Titanium-catalyzed heterogeneous oxidations of silanes, chiral allylic alcohols, 3-alkylcyclohexanes, and thianthrene 5-oxide: A comparison of the reactivities and selectivities for the large-pore zeolite Ti-β, the mesoporous Ti-MCM-41, and the layered alumosilicate Ti-ITQ-2

Article abstract of DOI:10.1006/jcat.2000.3043

A comparative study of silane oxidation, olefin epoxidation, and thianthrene 5-oxide sulfoxidation with the oxidants Ti-β/H₂O₂, Ti-MCM-41/t-BuOOH, and Ti-ITQ-2/t-BuOOH provides the catalytic reactivity order Ti-β > Ti-MCM-41 > Ti-ITQ-2. The steric constraints of the narrow channels make the Ti-β zeolite the most selective. For the more open structures of the Ti-MCM-41 and Ti-ITQ-2 hosts, such steric constraints are less pronounced and, therefore, a lower selectivity is exhibited by these heterogeneous catalysts. Both activate t-BuOOH for oxygen transfer through a transition structure analogous to the homogeneous Ti(Oi-Pr)₄/t-BuOOH oxidant.

Full text of DOI:10.1006/jcat.2000.3043

Journal of Catalysis 196, 339–344 (2000)
doi:10.1006/jcat.2000.3043, available online at http://www.idealibrary.com on
Titanium-Catalyzed Heterogeneous Oxidations of Silanes, Chiral Allylic
Alcohols, 3-Alkylcyclohexanes, and Thianthrene 5-Oxide: A Comparison
of the Reactivities and Selectivities for the Large-Pore Zeolite Ti- , the
Mesoporous Ti-MCM-41, and the Layered Alumosilicate Ti-ITQ-2
,2
Waldemar Adam, ,1 Avelino Corma,† Hermenegildo Garc´ıa,† and Oliver Weichold
Institut fu¨r Organische Chemie der Universita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany; and †Instituto de Tecnolog´ıa Qu´ımica,
UPV-CSIC, Avda. de los Naranjos s/n, E-46071 Valencia, Spain
Received April 28, 2000; revised August 21, 2000; accepted August 21, 2000
capable of oxidizing large organic substrates. Recently, a
new layered alumosilicate has been prepared, which has no
A comparative study of silane oxidation, olefin epoxidation, and
thianthrene 5-oxide sulfoxidation with the oxidants Ti- /H2O2, Ti-
MCM-41/t-BuOOH, and Ti-ITQ-2/t-BuOOH provides the catalytic
reactivity order Ti- > Ti-MCM-41 > Ti-ITQ-2. The steric con-
straints of the narrow channels make the Ti- zeolite the most
selective. For the more open structures of the Ti-MCM-41 and
Ti-ITQ-2 hosts, such steric constraints are less pronounced and,
therefore, a lower selectivity is exhibited by these heterogeneous cat-
alysts. Both activate t-BuOOH for oxygen transfer through a transi-
tion structure analogous to the homogeneous Ti(Oi-Pr)4/t-BuOOH
c
˚
channels or cavities, but consists of a layer of 25-A-thick
sheets (6). This material has been grafted with titanium
atoms and, therefore, exhibits catalytic oxidation activity
(7). These active Ti atoms are predominantly located at the
rims of the open cups of the ITQ-2 structure and are not
significantly shielded by the zeolite framework.
While Ti- -catalyzed oxidations are well studied (4, 8),
only a few reports for Ti-MCM-41 (9) and Ti-ITQ-2 exist (6,
7). Furthermore, no efforts have been made so far to assess
the reactivity and selectivity of these heterogeneous cata-
lysts and compare them with those of the heterogeneous
Ti- /H2O2 (4) and the homogeneous Ti(Oi-Pr)4/t-BuOOH
(10) oxidants. Herewith we fill this gap by reporting our re-
cent results for the catalytic oxidation of phenyldimethyl-
silane, the epoxidation of 3-alkylcyclohexanes and chiral
allylic alcohols, as well as the sulfoxidation of thianthrene 5-
oxide with Ti- /H2O2, Ti-MCM-41/t-BuOOH, and Ti-ITQ-
2/t-BuOOH.
oxidant.
