Mass-spectrometric and kinetic studies on the mechanism and degradation pathways of titanium salalen catalysts for asymmetric epoxidation with aqueous hydrogen peroxide

Article abstract of DOI:10.1002/adsc.200700601

The composition and degradation of a highly active and enantioselective titanium salalen in situ catalyst for the asymmetric epoxidation of olefins with aqueous hydrogen peroxide was investigated. Kinetic data and ESI-MS studies point to a mononuclear tit

Full text of DOI:10.1002/adsc.200700601

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DOI: 10.1002/adsc.200700601
Mass-Spectrometric and Kinetic Studies on the Mechanism
and Degradation Pathways of Titanium Salalen Catalysts for
Asymmetric Epoxidation with Aqueous Hydrogen Peroxide
Albrecht Berkessel,a,* Marc Brandenburg,a and Mathias Schäfera
a
Institut für Organische Chemie der Universität zu Kçln, Greinstr. 4, 50939 Kçln, Germany
Phone: (+49)-221-470-3283; fax: (+49)-221-470-5102; e-mail: berkessel@uni-koeln.de
Received: December 19, 2007; Published online: April 11, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The composition and degradation of a hydrogen peroxide, the amine functionality of the
highly active and enantioselective titanium salalen in salalen ligand is converted to the hydroxylamine, fol-
situ catalyst for the asymmetric epoxidation of ole- lowed by loss of water and generation of the inactive
fins with aqueous hydrogen peroxide was investigat- titanium-salen complex. This transformation limits
ed. Kinetic data and ESI-MS studies point to a the activity of the catalyst in the epoxidation of less
mononuclear titanium salalen as the catalytically electron-rich olefins, such as 1-octene.
A
active species. By means of ESI-MS and selective
monodeuteration of the salalen ligand, the oxidative Keywords: alkenes; asymmetric catalysis; catalyst
degradation was studied. Upon exposure to aqueous design; epoxidation; hydrogen peroxide; titanium
Introduction
Recently, our group has developed a practical and
versatile method for the preparation of salalen ligands
Asymmetric epoxidation (AE) of olefins is the most A (and complexes) which allows easy access to a vari-
direct approach for the preparation of enantiomeri- ety of both “symmetrical (R1 =R3 and R2 =R4)” and
cally enriched epoxides.[1] In terms of convenience “non-symmetrical” salalen ligands A, with easily tun-
and atom efficiency, aqueous hydrogen peroxide is able steric and electronic properties.[5] In our study,
the oxidant of choice, as it is cheap, safe, easy to we exploited the versatility of our method for the op-
handle, and produces water as the sole by-product.[2] timization of simple salalen ligands A in the Ti-cata-
Significant effort has been devoted to the develop- lyzed AE with H2O2. Furthermore, improved practica-
ment of AE methods using hydrogen peroxide as the bility results from the fact that the Ti catalyst can be
oxidant.[3] Recent work by Katsuki et al. is based on generated in situ by just combining stock solutions of
the use of titanium in combination either with a semi- titanium tetraisopropoxide [Ti(O-i-Pr)4] and of the
G
reduced salen-type ligand (a so-called salalen, A, chiral ligand. We obtained good to excellent yields
Scheme 1), or a fully reduced salen (salan, B, and enantioselectivities with phenyl-substituted sala-
Scheme 1). High yields and enantioselectivities were len 1 (R1 =R3 =Ph; R2 =R4 =H) as ligand for non-
achieved for a number of conjugated olefins.[4]
functionalized olefins [such as 1,2-dihydronaphthalene
(91% yield and 96% ee) and indene (88% yield and
97% ee)].
Besides the investigations on the catalysis perfor-
mance, we also determined the structure of the titani-
um complex [after exposure to air (moisture)] as a
dimer ([Ti(salalen 1)O]2) by X-ray crystallography
(Figure 1, left). High resolution mass spectrometry
from the catalytically active solution indicated mono-
nuclearity. On the basis of these results we formulated
a proposal for a mononuclear active peroxotitanium
species, which is activated for the oxygen transfer by
intramolecular hydrogen bonding (dashed green lines,
Scheme 1. Reduction stages of salens: half-reduced (salalen
A), fully reduced (salan B).
