Iodoarylation of arylalkynes with molecular iodine in the presence of hypervalent iodine reagents

Article abstract of DOI:10.3390/molecules14093132

Iodoarylation of arylacetylenes was performed using a simple reagent system composed of molecular iodine and [bis(benzoyloxy)iodo]benzene. Most arylacetylenes efficiently underwent the iodoarylation reaction with electron-rich arenes to give trans 1,1-dia

Full text of DOI:10.3390/molecules14093132

Molecules 2009, 14, 3132-3141; doi:10.3390/molecules14093132
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Iodoarylation of Arylalkynes with Molecular Iodine in the
Presence of Hypervalent Iodine Reagents
Md. Ataur Rahman and Tsugio Kitamura *
Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga
University, Honjo-machi, Saga, 840-8502, Japan
* Author to whom correspondence should be addressed; E-mail: kitamura@cc.saga-u.ac.jp.
Received: 6 August 2009; in revised form: 18 August 2009 / Accepted: 24 August 2009 /
Published: 24 August 2009
Abstract: Iodoarylation of arylacetylenes was performed using a simple reagent system
composed of molecular iodine and [bis(benzoyloxy)iodo]benzene. Most arylacetylenes
efficiently underwent the iodoarylation reaction with electron-rich arenes to give trans
1,1-diaryl-2-iodoethene adducts regio- and stereoselectively. As an exception, the
iodoarylation of p-methoxyphenylacetylene resulted in a mixture of E- and Z-isomers of
the corresponding product.
Keywords: iodoarylation; alkyne; molecular iodine; hypervalent iodine; iodoethene
1. Introduction
Alkenyl halides, especially iodides, are synthetically useful compounds and have attracted
considerable attention of organic chemists because the iodo group is useful for further elaboration by
subjecting the iodoalkene units to metal-catalyzed cross-coupling reactions [1-3]. However, direct
iodination of alkenes to iodoalkanes using molecular iodine (I2) is generally impractical because of the
low reactivity of iodine.
We have found that the iodination reaction of aromatic compounds with I2 proceeds smoothly in the
presence of potassium peroxodisulfate (K2S2O8) as an oxidant to give iodoarenes in good yields [4].
To further explore the use of I2, we extended the iodination reaction of arenes to iodoarylation of
alkynes with arenes. It was assumed that an alkyne would be activated by iodine to form a bridged
iodonium ion which would then undergo electrophilic substitution with an arene. However, there are

Molecules 2009, 14
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no reports in which molecular iodine is used as an iodine source for the iodoarylation of alkynes.
Recently, Barluenga et al. reported the iodoarylation of arenes using bis(pyridine)iodonium(I)
tetrafluroborate as an iodine source [5]. This reagent is commercially available, but very expensive,
and its preparation requires the use of toxic mercury oxide [6]. Therefore, a convenient, safe, and
simple iodine reagent for the iodoarylation of alkynes is strongly required. Preliminarily, we have
found that the iodoarylation of alkynes proceeds effectively in the presence of a simple reagent system:
I2 and a hypervalent iodine species like PhI(OCOPh)2 [7]. Herein we report the details on the
iodoarylation of alkynes with the reagent system consisting of molecular iodine and hypervalent iodine
compounds. The results demonstrate a convenient and simple carbon-carbon bond formation process
between electron rich arenes and aryl-substituted alkynes using molecular iodine in the presence of
hypervalent iodine reagents.
2. Results and Discussion
Molecular iodine and hypervalent iodine compounds have witnessed a large growth in recent years
[8-20]. In the present study we confined our attention to the formation of carbon-carbon bonds
between arenes and aryl-substituted alkynes in the presence of molecular iodine and hypervalent
iodine reagents. Initially, the present work concentrated on the efficiency of the formation of a carbon-
carbon bond between pentamethylbenzene (1a) and p-methylphenylacetylene (2) in the presence of I2
and (diacetoxyiodo)benzene, PhI(OAc)2 (Scheme 1). PhI(OAc)2 is one of the most widely available
commercial hypervalent iodine compounds and has been widely used in organic synthesis [8,9],
making it the first choice among hypervalent iodine reagents tested .
Scheme 1. Reaction of alkyne 2 with arene 1a.
I2, PhI(OAc)2
I
+
1a
2
3a
2.1. Optimization of Reaction Conditions
To optimize the reaction conditions the reaction of 1a and 2 using I2 in the presence of PhI(OAc)2
was investigated (Table 1). The iodoarylation reaction occurred in different solvent systems: 1,2-
dichloroethane (DCE), AcOH, and MeCN to give 1-iodo-2-(4-methylphenyl)-2-(pentamethylphenyl)-
ethene (3a; Entries 1-3). MeCN (Entry 3) gave the best results. Increasing the amount of 1a resulted
in a better yield of 3a (Entry 4). Elevation of the reaction temperature also improved the yield of 3a
o
(Entry 5). The best overall result (78% yield) was obtained by the reaction at 82 C using
10 equivalents of 1a (Entry 6). The reaction in EtOAc instead of MeCN resulted in a low yield (13%)
of product 3a (Entry 7).