2000 Academic Press
Key Words: zeolites; heterogeneous oxidation; silanes; allylic al-
cohols; thianthrene 5-oxide.
INTRODUCTION
The application of metal-doped zeolites in organic syn-
thesis has received increased attention during the past
decade (1). The structural variety of metal-doped zeolites
has made possible manipulations of their surface charac-
teristics and tuning of their acid/base properties, such that
these materials have become powerful synthetic tools. Since
the sites responsible for the oxidation activity are located
within highly uniform channels and cavities, the zeolite
oxidants offer the opportunity to conduct shape-selective
catalysis (2). The most prominent example is the titanium
METHODS
Preparation of Ti-ITQ-2. The titanium grafting was car-
ried out by pouring a solution of titanocene dichloride in
60 ml of CHCl3 onto 10 g of thermally dehydrated ITQ-2
(6), heated at 300 C under 10 3 Torr for 2 h. The suspension
was stirred at room temperature (20 C) under an atmo-
sphere of N2 gas for 1 h. Subsequently, a solution of triethy-
lamine in 7 ml of CHCl3 (1 : 1 ratio of NEt3 and TiCp2Cl2)
was added. The suspension changed from red to yellow-
orange, an indication of Ti coordination. The solid was col-
lected by filtration, exhaustively washed with CH2Cl2, and
calcined at 540 C in a nitrogen gas stream for 1 h and then
in air for an additional 6 h.
˚
silicalite TS-1 (channel diameter 5.5 A); however, only rela-
tively small organic molecules penetrate to reach the active
sites of this useful catalyst (3). To circumvent these limi-
tations, titanium-doped micro- and mesoporous materials
˚
with larger channel diameters, e.g., Ti- (6.4
7.5 A) (4)
˚
or Ti-MCM-41 (35 A) (5), have been developed, which are
1 To whom correspondence should be addressed. Fax: +49 931 888 4756.
E-mail: adam@chemie.uni-wuerzburg.de. Internet: http://www-organik.
chemie.uni-wuerzburg.de.
General oxidation procedure. To 150 mol of the sub-
strate in 0.3 ml of the appropriate solvent was added
2 E-mail: hgarcia@chim.upv.es.
339
0021-9517/00 $35.00
c
Copyright
2000 by Academic Press
All rights of reproduction in any form reserved.

340
ADAM ET AL.
TABLE 1
1.1 equiv of the oxygen donor (85% H2O2 or 8.0 M t-
BuOOH in CH2Cl2), followed by 15 mg of the catalyst.
The mixture was stirred for 24 h at the stated tempera-
ture, and the zeolite was removed by means of a mem-
brane filter (0.45 m pore size), washed with 0.4 ml of
MeOH (d4-MeOH in the case of the allylic alcohols and
cyclohexene derivatives to enable direct NMR analysis of
the reaction mixture), and two times with 0.4 ml of the
solvent in which the reaction was carried out. The prod-
uct analysis was conducted after workup as follows: the
silane oxidation was performed in CH2Cl2 (CH3CN for Ti-
) at ambient (ca. 20 C) temperature and the product com-
position determined by GC analysis {HP-5 column (5%
diphenylpolysiloxane and 95% dimethylpolysiloxane),
80(5 min) → [30 C/min] → 130(1 min) → [30 C/min] →
Mass Balances, Conversions, and Product Selectivities for the
Heterogeneous Oxidation of Phenyldimethylsilanea
catalyst
PhMe2SiH
→ PhMe2SiOH + [PhMe2Si]2O
solvent,20 C,24 h
(SiOH)
(Si2O)
mb
convn
(% )
Selectivity
[SiOH] : [Si2O]
Catalyst
Solvent
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
(% )
b
Ti- /H2O2
89
80
>99
74
99
48
29
38
>99 :1
90 : 10
95 : 5
Ti-MCM-41/t-BuOOH
Ti-ITQ-2/t-BuOOH
Ti(Oi-Pr)4/t-BuOOHb
39 : 61
a Mass balances, conversions, and chemoselectivities were determined
by gas chromatography after workup, with n-hexadecane as internal stan-
2
240(2 min), N2 pressure = 1.5 kg/cm }, with n-hexadecane
dard.
as internal standard; the allylic alcohols were epoxidized
in CDCl3 (CD3CN for Ti- ) at ambient (ca. 20 C) tem-
perature and the product composition was determined by
1H NMR spectroscopy, with naphthalene as internal stan-
dard; the cyclohexene derivatives (11) were epoxidized in
CDCl3 (CD3CN for Ti- ) at 40 C in sealed ampoules and
the product composition was determined as stated for the
allylic alcohols; the sulfoxidation of thianthrene 5-oxide
(SSO) was performed as described previously (12). The
use of an internal standard allowed the conversions (% )
and mass balances (mb, % ) to be assessed quantitatively.
