Application of iodanes to the dimerization of terminal alkynes

Article abstract of DOI:10.1055/s-2007-966008

The dimerization of terminal acetylenes was studied using iodanes as oxidants under palladium-catalyzed conditions. It was found that a number of iodanes were useful in the reaction, with (diacetoxyiodo)benzene and iodosylbenzene being the best oxidants.

Full text of DOI:10.1055/s-2007-966008

SHORT PAPER
1301
Application of Iodanes to the Dimerization of Terminal Alkynes
A
pplicatio
i
n of
I
o
e
danes in Dimeriza
Y
tion an,*a Feng Lin,b Zhenping Yanga
a
College of Chemical Engineering and Materials Sciences, Zhejiang University of Technology, Hangzhou 310032, Zhejiang,
P. R. of China
Fax +86(571)88320238; E-mail: jieyan87@zjut.edu.cn
Department of Chemistry, Zhejiang University, Hangzhou 310027, Zhejiang, P. R. of China
b
Received 8 January 2007; revised 26 February 2007
PdCl2, CuI, Et3N, THF
PhI(OAc)2
R
R
Abstract: The dimerization of terminal acetylenes was studied us-
ing iodanes as oxidants under palladium-catalyzed conditions. It
was found that a number of iodanes were useful in the reaction, with
(diacetoxyiodo)benzene and iodosylbenzene being the best oxi-
dants. Diynes were prepared in the dimerization in good yields in a
short period of time at room temperature.
R
Scheme 1
In order to extend the scope of the dimerization and to find
more mild, efficient oxidants, we investigated other hy-
pervalent iodine compounds as oxidants in the dimeriza-
tion of phenylacetylene. It was found that a number of
iodanes were useful oxidants in the dimerization under the
following reaction conditions:11 phenylacetylene (1.0
mmol), iodane (0.6 mmol), triethylamine (1.1 mmol), pal-
ladium(II) chloride (2.0 mol%) and copper(I) iodide (2.0
mol%) stirred for 30 minutes in tetrahydrofuran (3 mL).
When catalytic amounts of triphenylphosphine was added
to the mixture in a triphenylphosphine-to-palladium(II)
chloride ratio of 3:1, the yields of the dimerization were
almost quantitative when DIB or iodosylbenzene were
used. Under the optimum conditions, the dimerization of
phenylacetylene with a range of iodanes is shown in
Scheme 2 and the results are summarized in Table 1.
Key words: dimerization, diyne, iodane, (diacetoxyiodo)benzene,
iodosylbenzene
Diynes are very important compounds in terms of their
chemistry and the solid state properties of their homopoly-
mers.1 This type of compound can be synthesized by the
traditional Glaser oxidative dimerization of terminal
acetylenes2 and can also can be formed by Pd(0)–Cu(I)
catalyzed self-coupling of terminal alkynes in the pres-
ence of chloroacetone,3 ethyl bromoacetate,4 ally
bromide5 and iodine.6 The dimerization of terminal alky-
nyl halides with terminal alkynes is another route to
diynes under palladium-catalyzed conditions.7 Fairlamb
et al. reported the preparation of diynes under standard
Sonogashira cross-coupling conditions in the absence of
an obvious oxidant.8 Because palladium-catalyzed homo-
coupling of alkynes is quick, simple, mild and environ-
mentally more benign, research towards exploring the
scope of its use is still required.
PdCl2, CuI, PPh3, Et3N
Ph
Ph
Ph
iodane, THF
Scheme 2
Our recent interest has been in the development of new
synthetic methods using hypervalent iodine compounds
because their chemical properties and reactivity are simi-
lar to those of Hg(II), Tl(III) and Pb(IV), but without the
toxic and environmental problems associated with such
heavy metal congeners. (Diacetoxyiodo)benzene (DIB) is
one of the most important, well-studied and practically
useful, commercially available hypervalent iodine com-
pounds.9 As a general, universal oxidizing reagent, DIB
has been widely used for the oxidation of, for example,
phenols, enolizable ketones, alkenes and alkoxyallenes.10
Recently, we investigated the dimerization of terminal
acetylenes using DIB as an oxidant and found that this re-
action could be conveniently carried out in the presence of
catalytic amounts of palladium(II) chloride and copper(I)
iodide in tetrahydrofuran (Scheme 1).11
Table 1 Dimerization of Phenylacetylene with Iodanes
Entry
Iodane
Yield (%)a
1
2
3
4
PhI(OAc)2
PhI=O
96
96
92
72
p-ClC6H4I(OAc)2
OH
Ph
I
OTs
OH
5
68
O
C
O
6
PhI(CO2CF3)2
58
a Isolated yields.
SYNTHESIS 2007, No. 9, pp 1301–1303
x
x
.
x
x
.2
0
0
7
Advanced online publication: 05.04.2007
DOI: 10.1055/s-2007-966008; Art ID: P00207SS
© Georg Thieme Verlag Stuttgart · New York

