SCIENCE CHINA Chemistry • ARTICLES •

February 2012 Vol.55 No.2: 223–228 doi: 10.1007/s11426-011-4427-3

Synthesis and host-guest properties of pillar[6]arenes TAO HongQi1, CAO DeRong1,2*, LIU LuZhi1, KOU YuHui1, WANG LingYun1 & MEIER Herbert3 1

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School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China State Key Laboratory of Luminescent Materials and Devices; South China University of Technology, Guangzhou 510640, China 3 Institute of Organic Chemistry, University of Mainz, 55099 Mainz, Germany Received June 19, 2011; accepted July 12, 2011; published online November 4, 2011

A facile method for the synthesis of pillar[6]arenes was developed. A series of pillar[6]arenes were prepared with FeCl3 as catalyst and chloroform as solvent at room temperature in moderate yields (30%–40%). Their host-guest properties with n-cetyltrimethyl ammonium bromide were investigated by 1 HNMR. The results showed that high selectivity in the host-guest relationship became apparent between pillar[6]arenes and pillar[5]arenes based on the different size of the inner cavity. calixarene, pillararene, FeCl3, supramolecular chemistry, host-guest property

1 Introduction The preparation of novel macrocyclic hosts is very important since it is one of the major driving forces to accelerate the development of supramolecular chemistry [1, 2]. Among many calixarenes 1n, the meta-bridged cyclooligomers have been developed in supramolecular chemistry [36]. Recently, the para-bridged analogues, named pillararenes 2n, have also been reported (Figure 1) [712]. For example, Ogoshi reported the BF3 catalyzed condensation of 1,4-dimethoxybenzene and paraformaldehyde [7] 25 for the synthesis of pillar[5]arene 25 with 5-member rings. Pillar[5]arenes 25 are a new class of macrocyclic hosts, and have attracted considerable attention because of their applications in host-guest chemistry [713]. Previously we reported the synthesis of the first pillar[6]arenes 26 which contained 6-member rings via the p-toluenesulfonic acid catalyzed condensation reaction of 2,5-bis(benzyloxymethyl)-1,4-dialkoxybenzene [8]. Although 25 were obtained in good yield, the yields of 26 were very low (< 11%) [8, 9].

*Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2011

The size of the inner cavity of pillar[n]arenes is certainly an important feature for the selectivity of host-guest relationships. Therefore, it is necessary to synthesize new pillar[n]arenes containing bigger inner cavity, such as 26. However, current methods for the synthesis of 26 are limited, which restricted their application as molecular hosts. Thus, it becomes an urgent work to develop new methods for the preparation of 26. It is well known that iron salts have been used in various CC bond formations [14, 15]. Bearing this in mind, we tried to use FeCl3 as catalyst and vary the reaction conditions so that the generation of the cyclohexamers 26 was favored. After many attempts with various acids and solvents, we report here an efficient one-pot synthetic method for the synthesis of 26 on the basis of 1,4-dialkoxybenzenes 3ae, paraformaldehyde, and FeCl3 (Scheme 1 and Tables 1 and 2).

2 Experimental 2.1

Materials and measurements

Melting points were measured with a Tektronix X4 apparatus and uncorrected. 1H NMR (400 MHz) and 13C NMR chem.scichina.com

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purified by column chromatography [2 × 20 cm SiO2, petroleum ether (bp 60–90 °C)/ethyl acetate 30:1 (v/v)], to give 25 and 26 products as colorless crystals.

Figure 1

Structures of calixarenes 1n and pillararenes 2n.

