Polyoxaalkyl esters of phenylboronic acids as new podands

Article abstract of DOI:10.1016/j.molstruc.2006.01.013

Three oxaalkyl esters of phenylboronic acid and one oxaalkyl ester of phenyldiboronic acid have been synthesized and their ability to form complexes with Li⁺ and Na⁺ cations has been studied using multinuclear NMR, ESI mass spectrome

Full text of DOI:10.1016/j.molstruc.2006.01.013

Journal of Molecular Structure 791 (2006) 111–116
www.elsevier.com/locate/molstruc
Polyoxaalkyl esters of phenylboronic acids as new podands
a
a
b
b
b
A. Sporzynski , A. Miskiewicz , B. Gierczyk , R. Pankiewicz , G. Schroeder , B. Brzezinski
b,
*
´
´
a Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warszawa, Poland
b Faculty of Chemistry, Adam Mickiewicz University, ul Grunwaldzka 6, 60-780 Poznan, Poland
Received 12 December 2005; accepted 5 January 2006
Available online 21 February 2006
Abstract
Three oxaalkyl esters of phenylboronic acid and one oxaalkyl ester of phenyldiboronic acid have been synthesized and their ability to form
complexes with LiC and NaC cations has been studied using multinuclear NMR, ESI mass spectrometric and PM5 semiempirical methods. It has
been demonstrated that the esters of phenylboronic acid form 1:1 complexes with LiC and NaC cations whereas the ester of phenyldiboronic acid
can additionally form complexes of 1:2 stoichiometry. The stability constants of these complexes have been determined to show that with
increasing number of oxygen atoms in the oxaalkyl chains the complexes of increasing stability are formed. The exemplary 1:1 and 1:2 structures
of the complexes of esters with LiC and NaC cations, respectively, are given. They show that the cations are coordinated within a pseudo-crown
structures formed by oxaalkyl chains.
q 2006 Elsevier B.V. All rights reserved.
1
Keywords: Phenylboronic acids; Oxaalkyl esters; H NMR; 13C NMR; 7Li NMR; 23Na NMR; ESI MS; Complexes; Stability constants; Pseudo-crown structure
1. Introduction
method. The stability constants of the complexes are
determined and the structure of the complexes is discussed.
Arylboronic acids have found a wide range of applications
in organic synthesis, of which the most important reactions are
Suzuki coupling [1] and Petasis synthesis [2]. Another rapidly
developing field is the biological and pharmaceutical appli-
cation of boron compounds [3,4].
2. Experimental
Ethylene glycol monomethyl ether, diethylene glycol
monomethyl ether and triethyleneglycol monomethyl ether
(Aldrich) were commercial products of Aldrich and were used
without any purification. Phenylboronic and phenylenediboro-
nic acids were synthesized following the procedures given in
Ref. [10].
In recent years podands, open-chain ligands in contrast to
the crowns, have attracted increasing interest as anion
activators in homogeneous and heterogeneous systems. Podand
molecules including polyoxaalkyl chains (inorganic esters of
inorganic acids with ethylene glycol) show the ability to form
complexes with metal cations [5]. Further, It has been
demonstrated that tris(polyoxaalkyl) borates and tris(polyox-
aalkyl) phosphates form channels with protons or cations
[6–9]. Within these channels the protons or cations show large
polarizability due to their fast fluctuations.
2.1. Preparation of esters 1–4
Phenylboronic and phenylenediboronic esters (1–4) were
obtained in the reactions shown in Scheme 1. To (0.0517 mol)
of phenylboronic acid, (0.110 mol) of respective ethylene
glycol monomethyl ether was added. After addition of 50 cm3
of benzene the vessel was connected to a Dean-Stark trap,
placed in an oil bath and heated for 10 h. The excess of alcohol
was distilled off in vacuum (water pump). The products were
purified by bulb-to-bulb distillation at ca. 10K3 Torr. Yields: 1:
61%, 2: 57%, 3: 54%, 4: 38%.
A new class of borate podands such as polyoxaalkyl esters
of phenylboronic and phenylenediboronic acids have been
synthesized and their ability to form complexes with metal
cations has been studied using NMR spectroscopic and ESI MS
*
Corresponding author. Tel.: C48 61 8291330; fax: C48 61 8658008.
E-mail address: bbrzez@main.amu.edu.pl (B. Brzezinski).
2.2. Preparations of complexes
LiClO4, NaClO4 and acetonitrile (Aldrich) were used for
the preparations of complexes. The solutions were obtained
0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.01.013