The lower mb values (<80% ) in Tables 1 and 2 are due
to incomplete product recovery from the zeolite in the ex-
traction process, presumably due to covalent grafting of the
sensitive epoxides.
b See Ref. (8d).
(entry 4) (8d). The reactivity, therefore, follows the order
Ti- /H2O2 > Ti-MCM-41/t-BuOOH > Ti-ITQ-2/t-BuOOH
> Ti(Oi-Pr)4/t-BuOOH.
Chemoselectivity
In addition to the reactivity, the silane oxidation of-
fers also the possibility to test for chemoselectivity (i.e.,
silanol versus disiloxane formation) of these oxidants. For
Ti- /H2O2, we have previously found exclusively the silanol
product (8d). Despite their more open structure, Ti-MCM-
41/t-BuOOH and Ti-ITQ-2/t-BuOOH still display a high
chemoselectivity for the silanol. This is in stark contrast to
the homogeneous Ti(Oi-Pr)4/t-BuOOH, for which the dis-
iloxane product dominates. For the silane oxidation with
Ti- /H2O2, we have postulated that steric constraints inside
RESULTS
˚
the 7-A-wide channel of Ti- prevents the silanol conden-
Reactivity
˚
sation (8d). However, this does not apply to the 35-A-wide
channels of the mesoporous Ti-MCM-41 or on the surface
of the layered alumosilicate Ti-ITQ-2. We have shown re-
cently (12) that even inside the mesoporous Ti-MCM-41,
the metal is sufficiently shielded from coordination. It is
likely, therefore, that the good silanol selectivities for Ti-
MCM-41 are due to the inhibition of the simultaneous co-
ordination of two silanol molecules at the same Ti center.
For Ti-ITQ-2, the product selectivity may be rationalized
by its low acidity compared to that of Ti- , as supported
by the lack of acid-catalyzed ring-opening products in the
former case (6). The chemoselectivity, therefore, parallels
essentially the reactivity order.
We demonstrated previously that Ti- efficiently cata-
lyzes the selective oxidation of silanes to silanols with H2O2;
the condensation of the silanol product to its disiloxane
is prevented due to space limitations (8d). This oxidation
process has been used herein to assess the reactivity and
selectivity of the heterogeneous catalysts Ti-ITQ-2 and Ti-
MCM-41 (Table 1). As oxidant, t-BuOOH was used with
Ti-MCM-41 and Ti-ITQ-2, since these catalysts do not ex-
hibit sufficient oxidation activity with H2O2. This is in con-
trast to Ti- , which poorly activates t-BuOOH (data not
shown). While with Ti- /H2O2 complete silane conversion
was reached within 24 h (entry 1) (8d), Ti-ITQ-2 and Ti-
MCM-41 displayed substantially lower conversions (en-
tries 2 and 3), as indicated in the drop of the percent con-
version for Ti-MCM-41 and Ti-ITQ-2. Additionally, small
Diastereoselectivity
By use of the concept of allylic strain (13), we recently
amountsofthe disiloxane were also formed in the case ofTi- have refined the assumed transition structure for the Ti- -
ITQ-2 and Ti-MCM-41. These results contrast those of catalyzed epoxidation (8c, 14). This mechanistic probe is
Ti(Oi-Pr)4/t-BuOOH, for which a lowconversion wasfound now to be applied to the alcohols 1a–d, which were chosen

TITANIUM-CATALYZED HETEROGENEOUS OXIDATIONS
341
TABLE 2
Comparison of the Diastereoselectivities (threo/erythro) for the Epoxidation of Allylic Alcohols 1 with Various
Heterogeneous and Homogeneous Titanium-Based Oxidants
Ti-ITQ-2/t-BuOOH
Ti-MCM-41/t-BuOOH
(in CDCl3)a
(in CDCl3)a
Ti- /85% H2O2
(in CD3CN),a,b
dr
Ti(Oi-Pr)4/t-BuOOH
mb
(% )
convn
(% )
mb
(% )
convn
(% )
(in CDCl3),c
dr
Substrate
dr
dr
80
93
65
75
95 : 05
95 :05
>95
31
86 :16
91 :09
89 :11
91 :09
83 :17
82
78
83 :17
82
80
34
60
35 :65
78 :22
72
72
50
63
31 :69
78 :22
56 :44
70 :30
22 :78
57 :43
a Heterogeneous reaction; mass balances, conversions, and threo/erythro diastereoselectivities (dr) determined by 1H NMR
spectroscopy after workup; error 5% of the given value.
