Benzotrifuran (BTFuran): A building block for π-conjugated systems

Article abstract of DOI:10.1039/c7cc04505k

Reported here is the first synthesis, X-ray crystal structure, and derivatization of benzotrifuran (BTFuran). Single crystal X-ray analysis of BTFuran shows a tight hexagonal packing stabilized by π-stacking interactions and C-H···O contacts. α-Lithiation of BTFuran enables the preparation of reactive intermediates suitable for cross-coupling reactions, allowing access to representative BTFuran-containing π-conjugated systems.

Full text of DOI:10.1039/c7cc04505k

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Benzotrifuran (BTFuran): a building block for π-conjugated
systems†
Received 00th January 20xx,
Accepted 00th January 20xx
Renan B. Ferreira, Jose M. Figueroa, Danielle E. Fagnani, Khalil A. Abboud and Ronald K.
Castellano*
DOI: 10.1039/x0xx00000x
www.rsc.org/
R
BTT (1)
BTFuran (2) X = O; R = H
3
4
5
X = S; R = H
R
F
R
Reported here is the first synthesis, X-ray crystal structure, and
derivatization of benzotrifuran (BTFuran). Single crystal X-ray
analysis of BTFuran shows a tight hexagonal packing stabilized by
π-stacking interactions and C–H···O contacts. α-Lithiation of
BTFuran enables the preparation of reactive intermediates
suitable for cross-coupling reactions, allowing access to
representative BTFuran-containing π-conjugated systems.
8
X
X = O; R = OCO2CH3
X = O; R = OCOCH3
X = O; R = Ar
2
R
F
F
X
X
R
6
a R = Ar
R = TMS
CsOH—H2O (10 equiv),
H2O (10 equiv)
b
5
6b
R
DMAc, 175 °C, 2 h
54%
Fig. 1 Structures of benzotrithiophene (BTT, 1) and benzotrifuran (BTFuran, 2). The
optimized synthesis of BTFuran as reported in this work is also shown.
Fused C3h-symmetric (hetero)aromatic molecules are highly
sought building blocks for π-conjugated materials since their
The production of hexafunctionalized BTFurans from
condensation reactions with phloroglucinol was first reported
by Lang in 188625 and revisited later by other groups.26-29
While we introduced the first 2,5,8-trifunctionalized
delocalized
π-surfaces and “star” or “propeller” shapes can
confer useful optoelectronic and self-assembly properties.1
Notable members of this class include the benzotrithiophenes
(BTTs),2-6 triazacoronenes,7 and triazatruxenes (TATs).8-12 BTT
benzotrifurans (
compliments of Nakamura and coworkers, that the first π-
conjugated trifunctionalized BTFuran derivatives
appeared.31
The molecules display enhanced octopolar
3 and 4
, Fig. 1) in 2009,30 it was not until later,
(1, Fig. 1), for example, constitutes π-conjugated systems
covering a broad functional spectrum: columnar liquid crystals
with high charge carrier mobilities,5, 13 small molecules14 and
mesoporous polymers15 as photoactive layers in organic
photovoltaic devices, hole-transporting materials for
perovskite solar cells with power conversion efficiencies up to
18.2%,6 and metal-organic frameworks with molecular
adsorption and sensing properties.4, 16
(5)
properties as well as good 2D and 3D organization on surfaces
and in the crystalline phase, respectively.31 Synthesis of the
targets came not from parent
2, but from appropriately
functionalized 1,3,5-triethynyl-2,4,6-trifluorobenzenes 6a
.
Given the relatively harsh cyclization conditions required (10
equivalents of CsOH in DMAc at 175 °C), we imagined that
Furan-based π-conjugated building blocks have emerged
over the past decade as useful complements to thiophene
derivatives.17-21 Despite an often higher-lying highest occupied
molecular orbital (HOMO) and increased propensity for air
oxidation, furan derivatives can present superior solubility and
processability, more rigid planar structures, tighter packing in
the solid phase, and sustainable access from biomass
precursors compared to their thiophene counterparts.22-24 The
synthesis and functionalization of parent benzotrifuran
parent BTFuran
2 might serve as a versatile building block for
accessing broader classes of BTFuran-containing π-conjugated
materials moving forward, by analogy to BTT. Toward this
goal, reported here is the first synthesis and X-ray crystal
structure of
and desymmetrization functionalization reactions of
together with photophysical and computational
2. Also described are representative symmetric
2
characterization of various targets to highlight the system’s
relevance to organic π-conjugated materials.
