Indium(I)-mediated radical carbon-carbon bond-forming reaction in aqueous media

Article abstract of DOI:10.1055/s-0029-1219921

The carbon-carbon bond-forming radical reactions using indium(I) chloride and copper(I) chloride proceeded effectively through a single-electron transfer process in aqueous media.

Full text of DOI:10.1055/s-0029-1219921

LETTER
1341
Indium(I)-Mediated Radical Carbon–Carbon Bond-Forming Reaction in
Aqueous Media
Radical
C
arbon–Carbo
a
nBond-Fo
s
R
ea
a
ction fumi Ueda,a Hideto Miyabe,b Masafumi Torii,a Takahiro Kimura,a Okiko Miyata,*a Takeaki Naito*a
a
Kobe Pharmaceutical University, Motoyamakita, Higashinada, Kobe 658-8558, Japan
Fax +81(78)4417554; fax +81(78)4417642; E-mail: miyata@kobepharma-u.ac.jp; E-mail: taknaito@kobepharma-u.ac.jp
b
School of Pharmacy, Hyogo University of Health Sciences, Minatojima, Chuo-ku, Kobe 650-8530, Japan
Received 12 March 2010
using the suitable combination of indium(I) chloride and
copper(I) chloride.11,12
Abstract: The carbon–carbon bond-forming radical reactions using
indium(I) chloride and copper(I) chloride proceeded effectively
through a single-electron transfer process in aqueous media.
We first report the results of an experiment to probe the
utility of InCl as a radical initiator. In views of our previ-
ous success in promoting radical reaction by indium(0)
metal4a and of recently reported Loh’s work,8 we began to
investigate the reaction of glyoxylic oxime ether 1 with
isopropyl iodide under several reaction conditions using
Key words: indium(I) chloride, copper(I) chloride, radical addi-
tion, oxime ether, domino reaction, aqueous media
The indium-mediated carbon–carbon bond-forming reac-
tions in aqueous media have been of great importance in
organic synthesis.1–3 Particularly, a low first ionization
potential of 5.79 eV makes indium(0) metal attractive for
electron-transfer reactions.2 However, indium-mediated
radical reaction has received much less attention,3–7 al-
though the use of indium has continued to increase in syn-
thetic organic chemistry. Recently, we started to
investigate the utility of indium(0) in radical reactions and
found that indium(0) has the potential to induce radical re-
actions as a radical initiator via single electron-transfer
process.4 As a novel related method, indium(0) and iodine
mediated reductive radical cyclization of iodoalkenes and
iodoalkynes has also been reported by Yanada and Take-
moto.5 Very recently, Loh’s group reported an efficient
method for the alkylation of imines and nitrones promoted
by indium–copper in aqueous media.8 On the other hand,
the utility of indium(I) salts as promoters of radical reac-
tions was much less explored, probably because the sec-
ond ionization potential for indium is much higher (18.86
eV).9,10 Therefore, indium(I) halides are only employed
for the reactions of a limited number of substrates and the
latent reactivities are not disclosed completely so far.9 Ad-
ditionally, indium-mediated reactions show considerable
potential but current vagaries limit their acceptance.1a
From synthetic point of view, it is necessary to broaden
the scope of indium chemistry by introducing structural
diversity on the starting radical acceptors which should
open routes to the efficient synthesis of a large amount of
compounds.
Table 1 Isopropyl Radical Addition to Oxime Ether 1
i-PrI, InXn
MeO2C
NHOBn
NOBn
MeO2C
r.t.
i-Pr
1
2
Entry
1b
InXn
InCl
InCl
InBr
InI
Solvent
Additive Yield (%)a
H2O–MeOH
CH2Cl2
none
none
none
none
CuCl
CuCl
CuBr
CuI
63
2b
no reaction
trace
3b
H2O–MeOH
H2O–MeOH
H2O–MeOH
CH2Cl2
4b
no reaction
97
5c
InCl
InCl
InCl
InCl
InCl
InBr
InI
6c
no reaction
74
7c
H2O–MeOH
H2O–MeOH
H2O–MeOH
H2O-MeOH
H2O–MeOH
H2O–MeOH
H2O–MeOH
H2O–MeOH
H2O–MeOH
8c
65
9c
CuCl2
CuCl
CuCl
CuCl
none
CuCl
CuCl
50
10d
11d
12e
13b
14c
84
18 (74)
no reaction
24 (50)
45 (22)
no reaction
none
InCl2
InCl2
InCl3
In this communication, we present an efficient method for 15c
promoting the aqueous-medium radical addition to a vari-
ety of substrates involving carbon–nitrogen or carbon–
carbon double bonds and the a,b-unsaturated analogue by
a Isolated yield; yields in parentheses are for the recovered starting
material 1.
b Reactions were carried out with i-PrI (5 equiv X 2) and InXn (10
equiv).
c Reactions were carried out with i-PrI (5 equiv X 2), InXn (10 equiv),
and CuXn (1 equiv).
d Reactions were carried out with i-PrI (5 equiv X 2), InX (10 equiv),
and CuCl (4 equiv).
SYNLETT 2010, No. 9, pp 1341–1344
x
x.
x
x.
2
0
1
0
Advanced online publication: 06.05.2010
DOI: 10.1055/s-0029-1219921; Art ID: U01810ST
e Reaction was carried out with i-PrI (5 equiv X 2) and CuCl (4 equiv).
© Georg Thieme Verlag Stuttgart · New York