b See Ref. (8d).
c Homogeneous reaction, see Ref. (8d).
since they display the largest differences in diastereose- one Ti(Oi-Pr)4/t-BuOOH, a mechanistically pertinent
lectivity between heterogeneous (Ti- /H2O2) and homo- result.
geneous [Ti(Oi-Pr)4/t-BuOOH)] oxidations. The epoxida-
tion of these chiral allylic alcohols with Ti-ITQ-2/t-BuOOH
and Ti-MCM-41/t-BuOOH afforded exclusively the corre-
Steric Constraints
sponding epoxides in moderate to good yields (Table 2).
The diastereomeric ratios (dr) follow a similar trend for
these catalysts; in fact, for the allylic substrates 1c,d the
dr values are within the error limits. For the alcohols 1a,b
(entries 1 and 2) with a cis-methyl group (1,3-allylic strain
present), the threo epoxide is preferentially formed with all
oxidants, even when a gem-methyl group (1,2-allylic strain
present) is also present. In contrast, for substrate 1c (en-
try 3) without a cis-methyl group (1,3-allylic strain absent),
the erythro epoxide prevails for Ti-ITQ-2, Ti-MCM-41, and
Ti(Oi-Pr)4, while an almost 1 : 1 mixture of the threo/erythro
epoxides is found for Ti- . For the tert-butyl substrate
1d (entry 4), without 1,3- and 1,2-allylic strain, the threo
epoxide is obtained for all heterogeneous oxidants exam-
ined herein, while the homogeneous Ti(Oi-Pr)4/t-BuOOH
yields a nearly 1 : 1 mixture of both diastereomers. Ex-
cept for the substrate 1d, the diastereoselectivities for
the heterogeneous Ti-MCM-41/t-BuOOH and Ti-ITQ-
2/t-BuOOH oxidants match those for the homogeneous
The epoxidation of the 3-alkyl-substituted cyclohexene
derivatives 3a–d (Table 3) was conducted to probe for
purely steric effects, in the absence of electronic and coor-
dination effects between the substrate and metal catalyst,
which usually operate in the case of the allylic alcohols 3.
With increasing bulk of the 3-alkyl substituent, the trans
selectivity increases. Again, no significant difference in the
cis/trans selectivity was observed for the mesoporous Ti-
MCM-41 and the layered alumosilicate Ti-ITQ-2. In con-
˚
trast, with the large-pore zeolite Ti- (7 A diameter), the
trans epoxide was found in large excess over the cis one,
even for the sterically more exposed methyl derivate 3a.
The other sterically more encumbered cyclohexenes 3b–d
afforded the trans epoxide exclusively. A semilogarithmic
plot of the diastereomeric ratios against Taft’s steric param-
eter ES afforded a good linear correlation for Ti-MCM-41
and Ti-ITQ-2 (Fig. 1); however, such a plot is not feasible
for the Ti- case due to the essentially exclusive formation
of the trans epoxide already for the methyl derivative 3a,

342
ADAM ET AL.
