A divergent synthesis of oligoarylalkanethiols with Lewis-basic N-donor termini

Article abstract of DOI:10.1039/c003795h

Araliphatic thiols are key molecules for the formation of self-assembled monolayers with high long-range order. If these monolayers shall act as bases for the attachment of other molecules, the respective thiols need to carry suitable functional groups, such as the amino or the pyridine group. Due to their Lewis-basicity, these groups are not compatible with the thiol group under most reaction conditions. Here, an entry into this versatile class of compounds is presented, by using fundamental building blocks in which the thiol groups are protected as triisopropylsilyl sulfides making them compatible with many reagents including Grignard reagents and palladium catalysts. With this strategy at hand, six thiols with bi- and terphenyl backbones, one to three methylene groups, and amino or pyridine head groups became accessible in short reaction sequences. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c003795h

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
A divergent synthesis of oligoarylalkanethiols with Lewis-basic N-donor
termini
Bjo¨rn Schu¨pbach and Andreas Terfort*
Received 4th March 2010, Accepted 18th May 2010
First published as an Advance Article on the web 9th June 2010
DOI: 10.1039/c003795h
Araliphatic thiols are key molecules for the formation of self-assembled monolayers with high
long-range order. If these monolayers shall act as bases for the attachment of other molecules, the
respective thiols need to carry suitable functional groups, such as the amino or the pyridine group. Due
to their Lewis-basicity, these groups are not compatible with the thiol group under most reaction
conditions. Here, an entry into this versatile class of compounds is presented, by using fundamental
building blocks in which the thiol groups are protected as triisopropylsilyl sulfides making them
compatible with many reagents including Grignard reagents and palladium catalysts. With this strategy
at hand, six thiols with bi- and terphenyl backbones, one to three methylene groups, and amino or
pyridine head groups became accessible in short reaction sequences.
lengths and substitution patterns and predictable SAM-forming
Introduction
properties.9 This approach was recently extended by the intro-
The attachment of functional groups to the surface of solids is
duction of short alkyl chains in between the p-oligophenyl units
the basis of numerous techniques, such as adsorption control,1
and the binding groups. The alkyl chains on one hand are flexible
sensing,2 or solid phase synthesis.3 While many of these appli-
enough to permit an adjustment between the adsorption site and
cations require only a more or less dense distribution of these
the preferred packing of the oligophenyl moieties, and on the other
functional groups, the most advanced ones such as microelec-
hand are stiff enough to communicate the binding situation at the
tronics or nano-architectures require also a very high density as
anchoring group up to the head group due to their preference of
well as excellent lateral order.4 A typical approach for this kind
of surface functionalisation is the deposition of self-assembled
monolayers (SAMs), which are formed by the self-terminating
reaction of suitable molecules with the solid surfaces. For the
archetypical example, the thiolate-on-gold system, it has been
established already 25 years ago that even flexible alkane chains
become densely packed in an ordered fashion within these surface
layers.5 Unfortunately, the introduction of any donor group at
the topmost position (therefore called the ‘head group’ by many
authors), almost always results in disorder in the respective SAM
due to significant and often directed interactions, such as hydrogen
bonding.6 These interactions primarily lead to the bending of
the alkyl chains under formation of so-called ‘gauche defects’
(in a well-ordered monolayer the alkyl chains adopt an all-
trans conformation), leading to a diminished packing density
accompanied by translational disorder. In the past, we7 and others8
could show that the use of stiff backbones, which cannot be
bent by the head group interactions, is a suitable strategy to
suppress this disorder mechanism. Advantageous backbones for
these applications, both from the viewpoint of accessibility as
well as conformational stiffness, are para-oligophenyls, such as
p-terphenyl. These moieties are not only rod-shaped themselves,
but can be co-axially extended in the respective 4-(4¢-, 4¢¢-,
…)positions – this is with the binding groups for the surface
attachment as well as with the desired head groups – permitting
the rational design of a plethora of molecules with different
the all-trans conformation.10 This ultimately results in monolayers
with exceptional long range order and a pronounced odd-even
effect on the tilt angle of the oligophenyl unit. The latter also
affects the thickness of the monolayers as well as the orientation
of any functional group attached to the oligophenyl unit.
