Direct formation of pinacols from olefins over various titano-silicates

Article abstract of DOI:10.1006/jcat.2002.3595

The epoxidation and successive pinacol formation of tri- and tetraalkyl-substituted olefins using Ti-β/H₂O₂/H₂O as the catalytic system has been investigated. Aluminum-free Ti-β exhibits better activity and pinacol selectivity than TS-1, TS-2, Ti-MCM-22, and mesoporous Ti-MCM-41. Pinacol (vic-diol) is obtained as the major product with small amounts of the side products pinacolone, alcohol (via hydration), and oligomers. The conversion and pinacol selectivity increase with an increase in reaction temperature and time. The change in product distribution with reaction time over Ti-β shows that the epoxide is the initial product which undergoes a secondary reaction to give pinacol as the major product. The conversion and H₂O₂ selectivity decrease with the bulkiness of the substituents at the C=C bond but the selectivity of pinacol is not significantly affected. The reactivity of cyclic 1-methyl-1-cyclohexene is considerably lower than that of the corresponding open-chain analogue 2-methyl-2-butene. The symmetrical tetramethyl-substituted 2,3-dimethyl-2-butene led to the formation of small amount of dimers over medium-pore titanium silicates TS-1, TS-2, and Ti-MCM-22. The epoxidation of these substituted olefins proceeded more rapidly when using acetonitrile as a cosolvent than under triphase conditions. Mechanistically, the primary epoxide product undergoes acid-catalyzed nucleophilic ring opening by H₂O molecules to give pinacol.

Full text of DOI:10.1006/jcat.2002.3595

Journal of Catalysis 209, 260–265 (2002)
doi:10.1006/jcat.2002.3595
RESEARCH NOTE
Direct Formation of Pinacols from Olefins over Various Titano–Silicates
M. Sasidharan, Peng Wu, and Takashi Tatsumi1
Division of Material Science and Chemical Engineering, Graduate School of Engineering, Yokohama National University,
79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
Received December 10, 2001; revised February 27, 2002; accepted February 27, 2002
in one pot by the initial epoxidation of the corresponding
The epoxidation and successive pinacol formation of tri- and olefin followed by nuclophilic reaction of the epoxide with
tetraalkyl-substituted olefins using Ti-β/H O /H O as the catalytic H O. The name pinacol denotes vic-diols with four alkyls
2
2
2
2
system has been investigated. Aluminum-free Ti-β exhibits better
activity and pinacol selectivity than TS-1, TS-2, Ti–MCM-22, and
mesoporous Ti–MCM-41. Pinacol (vic-diol) is obtained as the major
product withsmallamountsofthesideproductspinacolone, alcohol
groups; more specifically, vic-diol with four methyl groups
is called pinacol. Pinacol is an important starting material
for synthesizing compounds such as pinacolone (tert-butyl
methyl ketone by the well-known pinacol–pinacolone re-
arrangement) and pinacolone oxime (by ammoxidation).
These compounds are useful intermediates in the pro-
duction of pesticides, pharmaceuticals, fragrances, photo-
graphicchemicals, andcrop-protectionchemicals(3, 4). The
(
via hydration), and oligomers. The conversion and pinacol selectiv-
ity increase with an increase in reaction temperature and time. The
change in product distribution with reaction time over Ti-β shows
that the epoxide is the initial product which undergoes a secondary
reaction to give pinacol as the major product. The conversion and
H O selectivity decrease with the bulkiness of the substituents at conventional pinacol synthesis is by the hydrolysis of epox-
2
2
the C == C bond but the selectivity of pinacol is not significantly af- ides catalyzed by either a base or an acid, such as perchloric
fected. The reactivity of cyclic 1-methyl-1-cyclohexene is consid- acid (5). Pinacols are also directly synthesized from olefins
erably lower than that of the corresponding open-chain analogue
by reagents such as OsO4 and alkaline KMnO4 that add two
2
-methyl-2-butene. The symmetrical tetramethyl-substituted 2,3-
hydroxyl groups to a C == C bond (6). The main drawback of
these reagents is that OsO4 is not only expensive but also
highlytoxic. Furthermore, theuseofalkalineKMnO4 isalso
limited, as it leads to oxidation of glycols (7, 8). Another
strategy for making 1,2-diols is the reduction of aldehydes
and ketones with active metals, such as sodium, magnesium,
or aluminum (9), but it leads to many side products due to
coupling reactions.