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Figure 1. X-ray structure of the [Ti(salalen 1)O]2-complex (left) and the resultant putatitive mononuclear peroxotitanium
complex (right); dashed green lines indicate intramolecular hydrogen bonding.
Figure 1, right). Activation by “external protonation”
appears unlikely, as buffering of the reaction mixture
around neutral pH was found to be beneficial.[4d]
Results and Discussion
Our current kinetic study further supports our pro-
posal of a mononuclear catalytically active species.
We determined yield/time profiles for the epoxidation
of 1,2-dihydronaphthalene as a function of the Ti(O-i-
G
Pr)4/salalen 1 concentrations (5 mol% and 10 mol%).
All other experimental parameters were kept con-
stant. Under these conditions the velocity n of the re-
action is dependent only on the concentration of tita-
nium: n=k”[Ti]c.
From the yield/time profile, the velocities n were
determined for the two titanium loadings (Figure 2)
Double logarithmic analysis gives the reaction order c
Figure 2. Velocities of the reaction at different concentra-
tions of Ti(O-i-Pr)4.
AHCTREUNG
in titanium. The value of c=1.09 indicates that the re- cantly. These observations led us to the assumption
action exhibits first-order kinetics in titanium which is that the life-time of the active titanium salalen species
consistent with a mononuclear catalytically active tita- is limited in the presence of hydrogen peroxide, and
nium complex.
that its deactivation proceeds more rapidly than
In the course of our investigation, we encountered oxygen transfer to electron-poor olefins.
a limitation of the method, namely that it is particu-
In this paper, we report our investigation on the de-
larly well suited for electron-rich olefins. We also activation of the titanium complex catalyst Ti(salalen
tested non-conjugated olefins such as vinylcyclohex- 1) in the presence of aqueous hydrogen peroxide. For
ane (VCH) and 1-octene. Whereas the enantioselec- this purpose, we first monitored the Ti catalyst pre-
tivity was encouraging for VCH (84% ee), the epox- pared in situ from the salalen ligand 1 and Ti(O-i-Pr)4
G
ide yield (<10%) was unsatisfactory. Prolonging of in dichloromethane by ESI-MS, without addition of
the reaction time from 18 h to a couple of days or ad- hydrogen peroxide (Figure 3).
dition of hydrogen peroxide in portions and up to 10
The main peak of the spectrum was characterized
equivalents, did not increase the epoxide yield signifi- as a mononuclear species by HR-ESI-MS (m/z=
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Mass-Spectrometric and Kinetic Studies on the Mechanism and Degradation Pathways
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Figure 3. ESI mass spectrum of the Ti(salalen 1) complex in CH2Cl2/MeOH (spray solvents).
À
553.197: [Ti(salalen 1)·OMe]+, OMe from spray sol- the mass of m/z=1075.18 to a salen/salalen mixed ti-
vent). Besides this peak, two more signals are visible tanium complex 3.
and indicate dimeric species [Ti(salalen 1)O]2 +H+],
The positions of the ligand which can be mono-oxy-
and 2 {[Ti(salalen 1)1= O]2·OMe}+ (Figure 3). Upon genated most easily are the benzylic CH and the
2
addition of aqueous hydrogen peroxide (30%), the amine NH bonds (Scheme 2, top).[6] To distinguish be-
mass spectrometric appearance of the titanium com- tween these two possibilities, we substituted a benzyl-
plex solution changes dramatically (Figure 4).
ic proton for a deuterium atom. Based on the most
As shown in Figure 4, the most intensive signal reasonable assumption that, after hydroxylation, ex-
(m/z=553.13) still belongs to the mononuclear spe- change of hydroxylic protons with aqueous hydrogen
cies [Ti(salalen 1)·OMe]+. Also, the signal of the peroxide will take place, the level of deuterium con-
dimer 2 [Ti(salalen 1)1= O]2·OMe]+ (m/z=1091.25) is tent should not change after oxidation if the amine
2
still present. Additionally, the newly occurring mass functionality is oxidized. By the same token, the level
of m/z=477.20 clearly indicates incipient demetalla- of deuterium content should decrease if the oxidation
tion of the titanium salalen catalyst to the free ligand takes place at the benzylic position (Scheme 2,
1. The appearance of two new monomeric species is middle). Additionally, information concerning the re-
the most striking effect of H2O2 addition. The peak at gioselectivity of subsequent water elimination can be
m/z=569.12 has exactly the mass of the main peak obtained from the D content. If a salen is formed, the
plus one oxygen atom [Ti(salalen 1)·OMe+O]+ which D content should decrease. On the other hand, elimi-
is consistent with a mono-oxygenated monomeric tita- nation to an “iso-salen” should not be accompanied
nium complex. The second new signal with m/z= by loss of deuterium (Scheme 2, bottom).
