A Versatile One-Pot Access to Cyanoarenes from ortho- and para-Quinones: Paving the Way for Cyanated Functional Materials

Article abstract of DOI:10.1002/chem.201600004

A generally applicable direct synthesis of cyanoarenes from quinones is presented. Particular emphasis is placed on the preparation of precursors and target molecules relevant for organic materials, including halogenated cyanoarenes and larger cyanated acenes. The reaction and work-up protocols are adjusted for the challenges presented by the different substrates and products. Screening results of the initial reaction optimization are given to further facilitate adaptation to other synthetic problems. The universality of the reaction is finally highlighted by successful substitution of para-quinones by an ortho-quinone as the starting material.

Full text of DOI:10.1002/chem.201600004

DOI: 10.1002/chem.201600004
Full Paper
&
Fused-Ring Systems
A Versatile One-Pot Access to Cyanoarenes from ortho- and para-
Quinones: Paving the Way for Cyanated Functional Materials
Florian Glçcklhofer,*[a] Markus Lunzer,[a] Berthold Stçger,[b] and Johannes Frçhlich[a]
Abstract: A generally applicable direct synthesis of cyanoar-
enes from quinones is presented. Particular emphasis is
placed on the preparation of precursors and target mole-
cules relevant for organic materials, including halogenated
cyanoarenes and larger cyanated acenes. The reaction and
work-up protocols are adjusted for the challenges presented
by the different substrates and products. Screening results
of the initial reaction optimization are given to further facili-
tate adaptation to other synthetic problems. The universality
of the reaction is finally highlighted by successful substitu-
tion of para-quinones by an ortho-quinone as the starting
material.
Introduction
have also been employed as strong electron acceptors in sev-
eral investigations of bimolecular photoinduced electron trans-
fer reactions,[6] and for two-photon absorption.[7]
In the past few decades, polycyclic aromatic hydrocarbons,
and in particular acenes, have attracted tremendous attention
as organic functional materials both theoretically and experi-
mentally.[1] Among these materials, cyanated compounds take
on a particular role as promising candidates for air-stable n-
type and ambipolar organic field effect transistors (OFETs). This
is due to the low internal reorganization energies for electron
and hole transfer, large intermolecular electronic couplings,
high electron affinities, and overall small HOMO–LUMO gaps,
which are all characteristics predicted by computational stud-
ies.[2] These theoretical works on larger cyanated acenes are
complemented by only a single experimental investigation of
the material properties of 5,12-dicyanonaphthacene and 6,13-
dicyanopentacene, confirming the significantly low HOMO and
LUMO levels and demonstrating the fabrication of OFET devi-
ces with ambipolar properties.[3] The absence of further experi-
mental results can clearly be attributed to a lack of convenient
synthetic access to this type of molecules.
As the nitrile represents a versatile functional group in syn-
thetic chemistry, a large number of secondary products can be
synthesized from appropriately substituted cyanoarenes. For
1,4-dicyanobenzenes and 9,10-dicyanoanthracenes, the known
modifications include the preparation of the corresponding al-
dehydes,[8] thioamides,[9] various heterocyclic compounds,[10]
and nucleophilic benzylation by using readily available zinc re-
agents.[11] Moreover, dicyanoarenes have been applied as li-
gands in p-stacked metalloparacyclophanes.[12]
Despite these manifold applications of cyanoarenes, prepa-
rative approaches, until recently, have often involved either
several synthetic steps or labor-intensive purification procedur-
es.[3,10a,13] This was overcome when we reported a synthetic
method for the conversion of quinones to cyanoarenes via sily-
lated cyanohydrin intermediates in a one-pot reaction.[14]
A
benefit of using quinones as the starting materials is their
(commercial) availability in a wide range of different substitu-
tion patterns, including halogenated derivatives. In principle,
this novel synthesis allows the preparation of cyanoarenes
with substitution patterns not accessible so far or accessible
only by multi-step procedures.
Aside from the applicability of the larger cyanated acenes
for organic electronics, smaller cyanated acenes also play an
important role as photosensitizers in light-mediated reac-
tions.[4] In particular, 9,10-dicyanoanthracene has been exam-
ined as a photosensitizer in photooxidations of olefins and in
other photochemical reactions such as CÀC bond cleavage re-
actions of lignin b-1 model compounds and in the preparation
of the anti-malaria drug Artemisinin.[5] Moreover, cyanoarenes
However, the reported method has some disadvantages.