dimethyl-2-butene led to the formation of small amount of dimers
over medium-pore titanium silicates TS-1, TS-2, and Ti–MCM-22.
The epoxidation of these substituted olefins proceeded more rapidly
when using acetonitrile as a cosolvent than under triphase con-
ditions. Mechanistically, the primary epoxide product undergoes
acid-catalyzed nucleophilic ring opening by H O molecules to give
2
pinacol.
ꢀc 2002 Elsevier Science (USA)
Over the course of a decade extensive research has been
carried out to replace these stoichiometric reagents with
reusable heterogeneous catalysts that are more environ-
mentally friendly. Titanium silicalite and its combination
with the clean oxidant H2O2 is employed in several chemi-
cal industries. Clerici et al. compared the reactivity of small
olefins such as butenes and pentenes in the epoxidation
with hydrogen peroxide over TS-1 (10, 11). Relevant to
the present study is the uncatalyzed epoxidation of 2,3-
dimethyl-2-butene with molecular O2 at high temperature
reported by Waldmann et al. (12). However, the synthe-
sis of pinacol using heterogeneous oxidizing agents such
as titanium-silicate and hydrogen peroxide in liquid-phase
conditions has not been reported. In this paper, reactiv-
ity and pinacol selectivity has been studied for various
INTRODUCTION
Selective oxidation and epoxidation plays a major role
both in industry and in the laboratory for the synthesis of
many useful organic building blocks in total synthesis. Thus,
one can design the required product by taking advantage
of the highly reactive oxirane ring (epoxides) and choosing
the right conditions to promote reactions, such as isomer-
ization, condensation, andnucleophilicsubstitution. Forex-
ample, styrene epoxide undergoes nucleophilic substitution
with aniline over zeolite HY (1) and α-pinene oxide isomer-
izes to campholenic aldehyde over Ti-β (2). In our study,
pinacol (2,3-dimethyl-2,3-butane diol) has been prepared
1
trialkyl- and tetraalkyl-substituted olefins using Ti-β/H O2
To whom correspondence should be addressed. E-mail: ttatsumi@
ynu.ac.jp.
2
and water as reaction medium (a triphase system). Also,
260
0
ꢀ
021-9517/02 $35.00
c 2002 Elsevier Science (USA)
All rights reserved.

FORMATION OF PINACOLS FROM OLEFINS OVER TITANO–SILICATES
261
reactionparameterssuchastemperature, reactiontime, and conditions. Ti–MCM-22 was prepared according to the lit-
the activity of various zeolites have been studied in detail erature (16) using fumed silica (Cab-o-sil-M7D), tetra-
using 2,3-dimethyl-2-butene. For comparison, the epoxida- butyl orthotitanate (TBOT), boric acid, and piperidine as
tion of different olefins has been carried out using acetoni- a structure-directing agent. The mesoporous Ti–MCM-41
trile in a two-phase system.
was also synthesized according to the reported procedure
17).
The catalytic reactions were carried out using a glass
reactor fitted with a water condenser through which ice-
cold water was passed. The reaction mixture was vigorously
stirred using a magnetic stirrer. A typical reaction involved
(
EXPERIMENTAL
Aluminum-free Ti-beta was synthesized in basic medium
pH of 12.5) by modifying a procedure found in the lit-
(
10 mmol of substrate, 10 mmol of H2O2, 20 wt% catalyst
erature (13). The titanoperoxo complex was formed by
the addition of Ti(OBu)4 and H2O2, which was added
to TEAOH under vigorous stirring. Then the required
amount of precipitated Nipsil VN-3 silica was added and
the stirring was continued for a few hours. Finally, 3%
of dealuminated seed crystals was added and the crystal-
lization was conducted at 413 K in a PTFE-lined stain-
less steel 150-ml autoclave under tumbling (60 rpm).