Pillar[5]arene 25a [8] Colorless solid, yield: 30%, mp 157–158 °C (Lit. [8]: 154–156 °C); 1H NMR (400 MHz, CDCl3): δ 1.241.28 (t, 30H, CH3), 3.803.85 (q, 20H, OCH2), 3.77 (s, 10H, CH2), 6.72 (s, 10H, ArH); 13C NMR (100 MHz, CDCl3): δ 15.0 (CH3), 29.6 (CH2-bridge), 63.7 (O–CH2), 115.0 (Ar–H), 128.4, 149.8 (Ar–O). Pillar[5]arene 25b Colorless solid, yield: 28%, mp 139–140 °C; 1H NMR (400 MHz, CDCl3): δ 1.00–1.03 (t, 30H, CH3), 1.71–1.80 (m, 20H, CH2), 3.78–3.81 (m, 30H, CH2), 6.82 (s, 10H, Ar–H); 13 C NMR (100 MHz, CDCl3): δ 10.8 (CH3), 23.0 (CH2), 29.6 (CH2-bridge), 69.9 (O–CH2), 115.0 (Ar–H), 128.3, 149.8 (Ar–O); MS (MALDI-TOF): calcd [M]+ m/z 1030.65, found 1030.6 [M]+, 1053.6 [M+Na]+, 1069.6 [M+K]+; Anal. calcd for C65H90O10: C, 75.69; H, 8.80; found: C, 75.65; H, 8.82. Pillar[5]arene 25c Colorless solid, yield: 31%, mp 221–223 °C; 1H NMR (400 MHz, CDCl3): δ 1.08–1.10 (d, 60H, CH3), 3.72 (s, 10H, CH2-bridge), 4.20–4.23 (m, 10H, O–CH), 6.66 (s, 10H, Ar–H); 13C NMR (100 MHz, CDCl3): δ 22.1 (CH3), 30.6 (CH2–bridge), 70.4 (O–CH), 117.3 (Ar–H), 129.7, 148.8 (Ar–O); MS (MALDI–TOF): calcd [M]+ m/z 1030.65, found 1030.5 [M]+, 1053.5 [M+Na]+, 1069.5 [M+K]+；Anal. calcd for C65H90O10: C, 75.69; H, 8.80; found: C, 75.66; H, 8.81.

Scheme 1

Preparation of pillar[n]arenes with FeCl3 as catalyst.

(100 MHz) spectra were recorded on a Bruker DRX 400 spectrometer by using CDCl3 as solvent and TMS as an internal standard. Matrix-assisted laser desorption ionization time-of flight (MALDITOF) mass spectra were measured on an Autoflex III smart-beam spectrometer. Compounds 3ag were prepared according to the literature procedure [8, 16, 17]. 2.2 General procedure for the preparation of pillar[n] arenes A mixture of 3ag (3.0 mmol), paraformaldehyde (270 mg, 9 mmol) and FeCl3 (73 mg, 0.45 mmol) was stirred in dry CHCl3 (50 mL) at around 30 °C for 2–3 h. Water (50 mL) was added, and the water layer was extracted with CH2Cl2 (3 × 25 mL). The combined organic phases were dried (Na2SO4), concentrated and portionwise (up to five portions)

Pillar[5]arene 25d [8] Colorless solid, yield: 31%, mp 132–133 °C (Lit.[8]: 133–135 °C); 1H NMR (400 MHz, CDCl3): δ 0.950.99 (t, 30H, CH3), 1.481.55 (m, 20H, CH2), 1.741.81 (m, 20H, CH2), 3.76 (s, 10H, CH2-bridge), 3.843.87 (t, 20H, O-CH2), 6.83 (s, 10H, ArH); 13C NMR (100 MHz, CDCl3): δ 14.0 (CH3), 19.5 (CH2), 29.3 (CH2), 32.1 (CH2-bridge), 67.9 (O–CH2), 114.5 (Ar–H), 128.1, 149.7 (Ar–O). Pillar[5]arene 25e Colorless solid, yield: 28%, mp 94–95 °C; 1H NMR (400 MHz, CDCl3): δ 0.89–0.92 (t, 30H, CH3), 1.32–1.36 (m, 40H, CH2), 1.49–1.54 (m, 20H, CH2), 1.77–1.84 (m, 20H, CH2), 3.75 (s, 10H, CH2-bridge), 3.833.96 (t, 20H, O–CH2), 6.84 (s, 10H, Ar–H); 13C NMR (100 MHz, CDCl3): δ 14.1 (CH3), 22.6 (CH2), 26.1 (CH2), 29.5 (CH2-bridge), 29.9 (CH2), 31.8 (CH2), 68.4 (O–CH2), 114.8 (ArH), 128.2, 149.9 (ArO); MS (MALDI-TOF): calcd [M]+ m/z 1451.12, found 1451.0 [M]+; Anal. calcd for C95H150O10: C, 78.57; H, 10.41; found: C, 78.52; H, 10.44.

