A Suzuki Approach to Quinone-Based Diarylethene Photochromes

Article abstract of DOI:10.1021/acs.joc.9b02632

Diarylethene photochromes show promise for use in advanced organic electronic and photonic materials with burgeoning considerations for biological applications; however, these compounds typically require UV light for photoswitching in at least one direction, thus limiting their appeal. We here introduce a naphthoquinone-based diarylethene that switches between open and closed forms with visible light. The synthesis of this quinone diarylethene relies on Suzuki methodology, allowing for the inclusion of functional groups not otherwise accessible with current synthetic routes.

Full text of DOI:10.1021/acs.joc.9b02632

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Note
A Suzuki Approach to Quinone Based Diarylethene Photochromes
Dinesh G. Patel, Travis B. Mitchell, Shea Douglas Myers, Dorothy A. Carter, and Frank A. Novak
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b02632 • Publication Date (Web): 03 Jan 2020
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The Journal of Organic Chemistry
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A Suzuki Approach to Quinone Based Diarylethene
Photochromes
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Dinesh G. Patel, * Travis B. Mitchell, Shea D. Myers, Dorothy A. Carter, and Frank A. Novak
1Department of Chemistry, the Pennsylvania State University at Hazleton, Hazleton, Pennsylvania 18202, United
States
2Department of Chemistry, the State University of New York at Buffalo, Buffalo, New York 14260-3000, United
States
Supporting Information Placeholder
ABSTRACT: Diarylethene photochromes show promise for use in advanced organic electronic and photonic
materials with burgeoning considerations for biological applications; however, these compounds typically
require UV-light for photoswitching in at least one direction thus limiting their appeal. We here introduce a
naphthoquinone-based diarylethene that switches between open and closed forms with visible light. The
synthesis of this quinone diarylethene relies on Suzuki methodology allowing for the inclusion of functional
groups not otherwise accessible with current synthetic routes.
Diarylethenes (DAEs, Scheme 1) have enjoyed sustained investigation as p-type1-2
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photochromes with applications in optical data storage devices, photoresponsive quantum dots,
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photomechanical constructs,6-7 charge transport systems,8-10 vehicles for drug delivery,11
photodynamic therapy agents,12-13 and as linkers in metal organic frameworks.14-17 Many of these
systems are the result of work focusing on hexafluorocyclopentene scaffolds18-21 and ultraviolet
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light (UV) to achieve ring closure (Scheme 1). Given the limitations inherent to the use of UV for
photoswitching, research has moved toward developing DAEs that photoisomerize exclusively
with visible light.22-24
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Quinone based diarylethenes (qDAEs, compound 1a as an example), in contrast to their
hexafluorocyclopentene analogues, have received miniscule attention. The limited contributions
to the literature, however, have suggested that quinone based DAEs could exhibit near ideal
visible light facilitated switching.25-26 When one considers that quinones have great biological
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importance27-28 and rich redox chemistry, appropriately functionalized qDAEs could open the
door to photochromes that are biologically29-30 and electrochemically relavent.31-32
Scheme 1. (Left) Photoisomerization of a quintessential diarylethene. (Right) The structure of
various qDAEs shown in the ring open form.
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Katsumura synthesized a series of qDAEs but did not examine their optical properties.
Subsequently, Deng and Liebeskind25 demonstrated that complete photoconversion from ring-
open to ring-closed isomers was not possible with a similar set of qDAEs (for example 1a – 1c).
Near complete ring closure was, however, realized in the presence of strong Lewis acids. Most
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recently, both the groups of Tsuda and König showed that the ring-open and ring-closed forms
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of qDAEs exhibited stark differences in redox behavior while the work of König further showed
that qDAEs are capable of influencing biological systems.
Thus far, all reported synthetic approaches toward qDAEs involve Stille coupling
conditions. These Stille routes require the use of lithiation and stannylation steps thus limiting
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the scope of functional groups that can survive to the final photochrome product. As examples,
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these functional groups include esters (1d), carboxylic acids, phosphonates, and phosphonic
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acids, which are easily accessible in non-qDAE systems. The limitations on functional groups in
turn limit the possible applications of qDAEs. We seek to enter into the literature qDAEs that can
switch in both directions with visible light and that contain otherwise synthetically inaccessible
functional groups using current methods. These functional groups (our targets were thienyl
esters, carboxylic acids, and halides), additionally, are capable of further modification to allow for
their potential use in biological systems34-35 and main chain photochromic polymers,36-38 both of
which have received much attention.
