Two-step synthesis of hexaammonium triptycene: An air-stable building block for condensation reactions to extended triptycene derivatives

Article abstract of DOI:10.1021/jo200843v

A simple two-step synthesis of an air-stable hexaammoniumtriptycene is introduced, which can be used for a variety of transformations by condensation reactions, e.g., to benzimidazole, benzotriazole, and quinoxaline derivatives in high yields.

Full text of DOI:10.1021/jo200843v

NOTE
pubs.acs.org/joc
Two-Step Synthesis of Hexaammonium Triptycene: An
Air-Stable Building Block for Condensation Reactions to
Extended Triptycene Derivatives
Michael Mastalerz,*,† Stefanie Sieste,† Mila Ceniꢀc,† and Iris M. Oppel‡
†Department of Organic Chemistry II & Advanced Materials, Ulm University, 89081 Ulm, Germany
‡Department of Inorganic Chemistry, RWTH Aachen, Landoltweg 1, 52074 Aachen, Germany
S Supporting Information
b
ABSTRACT: A simple two-step synthesis of an air-stable hexaammo-
niumtriptycene is introduced, which can be used for a variety of
transformations by condensation reactions, e.g., to benzimidazole,
benzotriazole, and quinoxaline derivatives in high yields.
he D3h-symmetric triptycene (1)1 is a versatile molecular
building block for the synthesis of larger molecules, which
hexaminotriptycene 2, starting from 1, with an overall yield of
approximately 18% (Scheme 1, on the right).9a,16 All steps
together are time-consuming, and some intermediates have to
be purified via column chromatography, which is certainly a
disadvantage on larger scales.
Here, we present a simple two-step synthesis of hexaammo-
niumtriptycene hexachloride 4 (Scheme 1),17 an air-stable salt of
the hexaminotriptycene 2. This hexaammonium salt can be stored
on the bench for several weeks without precautions (exclusion of
air, cooling, etc.) and can be used as a synthon for hexamine 2 for
further transformations, giving high yields of products, which is
demonstrated for some selected examples (see below).
T
are frequently investigated in both supramolecular and materials
chemistry.2 This is due to two facts: homoconjugation of the
aromatic rings of the triptycene scaffold and molecular rigidity.3
The unique homoconjugation properties of the triptycene unit lead
to “uncommon” properties, e.g., conductivity of conjugated poly-
mers constructed thereof.4 For the construction of molecular
motors and rotors,5 rotaxanes,6 shape-persistent macrocycles,7
and cages,8 as well as for metal organic frameworks,9 it is mainly
the rigidity of the triptycene that is exploited on a molecular level.
The rigidity and predefined geometry of triptycene is also the
prerequisite of the formation of porous triptycene polymers.10
The 1,2-phenylenediamine unit is a very valuable synthetic
moiety, which can be converted into a number of different
functional groups, and the resulting molecules, such as metal
salphens,11 quinoxalines,12 benzimidazoles,13 etc., bear interest-
ing molecular as well as material properties. Therefore, efforts
were made to use hexaminotriptycene 2 as a precursor for the
synthesis of the corresponding quinoxaline derivatives,9a,14 rigid
triptycene salphens with high internal free volumes,15 or D3h-
symmetric trisphenanthroline derivatives.14b One major draw-
back of the hexamine 2 is the ease of oxidation,9a which occurs
immediately after the synthetic reduction step from the corre-
sponding nitro compound. Therefore, the hexamine has to be
handled under exclusion of oxygen (or air) and needs to be used
for subsequent reactions immediately after reduction of the
precursor, which indicates that 2 does not allow storage of larger
amounts. Another inconvenience is the five-step synthesis of
The two-step synthesis of the ammonium salt 4 starts with a
6-fold nitration of triptycene 1:18 Triptycene 1 was dissolved in
fuming nitric acid and heated at 80ꢀ85 °C for 4 h giving after
1
workup a pale yellow solid as crude product. From H NMR
spectroscopy it can be estimated that the desired hexanitrotrip-
tycene 3 was formed as the main product in approximately 38%
yield. By recrystallization from hot DMF we were able to separate
3 as yellow needles from the crude mixture in yields between 16
and 18% of sufficient purity (approximately 97% of the desired
1
regioisomer as quantified by H NMR spectroscopy, see the
Supporting Information). Differential scanning calometric
(DSC, see Figure S16, Supporting Information) investigation
of 3 DMF showed a minor broad endothermic peak between 87
3
and 117 °C, which corresponds to the melting point of the
Received: April 27, 2011
Published: June 20, 2011
r
2011 American Chemical Society
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|

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Scheme 1. Two-Step Synthesis of Air-Stable Hexaammoniumtriptycene Hexachloride 4 as a Synthetic Analogue of Hexamino-
triptycene 2 (Right Side)a
a Key: (a) fuming nitric acid, 85 °C, 4 h; (b) SnCl2 2H2O, EtOH, HClaq (conc), reflux, 17 h. The five-step synthesis of the previous route was in detail
3
described in refs 9a and 16.
