Pyrrolo[2,3-d:5,4-d′]bisthiazoles: Alternate Synthetic Routes and a Comparative Study to Analogous Fused-Ring Bithiophenes

Article abstract of DOI:10.1021/acs.joc.7b02570

New synthetic methods have been developed for the preparation of 4-alkyl- and 4-aryl-pyrrolo[2,3-d:5,4-d′]bisthiazole (PBTz) building blocks from 2,4-thiazolidinedione. The resulting PBTz products have been fully characterized via structural, electronic, and optical methods, thus allowing full comparison to the previously reported dithieno[3,2-b:2′,3′-d]pyrrole (DTP) analogues. Such comparisons then allow a detailed discussion of the relative electronic effects of the various methods utilized to tune the properties of the parent DTP building block.

Full text of DOI:10.1021/acs.joc.7b02570

Article
Cite This: J. Org. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/joc
Pyrrolo[2,3‑d:5,4‑d′]bisthiazoles: Alternate Synthetic Routes and a
Comparative Study to Analogous Fused-Ring Bithiophenes
Eric J. Uzelac, Casey B. McCausland, and Seth C. Rasmussen*
Department of Chemistry and Biochemistry, North Dakota State University, NDSU Dept. 2735, P.O. Box 6050, Fargo, North
Dakota 58108-6050, United States
*
S Supporting Information
ABSTRACT: New synthetic methods have been developed for the
preparation of 4-alkyl- and 4-aryl-pyrrolo[2,3-d:5,4-d′]bisthiazole
(
PBTz) building blocks from 2,4-thiazolidinedione. The resulting
PBTz products have been fully characterized via structural,
electronic, and optical methods, thus allowing full comparison to
the previously reported dithieno[3,2-b:2′,3′-d]pyrrole (DTP)
analogues. Such comparisons then allow a detailed discussion of
the relative electronic effects of the various methods utilized to tune
the properties of the parent DTP building block.
INTRODUCTION
inductive effects to tune the corresponding HOMO or LUMO
energies, while the central placement of the side chains allows
the use of fairly bulky groups with limited steric interactions
that can lead to reduced backbone planarity in the resulting
■
Conjugated organic materials continue to receive considerable
fundamental and technological interest, with particular focus
on their development for applications such as sensors,
electrochromic devices, field-effect transistors (FETs), organic
photovoltaics (OPVs), and organic light-emitting diodes
5
,6
materials.
Of the bridged 2,2′-bithiophenes given in Chart 1, the
nitrogen-bridged DTP building blocks have drawn increased
attention and have been incorporated into a variety of
materials to give high charge carrier mobilities, enhanced
solution and solid-state fluorescence, and materials with
1
−3
(
OLEDs).
One of the many strengths of these materials
has been the ability to tune their desirable electronic and
optical properties at the molecular level via synthetic
modification. In this respect, thiophene-based materials have
been found to be especially popular due to their ease of
synthetic manipulation. One popular approach to modulat-
ing thiophene-based materials through structural modification
has been the application of fused-ring units, particularly fused
6
−9
reduced and low band gaps.
One limitation of traditional
2
,3
N-alkyl- and N-arylDTP building blocks, however, is the high
energy of the DTP HOMO, which limits stability and the
effective application of DTP-based materials to various devices.
As a solution to this limitation, a new class of DTPs
incorporating N-acyl groups were introduced by Evenson
and Rasmussen in 2010, which exhibit stabilized HOMO and
2
(
,2′-bithiophenes such as cyclopenta[2,1-b:3,4-b′]dithiophene
2−6 3−5
CDT),
silolo[3,2-b:4,5-b′]dithiophene (SDT),
2−9
dithieno[3,2-b:2′,3′-d]pyrrole (DTP),
and phospholo[3,2-
3
−5
8
,9
b:4,5-b′]dithiophene (PDT),
germolo[3,2-b:4,5-b′]-
and arsolo[3,2-b:4,5-b′]-
as shown in Chart 1. The fused-
LUMO energy levels. An alternate approach to stabilize the
HOMO energy levels of such nitrogen-bridged systems was
3
,10−12
dithiophene (GDT),
13−15
dithiophene (ADT)
16
also introduced by Heeney and co-workers in 2010, which
replaced the thiophene backbone with thiazole to generate the
Chart 1. Fused 2,2′-Bithiophene Building Blocks
analogous pyrrolo[2,3-d:5,4-d′]bisthiazole (PBTz, Chart
16−19
2
).
It must be pointed out that all previous reports on PBTz
have given this fused-ring unit the name pyrrolo[3,2-d:4,5-
1
6−19
d′]bisthiazole,
which actually describes the inverted
isomer referred to as iso-PBTz here (Chart 2). Such oversights
can be easy to miss, particularly for those less familiar with the
subtleties of fused-ring nomenclature, and even the current
authors initially overlooked this inversion and also used the
ring nature of these species enhances the planar nature of the
ground state, leading to improved conjugation, increased
delocalization, and lower band gaps, while also reducing
contributions of interannular torsional vibrations, potentially
leading to increased emission quantum yields. In addition, the
Received: October 10, 2017
bridging unit (ER or ER ) can modulate the electronics via
2
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.joc.7b02570
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The Journal of Organic Chemistry
Article
Chart 2. Pyrrolobisthiazole Building Blocks
synthetic methods of both Heeney and Marder began with the
commercially available 2-bromothiazole (Scheme 2).
1
6−19
Scheme 2. Previous Synthetic Routes to TIPS-Protected
PBTz Units
incorrect nomenclature in previous conference presentations.
In order to assist the field with such complexities, an
educational guide to fused-ring nomenclature has been
20
recently published. Although the synthesis of iso-DTP
21,22
analogues have been recently described,
blocks have not yet been reported.
iso-PBTz building
Although the reported PBTz-based materials do successfully
16−19
exhibit the desired stabilization,
the electronic and optical
properties of the PBTz units themselves have not been
significantly characterized. To date, the only such reported
characterization includes the electrochemistry of the triisopro-
pylsilyl (TIPS) protected 4-dodecylPBTz, as well as that of the
18
dibromoPBTz derivatives. As such, this makes it difficult to
compare the two relative approaches for stabilization (N-
acylDTP vs PBTz) or to accurately quantify the electronic
effects of the substitution of thiophene by thiazole.