Yields are moderate, the catalyst for the first reaction step—
cesium fluoride—is highly hygroscopic and a large excess of
trimethylsilyl cyanide (TMSCN) is required as a reagent. More-
over, this excess of TMSCN has to be carefully controlled by in
situ IR spectroscopic measurements to enable a successful
one-pot conversion of 1,4-benzoquinone, which is prone to
side reactions such as conjugate addition and rearrangement.
Thus, optimization of the reaction conditions is required prior
to investigations of the substrate scope.
[a] F. Glçcklhofer, M. Lunzer, Prof. Dr. J. Frçhlich
Institute of Applied Synthetic Chemistry, TU Wien
Getreidemarkt 9/163, 1060 Vienna (Austria)
E-mail: florian.gloecklhofer@tuwien.ac.at
[b] Dr. B. Stçger
Institute of Chemical Technologies and Analytics, TU Wien
Getreidemarkt 9/164, 1060 Vienna (Austria)
As cyanoarenes are particularly useful for organic materials
chemistry, the subsequent focus on the synthesis of precursors
and substrates for this field of research is considered beneficial.
Supporting information and ORCIDs from the authors for this article are
available on the WWW under http://dx.doi.org/10.1002/chem.201600004.
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Promising molecules include halogenated cyanoarenes for
transition-metal-catalyzed cross-coupling reactions, which are
frequently used in the synthesis of functional organic materi-
als,[15] and silylated alkynyl-functionalized aromatic compounds
as they belong to an intensely studied substance class.[1b,c]
Moreover, after desilylation, the alkynyl substituents allow for
a modification by Sonogashira cross-coupling reactions or cop-
per(I)-catalyzed azide–alkyne cycloadditions,[16] which makes al-
kynyl-functionalized cyanoarenes interesting precursors for fur-
ther reactions. The applicability of the reaction conditions to
larger substrates can be tested and optimized with the exam-
ple of 6,13-pentacenequinone, and the universality can be
tested by replacing para-quinone with ortho-quinone as the
starting material.
Table 1. Catalyst, solvent, and temperature screening of the reaction
from 9,10-anthraquinone 1a to cyanohydrin intermediate 2 (Scheme 1,
i).[a]
Catalyst (equiv) TMSCN
[equiv]
Solvent
(m)[b]
T
Reaction time[c]
CsF (0.2)
CsF (0.2)
CsF (0.2)
CsF (0.2)
3.0
3.0
3.0
2.6
MeCN (0.5) rt
2 h
5 h
MeCN (0.5) 08C
MeCN (0.5) À158C 18 h[d]
MeCN (0.5) rt
incomp. conver-
sion
CsF (0.2)
CsF (0.1)
CsF (0.1)
Cs2CO3 (0.1)
KF (0.1)
2.6
3.0
3.0
3.0
3.0
3.0
3.0
3.0
2.1
3.0
2.2
3.0
2.6
3.0
3.0
3.0
2.05
2.05
2.05
MeCN (0.5) 08C
MeCN (0.5) rt
6 h
5 h
30 h
6 h
no conversion
27 h[d]
no conversion
no conversion
40 h
1 h
3 h
20 min
20 min
1 h
3 h
THF (0.5)
rt
MeCN (0.5) 08C
MeCN (0.5) 08C
MeCN (0.5) 08C
MeCN (0.5) 08C
CH2Cl2 (0.5) 08C
MeCN (0.5) 08C
K2CO3 (0.1
no catalyst
Cs2CO3 (0.1)
Cs2CO3 (0.01)
K2CO3 (0.05)
K2CO3 (0.02)
K2CO3 (0.05)
K2CO3 (0.05)
Cs2CO3 (0.05)
Cs2CO3 (0.05)
no catalyst
KCN (0.05)
KCN (0.05)
no catalyst
Results and Discussion
DMF (0.3)
DMF (2.0)
08C
08C
Reaction optimization
DMSO (0.3) rt
DMSO (0.3) rt
Both reaction steps from quinones to cyanoarenes (Scheme 1,
i and ii) were screened individually to improve the reaction
conditions and to develop a convenient and generally applica-
ble procedure. However, applicability as a one-pot procedure
was kept in mind. 9,10-Anthraquinone 1a was used for the sol-
vent and catalyst screening of the first reaction step
(Scheme 1, i) as conversion of this substrate was found to be
more ambitious than conversion of 1,4-benzoquinone 1b. The
results of this screening are given in Table 1.
