Evidence of enhanced conjugation in ortho -arylene ethynylenes with transition metal coordination

Article abstract of DOI:10.1021/jo300034h

The effective conjugation of ortho and ortho-alt-para-arylene ethynylenes, with appropriately positioned pyridine and pyrazine heterocycles, increases upon binding to Ag(I) and Pd(II) cations. Significant bathochromic shifts in the electronic spectra, wit

Full text of DOI:10.1021/jo300034h

Note
pubs.acs.org/joc
Evidence of Enhanced Conjugation in ortho-Arylene Ethynylenes
with Transition Metal Coordination
Qianwei Ren, Cole G. Reedy, Eric A. Terrell, Joshua M. Wieting, Robert W. Wagie, Jake P. Asplin,
Leah M. Doyle, Steven J. Long, Michael T. Everard, Jon S. Sauer, Cassandra E. Baumgart,
Jason S. D’Acchioli, and Nathan P. Bowling*
Department of Chemistry, University of Wisconsin-Stevens Point, 2001 Fourth Avenue, Stevens Point, Wisconsin 54481, United
States
S
* Supporting Information
ABSTRACT: The effective conjugation of ortho and ortho-alt-
para-arylene ethynylenes, with appropriately positioned
pyridine and pyrazine heterocycles, increases upon binding
to Ag(I) and Pd(II) cations. Significant bathochromic shifts in
the electronic spectra, witnessed upon introduction of these
metal bridges, are consistent with enhanced electron
delocalization in the unsaturated backbone. Control studies
suggest that this electronic behavior is attributable exclusively
(in the case of Ag(I)) or partially (in the case of Pd(II)) to conformational restrictions of the conjugated backbones.
p-Arylene ethynylenes, such as p-phenylene ethynylene, are
common targets for chemists as they exhibit interesting
optical,1 conducting,2 and chemosensing/biosensing proper-
ties3−5 that can be used in a variety of technological
applications.6 Ortho and meta variants, on the other hand,
receive much less attention in this field as they lack the
attractive electronic properties of para systems. Beyond
fundamental differences between delocalization pathways,7
conjugation deficiencies are a result of thermodynamically
favorable helix formation in ortho and meta arylene
Figure 1. o-Phenylene ethynylenes and o-alt-p-phenylene ethynylenes,
the simplest examples of o-arylene ethynylenes and o-alt-p-arylene
ethynylenes.8−11 Upon helix formation, the π system becomes
twisted, precluding efficient electron delocalization in the
unsaturated backbone. Para systems, on the other hand, can
achieve efficient orbital overlap via free rotation of the
conjugated backbone. Moreover, the electronics of para-arylene
ethynylenes can be optimized via synthetic manipulation so that
free rotation is restricted, locking the molecule in a coplanar
conformation.12
ethynylenes, respectively, have highly unsaturated backbones but low
effective conjugation due to their propensity to fold into helices.
Enforced coplanarity in p-arylene ethynylene systems can be
achieved through a number of strategies, including hydrogen
bonding,12,13 transition metal coordination,14−16 and steric
hindrance.17 Enforcing planarity in these structures enhances
electronic conjugation, making these molecules exciting targets
for technological development.18 Similar strategies have been
exploited for stabilizing helices in meta oligomers.19,20 Little
effort, however, has been put into enforcing planarity in ortho
and meta structures for the purposes of enhancing electronic
properties.
As an entry point into this field, our focus was to establish a
general strategy for enforcing planarity in ortho-type arylene
ethynylenes (i.e., ortho and ortho-alt-para systems) (Figure 1).
To this end, we identified 1,2-bis(2′-pyridinylethynyl)benzene
(Figure 2) as a structural element that contains an ortho
Figure 2. 1,2-Bis(2′-pyridinylethynyl)benzene binds in a bidentate
fashion to a variety of metals including Ag(I) and Pd(II).
orientation and can be conformationally restricted via transition
metal coordination. This well-studied ligand binds to a number
of different metal cations, including Ag(I) and Pd(II),21−24 and
can be used in metal sensing25 or as a ligand in the Heck
reaction.26 For our purposes, the transition metal cation serves
as a bridge, restricting rotation within the macrocyclic structure.
Received: January 5, 2012
Published: February 1, 2012
© 2012 American Chemical Society
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Scheme 1. Generation of Ligands 1 and 2
Two systems, 1 and 2, should provide insight into the
feasibility of enforcing coplanarity in o-arylene ethynylenes and
o-alt-p-arylene ethynylenes via transition metal coordination
(Scheme 1). Sonogashira coupling of terminal alkyne 8, derived
from 1,2-bis(dodecyloxy)-4,5-diiodobenzene, with 2,5-diiodo-
pyrazine and 2,3-diiodopyrazine yields 1 and 2, respectively.
The conditions shown provided the highest conversions to
their respective products. Low yields in the generation of 1 are
a reflection of difficult chromatographic separations of the
product from the homo- and monocoupled byproducts rather
than poor conversion. One concern in these reactions was the
potential of the products and intermediates to bind to Pd(II)
and Cu(I), perhaps interfering with their catalytic viability. It
was our hope that the ability of 1,2-bis(2′-pyridinylethynyl)-
benzene to function as a ligand for the Heck reaction26 would
carry over to these Sonogashira reactions. To our delight, these
reactions proceeded with acceptable conversion, despite likely
involvement of our ligands in the catalytic cycle.
To verify that conformational control (i.e., planarity) can be
achieved upon introduction of metals to 1 and 2, there must be
evidence of increased conjugation in the presence of metals.