We wanted to extend this principle to SAM-forming molecules
carrying Lewis-basic head groups, which shall permit the at-
tachment of further groups after SAM formation, either by
coordination (metals) or formation of covalent bonds (amides
etc.). For this, we chose the pyridine and the primary amino
group, since they are well-established in coordination chemistry
as well as organic chemistry, and should fit well into densely
packed aromatic monolayers due to their small size. Both of
these groups have been used successfully for a large bandwidth of
applications. Some very basic studies on the monolayer-forming
properties of pyridine-terminated thiols show that only under
special precautions highly ordered monolayers can be obtained.11
Some of these monolayers show a pronounced dependency of
their structure on the pH value.12 The typical use, as stated
above, is the attachment of metal ions by coordination, either
for analytical purposes13 or for the formation of thin metal
films.14 Pyridine-terminated monolayers have also been used for
the tight attachment of redox-active proteins to investigate their
electrochemical behaviour.15 Even two 4-pyridylbenzene thiols
with16 and without17 alkylchain have been reported. The latter have
been used as a template for the directed growth of glycine crystals.18
For amino-terminated monolayers, the focus of interest is clearly
on the covalent attachment of biological entities.19 While for the
latter application mostly aliphatic thiols have been employed, a
few reports describe the use of aminothiophenols with the basic
Institut fu¨r Anorganische und Analytische Chemie, Universita¨t Frankfurt,
Max-von-Laue-Straße 7, 60438, Frankfurt, Germany. E-mail: aterfort@
chemie.uni-frankfurt.de; Fax: +49 (0)69 798 29188
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idea to provide a better electronic coupling. The redox-active
probes then have been either attached covalently20 or electro-
statically.21
proceed under mild conditions. A general problem is that most
of these coupling reactions are catalyzed by metals, typically
palladium, which become poisoned by thiols. Therefore many
attempts have been made to render thiol chemistry and Pd(0)
catalysed cross coupling compatible.24 The most obvious strategy
is to protect the thiol group during the coupling step (pg in
Scheme 1) and remove the protecting group as a last step in
the synthetic sequence. Frequently the t-butyl group has been
employed, but its removal requires harsh or inconvenient (Hg(II),
H2S) conditions giving only low yields.17,25 Because of that, we
were looking for another sterically hindered protecting group
which is easy to introduce and which is also easily cleaved
under mild conditions. Soderquist et al. have introduced the
triisopropylsilyl (TIPS) group as protecting group for thiols, which
even is compatible with Grignard conditions, as the authors
demonstrated for a protected bromoalkanethiol,26 by hydrolysing
the respective organometallic compound with deuterium oxide
to obtain the deuterated alkylsilanethiol. We could recently
extend this approach by demonstrating that the Grignard reagent
obtained from 4-BrC6H4STIPS can be employed in Pd catalyzed
cross coupling reactions either directly or after transformation
in the corresponding organozinc reagent.27 The deprotection of
the thiol group can then be accomplished with acids or with
tetrabutylammonium fluoride.26,28
For the access to (substituted) oligophenylalkanethiols, we
needed to prepare the analogous 4-bromophenylalkyl TIPS-
thioethers 1a–c with varying chain length. As has been shown
in ref. 26, triisopropylsilanethiol (TIPSSH, 2) is a suitable reagent
not only for the introduction of the TIPS group but the sulfur atom
as well. We simplified the synthesis of 2 by the direct reaction of
triisopropylsilane with elemental sulfur29 (Scheme 2). Deproto-
nation with n-butyllithium or potassium-t-butoxide activates this
material for the reaction with alkyl halides. For the synthesis of the
synthon with only one methylene group (1a), 91% yield could be
obtained by using commercially available 4-bromobenzyl bromide.