DMF (0.3)
08C
DMSO (0.3) rt
DMSO (0.3) rt
no conversion
4 h
3 h
DMF (2.0)
DMF (2.0)
DMF (2.0)
08C
rt
rt
no conversion
[a] Reactions carried out on a scale of 0.5 mmol to 1.0 mmol. [b] Concen-
tration of starting material 1a given in brackets. [c] Reaction time until
full conversion of both carbonyl groups of starting material 1a, deter-
mined by GCMS measurements. [d] Incomplete conversion of cis isomer
into trans isomer.
Scheme 1. Individually screened reaction steps from quinones to cyanoar-
enes by using 9,10-anthraquinone 1a as the starting material: cyanohydrin
formation (i) (Table 1), reductive aromatization (ii) (Table 2); synthesis finally
carried out as a one-pot reaction (iii) (Table 3).
Initially, the temperature dependence of the reaction time
was investigated at otherwise identical conditions as in our
previous report (Table 1, top). It was found that the reaction
proceeds considerably faster at room temperature (rt) than at
08C and À158C; nevertheless, at all temperatures full conver-
sion of both carbonyl groups of starting material 1a was ob-
served by GCMS measurements. The experiments at lower
temperatures revealed that both the trans and the cis isomer
of 2 are formed, but the latter is converted to the trans isomer
as the reaction proceeds (stereochemistry confirmed by X-ray
diffraction, Figure 1). The rate of this isomerization strongly de-
pends on the reaction temperature. At À158C, the isomeriza-
tion remained incomplete even after a reaction time of 18 h.
When adding less trimethylsilyl cyanide (TMSCN, 2.6 equiv),
the reaction remained incomplete at room temperature. At
08C, full conversion was observed. A lower catalyst loading of
Figure 1. Molecular structure of the cyanohydrin intermediate 2. C (gray),
N (blue), O (red) and Si (yellow) atoms are represented by ellipsoids drawn
at the 50% probability levels, H atoms by white spheres of arbitrary radius.
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0.1 equivalents instead of 0.2 equivalents required longer reac-
tion times but still showed full conversion at room tempera-
ture. The same experiment was successfully carried out in tet-
rahydrofuran (THF) with a considerably longer reaction time of
30 h.
Table 2. Reagent, solvent, and temperature screening of the aromatiza-
tion step from 2 to 3a (Scheme 1, ii) by addition of 0.5 mmol reagent
(PCl3 or PBr3) and 0.25 mmol DMF to 0.5 mmol of 2 in 1 mL solvent.
Reagent
Solvent
T
Reaction time [h][a]
Isolated yield [%][b]
PCl3
PBr3
PBr3
PBr3
PBr3
PBr3
PBr3
MeCN
MeCN
MeCN
CH2Cl2
THF
rt
rt
08C
08C
08C
rt
5
2
8
55
71
78
55
–
In a subsequent catalyst screening, Cs2CO3 was found to per-
form almost equally as well as CsF. This finding is particularly
useful because of the difficult handling of CsF owing to its
strong hygroscopic behavior. Furthermore, K2CO3 also enabled
full conversion within 27 h.
31
@120
@120
@120
DMF
DMSO
–
–
rt
We attributed the long reaction time with K2CO3 to its low
solubility in acetonitrile (MeCN) and therefore tested dimethyl
formamide (DMF) and dimethyl sulfoxide (DMSO) as solvents.
In addition, we reduced the catalyst loading to 0.05 equiva-
lents or less. These reactions (except for the blank test)
showed full conversion in no longer than 3 h at 08C (DMF) and
20 min at room temperature (DMSO, m.p. 198C).
[a] Reaction time until full conversion of 2, determined by GCMS meas-
urements. [b] Isolation of 3a by dilution of the reaction mixture with
CH2Cl2 and subsequent filtration over a thick pad of silica with CH2Cl2 as
the eluent.
All of the above-mentioned catalysts are also known to act
as deprotection agents in desilylation reactions. To reduce the
required excess of TMSCN at room temperature, we tested
KCN as the catalyst. The application of this reagent is expected
to result in back-formation of TMSCN if desilylation occurs.
