Journal of the Korean Chemical Society (대한화학회지)

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[2]Pseudorotaxanes Containing the Tertiary Alkylammonium Salts as Strings and Cucurbit[6]uril as a Bead

3차 암모늄염과 쿠커비튜릴을 이용한 유사로택산의 형성에 대한 고찰

  • Published : 2005.02.20
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Abstract

Keywords

INTRODUCTION

One of the most significant challenges presently facing supramolecular chemists is to find ways of constructing nanometer-scale devices such as molecular machines and switches from molecular components. This challenge exists not only for designing and constructing of working devices, but also for developing new mechanically interlocked molecules using a self-assembling approach. One particularly interesting class of supramolecular aggregates (mechanically interlocked molecules) is the so-called pseudorotaxanes.1 These supramolecular entities are comprised of one (or more) threadlike component(s) which is (are) encircled by one (or more) beadlike component(s). They are promising prototypes for a variety of nano-wired molecular machines,2 such as switches3 and shuttles.4 Most pseudorotaxanes have been prepared by utilizing: (a) van der Waals interactions5 between cyclodextrin derivatives and hydrophobic guests, (b) metalligand interactions6 between metal ions, such as copper(I), and bipyridine-derived beads/threads, (c) attraction between π-electron-rich7 macrocyclic hosts and π-electron-deficient threadlike guests or vice versa,8 and (d) association9 between secondary dialkylammonium ions, such as the linear threadlike cationic salts, and macrocyclic polyethers, such as dibenzo[24]crown-8 (DB24C8) and bis-p-phenylene[34]crown-10 (BPP34C10). In the past decade, Kim et al. have developed10 simple self-assembling systems capable of forming pseudorotaxanes, olyrotaxanes, molecular necklace, and so on. These supramolecular systems are based upon non-covalent interactions between diammoniumalkane derivatives as the linear threadlike cationic salts and cucurbit[6]uril.

Cucurbit[6]uril (CB[6]) is a large cage compound composed of six glycoluril units interconnected via twelve methylene bridges with relatively rigid structure which was prepared nearly 100 years ago.11 Each of the two carbonyl-fringed portals at the upper and lower side of the molecule has a diameter of 4 Å. The internal cavity has a diameter of ca. 5.5 Å, while the distance between the portals is 6 Å. The interior of the molecule represents a hydrophobic region, whereas the two portals are hydrophilic. Consequently, the hydrophobic organic moiety of organic ammonium ions extends into the interior, and the ionic part coordinates to one of the planes spanned through the negatively polarized carbonyl groups. It is this potential for the formation of the supramolecular host-guest complexes which make this molecule interesting. A series of alkylammonium and alkyldiammonium ions formed host-guest complexes with CB[6] which was observed and analyzed by NMR spectroscopy.12 Although CB[6] has now been demonstrated to have the ability to form host-guest complexes with primary and secondary alkyldiammonium salts, there have been relatively few studies with tertiary alkylammonium salts.12b, 13 We were thus intrigued to find out whether the tertiary alkylammonium salts, such as aminoalkyliminodiacetic acid salts as ligands, could be threaded with CB[6] to form pseudorotaxanes. Here, we report that pseudorotaxane complexes between the tertiary alkylammonium salts and CB[6] are indeed capable of being formed and characterized.

 

RESULTS AND DISCUSSION

Our interesting ligand structures are aminoalkyliminodiacetic acids I which their syntheses are outlined in Scheme 1. Cbz-protectedaminoalkyl amines 114 were dialkylated with methyl bromoacetate to afford the tertiary amines 2 in excellent yields. Both Cbz group deprotection and ester group hydrolysis could be carried out simultaneously using HBr-H2O to afford ammoniumalkyliminodiacetic acids I in 94~95 % yields.

Scheme 1.Synthesis of iminoacetic acids I. Reagents and conditions: (a) Methyl bromoacetate (2.2 eq.), DIEA, CH3CN, rt, 20 h; (b) HBr-H2O, 80 ℃, 5 h.

Scheme 2.Synthesis of CB[6]-pseudorotaxanes II.

Keeping in mind the host-guest chemistry with CB[6] and ammonium salts, we have tried to thread CB[6] with ammoniumalkyliminodiacetic acids I (Scheme 2). Ammoniumbutyliminodiacetic acid salt Ia (n = 1; 4 methylene units between ammonium salts) and CB[6] in H2O did not form the expected CB[6]-threaded complex. However, slight interaction between CB[6] and ammonium salt could be observed. This result is in contrast to the primary and secondary diammoniumalkyl salts having 4 methylene units between moiety which readily forms a CB[6]-threaded complex.12 The diacetic acid in Ia might experience both steric hindrance and repulsion with carbonyl groups of CB[6] and disallow threading CB[6] with ligand Ia. Besides, this will make the 4 methylene units in ligand Ia nor long enough to coordinate a CB[6] molecule. On the other hand, ammoniumpentyliminodiacetic acid salt Ib (n = 2; 5 methylene units between ammonium salts) and ammoniumhexyliminodiacetic acid salt Ic (n = 3; 6 methylene units between ammonium salts) formed CB[6]-threaded complexes IIb and IIc, respectively, in quantitative yields. These CB[6]-threaded complexes were isolable and characterized by 1H-NMR and ESI-mass spectroscopy.

