Asymmetric benzoylation of meso-hydrobenzoin using a reusable tripodal imidazoline-pyridine-Cu catalyst

Article abstract of DOI:10.1002/ejoc.201101722

Chiral, self-supported, polytopic imidazoline-pyridine ligands were designed and synthesized for use in Cu catalysis. The tripodal imidazoline-pyridine L6-Cu(BF₄)₂ complex catalyzed the asymmetric p-(tert-butyl)benzoylation of meso-h

Full text of DOI:10.1002/ejoc.201101722

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201101722
Asymmetric Benzoylation of meso-Hydrobenzoin Using a Reusable Tripodal
Imidazoline–Pyridine–Cu Catalyst
Takayoshi Arai*[a] and Ken Sakagami[a]
Keywords: Asymmetric catalysis / Self-supported catalysts / Metal-organic frameworks / Benzoylation
Chiral, self-supported, polytopic imidazoline–pyridine li-
gands were designed and synthesized for use in Cu catalysis.
The tripodal imidazoline–pyridine L6-Cu(BF4)2 complex cat-
alyzed the asymmetric p-(tert-butyl)benzoylation of meso-
pletion of the asymmetric benzoylation reaction, the self-sup-
ported tripodal imidazoline–pyridine–Cu catalyst could be
easily recovered as a precipitate by adding hexane, and the
recovered catalyst could be reused several times while main-
hydrobenzoin to give the adduct in up to 85%ee. After com- taining the catalyst activity and selectivity.
Introduction
Metal–organic frameworks (MOFs) produced by the as-
sembly of organic and inorganic molecules have received
much attention as coordination polymers, because the re-
sulting architectural porosity and/or large surface area pro-
vide a basis for exploring functions such as molecular re-
cognition and gas storage.[1] In addition, chiral versions of
some coordination polymers have been utilized as self-sup-
ported catalysts. The heterogeneous character of coordina-
tion polymers becomes an advantage for recovery and reuse
of the precious and expensive asymmetric catalysts.[2,3]
Here, new, self-supported, polytopic, chiral imidazoline–
pyridine ligands were designed and synthesized for use in
asymmetric Cu catalysis.
Figure 1. Our efforts towards the synthesis of chiral tosyl imid-
azoline ligands.
aminophenol L3,[7] which was effective for the Cu-catalyzed
asymmetric Henry reaction.
On the basis of the successful results obtained by using
the tosylated imidazoline ligands in solution-phase cataly-
sis, polytopic chiral imidazoline–pyridine L4 was designed
and synthesized by conjoining imidazoline with 1,3-benz-
enesulfonyl chloride (Scheme 1).[8]
The reaction of imidazoline–pyridine A with 1,3-benz-
enesulfonyl chloride was smoothly promoted by Et3N and
DMAP to give L4 in 85% yield. Towards the construction
of the coordination polymer by using L4, formation of the
complexes of the chelating ligands and metal salts needed
to be considered (Scheme 2). After complexation of the first
ligand and metal salt, it may be difficult for complex B to
accept further chelation of additional ligands, and this
would hinder the construction of the coordination polymer
(Scheme 2a). This is often observed in the formation of neu-
tral complexes having structurally crowded ligands. A
strong Jahn–Teller effect also prohibits further complex-
ation at the apical site of the metal center. Actually, in ad-
dition to Cu(OAc)2, neither CuCl nor CuCl2 formed the
insoluble coordination polymer with L4.[9] To force the for-
mation of the coordination polymer, we decided to employ
the cationic salt Cu(BF4)2 (Scheme 2b). The reaction of
Cu(BF4)2 with imidazoline–pyridine ligand L4 (1 equiv.) in
Results and Discussion
In a program aimed to explore new catalysts, we have
succeeded in developing chiral imidazoline-containing li-
gands.[4] For example, a nitrogen-tethered bisimidazoline,
abbreviated as Nb-imidazoline (L1), was realized as an ef-
ficient catalyst for the asymmetric benzoylation of meso-
diols (Figure 1).[5]
Although the L1-CuCl catalyst was originally discovered
by using a polystyrene-supported catalyst system, once it
was immobilized onto a conventional polymer backbone it
showed lower activity than that of homogeneous unimolec-
ular catalyst L1-CuCl; moreover, the heterogeneous catalyst
was difficult to recycle.[5b] We have also studied imid-
azoline–pyridine L2[6] as a simple analogue of imidazoline–
[a] Graduate School of Science, Chiba University,
Inage 263-8522, Japan
Fax: +81-43-290-2889
E-mail: tarai@faculty.chiba-u.jp
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101722.
Eur. J. Org. Chem. 2012, 1097–1100
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1097

