Cycloaddition reactions of benzynes generated from benzobisoxadisilole, benzotrisoxadisilole and naphthoxadisilole

Article abstract of DOI:10.1016/S0040-4039(02)00239-3

Benzynes (7, 16) generated from benzobisoxadisilole and benzotrisoxadisilole were trapped as the furan cycloaddition adducts. Naphthoxadisilole and naphthobisoxadisilole were also synthesized.

Full text of DOI:10.1016/S0040-4039(02)00239-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 2259–2262
Cycloaddition reactions of benzynes generated from
benzobisoxadisilole, benzotrisoxadisilole and naphthoxadisilole
†
Ya-Li Chen, Hong-Kui Zhang, Wai-Yeung Wong and Albert W. M. Lee*
‡
Department of Chemistry and Central Laboratory of the Institute of Molecular Technology for Drug Discovery and Synthesis,
Hong Kong Baptist University, Kowloon Tong, Hong Kong, China
Received 5 December 2001; revised 25 January 2002; accepted 31 January 2002
Abstract—Benzynes (7, 16) generated from benzobisoxadisilole and benzotrisoxadisilole were trapped as the furan cycloaddition
adducts. Naphthoxadisilole and naphthobisoxadisilole were also synthesized. © 2002 Elsevier Science Ltd. All rights reserved.
Benzynes are important reactive intermediates. They
have been the subject of theoretical interest and mecha-
nistic studies. Benzynes have also been widely used in
publication, this hypervalent iodine approach to ben-
zyne generation has caught the attention of many
research groups. Various applications, particularly in
the syntheses of theoretically interesting molecules and
1
organic syntheses of functional materials, natural prod-
2
4
ucts and theoretically interesting molecules. There are
functional materials have been reported. Later on, the
many known precursors for the generation of benzynes.
A recent addition, (phenyl)[o-(trimethylsilyl)phenyl]-
iodonium triflate 1a, was reported by Kitamura and
same team reported another new benzyne precursor,
[2-(hydroxydimethylsilyl)phenyl](phenyl)iodonium trifl-
ate 1b, which was prepared from 1,3-dihydro-1,1,3,3-
3
5
co-workers. The precursor of 1a is 1,2-bis(trimethyl-
tetramethyl-2,1,3-benzoxadisilole 3, which could be
silyl)benzene 2 which can be prepared from 1,2-
dichlorobenzene with the use of HMPA. Since its
readily synthesized from 1,2-bromobenzene without the
use of HMPA.
Me
Me
Si(Me) X
SiMe3
SiMe3
Si
Si
2
O
I(Ph)OTf
Me
Me Me Me
Me
Me
Si
O
Si
Si
Si
Y = H
7%
1
a X = Me 1b X = OH
2
3
O
8
X
Y
Y
Me
Me Me
Me
Br
Br
SiHMe2
SiHMe2
4
Br
Br
Me2HSi
Me2HSi
i
ii
Me
O
Me Si
Me
X = H 70%
X = Br 63%
Me
Me
O
X
Y = SiHMe2
2%
Si
Si
Me
Me
3
Si
Si
Me
Me
Me
Si
Me
O
Me
5
Scheme 1. Reagents and conditions: (i) Me HSiCl, Mg, THF, reflux; (ii) (a) CH OH/CH ONa, rt, (b) H O, 50°C.
2
3
3
2
Keywords: cycloaddition; Diels–Alder reaction; benzyne; benzobisoxadisilole; benzotrisoxadisilole; naphthoxadisilole.
*
Corresponding author. Tel.: 852-2339 7064; fax: 852-2339 7348; e-mail: alee@hkbu.edu.hk
This paper is dedicated to Professor Barry Sharpless on the occasion of his 60th birthday.
An Area of Excellence of the University Grants Committee (Hong Kong).
†
‡
0
040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00239-3

