An alternative synthesis of alkynyl(phenyl)iodonium triflates using (diacetoxyiodo)benzene and alkynylsilanes

Article abstract of DOI:10.1055/s-1998-2171

Phenyl(trimethylsilylethynyl)iodonium triflate (1a) was prepared in high yields (76-88%) by using the PhI(OAc)₂/TfOH or PhI(OAc)₂/Tf₂O reagent systems. This procedure was also applied to the preparation of other alkynyliod

Full text of DOI:10.1055/s-1998-2171

SHORT PAPERS
1416
An Alternative Synthesis of Alkynyl(phenyl)iodonium Triflates Using
(Diacetoxyiodo)benzene and Alkynylsilanes
Tsugio Kitamura,* Masashi Kotani, Yuzo Fujiwara
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Hakozaki, Fukuoka 812-81, Japan
Fax +81(92)6515606; E-mail: tkitatcf@mbox.nc.kyushu-u.ac.jp
Received 4 February 1998; revised 30 March 1998
Abstract: Phenyl(trimethylsilylethynyl)iodonium triflate (1a) was
prepared in high yields (76–88%) by using the PhI(OAc)2/TfOH
or PhI(OAc)2/Tf2O reagent systems. This procedure was also
The PhI(OAc)2/TfOH reagent system was prepared by the
reaction of PhI(OAc)2 with TfOH. When a suspension of
PhI(OAc)2 in CH2Cl2 was treated with double the molar
amount of TfOH, it turned to a clear yellow solution indi-
cating the formation of reactive iodine species. To the so-
lution were added alkynylsilanes at 0 ˚C and the corre-
sponding alkynyl(phenyl)iodonium triflates 1 were ob-
tained as crystals after evaporation of the solvent and
successive crystallization (eq 2). In the case of bis(tri-
methylsilyl)acetylene, alkynyliodonium triflate 1a was
obtained in high yield (76%) (Table, entry 1) but the reac-
tion of trimethyl(phenylethynyl)silane gave alkynyliodo-
nium triflate 1b in 40% yield (Table, entry 2).
applied to the preparation of other alkynyliodonium triflates 1.
Key words: trimethylsilylethynyliodonium triflate, alkynyliodo-
nium triflates, (diacetoxyiodo)benzene, alkynylsilanes, triflic
anhydride
Alkynyl(phenyl)iodonium triflates 1 have been recently
recognized as versatile compounds in organic synthesis.1
They act as Michael acceptors, as synthons for alkynyl
cations, and as 1,3-dipolarophiles. When alkynyliodoni-
um triflates 1 are reacted with nucleophiles, alkylidene-
carbenes are generated as the reactive intermediates and
generally undergo 1,5-C–H insertion to give cyclopentene
derivatives or 1,2-migration to alkynes. In particular,
phenyl(trimethylsilylethynyl)iodonium triflate (1a) is ex-
pected to have high utility in organic synthesis concerning
useful silicon chemistry.2 Recently, Bachi et al. have re-
ported the application to the synthesis of functionalized
alkynyl-β-lactams.3
Next, PhI(OAc)2 was treated with triflic anhydride (Tf2O)
to make a mild reagent system. We reported that
self-condensation occurs on the reaction of PhIO with
double the molar quantity of TfOH to give
PhI(OH)C6H4I(OH)OTf.10 On the other hand, self-con-
densation was suppressed by the use of Tf2O. The reaction
of PhIO with half the molar amount of Tf2O leads to the
formation of Zefirov’s reagent, PhI(OTf)OI(OTf)Ph.11
These results mean that TfOH activates an iodine reagent
more strongly than Tf2O. This consideration suggests that
the replacement of TfOH by Tf2O leads to the formation of
a relatively mild iodine reagent system, PhI(OAc)2/Tf2O.
