Polyoxaalkyl esters of phenylboronic acids as new podands

Journal of Molecular Structure 791 (2006) 111–116 www.elsevier.com/locate/molstruc

Polyoxaalkyl esters of phenylboronic acids as new podands A. Sporzyn´ski a, A. Mis´kiewicz a, B. Gierczyk b, R. Pankiewicz b, G. Schroeder b, 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. Keywords: Phenylboronic acids; Oxaalkyl esters; 1H NMR; 13C NMR; 7Li NMR; 23Na NMR; ESI MS; Complexes; Stability constants; Pseudo-crown structure

1. Introduction 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 application of boron compounds [3,4]. 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(polyoxaalkyl) phosphates form channels with protons or cations [6–9]. Within these channels the protons or cations show large polarizability due to their fast fluctuations. 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: [email protected] (B. Brzezinski).

0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.01.013

method. The stability constants of the complexes are determined and the structure of the complexes is discussed. 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 phenylenediboronic acids were synthesized following the procedures given in Ref. [10]. 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%. 2.2. Preparations of complexes LiClO4, NaClO4 and acetonitrile (Aldrich) were used for the preparations of complexes. The solutions were obtained

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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 the concentration of the salt was constant (0.2 mol dmK3) and the concentration of the ester was varied, whereas for the 1H 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 ˚ molecular sieve. 3A All preparations and transfers of solutions were carried out in a carefully dried glove box under nitrogen atmosphere.

2.4. ESI mass spectrometry 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

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. 13 C 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. 7 Li 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. 23 Na 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.

A. Sporzyn´ski et al. / Journal of Molecular Structure 791 (2006) 111–116 Table 1 Mass spectrometry data (ESI) for complexes of esters 1–4 with LiC and NaC cations Ester

Cation

1:1 Complex peaks (m/z)a

1:2 Complex peaks (m/z)

Fragment complex peaks (m/z)b

1 2 3 4 1 2 3 4

LiC LiC LiC LiC NaC NaC NaC NaC

245 333 421 421 261 349 437 437

– – – 214 – – – 230

187 231 275 363, 305 203 247 291 379, 321

a b

Exemplary structures given in Scheme 4. Structures given in Scheme 3.

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. 3. Results and discussion The structures and the numbering of the atoms of polyoxaalkyl esters of phenylboronic acid (1–3) and 1,4phenylenediboronic acid (4) are shown in Scheme 2. 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

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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. 3.2. NMR measurements 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. In the 1H 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. 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

Scheme 3. General fragmentation pathway of the 1:1 complexes of esters 1–4 with MC (MCZLiC or NaC) cations.

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Table 2 1 H NMR chemical shifts (ppm) of 1–4 esters and their 1:1 complexes with LiClO4 and NaClO4 salts Compound 1

1

Salt

2

3

4

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) 3.52 (m) 3.52 (m) 3.52–3.59 (m) 3.55–3.61 (m) 3.55–3.60 (m)

3.32(s) 3.31(s) 3.31(s)

– – – 3.47 (m) 3.46 (m) 3.45 (m)

– – – 3.29 (s) 3.22 (s) 3.22 (s)

– – – – – – 3.48 (m)

3.45–3.70 (m) 3.45–3.70 (m) – – –

– – –

– – –

– – – – – – 3.28 (s) 3.25 (s) 3.24 (s) – – –

3.50–3.70 (m)

3.51 (m) 3.50 (m) 3.49 (m)

3.31 (s) 3.30 (s) 3.29 (s)

7

(m), multiplet; (s), singlet.

Table 3 13 C NMR chemical shifts (ppm) of 1–4 esters and their 1:1 complexes with LiClO4 and NaClO4 salts Compound 1

Salt – LiClO4 NaClO4 – LiClO4 NaClO4 – LiClO4 NaClO4 – LiClO4 NaClO4

2

3

4

a

Signal not found.