In order to better understand the contributing structure−
function relationships, and to make these new PBTz building
blocks more readily available, the current report provides new
synthetic methods and full optical and electronic character-
ization of the resulting PBTz products. In the process, the
known family of PBTz building blocks has been suitably
expanded, including the first reported examples of N-arylPBTz
units. Lastly, the optical, electronic, and structural properties of
the PBTz units have been compared to the analogous N-alkyl-
and N-acylDTPs in order to fully quantify the relative
electronic effects of the various methods utilized to tune the
parent DTP building block.
Marder and co-workers specifically point out that 2-
bromothiazole is a preferable precursor over 2,4-dibromothia-
23
zole (1) due to the high cost of the latter. While it is correct
that commercial sources of 1 are nearly 10 times the cost of 2-
bromothiazole, this cost can be reduced to less than half that of
commercial 2-bromothiazole via the simple production of 1
24
from 2,4-thiazolidinedione as utilized here (Scheme 1).
All three of the synthetic routes given in Schemes 1 and 2
converge at the synthesis of 4,4′-dibromo-2,2′-bis(triisopropyl-
silyl)-5,5′-bisthiazole (3) and differ primarily in the steps to
generate this critical intermediate. The TIPS protecting group
has been found to be critical, as Heeney and co-workers
confirmed that the parent PBTz could not be generated
RESULTS AND DISCUSSION
■
1
6
directly from the unprotected 4,4′-dibromo-5,5′-bisthiazole.
Synthesis. The new synthetic methods for the generation
of N-alkyl- and N-arylPBTz monomers from 2,4-thiazolidine-
dione is given in Scheme 1. In comparison, the previous
Although they attributed this to competing Pd-catalyzed
arylation reactions at the 2-positions of the thiazole ring, this
is most likely due to the acidic nature of the 2-position of
t
thiazole which would be readily deprotonated by the NaO Bu,
Scheme 1. New Synthetic Routes to PBTz Monomers
thus potentially leading to various unwanted byproducts.
Tangentially, the methods reported here also allowed the
simple production of 4,4′-tetrabromo-2,2′-bis(trimethylsilyl)-
5
,5′-bisthiazole from 2, but any attempts to produce
trimethysilyl (TMS) protected PBTz units via this inter-
mediate were unsuccessful. This was attributed to nucleophilic
attack on the TMS silyl center by t-butoxide under the harsh
reflux temperatures of xylene, leading to an anionic thiazole
unit which can undergo additional chemistry or polymer-
ization. It should be pointed out that, while the TIPS
protecting groups enhance the stability of the PBTz unit,
side reactions via oxidative coupling are still possible as
evidenced by the isolation of the dimeric species 6,6′-
bis(triisopropylsilyl)-2,2′-bis(4-hexylphenylpyrrolo[2,3-d:5,4-
d′]bisthiazole) (6, see the Supporting Information).
The two previous routes to 3 both exploit the halogen
16,23,25
dance
combined with an oxidative coupling step, with
the methods largely differing in the order of which these two
processes occur. In contrast, the methods reported here
involved the generation of 2,2′,4,4′-tetrabromo-5,5′-bisthiazole
B
DOI: 10.1021/acs.joc.7b02570
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The Journal of Organic Chemistry
Article
Figure 1. Face and edge ellipsoid plots of PBTz 5a at the 50% probability level.
(
2) via conditions developed by Evenson and Rasmussen for
Crystallography. X-ray quality crystals of the TIPS-
protected PBTz 4d were grown by slow evaporation of a
diethyl ether solution, while crystals of the deprotected parent
PBTz 5a were grown by melting solid 5a on the walls of a vial
and allowing the melt to crystallize. Thermal ellipsoid plots of
8
the production of 3,3′-dibromo-2,2′-bithiophene, followed by
exchange of the α-bromides for TIPS protecting groups.
Compound 2 is the second tetrabromobisthiazole to be
reported, following the analogous 4,4′-bisthiazole isomer
reported in 2000. These new methods allow the production
of 3 from 2,4-thiazolidinedione with an overall yield of 52−
6
bromothiazole, this is an improvement over the methods of
Marder and co-workers (overall yield of 41−55%), although it
does fall short of the methods of Heeney and co-workers
2
6
5
4
a are given in Figure 1, and selected bond lengths of 5a and
d are given in Table 1, along with previously reported data for
4%. In comparison to the previous routes from 2-
Table 1. Experimental Bond Lengths of Pyrrolo[2,3-d:5,4-
d′]bisthiazoles 5a, 4d, and 4a, Compared to Dithieno[3,2-
b:2′,3′-d]pyrroles 7 and 8
(
overall yield of 64−73%). However, in addition to the
reduction in cost, the new methods can be accomplished with
common and readily available reagents.
All three routes use nearly identical methods for the
conversion of 3 to the desired PBTz units via the TIPS-
protected PBTz monomers 4 generated through Buchwald−
Hartwig amination. The initial amination step is based on fairly
standard conditions previously developed for the production of
a
b
c
bond
5a
4d
4a
7
8
S1−C1
S1−C3
C1−E1
E1−C2
C2−C3
N1−C2
C3−C4
N1−C7
1.761(8)
1.703(7)
1.31(1)
1.36(1)
1.39(1)
1.38(1)
1.43(1)
1.768(2)
1.703(3)
1.318(3)
1.363(3)
1.397(3)
1.394(3)
1.418(3)
1.768(2)
1.718(2)
1.325(3)
1.361(3)
1.387(3)
1.383(3)
1.413(3)
1.457(3)
1.719(3)
1.716(3)
1.349(6)
1.416(5)
1.384(4)
1.379(5)
1.420(4)
1.451(4)
1.720(2)
1.709(2)
1.341(3)
1.422(3)
1.369(3)
1.402(2)
1.431(2)
1.392(2)
6
DTPs, with the primary differences between the various
routes being the solvent and corresponding temperatures
applied. The methods of Heeney and co-workers utilize the
most common solvent (toluene), but require a pressurized vial
and microwave heating to achieve a temperature of 170 °C,
16
1.45(1)
1.495(3)
with average yields of ca. 60%. The methods of Marder and
a
b
c
co-workers employed mesitylene at reflux to achieve temper-
Reference 18. Reference 7. Reference 8.
18
atures of ca. 165 °C, but with much lower yields. In
comparison, the methods here utilize xylenes and modified
conditions of those initially developed by Rasmussen and co-
18
7
the TIPS-protected PBTz 4a, N-octylDTP (7), and N-
octanoylDTP (8) for comparison. As with previously reported
structures of DTPs, the fused-ring PBTz 5a is completely flat
with no evidence of bowing.