Increased conjugation results in a decrease in the HOMO−
LUMO gap, which is most often observed as a bathochromic
shift for the lowest energy absorption band. Indeed, when
dilute samples of ligands 1 and 2 were titrated with silver triflate
(AgOTf), significant bathochromic shifts were observed (Δλmax
= 75 nm for 1, Δλmax = 50−60 nm for 2) (Figure 3). As one
might expect, an end point is reached once two equivalents of
Ag(I) are added to 1 (Figure 3a). Additional metal elicited no
Figure 3. Significant bathochromic shifts are observed for the lowest
energy absorption bands when (a) 1 and (b) 2 are treated with
AgOTf.
further changes in the absorbance profile. With compound 2
(Figure 3b), more metal was required to reach the end point.
The origin of this difference in behavior is not clear at this
point, but it may indicate that planarity is less favorable in 2 as
this could lead to steric hindrance between alkoxy groups or the
creation of a very strong dipole in the metal−ligand complex.
Nonetheless, both compounds display behavior indicative of
increased conjugation upon metal coordination.
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Scheme 2. Generation of Ligands 3−6
To be certain that observed electronic changes are a
symptom of increased conjugation rather than electronic
interactions with the metal or formation of coordination
polymers and/or aggregates, we generated 3−6 (Scheme 2),
which are very similar to 1 and 2 in terms of their electronic
structure and ability to form coordination polymers and
aggregates but are mismatched for coordination-driven
planarization. The synthetic strategies for these isomers were
similar to those employed for 1 and 2. Careful deprotection of
appropriate silyl-protected precursors (9 and 11) yielded the
desired terminal alkynes (10 and 12) necessary for construction
of 3−6. Coupling of these alkynes with either 2,3-
diiodopyrazine or 2,5-diiodopyrazine yielded the four targeted
arylene ethynylenes (3−6) in reasonable to good yields.
Titration of dilute solutions of 3−6 in THF with AgOTf
leads to only negligible changes (Δλmax < 15 nm) in the
electronic spectra (see Supporting Information), consistent
with our expectation that metal binding in these compounds
does not lead to increased conjugation. Moreover, a lack of
electronic changes upon attempted saturation of potential
pyridine/pyrazine binding sites with excess Ag(I) supports our
assertion that the bathochromic shifts observed with 1 and 2
are at least partly due to conformation restrictions rather than
electronic metal/ligand interactions.
Attempts to further characterize complexes of 1−2 by NMR
and GPC were plagued by low solubility in appropriate organic
solvents. Lack of solubility could signify enhanced π−π stacking
from planarization of the conjugated backbone as complexes of
3, 4 and 6 are much more soluble than 1 and 2 (ligand 5 is
insoluble in most solvents). Nonetheless, ligand/metal/solvent
combinations that would allow better characterization of
complex formation were sought. PdCl2(PhCN)2 proved to be
a valuable tool for tracking sequential binding of metal cations.
UV−vis titration studies of 1 and 1H NMR titration studies of 2
provide evidence of distinct unbound (1, 2), singularly bound
(1-Pd1, 2-Pd1) and doubly bound (1-Pd2, 2-Pd2) ligands.
Upon initial introduction of metal to 1 (Figure 4), the signal
for the unbound ligand (λmax = 395 nm) decreases, while a
signal for what appears to be singly bound ligand (1-Pd1) grows
in with a λmax = 457 nm. As 2.0 equivalents of metal are
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Figure 4. UV−Vis titration study of ligand 1 with Pd(II) indicates
three distinct species: unbound 1, singly bound 1 (1-Pd1) and doubly
bound 1 (1-Pd2).
reached, this intermediate signal goes away while an absorption
for doubly bound ligand (1-Pd2) grows in with a λmax = 505
nm. It is worth noting that the control molecules (3−6) also
red-shift when PdCl2(PhCN)2 is introduced. Based on some
model calculations with 1,2-bis(2′-pyridinylethynyl)-benzene
(see Supporting Information), we tentatively attribute this
behavior to mixing of Cl p-orbitals with the π/π* system of the
pyridines.27 However, because complexation does not increase
conjugation for these systems, the shifts are less pronounced for
3 (Δλmax = 55 nm), 4 (Δλmax = 50 nm), and 5 (Δλmax = 30 nm)
than 1 (Δλmax = 110 nm) and 2 (Δλmax = 85 nm). Titration
studies with PdCl2(PhCN)2 were further complicated by the
fact that at high metal concentrations all electronic features for
ligands 1-6 were washed out, yielding broad featureless spectra.
This behavior, likely attributable to aggregation, is most evident
in the 4-ethynyl pyridine derivatives (5 and 6), where this
broadening is observed immediately upon Pd(II) introduction
(precluding an estimated Δλmax for 6).
1
Figure 5. H NMR titration study of ligand 2 with Pd(II) reveals a
progression from (a) just 2 with no metal, to (b) a mix of 2, 2-Pd1 and
2-Pd2 with 1.0 equiv of PdCl2(PhCN)2, to (c) just 2-Pd2 with 2.0
equiv of PdCl2(PhCN)2.