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Pillar[5]arene 25f Colorless solid, yield: 22%, mp 98–99 °C; 1H NMR (400 MHz, CDCl3): δ 0.84–0.87 (t, 30H, CH3), 1.20–1.25 (dd, 60H, CH2), 1.29–1.35 (m, 20H, CH2), 1.49–1.55 (m, 20H, CH2), 1.82 (s, 20H, CH2), 3.77 (s, 10H, CH2-bridge), 3.87 (s, 20H, O–CH2), 6.86 (s, 10H, Ar–H); 13C NMR (100 MHz, CDCl3): δ 14.1 (CH3), 22.6 (CH2), 26.4 (CH2), 29.3 (CH2), 29.4 (CH2-bridge), 29.6 (CH2), 29.9 (CH2), 31.8 (CH2), 68.3 (O–CH2), 114.7 (ArH), 128.1, 149.8 (ArO); MS (MALDI–TOF): calcd [M]+ m/z 1731.44, found 1731.5 [M]+, 1754.5 [M+Na]+, 1771.5 [M+K]+; Anal. calcd for C115H190O10: C, 79.71; H, 11.05; found: C, 79.68; H, 11.08. Pillar[5]arene 25g [8] Colorless solid, yield: 29%, mp 194–195 °C (Lit. [8]: 194–195°C); 1H NMR (400 MHz, CDCl3): δ 3.65 (s, 30H, CH3), 3.78 (s, 10H, CH2-bridge), 6.77 (s, 10H, Ar–H); 13C NMR (100 MHz, CDCl3): δ 29.6 (CH3), 55.7 (CH2-bridge), 113.9 (Ar–H), 128.2, 150.7 (Ar–O). Pillar[6]arene 26a [8] Colorless solid, yield: 34%, mp 172–173 °C (Lit. [8]: 172–173°C); 1H NMR (400 MHz, CDCl3): δ 1.271.31 (t, 36H, CH3), 3.803.85 (m, 36H, CH2), 6.70 (s, 12H, ArH); 13 C NMR (100 MHz, CDCl3): δ 15.2 (CH3), 31.0 (CH2-bridge), 64.0 (O–CH2), 115.2 (Ar–H), 127.9, 150.4 (Ar–O). Pillar[6]arene 26b Colorless solid, yield: 35%, mp 119–121 °C; 1H NMR (400 MHz, CDCl3): δ 0.92–0.96 (t, 36H, CH3), 1.68–1.73 (dd, 24H, CH2), 3.69–3.72 (t, 24H, O–CH2), 3.81 (s, 12H, CH2–bridge), 6.70 (s, 12H, Ar–H); 13C NMR (100 MHz, CDCl3): δ 10.6 (CH3), 23.0 (CH2), 30.7 (CH2-bridge), 70.1 (O-CH2), 115.0 (Ar–H), 127.9, 150.4 (Ar–O); MS (MALDI–TOF): calcd [M]+ m/z 1236.78, found 1236.8 [M]+, 1259.8 [M+Na]+, 1275.8 [M+K]+; Anal. calcd for C78H108O12: C, 75.69; H, 8.80; found: C, 75.65; H, 8.81. Pillar[6]arene 26c Colorless solid, yield: 30%, mp 209–211 °C; 1H NMR (400 MHz, CDCl3): δ 1.16–1.17 (d, 72H, CH3), 3.76 (s, 12H, CH2), 4.24–4.28 (dd, 12H, CH), 6.73 (s, 12H, Ar–H); 13C NMR (100 MHz, CDCl3): δ 22.4 (CH3), 23.0 (CH2), 30.9 (CH2-bridge), 70.5 (O–CH2), 117.1 (Ar–H), 129.2, 149.2 (Ar–O); MS (MALDI–TOF): calcd [M]+ m/z 1236.78, found 1259.6 [M+Na]+, 1275.6 [M+K]+; Anal. calcd for C78H108O12: C, 75.69; H, 8.80; found: C, 75.66; H, 8.81. Pillar[6]arene 26d [8] Colorless solid, yield: 43%, mp 89–91°C (Lit. [8]: 87–89 °C); 1 H NMR (400 MHz, CDCl3): δ 0.900.93 (t, 36H, CH3), 1.401.45 (m, 24H, CH2), 1.631.73 (m, 24H, CH2), 3.753.80 (m, 36H, CH2), 6.72 (s, 12H, ArH); 13C NMR

