A novel and efficient catalytic epoxidation of olefins with adducts derived from methyltrioxorhenium and chiral aliphatic amines

Article abstract of DOI:10.1016/j.jcat.2008.05.004

Nitrogen-based adducts derived from methyltrioxorhenium(VII) and chiral aliphatic amines have been synthesized and applied to the efficient catalytic epoxidation of olefins with urea hydrogen peroxide complex as the primary oxidant. These complexes retain their catalytic activity when microencapsulated in polystyrene. A moderate steroinduction was obtained in the epoxidation of prochiral olefins with complexes between methyltrioxorhenium and chiral trans-1,2-cyclohexyldiamine. The values of steroinduction were found to increase after the microencapsulation process.

Full text of DOI:10.1016/j.jcat.2008.05.004

Journal of Catalysis 257 (2008) 262–269
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Journal of Catalysis
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A novel and efficient catalytic epoxidation of olefins with adducts derived
from methyltrioxorhenium and chiral aliphatic amines
Stefano Vezzosi a, Anna Guimerais Ferré a,1, Marcello Crucianelli a,∗, Claudia Crestini b, Raffaele Saladino c,∗
a
Dipartimento di Chimica, Ingegneria Chimica e Materiali, Università dell’Aquila, via Vetoio, 67010, Coppito (AQ), Italy
Dipartimento di Scienze e Tecnologie Chimiche, Università di Tor Vergata, via della Ricerca Scientifica, 00133 Roma, Italy
Dipartimento di Agrobiologia e Agrochimica, Università della Tuscia, via S. Camillo de Lellis, 01100 Viterbo, Italy
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Nitrogen-based adducts derived from methyltrioxorhenium(VII) and chiral aliphatic amines have been
synthesized and applied to the efficient catalytic epoxidation of olefins with urea hydrogen peroxide
complex as the primary oxidant. These complexes retain their catalytic activity when microencapsulated
Received 21 March 2008
Revised 2 May 2008
Accepted 6 May 2008
Available online 17 June 2008
in polystyrene.
A moderate steroinduction was obtained in the epoxidation of prochiral olefins
with complexes between methyltrioxorhenium and chiral trans-1,2-cyclohexyldiamine. The values of
steroinduction were found to increase after the microencapsulation process.
© 2008 Elsevier Inc. All rights reserved.
Keywords:
Catalysis
Methyltrioxorhenium
Chiral amines
Oxidation
Hydrogen peroxide
1. Introduction
properties of MTO. MTO/Ln adducts exist in a fluxional equilib-
rium, and their stability depends on the physical and chemical
properties of the ligand and of the reaction medium. Recently,
Methyltrioxorhenium(VII) (CH3ReO3, MTO) [1] is one of the
most active catalysts for the activation of hydrogen peroxide
(H2O2). The active catalytic forms are a monoperoxo metal [Me-
heterogeneous rhenium catalysts based on microencapsulation of
MTO-derived adducts on polystyrene (2% cross-linked with di-
vinylbenzene) were prepared. These compounds are efficient and
selective catalysts in wide variety of oxidative reactions, maintain-
ing their stability for successive recycling experiments [3,4]. Of
note, the reactivity and selectivity of MTO/Ln adducts inside the
microcapsule can be modified with respect to homogeneous con-
ditions due to the effect of the polymeric matrix on the stability
of the complex.
Despite the large number of MTO/Ln adducts described in the
literature, only a few examples of complexes with aliphatic amines
have been reported, probably because of their low stability [5].
On the other hand, the preparation of adducts between MTO and
aliphatic amines is highly desirable, as in, for example, the synthe-
sis of chiral rhenium complexes useful for stereoselective oxida-
tions. To date, this represents one of the most stimulating chal-
lenges in MTO chemistry [1c]. With the aim of evaluating this
topic in depth, herein we report the synthesis of homogeneous
and heterogeneous rhenium catalysts based on the formation of
stable adducts between MTO and chiral aliphatic amines. The prop-
erties of the aliphatic amine necessary for the formation of isolable
MTO adducts were defined by comparing different monodentate
and bidentate amines as ligands for the rhenium atom. Data on
the efficient oxidation of some representative prochiral olefins af-
fording sensitive epoxides also are reported.
