free mesoporous material is lacking in acidic sites and also that
the presence of titanium(IV) in a siliceous matrix gives rise by
itself to a material containing both redox and acidic properties.9
Besides, the introduction of trivalent metal ion, although widely
used in bifunctional catalysts,10,11 appears to be redundant. The
use of a MCM-41 sample containing both titanium (1.91 wt.%
grafted from titanocene) and aluminium (0.59 wt.% inserted by
isomorphous substitution for Si) did not lead to a remarkable
improvement in catalytic features: the Al insertion shortens the
first-step reaction time (cyclization is complete after 2 h),
leaving the second-step reaction time and selectivity (74%
conversion of 2 and 88% selectivity to 3 after 18 h in toluene +
acetonitrile) practically unaffected.
In this reaction, employing Ti-MCM-41 obviates the use of
homogeneous catalysts in both steps, namely zinc halides in the
first and transition metal complexes or peroxy acids in the
second, thereby limiting the waste production to the work-up
phase, when the TBHP in excess has to be quenched. Such
bifunctional behaviour is to be attributed to the high dispersion
of TiIV onto the surface of the mesoporous solid, which brings
acidity into the silica matrix and causes the excellent activity in
the epoxidation step.
The authors gratefully acknowledge the CNR-MURST for
financial support through the ‘Program Chemistry Law 95/95-I
year’.
Both toluene and n-heptane exhibit a complete and selective
conversion of 1 into 2 within 6 h and this is consistent with the
results obtained on amorphous mixed-oxide TiO2–SiO2.1 The
reaction time is rather longer, but is necessary to avoid the
presence of unreacted citronellal in the second step.
In the epoxidation step, as in the tests on other terpenic
unsaturated alcohols,2 acetonitrile (test 7) showed better
performances than ethyl acetate (test 8). This behaviour might
be attributed to the weakly basic character of acetonitrile that
inhibits the formation of acid-catalysed secondary products
from isopulegol epoxide 3.12 The main by-product formed in
the second-step reaction is isopulegone, i.e. 5-methyl-
2-(1-methylethenyl)cyclohexanone.
With the purpose of obtaining a deeper insight about the
heterogeneity of the system,13 the reaction mixture was
carefully filtered at ca. 50% conversion (2 h reaction) in the
second step. Testing the colourless filtered solution for further
reaction, no activity of the liquid mixture was observed. From
this behaviour, the observed catalyst is presumably heteroge-
neous. Because of the intrinsic low chemical and mechanical
stability of MCM-41-based materials,14 work is in progress to
verify whether or not the catalyst is stable and recyclable.
In summary, the best result for the one-pot conversion of
citronellal into isopulegol epoxide with a ca. 68% global yield
was achieved on Ti-MCM-41 as follows. Toluene was used as
the solvent for the 6 h long cyclization step during which all the
citronellal 1 was completely and selectively converted into
isopulegol 2. Then, after the addition of TBHP and acetonitrile,
over the following 18 h isopulegol 2 was epoxidised to 3 with a
76% conversion and a 90% selectivity (test 7).
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