2-ethynyl, 3-ethynyl, or 4-ethynyl). Ligands 1 (λabs = 395 nm,
λem = 458 nm), 3 (λabs = 406 nm, λem = 463 nm) and 5 (λabs
406 nm, λem = 457 nm), for instance, have very similar
absorbance and emission profiles. Likewise, ligands 2 (λabs
∼350 nm, λem = 437 nm), 4 (λabs ≈ 350 nm, λem = 434 nm) and
6 (λabs ≈ 350 nm, λem = 434 nm) have nearly identical
electronic profiles. In all isomers, fluorescence is quenched
upon introduction of Ag(I) and Pd(II). This is consistent with
what has been reported for other 1,2-bis(2′-pyridinylethynyl)-
benzene-type systems.28
=
=
Solubility issues of metal−ligand complexes prevented
systematic NMR studies of metal titrations. Fortunately,
compound 2 and its Pd(II) complexes are sparingly soluble
in CDCl3, allowing characterization of coordination behavior.
With no metal present (Figure 5a), there are two signals for 2
above 8.3 ppm: a singlet (8.52 ppm) corresponding to the
pyrazine hydrogen atoms (HB) and a doublet (8.49 ppm)
corresponding to the pyridine hydrogen atoms (HA). At two
equivalents (Figure 5c) of Pd(II), this singlet (HH: 8.67 ppm)
is shifted downfield along with the doublet (HH: 8.84 ppm). At
one equivalent (Figure 5b), all three species (2, 2-Pd1 and 2-
Pd2) are present, with four resonances for 2-Pd1 at 8.88 ppm
(HC), 8.72 ppm (HD), 8.48 ppm (HE) and 8.37 ppm (HF). The
asymmetry of 2-Pd1 is evident in not only the number of
resonances but also splitting of the pyrazine hydrogens (HD: d,
8.72 ppm, HE: d, 8.48 ppm).
Compounds 1−6 are stable molecules that show no signs of
decomposition either in the solid state or in solution. Metal
complexes of these ligands, however, show slow discoloration
over several days in the solid state. This instability may be
indicative of a susceptibility to oxidation/reduction at the metal
centers, which would be consistent with our inability to
characterize metal complexes of 1−6 via ESI-MS. All six
reported compounds (1−6) are emissive (see Supporting
Information for spectra). The lowest energy electronic
transitions of the six isomers depends on the substitution
pattern around the central pyrazine ring and does not seem to
be affected by the arrangement of the pyridinyl end groups (i.e.,
In summary, two arylene ethynylene structures (1 and 2)
bind to Ag(I) and Pd(II) cations, eliciting behavior that is
consistent with increased effective conjugation. Support for this
conclusion comes from comparisons of the metal binding
behaviors of 1 and 2 to isomeric 3, 4, 5 and 6, which show no
signs of increased conjugation upon metal complexation.
Continuing studies of these systems will address how this
behavior applies to longer arylene ethynylene oligomers with
the same pattern of alternating benzene/heterocyclic rings.
EXPERIMENTAL SECTION
■
Transition Metal Titration Studies. Solutions for UV−vis and
fluorescence studies were made by combining dilute solutions of
ligands in HPLC grade THF with dilute solutions of AgOTf or
PdCl2(PhCN)2 in HPLC grade THF so that the final ligand
concentrations were between 10−5 and 10−6 M (exact concentrations
are listed in Supporting Information). Each of the metal/ligand
samples (e.g., 1.0 equiv of AgOTf, 2.0 equiv of AgOTf, etc.) was
prepared separately in advance in volumetric glassware, allowing for
ample mixing/equilibration time before analysis.
General Coupling Conditions. All Pd coupling reactions were
performed under an Ar atmosphere. Generally, three vacuum/purge
cycles were applied to the reaction tube to remove air and refill with
Ar. Air was displaced from the solvents by bubbling either N2 or Ar
through the liquids for at least 15 min.
2,5-Diiodopyrazine. Commercially available 2,5-dibromopyrazine
(1.84 g, 7.75 mmol) was dissolved with sodium iodide (5.81 g, 38.74
mmol) in acetonitrile (15 mL). Small amounts of concentrated H2SO4
(8 drops) and glacial acetic acid (20 drops) were added, and the
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mixture was refluxed for 24 h. The reaction mixture was cooled, then
diluted with CH2Cl2 (50 mL). This organic phase was washed with a
saturated NaHCO3 solution (3×), and a saturated sodium bisulfite
solution (2×). The remaining organic phase was dried with anhydrous
Na2SO4, filtered and concentrated to reveal the product as a white
solid (2.14 g, 6.45 mmol, 83% yield). 1H NMR (400 MHz, CDCl3): δ
= 8.64 (s, 2H) ppm. 13C NMR (100.6 MHz, CDCl3): δ = 154.0, 116.5
ppm. ESI-MS (m/z) calcd for C4H2I2N2H+ 332.8386; found 332.8359
(small signal due to low solubility in 2-propanol mobile phase).
2,3-Diiodopyrazine. Commercially available 2,3-dichloropyrazine
(1.0 g, 6.72 mmol) was refluxed with NaI (5.04 g, 33.6 mmol), glacial
acetic acid (0.76 mL), and concentrated sulfuric acid (0.04 mL) in
acetonitrile (10 mL) for one day. Upon cooling, the solvent was
removed under reduced pressure. The remaining reddish-brown solid
was taken up in CH2Cl2 (100 mL) and washed sequentially with a
saturated sodium bicarbonate solution (50 mL) and a saturated
sodium metabisulfite solution. The organic phase was dried with
anhydrous Na2SO4, filtered and concentrated to reveal a light brown
solid (1.82 g, 5.48 mmol, 82% yield). NMR (400 MHz, CDCl3): δ =
8.30 (s, 2H) ppm. 13C NMR (100.6 MHz, CDCl3): δ = 142.8, 131.5
ppm. ESI-MS (m/z) calcd for C4H2I2N2H+ 332.8386; found 332.8367
(small signal due to low solubility in 2-propanol mobile phase).