Chemical shifts (ppm) 1

2

3

4

5

6

8

9

131.89 131.91 131.91 131.92 131.95 131.94 131.88 131.87 131.86 –a –a –a

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 58.84 – – 73.44 58.93 – – 73.45 58.91 – – 72.06; 71.87; 71.01 58.73 71.88; 71.20; 70.87 58.98 71.82; 71.28; 70.92 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 73.53 58.84 – – 73.45 58.90 – – 73.44 58.91 – –

10

11

– – – – – –

– – – – – – 58.71 58.98 58.92 – – –

– – –

A. Sporzyn´ski et al. / Journal of Molecular Structure 791 (2006) 111–116

– LiClO4 NaClO4 – LiClO4 NaClO4 – LiClO4 NaClO4 – LiClO4 NaClO4

H NMR chemical shifts (ppm)

2

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Table 4 7 Li 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

– 1 1 2 2 3 3 4 4 a

Ratio ester/ MClO4

d (ppm)

Dn1/2 (Hz)

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

MZLi

pK

– 0.5C0.1 – 0.6C0.1 – 0.8C0.1 – 0.4C0.1 0.2C0.1

MZNa

pK

d (ppm)

Dn1/2 (Hz)

K6.35 K6.25 K6.37 K6.22 K6.36 K6.12 K6.35 K6.20 K6.21

20 60 100a 65 100a 70 100a 70 70

– 0.6C0.1 – 0.9C0.1 – 1.1C0.1 – 0.5C0.1 0.3C0.1

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 structure formed by two oxaalkyl chains. The 7Li and 23Na 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 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 shifts of 7Li and 23Na signals become less negative and more 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]. 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

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. 3.3. PM5 calculations 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 Table 5 The heats of formation (HOF) of 1–4 esters and their complexes with LiC and NaC cations Compound

HOF (kJ/mol)

DHOF (kJ/mol)

1 1CLiC(complexed) 1CLiC(uncomplexed) 1CNaC(complexed) 1CNaC(uncomplexed) 2 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)

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

– K252.12

DHOFZHOF(complexed)KHOF(uncomplexed).

K256.65 – K386.11 K384.58 – K463.27 K403.00 K433.22 K420.27 – K275.37 K366.64 K284.27 K378.55

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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 crownlike 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]. References [1] [2] [3] [4] [5] [6] [7] Scheme 4. The calculated structures of (a) 1:1 complex of ester 3 with LiC cation, and (b) 1:2 complex of ester 4 with NaC cations.

same experimental conditions as those of ESI measurements) 1–4 esters are able to form stable 1:1 complexes with LiC and NaC cations. This result is in very good agreement with the ESI data discussed above. The DHOF values also demonstrate that with increasing number of the oxygen atoms in the oxaalkyl chains the complexes become increasingly stable. This result is in very good agreement with the stability constants of the 1:1 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

[8] [9] [10] [11] [12] [13] [14] [15] [16]

N. Miyaura, A. Suzuki, Chem. Rev. 95 (1995) 2457. N.A. Petasis, I.A. Zavialov, J. Am. Chem. Soc. 119 (1997) 445. W. Yang, X. Gao, B. Wang, Med. Res. Rev. 21 (2001) 245. T.D. James, S. Shinkai, Top. Curr. Chem. 218 (2002) 159. G. Schroeder, B. Gierczyk, B. Łe˛ska, J. Incl. Phenom. Macroc. Chem. 35 (1999) 327. B. Brzezinski, B. Rozalski, G. Schroeder, F. Bartl, G. Zundel, J. Chem. Soc. Faraday Trans. 94 (1999) 2093. B. Gierczyk, G. Schroeder, G. Wojciechowski, B. Ro´z˙alski, B. Brzezinski, G. Zundel, Phys. Chem. Chem. Phys. 1 (1999) 4897. B. Gierczyk, G. Schroeder, B. Nowak-Wydra, G. Wojciechowski, B. Brzezinski, J. Mol. Struct. 513 (1999) 149. B. Brzezinski, B. Gierczyk, B. Rozalski, G. Wojciechowski, G. Schroeder, G. Zundel, J. Mol. Struct. 519 (2000) 119. Methoden der Organischen Chemie (Houben-Weyl), Organobor Verbindungen I, Georg Thieme Verlag, Stuttgart, 1982, p. 635. G. Schroeder, B. Gierczyk, B. Brzezinski, B. Ro´z˙alski, F. Bartl, G. Zundel, J. Sos´nicki, E. Grech, J. Mol. Struct. 516 (2000) 91. B. Gierczyk, G. Schroeder, P. Przybylski, B. Brzezinski, F. Bartl, G. Zundel, J. Mol. Struct. 738 (2005) 261. R. Pankiewicz, G. Schroeder, B. Gierczyk, B. Brzezinski, F. Bartl, Biopolymers (Biospectroscopy) 65 (2002) 95. G. Zundel, B. Brzezinski, J. Olejnik, J. Mol. Struct. 300 (1993) 573. B. Brzezinski, G. Zundel, J. Phys. Chem. 98 (1994) 2271. B. Brzezinski, G. Schroeder, A. Rabold, G. Zundel, J. Phys. Chem. 99 (1995) 8519.