In the 1H-NMR spectrum, CB[6] positions in CB[6]-threaded complexes IIb and IIc can be distinguish from other peaks. In other words, the location of CB[6] leans to the primary ammonium salt in complexes IIb and IIc. As shown in Fig. 1, the 1H-NMR shift of complex IIb exhibits five CH2 peaks of which four methylene groups (a, b, c, and d in complex IIb) showing upfield shifts while one (e in complex IIb) showing a downfield shift.12 These observations can be attributed as follows. First of all, the methylene unit a which is adjacent to primary ammonium group is included in the cavity of CB[6]. The methylene unit e is outside the cavity due to steric hindrance offered by the substituents attached to the tertiary ammonium nitrogen adjacent to e. Secondly, the methylene units b and d in complex IIb stay at different shielding zones within CB[6]; the unit b is located at strong shielding zone due to strong interaction of the primary ammonium salt and the unit d is located at weak shielding zone resulting from weak interaction of the tertiary ammonium salt. The methylene unit c, however, remains at strong shielding zone. The 1H-NMR spectrum of complex IIc exhibits six CH2 peaks where five methylene groups (a, b, c, d, and e in complex IIc) show upfield shifts and one methylene group (f in complex IIc) shows a downfield shift (Figure 1). The location of CB[6] in complexes IIc leans to the primary ammonium salt as in case of complexes IIb. The chemical environment of methylene units a, b, e, and f in complexes IIc correspond similar to that of methylene units a, b, d, and e in complex IIb, respectively, even though they show different magnitude of chemical shifts due to different alkane chain lengths. We can assume that complex IIb is stronger compared to complex IIc considering the lengths of methylene chain and the different strengths of interactions of the tertiary ammonium part. For ligand substrates I, pentyldiammonium salt Ib forms the most stable complex IIb (five methylene units) and the order of affinities for CB[6] follows the trend; number of methylene unit = 4 〈〈 6 〈 5. The plausible explanation is that the pentyl substituent optimally fills the cavity, the butyl chain is short to form a stable complex, while the hexyl substituent is obliged to protrude into the second occlus of CB[6].

Fig. 1.Chemical shift of methylene peaks in 1H-NMR (in D2O) ligands Ib and Ic and complexes IIb and IIc.

In summary, we have demonstrated the hostguest chemistry using tertiary ammonium salts such as aminoalkyliminodiacetic acids as ligands with cucurbit[6]uril. From the formation of pseudorotaxane complex between aminoalkyliminodiacetic acids salt and CB[6], it was proven that five methylenes unit is best to form stable complex but four methylenes unit is short to form complex II, and the location of CB[6] leans to the primary ammonium salt in complexes II.

 

EXPERIMENTAL SECTION

General Methods. Materials were obtained from commercial supplier and used without further purification. Thin-layer chromatography (TLC) was carried out on glass coated with silica-gel 60 (Merck 5554). Column chromatography was performed on silica-gel 60 (Merck 9385, 230-400 mesh). Electrospray ionization (ESI) mass spectra were measured on a VG Prospec mass spectrometer. 1H-NMR spectra were recorded on a Bruker AC300 (300 MHz) or a Bruker AMX500 (500 MHz) spectrometer using deturiated solvent. 13C-NMR spectra were recorded on a Bruker AC300 (75 MHz) spectrometer. All chemical shifts are quoted on the δ scale, and all coupling constants are expressed in Hertz (Hz).

General Procedure for dialkylation of primary amine. To a solution of primary amine (4 mmol) and diisopropylethylamine (8.1 mmol) in CH3CN (40 mL) was added methyl bromoacetate (8.8 mmol) under ice bath and then the resulting solution was stirred for 20 h at room temperature. The reaction solution was concentrated and dissolved with methylene chloride (80 mL). The resulting solution was washed with NaHCO3 aqueous solution, dried, and evaporated to give crude which was purified by column chromatography using EtOAc/n-hexane system as an eluent.

N-Cbz-aminobutyliminodiacetic ester 2a. Amine 1a gave 2a (98 %): 1H-NMR (CDCl3) δ 1.51 (m, 4H), 2.70 (t, J = 6.8 Hz, 2H), 3.19 (distorted t, 2H), 3.52 (s, 4H), 3.68 (s, 6H), 4.91 (br s, 1H), 5.08 (s, 2H), 7.32 (m, 5H).