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Given that borylation, unlike stannylation, can be accomplished without the need for a
lithiation step, we supposed that Suzuki methodology would allow access to qDAEs with
increasingly diverse functional groups.39 To date, Suzuki reactions have been employed for the
synthesis of DAEs containing hexafluoro- and perhydro-cyclopentene components in addition to
some benzothiophene examples.40-44 Suzuki methodology, curiously, has seen negligible use in the
synthesis of qDAEs. Previous reports have suggested that decomposition of the halogenated
quinone coupling partner prevents the reaction.39, 45
Scheme 2. Selected arylated quinones obtained by Suzuki, direct arylation, and Meerwein
coupling reactions.
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According to the literature, several non-photochromic monoaryl and tetraaryl quinone
compounds have been synthesized by a Suzuki approach, but 2,3-dichloro-5,6-dicyano-1,4-
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benzoquinone (DDQ) was needed as a final step before the desired products were obtained.46-47
When not employed, it is assumed that oxidation by atmospheric oxygen during the work up and
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purification helped achieve the same results as DDQ. Direct arylation using boronic acids has
also been explored, but the resulting quinones were only monosubstituted, and FeCl was
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required to re-oxidize the product mixture, which contained a reduced side product just as in
the reports by Langer and coworkers.46-47 The Meerwein arylation has also been explored but
afforded products in low yield. In all of these reports (Scheme 2), only benzene derived boronic
acids or azides were used and thus gave non-photochromic qDAEs. We have found no instances of
thiophene boronic acids or thiophene boronic acid esters51 participating in coupling reactions
thus affording photochromic qDAEs.
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We sought conditions that would obviate the need for DDQ or other oxidants. Anhydrous
Suzuki conditions, we hypothesized, would give products without any needed post-synthetic
oxidation and would avoid decomposition of the halogenated quinone precursors. Experiments
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using thallium carbonate in the absence of water have been examined previously; however, we
turned to fluoride as the activating base.53
Ester containing compound 1d served as our initial target owing to the ease with which the
necessary 5-methyl-2-thiophene carboxylic acid (2) can be protected as the methyl ester (3a,
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Scheme 3). We proceeded with the assumption that protection would prevent side reactions and
that deprotection would give the desired carboxylic acid functional group. Unfortunately,
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attempts at deprotection of 1d using standard methods either led to decomposition or returned
only starting material.
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Scheme 3. Synthesis of required boronic acid 5.
Given the difficulty encountered in hydrolyzing the methyl ester, we turned our attention
to synthesis of the more labile t-butyl analogue (Scheme 3). Starting with compound 2, we
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employed the method of Wright to protect the acid functionality to give t-butyl ester 3b. Using
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the methods of Hartwig and Tatsuo, the 4-position was borylated to give compound 4b. While
Hartwig has shown that boronic acids can be obtained by treating pinacol esters with sodium
periodate and aqueous hydrochloric acid, we were concerned that these conditions would lead to
unwanted deprotection of the t-butyl ester. To our delight, the use of a weaker acid, oxalic acid, in
place of hydrochloric acid gave desired compound 5b with the t-butyl protecting group intact.
The corresponding methyl ester compounds 4a and 5a are obtained using the same procedures as
their t-butyl analogues.
To access our desired qDAEs (compounds 7 – 9; Scheme 4), we started by treating 2,3-
dibromo-1,4-naphthoquinone (6)59 with compound 5b, cesium fluoride, and catalytic PEPPSI-
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IPr in anhydrous dioxane. Compound 1d is obtained in similar fashion. While several traditional
methods for deprotection of the t-butyl group exist, we found that heating 7 in diphenyl ether (bp
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259 °C) for 2h afforded dicarboxylic acid 8. Treatment of 8 with tetrabutylamine tribromide61
affords dibromide 9. Both 8 and 9 can undergo further synthetic manipulation at either the
carboxylate or aryl-bromide positions. It is worth noting that compound 9 has previously been
obtained by brominating 1a, which is available through a Stille coupling.25
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Scheme 4. Synthetic route to the ester, carboxylic acid, and halogen functionalized photochromic
qDAEs in this study.
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We firstly examined 1d, 7, 8, and 9 by H NMR spectroscopy. All show the expected
number of and multiplicity for peaks in the open forms. Using 1d as a model compound,
irradiation of an NMR sample with short wavelength visible light (405 – 410 nm) gave ~22% of
the ring closed form (Supporting Information). This result is similar to the work of Deng and
Liebeskind,25 and no attempt was made to force ring closure using strong Lewis acids.