Indeed, by using only 0.05 equivalents KCN, we were able to
achieve full conversion to the cyanohydrin intermediate 2
even at room temperature and with as little as 0.05 equivalents
excess of TMSCN. DMF was used as the solvent because of its
expected suitability for one-pot reactions: some DMF was
found necessary for a successful aromatization in the one-pot
reactions in our previous work. However, to enable an efficient
aromatization, the amount of DMF has to be kept low.
By using these final conditions, the trans intermediate 2 was
isolated in yields of 84%. X-ray diffraction of single crystals
grown from fluorobenzene confirmed the expected stereo-
promising results, especially in MeCN (Table 2). By using PBr3
instead of PCl3 the yield of 3a was increased to 71% at room
temperature and 78% at 08C. Carrying out the reaction in di-
chloromethane (CH2Cl2) resulted in a yield of 55%. The reaction
in THF did not afford any product, although PBr3 has been
used as an aromatization reagent in THF before.[18] Aromatiza-
tion in DMF and DMSO was also not successful.
Further investigations focused on the one-pot conversion of
the quinones to cyanoarenes (Scheme 1, iii), because insuffi-
cient stability of (silylated) cyanohydrins has been observed
during work-up. The first reaction step was carried out by
using KCN as the catalyst and as little DMF as possible. The re-
action was diluted with MeCN for the second reaction step
and PBr3 was added. Without the dilution with MeCN no prod-
uct formation was achieved. When using DMSO for the first re-
action step, no cyanoarene formation was observed in the
second step.
¯
chemistry. Compound 2 crystallizes in space group P1. One
crystallographically unique molecule (Figure 1) is located on
a center of inversion, reflecting the trans configuration of the
molecule. The annulated ring system deviates distinctly from
planarity [distance of the central C4 atom to the least squares
(LS) plane defined by the atoms of the benzene ring:
0.0821(11) Š], in contrast to the cyclohexadiene analog,[17]
which is virtually flat. The tetrahedral angles around C4 are dis-
torted owing to ring strain [O-C-CN: 104.51(9)8, cyclohexadiene
analog: 104.95(8)8]. Neither inter-molecular non-classical hy-
drogen-bonding, nor p–p stacking was observed, owing to the
bulky TMS groups.
In contrast to the screening reactions in pure solvents, the
one-pot reactions had to be heated to 508C after the first reac-
tion step for efficient aromatization. Work-up was again ach-
ieved by diluting the reaction with CH2Cl2 and filtering over
a thick pad of silica with CH2Cl2 as the eluent.
The new conditions not only enabled significantly higher
yields for the conversion of both 9,10-anthraquinone 1a and
1,4-benzoquinone 1b to 9,10-dicyanoanthracene 3a and 1,4-
dicyanobenzene 3b than previously reported (3a: 27%, 3b:
30%), but also helped to reduce the reaction times and to
avoid the formation of inseparable byproducts in the reaction
towards 3b (Table 3).
In the first report on the synthesis of cyanoarenes by reduc-
tive aromatization, we described the possibility of avoiding the
formation of reaction byproducts by using PCl3 instead of
POCl3 as the aromatization reagent for the second reaction
step (Scheme 1, ii).[14] However, increasing amounts of DMF in
the solvent mixture spoil this effect. This is particularly obstruc-
tive as the addition of DMF was found to be necessary for the
aromatization step in one-pot reactions. Furthermore, the re-
sulting byproducts when converting 1,4-benzoquinone 1b into
1,4-dicyanobenzene 3b could not be separated by column
chromatography. We therefore tested the weaker Lewis acid
PBr3 for aromatization of intermediate 2 and indeed obtained
Synthesis of halogenated and borylated cyanoarenes and
their applications in Suzuki coupling reactions
Despite the promising properties of 9,10-dicyanoanthracenes,
aryl-substituted derivatives have only been reported in two
patents, which claim the (multi-step) synthesis of such com-
pounds.[19] This is attributed to the lack of easily accessible
halogenated dicyanoanthracene precursors for cross-coupling
reactions. So far, 2-chloro-9,10-dicyanoanthracene is the only
known halogenated dicyanoanthracene.[19a]
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the synthesis of substance libraries of aryl-substituted dicya-
noanthracenes. That way, the preparation of larger numbers of
different boronic acids or esters can be avoided.
