Convenient access to readily soluble symmetrical dialkyl-substituted α-oligofurans

Article abstract of DOI:10.1039/c4ob00898g

An expedient approach to the synthesis of well soluble symmetrical dialkyl-substituted α-oligofurans containing up to 8 π-conjugated furan heterocycles is reported. An ultimate symmetry and high solubility of these α-oligofurans were guaranteed using the 3,3′-diheptyl-2,2′- bifuran core and its symmetrical elongation through Suzuki-Miyaura or Stille cross-couplings. 3,3′-Diheptyl-2,2′-bifuran was prepared from 2,2′-bifuran-3,3′-dicarbaldehyde by the Wittig olefination and subsequent Pd/C-catalyzed transfer hydrogenation. The most appropriate access to 2,2′-bifuran-3,3′-dicarbaldehyde was achieved through a regioselective lithiation of 3-furanaldehyde acetal followed by CuCl ₂-induced homocoupling and deprotection. Single crystal X-ray analysis of 2,2′-bifuran-3,3′-dicarbaldehyde revealed anti-arrangement of the furan rings in planar molecules and an unexpected tight herringbone-type packing in crystals. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob00898g

Organic &
Biomolecular Chemistry
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Convenient access to readily soluble symmetrical
dialkyl-substituted α-oligofurans†‡
Edward E. Korshin,*a Gregory M. Leitusb and Michael Bendikova
Cite this: Org. Biomol. Chem., 2014,
12, 6661
An expedient approach to the synthesis of well soluble symmetrical dialkyl-substituted α-oligofurans con-
taining up to 8 π-conjugated furan heterocycles is reported. An ultimate symmetry and high solubility of
these α-oligofurans were guaranteed using the 3,3’-diheptyl-2,2’-bifuran core and its symmetrical
elongation through Suzuki–Miyaura or Stille cross-couplings. 3,3’-Diheptyl-2,2’-bifuran was prepared
from 2,2’-bifuran-3,3’-dicarbaldehyde by the Wittig olefination and subsequent Pd/C-catalyzed transfer
hydrogenation. The most appropriate access to 2,2’-bifuran-3,3’-dicarbaldehyde was achieved through a
regioselective lithiation of 3-furanaldehyde acetal followed by CuCl2-induced homocoupling and de-
protection. Single crystal X-ray analysis of 2,2’-bifuran-3,3’-dicarbaldehyde revealed anti-arrangement of
the furan rings in planar molecules and an unexpected tight herringbone-type packing in crystals.
Received 1st May 2014,
Accepted 27th June 2014
DOI: 10.1039/c4ob00898g
www.rsc.org/obc
α-oligofurans.6 In 2001 Curtis et al. reported that head-to-tail
regioregular α-poly(3-octylfuran) exhibited a conductivity com-
Introduction
Since the discovery of highly conducting polyacetylene in parable to that of the corresponding polythiophenes.7
1977,1 π-conjugated organic polymers and oligomers featured However, a presumption of the intrinsic instability of α-oligo-
with electron delocalization along the conjugated backbone furans comprising more than four furan rings8 survived until
have attracted immense attention due to their important appli- the year 2010 when Bendikov et al. disclosed synthesis of
cations in numerous types of organic electronic devices.2,3 unsubstituted α-oligofurans in up to nine furan units
Advantageous electronic properties and good synthetic avail- length.9,10 Computational studies predicted elevated planar
ability of variously substituted α-oligothiophenes and α-poly- rigidity of the α-oligofuran chain compared to α-oligothio-
thiophenes led these compounds to be the most popular phenes.9,11 Indeed, unsubstituted α-oligofurans are highly
π-conjugated materials.3 However, unsubstituted α-poly- and fluorescent and have structured absorption and emission
α-oligothiophenes starting from α-sexithiophene are practically spectra, suggesting the coplanar-type conformation in solu-
insoluble and hardly processable.3,4 Attachment of solubilizing tion.9,10 Raman spectroscopy evidenced that π-conjugation in
substituents at positions 3 and/or 4 results in twisting of the α-oligofurans does not reach saturation up to α-octifuran.12 In
thiophene–thiophene chain from a coplanar conformation the planar ferrocene-capped α-oligofurans an excellent charge
that is ideal for π-conjugation.5 Thus, an inherent low planar delocalization was observed.13 The first experimental organic
rigidity hampers significant optimization of the thiophene- field-effect transistors with the active layer fabricated from
based electronic materials.
unsubstituted α-octifuran, hexyl-capped α-sexi- and α-septi-
Despite chemical dissimilarity between aromatic thiophene furans as well as from styryl-capped α-quaterfurans demonstrated
and more cyclic diene-type furan heterocycles, theoretical ana- high hole mobilities and on/off ratios akin to the observed in the
lysis predicted an essential similarity in the structural and corresponding oligothiophene-based devices.14,15
electronic properties of the long α-oligothiophenes and
One of the major practical benefits of π-conjugated furan-
based materials consists of enhanced solubility in comparison
with the thiophene-based materials. For example, unsubsti-
tuted α-sexifuran is 20 times more soluble in chloroform than
α-sexithiophene, but the solubility of α-septifuran and the
higher analogs is still transient.9 Taking advantage of compar-
able electronic properties of the thiophene- and furan-based
polymers with better solubilization of the latter’s, Fréchet’s16
and Janssen’s groups recently applied oligofuran-diketopyrrole
copolymers for fabrication of remarkably efficient bulk
aDepartment of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100,
Israel. E-mail: edward.korshin@weizmann.ac.il, edward.korshin@gmail.com
bDepartment of Chemical Research Support, Weizmann Institute of Science, Rehovot
76100, Israel
†In memory of Professor Michael Bendikov (deceased July 2, 2013).
‡Electronic supplementary information (ESI) available: Instrumentation, experi-
mental procedures, details of X-ray measurement and refinement, and copies of
the NMR spectra. CCDC 970220. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4ob00898g
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heterojunction solar cells with 3–5% power conversion revealed a comparable usefulness of both processes with a
performance.17
superior simplicity of the product purification in the first case.
