A novel coupling reaction of α-halo ketones promoted by SmI3/CuI

Article abstract of DOI:10.1039/c5ra26604a

With SmI₃ as the Lewis acid and catalyzed by CuI in DMF, α-haloketones were transformed unexpectedly into α-hydroxy-1,4-diketones in good to moderate yields. The mechanism was probed and a plausible reaction pathway was proposed. DMF was assumed to play a dual role both as a hydroxyl source and as a solvent.

Full text of DOI:10.1039/c5ra26604a

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A novel coupling reaction of α-halo ketones promoted by SmI3/CuI
Yongjun Liu,a Hengmin Zhao,a Guang Tian,a Feng Du,a Yan Qi,a Yonghong Wena
Abstract: With SmI3 as the Lewis acid and catalyzed by CuI in DMF, α-haloketones were
transformed unexpectedly into α-hydroxy-1,4-diketones in good to moderate yields. The
mechanism was probed and a plausible reaction pathway was proposed. DMF was assumed
to play dual roles both as a hydroxyl source and as a solvent.
Introduction
Samarium reagents in the past three decades have emerged as one of the reagents
extensively exploited in a wide range of applications in synthetic strategies1-4 since the
pioneering work of Kagan.1 In contrast to the ever-increasing explorations of SmI2 owing to
its powerful reducing reactivity,2,3 however, SmI3 draws relatively fewer attention and in
most cases acts as a moderate Lewis acid in organic synthesis.4c,5 For example, catalyzed by
SmI3, 3-indolylarylmethanamides were synthesized via Friedel-Crafts amidoalkylation,5a
while a similar amidoalkylation of 1,3-dicarbonyls afforded Mannich-type products.5b
SmI3-catalyzed Michael addition of indoles with electron-deficient olefins afforded
3-substituted indoles.5c-e By using SmI3, simple access to (E)-α-hydroxy-β,γ-unsaturated
amides starting from α,β-epoxyamides has been achieved with high selectivity.5f Besides
6a
SmI3, the utilization of other Sm(III) reagents as Lewis acids such as SmCl3 and
aState Key Laboratory Base of Eco-chemical Engineering; College of Chemistry and Molecular Engineering, Qingdao
University of Science and Technology, Qingdao 266042, P. R. China.
*Corresponding author. Email address: yongjunliu@qust.edu.cn
1
† Electronic Supplementary Information (ESI) available: CIF files of crystal data, H NMR and 13C NMR data for
products. See DOI:

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Sm(OTf)36b-d were also developed in recent studies.
In certain cases, SmI3 was used as a precursor for a reducing agent and also played its
role as a Lewis acid.7a-c Occasionally, SmI3 itself also exhibits reducing reactivity
independently due to the reducing ability of I-.7d,e
The incorporation of copper salts into other metals for the achievement of specific
purposes is impressive. In palladium-catalyzed carbon-carbon bond formation reactions,
the utility of Cu(I)X as cocatalysts is known as the "copper effect".8a Copper salt was
recenlty incorporated into organomagnesium to achieve the chanllenging
cross-coupling8b of secondary alkyl Grignard reagents with non-activated alkyl halides.
Catalyzed by copper(I), zinc mediated conjugate addition was able to proceed in water,8c
while a cyclopropylzinc species was afforded by the addition of diorganozinc to
cyclopropenes.8d The combination of copper and titanium also efficiently catalyzed the
three-component coupling of unactivated ketones with amines and terminal alkynes
under solvent-free conditions.8e
By virtue of copper’s strong coupling ability, we assume samarium reagents
incorporated with copper should exhibit exceptional reactivity upon some classical reactions.
As reported, SmI3 alone in THF only led to simple dehalogenation.7f In this paper, the
SmI3/Cu(I) combination was applied in the coupling of α-haloketones to afford
α-hydroxy-1,4-diketones in good efficiency (Scheme 1).
O
SmI3
reported work7f
Ar C CH3
O
THF
Ar C CH2Br
O
OH O
SmI3/CuI
DMF
Ar C CH2 CH C Ar
this work
Scheme 1. SmI3 promoted different reactions of α-haloketones
Results and Discussion
A series of reaction conditions were screened to optimize the reaction (Table 1). The

