O- and N-alkylated diketopyrrolopyrrole derivatives

Article abstract of DOI:10.1016/j.tetlet.2011.08.113

The alkylation of 3-phenyl-6-(2-thienyl)pyrrolo[3,4-c]pyrrole-1,4-dione with 1-bromo-2-ethylhexane was performed. Besides the expected N,N0-dialkylated product, both possible N,O-dialkyl derivatives were isolated and identified for the first time. The pos

Full text of DOI:10.1016/j.tetlet.2011.08.113

Tetrahedron Letters 52 (2011) 5769–5773
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
O- and N-alkylated diketopyrrolopyrrole derivatives
a,b,
⇑
a,c
b
a
c
ˇ
ˇ ˇ ˇ ˇ
, Zdenek Eliáš , Antonín Lycka , Stanislav Lunák Jr. , Jan Vynuchal ,
Štepán Frebort
ˇ a,b
Lubomír Kubác , Radim Hrdina a, Ladislav Burgert a
a Faculty of Chemical Technology, University of Pardubice, Studentská 573, CZ-532010 Pardubice, Czech Republic
b Centre for Organic Chemistry Ltd, Rybitví 296, CZ-533 54, Czech Republic
c Synthesia, a. s., Semtín 103, CZ-532 17 Pardubice, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
The alkylation of 3-phenyl-6-(2-thienyl)pyrrolo[3,4-c]pyrrole-1,4-dione with 1-bromo-2-ethylhexane
was performed. Besides the expected N,N0-dialkylated product, both possible N,O-dialkyl derivatives
were isolated and identified for the first time. The position of the alkyl substituents in all three dialkylat-
ed isomers was determined by 2D 1H and 13C NMR spectroscopy.
Received 6 May 2011
Revised 5 August 2011
Accepted 19 August 2011
Available online 26 August 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
1,4-Diketo-pyrrolo[3,4-c]pyrroles
Alkylation
O-Alkyl
NMR
Symmetrical pyrrolo[3,4-c]pyrrole-1,4-diones1 (also known as
1,4-diketo-pyrrolo[3,4-c]pyrroles, abbreviated DPP) with the same
(substituted) aryl group at the 3 and 6 positions have been used as
high-performance pigments for lacquers and plastics.2 The consid-
erably higher solubility of their N,N0-dialkylated derivatives3 has
led to broader applications, especially in electronics. Among these,
DPPs with substituted 2-thiophenes at the 3 and 6 positions as a
component of organic photovoltaic devices of both bulk hetero-
junction (BHJ)4 and dye-sensitised solar cells (DSSC)5 types seem
to be the most promising.
by a more straightforward route from ethyl-4,5-dihydro-5-oxo-2-
aryl(1H)pyrrole-3-carboxylate and thiophene-2-carbonitrile in
analogy with other unsymmetrical DPP derivatives.11
The reaction was carried out according to Scheme 112 and was
monitored by TLC.13 No starting compound 1 was detected in the
final reaction mixture. The reaction product was subjected to col-
umn chromatography14 and six fractions a–f were separated. The
TLC retardation factors of the main components are summarized
in Table 1.
The purity of fractions b and f was insufficient for further anal-
ysis. The four main products were contained in fractions c–f and
that the two minor products were in fraction a and b. MS analysis15
of fraction a gave a molecular peak (m/z = 406) corresponding to a
monoalkylated derivative, while the molecular peaks of fractions
c–e (m/z = 518) corresponded to dialkylated compounds. The iden-
tification of the major products in these four fractions was accom-
plished by NMR techniques.16
Sample a was dissolved in hexadeuteriodimethyl sulfoxide. The
existence of one CONH proton and appropriate relative intensities
of the aromatic protons related to the 2-ethylhexyl group were
consistent with the MS data giving evidence for a mono substituted
derivative. The proton signals were assigned using gradient-
selected (gs)-COSY. Protonated carbons were assigned by gs-HMQC
and quaternary carbons by gs-HMBC.17 There were four possible
locations for the 2-ethylhexyl group in the monoalkylated com-
pound (taking N(H)–C@O versus N@C(OH) tautomerism into ac-
count), two having an NCH2CH(C2H5)C4H9 arrangement and two
with an OCH2CH(C2H5)C4H9 arrangement. It is well known that
there are large differences in NCH2– versus OCH2– resonances.18
Base-catalysed N,N0-dialkylation is usually carried out as a one-
pot reaction, in which the N-monoalkylated intermediate is only
rarely isolated and reported,6 although recently it was found to be
efficient in selective anion sensing. The possibility of O-alkylation
was not considered in the original papers,3,7 but was recently pro-
posed8 as a possible explanation for the side product formation dur-
ing alkylation of 3,6-di-(2-thienyl)pyrrolo[3,4-c]pyrrole-1,4-dione.
However, no experimental evidence for this was provided (only
on the basis of the general behaviour of amides during alkylation9).
The aim of this Letter is to prove or rebut this idea.
We used 3-phenyl-6-(2-thienyl)pyrrolo[3,4-c]pyrrole-1,4-dione
(1) as the initial substrate, in order to take into account the eventual
different effects of the phenyl and thiophene rings at positions 3 and
6 on alkylation, and 1-bromo-2-ethylhexane (2) as the alkylating
agent, as it is frequently used in BHJ devices4a to increase the solubil-
ity of DPP derivatives. Compound 1 is known,10 butwe synthesized it
⇑
Corresponding author.
E-mail address: stepan.frebort@cocltd.cz (Š. Frebort).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.08.113