Alkynyl(phenyl)iodonium salts have been prepared, so
far, from terminal alkynes, alkynylsilanes, and alkynyl-
stannanes. In these preparations, various hypervalent io-
dine reagents have been used, for example, (di-
chloroiodo)benzene (PhICl2),4 iodosylbenzene (PhIO),5
[hydroxy(tosyloxy)iodo]benzene [Koser’s salt, PhI(OH)-
OTs],6 µ-oxobis[(phenyl)(trifluoromethylsulfonyloxy)-
iodine] (Zefirov’s reagent, PhI(OTf)OI(OTf)Ph],7 and
[(cyano)(trifluoromethylsulfonyloxy)iodo]benzene
[Stang’s reagent, PhI(CN)OTf].8
Thus, we conducted the reaction using Tf2O instead of
TfOH. PhI(OAc)2 was treated with half the molar quantity
of Tf2O at 0 ˚C in CH2Cl2 and reacted with alkynyl(tri-
methyl)silanes. After evaporation and crystallization, alky-
nyl(phenyl)iodonium triflates 1 were obtained as crystals
(eq 3). The results are given in the Table (entries 3–6). As
seen from the Table, the preparation of alkynyliodonium
triflate 1a has been improved by using the PhI(OAc)2/
Tf2O reagent system. This simple procedure was also ap-
plied to the preparation of alkynyliodonium triflates 1b–d.
Alkynyl(phenyl)iodonium triflates 1 have been prepared
by reaction with Zefirov’s and Stang’s reagents. These re-
agents are very useful for this purpose, but the preparation
of these reagents requires some special precautions be-
cause of their instability. (Diacetoxyiodo)benzene,
PhI(OAc)2, is the most stable hypervalent iodine reagent
and is commercially available. The use of PhI(OAc)2 in
the preparation of alkynyliodonium triflates 1 is syntheti-
cally preferable and convenient. Previously we have
found that a hypervalent iodine reagent prepared from
PhI(OAc)2 and triflic acid (TfOH) is highly reactive to-
ward aromatic compounds and is useful for the prepara-
tion of diaryliodonium triflates (eq 1).9 Thus, we exam-
ined the application of the PhI(OAc)2/TfOH reagent sys-
tem to the preparation of alkynyliodonium triflates 1.

SYNTHESIS
October 1998
1417
Table. Preparation of Alkynyl(phenyl)iodonium Triflates 1
reagent was added bis(trimethylsilyl)acetylene (8.60 g, 50 mmol) at
0°C and the mixture was stirred for 2 h. After the same workup of
the mixture, 1a was obtained; yield: 15.57 g (88%). Other alky-
nyl(phenyl)iodonium triflates 1b–d7 were obtained in a similar man-
ner.
Entry Alkynylsilane
Reagent System
Product Yield
(%)
1
2
3
4
5
6
Me3SiC≡CSiMe3
PhC≡CSiMe3
PhI(OAc)2/TfOH
PhI(OAc)2/TfOH
PhI(OAc)2/Tf2O
PhI(OAc)2/Tf2O
PhI(OAc)2/Tf2O
PhI(OAc)2/Tf2O
1a
1b
1a
1b
1c
1d
76
40
88
56
41
43
Phenyl[2-(trifluoromethylsulfonyloxy)hex-1-enyl]iodonium Tri-
flate (2); Reaction of Hex-1-yne with the PhI(OAc)2/Tf2O Reagent
System:
Me3SiC≡CSiMe3
PhC≡CSiMe3
tBuC≡CSiMe3
nBuC≡CSiMe3
To a stirred suspension of PhI(OAc)2 (0.652 g, 2.0 mmol) in CH2Cl2
(5 mL) was added Tf2O (0.282 g, 1.0 mmol) dropwise at 0°C. After
stirring for 30 min, hex-1-yne (0.196 g, 2.0 mmol) was added and the
mixture was stirred for 2 h. After evaporation of the solvent, Et2O was
added to crystallize the product. The crystals were collected, washed
with Et2O, and dried in vacuo to give 2; yield: 0.713 g (61%); mp
124–128°C (Lit.12 mp 125–128°C).
