A practical and versatile access to dihydrosalen (salalen) ligands: Highly enantioselective titanium in situ catalysts for asymmetric epoxidation with aqueous hydrogen peroxide

Article abstract of DOI:10.1002/adsc.200700221

A modular synthesis for chiral and nonsymmetrical salalen ligands (i.e., mono-reduced salens), carrying an imine and an amine functionality has been developed. The ligands can be assembled in situ by Schiff base formation of an N-(2-hydroxybenzyl)diamine with a salicylic aldehyde, thus allowing rapid and easy variation/optimization of the ligand structure. Aiming at optimal activity and enantioselectivity in the titanium-catalyzed asymmetric epoxidation of non-functionalized olefins, a positional scanning of the two ligand halves was carried out. High epoxide yields were achieved, and up to 98% ee. The composition of the titanium complex catalysts was determined by high resolution mass spectrometry and X-ray crystallography for one selected example.

Full text of DOI:10.1002/adsc.200700221

UPDATES
DOI: 10.1002/adsc.200700221
A Practical and Versatile Access to Dihydrosalen (Salalen)
Ligands: Highly Enantioselective Titanium In Situ Catalysts for
Asymmetric Epoxidation with Aqueous Hydrogen Peroxide
Albrecht Berkessel,a,* Marc Brandenburg,a Eva Leitterstorf,a Julia Frey,a
Johann Lex,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: May 2, 2007
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: A modular synthesis for chiral and non-
symmetrical salalen ligands (i.e., mono-reduced
salens), carrying an imine and an amine functionali-
ty has been developed. The ligands can be assem-
bled in situ by Schiff base formation of an N-(2-hy-
droxybenzyl)diamine with a salicylic aldehyde, thus
allowing rapid and easy variation/optimization of
the ligand structure. Aiming at optimal activity and
enantioselectivity in the titanium-catalyzed asym-
metric epoxidation of non-functionalized olefins, a
positional scanning of the two ligand halves was
carried out. High epoxide yields were achieved, and
up to 98% ee. The composition of the titanium
complex catalysts was determined by high resolu-
tion mass spectrometry and X-ray crystallography
for one selected example.
handle, and produces water as the sole by-product.[6]
Significant effort has been devoted to the develop-
ment of AE methods using hydrogen peroxide as the
oxidant.[4,7] In this context, Shi et al. combined carbo-
hydrate-derived organocatalysts with acetonitrile as
H2O2 activator.[8] Beller et al. and Strukul et al. em-
ployed chiral ruthenium and platinum catalysts for
the AE, including terminal olefins as substrates.[9] In
contrast, the recent work by Katsuki et al. is based on
titanium in combination either with a semi-reduced
salen-type ligand (a so-called salalen, A, Scheme 1),
or a fully reduced salen (salan, B, Scheme 1). Salan li-
gands were prepared by borohydride (full) reduction
of ordinary salens, and their titanium complexes can
be conveniently synthesized in situ by addition of Ti-
(O-i-Pr)4. High enantioselectivity was achieved for a
number of conjugated olefins, and even for non-con-
jugated terminal olefins such as 1-octene.[10]
Keywords: asymmetric catalysis; epoxidation; in
situ catalysts; salalen ligands; titanium
Ti-salalen complexes of ligands such as A were ac-
cessed by Katsuki et al. via Meerwein–Ponndorf–
Verley reduction of the fully assembled salen com-
plexes, such as those derived from C (Scheme 1).
Based on our earlier work on Mn- and Ni-salalen
complexes,[11] we reasoned that a more convenient
and versatile method for the preparation of salalen li-
Introduction
Optically active epoxides are versatile intermediates gands (and complexes) may consist in the procedure
that can be converted into a wide variety of enantio- shown in Scheme 2. The salan ligands B are, by
merically pure products. Various methods have been nature of their synthesis from salens, “symmetrical”
developed for the preparation of enantiomerically en- (R1 =R3, R2 =R4). Salalen ligands such as 7 can either
riched epoxides, with asymmetric epoxidation (AE) be prepared in a “symmetrical” fashion using the
of olefins being the most direct approach.[1] Estab- same salicylic aldehyde twice (3=6, Scheme 2), or in
lished methods include the Ti-catalyzed AE of allylic an “non-symmetrical” fashion, using two different sal-
alcohols,[2] the transition metal-catalyzed epoxidation icylic aldehydes 3 and 6. The purpose of our study
of non-functionalized olefins,[3] chiral ketone-cata- was (i) to prove the versatility of our novel synthesis
lyzed epoxidation[4] and the nucleophilic epoxidation of salalen ligands 7 (Scheme 2), and (ii) to exploit this
of electron-deficient olefins.[5] In terms of convenience versatility for the optimization of simple salalen li-
and atom efficiency, aqueous hydrogen peroxide is gands 7 in the Ti-catalyzed AE with H2O2. We fo-
the oxidant of choice, as it is cheap, safe, easy to cussed the latter part of our study on non-functional-
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Albrecht Berkessel et al.