TABLE 3
Diastereoselectivities (trans/cis) of the Heterogeneous Oxidation of 3-Alkyl-Substituted Cyclohexenes
by Ti-ITQ-2/t-BuOOH, Ti-MCM-41/t-BuOOH, and Ti- /H2O2
Ti-ITQ-2/t-BuOOH
Ti-MCM-41/t-BuOOH
Ti- /H2O2
(in CDCl3)a
(in CDCl3)a
(in CD3CN)a
mb
(% )
convn
(% )
mb
(% )
convn
(% )
mb
(% )
convn
(% )
Substrate
dr
dr
dr
75
80
81
79
68
79
31
54
61 :39
63 :37
76
71
82
70
43
58 :42
79
80
83
89
47
52
55
73
92 :08
86
84
82
61 :39
69 :31
92 :08
>95 :05
>95 :05
>95 :05
71 :29
>95 :05
a Mass balances, conversions, and trans/cis diastereoselectivities (dr) determined by 1H NMR spectroscopy after
workup; error 5% of the given value.
such that the steric effects between the substrate and cata- 41/t-BuOOH by employing the thianthrene 5-oxide (SSO)
lytic site are not sensed for this catalyst. Nevertheless, the probe for electronic effects (12). Since we had observed for
constraint-controlled selectivity, i.e., Ti- > Ti-MCM-41
these heterogeneous oxidants significant differences in the
Ti-ITQ-2, follows the expected order of steric effects, which XSO parameter [a quantitative measure of sulfone (SSO2)
are most pronounced for Ti- .
versus sulfoxide (SOSO) formation] (15) when compared
to the homogeneous oxidant Ti(Oi-Pr)4/t-BuOOH, it was
mechanistically relevant to determine also the electronic
nature of the Ti-ITQ-2/t-BuOOH oxidant (Scheme 1). As
for the heterogeneous oxidations described above, the re-
sults for Ti-ITQ-2/t-BuOOH (XSO = 0.12) parallel those for
Electronic Nature
We have recently assessed the electrophilic character
of the heterogeneous oxidants Ti- /H2O2 and Ti-MCM-
Ti-MCM-41/t-BuOOH (XSO = 0.18) and Ti- /H2O2 (XSO
=
0.07), but contrast those for the homogeneous oxidant
Ti(Oi-Pr)4/t-BuOOH (XSO = 0.59) (12). Therefore, the het-
erogeneous Ti-ITQ-2/t-BuOOH oxidant displays the ex-
pected electrophilic oxidizing properties, as previously
found for the Ti-MCM-41/t-BuOOH and Ti- /H2O2 cases.
MECHANISTIC DISCUSSION
Unquestionably, the overall catalytic activity of the three
porous titanosilicates (H2O2 or t-BuOOH as oxygen
sources) is primarily governed by the zeolite pore size and
by their relative hydrophilicity/hydrophobicity (1); how-
ever, mechanistic information on the transition structures
may be gained by means of stereochemical probes (13).
For this purpose, a detailed comparison of the herein ac-
quired stereochemical data of the Ti-ITQ-2/t-BuOOH and
FIG. 1. Diastereoselectivity (trans/cis) for the epoxidation of the chi-
ral 3-alkylcyclohexenes 3a–d as a function of Taft’s steric parameter Es.

TITANIUM-CATALYZED HETEROGENEOUS OXIDATIONS
343
SCHEME 1. Chemoselectivities for the sulfoxidation of thianthrene 5-oxide (SSO) with various heterogeneous and homogeneous titanium-
mediated oxidants.
Ti-MCM-41/t-BuOOH systems with those of the previously strate is involved in the case of Ti- /H2O2, which leads to a
reported Ti- /H2O2 (8c) and Ti(Oi-Pr)4/t-BuOOH (8c) more obtuse dihedral angle of ca. 120 (Figure 2, B) (8c).
shall now be made, which allows us to propose a detailed Consequently, 1,3A strain dominates for this oxidant and
mechanism for the oxygen transfer by these oxidants. Al- threo diastereoselectivity is observed for substrates 1a and
lylic strain in the chiral allylic alcohols 1a–c, i.e., the steric 1b (Table 2, entry 1 and 2); the latter stereochemical probe
interactions between the two geminal groups at a double (17) clearly demonstrates that 1,2A strain is negligible.
