material in very short times of irradiation (typically 2 min).
A complete selectivity to the oxathioacetal was also observed.
Blank runs (without catalyst and with the parent silica
helical material) gave virtually no conversion of starting
material after 15 min of microwave irradiation. Interestingly,
the catalytic activities of Al-HMM (Si/Al = 95) were compar-
able to those of sulfuric acid and significantly superior to those
obtained with related solid acids including zeolites and acidic
resins (Fig. 3). Al-HMM (Si/Al = 95) also gave slightly higher
conversions under the investigated conditions when compared
to a commercial Al-MCM-41 even with higher Al content
(Al-41, Si/Al = 60) (Fig. 3). These interesting findings may
imply the helical mesostructure can slightly affect the activity
in the materials for particular applications.2 Another explana-
tion for this unusual activity might as well be related to the
observed Al in octahedral coordination. Al in Oh coordination
has been shown to have excellent catalytic properties in
different acid catalysed processes.10
the investigated reaction conditions, preserving over 90% of
their initial activity after 4 uses (ESIw).
In conclusion, we have reported the introduction of catalytic
sites on helical mesoporous silicas and their use as highly
active and reusable materials in important acid catalysed
reactions, such as the oxathioacetalisation of carbonyl
compounds, for a wide variety of substrates. We envisage
these materials to be applicable to related acid and/or redox
catalysed processes. These reactions, including oxidations of
alcohols and Friedel–Crafts alkylation and acylations, are
currently under investigation in our laboratories.
RL gratefully acknowledges support from the Spanish
MICINN via the concession of a RyC contract (RYC-2009-
04199). ES acknowledges financial support from Spanish
MICINN through the JdC Program (ref. JCI-2008–2165).
Dr Carlos Untiedt is especially thanked for his continuous
support.
Notes and references
The scope of the protocol was also extended to other
substrates to prove the usefulness of the most active
Al-HMM catalyst under the investigated conditions.
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Table 2 shows the system was amenable to a range of
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Time of
reaction (min) (mol%)
Conversion
Entry Substrate
1
Product
2
5
5
2
2
94
89
96
99
90
2
3
4
5
6
2
99
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a
5 mmol substrate, 10 mmol 2-mercaptoethanol, 0.05 g catalyst,
microwaves, 300 W (60–80 1C maximum temperature reached).
Carbon balance of products found was >99% of the starting
materials.
ꢀc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 5163–5165 | 5165