8
2
7
workers for the amination of 3-bromothiophene. The
modifications consist primarily of a more rigorous reflux,
allowing temperatures of ca. 140 °C, and longer reaction times
to give average yields of ca. 60%. As such, these conditions
prove as effective as the previous methods of Heeney and co-
workers, but do not require any specialized instrumentation. It
should be pointed out that these conditions failed to
The bond lengths and angles of all three PBTz species are in
good agreement, although the PBTz structures exhibit some
marked differences from DTPs 7 and 8. Most notably are a
considerable shortening of both N−C bonds of the thiazole
ring in comparison to the analogous C−C bonds of the DTP
thiophenes. This shortening of these two exterior bonds of the
PBTz structure then results in a considerable elongation of the
external S−C bond (i.e., S1−C1). These structural differences
are consistent with the differences between the parent
successfully produce the t-butyl analogue 4d, although 4d
t
could be successfully produced via the application of ( Bu) P
3
rather than BINAP as the ligand. This is consistent with
previous limitations of BINAP in the production of various
6
28
DTPs, including the t-butylDTP. In all cases, the TIPS-
thiophene and thiazole heterocycles, although the elongation
protected PBTz monomers could be deprotected via
tetrabutylammonium fluoride (TBAF) in near quantitative
yields.
of the PBTz S−C bond is significantly greater than that
observed in the parent thiazole. In contrast, the central pyrrole
ring of PBTz exhibits quite good agreement with that of DTP.
C
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Article
a
The greatest effect of these structural differences is a vertical
displacement of the α-carbons of the PBTz unit relative to the
fused backbone. As a consequence, exterior bonds to these α-
positions adopt a near-perfect coplanar orientation with the
C3−C4 bond connecting the two thiazole rings of the PBTz
Table 2. Electrochemical Data for PBTz and DTP Units
a
b
compound
R
Ep (V)
Eonset (V)
EHOMO (eV)
5
5
5
5
5
5
7
8
a
b
c
d
e
C H
0.98
0.98
0.98
0.96
1.05
1.04
0.51
0.73
0.87
0.87
0.87
0.86
0.97
0.95
0.45
0.67
−6.0
−6.0
−6.0
−6.0
−6.1
−6.1
−5.6
−5.8
6
13
17
C H
8
C H
(
see Figure S25 in the Supporting Information). In contrast,
12 25
tBu
the exterior bonds of DTP exhibit a downward cant of ca. 4°
relative to the DTP backbone. As a result, PBTz-based
conjugated materials should exhibit a more linear rodlike
structure than the analogous DTPs. An example of this linear
rodlike structure can be seen in the PBTz dimer 6 (Figure
S24), and the enhanced linear nature of PBTz materials could
lead to enhanced packing effects in the solid state. Lastly, the
observed structural differences also result in the PBTz unit
being slightly smaller than the corresponding DTP.
Ph
f
c
C H Ph
6
13
C H
8
17
c
COC H
7
15
+
a
b
+
All potentials vs Fc/Fc . EHOMO = −(E[onset, ox vs Fc /Fc] + 5.1) (eV),
c
ref 31. ref 8.
oxidation of the conjugated backbone. Without the steric
protection of the TIPS groups, the radical cations generated
undergo rapid coupling typical of thiophene species, thus
accounting for the irreversible nature of the oxidation. As the
The packing of 5a consists of one-dimensional ribbons via
29
S···N short contacts, as well as associated S···H−C contacts.
These ribbons arrange in parallel arrangements separated by
ca. 11.43 Å, the space between which are occupied by the hexyl
side chains. Above and below each ribbon is another layer of
parallel ribbons, with each layer roughly perpendicular to the
next. The layers of perpendicular ribbons are again connected
via S···N and S···H−C short contacts, with the same sulfur on
each PBTz providing both linear and perpendicular contacts.
The linear S···N contacts are 3.064 Å (less than the sum of the
32,33
TIPS groups are also electron donating,
their removal
results in a positive shift of ca. 200 mV in the peak potentials
(
E ) of the deprotected PBTz series 5a−f (Table 2). No
pa
reduction processes were observed within the electrochemical
solvent window utilized, which is consistent with the predicted
value of ca. −2.9 V for the PBTz reduction.
As the TIPS groups of 4a−f alter the electronic nature of the
PBTz core, the deprotected PBTz units provide the most
accurate measure of the electronic stabilization in comparison
to DTP. As shown in Table 2 and Figure 2, direct comparison
30
van der Waals radii at 3.35 Å ) with a C−S···N angle of 164°,
while the perpendicular S···N contacts are slightly longer at
3
.078 Å with a C−S···N angle of 176°. Such S···N contacts
have also been observed for the 2,6-dibromo derivative of 5a,
18
although to a lesser degree (S···N contacts are 3.241 Å). The
sulfur of each S···N contact is simultaneously involved in a
short contact to the hydrogen on the α-carbon adjacent to the
nitrogen, in which the linear S···H−C contacts are 2.918 Å and
the perpendicular S···H−C contacts are slightly shorter at
2
.872 Å. However, as the S···H−C angles are 107° and 109°,
respectively, these interactions are expected to be extremely
weak and are most likely just an artifact of the more prominent
S···N interaction. The presence of such S···N interactions, in
addition to the typical S···S commonly seen in thiophene-
based materials, could again contribute modified packing
motifs for PBTz-based materials in comparison to DTP-based
materials.
Electrochemistry. In order to quantify the extent of
electronic stabilization resulting from the replacement of the
thiophene rings of DTP with thiazole, the TIPS-protected
PBTz series 4a−f and the parent PBTz series 5a−f were
characterized by cyclic voltammetry. The electrochemical data
of the PBTz series 5a−f are given in Table 2, as well as the data
for the N-alkyl- and N-acylDTPs 7 and 8 for comparison. The
data for the TIPS-protected PBTz series 4a−f are given in
Table S2 in the Supporting Information.
