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7Calixarenes Каликсарены
Макрогэтэроцмклы
http://macroheterocycles.isuct.ru
Paper Статья
DOI: 10.6060/mhc131268c
Synthesis of New meso-Substituted Heterocyclic Calix[4]arenes via SNH Approach
Mikhail V. Varaksin,ab Irina A. Utepova,ab Oleg N. Chupakhin,ab@ and Valery N. Charushina,b
Dedicated to Academician of the Russian Academy of Sciences A. I. Konovalov
on the occasion of his 80th birthday
Ural Federal University, 620002 Ekaterinburg, Russia
bInstitute of Organic Synthesis, Ural Branch of the RAS, 620041 Ekaterinburg, Russia @Corresponding author E-mail: chupakhin@ios.uran.ru
An effective synthetic approach to heterocyclic derivatives of meso-substituted calixarenes has been suggested by using the SNH methodology based on the direct, non-catalyzed by transition metals, C-C coupling of 1,2,4-triazines with the lithium salts of tetramethoxycalix[4]arenes.
Keywords: Calix[4]arenes, azines, C-C coupling, nucleophilic substitution of hydrogen (SNH).
Синтез новых мезо-замещенных гетероциклических каликс[4]аренов на основе подхода
М. В. Вараксин,аЬ И. А. Утепова,аЬ О. Н. Чупахин,аЬ@ В. Н. ЧарушинаЬ
Посвящается академику РАН А. И. Коновалову по случаю его 80-летнего юбилея
аУральский федеральный университет, 620002 Екатеринбург, Россия ьИнститут органического синтеза УрО РАН, 620041 Екатеринбург, Россия ®Е-тай: chupakhin@ios.uran.ru
Предложен эффективный синтетический подход к синтезу новых мезо-замещенных гетероциклических производных каликс[4]аренов, основанный на некатализируемом переходными металлами С-С сочетании 1,2,4-триазинов с литиевыми солями тетраметоксикаликс[4]аренов.
Ключевые слова: Каликс[41арены, азины, С-С сочетание, нуклеофильное замещение водорода
Introduction
Calixarenes are known to be one of the key structural units in supramolecular chemistry.[1] An enhanced interest in this promising class of macrocyclic compounds is due to wide opportunities of their practical use.[2] Indeed, calixarenes proved to be effective receptors for selective extraction of metal ions,[3] catalysts,[4] chemosensors,[5,6] transmembrane ion transporters,[7,8] materials for nonlinear optics,[9] biologically active substances,[10] etc.[11]
The structural organization of calixarenes has an effect on their complexing properties.[12] A common approach in the design of functionally substituted calixarenes is modification of these macrocyclic molecules either at the lower rim via the reactions of OH groups, or at the upper rim through functionalization of C(sp2)-H bonds of the aromatic rings. As far as chemical transformations, that result in modification of the CH2 groups via functionalization of C(sp3)-H bonds, are concerned, the data on these reactions have scarcely been presented in the literature.[13-15] Only a few examples
have recently been described, and all of them are associated with the ability of the bridging CH2 groups to undergo lithiation.[16"19] The reactions of the lithiated calixarenes with electrophiles afford the corresponding meso-substituted derivatives.[20] It should be noted that the data regarding the direct modification of the lithium salts of calixarenes with heterocyclic fragments have never been reported. Since a lot of data on heterocyclic compounds concern their applications in catalysis, coordinational, analytical, and medicinal chemistry, azinyl derivatives of calixarenes appear to be of great interest.[21-27]
In this communication we would like to report for the first time, that the direct heteroarylation of calixarenes at the bridging CH2-fragment is possible by using the methodology of nucleophilic substitution of hydrogen (SNH) in azines.[28-34] The SNH process is known to be a specific kind of cross-coupling reactions, that does require neither transition metal catalysis, nor the presence of halogen or other good-leaving groups in the structure of reactants. Due to these features SNH reactions demonstrate a high atom efficiency, [35,36] and appear to be environmentally benign processes.