(BTFuran, 2, Fig. 1), the oxygen analog of BTT, is reported here
for the first time.
Inspired by Nakamura’s synthetic approach to 2,5,8-
trifunctionalized benzotrifurans,31 we entertained 6b (Fig. 1)
bearing three trimethylsilyl (TMS) groups as an entry to
BTFuran (2). The cyclization precursor could be prepared in
Department of Chemistry, P.O. Box 117200, University of Florida, Gainesville, FL
32611, USA
† Electronic Supplementary Informaꢀon (ESI) available: Experimental procedures,
UV-Vis absorption and NMR spectra, computational data (energies and coordinates
of geometry-minimized structures), and details of X-ray data collection and
structure refinement. CCDC 1551219. See DOI: 10.1039/x0xx00000x
two steps from 1,3,5-trifluorobenzene, first through
modification of an exhaustive iodination procedure reported
by Wenk and Sander (see the SI for details),32 followed by
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Sonogashira coupling with TMS-acetylene as described by
Fourmigué and coworkers.33 Alkyne 6b was first subjected to
the conditions previously reported for the formation of aryl
functionalized BTFurans.31 Fortuitously, the strongly basic
reaction medium and perhaps the generation of fluoride ions
in solution led to in situ deprotection of the TMS groups and
direct access to the BTFuran core
2 in up to 41% yield (Table
S1). Different reaction times as well as additives as fluoride or
proton sources were evaluated; an optimized yield of 54% was
realized after two hours when 10 equivalents of water was
used (Fig. 1). Other bases (e.g., LiOH, Cs2CO3, NaOAc, KOAc,
NaOMe, and KOEt) were also evaluated (not shown) but failed
to produce
2, providing instead mixtures of partially cyclized
compounds (e.g., benzodifurans). BTFuran (
2) is isolated as a
white powder and presents good solubility in common organic
solvents. Also, while bench stable for several weeks (after
which it shows some coloration),
if stored in the refrigerator.
2 remains stable for months
Fig. 2 X-ray crystal structure of BTFuran (2): (a) ORTEP plot of BTFuran (thermal
ellipsoids shown at the 50% probability level); (b) the packing structure contains 65
screw axes localized in each vertex of the unit cell (only two columnar stacks are shown
for clarity); (c) the slipped stacking favors the partial overlap between benzene and
furan rings among the planar units that are not perfectly parallel (centroids indicated
Slow diffusion of water in a DMSO solution of BTFuran (2)
provided crystals suitable for single crystal X-ray analysis (Fig.
2). The heterocycle is highly planar; the maximum deviation of
any atom from the root mean square plane of the molecule is
0.02 Å (versus up to 0.09 Å for crystalline BTT2). Taken
together with nearly identical C–C bond lengths within the
core benzene ring (Fig. 2a), BTFuran can be considered
aromatic. The same conclusion was reached in earlier work for
by
a sphere, distances in Å); (d) lateral packing of the molecules is directed by
intermolecular O….H—Cα hydrogen bonding interactions, as observed by the
intermolecular distances (in Å) and the superposition of H and O atoms in the space-
filling model.