compound (see the Experimental Section) accompanied by the
loss of enclathrated DMF molecules. A further sharp and
exothermic peak (ꢀ384 kJ/mol) appears at 420 °C, which
corresponds to combustion at that temperature. Caution!
Although we never experienced any serious accidents with hexani-
trotriptycene 3 on scales described within this article, we cannot
exclude a spontaneous combustion of 3 on larger scales, e.g., by friction
or shock, and therefore, the compound has to be treated as a potential
safety hazard. We strongly recommend that adequate safety
precautions be taken into account done when experiments are
performed! The structure of 3 was additionally confirmed by
single-crystal X-ray diffraction of suitable crystals obtained
from DMF. Compound 3 crystallizes in the triclinic space
group P-1 (see Figure S17 in the Supporting Information).19
The asymmetric unit contains one molecule 3 and one DMF
molecule. The molecules themselves self-assemble in a hexa-
gonal fashion, with DMF molecules enclathrated inside the
framework voids.
For the subsequent 6-fold transformation of the hexanitro-
triptycene 3 to the corresponding hexaammonium hexachloride
4, we carried out the reduction using tin(II) chloride in aqueous
hydrochloride/ethanol solution to give the pale-yellow ammo-
nium salt 4 as a heptahydrate, as was determined by elemental
analysis, in quantitative yield. We found this to be superior to
methods usingPd/CandH2 orRaney-Ni/hydrazine. The reaction
was performed under air, and no precautions were found to
be necessary. More important to us than the high yield is the
stability of 4 toward oxidation, which was known before for similar
structures17 containing electron-rich o-phenylenediamine units.
This stability makes the handling for further transformations more
convenient, which is exemplified in some selected condensation
reactions (Scheme 2).
For example, with triethyl orthoformate the benzimidazole
analogue of triptycene (5a) is accessible in almost quantitative
yield (98%)17 by using directly the ammonium salt 4 as reactant
in water as solvent. Similarly, a benzotriazole analogue (5b) is
accessible in 61% yield by reacting 4 with sodium nitrite and
potassium acetate at room temperature.20 Quinoxaline derivative
6a can be prepared by reaction of the hexaammonium salt 4 with
diethyloxalate in water at 100 °C, giving the product as a pale
yellow solid in high yield (95%). For the reaction of 4 with anisil
or dihydroxydioxane to the corresponding quinoxalines 6b and
6c the addition of stoichiometric amounts of potassium acetate is
crucial: Treatment under similar conditions without potassium
acetate gave no reactions while with potassium acetate quinoxa-
line 6b was accessible in 75%21 and quinoxaline 6c in almost
quantitative yield (95%).