2-((4,5-Bis(dodecyloxy)-2-((trimethylsilyl)ethynyl)phenyl)-
ethynyl)pyridine (7). An oven-dried, side arm storage tube was
evacuated and filled with Argon. After addition of Pd(PPh3)2Cl2
(0.151 g, 0.22 mmol), CuI (0.041 g, 0.22 mmol) and 1,2-
bis(dodecyloxy)-4,5-diiodobenzene29,30 (3.00 g, 4.3 mmol), the tube
was purged with Ar (3×). Following the addition of freshly distilled
triethylamine (40 mL) and trimethylsilylacetylene (0.79 mL, 5.6
mmol) via syringe, the reaction tube was sealed and the contents
stirred at room temperature for one day. The contents were diluted
with CH2Cl2 and washed with distilled water until the aqueous layers
were colorless (5 washes) then with brine (3×). The organic layer was
dried with anhydrous Na2SO4, filtered and concentrated. This crude
mixture was dissolved in freshly distilled NEt3 and added to a storage
tube containing Pd(PPh3)2Cl2 (0.12 g, 1.7 mmol) and CuI (0.033 g,
0.17 mmol) under Ar. After argon was bubbled through this sample for
five minutes, 2-ethynylpyridine (0.45 mL, 4.47 mmol) was added via
syringe and the tube was sealed and heated at 70 °C for one day. The
contents were cooled, diluted with CH2Cl2 and washed with distilled
water until the aqueous layers were colorless, then with brine. The
organic layer was dried with anhydrous Na2SO4, filtered and
concentrated. The remaining oil was purified via flash chromatography
(silica gel) using a gradient mobile phase starting with a 20% CH2Cl2/
80% hexane mixture with incremental increases in polarity up to 100%
CH2Cl2. Appropriate fractions were concentrated to reveal the product
as a yellowish oil (1.66 g, 2.58 mmol, 60% from 1,2-bis(dodecyloxy)-
4,5-diiodobenzene). 1H NMR (400 MHz, CDCl3): δ = 8.61 (d, J = 4.6
Hz, 1H), 7.65 (td, J = 7.7, 1.5 Hz, 1H), 7.51 (dd, J = 7.7, 0.8 Hz), 7.21
(m, 1H), 7.07 (s, 1H), 6.96 (s, 1H), 4.00 (q, J = 6.4 Hz, 4H), 1.82 (p, J
= 6.4 Hz, 4H), 1.4−1.2 (m, 36H), 0.88 (t, J = 6.6 Hz, 6H), 0.27 (s,
9H) ppm. 13C NMR (100.6 MHz, CDCl3): δ = 150.4, 149.8, 149.5,
144.1, 136.3, 127.4, 122.8, 119.2, 118.2, 116.4, 116.4, 104.0, 97.1, 91.3,
88.7, 69.4, 32.3, 30.0, 30.0, 29.9, 29.7, 26.3, 26.3, 23.0, 14.4, 0.4 ppm.
ESI-MS (m/z) calcd for C42H65NO2SiH+ 644.4863; found 644.4790.
2-((4,5-Bis(dodecyloxy)-2-ethynylphenyl)ethynyl)pyridine
(8). Protected alkyne 7 (0.25 g, 0.38 mmol), dissolved in THF (10
mL), was cooled to −84 °C (EtOAc/N2) under an argon atmosphere.
Tetrabutylammonium fluoride (0.34 mL of 1.0 M solution in THF,
0.34 mmol) was added dropwise via syringe. After 30 min of stirring at
this temperature, the reaction flask was allowed to warm to room
temperature. Volatile solvents were gently removed (bath temp. < 40
°C) under reduced pressure. The remaining oil was dissolved in
CH2Cl2. The resulting organic layer was washed with water (2×),
dried with Na2SO4, filtered and concentrated to reveal 8 as a white
solid (0.202 g, 0.35 mmol, 92% yield). 1H NMR (400 MHz, CDCl3): δ
= 8.62 (dd, J = 4.8, 0.6 Hz, 1H), 7.66 (td, J = 7.7, 1.8 Hz, 1H), 7.54 (d,
J = 7.7 Hz, 1H), 7.22 (ddd, J = 7.7, 4.8, 1.1 Hz, 1H), 7.08 (s, 1H), 6.98
(s, 1H), 4.00 (t, J = 6.6 Hz, 2H), 3.99 (t, J = 6.6 Hz, 2H), 3.30 (s, 1H),
1.82 (p, J = 6.8 Hz, 4H), 1.5−1.2 (m, 36H), 0.88 (t, J = 6.6 Hz, 6H)
ppm. 13C NMR (100.6 MHz, CDCl3): δ = 150.1, 149.6, 149.5, 143.7,
136.0, 127.3, 122.6, 118.1, 117.8, 116.5, 116.3, 91.0, 88.2, 82.3, 79.7,
69.2, 69.2, 31.9, 29.7, 29.7, 29.6, 29.4, 29.0, 29.0, 26.0, 22.7, 14.1, 1.0
ppm. ESI-MS (m/z) calcd for C39H57NO2H+ 572.4468; found
572.4470.