Molecules 2009, 14
Table 1. Optimization of reaction conditions for the reaction of 1a and 2 in the presence of
3134
PhI(OAc)2.
1a
PhI(OAc)2
Temp.
(oC)
45
Yield of
3a (%)*
12
Entry
Solvent (Amt.)
Time (h)
(mmol)
(mmol)
1
2
3
4
5
6
7
1
5
1.25
3
DCE (2 mL)
28
48
48
48
56
65
65
AcOH (2 mL)
MeCN (2 mL)
MeCN (2 mL)
MeCN (4 mL)
MeCN (4 mL)
EtOAc (4 mL)
60
23
1.5
5
3
60
31
3
60
52
5
3
78
67
10
10
3
82
78
3
78
13
Reaction conditions: 1a, 2 (1 mmol), I2 (1.25 mmol), PhI(OAc)2 and solvent. * Isolated yield based
on 2.
To improve the yield of iodoarylation product 3a, we further examined the effect of different
[bis(acyloxy)iodo]benzenes, PhI(OH)OTs, and AgOCOPh on the iodoarylation reaction. Several
[bis(acyloxy)iodo]benzenes were prepared from benzoic acid, m- and p-chlorobenzoic acids, p-nitro-
benzoic acid, p-toluic acid, and p-anisic acid, according to the literature method [21]. The results are
given in Table 2. PhI(OCOPh)2 showed the highest activity among all other reagents, giving the
product 3a in 86% yield (Entries 1-6). [Bis(4-nitrobenzoyloxy)iodo]benzene also gave 3a in good
yield (80%), but we chose [bis(benzoyloxy)iodo]benzene as the activator because of the ready
availability of benzoic acid. Koser’s salt, PhI(OH)OTs, and silver benzoate show little effect in the
iodoarylation reaction (Entries 7 and 8).
Table 2. Effect of hypervalent iodine reagents on the reaction of 1a with 2.
Entry Hypervalent iodine reagent
Yield of 3a (%)
1
2
3
4
5
6
7
8
PhI(OCOPh)2
86
78
[Bis(m-chlorobenzoyloxy)iodo]benzene
[Bis(p-chlorobenzoyloxy)iodo]benzene
[Bis(p-nitrobenzoyloxy)iodo]benzene
[Bis(p-methylbenzoyloxy)iodo]benzene
[Bis(p-methoxybenzoyloxy)iodo]benzene
PhI(OH)OTs
65
80
74
70
33a,b
12c
AgOCOPh
Reaction conditions: 1a (10 mmol), 2 (1 mmol), I2 (1.25 mmol), a hypervalent iodine regent
(3 mmol), MeCN (6 mL), 82 °C, and 65 h. a Iodoarylation product 3a was contaminated with
b
hydroarylation product. The reaction was conducted by using 1a (3 mmol) in MeCN (4 mL) at
c
45 °C for 28 h. 1a (2 mmol), 2 (1 mmol), I2 (1 mmol), AgOCOPh (1 mmol), MeCN (2 mL),
40 °C and 36 h.

