Polymer-supported syntheses of thiophene-containing compounds using a new type of traceless linker

Article abstract of DOI:10.1039/c2ob06714e

A new type of traceless linker is described for use in polymer-supported (PS) syntheses of thiophene-containing compounds. It is based on the cleavage of PS aryl 2-thienyl ketones by a mixture of potassium t-butoxide and water (typical mol ratio 10:3) in an ethereal solvent. Cleavage occurs to give the soluble thiophene-containing product. The method is used to prepare a range of eight thiophene-containing compounds including a terthiophene and a dialkylquaterthiophene. PS unsymmetrical diaryl ketones incorporating, for example, ortho-methoxyphenyl or pyrrole moieties could also serve as traceless linkers.

Full text of DOI:10.1039/c2ob06714e

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PAPER
Polymer-supported syntheses of thiophene-containing compounds using a
new type of traceless linker
Abderrazak Ben-Haida and Philip Hodge*
Received 10th October 2011, Accepted 21st November 2011
DOI: 10.1039/c2ob06714e
A new type of traceless linker is described for use in polymer-supported (PS) syntheses of thiophene-
containing compounds. It is based on the cleavage of PS aryl 2-thienyl ketones by a mixture of potassium
t-butoxide and water (typical mol ratio 10 : 3) in an ethereal solvent. Cleavage occurs to give the soluble
thiophene-containing product. The method is used to prepare a range of eight thiophene-containing
compounds including a terthiophene and a dialkylquaterthiophene. PS unsymmetrical diaryl ketones
incorporating, for example, ortho-methoxyphenyl or pyrrole moieties could also serve as traceless linkers.
moieties. We exemplify its use by the synthesis of a range of
eight thiophene-containing compounds with conjugated π-elec-
Introduction
Polymer-supported (PS) organic syntheses, that is PS syntheses
other than those of natural oligomers or polymers, have been of
great interest since the 1970s.1–3 An important strategy with
many of these syntheses is the use of traceless linkers.4–6 With
traceless linkers, when the target molecule is released from the
support, the site by which it was originally attached to the
support is not apparent. This can be achieved, for example, by
the site of attachment on the target molecule being transformed
into a C–H bond,4,7–13 or by a combined cyclization-cleavage
reaction.4,14–19
Small molecules or oligomers with conjugated π-electron
systems are of interest because of their potential applications in
molecular electronics.7–9,10,13,20–30 They can also serve as valu-
able models for the corresponding polymers.30,31 The purity of
such compounds is often an issue, but their purification can be
difficult. Because using insoluble supports in syntheses can
assist with the purification of the products, this has prompted the
investigation of PS syntheses of small molecules or oligomers
with conjugated π-electron systems.7–9,10,13,20–25 Some of these
syntheses have used conventional linkers, such as ester lin-
kages,22,25 whilst others have used traceless linkers,7–13 such as
silicon-based,7–9 germanium-based,10,13 bismuth-based32 or acti-
vated-sulfonate linkers.11,12 The target products from many of
these syntheses are oligothiophenes with unsubstituted and/or 3-
substituted aromatic units.7–10,13,21,22,25,28,29 Oligothiophenes
have recently also found application as fluorescent ligands for
the detection of protein aggregates and a library of oligothio-
phenes has been prepared.33
tron systems.
Results and Discussion
The present traceless linker is based on the cleavage of non-eno-
lizable ketones, see Reaction 1, by treatment with the hetero-
geneous mixture formed by adding small amounts of water to
potassium t-butoxide (typical mol ratio 3 : 10) in an ethereal
solvent such as 1,2-dimethoxyethane or tetrahydrofuran (THF) :
the so-called butoxide–water reagent (BWR).34–36 It is believed
that in these reactions initially the carbonyl group of the ketone
is attacked by the hydroxide anion and then the tetrahedral inter-
mediate formed is deprotonated by t-butoxide to give dianion
1.35 The latter cleaves to give a carboxylate anion and a carba-
nion: Reaction 2.35 This cleavage reaction is clearly closely
related to the classical Haller–Bauer reaction which typically
uses a suspension of sodamide in a high boiling hydrocarbon
solvent to cleave non-enolizable ketones: Reaction 3.37 The
BWR cleaves diaryl ketones cleanly and in high yields under
much milder conditions.
ð1Þ
We now report a new type of traceless linker, a linker that can
be used for the synthesis of molecules containing thiophene
ð2Þ
Department of Chemistry, University of Manchester, Oxford Road,
Manchester, M13 9PL, UK. E-mail: Philip.Hodge@manchester.ac.uk;
Fax: (+44) 1524 793 252; Tel: (+) 44-01524-791728
ð3Þ
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The cleavage of unsymmetrical diaryl ketones is a key part of
the present work. They can cleave in two directions, the favoured
direction being that which produces the most stable aryl carba-
nion and/or relieves steric congestion resulting from the presence
of an ortho-substitutent.35 The unsymmetrical ketones of interest
in the present work are PS aryl 2-thienyl ketones 2 and these
must cleave mainly, if not entirely, at the bond neighbouring the
2-thienyl moiety as shown in 2. Two results from previous work
indicate that this will be the case.38,39 First, 2-benzoylthiophene
(3), when treated with the BWR in ether at 21 °C for 5 min,
cleaves in high yield and gives benzoic acid as the only isolated
acid, i.e. cleavage occurs exclusively as shown in 3.38 Second,
ketone 4 cleaves 90% at ‘a’ and 10% at ‘b’.38 This latter result is
of particular interest because in most cases 2-chlorophenyl aryl
ketones cleave almost entirely at the bond neighbouring the 2-
chlorophenyl moiety. Indeed, 2-chlorobenzoylation followed by
cleavage using the BWR is a convenient method to introduce
carboxylic acid groups into aromatic compounds.39 The result
with ketone 4 indicates that the 2-thienyl group promotes
cleavage of the neighbouring bond even more effectively than
the 2-chlorophenyl group.