In the reaction with the PhI(OAc)2/Tf2O reagent system,
we assume that a reactive species, such as PhI+OAc, is
generated and reacts with alkynylsilanes to give alkynyl-
iodonium triflates 1. When the PhI(OAc)2/Tf2O reagent
system was treated with hex-1-yne in order to confirm the
reactive species, phenyl[2-(trifluoromethylsulfonyloxy)-
hex-1-enyl]iodonium triflate (2)12 was obtained in 61%
yield (eq 4). This reaction suggests that the reactive spe-
cies generated in the present reagent system is different
from Zefirov’s reagent which affords an oxo-bridged
product 3 in the reaction with hex-1-yne.12
1H NMR (300 MHz, CDCl3): δ = 0.91 (t, J = 7 Hz, Me, 3 H), 1.30–
1.42 (m, CH2, 2 H), 1.47–1.57 (m, CH2, 2 H), 2.80 (t, J = 7 Hz, CH2,
2 H), 7.07 (s, =CH, 1 H), 7.48–7.53 (m, ArH, 2 H), 7.63–7.67 (m,
ArH, 1 H), 7.94–7.97 (m, ArH, 2 H).
13C NMR (75 Hz, CDCl3): δ = 13.52, 22.00, 27.95, 34.57, 92.31,
113.85, 132.33, 132.64, 134.62, 162.56.
The authors are grateful to Central Glass Co. (TfOH) and Shin-Etsu
Co. [bis(trimethylsilyl)acetylene] for a generous gift. This work was
partly supported by Grants-in-Aid for Scientific Research from the
Ministry of Education, Science, Sports and Culture, Japan.
(1) Kitamura, T.; Fujiwara, Y. Org. Prep. Proced. Int. 1997, 29,
409.
Varvoglis, A. Hypervalent Iodine in Organic Synthesis; Aca-
demic: San Diego, 1997.
Stang, P. J.; Zhdankin, V. V. Chem. Rev. 1996, 96, 1123.
Stang, P. J. In Modern Acetylene Chemistry; Stang, P. J.; Diede-
rich, F. Eds.; VCH: Weinheim, 1995, Chapter 3.
Kitamura, T. J. Synth. Org. Chem. Jpn. 1995, 53, 893.
Koser, G. F. In Supplement D2: The Chemistry of Halides,
Pseudo-Halides and Azides; Patai, S.; Rappoport, Z. Eds.; Wi-
ley: Chichester, 1995; Chapter 21.
Stang, P. J. In Supplement C 2: The Chemistry of Triple-Bonded
Functional Groups; Patai, S. Ed.; Wiley: Chichester, 1994;
Chapter 20.
In summary, we have found a simple one-pot synthesis of
alkynyliodonium triflates which uses PhI(OAc)2, a stable
and commercially available reagent. This method is espe-
cially useful for the preparation of trimethylethynyliodo-
nium triflate 1a which has potential synthetic utility.
Kita, Y.; Tohma, H.; Yakura, T. Trends Org. Chem. 1992, 3,
113.
Varvoglis, A. The Organic Chemistry of Polycoordinated Iodi-
ne; VCH: New York, 1992.
Phenyl(trimethylsilylethynyl)iodonium Triflate (1a); Typical
Procedure Using the PhI(OAc)2/TfOH Reagent System:
To a stirred suspension of PhI(OAc)2 (10.63 g, 33 mmol) in CH2Cl2
(50 mL) was added TfOH (5.4 mL, 60 mmol) dropwise at 0°C and the
mixture was stirred for 30 min. During this time, the mixture became
a clear yellow solution. To the resulting reagent was added bis(tri-
methylsilyl)acetylene (5.11 g, 30 mmol) at 0°C and the mixture was
stirred for 2 h. After evaporation of the solvent, Et2O was added to
crystallize the product. The crystals were collected, washed with
Et2O, and dried in vacuo to give 1a; yield: 10.33 g (76%); mp 143–
146°C (Lit.7 mp 138–139°C).
Stang, P. J. Angew. Chem. Int. Ed. Engl. 1992, 31, 274.
Moriarty, R. M.; Vaid, R. K.; Koser, G. F. Synlett 1990, 365.
Moriarty, R. M.; Vaid, R. K. Synthesis 1990, 431.
Ochiai, M. Rev. Heteroatom Chem. 1989, 2, 92.
Merkushev, E. B. Russ. Chem. Rev. (Eng. Transl.) 1987, 56,
826.
Ochiai, M.; Nagao, Y. J. Synth. Org. Chem. Jpn. 1986, 44, 660.
Moriarty, R. M.; Prakash, O. Acc. Chem. Res. 1986, 19, 244.
Varvoglis, V. Synthesis 1984, 709.