Molecules 2009, 14
3135
2.2. Scope of the iodoarylation reaction using I2 and PhI(OCOPh)2
The iodoarylation reaction of p-methylphenylacetylene (2) in the presence of PhI(OCOPh)2 was
examined with different electron-rich arenes 1 (Scheme 2 and Table 3). The results of the reactions
showed that the reaction with electron-rich arenes gave iodoarylation products in good yields. In
particular the electron rich arene mesitylene (1b) gave iodoarylation product 3b in high yield
(Entry 1). The reaction with durene (1c) and bromomesitylene (1d) gave iodoarylation products 3c
and 3d in 56 and 42% yields, respectively (Entries 2 and 3). The reaction with p-xylene (1e) gave a
low yield of iodoarylation product 3e (Entry 4).
Scheme 2. Reaction of alkyne 2 with arenes 1.
I2, PhI(OCOPh)2
+
Ar H
I
MeCN, 82 oC
1
2
Ar
3
Table 3. The reaction of alkyne 2 with arenes 1 in the presence of PhI(OCOPh)2.
Entry Arene
Time (h)
Product
Isolated yield (%)
1
2
3
4
Mesitylene (1b)
Durene (1c)
65
67
3b
3c
3d
3e
75
56
42a
33a
Bromomesitylene (1d) 72
p-Xylene (1e) 72
Reaction conditions: Arene 1 (10 mmol), p-methylphenylacetylene (2, 1 mmol), I2 (1.25 mmol),
PhI(OCOPh)2 (3 mmol), MeCN (6 mL), and 82 °C. a A mixture of E- and Z-isomers.
Next, we examine the reaction of phenylacetylene (4) with different electron-rich arenes 1 under the
same reaction conditions as used for 2 (Scheme 3). The results are given in Table 4. The reaction of
arenes such as 1a-1c gave iodoarylation products 5a-5c in good yields (Entries 1-3). Moderately
activated arenes 1d and 1e showed a very low reactivity, giving low yields (32 and 24%) of
iodoarylation products 5d and 5e, respectively.
Scheme 3. Reaction of alkyne 4 with arenes 1.
I2, PhI(OCOPh)2
I
+
Ar H
MeCN, 82 oC
Ar
1
4
5

Molecules 2009, 14
Table 4. The reaction of alkyne 4 with arenes 1 in the presence of PhI(OCOPh)2.
3136
Entry
Arene
1a
Time (h)
Product
Isolated yield (%)
1
2
3
65
70
73
5a
5b
5c
71
61
59
1b
1c
4
5
1d
1e
76
72
5d
5e
32
24
Reaction conditions: Arene 1 (10 mmol), phenylacetylene 4 (1 mmol), I2 (1.25 mmol),
PhI(OCOPh)2 (3 mmol), and MeCN (6 mL) at 82 °C.
The iodoarylation reaction of arylacetylenes with arenes proceeded regio- and stereospecifically to
give a single isomer of the possible E- and Z-1,1-diaryl-2-iodoethenes 3 and 5. We attempted to
determine the stereochemistry of iodoarylation product 3a by NOE experiments, but only a small
enhancement (3%) of the vinylic proton was observed when the ortho methyl proton was irradiated.
This may be attributed to the deviation of the pentamethylphenyl ring from the olefinic plane. Next, we
estimated the chemical shift of the vinylic proton according to the literature [22] and compared it with
the observed value. The calculation of the chemical shift concerning the vinylic proton for iodoethenes
3 and 5 gives 6.51 ppm for the E isomer and 6.84 ppm for the Z isomer. In the 1H-NMR spectra of 3
and 5, a singlet peak of the vinylic proton was observed at the range of 6.30–6.55 ppm. Therefore, the
stereochemistry of iodoethenes 3 and 5 is considered to be E. This is in accord with the results
obtained by Barluenga et al. [5].
The reaction of an internal alkyne, diphenylacetylene (6), with arenes 1 was further examined under
the above reaction conditions (Scheme 4). The reaction of electron rich arenes such as 1a and 1b with
6 yielded the expected iodoarylation products 7a and 7b, but the yields were low (14 and 8%,
respectively). The low yield of the reaction may be attributable to the low reactivity of
diphenylacetylene.
Scheme 4. Reaction of alkyne 6 with arenes 1.
I2, PhI(OCOPh)2
Ph
Ar
I
Ar H
+ Ph
Ph
MeCN
82 oC, 76 h
Ph
1
6
7
Moreover, we examined the reaction of electron-rich p-methoxyphenylacetylene (8) with arenes 1
(Scheme 5). The results are shown in Table 5. The reaction of electron-rich arenes 1a-1c gave
iodoarylation products 9 in good yields (Entries 1-3). Moderately activated arenes such as 1d and 1e
showed a low reactivity and the yields were 32 and 16%, respectively. In the case of arylacetylene 8,
the iodoarylation reaction lost its stereospecificity to give a mixture of E- and Z-isomers of iodoethenes
9. This behavior is different from that observed in the cases of other arylacetylenes 2 and 4.