The gel composition was SiO2:0.025 TiO2:0.337 H2O2:0.55
TEAOH:6.6 H2O. After the crystallization, the contents
were centrifuged and extensively washed with distilled
water. The aluminum-containing Ti–Al-β material was syn-
thesized using Al (NO3)3 · 9 H2O from the gel composi-
tion SiO2:0.0044 Al2O3:0.337 H2O2:0.55 TEAOH:16 H2O.
The as-synthesized materials were dried at 373 K and
calcined in a flow of O2 for 12 h at 793 K. The cal-
(
Ti-β, Si/Ti = 43) with respect to substrate, and 10 ml of
water or acetonitrile. For triphase reactions involving water
as a dispersion medium, the reaction products are homog-
enized using the solvent acetone and the reaction prod-
ucts were analyzed by using a capillary gas chromatograph
(
Shimadzu 14A, OV-1 columns with flame ionization detec-
tors). The products were identified with authentic samples
and GC-MS splitting patterns, in combination with thin-
layerandcolumnchromatographytechniquesand HNMR
spectroscopy.
1
RESULTS AND DISCUSSION
Activity of Various Titanium–Silicates
Table 1 shows the reactivity of 2,3-dimethyl-2-butene to
cined Ti-β samples thus obtained were thoroughly char- 2,3-dimethyl-2,3-butane diol (pinacol) with H2O2 over a
acterized by XRD, FTIR, and UV–vis spectroscopies and rangeofcatalystssuchasTS-1, TS-2, Ti–MCM-22(medium-
surface area measurements. TS-1 and TS-2 were synthe- pore zeolites), Ti-β (large-pore zeolite), and Ti–MCM-41
sized according to the literature (14, 15). In a typical syn- (mesoporous). As our objective is to make pinacol in a
thesis of TS-1, 42 g of tetraethyl orthosilicate was hy- single step, the reaction has been carried out using water
drolyzed with 67.7 g of tetrapropylammonium hydroxide as a dispersion medium (triphase). As expected, pinacol is
(
2
20% aqueous, Aldrich) with vigorous stirring for 2 h. Then, formed selectively with the coexistence of a small amount of
.19 g of Ti(OBu)4 in anhydrous isopropanol and 54 g of the primary epoxide product for all the catalysts, immaterial
water were added and the clear solution was crystallized of their activity. The large-pore Ti-β(Si/Ti = 43; crystal size,
at 443 K for 30 h under static condition. In the case of 0.5–1 µm) exhibits better activity and selectivity for pina-
TS-2, tetrabutylammonium hydroxide was used and the col than the other zeolites studied. However, since Ti-β is
crystallization was carried out at 443 K for 72 h under static partly acidic, acid-catalyzed side products are formed, such
TABLE 1
Formation of Pinacol over Various Titanium–Silicatesa
Product selectivities (%)
Conv.
H2O2
Entry
Catalyst
Ti-β (43)c
Ti–Al-β (40)
TS-1 (33)
TS-2 (46)
Ti–MCM-22 (51)
Ti–MCM-41 (50)
(mol%)
sel. (%)
Epoxide
Pinacol
Pinacolone
DMBb
Others
1
2
3
4
5
6
55.3
51.2
39.2
21.2
22.6
48.2
80.1
76.5
61.5
57.0
54.5
65.0
1.3
1.1
3.9
4.0
3.4
1.6
92.9
82.6
88.0
83.6
86.0
96.3
1.9
3.7
1.3
1.2
2.0
1.1
4.3
15.6
1.0
1.9
5.4
0.5
0.4
5.9
9.0
5.1
0.3
0.7
a
Reaction conditions: 10 mmol of 2,3-dimethyl-2-butene; 10 mmol of H2O2 (31 wt% aqueous solution); catalyst, 20 wt%
with respect to substrate; 5 ml water as a dispersion medium; temperature, 333 K; and reaction time, 6 h.
b
DMB is 2,3-dimethyl-2-butanol and “others” include oligomers.