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suitable reaction temperature is found to be 100–110°C (Table 1, entries 1-3). The results
also show that the appropriate molar ratio of SmI3 to substrate 1 is 1 : 1 (entries 3-6), and 5
mol% of CuI is sufficient to catalyze the coupling reaction while excess CuI (such as
100mol%) leads to decreased result (entry 7). For comparison, the effect of SmI2 was also
examined. As expected, the reaction mainly afforded the simple dehalogenation product.
(entries 8-9).
Table 1. Optimization of the coupling of α-bromoacetophone
O
O
OH
SmI3/CuI
Br
Ph
Ph
Ph
O
1a
2a
entrya Lewis acid (mmol) solvent
temp (°C) time (h) yieldb
1
SmI3
SmI3
SmI3
SmI3
SmI3
SmI3
SmI3
Sm/I2
SmI2
SmI3
FeCl3
ZnI2
2
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
THF
80
5
47
--c
2
2
reflux
105
1
3
2
2
71
71
57
38
56d
13f
--h
--g
--c
--c
--c
4
4
105
2
5
1
105
2
6
0.5
2
105
2
7
105
2
e
8
2
105
2
9
2
reflux
reflux
105
0.1
2.5
2
10
11
12
13
2
THF
2
DMF
DMF
DMF
2
105
2
AlCl3
2
105
2
b
c
aConditions: 2 mmol 1a in the presence of 0.1 mmol CuI (5 mol%). Isolated yield. No desired
d
e
f
product detected. 2 mmol of CuI. Samarium metal (2 mmol) with I2 (1 mmol). Acetophone was the
g
h
major product (61%). The reductive coupling product 3 was obtained in 73% yield. Acetophone was
isolated in 68% yield.

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The influence of DMF is critical to this coupling reaction.9 When THF was used as the
solvent, the reaction proceeded in an alternative way to give 1,4-diketones 3 (entry 10). The
attempts with other Lewis acids such as FeCl3, ZnI2 and AlCl3 were not successful, where
complicated reaction mixture was observed (entries 11-13).
In view of these results, the scope and limitation of this reaction was thereby
investigated and the results were summarized in Table 2. A variety of α-haloketones were
compatible with the reaction conditions, resulting in the formation of products 2 in
moderate yields.
Table 2. Reactions of α-haloketones promoted by SmI3/CuI in DMF
O
O
OH
SmI3/CuI
X
Ar
Ar
Ar
DMF, 105ºC
O
X= Cl, Br, I
1
2
entrya Ar-
X in 1
time (h)
yield (%)b
1
C6H5-
C6H5-
Br (1a)
Cl (1b)
I (1c)
2
71 (2a)
66 (2a)
77 (2a)
62 (2b)
60 (2b)
72 (2c)
65 (2c)
67 (2d)
71 (2e)
61 (2f)
78 (2g)
72 (2h)
75 (2i)
2
2.5
1
3
C6H5-
4
4-CH3-C6H4-
4-CH3-C6H4-
4-Cl-C6H4-
Br (1d)
Cl (1e)
Br (1f)
Cl (1g)
Br (1h)
Br (1i)
Br (1j)
Cl (1k)
Cl (1l)
Cl (1m)
2
5
2.5
2
6
7
4-Cl-C6H4-
2.5
2
8
4-Br-C6H4-
9
4-F-C6H4-
2
10
11
12
13
4-CH3O-C6H4-
3,4-diCH3-C6H3-
4-C2H5-C6H4-
4-i-C3H7-C6H4-
3
2
2
2