Figure 2. Comparative voltammograms of PBTz 5b, N-octyl DTP 7,
and N-octanoyl DTP 8.
of PBTz 5b to DTPs 7 and 8 reveals that PBTz provides
substantial stabilization even over the N-acylDTP 8. In fact, the
stabilization of the 4-alkylPBTz in comparison to the
analogous N-alkylDTP is essentially twice that of the
stabilization afforded by the N-acylDTP. Thus, a near linear
trend is observed in the corresponding HOMO energies, with
values of −5.6 eV for the N-alkylDTP, −5.8 eV for the N-
acylDTP, and −6.0 eV for the 4-alkylPBTz. In addition, all of
these values can be further stabilized by ca. 0.1 eV by using the
corresponding aryl analogues, thus allowing continuous tuning
of the HOMO energies over the range of −5.6 to −6.1 eV.
UV−vis Spectroscopy. The spectroscopic data for the
PBTz series 5a−f are given in Table 3, as well as the data for
the N-alkyl- and N-acylDTPs 7 and 8 for comparison.
Comparative UV−visible spectra of 5b, 7, and 8 are shown
in Figure 3. All of the 4-alkylPBTz species exhibit a single
transition centered at ca. 305 nm, although weakly defined
All of the TIPS-protected PBTz species exhibit a
quasireversible oxidation assigned as the oxidation of the
conjugated backbone. The quasireversible nature here is due to
32
the steric bulk of the TIPS groups which prevents coupling of
the radical cation generated by the oxidative process. The
alkyl-functionalized series 4a−d exhibit an E1 of 0.78 V,
/2
which agrees well with the previously reported voltammogram
1
8
of 4c. The 4-arylPBTz species 4e and 4f, however, exhibit a
further stabilization of ca. 100 mV, which is consistent with the
stabilization of N-arylDTPs in comparison to the N-alkyl
7
analogues. In comparison, the deprotected PBTz series 5a−f
exhibit a well-defined irreversible oxidation, again assigned to
D
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Article
Table 3. UV−Visible Absorption Data for PBTz and DTP
in reduced absorptivity has been previously reported for other
cases in which thiophene has been replaced by thiazole,
a
34−36
Units
and computational studies have shown that the transition
oscillator strength continues to decrease with each additional
ε (M− cm−1
1
)
λmax (nm)
ε (M−1 cm−1
)
compound
λmax (nm)
3
7
5
a
306
305
304
306
300
305
310
305
15100
14700
14200
16600
18000
12700
26100
substitution by thiazole. Even the parent heterocycles
themselves exhibit this difference in absorptivity. Although
the absorbance energies of thiophene and thiazole are very
similar (231 vs 233 nm), the molar absorptivity of thiazole is
5
b
5
c
5
d
5
e
245
246
298
289
15700
10300
29300
27600
−1
−1
−1
−1
only 3700 M cm in comparison to 7400 M cm for
thiophene.
38
5
7
8
f
b
c
No significant explanation for this difference in absorptivity
has been previously reported, yet what is known is that molar
absorptivity is related to both the allowedness of the transition
15400
a
b
c
In CHCl . In CH Cl , ref 7. In CH CN, ref 8.
3
2
2
3
39
and the cross-sectional area of the chromophore. While it is
likely that the transition allowedness is influenced by electronic
differences between thiophene and thiazole, it is well
40
established that the thiazole ring is smaller than thiophene,
which should lead to a reduction in the maximum possible
cross-sectional area of thiazole analogues and lower absorptiv-
ity. As can been seen in the overlay of the DTP and PBTz
crystal structures given in Figure S25, PBTz is smaller than
DTP, but not by enough to account for the extent of the
differences in the corresponding molar absorptivities. Never-
theless, this reduced absorptivity could limit the effectiveness
of PBTz-based materials in applications for which the extent of
light absorbance is critical, such as photovoltaics.
CONCLUSIONS
■
Figure 3. Comparative UV−visible spectra of PBTz 5b (in CHCl ),
An alternate and practical synthetic route to pyrrolo[2,3-d:5,4-
d′]bisthiazole (PBTz) building blocks has been presented,
starting from the inexpensive 2,4-thiazolidinedione. In the
process, the scope of the known PBTz family has been
expanded with four new members, including the first reported
3
and DTPs 7 (in CH Cl ), and 8 (in CH CN).
2
2
3
shoulders can be observed at both lower and higher energy.
Due to the close energetic spacing of these transitions (ca.
4
-arylPBTz units. More critically, the significant character-
−
1
1
400 cm ), these shoulders are assigned as vibrational
ization of the PBTz unit has been presented for the first time,
including the first crystal structure of a deprotected parent
PBTz unit. As such, it has allowed the direct comparison of the
two relative approaches for stabilization of the more commonly
applied dithieno[3,2-b:2′,3′-d]pyrrole (DTP) building blocks
components of the same electronic transition. The 4-aryl
analogues 5e and 5f exhibit similar transitions near 305 nm,
although with a less defined structure. In addition, the aryl
analogues exhibit a second higher-energy transition at ca. 245
nm, which is assumed to involve the π-system of the aryl
substituent.
(
i.e., N-acyl functionalization of DTP vs PBTz) and to
accurately quantify the electronic effects of the substitution
of thiophene by thiazole. This has revealed that the PBTz unit
allows the greatest extent of stabilization of the HOMO energy
level and, when combined with the currently known family of
DTP building blocks, these HOMO levels can be finely tuned
in increments of ca. 0.1 eV over the range of −5.6 eV for N-
alkylDTPs to −6.1 eV for 4-arylPBTz units. However, although
PBTz units provide the greatest extent of HOMO stability, the
optical characterization of these species has also revealed that
they exhibit the greatest reduction in molar absorptivity. As
such, the limited absorbance efficiency of these building blocks
could limit the effectiveness of PBTz-based materials in
applications such as photovoltaics.
The overall structure of absorbance for the 4-alkylPBTz
species is similar to that of N-alkylDTPs, although not as
strongly defined and with onsets red-shifted by ca. 10 nm. As it
has already been established above, that the HOMO energy
levels of the PBTz units are significantly stabilized in
comparison to DTP. Thus, this red shift indicates that the
PBTz LUMO is similarly stabilized, but to a greater extent. In
comparison to the N-acylDTPs, the 4-alkylPBTz absorbance
onset is very slightly blue-shifted with a very similar shape to
lower-energy transition of the N-acylDTP. This is notable as
the N-acylDTP transition has been assigned to be of at least
8
partial charge transfer (CT) character, although there is
currently no evidence of any CT character in the PBTz
transition. Marder and co-workers reported the DFT-
calculated molecular orbital surfaces for the PBTz HOMO
EXPERIMENTAL SECTION
■
1
8
General Information. 2,4-Dibromothiazole (1) was prepared as
and LUMO, which were found to be nearly identical to that
previously determined for DTP.