5770
Š. Frebort et al. / Tetrahedron Letters 52 (2011) 5769–5773
CH3
CH3
O
O
1
O
4
Br 2
6
3
, NMP, K 2CO3, 80 °C
5
HN
HN
NH
N
2N
NH
+
+
2
5
4
1
3
6
O
O
O
S
S
S
H3C
H3C
1
3
4
CH3
CH3
CH3
CH3
O
4
3
O
4
3
2N
NH
O
2N
N
6
5
1
1
+
+
+
+
6
5
5
HN
1
N 2
O
6
S
4
O
3
S
H3C
H3C
O
S
H3C
H3C
5
7
6
CH3
CH3
CH3
CH3
CH3
CH3
O
4
O
1
O
3
6
3
4
2N
N 5
5N
N
2N
N
+
+
+
2
5
1
4
1
6
3
6
O
O
O
S
S
S
H3C
H3C
H3C
H3C
H3C
H3C
8
10
9
Scheme 1. Possible products 3–7 of the alkylation of compound 1. Products in brackets were not isolated.
group had to bound to the nitrogen closer to the thiophene ring.
Thus, sample a corresponded to structure 3, that is, 2-(2-ethyl-
hexyl)-6-phenyl-3-(2-thienyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-
dione. No further monoalkylated derivatives were separated in
pure form.
Table 1
TLC retardation factors of the compounds under study
Fraction Compound Rf
hexane/EtOAc
Rf
Rf
CH2Cl2
hexane/acetone
(3:2)
(10:1)
The three other samples were dissolved in deuteriochloroform.
Sample c had two NCH2CH(C2H5)C4H9 fragments in its structure,
the observed 13C NMR chemical shifts of NCH2– being d 45.7 and d
45.0. The long-range correlations of the NCH2 (d 3.82) and thiophene
C(3)H protons with carbon C(3) [via 3J(13C, 1H)] indicated that the
NCH2CH(C2H5)C4H9 group was attached to the nitrogen closer to
thiophene ring, while long-range correlations of the NCH2 [d(1H)
= 3.96] and ortho phenyl protons with carbon C(6) via [3J(13C, 1H)]
proved that the NCH2CH(C2H5)C4H9 group was at position 5. Thus
sample c was compound 7, 2,5-bis(2-ethylhexyl)-3-phenyl-6-(2-
thienyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione.
f
—
9
8
7
—
3
0.85
0.77
0.74
0.66
—
—
—
—
—
0.70
0.62
0.80
0.71
0.60
0.58
0.21
0.20
e
d
c
b
a
—
The observed 13C NMR chemical shift (d 45.2) indicated clearly that
the NCH2CH(C2H5)C4H9 group was present in the compound stud-
ied, that is, either compound 3 or 4. The determination of the exact
NCH2CH(C2H5)C4H9 position was based on a detailed analysis of
the 2D 1H–13C HMBC spectrum: we observed long-range correla-
tions between both of the NCH2 and thiophene C(3)H protons with
the carbon C(3) [via 3J(13C, 1H)] and hence the NCH2CH(C2H5)C4H9
Samples d and e were found to possess one NCH2CH(C2H5)C4H9
fragment and one OCH2CH(C2H5)C4H9 motif each from the 13C
NMR chemical shifts of the NCH2– (d 44.8; 45.6) and OCH2– (d