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14
15
Cl (1n)
Br (1o)
2
4
76 (2j)
4-Ph-C6H4-
--c
b
aConditions: substrate 1 (2 mmol), SmI3 (2 mmol) and CuI (0.1 mmol) in DMF at 105°C. Isolated
yield. cNo desired product obtained.
α-Haloketones, including chloro, bromo and iodoketones all underwent the coupling
reaction smoothly. Among them, α-chloroketones were relatively inactive and afforded
slightly lower yield of products in longer reaction time (Table 2, entries 2, 5, 7). In contrast,
the iodoketone was the most reactive (entry 3). No obvious difference in the reactivity was
observed between electron-donating (entries 4-5, 10-14) and electron-withdrawing
substituents (entries 6–9) on aryl rings. The reductive dehalogenation was unavoidable
therefore acetophones were the byproducts which lowered the yields of 2. For substrate
1-(biphenyl-4-yl)-2-chloroethanone (1o), the reductive dehalogenation predominated and it
failed to undergo the coupling reaction (entry 15).
Besides the NMR, IR, and HRMS spectral data for the identification of 2,
single-crystal X-ray diffraction of 2c was also given.10a It determines unambiguously that 2c
has a hydroxyl group in the α-position of carbonyl.
Figure 1. The crystal structure of 2c with 50% thermal ellipsoids.
To have an understanding of the reaction mechanism, KI was introduced as an
additional source of iodine ion and its influence upon the reaction were investigated (Table
3). It is found that although SmI3 is essential to the reaction (Table 3, entry 1), the
employment of KI can accelerate the reaction and increase the yield of 2 remarkably
(entries 2-3). However, too much KI did not further increase the yield of 2, although the

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reaction time became shorter (entries 4-5).
Table 3. The influence of KI on the coupling reaction
O
O
OH
SmI3/CuI/KI
DMF, 105ºC
Br
Ph
Ph
Ph
O
1a
2a
a
Entry
SmI3 / mmol KI (mmol)
Time (h)
Yield (%)b
1
2
3
4
5
0
6
10
10
5
0
0.1
0.5
1
6
31
63
70
72
6
6
4
2
10
1.5
a2 mmol 1a and 0.1 mmol CuI. bIsolated yields.
When α-haloketones were treated with SmI3/CuI in THF, however, an intermolecular
reductive coupling reaction occurred and thus 1,4-diketones were obtained (Table 4). The
α-hydroxyl products 2 that were formed in DMF could not be observed in such a reaction
manner.
Table 4. The formation of 1,4-diketones promoted by SmI3/CuI in THF
O
O
SmI3/CuI
X
Ar
Ar
Ar
THF, reflux
O
1
3
entrya
R
X
time (h)
2
yieldb
1
2
3
4
5
6
C6H5- (1a)
C6H5- (1b)
Br
Cl
Cl
Cl
Cl
Cl
73 (3a)
68 (3a)
71(3b)
66 (3c)
70 (3d)
74 (3e)
2.5
4-Cl-C6H4- (1g)
2.5
4-CH3-C6H4- (1e)
4-C2H5-C6H4- (1l)
3,4-diCH3-C6H4- (1k)
2.5
2.5
2.5
aConditions: 1 (2 mmol), SmI3 (2 mmol) and CuI (0.1 mmol) in THF at reflux. bIsolated yields.

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All α-haloketones in Table 4 underwent the homocoupling reaction smoothly in
moderate yields. Products 2 can not be detected at all in this case. Nevertheless, there was
another unknown by-product which decreased the yields of 3. The by-product that was
separated from the homocoupling mixture of 1e is proved to be the unexpected
1,2,3-tri(4-toloyl)cyclopropane (4a) in 21% yield (Scheme 2).
H3C
O
O
Cl
O
SmI3/CuI
THF, reflux
H3C
O
CH3
H3C
1e
4a (21%)
Scheme 2. The formation of 1,2,3-tri(4-toloyl)cyclopropane in THF
The structure of 4a was also identified by a single crystal X-ray diffraction,10b which
shows that three toloyl in compound 4a is in the trans-configuration.
Figure 2. The crystal structure of 4a with 50% thermal ellipsoids.
It was reported that a cross-coupling reaction between α-chloroketones and organotin
enolates in the presence of ZnX2 afforded a method for the synthesis of 1,4-diketones,11a