24
previously reported. Xylenes and THF were distilled from sodium/
benzophenone prior to use. CHCl and CH Cl were dried with
3
2
2
The most notable aspect of the PBTz absorbance, however,
is the significant reduction in molar absorptivity (ε) compared
to the DTPs. Typical values for N-alkylDTPs are 25 000−
MgSO and filtered through a silica plug prior to use. ZnCl and
CuCl were dried in vacuo, and all other materials were reagent-grade
and used without further purification. All glassware was oven-dried,
assembled hot, and cooled under a N atmosphere. Chromatography
4
2
2
−
1
−1
3
0 000 M cm , while the analogous 4-alkylPBTz values
2
−1
−1
given in Table 3 are only 14 000−16 000 M cm . This trend
was performed using standard methods, with 230−400 mesh silica gel
E
DOI: 10.1021/acs.joc.7b02570
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
in 1 in. diameter columns. Melting points were obtained with a digital
36H) 0.85 (t, J = 6.7 Hz, 3H). 13C NMR: δ 166.4, 158.5, 106.7, 45.3,
31.8, 29.8, 29.1(3), 29.1(1), 26.8, 22.7, 18.6, 14.1, 11.8. HRMS (m/
thermocouple, accurate to 0.1 °C resolution. NMR spectroscopy was
+
performed on a Bruker 400 MHz spectrometer in CDCl solvent. All
z): calcd for C H N S Si [M + H] 606.3762, found 606.3776.
3
32 60
3
2
2
NMR spectra are referenced to the chloroform resonance at 7.26
ppm, and multiplicity is as follows: s = singlet, d = doublet, t = triplet,
quin = quintet, sep = septet, m = mulitplet. HRMS (ESI-TOF) was
performed in-house.
4-Dodecyl-2,6-bis(triisopropylsilyl)-4H-pyrrolo[2,3-d:5,4-d′]-
1
bisthiazole (4c). 364−406 mg (55−60% yield); mp 35.8−37.1 °C. H
NMR: δ 4.61 (t, J = 6.7 Hz, 2H), 2.04 (quin, J = 6.7 Hz, 2H) 1.48
(sep, 6H, J = 7.5 Hz), 1.28 (m, 18H), 1.19 (d, J = 7.5 Hz, 36H), 0.90
1
3
2
,2′,4,4′-Tetrabromo-5,5′-bisthiazole (2). A 250 mL three-
(t, J = 6.7 Hz, 3H). C NMR: δ 166.4, 158.5, 106.7, 45.3, 31.9, 29.8,
neck round-bottom flask was equipped with a 125 mL addition
29.6(8), 29.6(5), 29.6(4), 29.5, 29.4, 29.1, 26.7, 22.7, 18.6, 14.1, 11.8.
18
funnel, to which the solids 2,4-dibromothiazole (3.64 g, 15 mmol)
All NMR values agree with previously reported values.
and ZnCl (2.45 g, 18 mmol) were added. The complete system was
4-Phenyl-2,6-bis(triisopropylsilyl)-4H-pyrrolo[2,3-d:5,4-d′]-
2
then evacuated and backfilled with N three times. THF (75 mL) was
bisthiazole (4e). 262−331 mg (46−58% yield); mp 87.7−89.2 °C.
2
1
added to the flask and cooled to 0 °C. Diisopropylamine (2.55 mL,
H NMR: δ 8.55 (d, J = 8.7 Hz, 2H), 7.60 (t, J = 7.6 Hz, 2H), 7.34 (t,
1
8.0 mmol) and butyllithium (7.2 mL, 2.5 M in hexanes, 18 mmol)
J = 7.6 Hz, 1H), 1.52 (sep, J = 7.4 Hz, 6H) 1.22 (d, J = 7.4 Hz, 36H).
1
3
were added, and the solution stirred for 30 min at 0 °C before cooling
C NMR: δ 167.2, 157.1, 138.7, 128.9, 124.6, 120.9, 109.7, 18.6, 11.8.
to −78 °C via an acetone/CO bath. THF (75 mL) was then added
+
2
HRMS (m/z): calcd for C H N S Si [M + H] 570.2823, found
30 48 3 2 2
to the addition funnel, and the funnel contents were added dropwise
to the lithium diisopropylamide solution, which was then stirred for 1
h 45 min. The flask was warmed to room temperature over 15 min,
5
70.2833.
-(4-Hexylphenyl)-2,6-bis(triisopropylsilyl)-4H-pyrrolo[2,3-d:5,4-
d′]bisthiazole (4f). 262−314 mg (40−48% yield); mp 36.3−36.8 °C.
4
and then cooled again to −78 °C. CuCl was added (2.42 g, 18.0
1
2
H NMR: δ 8.57 (d, J = 8.7 Hz, 2H), 7.34 (d, J = 8.7 Hz, 2H), 2.67 (t,
J = 7.7 Hz, 2H), 1.69 (quin, J = 7.7 Hz, 2H), 1.52 (sep, J = 7.7 Hz,
mmol) and the solution was stirred for 30 min. Dry air was bubbled
into the reaction for 2 min, and the flask was left in the cryogenic bath
overnight, warming slowly to room temperature. The following day,
6
3
3
H), 1.40 (m, 6H), 1.19 (d, J = 7.7 Hz, 36H), 0.90 (t, J = 7.7 Hz,
13
H). C NMR: δ 167.0, 157.1, 139.3, 136.4, 128.7, 120.7, 109.4,
saturated aqueous NH Cl was added and the mixture was extracted
4
5.6, 31.8, 31.5, 28.9, 22.6, 18.6, 14.1, 11.8. HRMS (m/z): calcd for
with CHCl . The organic fractions were combined and dried with
+
3
C H N S Si [M + H] 654.3762, found 654.3792.
3
6
60
3
2
2
MgSO , and the solvent was removed in vacuo to give a brown solid.
4
4
-tert-Butyl-2,6-bis(triisopropylsilyl)-4H-pyrrolo[2,3-d:5,4-d′]-
The solid was then washed well with methanol to give 2.96−3.17 g of
bisthiazole (4d). Compound 4b was prepared as above substituting
1
3
a light tan powder (82−88% yield). mp 224.1−225.6 °C. C NMR: δ
t
[
( Bu) P]BF for BINAP to give 330−347 mg (60−63% yield); mp
3
4
7
9
+
1
38.0, 126.5, 125.4. HRMS (m/z): calcd for C N S Br [M + H]
1
6
2
2
4
82.4−84.1 °C. H NMR: δ 2.01 (s, 9H), 1.45 (sep, J = 7.4 Hz, 6H),
4
80.6315, found 480.6291.