Experimental
The 1H (400 MHz) and 13C (100 MHz) spectra were recorded on a Bruker Avance II spectrometer in a mixture of CD3CN and CDCl3 (2:8). The assignment of 1H and 13C signals was performed by a combination of 2D homonuclear 1H-1H (COSY) and heteronuclear 1H-13C (HSQC and HMBC) experiments. The mass spectra were recorded on a Shimadzu GCMS-QP2010 Ultra EI mass spectrometer with sample ionization by electron impact (EI). The IR Spectra were recorded on an IR Fourier spectrophotometer Bruker Alpha equipped with a device for measuring incomplete internal reflection. The elemental analysis was carried out on a Perkin Elmer 2400-II CHNS/O analyzer. The course of the reactions was monitored by TLC on 0.25 mm silica gel plates (60F 254). Column chromatography was performed on silica gel (60, 0.0350.070 mm (220-440 mesh)). The solvents were purified and dried by standard procedures. TMEDA, DDQ, n-BuLi (1.6 M solution in hexane) were purchased from Sigma-Aldrich. 25,26,27,28-Tetramethoxycalix[4]arene 1a, 5,11,17,23-tetra-tert-butyl-25,26, 27,28-tetramethoxycalix[4]arene 1b,[37] 3,6-diphenyl-1,2,4-triazine 3[38] were prepared according to the published procedures.
General method for the synthesis of calixarenes 5a,b. To a vigorously stirred solution of TMEDA (0.43 ml, 2.9 mmol) in dry THF (4 ml) cooled to -78 oC 1.6 M solution of «-BuLi in hexane (1.44 ml, 2.3 mmol) was added under argon. After 40 min a solution of the corresponding 25,26,27,28-tetramethoxycalix[4]arene 1 (1 mmol) in anhydrous THF was added. The resulting cherry red mixture containing 2-lithio-25,26,27,28-tetramethoxycalix[4]arene 2[17] was allowed to warm up to ambient temperature and then was stirred for additional 2 h. Then the mixture was cooled down to -78 oC and a solution of 3,6-diphenyl-1,2,4-triazine 3 (464 mg, 2.0 mmol) in dry THF (6 mL) was added. The resulting yellow solution was allowed to warm up to ambient temperature, and then water (0.04 m1, 2.0 mmol) was added and the mixture was concentrated in vacuo. The residue was subjected to silica gel column chromatography with the EtOAc-CHCl3 (4:6) mixture as an eluent, and the resulting eluate was concentrated to dryness under reduced pressure.
2-(3,6-Diphenyl-4,5-dihydro-1,2,4-triazin-5-yl)-25,26,27,28-tetramethoxycalix[4]arene, 5a (570 mg, 0.8 mmol, 80 % on calix-arene 1a). M.p. 188-190 oC. Found: C 78.91, H 6.04, N 5.83 %. C47H43N3O4 requires C 79.08, H 6.07, N 5.89. m/z (EI) (%) 713 [M+] (100). IR (DRA) vmax cm-1: 693, 760, 811, 838, 916, 974, 1020,
1157, 1204, 1221, 1247, 1276, 1294, 1338, 1427, 1464, 1506, 2820, 2866, 2916, 2933, 2977, 3027, 3061, 3133, 3376.1H NMR (CDCl3-CD3CN 8:2, 298 K) 8H ppm: 9.67 (1H, s, N(4')H), 7.62-7.66 (1H, m, Ar), 7.50-7.54 (1H, m, Ar), 7.46-7.48 (1H, m, Ar), 7.36-7.40 (1H, m, Ar), 7.27-7.33 (1H, m, Ar), 7.10-7.14 (4H, m, Ar), 7.007.09 (5H, m, Ar), 6.75-6.86 (3H, m, Ar), 6.40-6.46 (1H, m, Ar), 6.01-6.06 (1H, m, Ar), 6.03 (1H, dd J=11.46 Hz, J=11.36 Hz, C(5') H), 5.24 (1H, dd J=11.46 Hz, J=11.36 Hz, C(2)H), 4.05-4.23 (12H, m, 3H ArCH2Ar + 6H OMe), 3.78-3.82 (3H, m, OMe), 3.27-3.39 (3H, m, ArCH2Ar). 13C NMR (CDCl3-CD3CN 8:2, 298 K) 8c ppm: 153.8, 152.9, 152.2, 147.2, 136.2, 135.7, 135.2,135.0, 134.9, 134.8,
131.8, 131.2, 129.2, 129.1, 129.0, 128.9, 128.8, 128.7, 128.2, 127.9,
126.9, 126.3, 126.0, 125.2, 65.2, 65.0, 64.9, 64.8, 64.3, 52.0 (C(5')), 34.0 (C(2)H), 29.6, 29.4.