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(100 MHz, CDCl3): δ 13.9 (CH3), 19.4 (CH2), 30.7 (CH2), 31.9 (CH2-bridge), 68.2 (O–CH2), 115.0 (Ar–H), 127.8, 150.4 (Ar–O). Pillar[6]arene 26e Colorless solid, yield: 45%, mp 85–86 °C; 1H NMR (400 MHz, CDCl3): δ 0.88–0.91 (t, 36H, CH3), 1.26–1.31 (m, 48H, CH2), 1.42–1.45 (m, 24H, CH2), 1.68–1.75 (m, 24H, CH2), 3.74–3.78 (m, 36H, O–CH2 + CH2-bridge), 6.73 (s, 12H, Ar–H); 13C NMR (100 MHz, CDCl3): δ 14.1 (CH3), 22.7 (CH2), 26.0 (CH2), 29.8 (CH2), 30.9 (CH2-bridge), 31.8 (CH2), 68.5 (O–CH2), 114.9 (Ar–H), 127.7, 150.4 (Ar–O); MS (MALDI-TOF): calcd [M]+ m/z 1741.35, found 1764.3 [M+Na]+, 1780.3 [M+K]+; Anal. calcd for C114H180O12: C, 78.57; H, 10.41. found: C, 78.54; H, 10.42. Pillar[6]arene 26f Colorless solid, yield: 45%, mp 71–72 °C; 1H NMR (400 MHz, CDCl3): δ 0.89–0.91 (t, 36H, CH3), 1.30–1.31 (m, 96H, CH2), 1.41–1.46 (m, 24H, CH2), 1.69–1.74 (m, 24H, CH2), 3.74–3.78 (m, 36H, O–CH2 + CH2-bridge), 6.74 (s, 12H, Ar–H); 13C NMR (100 MHz, CDCl3): δ 14.1 (CH3), 22.7 (CH2), 26.4 (CH2), 29.4 (CH2), 29.7 (CH2), 30.0 (CH2), 31.0 (CH2-bridge), 32.0 (CH2), 68.5 (O–CH2), 115.0 (ArH), 127.7, 150.5 (ArO); MS (MALDI–TOF): calcd [M]+ m/z 2077.72, found 2100.7 [M+Na]+; Anal. calcd for C138H228O12: C, 79.71; H, 11.05; found: C, 79.69; H, 11.08.

3

Results and discussion

3.1 Influencing factors of the formation of pillar[6] arenes 3.1.1 Catalyst A variety of catalysts were screened under the same reaction conditions: a mixture of 3a (3.0 mmol), paraformaldehyde (9 mmol), CH2Cl2 and catalyst (0.45 mmol) was stirred at 29–32 °C. The isolated yields are summarized in Table 1. The results showed that the target product was prepared in a better yield in the cases of FeCl3 or SnCl4 as the catalyst (Table 1, entries 6 and 7). FeCl3 was proved to be the best catalyst with shorter reaction time. p-Toluenesulfonic acid (TsOH) (Table 1, entry 8) could also be used as a catalyst, but its catalyst efficiency was low. In contrast, no reaction was observed when other catalysts (Table 1, entries 15, 9 and 10) or no catalyst (Table 1, entry 11) was used. FeCl3 was chosen as catalyst owing to its outstanding advantages such as easy availability, low cost, low toxicity, environmental hospitality, and high activities. 3.1.2 Solvents The effect of solvents on this reaction was also studied (Table 2). Compared with other solvents, dry chloroform (Table 2, entry 4) was proved to be the best solvent to prepare

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Table 1 Entry 1 2 3 4 5 6 7 8 9 10 11

Table 2 Entry 1 2 3 4 5 6 7 8 9 10

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Preparation of 26a by different catalysts Cat. HCl CH3CO2H H2SO4 HCO2H CF3CO2H FeCl3 SnCl4 TsOH BF3-OEt2 AlCl3 none

Temperature (°C) rt41 rt41 rt rt41 rt rt rt rt rt rt41 rt41

Table 3 Preparation of pillar[6]arene with different alkoxysubstituents R Time (h) 22 45 1 55 30 23 68 52 1 30 60

6

Yield of 2 a (%) 0 0 0 0 0 10 10 5 0 0 0

Preparation of 26a in different solvents a) Solvent CH2Cl2 BrCH2CH2Br ClCH2CH2Cl CHCl3 CCl4 THF C2H5OH C6H5CH3 CH3CN DMF

Temperature (°C) rt rt53 rt rt rt60 rt60 rt80 rt rt84 rt110

Time (h) 23 36 5 3 40 30 30 24 30 60

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Yield of 25a Yield of 26a (%) (%) 88 510 0 0 69 5 30 34 0 0 0 0 0 0 0 0 0 0 0 0

a) Isolated yield.