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A. Sporzynski et al. / Journal of Molecular Structure 791 (2006) 111–116
112
Scheme 1. Synthesis of the esters 1–4.
by dissolving the respective esters (1–4) and the salt in
analytical concentrations. For 7Li and 23Na measurements
2.4. ESI mass spectrometry
the concentration of the salt was constant (0.2 mol dmK3
and the concentration of the ester was varied, whereas for
)
The ESI (electrospray ionization) mass spectra were
recorded on a Waters/Micromass (Manchester, UK) ZQ mass
spectrometer equipped with a Harvard apparatus syringe pump.
The sample solutions were prepared in acetonitrile (1:1) at a
concentration of approximately 10K4 M. The samples were
infused into the ESI source using a Harvard pump at the flow
rate of 20 ml minK1. The ESI source potentials were: capillary
3 kV, lens 0.5 kV, extractor 4 V. In the case of standard ESI
1
the H and 13C measurements the concentration of the ester
was constant (0.2 mol dmK3) and the concentration of the
salt was varied.
All solvents were spectroscopic grade and were dried over
˚
3 A molecular sieve.
All preparations and transfers of solutions were carried out
in a carefully dried glove box under nitrogen atmosphere.
2.3. NMR measurements
The NMR spectra were recorded in CD3CN using a Varian
Gemini 300 MHz spectrometer. All spectra were locked to
deuterium resonance of CD3OD or CD3CN, respectively. The
error in parts per million values was 0.01.
All 1H NMR measurements were carried out at the operating
frequency 300.075 MHz; flip angle, pwZ458; spectral width,
swZ4500 Hz; acquisition time, atZ2.0 s; relaxation delay,
d1Z1.0 s; TZ293.0 K and TMS as the internal standard. No
window function or zero filing was used. Digital resolutionZ
0.2 Hz/point.
13C NMR spectra were recorded at the operating frequency
75.454 MHz; pwZ608; swZ19,000 Hz; atZ1.8 s; d1Z1.0 s;
TZ293.0 K and TMS as the internal standard. Line broadening
parameters were 0.5 or 1 Hz.
7Li NMR measurements were made in the following
conditions: LiCl in D2O (1 mol dmK3) as external standard,
sfrqZ116.621 MHz; pwZ258; swZ20,000 Hz; atZ1.0 s;
d1Z0.5 s; TZ293.0 K. Digital resolution 0.6 Hz/point. No
window function or zero filing was used.
23Na NMR spectra were taken at: sfrqZ79.373 kHz; swZ
20,000 Hz; pwZ708; atZ1.0 s; d1Z1.0 s; TZ293.0 K and
1 mol dcmK3 solution of NaCl/D2O as the external standard.
Digital resolutionZ0.7 Hz/point. No window function or zero
filing was used.
Scheme 2. Structures and atom numbering of the esters 1–4 studied.