As a last example to demonstrate the versatility of hexaammo-
nium salt 4 in condensation reactions, the Schiff base condensa-
tion to a trinuclear nickel salphen (7) was tested. Trissalphen 7 is
comparable to the compounds MacLachlan et al. reported and
accessible in 68% yield as a wine-red solid.15
In summary, we have developed a facile two-step synthesis of
hexaammoniumtriptycene hexachloride 4 that is air-stable and
can be stored on the bench for several weeks without special
precautions (e.g., excluding air, light etc.). An additional advan-
tage is that for the purification of both compounds, 3 and 4, no
column chromatography is needed. Compound 4 can be used as
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NOTE
Scheme 2. Reactions of Hexaammonium Salt 4 in Various Condensation Reactionsa
a Key: (a) CH(OEt)3, DMF, CH2Cl2, rt, 22h, 98% of 5a; (b) KOAc, NaNO2, H2O, rt, 16h, 61% of 5b; (c) diethyl oxalate, H2O, 100 °C, 20 h, 95% of 6a;
(d) KOAc, 2,3-dihydroxydioxane, THF, rt, 19 h, 75% of 6b; (e) anisil, KOAc, EtOH, 100 °C, 3 d, 95% of 6c; (f) 3,5-di-tert-butylsalicylaldehyde,
Ni(OAc)2 4H2O, KOAc, 16 h, 90 °C, 68% of 7.
3
precursor for a variety of condensation reactions, resulting in a
number of functionalized extended triptycene derivatives in
high yields. With air-stable hexaammonium salt 4 we are provid-
ing a precursor that will serve as versatile building block for the
synthesis of larger molecules and new materials based on D3h-
symmetric subunits. This synthetic approach will be adopted for
similar compounds containing electron-rich o-phenylenediamine
subunits, which are prone to fast oxidation, such as tribenzo-
triquinacene22 or centrohexaindane derivatives.23
(M + 2+), 525 (M + H+). Anal. Calcd for C20H8N6O12 DMF (597.40):
3
C, 46.24; H, 2.53; N, 16.41. Found: C, 46.20; H, 2.67; N, 16.59.
2,3,6,7,14,15-Hexaammoniumtriptycene Hexachloride
(4). A suspension of 3 (1.25 g, 2.4 mmol) and tin(II) chloride dihydrate
(18 g, 79 mmol) in ethanol (140 mL) and concentrated hydrochloric
acid (60 mL) was refluxed for 24 h. The reaction mixture was cooled to
room temperature and the white precipitate collected by filtration,
washed with concd hydrochloric acid (3 ꢁ 15 mL), and dried in vacuum
to give 4 7H2O as a pale yellow solid (1.70 g (quant): mp 288 °C dec;24
3
1H NMR (400 MHz, DMSO-d6) δ 7.11 (s, 6H), 5.38 (s, 2H); 13C NMR
(126 MHz, DMSO-d6) δ 141.9, 125.6, 117.9, 50.3. FT-IR (KBr) 3401,
2856, 2567, 1627, 1556, 1478, 1278, 1205, 1092, 885, 829, 587, 457,
’ EXPERIMENTAL SECTION
229 cmꢀ1. Anal. Calcd for C20H26N6Cl6 7H2O (689.2): C, 34.85; H,
3
5.85; N, 12.19. Found: C, 34.65; H, 5.60; N, 11.83.
2,3,6,7,14,15-Hexanitrotriptycene (3). Triptycene (5.15 g,
20.2 mol) was suspended in fuming nitric acid (150 mL, 100%) and
heated to 85 °C for 4 h. The reaction mixture was cooled to room
temperature, poured into water (1 L), and stirred for 1 h. The pale yellow
(slightly pink) precipitate was collected by suction filtration, washed
with water, and dried in air to give approximately 11 g of crude product.