Interestingly, the thienyl proton on 1d, which appears as a broad singlet centered at 7.42 ppm in
the open form shifts to 8.29 ppm, now as a sharp singlet, in the closed form. The downfield shift is
expected as the conjugation imparted by cyclization allows for effective electron withdrawal by
the quinone ring. Similarly, both the ester methyl group and thienyl methyl group experience
downfield shifts, though much less pronounced. Compounds 1d and 7 have nearly identical
spectra; only the ester alkyl groups have differing chemical shifts.
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Figure 1. The absorption spectrum of 7 in acetone (5.7 x 10 M) before and after irradiation with
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05 – 410 nm light.
UV-Vis spectroscopy of compounds 7 and 8 showed distinct spectra between ring open and
ring closed forms. The open form of 7 (Figure 1) has peak absorption at 412 nm and an onset of
absorption at ~500 nm, longer than most previously reported qDAEs. Upon irradiation, a new
absorption band appears. The closed form of compound 7 exhibits a long wavelength absorption
band centered at 612 nm; a second absorption band at 441 nm is due to the presence of both the
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ring open and ring closed forms given that complete ring closure was not observed by H NMR.
Onset of absorption for the closed form of 7 occurs at ~762 nm. Compound 8, upon ring closure,
exhibits significant broadening of the long wavelength absorption band (Supporting Information),
presumably due to hydrogen bonding in solution as this broadening is not observed in the
spectrum of 7 in the closed form.
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The absorption spectrum for brominated compound 9, in contrast to that of 7 and 8, shows
only minor changes upon irradiation (see Supporting Information); the reason for this diminished
response is not immediately apparent though it may be due to some deactivation pathway
available to the excited state as a consequence of the heavy atom effect. The long wavelength
absorption due to the closed form has a maximum near 560 nm, much shorter than that for
compounds 7 and 8. This hypsochromic shift informs that the electron withdrawing nature of the
thienyl substituents greatly impacts the absorption spectrum and electronics of the closed form
qDAE. Bromine is not as strong an electron-withdrawing group when compared to ester or
carboxylic acid functionalities. Given this structure-property relationship, we theorize that even
stronger electron withdrawing groups would lead to more prominent changes in absorption
spectra between open and closed forms while also shifting the long-wavelength closed form
absorption band further into the near IR.
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Single crystals of compound 8 were obtained by slow evaporation of chromatographic
fractions leading to the formation of two different morphologies: the photoinactive form as
orange prisms (8a, Figure 2) and the photoactive form as yellow plates (8b, Figure 3). A suitable
crystal from each morphology was analyzed by X-ray diffraction.
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Figure 2. (a) Molecular structure of qDAE 8a. Aromatic and alkyl hydrogen atoms are omitted
for clarity. (b) Face-to-face π···π stacking viewed down [1 1 0]. (c) Hydrogen bonding between
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π···π stacked dimers viewed down .
[ ]
The orange prisms of 8a crystallize in the centrosymmetric space group 푃1 with one qDAE
molecule in the asymmetric unit. The crystal structure of 8a assumes the parallel conformation
leading to photo-inactivity in the solid state.62-63 Dimers of this qDAE form parallel to the
1 1 0
[ ]
through face-to-face π···π stacking between the quinoid and benzenoid rings separated by a
distance of 3.5336(5) Å (Figure 2b). Each dimer hydrogen bonds to four neighboring dimers
through the carboxylic acid groups forming 2-D sheets (Figure 2c, Table S1) that possess solvent
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accessible pores parallel to . Attempts to model the electron density within the pores,
[ ]
presumably arising from highly disordered solvent molecules, were unsuccessful. The electron
density was subsequently removed using the SQUEEZE routine in PLATON.64
The yellow plates of 8b crystallize in the non-centrosymmetric space group Pna2 with one
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qDAE molecule in the asymmetric unit. The crystal structure of 8b assumes the anti-parallel
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conformation with C–C separation distance of 3.284(6) Å for the carbon atoms involved in
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photocyclization; this value is characteristic of photoactive DAEs in the solid state. The solid-
state photochromism of 8b will be examined in future work. Each qDAE molecule parallel to the
n-glide plane (or molecule parallel to [0 ½ ½]) undergoes an electrostatic interaction between the
carbonyl carbon adjacent to the thiophene ring of one molecule and the quinoidal oxygen on the
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backbone of a neighboring molecule (Figure 3b, C14 O1 = 3.048(6) Å, (i) = ½-x, -½-y, ½+z). In
addition, each 8b qDAE molecule hydrogen bonds with adjacent qDAE molecules through the
carboxylic acid groups forming infinite chains with two-fold helical symmetry along the
crystallographic a axis (Figure 3c, Table S2).67
Figure 3. (a) Molecular structure of qDAE 8b. Aromatic and alkyl hydrogen atoms are omitted for
clarity. (b) Electrostatic interaction between quinoidal oxygen and carboxyl carbon. (c) Infinite
chain imposed by hydrogen bonds between carboxylic acids on adjacent qDAE molecules.
