the presence of potassium iodide, the higher temperature and longer time it required
together with the relatively lower yield make it less attractive (Table 1, entry 6). The
results also showed that either benzaldehyde or samarium alone is unreactive to copper(I)
iodide even under reflux for 5 hours.
Finally, Sm/CuCl/KI was established as the standard conditions. To explore the scope
of the reaction, a series of aromatic aldehydes were examined. Gratifyingly, most
substrates afforded the desired benzoins smoothly. In certain cases, unexpectedly,
benzils were also obtained as the byproducts (Scheme 2). The results of the reactions
are summarized in Table 2.
In the presence of Sm, CuCl and KI (compared with CuI alone), the reaction proceeded
more readily and afforded higher yields (Table 2, make a comparison between entries 1
and 1', 5 and 5', 10 and 10', 13 and 13', 19 -21 and 19' - 21'). Even remarkable concerning
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Sm/CuCl/KI is that 4-cyanobenzaldehyde (1m) can undergo the condensation smoothly
to afford the corresponding benzil, while no reaction occur under CuI/Sm alone (Table 2,
entries 13 and 13'). The condensations of 1a, 1f, 1g, 1h, 1j, 1k, 1l, 1o and 1p possessing
electron-donating groups afforded the bezoins in good yields (Table 2, entries 1, 6-8,
1
0-12, 15 and 16). In contrast, the condensations of 1b, 1c, 1d and 1e possessing
electron-withdrawing groups require relatively longer reaction time and give lower total
yields (Table 2, entries 2-5). Nevertheless, the formation of either 2h, 2o, 3i or 3m is
noteworthy since the vitamin B
1
catalyzed benzoin condensation was not applicable for
the aromatic aldehyde substrates with strong electron-withdrawing or electron-donating
[9]
groups attached on the aryl, while other methods could not afford the benzoins from
aldehydes in such a direct way.
The reaction of o-substituted aromatic aldehyde such as 2-chlorobenzaldehyde (1d) only
afforded 2,2'-chlorobenzil 3d (Table 2, entry 4). The same results were observed with
heterocyclic aldehydes (1s, 1t) and fused-ring aromatic aldehyde (1u), which underwent
the condensation successfully (Table 2, entries 19-21) and afforded benzil as the only
products after work-up. The reaction of aliphatic aldehyde (1r), however, resulted in a
complex mixture and the desired benzoin was not isolated in this case (Table 2, entry 18).
The condensations of 4-nitrobenzaldehyde (1n, Table 2, entry 14) and
1
,4-phthalaldehyde (1q, Table 2, entry 17) failed to generate benzoins. However, it
was surprising that the nitro group, labile to most samarium regeants, was tolerated
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under the reaction conditions. Attempts to expand the condensation to
4
-hydroxybenzaldehyde or 2-hydroxybenzaldehyde were not successful, which may
indicate the process is incompatible with proton donor.
Elucidation of the condensation mechanism is challenging since apparently no
reducing process occurs during the transformation from benzaldehydes to benzions.
Considering the reaction was operated under inert nitrogen atmosphere, we propose a
dehydrogenation step may be involved during the condensation process since copper
salts are efficient catalysts for dehydrogenation.[10] Moreover, based on the
experimental results and other studies on the copper catalyzed reactions,[2,3] we
suggest a Cu(III) species may be involved in the reaction. Thus, aromatic aldehydes
firstly obtain an electron from samarium to form ketyl A, which then transforms into
a six-membered Cu(III) species[3d] with the cooperation of iodine ion to increase its
stability and solubility. Subsequently a complicated dehydrogenation of the
intermediate may occur thus affords enol B, which would tautermerizes to give
benzoin 2 in the work-up.
An alternative mechanism may also be proposed, which could rationale the formation
of benzoin by dehydrogenation of hydrobenzoin in the presence of copper salts. A
controlled experiment was conducted, where 1,2-diphenylethane-1,2-diol was also
subjected to the same reaction conditions. It was only to find no reaction occurred at
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all. Thus the dehydrogenation of hydrobenzoin under the conditions was excluded.