13
1
1
5
.18 (d, J = 7.4 Hz, 36H). CNMR: δ 164.8, 158.2, 107.5, 59.3, 30.5,
4
,4′-Dibromo-2,2′-bis(triisopropylsilyl)-5,5′-bisthiazole (3).
8.6, 11.7. HRMS (m/z): calcd for C H N S Si [M + H]+
2
8
52
3
2
2
Bisthiazole 2 (1.45 g, 3.0 mmol) was added to a 500 mL three-
neck round-bottom flask, which was then placed under a N2
atmosphere. THF (250 mL) and triisopropylsilyl chloride (1.41
mL, 6.6 mmol) were then added, and the mixture cooled to −78 °C in
50.3057, found 550.3166.
General Deprotection Methods to Generate 4-Function-
alized 4H-pyrrolo[2,3-d:5,4-d′]bisthiazoles (5a−f). The TIPS-
protected PBTz (0.4 mmol) was added a 50 mL round-bottom flask,
followed by 15 mL of dry THF. Tetrabutylammonium fluoride (1.6
mL, 1.0 M in THF, 1.6 mmol) was added, and the reaction was stirred
at room temperature for 4 h. A saturated aqueous NaCl solution was
added to the reaction mixture, which was then extracted with diethyl
an acetone/CO bath. Butyllithium (2.64 mL, 2.5 M in hexanes, 6.6
2
mmol) was added, the solution was stirred at −78 °C for 2 h, and the
reaction was allowed to warm to room temperature overnight.
Saturated aqueous NH Cl was then added, and the mixture was
4
extracted with CHCl . The combined organic layers were dried with
3
ether. The combined organic layers were then dried with MgSO and
4
MgSO and concentrated to give a brown oily solid that was purified
4
concentrated in vacuo. The resulting solid was purified via column
chromatography (5% diethyl ether in hexanes) to afford the
deprotected PBTz as a dark yellow microcrystalline solid.
via column chromatography (20% CHCl in hexanes) to yield 1.43−
3
1
1
3
.53 g of a yellow microcrystalline solid (75−80% yield). mp 109.7−
16
1
10.9 °C (Lit. 112−114 °C). H NMR: δ 1.47 (sept, J = 7.4 Hz,
13
4
-Hexyl-4H-pyrrolo[2,3-d:5,4-d′]bisthiazole (5a). 99−102 mg
6H), 1.18 (d, J = 7.4 Hz, 6H). C NMR: δ 172.5, 130.3, 125.0,
1
(
93−96% yield). mp 82.5−83.6 °C. H NMR: δ 8.59 (s, 2H), 4.57
(t, J = 7.3 Hz, 2H) 2.02 (quin, J = 7.3 Hz, 2H), 1.31 (m, 6H), 0.85 (t,
J = 7.3 Hz, 3H). C NMR: δ 154.7, 149.0, 103.9, 45.6, 31.3, 30.1,
6.5, 22.5, 14.0. H NMR values agree with previously reported
1
8.44, 11.56. All NMR values agree with previously reported
16,21
values.
1
3
General Synthesis of 4-Functionalized 2,6-Bis(triisopropyl-
1
2
silyl)-4H-pyrrolo[2,3-d:5,4-d′]bisthiazoles. Sodium tert-butoxide
18
values.
(
(
5
(
0.461 g, 4.8 mmol), Pd (dba) (0.046 g, 5 mol %), bisthiazole 3
2 3
4
-Octyl-4H-pyrrolo[2,3-d:5,4-d′]bisthiazole (5b). 113−114 mg
0.638 g, 1.0 mmol), and BINAP (0.125 g, 20 mol %) were added to a
0 mL round-bottom flask equipped with a reflux condenser. Xylenes
25 mL) were added, and the mixture stirred for 20 min. The
1
(
(
6
3
96−97% yield). mp 69.1−69.9 °C. H NMR: δ 8.60 (s, 2H), 4.57
t, J = 7.4 Hz, 2H), 2.02 (quin, J = 7.4 Hz, 2H) 1.35 (m, 4H) 1.25 (m,
H), 0.85 (t, J = 7.4 Hz, 3H). C NMR: δ 154.7, 149.0, 103.9, 45.6,
1
3
appropriate amine was added (1.4 mmol), and the solution heated at
vigorous reflux for 20 h. Water was then added and the mixture was
extracted with diethyl ether. The combined organic layers were then
1.7, 30.1, 29.1(4), 29.1(2), 26.8, 22.6, 14.1. HRMS (m/z): calcd for
+
C H N S [M + H] 294.1099, found 294.1089.
14 20
3 2
4
-Dodecyl-4H-pyrrolo[2,3-d:5,4-d′]bisthiazole (5c). 131−133 mg
dried with MgSO and concentrated in vacuo, and the resulting solid
4
1
(
94−95% yield). mp 48.6−48.8 °C. H NMR: δ 8.59 (s, 2H), 4.57 (t,
J = 7.2 Hz, 2H) 2.02 (quin, J = 7.2 Hz, 2H), 1.34 (m, 5H), 1.22 (m,
3H), 0.88 (t, J = 7.2 Hz, 3H). C NMR: δ 154.7, 149.0, 103.9, 45.6,
was purified via column chromatography (hexanes) to give a dark
yellow microcrystalline material.
1
3
1
4
-Hexyl-2,6-bis(triisopropylsilyl)-4H-pyrrolo[2,3-d:5,4-d′]-
bisthiazole (4a). 340−370 mg (59−64% yield); mp 74.9−76.0 °C.
31.9, 30.1, 29.6 (two carbons), 29.5(3), 29.4(7), 29.3, 29.2, 26.8, 22.7,
1
H NMR: δ 4.62 (d, J = 6.7 Hz, 2H) 2.04 (quin, J = 6.7 Hz, 2H), 1.47
14.1.