In conclusion, we have demonstrated the use of Suzuki coupling methodology to obtain
photochromic quinone-based diarylethenes with previously inaccessible functional groups. These
compounds show ideal visible light absorption characteristics and photoswitchability and are
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amenable to further synthetic modification. Crystal structure data on a carboxylic acid terminated
qDAE show extensive hydrogen bonding and that potentially photoactive crystals can be obtained.
Future work will focus on computations, photocrystallography, and the realization of qDAE based
polymers and metal-organic frameworks.
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7
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9
0
1
2
3
4
5
6
7
8
9
0
Experimental Section
All reagents were obtained from commercial sources and used as received unless
otherwise noted in the text. Flash column chromatography was accomplished using silica gel
purchased from Fisher Scientific (230 – 400 mesh, grade 60, catalog number S825-212).
Polyester backed TLC silica plates (grade 60 with UV indicator) were obtained from EMD
Millipore (catalog number 1.05735.0001). NMR spectra were obtained on a Bruker AVIII-500
1
13
spectrometer operating at 500 MHz for H NMR spectra and 125 MHz for C NMR spectra or on a
1
13
Bruker DRX-400 spectrometer operating at 400 MHz for H NMR spectra and 100 MHz for C
1
NMR spectra; the H NMR spectrum for 1d after irradiation (Supporting Information) was
acquired on a Varian Inova spectrometer operating at 500 MHz. NMR peaks are referenced to
68
1
13
residual solvent signals in either CDCl3 (7.26 ppm for H, 77.0 ppm for C) or acetone-d (2.05
6
1
13
ppm for H, 29.84 for C). Solution UV-Vis experiments were preformed in methylene chloride or
acetone using a Shimadzu UV-1800 spectrometer. Solvents for these experiments were used as
received and not degassed prior to use. Mass spectrometric analyses were performed at the
Pennsylvania State University Proteomics and Mass Spectrometry Core Facility, University Park,
PA with high-resolution mass spectra (HRMS) obtained on a Waters Q-TOF quadrupole time-of-
flight mass spectrometer. Data were obtained in either positive or negative mode as denoted in
each compound’s characterization. X-ray diffraction data was collected on either a home source
or a synchrotron source as described in the supporting information.
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5
4
5
-methyl-2-thiophene carboxylic acid methyl ester (3a). The procedure by Rückle was
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
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2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
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3
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5
5
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5
5
6
followed; in a 500 mL round bottom flask was placed methanol (250 mL). Concentrated sulfuric
acid (11 mL, 18 M) was added slowly and with vigorous stirring. Heat was evolved during the
addition; without cooling, 5-methyl-2-thiophene carboxylic acid (9.300 g, 65.42 mmol) was added
in a single portion followed by reflux for 24 hours. Upon cooling to room temperature,
neutralization was achieved by addition of small portions of sodium carbonate. Some solids were
present after complete neutralization and were removed by suction filtration. The filtrate was
concentrated to approximately 30 mL, then diluted with water (200 mL) and the product
extracted with hexanes (3 x 100 mL). The combined hexane phases were dried over magnesium
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
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1
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4
5
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7
8
9
0
sulfate, filtered, and solvent removed under reduced pressure to afford a faint yellow oil (8.910 g,
1
8
3
1
7%). H NMR (400 MHz, chloroform-d), δ 7.59 (d, 1H, J = 3.8 Hz), 6.74 (d, 1H, J = 3.3 Hz), 3.83 (s,
13
1
H), 2.49 (s, 3H); C{ H} NMR (100 MHz, chloroform-d), δ 162.6, 147.8, 133.8, 130.8, 126.3, 51.8,
