S. Antoniotti, E. Dun˜ach
FULL PAPER
Preparation of Bi(OTf)3 by Oxidative Dissolution of Bi0: Metallic
bismuth powder (10 mmol, 2.1 g) was placed in a 25-mL round-
bottomed flask. After flushing with nitrogen, freshly distilled
DMSO (10 mL) was added. The flask was then cooled in an ice-
bath and kept cool during the addition of triflic acid (30 mmol,
4.5 g) (CAUTION!) in three aliquots. The flask was then connected
to a condenser capped with an oxygen bag, and the mixture was
heated to 100 °C with stirring. Reaction progress was monitored
by observation of the slow dissolution of the black bismuth powder.
After completion, DMSO was removed by rectification using a
bulb-to-bulb oven (100 °C, 0.1Ϫ1 mbar). The white solid obtained
bismuth() species. In the overall reaction, the bismuth
catalyst plays the double role of Lewis acid and active redox
agent. Reduced states of the bismuth catalytic species are
then re-oxidised to the ϩ oxidation state by O2 in the pres-
ence of protons. Under nitrogen, stoichiometric consump-
tion of the initial bismuth() salt and gradual formation
of a black bismuth(0) precipitate occurs, probably after a
dismutation reaction of a bismuth() intermediate spec-
ies.[20]
was washed with
under nitrogen and stored under nitrogen. 19F NMR (200 MHz,
CDCl3, 20 °C): Ϫ79.6 (s) ppm. Bi(OTf)3·7.8DMSO
a small volume of cold dichloromethane
Experimental Section
δ
ϭ
(C18.6H46.8BiF9O16.8S10.8; 1455.63): calcd. C 17.6, H 3.7, Bi 16.5, F
13.5, S 27.4; found: C 17.5, H 3.7, Bi 16.2, F 13.2, S 27.7.
Materials: Cyclohexene oxide (1), 1,2-cyclohexanedione (2), 2-
hydroxycyclohexanone (3), Bi0 powder, triflic acid, bismuth()
chloride, bismuth() bromide and the solvents used in this study
were purchased from Aldrich. Bismuth() triflate was prepared ac-
cording to a literature procedure.[10]
Measurement of Oxygen Consumption During Oxidation Reactions:
The procedure was identical to the one presented above except that
the O2 source bag is also connected through a silica gel trap to an
open graduated U-shaped cylinder containing water. Direct obser-
vation of the change of the water level in the graduated tube allows
calculation of the amount of gas consumed during the reaction.
Equipment: GC analyses were carried out using a Varian CP 3380
gas chromatograph with an FID detector, under the following op-
erating conditions: vector gas N2; injector and detector tempera-
tures 250 °C; split ratio 1:50; constant flow 1 mL/min; Chrompack
column WCOT fused silica (25 m ϫ 0.20 mm i.d., film thickness
0.25 µm). GC-MS analyses were performed using an HP 5890 gas
chromatograph (conditions: carrier gas He; injector and detector
temperatures 250 °C; injected volume 0.5 µL; split ratio 1:100;
pressure 180 kPa; HP1 column polydimethylsiloxane (50 m ϫ
0.20 mm i.d., film thickness 0.33 µm); coupled to an HP 5970A
mass-selective detector. Mass spectra were obtained by electron
ionisation at 70 eV, m/z ϭ 35Ϫ400, source temperature 250 °C. 1H,
19F and 13C NMR spectra were recorded with a Bruker AC 200
FT spectrometer at 20 °C in CDCl3 with TMS as internal standard.
Analyses at 100 °C were recorded with a Bruker Avance 500 spec-
trometer equipped with a thermostat. Cyclic voltammetry curves
were obtained with a Potentiostat/Galvanostat EG&G Princeton
Applied Research, model 273 A (electrodes: graphite, platinum; ref-
erence Ag/AgCl).
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General Procedure for the Oxidation Catalysed by Bi/O2 in the Pres-
ence of Triflic Acid: A mixture of the appropriate amount of bis-
muth(0) (0.25 to 0.50 mmol) and the additive (CAUTION: triflic
acid is highly corrosive!), or bismuth triflate (0.25 to 0.50 mmol),
in freshly distilled DMSO (15 mL) was heated at 100 °C under O2
(1 atm). The substrate (5 mmol) in DMSO (5 mL) was introduced
through a serum cap and the mixture was stirred at this tempera-
ture until completion (monitored by GC). The reaction mixture
was then hydrolysed with brine (50 mL) and extracted with diethyl
ether (3 ϫ 50 mL). The combined organic layers were dried with
MgSO4 and the solvents evaporated under reduced pressure. The
crude product was purified by column chromatography on silica
gel with a gradient pentane/dichloromethane as the eluent. Five-
and six-membered ring α-diketones were only observed in their
enol form under analytical conditions. All compounds were charac-
terised by comparison with authentic samples.
[12]
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T. Cohen, T. Tsuji, J. Org. Chem. 1961, 26, 1681.
X. Creary, E. A. Burtch, J. Org. Chem. 2004, 69, 1227Ϫ1234.
Bismuth() species are rare but have already been reported in
the literature: J. Lorberth, W. Massa, S. Wocadlo, I. Sarraje,
S.-H. Shin, X.-W. Li, J. Organomet. Chem. 1995, 485, 149Ϫ152.
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[20]
Received March 23, 2004
3464
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2004, 3459Ϫ3464