Re(O)2(O2)] and
a bisperoxo metal [MeRe(O)(O2)2] complexes
and/or their adducts with solvent molecules [2]. Among the syn-
thetic applications of MTO, the epoxidation of olefins is receiving
increased attention. The most relevant drawback of this reaction
is the concomitant ring-opening of the newly formed oxiranyl ring
to diols due to the acidic properties of rhenium compounds. To
date, the procedures developed to avoid this side reaction require
the use of an anhydrous reaction medium and urea hydrogen per-
oxide adduct (UHP) as the primary oxidant. Low-molecular-weight
compounds bearing one or more nitrogen atoms also have been
used as mediators for oxidation. In this latter case, adducts of MTO
with Lewis bases, of general formula MTO/Ln [where L = ligand,
n = 1 (monodentate) or 2 (bidentate)], have been prepared by re-
action with aromatic amines, such as pyridine, pyridine derivatives,
quinuclidine, anilines, toluidines, pyrazoles, and others [1b]. These
adducts inhibit the formation of diols by tuning the electronic
Corresponding authors. Faxes: +39 0 862 433753; +39 0 761 357242.
E-mail addresses: marcello.crucianelli@univaq.it (M. Crucianelli),
*
saladino@unitus.it (R. Saladino).
1
On leave from Department of Organic Chemistry, University of Barcelona, Martí
i Franqués 1, 08028 Barcelona, Catalonia, Spain.
0021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.05.004

S. Vezzosi et al. / Journal of Catalysis 257 (2008) 262–269
263
Fig. 1. Structures of chiral amines a–h.
Scheme 1. Formation of perrhenate salts I–III from reaction of MTO (or perrhenic acid) with chiral amines a–c.
2. Experimental
film thickness BETA-DEX-325 fused silica capillary column. When
necessary, 1H and 13C-NMR analyses of products were performed
after flash-chromatographic purification on columns packed with
silica gel (230–400 mesh), and compared with authenticated sam-
ples. Mass spectra were recorded with an electron beam of 70 eV.
For the scanning electron microscopy (SEM) photographs, samples
were spitter-coated with gold (20 nm). Elemental analyses (C, H,
N) were performed with a Fisons Instruments 1108 CHNS-O ele-
mental analyzer.
All commercial products were purchased in the highest purity
grade available and were used as such. Methyltrioxorhenium(VII),
hydrogen peroxide (35% aqueous solution), aliphatic amines (a–h),
olefins 1–4, and perrhenic acid were purchased from a commer-
cial source (Aldrich). 1H and 13C NMR spectra were recorded
on a Bruker (AC-200 MHz) instrument. Attenuated total reflec-
tion infrared (ATR-IR) spectroscopic analyses were performed, at
room temperature, with a Perkin–Elmer Spectrum One spectrome-
ter equipped with an ATR-IR cell. IR spectra were recorded by aver-
aging 32 scans, with a resolution of 4 cm−1. Gas chromatography-
mass spectrometry (GC-MS) analyses of the reaction products
were performed using a Varian 2000 GC-MAS instrument, with
a 30 m × 0.32 mm × 0.25 μm film thickness (cross-linked 5%
phenylmethylsiloxane) column and chromatography-grade helium
as the carrier gas. Yields and conversions of the reactions were
quantified using n-octane as internal standard. In GC calculations,
all peaks amounting to at least 0.5% of the total products were
taken into account. Enantiomeric excess (ee) was evaluated us-
ing a Hewlett–Packard 6890 series gas chromatograph equipped
with a flame ionization detector, using 30 m × 0.25 mm × 0.25 μm
2.1. General procedure for the preparation of perrhenate salts I–V
To a methanol (1.0 mL) solution containing 0.1 mmol of appro-
priate amine (a–e, Fig. 1), 0.1 mmol of perrhenic acid were added
under stirring, and the reaction was maintained at room temper-
ature for 1 h. After evaporation of the solvent, the residue was
dried under high vacuum, affording a quantitative yield of the cor-
responding perrhenate salt (Schemes 1–3) as a white powder.
2.1.1. Hydrocinchonine perrhenate salt (I)
IR (KBr disc) (cm−1) 3300, 2940, 1500, 1450, 890, 870. 1H NMR
(CD3OD) δ 8.85–8.75 (m, 1H), 8.10–7.95 (m, 2H), 7.80–7.60 (m, 3H),
5.93 (s, 1H), 3.95–3.82 (m, 1H), 3.65–3.15 (m, 4H), 2.35–2.20 (m,

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S. Vezzosi et al. / Journal of Catalysis 257 (2008) 262–269
1H), 1.90–1.68 (m, 4H), 1.60–1.40 (m, 2H), 1.15–1.00 (m, 1H), 0.85
(t, J = 7.2 Hz, 3H). 13C NMR (CD3OD) δ 151.02; 148.72; 148.16;
131.16; 130.27; 129.02; 126.31; 123.67; 120.27; 68.94; 61.69; 52.05;
50.79; 36.16; 26.28; 25.32; 24.52; 19.04; 11.74.
δ 150.76; 148.77; 148.28; 139.04; 131.43; 129.94; 129.20; 126.35;
123.77; 120.41; 117.29; 68.76; 61.93; 55.83; 45.83; 38.43; 28.09;
25.18; 19.70.