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Scheme 1. Reduction stages of salens C: half-reduced (salalen A), fully reduced (salan B), non-reduced (salen C).
Scheme 2. Synthesis of DACH-salalen ligands 7a–m.
ized olefins (such as 1,2-dihydronaphthalene) as they “non-symmetrical” salalen ligands 7, with easily tuna-
pose a major challenge in AE, and to assure direct ble steric and electronic properties.
comparability with the work of Katsuki et al.
With the above synthetic route in hand, we pro-
ceeded to a positional scanning of the “left” (i.e., sub-
stituents R1 and R2) and “right” (i.e., substituents R3
and R4) halves of the salalen ligand. The novel and
non-symmetrical salalen ligands 7a–m were prepared
Results and Discussion
First, the diamine component (e.g., trans-1,2-diamino- and examined for their activity and asymmetric induc-
cyclohexane, DACH; 1), in the form of its anhydrous tion in the epoxidation of several olefins with aqueous
monohydrochloride 2,[12] is condensed with one equiv- hydrogen peroxide. The Ti complexes were generated
alent of a salicylic aldehyde 3, and the resulting imine in situ from the salalen ligands 7 and Ti
ACHTREUNG
4
is reduced to the monoalkylated diamine the absence of either the salalen ligands 7 or Ti
5
ACHTREUNG
(Scheme 2). For the reduction step, sodium cyanoboro- Pr)4, no epoxidation took place. As also observed by
hydride in pure acetic acid proved best. With the Katsuki et al., the corresponding non-reduced salens
amine component 5 in hand, condensation with one were catalytically ineffective.[10]
equivalent of a second salicylic aldehyde 6 affords the
We first investigated the effect of the “left” half of
salalen ligand 7. Our synthetic method thus allows the ligands 7 by varying the salicylic aldehyde 3. The
facile access to a variety of both “symmetrical” and condensation of the resulting amines 5 with unsubsti-
tuted salicylic aldehyde (R3 =R4 =H) afforded the
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A Practical and Versatile Access to Dihydrosalen (Salalen) Ligands
UPDATES
Table 1. Positional scanning of the “left” half of the salalen ligands 7: variation of the salicylic aldehyde 3.
Substituents
Entry Ligand no.
Yield[a] [%] ee[b] [%] Yield[c] [%] ee[b] [%] Yield[c] [%] ee[b] [%]
1
2
3
4
5
6
7a
7b
7c
7d
7e
7f
F
Cl
IHI
OMe
t-Bu
Ph
F
Cl
H
H
t-Bu
H
H
H
6
H
H
H
H
H
0
-
91
0
23
-
2
4
-
75
traces
75
0
4
-
-
66
69
36
25
45
76
1
H
H
H
93
96
98
traces
2
5
-
47
73
74
77
24
[a]
[b]
[c]
Determined by HPLC analysis.
See Supporting Information for details of ee analysis.
Determined by GC analysis.
salalen ligands 7a–f. As test substrates, we chose 1,2- R4 =F; entry 2). Iodo- and in particular chloro-sub-
dihydronaphthalene (8), styrene (9) and non-conju- stituents in the positions R3 and R4 led to good yields
gated vinylcyclohexane (VCH, 10). The results are and high enantioselectivities in the epoxidation of 1,2-
summarized in Table 1.
dihydronaphthalene (8) (Table 2, entries 3 and 4).
The first ligand examined (7a) was based on 3,5-di- Changing to electron-withdrawing nitro groups led to
fluorosalicylic aldehyde, and did not show significant diminished activity (ligand 7j, entry 5), albeit at high
catalytic activity (entry 1). Good yields and high ee. Similarly, methoxy substitution (R3 =OMe, R4 =
enantioselectivities were obtained with chloro sub- H) gave moderate yields (ligand 7k; entry 6), but high
stituents in the positions R1 and R2 (ligand 7b, enantioselectivity. Changing to sterically demanding
entry 2). The diiodide 7c showed lower activity and tert-butyl groups in positions R3 and R4 proved detri-
selectivity (entry 3). Changing to an electron-donating mental for the catalyst activity (entry 7). Apparently,
methoxy substituent in the 3-position (7d) led to di- the catalytic system is highly sensitive to steric con-
minished yields (entry 4). We furthermore synthesized gestion around the R3 position. By far the best results
the 3,5-di-tert.-butylated salalen 7e and obtained mod- were obtained with a phenyl substituent in position
erate yield and excellent enantiomeric excess in the R3 (entry 8).
