Spiroborate-based dimeric assembly of oligo(ethylene glycol) end-capped with phenolic gallates

Article abstract of DOI:10.1016/j.tetlet.2016.05.008

The synthesis of oligo(ethylene glycol) with biphenolic gallates as termini and their assembly to dimeric structures upon spiroborate formation are described. Complexation of these dimeric structures with alkali metal ions showed selectivity to lithium and potassium by tuning the length of the ethyleneoxy units. The methoxycarbonyl group on the dimeric compound could be manipulated in harsh conditions without destruction of the dimeric structure.

Full text of DOI:10.1016/j.tetlet.2016.05.008

Tetrahedron Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Spiroborate-based dimeric assembly of oligo(ethylene glycol) end-
capped with phenolic gallates
Jian-Ping Yang , Xiao-Yu Wu , Wan-Jun Wang , Zuo-Gang Huang , Biao Jiang b,c,
a,b
a
c
b,
⇑
⇑
a
Department of Chemistry, Shanghai University, 99 Shangda Road, Shanghai 200444, China
Shanghai Advanced Research Institute, Chinese Academy of Sciences, 99 Haike Road, Shanghai 201210, China
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of oligo(ethylene glycol) with biphenolic gallates as termini and their assembly to dimeric
structures upon spiroborate formation are described. Complexation of these dimeric structures with
alkali metal ions showed selectivity to lithium and potassium by tuning the length of the ethyleneoxy
units. The methoxycarbonyl group on the dimeric compound could be manipulated in harsh conditions
without destruction of the dimeric structure.
Received 16 March 2016
Revised 3 May 2016
Accepted 4 May 2016
Available online xxxx
Ó 2016 Published by Elsevier Ltd.
Keywords:
Spiroborate
Oligo(ethylene glycol)
Alkali metal ions
Self assembly
Post modification
Introduction
motifs with 2 M aq HCl in methanol afforded the oligo-ethyleneoxy
chain-linked bis-biphenolic compound 4a–e in nearly quantitative
Since the classical crown ethers, cryptands, and calixarenes pio-
yields.
To examine whether the assembly of the single stand ligands
with boron and alkali metal ions could occur, the simple mixing
experiments of 4a–e with Na were conducted and measured
1
neered by Pedersen, Lehn, and Cram, boron-assisted crown
2
3
ethers, borocryptands, and other boron-based molecular assem-
4
blies also have been attracting much attention. For example,
2 4 7
B O
spiroborate linkage has been utilized for the synthesis of double-
stranded supramolecular architectures,5 which consist of
ortho-linked oligophenol chains bearing bipyridine units. Herein,
we would like to report the preparation of oligo-ethyleneoxy ether
strands bearing two gallic acid esters as termini, and their assem-
bly to dimeric structures (Fig. 1. 2L+2B+M) upon spiroborate for-
mation on the catechol-like motifs, and their complexation with
alkali metal ions.
by ESI-MS (negative mode) with results shown in Table 1. The data
revealed that m/z peaks bearing negative charges corresponding to
2
À
À
[2L+2B] and [2L+2B+Na] had been detected. Therefore, we pos-
tulated that dimeric structures of 4a–e upon spiroborate linkage
formation could be generated and their complexes with alkali
metal ions would be attainable. As shown in Scheme 2, when
refluxing 4a–e with boric acid (1.03 equiv) in THF and toluene
(v/v = 4/1) in the presence of 4A molecular sieves, the dimeric
compound 5a–e were obtained in 72–88% yields.
Results and discussions
As shown in Scheme 1, the synthesis was straightforward from
gallic acid methyl ester 1, which was converted into compound 2
in 61% yield by reacting with triethyl orthoformate in the presence
of p-toluenesulfonic acid,6 and subsequent alkylation with bis-
MeOOC
COOMe
O
O
O
n
O
O
O
O
O
O
O
O
B-
B-
M+
O
?
tosylates of oligo(ethylene glycol) with K
2
CO
3
as the base in DMF
O
O
gave 3a–e in 80–89% yields.7 Liberation of the ortho-biphenolic
n
MeOOC
COOMe
⇑
Corresponding authors. Tel.: +86 21 2032 5000; fax: +86 21 2032 5039.