The figures in the parentheses represent Si/Ti ratios.
c

2
62
SASIDHARAN, WU, AND TATSUMI
O
CH3
CH3
OH OH
Effect of Reaction Temperature and Time
H3C
H3C
CH3
H3C
H3C
H C
Ti-beta
CH3 H2O2 / H2O
+ H2O
3
CH3
CH3
H3C
The influence of temperature on activity and product
distribution is presented in Fig. 1 for large-pore Ti-β; sep-
arate experimental runs were carried out for each point
of the graph in order to get a better mass balance. The
temperature has a marked effect on the conversion of
the bulky 2,3-dimethyl-2-butene. As expected, the reac-
tivity of 2,3-dimethyl-2-butene increases with temperature
while the selectivity to pinacol increases from 87 to 92%.
Since even in the absence of aluminum the Ti-β is par-
tially acidic due to the presence of structural defects as well
as a large number of silanol groups, it gives rise to acid-
catalyzed reactions, such as hydration and rearrangement.
Pinacolone, ultimately the most useful product in this re-
action, does not increase considerably with temperature,
since the Brønsted acid strength is not strong enough for
the facile pinacol-to-pinacolone rearrangement. The hydra-
tion of 2,3-dimethyl-2-butene to 2,3-dimethyl-2-butanol in-
creases from 4 to 5.1% at 333 K. In Fig. 1 “others” refers to
a mixture of epoxides, pinacolone, and oligomers (formed
in negligible amounts over large-pore Ti-β). The H2O2 se-
lectivity increases slightly with temperature; this is mainly
due to the relatively easy diffusion of bulky 2,3-dimethyl-2-
butene at high temperature, leading to the increased con-
version. As the temperature increases, the epoxide is almost
completely converted to pinacol.
Figure 2 exhibits the effect of reaction time on conversion
and selectivity of the various products. Pinacol is the ma-
jor product throughout the course of the reaction and the
epoxide selectivity in the first hour is 4.5%, which gradually
decreased to 1.3% after 6 h. The selectivity of pinacol in-
creased slightly with time and ranged between 86 and 92%.
Thus, the primary epoxide product readily undergoes cleav-
age to the 1,2-diol (pinacol), possibly due to steric crowd-
ing of the four-methyl substituents attached to the oxirane
ring as well as to the presence of a large excess of water
as a dispersion medium. The acid-catalyzed hydration of
Pinacol
2
,3-dimethyl-2-butene
Epoxide
Path A
+
H
-
H2O Path B
+
H
OH
H3C
H3C
CH3- shift
H C
O
CH3
3
+
CH3
CH3
+
H C
-
H
3
CH3
Pinacolone
SCHEME 1
as pinacolone (tert-butyl methyl ketone via acid-catalyzed
rearrangement), 2,3-dimethyl-2-butanol (via hydration of
olefin), and small amounts of oligomers. The various prod-
ucts formed in this reaction are depicted in Scheme 1.
Though the activities is of Ti-β and Ti–Al-β (Si/Ti = 40;
crystal size, 1 µm) are similar, there is a marked difference
in the amount of acid-catalyzed products formed; in par-
ticular, Ti–Al-β produces a large amount of 2,3-dimethyl-
-butanol via acid-catalyzed hydration of the reactant
olefin.