537.06 corresponds to a titanium salen complex [Ti-
Deuterium was incorporated into the benzylic posi-
ACHTREUNG
sults from the use of aqueous H2O2. In the mass range First, trans-1,2-diaminocyclohexane 4 (DACH), in the
of the dimers, similar species can be identified. The form of its anhydrous monohydrochloride 5,[8] was
signal with m/z=1109.21 points at a doubly oxygenat- condensed with one equivalent of 3-phenylsalicylic al-
ed titanium dimer {[Ti(salalen 1)O]2 +2O+H+}, and dehyde 6, and the resulting imine 7 was reduced to
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Figure 4. ESI mass spectrum of the Ti(salalen 1) complex in the presence of 50 equivalents of aqueous hydrogen peroxide in
CH2Cl2/MeOH (spray solvents).
Scheme 2. Hydroxylation reactions of the catalytically active mononuclear Ti(salalen 1)-species (top), and strategy of the
deuterium labelling experiment for distinguishing between possible pathways (bottom).
the monoalkylated diamine mix-8. The diastereomeric amine component mix-8 in hand, condensation with
ratio 8a:8b was determined as 4:1 by NMR spectros- one equivalent of 3-phenylsalicylic aldehyde 6 afford-
copy (see Supporting Information for details). For the ed the mixture of diastereomeric deuterio-salalen li-
reduction step, sodium cyanoborodeuteride in pure gands mix-9.
acetic acid proved best. With the monodeuterated
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Mass-Spectrometric and Kinetic Studies on the Mechanism and Degradation Pathways
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Scheme 3. Synthesis of the monodeuterated DACH-salalen ligand mix-9.
Analogous to the nondeuterated ligand 1, the tita- meric structure 10 (Figure 5, left). After addition of
nium deuterio-salalen complex was prepared in situ hydrogen peroxide, the spectrum shows identical
from the deuterio-salalen ligand mix-9 and Ti(O-i- changes as in the case of the non-deuterated Ti com-
G
Pr)4 in dichloromethane. The mass spectra of the re- plex (Figure 4). Besides the signal at m/z=554.19 for
sultant complex before and after addition of aqueous [Ti(salalen mix-9)·OMe]+, the masses for a mono-oxy-
hydrogen peroxide are shown in Figure 5. Before ad- genated titanium complex [Ti(salalen mix-9)·OMe+
dition of hydrogen peroxide, two well-defined signals O]+ (m/z=570.18) and a titanium salen complex [Ti-
can be identified analogous to the spectrum of the
A
non-deuterated Ti complex (Figure 3). The signal at (Figure 5, right). Analogous to the spectrum of the
m/z=554.20 once again points to the mononuclear non-deuterated Ti complex (Figure 4) the peaks at
species: [Ti(salalen mix-9)·OMe]+ (ÀOMe from spray m/z=1077.24 and m/z=1095.18 can be assigned to di-
solvent) and the second with m/z=1093.30 to the di- meric species.
Figure 5. ESI mass spectra of the Ti(deuterio-salalen mix-9) complex before (left) and after (right) addition of H2O2
(50 equiv.) in CH2Cl2/MeOH (structures of the dimeric species are omitted for clarity).
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Figure 6. Isotopic patterns of the peaks belonging to [Ti(salalen mix-9)·OMe]+ (left) before addition of H2O2 and [Ti(salalen
mix-9)·OMe+O]+ after addition of H2O2 (middle) as well as the [Ti(salen)·OH]+ (right). The peak heights relative to the
G
100% signals reflect the H content.