Molecules 2009, 14
3137
Scheme 5. Reaction of alkyne 8 with arenes 1.
MeO
I2, PhI(OCOPh)2
+
Ar H
MeO
I
MeCN, 82 oC
Ar
1
8
9
Table 5. The reaction of alkyne 8 with arenes 1 in the presence of PhI(OCOPh)2.
Entry
Arene
1a
Time (h)
Producta Isolated yield (%)
1
2
3
4
5
72
72
73
76
76
9a
9b
9c
9d
9e
75
63
62
32
16
1b
1c
1d
1e
Reaction conditions: Arene 1 (10 mmol), alkyne 8 (1 mmol), I2 (1.25mmol), PhI(OCOPh)2
(3 mmol), and MeCN (6 mL) at 82 °C. a Products 9 were obtained as a mixture of E- and Z-
isomers.
Finally, we examine the reaction of p-fluorophenylacetylene (11) with arenes 1 under the above
reaction conditions. The results are given in Table 6. The reaction with electron-rich arenes 1a-1c
gave iodoarylation products 12 in good yields (Entries 1-3). The similar reaction of 1d and 1e resulted
in low yields (27 and 23%, respectively) of products 12. Arylacetylene 11 showed a similar behavior
to 2 and 4 to undergo stereospecific iodoarylation reaction.
Scheme 6. Reaction of alkyne 11 with arenes 1.
F
I2, PhI(OCOPh)2
+
Ar H
F
I
MeCN, 82 oC
Ar
1
11
12
Table 6. The reaction of alkyne 11 with arenes 1 in the presence of PhI(OCOPh)2.
Entry
ArH
1a
Time (h)
Product
12a
Isolated yield (%)
1
2
3
4
5
72
72
73
76
76
69
67
56
27
23
1b
1c
12b
12c
1d
1e
12d
12e
Reaction conditions: Arene 1 (10 mmol), p-fluorophenylacetylene 11 (1 mmol), I2 (1.25 mmol),
PhI(OCOPh)2 (3 mmol), and MeCN (6 mL) at 82 °C.

Molecules 2009, 14
3138
No iodoarylation reaction occurred in the case of 1-hexyne and 3-butyn-2-one, suggesting that this
iodoarylation reaction is applicable only to relatively electron-rich alkynes.
2.3. Mechanistic consideration
The proposed mechanism of the iodoarylation reaction of arylacetylenes is shown in Scheme 7.
Initially iodine reacts with PhI(OCOPh)2 to form a hypoiodite, IOCOPh [23,24], which actually
undergoes iodination of an arylacetylene. The in situ-generated IOCOPh adds the arylacetylene to give
a cyclic iodonium benzoate. In most cases, the cyclic iodonium form is stable compared with an open
vinyl cation due to a significant neighboring participation of iodo group [25]. However, the open vinyl
cation governs the reaction process in the case of p-methoxyphenylacetylene, where the vinyl cation is
strongly stabilized by p-methoxyphenyl group and exists as the open form. The cyclic iodonium ion
undergoes aromatic electrophilic substitution with an electron-rich arene to afford an iodoarylation
product stereoselectively. The presence of the cyclic iodonium ion causes trans addition giving the
stereochemically defined product. On the other hand, the open vinyl cation has a linear sp-hybridized
structure and can react with an arene at both sides of the vacant p orbital. Accordingly, in the case of
p-methoxyphenylacetylene, a mixture of E- and Z-isomers is formed. Benzoate anion can trap a proton
generated by aromatic electrophilic substitution and prevent the reaction from occurring by
protonation.
Scheme 7. Proposed mechanism of iodoarylation reaction of alkynes.