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A. Sporzynski et al. / Journal of Molecular Structure 791 (2006) 111–116
113
Table 1
Mass spectrometry data (ESI) for complexes of esters 1–4 with LiC and NaC
cations
detected by the signals at m/zZ214 and m/zZ230, respectively.
In addition to the signals assigned to the 1:1 complexes always
another signal indicating the fragmentation process of the basic
1:1 complex is observed. As shown in Scheme 3 the general
fragmentation pathway for 1:1 complexes of esters 1–4 with
monovalent cations at cvZ30 V generally involves the
abstraction of one oxaalkyl chain at each boric acid group.
Ester
Cation
1:1 Complex
peaks (m/z)a
1:2 Complex
Fragment complex
peaks (m/z)b
peaks (m/z)
1
2
3
4
1
2
3
4
LiC
LiC
LiC
LiC
NaC
NaC
NaC
NaC
245
333
421
421
261
349
437
437
–
187
–
231
–
275
214
–
363, 305
203
3.2. NMR measurements
–
247
–
291
The 1H and 13C NMR data of the esters and their 1:1
complexes with LiC and NaC cations in acetonitrile are
collected in Tables 2 and 3, respectively. The assignments the
proton and carbon spectra of the esters and their complexes
were made with the aid of 2D experiments.
230
379, 321
a
Exemplary structures given in Scheme 4.
Structures given in Scheme 3.
b
mass spectra the cone voltage was 30 V. Source temperature
was 120 8C and dessolvation temperature was 300 8C. Nitrogen
was used as the nebulizing and dessolvation gas at flow-rates of
100 and 300 1 hK1, respectively.
1
In the H NMR spectra of 1–4 esters the signals of the
protons in the ortho position to the boron substituent in the
phenyl ring (H-2) are shifted towards higher parts per million
values in comparison with those of H-3 and H-4 due to the
anisotropic effect of the boron atom. Due to the anisotropic
effect of the phenyl ring, the signals of the protons of the
methylene groups (H-5) are also clearly shifted to ca. 4.10 ppm
in comparison with those of all other methylene protons which
are observed in a small region at ca. 3.50 ppm.
3. Results and discussion
The structures and the numbering of the atoms of
polyoxaalkyl esters of phenylboronic acid (1–3) and 1,4-
phenylenediboronic acid (4) are shown in Scheme 2.
The signals of the CH3 protons in CD3CN are observed
always at ca. 3.30 ppm. After addition of the corresponding
MClO4 salts to the esters all proton signals of the CH3 and CH2
groups are only slightly shifted and become clearly broadened
(6–15 Hz) indicating strong interactions between the oxaalkyl
ligands and the cations.
The 13C NMR chemical shifts in the spectra of 1–4 esters
and their mixtures with LiC and NaC cations are collected in
Table 3. A comparison of the chemical shifts of the 13C NMR