2
260
Y.-L. Chen et al. / Tetrahedron Letters 43 (2002) 2259–2262
In view of the relatively mild conditions for the genera-
tion of benzyne from the benzoxadisilole precursor 3
homogeneous reaction mixture was reacted with
another equivalent of iPr NH, nBu NF and excess
2
4
(
via 1b), we began a study of the generation of benzy-
furan. The Diels–Alder product 8 was formed in 63%
isolated yield. Repeating the same reaction sequence in
the absence of furan, dimer 9 of benzyne 7 was isolated
in 40% yield. In addition to standard spectroscopic
characterization, the structures of the cycloaddition
products 8 and 9 were further confirmed by X-ray
analyses (Fig. 1).
nes from benzobisoxadisilole 4 and benzotrisoxadisilole
. In particular, we wanted to discover whether bis-ben-
5
zynes or any other related intermediates could be
detected.
The syntheses of 4 and 5 according to the literature
6
procedures are outlined in Scheme 1. The X-ray struc-
7
ture of 4 had been reported. Herein, we report the first
It seems that only one of the oxadisilole rings of 4
could be open up to the benzyne precursor 6 under
these conditions. Slightly more forcing conditions were
then tried. The first step of the reaction sequence was
carried out in CH Cl at 40°C followed by the furan
X-ray structure of the benzotrisoxadisilole 5 (Fig. 1).
Benzotrisoxadisilole 5 was almost planar including the
benzene ring and the silicon and oxygen atoms.
2
2
For the generation of benzyne from 1,2-bis(trimethyl-
silyl)benzene (via 1a), it had been reported that the
addition of diisopropyl amine during the preparation of
trapping reaction at room temperature. A good yield of
8 was again obtained but with no indication of the
formation of any bis-benzyne cycloadduct. It is also
worth noting that both 8 and 9 were completely inert
under the phenyliodination conditions (room tempera-
ture or 40°C in CH Cl ). No benzyne intermediate
1
a and subsequent trapping reaction with diene was
4e
advantageous. Benzobisoxadisilole 4 was treated with
excess PhI(OAc) /TfOH/iPr NH (3:6:4) in CH Cl
2
2
2
2
2
2
(
Scheme 2). Without isolation of any intermediate, the
would be generated from these two compounds.
Figure 1. X-Ray structures of 5, 8 and 9.
Me
Me Me
Si
Me
Me
O
Me
Me
Me
Me
Me
Si
i
Si
Si
Si(OH)Me2
I(Ph)OTf
ii
Si
Si
O
O
O
Si
Si
Me Me Me
Me
Me
Me
4
6
7
Me
O
Me
Si
Si
furan
O
63%
Me
Me
Me
7
8
Me
Me
Me
O
Si
Si
Si
Si
O
40%
Me
Me
Me
Me
9
Scheme 2. Reagents and conditions: (i) PhI(OAc) , TfOH, iPr NH, CH Cl , 0°C to rt; (ii) nBu NF/THF, iPr NH, rt.
2
2
2
2
4
2