The valuable optoelectronic properties and a reasonable
processability of α-oligo- and α-polyfurans prompted to con-
sider this type of compounds as an emerging alternative to the
thiophene-based π-conjugated materials.9–17 Yet, an assort-
ment of the suitable furan-based π-conjugated compounds for
organic electronic applications and convenient methods for
their preparation is very limited and should be essentially
extended. To address this quest, a study on the synthesis of
the well soluble symmetrical disubstituted long α-oligofurans
was initiated. This year, the first variant of the synthesis of
dihexyl-substituted long α-oligofurans A, which was based on
the Ni(II)-catalyzed Kumada–Corriu cross-coupling of 3,3′-
dibromo-2,2′-bifuran (1) with hexylmagnesium bromide fol-
lowed by bromination and the subsequent Stille cross-coup-
ling, has been published (Scheme 1).18 Herein, we disclose the
details of an expedient complementary approach to the syn-
thesis of similar symmetrical dialkyl-substituted α-oligofurans
A, for example di-n-heptyl-derivatives, starting from furan-
3-aldehyde (2). We demonstrate the possibility to synthesize
these promising well-soluble π-conjugated oligomers by avoid-
ing highly toxic Ni(II)-complexes and organic tin reagents
without essentially affecting the overall synthetic efficiency. A
direct comparison of a scarcely exploited Pd(II)-catalyzed
Suzuki–Miyaura cross-coupling17,19 for formation of the α,α′-
furan–furan junction and the only applied Stille cross-
coupling8,9,12–16,18,20 for assembling of the α-oligofuran chain
Results and discussion
Our study was directed toward elaboration of a general
approach to synthesis of symmetrical disubstituted α-oligo-
furans from simple commercially available furan reagents. In
this paper, we describe preparation of the representative di-
n-heptyl-substituted derivatives, which contain linear alkyl
substituents suitable for efficient solubilization of the mole-
cules. From the very beginning, we considered previously
unknown 3,3′-dialkyl-2,2′-bifurans of type B (Scheme 1) as valu-
able central entities, which could be symmetrically elongated
to give the soluble long α-oligofurans A. However, no reliable
methods for preparation of the bifurans B have been reported
in the beginning of this work. Thus, though dibromide 1 was
reported in the year 1978,21 any reactivity of this compound
was unrevealed before the year 2014.18 As a model, Kumada–
Corriu cross-coupling of the parent 3-bromofuran (5) with
alkylmagnesiums to form the corresponding 3-alkylfurans was
documented to be inefficient.7,22 A complementary bromine–
lithium exchange in 5 followed by alkylation with alkyl halides
afforded 3-alkylfurans in modest yield.23
Based on the precedent of a viable CAN-induced head-to-
head dimerization of 3-(pyrrol-2-yl)furans to 3,3′-bis(pyrrol-
2-yl)-2,2′-bifurans,24 at the outset of this work we considered
an option of the most straightforward access to the bifuran B
directly from 3-alkylfuran 3. Following the reported low-yield-
ing procedures for 3-octyl-7 and 3-propylfurans,22 the starting
3-n-heptylfuran (3) was initially prepared by cross-coupling of
3-bromofuran (5) with a freshly generated n-heptylmagnesium
bromide in the presence of (dppp)NiCl2 or (dppe)NiCl2 in 20%
and 24% yield respectively (Scheme 2). Alkylation of 5 through
a low-temperature bromine–lithium exchange with n-BuLi in
THF–HMPA followed by alkylation with alkyl halide23 failed
completely with n-heptyl bromide and gave only 27% of 3 with
n-heptyl iodide. On the other hand, Wittig olefination of the
aldehyde 2 with n-hexyltriphenylphosphonium bromide (6)
and LDA with subsequent Pd/C-catalyzed transfer hydrogen-
ation25,26 of the 3-alkenyl-intermediate 7 provided 3-n-heptyl-
furan (3) in reasonable yield (Scheme 2).
Scheme 2 Synthesis of 3-n-heptylfuran (3). Reagents and conditions:
a, n-C7H15MgBr (1.4 equiv.), LNiCl2 (5 mol%), ether, reflux, 16 h, 20% if
L = dppp, 24% if L = dppe; b, (i) n-BuLi (1.05 equiv.), THF, −78 °C, 1 h;
(ii) n-C7H15I (1.3 equiv.), THF–HMPA, −60 °C, 14 h, 27%; c, (i) (n-C6H13
)
Ph3P+ Br− (6) (1.5 equiv.), LDA (1.5 equiv.), THF, −78 °C to r.t.; (ii) r.t.,
+
14 h; d, HCOO− NH4 (3.2 equiv.), 5% Pd/C (1.5 mol% Pd), MeOH, reflux,
Scheme 1 General routes to dialkyl-substituted α-oligofurans A.
6662 | Org. Biomol. Chem., 2014, 12, 6661–6671
2 h, 66% of 3 in two steps.
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However, all our efforts toward preparatively suitable homo-
coupling of 3-heptylfuran (3) to form either the head-to-head
dimer 8 = B (R = n-C7H15) or other possible dimers were unsuc-
cessful. In contrast to the recently reported homocoupling of
3-(pyrrol-2-yl)furans to 3,3′-bis(pyrrol-2-yl)-2,2′-bifurans,24
similar treatment of 3 with CAN in aqueous acetonitrile as well
as in the dry solvent at −20 °C led to complex reaction mix-
tures containing <5% of the desired bifuran 8. It is known that
3-alkyl-substituents do not provide a valid differentiation of
C-2 and C-5 positions in furans upon direct lithiation.27
Indeed, lithiation of 3 with n-BuLi in THF followed by treat-
28
29
ment with anhydrous NiCl2 or CuCl2 resulted in regio-
random couplings to give inseparable mixtures of all three
possible bis-heptyl-bifurans in 18% and 32% total yields
respectively and ca. 20% content of the head-to-head bifuran 8
in each mixture. Hence, further attempts on preparation of the
bifuran 8 from 3-heptylfuran (3) were abandoned.
At this stage, we considered an alternative formation of B
from 2,2′-bifuran-3,3′-dialdehyde (4) using the same olefin-
ation/hydrogenation sequence as for the preparation of 3 from
aldehyde 2 (Scheme 2). The dialdehyde 4 was assumed to be
derived from some acetal of the aldehyde 2 taking advantage
of the heteroatom directed ortho-lithiation/coupling method-
ology.30,31 Indeed, lithiation of the cyclic acetal 9, which is
readily available from the aldehyde 2,32 in boiling ether fol-
lowed by stannylation led to the 2-tributylstannyl-derivative 10
exclusively.33 Similar to that, we prepared the supplementary
2-iodofuran-acetal 11 by lithiation of the acetal 9 in ether fol-
lowed by the treatment of the corresponding 2-lithiofuran
intermediate with molecular iodine. In the next step, a tra-
ditional (Ph3P)4Pd-catalyzed Stille cross-coupling8,9,12–16,18,20 of
stannylfuran 10 with iodofuran 11 in boiling toluene afforded
head-to-head dimer 12 in 54% yield (Scheme 3).
Scheme 3 Synthesis of 2,2’-bifuran-3,3’-dicarbaldehyde (4). Reagents
and conditions: a, (i) n-BuLi (1.1 equiv.), ether, −78 °C to r.t.; (ii) I2 (1.1
equiv.), THF, −78 °C to r.t., 57%; b, 10 (1.45 equiv.), (Ph3P)4Pd (5 mol%),
toluene, reflux, 8 h, 54% of 12; c, (dppp)NiCl2 (30 mol%), KI (2 equiv.), Zn
powder (4 equiv.), HMPA, 100–110 °C, 14 h; 20% of 12 and 6% of 13; d,
(i) n-BuLi (1.05 equiv.), THF, −78 °C to 0 °C; (ii) CuCl2 (1.05 equiv.),
−78 °C to r.t., 63% of 12 and 4% of 13; e, glyoxalic acid monohydrate
(12 equiv.), p-TsOH monohydrate (6 mol%), dichloromethane, r.t., over-
night, 98%.
