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N-tert-butyl Sulfinyl Squaramide Receptors for Anion Recognition through Assisted tert-butyl C-H Hydrogen Bonding Yao Li, Guo-Hui Yang, Ye-Ye Shen, Xiao-Song Xue, Xin Li, and Jin-Pei Cheng J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b01634 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 3, 2017
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The Journal of Organic Chemistry
N-tert-butyl Sulfinyl Squaramide Receptors for Anion Recognition through Assisted tert-butyl C-H Hydrogen Bonding Yao Li, Guo-Hui Yang, Ye-Ye Shen, Xiao-Song Xue, Xin Li* and Jin-Pei Cheng State key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, China. Supporting Information Placeholder
ABSTRACT: A series of novel N-tert-butyl sulfinyl squaramide receptors have been designed and synthesised. Their interactions with a range of inorganic anions were studied in solution by 1H NMR spectroscopy. This new type squaramides displayed better anion binding ability over conventional diphenyl squaramides. The single crystals and DFT calulations implied the role of the tertbutyl C-H protons in binding Cl-. The energies of C-H···Cl- interactions were estimated to be 1.4-1.6 kcal/mol in solution.
INTRODUCTION INTRODUCTION In recent years, anion recognition, sensing, extraction, transport and related catalysis based on artificial receptors have become important themes of chemistry.1 The interest is rooted in the roles that the anions play in biological, medical and environmental science.2 The design of neutral receptors are usually based on hydrogen-bonding, halogen bonding, chalcogen bonding and hydrophobic et al. noncovalent interactions.3 Among them, hydrogen bond interaction is the one that applied widely in anion recognition. Receptors containing polarized N-H groups, such as amide, sulfonamide, urea, thiourea and pyrrole, have especially been extensively employed.4 Compared to the more traditional N-H and O-H donors, the hydrogen bonding donor abilities of C-H bonds are relative weaker.5 Although alkyl and aryl C-H bonds are present in the overwhelming majority (97%) of organic compounds,6 receptors utilizing C-H hydrogen bonds are rare.6-8 Neutral C-H and cationic C-H donors, such as alkyl C-H donors, phenyl C-H donors, triazole-based C-H donors, imidazolium and triazolium C-H donors, have been applied in binding anions.6-8 Among the C-H donors, receptors containing alkyl C-H donors are very rare.8 Recently, Hay and Pedzisa used electronic structure calculations to determine C-H···Cl- hydrogen bonding energies for a series of XCH3 donor groups.5c They showed that the aliphatic C-H···Cl- binding energies ranged in strength from -3.6 to -21.9 kcal/mol. It suggested that the aliphatic C-H bonds should be considered in anion recognition. However, in previous reports, aliphatic C-H bond were usually polarized by adjacent electron-withdrawing groups to enhance the donor ability.
On the other hand, C-H donors have been recently used in anion-binding catalysis.9 What‘s more, it has been found that the C-H···O hydrogen bonding interactions could serve as a key factor to control of stereoselectivity.10 Recently, our group developed a novel N-tert-butyl sulfinyl squaramide,11 in which the N-tert-butyl sulfinyl group enhances the lipophicity and acidity of squaramide.12 The novel N-tert-butyl sulfinyl squaramide successfully catalyzed the enantioselective Friedel−Crafts alkylations of indoles and acyl phosphonate. DFT calculations indicated that C−H···O hydrogen-bonding is crucial to induction of the observed enantioselectivity. Since this new type squaramides have shown better catalytic effects over conventional squaramides, it is highly desirable to examine the anion binding abilities of the novel squaramides in solutions. Here, we reported our recent study about the anion recognition by N-tert-butyl sulfinyl squaramides. An unconventional weak tert-butyl C-H hydrogen bonding interaction was observed for the assistance of the anion recognition (Figure 1), which gave an experimental evidence to support our previous work. Besides, this new type squaramides displayed better anion binding ability over conventional diphenyl squaramides.13 O O S
N H
O N H
O S
Cl-
N-tert -butyl sulfinyl squaramide better recognition ability than squaramide unconventional C-H Cl - interaction
Figure 1. Anion recognition through N-H and assisted tert-butyl C-H hydrogen bonding
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The Journal of Organic Chemistry
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RESULTS AND DISCUSSION Scheme 1. N-tert-butyl sulfinyl squaramide receptors.
Figure 2. 1H NMR stack plot of 1 with TBACl in CDCl3 at 298 K.
