Unexpected formation of a cyclic vinylene sulfate in the synthesis of ethynyl-substituted acenes

Article abstract of DOI:10.1039/c2cc32805d

(E)-2-Styrylanthracene derivatives containing triisopropylsilylacetylene groups at the 9 and 10 positions were synthesized and characterized. The electronic properties have been studied by DFT calculations, spectroscopy and electrochemistry, revealing asymmetric resonance stabilization effects that result in the regioselective formation of an unusual cyclic vinylene sulfate.

Full text of DOI:10.1039/c2cc32805d

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COMMUNICATION
Unexpected formation of a cyclic vinylene sulfate in the synthesis of
ethynyl-substituted acenesw
Brandon Djukic and Dmitrii F. Perepichka*
Received 19th April 2012, Accepted 4th May 2012
DOI: 10.1039/c2cc32805d
(E)-2-Styrylanthracene derivatives containing triisopropylsilyl-
acetylene groups at the 9 and 10 positions were synthesized and
characterized. The electronic properties have been studied by
DFT calculations, spectroscopy and electrochemistry, revealing
asymmetric resonance stabilization effects that result in the
regioselective formation of an unusual cyclic vinylene sulfate.
from (E)-2-styrylanthracene derivative 3, which is also capable of
high field-effect mobility and strong solid state luminescence.7
Stemming from these considerations, our aim was to prepare an
(E)-2-styrylanthracene derivative 6 with increased solubility
through the incorporation of TIPSE groups. However, during
the course of the synthesis of 6 from corresponding anthraquinone
4, under standard and widely employed conditions, we discovered
an intriguing new reaction that resulted in the isolation of an
unusual cyclic vinylene sulfate 7 as the major product.
The ability to tune properties of organic semiconductors by
means of organic synthesis has led to an active exploration in
using these materials for organic field effect transistors
(OFETs) and related devices.1 Among the most promising
candidates for ‘‘solution processable’’ OFETs are acenes and
heteroacenes bearing triisopropylsilylethynyl (TIPSE) groups.2,3
The most widely studied of these derivatives, TIPSE-pentacene
[6,13-bis(triisopropylsilylethynyl)pentacene] 1, shows excellent
Less than a handful of cyclic vinylene sulfate examples have
been reported to date.8 One of these was also prepared from
an acetylene derivative, but this reaction required extreme condi-
tions – heating in 60% oleum – to achieve this transformation.8d
Although rare, cyclic vinylene sulfates offer unique opportunities
in organic synthesis,9 readily facilitating transformations such as
the conversion into the corresponding diketone through pericyclic
SO2 elimination.8d The selective introduction of a cyclic sulfate
group in an anthracene derivative, therefore, offers a potential
approach for the asymmetric functionalization of acene based
materials and provides a general insight into the role of resonance
stabilization effects in the synthesis of these materials.
charge mobility (>1 cm2 Vꢀ1 ꢀ1) in solution-casted thin films
s
owing to incorporation of the TIPSE groups.4 Derivatives of
TIPSE-anthracene 2 have also shown high solubility but only
moderate OFET performance for solution processed films.5
Anthracenes, however, are particularly appealing to use as organic
semiconductors due to higher oxidative stability6 and efficient
luminescence in comparison to pentacenes. Recently we have
demonstrated these advantages in the fabrication of devices made
The synthesis of 6 followed the established methodology
(Scheme 1) and is described in the ESI.10w (E)-2-Styrylanthracene-
9,10-dione 4 was reacted with TIPSE-lithium, producing the
intermediate 5. Reduction of 5 using SnCl2 in the presence of a
dilute (10%) aqueous H2SO4 solution provided a small amount of
6, which was purified by column chromatography, using hexanes
to elute it as a vibrant yellow and fluorescent band (PLQY6 =
39%, ESIw). Surprisingly, a second, larger and less fluorescent
band (PLQY7 = 10%) was also isolated from the column and
later identified as product 7. The purified yields of 6 and 7 were
24% and 76% respectively. Curiously, we then attempted
the same procedure in a reaction with pentacenequinone
(the precursor of 1), using an identical H2SO4 solution, but
Department of Chemistry and Centre for Self-Assembled Chemical
Structures, McGill University, Montreal, Qc, Canada.
E-mail: dmitrii.perepichka@mcgill.ca; Fax: +1 514 3983797;
Tel: +1 514 3986233
w Electronic supplementary information (ESI) available: Experimental
section; 1H-NMR, 13C-NMR, MS, FT-IR, UV-Vis and fluorescence
spectra for synthesized compounds; calculation details. CCDC
877734–877736 (4, 6 and 7). For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2cc32805d
c
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Chem. Commun., 2012, 48, 6651–6653 6651