26a (34%). CH2Cl2 (Table 2, entry 1) and ClCH2CH2Cl (Table 2, entry 3) could also be used as solvents, but the product was 25a (69%88%), with 26a being formed as the minor product in 5%10% yield. Pillararenes 2n could not be obtained in other solvents such as CCl4 (Table 2, entries 2–10). According to the results in table 2, only polar organic solvents containing chlorine, such as CHCl3, CH2Cl2 could produce 2n. In a broad context, solvent inclusion may be considered as polymorphic modifications of the host component, which are stabilized by the presence of a template guest. Li et al. reported that the crystal formation of pseudopolymorph of hmcyCuc6 is highly selective to CH2Cl2, CHCl3, and CCl4 [18]. In addition, the influence of the solvent on the organization of supramolecular systems is well-known. For example, the denaturation of proteins resulting in dramatic changes in their biological activities was different in different solvent [19]. The assembly of metal-organic frameworks (MOFs) [20], inorganic and organic functional nanomaterials [21], and supramolecular structures show solvent dependencies [22]. The solvent effects might play a significant role in this reaction system. 3.1.3 Substituents In order to expend the reaction scope, we studied the effect of the chain length of alkoxysubstituents on the reaction.

Starting compds. 3a 3b 3c 3d 3e 3f

R Et n-Pr i-Pr n-Bu n-hexyl n-octyl

Time (h) 2 23 23 23 2 3

Yield of 26 (%) a) 26a (34) 26b (35) 26c (30) 26d (43) 26e (45) 26f (45)

Yield of 25 (%) a) 25a (30) 25b (28) 25c (31) 25d (31) 25e (28) 25f (22)

a) Isolated yields.

The results showed that the overall yield and the ratio 26/25 increased with increasing the chain length of alkoxy, which implied that a long chain linked to hydropuinone was helpful to form pillar[6]arenes. The isolated yields of pillar[6] arenes 26af were around 30%50% (Table 3), significantly improved from the previous results [8, 9]. Due to the fact that only acids with a certain oxidation potential work as catalysts, we suggested a radical cation mechanism for the cyclooligomerization [8]. We propose that the catalytic system FeCl3/CHCl3 arranges the reactive components around itself, so that the generation of hexamers is favored in comparison to the generation of pentamers. 3.2

Host-guest properties

Pillar[n]arenes as host systems have the possibility to include guests in their cavities. Apart from the van der Waals forces, polar interactions are important. Since pillararenes have high electron density in the pillar structure, electron deficient guests are “welcome”. ,-Dibromoalkanes are firmly encapsulated in 25a, as evidenced by the up-field shift  of up to 3.5 ppm of the guest protons in the 1H NMR spectra. On the other hand, the larger host 26a does not show such behavior at all, illustrating a high selectivity in the host-guest relationship. On the contrary, quaternary ammonium salts are suitable guests for 26. The 11 complex of 26b and n-cetyltrimethyl ammonium bromide (N) was found in the ESI-MS which had a peak at m/z 1352.4 [26b⊃N−Br]+ corresponding to 26b-N [9]. Figure 2 depicts the 1H NMR spectra of 26b, N, and the complex in CDCl3. The encapsulation of N in the cavity of 26b is evidenced by the up-field shift of the proton H9. A possible explanation for the shift is that the guest is located in the cavity of 26b and shielded by its electron-rich cyclic structure after the formation of the threaded structure between 26b and N [9]. 26b had an excellent host-guest property with N. To examine the influence of the substituents of pillar[6]arenes on the binding interaction, similar investigations were carried out by using other pillar[6]arenes (26a, 26d, 26e and 26f) as hosts. Table 4 shows that the observed 1H NMR up-field shift  up to 0.35 ppm of H9 in the complex of guest N with 26a in CDCl3 ([N] = [26] = 20 mM, 298 K). For 26b

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Figure 2

Table 4

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H NMR in CDCl3. (a) Host 26b; (b) host-guest complex 26b + N; (c) guest N.

The up-field shift  of H9 of N in the complex

Complex N 26a + N 26b + N 26d + N 26e + N 26f + N

H9 (ppm) 3.466 3.107 3.196 3.256 3.148 3.149

 (ppm) 0 0.359 0.270 0.210 0.318 0.317

This work was supported by the National Natural Science Foundation of China (20872038, 21072064). 1 2 3

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and 26d, their binding abilities were not as strong and sensitive as 26a based on the chemical shift changes (∆δ of H9). For 26e and 26f, the proton H9 of 26e-N and 26f-N showed larger upfield shift than 26b-N and 26d-N, but less upfiled shift than 26a-N. These results indicated that the substituents of pillar[6]arenes have little influence on the binding ability with quaternary ammonium salts.

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4 Conclusion In conclusion, a simple and efficient method for the synthesis of pillar[6]arenes was developed. The FeCl3/CHCl3 system favors the formation of cyclohexamers over the usually dominating cyclopentamers. Due to the different sizes of their cavities, pillar[5]arenes and pillar[6]arenes show specific encapsulations respectively. The promising results would stimulate further studies on the host-guest chemistry of pillar[n]arenes.

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