An access to the head-to-head dimeric acetal 12 might be
additionally simplified if a single precursor could be used. So,
we attempted to dimerize the iodide 11 through the conven-
tional Ullmann biaryl synthesis.34 While all our attempts to
accomplish a copper powder promoted reductive homocou-
pling of the iodoacetal 11 were unsuccessful, 14 h heating of
11 with excess zinc powder and KI in the presence of (dppp)
NiCl2 (30 mol%) in HMPA at 100–110 °C led to the diacetal 12
(20%) along with the aldehyde–acetal dimer 13 (6%). Finally,
we were delighted to find that the diacetal 12 (63%) along with
the minor aldehyde–acetal 13 (4%) could be prepared directly
from the acetal 9 through its lithiation in THF followed by
CuCl2-promoted oxidative homocoupling (Scheme 3). Surpris-
ingly, no head-to-tail and tail-to-tail dimers related to 12 were
detected in the reaction mixture. A recently reported appli-
cation of NiCl2 instead of CuCl2 almost arrested the coup-
ling leading to poor yields of 12 (5%) and 13 (1%). When
lithiation of 9 and subsequent CuCl2-induced coupling were
carried out in ether, poor solubility of CuCl2 and the copper-
containing intermediates in this medium affected the feasi-
bility of the process, resulting in 16% yield of 12 and 2% yield
of 13. In the next step, the diacetal 12 was mildly deprotected
with excess glyoxalic acid monohydrate35 and a catalytic
amount of p-TsOH monohydrate in methylene chloride to give
the bis-aldehyde 4 in 98% yield (Scheme 3). A similar deprotec-
tion of the aldehyde–acetal 13 afforded the dialdehyde 4 quan-
titatively. The molecular structure of the head-to-head dimeric
dialdehyde 5 was determined by 1H, 13C NMR, and IR-spec-
troscopy, HRMS, and characterized by single crystal X-ray ana-
lysis (Fig. 1).36
Unlike the only reported X-ray structure of 3,3′-bis(pyrrol-
2-yl)-2,2′-bifuran, which is characterized by a highly twisted
syn-conformation of the furan rings,24 molecules of the di-
aldehyde 4 in the solid state are C2-symmetrical with a planar
anti-conformation of the furan rings (Fig. 1a). A planar anti-
conformation is typical for all the previously X-ray studied
α-oligofurans9,13,15 and α-bifurans unsubstituted in positions 3
and 4.20,37–39 It is noteworthy that the planarity of the oligo-
furan backbone is almost unaffected even by the head-to-head
junction in 3′′′,4″-dihexyl α-sexifuran.18 The inter-furan C2–C2′
bond in bifuran-dialdehyde 4 is 1.4370(11) Å. This bond
length is similar to that of longer α-oligofurans
(1.431–1.439 Å),9 5,5′-di(thien-2-yl)-2,2′-bifuran (1.434 Å),37a
and 2,2′-bithieno[3,2-b]furan (1.432 Å),38 and slightly shorter
compared to the C5″–C2′′′ bond length in 3′′′,4″-dihexyl α-sexi-
28
29
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angle of 33.8° is much more acute than that of α-sexifuran.9
Two adjacent chains are connected with O2⋯H5 bonds of
2.5396(4) Å. The distance between two parallel chains is
3.320(1) Å. Overall, the molecules of 2,2′-bifuran-3,3′-dicarb-
aldehyde (4) are surprisingly tightly packed in crystals, and
such a tight packing is reflected in an unpredictably high
melting point (184 °C) and density (Dc = 1.574 g cm−3).
Having an optimized access to the dialdehyde 4, we pro-
cessed it further to 3,3′-di-n-heptyl-2,2′-bifuran (8) (Scheme 4).
Thus, Wittig olefination of the dialdehyde 4 with an excess of
the phosphorane, generated in situ from the phosphonium
salt 6 and LDA, led to the corresponding 3,3′-bis-alkenyl-
derivative 14 as a mixture of E- and Z-isomers almost
quantitatively. Subsequent Pd/C-catalyzed transfer hydrogen-
ation of 14 with excess ammonium formate as a hydrogen
source in boiling methanol afforded 90% of the desired 3,3′-
di-n-heptylbifuran (8) as a colorless mobile oil. Being solidified
at −25 °C, the bifuran 8 could be stored for years as a stock
precursor for longer α-oligofurans. Apparently, application of
other reagents to olefination of the dialdehyde 4 and the fol-
lowing hydrogenation would enable access to a variety of 3,3′-
disubstituted-2,2′-bifurans structurally related to 8.
Following a traditional approach to the synthesis of α-oligo-
and α-polyfurans through the Stille coupling of the corres-
ponding 2-furylstannanes and 2-bromofurans,8,9,13–16,18,20 we
performed a bromination of the bifuran 8 with NBS to obtain
the corresponding 5,5′-dibromo-2,2′-bifuran 15 in up to 98%
yield (Scheme 5). A freshly generated dibromide 15 could be
purified by flash chromatography (FC) on silica gel in the dark
and isolated in the NMR pure state as yellowish oil solidified
at −25 °C. However, the inherent instability of the dibromide
15 required its use in the next transformations as soon as poss-
ible, without any storage.43 In the next step, (Ph3P)4Pd-cata-
lyzed reaction of the freshly prepared dibromide 15 with
2-furylstannane 16 in boiling toluene afforded the diheptyl-
substituted quaterfuran 17 in high yield. However, huge
amounts of hardly removable tin-contaminants are ultimately
formed in the Stille cross-coupling, and at least three sequen-
tial FC were required for isolation of 17 of ≥95% purity.44
Since both the stability and yield of α-oligofurans are essen-
tially decreased with elongation of the π-conjugated system,8,9
it was highly desirable to identify a viable synthetic method
which allowed an easier purification of these sensitive com-
Fig. 1 (a) Crystal structure (ellipsoid presentation at 50% probability) of
a 2,2’-bifuran-3,3’-dicarbaldehyde (4) molecule.36 Hydrogen bonds are
shown: intramolecular O1⋯H6 bond 2.4612(5) Å and intermolecular
O2⋯H4’ bond 2.4061(5) Å. The O2⋯H4’ and O2’⋯H4 intermolecular
bonds combine molecule 4 in infinite plane chains lying in planes (111)
and (1–11). (b) Herringbone-type packing of the molecule 4 chains (view
along the [10 − 1] direction).
furan (1.449–1.451 Å).18 In the solid state planar structure of 4
short intramolecular contacts between hydrogen atoms of the
aldehyde groups and the oxygen atoms of the neighboring
furan rings (H6⋯O2′ 2.4612 Å) are observed. According to the
NMR data, the planar anti-conformation of 4 is likely preserved
in CDCl3 solution. Indeed, in the 1H NMR spectra of 4 the
aldehyde proton is detected as
a broadened singlet at
δ 10.50 ppm. This proton is considerably deshielded compared
to the corresponding aldehyde proton in the additional furan
ring lacking 3-formyl- and in 2-methyl-3-formylfuran
(δ 9.90–9.95 ppm)40 as well as in 2-phenyl-3-formylfuran
(δ 10.15 ppm).41 A noticeable low field chemical shift of the
aldehyde proton in 4 probably resulted from a through the
space deshielding by a closely positioned electronegative
oxygen atom, which could be effective at such a short distance
between the atoms as shown in Fig. 1a. We earlier observed a
similar strong through the space deshielding of methylene
protons by proximal oxygen atoms in the 2,3-dioxabicyclo
[3.3.1]nonane series.42
Unexpectedly, the packing of 4 is more similar to the
packing of unsubstituted α-sexifuran than to the less regular
packing of the more structurally related α-terfuran.9 The
bifuran dialdehyde molecules 4 are combined by two hydrogen
O2⋯H4′ bonds 2.4061(5) Å in planar chains. The chains lie in
planes (111) and (1–11), forming a herringbone structural
motif (Fig. 1b) with an angle of 146.22(4)°. The herringbone
Scheme 4 Synthesis of 3,3’-di-n-heptyl-2,2’-bifuran (8). Reagents and
conditions: a, (i) (n-C6H13)Ph3P+ Br− (6) (5.75 equiv.), LDA (5.8 equiv.),
THF, −78 °C to r.t.; (ii) 50–60 °C, 4 h, 98% (E-/Z- ca. 1 : 1); b, HCOO−
NH4+ (10 equiv.), 5% Pd/C (2 mol% Pd), MeOH, reflux, 1 h, 90%.
