Recyclable Poly-Zn3(OAc)4-3,3′-Bis(aminoimino)binaphthoxide Catalyst for Asymmetric Iodolactonization

Article abstract of DOI:10.1002/cctc.201500842

On the basis of the structure of the unimolecular Zn₃(OAc)₄-3,3′-bis(aminoimino)binaphthoxide complex, a poly-Zn₃(OAc)₄-3,3′-bis(aminoimino)binaphthoxide (poly-Zn) complex was prepared from 3,3′-diformylbinaphth

Full text of DOI:10.1002/cctc.201500842

DOI: 10.1002/cctc.201500842
Communications
Recyclable Poly-Zn3(OAc)4–3,3’-
Bis(aminoimino)binaphthoxide Catalyst for Asymmetric
Iodolactonization
Takayoshi Arai,*[a] Takahiro Kojima,[a] Ohji Watanabe,[a] Tsutomu Itoh,[b] and Hirofumi Kanoh[c]
On the basis of the structure of the unimolecular Zn3(OAc)4–
3,3’-bis(aminoimino)binaphthoxide complex, a poly-Zn3(OAc)4–
3,3’-bis(aminoimino)binaphthoxide (poly-Zn) complex was pre-
pared from 3,3’-diformylbinaphthol, tetramine, and Zn(OAc)2.
The first-generation poly-Zn catalyst (poly-Zn1) was prepared
from poly(aminoiminobinaphthol) and Zn(OAc)2. Although
poly-Zn1 showed high catalytic activity for iodolactonization,
the catalyst could not be reused. The second-generation poly-
Zn catalyst (poly-Zn2) was prepared by the self-organization of
3,3’-diformylbinaphthol, tetramine, and Zn(OAc)2. This pro-
duced a stable and active poly-Zn2 catalyst for asymmetric
iodolactonization that was reused over five cycles.
these soluble recyclable catalysts, heterogeneous catalysts are
still fundamentally advantageous, because they can be sepa-
rated from the product by simple filtration. Polymeric ligands
not containing a polymer backbone would be an ideal insolu-
ble recyclable catalyst system, though successful examples are
limited.[5] This report describes the development of poly-
Zn3(OAc)4–3,3’-bis(aminoimino) binaphthoxide (poly-Zn), based
on the unimolecular Zn3(OAc)4–3,3’-bis(aminoimino)binaphth-
oxide complex,[6] as a recyclable catalyst for asymmetric iodo-
lactonization.[7]
In 2014, we reported
a Zn3(OAc)4–3,3’-bis(aminoimino)
binaphthoxide complex for catalytic asymmetric iodolactoniza-
tion.[6] It was shown that trinuclear Zn3(OAc)4–3,3’-bis(amino-
imino)binaphthoxide (tri-Zn, 1 mol%) could catalyze asymmet-
ric iodolactonization with up to 99.9% ee (Scheme 1).
The construction of reusable catalyst systems is important for
sustainable organic syntheses, especially for expensive asym-
metric catalysts. Although polymer-supported catalysts[1] have
been used widely for the recov-
The 3,3’-bis(aminoimino)binaphthol ligand (C) was synthe-
sized by condensation of (R)-3,3’-diformylbinaphthol (A) with
ery and reuse of asymmetric cat-
alysts, the polymer backbone
often causes significant negative
effects on the activity of the cat-
alyst and the stereoselectivity of
the target reaction. To overcome
these drawbacks, approaches
such as ion-supported cata-
lysts,[2] fluorous catalysts,[3] and
dendritic catalysts[4] have been
Scheme 1. The unimolecular Zn3(OAc)4–3,3’-bis(aminoimino)binaphthoxide (tri-Zn) catalyst for asymmetric iodo-
lactonization.
developed to maintain the origi-
nal stereoselectivity. Comparing
[a] Prof. Dr. T. Arai, T. Kojima, O. Watanabe
Molecular Chirality Research Center
Synthetic Organic Chemistry
(1R,2R)-2-(isoindolin-2-yl)-1,2-diphenylethyrene diamine (B)
(Scheme 2a). Polymeric ligand poly-L1 was designed by joining
the isoindoline ring of D to the original 3,3’-bis(aminoimino)-
binaphthol ligand (Scheme 2b). Thus, condensation of A with
(R,R)-diphenylethylenediamine-derived tetramine D in EtOH
under reflux for 38 h gave soluble polymeric 3,3’-bis(aminoimi-
no)binaphthol ligand poly-L1a (Table 1, entry 1).