+
5.6. HRMS (ESI-TOF) m/z: [M + H] calcd for C H O S 157.0323; found 157.0318.
7
9 2
5
-methyl-2-thiophene carboxylic acid t-butyl ester (3b). This compound has been
6
9
previously reported, obtained by a different route than presented here, and has not been fully
56
characterized; based on the procedure by Wright and coworkers, a 500 mL round bottom flask
was charged with magnesium sulfate (42.33 g, 351.7 mmol), methylene chloride (200 mL), and
concentrated sulfuric acid (6.90 g, 3.75 mL, 70.3 mmol). After stirring the suspension for 5
minutes, 5-methyl-2-thiophene carboxylic acid (2, 10.000 g, 70.33 mmol) was added in a single
portion and immediately followed by the addition of t-butanol (20.85 g, 28 mL, 281.3 mmol). The
reaction was allowed to stir, tightly capped (a rubber septum was used), for 48 hours. At this
point, the magnesium sulfate was removed by suction filtration and the filter cake was washed
with hexanes (150 mL). Owing to the small particulate size of the magnesium sulfate resulting
from stirring for 48 h, the filtration process took several hours. The washings and filtrate were
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The Journal of Organic Chemistry
combined, and then washed with aqueous sodium hydroxide (1 M, 2 x 40 mL). Upon drying the
remaining organic phase over magnesium sulfate and filtering, solvent was removed under
reduced pressure. The resulting oil was chromatographed on silica eluting with 100% hexanes to
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
0% ethyl acetate (EtOAc) in hexanes. Removal of solvent afforded a clear, faint yellow oil (9.348
1
g, 67%). H NMR (500 MHz, CDCl ), δ 7.52 (d, 1H, J = 3.7 Hz), 6.72 (d, 1H, J = 3.7 Hz), 2.49 (s, 3H),
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
1
3
1
1.55 (s, 9H); C{ H} NMR (125 MHz, CDCl ), δ 161.5, 147.0, 133.1, 133.0, 126.0, 81.2, 28.1, 15.6.
3
+
HRMS (ESI-TOF) m/z: [M + Na] calcd for C H O SNa 221.0612; found 221.0602.
1
0
14 2
Methyl 5-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophene-2-carboxylate
7
0
(4a). As in the procedure by Sawamura and coworkers, a 250 mL Schlenk tube was charged
with compound 3a (7.000 g, 44.81 mmol), bis(pinacolato)diboron (B pin , 6.828 g, 26.89 mmol),
2
2
and 4,4’-di-tert-butylbipyridine (dtbpy, 0.024 g,
and the solution subjected to three freeze-pump-thaw cycles followed by a nitrogen backfill. Upon
warming to room temperature, (1,5-cyclooctadiene)(methoxy)iridium(I) dimer
[Ir(COD)(OMe)] , 0.030 g, 0.04 mmol) was added under a stream of nitrogen. After
0.09 mmol). Heptane (40 mL) was added
(
2
approximately 1 – 2 minutes, the initially yellow solution turned dark brown; heating at 100 °C
was carried out for 24 h. Upon cooling, a white precipitate was visible. The reaction was
transferred to a separatory funnel and a 1:1 solution of hexanes/methylene chloride was added.
This organic phase was washed with potassium acetate (0.10 M aqueous, 2 x 50 mL) and brine (50
mL). The remaining organic phase was passed through a short plug of silica eluting with 1:1
hexanes/methylene chloride. A white solid was obtained upon removal of solvent (12.356 g,
9
3
1
2
8%); spectral data matched that in the literature.70 H NMR (400 MHz, CDCl ), δ 7.95 (s, 1H),
1
3
1
3
1
.86 (s, 3H), 2.72 (s, 3H), 1.34 (s, 12H); C{ H} NMR (100 MHz, CDCl ), 162.7, 159.7, 140.4, 130.1,
3
+
29.1, 83.6, 51.9, 24.9, 16.3. HRMS (ESI-TOF) m/z: [M + H] calcd for C H BO S 283.1178; found
1
3
20
4
83.1167.
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tert-butyl
5-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophene-2-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
carboxylate (4b). Following a previously reported procedure,57-58 in a 250 mL Schlenk flask was
combined compound 3b (5.000 g, 25.21 mmol), B pin (3.665 g, 0.57 mmol), dtbpy (0.010 g, 0.04
2
2
mmol), and heptane (50 mL). The solution was degassed by three freeze-pump-thaw cycles and
then backfilled with nitrogen. Under a stream of nitrogen, [Ir(COD)OMe] (0.012 g, 0.02 mmol)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
was added quickly and the reaction heated to 100 °C and followed until TLC (silica, 5% EtOAc in
hexanes) showed that no starting material remained. The reaction solution was passed through a
short silica column followed by elution with 100% hexanes to 5% EtOAc in hexanes. The fractions
containing product were collected affording a white solid upon removal of solvent (6.743 g, 82%).
1
13
1
H NMR (500 MHz, CDCl ), δ 7.80 (s, 1H), 2.67 (s, 3H), 1.55 (s, 9H), 1.32 (s, 12 H); C{ H} NMR
3
(
125 MHz, CDCl ), δ 161.6, 159.1, 139.1, 132.7, 128.7, 83.4, 81.2, 28.2, 24.8, 16.3. HRMS (ESI-TOF)
3
+
m/z: [M + Na] calcd for C H BO SNa 347.1464; found 347.1437.