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Scheme 6 Preparation of 2-(α-oligofur-5-yl)-1,3,2-dioxaborolanes 24
and 25. Reagents and conditions: i, n-BuLi (1.15 equiv.), THF, −78 °C to
0 °C; ii, 27 (1.4 equiv.), −78 °C to r.t., overnight at r.t.; 49% of 24 and 33%
of 25.
was isolated by sequential FC in 41% yield (Scheme 7). Upon
using 5-stannylated terfuran 23 in the Stille coupling with
dibromide 15, diheptyl-substituted α-octifuran 21 was pre-
pared in 23% yield. The supplementary Pd(OAc)2/XPhos-cata-
lyzed Suzuki–Miyaura cross-coupling of 15 with an excess of
the bifurane-pinacolboronate 24 gave 35% yield of α-sexifuran
20. α-Octifuran 21 (19%) was also synthesized through a
similar Suzuki–Miyaura cross-coupling of 15 with 5-terfuryl-
boronate 25 (Scheme 7).
All the cross-couplings depicted in Scheme 7 are evidently
accompanied with a number of competing processes. Particu-
larly, protodeboronation47 of the α-oligofurylboronates 24 and
25 resulting in the release of α-bifuran 26 (n = 1) or α-terfuran
26 (n = 2) was detected in the syntheses of long α-oligofurans
20 and 21 by Suzuki–Miyaura reactions. TLC monitoring
revealed a premature consumption of the boronates 24 and 25
after 8 h warming at 55–60 °C, while a considerable amount of
dibromide 15 still remained in the reaction mixture. So,
second portions of boronates 24 and 25 were added for com-
Scheme 5 Synthesis
of 3’,3’’-di-n-heptyl-2’,2’’-quaterfuran (17).
Reagents and conditions: a, NBS (2.03 equiv.), benzene–CH2Cl2, r.t., 2 h,
98%; b, 16 (2.5 equiv.), (Ph3P)4Pd (10 mol%), toluene, reflux, 8 h, 90%; c,
18 (3 equiv.), Pd(OAc)2 (7 mol%), XPhos (14 mol%), K3PO4 (6 equiv.),
1,4-dioxane–H2O (10 : 1), 55–60 °C, 12 h, 73%.
pounds. We were delighted to find that the quaterfuran 17
could be efficiently synthesized through a Suzuki–Miyaura
cross-coupling, which is almost uncharted for the preparation
of α-oligofurans.17,19 Indeed, after fruitless experiments with 2-
furylboronic acid and potassium 2-furyltrifluoroborate, a mild
heating of the dibromide 15 with 2-(2-furyl)-1,3,2-dioxa-
borolane 1845,46 in dioxane–water (10 : 1) in the presence of a
catalytic amount of Pd(OAc)2 and XPhos afforded a pure qua-
terfuran 17 in 73% yield after one FC only (Scheme 5).
Bromination of the quaterfuran 17 with NBS afforded
5,5′′′-dibromoquaterfuran 19, which was isolated by FC as an
unstable yellow solid.43 Being considerably more labile com-
pared to dibromobifuran 15, a purified 19 spontaneously
exothermically decomposed over less than an hour at r.t.
Though dibromoquaterfuran 19 could be used without iso-
lation in the Stille coupling with a large excess of 16 to give
diheptylsexifuran 20 in 12% yield after tedious FC purifi-
cations,‡ for more reasonable access to 20 and to diheptylocti-
furan 21 we turned to the cross-couplings of more stable
dibromobifuran 15. In fact, all the previously reported synth-
eses of α-sexifurans and α-octifurans were achieved using Stille
coupling of 5,5′-dibromo-2,2′-bifurans with 5-tributyltin-2,2′-
bifuran (22) and 5-tributyltin-2,2′:5′,2″-terfuran (23) respect-
ively.9,18 For elongation of the α-oligofuran chain through the
Suzuki–Miyaura reaction α-oligofuran pinacolboronates 24 (n =
1) and 25 (n = 2) were prepared by lithiation of bifuran 26 (n =
1) or terfuran 26 (n = 2) followed by borylation with 2-iso-
propoxy-1,3,2-dioxaborolane 27 (Scheme 6).
Scheme 7 Synthesis of long α-oligofurans 20 and 21. Reagents and
conditions: a, 22 or 23 (3 equiv.), (Ph3P)4Pd (10 mol%), toluene, reflux,
12 h, 41% yield of 20 or 23% of 21; b, 24 or 25 (3 + 2 equiv.), Pd(OAc)2
(8 mol%), XPhos (16 mol%), K3PO4 (6 equiv.), 1,4-dioxane–H2O (10 : 1),
(Ph3P)4Pd-catalyzed Stille cross-coupling of the dibromobi-
furan 15 with 3 equiv. of 5-stannylbifuran 22 in boiling
toluene afforded the diheptyl-substituted α-sexifuran 20, which 55–60 °C, 8 + 8 h, 34% yield of 20 or 21% of 21.
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pletion of the Suzuki–Miyaura cross-couplings, and heating of Pd/C-catalyzed transfer hydrogenation. Subsequent bromina-
the reaction mixtures was continued for additional 8 h. In tion led to the corresponding 5,5′-dibromo-2,2′-bifuran. It was
another vein, substantial amounts of α-quaterfuran and α-sexi- converted to the final diheptyl-substituted long α-oligofurans
furan resulting from the Pd-catalyzed deboronative homo- using either rarely applied, for assembly of the α-oligofuran
coupling48,49 of α-oligofurylboronates 24 and 25 were also junction, Suzuki–Miyaura cross-coupling or conventional Stille
isolated in the studied Suzuki–Miyaura reactions. Similar cross-coupling. Both processes resulted in similar yields of
destannylative homocouplings,50 albeit to a less extent, were α-oligofurans, but much less laborious purifications of the pro-
observed in the Stille cross-couplings of the dibromide 15 with ducts were required in the case of Suzuki–Miyaura reactions.
5-stannyl-α-oligofurans 22 and 23. For the time being, we did
not succeed in suppressing these competing processes without
affecting the desired cross-couplings.