A comparison of the isotopic patterns of the peaks the expected mode of reaction, as imine formation
belonging to the deuterated mononuclear titanium from alkylhydroxylamines is known in the literature,
complex [Ti(salalen·mix-9)·OMe]+ (Figure 6, left) and and salen formation should additionally be favoured
its oxygenated form [Ti(salalen mix-9)·OMe+O]+ by the generation/extension of the conjugated
(Figure 6, middle) indicate that the deuterium level system.[6,7] It is especially worthy of note that imine
did not change upon incorporation of an oxygen atom formation from N,N-dialkylated amines was reported
[compare the horizontal lines for m/z=553.18 and to proceed already at very low temperatures in the
m/z=569.16, the peak heights relative to the 100% presence of titanium halides.[7]
signals (554.14, 570.14) reflect the H content].
In the case of deuterium loss, the intensity of the
signal at m/z=569.16 would have increased. Conse- Conclusions
quently, oxygenation of the ligand in [Ti(salalen mix-
9)·OMe]+ is not accompanied by loss of deuterium. The results of our study can be summarized as follows
This, in turn, indicates that the benzylic position is (Scheme 4). One of the deactivation pathways of the
not hydroxylated but the amine functionality, afford- Ti catalyst derived from salalen ligand 1, in the pres-
ing a hydroxylamine. The signal at m/z=537.14 ence of aqueous hydrogen peroxide, consists in hydro-
(Figure 6, right) which corresponds either to the mon- lytic demetallation (path A), affording the free ligand
onuclear Ti-salen or Ti-“iso-salen” complex shows a 1 and catalytically inactive titanium dioxide. More im-
diminished level of deuterium. Consequently, dehy- portantly, the amine functionality of the Ti(salalen 1)
dration resulted in a salen-complex, and not in an catalyst is oxidized to the hydroxylamine (path B).
“iso-salen” Ti complex (Scheme 2, bottom). In fact, Subsequent loss of water leads to the inactive titani-
dehydration of the hydroxylamine to form the salen is um salen complex.
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Mass-Spectrometric and Kinetic Studies on the Mechanism and Degradation Pathways
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Scheme 4. Degradation pathways of the Ti(salalen 1)-catalyst in the presence of hydrogen peroxide (these pathways do not
necessarily occur on the stage of the peroxo-species).
The life time of the Ti(salalen 1) catalyst and its droxybenzaldehyde 6 (252 mg, 1.27 mmol) was added in
small portions. The mixture was stirred overnight. The sol-
vent was removed, and the remaining yellow solid was dried
under vacuum over phosphorus pentoxide at 408C. The
product mix-9 was obtained as a pale yellow powder; yield:
active peroxotitanium species in the presence of hy-
drogen peroxide is long enough for the epoxidation of
electron-rich, conjugated olefins, such as 1,2-dihydro-
naphthalene. However, it seems to be too short for ef-
fectively transferring an oxygen atom to less reactive
(i.e., electron-poor), non-conjugated olefins, such as
vinylcyclohexane.