epoxidation of 1,2-dihydronaphthalene (8). Unfortu-
nately, the yields for styrene (9) and VCH (10) were formation of the Ti catalyst from a pre-formed ligand
comparatively low (entry 5). The best activity and (7a–m) and Ti(O-i-Pr)4. Please note, however, that
All results reported so far were obtained by in situ
AHCTREUNG
enantioselectivity for all three substrates 8–10 was the synthesis of the ligand itself can be performed in
achieved with the 3-phenyl-substituted DACH-salalen situ as well. In other words, by just mixing an amine 5
7f (entry 6). The amines 5 (Scheme 1), when em- (Scheme 1) with a salicylic aldehyde 6 and subse-
ployed as tridentate ligands with Ti
afford active epoxidation catalysts.
N
G
After identification of 3-phenyl-substitution as most verse Ti-salalen catalysts is thus reduced to the simple
effective for the “left” half of the ligands 7a–f, we combination of stock solutions of the amines 5, the
turned to the scanning of the “right” half of the sala- salicylic aldehydes 6 and of Ti(O-i-Pr)4. The efficiency
A
len ligands 7. While keeping R1 =Ph and R2 =H of of this “total in situ” approach is demonstrated by
the amine 5 constant (Scheme 1), the substituents R3 entry 8 of Table 2 (values in parentheses): in fact, the
and R4 were varied. Thus, a second set of salalen li- “total in situ” catalyst even showed enhanced enantio-
gands 7g–m was prepared by varying the salicylic al- selectivity.
dehyde 6 in our synthetic route (Scheme 1). The re-
sults are summarized in Table 2.
With ligand 7m identified by positional scanning,
we investigated the substrate scope of its Ti complex
For comparison, entry 1 reiterates the results ob- and further optimized the epoxidation process
tained with ligand 7f, with no substituents on the (Table 3).
“right” side, that is, R3 =R4 =H. Again, introduction
For example, we found that the epoxidation of 1,2-
of fluoro substituents led to inactivity (ligand 7g, R3 = dihydronaphthalene (8) is completed already after
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Table 2. Positional scanning of the “right” half of the salalen ligands 7: variation of the salicylic aldehyde 6.
Substituents
Entry Ligand no.
Yield[a] [%] ee[b] [%] Yield[d] [%] ee[b] [%] Yield[d] [%] ee[b] [%]
1
2
3
4
5
6
7
8
7f
7g
7h
7i
7j
7k
7l
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
H
H
H
H
H
H
H
H
F
Cl
II26
NO2
OMe
t-Bu
Ph
H
F
Cl
76
0
54
86
98
-
98
24
0
13
59
2
8
77
-
73
5
0
3
-
2
2
73
-
68
3
traces
73
NO2 12
44
t-Bu traces
91 (81[c])
95
96
-
11
54
-
H
75
-
78
traces
traces
8
7m
H
96 (98[c]) 35
81
[a]
[b]
[c]
[d]
Determined by HPLC analysis.
See Supporting Information for details of ee analysis.
“total in situ”.
Determined by GC analysis.
Table 3. Asymmetric epoxidation catalyzed by 7m.
Entry
1
Substrate
No.
t [h]
Yield[a] [%]
ee[b] [%]
8
18
91
96
2
3
4
5
8
3
90
95
11
9
18
18
18
88
97
35 (60[c])
5
78 (76[c])
77
12
6
7
8
13
10
18
18
17
30
8 (14[c])
81 (84[c])
14
15
18
18
9
6
49
60
9
[a]
Determined by HPLC and GC analysis, respectively.
See Supporting Information for details of ee analysis.
4.5 equivalents of H2O2 were used.
[b]
[c]
3 h. Work-up after this short reaction time affords enantioselectivities were obtained for indene (11) as
yields and ees comparable to those obtained after well (entry 3). In the case of styrene (9), we were able
18 h (Table 3, entries 1 and 2). Excellent yields and to increase the epoxide yield from 35% to 60% by
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A Practical and Versatile Access to Dihydrosalen (Salalen) Ligands
UPDATES
using 4.5 equivalents of aqueous hydrogen peroxide
(entry 4, values in parentheses). For both 1-phenylcy-
clohexene (12) and trans-b-methylstyrene (13), the
epoxide yields were relatively low (entries 5 and 6).
We also tested non-conjugated olefins such as VCH
(10), 1-methylcyclohexene (14) and 1-octene (15)
(Table 3, entries 7–9). Whereas the enantioselectivity
was encouraging for VCH (10, 84% ee, entry 7), the
epoxide yield was unsatisfactory in all three cases. A
somewhat higher yield resulted from increasing the
amount of hydrogen peroxide (entry 7, values in par-
antheses).