Synthesis of receptors. As shown in Scheme 1, N-tertbutyl sulfinyl squaramide receptors 1-3 could be easily prepared from commercial avaliable starting materals. Receptor 1 was prepared from diethyl squarate, (R)-tertbutanesulfinamide and (n-Bu)2Mg.14 Stirring diethyl squarate with (R)-tert-butanesulfinamide and EtONa in EtOH gave the first substitution product 2a.15 Receptor 2 and 3 were prepared from corresponding anilines and 2a using Zn(OTf)2 as a catalyst.16 The novel receptors give moderate to good solubility in CHCl3. The better solubility makes it possible to examine the anion binding ability of the novel squaramides in low polar solvents. Anions binding studies. In order to evaluate the anion binding properties of receptors 1-3, 1H NMR titration studies were initially conducted in CDCl3. Upon addition of Cl-, Br-, I, NO3- and HSO4- to receptors 1-3, significant downfield shifts (1-2.6 ppm) were observed for the N-H protons of squaramide moiety (Figure 2, for receptor 1 as example). Besides, slightly downfield shifts (0.01-0.1 ppm) were also observed for the protons of the tert-butyl group (Figure 2, for receptor 1 as example). When strong basic anions (F-, AcO-, H2PO4-) were added, the signal of N-H protons became broadened or disappeared, suggesting that the deprotonation.13,16-17 Upfield shifts were observed for the signal of the tert-butyl group, the opposite changes of shifts indicated that the examined receptors 1-3 might be deprotonated. Upon addition of NO3-, the color of solution turned from colorless to yellow, and the 1 H NMR spectra gave a complex mixture, which implied a possible decomposition of the receptors.18 Job’s plot indicated a 1:1 binding stoichiometry between receptors and anions.18 The binding constants of receptors 1-3 with different anions in CDCl3 were calculated by a nonlinear curve fitting with the 1:1 model (Table 1).
Table 1. Binding constants Ka (M-1) of receptors 1-3 with various anions determined by 1H NMR titrations in CDCl3.a anionb -
receptors 1 c
F
Cl-
Br I-
-
receptors 2 c
receptors 3 --c
--
--
>104
>104
>104
4
>104
4200±800
>10
426±42
816±68
2540±330
3480±540
570±80
690±68
NO3-
--d
--d
--d
AcO-
--c
--c
--c
H2PO4-
--c
--c
--c
HSO4
a
Data was best fitted in 1:1 binding stoichiometry. bAnions used as tetrabutylammonium salts. cDeprotonation behavior was observed. dDecomposition was observed.
Since the sulfinyl group is more acidifying than the frequently used 3,5-di(trifluoromethyl)phenyl group,19 the order of acidities for the receptors are 1>3>2. However, the order of binding constants of receptors with I- are 3>2>1, which is not consistent with the order of acidities. Besides, the order of binding constants of receptors with HSO4- are 1>3>2. It seems that the acidities and structures of receptors both affect the anion binding affinities. We proposed that phenyl group might enhance the halide ions binding affinities of receptors through aryl C-H···anion interaction.5a-b,13a Since the Lewis base properties of S=O group,20-21 the tert-butyl sulfinyl group may enhance HSO4- binding affinities through hydrogen bonding interaction between the hydroxyl of HSO4- anion and the S=O group of the receptor. Fortunately, the 1:1 binding stoichiometry between receptor 1 and Cl- was further revealed in the solid by X-ray diffraction.22 Single crystals of 1·Cl- complex were grown from a 1:2 mixture of 1 and tetrabutylammonium chloride in chloroform and tert-butyl methyl ether. The ORTEP drawing of 1·Clcomplex is shown in Figure 3. In particular, the crystal contains two asymmetrically equivalent hydrogen-bond complexes and the corresponding two counterions. The Cl- is binding with the dual hydrogen bonding of squaramide N-H motifs. Besides, unique and unconventional weak C-H
2
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hydrogen bondings play a role in Cl- recognition. The distances between Cl(1a) and H(6aa) and H(11a) are 2.884 and 2.893 Å, respectively. In the other complex, the distances between Cl(1) and H(6a) and H(11d) are 3.051 and 2.848 Å, respectively. The H···Cl distances are shorter than the sum of the van der Waals radius of hydrogen and chloride,23 indicating the hydrogen bond formation. This results is in agreement with the observed downfield shifts changes of the tert-butyl C-H protons upon addition of Cl-.
O
O
N H
F3C
O
N H
N H 4
CF3
O
CF3
N H
F3C
O
N H
5
K a : 260 M -1
O
CF3
N H
CF3
6 K a : 643 M-1
K a : 458 M-1
Table 2. Binding constants Ka (M-1) of receptors 1-3 with various anions determined by 1H NMR titrations in DMSO-d6 with 0.5 % water.a anionb -
receptors 1
receptors 2
receptors 3
Cl
592±35
530±37
820±67
Br-
12.2±1.7
24.8±1
34.3±2
I-