1
6
25
4
(
5-(methoxycarbonyl)-2-methylthiophen-3-yl)boronic acid (5a). A modification of the
procedure by Hartwig and coworkers was followed.57 A 250 mL flask was charged with
compound 4b (7.000 g, 24.81 mmol), oxalic acid dihydrate (6.253 g, 49.62 mmol), water (30 mL),
and acetonitrile (150 mL). After stirring for 1 min, NaIO (7.960, 37.21 mmol) was added in a
4
single portion and the resulting mixture stirred for 2 hours. During this time, the reaction
increased in cloudiness; a solid mass was apparent when pouring the reaction into a separatory
funnel. The material in the separatory funnel was diluted with water (150 mL) and extracted with
diethyl ether (5 x 100 mL). A white precipitate remained at the aqueous/organic interface during
the extraction process. The combined organic phases were washed with brine (100 mL), dried
over magnesium sulfate, filtered, and solvent removed under reduced pressure. The obtained
solid was washed with hexanes and the resulting suspension collected by suction filtration to
1
afford a white solid (4.280 g, 86%). H NMR (500 MHz, acetone-d ), δ 7.97 (s, 1H), 7.22 (br s, 2H),
6
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The Journal of Organic Chemistry
1
3
1
3
.80 (s, 3H), 2.71 (s, 3H); C{ H} NMR (125 MHz, acetone-d ), 163.0, 159.9, 158.6, 141.1, 130.0,
6
1
2
-
5
2.1, 16.4. HRMS (ESI-TOF) m/z: [M – H] calcd for C H BO S 199.0238; found 199.0229.
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
7
8
4
(5-(tert-butoxycarbonyl)-2-methylthiophen-3-yl)boronic acid (5b). A modification of the
procedure by Hartwig and coworkers was followed.57 In a 100 mL round bottom flask was
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
combined compound 4b (3.00 g, 9.25 mmol), oxalic acid dihydrate (2.332 g, 18.50 mmol),
acetonitrile (50 mL), and water (10 mL). After stirring for 1 minute NaIO (2.968 g, 13.88 mmol)
4
was added in a single portion. After stirring at room temperature for 7 h, TLC (silica, 1:1
hexanes/EtOAc) showed a new spot of low R (~ 0.1 – 0.2) and some starting material; stirring
f
was continued for an additional 9 h at which point TLC showed no starting material. Upon
dilution with water (50 mL), the product was extracted with diethyl ether (3 x 50 mL). The ether
phases were washed with brine (50 mL), dried over MgSO ; upon removal of solvent a white solid
4
was obtained. This solid was washed with hexanes, water, and dried under vacuum (1.880 g,
1
8
4%). H NMR (500 MHz, acetone-d ), δ 7.90 (s, 1H), 7.19 (s, 2H), 2.69 (s, 3H), 1.53 (s, 9H);
6
1
3
1
C{ H} NMR (125 MHz, CDCl ), δ 162.0, 158.0, 140.4, 132.3, 81.4, 28.4, 16.4. HRMS (ESI-TOF)
3
-
–
m/z: [M – H] calcd for C H BO S [M-H] 241.0708; found 241.0698.
1
0
14
4
2
,3-dibromonaphthalene-1,4-dione (6). According to the slightly modified procedure by
7
1
Anuratha and coworkers, a 250 mL round bottom flask was charged with 1,4-naphthoquinone
(5.000 g, 31.62 mmol) and sodium acetate (26.934 g, 316.15 mmol). After holding under high
vacuum for 30 minutes and then backfilling with nitrogen, acetic acid (AcOH, 125 mL) was added.