Initially in the present work aryl 2-thienyl ketone moieties (5)
were created on 1% crosslinked polystyrene beads by reacting
chloromethylated beads with 4-hydroxyphenyl 2-thienyl ketone
(6), potassium carbonate and potassium iodide in N-methylpyr-
rolidone (NMP) at 140 °C for 48 h. The required phenol 6 was
readily obtained by demethylating the commercially available
ketone 7. The loading of moieties 5, as estimated by the weight
gain, was 2.14 mmol g−1. Cleavage of ketone 7 gave 4-methoxy-
benzoic acid in an isolated yield of >95%, so indicating that the
presence of the 4-alkoxy group does not significantly affect the
direction of cleavage.35
As it was intended to elaborate the 2-thienyl moiety of PS 5
using Suzuki couplings, it was necessary to introduce a 5-bromo
substituent into the thiophene ring, i.e. prepare PS 8. Model reac-
tions were carried out to help to identify a suitable bromination
procedure. Initially an attempt was made to selectively and
efficiently brominate ketone 7 with one equivalent of N-bromo-
succinimide (NBS) in dry N,N-dimethylformamide (DMF) at
153 °C. This was not successful: a mixture of products was
obtained. Compound 7 was therefore treated with ethylene
glycol in the presence of a catalytic amount of p-toluenesulfonic
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Chart 1
Table 1 Summary of PS syntheses of thiophenes 20–23, 29 and 30a
Suzuki reactions
PS
Cleavage reactions
Crude Yield
Entry
Linker
Boronic acid
Product
Yield
Yield
Product
after purification
1
2
3
4
5
6
7
8
8a
8a
8a
8a
8b
8b
8b
8b
11
12
13
14
11
12
25
26b
15a
16a
17
80%
89%
86%
86%
81%
79%
71%
83%
78%
59%
79%
81%
92%
79%
96%
84%
65%
—
67%
72%
74%
—
20
21
22
23
20
21
29
30
18
15b
16b
27
88%
—
28
a Full details of the reaction conditions are given in the Experimental Section. b Compound 26 is a boronate ester.
acid (PTSA) in refluxing toluene to give the ketal 9 (93% iso-
lated yield). Bromination of the latter with NBS in dry DMF fol-
lowed by deketalization gave the desired bromo compound 10 in
an isolated yield of 90%. Repetition of the reactions with the PS
ketone 5 gave PS 8a in an overall yield of 74%, as estimated by
weight changes and elemental analyses for S and Br. The
loading of residues 8 was 1.59 mmol g−1. It is labeled 8a to dis-
tinguish it from a similar sample of PS 8 prepared by another
route which is described below.
To elaborate PS 8a prior to cleavage it was treated separately
with boronic acids 11 to 14 in THF and potassium carbonate in
the presence of tetrakis(triphenylphosphine)palladium(0) at
60 °C for 72 h: see Chart 1. As judged by weight gains the
Suzuki couplings occurred to give PS 15 to 18 in yields of 80 to
89%: see Table 1 entries 1 to 4.
Cleavage of the target molecules from the support is compli-
cated by the fact that both the BWR and PS reactions are hetero-
geneous. This has been negotiated successfully before in a
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Chart 2
procedure for introducing carboxylic acid groups into cross-
linked polystyrene beads.40 In this the beads were first 2-chloro-
benzoylated in a Friedel–Crafts reaction then the unsymmetrical
diaryl ketone moieties were cleaved by treatment with the BWR
generated in situ in the beads. The latter was achieved by treating
the acylated beads with a solution of potassium t-butoxide in
1,2-dimethoxyethane for 1 h, then the appropriate amount of
water was added to the bead suspension. As the water diffused
into the beads the heterogeneous BWR was formed within the
beads. Cleavage was then achieved by allowing the PS acyl
groups to react with the BWR at 85 °C for 4–10 h. Cleavage
yields were essentially quantitative though with the more highly
loaded beads it was sometimes necessary to retreat the beads
with the BWR. In an extension of this earlier work it was found
that under very similar reaction conditions the cleavage yield,
based on elemental analyses for chlorine, was 75% in 8h, and
>90% in 24 h.41 In the present work the reaction time was stan-
dardised at 72 h simply in order to obtain maximum yields in all
cases. Applying these conditions to the cleavage of PS 5 gave
polymer beads containing 0.15% of sulfur. This corresponds to a
cleavage yield of 97% to give PS 19.
and 2 and Table 1 entries 5 to 8. Cleavage in the usual manner
gave products 20, 21, 29 and 30 in 79–96% yields. It is evident
that PS 8a and PS 8b behave similarly.
In the final part of the project compounds, 31 and 32, each
containing four conjugated aromatic rings, were prepared using
the new linker PS 33. The latter was prepared using standard
reactions as outlined in Scheme 1. Thus, 2-bromo-3-hexylthio-
phene (34)42 was treated with 4-methoxybenzoyl chloride under
Friedel–Crafts conditions to give diaryl ketone 35. Demethyla-
tion of this gave compound 36. Reaction of compound 36 with
chloromethylated 1% crosslinked polystyrene beads, potassium
carbonate and potassium iodide in NMP at 140 °C for 2 days
gave PS 33. From the gain in weight and elemental analyses for
bromine and sulfur the beads contained 1.86 mmol g−1 of resi-
dues 33.