Š. Frebort et al. / Tetrahedron Letters 52 (2011) 5769–5773
5771
Table 2
indicated that the NCH2CH(C2H5)C4H9 group was bound to the
nitrogen closer to the thiophene group in the compound 9. Thus,
sample e is compound 9, that is, 2-(2-ethylhexyl)-4-(2-ethyl
hexyloxy)-6-phenyl-3-(2-thienyl)pyrrolo[3,4-c]pyrrol-1(2H)-one.
All the proton and carbon chemical shifts are reported in Table 2.
The composition of fraction f did not allow convincing determi-
nation of the entire structure of its main component 10. Besides
impurities, the 13C NMR chemical shifts of the OCH2– group were
detected, but no 13C NMR chemical shifts of an NCH2– moiety were
evident.
Chromatographic fractions a, c, d and e were strongly fluores-
cent. The fluorescence excitation spectra of fractions a, c and d
(Fig. 1) coincide with the absorption spectra. Thus compounds 3,
7 and 8 are the only coloured components of the corresponding
fractions. On the other hand, a considerable difference between
absorption and excitation of fraction e was observed (Fig. 2).
Consequently, compound 9 in fraction e may not be the only com-
ponent absorbing in the visible region.
In order to understand better the differences between the N-
and O-alkylated derivatives, molecular modelling based on density
functional theory (DFT)19,20 was carried out. The optimized geom-
etries of the four model isomeric dialkylated derivatives 7m–10m,
in which a methyl was inserted instead of 2-ethylhexyl as the N- or
O-substituent, are shown in Figure 3. O-Methylation did not force
the molecule to be non-planar, while the N-methyl steric effects
rotate considerably the phenyl, but not the 2-thienyl, out of the
diketo-pyrrolo-pyrrole plane. The computed excitation energies
are summarized in Table 3. Only one strong (allowed) spectral
transition of HOMO ? LUMO character falls into the visible area,
covering a relatively narrow region (511 – 531 nm) for all four iso-
mers. On the other hand, the energy of weak (forbidden) transi-
1H and 13C chemical shifts in compounds 3 and 7–9. The numbering of atoms is
according to standard nomenclature
3a,b
13C
7c,d
13C
8c,e
13C
9c,f
13C
1H
1H
1H
1H
1
—
—
—
—
—
—
4.00
1.78
—
161.4
139.4
109.3
162.0
144.6
108.3
45.2
—
—
—
—
—
—
3.82
1.51
—
162.1
146.9
107.9
162.3
142.1
109.5
45.0
—
—
—
—
—
—
3.75
1.48
—
162.2
149.6
111.2
167.7
150.7
116.0
44.8
—
—
—
—
—
—
3.93
1.79
—
162.2
143.6
114.8
165.9
156.2
112.4
45.6
3
3ag
4
6
6ag
N(2)CH2
N(2)CH2CH
C(3)-1
C(3)-2
C(3)-3
C(3)-4
O(4)CH2
O(4)CH2CH
N(5)CH2
N(5)CH2CH
C(6)-1
C(6)-2
C(6)-3
C(6)-4
38.5
127.7
38.5
129.7
38.7
129.0
39.0
129.6
8.82 134.6
7.44 128.5
8.12 132.6
7.74 128.6
7.50 128.7
7.50 130.6
7.66 128.8
7.51 128.6
7.51 130.9
8.26 135.3
7.23 127.9
7.70 131.9
—
—
—
—
—
129.2
—
—
3.96
1.82
—
—
—
45.7
39.0
128.6
4.38
1.63
—
—
—
72.0
39.1
—
—
138.5
4.54
1.81
—
—
—
72.2
39.2
—
—
133.7
—
—
—
—
8.52 127.7
7.62 129.2
7.62 132.1
8.90 135.5
7.27 128.4
7.65 130.8
8.47 131.8
7.20 128.9
7.52 130.3
8.59 128.7
7.48 128.4
7.42 130.5
a
DMSO-d6.
b
d(1H) = 1.15–1.40 (CH2) and 0.80–0.90 (CH3), d(13C) = 29.7, 27.8, 23.2, 22.5 (all
CH2), 13.9 and 10.4 (both CH3).
c
CDCl3.
d
d(1H) = 0.65–1.45, d(13C) = 30.3, 30.2, 28.4, 28.2, 23.7, 23.6, 23.0, 22.8 (all CH2),
13.9, 13.8, 10.3 and 10.2 (all CH3).
e
d(1H) = 0.65–1.46 , d(13C) = 30.5, 30.3, 28.9, 28.2, 23.9, 23.6, 23.0, 22.8 (all CH2),
14.0, 13.9, 11.1 and 10.3 (all CH3).
f
d(1H) = 0.65–1.46, d(13C) = 30.6, 30.1, 29.0, 28.7, 24.0, 23.5, 23.0, 22.9 (all CH2),
14.0, 13.9, 11.2 and 10.4 (all CH3).
g
The assignment can be interchanged.
tions of HOMO-1
? LUMO nature depend strongly on the
position of the methyl substituent, ranging from 388 nm for 7m
to 437 nm and 446 nm for 8m and 9m and finally 536 nm for the
O,O-disubstituted compound 10m, for which it is the lowest en-
ergy transition, respectively. The computed excitation maxima of
model compounds 7m–9m are in quite good agreement with the
experimental maxima of compounds 7–9 (Table 3). It is known that
resolution of the vibronic structure in absorption and fluorescence
spectra strongly depend on planarity, being totally unresolved for
N,N-dialkylated symmetrical 3,6-diphenyl-DPPs6a and well-re-
solved for 3,6-dithienyl-DPPs.21 The considerably better resolved
72.0; 72.2) moieties, respectively. The differentiation was again
based on the procedure mentioned above: in sample d, we observed
long-range correlations of the NCH2 and ortho phenyl protons with
carbon C(3) [via 3J(13C, 1H)] and thus, the NCH2CH(C2H5)C4H9 group
had to be bound to the nitrogen closer to the phenyl group. Thus
sample d corresponded to compound 8, that is, 2-(2-ethylhexyl)-
4-(2-ethylhexyloxy)-3-phenyl-6-(2-thienyl)pyrrolo[3,4-c]pyrrol-
1(2H)-one. By contrast, the long-range correlations of the NCH2
and thiophene C(3)H protons with carbon C(3) [via 3J(13C, 1H)]
Figure 1. Absorption, fluorescence excitation and emission spectra of compound 8 in DMSO.