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while catalyzed by NiCl2 zinc can reduce the unactivated alkylbromides to homocoupling
products.11b Illuminated by these reports and considering the reducibility exhibited by
SmI3,7e,f the homo-coupling pathway is proposed in Scheme 3.
O
O
O
O
OSmI2
CH2
Ar
C
SmI3
Ar
C
ICH2
Ar C CH2 CH2 C Ar
Ar C CH2Br
- IBr
[CuI]
1
3
Scheme 3. The debromination and homocoupling pathway
However, the formation of the hydroxyl group in compound 2 in Table 2 is
considerably surprising. To have a probe into the reaction mechanism, a series of
investigations were carried out. Firstly, an in situ dehydration reaction was attempted after
the completion of the coupling reaction. Thus, 2 mmol of 4-toluenesulfonic acid was added
to the reaction mixture under one-pot conditions after the completion of the coupling
reaction of 1j. The in situ formed product 2f disappeared slowly in 2 hours and the desired
dehydration product 1,4-bis(4-methoxyphenyl)butene-1,4-dione 5a was obtained in
(E)-configuration (Scheme 4). Such a result was assumed to confirm that the α-hydroxyl
group should come from the reaction system itself, not from the workup.
O
O
O
SmI3/CuI
Br
p-TolSO3H
DMF, 105ºC, 2h
DMF, 105ºC, 2h
O
O
O
1j
5a (46%, based on 1j)
Scheme 4. The formation of dehydrated product in one pot
To investigate if the hydroxyl in product 2 comes from the moisture in DMF, the
influence of water has to be examined. Several parallel experiments were carried out with
DMF that is contaminated with different amounts of water (Table 5). The results showed
that no matter how strict the anhydrous conditions are, the efficiency for the formation of 2a
remained almost the same (entries 1-3). Further, no difference can be observed yet when a
small amount of water was introduced into the reaction system (entry 4). However, the yield

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of 2a decreased obviously and the reaction became sluggish or even ceased to occur when
large amount of water was introduced (entries 5-6).
Table 5. The influence of water or additives upon the formation of 2a in DMF
O
O
OH
SmI3/CuI
Br
Ph
Ph
Ph
DMF(pretreated), 105ºC
O
1a
2a
entrya substrate
pretreatment of DMF (10 mL)
pre-dried over molecular sieve
reflux over CaH2
time (h)
yield of 2ab
1
2
3
4
5
6
7
8
9
2
2
2
2
2
71
1a
1a
1a
1a
1a
72
reflux over P2O5
70
0.05 mL H2O was added
0.5 mL H2O was added
5 mL H2O was added
70
49
2
2
--c
1a
1a
0.5 mL H2O18 was added
0.5 mL NH3·H2O was added
0.5 mL CH3OH was added
48d
24d
53d
2
2
1a
1a
aConditions: 1a (2 mmol), SmI3 (2 mmol) and CuI (0.1 mmol) in DMF at 105 °C. bIsolated yield. cNo
product observed.dNo products with -O18H, -NH2 or -OCH3 substituted at α-position observed.
A further O18 labeling experiment (entry 7) showed that the O18 can not be detected in
the product 2a when the reaction was carried out in the presence of H2O18 which was often
used to identify the source of oxygen in the product.12 Besides, the investigation also
showed that no byproduct can be observed yet when the reaction was carried out in the
presence of some additional additives such as NH3·H2O (entry 8) or CH3OH (entry 9)
respectively. These results should be crucial to demonstrate the role of DMF in the
mechanism.
A recent study by Liang and coworkers showed that DMSO acted as the hydroxyl
source in the reaction of α‑hydroxylation of ketones.13 Considering the structural similarity
of DMF to DMSO and the above experimental results, it is probable that DMF is the source

