Carbonylation of organic halides with carbon monoxide mediated by samarium diiodide. Improvement and mechanistic investigations

Article abstract of DOI:10.1246/bcsj.79.1444

Under an atmospheric pressure of carbon monoxide, photoinduced reductive carbonylation of organic halides (RX) with samarium diiodide (SmI₂) occurs to afford the corresponding unsymmetrical ketones (RC(O)CH₂R) in good yields. Mechanistic insight into this carbonylation, especially the pathway for the generation of an acylsamarium species (RC(O)SmI₂) as a key intermediate, is also investigated in detail.

Full text of DOI:10.1246/bcsj.79.1444

1444 Short Articles
Bull. Chem. Soc. Jpn. Vol. 79, No. 9, 1444–1446 (2006)
Table 1. Atmospheric Carbonylation of Organic HalidesaÞ
Carbonylation of Organic Halides
with Carbon Monoxide Mediated by
Samarium Diiodide. Improvement
and Mechanistic Investigations
SmI -hν
2
O
2 R X
2 CO
R
R
1 atm
Entry
R–X
Yield/%
1
2
3
4
5
nC12H25Cl
nC12H25Br
nC12H25I
cC6H11I
45
50
58
85
68
Yuri Tomisaka,2 Nami Harato,3 Masahiro Sato,2
CH2=CH(CH2)4Br
ꢀ1
Akihiro Nomoto,1 and Akiya Ogawa
a) R–X (0.5 mmol), SmI2 (2 mmol), Sm (2 mmol), THF
(15 mL), CO (1 atm), r.t., 6 h, hꢁ: Xe lamp (500 W), Pyrex.
1Department of Applied Chemistry, Graduate School
of Engineering, Osaka Prefecture University,
1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531
Results and Discussion
2Advanced Materials Research Lab., KRI, Inc.,
Kyoto Research Park, 134 Chudoji Minami-machi,
Shimogyo-ku, Kyoto 600-8813
We initially investigated the influence of the visible-light ir-
radiation on this reductive carbonylation under an atmosphere
of carbon monoxide. When the reaction of 1-iodododecane
(0.5 mmol) with CO (1 atm) was carried out using a SmI2/
Sm (2 mmol/2 mmol)2 mixed system upon irradiation with a
xenon lamp through Pyrex, reductive carbonylation took place
with a 58% yield of dodecyl tridecyl ketone, which consisted
of two dodecyl and two CO units (Eq. 2 and Entry 3 in
Table 1). In contrast, no reductive carbonylation took place
at all in the dark. A deep blue-colored solution of SmI2
in THF has an absorption maximum in the visible region
(ꢀmax ¼ 565 and 617 nm), which is assigned to a 4f6 !
4f55d1 transition. In a previous paper, we reported that irradi-
ation at wavelengths between 500 and 700 nm is effective for
the excitation of SmI2, which dramatically enhances its reduc-
ing ability compared to SmI2 in the ground state.3
3Department of Chemistry, Faculty of Science,
Nara Women’s University, Kitauoyanishi-machi,
Nara 630-8506
Received January 18, 2006; E-mail: ogawa@chem.osakafu-u.
ac.jp
Under an atmospheric pressure of carbon monoxide,
photoinduced reductive carbonylation of organic halides
(RX) with samarium diiodide (SmI2) occurs to afford the
corresponding unsymmetrical ketones (RC(O)CH2R) in good
yields. Mechanistic insight into this carbonylation, especially
the pathway for the generation of an acylsamarium species
(RC(O)SmI2) as a key intermediate, is also investigated in
detail.
O
SmI /Sm
2
2 C
H I
12 25
2CO
C
H
12 25
C H
12 25
ð2Þ
1 atm
58 %
hν
dark
−
Transition-metal-catalyzed carbonylation of organic halides
with carbon monoxide via oxidative addition of the organic
halides to the transition-metal catalysts is a synthetically useful
process in organic synthesis. In contrast, examples of reductive
carbonylation of organic halides involving electron-transfer
process are rare. Recently, we have developed a novel re-
ductive carbonylation reaction of organic halides (RX) with
carbon monoxide (50 atm) mediated by samarium diiodide
(SmI2), which occurs upon visible-light irradiation, affording
the corresponding unsymmetrical ketones (RC(O)CH2R) in
good yields.1 Further detailed study on this carbonylation has
led to an interesting finding that this SmI2-induced reductive
carbonylation of haloalkanes does proceed under an atmo-
spheric pressure of CO upon visible-light irradiation (Eq. 1).
Moreover, atmospheric carbonylation is advantageous to get
insight into the present carbonylation pathway. In this paper,
we wish to report the atmospheric carbonylation of organic
halides with SmI2 upon visible-light irradiation and its detailed
mechanistic aspects.
Table 1 summarizes the results of the photoinduced carbon-
ylation of several organic halides with SmI2/Sm under an
atmosphere of CO. Primary alkyl halides, such as chloro-, bro-
mo-, and iodododecanes (Entries 1–3), produced the corre-
sponding unsymmetrical ketones in moderate yields. Similarly,
cyclohexyl iodide as a secondary alkyl iodide underwent atmo-
spheric carbonylation to afford an unsymmetrical ketone in
(pathB)
SmI -h
ν
2
SmI
2
RSmI
R
X
R
2
-
- X
(path C)
(path A)
SmI (CO)
CO
CO
2
n
O
SmI
O
2
R
SmI
2
R
O
SmI2
ð1Þ
2 RX
2 CO
Scheme 1. Possible pathways for the formation of acylsama-
rium species.
R
R
Published on the web September 5, 2006; doi:10.1246/bcsj.79.1444