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Scheme 1 Synthesis of (E)-2-styrylanthracene derivatives 6 and 7.
Fig. 2 Cyclic voltammograms of 6 and 7 (red line shows repeated
scanning).
ox
TIPSE group (E = 0.53 V). During reductive scanning, 6
1/2
shows one reversible reduction wave at E1/2 = ꢀ2.20 V. For 7,
two reductive processes are observed. The first, at ꢀ2.01 V, is
an irreversible process that can be attributed to the reductive
cleavage of the cyclic sulfate group. The second is a reversible
reduction at E1/2 = ꢀ2.22 V which closely resembles the wave
observed for 6. Repeated reductive scanning of 7 results in the
progressive disappearance of its oxidation and reduction peaks,
reproducing a voltammogram identical to 6. Lability of the
sulfate group is further supported by high resolution APCI-MS,
which demonstrates its cleavage upon chemical ionization.
To account for the formation of 7, we proposed a reaction
mechanism that is consistent with those described for the
synthesis of other cyclic sulfates (Scheme 2).8d Protonation and
cleavage of a hydroxy group occur first, followed by the addition
of the sulfate anion. Protonation of the second hydroxy group
induces its cleavage, and subsequent intramolecular nucleophilic
attack on the allene carbon forms a sulfate ring and aromatizes
the anthracene core.
Fig. 1 Crystal structures of 6 (left) and 7 (right).
observed only the formation of 1 with a 91% overall yield. We
also repeated the reaction with 5, substituting aqueous HCl in
place of the H2SO4 solution, and obtained 6 as the only
product in 83% yield.
Both 6 and 7 were isolated as highly soluble, crystalline solids
and slow evaporation of either in CH2Cl2/MeOH provided
single crystals for X-ray analysis, with crystal structures shown
in Fig. 1. The crystal packing of 6 is dimeric with geometry that
does not allow any p-stacking (ESIw). 6 features TIPSE groups
with Si–Csp bond distances between 1.84 and 1.85 A that are in
agreement with those reported for related acenes.4,11 For 7, the
crystal structure reveals one TIPSE group, with a Si–Csp bond
distance of 1.85 A.12 In place of the other TIPSE group, we
found that the alkyne had been replaced with a cyclic vinylene
2
sulfate ring. The Si–Csp bond distance of 7 increased to 1.90 A,
indicating a weakened carbon–silicon bond (as compared to 6).
Other bond distances and angles observed for the vinylene
sulfate ring are consistent with those reported in the literature.8b
The FT-IR spectrum of 7 displays strong peaks at 1216 and
1402 cmꢀ1 that arise from characteristic sulfate vibrations.
Analysis of the 1H- and 13C-NMR of 7 does not suggest the
presence of other isomers (even in the crude NMR), indicating
that the sulfate formation occurs selectively, on one side of
the anthracene component. The UV/Vis spectrum of TIPSE
derivative 6 reveals a vibronically structured band with peaks
at 405, 429 and 456 nm. This band is slightly shifted into the
blue region for vinylene sulfate 7 (peaks at 421 and 446 nm).
The corresponding fluorescence bands have lmax of 463 nm
and 472 nm for 6 and 7, respectively; the larger Stokes shift
(0.15 eV) demonstrated by 7 is in line with its more polar and
less rigid structure.
Next we performed DFT calculations to explain the unusual
reactivity of 5, which resulted in the selective synthesis of 7
(ESIw). Molecular structures of both 6 and 7 were optimized at
the B3LYP/6-31G(d,p) level. Calculated geometry and energy
levels are corroborated by X-ray analysis, spectroscopic and
The electrochemical properties of 6 and 7 were investigated
with cyclic voltammetry (Fig. 2). Oxidative scanning of 6
reveals a reversible peak at 0.37 V vs. ferrocene (Fc/Fc+). A
higher oxidation potential of 0.67 V measured for the related
compound 37 reflects an electron-donating effect of TIPSE
groups stabilizing the radical cation. A similar, but less
prominent, effect was observed for 7, which bears only one
Scheme 2 Proposed mechanism for the selective synthesis of 7.
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6652 Chem. Commun., 2012, 48, 6651–6653
