Synthesis of p-tert-butylcalix[6]-1,4-2,5-bis-crowns

Article abstract of DOI:10.1016/S0040-4039(99)02332-1

The synthesis and complexation properties of the first example of p- tert-butylcalix[6]-1,4-2,5-bis-crowns are reported. Their complexation abilities and selectivities are different from that of analogous calix[6]crown. Compound 2b exhibits high complexat

Full text of DOI:10.1016/S0040-4039(99)02332-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1571–1574
Synthesis of p-tert-butylcalix[6]-1,4-2,5-bis-crowns
Yuanyin Chen,∗ Fafu Yang and Xueran Lu
Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China
Received 1 September 1999; accepted 16 December 1999
Abstract
The synthesis and complexation properties of the first example of p-tert-butylcalix[6]-1,4-2,5-bis-crowns are re-
ported. Their complexation abilities and selectivities are different from that of analogous calix[6]crown. Compound
+
2b exhibits high complexation selectivity towards n-PrNH3 . © 2000 Elsevier Science Ltd. All rights reserved.
Calixcrowns are a family of macropolycyclic molecules in which the subunits of calixarene and crown
ethers are combined through the bridging of phenolic oxygen atoms of the calixarene by polyoxyethylene
chains. The first member of this family was reported by Alfieri et al. as early as 1983.1 As calixcrowns
possess well preorganized structures and more rigid binding sites in comparison with calixarenes and
crown ethers, they exhibited superior recognition ability toward alkali metal cations. For example, the
Na+/K+ selectivity attainable with crown ethers has been saturated at the 102 order; with calixarenes
the selectivity can reach 103.1, but it is 105.3 for diethoxycalix[4]crown-4 in a partial cone conformation.2
Therefore, much attention has been paid to more sophisticated molecules: calixa-bis-crowns. All possible
types of calix[4]-bis-crowns have been synthesized,3–9 and their recognition properties toward Cs+, K+
etc. have been studied.10–14 Two types of calix[8]-bis-crowns have also been reported.15,16 However, no
paper concerning the synthesis of calix[6]-bis-crowns has appeared to date. Even for calix[6]crowns, only
1,4-p-tert-butylcalix[6]crown-5 and 1,4-calixa[6]crown-517 as well as 1,3-p-tert-butylcalix[6]crown-318
have been synthesized.
In this paper, we wish to report the synthesis of the first calix[6]-bis-crowns: p-tert-butylcalix[6]-1,4-
2,5-bis-crown-4 2a and p-tert-butylcalix[6]-1,4-crown-4-2,5-benzocrown-4 2b as well as their extraction
abilities toward some cations. The synthetic route is depicted in Scheme 1.
Reacting p-tert-butylcalix[6]arenes with triethylene glycol ditosylates or 1,2-bistosyloxyethoxybenzene
in the presence of 9 equiv. of K2CO3 in toluene for 36 h under a nitrogen atmosphere, gave 1,4-p-tert-
butylcalix[6]crown-4 1a and 1,4-p-tert-butylcalix[6]benzocrown-4 1b.19 To the DMF solution of 1, NaH
was added at room temperature, followed by 1.2 equiv. of triethylene glycol ditosylate with stirring at
70°C for 7 h. The excess of NaH was quenched by addition of a minimal quantity of methanol (caution!).
Distilling off the solvent, the residue was treated with HCl (10%, v/v) and then extracted with ethyl
∗
Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02332-1
tetl 16260

1572
Scheme 1. Reagents and conditions: (i) K2CO3, toluene, reflux, 36 h; (ii) NaH/DMF, 70°C, 7 h
acetate. After purification by column chromatography, the p-tert-butylcalix[6]-1,4-2,5-bis-crowns-4 2a
and p-tert-butylcalix[6]-1,4-crown-4-2,5-benzocrown-4 2b were isolated as white solids in 66% yield,
m.p. 322–324°C and in 70% yield, m.p. 308–310°C, respectively.
Two of the synthesized compounds gave satisfactory elemental analysis results and exhibited the
expected molecular ion peaks. The structures of these compounds were also confirmed by NMR
spectra.20 In the NMR spectrum of 2a, two singlets for the tert-butyl groups (2:1), a pair of doublets (one
overlaps with ethylene protons) and one singlet for methylene bridges (2:1), and a singlet for hydroxy
protons can be assigned. This is in good accordance with the structure of p-tert-butylcalix[6]-1,4-2,5-bis-
crowns 2a and not in accordance with other structures such as p-tert-butylcalix[6]-1,4-2,3-bis-crowns 3a
or p-tert-butylcalix[6]-1,4-2,6-bis-crowns 4a which should have three singlets for the tert-butyl groups
(1:1:1) as shown in Scheme 1. As 2a and 2b are synthesized by the same method, it is reasonable
to assume that they possess similar structures. This is supported by their NMR spectra. In the NMR

