Organic Syntheses, Vol. 78, pp. 104-112
[Iodonium, phenyl-, 2-(trimethylsilyl)phenyl-, salt with trifluoromethane-sulfonic
acid]
Submitted by Tsugio Kitamura, Mitsuru Todaka, and Yuzo Fujiwara
1.
Checked by Ralf Demuth and Rick L. Danheiser.
1. Procedure
A. 1,2-Bis(trimethylsilyl)benzene
(
1). A
dry, 500-mL, three-necked, round-bottomed flask is equipped
with a large Teflon-covered magnetic stir bar,
100-mL pressure-equalizing
addition funnel,
Dimroth condenser (Note
1)
fitted with a
drying tube, and a
glass stopper.
The flask is charged with
9.72 g (0.400
mol) of magnesium turnings (Note
2),
70 mL of hexamethylphosphoramide
(HMPA) (Note
3),
11.25
mL (0.100 mol) of 1,2-dichlorobenzene
(Note
4), and
0.254 g
(1.00 mmol) of iodine (I
2)
(Note
5). The addition funnel is charged with
51.0
mL (0.400 mol) of freshly distilled chlorotrimethylsilane
(Note
6). The flask is immersed in an
oil bath
at 70°C, stirring is initiated, and
chlorotrimethylsilane is
added slowly, dropwise with vigorous stirring. After completion of the addition, the
oil bath is heated to 100°C and the reaction mixture is stirred at this temperature
for 2 days. During this time, the reaction mixture becomes viscous and finally separates
into two phases. The reaction mixture is cooled to ca. 40°C (Note
7)
and poured into a
1-L beaker containing
saturated
sodium bicarbonate (NaHCO3) solution (200
mL),
diethyl ether
(100 mL), and ice (ca. 100 g). Solids and unreacted
magnesium
metal are separated by suction filtration, the filtrate is transferred to a
separatory
funnel, and the aqueous phase is extracted with three 150-mL portions
of ether. The combined ethereal extracts are washed with water (500 mL) and
saturated sodium chloride (500 mL),
dried over
anhydrous sodium sulfate,
and filtered. The solvent is evaporated under reduced pressure and the residue is
distilled from a
100-mL, round-bottomed flask with a magnetic stir bar
through a 20-cm Vigreux column at reduced pressure. The fraction boiling at 128-133°C
(20 mm) is collected to afford
16.5-16.7
g (
74-75%) of
1,2-bis(trimethylsilyl)benzene (
1) (Note
8)
as a colorless liquid.
C. Generation of benzyne and trapping with furan.
A
50-mL round-bottomed flask fitted with a
pressure-equalizing
addition funnel equipped with an
argon inlet adapter
and a
magnetic stir bar is charged with
1.51
g (3.00 mmol) of (phenyl)[2-(trimethylsilyl)phenyl]iodonium
triflate (
2),
10 mL
of dichloromethane, and
1.10
mL (15.1 mmol) of furan (Note
14). The addition funnel is charged with
3.6
mL (3.6 mmol) of 1.0 M tetrabutylammonium fluoride
(Bu4N+F−) in
tetrahydrofuran
(THF) (Note
15). The flask is placed in an
ice
bath and the
tetrabutylammonium fluoride solution
is added dropwise over ca. 5 min. The reaction mixture is stirred at room temperature
for 30 min, and water (20 mL) is added. The aqueous phase is separated and extracted
with three
10-mL portions of dichloromethane.
The combined organic extracts are washed with 15 mL of water, dried over
anhydrous sodium sulfate, and filtered.
The solvent is evaporated by rotary evaporation under reduced pressure, and the residual
oil is purified by column chromatography through
60
g of silica gel packed in a 4-cm diameter column (elution with
dichloromethane)
to give
0.415-0.418 g (
96-97%) of
1,4-dihydronaphthalene
1,4-oxide (
3) as colorless crystals,
mp 52-55°C (Notes
16,
17).
2. Notes
1. A highly efficient
reflux condenser is
required to avoid the loss of
chlorotrimethylsilane by evaporation
during the reaction.
2.
Magnesium turnings
were purchased from Nacalai Tesque, Inc. or Fisher Scientific Company.
4.
1,2-Dichlorobenzene
was purchased by the submitters from Tokyo Kasei Kogyo Co.
and distilled under reduced pressure. The checkers used
99%
anhydrous 1,2-dichlorobenzene from Aldrich Chemical
Company, Inc., without further purification.
5.
Iodine was purchased
from Tokyo Kasei Kogyo Co. or Aldrich Chemical Company,
Inc., and used as received.
6. The submitters purchased
chlorotrimethylsilane
from Shin-Etsu Chemicals and distilled it prior to
use. The checkers used
99+% chlorotrimethylsilane
from Aldrich Chemical Company, Inc., without further
purification.
7. If cooled to room temperature, the lower layer solidifies.
8.
Bis(trimethylsilyl)benzene (
1) has
the following spectral properties:
1H
NMR (300 MHz, CDCl
3) δ: 0.36 (s, 18 H), 7.28-7.34
(m, 2 H), 7.64-7.68 (m, 2 H);
13C NMR (75 MHz, CDCl
3) δ:
2.0, 127.8, 135.2, 146.0.