1H NMR (300 MHz, CDCl3): δ = 0.22 (s, Me, 9 H), 7.49–7.54 (m,
ArH, 2 H), 7.62–7.68 (m, ArH, 1 H), 8.06 (d, J = 8.7 Hz, ArH, 2 H).
13C NMR (75 Hz, DMSO-d6): δ = –0.96, 53.09, 113.25, 118.62,
132.00, 132.41, 134.72.
Koser, G. F. In The Chemistry of Functional Groups, Supple-
ment D; Patai, S.; Rappoport, Z. Eds.; Wiley: New York, 1983;
Chapters 18 and 25.
(2) Magnus, P. D.; Sarkar, T.; Djuric, S. Comprehensive Organo-
metallic Chemistry; Wilkinson, G.; Stone, G. G. A.; Abel, E. W.
Eds.; Pergamon: Oxford, 1982; Vol. 7; Chapter 48.
Colvin, E. W. Silicon in Organic Synthesis; Butterworth: Lon-
don, 1981.
Phenyl(trimethylsilylethynyl)iodonium Triflate (1a); Typical
Procedure Using the PhI(OAc)2/Tf2O Reagent System:
To a stirred suspension of PhI(OAc)2 (16.10 g, 50 mmol) in CH2Cl2
(50 mL) was added Tf2O (4.2 mL, 25 mmol) dropwise at 0°C and the
mixture was stirred for 30 min. To the resulting yellow solution of the
(3) Bachi, M. D.; Bar-Ner, N.; Stang, P. J.; Williamson, B. L.
J. Org. Chem. 1993, 58, 7923.

SYNTHESIS
Short Papers
1418
(4) Beringer, F. M.; Galton, S. A. J. Org. Chem. 1965, 30, 1930.
(5) Kitamura, T.; Stang, P. J. J. Org. Chem. 1988, 53, 4105.
Ochiai, M.; Kunishima, M.; Sumi, K.; Nagao, Y.; Fujita, E.;
Arimoto, M.; Yamaguchi, H. Tetrahedron Lett. 1985, 26, 4505.
(6) Stang, P. J.; Surber, B. W.; Chen, Z. C.; Roberts, K. A.; Ander-
son, A. G. J. Am. Chem. Soc. 1987, 109, 228.
(9) Kitamura, T.; Matsuyuki, J.; Taniguchi, H. Synthesis 1994, 147.
(10) Kitamura, T.; Furuki, R.; Nagata, K.; Zheng, L.; Taniguchi, H.
Synlett 1993, 193.
Kitamura, T.; Furuki, R.; Nagata, K.; Taniguchi, H.; Stang, P. J.
J. Org. Chem. 1992, 57, 6810.
(11) Hembre, R. T.; Scott, C. P.; Norton, J. R. J. Org. Chem. 1987,
52, 3652.
Stang, P. J.; Surber, B. W. J. Am. Chem. Soc. 1985, 107, 1452.
Rebrovic, L.; Koser, G. F. J. Org. Chem. 1984, 49, 2462.
Koser, G. F.; Rebrovic, L.; Wettach, R. H. J. Org. Chem. 1981,
46, 4324.
Zefirov, N. S.; Zhdankin, V. V.; Dankov, Yu. V.; Sorokin, V.
D.; Semerikov, V. N.; Kozmin, A. S.; Caple, R.; Berglund, B.
A. Tetrahedron Lett. 1986, 27, 3971.
(7) Bachi, M. D.; Bar-Ner, N.; Crittell, C. M.; Stang, P. J.; William-
son, B. L. J. Org. Chem. 1991, 56, 3912.
(8) Williamson, B. L.; Stang, P. J.; Arif, A. M. J. Am. Chem. Soc.
1993, 115, 2590.
Zefirov, N. S.; Zhdankin, V. V.; Dankov, Yu. V.; Kozmin, A. S.
J. Org. Chem. USSR (Engl. Transl.) 1984, 20, 401.
(12) Kitamura, T.; Furuki, R.; Taniguchi, H.; Stang, P. J. Tetrahe-
dron 1992, 48, 7149.
Stang, P. J.; Williamson, B. L.; Zhdankin, V. V. J. Am. Chem.
Soc. 1991, 113, 5870.