(
(
2
sep, J = 7.5 Hz, 6H), 1.28 (m, 6H), 1.19 (d, J = 7.5 Hz, 36H), 0.90
4-tert-Butyl-4H-pyrrolo[2,3-d:5,4-d′]bisthiazole (5d). 90−91 mg
13
1
t, J = 6.7 Hz, 3H). C NMR: δ 166.4, 158.5, 106.7, 45.3, 31.3, 29.7,
(95−96% yield). mp 168.4−169.9 °C (dec). H NMR: δ 8.56 (s,
1
13
6.4, 22.5, 18.6, 14.0, 11.8. H NMR values agree with previously
2H), 2.01 (s, 9H). C NMR: δ 154.6, 147.6, 104.8, 59.7, 30.4. HRMS
18
+
reported values.
(m/z): calcd for C10
H
12
N
3
S
2
[M + H] 238.0473, found 238.0464.
4
-Octyl-2,6-bis(triisopropylsilyl)-4H-pyrrolo[2,3-d:5,4-d′]-
4-Phenyl-4H-pyrrolo[2,3-d:5,4-d′]bisthiazole (5e). 95−97 mg
1
bisthiazole (4b). 393−412 mg (65−68% yield); mp 58.4−60.3 °C.
(92−94% yield). mp 129.1−131.6 °C. H NMR: δ 8.69 (s, 2H),
1
H NMR: δ 4.61 (t, J = 6.7 Hz, 2H), 2.05 (quin, J = 6.7 Hz, 2H) 1.46
8.21 (d, J = 7.9 Hz, 2H), 7.57 (t, J = 7.9 Hz, 2H), 7.35 (t, J = 7.5 Hz,
1
3
(
sep, J = 7.4 Hz, 6H), 1.31 (m, 4H), 1.21 (m, 6H) 1.18 (d, J = 7.4 Hz,
1H). C NMR: δ 153.8, 149.6, 137.4, 129.4, 126.2, 122.3, 106.4.
F
DOI: 10.1021/acs.joc.7b02570
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The Journal of Organic Chemistry
Article
HRMS (m/z): calcd for C H N S [M + H]+ 258.1060, found
(2) Rasmussen, S. C.; Ogawa, K.; Rothstein, S. D. In Handbook of
Organic Electronics and Photonics; Nalwa, H. S., Ed.; American
Scientific Publishers: Stevenson Ranch, CA, 2008; Vol. 1, Chapter 1.
(3) Handbook of Thiophene-based Materials; Perepichka, I. F.,
Perepichka, D. F., Eds.; John Wiley & Sons: Hoboken, NJ, 2009.
(4) Baumgartner, T. J. Inorg. Organomet. Polym. Mater. 2005, 15,
389−409.
1
2
8
3 2
2
58.1064.
-(4-Hexylphenyl)-4H-pyrrolo[2,3-d:5,4-d′]bisthiazole (5f). 124−
4
1
1
8
2
3
3
27 mg (91−93% yield). mp 68.9−70.3 °C. H NMR: δ 8.67 (s, 2H),
.03 (d, J = 8.5 Hz, 2H), 7.37 (d, J = 8.5 Hz, 2H), 2.68 (t, J = 7.7 Hz,
H) 1.65 (quin, J = 7.7 Hz, 2H), 1.31 (m, 6H), 0.89 (t, J = 7.7 Hz,
H). 13C NMR: δ 153.8, 149.5, 141.2, 134.9, 129.3, 122.4, 106.0,
5.6, 31.8, 28.9, 22.7, 18.1, 14.1. HRMS (m/z): calcd for C H N S
1
8
20
3
2
(5) Rasmussen, S. C.; Evenson, S. J.; McCausland, C. B. Chem.
Commun. 2015, 51, 4528−4543.
+
[
M + H] 342.1099, found 342.1082.
UV−Visible Spectroscopy. UV−visible spectra were measured
on a dual beam scanning spectrophotometer using samples prepared
(6) Rasmussen, S. C.; Evenson, S. J. Prog. Polym. Sci. 2013, 38,
1
773−1804.
(7) Ogawa, K.; Rasmussen, S. C. J. Org. Chem. 2003, 68, 2921−
928.
as dilute CHCl solutions in matched 1 cm quartz cuvettes.
3
Electrochemistry. All electrochemical methods were performed
2
utilizing a three-electrode cell consisting of a platinum disc working
(
(
8) Evenson, S. J.; Rasmussen, S. C. Org. Lett. 2010, 12, 4054−4057.
9) Evenson, S. J.; Pappenfus, T. M.; Delgado, M. C. R.; Radke-
+
electrode, a platinum wire auxiliary electrode, and a Ag/Ag reference
electrode (0.10 M AgNO3 in CH CN). Supporting electrolyte
3
Wohlers, K.; Navarrete, J. T. L.; Rasmussen, S. C. Phys. Chem. Chem.
Phys. 2012, 14, 6101−6111.
consisted of 0.10 M tetrabutylammonium hexafluorophosphate
(
TBAPF ) in dry CH Cl . Solutions were deoxygenated by sparging
6 2 2
(10) Amb, C. M.; Chen, S.; Graham, K. R.; Subbiah, J.; Small, C. E.;
So, F.; Reynolds, J. R. J. Am. Chem. Soc. 2011, 133, 10062−10065.
with argon prior to each scan and blanketed with argon during the
measurements. All measurements were collected at a scan rate of 100
(11) Fei, Z.; Kim, J. S.; Smith, J.; Domingo, E. B.; Anthopoulos, T.
mV/s and final potentials all reported relative to an internal ferrocene
D.; Stingelin, N.; Watkins, S. E.; Kim, J.-S.; Heeney, M. J. Mater.
Chem. 2011, 21, 16257−16263.
+
standard (50 mV vs Ag/Ag ). E
values were estimated from the
HOMO
onset of oxidation in relation to ferrocene, using the value of −5.1 eV
(12) Gendron, D.; Morin, P.-O.; Berrouard, P.; Allard, N.; Aich, B.
31
vs vacuum for ferrocene.
R.; Garon, C. N.; Tao, Y.; Leclerc, M. Macromolecules 2011, 44,
188−7193.
13) Green, J. P.; Han, Y.; Kilmurray, R.; McLachlan, M. A.;
Anthopoulos, T. D.; Heeney, M. Angew. Chem., Int. Ed. 2016, 55,
148−7151.
14) Kato, T.; Imoto, H.; Tanaka, S.; Ishidoshiro, M.; Naka, K.
Dalton Transactions 2016, 45, 11338−11345.
15) Green, J. P.; Cryer, S. J.; Marafie, J.; White, A. J. P.; Heeney, M.
Organometallics 2017, 36, 2632−2636.