For the synthesis of the respective homolog with an ethylene group,
first the carbon backbone had to be assembled, starting from
1,4-dibromobenzene. This was converted to the mono-Grignard
reagent in ether (in THF the reaction does not proceed selectively),
before being reacted with oxirane in THF. The presence of catalytic
amounts of Cu(I) salts helped the ring-opening addition to give
2-(4-bromophenyl)ethanol 3 in 64% yield. Treatment of 3 with
thionyl chloride yielded the respective chloride (4, 68%), which
then was reacted with the aforementioned lithium salt of 2 under
formation of 67% of 1b. As an alternative route, the photo-
induced addition of thiols to alkenes, which due to its cleanliness
and high yields now is considered to be a ‘click’ reaction,30 has
been tested. Nevertheless, the addition of 2 to 4-bromostyrene
proceeded sluggish and yielded only about 50% of 1b in an impure
state. We therefore prefer the multistep process, in spite of its
overall yield of only 29%, since this method utilizes much cheaper
reagents (permitting its performance on a multi-gram scale) and
yields pure products.
Due to the numerous applications, we figured it necessary to
develop a family of molecules, which provides the access to highly
ordered amino- and pyridyl-terminated monolayers on gold. The
major challenge for the synthesis of thiols bearing Lewis-basic
groups is that the thiol groups become very air-sensitive under
basic conditions, making their handling difficult, in particular
during the purification or the deposition of the molecules. This is
especially true for molecules carrying primary alkylamino groups,
the corresponding ammonium ions of which have comparable pKa
values as alkanethiol groups (about 10–11). If these groups are
present in the same molecule, most of the thiol groups will become
deprotonated, thus being not only less suited for the formation
of SAMs22 but also becoming almost instantly oxidized upon
contact with air. It is therefore viable to use a suitable protecting
group chemistry for the thiol groups during the construction of
the molecule framework, setting the sensitive combination only
free at the very end of the synthesis.
Results and discussion
Several strategies for the synthesis of 4-substituted bi- and
terphenyl derivatives have been employed in the past.23 Since
in most cases only one or two compounds were targeted, the
development of divergent synthetic strategies mostly was not
necessary. In contrast to this, our extended study on the behaviour
of substituted oligophenylalkanethiols in SAMs required fast and
efficient access to a variety of molecules not only with different
head groups but also with different lengths of the alkyl chains. We
therefore decided to develop a building block approach in which
benzene or biphenyl derivatives, which already carry the required
head group, become attached to more or less universal synthons
(1) that permit the introduction of (protected) w-mercaptoalkyl
groups of different lengths (Scheme 1).
Scheme 1 Retrosynthetic approach to the N-terminated oligophenyls.
‘dg’ signifies the donor group (N or C-NH2), ‘M’ is a metal–organic group,
and ‘pg’ represents a suitable protective group for the coupling chemistry.
m and n are 0–1 and 1–3 in this manuscript, respectively.
For the synthesis of the third synthon (1c, with three methy-
lene groups), 4-bromo(prop-2-enyl)benzene was used as starting
material, which itself can be obtained from 1,4-dibromobenzene
according to a literature procedure.31 The attempted addition
of 2 using AIBN or light as initiators proceeded even more
sluggish than in case of the respective styrene. In contrast to
The synthons also already carry a phenyl group, since C–C
coupling reactions between arenes are well established and can
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Scheme 2 Syntheses of the building blocks 1a–c with different lengths of the alkyl chain between the phenyl rings and the sulfur atoms. The latter are
already protected for the coupling chemistry (TIPS = triisopropylsilyl).
this, the addition of thioacetic acid to 1c proceeded smoothly
to give S-3-(4-bromophenyl)propyl thioacetate 5 in 80% yield.
Surprisingly, this reaction even started in the absence of AIBN
when the reaction is carried out in air. The resulting thioacetate
5 was saponificated with KOH to the respective thiol 6 in almost
quantitative yield (Scheme 2). This was converted to the TIPS
thioether by the action of triisopropylsilylchloride (TIPSCl). For
the success of the reaction the choice of the right base turned
out to be essential: While for the silylation of thiophenols with
trimethylsilylchloride (TMSCl) triethylamine was reported to be
useful,32 the reaction of 6 with TIPSCl in presence of this base
proceeded unsatisfactorily (50% conversion after one week at
reflux temperature). The use of n-BuLi, as has been successful
for the inverse etherification steps above, gave full conversion
after 24 h at room temperature, but undesired byproducts were
formed, most likely due to halogen-lithium exchange in 6. In
this case we therefore could only isolate 1c in a yield of 52%.