Experimental section‡
Synthesis of 3,3′-di(1,3-dioxolan-2-yl)-2,2′-bifuran (12) and 3′-
Both applied Stille and Suzuki–Miyaura cross-couplings
provided very moderate yields of the soluble long α-oligofurans
(1,3-dioxolan-2-yl)-[2,2′-bifuran]-3-carbaldehyde (13) through a
20 and 21 (Scheme 7). On the other hand, the previously
one-pot lithiation of 3-(1,3-dioxolan-2-yl)furan (9) followed by
reported hardly soluble unsubstituted α-oligofurans were iso-
CuCl2-induced homocoupling
lated in even lower yields using a time-consuming vacuum
sublimation as a purification technique.9 The isolated yields
To a solution of 9 (2.170 g, 15.485 mmol) in dry THF (20 mL)
at −78 °C a solution of n-BuLi (6.5 mL of 2.5 M in hexane,
16.25 mmol, 1.05 equiv.) was added. The heterogeneous reac-
tion mixture was allowed to warm to ambient temperature and
stirred for 1 h. The reaction mixture was cooled to −78 °C, and
solid CuCl2 (2.189 g, 16.28 mmol, 1.05 equiv.) was added in
3 portions (with 10 min interval between additions) under
overpressure of argon and intensive stirring. The reaction
mixture was stirred for 1 h at −78 °C, for 2 h at −35 °C, for 1 h
at 0 °C and for 2 h at r.t. After cooling to 0 °C, the reaction
mixture was quenched by addition of solid NH4Cl (883 mg,
16.5 mmol), stirred for 30 min at 0 °C and for 30 min at r.t.
The mixture was diluted with ether (100 mL) and stirred
for 1 h for better sedimentation of the inorganic black precipi-
tate. A supernatant solution was filtered via a plug of anhy-
drous Na2SO4, and the remaining black precipitate was
thoroughly extracted with ether–EtOAc (3 : 7). All the extracts
were filtered; the combined organic solution was washed with
10% aq. solution of glycine (2 × 75 mL), water, satd NaHCO3
and brine, and dried over MgSO4. The solution was filtered,
evaporated, and then the residue was subjected to flash
chromatography (FC) on silica gel (gradient elution, from 5%
to 40% EtOAc–hexane) to give the acetal-dimer 12 (1.362 g,
63%) and the aldehyde–acetal dimer 13 (78 mg, 4%).
of α-oligofurans 20 and 21 were slightly lower in the Suzuki–
Miyaura couplings than the Stille alternative. From a practical
point of view, faintly lower yields in the developed Suzuki–
Miyaura reactions are overcompensated by avoiding the poiso-
nous organic tin compounds as well as by the less laborious
isolation of the products (only 2–3 FC were required for purifi-
cation in the case of boron instead of 5–6 FC in the case of
tin).
The long α-oligofurans 20 and 21 are reasonably stable,
high melting point deep-yellow and yellow-orange solids
respectively, soluble in most organic solvents. For example, the
solubility of 20 and 21 in C6D6 is more than 10 mg mL−1. A
similar solubility was observed in 1,4-dioxane and in propylene
carbonate, which could also be used for a formulation of rela-
tively stable solutions.
Diheptyl-substituted sexifuran 20 and octifuran 21 were
characterized by 1D and 2D (COSY, HSQC and HMBC) NMR,
and by the field desorption (FD) HRMS, which gave intensive
[M]+-peaks. The UV/Vis and fluorescence spectra of diheptyl-
substituted α-sexifuran 20 and α-octifuran 21, which are essen-
tial for characterization of π-conjugated compounds, were
found to be very similar to the thoroughly discussed data for
dihexyl-substituted analogues.18 Evaluation of the synthesized
α-oligofurans 20 and 21 as potential organic electronic
materials will be reported in due course of the study.
Acetal-dimer 12
Colorless solid; m.p. 120–121 °C; Rf 0.26 (EtOAc–hexane =
2 : 3); 1H NMR (CDCl3, 500 MHz): δ 7.44 (d, J = 2.0 Hz, 2H),
6.60 (d, J = 2.0 Hz, 2H), 6.20 (s, 2H), 4.18–3.98 (sym m, 8H);
13C NMR (CDCl3, 126 MHz): δ 143.57 (C), 142.58 (CH), 121.12
(C), 110.04 (CH), 97.63 (CH), 65.30 (2 CH2); HRMS (FD): calcd
for C14H14O6 [M]+ 278.0790; found 278.0797.
Conclusions
In summary, we elaborated a convenient 7-step synthesis of
symmetrical and well soluble dialkyl-substituted long α-oligo-
furans from the commercially available furan-3-aldehyde. In
the key step of the synthesis, acetal of furan-3-aldehyde was
Aldehyde–acetal dimer 13
subjected to regioselective heteroatom-directed lithiation fol- Yellowish solid; m.p. 89–91 °C, Rf 0.30 (EtOAc–hexane = 2 : 3);
lowed by CuCl2-induced homocoupling to give the head-to- 1H NMR (CDCl3, 500 MHz): δ 10.41 (br s, 1H), 7.54 (br d, J =
head dimer. A mild deprotection of the latter afforded 2,2′- 1.8 Hz, 1H), 7.44 (dd, J = 2.0, 0.8 Hz, 1H), 6.88 (br d, J = 2.0 Hz,
bifuran-3,3′-carbaldehyde. High yielding attachment of the 1H), 6.70 (br d, J = 1.8 Hz, 1H), 6.29 (s, 1H), 4.20–4.02 (sym m,
n-heptyl substituents to give 3,3′-diheptyl-2,2′-bifuran was 4H); 13C NMR (CDCl3, 126 MHz): δ 186.13 (CHvO), 151.23 (C),
accomplished through
a Wittig olefination followed by 144.23 (CH), 143.20 (CH), 142.85 (C), 124.58 (C), 124.10 (C),
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110.99 (CH), 108.81 (CH), 97.25 (CH), 65.47 (2 CH2); IR 29.41, 29.39, 29.20, 29.15, 28.97, 28.95, 22.57 (CH2); 14.06
(CH2Cl2): ν 3157 (w), 3131 (w), 3060 (w), 2959 (m), 2929 (w), (CH3); MS (ESI): m/z (%) 327.2 (100%) [M + H]+, 349.1 (92%)
2891 (m), 1673 (vs) (CvO), 1481 (m, br), 1397 (m), 1172 (s), [M + Na]+, 365.2 (53%) [M + K]+.
1116 (s, br), 1077 (s), 1033 (s), 888 (s) cm−1; HRMS (FD): calcd
for C12H10O5 [M]+ 234.0528; found 234.0534.