2,5-Bis((4,5-bis(dodecyloxy)-2-(pyridin-2-ylethynyl)phenyl)-
ethynyl)pyrazine (1). To an air-free storage tube (under argon) was
added 8 (0.493 g, 0.86 mmol), 2,5-diiodopyrazine (0.152 g, 0.46
mmol), CuI (0.012 g, 0.062 mmol), PdCl2(PhCN)2 (0.0097 g, 0.025
mmol), and 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl (X-
Phos, 0.0095 g, 0.020 mmol). The powders were rinsed into the tube
with 20 mL freshly distilled triethylamine, and the tube was sealed and
heated at 80 °C for two days. After cooling to room temperature, the
contents were diluted with CH2Cl2. This organic mixture was washed
with saturated ammonium chloride solution (3×). The combined
aqueous phases were back-extracted with CH2Cl2 (3×). The combined
organic layers were then washed with distilled water and brine, then
dried with anhydrous Na2SO4, filtered and concentrated. Sequential
flash chromatography columns (silica gel, 1% MeOH/CHCl3, then
silica gel, 1% EtOAc/CHCl3) provided enough pure product (yellow
solid, 0.208 g, 0.17 mmol, 39% yield, mp = 153−155 °C) for further
studies. 1H NMR (400 MHz, CDCl3): δ = 8.91 (s, 2H), 8.65 (m, 2H),
7.67 (ddd, J = 7.8, 7.3, 1.7 Hz, 2H), 7.63 (m, 2H), 7.23 (ddd, J = 4.9,
1.7, 1.5 Hz, 2H, 7.14 (s, 2H), 7.12 (s, 2H), 4.04 (t, J = 6.6 Hz, 4H),
4.03 (t, J = 6.5 Hz, 4H), 1.85 (p, J = 6.7 Hz, 8H), 1.49 (m, 8H), 1.4−
1.2 (m, 64H), 0.88 (t, J = 6.6 Hz, 12H) ppm. 13C NMR (100.6 MHz,
CDCl3): δ = 150.3, 150.3, 149.7, 147.4, 143.5, 137.8, 136.2, 127.1,
122.8, 118.7, 117.3, 116.0, 94.4, 92.0, 89.3, 87.9, 69.3, 69.2, 31.9, 29.7,
29.7, 29.6, 29.4, 29.0, 29.0, 26.0, 22.7, 14.1 ppm. ESI-MS (m/z) calcd
for C82H115N4O4H+ 1219.8918; found 1219.8803. λmax nm (ε,
M−1cm−1) (THF) = 299 (21,920), 395 (8,240).
2,3-Bis((4,5-bis(dodecyloxy)-2-(pyridin-2-ylethynyl)phenyl)-
ethynyl)pyrazine (2). To an air-free storage tube (under argon) was
added 2,3-diiodopyrazine (0.230 g, 0.67 mmol), PdCl2(PhCN)2
−
(0.0195 g, 0.051 mmol), HP+(Bu)3BF4 (0.0295 g, 0.102 mmol)
and CuI (0.0070 g, 0.037 mmol). Freshly distilled triethylamine (20
mL) was added via syringe. In a separate container, terminal alkyne 8
(0.846 g, 1.48 mmol) was dissolved in triethylamine (5 mL) and was
transferred via syringe to the reaction tube. An additional five mL
triethylamine was added and the tube was sealed and heated at 80 °C
for one day. After cooling to room temperature, the contents of the
tube were diluted with ethyl acetate. This organic phase was washed
with distilled water (3 × 25 mL) and brine (3 × 30 mL), then dried
with anhydrous Na2SO4, filtered and concentrated under reduced
pressure. The crude sample was purified via flash chromatography
(silica gel, 40% EtOAc/60% hexane increased to 50% EtOAc/50%
hexane). Appropriate fractions were concentrated to reveal the
product as a yellow solid (0.576 g, 0.47 mmol, 70% yield, mp =
85−88 °C). 1H NMR (400 MHz, CDCl3): δ = 8.52 (s, 2H), 8.49 (d, J
= 4.5 Hz, 2H), 7.62 (d, J = 7.8 Hz, 2H), 7.54 (td, J = 7.6, 1.7 Hz, 2H),
7.11 (ddd, J = 7.5, 4.9, 1.2 Hz, 2H), 7.04 (s, 2H), 7.01 (s, 2H), 3.97 (t,
J = 6.6 Hz, 4H), 3.86 (t, J = 6.6 Hz, 4H), 1.9−1.7 (m, 8H), 1.5−1.2
(m, 72H), 0.88 (t, J = 6.6 Hz, 12H) ppm. 13C NMR (100.6 MHz,
CDCl3): δ = 150.0, 149.7, 149.4, 143.4, 142.2, 141.9, 135.9, 127.7,
122.4, 118.4, 117.2, 116.3, 115.9, 95.6, 92.0, 88.6, 88.0, 69.2, 69.0, 32.0,
29.8, 29.7, 29.7, 29.7, 29.5, 29.4, 29.4, 29.1, 29.0, 26.0, 26.0, 22.7, 14.1
ppm. ESI-MS (m/z) calcd for C82H115N4O4H+ 1219.8918; found
1219.8805. λmax nm (ε, M−1cm−1) (THF) = 210 (38,650), 303
(55,180), 350 (23,200).
3-((4,5-Bis(dodecyloxy)-2-((trimethylsilyl)ethynyl)phenyl)-
ethynyl)pyridine (9). To an oven-dried storage tube, under argon,
was added 1,2-bis(dodecyloxy)-4,5-diiodobenzene (2.0 g, 2.86 mmol),
CuI (0.27 g, 0.143 mmol) and Pd(PPh3)2Cl2 (0.100 g, 0.143 mmol).