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for the hydroxyl in product 2. Besides as a solvent with strong polarity, DMF was used in
many cases both as a reaction agent and as a catalyst in synthetic organic chemistry by
virtue of its unexpected reactivity.14,15 For examples, Duan synthesized α-ketoamides from
α-oxocarboxylic acids and DMF by the nucleophilicity of the nitrogen on formamide.14a
The oxygen on DMF attacked POCl3 and then underwent elimination to form the famous
Vilsmeier reagent.15 What’s more, DMF can coordinate effectively with metal ion such as
copper16a-b or samarium16c-d as well known, and thus often exhibits unexpected reactivity 17a
and electrochemical property17b which probably lead to totally novel organic reactions as
well-documented in reports.17c-g
Further investigation found that when 1,2,3-tri(4-toloyl)cyclopropane (4a) was
subjected to the same reaction conditions as described in Table 2, product 2b was obtained
albeit in relatively lower yield, as shown in Scheme 5. This result implies that the reaction
may proceed through 4a.
H3C
O
CH3
O
O
OH
SmI3/CuI
DMF, 105ºC
O
O
H3C
CH3
H3C
4a
2b
(31%)
Scheme 5. The formation of 2b from 1,2,3-tri(4-toloyl)cyclopropane
At this stage, taking together the experimental results above and the references
involved, a reasonable reaction mechanism may be proposed (Scheme 6), in which DMF
acted not only a solvent but also a hydroxyl source. According to the pathway,
tribenzoylcyclopropane was firstly formed via three times of coupling between enolate and
haloketones with the assistance of Sm3+. Then the ring of tribenzoylcyclopropane
underwent opening via the attack of DMF and formed intermediate A, which finally
transformed into 2a in the presence of copper.

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OSmI2
O
O
O
I-
SmI3
-HI
I
Br
I
I
O
Ph
Ph
Ph
O
Ph
Ph
O
O
Ph
O
OSmI2
Ph
I
Ph
I
Ph
I
O
Ph
I-
Ph
Sm3+
O
O
Ph
Sm3+
H
Ph
Ph
Ph
OSmI2
Ph
O
O
Me2N
O
O
O
I
[Cu]
O
SmI2
Ph
O
Ph
Ph
Ph
A
Me2N
H
O
O
SmI
H
Me2N
Ph
Ph
O
O
O
workup
O
OH
2a
Me2N
H
Ph
DMF
Scheme 6. Tentative mechanism
Products 2 containing the α-hydroxyl-1,4-dicarbonyl moieties, which are found in
natural products and other compounds of biomedical relevance.18a-c So far, the efficient
synthesis for such compounds are mainly realized via Mukaiyama aldol from the
α-dicarbonyl compounds and enamides (or enolsilanes), and catalyzed usually by metals
such as copper,18d-g nickel18h and magnesium18i during an asymmetric manner.18d-j
Considering the relatively complicated Mukaiyama-reagents, the investigation reported in
this paper should be an alternative method of special importance to achieve the useful
α-hydroxyl-1,4-dicarbonyl skeletons from a novel perspective.
Conclusions
A new application of SmI3 cooperated by Cu(I) was demonstrated. The condensation of
α-haloketones promoted by SmI3/Cu(I) provides an efficient method for the synthesis of
α-hydroxy-1,4-diketones from very readily available materials in a facile and novel way.
Further, the mechanism for the unexpected condensation was probed and a reasonable
mechanism was proposed.

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Acknowledgments
The project sponsored by the Promotive Research Fund for Excellent Young and
Middle-Aged Scientists of Shandong Province (BS2011CL019), and the Scientific Research
Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (Project
No. [2007]1108).
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Graphic Abstract
O
O
SmI3 / THF
CuI (5%)
X
Ar
Ar
Ar
Ar
Ar
O
O
O
Ph
O
O
Ph
SmI3 / DMF
CuI (5%)
O
O
H
NMe2
OH
Ph
In the presence of SmI3 and catalyzed by CuI in DMF, α-haloketones were transformed
unexpectedly into α-hydroxy-1,4-diketones. The mechanism was probed and DMF was
assumed to play dual roles both as a hydroxyl source and as a solvent.