With vigorous stirring, bromine (15.157 g, 4.88 mL, 94.85 mmol) was introduced in portions over
2 – 3 minutes. After complete addition of bromine, the reaction was refluxed for 2 hours followed
by cooling to room temperature overnight. The resulting orange suspension was poured into
water (300 mL) and the solids collected by suction filtration. The resulting filter cake was
collected and suspended in a 5 wt. % sodium thiosulfate solution with gentle swirling; the solids
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Page 16 of 31
were again collected and suspended, with stirring, in hot ethanol. Upon cooling to room
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
temperature, a bright yellow solid was collected by suction filtration and dried under vacuum
1
(6.656 g, 67%). H NMR (500 MHz, CDCl ), δ 8.19 (dd, 2H, J = 5.7 Hz, 3.3 Hz), 7.79 (dd, 2H, 5.8 Hz,
3
13
1
3
.3 Hz). C{ H} NMR (125 MHz, CDCl ), δ 175.8, 142.6, 134.5, 130.8, 128.2. HRMS (ESI-TOF) m/z:
3
-
[
M] calcd for C H Br O 313.8578; found 313.8615.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
10
4
2 2
4,4'-(1,4-dioxo-naphthalene-2,3-diyl)bis(5-methylthiophene-2-carboxylic acid tert-butyl
ester) (7). In a 50 mL Schlenk tube was combined Compound 6 (0.746 g, 2.36 mmol), compound
b (1.200 g, 4.96 mmol), cesium fluoride (1.510 g, 9.91 mmol), and 1,4-dioxane (10 mL). The
5
yellow mixture was subjected to three freeze-pump-thaw cycles, then backfilled with nitrogen.
PEPPSI-IPr (0.064 g, 0.09 mmol) was quickly added under a stream of nitrogen and the reaction
refluxed for 15 h, then cooled to room temperature. After dilution with EtOAc (200 mL), the
organic phase was washed with water (3 x 50 mL) followed by brine (50 mL). The remaining
organic phase was dried over MgSO , then adsorbed onto silica followed by chromatography on
4
silica eluting with 100% hexanes to 25% EtOAc in hexanes. The pure fractions were collected;
upon removal of solvent under reduced pressure a dark green solid was obtained (0.954 g, 72%).
1
H NMR (500 MHz, CDCl ), δ 8.18 (dd, 2H, J = 5.7 Hz, 3.3 Hz), 7.81 (dd, 2H, J = 5.7 Hz, 3.3 Hz), 7.30
3
1
3
1
(
br s, 2H), 2.08 (s, 6H), 1.53 (s, 18H); C{ H} NMR (125 MHz, CDCl ), 183.6, 161.0, 145.9, 141.8,
3
+
134.9, 134.1, 132.1, 132.0, 130.6, 126.8, 81.7, 28.2, 15.2. HRMS (ESI-TOF) m/z: [M + Na] calcd for
C H O S Na 573.1376; found 573.1355.
3
0
30 6 2
4
,4'-(1,4-dioxonaphthalene-2,3-diyl)bis(5-methylthiophene-2-carboxylic acid) (8). In a 50
mL flask with reflux condenser was combined diphenyl ether (15 mL) and compound 7 (0.800 g,
.45 mmol). The dark green solution was refluxed (259 °C) for 2h. After cooling, the product was
1
precipitated by slow addition of the reaction solution to stirring hexanes. The resulting solid was
collected by suction filtration, dissolved in acetone, then adsorbed onto silica followed by
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The Journal of Organic Chemistry
chromatography on silica eluting with 1:1 hexanes/EtOAc to 100% EtOAc to 5% AcOH in EtOAc.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
The photochromic fractions were collected; upon removal of solvent under reduced pressure, a
1
dark green solid was obtained (0.362 g, 57%). H NMR (400 MHz, acetone-d ), δ 8.15 (dd, 2H, J =
6
13
1
5
.7 Hz, 3.3 Hz), 7.92 (dd, 2H, J = 5.7 Hz, 3.3 Hz), 7.54 (s, 2H), 2.24 (s, 6H); C{ H} NMR (100 MHz,
acetone-d ), 184.3, 162.8, 147.3, 142.8, 137.1, 134.8, 133.2, 132.7, 130.8, 127.1, 15.1. HRMS (ESI-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
6
+
TOF) m/z: [M + H] calcd for C H O S 439.0310; found 439.0281.
2
2
15 6 2
2
,3-bis(5-bromo-2-methylthiophen-3-yl)naphthalene-1,4-dione (9). In a 35 mL pressure
tube was combined 8 (0.100 g, 0.23 mmol), anhydrous K PO (0.102 g, 0.48 mmol), and
3
4
tetrabutylammonium tribromide (0.231 g, 0.48 mmol). Acetonitrile (5 mL) was added and the
capped tube was heated to 100 °C. During the first few minutes, the reaction clarified slightly and
went from a white-green mixture to orange. After stirring for 24 h, the reaction was cooled to
room temperature and diluted with methylene chloride (50 mL). The cloudy mixture was then
washed with water (2 x 30 mL), saturated aqueous NaHCO (30 mL), sodium thiosulfate (10%, 30
3
mL), and finally with brine (30 mL). The remaining organic phase was dried over magnesium
sulfate, filtered, and adsorbed onto silica followed by chromatography on silica eluting with 100%
hexanes to 20% EtOAc in hexanes. The yellow band was collected and those fractions deemed
pure were combined and solvent removed to give a brown solid with a metallic luster (0.043 g,
1
37%). H NMR (400 MHz, CDCl ), δ 8.17 (dd, 2H, J = 5.8 Hz, 3.3 Hz), 7.80 (dd, 2H, J = 5.8 Hz, 3.3
3
13
1
Hz,), 6.64 (s, 2H), 2.02 (s, 6H); C{ H} NMR (100 MHz, CDCl ), 183.6, 141.5, 140.9, 134.1, 132.0,
3
+
1
31.5, 130.4, 126.8, 108.0, 14.8. HRMS (ESI-TOF) m/z: [M + Na] calcd for C H Br O S Na
20 12
2 2 2
528.8543; found 528.8516.