Freshly distilled triethylamine (30 mL) was added via syringe, followed
by trimethylsilylacetylene (0.45 mL, 3.15 mmol). The tube was sealed
and the contents stirred at room temperature for one day. After
dilution with CH2Cl2, the organic phase was washed with saturated
NH4Cl solution (2 × 20 mL), distilled water (3 × 30 mL), and brine
(2 × 20 mL), dried with anhydrous Na2SO4, filtered and concentrated
under reduced pressure. This crude mixture was taken up in freshly
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distilled triethylamine (20 mL) then added to an air-free (under Ar)
storage tube containing Pd(PPh3)2Cl2 (0.100 g, 0.143 mmol), CuI
(0.027 g, 0.143 mmol), and 3-ethynylpyridine (0.38 g, 3.72 mmol).
Additional triethylamine (10 mL) was added before the tube was
sealed and heated at 80 °C for one day. After cooling and diluting
contents with CH2Cl2, the organic phase was washed with saturated
NH4Cl solution (2 × 30 mL), distilled water (3 × 20 mL), and brine
(2 × 20 mL), dried with anhydrous Na2SO4, filtered and concentrated
under reduced pressure. The crude sample was purified via flash
chromatography (silica gel, CH2Cl2). Appropriate fractions were
concentrated to reveal the product as a yellow oil (0.905 g, 1.4 mmol,
49% yield from 1,2-bis(dodecyloxy)-4,5-diiodobenzene). 1H NMR
(400 MHz, CDCl3): δ = 8.78 (d, J = 1.3 Hz, 1H), 8.53 (dd, J = 4.9, 1.4
Hz, 1H), 7.80 (dt, J = 7.9, 1.9 Hz, 1H), 7.27 (m, 1H), 6.98 (s, 1H),
6.97 (s, 1H), 4.00 (m, 4H), 1.83 (p, J = 6.7 Hz, 4H), 1.5−1.2 (m,
36H), 0.88 (t, J = 6.3 Hz, 6H), 0.26 (s, 9H) ppm. 13C NMR (100.6
MHz, CDCl3): δ = 152.6, 149.9, 149.7, 148.7, 138.5, 123.3, 121.1,
119.2, 118.5, 116.8, 116.3, 104.0, 97.3, 92.2, 88.6, 69.6, 69.6, 32.3, 30.0,
30.0, 29.9, 29.7, 29.4, 26.3, 23.0, 14.4, 1.3 ppm. ESI-MS (m/z) calcd
for C42H65NO2SiH+ 644.4863; found 644.4880.
3-((4,5-Bis(dodecyloxy)-2-ethynylphenyl)ethynyl)pyridine
(10). Protected alkyne 9 (0.90 g, 1.4 mmol) was dissolved in THF (20
mL) and the solution cooled to −84 °C (EtOAc/N2). Tetrabuty-
lammonium fluoride (2.0 mL, 1.0 M solution in THF, 2.0 mmol) was
added dropwise via syringe. The reaction mixture was allowed to
slowly warm to room temperature, then stir for one day. After gentle
removal of the solvent (bath temp. < 40 °C) under reduced pressure,
the crude product was purified via flash chromatography (silica gel,
CH2Cl2). Concentration of appropriate fractions revealed the product
as a greyish/white solid (0.61 g, 1.1 mmol, 75% yield). 1H NMR (400
MHz, CDCl3): δ = 8.78 (d, J = 1.4 Hz, 1H), 8.53 (dd, J = 4.8, 1.6 Hz,
1H), 7.81 (dt, J = 7.9, 1.8 Hz, 1H), 7.27 (m, 1H), 6.99 (s, 2H), 4.01
(q, J = 6.6 Hz, 4H), 3.29 (s, 1H), 1.82 (m, 4H), 1.46 (m, 4H), 1.4−1.2
(m, 32H), 0.88 (m, 6H) ppm. 13C NMR (100.6 MHz, CDCl3): δ =
152.6, 149.8, 149.8, 148.7, 138.6, 123.3, 121.0, 118.6, 118.0, 116.9,
116.1, 91.8, 88.7, 82.6, 80.1, 69.5, 32.2, 30.0, 30.0, 29.9, 29.7, 29.7,
29.4, 26.3, 23.0, 14.4, 1.3 ppm. ESI-MS (m/z) calcd for C39H57NO2H+
572.4468; found 572.4471.