Bull. Chem. Soc. Jpn. Vol. 79, No. 9 (2006)
Ó 2006 The Chemical Society of Japan 1445
good yield (Entry 4).
Br
The present photoinduced carbonylation may involve the
formation of acylsamarium species (RC(O)SmI2) as a key in-
termediate from haloalkane (RX), CO, and SmI2. Kagan et al.
previously reported that the reduction of an acyl chloride,
(RC(O)Cl), by an excess amount of SmI2 gave the correspond-
ing unsymmetrical ketones (RC(O)CH2R) and suggested that a
possible reaction pathway might involve the dimerization of an
acylsamarium species and then further reduction with SmI2
(Eq. 3).4
O
CO
SmI2
CO
SmI2
h
ν
kCO
SmI2
SmI2
kSm
5-exo
O
k1
O
O
2
SmI2
R
+
SmI
2
SmI2
I2Sm
CO
R
R
Cl
THF
4molar amounts
ð3Þ
SmI
SmI
THF
2
2
O
O
OSmI
2
dimer.
R
O
R
SmI
R
2
OSmI
2
O
In this paper, the mechanistic study is focused on the
not detected
68%
carbonylation step to generate the acylsamarium species from
haloalkanes and CO in the presence of low-valent samarium
reagent (SmI2/Sm). Scheme 1 shows some possible reaction
pathways for the formation of the acylsamarium species
(RC(O)SmI2).
kCO = 6.3 x 105 M-1s-1 (80 °C)6b, [CO] = 1.01 x 105 / 8.31 x 103 x 298
= 4 x 10-2 mol/L, kCO[CO] = 6.3 x 105 x 4 x 10-2 = 103; kSm = ~105
M-1s-1 (40 °C, SmI2-THF)6c, [SmI2] = 0.1 mol/L, kSm[SmI2] = 105 x 10-1
= 104; k1 = 2.3 x 105 s-1 (25 °C)6a
At first, we presumed that an in situ generated an alkyl radi-
cal (Rꢁ), reacted with CO to give an acyl radical species
(RC(O)ꢁ),5 and the subsequent reduction of the acyl radical
with SmI2 formed the acylsamarium species (Path A). To elu-
cidate whether atmospheric carbonylation includes the forma-
tion of acyl radical intermediates, we examined the photoin-
duced carbonylation of 5-hexenyl bromide with SmI2 under
an atmospheric pressure of CO. Scheme 2 summarizes the
kinetic data related to this reaction system.6 From Scheme 2,
the rate for the 5-exo cyclization of 5-hexenyl radical is
k1 ¼ 2:3 ꢂ 105 sꢃ1 (25 ꢄC). However, the rate for the reduction
of 5-hexenyl radical and the rate for the carbonylation of 5-hex-
enyl radical can be estimated roughly to be kSm[SmI2] ¼ ꢅ104
sꢃ1 and kCO[CO] ¼ ꢅ103 sꢃ1, respectively. The kinetic data
strongly suggest that the 5-exo cyclization of 5-hexenyl radical
takes place preferentially and, in contrast, the direct carbonyl-
ation of 5-hexenyl radical is an unfavorable process at least
under a CO atmosphere. Surprisingly, however, the expected
cyclic carbonylated product was not obtained, and instead, an
acyclic ketone was obtained in 68% yield. This result can not
be explained by the free radical pathway (Path A in Scheme 1).
Since the free radical pathway (Path A) is not suitable for
the formation of acylsamarium species, we next examined
the organosamarium pathway, which consists of the reduction
of haloalkanes (RX) with SmI2 to generate an organosamarium
species (RSmI2), followed by the carbonylation of RSmI2 with
CO to give an acylsamarium species (Path B in Scheme 1).
Dodecylsamarium diiodide was prepared by the reaction of
1-iodododecane (0.5 mmol) with SmI2/Sm (2 mmol/2 mmol)
at room temperature upon irradiation with xenon lamp through
Pyrex for 1.5 h. Then, CO (1 atm) was introducted into the so-
lution upon photoirradiation (rt, 4.5 h). However, the desired
unsymmetrical ketone was not obtained (Eq. 4).
Scheme 2. Photoinduced atmospheric carbonylation of 5-
hexenyl bromide with SmI2.
SmI -h
ν
2
n
n
2 C
H
12 25
I
2
C
H
SmI
H
12 25
2
r.t., 1.5 h
O
ð4Þ
CO (1 atm), h
ν
C H
12 25
C
12 25
r.t., 4.5 h
trace
To determine if dodecylsamarium diiodide is formed in this
reaction system,7 we attempted to trap dodecylsamarium di-
iodide with excess amounts of diethyl ketones. However, the
Grignard-type reaction did not occur, and instead dodecane
and dodecene were obtained in 45 and 38% yields, respective-
ly, as a result of reduction and disproportionation (Eq. 5). Fur-
thermore, the same reaction was carried out between ꢃ15 to
5 ꢄC, but the addition product was not detected.
SmI -h
ν
Et CO (3 equiv), h
ν
2
2
n
n
C
H
12 25
I
C H SmI
12 25 2
r.t., 1.5 h
r.t., 2 h
OH
ð5Þ
n
n
n
C
H
12 26
C H
12 24
C
H
12 25
Et
Et
45%
38%
not formed
These results suggest that alkylsamarium diiodide is unsta-
ble under these conditions and, thus, decomposed before the
reaction with the CO. Accordingly, the pathway through the
generation of the alkylsamarium species (Path B) is unfavora-
ble for this carbonylation reaction, because of the instability of
alkylsamarium species as an intermediate.