N-Cbz-aminopentyliminodiacetic ester 2b. Amine 1b gave 2b (98 %): 1H-NMR (CDCl3) δ 1.32 (m, 2H), 1.48 (m, 4H), 2.68 (t, J = 7.2 Hz, 2H), 3.18 (distorted t, 2H), 3.53 (s, 4H), 3.69 (s, 6H), 4.92 (br s, 1H), 5.08 (s, 2H), 7.34 (m, 5H); 13C NMR (CDCl3) δ 24.5, 27.8, 30.1, 41.3, 51.9, 54.5, 55.2, 66.9, 128.4, 128.9, 137.1, 156.8, 172.1.

N-Cbz-aminohexyliminodiacetic ester 2c. Amine 1c gave 2c (99%): 1H-NMR (CDCl3) δ 1.31 (m, 4H), 1.47 (m, 4H), 2.67 (t, J = 7.2Hz, 2H), 3.18 (distorted t, 2H), 3.53 (s, 4H), 3.70 (s, 6H), 4.90 (br s, 1H), 5.08 (s, 2H), 7.33 (m, 5H); 13C NMR (CDCl3) δ 26.8, 27.0, 28.1, 30.2, 41.3, 51.9, 54.5, 55.2, 66.9, 128.4, 128.5, 128.9, 137.1, 156.8, 172.1.

General Procedure for the preparation of substituted-iminodiacetic acids by Cbz deprotection and simultaneous hydrolysis. A solution of Cbz-protected aminoester (1 mmol) in HBr-H2O (10 mL) was stirred for 5 h at 80 ℃. The resulting solution was concentrated and solidified from EtOAc which the generated solid was filtered to collect and washed with ether extensively.

Aminobutyliminodiacetic acid Ia. Compound 2a gave Ia (94 %): 1H NMR (D2O) δ 1.72 (m, 2H), 1.84 (m, 2H), 3.03 (t, J = 7.5Hz, 2H), 3.41 (t, J = 7.7Hz, 2H), 4.19 (s, 4H); 13C NMR (D2O) δ 21.2, 24.1, 39.1, 55.5, 56.3, 168.9.

Aminopentyliminodiacetic acid Ib. Compound 2b gave Ib (95 %): 1H NMR (D2O) δ 1.42 (m, 2H), 1.69 (m, 2H), 1.79 (m, 2H), 2.98 (t, J = 7.4 Hz, 2H), 3.37 (t, J = 8.2 Hz, 2H), 4.20 (s, 4H); 13C NMR (D2O) δ 23.0, 23.4, 26.5, 39.5, 55.2, 56.8, 168.8.

Aminohexyliminodiacetic acid Ic. Compound 2c gave Ic (94 %): 1H NMR (D2O) δ 1.40 (m, 4H), 1.65 (m, 2H), 1.75 (m, 2H), 2.97 (t, J = 7.5 Hz, 2H), 3.35 (t, J = 8.3 Hz, 2H), 4.18 (s, 4H); 13C NMR (D2O) δ 23.5, 25.4, 26.8, 39.7, 55.2, 57.0, 168.9.

General Procedure for the Preparation of CBthreaded Complexes. To a solution of polyamine ligand (0.3 mmol) in 30 mL of H2O was added cucurbit[6]uril (0.4 mmol) in a small portions and stirred for 2~5 h at room temperature. Excess cucurbit[6]uril was filtered out to provide clear solution which was evaporated to leave ca. 1 mL. 30 mL of EtOH was added to the resulting solution to make the precipitate which was isolated by the filteration to afford the desired CB-threaded complex quantitatively.

CB-threaded complex Ib. Ligand Ib gave Ib: 1H NMR (D2O) δ 0.35 (m, 2H), 0.62 (m, 2H), 0.90 (m, 2H), 2.22 (t, J = 9.0 Hz, 2H), 3.43 (t, J = 8.5 Hz, 2H), 4.33 (d, J = 8.5Hz, 6H), 4.38 (d, J = 8.6Hz, 6H), 4.80 (dd, J = 25.9Hz and 17.7Hz, 4H), 5.61 (s, 12H), 5.75 (d, J =15.8Hz, 6H), 5.81 (d, J = 15.6Hz, 6H); ESIMS: m/z 608.3 ([M + 2H]2+), 1215.5 ([M + H]+).

CB-threaded complex IIc. Ligand Ic gave IIc: 1H NMR (D2O) δ 0.41 (m, 4H), 0.65 (m, 2H), 0.17 (m, 2H), 2.65 (m, 2H), 3.48 (t, J = 8.1 Hz, 2H), 4.30 (d, J = 9.1 Hz, 6H), 4.35 (d, J = 9.2 Hz, 6H), 4.61 (dd, J = 25.3 Hz and 17.6 Hz, 4H), 5.58 (s, 12H), 5.76 (d, J = 15.1 Hz, 6H), 5.81 (d, J = 14.9 Hz, 6H); ESIMS: m/z 615.3 ([M+ 2H]2+), 626.2 ([M+ H + Na]2+), 1229.4 ([M + H]+).

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