2-(3,6-Diphenyl-4,5-dihydro-1, 2,4-triazin-5-yl) -5,11,17,23-tetra-tert-butyl-25,26,27,28-tetra-methoxycalix[4] arene, 5b (768 mg, 0.82 mmol, 82 % on calixarene 1b). M.p. 172174 oC. Found: C 80.31, H 8.24, N 4.23 %. C63H75N3O4 requires C 80.64, H 8.06, N 4.48. m/z (ESI) (%) 937 (100) [M+]. IR (DRA) v cm-1: 691, 771, 800, 874, 1020, 1121, 1178, 1203, 1261, 1319,
max 5 5 5 5 55 5 5 55
1362, 1393, 1419, 1436, 1456, 1482, 1506, 1578, 1602, 1626, 2819, 2869, 2932, 2960, 3394. 1H NMR (CDCl3-CD3CN 8:2, 298 K) 8H ppm: 9.68 (1H, s, N(4' )H), 7.59-7.62 (1H, m, Ar), 7.46-7.52 (4H, m, Ar), 7.37-7.39 (1H, m, Ar), 7.29-7.31 (2H, m, Ar), 7.12-7.17 (5H, m, Ar), 7.07-7.09 (1H, m, Ar), 7.02-7.04 (1H, m, Ar), 6.926.94 (1H, m, Ar), 6.07 (1H, d J=11.50 Hz, C(5' )H), 5.23 (1H, d J=11.50 Hz, C(2)H), 4.08-4.16 (3H, m, ArCH2Ar), 4.03-4.06 (9H, m, OMe), 3.73 (3H, s, OMe), 3.25-3.33 (m, 3H, ArCH2Ar), 1.25 (9H, s, t-Bu), 1.12 (9H, s, t-Bu), 1.09 (9H, s, t-Bu), 0.91 (9H, s, t-Bu). 13C NMR (CDCl3-CD3CN 8:2, 298 K) 8c ppm: 153.6, 151.7, 151.0, 150.8, 150.7, 148.8, 1438.7, 148.5, 147.9, 147.4, 136.5, 134.6, 134.5, 134.4, 134.2, 131.8, 131.1, 128.7, 128.6, 126.8, 126.1, 126.0, 125.9, 125.8, 123.7, 123.2, 65.03, 64.9, 64.7, 64.4, 55,9 (C(5')), 35.7 (C(2)H), 34.5, 34.2, 34.1, 33.8, 31.8, 31.2, 31.1, 31.0, 30.8, 30.2, 29.9, 29.8.
General method for the synthesis of calixarenes 6a,b. To a vigorously stirred solution of calixarenes 5a,b (1 mmol) in dry THF (4 ml) DDQ (0.272 mg, 1.2 mmol) was added at ambient temperature. The mixture was stirred for 15 min, then filtered through neutral alumina, washed several times with EtOAc, and concentrated in vacuo. The residue was subjected to silica gel column chromatography with the EtOAc-hexane mixture (2:8) as an eluent, and the resulting eluate was concentrated to dryness under reduced pressure.
2-(3,6-Diphenyl-1,2,4-triazin-5-yl) -25,26,27,28-tetra-methoxycalix[4]arene, 6a (576 mg, 0.95 mmol, 95 % on calixarene 5a). M.p. 216-218 oC. Found: C 79.51, H 6.02, N 5.73 %. C47H43N3O4 requires C 79.30, H 5.85, N 5.90. m/z (ESI) (%) 711 (1004 [M+]. IR (DRA) vmax cm-1: 646, 690, 707, 737, 794, 831, 917, 961, 1018, 1089, 1205,T246, 1291, 1322, 1339, 1391, 1437, 2817, 2866, 2931, 2977, 3013, 3060. 1H NMR (CDCl3-CD3CN 8:2, 298 K) 8H ppm: 8.56 (2H, m, Ar), 7.40-7.58 (8H, m, Ar), 7.03-7.22 (2H, m, Ar), 6.83-6.94 (3H, m, Ar), 6.61-6.73 (6H, m, Ar), 6.346.54 (1H, m, Ar), 5.92,6.34 (1H, s, C(2)H), 4.25-4.32, 4.12-4.17 (2H, m, ArCH2Ar), 3.72-3.77 (4H, m, 1 H ArCH2Ar + 3H OMe), 3.65 (3H, s, OMe), 3.57 (1H, m, ArCH2Ar), 3.42-3.48 (4 H, m, 1 H ArCH2Ar + 3H OMe), 3.16 (3H, m, 3H OMe), 2.85 (1H, m, ArCH2Ar). 13C NMR (CDCl3-CD3CN 8:2, 298 K) 8c ppm: 161.3, 159.4, 159.4, 158.5, 158.3, 157.9, 157.2, 157.1, 145.1, c143.8, 143.5, 137.9, 137.2, 135.3, 135.1, 134.9, 134.8, 131.6, 129.7, 129.4, 129.2, 129.0, 128.8, 128.6, 128.3, 128.2, 128.1, 125.9, 122.5, 121.7, 61.6, 60.7, 59.4, 59.0, 41.0 (C(2)H), 31.5, 30.6, 30.4.
