Efficient electroorganic synthesis of 2,3,6,7,10,11-Hexahydroxytriphenylene derivatives

Article abstract of DOI:10.3762/bjoc.8.196

2,3,6,7,10,11-Hexahydroxytriphenylene of good quality and purity can be obtained via anodic treatment of catechol ketals and subsequent acidic hydrolysis. The electrolysis is conducted in propylene carbonate circumventing toxic and expensive acetonitrile.

Full text of DOI:10.3762/bjoc.8.196

Efficient electroorganic synthesis of
2,3,6,7,10,11-hexahydroxytriphenylene derivatives
Carolin Regenbrecht and Siegfried R. Waldvogel*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1721–1724.
Institute for Organic Chemistry, Mainz University, Duesbergweg
10–14, 55128 Mainz, Germany
doi:10.3762/bjoc.8.196
Received: 15 August 2012
Accepted: 07 September 2012
Published: 10 October 2012
Email:
Siegfried R. Waldvogel* - waldvogel@uni-mainz.de
* Corresponding author
Associate Editor: C. Stephenson
Keywords:
© 2012 Regenbrecht and Waldvogel; licensee Beilstein-Institut.
License and terms: see end of document.
catechol; electrochemical oxidation; hexahydroxytriphenylene; ketals;
propylene carbonate
Abstract
2,3,6,7,10,11-Hexahydroxytriphenylene of good quality and purity can be obtained via anodic treatment of catechol ketals and
subsequent acidic hydrolysis. The electrolysis is conducted in propylene carbonate circumventing toxic and expensive acetonitrile.
The protocol is simple to perform and superior to other chemical or electrochemical methods. The key of the method is based on the
low solubility of the anodically trimerized product. The shift of potentials is supported by cyclic voltammetry studies.
Introduction
The unique spectroscopic and geometric features of triphenyl- The anodic oxidation of catechol derivatives was first demon-
enes give rise to a variety of applications for this very common strated by Parker et al. [13]. Applying this methodology in a
structural motif. The use of triphenylenes in discotic liquid crys- specific electrolysis cell, Simonet et al. established a trimeriza-
tals [1,2] as building blocks in supramolecular chemistry [3,4] tion protocol [14-16]. Usually, yields are in a moderate range
as well as in solid-state chemistry [5] is well documented. (≤35%) [15] when the electrolysis is performed on platinum or
Furthermore, triphenylenes are applied as components of func- graphite anodes in anhydrous and non-nucleophilic electrolytes
tional polymers [6,7] and fluorescent labels [8,9]. Typically, the [14]. Poor yields are caused by the low oxidation potential of
oxidative trimerization of catechol derivatives can be induced the products and the preference for over-oxidation followed by
by metal salts in high oxidation states, for example by molyb- decomposition of the generated radical cations [11,16].
denum pentachloride [4,10]. Electrochemical methods can be
applied to realize the oxidative trimerization since the forma- Since we have a specific interest in a large-scale access to
tion of metal waste is avoided [11,12]. Furthermore, no tran- derivatives of 2,3,6,7,10,11-hexahydroxytriphenylene, we were
sition metal cations, which promote the cleavage of the ketal prompted to develop a sustainable protocol providing the target
moiety [11], are involved.
compound by electroorganic methods. Precipitation during
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Beilstein J. Org. Chem. 2012, 8, 1721–1724.
electrolysis avoided over-oxidation of the electron-rich triphen- the electrolysis. Usually, the precipitate floats as fine particles
ylene derivatives [12]. Yields up to 62% were obtained when in the cell and is prevented from over-oxidation. The best
tetrabutylammonium tetrafluoroborate (TBABF4) in acetoni- results for the individual catechol ketals are listed in Table 1.
trile (ACN) was used as electrolyte [11]. To our delight, we
found that the electrolysis is accomplished under practical
Table 1: Anodic trimerization of catechol ketals.a
galvanostatic conditions using platinum sheets as electrode
Entry Catechol ketal Method Triphenylene ketal
[%]
CE
[%]
materials. Due to the large potential window and outstanding
solubility for common conducting salts, ACN is among the
standard solvents for electrochemical purposes [17]. However,
the application of ACN in electrochemical processes is
adversely affected by its severe toxicity and significant costs.
Therefore, switching to an environmentally benign and inexpen-
sive solvent is highly desired. This work demonstrates that
propylene carbonate (PC) successfully replaces ACN by fine-
tuning of the supporting electrolytes.
1a
1a
A
B
2a
2a
80
70
52
45
1
2
3
1b
1b
A
B
2b
2b
61
37
39
24
1c
1c
A
B
2c
2c
61
50
39
32
4
5
1d
1e
B
B
2d
2e
29
39
25
19
aConditions: Pt electrodes, T = 20 °C, J = 16 mA cm−2, 3.1 F/catechol
ketal, argon.
Results and Discussion
Anodic oxidation of catechol ketals
We report a simple protocol for the anodic oxidation of cate-
chol ketals forming triphenylene ketals (Scheme 1).
Yields up to 80% are obtained for the dehydrotrimers 2a–c in
PC and TMABF4 as electrolyte. The solvation effects and
Best results were obtained in PC and TMABF4 (method A) or polarity of the solvent greatly influence the precipitation of the
TBABF4 (method B) as electrolytes. Performing the reaction in desired products. Since the solvation increases with a