1342
M. Ueda et al.
LETTER
indium(I) salts.13 On our initial attempt, the reaction using
InCl in H2O–MeOH gave the desired adduct 2 in 63%
yield (Table 1, entry 1). Aqueous solvent was essential to
achieve good conversions; when the reaction was carried
out in organic solvent such as CH2Cl2, no desired product
was observed (entry 2). In contrast to InCl, InBr and InI
were less effective under the similar reaction conditions
(entries 3 and 4). It is important to emphasize that InCl
worked well in aqueous media, in spite of a high second
ionization potential of indium. Therefore, as one possible
reaction pathway, we propose that the first step of the
present reaction would be the disproportionation of InCl
giving indium(0) metal (Scheme 1).14 Another role of
InCl would be the activation of oxime ether group as
Lewis acid.10 Further experiments showed that the addi-
tion of CuCl led to an enhancement in chemical yield (en-
try 5). The monophasic reaction of 1 in H2O–MeOH gave
the isopropylated product 2 in 97% yield without forma-
tion of significant byproducts such as a reduced product.
At this stage, the role of CuCl in our reactions was ques-
tioned. We assume that one role would involve the activa-
tion of InCl or the promotion of disproportionation by
redox reaction between indium(I) and copper(I).15 Other
copper(I) salts were also investigated but provided poor
results relative to CuCl (entries 7 and 8). The addition of
CuCl2 did not affect the chemical efficiency (entry 9). The
combination of InBr and CuCl accelerated the reaction
with good activity (entry 10), in contrast, the reaction us-
ing InBr in the absence of CuCl gave only a trace amount
of product (entry 3). It was reported that copper(I) salts
promoted radical reactions;16 thus, we tried the reaction
with CuCl in the absence of indium(I) salts, however, no
reaction took place (entry 12). More interestingly, InCl2
promoted the reaction to form the product 2 in moderate
yields (entries 13 and 14). However, InCl3 did not work
(entries 15).7 On the basis of these results, we concluded
that the best conditions are shown in entry 5. As an alkyl
radical precursor, primary iodide such as methyl iodide
did not work and tert-butyl iodide gave a complex mix-
ture. It is important to note that the reaction using InCl and
CuCl was suppressed by the addition of a galvinoxyl free
radical as radical scavenger. Thus, the present indium(I)-
induced reaction proceeded through a radical mechanism
based on a single-electron transfer process (Scheme 1),
which was analogously proposed by Loh’s group under
different reaction conditions but with no systematic inves-
tigation and discussion involving scavenger experiment.8
3 InCl
CuCl
H2O
Lewis acid
InCln
disproportionation
i-PrI InIn
•
MeO2C
NHOBn
NOBn
MeO2C
NOHBn
1
InCl3
2 In(0)
i-Pr
•
+
i-Pr
A
(InCl, CuCl)
