nation of the epoxide ring, epoxide C–O bond cleavage and
furano/pyrano bond termination. Under strongly acidic con-
ditions, the early transition state requires modest epoxide
cleavage and C–OH bond formation. Under this regime (as the
furano oxide is the kinetic product), the furano isomer is
formed preferentially. As the acid strength is decreased, attack
at the furano carbon becomes less favourable as the ring
forming process is under a much greater influence from the
strain energy released by cleavage of the 3-membered ring and
the lower strain energy of the 6-membered ring product.
TiMCM-41 only contains weak Lewis acid sites and this is
therefore consistent with the lowest furano : pyrano ratio
(0.9 : 1) observed with the catalysts studied.6 The acid strengths
in the TiAlβ–MeCN–TBHP, TiAlβ–MeOH–TBHP and TiAlβ–
MeOH–UHP systems are expected to be lower than the acid
strength in the TiAlβ–MeOH system because of neutralisation
of acid sites, as described previously. The actual ratios obtained
are in agreement with this observation (Table 4 entries 4–7).
Preparation of TiAlꢁ. TiAlβ with a Si : Ti ratio of 30 : 1 and a
TO2 : Al2O3 ratio of 800 : 1 (where T represents both Ti and Si)
was prepared following the method proposed by Corma and co-
workers.32 TEOS (12.5 g, Aldrich) was placed into a beaker to
which hydrochloric acid (4.5 ml 0.1 M, Fisher) was added and
the resulting mixture stirred for 20 min. The mixture was then
cooled to 273 K before the dropwise addition of a solution
containing titanium butoxide (TBOT) (0.68 g, Aldrich) in
propan-2-ol (9 g, Fisher). The mixture was then stirred for a
further 15 min at 273 K and the resulting clear yellow solution
was allowed to warm to room temperature before the dropwise
addition of a portion of tetraethylammonium hydroxide
(TEAOH) (10 ml, Alfa). Upon addition of ∼3 ml of TEAOH a
white gel was formed. The gel was dried (373 K, 6 h) before
aluminium isopropoxide (0.03 g) and the remainder of the
TEAOH (10 ml) were added. The resulting thick paste was
mixed thoroughly then placed into a Teflon lined autoclave and
heated at 408 K under autogeneous pressure without stirring
for a period of 7 d. The resulting white solid was isolated using
a centrifuge, dried at 373 K for 12 h and calcined at 823 K for
24 h prior to use.
Conclusions
Both TS-1 and TiAlβ were characterised by powder XRD
and FTIR and were found to be consistent with materials
reported in the literature.
For the oxidation of monoterpenes, it has been shown that
titanosilicates are effective catalysts when using a variety of
oxidants. Dihydromyrcene is selectively epoxidised at the more
electron rich double bond and significant amounts of ring
opened products are observed with both TS-1 and TiAlβ with
aqueous hydrogen peroxide. Upon replacing the aqueous
hydrogen peroxide with urea–hydrogen peroxide complex or
tert-butyl hydroperoxide, the epoxide can be obtained as the
major product. The epoxidation of geraniol can occur at either
double bond. As observed in the dihydromyrcene experiments,
the subsequent ring opening reaction can be prevented by using
tert-butyl hydroperoxide, urea–hydrogen peroxide complex, or
aqueous hydrogen peroxide in conjunction with acetone. With
aqueous hydrogen peroxide in methanol, the epoxides rapidly
undergo hydrolysis to ether diols or triol. Indirect evidence of
leaching in the TiAlβ–H2O2 system was observed. Reactions
carried out using citronellol show this is entirely due to the
allylic alcohol functional group. Finally, the epoxidation of
linalool occurred exclusively at the more electron rich double
bond. This was found to undergo rapid intra-molecular cyclisa-
tion to a five and a six membered heterocycle. Although the
catalysts used for this transformation are porous, the ratio of
the two products was thought to be dependent on the catalyst
pore size, but data obtained in this study suggest the acid
strength of the support plays a far greater role.
Catalytic reactions
The catalytic reactions were carried out in a 50 ml two-necked
round bottomed flask fitted with a condenser and rubber
septum for sampling. The mixtures were stirred and heated
using a hotplate stirrer, magnetic follower and oil bath.
Reactions were typically carried out as follows; substrate
(20 mmol), catalyst (0.1 g), and solvent (24 g) were added to the
round bottomed flask, followed by the oxidant (10 mmol). This
mixture was then heated to 333 K when methanol or acetone
was used as the solvent or 353 K when acetonitrile was used as
the solvent. Samples were taken at timed intervals and diluted
in acetone prior to analysis. Analysis was by GC (Varian 3400)
fitted with a split/splitless injector (split ratio 1 : 50) and a
FID. The column used was an HP1 (30 m, id 0.23 mm) with
helium as the carrier gas. Conversions were based on mmol
of oxidant (i.e. 100% conversion corresponds to 50% conver-
sion of reactant and 100% conversion of oxidant). Reaction
products were analysed by GCMS (HP5890 GC coupled to a
TRIO 1 mass spectrometer) using the column and carrier gas
mentioned above and identified by means of library fitting and
comparison with authentic samples.
Experimental
Acknowledgements
Catalyst preparation
We thank Quest International for financial support.
Preparation of TS-1. TS-1 with a Si : Ti ratio of 50 : 1 was
prepared following the method proposed by Taramasso et al.21
Tetraethyl orthosilicate (TEOS) (86.4 g, Aldrich) was placed
into a beaker into which titanium ethoxide (1.92 g, Aldrich) was
carefully added with continuous stirring. The mixture was then
covered and stirred for a period of 2 h. A portion (∼10 ml) of
this mixture was then added dropwise to tetrapropylammonium
hydroxide (TPAOH) (40 wt% in water, 96 ml, Alfa) followed by
the addition of deionised water (20 ml). The remainder of
the mixture was then carefully added to the TPAOH solution
and the total volume of the resulting mixture was noted. This
volume was maintained via the addition of water whilst the
mixture was stirred for a period of 3 h at room temperature.
The solution was heated to 333 K for a period of 3 h with
continuous stirring, then aged for 18 h. The gel was then heated
at 448 K under autogeneous pressure in a Teflon lined autoclave
for 2 d under static conditions. The white solid obtained after
this period was isolated by filtration, dried at 373 K for ∼8 h and
calcined at 823 K in air for 24 h prior to use.
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