Because both methods were unsatisfactory, we were looking for
an alternative strong base which is non-nucleophilic and does not
undergo halogen/metal exchange. We found that sodium hydride
is the base of choice for the deprotonation of 6. The resulting
sodium thiolate reacted very smoothly with TIPSCl permitting
the isolation of the third synthon 1c in a yield of 83%.
With the synthons 1a–c at hand, the synthesis of the donor-
substituted oligophenylalkanethiols could be approached. For the
attachment of the 4-pyridyl head groups, either the commer-
Scheme 3 Assembly of the 4-pyridyl-terminated oligophenylalkanethiols
using the Kumada coupling with the Grignard reagents of 1a–c, followed
by the deprotection of the thiol group.
cially available 4-bromopyridine (7a, as hydrochloride) or 4-(4-
bromophenyl)pyridine (7b, ref. 33) have been used. These were
reacted with the Grignard reagents of 1a–c in the presence of
Pd(dppf)Cl2 (Kumada reaction) to form the coupling products
8a–d in 34–74% yield (Scheme 3). In case of 7a, the deprotonation
of the hydrochloride was achieved by the action of one equivalent
of ethylmagnesium chloride before the other reagents were added.
In the final step, the TIPS group was removed protolytically using
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HCl. The free molecules 9a–d were obtained by adjusting the pH to
about 8 to deprotonate the pyridyl ring but not the thiol group. The
pyridinethiols are air-stable as crystalline compounds but become
easily oxidized in solution.
carbamic-carbonic anhydride 11c directly or via the respective
isocyanate 11d, by the action of unaltered 10.36 By reaction of
11a with excess Boc2O, 11b is obtained (Scheme 4). The respective
mono-Boc compound could not be isolated. To improve the yield
of 11, we attempted to protect 10 with Boc2O in the presence of
sodium hydride but without Steglich’s catalyst. Surprisingly, the
sodium hydride itself could not push the reaction directly to the
di-Boc compound 11, but when DMAP was added, we finally were
able to obtain 11 in a more acceptable yield of 51% with 11b as
only by-product (34%).
The thus protected amine 11 was then used for the Negishi cross
coupling in the presence of Pd(dppf)Cl2 as catalyst (Scheme 5).
Presumably due to the formation of Lewis-acidic zinc salts, no
coupling products still carrying the diBoc imido group could be
isolated but rather the terphenyl derivatives with no (12a,b) or
one Boc group (13a,b) remaining. The two classes of compounds,
which formed in a ratio of ~ 2 : 3 (12:13), could be easily separated
by chromatography giving total yields of 68% (12a,13a) and
66% (12b,13b), respectively. For the removal of the protective
groups, acidic conditions similar to the ones used for the pyridine-
terminated thiols have been chosen, regardless on whether the
materials with or without Boc group have been used. The final
amino thiols 14a,b could be obtained with good to excellent yields,
if some precautionary measures were taken, since the amino-
terminated thiols are even more air sensitive than the pyridine
thiols, in particular in solution.
For the extension of this approach to the amino-terminated
thiols, suitable building blocks had to be found. The amino group
is inherently not compatible with most metal–organic reagents,
typically leading to the protonation of the reagents. It is therefore
necessary to introduce protective groups which are compatible
with the coupling chemistry. For this, we decided to switch from
the magnesium-based coupling chemistry (Kumada coupling) to
the respective zinc-based chemistry (Negishi coupling) known
to be compatible with carbonyl groups so we could use the
t-butyloxycarbonyl (Boc) group for the protection of the amino
group. This well-established group has the advantage – among
others – to be removable under acidic conditions, that is, the
same conditions under which the TIPS group can be removed.