12. Product
2 has the following spectral properties:
1H NMR (400 MHz, CDCl
3)
δ: 0.42 (s, 9 H), 7.26-8.13 (m, 9 H);
13C NMR (100 MHz, CDCl
3)
δ: 0.1, 114.0, 121.2, 132.2,
133.2, 133.4, 138.5, 139.1, 147.3.
13. The submitters obtained
2 in
86% yield.
14.
Furan was purchased
from Tokyo Kasei Kogyo Co. or Aldrich Chemical Company,
Inc., and distilled prior to use.
16. If the product is obtained as an oil, it is cooled in a −78°C
bath to induce crystallization.
17. Product
3 has the following spectral properties:
1H NMR (400 MHz, CDCl
3)
δ: 5.69 (s, 2 H), 6.94-6.96 (m, 2 H), 7.00 (s,
2 H), 7.22-7.24 (m, 2 H);
13C NMR (100 MHz, CDCl
3) δ:
82.2, 120.2, 124.9, 142.9, 148.9.
Waste Disposal Information
All toxic materials were disposed of in accordance with "Prudent Practices in the
Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
Benzyne is one of a group of reactive intermediates widely applicable to organic
synthesis.
2 3 4
5 The title hypervalent
iodine-benzyne
precursor,
(phenyl)[2-(trimethylsilyl)phenyl]iodonium triflate
2,
6 is prepared by only
two steps from commercially available reagents. Products
1 and
2 are
stable and easily purified. The hypervalent
iodine-benzyne precursor
2 is obtained as a stable solid and handled without any precautions. More importantly,
benzyne is generated by using
tetrabutylammonium fluoride under
mild and neutral conditions. Therefore, compound
2 is useful for reactions
of substrates that cannot be conducted at high temperatures or under basic conditions.
The advantages of the use of this hypervalent
iodine-benzyne
precursor
2 are as follows: (1) The benzyne precursor
2 is a stable crystalline compound up to its
melting point, usually to
130°C.
(2) The benzyne precursor
2 is not hygroscopic and is stable to air; it can
be handled without any special precautions. (3) The generation of benzyne can be conducted
under neutral conditions and at room temperature.
The high efficiency of the present precursor
2 is demonstrated by comparison
with a similar precursor,
2-(trimethylsilyl)phenyl triflate (
4),
which generates benzyne under mild conditions (room temperature and neutral).
7 Benzyne precursor
2 gives the
adduct,
1,4-epoxy-1,4-dihydronaphthalene 3, quantitatively
in the reaction with
furan, while the reaction of benzyne precursor
4 under the same conditions leads to a lower yield of adduct
3 and needs
longer reaction time.
The reaction of thiobenzophenones with benzyne shows the superiority of the present
iodine precursor
2 over
benzenediazonium-2-carboxylate
(
5), which is widely used.
8 The reaction of the hypervalent
iodine
precursor
2 with thiobenzophenones affords [4+2] cycloadducts from benzyne
and thiophenzophenones under mild conditions. However, the reaction with
benzenediazonium-2-carboxylate
5 gives no benzyne adducts, but benzoxathianones, which are presumably derived
from the reaction of 2-carboxyphenyl cation and cyclization.


Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
(Phenyl) [2-(trimethylsilyl)phenyl]iodonium triflate:
Iodonium, phenyl-, 2-(trimethylsilyl)phenyl-, salt with trifluoromethanesulfonic
acid (1:1) (13); (164594-13-2)
1,2-Bis(trimethylsilyl)benzene: Silane, o-phenylenebis[trimethyl-
(8); Silane, 1,2-phenylenebis[trimethyl- (9); (17151-09-6)
Magnesium (8,9); (7439-95-4)
Hexamethylphosphoramide: HIGHLY TOXIC: CANCER SUSPECT
AGENT: Phosphoric triamide, hexamethyl- (8,9) (680-31-9)
l,2-Dichlorobenzene; Benzene, o-dichloro-
(8); Benzene, 1,2-dichloro- (9); (95-50-1)
Iodine (8,9); (7553-56-2)
Chlorotrimethylsilane: Silane, chlorotrimethyl-
(8,9); (75-77-4)
(Diacetoxyiodo)benzene: Aldrich: Iodobenzene
diacetate: Benzene, (diacetoxyiodo)- (8); Iodine,
bis(aceto-O)phenyl- (9); (3240-34-4)
Trifluoromethanesulfonic acid: HIGHLY CORROSIVE: Methanesulfonic
acid, trifluoro- (8,9); (1493-13-6)
Furan (8,9); (110-00-9)
Tetrabutylammonium fluoride: Ammonium, tetrabutyl-,
fluoride (8); 1-Butanaminium, N,N,N-tributyl-, fluoride
(9); (429-41-4)
1,4-Dihydronaphthalene 1,4-oxide: 1,4-Epoxy-1,4-dihydronaphthalene:
1,4-Epoxynaphthalene, 1,4-dihydro- (8,9); (573-57-9)
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