PhI
I2
+
PhI(OCOPh)2
2 IOCOPh
Ar2
H
Ar1
Ar2
I
I
I
OCOPh
IOCOPh
Ar1

PhCOOH
Ar1
Ar2
H
Ar1
Ar2
I
Ar2
Ar1
I
Ar1
+
IOCOPh

PhCOOH
OCOPh
3. Conclusions
We have demonstrated that arylacetylenes undergo iodoarylation reaction in the presence of a
simple reagent system composed of I2 and PhI(OCOPh)2. The iodoarylation reaction of most
arylacetylenes with electron-rich arenes proceeds regio- and stereoselectively to give trans 1,1-diaryl-
2-iodoethene adducts in good to high yields. In the case of p-methoxyphenylacetylene, the
iodoarylation reaction affords a mixture of E- and Z-isomers of 1,1-diaryl-2-iodoethenes 9. The
procedure involves a simple and convenient reagent system and provides synthetically valuable
iodoalkenes.

Molecules 2009, 14
3139
4. Experimental
4.1. General
All solvents and starting materials were used as received without further purification unless
1
otherwise indicated. H-NMR (300 MHz) and 13C-NMR (75 MHz) were recorded on a JEOL JNM-Al
300 FT-NMR spectrometer in CDCl3 solution (TMS as an internal standard). Melting points were
measured with a YANACO micro melting apparatus and are uncorrected. Column chromatographic
separations were carried out using silica gel as the stationary phase. Pre-coated plates (silica gel 60
F254, MERCK) were used for TLC examination. All [bis(acyloxy)iodo]benzenes were prepared using a
literature procedure [21]. Elemental analysis was performed by the Service Center of the Elementary
Analysis of Organic Compounds, Faculty of Science, Kyushu University.
4.2. General procedure for the iodoarylation of alkynes
A mixture of an arene (10 mmol), an arylacetylene (1 mmol), I2 (1.25 mmol), PhI(OCOPh)2
(3 mmol) and MeCN (6 mL) was placed in a 25 mL round-bottom flask. The reaction mixture was
stirred for about 5 min at room temperature and then refluxed at 82 ºC with stirring until the
completion of the reaction. The reaction mixture was dissolved in CH2Cl2, (20 mL) and water (20 mL)
was added to the CH2Cl2 solution. The mixture was then washed with 1M aqueous sodium
thiosulphate solution to remove the unreacted iodine. The aqueous reaction mixture was extracted with
CH2Cl2 (4 × 10 mL) and the CH2Cl2 extract was dried over anhydrous sodium sulfate. Finally, the
solvent was removed under reduced pressure below 40 ºC. Individual pure compounds were isolated
from the reaction mixture by column chromatography on silica gel using hexane and CH2Cl2 as eluent.
Data of 1,1-diaryl-2-iodoethenes 3a-e, 5a-e, and 12a-e were previously reported in the literature [7].
1-Iodo-2-(pentamethylphenyl)-1,2-diphenylethene (7a): Pale yellow crystalline solid; mp 194.5-
1
197 °C; H-NMR  7.44-6.99 (m, 10H, ArH), 2.20 (s, 6H, 2xMe), 2.08 (s, 3H, Me), 2.03 (s, 6H,
13
2xMe); C-NMR  149.34, 144.94, 144.66, 137.66, 133.88, 132.35, 130.48, 129.37, 128.35, 127.64,
127.40, 127.31, 100.50, 18.80, 16.67, 16.49 (one peak overlapped); Anal. calcd. for C25H25I: C, 66.38;
H, 5.57. Found: C, 66.34, H, 5.59.
1-Iodo-1,2-diphenyl-2-(2,4,6-trimethylphenyl)ethene (7b): Pale yellow crystalline solid; mp 154-
1
156 °C; H-NMR  7.42-7.06 (m, 10H, ArH), 6.64 (s, 2H, ArH), 2.16 (s, 6H, 2x Me), 2.13 (s, 3H,
Me); 13C-NMR  147.55, 144.50, 144.19, 137.29, 136.77, 135.40, 129.52, 128.93, 128.39, 127.66,
127.60, 127.56, 127.42, 101.39, 20.92, 20.75; HRMS-EI: m/z calcd. for C23H21I [M]+: 424.0688;
found: 424.0689.
1-Iodo-2-(4-methoxyphenyl)-2-(pentamethylphenyl)ethene (9a): Pale yellow crystalline solid; mp 110-
1
111.5 °C; a mixture of E- and Z-isomers (92:8); H-NMR  7.42 (d, J = 9.0 Hz, ArH), 7.14 (d,
J = 9.0 Hz, ArH), 7.02 (s, =CH), 6.82 (d, J = 9.0 Hz, ArH), 6.76 (d, J = 9.0 Hz, ArH), 6.24 (s, =CH),
3.78 (s, OMe), 3.76 (s, OMe), 2.28 (s, Me), 2.23 (s, 2xMe), 2.19 (s, 2xMe), 2.14 (s, Me), 2.04 (s,
2xMe); 13C- NMR  159.29, 159.01, 152.24, 150.80, 139.69, 138.94, 134.20, 132.68, 132.46, 132.09,