1446 Y. Tomisaka et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 9 (2006)
Table 2. SmI2-Induced Reduction of 2-OctanoneaÞ
diiodide. Further mechanistic studies of this carbonylation
are now under investigation.
O
SmI2/Sm
nC6H13
CH3
OH
CH3
Experimental
r.t., 1 h
Atmospheric Carbonylation. In a 20 mL three-necked glass
flask (Pyrex) equipped with a dropping funnel, a cooler, and an in-
let, were placed, under nitrogen, Sm powder (4 mmol, 0.601 g),
1,2-diiodoethane (2 mmol, 0.564 g), and THF (10 mL). The result-
ing mixture was stirred magnetically for 1.5 h to prepare a SmI2/
Sm reagent, and then, carbon monoxide was introduced into the
vessel. Upon irradiation with a Xe lamp (500 W), the haloalkane
(0.5 mmol) was added dropwise to the mixture over 1 h, and then,
the reaction was continued for another 5 h. After the decomposi-
tion of the excess SmI2/Sm with air, followed by 1 mol Lꢃ1
HCl, the product was extracted with ether. The extracts were dried
(MgSO4) and concentrated in vacuo. Purification of the product
was performed by preparative HPLC or PTLC.
O
OH
CH3
C6H13
n
n
C6H13
+
+
nC6H13
nC6H13
nC6H13
H
H3C CH3
H3C
OH
1
2
3
Yield/%
Reaction
condition
Entry
2-Octanone
1
2
3
1
2
3
hꢁ
dark
hꢁ þ CO
15
25
80
72
62
10
7
10
7
6
3
3
a) 2-Octanone (0.5 mmol), SmI2 (2 mmol), Sm (2 mmol), THF
(15 mL), CO (1 atm), hꢁ: Xe lamp (500 W), Pyrex.
References
During the course of our study on this carbonylation reac-
tion, we have found that the reducing ability of SmI2 is greatfly
reduced in the presence of CO (Table 2). For example, when
the photoinduced reduction of 2-octanone by SmI2 was con-
ducted at room temperature for 1 h in the absence of CO, 2-
octanol (1) was obtained as the major product, along with small
amounts of vic-diol (2), as a pinacol-type product, and ketone
(3) as a pinacol rearrangement product (Entry 1). The same
reaction without photoirradiation led to similar results as the
case with photoirradiation (Entry 2). In the presence of CO,
however, the photoinduced reaction of 2-octanone afforded
the desired alcohol (1) in very poor yield, and instead, the start-
ing material was mainly recovered (Entry 3). It is clear that the
existence of CO decreases the reducing ability of SmI2, most
probably by the coordination of CO to samarium species.8
With these results in mind, a possible pathway for the for-
mation of acylsamarium species (Path C in Scheme 1) is as
follows: Dodecyl iodide undergoes single-electron transfer
from SmI2 to generate a dodecyl radical, which may be cap-
tured rapidly by the CO coordinated to the samarium species
and simultaneously undergoes single electron reduction by
SmI2(CO)n generating acylsamarium species (Eq. 6).
1
a) A. Ogawa, Y. Sumino, T. Nanke, S. Ohya, N. Sonoda,
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2
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the yield of dodecyl tridecyl ketone was lower compared to the
reaction using the SmI2/Sm-mixed system. We already reported
the combination of SmI2 and Sm metal exhibited higher reducing
power compared with SmI2 or Sm metal itself. See: A. Ogawa,
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4
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5
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6
a) A. L. Beckwith, C. H. Schiesser, Tetrahedron 1985, 41,
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7 T. Hanamoto, Y. Sugimoto, A. Sugino, J. Inanaga, Synlett
1994, 377.
-
O
e
SmI (CO)
2 n
R-I
R
ð6Þ
R
SmI (CO)
2 n-1
-
I
In conclusion, we have demonstrated the photoirradiated
atmospheric carbonylation of organic halides with samarium
8
The existence of CO also decreases the reducing ability of
SmI2 in the reduction of 1-iodododecane. See: Ref. 1b.