PS 33 was reacted in a Suzuki reaction with biphenyl-4-
boronic acid (11) to give PS 37: see Scheme 2. This was bromi-
nated by treatment with NBS in DMF to give PS 38. Though it
was possible that PS 37 could brominate at more than one site,
1
the H NMR spectrum of the final soluble product indicated that
Cleavage of PS 15 to 18 using the standard conditions gave
products 20 to 23, see Chart 1, in crude yields of 59–81% and,
in most cases, in yields of 65% to 72% after recrystallisation.
The products were identified by 1H NMR spectroscopy and
melting point or elemental analyses.
In an alternative and shorter procedure to obtain PS 8, ether
10 was demethylated to give phenol 24 and the latter was
directly attached to the chloromethylated 1% crosslinked poly-
styrene beads to give PS 8b. Based on the gain in weight of the
beads and elemental analyses for S and Br, the loading of resi-
dues 8 was 1.89 mmol g−1, i.e. slightly higher than in PS 8a.
Using PS 8b Suzuki reactions were carried out as before with
boronic acids 11, 12, and also with 25 and boronate 26,42,43 to
give PS ketones 15b, 16b, 27 and 28 respectively: see Charts 1
Scheme 1 (i) 4-Methoxybenzoyl chloride/SnCl4/dichloromethane,
59% yield. (ii) BBr3 in dichloromethane, 93% yield. (iii) Potassium car-
bonate/potassium iodide/NMP.
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Scheme 2 (i) Suzuki coupling, 83% yield. (ii) Bromination, 85%
yield. (iii) Suzuki coupling, 94% yield. (iv) Cleavage with BWR, 68%
yield.
Scheme 3 (i) Suzuki coupling, 61% yield. (ii) Bromination, 90%
yield. (iii) Suzuki coupling, 87% yield. (iv) Cleavage with BWR, 54%
yield.
the main bromination product was PS 38. As judged by elemen-
tal analyses the bromination yield was 85%. PS 38 was then
reacted with boronate ester 26 in a further Suzuki reaction to
give PS 40 in 94% yield. Cleavage from the support in the usual
way gave, after column chromatography, compound 31 in 68%
yield. The final product was characterized by 1H NMR spec-
troscopy, mass spectroscopy and elemental analyses.
chloromethylated polystyrene beads were 1% crosslinked, of
200–400 mesh, with 4.11 mmol of chloromethyl groups per
gram. Solids were generally dried overnight in a vacuum oven
using a pressure of approximately 1 mm of Hg and a temperature
of 60 °C except that in the case of low-melting solids drying was
achieved at ambient temperature. Organic solutions were dried
over magnesium sulfate. Column chromatography used silica gel
as the stationary phase.
Using similar procedures, but replacing boronic acid 11 with
boronic acid 14, PS 33 was converted, via PS 41 and 42 into PS
43: see Scheme 3. The bromination yield was 90%. Cleavage of
PS 43 gave compound 3244 in 54% yield after column chromato-
Melting points (Mps) were recorded using a Gallenkamp Hot
Stage Melting Point Apparatus. Infrared (IR) absorption spectra
were measured on an ATI Mattson Genesis series Fourier Trans-
form Infrared (FTIR) spectrometer. UV-Vis measurements were
1
graphy. It was characterized by melting point, H NMR spec-
troscopy, mass spectroscopy and elemental analyses.
1
obtained on a Varian Cary 5000 UV-vis-NIR spectrometer. H
NMR Spectra were recorded on either a Varian Inova 300 or a
Bruker AMX 500 spectrometer. Chemical shifts (δ) are quoted
in units of parts per million (ppm). Tetramethylsilane (TMS)
was used as internal reference. The quoted splitting patterns are
abbreviated to singlet (s), doublet (d), triplet (t), multiplet (m)
and broad (b). Coupling constants (J) are reported in Hertz.
Assignments, when they can be made with confidence, are ident-
ified using the numbers shown on the respective formulae. All
mass spectra (MS) are EI/CI spectra and were obtained using a
VG Trio-2000 instrument. For the CI spectra the reagent gas was
ammonia. All elemental analyses were obtained in house. C and
H analyses were made using a Carlo Erba Instruments EA1108
Elemental Analyser.
Conclusions
Thiophene-containing compounds 20–23 and 29–32 have been
prepared successfully using PS reactions including a traceless
linker based on Reaction 1. In each synthesis the key step is clea-
vage of an aryl 2-thienyl ketone, using the BWR, to give the
thiophene-containing compound in solution. Previous work
suggests that PS unsymmetrical diaryl ketones bearing, for
example, ortho-methoxy,35 ortho-chloro,35,39 pyrrole,45 and
possibly furan or pyridine moieties could also function success-
fully as traceless linkers.
Experimental Section
All reagents, solvents and chromatographic materials were
obtained from Sigma-Aldrich Co. The chemicals were of the
highest purity available and were used as received. The
Demethylation of ketone 7. Sodium thiomethoxide (2.95 g,
42.1 mmol) was added to ketone 7 (3.00 g, 13.8 mmol) in DMF
(40 mL). The mixture was stirred under nitrogen and heated
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under reflux for 10 h, then cooled to 20 °C. Aqueous phosphoric
acid (70 mL of 10%) was added and the mixture extracted with
ethyl acetate (4 × 25 mL). The combined extracts were washed
with aqueous silver nitrate (0.5 N, 2 × 25 mL) and water (2 ×
25 mL) then dried. The solvent was evaporated and the residue
subjected to column chromatography. Elution with chloroform
gave phenol 6 (2.64 g, 94%). It had Mp 116–117 °C (lit.,46
116 °C); IR (cast film, cm−1) 1620; 1H NMR (500 MHz, CDCl3,
ppm) δ 7.83 (2H; m; H-2 and 4), 7.68 (1H; m; H-7), 7.64 (1H;
m; H-5), 7.14 (1H; m; H-6), 6.93 (2H; m; H-1 and 3), 6.84 (1H;
bs; OH).
moieties to hydrolyze. The crude product was purified by
column chromatography. Elution with a mixture of hexane and
ethyl acetate (4 vo l : 1 vol) gave bromo compound 10 (1.02 g,
90%). It had Mp 104–105 °C (lit.,46 103 °C); IR (film, cm−1
)
1
1616; H NMR (500 MHz, CDCl3, ppm) δ 7.94 (2H; m; H-2
and 4), 7.61 (1H; m: H-5), 7.39 (1H; m; H-6), 7.03 (2H; m; H-1
and 3), 3.93 (3H; s; H-7); MS (EI) m/z (%): 296 (70) and 298
(74), [M+].