Table 3. One-pot conversion of 9,10-anthraquinone 1a and 1,4-benzoqui-
none 1b to 9,10-dicyanoanthracene 3a (Scheme 1, iii) and 1,4-dicyano-
benzene 3b.[a]
The reactions from 4a to 7 represent a versatile general
strategy to aryl-substituted dicyanoanthracenes. Halogenated
anthraquinones like 4a and 4b can be prepared from amino-
and diamino-9,10-anthraquinones,[20] which are commercially
available with any possible substitution pattern.
Starting material Product Reagent Reaction time[b] Isolated yield [%]
1a
1a
1b
1b
3a
3a
3b
3b
PCl3
PBr3
PCl3
PBr3
3 h+2 h
3 h+3 h
44
53
10 min+30 min no pure product[c]
10 min+30 min 40
By using 2,5-dibromobenzoquinone 8, which can be easily
obtained on a multigram scale,[21] the novel procedure also al-
lowed for an efficient one-pot synthesis of 1,4-dibromo-2,5-di-
cyanobenzene 9 (Scheme 3, i). The short total reaction time
and the facile work-up by direct filtration over silica enabled
preparation from start to finish in less than 3 h. Suzuki cou-
pling with phenylboronic acid was again carried out in excel-
lent yields by applying the same reaction conditions as for the
coupling of 5a (Scheme 3, ii).
[a] Carried out on
a 0.50 mmol scale by using 2.05 equiv TMSCN,
0.05 equiv KCN, and 0.25 mL DMF for the first reaction step (at rt) and
adding 1.50 mL MeCN and 1.20 equiv reagent (PCl3 or PBr3) for the
second reaction step (at 508C). [b] Reaction time of first and second step.
[c] Inseparable byproduct.
Following the optimized one-pot protocol enabled the syn-
thesis of novel brominated and iodinated 9,10-dicyanoanthra-
cenes 5a and 5b from the respective anthraquinones
(Scheme 2, i). The work-up procedure, however, had to be
modified to achieve good yields for these weakly soluble com-
pounds: purification was carried out by repeated washing of
the solids with MeCN in the reaction flask. The crude products
were then collected by filtration and purified by trituration in
boiling toluene. Omitting the washing of the solids results in
impurities, which we assume originate from polymerization of
phosphoric side products and which cannot be removed by
trituration in toluene.
Scheme 3. i) Synthesis of 1,4-dibromo-2,5-dicyanobenzene 9 from 2,5-dibro-
mobenzoquinone 8; conditions: TMSCN, KCN, DMF, 40 min, rt; PBr3, MeCN,
1 h, 508C; 93 mg. ii) Suzuki coupling of 9 with phenylboronic acid; condi-
tions: [Pd(PPh3)4], aq. K2CO3, THF, argon, 6 h, reflux, 69 mg.
Synthesis of alkynyl-substituted cyanoarenes
Moving from aryl- to alkynyl-substituted 9,10-dicyanoanthra-
cenes, there has not been a single compound reported in the
scientific literature so far. In our opinion, this is also owing to
the lack of easily accessible halogenated dicyanoanthracene
precursors for cross-coupling reactions.
We first tried to synthesize alkynyl-substituted 9,10-dicyano-
anthracene 12 from the suitably substituted anthraquinone 11
(Scheme 4, ii), which was obtained from 4a in excellent yields
(Scheme 4, i). Indeed, by following the optimized one-pot pro-
tocol, 12 was obtained. However, impurities of starting materi-
al 11 could not be removed, neither by column chromatogra-
phy nor by crystallization.
Scheme 2. i) Synthesis of 2,6-dibromo- and 2,6-diiodo-9,10-dicyanoarenes 5a
and 5b; conditions: TMSCN, KCN, DMF, 3 h, rt; PBr3, MeCN, 3 h, 508C; 153/
140 mg. ii) Preparation of dicyanoanthraceneboronic ester 6 by cross-cou-
pling of 5a with bis(pinacolato)diboron; conditions: [Pd(PPh3)4], potassium
acetate, DMSO, argon, 6 h, 858C, 48 mg. iii and iv) Suzuki coupling of 5a
with phenylboronic acid (conditions: [Pd(PPh3)4], aq. K2CO3, THF, argon, 3 h,
reflux, 87 mg) and 6 with bromobenzene (conditions: [Pd(PPh3)4], aq. K2CO3,
THF, argon, 3 h, 658C, 15 mg).
Fortunately, the Sonogashira coupling approach using 5a as
the substrate also resulted in 12 (Scheme 4, iii) when using
a 5:1 mixture of THF and diisopropylamine as solvent.