MeO2C
–
H3O+
MeO2C
NOBn
i-Pr
i-Pr
2
B
Scheme 1 Proposed reaction pathway from 1 to 2
RI (5 equiv X 2)
InCl (10 equiv), CuCl (1 equiv)
MeO2C
NHNPh2
NNPh2
MeO2C
H2O–MeOH, r.t., 2 h
R
3
4a R = i-Pr (93%)
4b R = s-Bu (96%)
4c R = c-Pent (89%)
O
N
O
NNPh2
NHNPh2
i-Pr
N
SO2
i-PrI (5 equiv X 2)
SO2
InCl (10 equiv), CuCl (1 equiv)
H2O–MeOH, r.t., 2 h
5
6 (79%, 87% de)
Scheme 2 Alkyl radical addition to hydrazones 3 and 5
Further application of InCl–CuCl system into the a,b-
unsaturated imines 7a and 7b led us very interesting result
as follows. Treatment of conjugated oxime ether 7a with
isopropyl iodide in the presence of InCl and CuCl in H2O–
EtOH gave regioselectively b-addition product 9a in 58%
yield (Table 2, entry 1). Interestingly, lacking of CuCl
gave a mixture of two products 8a and 9a in 18% and 22%
yields, respectively, with 4% yield recovery of 7a (entry
2). The former product 8a is b-isopropyl and a-hydroxy
adduct.17c When H2O–Et2O was employed as a solvent,
two products 8a and 9a were produced more efficiently in
40% and 38% yields, respectively (entry 3). In order to ex-
plain the origin of hydroxy group on adduct 8a, oxygen
saturated ether was employed as H2O–Et2O system, but
both alkyl adduct 9a and hydroxylated alkyl adduct 8a
were formed in less yields indicating that too much
amount of oxygen retards the reaction (entries 4 and 5).
As an alternative solvent system, H2O–CH2Cl2 combina-
tion in the absence of CuCl gave both products 8a and 9a
in better yields (entry 6). The corresponding conjugated
hydrazone 7b worked well to afford isopropyl adduct 9b
in moderate yield with a trace amount of hydroxylated
alkyl adduct 8b under InCl–CuCl conditions in H2O–Et2O
or H2O–EtOH (entries 7 and 8).
To survey the scope and limitations of the present method,
we next investigated the reaction of hydrazones 3 and 5
(Scheme 2).17 Under analogous reaction conditions using
InCl–CuCl system, the reaction of 3 with isopropyl radi-
cal was facile to give product 4a in 93% yield. The sec-
butyl and cyclopentyl iodides worked well. Good diaste-
reoselectivity was observed in the reaction of hydrazone
5, although its reaction was slightly less effective, which
was attributed to the greater steric bulk. In general all re-
actions found here took place effectively as in the case of
In(0)-mediated reaction.4
Reaction pathway from conjugated oxime ether 7a to iso-
propyl adduct 9a is reasonably explained through two step
process of SET via a carbanion intermediate D, while the
pathway from 7a to b-isopropyl and a-hydroxy adduct 8a
is not clear but proposed as shown in Scheme 3.
To gain further insight into the utility of InCl–CuCl sys-
tem, we next examined the reaction of several radical ac-
ceptors which were employed in our previous studies
Synlett 2010, No. 9, 1341–1344 © Thieme Stuttgart · New York