Besides, aldehydes were found inert to either samarium metal or CuI, which indicates
a possible synergistic effect among aldehyde, samarium and copper salt.
The formation of benzils may result from auto-oxidation of benzoins. The reports that
some metal salts such as copper(I)[11a] and Ce(IV)[11b] can easily catalyze the
autoxidation of benzoins to the corresponding benzils make it reasonable to deduce
that during the work-up, the presence of copper(I) and samarium salts may accelerate
the formation of benzils from benzoins. An experiment was also designed to confirm
the auto-oxidation process. Thus when the condensation reaction of benzaldehyde was
complete, the reaction mixture was placed in air at room temperature and then
monitored by TLC in regular intervals. The results showed the content of benzoins
became less and less, while that of benzils increased as the standing time prolonged.
Finally, the benzoins almost disappeared in 10-20 hours, thus validating the
auto-oxidation process (Scheme 3). The results are summarized in Table 3.
In conclusion, Cu(I) salts were used to combine with samarium metal to construct a new
system and its reactivity was preliminarily examined. With aromatic aldehydes as the
substrates, the system did show unique reactivity and afforded benzoins and benzils as
the products instead of pinacols. Further investigation concerning clarification of the
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mechanism and search for more applications of the system in organic synthesis are
currently underway in our lab.
EXPERIMENTAL
General
All NMR spectra were measured in CDCl
3
and recorded on Bruker Avance-500 (500
MHz) spectrometer with TMS as the internal standard. Chemical shifts are expressed in
ppm and J values are given in Hz. IR spectra were recorded on a Bruker Tensor-27
spectrometer. Melting points are uncorrected. All the reactions in this paper were
performed under nitrogen atmosphere. THF was refluxed and got rid of water before use.
Typical Procedure For The Self-Condensation Of Aromatic Aldehydes
To a mixture of Sm powder (0.3 g, 2 mmol), CuCl (0.2 g, 2 mmol) and KI (0.33 g, 2
mmol) in anhydrous tetrahydrofuran (THF, 10 mL), benzaldehyde (0.4 mL, 4 mmol) was
added at room temperature under a nitrogen atmosphere with magnetic stirring. After
completion of the reaction (about 2 h, monitored by TLC), dilute hydrochloric acid (2 M,
5
mL) was added and the resulting mixture was extracted with ethyl acetate (3 × 20 mL).
The combined organic layer was washed with brine and saturated sodium thiosulfate
solution, dried over anhydrous sodium sulfate, and concentrated under reduced pressure.
The crude product were purified with flash chromatography (silica/hexane - ethyl acetate
5
: 1 v/v) to afford 0.36 g of the corresponding benzoin (2a) with 84 % yield.
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7
f
1
2
-Hydroxy-1,2-diphenylethanone (2a): white solid; mp 135-136°C, lit. 134-136°C; H
NMR: δ 7.92 - 7.93 (m, 2 H), 7.52 -7.55 (m, 1 H), 7.39 -7.42 (m, 2 H), 7.29 -7.34 (m, 5
H), 5.96 (d, 1 H, J = 6.0 Hz), 4.56 (d, 1 H, J = 6.0 Hz); IR (KBr): 3416, 3028, 2931, 1680,
-1 13
1
1
596, 1491, 1449 cm ; C NMR: δ 76.3, 127.9, 128.7, 128.8, 129.2, 133.5, 134.0, 139.1,
99.0.
ACKNOWLEDGMENTS
The project was supported by the Promotive Research Fund for Excellent Young and
Middle-Aged Scientists of Shandong Province (BS2011CL019), and the Natural Science
Foundation of Shandong Province (Project No. 2009ZRA02039).