T. Arai, K. Sakagami
SHORT COMMUNICATION
sponding unimolecular catalyst [L2-Cu(BF4)2] gave the
product in racemic form (Table 1, Entry 1). Stimulated by
the positive effect on the asymmetric induction by using
self-supported catalysts, a family of polytopic imidazoline–
pyridine ligands was prepared (Scheme 1).[11] The reaction
of telephthaloyl chloride with A gave L5 in 65% yield. Ben-
zoylated imidazoline–pyridine L5 should show a stronger
coordinating ability to copper metal than L4. The asym-
metric benzoylation of meso-hydrobenzoin by using L5-
Cu(BF4)2 gave the product with 42%ee (Table 1, Entry 5),
and the reaction using o-methylbenzoyl chloride improved
the enantioselectivity up to 73%ee. The positive effects of
substituted benzoyl chlorides have been partially observed
in the Nb-imidazoline-catalyzed asymmetric benzoylation
of meso-diols.[5b,5c] At this stage, the effects of cationic cop-
per salts on the asymmetric benzoylation were examined.
Coordination polymers having a pale blue color were pre-
pared from both Cu(OTf)2 and Cu(ClO4)2 by using the
method for the preparation of L5-Cu(BF4)2 and showed the
same level of catalyst activity to that of L5-Cu(BF4)2
(Table 1, Entries 6–8).
Table 1. Asymmetric benzoylation of meso-diols catalyzed by self-
supported imidazoline–pyridine–Cu complexes.
Scheme 1. Synthesis of polytopic imidazoline–pyridine ligands L4–
L6.
EtOH occurred smoothly to give a pale blue insoluble resi-
due. The solid was isolated by simple filtration and used as
a self-supported polymer catalyst after washing with EtOH. Entry
R1
R2
CuX2
L
t
[h]
Yield
[%]
ee
[%]
1
2
3
4
5
6
7
8
Ph
Ph
Ph
CH3
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
H
o-Me
H
Cu(BF4)2
Cu(BF4)2
Cu(BF4)2
Cu(BF4)2
Cu(BF4)2
Cu(BF4)2
Cu(OTf)2
Cu(ClO4)2
Cu(BF4)2
Cu(BF4)2
Cu(BF4)2
Cu(BF4)2
Cu(BF4)2
Cu(BF4)2
L2
L4
L4
L4
L5
L5
L5
L5
L6
L6
L6
L6
L6
L6
L6
L6
L6
43
66
66
64
66
64
40
40
32
43
32
17
40
33
49
40
70
57
69
92
79
90
rac
46
46
50
42
73
73
73
62
79
60
67
62
58
67
61
85
H
o-Me
o-Me
o-Me
H
o-Me
m-Me
p-Me
o-Br
o-I
9
10
11
12
13
14
15
16
17
21 Ͼ99
25
26
21 Ͼ99
21
37
90
92
o-NO2 Cu(BF4)2
p-Br
p-tBu
Cu(BF4)2
Cu(BF4)2
94
82
Cationic copper salts should form similar coordination
polymers on the basis of the hypothesis studied in
Scheme 2b. Because the main purpose for the preparation
of a heterogeneous self-supported catalyst is to provide a
practical method for its reuse, we then tried to recover and
reuse the L5-Cu(BF4)2 catalyst. However, we soon realized
Scheme 2. Classification of complexation for MXn with polytopic
ligands.
In the asymmetric benzoylation of meso-hydrobenz- that some significant amount of Cu metal was leached by
oin,[10] tosyl-substituted imidazoline–pyridine L4-Cu- the addition of amine under the reaction conditions. The
(BF4)2 gave the monobenzoylated product in 40% yield solution phase took on a pale green color upon the addition
with 46%ee (Table 1, Entry 2). Interestingly, the corre- of the amine.
1098
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Eur. J. Org. Chem. 2012, 1097–1100