bond (1,2-allylic strain, 1,2A) or between the cis substituents
A comparison of the stereochemical results for the ox-
(1,3-allylic strain, 1,3A), leads to a preferred conformation idation of the chiral allylic alcohols 1a–c by Ti-ITQ-2/t-
of the substrate–oxidant complex with the hydroxy group BuOOH and Ti-MCM-41/t-BuOOH (Table 2) with the
favorably disposed for interaction with the oxidant. Conse- known data for Ti(Oi-Pr)4/t-BuOOH (8c) and Ti- /H2O2
quently, the two faces in the allylic substrate are appro- (8c) reveals better correspondence with the homogeneous
priately differentiated for the oxygen transfer and charac- Ti(Oi-Pr)4/t-BuOOH oxidant (metal–alcoholate bonding)
teristic diastereoselectivities are observed (hydroxy group than the heterogeneous one Ti- /H2O2 (hydrogen bond-
directivity) (13).
ing). Therefore, we propose that the transition structure
In the case of Ti(Oi-Pr)4/t-BuOOH, the substrate is fixed for the oxidation by the heterogeneous catalysts Ti-ITQ-2
by a metal–alcoholate bond (Fig. 2, structure A) (16), which and Ti-MCM-41 is more akin to the one for Ti(Oi-Pr)4/t-
== – –
requires a more acute dihedral angle (C C C O) of ca. BuOOH, i.e., the substrate is fixed by a metal–alcoholate
70 to 90 in the oxygen transfer, such that a pronounced bond (Figure 2, C) and a dihedral angle of 70 < < 90
influence of 1,2A strain operates in the control of the di- applies in this oxygen-transfer process.
astereoselectivity, as observed for substrates 1a,c (Table 2,
The appreciable threo selectivity for the epoxidation of
entries 1 and 3). In contrast, hydrogen bonding between the the tert-butyl derivative 1d with the heterogeneous oxida-
metal-based oxidizing species and the allylic alcohol sub- tion systems Ti-ITQ-2/t-BuOOH, Ti-MCM-41/t-BuOOH,
and Ti- /H2O2 (Table 2, entry 4) seems to contradict the
proposed transition-structure model in Fig. 2. If at all, an
erythro preference would be expected in the diastereose-
lectivity since metal–alcoholate bonding operates and the
substrate does not possess allylic strain (13). However, for
such sterically encumbered chiral allylic substrates as the
tert-butyl derivative 1d, additional repulsions come into
play, which are divulged in Scheme 2. Thus, in the erythro
transition structure D, the steric interactions of the large
t-Bu group with the zeolite lattice disfavor this arrange-
ment, while such interactions are absent in the threo transi-
tion structure E. In the homogeneous Ti(Oi-Pr)4/t-BuOOH
oxidant no such steric constraints are present and, conse-
quently, a ca. 1 : 1 erythro/threo mixture of epoxides is ob-
tained. In this respect, it is mechanistically relevant to note
FIG. 2. Transition structures for the oxygen transfer by the homo-
geneous Ti(Oi-Pr)4/t-BuOOH (A) and the heterogeneous Ti- /H2O2
(B), Ti-ITQ-2/t-BuOOH (C), and Ti-MCM-41/t-BuOOH (C) oxidants.
Note the difference between the oxygen sources in the activated com-
that for the epoxidation of the alcohol 1d by m-CPBA, for
plexes B (H2O2) and C (t-BuOOH).

344
ADAM ET AL.
SCHEME 2. Transition structures erythro (D) and threo (E) for the epoxidation of the chiral tert-butyl-substituted allylic alcohol 1d with the
heterogeneous Ti-ITQ-2/t-BuOOH and Ti-MCM-41/t-BuOOH oxidants.
which the transition structure analogous to that of Ti- was
postulated (8c), also a ca. 1 : 1erythro/threo mixture ofepox-
ides was found. All these results taken together substanti-
ate that the preferred threo selectivity for the oxidation of
alcohol 1d by Ti-ITQ-2/t-BuOOH, Ti-MCM-41/t-BuOOH,
and Ti- /H2O2 derives from steric constraints at the zeolite
surface of these solid-supported titanium-based catalysts.
(1998); (c) Corma, A., Chem. Rev. 97, 2373 (1997); (d) Sheldon, R. A.,
Stud. Surf. Sci. Catal. 110, 151 (1997); (e) Notari, B., Adv. Catal. 41,
253 (1996).
2. (a) Venuto, R. B., and Landis, P. S., Adv. Catal. 18, 259 (1968);
(b) Csicsery, S. M., Zeolites 4, 202 (1984).