2-(3,6-Diphenyl-1,2,4-triazin-5-yl)-5,11,17,23-tetra-tert-butyl-25,26,27,28-tetramethoxycalix[4]arene, 6b (860 mg, 0.92 mmol, 92 % on calixarene 5b). M.p. 279-281 oC. Found: C 80.44, H 8.02, N 4.21 %. C63H73N3O4 requires C 80.82, H 7.86, N 4.49. m/z (ESI) (%) 935 (100) [M+]. IR (DRA) v cm-1: 637, 692, 762, 797,
871, 943, 975, 1014, 1110, 1122, 1200, 1242, 1296, 1315, 1391, 1427, 1459, 1498, 2819, 2866, 2901, 2932, 2955. 1H NMR (CDCl3-CD3CN 8:2, 298 K) 8H ppm: 8.65-8.74 (2H, m, Ar), 8.09-8.22 (2H, m, Ar), 7.51-7.79 (8HH, m, Ar), 7.18-7.24 (6H, m, Ar), 5.84, 6.43 (1H, s, C(2)H), 4.17-4.30 (9H, m, 3H ArCH2Ar + 3H OMe), 3.83 (6H, s, OMe), 3.44-3.53 (m, 3H, ArCH2Ar), 1.20-1.28 (36H, m, t-Bu). 13C NMR (CDCl3-CD3CN 8:2, 298 K) 8c ppm: 161.0, 160.9,
158.8, 158.6, 158.3, 155.1, 150.8, 150.7, 148.9,'148.6, 144.7, 135.4, 135.1, 135.0, 134.6, 134.3, 133.9, 132.0, 131.6, 130.7, 129.9, 129.5,
128.9, 128.7, 128.6, 128.4, 128.3, 127.2, 126.2, 125.9, 125.8, 124.6, 65.2, 65.1, 41.2 (C(2)H), 36.3, 34.6, 34.6, 34.3, 33.8, 31.9, 31.5, 31.4, 31.2, 31.1, 30.4, 30.0, 29.0.
Results and Discussion
Novel calix[4]arenes bearing azinyl fragments at the meso-position have been synthesized through the direct, non-catalyzed by transition metals, cross-coupling reaction of n-deficient azaaromatic compounds with the lithium salts of tetramethoxycalixarenes. The approach is based on using the methodology of nucleophilic substitution of hydrogen (SNH) in heteroaromatic systems.[28-33]
According to the current concept of the SNH reactions, one of the most plausible mechanisms of nucleophilic substitution of hydrogen in (hetero)arenes is the two-steps
relatively stable form. The second step of the SNH process is oxidative aromatization of cH-adducts, which takes place by means of air oxygen or another external oxidant to give SNH products D.
2-Lithium-25,26,27,28-tetramethoxycalix[4]arenes 1a,b have been obtained according to the known procedure, based on deprotonation of a calixarene methylene group by action of n-BuLi under argon atmosphere (Scheme 2).[17] In order to stabilize lithioderivatives 2a,b, TMEDA was used as a chelating additive. The reaction is initiated at -78 °C, as indicated by the observed pink colour of the reaction mixture. As the reaction temperature is enhanced to ambient, the reaction mixture colour being gradually changed into bright red. It should be noted that both unsubstituted at the upper rim calix[4]arene 1a and 5,11,17,23-tetra-tert-butyl-substituted analogue 1b undergo the meso-lithiation.