Entries 3–5 (Table 1) show the reactivity and pinacol se-
lectivity over the medium-pore TS-1, TS-2, and Ti–MCM-
2 catalysts. The activity of TS-1, with 10-membered ring
2
2
pore openings, is considerably lower than that of Ti-β,
which has large 12-membered ring pore openings. Thus,
the difference in activity between the medium-pore TS-1
(
Si/Ti = 33) and large-pore Ti-β (Si/Ti = 43) may be due to
the diffusion limitation of 2,3-dimethyl-2-butene exerted by
medium-pore zeolites, although the former contains more
titanium. Furthermore, the activity of TS-2 (Si/Ti = 46) is
only one-half that of TS-1, although there is not much dif-
ference in their crystallographic structure. The lower ac-
tivity of TS-2 suggests that the higher Si/Ti ratio coupled
with the large particle size (TS-1 and TS-2 crystal sizes are
0
.1–0.2 and 2–3 µm, respectively) and the disordered en-
IV
vironment around Ti species also influence the reactiv-
ity. The above results clearly suggest that the morphology
of the catalysts and the diffusion of reactants and prod-
ucts play an important role in addition to the pore size of
these titano-silicates. The observed activities of TS-2 and
Ti–MCM-22 (Si/Ti = 50; crystal size, 0.2–0.5 µm) are simi-
lar. All these medium-pore titanium-silicates yield pinacol
as the major product, but a small amount of oligomers (par-
ticularly dimers) is formed over these zeolites. The meso-
porous Ti–MCM-41 (Si/Ti = 51; crystal size, 0.2–0.5 µm)
shows good activity and the best selectivity of 96.3% with
negligible side products. Furthermore, Ti-β and Ti–MCM-
1
00
90
80
70
60
50
40
30
20
Conversion
Pinacol
DMB
Others
H2O2 Sel.
10
0
4
1 show higher H2O2 selectivity than the medium-pore
303
313
323
323
titanium-silicates. Thus, the observed activities of the large-
pore, medium-pore, and mesoporous catalysts clearly sug-
gest that diffusion of reactants and products controls the
rate of reaction and therefore Ti-β was used for further
study.
Temperature, K
FIG. 1. Reactivity of 2,3-dimethyl-2-butene and product selectivity
over Ti-β at different temperatures using H2O2 as oxidant and water as
reaction medium.

FORMATION OF PINACOLS FROM OLEFINS OVER TITANO–SILICATES
263
1
00
pinacol formation. Entries 2–4 show the effect of methyl,
isopropyl, and tert-butyl substituents on the olefinic dou-
ble bond, respectively. The reactivity of 2,3-dimethyl-2-
butene (entry 2) in which four methyl groups are at-
tached to the double bond was less than that observed
for 2-methyl-2-butene. When one of the methyl groups
in 2,3-dimethyl-2-butene was replaced by an isopropyl
substitutent, the activity decreased remarkably (entry 3).
Similarly, the presence of a tert-butyl substituent decre-
ased the reactivity considerably even when the double
bond was triply substituted (entry 4). However, the pinacol
was obtained in a narrow selectivity range (86–92%). In
addition to the normal acid-catalyzed products pinacolone
and alcohol, the bulky olefins give appreciable amounts of
oligomers. It is generally expected that the rate of reaction
should increase with the number of alkyl substituents since
they increase the electron density of the double bond. How-
ever, the observed reactivity follows the order 2-methyl-
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
Epoxide
DMB
Pinacolone
Conv.
Pinacol
H2O2 sel.
1
2
3
4
5
6
Reaction Time, h
FIG. 2. Distribution of various products with reaction time over a
Ti-β/H2O2/H2O catalytic system.
2
4
,3-dimethyl-2-butene also increased slightly with time, to
.7%. The selectivity of H2O2 dropped slightly as the reac-
tion progressed.