E-mail addresses: huangzg@sari.ac.cn (Z.-G. Huang), jiangb@sari.ac.cn (B. Jiang).
Figure 1. 2L+2B+M.
http://dx.doi.org/10.1016/j.tetlet.2016.05.008
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0
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J.-P. Yang et al. / Tetrahedron Letters xxx (2016) xxx–xxx
OEt
O
OH
OH
OH
O
O
HC(OEt)
3
TsO
OTs
n
p-TsOH
MeOOC
MeOOC
MeOOC
OH
2 3
K CO
1
2
OEt
O
EtO
O
O
OH
OH
O
O
OH
HO
O
2
M HCl (aq.)
O
O
O
COOMe
MeOOC
O
COOMe
n
n
3
a-e (n = 0-4)
4a-e (n = 0-4)
Scheme 1. Synthesis of oligo(ethylene glycol) with end-functionalities.
Table 1
2 4 7
ESI-MS (negative mode) of 4a–e with Na B O
Entry
M
H
4
L1-5 (n = 0–4)
MS-1
m/z
Composition
[2L1+2B]2
MS-2
m/z
Composition
À
À
À
À
À
À
1
2
3
4
5
Na
Na
Na
Na
Na
4a (n = 0)
4b (n = 1)
4c (n = 2)
4d (n = 3)
4e (n = 4)
401
445
489
533
577
825
[2L1+2B+Na]
[2L2+2B+Na]
[2L3+2B+Na]
[2L4+2B+Na]
[2L5+2B+Na]
2
À
À
À
À
[2L2+2B]
913
1001
1089
1177
[2L3+2B]2
2
[2L4+2B]
[2L5+2B]2
Then ion-exchange experiments of the dimeric 5a–e with
MClO (M = Li, Na, K) were carried out. The reactions were moni-
tored by NMR analysis, and significant changes in chemical shifts
for combinations of 5c with LiClO and 5d with KClO compared
to those of 5c and 5d were observed. As shown in Figure 2, addition
of 2 equiv of LiClO to 5c resulted in downfield shifts for the aro-
matic Ha and Hb of 5c (0.017 and 0.71 ppm), while the NMR spec-
tra remained essentially the same upon addition of NaClO and
KClO . The lithium ion adduct was measurable by ESI-MS (negative
mode) with m/z 489 and 985, which could be assigned to
X-ray crystallographic data8 unambiguously revealed that the
[5dÀ2H+2K] complex adopts a pseudo-D -symmetry (Fig. 4). Each
4
2
spiroborate bridges two catechol-like biphenolic aromatic rings
perpendicular to each other (O23/O24, B1, O25/O26; O19/O20,
B2, O21/O22). Each potassium ion is embraced in the center of
the tetra(ethylene glycol) loop and coordinated by five oxygen
atoms (K1, O6–O10; K2, O1–O5) of the oligoether chain and two
oxygen atoms (K1, O21/O25; K2, O19/O23) of the spiroborate link-
ages. According to the stereochemical arrangement, the [5dÀ2H
+2K] complex should be in racemic form.
4
4
4
4
4
2
À
À
1
[5cÀ2H] and [5cÀ2H+Li] , respectively.
As for 5a, 5b, and 5e, their H NMR spectra showed no changes
When mixing of 5d with KClO , upfield shifts were observed as
4
4 4 4
in chemical shifts upon addition of LiClO , NaClO , or KClO . We
shown in Figure 3 for the aromatic Hb (À0.08 ppm), and the ethy-
lene Hc (À0.068 and À0.175 ppm), and Hd (À0.041 ppm), which
probably resulted from the inclusion of potassium-ions and their
coordination to oxygen atoms. The potassium ion adduct was mea-
surable by ESI-MS (negative mode) with m/z: 533 and 1105, corre-
deduced that the above differences in binding affinities might have
resulted from the variable cavity spaces, being limited in 5a (n = 0)
and 5b (n = 1), loose in 5e (n = 4), and optimal in 5c (n = 2) for
lithium ions and 5d (n = 3) for potassium ions.