(ii) Hydrogenation of 3,3′-di-(n-hept-1-en-1-yl)-2,2′-bifuran
(14). A mixture of E/Z-14 (840 mg, 2.573 mmol), ammonium
formate (1.622 g, 25.73 mmol, 10 equiv.) and 5% Pd/C
(105 mg; 0.049 mmol Pd, 1.9 mol%) in MeOH (40 mL) was
2,2′-Bifuran-3,3′-dicarbaldehyde (4)
A heterogeneous mixture of the acetal-dimer 12 (1.285 g, intensively stirred for 1.5 h at r.t. under overpressure of argon
4.618 mmol), glyoxalic acid monohydrate (5.10 g, and gently refluxed for 1 h. The reaction mixture was diluted
55.415 mmol, 12.0 equiv.) and p-TsOH monohydrate (57 mg, with ether (100 mL), filtered via a cotton wool and concen-
0.300 mmol, 0.065 equiv.) in CH2Cl2 (50 mL) was stirred for trated to ca. 5 mL volume. The concentrate was diluted with
12 h at r.t. Water (40 mL) was added, and the mixture was water (60 mL) and extracted with 25% EtOAc–hexane. The com-
extracted with CH2Cl2. The combined organic layer was bined organic phase was washed with satd NaHCO3 and brine,
washed with satd NaHCO3 and dried over MgSO4. Filtration, and then dried over MgSO4. Filtration and evaporation fol-
evaporation, followed by purification by FC on silica gel (gradi- lowed by FC on silica gel (hexane) afforded bifuran 8 (763 mg,
ent elution, from 25% to 50% EtOAc in hexane) afforded the 90%) as a colorless mobile oil; Rf 0.50 (hexane). 1H NMR
title dialdehyde 4 (860 mg, 98%) as pale yellow shining crys- (C6D6, 500 MHz): δ 7.10 (d, J = 1.8 Hz, 2H), 6.16 (d, J = 1.8 Hz,
tals; m.p. 184 °C; Rf 0.29 (EtOAc–hexane = 3 : 7). 1H NMR 2H), 2.76 (t, J = 7.7 Hz, 4H), 1.60 (br tt, J ≈ 7.5, 7.5 Hz, 4H),
(CDCl3, 500 MHz): δ 10.50 (br s, 2H), 7.57 (dd, J = 2.0, 0.6 Hz, 1.35–1.28 (m, 4H), 1.28–1.16 (m, 12H), 0.87 (t, J = 7.0 Hz, 6H);
2H), 6.98 (br d, J = 2.0 Hz, 2H); 13C NMR (CDCl3, 126 MHz): 13C NMR (C6D6, 126 MHz): δ 143.05 (C), 141.32 (CH), 123.03
δ 185.38 (CHvO), 149.50 (C), 144.85 (CH), 126.08 (C), 109.69 (C), 113.05 (CH), 32.17 (CH2), 30.79 (CH2), 29.67 (CH2), 29.52
(CH); IR (CH2Cl2): ν 3157 (w), 3133 (w), 3060 (w), 2927 (w), (CH2), 25.37 (CH2), 23.03 (CH2), 14.29 (CH3); HRMS (FD): calcd
2890 (w), 2863 (w), 1673 (vs) (CvO), 1540 (m), 1478 (m), for C22H34O2 [M]+ 330.2559; found 330.2564.
1397 (s), 1172 (s), 1034 (m), 888 (s) cm−1; HRMS (FD): calcd for
5,5′-Dibromo-3,3′-di-n-heptyl-2,2′-bifuran (15)43
C10H6O4 [M]+ 190.0266; found 190.0271.
To
a solution of 3,3′-diheptyl-2,2′-bifuran (8) (305 mg,
3,3′-Di-n-heptyl-2,2′-bifuran (8)
0.923 mmol) in dry benzene (6 mL) and CH2Cl2 (1 mL) solid
(i) Preparation of 3,3′-di-(n-hept-1-en-1-yl)-2,2′-bifuran NBS (334 mg, 1.876 mmol, 2.03 equiv.) was added. The reac-
(14). To a cold (−78 °C) suspension of phosphonium salt 7 tion mixture covered with aluminum foil was stirred for 2 h at
(6.656 g, 15.22 mmol, 5.74 equiv.) in dry THF (40 mL) a solu- r.t. The reaction mixture was poured into 10% aqueous
tion of LDA (7.7 mL of 2 M sol-n in THF–heptane–ethyl Na2S2O3 (20 mL), and extracted with EtOAc–hexane (1 : 2). The
benzene, 15.4 mmol) was added dropwise. The deep orange organic extract was washed with satd NaHCO3 and brine, and
reaction mixture was stirred at r.t. for 1 h and cooled again to then dried over MgSO4. Filtration and evaporation followed by
−78 °C. Solid bis-aldehyde 4 (504 mg, 2.65 mmol) was added FC on silica gel (hexane) gave dibromide 15 (442 mg, 98%) as
1
in one portion under overpressure of dry argon. The reaction an unstable colorless solid; Rf 0.56 (hexane). H NMR (CDCl3,
mixture was stirred at r.t. overnight and heated at 50–60 °C for 300 MHz): δ 6.26 (s, 2H), 2.52 (t, J = 7.6 Hz, 4H), 1.53 (br tt, J ≈
4 h. The reaction was quenched by addition of solid NH4Cl 7.5, 7.5 Hz, 4H), 1.38–1.22 (m, 16H), 0.88 (t, J = 6.9 Hz, 6H);
(1.07 g, 20.0 mmol); the mixture was stirred for 30 min at r.t. 13C NMR (CDCl3, 75.5 MHz): δ 142.76 (C), 126.55 (C), 121.21
and for 1 h at 50–60 °C. The mixture was diluted with ether (C), 114.18 (CH), 31.78 (CH2), 29.93 (CH2), 29.13 (CH2), 29.01
(100 mL). After sedimentation of the precipitate the mixture (CH2), 24.89 (CH2), 22.65 (CH2), 14.09 (CH3); HRMS (FD): calcd
was filtered through a plug of Na2SO4 and evaporated. FC on for C22H3279Br81BrO2 [M]+ 488.0749; found 488.0744 (correct
silica gel (hexane) gave the title bis-alkenyl-derivative 14 isotope pattern).
(851 mg, 98%, a mixture of E- and Z-isomers ca. 45 : 55) as a
3″,4′-Di-n-heptyl-2,2′:5′,2″:5″,2′′′-quaterfuran (17)
yellowish mobile oil; Rf 0.36 (hexane). 1H NMR (CDCl3,
500 MHz): δ 7.43 (br dd, J = 3.5, 1.9 Hz, 1.1H), 7.31 (br dd, J ≈
2.2, 2.2 Hz, 0.9H), 6.69 (br dd, J = 15.8, 1.2 Hz, 0.9H), 6.62 (dd, solution of
(a) Synthesis of 17 through the Stille cross-coupling. A
freshly prepared dibromide 15 (383 mg,
a
J = 3.5, 1.9 Hz, 1.1H), 6.60 (br dd, J ≈ 2.2, 2.2 Hz, 0.9H), 6.52 0.784 mmol), 2-tributylstannylfuran (16) (700 mg, 1.960 mmol,
(br d, J = 11.5 Hz, 1.1H), 6.02 (dt, J = 15.8, 7.0 Hz, 0.9 H), 5.61 2.5 equiv.) and (Ph3P)4Pd (91 mg, 0.0784 mmol, 10 mol%) in
(dt, J = 11.5, 7.2 Hz, 1.1H), 2.32 (dtd, J = 7.2, 7.2, 1.8 Hz, 2.2 H), dry toluene (8.0 mL) was gently refluxed for 8 h. The resulting
2.17 (br dtdd, J ≈ 7.0, 7.0, 1.6, 1.6 Hz, 1.8H), 1.51–1.42 (m, 4H), mixture was evaporated, and subjected to two sequential FC
1.37–1.30 (m, 8H), 0.92–0.87 (m, 6H); 13C NMR (CDCl3, on silica gel (hexane) and, finally, on Et3N-pretreated silica
