Brønsted Acid-Catalyzed Dihydroxylation of Olefins in Aqueous Medium
5 min, then the substrate (25 mmol) was added. After 21 h
stirring at 508C, the reaction mixture was neutralized with
sodium bicarbonate, and reduced with Na2SO3, and then ex-
tracted with diethyl ether.
General Procedure C (Dihydroxylation of
Cyclohexene in the Presence of Substrates bearing
Different Functional Groups)
In a sealed flask was added PTSA (20 mol%) and H2O2
(30% aqueous solution, 2 equiv.) that were stirred for 5 min,
then the cyclohexene (1–2.5 mmol) and the substrate (50 or
100 mol%) were added. After 21 h stirring at 508C, the re-
action mixture was extracted with diethyl ether, and the
mixture was separated by column chromatography on silica
gel. When the substrate was not soluble in the aqueous
phase, or the olefin phase (cyclohexene), 1-propanol was
added as co-solvent.
Scheme 1. Proposed mechanism for the trans-dihydroxyla-
tion via epoxide formation.
catalyzed ring opening (Scheme 1). In addition, cyclo-
hexene oxide afforded quantitatively the correspond-
ing diol in only 5 min, at room temperature with
20 mol% of PTSA in H2O. This result explains why
cyclohexene oxide was never detected when following
1
the cyclohexene dihydroxylation by H and 13C NMR
General Procedure D (Recycling the Catalyst)
spectroscopy under different experimental conditions.
By using the catalyst PTSA enriched in 18O under our
optimized dihydroxylation conditions, it was observed
by MS that 18O was being retained exclusively in the
recovered catalyst and not in the formed diol. The
collected information supported an expected mecha-
nism comprising epoxidation followed by fast ring
opening by water in which both steps are catalyzed by
PTSA.
In a sealed flask was added PTSA (20 mol%, 951.1 mg),
H2O2 (2 equiv., 30% aqueous solution, 5.66 g) and the sub-
strate (25 mmol, 2.5 mL), at 508C. After the required reac-
tion time the crude aqueous phase was extracted with dieth-
yl ether (3ꢃ100 mL). The aqueous phase was concentrated
by water evaporation, and the next cycle started by adding
H2O2 (2 equiv, 30% aqueous solution, 5.66 g) and the sub-
strate (25 mmol, 2.5 mL). Diethyl ether was removed from
the organic layer, affording the diol 2 in very high purity.
In summary, we have described a simple, robust
and mild Brønsted acid-catalyzed and metal-free di-
hydroxylation method of olefins in aqueous media
that is also compatible with a considerable range of
organic functional groups.
Acknowledgements
We thank the Fundażo para a CiÞncia e Tecnologia (POCI
2010) and FEDER (POCI/QUI/66695/2006, PTDC/QUI-
QUI/119823/2010, SFRH/BD/28242/2006 and SFRH/BPD/
75045/2010) for financial support.
Experimental Section
Caution: We have performed experiments with 30% hydro-
gen peroxide at 508C more than 50 times and no accident
was observed, although performing reactions with this oxi-
dant at this temperature always requires special attention, due
to instability and the possibility of explosion.
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General Procedure A (Dihydroxylation of
Cyclohexene using Different Promoters or Oxidants)
In a sealed flask was added the promoter and the oxidant
that were stirred for 5 min, and then the cyclohexene (1–
2.5 mmol) was added. After 21 h stirring at 208C (or 508C)
chloroacetic acid (50 mol% approximately) was added. The
1
mixture was analysed by H NMR and the yield of 1,2-cyclo-
hexenediol was calculated based on the amount of the inter-
nal standard (for more information see the Supporting In-
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General Procedure B (Dihydroxylation of Several
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In a sealed flask was added PTSA (20 or 100 mol%) and
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Adv. Synth. Catal. 2011, 353, 2920 – 2926
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