1573
Table 1
Percentage extraction (%E) of picrate salts from water into CHCl3 at 25°C.a Arithmetic mean of
several experiments–standard deviation on the mean: óN–1<1
spectrum of 2b, three singlets for the tert-butyl groups (1:1:1), three pairs of doublets (a pair of doublets
overlaps with ethylene protons) for methylene bridges, and a singlet for the hydroxy protons can be
distinguished. This is also consistent with the structure of 2b and not with 3b which should have four
pairs of doublets for methylene bridges at least or 4b which should have four singlets for tert-butyl
groups. It is noteworthy that although the conformations of 2a and 2b are difficult to define, they are
different from cone or 1,2,3-alternate conformations which have been studied thoroughly.
The percentage extraction of hosts 2a and 2b as well as 1a and 1b toward some picrate salts are shown
in Table 1, the extraction abilities of 2a toward the tested cations are higher than that of 2b except for
+
+
n-PrNH3 . The host 2b shows very high selectivity toward n-PrNH3 . To the best of our knowledge,
+
this is the most effective ionophore for n-PrNH3 reported in calixarene chemistry. Compared to 1a and
1b, the extraction levels of 2a and 2b toward K+, NH4 , n-PrNH3 etc. are much higher, but the reverse
+
+
phenomena are observed for small alkali metal cations, i.e. Li+ and Na+.
Acknowledgements
Financial support from the Natural Science Foundation of China is gratefully acknowledged.
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1
20. H NMR (250 MHz, CDCl3): 2a: 1.16 (s, 36H, ArC(CH3)3), 1.33 (s, 18H, ArC(CH3)3), 2.86 (t, 4H, J=9.0 Hz, OCH2CH2),
3.25 (q, 4H, J=10.5 Hz, OCH2CH2), 3.34–3.50 (m, 16H, OCH2CH2 and ArCH2Ar), 3.8 (q, 4H, J=10.5 Hz, OCH2CH2), 4.07
(s, 4H, ArCH2Ar), 4.39 (d, 4H, J=15.0 Hz, ArCH2Ar), 6.91 (s, 4H, ArH), 7.03 (s, 2H, ArOH), 7.04 (s, 4H, ArH), 7.11 (s, 4H,
ArH). MS (FAB): 1239 (MK+, 100%), 1200 (M+, 20%). Anal calcd for C78H104O10: C, 77.96; H, 8.72; found: C, 78.10; H,
8.60; 2b: 1.14 (s, 18H, ArC3(CH3)), 1.25 (s, 18H, ArC(CH3)3), 1.28 (s, 18H, ArC(CH3)3), 2.44 (t, 2H, J=9.7Hz, OCH2CH2),
2.74 (q, 2H, J=7.1 Hz, OCH2CH2), 3.15 (q, 2H, J=7.1 Hz, OCH2CH2), 3.26–3.86 (m, 18H, OCH2CH2 and ArCH2Ar), 3.99
(d, 2H, J=17.4 Hz, ArCH2Ar), 4.19 (d, 2H, J=13.5 Hz, ArCH2Ar), 4.23 (d, 2H, J=13.5 Hz, ArCH2Ar), 4.64 (d, 2H, J=17.4
Hz, ArCH2Ar), 6.54 (t, 4H, J=2.4 Hz, ArH), 6.72 (t, 4H, J=2.4 Hz, ArH), 6.95 (d, 2H, J=2.5 Hz, ArH), 7.04 (s, 2H, ArOH),
7.10 (d, 2H, J=2.5 Hz, ArH), 7.17 (d, 2H, J=2.5 Hz, Ar), 7.24 (d, 2H, J=2.5 Hz, ArH). MS (FAB): 1287 (MK+, 100%), 1248
(M+, 20%). Anal. calcd for C82H104O10: C, 78.81; H, 8.39; found: C, 78.85; H, 8.4.