LETTER
Radical Carbon–Carbon Bond-Forming Reaction
1343
Table 2 Isopropyl Radical Reaction to a,b-Unsaturated Imines 7a
and 7ba
i-PrI (5 equiv X 2)
InCl (7 equiv), CuCl (1 equiv)
Ph
NHTs
i-Pr
NTs
O
Ph
O
H2O–MeOH, r.t., 2 h
OH
10
β
α
11 (78%)
NR
EtO2C
O
O
O
i-Pr
i-PrI (5 equiv X 2)
InCl (7 equiv), CuCl (1 equiv)
i-Pr
8a R = OBn
8b R = NPh2
i-PrI (5 equiv X 2)
InCl (10 equiv)
O
N
N
H2O–MeOH, r.t., 2 h
α
NR
EtO2C
+
13 (75%)
12
14
r.t.
NR
EtO2C
β
i-PrI (5 equiv X 2)
InCl (7 equiv), CuCl (1 equiv)
7a R = OBn
7b R = NPh2
i-Pr
PhO2S
O2S
PhO2S
i-Pr
9a R = OBn
9b R = NPh2
H2O–MeOH, r.t., 2 h
15 (72%)
RI (5 equiv X 2)
InCl (4 equiv), CuCl (1 equiv)
R
I
Entry Substrate Solvent
Additive Time Yield of Yield of
8a,bb
9a,b
O2S
(1 equiv) (h)
N
H2O, r.t., 2 h
N
(%)c
(%)c
CHPh2
16
CHPh2
17a R = i-Pr
1
2
3
4
5f
6
7
8
7a
7a
7a
7a
7a
7a
7b
7b
H2O–EtOH
H2O–EtOH
H2O–Et2O
H2O–Et2Oe
H2O–Et2Oe
CuCl
none
CuCl
none
none
3
5
–
58
(83%, trans/cis = 1:1.7)
17b R = c-Pent
(65%, trans/cis = 1:1.7)
18
22d
38
7
40
Scheme 4 Radical addition to various substrates 10, 12, 14, and 16
5
28
22d
10d
47
worked well without significant polymerization. The
InCl–CuCl system was successfully applied to the domino
radical addition–cyclization–trapping reaction of 16.19
6
19
H2O–CH2Cl2 none
24
5
30
In conclusion, we have disclosed an efficient InCl–CuCl
system promoting the radical reactions in aqueous media
which are amenable to green chemistry. In conjunction
with Loh’s group works,8 our found InCl–CuCl-mediated
reaction is proved to be quite general and applicable to a
substrate of wide scope, because it does not require the
use of moisture-sensitive irritant tin hydride and any spe-
cial precautions such as drying, degassing, and purifica-
tion of solvents.
H2O–Et2O
H2O–EtOH
CuCl
CuCl
trace
trace
50d
61
5
a Reactions were carried out with i-PrI (5 equiv X 2) and InCl (10
equiv) under air atmosphere.
b Obtained as 3.7–2.3:1 ratio of anti/syn isomers.
c Isolated yields.
d A small amount of substrate was recovered.
e Pretreatment of Et2O with O2 gas.
f Reaction was carried out under O2 gas.
3 InCl
Acknowledgment
H3O+
–
NOBn
EtO2C
CuCl H2O
9a
This work was supported in part by Grant-in-Aid for Young Scien-
tists (B) (M.U.), for Scientific Research (C) (H.M.), for Scientific
Research (B) (T.N.) from the Ministry of Education, Culture,
Sports, Science and Technology of Japan. Our thanks are also direc-
ted to the Science Research Promotion Fund of the Japan Private
School Promotion Foundation for a research grant.
i-Pr
D
InCl3
+
2 In(0)
SET (InCl, CuCl)
i-PrI
InIn
7a
SET
I
•
NOBn
NOBn
NOBn
EtO2C
EtO2C
i-PrI
i-Pr
•
i-Pr
F
i-Pr
C
References and Notes
O2
H2O
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O
OH
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(InCl, CuCl)
i-Pr
E
i-Pr
8a
Scheme 3 Proposed reaction pathway from 7a to 8a and 9a
(Scheme 4).4,18,19 Reaction of N-Ts imine 10 proceeded
smoothly to give the desired product 11 in good yield.18
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method in aqueous media. Phenyl vinyl sulfone 14 also
Synlett 2010, No. 9, 1341–1344 © Thieme Stuttgart · New York

1344
M. Ueda et al.
LETTER
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To a microtube containing oxime ether 1 (50 mg, 0.26
mmol), i-PrI (1.3 mL, 1.3 mmol), indium salt (2.6 mmol),
copper salt (0.26 mmol), and MeOH (0.2 mL) H2O was
added dropwise (0.4 mL) at r.t. After the reaction mixture
was vigorously stirred at the same temperature for 1 h, i-PrI
(1.3 mL, 1.3 mmol) was added to the reaction mixture. After
being vigorously stirred at the same temperature for 1 h, the
reaction mixture was diluted with aq 36% potassium sodium
(+)-tartrate and then extracted with EtOAc. The organic
phase was dried over MgSO4 and concentrated under
reduced pressure. Purification of the residue by preparative
TLC (hexane–EtOAc, 4:1) afforded the adduct 24b in the
yields shown in Table 1.
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