2.1.4. (R)-1-Phenethylamine perrhenate salt (IV)
IR (KBr disc) (cm−1) 3440, 3020, 1590, 1480, 890. 1H-NMR
(CD3OD) δ 7.34 (s, 5H), 4.35 (q, J = 7 Hz, 1H), 1.52 (d, J = 7 Hz,
3H). 13C NMR (CD3OD) δ 139.48; 130.31; 130.20; 127.60; 52.39;
20.64.
2.1.2. Cinchonine perrhenate salt (II)
IR (KBr disc) (cm−1) 3300, 2940, 1500, 890, 860. 1H-NMR
(CD3OD) δ 8.82 (br. s, 1H), 8.15–7.90 (m, 2H), 7.85–7.60 (m, 3H),
6.10–5.90 (m, 2H), 5.30–5.10 (m, 2H), 4.25–4.10 (m, 1H), 3.70–
3.15 (m, 3H), 2.75–2.60 (m, 1H), 2.45–2.20 (m, 1H), 1.95–1.65 (m,
3H), 1.14 (br. signal, 1H), 0.90–0.82 (m, 1H). 13C NMR (CD3OD)
δ 150.50; 149.04; 147.88; 137.90; 131.61; 129.58; 129.31; 126.40;
123.84; 120.33; 117.69; 69.04; 61.63; 50.80; 50.37; 38.23; 28.56;
23.96; 19.27.
2.1.5. (R)-N,N-Dimethylphenethylamine perrhenate salt (V)
IR (KBr disc) (cm−1) 3410, 2970, 2700, 1440, 900, 855. 1H-NMR
(CD3OD) δ 7.43 (s, 5H), 4.42 (q, J = 7.2 Hz, 1H), 2.80 (s, 3H),
2.64 (s, 3H), 1.66 (d, J = 7.2 Hz, 3H). 13C NMR (CD3OD) δ 136.00;
131.26; 130.51; 129.78; 67.60; 41.28; 16.42.
2.1.3. Cinchonidine perrhenate salt (III)
IR (KBr disc) (cm−1) 3320, 2950, 1500, 1450, 890, 860. 1H-NMR
(CD3OD) δ 8.81 (br. s, 1H), 8.14–7.90 (m, 2H), 7.80–7.65 (m, 3H),
5.90 (s, 1H), 5.80–5.55 (m, 1H), 5.10–4.85 (m, 2H), 4.17 (br. sig-
nal, 1H), 3.70–3.40 (m, 2H), 3.35–3.10 (m, 2H), 2.72 (br. signal,
1H), 2.25–1.75 (br. m, 4H), 1.48 (br. signal, 1H). 13C NMR (CD3OD)
2.2. In situ 1H and 13 C NMR analyses
For the amines (R)-1-phenylethylamine (d) and (R)-N,N-di-
methyl-1-phenylethylamine (e), formation of the corresponding
adducts with MTO was monitored by 1H and 13C NMR analyses.
As a general procedure, to a 0.5-mL benzene-d6 solution contain-
ing 0.1 mmol of MTO, 0.1 mmol of appropriate amine was added,
at room temperature, and the sample was submitted to sequential
1H and 13C NMR analyses.
2.2.1. MTO/(R)-1-phenylethylamine adduct (not isolated)
1H-NMR (benzene-d6) δ 6.95–6.85 (m, 3H), 6.70–6.60 (m, 2H),
3.49 (q, J = 6.2 Hz, 1H), 1.45 (br. s, 2H), 1.30 (s, 3H), 0.95 (d,
J = 6.2 Hz, 3H). 13C-NMR (benzene-d6) δ 146.19; 129.51; 128.23;
126.72; 51.81; 25.81; 23.79.
2.2.2. MTO/(R)-N,N-Dimethylphenylethylamine adduct (not isolated)
1H-NMR (benzene-d6) δ 7.07 (s, 5H), 3.26 (q, J = 6.8 Hz, 1H),
1.92 (s, 6H), 1.37 (s, 3H), 1.32 (d, J = 6.8 Hz, 3H). 13C-NMR
(benzene-d6) δ 134.55; 128.86; 128.70; 128.38; 66.56; 43.25; 20.13;
17.56.
Scheme 2. Formation of perrhenate salt IV from reaction of MTO (or perrhenic acid)
with (R)-1-phenylethylamine (d).
2.3. General procedure for the preparation of MTO/Ln adducts VI–VIII
MTO/Ln adducts VI, VII, and VIII, containing the bidentate
aliphatic amines trans-(1R,2R)-1,2-cyclohexyldiamine (f), trans-
(1S,2S)-1,2-cyclohexyldiamine (g), and cis-1,2-cyclohexyldiamine
(h), respectively, were prepared following a synthetic route de-
scribed previously (Fig. 1, Schemes 4 and 5) [6]. In the general pro-
cedure, 0.2 mmol of the appropriate ligand was added to 0.2 mmol
of MTO in toluene (2.0 mL) at room temperature. Rapid forma-
tion of a yellow precipitate was obtained in quantitative yield.