signals of oxaalkyl chains of free esters with those of in the
spectra of their complexes show that all these signals are
3.1. ESI mass spectrometry
ESI mass spectrometric m/z data obtained for the complexes
formed between MClO4 (MZLi, Na and K) and the esters (1–4)
are collected in Table 1. The m/z signals observed in the spectra
at the low cone voltage value of cvZ30 of LiC and NaC
complexes with esters 1–3 indicate than only 1:1 complexes are
always formed, irrespectively of the stoichiometry of ester
mixtures: the cation (1:1, 1:2) used. In the case of ester 4 also the
formation of 1:2 complexes with of LiC and NaC cations is
Scheme 3. General fragmentation pathway of the 1:1 complexes of esters 1–4 with MC (MCZLiC or NaC) cations.

Table 2
1H NMR chemical shifts (ppm) of 1–4 esters and their 1:1 complexes with LiClO4 and NaClO4 salts
Compound
1
Salt
1H NMR chemical shifts (ppm)
2
3
4
5
6
7
8
9
10
11
–
7.64 (m)
7.59 (m)
7.60 (m)
7.64 (m)
7.63 (m)
7.64 (m)
7.62 (m)
7.62 (m)
7.62 (m)
7.60 (s)
7.58 (s)
7.56 (s)
7.40 (m)
7.40 (m)
7.40 (m)
7.40 (m)
7.40 (m)
7.40 (m)
7.40 (m)
7.40 (m)
7.40 (m)
–
7.40 (m)
7.40 (m)
7.40 (m)
7.40 (m)
7.40 (m)
7.40 (m)
7.40 (m)
7.40 (m)
7.40 (m)
–
4.13 (m)
4.11 (m)
4.12 (m)
4.11 (m)
4.12 (m.)
4.13 (m)
4.12 (m)
4.13 (m)
4.13 (m)
4.12 (m)
4.10 (m)
4.10 (m)
3.53 (m)
3.32(s)
3.31(s)
3.31(s)
–
–
–
–
LiClO4
NaClO4
–
3.52 (m)
–
–
–
–
3.52 (m)
–
–
–
–
2
3
4
3.52–3.59 (m)
3.55–3.61 (m)
3.55–3.60 (m)
3.47 (m)
3.46 (m)
3.45 (m)
3.29 (s)
3.22 (s)
3.22 (s)
–
–
LiClO4
NaClO4
–
–
–
–
–
3.50–3.70 (m)
3.48 (m)
3.28 (s)
LiClO4
NaClO4
–
3.45–3.70 (m)
3.25 (s)
3.45–3.70 (m)
3.24 (s)
3.51 (m)
3.50 (m)
3.49 (m)
3.31 (s)
3.30 (s)
3.29 (s)
–
–
–
–
–
–
–
–
–
–
–
–
LiClO4
NaClO4
–
–
–
–
(m), multiplet; (s), singlet.
Table 3
13C NMR chemical shifts (ppm) of 1–4 esters and their 1:1 complexes with LiClO4 and NaClO4 salts
Compound
Salt
Chemical shifts (ppm)
1
2
3
4
5
6
7
8
9
10
11
1
–
131.89
131.91
131.91
131.92
131.95
131.94
131.88
131.87
131.86
134.09
134.04
134.07
134.10
134.09
134.10
134.11
134.00
134.01
133.20
133.18
132.21
128.30
128.34
128.34
128.27
128.55
128.41
128.28
128.49
128.41
–
130.26
130.38
130.37
130.22
130.30
130.29
130.23
130.56
130.50
–
64.52
64.41
64.56
64.49
64.52
64.55
64.66
64.68
64.61
64.54
64.47
64.58
73.54
73.44
73.45
58.84
58.93
58.91
–
–
–
–
–
–
–
–
–
–
–
–
–
LiClO4
NaClO4
–
–
–
2
3
4
72.06; 71.87; 71.01
58.73
–
LiClO4
NaClO4
–
71.88; 71.20; 70.87
71.82; 71.28; 70.92
58.98
58.90
–
–
72.40; 72.05; 71.08; 70.95; 70.82
71.85; 71.50; 70.21; 69.74; 69.62
71.82; 71.57; 70.22; 69.70; 69.62
58.71
58.98
58.92
–
LiClO4
NaClO4
–
a
–
73.53
73.45
73.44
58.84
58.90
58.91
–
–
–
–
–
–
–
–
–
a
LiClO4
NaClO4
–
a
–
–
–
–
–
–
–
a
Signal not found.

