Synthesis and reactions of a hypervalent iodine-benzyne precursor possessing an ester group

Article abstract of DOI:10.1055/s-1999-2734

The Diels-Alder reaction of methyl 2-pyrone-5-carboxylate and bis(trimethylsilyl)acetylene gave methyl 3,4-bis(trimethylsilyl)-benzoate which was transformed into [5-(methoxycarbonyl)2- (trimethylsilyl)phenyl](phenyl)iodonium triflate by the reaction with

Full text of DOI:10.1055/s-1999-2734

LETTER
731
Synthesis and Reactions of a Hypervalent Iodine-Benzyne Precursor
Possessing an Ester Group
Tsugio Kitamura,* Kanako Wasai, Mitsuru Todaka, Yuzo Fujiwara
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan
Fax:+81-92-651-5606; E-mail: tkitatcf@mbox.nc.kyushu-u.ac.jp
Received 2 March 1999
O
SiMe3
SiMe3
Abstract: The Diels-Alder reaction of methyl 2-pyrone-5-carboxy-
late and bis(trimethylsilyl)acetylene gave methyl 3,4-bis(trimethyl-
silyl)-benzoate which was transformed into [5-(methoxycarbonyl)-
2-(trimethylsilyl)phenyl](phenyl)iodonium triflate by the reaction
with a hypervalent iodine reagent PhI(OAc)2/TfOH. The generation
and trapping reactions of 4-(methoxycarbonyl)-1,2-didehydroben-
zene were successfully conducted by the reaction of the iodonium
triflate with Bu4NF.
- CO2
+
Me3SiC CSiMe3
O
MeO2C
MeO2C
2
1
3
Table 1 Synthesis of Methyl 3,4-Bis(trimethylsilyl)benzoate 3
Conditionsa
PhCl, reflux
Entry
Time (h)
Yield of 3 (%)
Key words: hypervalent iodine compound, benzyne, Diels-Alder
reaction, methyl 2-pyrone-5-carboxylate, bis(trimethylsilyl)acetyl-
ene
1
2
3
4
5
6
24
44
12
9
27
36
19
53
93
98
PhBr, reflux
In a sealed tube, 170 °C
In a sealed tube, 200 °C
In a sealed tube, 200 °C
In a sealed tube, 200 °C
Hypervalent iodine compounds are very useful reagents in
organic synthesis since their reactions occur efficiently
under mild conditions.1 Recently we have found that phe-
nyl[2-(trimethylsilyl)phenyl]iodonium triflate reacts with
Bu4NF to generate benzyne under very mild conditions.2
The benzyne generated by this method is efficiently
trapped with dienes in high yield. Synthesis of the
benzyne precursor is readily performed by using 1,2-
bis(trimethylsilyl)benzene and a hypervalent iodine re-
agent PhI(OAc)2/TfOH. However, the preparation of 1,2-
bis(trimethylsilyl)benzene3 requires a high temperature
and a long reaction time, in addition to the use of HMPA
that is a toxic solvent. Furthermore, the procedure suffers
the disadvantage that this method cannot be applied to the
substrate having functional groups that are sensitive to
bases or reagents. To remove the disadvantage, we have
studied the new procedure for the synthesis of hypervalent
iodine-benzyne precursor possessing an ester group.
12
24
a 1 (1 mmol) and 2 (2 mmol). In entries 1 and 2, a halobenzene (10
ml) was used.
When methyl 3,4-bis(trimethylsilyl)benzoate 3 was react-
ed with a hypervalent iodine reagent PhI(OAc)2/TfOH5 in
CH2Cl2,
[5-(methoxycarbonyl)-2-(trimethylsilyl)phe-
nyl](phenyl)iodonium triflate (4) was obtained in 73%
yield.6 The iodonium triflate 4 is a stable crystalline com-
pound and melts at 143-144 °C. The phenyliodination of
3 occurs selectively at the meta position to the ester group.
No other products were detected.
The previous method3 using haloarenes and Me3SiCl/Mg
reagent system is not applicable to the substrate having
methoxycarbonyl group since the methoxycarbonyl group
reacts with the Grignard reagent. We have adopted the
Diels-Alder cycloaddition of 2-pyrone and bis(trimethyl-
silyl)acetylene.4 This method involves the reaction condi-
tions at 150 °C for 4.5 days in bromobenzene. In the
reaction conditions described in the literature (Table 1,
entries 1 and 2), however, satisfactory results were not ob-
tained. Then, the mixture of methyl 2-pyrone-5-carboxy-
late (1) and bis(trimethylsilyl)acetylene (2) were placed in
a sealed stainless steel tube and heated. Heating at 200 °C
for more than 12 hr gave methyl 3,4-bis(trimethylsi-
lyl)benzoate (3) in almost quantitative yield (Table 1, en-
tries 5 and 6). In addition, 3 was obtained in 80% yield on
a larger scale: 1 (8 mmol) and 2 (16 mmol) at 200 °C for
48 h.
SiMe3
+
PhI(OAc)2
MeO2C
SiMe3
3
SiMe3
TfOH, CH2Cl2, 0 °C→r.t.
73%
MeO2C
IPh OTf
4
The generation and reaction of 4-(methoxycarbonyl)-1,2-
didehydrobenzene (5) could be conducted by simply add-
ing Bu4NF to the solution of the [5-(methoxycarbonyl)-2-
(trimethylsilyl)phenyl]iodonium triflate 4 in the presence
of trapping agents (Scheme).7 Treatment of [5-(meth-
oxycarbonyl)-2-(trimethylsilyl)phenyl]iodonium triflate
4 with Bu4NF in the presence of furan and 2,5-dimethyl-
Synlett 1999, No. 6, 731–732 ISSN 0936-5214 © Thieme Stuttgart · New York