Molecules 2009, 14
3140
131.34, 130.45, 130.30, 130.18, 129.52, 127.65, 113.84, 113.07, 79.28, 75.80, 55.20, 55.14, 18.06,
17.60, 16.94, 16.89, 16.80, 16.58; HRMS-EI: m/z calcd. for C20H23IO [M]+: 406.0794; found:
406.0791.
1-Iodo-2-(4-methoxyphenyl)-2-(2,4,6-trimethylphenyl)ethene (9b): Pale yellow highly viscous liquid; a
mixture of E- and Z-isomers (87:13); 1H-NMR  7.36-6.76 (m, ArH and =CH), 3.77 (s, 2xOMe), 2.33
(s, Me), 2.27 (s, Me), 2.21 (s, 2xMe), 2.06 (s, 2xMe); HRMS-EI: m/z calcd. for C18H19IO [M]+:
378.0481; found: 378.0479.
1-Iodo-2-(4-methoxyphenyl)-2-(2,3,5,6-tetramethylphenyl)ethene (9c): Pale yellow crystalline solid;
1
mp 108-110 °C; a mixture of E- and Z-isomers (51:49); H-NMR  7.36-6.77 ( m, ArH and =CH),
3.78 (s, 2xOMe), 2.25 (s, 2xMe), 2.21 (s, 2xMe), 2.14 (s, 2xMe), 1.99 (s, 2xMe); 13C-NMR  159.37,
159.06, 157.94, 151.51, 144.75, 141.42, 134.16, 133.68, 133.40, 131.69, 131.05, 130.91, 130.83,
130.51, 130.15, 127.59, 113.89, 113.12, 79.04, 55.22, 55.16, 20.12, 20.11, 16.47, 15.82; HRMS-EI:
m/z calcd. for C19H21IO [M]+: 392.0637; found: 392.0637.
1-(3-Bromo-2,4,6-trimethylphenyl)-2-iodo-1-(4-methoxyphenyl)ethene (9d): Pale yellow highly
viscous liquid; a mixture of E- and Z-isomers (58:42); 1H-NMR  7.81-6.68 ( m, ArH and =CH), 3.86
(s, OMe), 3.79 (s, OMe), 2.45 (s, Me), 2.38 (s, Me), 2.37 (s, Me), 2.21 (s, Me), 2.16 (s, Me), 2.01 (s,
Me); HRMS-EI: m/z calcd. for C18H18BrIO [M]+: 455.9586; found: 455.9583.
1-(2,5-Dimethylphenyl)-2-iodo-1-(4-methoxyphenyl)ethene (9e): Pale yellow highly viscous liquid; a
mixture of E- and Z-isomers (60:40), 1H-NMR  7.76-6.73 (m, ArH and =CH), 3.86 (s, OMe), 3.79 (s,
OMe), 2.33 (s, Me), 2.32 (s, Me), 2.21 (s, 2xMe); 13C-NMR  159.08, 157.55, 157.22, 144.44, 143.91,
139.27, 136.44,135.76, 131.30, 130.55, 130.43, 129.98, 129.93, 129.02, 128.78, 128.39, 113.40,
110.02, 85.30, 77.21, 56.33, 55.20, 20.97, 19.14, 19.09, 17.74; HRMS-EI: m/z calcd. for C17H17IO
[M]+: 364.0324; found: 364.0327.
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Sample Availability: Samples of the compounds 3a-e, 5a-e, 9a-e and 12a-e are available from the
authors.
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