1
607 mg (1.27 mmol, 49%); mp 718C; H NMR (CDCl3): d=
1.23–1.35 (m, 1H), 1.26–1.41 (m, 1H), 1.29–1.41 (m, 1H),
1.55–1.67 (m, 1H), 1.61–1.82 (m, 1H), 1.73–1.82 (m, 1H),
1.76–1.84 (m, 1H), 2.14–2.28 (m, 1H), 2.72–2.84 (m, 1H),
3.01–3.13 (m, 1H), 3.83–3.89 (m, 1H), 6.76–6.88 (m, 1H),
6.88–6.99 (m, 1H), 6.91–6.99 (m, 1H), 7.17–7.26 (m, 1H),
7.20–7.29 (m, 1H), 7.25–7.32 (m, 1H), 7.29–7.35 (m, 1H),
7.34–7.40 (m, 2H), 7.37–7.43 (m, 1H), 7.40–7.46 (m, 1H),
7.52–7.55 (m, 1H), 7.61–7.67 (m, 2H), 8.46 (s, 1H), 13.78
(br. s, 2H); 13C NMR (CDCl3): d=24.4 (t), 24.5 (t), 30.0 (t),
34.0 (t), 49.9 (d, JC-D =19.5 Hz), 61.4 (s), 73.4 (d), 118.8 (s),
118.9 (d), 119.1 (d), 123.6 (s), 126.8 (d), 127.2 (d), 127.5 (d),
128.1 (d), 128.2 (d), 129.2 (s), 129.3 (d), 129.4 (d), 129.8 (d),
129.9 (s), 131.1 (d), 133.6 (d), 137.7 (s), 138.5 (s), 155.1 (s),
Experimental Section
Deuterio-(1R,2R)-Ph,H;Ph,H-DACH-salalen (mix-9)
Anhydrous HCl (1.31 mL, 2.61 mmol, 2M in diethyl ether)
was added to a solution of (1R,2R)-trans-1,2-diaminocyclo-
hexane 4 (298 mg, 2.61 mmol) in dry diethyl ether (10 mL)
over a period of 15 min. The mixture was stirred at room
temperature for 18 h. The precipitate was collected by
vacuum filtration, washed with diethyl ether and dried
under vacuum. The colourless solid was then dissolved in a
methanol/ethanol mixture (50/50 v/v, 20 mL), and 3-phenyl-
2-hydroxybenzaldehyde 6 (466 mg, 2.35 mmol) was added in
small portions. The reaction mixture was stirred overnight.
After removal of the solvent, the remaining yellow solid was
washed twice with diethyl ether and suspended in glacial
acetic acid (15 mL). Sodium cyanoborodeuteride (132 mg,
2.00 mmol) was added in one single portion. After one hour,
additional NaBD3CN (68 mg, 1.04 mmol) was added, and
the solution was stirred for three hours to ensure complete
reduction. 6M KOH (aqueous) was added until pH 8 was
reached. The resulting suspension was extracted with
CH2Cl2, and the combined organic phases were washed with
saturated aqueous NaHCO3. After drying over MgSO4, fil-
tration and evaporation, the remaining oil was dissolved in a
methanol/ethanol mixture (50/50 v/v, 50 mL). 3-Phenyl-2-hy-
˜
158.3 (s), 166.1 (d); IR (ATR): n=3054, 2930, 2856, 1623,
1623, 1496, 1456, 1428, 1289, 1098, 1070, 1029, 831, 757,
697 cmÀ1; HR-MS (ESI): m/z [u]=478.260, exact mass calcd.
for [C32H32DN2O2]+ ([M+H]+): 478.261.
[Ti(salalen mix-9)O]2
Titanium isopropoxide (5.9 mL, 20 mmol) was added to a so-
lution of the deuterio-(1R,2R)-Ph,H;Ph,H-DACH-salalen
mix-9 (9.6 mg, 20 mmol) in 1 mL of dry CH2Cl2. The result-
ing yellow solution was stirred for two hours and exposed to
air for 24 h. Drying under vacuum afforded the Ti complex
as a yellow solid; yield: 10.8 mg (10 mmol, quant.). HR-MS
(ESI; CH2Cl2, MeOH): m/z [u]=554.204, exact mass calcd.
for [C33H32DN2O3Ti]+ ([M+OMe]+): 554.203.
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General Procedure for the Mass Spectrometric
Monitoring of Titanium Salalen Complexes in the
Presence of Aqueous Hydrogen Peroxide
[Ti(salalen 1)O]2 (2.0 mg, 1.9 mmol, 2 equiv.) and [Ti(salalen
mix-9)O]2 (2.0 mg, 1.9 mmol, 2 equiv.), respectively, were dis-
solved in a solvent mixture consisting of 200 mL of dry
CH2Cl2 and 200 mL of dry MeOH. Fifty equivalents
(18.9 mL, 186 mmol) of aqueous hydrogen peroxide (30%)
were added. An aliquot of the solution was analyzed by ESI
mass spectrometry on a Finnigan MAT 900S instrument (for
further details concerning ESI-MS experiments: see Sup-
porting Information).
Acknowledgements
This work was supported by the Fonds der Chemischen In-
dustrie. In particular, a doctoral fellowship to M. B. is grate-
fully acknowledged.
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