Bromination of PS-thiophene 5. (i) Ketal formation: A
mixture of PS-thiophene 5 (7.500 g, 16.1 mmol of thiophene
residues), ethylene glycol (50 mL), PTSA (0.10 g) and toluene
(30 mL), in a flask equipped with a Dean–Stark apparatus, was
heated and stirred under gentle reflux. After 12 h no more water
was produced, so the resin beads were filtered off, washed
copiously with dry THF, then dried. This gave the PS-ketal
(8.150 g). The IR (KBr disc) showed only a small band at
1625 cm−1 in the carbonyl region. The weight gain (0.650 g)
corresponds to a yield of 92%, and 1.81 mmol of the ketal per g.
(ii) Bromination followed by deketalization: NBS (2.30 g,
12.8 mmol) was added to a stirred mixture of the PS-acetal
(8.000 g) in dry DMF (60 mL) at 20 °C under nitrogen and then
the suspension was heated at 90 °C for 72 h. Aqueous tartaric
acid (20 g in 30 mL) was added via a dropping funnel, and the
reaction was continued under the same conditions for another
10 h. Finally the solids were collected by filtration and washed
successively with DMF (3 × 25 mL), water (3 × 25 mL) and
acetone (3 × 25 mL) before being dried. This gave PS 8
(8.460 g). The weight gain (460 mg) corresponds to 12.77 mmol
of bromothiophene derivative, equal to 1.56 mmol of bromothio-
phene residues per g. It had IR (KBr disc, cm−1) 1632; elemental
analyses gave bromine 12.72% and sulfur 6.34%. The bromine
content corresponds to a loading of 1.59 mmol of PS 8 per g: the
sulfur to a loading of 1.98 mmol of thiophene residues per
g. This corresponds to a bromination yield of 80%. The product
is labeled PS 8a to distinguish it from PS 8 prepared by another
route (see PS 8b below).
General procedure for attaching phenols to chloromethylated
polystyrene beads: attachment of phenol 6 to give PS 5. Chloro-
methylated 1% crosslinked polystyrene beads (3.650 g,
15.0 mmol of chloromethyl groups) were added to a mixture of
phenol 6 (3.33 g, 16.3 mmol), anhydrous potassium carbonate
(1.23 g, 9.0 mmol) and potassium iodide (0.99 g, 6.0 mmol) in
NMP (30 mL) under nitrogen. The suspension was stirred and
heated at 140 °C for 48 h. It was then cooled to 20 °C, the beads
filtered off and washed successively with NMP (2 × 25 mL),
water (3 × 25 mL) and acetone (2 × 25 mL) before being dried
to give PS 5 (5.691 g). The weight gain (2.041 g) corresponds to
12.2 mmol of thiophene derivative 5 (81% yield), equal to
2.14 mmol of residues 5 per g. It had IR (KBr disc, cm−1) 1629;
microanalysis: found sulfur 7.21%: this corresponds to a loading
of 2.25 mmol of thiophene derivative 5 per g.
Synthesis of bromo compound 10. i) Direct bromination of
compound 7: Compound 7 (1.50 g, 6.88 mmol) was dissolved in
DMF (20 mL) at 50 °C. NBS (1.23 g, 6.87 mmol) was added
slowly to the stirred solution and the mixture was heated under
reflux for 12 h. A solution of aqueous tartaric acid (10 g of acid
in 25 mL of water) was then added and the mixture stirred at
60 °C for 4 h. The resulting hot solution was filtered and the
filter cake washed with dichloromethane (50 mL). The combined
filtrates were evaporated to dryness under reduced pressure. The
residue was shown by thin layer chromatography to contain three
major components. The 1H NMR spectrum indicated that the
sample contained several components.
General Suzuki coupling procedure: coupling of boronic acid
11 to the PS 8a. The results of this experiment are summarized
in Table 1, entry 1. PS 8a (1.500 g, 2.39 mmol) and 4-biphenyl-
boronic acid (11) (0.590 g, 3.00 mmol) were charged into a
round-bottomed flask and flushed with nitrogen for 1 h prior to
successive additions, via a syringe, of degassed THF (20.0 mL),
tetrakis(triphenylphosphine)palladium(0) (0.209 g, 0.180 mmol)
in degassed THF (4.0 mL) and aqueous potassium carbonate
(2.0 M, 8.48 mL, 17 mmol). The reaction mixture was stirred at
60 °C for 72 h, then it was cooled and the beads collected by fil-
tration. They were washed successively with THF (3 × 25 mL),
water (3 × 25 mL) and acetone (3 × 25 mL), then dried. This
gave PS 15a (1.640 g). It is labeled PS 15a to distinguish it from
a sample of PS 15 prepared by another route (see PS 15b
below). The weight gain (140 mg) corresponds to 1.91 mmol
(80% yield) of residues 15, equal to 1.17 mmol per gram. It had
IR (KBr disc, cm−1) 1627. Elemental analysis for bromine gave
0.18% (2.1% recovered).
ii) Via cyclic ketal 9: a) Synthesis of ketal 9: A mixture of
ketone (7) (1.74 g, 8.00 mmol), 1,2-ethanediol (0.74 g,
12 mmol), PTSA (80 mg) and toluene (25 mL) was heated
under reflux in a flask equipped with a Dean–Stark apparatus.