3,6-Disubstituted 1,2,4-triazines were selected as model compounds in the SNH reactions of lithiocalixarenes 2a,b with azines. The choice is due to an enhanced electrophilic character and a high reactivity of these azaaromatic heterocycles. It is well known that 1,2,4-triazines have a profound tendency to undergo nucleophilic addition at C(5) to give rather stable intermediates, 4,5-dihydrotriazines, [39] which could be considered as an experimental proof of the
"addition-oxidation" SNH(AO) protocol (Scheme 1). The SNH(AO) mechanism, as evidenced, for instance, by isolation
first step involves addition of lithiocalixarene, acting as a nucleophilic reagent B, to the C=N bond of (hetero)arenes A. The resulting intermediates C, so-called cH-adducts, have a broad scale of stability - from an extremely low state to a
of CTH-adducts from the reaction of organolithium reagents with 1,2,4-triazines.
Indeed, dihydrotriazine derivatives 5a,b were obtained in 80-82 % yields from the reaction of lithioca-
\ Nlf — rgA .0! -
EWG B EWG EWG
-2e
A ----C D
EWG - electron-withdrawing groups
Scheme 1.
n-BuLi, TMEDA THF
-78 °C-
rt
1,2: a (R = H), b (R = f-Bu)
Scheme 2.
lixarenes 2a,b with 3,6-diphenyl-1,2,4-triazine 3, followed by treatment of intermediates 4a,b with water (Scheme 3). Dihydro compounds 5a,b proved to be rather stable cH-adducts, which do not undergo oxidation during a prolonged storage in air. Good yields of dihydrotriazines 5a (80 %) and 5b (82 %) indicate that tert-butyl substituents at the upper rim of the calix[4]arene do not have a significant effect on the reactivity of meso -lithioderivatives 2a and 2b. The second step of the SNH(AO) process, oxidative aromati-zation of adducts 5a,b, was carried out in THF at ambient temperature with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) as an oxidant, thus giving the corresponding SNH products 6a,b in 92-95 % yields. It is worth noting that effectiveness of DDQ has also been shown in other SNH(AO) processes.139-421
Heterocyclic derivatives of calixarenes 5a,b and 6a,b were isolated by using SiO2 column chromatography. All new compounds were characterized by the data of elemental analysis, mass spectrometry, IR, 1H, and 13C NMR spectroscopy, including homonuclear 1H-1H (COSY) and heteronuclear 1H-13C (HSQC and HMBC) correlation experiments. In the IR spectra of calixarenes 5a,b characteristic absorption bands, corresponding to the NH group stretching vibrations are observed at v 33763394 cm-1 (contrary to the IR spectra of aromatic analogues 6a,b). Also the molecular ion peaks [M]+ were registered in
their mass spectra. The methoxy calix[4]arenes are likely to possess a high conformational flexibility in many organic solvents, imposing some restrictions on their studying by means of NMR spectroscopy. In order to solve this problem, a convenient preparative technique,[17] based on fixation of conformers through coordination of the methoxy calixarenes with Na+ ions was applied. The approach allows a number of the resonance signals in NMR spectra to be interpreted. The procedure involves addition of an excess amount of NaI, previously dissolved in CD3CN, to a calixarene solution in CDCl3. Typical NMR spectra of cH-adduct 5a and the corresponding SNH product 6a are presented in Figures 1 and 2, respectively.
A single set of signals in the 1H (Figure 1a) and 13C NMR spectra of cH-adducts 5a,b indicates that compounds 5a,b exist in the only conformational state. The resonance signals of CH2 and OMe groups are located at 5 3.27-4.23 ppm for 5a and at 5 3.25-4.16 ppm for 5b, respectively. The signals of aromatic protons are observed as multiplets at 5 6.39-7.66 ppm for 5a and at 5 6.92-7.62 ppm for 5b. The N(4')H protons are registered at 5 9.67 ppm for 5a and at 5 9.68 ppm for 5b. In the 'H NMR spectra of compounds 5a,b the C(2)H protons of the bridging sp3-hybridized fragment resonate at 5 5.24 ppm in case of compound 5a and at 5 5.23 ppm for 5b, while the C(5')H resonance signal of the dihydrotriazine moiety is observed at 5 6.03 ppm for 5a
Phu .N
N
* A
N Ph 3
THF -78 °C—rt
R
H20
R
R
R
5a,b (80-82%) 2,4-6: a (R = H), b(R= f-Bu)
R
6a,b (92-95%)
Scheme 3.