2-butene > 2,3-dimethyl-2-butene > 2,3,4-trimethyl-2-pen-
tene > 2,4,4-trimethyl-2-pentene. The titanium-silicate, as
an epoxidation catalyst characterized by electrophilic
character, should give the reverse order and this order
disagrees with what would be expected for the epoxidation
with peracids and other electrophilic oxidizing agents
Reactivity of Various Tri- and Tetraalkyl-Substituted
Olefins over Ti-β
Therateofoxidationisgenerallydependentontheolefin, (18). This must be due to the steric constraints imposed
i.e., thenumberandelectronicpropertiesofthesubstituents by the alkyl substitutents at the double bond, which
attached to the olefinic double bond. The observed reactiv- would play a major role in epoxidation and subsequent
ity and pinacol selectivity of various olefins with the cata- pinacol formation. Entry 5 exhibits the reactivity of cyclic
lytic system Ti-β/H2O2/H2O are given in Table 2. Among 1-methyl-1-cyclohexene, which is considerably less reactive
the different substrates studied, 2-methyl-2-butene with than its open-chain analogue 2-methyl-2-butene, proba-
three methyl substitutents attached to the olefinic double bly due to steric hindrance. In addition to epoxidation,
bond showed the highest activity for epoxidation and 1-methyl-1-cyclohexene also gives a small amount of allylic
TABLE 2
Reaction of Tri- and Tetraalkyl-Substituted Olefins over Ti-β under the Triphase Conditionsa
Product selectivities (%)
Conv.
(mol%)
H2O2
sel. (%)
Entry
Substrate
Epoxide
Pinacol
Pinacolone
Alcoholb
Others
1c
2
60.9
55.3
39.1
23.3
79.0
80.1
49.7
40.1
5.1
1.3
1.6
2.1
89.5
92.9
89.0
87.7
2.9
2.1
1.5
1.9
2.1
4.5
3.2
3.9
0.5
0.5
4.7
5.5
3
4
5
26.8
48.2
3.3
86.1
—
—
10.6d
a
Reaction conditions: 10 mmol of substrate; 10 mmol of H2O2 (31 wt% aqueous solution); Ti-β, 20 wt% with respect to
substrate; 5 ml water as dispersion medium; temperature, 333 K; and reaction time, 6 h.
b
Corresponding alcohol produced via hydration of olefin and “others” include oligomers.
c
The reaction was carried out in a closed stainless steel reactor (Parr-type reactor), as the substrate has a low boiling point
◦
(
boiling point 48 C).
d
Mixture of allylic oxidation products.

2
64
SASIDHARAN, WU, AND TATSUMI
TABLE 3
Reaction of Various Tri- and Tetraalkyl-Substituted Olefins over Ti-β Using Acetonitrile Cosolventa
Product selectivities (%)
Conv.
(mol%)
H2O2
sel. (%)
Entry
Substrate
Epoxide
Pinacol
Pinacolone
Alcoholb
Others
3.0
1c
2
42.5
38.0
44.5
39.0
48.1
43.5
1.1
1.7
1.3
28.6
57.8
1.9
9.8
3
4
28.2
14.9
36.1
19.0
16.0
43.0
76.0
51.0
0.5
1.0
0.4
0.9
7.4
4.1
5
18.7
25.6
48.5
43.1
—
2.1
6.1d
a
Reaction conditions: 10 mmol of substrate; 10 mmol of H2O2 (31 wt% aqueous solution); Ti-β, 20 wt% with respect to
substrate; 5 ml acetonitrile; temperature, 333 K; and reaction time, 6 h.
b
Corresponding alcohol produced via hydration of olefin and “others” include oligomers.
c
The reaction was carried out in a closed stainless steel reactor (Parr-type reactor), as the substrate has a low boiling point
◦
(
boiling point 48 C).
d
Mixture of allylic oxidation products.
oxidation products. The selectivity of H2O2 also decreased Mechanistic Aspects
considerably as the bulkiness of the substituents attached
Mechanistically, the primary epoxide product undergoes
to the double bond increased. Thus the pinacol formation
is controlled by diffusion of reactants rather than by an
electronic factor.