Adducts VI–VIII were isolated by filtration, washed with n-hexane,
and used with no further purification.
Scheme 3. Formation of perrhenate salt V from reaction of MTO (or perrhenic acid)
with (R)-N,N-dimethyl-1-phenylethylamine (e).
Scheme 4. Synthesis of MTO/Ln chiral adducts VI–VII.

S. Vezzosi et al. / Journal of Catalysis 257 (2008) 262–269
265
Scheme 5. Synthesis of MTO/Ln chiral adduct VIII.
Scheme 6. Synthesis of heterogeneous chiral rhenium catalysts PS/MTO/Ln mVI–mVII.
2.3.1. MTO/(1R,2R)-1,2-diaminocyclohexane adduct (VI)
2.5. Epoxidation of prochiral olefins 1–4
For 1H- and 13C-NMR spectra, see adduct VII below. Elemen-
tal analysis calcd (%) for C7H17 N2O3Re: C, 23.13; H, 4.71; N, 7.71.
Found: C, 23.60; H, 4.22; N, 7.85.
2.5.1. Epoxidation with MTO/Ln adducts VI–VIII
At room temperature, to the solution of the appropriate cata-
lyst (5.0% w/w) in 1.0 mL of solvent (CH2Cl2 or EtOH) was added
the olefin (1.0 mmol) to be oxidized (Schemes 7 and 8) and an
excess (3.0 equiv.) of oxidant (UHP). At the end of the reaction, a
small amount of MnO2 (2.0 mg) was added, to eliminate the excess
of primary oxidant. The reaction mixture was found to be un-
changed after the addition of MnO2. The suspension was filtered,
and the filtrate was dried over anhydrous Na2SO4. After the solvent
evaporated, the crude product was analyzed by GC-MS and, when
necessary, purified by flash-chromatography. Identity of products
was confirmed by 1H and 13C NMR analyses. Spectra were com-
pared with those of authenticated compounds.
2.3.2. MTO/(1S,2S)-1,2-diaminocyclohexane adduct (VII)
1H NMR (DMSO-d6) δ 4.95–4.65 (br. signal, 2H), 3.95–3.55 (br.
signal, 2H), 2.90–2.50 (br. signal, 1H), 2.50–2.10 (br. signal, 1H),
1.90–1.70 (m, 2H), 1.65–1.45 (m, 2H), 1.40–0.95 (m, 4H), 0.89 (s,
3H). 13C-NMR (DMSO-d6) δ 55.85, 33.63, 27.23, 24.43. Elemen-
tal analysis calcd (%) for C7H17 N2O3Re: C, 23.13; H, 4.71; N, 7.71.
Found: C, 23.87; H, 4.42; N, 7.75.
2.3.3. MTO/cis-1,2-diaminocyclohexane adduct (VIII)
1H NMR (DMSO-d6) δ 3.20–2.85 (br. signal, 4H), 2.27 (s, 3H),
1.70–0.70 (br signal, 10H). 13C-NMR (DMSO-d6) δ 51.96, 30.71,
27.68, 21.27. Elemental analysis calcd (%) for C7H17 N2O3Re: C,
23.13; H, 4.71; N, 7.71. Found: C, 23.43; H, 4.37; N, 7.58.
2.5.2. Epoxidation with microencapsulated PS/MTO/Ln adducts mVI
and mVII
At room temperature, to the suspension of the appropriate cat-
alyst mVI and mVII (5.0% w/w, loading factor 2.0) in 1.0 mL of
solvent (CH2Cl2 or EtOH) was added the olefin (1.0 mmol) to be
oxidized (Schemes 7 and 8) and an excess (3.0 equiv.) of UHP.
At the end of the reaction, the catalyst was recovered by filtra-
tion and washed with CH2Cl2. A small amount of MnO2 (2.0 mg)
was added to eliminate the excess of primary oxidant. Then the
suspension was filtered once again, with the filtrate dried over an-
hydrous Na2SO4. After the solvent evaporated, the crude product
was analyzed by GC-MS and, when necessary, purified by flash-
chromatography. Identity of products was confirmed by 1H and 13C
NMR analyses. Spectra were compared with those of authenticated
compounds.