Y.-L. Chen et al. / Tetrahedron Letters 43 (2002) 2259–2262
2261
Furan adduct 8 could be deoxygenated in good yield by
Nonetheless, without the amine, 5 disappeared slowly
(TLC monitoring). Without isolation of any intermedi-
ate, the homogeneous reaction mixture was treated with
8
TiCl –LiAlH –Et N in THF to 2,3-naphthoxadisilole 10
4
4
3
(
Scheme 3). Hydrogenation of 8 also took place readily to
afford 11 in good yield. 2,3-Naphthoxadisilole 10 was
completely inert to the phenyliodination conditions in the
nBu NF and excess furan (Scheme 4). The furan Diels–
4
Alder adduct 17 of benzyne 16 was isolated in 31% yield.
Once again, only one of the three oxadisilole rings of 5
could be opened up to the benzyne precursor 15 under
these conditions. Finally, deoxygenation with TiCl4–
LiAlH –Et N in THF completed the synthesis of 1,2,3,4-
presence of iPr NH. However, without the amine,
2
phenyliodination followed by benzyne formation
(
nBu NF) and furan trapping, the Diels–Alder adduct 14
4
of 2,3-naphthyne 13 was obtained. This represents a
synthesis of 2,3-naphthoxadisilole 10 and the use of it as
a naphthyne precursor. When compound 11 was treated
under the same phenyliodination conditions without
4
3
naphthobisoxadisilole 18.
In summary, we have demonstrated that benzynes 7 and
iPr NH followed by benzyne formation and furan trap-
2
1
6 could be generated from benzobisoxadisilole 4 and
ping, a trace amount (7%) of 14 was detected. We
speculate that 11 was transformed to 10 under these
conditions. Indeed, when the phenyliodination of 11 was
worked up after 1 h, a good yield (71%) of 10 was isolated.
benzotrisoxadisilole 5, respectively. They were trapped
as the Diels–Alder adducts of furan or the cycloaddition
dimer in case of 7. The synthesis of naphthoxadisilole 10,
which could serve as a precursor of 2,3-naphthyne, and
the preparation of naphthobisoxadisilole 18 are also
reported.
Benzotrisoxadisilole 5 was again completely inert to the
phenyliodination conditions in the presence of iPr NH.
2
Me
O
Me
Me
O
Me
Si
Si
i
Si
Si
iii
Me2(HO)Si
TfO(Ph)I
O
98%
Me
Me
Me
Me
8
12
10
iv
ii
iii
90%
(1hr) 71%
Me
O
Me
Si
Si
iii, iv
2
2%
O
O
7%
Me
Me
13
14
11
Scheme 3. Reagents and conditions: (i) TiCl , LiAlH , Et N–THF, reflux; (ii) H , Pd/C, rt; (iii) PhI(OAc) , TfOH, CH Cl , 0°C
4
4
3
2
2
2
2
to rt; (iv) nBu NF/THF, iPr NH, furan, rt.
4
2
Me
Me
Me
O
O
Me Si
Me Si
O
Me
Me
Me
Me
Si
Me
O
Me
Me
Si
Si
Si
Si
Si
Si
Me (HO)Si
Me
Me
i
2
ii
Me
Me
Me
Me
Si
Si
TfO(Ph)I
Me
Me
Me
Me
Si
Si
Me
Me
O
Me
O
Si
O
Me
Me
Me
5
15
16
Me
Me
O
O
Me Si
Me Si
Me
Me
Si
Si
Si
31%
iii
Me
Me
Me
Me
O
7
0%
Si
Me
Me
Si
Si
Me
O
Me
O
Me
Me
18
1
7
Scheme 4. Reagents and conditions: (i) PhI(OAc) , TfOH, CH Cl , 0°C to rt; (ii) nBu NF/THF, iPr NH, furan, rt; (iii) TiCl ,
2
2
2
4
2
4
LiAlH , Et N–THF, reflux.
4
3

2
262
Y.-L. Chen et al. / Tetrahedron Letters 43 (2002) 2259–2262
Y. J. Am. Chem. Soc. 1999, 121, 11674–11679; (c) Kita-
Acknowledgements
mura, T.; Todaka, M.; Fujiwara, Y. In Organic Syntheses;
Roush, W. R., Ed.; Wiley, 2000; Vol. 78, pp. 104–112.
4. (a) Ye, X.-S.; Li, W.-K.; Wong, H. N. C. J. Am. Chem.
Soc. 1996, 118, 2511–2512; (b) Ye, X.-S.; Wong, H. N. C.
J. Org. Chem. 1997, 62, 1940–1954; (c) Kitamura, T.;
Todaka, M.; Shin-machi, I.; Fujiwara, Y. Heterocycl.
Commun. 1998, 4, 205–208; (d) Okuma, K.; Shiki, K.;
Shioji, K. Chem. Lett. 1998, 79–80; (e) Winling, A.; Rus-
sell, R. A. J. Chem. Soc., Perkin Trans. 1 1998, 3921–3923;
We are grateful to the financial support from the
Research Grant Council of the Hong Kong SAR Gov-
ernment (HKBU 2048/97P).
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