1302
J. Yan et al.
SHORT PAPER
R
R
As can be seen in Table 1, yields were almost quantitative
when DIB or iodosylbenzene were used as oxidants (en-
tries 1 and 2), while when (diacetoxyiodo)-4-chloroben-
zene was used, the yield of 92% was somewhat lower
(entry 3). Only a moderate yield of 58% was obtained us-
ing [bis(trifluoroacetoxy)iodo]benzene as oxidant due to
partial decomposition of the reagent during the reaction
(entry 6). Two further iodanes used in the reaction gave
the product diphenylbutadiyne in good yields.
Pd (0)
PhI(OAc)2
PhI
R
R
Pd
Pd(OAc)2
Et3N⋅HCl
R
The excellent yields obtained confirmed that iodosylben-
zene was comparable to DIB as an oxidizing agent for the
palladium-catalyzed dimerization of terminal alkynes.
Under the optimum conditions described above, we stud-
CuI, Et3N
PdCl2
CuI, Et3N
R
Et3N⋅HOAc
ied the dimerization of a series of terminal alkynes using Scheme 3
iodosylbenzene as an oxidant (Table 2). It can be seen that
the dimerization of terminal aromatic alkynes gave diynes
In conclusion, the dimerization of terminal acetylenes was
studied, using iodanes as oxidants, under palladium-cata-
lyzed conditions. (Diacetoxyiodo)benzene and iodosyl-
benzene were found to be the best oxidants. Following
this protocol, diynes were prepared in good to excellent
yields rapidly, at room temperature, using iodosylbenzene
as oxidant.
in excellent yields (entries 1–3), while when aliphatic
alkynes were used, the yields were somewhat lower (en-
tries 4–6). The reduction in the yields and increased reac-
tion times required for the dimerization of aliphatic,
compared with aromatic alkynes, presumably stems from
the lower acidity of the acetylinic proton.12
Table 2 Dimerization of Terminal Alkynes using Iodosylbenzene
as Oxidant
1
IR spectra were recorded on a FT-170 SX instrument. H NMR
spectra (400 MHz) were measured on a Bruker AM-400 FT-NMR
spectrometer. Mass spectra were determined on a HP5989A mass
spectrometer. Iodanes were prepared according to literature proce-
dures.13–17 All terminal alkynes are commercially available.
PdCl2, CuI, PPh3, Et3N
R
R
R
PhI=O, THF
1
2
Entry
1
Alkyne
Diyne
Time (h) Yield (%)a
Terminal Alkyne Dimerization; General Procedure
2a
0.5
1.0
96
82
Ph
To a mixture of alkyne 1 (1.0 mmol), iodane (0.6 mmol), Et3N (1.1
mmol), PdCl2 (2.0 mol%), CuI (2.0 mol%) and PPh3 (6.0 mol%),
THF (3mL) was added. The mixture was stirred at r.t. for 0.5–2.0 h
and then separated on a silica gel plate using petroleum ether as sol-
vent. Diyne 2 was afforded in good to excellent yield.
1a
p-MeC6H4
2
2b
1b
p-EtC6H4
3
4
5
6
2c
2d
2e
2f
1.0
2.0
2.0
2.0
84
80
70
72
Diphenylbutadiyne (2a)6
1c
n-Bu
IR (film): 3049, 2978, 2145, 1261, 730 cm–1.
1H NMR (CDCl3): d = 7.32–7.36 (m, 6 H), 7.50–7.55 (m, 4 H).
MS (EI, 70 eV): m/z (%) = 202 (100) [M+].
1d
n-C5H11
1e
n-C6H13
Bis(4-methylphenyl)butadiyne (2b)18
IR (film): 3060, 2988, 2133, 1267, 751 cm–1.
1f
a Isolated yields.
1H NMR (CDCl3): d = 2.39 (s, 6 H), 7.25 (d, J = 11.0 Hz, 4 H), 7.42
(d, J = 11.0 Hz, 4 H).
MS (EI, 70 eV): m/z (%) = 230 (100) [M+].
The proposed mechanism for the dimerization of alkynes
is shown in Scheme 3, using DIB as a representative io-
dane. Firstly, the terminal alkyne reacted with palladi-
um(II) chloride to form the dialkynylpalladium, in the
presence of copper(I) iodide and triethylamine, which is
then transformed to the diyne via a reductive elimination.
DIB then regenerates the Pd(II) catalyst by oxidative ad-
dition of the Pd(0) formed in the catalytic cycle, making
further reductive elimination possible.
Bis(4-ethylphenyl)butadiyne (2c)4
IR (film): 3051, 2991, 2152, 1268, 743 cm–1.
1H NMR (CDCl3): d = 1.23 (t, J = 7.6 Hz, 6 H), 2.65 (q, J = 7.6 Hz,
4 H), 7.14 (d, J = 8.0 Hz, 4 H), 7.42 (d, J = 8.0 Hz, 4 H).
MS (EI, 75 eV): m/z (%) = 258 (100) [M+].
5,7-Dodecadiyne (2d)18
IR (film): 2966, 2140, 1259, 736 cm–1.
1H NMR (CDCl3): d = 0.90 (t, J = 7.2 Hz, 6 H), 1.36–1.44 (m, 4 H),
1.45–1.54 (m, 4 H), 2.24 (t, J = 6.8 Hz, 4 H).
MS (EI, 70 eV): m/z (%) = 162 (100) [M+].
Synthesis 2007, No. 9, 1301–1303 © Thieme Stuttgart · New York