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A. Sporzynski et al. / Journal of Molecular Structure 791 (2006) 111–116
115
Table 4
7Li and 23Na NMR data of LiClO4 and NaClO4 and their complexes with 1–4 esters in CD3CN at 293 K dependent on salt concentration as well as the stability
constants of the complexes
Ester
Ratio ester/
MClO4
MZLi
pK
MZNa
d (ppm)
pK
d (ppm)
Dn1/2 (Hz)
Dn1/2 (Hz)
–
1
1
2
2
3
3
4
4
0:1
1:1
1:2
1:1
1:2
1:1
1:2
1:1
1:2
K1.70
K1.61
K1.68
K1.61
K1.68
K1.60
K1.68
K1.62
K1.62
0.7
1.0
0.8a
1.0
0.8a
1.0
0.8a
1.0
1.0
–
K6.35
K6.25
K6.37
K6.22
K6.36
K6.12
K6.35
K6.20
K6.21
20
–
0.5C0.1
–
60
100a
0.6C0.1
–
0.6C0.1
–
65
0.9C0.1
–
100a
70
0.8C0.1
–
1.1C0.1
–
100a
0.4C0.1
0.2C0.1
70
0.5C0.1
0.3C0.1
70
a
Mean values for free MC cation and its 1:1 complex with the respective ester.
slightly shifted toward lower frequency pointing to strong
interactions of LiC and NaC cations with all oxygen atoms of
the oxaalkyl chains. Such interactions can be only realised by
fast fluctuations of LiC or NaC cations in a crown ether-like
this type of complexes between the ester 4 and LiC or NaC
cations is less favourable than the formation of those of the 1:1
stoichiometry.
7
structure formed by two oxaalkyl chains. The Li and 23Na
3.3. PM5 calculations
NMR and PM5 semiempirical calculations studies confirm the
suggestion of fast fluctuations of the LiC and NaC cations in a
crown ether-like structure.
The heats of formation (HOF) of the complexed and
uncomplexed species and the differences between these values
(DHOF) of 1–4 esters with LiC and NaC monovalent cations
are summarized in Table 5. These data show that the formation
of the respective 1:1 complexes is energetically favourable.
The DHOF values demonstrate that in the gas phase (in the
The 7Li and 23Na NMR signals and their line widths for the
mixtures of various concentrations of LiClO4 and NaClO4 salts
with a constant amount of 1–4 esters in acetonitrile together
with the pK values of the 1:1 complexes calculated from the
NMR titration experiments are shown in Table 4. For
comparison, the respective 7Li and 23Na NMR signals of
salts alone are also given. In the spectra of the 1:1 mixtures of
esters with LiClO4 or NaClO4 salts the corresponding chemical
Table 5
The heats of formation (HOF) of 1–4 esters and their complexes with LiC and
NaC cations
7
shifts of Li and 23Na signals become less negative and more
Compound
HOF (kJ/mol)
DHOF (kJ/mol)
broadened in comparison with the signals of the salts alone. In
the spectra of the 1:2 mixtures only for ester 4 no changes in the
chemical shift is observed indicating that only this ester is able
to form the complexes of 1:2 stoichiometry with LiC or NaC
cations. In other spectra of the 1:2 mixtures only medium parts
per million values between the chemical shifts of complexed
and uncomplexed species are observed. All these changes of
the signals and their line widths in the respective NMR spectra
are additional evidence of formation of the 1:1 stoichiometry
complexes between 1 and 4 esters and LiC or NaC cations. It is
interesting to note that the direction of changes in the chemical
shifts of the LiC and NaC respective NMR signals is the same
as that of the changes observed for LiC and NaC complexes
with lasalocid in methanol and oligomycin A in acetonitrile as
well as the opposite to that of the changes in pyridine solutions
[11–13].
1
K818.19
K554.66
K302.54
K480.45
K223.80
K1130.13
K1000.59
K614.48
K920.32
K535.74
K1443.29
K1390.91
K927.64
K814.99
K411.99
K1282.12
K848.90
K674.78
K254.51
K1753.66
K1513.38
K1238.01
K1089.00
K722.36
K1443.54
K1159.27
K943.43
K564.88
–
1CLiC(complexed)
1CLiC(uncomplexed)
1CNaC(complexed)
1CNaC(uncomplexed)
2
K252.12
K256.65
–
2CLiC(complexed)
2CLiC(uncomplexed)
2CNaC(complexed)
2CNaC(uncomplexed)
3
3CLiC(complexed)
3CLiC(uncomplexed)
3C2Li(complexed)
3C2Li(uncomplexed)
3CNaC(complexed)
3CNaC(uncomplexed)
3C2NaC(complexed)
3C2NaC(uncomplexed)
4
4CLiC(complexed)
4CLiC(uncomplexed)
4C2LiC(complexed)
4C2LiC(uncomplexed)
4CNaC(complexed)
4CNaC(uncomplexed)
4C2NaC(complexed)
4C2NaC(uncomplexed)
K386.11
K384.58
–
K463.27
K403.00
K433.22
K420.27
–
The values of stability constants for the 1:1 complexes of
esters with LiC or NaC cations, collected in Table 4, increase
slightly with increasing length of the oxaalkyl chains, i.e. with
the increasing number of the oxygen atoms able to coordinate
of these cations. The values of the stability constants for the 1:1
complexes of 1–4 esters with NaC cations are slightly higher
than those of the respective complexes with LiC cations,
although their order is comparable. The values of the stability
constants for 1:2 complexes demonstrate that the formation of
K275.37
K366.64
K284.27
K378.55
DHOFZHOF(complexed)KHOF(uncomplexed)
.

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A. Sporzynski et al. / Journal of Molecular Structure 791 (2006) 111–116
116
formation of such 1:2 complexes of ester 3 with LiC and NaC
cations was not experimentally (ESI MS, NMR) detected. In
contrast to this result the formation of 1:2 complexes with ester
4 is energetically favourable for LiC as well as NaC cations
and also is experimentally evidenced.
The calculated exemplary structures of 1:1 and 1:2
complexes of esters are given in Scheme 4a and b, respectively.
In both types of complexes the oxaalkyl chains form crown-
like structure in which all oxygen atoms are involved in the
coordination process. This result suggests that with increasing
numbers of oxygen atoms in the oxaalkyl chains the LiC and
NaC cations fluctuate very fast conserving the crown-like
structure and demonstrating the so-called cation polarizability,
which was in detail discussed previously [7,14–16].
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same experimental conditions as those of ESI measurements)
1–4 esters are able to form stable 1:1 complexes with LiC and
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data discussed above. The DHOF values also demonstrate that
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chains the complexes become increasingly stable. This result is
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complexes determined. The DHOF values obtained for the 1:2
complexes show that for ester 3 the formation of
such complexes is energetically possible but less favourable
than that of the 1:1 complexes. It is interesting to note that the
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