2.4. General procedure for the preparation of microencapsulated
MTO/Ln adducts mVI and mVII
Microencapsulated PS/MTO/Ln catalysts mVI and mVII were
prepared following a modified procedure reported previously for
the synthesis of polystyrene/MTO catalyst (Scheme 6) [7]. In brief,
to a suspension of 100 mg of polystyrene (PS) in 2 mL of tetrahy-
drofuran (THF) was added 0.2 mmol of the appropriate MTO/Ln
adduct VI or VII to obtain a value of the loading factor (i.e., mmol
of MTO per g of support) of 2.0. Then the mixture was stirred
for 1 h with a magnetic stirrer. Hexane (5.0 mL) was added to
harden the capsule walls. Newly formed PS/MTO/Ln catalysts mVI
and mVII were recovered by filtration, dried, and used with no
further purification. In each case, the adducts VI and VII were
completely bound to the polymer. This result was confirmed by
spectroscopic analysis of the residue obtained after evaporation of
the organic layers. The catalysts were used with no further purifi-
cation.
3. Results and discussion
3.1. Synthesis and characterization of MTO/Ln adducts between MTO
and aliphatic amines: Effects of the ligand and the reaction solvent
Chiral monodentate and bidentate aliphatic amines a–h (Fig. 1)
were used for the synthesis of MTO/Ln adducts, following a synthe-

266
S. Vezzosi et al. / Journal of Catalysis 257 (2008) 262–269
Scheme 7. Catalytic epoxidation of olefins 1–3, in dichloromethane.
Scheme 8. Catalytic epoxidation of olefins 1–4, in ethanol.
sis procedure reported previously [5a,6]. In particular, the alkaloids
hydrocinchonine (a), cinchonine (b), and cinchonidine (c) did not
yield the corresponding MTO/Ln adducts, irrespective of the use of
nonpolar solvents, such as toluene or benzene, or slightly more po-
lar solvents, such as tetrahydrofuran (THF) or dioxane, working ei-
ther at room temperature or lower temperatures. The hindrance at
the tertiary nitrogen atom of the quinuclidine ring likely prevented
the formation of isolable adducts. Any attempt to force the experi-
mental conditions led to the decomposition of MTO with formation
of the corresponding perrhenate salts I–III (Scheme 1, path 1).
These compounds did not exhibit the Re–CH3 signal in their 1H-
equimolar amount of MTO in toluene at room temperature resulted
in the rapid formation of a pale-yellow precipitate, with the cor-
responding MTO/Ln adducts VI and VII recovered in quantitative
yield (Scheme 4). The adducts VI and VII were fully characterized
by spectroscopic and spectrometric analyses. These compounds
were found to be stable for longer than 6 months when stored
◦
at 4 C. To better investigate the effect of the configuration of the
diamino moiety on the formation of stable MTO/Ln adducts, we
also studied the reaction of MTO with the isomeric meso form cis-
1,2-cyclohexyldiamine (h). In this latter case, the MTO/(h) adduct
VIII also was recovered from the reaction mixture as a pale-yellow
precipitate in quantitative yield (Scheme 5).
These data indicate that irrespective of the stereochemistry of
the amino ligand, the 1,2-diamino moiety was a crucial require-
ment for stable and recoverable MTO/Ln adducts. To the best of our
knowledge, this is the first study reported to date dealing with the
preparation, isolation, and characterization of stable and easily re-
coverable MTO adducts with aliphatic amines. Of most relevance
appears to be the possibility of synthesizing stable chiral MTO
complexes. The chiral adducts VI and VII also have been used for
the successive preparation of MTO heterogeneous catalysts.
NMR spectra but did have the characteristic symmetric and asym-
−1
metric ν(Re=O) stretching vibration mode at ca. 900 cm
in
their FT-IR spectra. The structure of perrhenate salts I–III was
unambiguously assigned through comparison with authenticated
samples prepared by reaction of amines a–c with perrhenic acid
(Scheme 1, path 2). The reaction of (R)-1-phenylethylamine (d)
with MTO was monitored by in situ 1H NMR spectroscopy in
◦
benzene-d6 at 25 C and compared with the spectra of authenti-
cated samples of both amine (d) and MTO recovered under similar
experimental conditions. The presence of a downfield shift for both
the NH2 protons (ꢀδ +0.82 ppm) and ReCH3 group (1H NMR: ꢀδ
+0.30 ppm; 13C NMR: ꢀδ +6.69 ppm) and of an upfield shift
for two aromatic protons (ꢀδ −0.18 ppm) suggested the forma-
tion of the MTO/(d) complex. Unfortunately, any attempt to isolate
this complex led only to the formation of a dark-brown, sticky
gum containing, along with perrhenate salt IV (Scheme 2, path 1),
other unidentified side products likely derived from the known ox-
idation processes of primary amines in the presence of rhenium
catalysts [8]. In the case of (R)-N,N-dimethyl-1-phenylethylamine
(e), rapid decomposition of the MTO/(e) adduct to perrhenate salt
V was observed, probably due to the high basic character of the
tertiary amine (e) (Scheme 3, path 1). Again, the structures of
perrhenate salts IV and V were unambiguously assigned through
comparison with authenticated samples prepared by reaction of
amines (d) and (e) with perrhenic acid (Schemes 2 and 3, path 2).