SHORT PAPER
Application of Iodanes in Dimerization
1303
6,8-Tetradecadiyne (2e)18
(8) Fairiamb, I. J. S.; Bauerlein, P. S.; Marrison, L. R.;
IR (film): 2971, 2152, 1256, 744 cm–1.
Dickinson, J. M. Chem. Commun. 2003, 632.
(9) (a) Varvoglis, A. Hypervalent Iodine in Organic Synthesis;
Academic Press: London, 1997. (b) Varvoglis, A.
Tetrahedron 1997, 53, 1179. (c) Stang, P. J.; Zhdankin, V.
V. Chem. Rev. 1996, 96, 1123. (d) Zhdankin, V. V.; Stang,
P. J. Chem. Rev. 2002, 102, 2523. (e) Ochiai, M. In
Chemistry of Hypervalent Compounds; Akibe, K., Ed.;
Wiley-VCH: Weinheim, 1999, Chap. 13, 359. (f) Ochiai,
M. J. Organomet. Chem. 2000, 611, 494. (g) Okuyama, T.
Acc. Chem. Res. 2002, 35, 12. (h) Grushin, V. V. Chem.
Soc. Rev. 2000, 29, 315.
(10) (a) Prakash, O.; Saini, N.; Sharma, P. K. Synlett 1994, 221.
(b) Wipf, P.; Kim, Y.; Fritch, P. C. J. Org. Chem. 1993, 58,
7195. (c) Hara, H.; Inoue, T.; Nakamora, H.; Endoh, M.;
Hoshino, O. Tetrahedron Lett. 1992, 34, 6419.
(d) Moriarty, R. M.; Hopkins, T. E.; Vaid, R. K.; Vaid, B. K.;
Levy, S. G. Synthesis 1992, 847. (e) Pelter, A.; Elgendy, S.
M. A. J. Chem. Soc., Perkin Trans. 1 1993, 1891.
(f) Mariarty, R. M.; Penmasta, R.; Awasthi, A. K.; Prakash,
I. J. Org. Chem. 1988, 53, 6124.
1H NMR (CDCl3): d = 0.89 (t, J = 7.2 Hz, 6 H), 1.24–1.40 (m, 8 H),
1.48–1.55 (m, 4 H), 2.24 (t, J = 7.2 Hz, 4 H).
MS (EI, 70 eV): m/z (%) = 190 (100) [M+].
7,9-Hexadecadiyne (2f)19
IR (film): 2982, 2133, 1263, 741 cm–1.
1H NMR (CDCl3): d = 0.89 (t, J = 7.2 Hz, 6 H), 1.18–1.40 (m,
12 H), 1.45–1.56 (m, 4 H), 2.22 (t, J = 6.8 Hz, 4 H).
MS (EI, 70 eV): m/z (%) = 218 (100) [M+].
Acknowledgment
Financial support from the Natural Science Foundation of China
(Project 20672100) is greatly appreciated.
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Synthesis 2007, No. 9, 1301–1303 © Thieme Stuttgart · New York