5772
Š. Frebort et al. / Tetrahedron Letters 52 (2011) 5769–5773
Figure 2. Absorption, fluorescence excitation and emission spectra of compound 9 in DMSO.
Table 3
Theoretical excitation energies of 7m–10m recomputed to wavelengths (kex), the
experimental fluorescence excitation (kex) and emission (kem) maxima of compounds
7–9
Derivative
Theoretical kex (nm)
Experimental
kex (nm) kem (nm)
HOMO-1 ? LUMO
HOMO ? LUMO
7m
8m
9m
10m
388 (0.000)
437 (0.007)
446 (0.008)
536 (0.001)
514 (0.558)
511 (0.631)
524 (0.637)
531 (0.753)
493a
504
527
—
555
574
551
—
a
A maximum corresponds to a 0–1 vibronic transition.
doubt. Only one monoalkylated product (N-alkylated compound
3) was confirmed, which may be the intermediate for compounds
7 and 10. Compound 8 should arise from compound 4, which may
be present in non-analysable fraction b. As no O-alkylated and
O,O0-dialkylated products were identified, it is impossible to con-
clude whether direct O-alkylation of compound 1 can proceed or
whether O-alkylation of N-alkylated compounds is the only path-
way to obtain N,O-dialkylated derivatives.
Acknowledgements
The support of this work by the Grant Agency of the Czech
Republic under project P205/10/2280 is acknowledged. J.V. and
Z.E. were supported by a project of the Grant Agency of the Czech
Republic No. 203/08/1594.
Figure 3. Optimized geometries of four model dimethylated isomers (dihedral
References and notes
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12. 6-Phenyl-3-(2-thienyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione
(1)
(3.0 g,
0.010 mol) was suspended in NMP (150 ml). The mixture was heated and
maintained at 80 °C during the reaction. K2CO3 (5.6 g, 0.041 mol) was added
and the mixture became darker and lost its yellow fluorescence. 1-bromo-2-
ethylhexane (2) (17.9 g, 0.093 mol) was added slowly over 48 h. After cooling
to room temperature, the mixture was poured into demineralized H2O (1 L) to
form a stable emulsion. The emulsion was extracted with CHCl3 (3 ꢀ 100 mL).
Separation of the phases was possible only after addition of NaCl to the H2O
phase. The organic solution was dried over MgSO4 and the CHCl3 was removed
by distillation. The liquid residue was a mixture of alkylated product and
unreacted 1-bromo-2-ethylhexane.
13. ALUGRAM SIL/UV254, silica gel 60, mobile phase = hexane/EtOAc, 10:1 and
hexane/acetone, 3:2.
14. The liquid mixture of alkylated products and unreacted alkylating agent was
adsorbed onto silica gel (MERCK 9385, 230–400 mesh) which was added to the
top of a previously prepared column (4.5 cm in diameter, 30 cm high). Hexane/
EtOAc (20:1) mixture was used for elution of the first group of fractions f, e, d, c
at the beginning of separation. After elution of the first group of products, the
mobile phase was enriched with EtOAc to a final ratio of 2:1. This allowed
elution of samples b and a. All collected fractions were dried over anhydrous
CaCl2 and the solvents were evaporated.
21. (a) Tamayo, A. B.; Tantiwiwat, M.; Walker, B.; Nguyen, T. Q. J. Phys. Chem. C
2008, 112, 15543; (b) Bürckstümmer, H.; Weissenstein, A.; Bialas, D.;
Würthner, F. J. Org. Chem. 2011, 76, 2426.