nucleophilic ring opening of oxirane to give the final prod-
uct pinacol over the catalytic Ti-β/H2O2/H2O system. As
depicted in Scheme 1, pinacolone can be found in two
reaction paths. The primary epoxide can interact with a
Brønsted acid site to a tert-carbocation (Path A), which
undergoes 1,2-migration of the methyl group as carbanion
Epoxidation of Various Tri- and Tetraalkyl-Substituted
Olefins Using Acetonitrile Cosolvent
Table 3 shows the epoxidation of various tri- and tetra- followed by proton elimination to give pinacolone. Alterna-
alkyl-substituted olefins using acetonitrile as a cosolvent tively, the pinacol can interact with a Brønsted acid site and
(
biphasic system) over the Ti-β/H2O2 catalytic system and undergo dehydration (Path B) to give tert-carbocation sim-
the activity is considerably lower than that observed under ilar to Path A and the remaining steps would be the same.
triphase conditions. Due to their hydrophobicity, titanium- The amount of pinacolone formed is not increased with an
silicates will selectively adsorb the organic molecules increase in temperature and is always less than 3% (for clar-
in triphase operation, leading to a high conversion, in ity it is not given in the graph). As the reaction was carried
accordance with our earlier report (19). Thus, the least out using excess water, the formation of the carbocation by
substituted 2-methyl-2-butene (entry 1) shows the highest dehydration (Path B) is not favored. Thus, it is suggested
activity, as in the triphase medium, and the reactivity de- that pinacolone is formed directly from the epoxide rather
creases with increasing number and cross section of the than from pinacol. Further, 2,3-dimethyl-2-butanol can be
substituents (entries 2 and 3). The tert-butyl-substituted formed either via direct olefin hydration or by cation for-
(
entry 4) and cyclic (entry 5) olefins react very slowly. Fur- mation from pinacol followed by hydride abstraction from
thermore, olefins with four alkyl substitutents (entries 3 and another molecule.
) give less epoxide selectivity than the olefins with three
Figure 2 exhibits the existence of appreciable amounts
4
alkyl substituents (entries 1, 4, and 5), possibly due to the of epoxide in the initial period, which decreased gradu-
steric strain of the oxirane ring. The Ti-β catalyst used in ally with reaction time. However, the distribution of pina-
the triphase condition was activated at 673 K for 2 h under col and epoxide clearly reveals that pinacol is formed in-
a flow of air and reused for the oxidation of 2,3-dimethyl- stantaneously as soon as the epoxide is formed, probably
2
-butene. The catalyst retained 81% of its original activity because of the strain of the epoxide as well as the high
after three successive uses. The loss of activity could be par- solubility of pinacol in water. Like other oxidation reac-
tial leaching of titanium from the frameworks either by the tions using titanium-silicates, the facile formation of pinacol
action of H2O2 or by the side products (20, 21).
would be facilitated by successive epoxidation and cleavage

FORMATION OF PINACOLS FROM OLEFINS OVER TITANO–SILICATES
265
by H2O molecules assisted both by titanium-hydroperoxo
complexes formed by the interaction of TiIV species with
H2O2 and H2O, as proposed in the literatures (22, 23), and
by Brønsted acid sites.
2. Kunkeler, P. J., van der Waal, J. C., Bremmer, J., Zuurdeeg,
B. J., Downing, R. S., and van Bekkum, H., Catal. Lett. 135
(
1998).
. Schossig, F., “Ullmann’s Encyclopedia of Industrial Chemistry”
F. Thomas Campbell, Rudolf Pfefferkorn, and James F. Rounsaville,
Eds.), Vol. A1, p. 305. Fed. Rep. of Germany, 1985.
3
(
CONCLUSIONS
4. Sheldon, R. A., Elings, J. A., Lee, S. K., Lempers, H. F. B., andDowning,
R. S., in “Coll. Abstr., 4th Int. Symp. Heterog. Catal. and Fine Chem-
icals, Basel,” p. 80 (1996).