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currently attempting to utilize this reactivity for synthesizing
more sophisticated organic semiconductor materials.
This work was supported by NSERC of Canada. The authors
are thankful to Francine Belanger (University of Montreal) for
X-ray analysis.
Notes and references
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3 D. Lehnherr, R. Hallani, R. McDonald, J. E. Anthony and
R. R. Tykwinski, Org. Lett., 2012, 14, 62.
Fig. 3 Molecular orbital diagrams depicting the HOMOs of carbo-
cations 70, 80 and 10.
4 S. K. Park, T. N. Jackson, J. E. Anthony and D. A. Mourey, Appl.
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8 (a) G. Rio and J.-P. Cornu, Bull. Soc. Chim. Fr., 1958, 1540;
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9 For a general review on application of cyclic sulfates in synthesis,
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10 J. E. Anthony, J. S. Brooks, D. L. Eaton and S. R. Parkin, J. Am.
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11 Crystal data for 6: C44H56Si2, M = 641.07, monoclinic, a =
14.5106(6) A, b = 18.8272(7) A, c = 29.1708(12) A, a = 901,
b = 100.686(2)1, g = 901, V = 7831.1(5) A3, T = 100 K, space
electrochemical measurements supporting B3LYP/6-31G(d,p)
as an appropriate level of theory. The proposed mechanism
(Scheme 2) identifies cation 70 as the key intermediate in
formation of cyclic sulfate 7 and we compared its stability to
that of cation 80 (Fig. 3), where cleavage of the hydroxyl group
occurs on the other side of the anthracene ring. Formꢀa1tion of
carbocation 70 from 5 was found to be 7.1 kcal mol more
favorable than the formation of 80 from 5, thus accounting for
the observed selectivity. The difference can be reasonably
attributed to the lack of resonance stabilization13 of the
carbocation by the styryl group in 80 (meta-connection).
Indeed, analysis of molecular orbital (MO) diagrams shows
that the HOMO of 70 spans throughout the entire molecule,
effectively stabilizing the carbocation (Fig. 3). Whereas in 80, the
HOMO is localized on the stilbene fragment, with almost no
density on the carbocation center. Our calculations also show
that formation of the corresponding pentacene cation 10 from the
dihydroxy precursor 9 (ESIw) is enthalpically less favorable than
formation of 70 from 5 by 3 kcal molꢀ1. This likely explains why
no vinylene sulfate is observed in synthesis of 1.
group P21/n,
Z = 8, 165 570 reflections measured, 14 770
independent reflections (Rint = 0.061). The final R1 values were
0.0979 (I > 2s(I)). The final wR(F2) values were 0.2740 (I > 2s(I)).
The final R1 values were 0.1163 (all data). The final wR(F2) values
were 0.2912 (all data). CCDC 877735.
In summary, we have described a new reaction leading to
the unexpected formation of a vinylene sulphate moiety in the
synthesis of 9,10-diethynyl substituted anthracene derivatives. The
reaction is enabled by resonance stabilization of the carbocation
intermediate by a p-donating substituent (styryl group) and there-
fore is highly regioselective. We have structurally characterized
these molecules and studied their electronic structures through
optical and electrochemical experiments. The selective introduc-
tion and subsequent chemical and electrochemical cleavage of the
vinylene sulfate group suggest a new method for the asymmetric
functionalization of acene based materials.3 In this regard, we are
12 Crystal data for 7: C44H56O4SSi2, M = 737.13, triclinic, a =
8.4790(4) A, b = 15.5718(7) A, c = 15.9893(7) A, a = 86.192(2)1,
b = 79.705(2)1, g = 76.941(2)1, V = 2022.66(16) A3, T = 100 K,
%
space group P1, Z = 2, 85 141 reflections measured, 7572
independent reflections (Rint = 0.029). The final R1 values were
0.0357 (I > 2s(I)). The final wR(F2) values were 0.1011 (I > 2s(I)).
The final R1 values were 0.0380 (all data). The final wR(F2) values
were 0.1039 (all data). CCDC 877736.
13 J. P. Richard, M. M. Toteva and J. Crugeiras, J. Am. Chem. Soc.,
2000, 122, 1664; M. Pittelkow, J. B. Christensen and T. I. Solling,
Org. Biomol. Chem., 2005, 3, 2441.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6651–6653 6653