(a)
I C(51H
C(2)H
H oo i m icifl OH
in rnic in '—o n fM
in m rg o iD^-nT-H
OOOO rN rsj fNJ fN
" " " " in L/i in in
C(2)H
C(50H
10 0 10-0 9-5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5
(ppm)
Figure 1. NMR spectra of 5a with Nal: 'H (a), 2D 1H-13C HSQC (b), and 2D 1H-1H COSY (c) in the mixture of CD3CN and CDCl3 at
295 K.
and 5 6.07 ppm for 5b. In the 13C NMR spectra of 5a,b the carbon-13 resonance signals of C(2) and C(5') are registered at 5 34.0 and 52.0 ppm for 5a, 35.7 and 55.9 ppm for 5b, respectively. The presence of cross-peaks (5.24, 34.0) and (6.03, 52.0) in the 2D 'H-13C HSQC spectra of 5a, as well as (5.23, 35.7) and (6.07, 55.9) for 5b, indicates at the direct links between protons and the corresponding carbon nuclei (Figure 1b). Symmetrical cross-peaks (6.03, 5.24) in the 2D 'H-'H COSY correlation spectra of 5a, as well as (5.24, 34) for 5b, confirm the vicinal location of C(2)H and C(5') H protons (Figure 1c). The resonance signals of tert-butyl substituents for 5b are recorded at 5 0.91-1.25 ppm.
Contrary to the cH-adducts 5a,b, several sets of signal are registered in the 1H and 13C NMR spectra of aromatic products 6a,b, thus indicating that these calixarenes exist in several conformational states. In the 1H NMR spectra the resonance signals of CH2 and OMe protons are located in the field of 5 2.85-4.32 ppm for 6a and 5 3.44-4.30 ppm for 6b, respectively (Figure 2a). The resonance signals of aromatic protons are observed as multiplets at 5 6.36-8.56 ppm for 6a and at 5 7.18-8.74 ppm for 6b. There are no signals of the dihydrotriazine ring (neither sp3-hybridized CH nor NH). The C(2)H meso-proton signals resonate at 5 5.92 ppm and at 5 6.34 ppm for 6a, as well as at 5 5.84 ppm and 5 6.43 ppm for 6b. In the 13C NMR spectra of 6a,b the signals of C(2) carbons are registered at 5 41.0 ppm for 6a
and at 5 41.2 ppm for 6b, respectively. In the 2D 'H-13C HSQC spectra of 6a,b the presence of two cross-peaks (6.34, 41.0) and (5.92, 41.0) for 6a, as well as (5.84, 41.2) and (6.43, 41.2) for 6b (Figure 2b), corresponding to direct interactions of C(2)H protons with C(2), indicates that the calixarenes 6a,b exist in multiple conformational states. The resonance signals of tert-butyl substituents for 6b are recorded at 5 1.20-1.28 ppm.
Also it should be noted that the and 13C NMR spectral data for calixarenes 5a,b and 6a,b are correlated nicely with those obtained for 25,26,27,28-tetramethoxycalix[4]arenes bearing alkyl, benzyl, and carboxyl groups at the meso-position.[16]
Conclusions
A principal opportunity for the direct modification of calixarenes at the meso-position with triazinyl fragments has been presented. The suggested SNH approach allows to obtane calixarenes, bearing either dihydrotriazine or triazine fragments at the meso-position.
Acknowledgements. The research was financially supported by the Russian Foundation for Basic Research (13-03-90606, and 13-03-01271), the UrFU development program.
(a)
C(2)H
6.45 6.« 6.35 6.30 6.25 6.20 6.15 6.10 6.05 6.00 5.95 5.90 (PPm)
Ol 00
O th
rM
rn Ol
KD 1/1
I I
Pti.6' N, 2'
y? N
^ M
" N4' Ph
u
10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0
(dditiI
2.5
Figure 2. NMR spectra of 6a with Nal: 'H (a), and 2D 'H-^C HSQC (b) in the mixture of CD3CN and CDCl3 at 295 K.
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Received 16.12.2013 Accepted 27.12.2013