After 12 h no more water was produced so the reaction mixture
was cooled then evaporated to dryness under vacuum. The
residue was purified by column chromatography. Elution with a
mixture of hexane and ethyl acetate (4 vo l : 1 vol) gave the
cyclic ketal 9 (1.94 g, 93% yield). It had Mp 65–67 °C; IR
1
(film, cm−1) 2958, 1603; H NMR (500 MHz, CDCl3, ppm) δ
7.95 (2H; m), 7.82 (2H; m), 7.37 (1H; m; H-7), 6.96 (2H; m;
H-1 and 3), 4.50 (2H; m; H-9 or 10), 4.38 (2H; m; H-9 or 10)
and 3.85 (3H; s; H-8); MS (EI) m/z (%): 262 (82), [M+]; microa-
nalyses: calc: for C14H14O3S C, 64.11%, H, 5.38%, S, 12.20%;
found: C, 63.88%, H, 5.27%, S, 12.10%.
b) Bromination of ketal 9 followed by cleavage of the acetal:
Following the procedure described above for compound 7, ketal
9 (1.00 g, 3.81 mmol) in DMF (20 mL) was brominated using
NBS (0.680 g, 3.83 mmol). The procedure also caused the ketal
Suzuki couplings of boronic acids 12–14 to PS 8a. These were
carried out on the same scale (1.500 g, 2.39 mmol, of PS 8a)
and using the same procedure given above. This gave PS 16a,
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17 and 18 respectively. The results are summarized in Table 1,
entries 2–4 respectively.
Entry 3, cleavage of PS 17. The cleavage gave resin beads
(0.760 g), a weight loss of 0.240 g. Chromatography of the
soluble products gave a fraction (211 mg, crude yield 79%) that
Entry 2: PS 8a gave PS 16a (2.160 g). The weight gain
(660 mg), corresponds to 2.13 mmol of thiophene-containing
residues (89% yield), equal to 0.99 mmol per g. It had IR (KBr
disc, cm−1) 1630. Elemental analysis gave bromine 0.14% (1.6%
recovered).
Entry 3: PS 8a gave PS 17 (1.596 g). The weight gain
(96 mg) corresponds to 2.04 mmol of naphthalene-containing
residues (86% yield), equal to 1.28 mmol of derivative per g. It
had IR (KBr disc, cm−1) 1633. Elemental analysis gave bromine
0.22% (2.5% recovered).
Entry 4: PS 8a gave PS 18 (1.670 g). The weight gain
(174 mg) corresponds to 2.05 mmol of the dithienyl-containing
derivative (86% yield), equal to 1.23 mmol per gram of resin. It
had IR (KBr disc, cm−1) 1632. Elemental analysis gave bromine
0.13% (2.5% recovered).
1
from the H NMR spectrum was mainly (>95%) compound 22.
Recrystallization from methanol gave compound 22 (0.179 g,
0.852 mmol, 67%) with Mp 105–106 °C (lit.,49 104–105 °C);
1H NMR (500 MHz, CDCl3, ppm) δ 8.05 (1H; m), 7.90–7.65
(4H; m), 7.45–7.30 (3H; m), 7.22 (1H; m), 7.05 (1H; m); MS
(CI: m/z (%): 211 (90), [MH+].
Entry 4, cleavage of PS 18: The cleavage gave resin beads
(0.730 g), a weight loss of 270 mg. Chromatography gave a frac-
tion (247 mg, crude yield 81%) that from the 1H NMR spectrum
was mainly (>95%) compound 23. Recrystallization from
hexane gave compound 23 (219 mg, 0.883 mmol, 72%) with
1
Mp 95–96 °C (lit.,50 94–95.5 °C); H NMR (500 MHz, CDCl3,
ppm) δ 7.19 (2H; m), 7.15 (2H; m), 7.10 (2H; m), 6.95 (2H; m);
MS (CI): m/z (%): 249 (80), [MH+].
General cleavage procedure:.40 Cleavage of the PS 5 to give
PS 19. PS 5 (1.000 g, 2.14 mmol) was added to a stirred sol-
ution of potassium t-butoxide (2.01 g, 17.9 mmol) in THF
(10 mL) at 20 °C under nitrogen. Stirring was continued for 1 h
to allow the alkoxide to diffuse into the beads, then water
(0.09 mL, 5.0 mmol) was added. The resulting heterogeneous
mixture was stirred and heated under reflux. After 72 h the
mixture was cooled, the beads filtered off and washed succes-
sively with dichloromethane (2 × 100 mL), hydrochloric acid
(10 mL of 2 N) in THF (25 mL), water (2 × 100 mL) and
acetone (2 × 50 mL) then dried. This gave PS 19 (0.903 g): a
weight loss of 97 mg. It had IR (KBr disc, cm−1) 1730, 1683.
Elemental analysis indicated 0.15% of sulfur (2.3% recovered).
Demethylation of ether 10. Using the procedure described
above for the demethylation of ether 7, ether 10 was converted
into phenol 24. It was obtained as a white solid (88% yield) with
Mp 134 °C (lit.,46 133 °C); IR (KBr disc, cm−1) 1633; 1H NMR
(300 MHz, CDCl3, ppm) δ 7.85 (2H; m; 2H; H-2 and 4), 7.62
(1H; m; H-5), 7.41 (1H; m; H-6), 6.95 (2H; m; H-1 and 3), 6.80
(1H; br s; OH).
Attachment of phenol 23 to chloromethylated beads to give PS
8b. Using the general procedure given above, phenol 24 was
attached to the chloromethylated beads (5.000 g, 20.55 mmol).