Asymmetric Benzoylation of meso-Hydrobenzoin
To improve the stability of the coordination polymer un-
der the reaction conditions, tripodal imidazoline–pyridine
L6 was also synthesized by using trimesoyl chloride in 72%
yield.[4,12] To our delight, tripodal imidazolidine–pyridine
L6 showed the best catalyst activity for the simple benzo-
ylation reaction and gave the product with 62%ee (Table 1,
Entry 9 vs. Entries 2 and 5). As observed with L5-Cu-
(BF4)2, the use of the substituted benzoyl chloride was also
beneficial and improved the enantioselectivities when L6-
Cu(BF4)2 was used. For example, o-methylbenzoyl chloride
gave the product with 79%ee (Table 1, Entry 10). When the
p-(tert-butyl)benzoyl chloride was utilized, the enantio-
selectivity increased to 85%ee (Table 1, Entry 17). After op-
timization of the procedure to recover the catalyst, we
found that the addition of hexanes after completion of the
reaction allowed the quantitative precipitation of L6-
Cu(BF4)2. The precipitated catalyst was separated by centri-
fugation and retained its activity. The results of several re-
use experiments are summarized in Table 2. The reuse ex-
periment was examined with the use of 10 mol-% of the
catalyst, and the whole recovered catalyst was applied in the
next run.
Experimental Section
Asymmetric Benzoylation of meso-Hydrobenzoin Using the L6-Cu
Catalyst: Under an argon atmosphere, L6 (10.5 mg,
1.00ϫ10–2 mmol) and Cu(BF4)2·6H2O (5.6 mg, 1.50ϫ10–2 mmol)
were stirred in EtOH (2 mL) for 3 h to give a pale-blue precipitate.
The precipitate was washed with EtOH (3ϫ10 mL) and dried un-
der reduced pressure. To the self-supported catalyst (14.1 mg,
1.00ϫ10–2 mmol as L6 and 1.50ϫ 10–2 mmol as Cu) was added
meso-hydrobenzoin (43.3 mg, 0.20 mmol) in CH2Cl2 (2 mL) and
NiPr2Et (68.5 μL, 0.4 mmol). Then, o-methylbenzoyl chloride
(26.5 mL, 0.20 mmol) was added to the reaction mixture at –40 °C.
After stirring for 40 h, the reaction was quenched by adding H2O.
The organic layer was extracted with CH2Cl2 (3ϫ20 mL), dried
with anhydrous Na2SO4, filtered, and concentrated in vacuo. The
residual crude product was purified by silica gel column
chromatography to give the monobenzoylated hydrobenzoin
(52.5 mg, 78% yield; Table 1, Entry 10). The enantiomeric excess
of the product was determined by chiral stationary phase HPLC
analysis.
Reuse of L6-Cu Catalyst: After completion of the asymmetric ben-
zoylation of meso-hydrobenzoin, the reaction mixture (ca. 2 mL)
was warmed to room temperature. After the addition of hexane
(8 mL), the mixture was stirred for 5 min to give a precipitate of
the self-supported catalyst. The precipitate was washed with AcOEt
(10 mL) and dried under reduced pressure. The recovered catalyst
was used for the next asymmetric benzoylation of meso-1,2-hydro-
benzoin.
Table 2. Reuse of the L6-Cu(BF4)2 catalyst.
Supporting Information (see footnote on the first page of this arti-
cle): Detailed experimental procedures and analytical data of the
products.
Run
Yield [%]
ee [%]
Leaching of Cu
[%][a]
Acknowledgments
1
2
3
4
5
6
7
8
78
77
77
76
76
76
75
75
75
77
79
77
79
75
77
77
1.2
3.1
3.8
1.3
2.5
1.5
1.6
1.3
This work was supported by a Grant-in-Aid for Scientific Research
(B) from the Japan Society for the Promotion of Science (JSPS).
We appreciate the Shimadzu Analytical & Measuring Center for
ICP analysis. We thank Ms. A. Awata in Chiba University for her
technical support.
[a] Determined by ICP analysis.
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Received: November 30, 2011
Published Online: January 17, 2012
1100
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Eur. J. Org. Chem. 2012, 1097–1100