126 MHz): δ 143.48, 143.29 and 141.63 (C); 142.44, 142.21, gel44 to give the title quaterfuran 17 (327 mg, 90%).
141.95 and 141.74 (CH); 132.77 and 132.72 (CH); 132.04 and
(b) Synthesis of 17 through the Suzuki–Miyaura cross-
131.98 (CH); 121.94, 121.52, 120.36 and 119.95 (C); 120.14 and coupling. A mixture of a freshly prepared dibromide 15
120.10 (CH); 119.03 and 119.02 (CH); 112.11 and 112.08 (CH); (251 mg, 0.514 mmol), 2-furylpinacolborolane (18) (300 mg,
108.84 and 108.82 (CH); 33.15, 33.12, 31.63, 31.47, 31.45, 1.545 mmol, 3 equiv.), Pd(OAc)2 (8.2 mg, 0.036 mmol,
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7 mol%), XPhos (34.3 mg, 0.072 mmol, 14 mol%) and K3PO4 (CDCl3, 500 MHz): δ 7.43 (dd, J = 1.8, 0.7 Hz, 1H), 7.13 (d, J =
(658 mg, 3.10 mmol, 6 equiv.) in 1,4-dioxane–water (10 : 1, 3.5 Hz, 1H), 6.80 (d, J = 3.6 Hz, 1H), 6.66 (d, J = 3.5 Hz, 1H),
total 6.6 mL) was stirred at 55–60 °C for 12 h. The resulting 6.63 (d, J = 3.4 Hz, 1H), 6.61 (d, J = 3.6 Hz, 1H), 6.47 (dd, J =
mixture was diluted with water (50 mL) and extracted with 3.4, 1.8 Hz, 1H), 1.36 (s, 12H); 13C NMR (CDCl3, 126 MHz):
10% EtOAc in hexane. The combined organic extract was δ 150.56 (C), 146.19 (C), 146.13 (C), 145.55 (C), 142.06 (CH),
washed with water and brine, and then dried over MgSO4. Fil- 125.34 (CH), 111.50 (CH), 108.64 (CH), 107.05 (CH), 106.25
tration and evaporation followed by FC on Et3N-pretreated (CH), 105.71 (CH), 84.26 (C), 24.73 (CH3). The resonance
silica gel44 (hexane) afforded the quaterfuran 17 (173 mg, signal of the boron-bound carbon atom was not observed
73%).
since it is significantly broadened by a quadrupolar boron
nucleus. HRMS (FD): calcd for C18H19BO5 [M]+ 326.1326;
found 326.1320.
Diheptyl quaterfuran 17
A colorless oil; Rf 0.22 (hexane); 1H NMR (C6D6, 500 MHz):
δ 7.04 (dd, J = 1.8, 0.7 Hz, 2H), 6.58 (s, 2H), 6.55 (br d, J =
3.4 Hz, 2H), 6.12 (dd, J = 3.4, 1.8 Hz, 2H), 2.80 (t, J = 7.7 Hz,
4H), 1.64 (br tt, J ≈ 7.5, 7.5 Hz, 4H), 1.41–1.35 (sym m, 4H),
3′′′,4″-Di-n-heptyl-2,2′:5′,2″:5″,2′′′:5′′′,2′′′′:5′′′′,2′′′″-sexifuran (20)
(a) Synthesis of 20 through the Stille cross-coupling of
1.31–1.18 (m, 12H), 0.88 (t, J = 7.0 Hz, 6H); 13C NMR (C6D6, dibromobifuran 15. A solution of the freshly prepared
126 MHz): δ 147.22 (C), 145.43 (C), 142.23 (C), 142.03 (CH), dibromobifuran 15 (255 mg, 0.522 mmol), tributylstannylbi-
125.01 (C), 111.69 (CH), 109.18 (CH), 105.33 (CH), 32.26 (CH2), furan 22 (666 mg, 1.574 mmol, 3.0 equiv.) and (Ph3P)4Pd
30.62 (CH2), 29.85 (CH2), 29.71 (CH2), 25.63 (CH2), 23.04 (61 mg, 0.0527 mmol, 10 mol%) in dry toluene (6 mL) was
(CH2), 14.32 (CH3); HRMS (FD): calcd for C30H38O2 [M]+ gently refluxed for 12 h. The reaction mixture was evaporated
462.2770; found 462.2777.
and subjected to three sequential FC on silica gel (gradient
elution, from hexane to 3% EtOAc–hexane) followed by
the final FC on Florisil44 (hexane) to give the sexifuran 20
2-([2,2′-Bifuran]-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (24)
To a solution of 2,2′-bifuran (26, n = 1) (975 mg, 7.274 mmol) (126 mg, 41%).
in dry THF (12 mL) at −78 °C a solution of n-BuLi in hexane
(b) Synthesis of 20 through the Suzuki–Miyaura cross-
(5.2 mL of 1.6 M solution, 8.3 mmol, 1.15 equiv.) was added. coupling. A mixture of the freshly prepared dibromide 15
The reaction mixture was allowed to warm to r.t., stirred for (232 mg, 0.475 mmol), bifuryl pinacolboronate 24 (372 mg,
2 h and cooled again to −78 °C. A solution of 2-isopropoxy- 1.430 mmol, 3 equiv.), palladium acetate (8.5 mg, 0.038 mmol,
pinacolborolane (27) (1.98 g, 10.20 mmol, 1.4 equiv.) in THF 8 mol%), XPhos (36.2 mg, 0.076 mmol, 16 mol%) and K3PO4
(5 mL) was added over 5 min. The reaction mixture was stirred (605 mg, 2.85 mmol, 6 equiv.) in 1,4-dioxane–water (10 : 1,
at −78 °C for 1 h and at r.t. overnight. After cooling to 0 °C the total 6.0 mL) was heated at 55–60 °C for 8 h. At that time, the
reaction was quenched by addition of AcOH (0.60 g, second portion of 24 (247 mg, 0.950 mmol, 2.0 equiv.) was
10.0 mmol) in ether (5 mL). The resulting mixture was stirred added, and heating at 55–60 °C was continued for additional
for 30 min at 0 °C, poured into cold water, and extracted with 8 h. The reaction mixture was diluted with water and extracted
50% EtOAc–hexane. The combined organic phase was washed with 25% EtOAc in hexane. The combined organic extract was
with water and brine, and then dried over MgSO4. Filtration washed with water and brine, and then dried over MgSO4. Fil-
and evaporation followed by sequential FC on silica gel (gradi- tration, evaporation, and FC on silica gel (gradient elution,
ent elution, from 5% to 30% EtOAc in hexane) gave the title from hexane to 3% EtOAc–hexane) followed by FC on Florisil44
boronate 24 (920 mg, 49%) as a colorless solid; m.p. 68–69 °C. (hexane) gave the sexifuran 20 (95 mg, 34%).