This gave PS 8b (9.372 g), a weight gain of 4.372 g. This corre-
sponds to 17.7 mmol (86% yield) of residues 8, equal to
1.89 mmol per g. It had IR (KBr disc, cm−1) 1629. Elemental
analysis gave 5.90% sulfur and 14.81% bromine. Based on the
sulfur content this corresponds to a loading of 1.84 mmol of
thiophene residues 8 per gram of beads; based on the bromine
analysis 1.85 mmol per g.
Cleavages of the PS 15a, 16a, 17 and 18. These were carried
out on the same scale and using the same procedure as given
above. This gave 20, 21, 22 and 23 respectively. The results are
summarized in Table 1, entries 1–4 respectively.
Entry 1, cleavage of PS 15a. PS 15a (1.000 g, 1.17 mmol)
gave polymer beads (0.781 g), a weight loss of 219 mg. The
organic washings were combined, dried and the solvent evapor-
ated off. The residue in dichloromethane (2.0 mL) was chro-
matographed over a short column of silica gel. Elution with
dichloromethane gave a fraction (215 mg, crude yield 78%) that
Suzuki couplings of boronic acids 11, 12 and 25 and boronate
26 to PS 8b. These were carried out using the general procedure
and on the same scale (1.500 g, 2.84 mmol of PS 8b) as above.
This gave PS 15b, 16b, 27 and 28 respectively. The results are
summarized in Table 1, entries 5–8 respectively.
Entry 5: Reaction with boronic acid 11 gave PS 15b (1.668 g),
a weight gain of 168 mg, corresponding to 2.30 mmol of resi-
dues 15 (81% yield), equal to 1.38 mmol per gram. It had IR
(KBr disc, cm−1) 1631. Elemental analysis gave bromine 0.10%.
Entry 6: Reaction with boronic acid 12 gave PS 16b
(2.196 g), a weight gain of 696 mg, corresponding to 2.45 mmol
of residues 16 (79% yield), equal to 1.12 mmol of derivative per
gram of resin. It had IR (KBr disc, cm−1) 1630. Elemental analy-
sis gave bromine 0.27%.
Entry 7: Reaction with boronic acid 25 gave PS 27 (1.677 g),
a weight gain of 177 mg, corresponding to 2.01 mmol (71%
yield) of residues 27, equal to 1.20 mmol per gram of resin. It
had IR (KBr disc, cm−1) 1633. Elemental analysis gave bromine
0.14%.
Entry 8: Reaction with boronate 26 gave PS 28 (1.495 g), a
weight loss of 5 mg. Elemental analysis gave bromine 0.19%,
suggesting that >99% of the bromo groups had reacted. If they
had been replaced by hydrogen the expected weight of the
1
from the H NMR spectrum was mainly (>95%) compound 20.
Recrystallization from hexane gave a sample of compound 20
(180 mg, 0.762 mmol, 65% yield) with Mp 171–173 °C (lit.,47
1
172 °C); H NMR (500 MHz, CDCl3, ppm) δ 7.63 (m, 2 H),
7.54 (2H; m), 7.39–7.36 (5H; m), 7.29 (1H; m), 7.28 (1H; m),
7.03 (1H; m); MS (CI): m/z: 237 (100), [MH+].
Entry 2, cleavage of PS 16a. The cleavage gave beads
(0.640 g), a weight loss of 0.360 g. Chromatography of the
organic extracts gave compound 21 as a colourless oil48
(274 mg, 0.581 mmol, 59%). It had 1H NMR (500 MHz,
CDCl3, ppm) δ 7.69 (2H; m), 7.56 (2H; m), 7.30–7.46 (4H; m),
7.25–7.10 (2H; m), 1.88 (4H; m; CH2), 1.25–0.89 (20H; m;
CH2), 0.76 (6H; t, J = 7.5; CH3), 0.60 (4H; q; CH2). This spec-
trum is in good agreement with that reported by Belletête et al.48
MS (CI) m/z (%): 473 (100), [MH+]. Elemental analysis calcu-
lated (%) for C33H44S: C 83.84%, H 9.39%, S 6.77%; found: C
83.68%, H 9.30%, S 6.69%.
1760 | Org. Biomol. Chem., 2012, 10, 1754–1763
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recovered beads would be 1.299 g. On the basis that the gain in
weight (196 mg) is due to the Suzuki coupling, this corresponds
to 2.36 mmol of residues 27 (83% yield), equal to 1.58 mmol
per g. It had IR (KBr disc, cm−1) 1631.
C, 56.69%, H, 5.56%, S, 8.36%, found C, 56.88%, H, 5.35%, S,
8.50%.
(ii) Synthesis of 2-bromo-3-n-hexylthien-5-yl 4-hydroxyphenyl
ketone (36): 2-Bromo-3-n-hexyl-5-thienyl 4-methoxyphenyl
ketone (35) (6.00 g, 15.7 mmol) in dichloromethane (60 mL)
under nitrogen was cooled in an acetone-dry ice bath. A solution
of boron tribromide (15.9 g, 3.0 mL, 32 mmol) in dichloro-
methane (40 mL) was added dropwise with stirring. When the
addition was complete, the reaction mixture was allowed to
attain 20 °C and stirred overnight. It was then poured over a
mixture of ice and water (100 mL) and after 30 min extracted
with dichloromethane (3 × 50 mL). The extracts were dried and
the solvent evaporated off. The residue was purified by column
chromatography over silica gel. Elution with chloroform to give
Cleavages PS 15b, 16b, 27 and 28. These were carried out
using the general procedure and on the same scale (1.000 g of
starting material) as above. This gave 20, 21, 29 and 30 respect-
ively. The results are summarized in Table 1, entries 5–8
respectively.
