Iron(iii) Fluoride for the Protection of Carbonyl Compounds
609
using iron(iii) fluoride (0.05 mmol) under solvent-free condi-
tions as an alternative to existing methods. Important features
of the present method are chemoselective protection of alde-
hydes in the presence of ketones, high yields, operational
simplicity, and mild reaction conditions.
S
O
CHO
FeF3 (5 mol%)
rt, neat, 30 min
HS
HO
ϩ
COCH3
COCH3
95%
Experimental
S
S
CHO
Br
COCH3
HS
COCH3
Iron(iii) fluoride was purchased from Acros Organics and used with-
out further purification. All reactions were carried out in oven-dried
flasks. All chemicals were of analytical grade. IR spectra were recorded
on a Bomem MB 104 FT-IR spectrometer and 1H NMR spectra were
recorded on an AC 300 F spectrometer (300 MHz). Mass spectra were
recorded on a Bruker ion trap spectrometer.
SH, FeF3 (5 mol%)
rt, neat, 35 min
ϩ
ϩ
Br
94%
97%
In a typical procedure, a mixture of benzaldehyde (10 mmol), 1,2-
mercaptoethanol (10 mmol), and iron(iii) fluoride (0.05 mmol) was
stirred at room temperature for 5 min. After completion of the reaction,
as indicated by TLC, the odorless mixture was directly distillated under
reduced pressure of 0.5 mmHg at an oven temperature of 170–190◦C
to furnish 2-phenyl-1,3-oxathioacetal (95%). The iron(iii) fluoride
remained in the distilling flask as a white solid (97%) and was used
directly for the next reaction without further purification.
Scheme 2.
As aldehydes are converted faster than ketones, this pro-
cedure affords chemoselective protection of aldehydes in the
presence of ketones (Scheme 2).
Further, we studied the catalytic ability of other iron salts
such as FeCl3, FeBr3, and FeI3 for the synthesis of oxathioac-
etals. By using 5 mol% of FeCl3, FeBr3, and FeI3, benzalde-
hyde was converted into the corresponding oxathioacetal in
58%, 40%, and 35% yield, respectively, when the reaction
mixture was stirred for 30 min. This result shows FeF3 is a
stronger catalyst than FeCl3, FeBr3, and FeI3.
References
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Iron(iii) fluoride obtained as a white solid (97%) after dis-
tillation was used directly for the next reaction. The first run
in the reaction of benzaldehyde with 1,2-mercaptoethanol
gave 90% yield and the second run gave 87% yield of
the corresponding oxathioacetal. This result indicates that
iron(iii) fluoride is expected to solve some severe environ-
mental problems induced by Lewis acid-promoted reactions
in industry. As compared with methods that require sto-
ichiometric amounts of reagent,[11,27] the present method
required a catalytic amount of iron(iii) fluoride (0.05 mmol)
for the reaction to proceed cleanly. Moreover, neither use
of dehydrating agent,[11,27c,27e,28] nor the azeotropic removal
of water,[27d,29] are necessary for the present method.
Although various methods have been reported for this con-
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study shows iron(iii) fluoride is superior as a catalyst for
the protection of carbonyl compounds as oxathioacetals and
dithioacetals, with regard to toxicity, handling, selectivity,
and recyclability. The superiority of this procedure is illus-
trated by the use of entirely solvent-free conditions and is
applicable to distillable compounds. Although the synthesis
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the work-up process.
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In summary, the present procedure demonstrates the con-
version of aldehydes into oxathioacetals and dithioacetals