2,5-Bis((4,5-bis(dodecyloxy)-2-(pyridin-3-ylethynyl)phenyl)-
ethynyl)pyrazine (3). Terminal alkyne 10 (0.300 g, 0.52 mmol) was
dissolved in freshly distilled triethylamine (25 mL) and transferred via
syringe to an air-free storage tube under argon. 2,5-Diiodopyrazine
(0.083 g, 0.24 mmol), CuI (0.0186 g, 0.098 mmol), PdCl2(PhCN)2
(0.0053 g, 0.0138 mmol), and 2-dicyclohexylphosphino-2′,4′, 6′-
triisopropylbiphenyl (X-Phos, 0.0036 g, 0.0076 mmol) were added
to the reaction tube and rinsed into solution with 5 mL NEt3. The
tube was sealed and heated at 80 °C for one day. After cooling to room
temperature, the mixture was diluted with CH2Cl2. This organic
mixture was washed with saturated NH4Cl solution until the aqueous
layer was no longer blue. The aqueous phases were then extracted with
CH2Cl2 (3×). The combined organic layers were washed with distilled
water and brine then concentrated under reduced pressure. This crude
mixture was purified using consecutive flash chromatography columns
(1st: silica gel, CH2Cl2 then 5% MeOH/95% CH2Cl2. second: silica
gel, 1% MeOH in CHCl3). Appropriate fractions were concentrated to
reveal the product as a brownish solid (0.124 g, 0.10 mmol, 42% yield,
mp = 160−163 °C). 1H NMR (400 MHz, CDCl3): δ = 8.86 (d, J = 1.4
Hz, 2H), 8.72 (s, 2H), 8.55 (dd, J = 4.9, 1.5 Hz, 2H), 7.91 (dt, J = 7.7,
1.9 Hz, 2H), 7.29 (dd, J = 7.8, 4.9 Hz, 2H), 7.11 (s, 2H), 7.05 (s, 2H),
4.05 (m, 8H), 1.86 (m, 8H), 1.6−1.2 (m, 72H), 0.88 (t, J = 6.8 Hz,
12H) ppm. 13C NMR (100.6 MHz, CDCl3): δ = 152.2, 150.4, 149.6,
148.6, 147.0, 138.4, 137.8, 123.1, 120.5, 118.9, 116.9, 116.1, 115.6,
94.4, 91.3, 89.4, 89.1, 69.3, 31.9, 29.7, 29.7, 29.6, 29.4, 29.0, 26.0, 22.7,
14.1 ppm. ESI-MS (m/z) calcd for C82H115N4O4H+ 1219.8918; found
1219.8794. λmax nm (ε, M−1cm−1) (THF) = 299 (67700), 406
(33900).
(0.0801 g, 0.24 mmol), CuI (0.0030 g, 0.016 mmol), PdCl2(PhCN)2
(0.0047 g, 0.012 mmol), and 2-dicyclohexylphosphino-2′,4′,6′-
triisopropylbiphenyl (X-Phos, 0.0034 g, 0.0072 mmol) were added
and rinsed into the container with additional triethylamine (5 mL).
The tube was sealed and heated at 80 °C for one day. After cooling to
room temperature, the contents of the tube were diluted with CH2Cl2.
This organic layer was washed with saturated NH4Cl solution (2 × 30
mL), distilled water (3 × 30 mL) and brine (2 × 30 mL), then dried
with anhydrous Na2SO4, filtered and concentrated under reduced
pressure. This crude mixture was purified by flash chromatography
(silica gel, 50% EtOAc/50% hexane increasing to 90% EtOAc/10%
hexane). Appropriate fractions were concentrated to reveal the
product as a brown solid (0.220 g, 0.18 mmol, 75% yield, mp =
1
85−88 °C). H NMR (400 MHz, CDCl3): δ = 8.69 (s, 2H), 8.53 (s,
2H), 8.42 (dd, J = 4.8, 1.2 Hz, 2H), 7.74 (dt, J = 7.9, 1.8 Hz, 2H), 7.13
(dd, J = 7.8, 4.9 Hz, 2H), 7.00 (s, 2H), 6.92 (s, 2H), 4.00 (t, J = 6.6
Hz, 4H), 3.85 (t, J = 6.6 Hz, 4H), 1.84 (p, J = 7.0 Hz, 4 H), 1.78 (p, J
= 7.0 Hz, 4H), 1.5−1.2 (m, 72H), 0.88 (t, J = 6.8 Hz, 12H) ppm. 13C
NMR (100.6 MHz, CDCl3): δ = 152.1, 150.1, 149.3, 148.2, 142.4,
141.8, 138.4, 122.8, 120.4, 118.7, 117.2, 116.3, 115.3, 95.4, 91.4, 89.4,
88.7, 69.3, 69.0, 32.0, 31.9, 29.8, 29.7, 29.7, 29.7, 29.5, 29.4, 29.4, 29.1,
26.0, 26.0, 22.7, 14.1 ppm. ESI-MS (m/z) calcd for C82H115N4O4H+
1219.8918; found 1219.8808. λmax nm (ε, M−1cm−1) (THF) = 273
(19800), 303 (21600), 350 (9600).
4-((4,5-Bis(dodecyloxy)-2-((triisopropylsilyl)ethynyl)phenyl)-
ethynyl)pyridine (11). Pd(PPh3)2Cl2 (0.022 g, 0.031 mmol) and
CuI (0.006 g, 0.031 mmol) were added to a storage tube under Ar.
After three vacuum/purge cycles (Ar), 4-bromopyridine hydrochloride
(0.120 g, 0.616 mmol) was added and rinsed into the container with
degassed DMF (2 mL). The terminal alkyne precursor (obtained by
methods reported by Zhao)31 was taken up in degassed diisopropyl-
amine (4 mL) then transferred to the reaction tube, followed by
additional DIPA (2 mL). The tube was sealed and heated at 60 °C for
20 h. After cooling to room temperature, the contents were transferred
to a separatory funnel with diethyl ether and sat. NH4Cl solution. The
layers were separated and the organic phase washed sequentially with
sat. NH4Cl solution then brine, dried with anhydrous Na2SO4, filtered
and concentrated. The resulting brown oil was purified via flash
chromatography (silica, 25% ether/75% hexane eluent) to reveal a
light brown oil (0.261 g, 0.36 mmol, 58% yield). 1H NMR (400 MHz,
CDCl3): δ = 8.57 (m, 2H), 7.35 (m, 2H), 6.98 (s, 1H), 6.96 (s, 1H),
4.01 (m, 4H), 1.83 (m, 4H), 1.5−1.2 (m, 36H), 1.12 (s, 21H), 0.88 (t,
J = 6.9 Hz, 6H) ppm. 13C NMR (100.6 MHz, CDCl3): δ = 149.8,
149.6, 149.2, 131.8, 125.6, 119.3, 117.5, 116.7, 116.3, 105.2, 93.6, 93.4,
88.6, 69.3, 69.2, 31.9, 29.7, 29.7, 29.6, 29.4, 29.1, 29.1, 26.0, 26.0, 22.7,
18.7, 14.1, 11.4 ppm. ESI-MS (m/z) calcd for C48H77NO2SiH+
728.5802; found 728.5739.