The large-pore Ti-β with a three-dimensional channel
structure exhibits a high activity and selectivity in the
epoxidation and subsequent pinacol formation of tri- and
tetraalkyl-substituted olefins using dilute H2O2 as oxidant.
The reactivity of these olefins increases as the temperature
increases from 303 to 333 K. The distribution of products
with time reveals that epoxide is the initial reaction prod-
uct, which is transformed to pinacol by reaction with H2O.
The reactivity of mesoporous porous Ti–MCM-41 is as high
5
. Fieser, F., “Reagents for Organic Synthesis,” Vol. 1. Wiley, New York,
967.
. Hudlicky, M., “Oxidations in Organic Chemistry.” Am. Chem. Soc.,
1
6
Washington, DC, 1990.
7. Taylor, G., Can. J. Chem. 63, 2777 (1985).
8
9
. Wolfe, I., J. Am. Chem. Soc. 103, 940 (1981).
. Cusk, F., and Weidmann, R., J. Chem. Soc. Perkin Trans. 1 1729
(
1988).
0. Clerici, M. G., Bellussi, G., and Romano, U., J. Catal. 129, 159
1991).
1
(
as Ti-β, but the medium-pore TS-1, TS-2, and Ti–MCM-22 11. Clerici, M. G., and Ingallina, P., J. Catal. 140, 71 (1993).
1
2. Waldmann, H., Wilms, K. G., Schmid, O., Siekmann, G., and Keiser,
H., Chem.-Ing.-Tech. 61 (12), 967 (1989).
give a lower conversion than Ti-β or Ti–MCM-41. Acidic
titanium-hydroperoxo complexes as well as Brønsted acid
sites would promote the nucleophilic ring opening of the
oxirane ring by a H2O molecule to give pinacol in high
13. Blasco, T., Camblor, M. A., Corma, A., Esteve, P., Guil, J. M., Martinez,
A., Perdigon-Melon, J. A., and Valencia, S., J. Phys. Chem. B 102, 75
(
1998).
selectivity. All the titano-silicates produce unwanted acid- 14. Thangaraj, A., Kumar, R., Mirajkar, S. P., and Ratnasamy, P., J. Catal.
1
30, 1 (1990).
5. Reddy, J. S., Kumar, R., and Ratnasamy, P., Appl. Catal. L1, 58
1990).
6. Peng, W., Tatsumi, T., Komatsu, T., and Yashima, T., Chem. Lett. 774
2000).
catalyzedsideproductsdependingupontheirBrønstedacid
strength. Ti–Al-β produces a large amount of alcohol as hy-
dration product. The activity of these olefins and the H2O2
selectivity decrease as the bulkiness or cross section of the
1
1
(
(
substituent increases. Thus, the least substituted 2-methyl- 17. Koyano, K. A., and Tatsumi, T., Microporous Mater. 10, 259
(
1997).
2
2
-buteneshowsthehighestactivitywhereas2,4,4-trimethyl-
-pentene shows the lowest activity.
1
1
2
8. Sheldon, R. A., J. Mol. Catal. 7, 107 (1990) and refs. therein.
9. Bhaumik, A., and Tatsumi, T., J. Catal. 176, 305 (1998).
0. Carati, A., Flego, C., Massara, E. P., Millini, R., Caruccio, L., Parker,
W. O., Jr., and Bellussi, G., Microporous Mesoporous Mater. 30, 137
ACKNOWLEDGMENTS
(
1999).
2
1. Davis, L. J., Mcmorn, P., Bethell, D., Bulman Page, P. C., King,
F., Hancock, F. E., and Hutchings, G. J., J. Mol. Catal. A 165, 243
M.S. thanks the Japan Society for the Promotion of Science for a post-
doctoral fellowship.
(
2001).
2
2
2. Bellusssi, G., Carati, A., Clerici, M. G., Maddinelli, G., and Millini, R.,
J. Catal. 133, 220 (1992).
3. Bhaumik, A., Kumar, P., and Kumar, R., Catal. Lett. 40, 47
(1996).
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1