Recrystallization from hot DMF (reflux temperature) gives after cooling
to room temperature 3 as yellow crystals (1.85 g, 18%): mp 113 °C; 1H
NMR (400 MHz, DMSO-d6) δ 8.43 (s, 6H), 6.68 (s, 2H); 13C NMR
(100 MHz, DMSO-d6) δ 147.9, 140.2, 122.6, 50.7; FT-IR (KBr): 3090,
3044, 2886, 1539, 1459, 1408, 1368, 1345, 1253, 1206, 1167, 913, 884,
860, 781, 758, 581, 533 cmꢀ1; UV/vis (CH3CN) λmax (log ε) 193 nm
2,6,12-Trihydrotripty[2,3-d:6,7-d0:12,13-d00]triimidazole (5a).25
To a suspension of hexammonium salt 4 7H2O (600 mg, 0.44 mmol)
3
in DMF (10 mL) and dichloromethane (25 mL) was added triethyl
orthoformate (2 mL) and the mixture stirred for 22 h at room
temperature. To the creamy yellow solution were added methanol
(50 mL) and triethylamine (8 mL), resulting in a green solution.
Solvents were removed by rotary evaporation until a yellow solid started
to precipitate. Water was added in large excess and the yellow precipitate
collected by suction filtration, washed with water (3 ꢁ 30 mL), and dried
in vacuo to give 5a as an off-white solid (321 mg, 98%): mp >400 °C; 1H
NMR (500 MHz, DMSO-d6) δ 8.13 (s, 3H), 7.62 (s, 6H), 5.75 (s, 2H),
13C NMR (126 MHz, 375 K, DMSO-d6) δ 140.7, 139.9, 134.9, 109.7,
+
(2.1), 230 (2.0), 274, sh (1.6); MS (CI) m/z 553 (M + C2H5 ), 526
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NOTE
53.0; FT-IR (KBr) 3614, 3109, 2949, 2809, 1719, 1628, 1580, 1500, 1474,
1449, 1406, 1361,1309, 1293, 1266, 1235, 1186, 1061, 1037, 1021, 953,
754, 623, 587, 455, 436, 411 cmꢀ1; UV/vis (MeOH) λmax (log ε) 243 nm
(0.9), 250,sh (0.9), 260,sh (0.7), 271, sh (0.3), 291 (1.1), 301 (1.2);
HRMS (ESI) m/z calcd for C20H14N6Na 397.11722, found 397.11649.
2,6,12-Trihydrotripty[2,3-d:6,7-d0:12,13-d00]triazole (5b).25
Tris-nickelsalphen 7. In a screw-capped vial, hexaammonium salt
4 (50 mg, 0.072 mmol), salicylaldehyde (130 mg, 0.55 mmol), nickel
acetate tetrahydrate (60 mg, 0.21 mmol), and potassium acetate (60 mg,
0.66 mol) were suspended in 6 mL of ethanol and the mixture heated for
16 h at 90 °C. After the mixture was cooled to room temperature, the
wine-red precipitate was collected on a B€uchner funnel, washed with
ethanol (3 ꢁ 2 mL), and dried in the vacuum stream to obtain 7 as a
Hexaammonium salt 4 7H2O (140 mg, 0.1 mmol) and potassium
3
1
wine-red solid (123 mg, 68%): mp >400 °C; H NMR (500 MHz,
acetate (120 mg, 0.6 mmol) were dissolved in water (4 mL), and a
solution of sodium nitrite (55 mg, 0.8 mmol, in 1 mL water) was added
dropwise at room temperature. Immediately, a tan solid started to
precipitate. After the reaction mixture was stirred for another 16 h at
room temperature, the precipitate was collected on a B€uchner funnel
and washed with water (3 ꢁ 4 mL). After drying in vacuo, 5b remained
DMSO-d6, 375 K) δ 8.62 (s, 6H), 8.24 (s, 6H), 7.37 (s, 6H), 5.75 (s,
2H), 1.41 (s, 54H), 1.33 (s, 54H). FT-IR (KBr) 2956, 2906, 2868, 1618,
1586, 1525, 1467, 1426, 1385, 1358, 1331, 1269, 1251, 1234, 1199, 1178,
1129, 1058, 1026, 938, 910, 864, 840, 788, 751, 676, 636, 574, 541,
506 cmꢀ1; UV/vis (DMF) λmax (log ε) 386 nm (2.0), 405,sh (1.9), 493
(1.6); HRMS (ESI) m/z calcd for C110H1134N6O6Ni3 1808.84198,
found 1808.84155. Anal. Calcd for C110H134N6O6Ni3: C, 61.74; H, 6.49.