Yields for the conversion of 1,4-dibromo-2,5-dicyanobenzene
9 to the corresponding alkynyl-substituted dicyanobenzene
derivative 13 (Scheme 4, iv) were high when using triethyla-
mine as the solvent. Single crystals of 13 were obtained by sol-
vent evaporation from a solution in chloroform and ethanol
and measured. Compound 13 crystallizes in space group P21/
c with one crystallographically unique molecule (Figure 2a) lo-
cated in a general position. The CꢀC-Si angles deviate signifi-
cantly from linearity [172.7(2)8 and 174.3(2)8]. The molecules
are arranged in layers parallel to (100), which connect through
the TMS groups (Figure 2b). Besides these van-der-Waals inter-
actions, no additional supramolecular features are observed.
Applying common Suzuki conditions for the coupling of 5a
with phenylboronic acid, we were able to demonstrate the
synthesis of 2,6-diphenyl-9,10-dicyanoanthracene 7 in excellent
yields (Scheme 2, iii). The same compound was obtained by
coupling of dicyanoanthraceneboronic ester 6 with bromoben-
zene (Scheme 2, iv). This formal exchange of the functional
groups is expected to be particularly useful if the boronic acid
or ester for a Suzuki coupling with 5a or 5b cannot be pre-
pared. Moreover, 6, which we obtained by coupling 5a with
bis(pinacolato)diboron (Scheme 2, ii), is useful, for example, for
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Facilitating the synthesis of 6,13-dicyanopentacene
In 2011, Katsuta et al. reported a five-step synthesis of 6,13-di-
cyanopentacene 15 starting from pentacene.[3] In their opinion,
a preparation of this air-stable ambipolar field-effect transistor
material by other research groups has “not been achieved due
to […] synthetic difficulties”.
We aimed for a simplified preparation of this compound
from 6,13-pentacenequinone 14 by following the optimized
one-pot protocol (Scheme 5). However, we experienced syn-
thetic difficulties:
Hardly any formation of the cyanohydrin intermediate was
observed when using KCN, CsF, Cs2CO3, or K2CO3 as the cata-
lyst. This was overcome by adding a suspension of LiCN in
TMSCN instead. The suspension was prepared by addition of
n-butyllithium to a slightly increased excess of TMSCN at room
temperature and was added to precooled 6,13-pentacenequi-
none 14 at 08C with some DMF. After 6 h at 08C complete in-
termediate formation was observed by TLC analysis.
Scheme 4. i) Synthesis of 2,6-bis[(trimethylsilyl)ethynyl]-9,10-anthraquinone
11; conditions: trimethylsilylacetylene, [PdCl2(PPh3)2], CuI, triethylamine,
argon, overnight, 408C; 368 mg. ii and iii) Preparation of 12 from 11 (condi-
tions: TMSCN, KCN, DMF, 24 h, rt; PBr3, MeCN, overnight, 508C; 42 mg, *cor-
rected yield) and from 5a (conditions: trimethylsilylacetylene, [PdCl2(PPh3)2],
CuI, 5:1 THF/diisopropylamine, argon, 1 h, reflux; 183 mg). iv) Synthesis of
1,4-dicyano-2,5-bis[(trimethylsilyl)ethynyl]benzene 13; conditions: trimethyl-
silylacetylene, [PdCl2(PPh3)2], CuI, triethylamine, argon, 6 h, 508C; 120 mg.
Scheme 5. Synthesis of 6,13-dicyanopentacene 15; conditions: TMSCN,
nBuLi, argon, 15 min, rt; 14, DMF, argon, 6 h, 08C; PBr3, CH2Cl2, argon, over-
night, 08C; 49 mg.
Yields for the subsequent aromatization were very low at
508C owing to back-formation of 14. However, no aromatiza-
tion was observed when we maintained the temperature at
08C for increased stability of the cyanohydrin intermediate. By
adding CH2Cl2, which is also a suitable solvent for the aromati-
zation (Table 2), instead of MeCN for increased solubility of the
intermediate, significantly better results were obtained. Even at
08C full conversion was observed overnight.
Purification was achieved by evaporation of the solvent and
volatile side products and subsequent column chromatogra-
phy. By following this protocol, 6,13-dicyanopentacene 15 was
obtained in yields of 30% (Katsuta et al.: 26% overall yield).