In order to test the stability of the dimeric assembly, as shown
in Scheme 3, under nitrogen atmosphere, compound 5d was trea-
2
À
À
sponding to [5dÀ2H] and [5dÀ2H+K] , respectively. The dimeric
structure was further supported by high resolution mass spectrum
4
ted with LiAlH at 0 °C in THF, and further reaction with NaH and
+
(
1
C
positive mode): calcd for C48
H
B
52 2
O
26
K
2
H : 1145.2296, found
CH I afforded compound 7 in 60% yield over two steps. The NMR
3
+
145.2299, corresponding to [5dÀ2H+2K+H] ; calcd for
analysis showed that the tetrakis(methoxycarbonyl) groups had
been converted into tetrakis(methoxymethyl) groups on the ben-
+
48
52 2 26 3
H B O K : 1183.1855, found 1183.1860, corresponding to
+
9
[5dÀ2H+3K] .
zene ring, and the mass spectroscopic data indicated that the
Slow evaporation of the solution of 5d and KClO
4
in water and
dimeric structure had survived the harsh conditions without
destruction during the post-modification process.
DMF precipitated single crystals for X-ray diffraction analysis. The
MeOOC
COOMe
MeOOC
COOMe
O
O
O
n
O
O
O
O
O
n
O
O
B(OH)
3
O
O
O
O
O
O
O
MClO
4
O
O
O
O
O
O
H+
B-
2 M+
B-
4a-e (n = 0-4)
B-
B-
2
O
O
O
THF/PhMe
reflux
DMSO-d6
M = Li,
Na, K
O
O
n
n
MeOOC
COOMe
MeOOC
COOMe
5a-e (n = 0-4)
6 (2L+2B+M)
Scheme 2. Preparation of dimeric 5a–e with spiroborate linkage and complexes with alkali metal ions.
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J.-P. Yang et al. / Tetrahedron Letters xxx (2016) xxx–xxx
3
Figure 2. 1H NMR spectra of (a) 5c-KClO
, (b) 5c-NaClO , (c) 5c-LiClO , (d) 5c.
4 4 4
Figure 3. 1H NMR spectra of (a) 5d-LiClO
, (b) 5d-NaClO , (c) 5d-KClO , (d) 5d.
4 4 4
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4
J.-P. Yang et al. / Tetrahedron Letters xxx (2016) xxx–xxx
Figure 4. X-ray crystal structure of [5dÀ2H+2K] complex (hydrogen atoms and solvent molecules omitted for clarity).
MeOOC
COOMe
MeOCH
2
2
CH OMe
O
O
O
n
O
O
O
O
n
O
O
O
O
O
O
O
O
(1) LiAlH
4
O
O
O
O
O
O
B-
H+
B-
B-
2 H+
O
B-
2
O
O
O
O
(2) NaH, MeI
O
O
n
n
MeOOC
COOMe
MeOCH2
2
CH OMe
5d (n = 3)
7 (n = 3)
Scheme 3. Post-modification of dimeric compound 5d.
Conclusion
References and notes
1
.
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In summary, we have prepared a series of oligo(ethylene glycol)
compounds with end-functionalities as gallate motifs from easily
available starting materials. The spiroborate linkage formation on
the catechol-like termini afforded dimeric structures. The flexible
and tunable structures of the oligoether compounds make them
potentially selective hosts for alkali metal ions. Further work on
the supramolecular structure of these boron-assisted dimeric
macrocyclic oligoethers and their applications for selective separa-
tion of alkali metal salts are underway in this laboratory.
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Acknowledgment
This project was supported by the ‘Strategic Priority Research
Program’ of the Chinese Academy of Sciences, grant No.
XDA02020102.
6.
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7
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Supplementary data
Crystallographic data (excluding structure factors) for the structures in this
paper have been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC 1468293.
Supplementary data (detailed experimental procedures and
identification of the intermediates and products) associated with
this article can be found, in the online version, at http://dx.doi.
org/10.1016/j.tetlet.2016.05.008.
9.
The ESI-MS (negative mode) analysis showed m/z: 505 and 1033, corresponding
2
À
À
to [7À2H] and [7À2H+Na] , respectively. The structure was further supported
+
by high resolution mass spectrum (positive mode): calcd for C48
1013.3997, found 1013.3996, corresponding to [7+H] .
H
60
B
2
O
22
H
3
:
+
Please cite this article in press as: Yang, J.-P.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.05.008