Gel-permeation chromatography (GPC) of poly-L1a indicat-
ed a molecular weight of approximately 5500, which corre-
sponds to the 6-mers of the monomeric structure described in
Department of Chemistry
Graduate School of Science
Chiba University
1-33 Yayoi, Inage, Chiba 263-8522 (Japan)
E-mail: tarai@faculty.chiba-u.jp
[b] Dr. T. Itoh
Center for Analytical Instrumentation
Chiba University
1-33 Yayoi, Inage, Chiba 263-8522 (Japan)
[c] Prof. Dr. H. Kanoh
Molecular Chemistry, Department of Chemistry
Graduate School of Science
Chiba University
Scheme 2b. Condensation conducted in dioxane under reflux
for 120 h gave insoluble poly-L1b corresponding to the 40-
mers (Table 1, entry 2). Using these polymeric ligands, corre-
1-33 Yayoi, Inage, Chiba 263-8522 (Japan)
sponding
poly-Zn3(OAc)4–3,3’-bis(aminoimino)binaphthoxide
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500842.
catalysts poly-Zn1a and poly-Zn1b were prepared, and their
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was also an efficient asymmetric catalyst for the iodo-
lactonization reaction, and it gave 2a in >99% yield
with 97%ee (Table 2, entry 4).
The crystalline nature of poly-Zn1b was suggested
by the SEM images (Figure 2). Comparison of the
SEM image of poly-L1b (amorphous structure) with
that of poly-Zn1b revealed that poly-Zn1b adopts
a plate structure, which results in a limited effective
surface of the insoluble catalyst.
Although the conditions for the catalytic asymmet-
ric iodolactonization reaction with the use of poly-
Zn1b were further optimized, it was difficult to im-
prove the stereoselectivity of the poly-Zn1b-cata-
lyzed iodolactonization.[8]
To obtain effectively workable poly-Zn catalysts,
the preparation of the polymeric catalyst was rein-
vestigated. Second-generation poly-Zn2 was then
prepared directly from a mixture of A, D, and
Zn(OAc)2, as shown in Scheme 3.
Scheme 2. a) Synthesis of the 3,3’-bis(aminoimino)binaphthol ligand, and b) design of
the polymeric 3,3’-bis(aminoimino)binaphthol ligand.
Table 1. Preparation of polymeric 3,3’-bis(aminoimino)binaphthol ligands.
(R)-A+(R,R,R,R)-D ! polymeric ligand (poly-L1)
Table 2. Asymmetric iodolactonization by using the poly-Zn3(OAc)4–3,3’-
bis(aminoimino)binaphthoxide (poly-Zn 1a and poly-Zn1b) catalysts.
[a]
[c]
Entry
Polymer
Solvent
t [h]
Mw (n[b]
)
Mw/Mn
1
2
poly-L1a
poly-L1b
EtOH
1,4-dioxane
38
120
5490 (6)
35780 (40)
6.6
13.1
Entry
Catalyst
t [h]
Yield [%]
ee [%]
[a] Analyzed by GPC. Mw =weight-average molecular weight. [b] Estimat-
ed degree of polymerization. [c] Mn =number-average molecular weight.
1
2
3
4
poly-Zn1a
poly-Zn1b
tri-Zn[a]
E-Zn[b]
15
22
20
25
99
>99
>99
>99
99
22
99.5
97
[a] 1 mol% catalyst. [b] Ligand E
catalyst activity was examined in the standard iodolactoniza-
tion of 5-phenylhex-5-enoic acid (1a) (Table 2).
Soluble catalyst poly-Zn1a prepared from poly-L1a smooth-
ly catalyzed the asymmetric iodolactonization reaction to give
product 2a in 99% yield with 99%ee, whereas insoluble cata-
lyst poly-Zn1b prepared from poly-L1b gave 2a with 22%ee.
Although asymmetric induction of catalyst poly-Zn1b was low,
analysis of poly-Zn1b was done by using XRD and SEM. The
XRD pattern (Figure 1) revealed several unique peaks for com-
plex poly-Zn1b, even though poly-L1b did not produce signif-
icant peaks, which suggests that the conformational lock was
provided in the formation of multinuclear Zn complexes on
poly-L1b. Figure 1c shows the peaks observed in parent
powder complex tri-Zn (green and yellow circles). The peak at
2q=25.518 (d=3.49 Š) was assigned to the distance between
a central Zn atom and an outer Zn atom, whereas the peak at
2q=12.848 (d=6.89 Š) was assigned to the distance between
two outer Zn atoms in the tri-Zn complex.
Poly-Zn2a was generated in 1,4-dioxane at 808C after
3 days, whereas poly-Zn2b was generated in 1,4-dioxane
under reflux after 7 days. Poly-Zn2b was insoluble in 1,4-diox-
ane. The catalytic activity of newly prepared poly-Zn2 was ex-
amined in the asymmetric iodolactonization reaction (Table 3).