The novel procedure enables us to obtain 15 in a single re-
action. Furthermore, costs for the starting material 14 are sig-
nificantly lower than for pentacene, which was used by Katsuta
et al. In general, we have demonstrated the great potential of
this reaction sequence in the synthesis of large cyanoarenes
for applications in organic electronics.
Single crystals of 15 were grown from a solution in a mixture
of petroleum ether (PE) and CH2Cl2. The long dark needles
were subjected to X-ray diffraction. Compound 15 crystallizes
¯
in space group P1. One crystallographically unique molecule
(Figure 3a) is located on a center of inversion. As expected,
the pentacene core is virtually flat, with a largest deviation
from the LS plane of 0.0222(8) Š for the C1 atom. The CN
group is slightly inclined with respect to the pentacene core
[distance to LS plane: N1: 0.1677(8) Š, C12: 0.0841(8) Š; angle
CꢀN to LS plane: 4.19(6)8]. The molecules are stacked in
Figure 2. a) Molecular structure and b) packing of 1,4-dicyano-2,5-bis[(trime-
thylsilyl)ethynyl]benzene 13 viewed down [010]. Atom designations as in
Figure 1.
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Figure 3. a) Molecular structure, b) rods of p-stacked molecules, and c) packing of 6,13-dicyanopentacene 15 viewed down [100]. d) Packing of 5,12-dicyano-
naphthacene viewed down [100] (coordinates taken from Katsuta et al.[3]). Atom designations as in Figure 1.
a face-to-face slipped manner to rods extending along the
[100] direction (Figure 3b). The small distance between the LS
planes of 3.385 Š indicates significant p–p interactions. Thus,
15 exhibits a packing motif that is desirable for organic elec-
tronics applications.[1b] The rods are arranged in a brick wall
pattern (Figure 3c), where adjacent rods are offset along the
[100] direction and the CN groups protrude into the free space
between adjacent rods. 5,12-Dicyanonaphthacene crystallizes
in an analogous structure (Figure 3d) with slightly enlarged LS
plane distances of 3.392 Š.[3]
C2/c. One crystallographically unique molecule (Figure 4a) is
located on a twofold rotation axis. In contrast to 15, the CN
groups are virtually coplanar with the phenanthrene ring
system [distance to LS plane: N1: 0.0044(10) Š, C8: 0.0029(9) Š;
angle CꢀN to LS plane: 0.08(7)8]. The molecules are stacked in
a face-to-face slipped manner to rods extending along the
[001] direction (Figure 4b). Adjacent molecules are related by
inversion and therefore the CN groups face opposite direc-
tions. The distance between the LS planes defined by the
phenanthrene atoms of the adjacent molecules is 3.414 Š,
larger than in the case of 15. The rods are arranged in a check-
erboard pattern, where adjacent rods are offset along the [001]
direction (Figure 4c).
Conversion of ortho-quinones to cyanoarenes
Considering the effort for the published syntheses of 9,10-di-
cyanophenanthrene 17,[11,22] it becomes clear that a direct con-
version of ortho-quinones to cyanoarenes is a valuable exten-
sion of the reaction scope of the presented one-pot protocol.
We were able to carry out such a synthesis of 17 from 9,10-
phenanthrenequinone 16 (Scheme 6) by using the same opti-
mized reaction protocol as for the preparation of dicyanoan-
thracene 3a. The achieved yields were similar to the synthesis
of 3a. Crystals of 17 were grown from toluene and subjected
to X-ray diffraction. Compound 17 crystallizes in space group
Conclusion
By using 9,10-anthraquinone as a model compound, we were
able to develop a convenient and reliable one-pot procedure
for the synthesis of cyanoarenes from quinones.
Halogenated cyanoarenes can be synthesized, purified, and
used for cross-coupling reactions despite the low solubility of
some of these compounds after minor adaptation of the pro-
cedure.
Preparation of alkynyl-substituted cyanoarenes is possible
either by Sonogashira coupling of the halogenated cyanoar-
enes or directly from the respective quinones. However, purifi-
cation is difficult after synthesis by the latter approach.
6,13-Dicyanopentacene can be obtained from 6,13-pentace-
nequinone at lower temperatures when using LiCN as the cata-
lyst. Single crystals of this compound (and three other target
molecules) were obtained by solvent evaporation. X-ray diffrac-
Scheme 6. Synthesis of 9,10-dicyanophenanthrene 17; conditions: TMSCN,
KCN, DMF, 1 h, rt; PBr3, MeCN, overnight, 508C; 58 mg.