3
and
4
based on X-ray crystallographic analysis and DFT
further
useful for cross-coupling reactions. Along these lines,
deprotonation of BTFuran with a slight excess of n-BuLi
followed by quenching with several common electrophiles
successfully affords 7a–d, equipped with halogen, trimethyltin,
and boronic ester groups, in moderate to good yields (Scheme
1). Of note, attempts to purify 7a and 7b by column
chromatography on silica led to recovery of non-functionalized
calculations, respectively.30 The molecules of
2
organize into helical columnar stacks (Fig. 2b-c), an
arrangement dominated by parallel-displaced π-π stacking
interactions and the partial overlap between benzene and
furan rings. The π-stacking distances (3.38–3.42 Å) are at the
van der Waals limit (2
×
rvdW(carbon) = 3.40 Å),34 and about 0.1
Å shorter than those observed in the crystal structure of BTT
(see Figs. S1–2 for additional details).2 C–H….O interactions35
define BTFuran’s lateral interactions in the solid state and the
intercolumnar associations (Fig. 2d). The O….H—Cα
arrangement is only slightly distorted from linearity and the
average intermolecular distance between the oxygen atom
and the α-carbon is 3.44 Å, shorter than the sum of the C–H
bond distance and the van der Waals radii of hydrogen and
oxygen (r(C—H)+rvdW(H)+rvdW(O) = (1.08+1.20+1.52) = 3.80
Å).34 Given the distances and angles of these interactions it is
reasonable to conclude that hydrogen bonding influences
molecular organization of BTFuran in the crystal. The
combination of face-to-face π-π and lateral hydrogen bonding
interactions leads to a tight hexagonal packing arrangement,
very different from the packing structure of BTT that is
dominated by edge-to-face interactions (Fig. S2). BTFuran’s
crystal packing features bode well for its usage in
optoelectronic device settings where charge transport through
π-stacks is important.
2
. Alternatively, simple crystallization from MeOH proved to be
an easy and efficient way to obtain compounds 7a–d in good
purity.
To evaluate the fitness of the reactive intermediates shown
in Scheme 1 to produce BTFuran-containing π-conjugated
systems, Pd-catalyzed cross-coupling reactions were
attempted using appropriate thiophene partners to obtain
novel tris(2-hexylthiophenyl) derivative
8
(Scheme 2). After
could be
routine optimization of the reaction conditions,
8
obtained in good yields for all combinations of 7a–c and the
thiophene derivatives in both Stille and Suzuki coupling
reactions (Scheme 2a). Compound
8 is a furan/thiophene
hybrid π-system, a new entry in a class of materials that has
X
O
O
i. nꢀBuLi, THF
X
ii. E–X
O
O
O
X
O
BTFuran (2)
The synthesis of π-conjugated systems relies extensively on
metal-mediated cross-coupling reactions due to their general
efficiency, modularity, and availability of coupling partners. To
7a: E–X
-PrOBPin, X BPin
= i
=
, 45%
7b: E–X = Me3SnCl, X = SnMe3, 69%
7c: E–X = CBr4, X = Br, 70%
7d: E–X = I2, X = I, 46%
establish BTFuran as
a building block for π-conjugated
Scheme 1 α-Lithiation of BTFuran (
2) to create cross-coupling precursors 7a–d.
materials, we have explored its derivatization to intermediates
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Table 1 Screening of Stille coupling conditions to produce desymmetrized 10 and 11.
been receiving increasing attention given combination of the
superior optical/electronic properties of thiophenes and
36-38
Additionally,
improved packing of furans.21,
halogenated7c–d function well in Sonogashira reactions with
terminal acetylenes to afford trialkynes 9a–b Of note, a
.
copper-free methodology was used in reactions of
phenylacetylene39 to afford 9a in moderate yields with no sign
of homocoupling by-product. While the same methodology
failed with TMS-acetylene, 9b was successfully prepared using
standard Sonogashira conditions (Scheme 2b).
T
10
(%)b
16
26
23
31
33
16
48
31
8
11
(%)c
37
44
39
37
19
33
10
-
8 (%)c
Entry
Solvent
7c (%)a,b
(°C)
80
60
50
40
30
110
100
90
80
66
1
DMF
DMF
DMF
DMF
DMF
PhMe
PhMe
PhMe
PhMe
THF
-
38
19
14
11
7
2
1
Desymmetrizing otherwise C3h-symmetric heteroaromatics
is one way to access more exotic structures, like dendrimers
and donor-acceptor constructs, with unique structural and
electronic features. The approach has been exploited with
BTT, where dendritic oligomers were synthesized and applied
as p-channel semiconductors in OFET devices.40 With this goal
in mind, we have explored ways to mono- and di-functionalize
3
1
4
4
5
4
6
12
32
52
71
74
15
-
7
8
-
9
-
-
BTFuran.