16) Al-Hashimi, M.; Labram, J. G.; Watkins, S.; Motevalli, M.;
Anthopoulos, T. D.; Heeney, M. Org. Lett. 2010, 12, 5478−5481.
17) Xia, R.; Al-Hashimi, M.; Tsoi, W. C.; Heeney, M.; Bradley, D.
D. C.; Nelson, J. Sol. Energy Mater. Sol. Cells 2012, 96, 112−116.
18) Getmanenko, Y. A.; Singh, S.; Sandhu, B.; Wang, C.-Y.;
Timofeeva, T.; Kippelen, B.; Marder, S. R. J. Mater. Chem. C 2014, 2,
24−131.
19) Patra, D.; Lee, J.; Lee, J.; Sredojevic, D. N.; White, A. J. P.;
X-ray Crystallography. X-ray quality crystals of 4d and 6 were
grown by slow evaporation of diethyl ether solutions, while crystals of
a were grown from cooling of molten samples. The X-ray intensity
7
(
5
data of the crystals were measured at 100 K on a CCD-based X-ray
diffractometer system equipped with a Cu X-ray tube (λ = 1.54178 Å)
operated at 2000 W of power. The detector was placed at a distance
of 5.047 cm from the crystal. Frames were collected with a scan width
of 0.3° in ω and exposure time of 10 s/frame and then integrated with
the Bruker SAINT software package using an arrow-frame integration
algorithm. The unit cell was determined and refined by least-squares
upon the refinement of XYZ-centeroids of reflections above 20σ(I).
The structure was refined using the Bruker SHELXTL (Version 5.1)
Software Package.
7
(
(
(
(
(
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.7b02570.
■
1
(
*
S
Bazzi, H. S.; Brothers, E. N.; Heeney, M.; Fang, L.; Yoon, M.-H.; Al-
Hashimi, M. J. Mater. Chem. C 2017, 5, 2247−2258.
(
(
20) Rasmussen, S. C. ChemTexts 2016, 2, 16.
21) Martin, R.; Larsen, C. H.; Cuenca, A.; Buchwald, S. L. Org. Lett.
NMR spectra for 2, 3, 4a−f, 5a−f (PDF)
Crystallographic data for 4d, 5a, 6 (CIF)
2
(
007, 9, 3379−3382.
22) Fortsch, S.; Bauerle, P. Polym. Chem. 2017, 8, 3586−3595.
̈
̈
AUTHOR INFORMATION
Corresponding Author
E-mail: seth.rasmussen@ndsu.edu.
(23) Getmanenko, Y. A.; Tongwa, P.; Timofeeva, T. V.; Marder, S.
■
R. Org. Lett. 2010, 12, 2136−2139.
(
24) Uzelac, E. J.; Rasmussen, S. C. J. Org. Chem. 2017, 82, 5947−
*
5951.
ORCID
(25) Schnurch, M.; Spina, M.; Khan, A. F.; Mihovilovic, M. D.;
̈
Seth C. Rasmussen: 0000-0003-3456-2864
Notes
The authors declare no competing financial interest.
Stanetty, P. Chem. Soc. Rev. 2007, 36, 1046−1057.
(
26) Nussbaumer, T.; Neidlein, R. Heterocycles 2000, 52, 349−364.
(27) Ogawa, K.; Radke, K. R.; Rothstein, S. D.; Rasmussen, S. C. J.
Org. Chem. 2001, 66, 9067−9070.
28) Katritzky, A. R.; Pozharskii, A. F. Handbook of Heterocyclic
Chemistry, 2nd ed.; Pergamon: Amsterdam, 2000; pp 61, 101.
(29) Politzer, P.; Murray, J. S.; Janjic, G. V.; Zaric, S. D. Crystals
2014, 4, 12−31.
30) Weast, R. C., Ed. Handbook of Chemistry and Physics, 68th ed.;
CRC Press, Inc.: Boca Raton, FL, 1987; p D-188.
31) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G.
(
ACKNOWLEDGMENTS
■
The authors thank North Dakota State University for support
of this research and NSF-CRIF (CHE-0946990) for the
purchase of departmental XRD instrument. We also wish to
thank Dr. Angel Ugrinov (NDSU) for assistance with the
collection of the X-ray crystal data and Dr. Bill Pennington
́
́
(
(
C. Adv. Mater. 2011, 23, 2367−2371.
(
Clemson University) for helpful discussions about sulfur-
nitrogen interactions.
(
(
32) Rucker, C. Chem. Rev. 1995, 95, 1009−1064.
33) Panek, J. S.; Prock, A.; Eriks, K.; Giering, W. P. Organometallics
1
990, 9, 2175−2176.
REFERENCES
■
(34) Josse, P.; Chavez, P.; Dindault, C.; Dalinot, C.; McAfee, S.;
Dabos-Seignon, S.; Tondelier, D.; Welch, G.; Blanchard, P.; Leclerc,
N.; Cabanetos, C. Dyes Pigm. 2017, 137, 576−583.
(
1) Handbook of Conducting Polymers, 3rd ed.; Skotheim, T. A.,
Reynolds, J. R., Eds.; CRC Press: Boca Raton, FL, 2007.
G
DOI: 10.1021/acs.joc.7b02570
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
(
35) Bronstein, H.; Collado-Fregoso, E.; Hadipour, A.; Soon, Y. W.;
Huang, Z.; Dimitrov, S. D.; Ashraf, R. S.; Rand, B. P.; Watkins, S. E.;
Tuladhar, P. S.; Meager, I.; Durrant, J. R.; McCulloch, I. Adv. Funct.
Mater. 2013, 23, 5647−5654.
(
36) Uzelac, E. J.; Rasmussen, S. C. Eur. J. Inorg. Chem. 2017, 2017,
3
(
878−3883.
37) Soto-Rojo, R.; Baldenebro-Lop
J. Photoenergy 2016, 2016, 6479649.
38) Katritzky, A. R.; Pozharskii, A. F. Handbook of Heterocyclic
Chemistry, 2nd ed.; Pergamon: New York, 2000; pp 68, 115.
́
ez, J.; Glossman-Mitnik, D. Int.
(
(
(
39) Braude, E. A. J. Chem. Soc. 1950, 379−384.
40) Katritzky, A. R.; Pozharskii, A. F. Handbook of Heterocyclic
Chemistry, 2nd ed.; Pergamon: New York, 2000; p 101.
H
DOI: 10.1021/acs.joc.7b02570
J. Org. Chem. XXXX, XXX, XXX−XXX