The activity and selectivity of the Ti-salalen com-
plexes are in marked contrast with the inefficiency of
the corresponding titanium-salen complexes. Katsuki
et al. proposed that the catalytic activity is due to hy-
drogen bonding of the amine NH to one of the
oxygen atoms of a putative Ti-peroxo intermediate
(Figure 1).[10]
In this context, it is important to note that we could
indeed characterize the catalyst prepared in situ from
Figure 2. X-ray crystal structure of [Ti(salalen 7m)O]2.
the salalen ligand 7m and Ti(O-i-Pr)4 as a mononu-
N
clear species by HR-ESI-MS (m/z=553.197: [Ti(sala- Conclusions
len 7m)·OMe]+, ÀOMe from spray solvent). To the
best of our knowledge, this is the first experimental In summary, we describe a versatile synthesis for the
result pointing to mono-nuclearity of the Ti-salalen salalen ligands 7 and their application in the titanium-
catalyst. Furthermore, we were able to obtain crystals catalyzed AE of non-functionalized olefins with hy-
suitable for X-ray analysis of the complex upon expo- drogen peroxide. The catalysts can be generated in
sure to air (moisture).[13] Under these conditions, a di- situ from DACH-derived salalen ligands and Ti
ACHTREUNG
We were also able to crystallize the titanium com- of one selected example. The ligands are prepared
plex derived from (S,S)-DACH-salalen ent-7m. As the from readily accessible starting materials. By position-
sense of chirality at the titanium centre is controlled al scanning, we identified the most active and selec-
by the absolute configuration of the DACH backbone tive ligand 7m (R1 =R3 =Ph, R2 =R4 =H). Good to
in the salalen ligands 7m and ent-7m, respectively, the excellent enantioselectivities were achieved for sever-
two complexes are mirror images.[13] For clarity, only al non-functionalized olefins. Future work will aim at
one monomeric unit of the complexes is shown in determining the substrate scope, potential limitations
Figure 3.
by, e.g., remote oxidizable groups or strongly metal-
Figure 3 also illustrates the hydrogen bonding be- coordinating substructures, and at further optimizing
tween the ligandꢁs NH moiety and one of the oxo both the catalyst design and the reaction conditions.
bridges at the titanium centre (dashed green line:
2.664 Š).
Experimental Section
Synthesis of the Salalen Ligand 7m
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 1 (298 mg, 2.61 mmol) in dry diethyl ether (10 mL)
over a period of 15 min. The mixture was allowed to stir at
room temperature for 18 h. The precipitate was collected by
vacuum filtration, washed with diethyl ether and dried
under vacuum. The white solid was then dissolved in a
methanol/ethanol mixture (50/50 v/v, 20 mL), and 3-phenyl-
2-hydroxybenzaldehyde (466 mg, 2.35 mmol) was added in
Figure 1. Putatitive peroxotitanium complex, activated by
intramolecular hydrogen bonding.
small portions. The reaction mixture was stirred overnight.
After removal of the solvent, the remaining yellow solid was
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Albrecht Berkessel et al.
UPDATES
Figure 3. Mononuclear units of the [Ti(salalen 7m)O]2 and [Ti(salalen ent-7m)·O]2 complexes; the dashed green lines indi-
cate NH-oxo hydrogen bonding.
washed twice with diethyl ether and suspended in glacial
acetic acid (15 mL). Sodium cyanoborohydride (126 mg,
2.00 mmol) was added in a single portion. After one hour,
additional NaBH3CN (65 mg, 1.04 mmol) was added, and
the solution was stirred for three hours to ensure complete
reduction. 6M aqueous KOH 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 disssolved in
a methanol/ethanol mixture (50/50 v/v, 50 mL). 3-phenyl-2-
hydroxybenzaldehyde (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 7m was obtained as a yellow powder; yield: 605 mg
(1.27 mmol, 49%); mp 788C. See Supporting Information
for analytical data.
dronaphthalene (97% ee) were obtained after flash chroma-
tography on silica gel (cyclohexane:ethyl acetate, 20:1, v/v).
Acknowledgements
This work was supported by the Fonds der Chemischen In-
dustrie. In particular, doctoral fellowships to M. B. and J. F.,
and a KekulØ-fellowshipto E. L. are gratefully acknowl-
edged.
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General Procedure for the Asymmetric Epoxidation
of Olefins using a Catalyst Prepared in situ
To the solution of a salalen ligand 7 (10 mmol) in 500 mL of
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A Practical and Versatile Access to Dihydrosalen (Salalen) Ligands
UPDATES
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