732
T. Kitamura et al.
LETTER
O
SiMe3
Bu4NF, CH2Cl2
O
100%
MeO2C
O
MeO2C
IPh OTf
MeO2C
Ph
O
5
4
6
O
Me
Me
Ph
Ph
100%
Ph
84%
Ph
Me
89%
Ph
Ph
Ph
O
Ph
Ph
O
MeO2C
MeO2C
MeO2C
Me
7
Ph
Ph
9
8
Scheme
Hoshina, Y.; Nogami, G.; Yokoyama, M. Yuki Gosei
Kyokaishi 1997, 55, 90. Varvoglis, A. Hypervalent Iodine in
Organic Synthesis; Academic Press: San Diego, 1997.
Kitamura, T.; Fujiwara, Y. Org. Prep. Proced. Int. 1997, 29,
409.
furan gave the corresponding adducts, methyl 1,4-dihy-
dro-1,4-epoxy-6-naphthalenecarboxylate (6) and methyl
1,4-dihydro-1,4-epoxy-1,4-dimethyl-6-naphthalenecar-
boxylate (7), respectively, in 100 and 89% yields. Similar
reaction of iodonium triflate 4 with Bu4NF in the presence
of tetraphenylcyclopentadienone afforded methyl 1,2,3,4-
tetraphenyl-6-naphthalenecarboxylate (8) in 100% yield.
Trapping reaction of the benzyne 5 with 1,3-diphenyl-
isobenzofuran was also successfully carried out to give
methyl 9,10-diphenyl-9,10-epoxy-2-anthracenecarboxy-
late (9) in 84% yield.
(2) Kitamura, T.; Yamane, M. J. Chem. Soc. Chem. Commun.
1995, 983.
(3) Bourgeois, P.; Calas, R. J. Organomet. Chem. 1975, 84, 165.
(4) Seyferth, D.; White, D. L. J. Organomet. Chem. 1972, 34,
119.
(5) Kitamura, T.; Matsuyuki, J.; Taniguchi, H. Synthesis 1994,
147.
(6) A typical procedure is as follows. To a suspension of
PhI(OAc)2 (1 mmol) in CH2Cl2 (5 ml) was added TfOH (2
mmol) dropwise at 0 °C, and the mixture was stirred for 30
min at room temperature. Methyl 3,4-bis(trimethylsilyl)-
benzoate 3 (1 mmol) was added at 0 °C, and the mixture was
stirred for 24 h at room temperature. After evaporation of the
solvent, the residue was crystallized by adding ether. The
crystals were collected, washed with ether, and dried in vacuo
to give [5-(methoxycarbonyl)-2-(trimethylsilyl)phenyl]-
(phenyl)iodonium triflate 4 (409 mg, 73%): mp 143-144 °C;
1H NMR (CDCl3) d 0.46 (s, Me, 9 H), 3.96 (s, OMe, 3 H),
7.44-7.50 (m, ArH, 2 H), 7.56-7.59 (m, 2 H, ArH), 7.79-7.82
(m, ArH, 3 H), 8.25-8.28 (m, ArH, 2 H), 8.65 (s, ArH, 1 H);
13C NMR (CDCl3) d -0.06, 52.786, 113.955, 121.513,
131.996, 132.235, 132.326, 133.183, 134.155, 138.219,
139.389, 152.617, 164.295.