PC is beneficial for several reasons: PC is hazard-free, inexpen- decreasing size of the tetraalkylammonium cations, the polarity
sive and suitable for large-scale reaction. Moreover, triphenyl- of the electrolyte increases as well [18,19]. The precipitation of
ene ketals have lower oxidation potentials than the corres- the non-polar triphenylenes is promoted in the strongly polar
ponding catechol ketals. Since the non-polar products exhibit a electrolyte. Triphenylene derivatives 2d and 2e were obtained
relative poor solubility in PC, precipitation is achieved during with 29 and 39% yield, respectively, using TBABF4 in PC. The
Scheme 1: Experimental conditions for the anodic oxidation of catechol ketals.
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Beilstein J. Org. Chem. 2012, 8, 1721–1724.
yields are higher compared to that of reported electrochemical
methods [15]. The simple reaction setup using an undivided cell
and galvanostatic conditions applying this moderate current
density allows a scale-up of this electroorganic synthesis.
Cyclic voltammetry
Cyclic voltammetry studies show that triphenylene ketal 2a gets
more easily oxidized at lower potentials than subunit 1a
(Figure 1).
Scheme 2: Acid-catalyzed cleavage of ketal moieties.
The hydrolysis is initiated by the water which is present in the
acetic acid. Commonly, 3 is prepared by dealkylation of
2,3,6,7,10,11-hexamethoxytriphenylene [20,21]. The resulting
product is usually a black solid being colorized by chinoide
byproducts [20,22] which are not present in our method.
However, our method provides easy access to pure 3 which can
easily be separated and purified by simple filtration.
Conclusion
Triphenylene ketals are easily available by anodic oxidation
using a simple galvanostatic protocol. Best results are obtained
when the synthesized dehydrotrimers are almost insoluble in the
electrolyte. Precipitation occurs during the electrolysis when
employing tetraalkylammonium salts in PC. This effectively
prevents the desired products from over-oxidation. Since PC is
environmentally benign and inexpensive, the procedure signifi-
cantly improved the access to the triphenylene derivatives.
Moreover, acid-catalyzed cleavage of triphenylene ketals
Figure 1: Cyclic voltammograms of catechol ketal 1a in ACN and PC
and triphenylene ketal 2a in ACN, for magnification of 2a see inner
rectangle (1a: 5∙10−3 M in 0.1 M TBAClO4/ACN or 0.1 M TBAClO4/PC
vs Ag/AgCl, 2a: 5∙10−3 M in 0.1 M TBAClO4/ACN vs Ag/AgCl; sweep
rates: 50 mV/s; third cycles).
The first reversible process at Eox = 1.22 V corresponds to the provides 2,3,6,7,10,11-hexahydroxytriphenylene almost
oxidation of 2a to a radical cation [13]. This SET is followed by quantitatively in very good quality.
the irreversible oxidation at Eox = 1.70 V. According to the
Experimental
Nernst equation, oxidation potentials should be increased due to
the insolubility of 2a in PC. Because of the insufficient solu- General protocol for anodic oxidation
bility in this solvent, cyclic voltammograms were measured The catechol ketal (10 mmol) was mixed with the electrolyte
solely in ACN. The oxidation of 1a was analyzed in PC and (method A: 2.012 g TMABF4 and 23.3 mL PC; method B:
ACN. Both cyclic voltammograms show an irreversible oxi- 4.116 g TBABF4 and 20.7 mL PC) in an undivided standard
dation at Eox = 1.61 and 1.58 V, respectively [15]. The increase electrolysis cell and stirred under argon for 5 min. At 20 °C a
of the current densities beginning at 2.14 V is caused by the galvanostatic electrolysis with a current density of 16 mA cm−2
decomposition of electrolyte which was confirmed by blank was performed on platinum foil as electrodes (2.2 cm × 3.2 cm).
experiments. The obtained data clearly indicate that triphenyl- The polarity of electrodes was reversed every 15 min. During
ene derivatives should precipitate during electrolysis to avoid the electrolysis, vigorous stirring was necessary and the forma-
over-oxidation.
tion of triphenylene products as light brown precipitates could
be observed. After application of 3000 C (3.1 F) the electrolyte
and precipitate were removed from the cell by dissolving in
dichloromethane. After removal of the solvent under reduced
Preparation of 2,3,6,7,10,11-hexahydroxytri-
phenylene
2,3,6,7,10,11-Hexahydroxytriphenylene (3) was obtained pressure, the crude product and PC were dissolved in 40 mL
almost quantitatively by acidic cleavage of the ketal moieties of of hot methanol. Water (17 mL) was slowly added to the hot
2b (Scheme 2).
solution and the mixture was allowed to cool to room tempera-
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Beilstein J. Org. Chem. 2012, 8, 1721–1724.
19.Fry, A. J. Tetrahedron 2006, 62, 6558–6565.
ture. Subsequently, the mixture was kept at 5 °C for several
hours. The precipitate was filtered off and washed with
methanol:water (7:3, 25 mL) and dried (45 °C, 8·10−3 mbar).
Triphenylene derivatives 2a–e were unequivocally identified.
doi:10.1016/j.tet.2006.03.057
20.Naarmann, H.; Hanack, M.; Mattmer, R. Synthesis 1994, 477–478.
doi:10.1055/s-1994-25505
21.Krebs, F. C.; Schiødt, N. C.; Batsberg, W.; Bechgaard, K. Synthesis
1997, 1285–1290. doi:10.1055/s-1997-3188
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Supporting Information
Supporting Information File 1
Characterization data and spectra of synthesized
compounds.
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