Of note, the addition of enantiomeric trans-(1R,2R)-1,2-cyclo-
hexyldiamine (f) and trans-(1S,2S)-1,2-cyclohexyldiamine (g) to an
3.2. Synthesis and characterization of heterogeneous chiral rhenium
catalysts PS/MTO/Ln based on microencapsulation of MTO/Ln VI and VII
adducts in polystyrene
The chiral heterogeneous catalysts were prepared by mi-
croencapsulation of MTO/Ln VI and VII adducts in polystyrene
(Scheme 6) [7b]. A set of scanning electron microscopy (SEM) pho-
tographs showing the morphology of the surface of particles of
catalysts mVI and mVII are reported in Figs. 2–4. Compounds mVI
and mVII are characterized by particles with a regular spherical
shape, with an average value of diameter of the order of 50 μm.
Small numbers of irregular fragments were observed for both cat-
alysts, likely due to mechanical damage of particles during sample
preparation. At higher magnification, the particles of mVI exhibit a
dense sheet paving formed by a compact aggregate of small spher-
ical grumes.

S. Vezzosi et al. / Journal of Catalysis 257 (2008) 262–269
267
Table 1
Selected vibrational frequencies [ν(ReO)] of catalysts VI,
VII, VIII, mVI and mVIIa
−1
Catalyst
[ν(ReO)] cm
VI
897.0; 837.0
VII
896.5; 836.5
898.0; 837.0
896.2; 839.6
896.5; 840.0
VIII
mVI
mVII
a
Data recovered at room temperature.
Table 2
Oxidation of olefins 1–3 in CH2Cl2 with UHP and MTO/Ln adducts VI–VIII or mi-
croencapsulated PS/MTO/Ln adducts mVI and mVII
Entry
Olefin
Catalysta
Conv.
(%)
Productb
Yield(s)c
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1
1
2
3
1
2
3
1
2
3
1
3
1
3
MTO
VI
VI
VI
VII
78
72
35
74
78
38
78
39
37
55
54
68
56
70
6a
6a
6b
6c
6a
6b
6c
6a
6b
6c
6a
6c
6a
6c
70d
72
30
73
72
34
76
39
37
55
52
65
54
68
Fig. 2. Scanning electron microscopy (SEM) micrograph of mVI catalyst.
VII
VII
VIII
VIII
VIII
mVI
mVI
mVII
mVII
a
Catalysts: MTO/(1R,2R)-1,2-cyclohexyldiamine, VI; MTO/1S,2S)-1,2-cyclohexyl-
diamine, VII; MTO/cis-1,2-cyclohexyldiamine, VIII; microencapsulated MTO/(1R,2R)-
1,2-cyclohexyldiamine, mVI; microencapsulated MTO/(1S,2S)-1,2-cyclohexyldi-
amine, mVII.
b
The reactions were performed in CH2Cl2, UHP (3 equiv.), 5% w/w of catalyst, at
r.t., running for 24 h.
c
The yields are referred to mmol of product per mmol of starting material.
Phenylacetaldehyde (8% yield) was also detected in the reaction mixture.
d
Because the assignment of the vibrational frequencies of the R–
ReO3 and R–ReO3/ligand systems has been described previously, IR
spectroscopy is also useful for analyzing novel MTO complexes [9].
Table 1 gives the values recovered for the ν(ReO) stretching vi-
bration modes of catalysts VI, VII, VIII, mVI, and mVII as revealed
by ATR-IR spectroscopic analysis. All of the complexes have lower
Fig. 3. Scanning electron microscopy (SEM) micrograph of mVII catalyst.
−1
ν(ReO) frequencies compared with MTO, equal to 998 cm
for
−1
the symmetric stretching vibration mode and to 959 cm
for the
asymmetric stretching vibration mode [10]. These data are in ac-
cordance with findings reported previously for both homogeneous
and heterogeneous MTO complexes.