Found: C, 61.61; H, 6.23.
1
as pale tan solid (46 mg, 61%): mp >400 °C; H NMR (400 MHz,
DMSO-d6) δ 15.57 (s, br, 3H), 8.09 (s, br, 3H), 7.87(s, br, 3H), 6.08 (s,
br, 2H); 13C NMR (100 MHz, DMSO-d6) δ 143.4 (br), 142.6, 140.0
(br),131.6, 113.4, 106.2, 52.2; FT-IR (KBr): 3388, 3161, 2985, 2905,
2796, 1630, 1587, 1506, 1446, 1389, 1293, 1267, 1243, 1195, 1078, 995,
882, 754, 587, 454, 404 cmꢀ1; UV/vis (DMF) λmax (log ε) 294 nm
(1.4), 303 (1.4); HRMS (ESI) m/z calcd for C20H11N9 378.12102,
found 378.12101.
’ ASSOCIATED CONTENT
Supporting Information. 1H and 13C NMR spectra of all
S
b
new compounds. MALDI-TOF MS spectra of 7 and crystal-
lographic data of 3. This material is available free of charge via the
Internet at http://pubs.acs.org.
2,3,6,7,12,13-Hexahydroxy-2,6,12-trihydrotripty[2,3-d:6,7-d0:
12,13-d00]tripyrazyl (6a):25. Hexaammonium salt 4 7H2O (140 mg,
3
0.19 mmol) and diethyl oxalate (0.1 mL) were dissolved in water
(6 mL) and vigorously stirred at 100 °C. After 20 h, the reaction mixture
was cooled to room temperature and the off-white solid collected on a
B€uchner funnel and washed with water (3 ꢁ 4 mL). After being dried in
vacuo, 6a remained as an off-white solid (105 mg, 95%): mp >400 °C;
1H NMR (500 MHz, DMSO-d6) δ 11.88 (s, 6H), 7.17 (s, 6H), 5.71
(s, 2H); 13C NMR (126 MHz, 375 K, DMSO-d6) δ 155.2, 139.9, 122.3,
110.7, 50.1; FT-IR (KBr) 3443, 3177, 3060, 2952, 1689, 1481, 1454,
1390, 1303, 1245, 1193, 1095, 890, 795, 646, 537, 462 cmꢀ1; UV/vis
(DMF) λmax (log ε) 329 nm (1.0), 341 (1.0), 356,sh (0.8); HRMS (ESI)
m/z calcd for C26H14N6O6Na 529.08670, found 529.08606.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: michael.mastalerz@uni-ulm.de.
’ ACKNOWLEDGMENT
The “Fond der Chemischen Industrie (FCI)” and the “Deutsche
Forschungsgemeinschaft (DFG, Project MA4061/4-1)” are grate-
fully acknowledged for financial support. We also thank the German
Academic Exchange Sevice (DAAD) for a scholarship for M.C. in
the frame of the IASTE-program (D/10/1068/1). Solvay Fluor
GmbH is acknowledged for the donation of chemicals. M.M. thanks
Prof. T. M. Klap€otke (LMU Munich) for helpful discussions.
2,6,12-Trihydrotripty[2,3-d:6,7-d0:12,13-d00]tripyrazyl (6b).25
Hexaammonium salt 4 7H2O (155 mg, 0.22 mmol), potassium acetate
3
(140 mg, 1.4 mmol), and 2,3-dihydroxydioxane (90 mg, 0.75 mmol) were
suspended in THF (6 mL) and stirred at room temperature for 19 h. The
reaction mixture was filtered through a plug of silica and washed with THF.