1H NMR (CDCl3, 300 MHz): δ 7.41 (dd, J = 1.8, 0.7 Hz, 1H),
7.11 (d, J = 3.5 Hz, 1H), 6.73 (dd, J = 3.4, 0.7 Hz, 1H), 6.59 (d,
J = 3.5 Hz, 1H), 6.44 (dd, J = 3.4, 1.8 Hz, 1H), 1.35 (s, 12H);
Diheptyl sexifuran 20
13C NMR (CDCl3, 75.5 MHz): δ 150.87 (C), 146.39 (C), 142.15 A yellow solid; m.p. 109–110 °C; Rf 0.25 (benzene–hexane =
(CH), 125.19 (CH), 111.44 (CH), 106.68 (CH), 105.86 (CH), 1 : 10); 1H NMR (C6D6, 400 MHz): δ 7.02 (dd, J = 1.8, 0.5 Hz,
84.17 (C), 24.67 (CH3). The resonance signal of the boron- 2H), 6.60 (s, 2H), 6.57 (br d, J = 3.4 Hz, 2H), 6.56 (d, J = 3.5 Hz,
bound carbon atom was significantly broadened by a quadru- 2H), 6.54 (d, J = 3.5 Hz, 2H), 6.09 (dd, J = 3.4, 1.8 Hz, 2H), 2.81
3
polar boron nucleus and was not observed. HRMS (FD): calcd (t, J = 7.6 Hz, 4H), 1.66 (br tt, J ≈ 7.4, 7.4 Hz, 4H), 1.44–1.38
for C14H17BO4 [M]+ 260.1220; found 260.1217.
(sym m, 4H), 1.34–1.20 (m, 12H), 0.87 (t, J = 6.9 Hz, 6H); 13C
NMR (C6D6, 101 MHz): δ 146.84 (C), 146.41 (C), 146.30 (C),
145.02 (C), 142.45 (C), 142.19 (CH), 125.29 (C), 111.72 (CH),
109.65 (CH), 107.56 (CH), 107.27 (CH), 105.77 (CH), 32.21
2-([2,2′:5′,2″-Terfuran]-5-yl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (25)
Prepared by following the procedure given for the synthesis of (CH2), 30.61 (CH2), 29.86 (CH2), 29.58 (CH2), 25.70 (CH2),
24 from terfuran (26, n = 2) (1.050 g, 5.243 mmol), 1.6 M solu- 23.04 (CH2), 14.28 (CH3); UV/Vis (1,4-dioxane): λmax 406,
tion of n-BuLi in hexane (3.8 mL, 6.1 mmol, 1.15 equiv.) and 448 (sh) nm; fluorescence spectrum (1,4-dioxane): λmax 446,
borolane 27 (1.424 g, 7.340 mmol, 1.4 equiv.) to yield 25 475 nm; HRMS (FD): calcd for C38H42O6 [M]+ 594.2981; found
1
(562 mg, 33%) as a cream color solid; m.p. 81–83 °C. H NMR 594.2971.
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3′′′′,4′′′-Di-n-heptyl-2,2′:5′,2″:5″,2′′′:5′′′,2′′′′:5′′′′,2′′′″:5′′′″, 2′′′′′′:5′′′′′′,-
2′′′′′′′-octifuran (21)
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(a) Synthesis of 21 through the Stille cross-coupling. Octi-
furan 21 was synthesized following the procedure given for the
synthesis of 20 from a freshly prepared dibromobifuran 15
(237 mg, 0.485 mmol), tributylstannylterfuran 23 (716 mg,
1.462 mmol, 3.0 equiv.) and (Ph3P)4Pd (57 mg, 0.0493 mmol,
10 mol%). Five sequential FC on silica gel (from hexane to 3%
EtOAc–hexane) followed by the final FC on Et3N-pretreated silica
gel44 (0.5% EtOAc–hexane) gave the title product 21 (80 mg, 23%).
(b) Synthesis of 21 through the Suzuki–Miyaura cross-
coupling. Octifuran 21 was synthesized as described for the
synthesis of 20 from a freshly prepared dibromobifuran 15
(161 mg, 0.330 mmol), terfuryl pinacolboronate 25 (total
540 mg, 1.655 mmol, 5 equiv.), Pd(OAc)2 (6.3 mg, 0.028 mmol,
8 mol%), XPhos (26.7 mg, 0.056 mmol, 16 mol%) and K3PO4
(425 mg, 2.0 mmol, 6 equiv.). Two sequential FC on silica gel
(from 1% to 3% EtOAc–hexane) followed by the final FC on
Florisil44 (0.5% EtOAc–hexane) gave the product 21 (51 mg, 21%).
Diheptyl octifuran 21
A yellow-orange solid; m.p. 140–141 °C; Rf 0.37 (benzene–
1
hexane = 1 : 5); H NMR (C6D6, 500 MHz): δ 6.99 (dd, J = 1.8,
0.5 Hz, 2H), 6.62 (s, 2H), 6.58 and 6.57 (AB-q, J = 3.6 Hz, 4H),
6.56 (d, J = 3.5 Hz, 2H), 6.53 (br d, J = 3.4 Hz, 2H), 6.51 (d, J =
3.5 Hz, 2H), 6.06 (dd, J = 3.4, 1.8 Hz, 2H), 2.83 (t, J = 7.6 Hz,
4H), 1.67 (br tt, J ≈ 7.5, 7.5 Hz, 4H), 1.42 (br tt, J ≈ 7.3, 7.3 Hz,
4H), 1.37–1.18 (m, 12H), 0.88 (t, J = 7.0 Hz, 6H); 13C NMR
(C6D6, 126 MHz): δ 146.69 (C), 146.57 (C), 146.48 (C), 145.97
(C), 145.90 (C), 144.98 (C), 142.51 (C), 142.23 (CH), 125.36 (C),
111.73 (CH), 109.81 (CH), 107.95 (CH), 107.79 (CH), 107.55
(CH), 107.42 (CH), 105.96 (CH), 32.22 (CH2), 30.62 (CH2), 29.86
(CH2), 29.59 (CH2), 25.71 (CH2), 23.05 (CH2), 14.31 (CH3);
UV/Vis (1,4-dioxane): λmax 428, 460 (sh) nm; fluorescence spec-
trum (1,4-dioxane): λmax 472, 503 nm; HRMS (FD): calcd for
C46H46O8 [M]+ 726.3193; found 726.3204.
10 For highlights on α-oligofurans, see: (a) U. H. F. Bunz,
Angew. Chem., Int. Ed., 2010, 49, 5037–5040; (b) O. Gidron
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12 C. C. Ferrón, M. C. R. Delgado, O. Gidron, S. Sharma,
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14 O. Gidron, A. Dadvand, Y. Sheynin, M. Bendikov and
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L. J. W. Shimon, M. Bendikov and D. F. Perepichka,
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M. M. Wienk, D. M. de Leeuw and R. A. J. Janssen, J. Mater.
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Acknowledgements
The authors thank Dr A. Tishby for recording HRMS,
Dr L. Konstantinovski and Dr O. Gidron for their help in the
measurements of 2D NMR spectra and fluorescence spectra
respectively. The authors acknowledge financial support from
the MINERVA Foundation (Germany) and from the Helen and
Martin Kimmel Center for Molecular Design (Weizmann Insti-
tute). E.E.K. and G.M.L. thank the Israel Ministry of Absorp-
tion for Kamea Excellence Fellowships. M.B. was a member
ad personam of the Lise Meitner – Minerva Center for Compu-
tational Quantum Chemistry.
Notes and references
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