3.3. Epoxidation of olefins with MTO/Ln adducts VI–VIII and
microencapsulated PS/MTO/Ln heterogeneous catalysts mVI and mVII
Table 2 and Scheme 7 summarize the epoxidation of a panel
of prochiral olefins as styrene 1, α-methylstyrene 2, and cis-β-
methyl styrene 3 with MTO/Ln adducts VI–VIII or microencapsu-
lated PS/MTO/Ln heterogeneous catalysts mVI and mVII, using the
urea hydrogen peroxide adduct as the primary oxidant. Irrespec-
tive of experimental conditions, the epoxide was the only product
recovered besides unreacted substrate. High mass balance values
were seen, in accordance with complete selectivity of the oxida-
tion. The epoxidation of olefin 1 with MTO also was performed
as a reference. In accordance with data reported previously in the
literature on the reactivity of rhenium derivatives, very low con-
version of substrate (<3%) was obtained when perrhenate salts I–V
were used as catalysts, even under forced experimental conditions
(i.e., high temperature and large excess of the primary oxidant)
[11]. In particular, the oxidation of 1 with VI afforded the epoxide
Fig. 4. Scanning electron microscopy (SEM) micrograph of mVI catalyst at largest
magnification.

268
S. Vezzosi et al. / Journal of Catalysis 257 (2008) 262–269
Table 3
under selected conditions (Table 3 and Scheme 8). Because EtOH
is a nucleophilic solvent, these oxidations were useful probes for
evaluating the stability of newly formed epoxides toward possible
solvolytic side processes. The results of the oxidations are reported
in Table 3 and Scheme 8. Again, the yields are referred to mmol
of product per mmol of starting material. The oxidation of 1 with
VI in EtOH proceeded with a quantitative conversion of substrate
(greater than that observed previously in CH2Cl2) to give a mix-
ture of 6a as the main reaction product (70%) and of the oxiranyl
ring-opened derivative 7a (28%) as a side product (Table 3, en-
try 2). Similar behavior was observed in the oxidation of 1 with
VII and VIII (Table 3, entries 5 and 8, respectively). Of note, the
yield of 6a with catalysts VI and VII was higher than that obtained
with MTO (see, e.g., Table 3, entry 2 vs entry 1), with VII being the
most efficient system. These data further confirm the capability of
aliphatic amines of tuning the reactivity of MTO. Irrespective of
experimental conditions, the oxidation of olefin 2 always afforded
the ring-opened derivative 7b in high or quantitative yield (Table 3,
entries 3, 6, and 9). In contrast, a quantitative conversion of 3 and
a quantitative yield of epoxide 6c was obtained with catalysts VI
and VII (Table 3, entries 4 and 7), with catalyst VIII showing a
slightly lower reactivity (Table 3, entry 10). Finally, when olefin 4
was treated with VII, a quantitative conversion of substrate and
quantitative yield of cis-stylbene oxide 6d was obtained (Table 3,
entry 11). It is interesting to note that the oxidation of 1 with
microencapsulated catalyst mVII afforded 6a in higher yield and
better selectivity than obtained with catalyst VII (Table 3, entry
12 vs entry 2), thus suggesting a role of the microcapsule envi-
ronment in the oxidation pathway. These data are in accordance
with our previous findings on the effect of the polymeric matrix
on the reactivity and selectivity of MTO [4]. Again, irrespective of
the catalyst used, the oxidation of 2 always afforded 7b as the
sole recovered product, with mVII being the most reactive system
(Table 3, entries 13 and 16). Of note, a quantitative conversion of
substrate and a quantitative yield of epoxide 6c were obtained in
the oxidation of 3 with both catalysts mVI and mVII (Table 3, en-
tries 14 and 17). The catalyst mVII also proved to be an efficient
system for the oxidation of 4 to give 6d in high yield (Table 3,
entry 18).
Oxidation of olefins 1–4 in EtOH with UHP and MTO/Ln adducts VI–VIII or microen-
capsulated PS/MTO/Ln adducts mVI and mVII
Entry
Olefin
Catalysta
Conv.
(%)
Product(s)b
Yield(s)
(%)c,d
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
1
1
2
3
1
2
3
1
2
3
4
1
2
3
1
2
3
4
MTO
VI
VI
VI
VII
99
99
99
99
99
99
99
72
87
88
97
82
78
99
85
88
99
85
6a, 7a
6a, 7a
7b
6c
6a, 7a
7b
6c
6a, 7a
7b
6c
6d
6a, 7a
7b
6c
6a, 7a
7b
6c
56 (44)
70 (28)
98
98
72 (27)
98
98
47 (25)
87
88
96
75 (7)
76
99
78 (6)
86
99
VII
VII
VIII
VIII
VIII
VII
mVI
mVI
mVI
mVII
mVII
mVII
mVII
6d
83
a
Catalysts: MTO/(1R,2R)-1,2-cyclohexyldiamine, VI; MTO/(1S,2S)-1,2 cyclo-
hexyldiamine, VII; MTO/cis-1,2-cyclohexyldiamine, VIII; microencapsulated MTO/
(1R,2R)-1,2-cyclohexyldiamine, mVI; microencapsulated MTO/(1S,2S)-1,2-cyclo-
hexyldiamine, mVII.
b
The reactions were performed in EtOH, UHP (3 equiv.), 5% w/w of catalyst, at
r.t., running for 24 h.
c
Yields in parentheses are referred to product 7a.