Solvent was removed at the rotary evaporator to obtain 230 mg of a pale
yellow solid. The solid was digested in 15 mL of light petroleum ether,
filtered, and washed with petroleum ether (3 ꢁ 5 mL). The residue was
dried in vacuo to give 6b as a pale yellow solid (67 mg, 75%): mp >400 °C;
1H NMR (400 MHz, CDCl3) δ 8.78 (s, 6H), 8.23 (s, 6H), 6.15 (s, 2H).
Analytical data are in accordance to those previous reported.9a,14a
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Hz, 12H), 6.84 (d, J = 8.8 Hz, 12H), 6.05 (s, 2H), 3.81 (s, 18H); 13C NMR
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110(2) K, C23H15N7O13, M = 597.42, triclinic space group P-1, a =
9.7248(3) Å, b = 10.2761(3) Å, c = 13.0872(3) Å, R = 92.220(2)°, β =
109.872(3)°, γ = 97.000(3)° V = 1216.29(6) Å3, Z = 2, DC = 1.631 g/
cm3, μ = 0.137 mmꢀ1, 3.16° < Θ < 26.25°, reflections collected/unique
29500/4902 [R(int) = 0.0295], data/restrains/parameters 4902/0/390,
GOF 1.049, final R[xI > 2σ(I)] R1 = 0.0395, wR2 (all data) = 0.1069,
residual density 0.475 and ꢀ0.265 e Aꢀ3
.
3
(20) Damshoder, R. E.; Peterson, W. D. Org. Synth. 1940, 20, 16–18.
(21) Note that with the free, air-sensitive hexamine 2 product 6b was
isolated in 29% yields (ref 9a).
(22) Tellenbr€ocker, J.; Kuck, D. Angew. Chem., Int. Ed. 1999, 111,
919–922.
(23) For a review of centropolyindanes, see: Kuck, D. Chem. Rev.
2006, 106, 4885–4925.
(24) The compound starts to darken at 260 °C, so we assume that
decomposition starts already at this temperature.
(25) The IUPAC names for the compounds are as follows: 5a:
5,7,15,17,24,26-hexaazaoctacyclo[9.9.9.0{2,10}.0{4,8}.0{12,20}.0{14,18}.0{21,29}
.
0{23,27}]nonacosa-2,4(8),5,9,12,14(18),15,19,21(29),22,24,27-dodecaene.
5b: 5,6,7,15,16,17,24,25,26-nonaazaoctacyclo[9.9.9.0{2,10}.0{4,8}.0{12,20}
.
0{14,18}.0{21,29}.0{23,27}]nonacosa-2,4(8),5,9,12,14(18),15,19,21(29),22,24,27-
dodecaene. 6a: 6,7,17,18,27,28-hexahydroxy-5,8,16,19,26,29-octaazaocta-
cyclo[10.10.10.0{2,11}.0{4,9}.0{13,22}.0{15,20}.0{23,32}.0{25,30}]dotriacosa-
2,4(9),5,7,10,13,15(20),16,18,21,23(32),24,26,28,30-pentadecaene.
6b: 5,8,16,19,26,29-octaazaocta-cyclo[10.10.10.0{2,11}.0{4,9}.0{13,22}.0{15,20}
.
0{23,32}.0{25,30}]dotriacosa-2,4(9),5,7,10,13,15(20),16,18,21,23(32),24,26,28,30-
pentadecaene. 6c: 6,7,17,18,27,28-hexa(4-methoxyphenyl)-5,8,16,19,26,29-
octaazaoctacyclo[10.10.10.0{2,11}.0{4,9}.0{13,22}.0{15,20}.0{23,32}.0{25,30}]-
dotriacosa-2,4(9),5,7,10,13,15(20),16,18,21,23(32),24,26,28,30-pentadecaene.
(26) Sheldrick, G. M. SHELXTL-97, Universit€at G€ottingen,
G€ottingen, Germany, 1997.
6393
dx.doi.org/10.1021/jo200843v |J. Org. Chem. 2011, 76, 6389–6393