Not only poly-Zn2a but also insoluble poly-Zn2b smoothly
catalyzed the iodolactonization reaction, and surprisingly, 2a
was obtained with 96%ee (Table 3, entry 2). Poly-Zn2 was
characterized by using XRD (Figure 3). The XRD pattern of
poly-Zn2a contains clear peaks, which suggest that poly-Zn2a
is more crystalline in nature than poly-Zn1b (Figure 3a,c),
owing to the smooth introduction of Zn(OAc)2 in the polymer
structure.
Identification of the other peaks (red circles) was accom-
plished by preparing symmetrically connected binaphthols E
on (R,R)-diphenylethylene diamine-derived tetramine D and by
obtaining the XRD pattern of E-Zn (Figure 1d). The XRD pat-
tern of poly-Zn1b can be explained as the summation of the
XRD patterns of tri-Zn and E-Zn. Interestingly, complex E-Zn
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Figure 3. XRD analysis of a) poly-Zn2a, b) poly-Zn2b, and c) poly-Zn1b.
Figure 1. XRD analysis of a) poly-L1b, b) poly-Zn1b, c) tri-Zn, and d) E-Zn.
Figure 2. SEM images of a) poly-L1b and b) poly-Zn1b.
Figure 4. SEM images of a) poly-Zn2a, b) poly-Zn2b, and c) poly-Zn2a (ex-
pansion) and d) HAADF-STEM image of a poly-Zn2a particle.
ure 4a,b). Poly-Zn2a was formed by the agglomeration of
small particles (Figure 4c). The high-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM)
image of poly-Zn2a reveals a uniform distribution of Zn atoms
in the particles (Figure 4d, enhanced by white color).
Scheme 3. Revised method for preparing poly-Zn catalysts.
Table 3. Asymmetric iodolactonization by using catalyst poly-Zn2.
Next, the generality of the asymmetric iodolactonization was
examined by using insoluble poly-Zn2b (2 mol%) (Table 4). For
5-arylhex-5-enoic acids, various substituents including electron-
withdrawing and electron-donating functionalities on the ben-
zene ring were successfully employed to produce iodolactones
2a–f with stereoselectivities ranging from 91 to 97%ee. Di-
methyl substituents on the alkyl chain resulted in 2g with
93%ee. Replacement of the phenyl group with alkyl groups
successfully provided 2h and 2i with high stereoselectivities of
79 and 95%ee, respectively.
Entry
Catalyst
t [h]
Yield [%]
ee [%]
1
2
poly-Zn2a
poly-Zn2b
15
15
>99
>99
97
96
Interestingly, the XRD pattern of poly-Zn2b shows broad
peaks, even though insoluble poly-Zn2b is an effective catalyst
for asymmetric iodolactonization. Thus, poly-Zn2b would be
formed via poly-Zn2a in refluxing 1,4-dioxane. These differen-
ces are also supported by the SEM images (Figure 4).
Finally, catalyst recovery and reuse were examined for cata-
lyst poly-Zn2b. After recovery of the catalyst by centrifugation
after the first use, the second use of poly-Zn2b gave 2a in
94% yield with 92%ee. Catalyst poly-Zn2b was transformed
into a paste after the first iodolactonization; the tiny amount
of poly-Zn2b that was used made its recovery difficult. To
overcome this difficulty, the addition of an external support
Poly-Zn2a was obtained as a rectangular block, whereas
poly-Zn2b was an amorphous solid having a large surface (Fig-
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Acknowledgements
Table 4. Asymmetric iodolactonization by using catalyst poly-Zn2b.
This work was supported by Tokuyama Science Foundation and
a Grant-in-Aid for Scientific Research from the Ministry of Educa-
tion, Culture, Sports, Science and Technology (Japan).
Keywords: asymmetric catalysis · heterogeneous catalysis ·
iodolactonization · polymers · zinc
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Yield [%]
ee [%]
1
2
3
4
5
>99
>99
>99
97
99
96
94
95
97
>99
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naphthoxide (poly-Zn2b) was prepared by the self-assembly of
3,3’-diformylbinaphthol, tetramine, and Zn(OAc)2 in one pot.
Poly-Zn2b was found to be a stable and active catalyst for
asymmetric iodolactonization. The development of additional
approaches for asymmetric catalysis by using self-organization
methods is underway.
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Received: July 28, 2015
[8] Trial on the recovery and reuse of soluble poly-Zn1a resulted in failure.
Published online on September 11, 2015
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