4
,4'-(1,4-dioxo-naphthalene-2,3-diyl)bis(5-methylthiophene-2-carboxylate methyl ester)
(
1d). Compounds 6 (0.376 g, 1.19 mmol) and 5a (0.500 g, 2.50 mmol) were combined in a 100
mL Schlenk tube followed by the addition of cesium fluoride (0.759 g, 5.00 mmol) and 1,2-
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The Journal of Organic Chemistry
Page 18 of 31
dimethoxyethane (10 mL). The mixture was degassed by three freeze-pump-thaw cycles, then
backfilled with nitrogen. Upon warming to room temperature, and under a stream of nitrogen,
PEPPSI-IPr (0.016 g, 0.02 mmol) was quickly added. After refluxing for 18 h, TLC (silica, 10%
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
EtOAc in hexanes) showed a photochromic spot of low R . Removal of solvent led to a brown solid
f
that was adsorbed onto silica and chromatographed on silica eluting with 100% hexanes to 30%
EtOAc in hexanes. The photochromic spots were collected and solvent removed to afford a yellow
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
powdery solid that was pure by H NMR (0.514 g, 57%). Alternatively, purification can be
achieved by passing the crude product (after aqueous workup extracting with chloroform)
through a short plug of silica eluting with chloroform. The solid obtained after removal of solvent
is heated in methanol until boiling. After storing cold overnight, the remaining solids can be
collected by suction filtration, washed with cold methanol, and dried under high vacuum. In this
case, a greenish solid is obtained in lower yield (36% yield) than achieved by chromatography
1
but at the same level of purity. H NMR (500 MHz, CDCl ), δ 8.17 (dd, 2H, J = 5.7, 3.3 Hz), 7.81 (dd,
3
1
3
1
2
1
H, J = 5.8, 3.3 Hz), 7.42 (br s, 2H), 3.84 (s, 6H), 2.09 (s, 6H); C{ H} NMR (125 MHz, CDCl ) 183.5,
3
62.1, 146.6, 141.6, 135.6, 134.2, 131.9, 130.8, 129.8, 126.8, 52.1, 15.3. HRMS (ESI-TOF) m/z: [M +
+
Na] calcd for C H O S Na 489.0443; found 489.0436.
2
4
18 6 2
Supporting Information.
The Supporting Information is available free of charge on the ACS Publications website at DOI:
1
13
Copies of H and C NMR data for all compounds, UV-Vis data for compounds 8 and 9,
hydrogen bonding tables for 8a and 8b, and additional experimental information (PDF).
Crystal structure of 8a (cif)
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Crystal structure of 8b (cif)
AUTHOR INFORMATION
Corresponding Author
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*E-mail: dgp15@psu.edu
ORCID
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Dinesh G. Patel: 0000-0003-0733-5383
Shea D. Myers: 0000-0002-9104-5305
ACKNOWLEDGMENTS
D.G.P. acknowledges support from Penn State Hazleton in the form of a research development
grant. Work by T.B.M. was supported by the National Science Foundation under Grant No. DMR-
1
455039. Access to NSF’s ChemMatCARS Sector 15 is supported by the Divisions of Chemistry
(CHE) and Materials Research (DMR), National Science Foundation, under grant number
NSF/CHE- 1834750. Use of the Advanced Photon Source, an Office of Science User Facility
operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National
Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. The
authors thank Dr. Tatiana Laremore for help with sample preparation, data interpretation, and
providing access to the Pennsylvania State University mass spectrometry facility. Dr. Carlos
Pacheco is acknowledged for assistance in the acquisition of NMR data, and Professor Jason
Benedict is acknowledged for providing advice on the crystal structure analyses. Dr. Yu-Sheng
Chen and Dr. SuYin Grass Wang of NSF’s ChemMatCARS are also thanked for their time, expertise,
and discussions with regards to the acquired beamline data.
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