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1
1
2
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Table 1. Reaction Results of Benzaldehyde in the Presence of Samarium / Copper Salts
entrya
metal salts
temp.
time (h) isolated yield
of benzoin
1
2
3
4
5
6
7
8
CuI
rt
5
5
5
2
2
5
5
5
72
CuCl
reflux
reflux
rt
--b
CuCl
2
-- b,c
CuCl/KI
84
CuBr/NaI
rt
81
CuCl
(AcO)
CuSO
2
/KI
reflux
12b
-- b,c
--b,c
2
Cu/KI reflux
4
/KI reflux
a.
b
c
The ratio of aldehyde : samarium : Cu (I) is 2 : 1 : 1. No reaction. The ratio of
aldehyde : samarium : Cu (II) is 2 : 2 : 1.
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Table 2. Reactions of Aromatic Aldehydes Promoted by Sm/CuCl/KI in THF
entrya Ar in substrate 1
temp.
ºC)
time
(h)
2
isolated yield of 2 isolated yield of 3
(
(%)
(%)
1
1
2
3
4
5
5
6
7
8
9
1
1
C
6
H
5
- (1a)
rt
84 (2a)
72 (2a)
68 (2b)
66 (2c)
'
'
(1a)
p-F-C
rt
5
6
H
4
- (1b)
rt
3
p-Cl-C
6
6
H
4
4
- (1c)
- (1d)
rt
2
o-Cl-C
H
rt
3
60 (3d)
13 (3f)
p-Br-C
6
H
4
- (1e)
reflux
reflux
rt
2
64 (2e)
48 (2e)
76 (2f)
77 (2g)
78 (2h)
(1e)
4
p-Me-C
m-Me-C
p-MeO-C
p-EtO-C
6
H
4
- (1f)
3
6
H
4
- (1g)
-(1h)
- (1i)
reflux
rt
3
6
H
4
3
15 (3h)
54 (3i)
6
H
4
rt
2
0
p-Me
(1j)
2
N-C
6
H
4
- (1j) rt
3
67 (2j)
53 (2j)
81 (2k)
86 (2l)
0'
rt
5
1
1
p-i-Pr-C
6
H
4
- (1k)
- (1l)
- (1m)
rt
2
12
13
13'
14
p-t-Bu-C
6
H
4
rt
2
p-CN-C
6
H
4
reflux
reflux
reflux
4
52 (3m)
(1m)
6
-- b
20c
-- b
p-NO
2
-C
6
H
4
-(1n)
24
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1
5
O
rt
2
76 (2o)
87 (2p)
O
(
1o)
1
6
Me
rt
3
Me
(
1p)
1
1
7
8
reflux
reflux
20
3
12b
-- b
OHC
(
1q)
-- b
CH CH
2 2
(
1r)
1
9
rt
3
63 (3s)
O
(1s)
(1t)
1
9'
0
(1s)
reflux
rt
6
3
40 (3s)
82 (3t)
2
S
2
0'
1
(1t)
reflux
rt
5
3
67 (3t)
71 (3u)
2
(
1u)
21'
(1u)
reflux
5
53 (3u)
a.Entries 1', 5', 10', 13', 19', 20', 21' are performed in the presence of CuI alone without
b.
c.
potassium iodide. A complex mixture was obtained. 4-Nitrobenzyl alcohol was
afforded.
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Table 3 The auto-oxidation of the benzoins to benzils under air
Entry R
Time (h) Isolated yield of 2 (%)a Isolated yield of 3 (%)a
1
2
3
4
5
H
12
10
15 (2a)
6 (2b)
-b
68 (3a)
61 (3b)
67 (3c)
65 (3e)
60 (3f)
p-F
p-Cl 10
p-Br 10
p-Me 20
-b
16 (2f)
a.
b
Based on aryl aldehydes. Not detected.
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Scheme 1 Reaction of benzaldehyde promoted by CuI/Sm
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17

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Scheme 2 Reactions of aromatic aldehydes promoted by Cu(I)/Sm
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18

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Scheme 3 Possible mechanism for the formation of benzoin
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19

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Scheme 4 Oxidation of benzoins to benzils in condensation mixture under air
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20