4-((4,5-Bis(dodecyloxy)-2-ethynylphenyl)ethynyl)pyridine
(12). TIPS-protected alkyne 11 (0.261 g, 0.36 mmol) was dissolved in
wet THF (10 mL) then cooled to −89 °C (2-propanol/liq. N2). TBAF
(1 M in THF, 0.36 mL, 0.36 mmol) was added dropwise via syringe.
The mixture stirred on the cold bath for 20 min, then was removed
from the bath and was allowed to stir for another 20 min. The mixture
was rinsed into a separatory funnel with sat. NH4Cl solution and
diethyl ether. The layers were separated and the organic layer was then
washed with H2O and brine, dried with anhydrous Na2SO4, filtered
and concentrated. The resulting residue was purified via flash
chromatography (silica, 50% ether/50% hexane) to yield a white
solid (0.166 g, 0.29 mmol, 81% yield). 1H NMR (400 MHz, CDCl3): δ
= 8.59 (m, 2H), 7.38 (m, 2H), 6.99 (s, 1H), 6.99 (s, 1H), 4.01 (m,
4H), 3.30 (s, 1H), 1.83 (m, 4H), 1.5−1.2 (m, 36 H), 0.88 (m, 6H)
ppm. 13C NMR (100.6 MHz, CDCl3): δ = 149.8, 149.7, 149.5, 131.7,
125.5, 118.1, 117.7, 116.5, 115.9, 93.0, 89.1, 82.2, 79.9, 69.3, 69.2, 31.9,
29.7, 29.7, 29.6, 29.4, 29.1, 29.0, 26.0, 22.7, 17.7, 14.1, 12.3 ppm. ESI-
MS (m/z) calcd for C39H57NO2H+ 572.4468; found 572.4404.
2,5-Bis((4,5-bis(dodecyloxy)-2-(pyridin-4-ylethynyl)phenyl)-
ethynyl)pyrazine (5). PdCl2(PhCN)2 (1.6 mg, 0.0042 mmol), X-
Phos (2.0 mg, 0.0042 mmol), 2,5-diiodopyrazine (0.046 g, 0.14 mmol)
and CuI (∼1 mg) were added to a reaction tube and oxygen was
removed via three vacuum/purge cycles (Ar). Degassed DMF (1 mL)
2,3-Bis((4,5-bis(dodecyloxy)-2-(pyridin-3-ylethynyl)phenyl)-
ethynyl)pyrazine (4). Terminal alkyne 10 (0.300 g, 0.52 mmol) was
dissolved in freshly distilled triethylamine (30 mL) and transferred via
syringe to an air-free storage tube under argon. 2,3-Diiodopyrazine
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The Journal of Organic Chemistry
Note
was added via syringe. Terminal acetylene 12 (0.166 g, 0.29 mmol)
was taken up in degassed diisopropylamine (20 mL) and was
transferred to the reaction tube via syringe, followed by additional
DIPA (2 mL). The reaction tube was sealed and the contents heated at
70 °C for three days. A yellow precipitate formed during the reaction.
This solid was removed via suction filtration and rinsed with ether, sat.
NH4Cl solution, water, then cold ether. The bright yellow solid was
air-dried and weighed (0.105 g, 0.086 mmol, 61% yield, mp = 157−
160 °C). Despite low solubility in common organic solvents, making
characterization difficult, the identity of this compound was verified via
1H NMR spectroscopy and was analyzed via UV−vis spectroscopy. 1H
NMR (400 MHz, CDCl3): (broad signals due to poor solubility) δ =
8.74 (s, 2H), 8.62 (br s, 4H), 7.48 (br s, 4H), 7.12 (s, 2H), 7.05 (s,
2H), 4.05 (q, J = 6.8 Hz, 8H), 1.86 (m, 8H), 1.49 (m, 8H), 1.5−1.2
(m, 72H), 0.88 (t, J = 6.3 Hz, 12H) ppm. λmax nm (ε, M−1cm−1)
(THF) = 300 (14400), 406 (7700).
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8.55 (s, 2H), 8.46 (m, 4H), 7.29 (m, 4H), 6.93 (s, 2H), 6.92 (s, 2H),
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ASSOCIATED CONTENT
■
S
* Supporting Information
NMR spectra (1H and 13C) for all compounds, UV−vis/
fluorescence spectra for all aryleneethynylene isomers (1−6)
and metal complexes thereof, and computational details are
provided. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Ichikawa, T.; Zhang, J.; Mamdouh, W.; Sonoda, M.; De Schryver, F.
C.; Tobe, Y. J. Am. Chem. Soc. 2006, 128, 16613.
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3643.
AUTHOR INFORMATION
■
Corresponding Author
*nbowling@uwsp.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by an award from Research
Corporation. Additional support was provided by the Philip &
Helen Marshall Chemistry Research Fund, a UWSP UPDC
grant and UWSP L&S UEI awards. NMR characterization was
based upon work supported by the National Science
Foundation under CHE-0957080. HR-MS characterization
was based upon work supported by the National Science
Foundation under CBET-0958711.
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dx.doi.org/10.1021/jo300034h | J. Org. Chem. 2012, 77, 2571−2577