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General procedure for the synthesis of readily soluble cya-
noarenes (3a, 3b, 9, 12, 17)
Starting material (1.00 equiv) and KCN (0.05 equiv) were suspend-
ed/dissolved in dry DMF (0.5 mLmmolÀ1 starting material) in
a sealed reaction vial equipped with a septum. TMSCN (2.05 equiv)
was added dropwise and the resulting mixture stirred at room
temperature until full conversion to the cyanohydrin intermediate.
The reaction was then diluted with dry MeCN (3.0 mLmmolÀ1 start-
ing material). PBr3 (1.20 equiv) was added in one portion and the
vial was heated to 508C until full conversion of the intermediate
occurred. The reaction was then allowed to cool to room tempera-
ture and CH2Cl2 (about 8 mLmmolÀ1 starting material) was added.
Direct filtration over a thick pad of silica (conditioned with CH2Cl2
and some drops of MeCN) with CH2Cl2 as the eluent afforded the
pure product after evaporation of the solvent.
General procedure for the synthesis of weakly soluble cya-
noarenes (5a, 5b)
Starting material (1.00 equiv) and KCN (0.05 equiv) were mixed
with dry DMF (0.5 mLmmolÀ1 starting material) in a sealed reaction
vial equipped with a septum. TMSCN (2.05 equiv) was added drop-
wise to the slurry and the resulting mixture stirred at room tem-
perature until full conversion to the cyanohydrin intermediate. The
reaction was then diluted with dry MeCN (3.0 mLmmolÀ1 starting
material). PBr3 (1.20 equiv) was added in one portion and the vial
was heated to 508C until full conversion of the intermediate oc-
curred. The reaction was then allowed to cool to room tempera-
ture. MeCN (about 4 mLmmolÀ1 starting material) was added and
stirring was stopped. After settling of the solid, the overlaying
liquid was removed, replaced with fresh MeCN, and stirred again
for 1 min. This was repeated another three times before the solid
was filtered off and washed once with MeCN. Trituration in boiling
toluene (50 mLmmolÀ1 starting material) and subsequent filtration
afforded the pure product.
Procedure for the synthesis of larger cyanoarenes (15)
n-Butyllithium (0.10 equiv, 2.5m in hexanes) was added to rigorous-
ly stirred TMSCN (2.20 equiv) in a sealed reaction vial equipped
with a septum under an argon atmosphere at room temperature.
After 15 min, the resulting suspension was added dropwise to
a second reaction vial, equipped with a septum under an argon at-
mosphere, charged with 6,13-pentacenequinone 14 and precooled
to 08C. Dry DMF (0.5 mLmmolÀ1 starting material) was used to
fully transfer the residues of the suspension. The resulting mixture
was stirred at 08C until full conversion to the cyanohydrin inter-
mediate occurred. After 6 h, the reaction was diluted with dry
CH2Cl2 (3.0 mLmmolÀ1 starting material). PBr3 (1.20 equiv) was
added in one portion and the reaction stirred overnight, still kept
at 08C. Solvents and volatile side products were then evaporated
in vacuo and the residue was purified by column chromatography
(PE/CH2Cl2, gradient from 40% to 70% CH2Cl2) to afford 6,13-dicya-
nopentacene 15 as a pure dark solid.
Figure 4. a) Molecular structure, b) rods of p-stacked molecules, and c) pack-
ing of 9,10-dicyanophenanthrene 15 viewed down [001]. Atom designations
as in Figure 1.
tion revealed a crystal structure promising for OFET applica-
tions.
Furthermore, conversion of ortho-quinones to cyanoarenes
is possible by following the same protocol as for the conver-
sion of the para-quinones.
CCDC 1443832–1443835 contain the supplementary crystallograph-
ic data for this paper. These data are provided free of charge by
The Cambridge Crystallographic Data Centre.
Experimental Section
Acknowledgements
Experimental and instrumental details for the synthesis and charac-
terization of all compounds are provided in the Supporting Infor-
mation.
We gratefully thank Emmanuel Reichsçllner, Kristina Hager,
Vera Kunz, and Patrick Fritz for contributing to synthetic ex-
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Keywords: arenes · cyanides · fused-ring systems · quinones ·
reductive aromatization
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Received: January 1, 2016
Published online on March 1, 2016
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