Entry 5, cleavage of PS 15b: The cleavage gave recovered
beads (0.695 g), a weight loss of 0.305 g. Chromatography of
the soluble products gave compound 20 as a gum (300 mg,
crude yield 92%). Crystallisation from hexane gave compound
20 (241 mg, 1.02 mmol, 74% yield) with Mp 170–172 °C. It
had an 1H NMR spectrum identical to that described above.
Entry 6, cleavage of PS 16b: The cleavage gave recovered
beads (0.501 g), a weight loss of 499 mg. Chromatography of
the soluble products gave compound 21 as an oil (418 mg, 79%
phenol 36 (5.37 g, 93% yield) as a gum. It had IR (film, cm−1
)
1
1631; H NMR (500 MHz, CDCl3, ppm) δ 7.60 (2H; m; H-2
and 4), 7.51 (1H; s; H-5), 6.84 (2H; m; H-1 and 3), 2.70 (2H; m;
H-7), 1.62 (2H; m), 1.42–1.22 (6H; m), 0.90 (3H; m; H-8); MS
(CI): m/z (%): 368 (100), [MH+]. Elemental analysis calculated
for C17H19BrO2S: C, 55.59%, H, 5.22%, S, 8.71%, Br, 21.77%,
found: C, 55.68%, H, 5.32%, S, 8.70%, Br, 22.00%.
1
yield). The H NMR spectrum was identical to that described
above for compound 21.
Entry 7, cleavage of PS 27: The cleavage gave recovered
beads (0.750 g), a weight loss of 250 mg. Chromatography of
the soluble products gave an oil (250 mg, crude yield 96%) that,
by 1H NMR spectroscopy, was >95% compound 29. Crystalliza-
tion from hexane gave plates (0.229 g, 88% yield), Mp
(iii) Attachment of phenol 36 to chloromethylated beads to
give PS 33: Using the general procedure given above, phenol 36
(5.700 g) was attached to the chloromethylated polystyrene
beads (3.700 g). This gave PS 33 (6.970 g). The weight gain
(3.270 g) corresponds to 13.00 mmol of thiophene-containing
residues, equal to 1.86 mmol per g. PS 33 had IR (KBr disc,
cm−1) 1633. Elemental analysis for sulfur gave 5.80% and for
bromine 14.56%. These correspond respectively to loadings of
1.81 and 1.82 mmol per g of thiophene-containing residues 33.
1
67–68 °C (lit.,51 68.0–68.4 °C); H NMR (300 MHz, CDCl3,
ppm) δ 7.04–7.70 (6H; m); MS (CI): m/z (%): 167 (100),
[MH+].
Entry 8, cleavage of PS 28: The cleavage gave recovered
beads (0.720 g), a weight loss of 0.280 g. Chromatography of
the soluble products gave compound 30 as an oil52 (252 mg,
1
84% yield). H NMR (300 MHz, CDCl3, ppm) δ 7.32 (1H; m),
Synthesis of compound 31. The reactions involved are sum-
marized in Scheme 2.
7.17 (1H; m), 7.10 (1H; m), 7.08 (1H; m), 6.91 (1H; m), 2.73
(2H; m), 1.65 (2H; m), 1.32 (2H; m), 1.28 (4H; m), 0.91 (3H;
m); MS (CI): m/z (%): 251(100), [MH+]. Elemental analysis cal-
culated for C14H18S2: C, 64.18%, H, 7.26%, S, 25.58%; found:
C, 63.88%, H, 7.27%, S, 25.30%.
(i) Suzuki coupling of boronic acid 11 to PS 33: Following the
general procedure a Suzuki reaction was carried out between PS
33 (2.000 g, 3.64 mmol) and boronic acid (11) (0.79 g,
4.00 mmol). This gave PS 37 (2.220 g). The weight gain
(0.220 g), corresponds to 3.02 mmol of the Suzuki product
(83% yield), equal to 1.36 mmol per gram of resin. It had IR
(KBr disc, cm−1) 1630. Elemental analysis for bromine gave
0.38%.
(ii) Bromination of the PS 37: PS 37 (2.000 g, 2.72 mmol)
was suspended in DMF (65 mL) and NBS (0.490 g, 2.76 mmol)
was added. The mixture was stirred at 90 °C for 72 h. It was
then cooled. The beads were collected and washed successively
with DMF (2 × 25 mL), water (2 × 25 mL), and acetone (2 ×
25 mL) before being dried. This gave PS 38 (2.180 g). The
weight gain (0.180 g) corresponds to 2.31 mmol of bromo resi-
dues (85% yield), equal to 1.05 mmol per gram. It had IR (KBr
disc, cm−1) 1631. Elemental analysis for bromine gave 8.72%
this corresponds to a loading of 1.09 mmol of residues 38 per g.
(iii) Suzuki coupling of boronate 26 to PS 38: Following the
usual Suzuki reaction procedure boronate 26 (0.880 g,
3.00 mmol) was reacted with PS 38 (2.000 g, 2.10 mmol). This
gave PS 39 (2.170 g). The weight gain (0.170 g) corresponds to
1.98 mmol of residues 39 (94% yield), equal to 0.91 mmol per
g. It had IR (KBr disc, cm−1) 1632. Elemental analysis for
bromine gave 0.27%.
Synthesis of PS 33. The reaction scheme is shown in
Scheme 1.