3. Reddy, J. S., and Sivasanker, S., Catal. Lett. 11, 241 (1991).
4. Camblor, M. A., Corma, A., Mart´ınez, A., and Pe´rez Pariente, J.,
J. Chem. Soc., Chem. Commun. 589 (1992).
5. (a) Corma, A., Navarro, M. T., and Pe´rez Pariente, J., J. Chem. Soc.,
Chem. Commun. 147 (1994); (b) Blasco, T., Corma, A., Navarro,
M. T., and Pe´rez Pariente, J., J. Catal. 156, 65 (1995).
6. Corma, A., Forne´s, V., Pergher, S. B., Maesen, Th. L. M., and Buglass,
J. G., Nature 396, 353 (1998).
CONCLUSION
It was demonstrated that the heterogeneous oxidants Ti-
ITQ-2/t-BuOOH and Ti-MCM-41/t-BuOOH exhibit simi-
lar reactivity and chemoselectivity due to their quite open
structure. A transition structure analogous to the one for
the homogeneous Ti(Oi-Pr)4/t-BuOOH oxidant applies, in
which the substrate is fixed by metal–alcoholate bonding
and with a dihedral angle of 70 < < 90 for the oxy-
gen transfer. The importance of allylic strain is evident for
the chiral allylic alcohols 1a–c, but additionally interactions
with the inorganic framework influence the transition-state
geometry for the sterically encumbered substrate 1d.
7. Corma, A., D´ıaz, U., Forne´s, V., Jorda´, J. L., Domine, M., and Rey, F.,
J. Chem. Soc., Chem. Commun. 779 (1999).
8. (a) Camblor, M. A., Corma, A., and Pe´rez-Pariente, J., Zeolites 13,
82 (1993); (b) Hulea, V., and Moreau, P., J. Mol. Catal. A: Chemical
113, 499 (1996); (c) Adam, W., Corma, A., Reddy, T. I., and Renz, M.,
J. Org. Chem. 62, 3631 (1997); (d) Adam, W., Garc´ıa, H., Mitchell,
C. M., Saha-Mo¨ller, C. R., and Weichold, O., J. Chem. Soc., Chem.
Commun. 2609 (1998).
9. (a) Corma, A., Navarro, M. T., and Pe´rez-Pariente, J., J. Chem. Soc.,
Chem. Commun. 147 (1994); (b) Chen, L. Y., Chuah, G. K., and
Jaenicke, S., Catal. Lett. 50, 107 (1998).
10. Rossiter, B. E., Verhoeven, T. R., and Sharpless, K. B., Tetrahedron
Lett. 4733 (1979).
11. Adam, W., Mitchell, C. M., and Saha-Mo¨ller, C. R., Eur. J. Org. Chem.
785 (1999).
ACKNOWLEDGMENTS
12. Adam, W., Mitchell, C. M., Saha-Mo¨ller, C. R., Selvam, T., and
Weichold, O., J. Mol. Catal. A: Chemical 154, 251 (2000).
13. Adam, W., and Wirth, T., Acc. Chem. Res. 32, 703 (1999).
14. (a) Bellussi, G., Carati, A., Clerici, M. G., Maddinelli, G., and Millini,
R., J. Catal. 133, 220 (1992); (b) Clerici, M. G., and Ingallina, P.,
J. Catal. 140, 71 (1993).
We thank the Deutsche Forschungsgemeinschaft (SFB 347 “Selek-
tive Reaktionen Metall-aktivierter Moleku¨le”) and the Fonds der
Chemischen Industrie for generous financial assistance. O.W. thanks the
Fonds “Hochschule International” for a research fellowship at the Insti-
tuto de Tecnologı´a Qu´ımica, Valencia.
15. (a) Adam, W., Haas, W., and Sieker, W., J. Am. Chem. Soc. 106, 5020
(1984); (b) Adam, W., and Golsch, D., Chem. Ber. 127, 1111 (1994).
16. Hoveyda, A. H., Evans, D. A., and Fu, G. C., Chem. Rev. 93, 1307
(1993).
REFERENCES
1. (a) Saxton, R. J., Top. Catal. 9, 43 (1999); (b) van der Waal, J. C.,
Rigutto, M. S., and van Bekkum, H., Appl. Catal. A: General 167, 391
17. Adam, W., and Nestler, B., J. Am. Chem. Soc. 115, 5041 (1993).