The yields are referred to mmol of product per mmol of starting material.
d
6a in acceptable conversion of substrate and high yield (Table 2,
entry 2). Catalyst VI was more selective than MTO; in fact, byprod-
ucts, such as phenylacetaldehyde, previously observed during the
oxidation with MTO (Table 2, note d), were not recovered in the
reaction mixture (Table 2, entry 2 versus entry 1). The position
of the methyl group on the isomeric olefins 2 and 3 (i.e., the
α- vs β-position on the vinyl moiety) played a relevant role in
the efficiency of the oxidation. Thus, although a low conversion of
substrate and a low yield of epoxide 6b were recovered from the
oxidation of 2, the oxidation of 3 afforded the epoxide 6c in high
conversion and yield (Table 2, entry 4 vs entry 3). Similar results
were obtained in the oxidations with VII, with epoxides 6a and
6c isolated at yields (72% and 76%, respectively) greater than that
of 6b (Table 2, entries 5–7). As a general reaction pattern, catalyst
VIII exhibited poorer performance compared with catalysts VI and
VII (Table 2, entries 8–10).
The oxidations with microencapsulated PS/MTO/Ln were succes-
sively performed with the most reactive catalysts, mVI and mVII,
and olefins 1 and 3. It is interesting to note that both catalysts af-
forded epoxides 6a and 6c in yields comparable to that of their
homogeneous counterpart, with olefin 3 being the most reactive
substrate (see, e.g., Table 2, entries 11 and 12 vs 2 and 4). Thus,
specific kinetic barriers to the approach of substrate to heteroge-
neous systems were not observed. To the best of our knowledge,
this is the first report dealing with the high efficacy of aliphatic
amines in tuning the reactivity of MTO, both in homogeneous and
heterogeneous conditions. This reactivity is comparable to that re-
ported previously for aromatic amines [12].
3.5. Evaluation of stereoselectivity for the epoxidation of prochiral
olefins
Despite the high reactivity and versatility of MTO in the epoxi-
dation of olefins, up to now, no relevant results in terms of stereos-
electivity have been obtained. To date, efforts have focused mainly
on using MTO in the presence of a large excess of chiral amines
added to the reaction mixture; for example, Corma et al. [14] re-
ported moderate stereoinduction values (accompanied by low con-
version values) during the epoxidation of prochiral olefins at low
◦
◦
temperature (from −5 C to −55 C), with MTO and added chiral
amines. Similar results have been reported by Herrmann and Kühn
et al. [1b,15] in the MTO-catalyzed epoxidation of prochiral olefins
in the presence of a large excess of chiral pyrazole based ligands.
In this latter case, the fluxionality property of MTO nitrogen-based
adducts (i.e., the rapid equilibrium between the coordinated and
the noncoordinated species) was suggested to explain the effect
of the temperature on the values of stereoinduction [1b,15]. With
the aim of evaluating the values of stereoselectivity induced by
our chiral MTO complexes, the epoxidation of 1 with VI and mVI,
performed in both CH2Cl2 and EtOH, was analyzed by GC in a chi-
ral capillary column (30 m × 0.25 mm × 0.25 μm film thickness
BETA-DEX-325 fused silica) as selected representative examples.
The results, collected in Table 4, show rather low ee in the oxi-
dation of 1 with VI in both reaction solvents (Table 4, entries 1
and 2). In contrast, the use of mVI as the catalyst led to a signif-
3.4. Effect of solvent on epoxidation
It is well known from the literature that the reactivity of MTO
can be increased by the use of polar protic solvents, such as alco-
hols [1a,13]. Based on these data, and with the aim of increasing
the conversion of substrate, we repeated the oxidation of olefins
1–3 with MTO/Ln adducts VI–VIII and PS/MTO/Ln adducts mVI and
mVII in ethanol (EtOH) under similar experimental conditions. In
this latter case, the cis-stylbene 4 also was evaluated as a substrate

S. Vezzosi et al. / Journal of Catalysis 257 (2008) 262–269
269
Table 4
It is noteworthy that the microencapsulation process increased the
stereoinduction values.
Asymmetric epoxidation of styrene 1 catalyzed by compounds VI and mVI
Entry
Catalyst
Solvent
Conversion
(%)
ee
Yield of
epoxide (%)
(%)a
Acknowledgments
1
2
3
4
VI
VI
mVI
mVI
CH2Cl2
EtOH
CH2Cl2
EtOH
72
99
54
82
15
13
20
24
72
70
52
75
Financial support for this work was provided by MIUR (FIRB
2006) and ASI.
a
The ee was evaluated by gas-chromatography in
a chiral capillary column
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