(i) Synthesis of 2-bromo-3-n-hexylthien-5-yl 4-methoxyphenyl
ketone (35): To a solution of 4-methoxybenzoyl chloride (5.12 g,
30.00 mmol) and 2-bromo-3-n-hexyl-thiophene (34)41 (7.41 g,
30.00 mmol) in dichloromethane (60 mL) tin tetrachloride
(2.49 g, 36 mmol) was added and the mixture stirred at 60 °C
for 10 h. It was then cooled and added to a mixture of water
(150 mL) and hydrochloric acid (20 mL of 5 M). The acidity
was neutralised (to litmus) by treatment with saturated aqueous
sodium bicarbonate. The aqueous layer was then extracted with
dichloromethane (3 × 30 mL). The combined extracts were
dried, evaporated to drynesss, and the residue subjected to
column chromatography. Elution with hexane/dichloromethane
(1 vo l : 4 vols) gave compound 35 (6.74 g, 59% yield) as a
1
gum. It had IR (film, cm−1) 1631; H NMR (500 MHz, CDCl3,
ppm) δ 7.61 (2H; m; H-2 and 4), 7.50 (1H; s; H-5), 6.90 (2H;
m; H-1 and 3), 3.84 (3H; s; H-6), 2.70 (2H; m; H-7), 1.62 (2H;
m; CH2), 1.41-1.22 (6H; m; CH2), 0.89 (3H; m; H-8); MS (EI)
m/z = 382 (M+). Elemental analysis calculated for C18H21BrO2S:
This journal is © The Royal Society of Chemistry 2012
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and Separations Using Functional Polymers, ed. D. C. Sherrington and
P. Hodge, Wiley, Chichester, 1988.
(iv) Cleavage of PS 39: The cleavage reaction was carried out
on PS 39 (1.000 g, 0.91 mmol) using the procedure described
above. This gave recovered beads (0.550 g), a weight loss of
0.450 g. The purified cleaved product was identified as 31
(0.340 g, 68%). It had Mp 35–37 °C; λmax (ethanol) 309 nm (ε
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1
40,100); H NMR (500 MHz, CDCl3, ppm) δ 7.71 (4H; m; H-3
or 4), 7.55 (4H; m; H-3 or 4), 7.29 (2H; d, J = 4.5; H-1), 7.03
(2H; d, J = 4.5; H-2), 2.75 (4H; t, J = 7.7; α-CH2), 1.65 (4H; m;
β-CH2), 1.30 (12H; m; CH2), 0.91 (6H; t, J = 7.7; CH3); MS
(CI): m/z (%): 487 (100), [MH+]. Elemental analysis calculated
for C32H38S2: C, 78.97%, H, 7.88%, S, 13.15%, found: C,
78.80%, H, 7.55%, S, 13.45%.
Synthesis of compound 32. The reactions involved are sum-
marized in Scheme 3.
(i) Suzuki coupling of boronic acid 14 to PS 33: Following the
usual procedure boronic acid 14 (0.86 g, 4.1 mmol) was reacted
with PS 33 (3.000 g, 5.46 mmol). This gave PS 40 (2.280 g) a
weight gain of 0.280 g, corresponding to 3.34 mmol of Suzuki
product (61% yield), equal to 1.46 mmol per g. It had IR (KBr
disc, cm−1) 1632. Elemental analysis for bromine gave 0.20%.
(ii) Bromination of the PS 40: PS 40 (2.000 g, 2.84 mmol)
was brominated using the procedure described for the prep-
aration of PS 38. This gave PS 41 (2.205 g) a weight gain of
0.205 g, corresponding to 2.56 mmol of residues 41 (90%
11 A. N. Cammidge and Z. Ngaini, Chem. Commun., 2004, 1914–1915.
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6499.
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yield), equal to 1.17 mmol per g. It had IR (KBr disc, cm−1
)
1629. Elemental analysis for bromine gave 9.75%. This corre-
sponds to a loading of 1.22 mmol of residues 41 per g.
(iii) Suzuki coupling of boronate 26 with PS 41: PS 41
(2.000 g, 2.42 mmol) was reacted with boronate 26 (0.884 g,
3.00 mmol) using the usual procedure. This gave PS 42
(2.183 g). The weight (0.183 g) corresponds to 2.10 mmol of
residues 42 (87% yield), equal to 0.96 mmol of per g. It had IR
(KBr disc, cm−1) 1632. Elemental analysis for bromine gave
0.19%.
20 J. M. Tour, Chem. Rev., 1996, 96, 537–553.
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(iv) Cleavage of PS 42: Following the usual procedure PS 42
(1.000 g, 0.98 mmol), gave recovered beads (0.624 g), a weight
loss of 0.384 g. The soluble product was purified by chromato-
graphy. This gave compound (265 mg, 54%). It had Mp
1
30–32 °C (lit.,44 28–30 °C); H NMR (500 MHz, CDCl3, ppm)
δ 7.09 (2H; d, J = 5.2; H-1), 7.04 (2H; m; H-3 or 4), 6.94 (2H;
m; H-3 or 4), 6.82 (2H; d, J = 5.2; H-2), 2.73 (4H; m; α-CH2),
1.60 (4H; m; β-CH2), 1.25 (12H; m; CH2), 0.81 (6H; m; CH3);
MS (EI): m/z (%): 498 (100), [M+]. Elemental analysis calcu-
lated for C28H34S4: C, 67.45%, H, 6.88%, S, 25.67%, found: C,
67.23%, H, 6.68%, S, 25.65%.
33 T. Klingstedt, A. Åslund, R. A. Simon, L. B. G. Johansson, J. J. Mason,
S. Nyström, P. Hammarström and K. P. R. Nilsson, Org. Biomol. Chem.,
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Acknowledgements
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41 M. Humadi, Ph D Thesis, University of Manchester, 2011.
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This work was carried out under Grant GR/S02280/01 of the
Carbon Based Electronics Programme. We thank Dr Giudi Barta-
lucci, Dr Arantxa Rodriguez-Menendez, Mohammad Humadi,
Douzhi Zhang and Mahdi Al’Makki for laboratory assistance
during student projects.
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