Initially attempts to produce desymmetrized
10
26
-
-
BTFuran derivatives through selective lithiation (by controlling
temperature and reagent stoichiometry) led to a mixture of
products requiring difficult purification. We turned to a
strategy described by Takimiya et al. in the synthesis of BTT-
containing materials40 involving the control of the
stoichiometry of the cross-coupling reactions as an alternative
way to create desymmetrized molecules. One advantage of
this approach is that the mono- and di-functionalized products
produced still contain reactive groups which can subsequently
engage in cross-coupling reactions with different reaction
partners. As proof-of-principle, Stille coupling of tribromo
species 7c was selected due to the stability of the bromides 10
and 11 upon purification by column chromatography (Table 1).
As expected, higher temperatures favored the production of
a
b
Recovery yield. Yield calculated based on NMR integration of the isolated
mixture of 7c and 10. c Isolated yield after purification.
reaction rates and lower conversion, causing the increasing
recovery of the starting material 7c. We also observed that
DMF favored the coupling reactions compared to toluene and
THF. By screening several conditions we could successfully
obtain 10 and 11 in modest yields (entries 2 and 7, Table 1).
UV analysis of BTFuran (
expectedly show bathochromic shifts in λmax with increasing
conjugation (e.g., 9a 9b), a trend generally mirrored in
2), 7c, and derivatives 8–11
8
>
>
the HOMO–LUMO gaps obtained through electronic structure
calculations at the B3LYP/6-31+G* level (Fig. 3 and Table S2).
Comparing the BTFuran-based compounds with their
published BTT-based congeners reveals higher energy λmax
the byproduct
8. Decreasing the temperature to favor
values for the former,2, again consistent with theoretical
3
formation of incomplete coupling products also led to slower
calculations that indicate slightly smaller HOMO-LUMO gaps
for the thiophenes (Table S2). Nonetheless, the absorption
differences are relatively small and the BTFuran derivatives are
typified by high molar extinction coefficients common to
organic π-systems. Perhaps most interestingly, comparison of
1.8
2
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
7c
8
9a
9b
10
11
200
250
300
Wavelength (nm)
Fig. 3. Absorption spectra of BTFuran and its derivatives (15 μM solutions in
CH2Cl2). Bathochromic shifts are observed upon increasing -conjugation.
350
400
π
Scheme 2 Pd-catalyzed cross-coupling reactions of BTFuran derivatives.
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the calculated HOMO and LUMO energy levels of BTT
1 vs.
BTFuran
2 (or alternatively 8 versus its BTT derivative, Table
S2) shows comparable HOMO energies and modulated LUMO
energies. To follow up on this comparison experimentally,
electrochemical
measurements
were
performed.
Electrochemical analysis of BTFuran (Fig. S3) revealed an
irreversible oxidation, Epa = 1.61 V (corresponding to an
estimated EHOMO of –6.1 eV), only slightly higher than that
determined for BTT, Epa = 1.55 V, under similar electrochemical
conditions.2 Currently we are exploring the consequences of
the improved solid-state packing ability of the benzotrifurans
on solid-state absorption and charge transport properties.
In conclusion, parent, C3h-symmetric BTFuran (2) has been
prepared for the first time. The molecule presents a tight
hexagonal packing arrangement in the solid state, with
structural features (e.g., short π-stacking distances) that are
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attractive
for
downstream
π-conjugated
materials
applications. Lithiation of BTFuran followed by quenching with
various electrophiles readily generates reactive intermediates
for a range of Pd-catalyzed cross-coupling reactions. Sample
BTFuran-containing π-conjugated molecules were produced in
moderate to good yields and show, based on UV-Vis
absorption measurements, electrochemistry, and DFT
calculations, comparable optical and electronic properties to
their BTT congeners. The protocols developed in this work are
expected to encourage the development of BTFuran-based π-
conjugated systems for solid-state optoelectronic applications.
We acknowledge the National Science Foundation (NSF)
(CHE-1507561) and the University of Florida (graduate
fellowship to R.B.F.) for supporting this research. The authors
are grateful to the UF High Performance Computing Center for
providing computational resources and both UF and the NSF
(CHE-0821346) for funding the X-ray equipment. We thank Dr.
Brian J. Cook and Prof. Leslie J. Murray for assistance with the
electrochemical measurements.
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Reported here is the first synthesis, X-ray crystal structure, and derivatization of C3h-symmetric
benzotrifuran (BTFuran), oxygen analog of the widely exploited benzotrithiophene.