Synthesis of a new hypervalent iodine compound, [2-(hydroxydimethylsilyl)phenyl](phenyl)iodonium triflate as a convenient approach to benzyne

Article abstract of DOI:10.1016/S0040-4039(00)01128-X

A new benzyne precursor, [2-(hydroxydimethylsilyl)phenyl](phenyl)iodonium triflate, is prepared from 1,2-dibromobenzene in good yield. This procedure avoids the use of carcinogenic HMPA and the severe reaction conditions. The reaction of the benzyne precursor with Bu₄NF in the presence of a trapping agent under very mild conditions efficiently generates benzyne and gives the benzyne adduct in high yield. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01128-X

Tetrahedron Letters 41 (2000) 6611±6614
Synthesis of a new hypervalent iodine compound,
[2-(hydroxydimethylsilyl)phenyl](phenyl)iodonium tri¯ate
as a convenient approach to benzyne
Tsugio Kitamura,* Zhaohong Meng and Yuzo Fujiwara
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Hakozaki,
Fukuoka 812-8581, Japan
Received 24 May 2000; revised 19 June 2000; accepted 30 June 2000
Abstract
A new benzyne precursor, [2-(hydroxydimethylsilyl)phenyl](phenyl)iodonium tri¯ate, is prepared from
1,2-dibromobenzene in good yield. This procedure avoids the use of carcinogenic HMPA and the severe
reaction conditions. The reaction of the benzyne precursor with Bu4NF in the presence of a trapping agent
under very mild conditions eciently generates benzyne and gives the benzyne adduct in high yield. # 2000
Elsevier Science Ltd. All rights reserved.
Benzyne (1) is an important reactive intermediate which has been widely used in organic
syntheses, mechanistic studies, and the synthesis of functional materials.1 Although many
benzyne precursors have been reported so far, there are only a few examples2,3 of stable
precursors that can be used under very mild reaction conditions that meet the requirements of
ambient temperature and neutral conditions.
Very recently we have reported a trimethylsilyl-substituted phenyliodonium salt as a stable
benzyne precursor that generates benzyne quantitatively in the reaction with Bu4NF under mild
conditions (room temperature and neutral condition), as shown in Scheme 1.3 This benzyne
precursor is very convenient because it is a stable crystalline compound and easy to handle.
Scheme 1.
* Corresponding author. Tel: +81 92 642 3549; fax: +81 92 651 5606; e-mail: tkitatcf@mbox.nc.kyushu-u.ac.jp
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01128-X

6612
The synthesis of the benzyne precursor consists of two reaction steps: (1) 1,2-bis-
trimethylsilylation of 1,2-dichlorobenzene with chlorotrimethylsilane and Mg by use of HMPA as
the solvent; and (2) phenyliodination of the prepared 1,2-bis(trimethylsilyl)benzene with a
PhI(OAc)2/TfOH reagent system.
However, the synthesis of 1,2-bis(trimethylsilyl)benzene su€ers the following disadvantages: (1)
the use of carcinogenic HMPA as the solvent is necessary; and (2) the reaction requires severe
conditions (high temperature and long reaction time). We have developed a new stable hypervalent
iodine±benzyne precursor that can be prepared without the use of a high temperature and HMPA
solvent.4
The new approach was undertaken with the bisdimethylsilylation of 1,2-dibromobenzene to
avoid the use of HMPA. 1,2-Bis(dimethylsilyl)benzene was prepared by the reaction of 1,2-
dibromobenzene with chlorodimethylsilane and Mg in THF according to the literature (Scheme 2).5
The hydrolysis of 1,2-bis(dimethylsilyl)benzene with MeOH and H2O in the presence of NaOMe
gave 1,3-dihydro-1,1,3,3-tetramethyl-2,1,3-benzoxadisilole (2)5 in a high yield.
Scheme 2.
Phenyliodination of benzoxadisilole 2 was conducted with a hypervalent iodine reagent system,
PhI(OAc)2/TfOH.6 Treatment of benzoxadisilole 2 with PhI(OAc)2/TfOH gave [2-(hydroxy-
dimethylsilyl)phenyl](phenyl)iodonium tri¯ate (3)7 as crystals in 87% yield (Eq. (1)). This 2-
(hydroxydimethylsilyl)phenyliodonium tri¯ate 3 was quite stable at room temperature up to
melting point, not sensitive to air and moisture, and not hygroscopic.
ꢀ1†
The generation and trapping reaction of benzyne 1 could be conducted by simply mixing the
precursor 3 with a ¯uoride source in the presence of trapping agents (Eq. (2)). The trapping
reaction of benzyne 1 with furan was carried out under various conditions. The benzyne adduct,
1,4-dihydro-1,4-epoxynaphthalene (4),8 was obtained in good to high yields. The results are given
in Table 1.
Table 1 shows that 2-(hydroxydimethylsilyl)phenyliodonium tri¯ate 3 behaves well as the ben-
zyne precursor. The best result (98% yield of benzyne adduct 4) was obtained when the reaction
was carried out by the use of Bu4NF as the ¯uoride source, CH2Cl2 as the solvent, and 5 equiv. of
furan. This result is the same as the previous one by the trimethylsilyl-substituted phenyl-
iodonium salt.3 Cesium ¯uoride also acts as a good desilylating agent and gives the benzyne
adduct 4 in 85% yield.

6613
ꢀ2†
Table 1
Generation of benzyne 1 and trapping with furana
Similar reactions of 2-(hydroxydimethylsilyl)phenyliodonium tri¯ate 3 with Bu4NF in the
presence of other trapping agents were conducted (Scheme 3). In the presence of tetraphenyl-
cyclopentadienone, the benzyne adduct, 1,2,3,4-tetraphenylnaphthalene (5),9 was obtained in
72% yield when the benzyne precursor 3 was reacted with Bu4NF in CH2Cl2. The benzyne 1
generated similarly from the precursor 3 was trapped with 1,3-diphenylisobenzofuran to give
9,10-dihydro-9,10-o-benzenoanthracene (6)10 in 78% yield.
Scheme 3.

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The above experiments illustrate that 2-(hydroxydimethylsilyl)phenyliodonium tri¯ate 3
generates benzyne 1 with a high eciency under mild conditions. In addition, the present benzyne
precursor 3 has a preferable procedure of the synthesis compared with the previously reported
2-(trimethylsilyl)phenyliodonium tri¯ate.3
In conclusion, we have developed a new approach to an excellent benzyne precursor. A new
benzyne precursor, 2-(hydroxydimethylsilyl)phenyliodonium tri¯ate 3, is readily prepared via the
bisdimethylsilylation of 1,2-dibromobenzene followed by the hydrolysis to benzoxadisilole 2 and
the phenyliodination. This procedure avoids the use of carcinogenic HMPA and the severe
reaction conditions required in the synthesis of [2-(trimethylsilyl)phenyl](phenyl)iodonium
tri¯ate. The new benzyne precursor 3 gives benzyne adducts in high yields in the reaction with
Bu4NF in the presence of trapping agents under very mild conditions.
Acknowledgements
We thank the Ministry of Education, Science, Sports and Culture, Japan, for ®nancial support
(Grants-in-Aid for Scienti®c Research).
References
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1
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