The hypervalent iodine precursor, [5-(methoxycarbonyl)-
2-(trimethylsilyl)phenyl]iodonium triflate 4, efficiently
generates 4-(methoxycarbonyl)-1,2-didehydrobenzene 5
in situ with action of Bu4NF. The ester group is not dam-
aged by this treatment at all. The 4-(methoxycarbonyl)-
1,2-didehydrobenzene 5 can be trapped effectively to give
the adducts in high yields. The present result indicates that
this alternate synthesis of [5-(methoxycarbonyl)-2-(trime-
thylsilyl)phenyl]iodonium triflate 4 is very excellent and
convenient for the procedure of hypervalent iodine-ben-
zyne precursors because of the high yields of the adducts
and no damage of the sensitive ester group.
(7) The representative trapping experiment is as follows. To a
solution of 4 (0.2 mmol) and furan (1.0 mmol) in CH2Cl2 (2
ml) was added a THF solution of Bu4NF (1 M, 0.25 ml) at 0
°C, and the mixture was stirred for 30 min. Water was added
to the mixture, and the product was extracted with CH2Cl2.
After evaporation of the solvent, the product was purified by
column chromatography on silica gel (hexane/CH2Cl2) to
yield crystals of the adduct 6 (100%): mp 97-98 °C; 1H NMR
(CDCl3) d 3.79 (s, OMe, 3 H), 5.66 (bs, CH, 2 H), 6.90-6.95
(m, 2 H, = CH), 7.20 (d, J = 7.4 Hz, ArH, 1 H), 7.67 (d, J = 7.4
Hz, ArH, 1 H), 7.78 (s, ArH, 1 H); 13C NMR (CDCl3) d 51.95,
81.98, 82.00, 119.79, 120.54, 127.08, 127.94, 142.26, 143.22,
149.44, 154.33, 166.77.
Acknowledgement
This work was partly supported by a Grant-in-Aid for Scientific Re-
search from the Ministry of Education, Science, Sports, and Culture
of Japan.
References and Notes
(1) For recent reviews and books, see: Koser, G. F. In The
Chemistry of Functional Groups, Supplement D; Patai, S.,
Rappoport, Z., Eds.; John Wiley & Sons: New York, 1983;
Chapters 18 and 25. Ochiai, M. Rev. Heteroatom Chem. 1989,
2, 92. Moriarty, R. M.; Vaid, R. K. Synthesis 1990, 431.
Varvoglis, A. The Organic Chemistry of Polycoordinated
Iodine; VCH Publishers: New York, 1992. Stang, P. J.;
Zhdankin, V. V. Chem. Rev. 1996, 96, 1123. Togo, H.;
Article Identifier:
1437-2096,E;1999,0,06,0731,0732,ftx,en;Y04999ST.pdf
Synlett 1999, No. 6, 731–732 ISSN 0936-5214 © Thieme Stuttgart · New York