Since the simple Boc amides are even more acidic than the
corresponding amines, we decided to rather use the diBoc imides
instead.
To obtain the building block for the synthesis of the amino-
terminated terphenyl thiols, we treated 4-amino-4¢-bromobiphenyl
1034 with an excess of di-tert-butyl dicarbonate (Boc2O) in
the presence of Steglich’s catalyst35 (4-dimethylaminopyridine,
DMAP). It turned out that, if the catalyst was added right from
the beginning of the reaction, three products were obtained, with a
yield of the desired imide 10 of only 16%. The other two products
were identified to be N,N¢-bis-(4¢-bromobiphenyl-4-yl)urea 11a
and N,N¢-diBoc-N,N¢-bis(4¢-bromobiphenyl-4-yl)urea 11b. It has
been suggested that these ureas either form from an intermediate
To obtain the amino thiols 14a,b with very high purity, gradient
sublimation in high vacuum is recommended. The resulting
colourless solids can be stored under exclusion of air for months,
but they slowly turn yellowish upon exposure to oxygen.
Scheme 4 The synthesis of the amino-terminated oligophenylalkanethiols requires a complete protection of the amino-group during the coupling step
to avoid protolysis of the metal–organic reagent. The di-Boc group is compatible with the zinc-organic reagent and can be removed under the same
conditions as the TIPS group, but its introduction is accompanied by the formation of the two ureas 11a and 11b. The formation of 11a can be suppressed
by the presence of NaH during the protection, resulting in an improved yield of 11. See text for details.
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acid, which were distilled prior to use. Unless otherwise stated,
all reactions were carried out using standard Schlenk procedures
and anhydrous and oxygen-free solvents. THF and diethyl ether
were destilled from Na/K alloy. DMF was distilled from CaH2.
‘Oxygen-free’, but not dry solvents were liberated of oxygen by
passing a stream of nitrogen through the respective solvent for
at least one hour. 4-(4-Bromophenyl)pyridine,33 4-bromo(prop-
2-enyl)benzene,31 4-amino-4¢-bromobiphenyl,34 and Pd(dppf)Cl2
37
were prepared according to literature procedures. The synthesis
of triisopropylsilanethiol (2) was carried out in analogy to a
literature known synthesis of triphenylsilanethiol.29 Melting points
were determined with an apparatus according to Dr Tottoli and
are uncorrected. The following NMR spectrometers were used:
Bruker Avance 300, Bruker DRX 400, Bruker Avance 400, Bruker
ARX 200, and Varian Gemini 2000. IR spectra were recorded
on a Bruker Vertex or a Nicolet 6700 Fourier-transform IR
spectrometer using a diamond ATR cell. Mass spectra were
recorded on a MAT 95 from Finnigan or a Finnigan LTQ-FT
from Thermo Fischer Scientific. For elemental analysis we used
a vario micro cube from elementar. GCMS analysis was carried
out using a Trace GC 2000 Series in combination with a Finnigan
Trace mass spectrometer from ThermoQuest CE Instruments.
4-Bromobenzyl(triisopropylsilyl)sulfide (1a)
To a solution of 2 (9.22 g, 48.4 mmol) in 100 mL of THF, n-BuLi
(30 ml of a 1.6 mol L-1 solution in hexanes, 48 mmol) was
added under external cooling with ice. Cooling was maintained
and 4-bromobenzyl bromide (10.54 g, 42.17 mmol) dissolved in
50 mL of THF was added slowly. After the reaction mixture
was stirred for 20 h at room temperature, 25 mL of water were
added and the THF was evaporated under reduced pressure.
Further 100 mL of water were added. The aqueous suspension
was extracted once with methylcyclohexane and three times
with CH2Cl2. Acidification with HOAc helped the separation of
the phases. The organic phases were collected, the solvent was
evaporated and the remaining oil distilled under reduced pressure
Scheme 5 The couplings of the diBoc imide 11 with the zinc reagents of
1b,c yield the terphenyls 12a,b and 13a,b under loss of one or both Boc
groups. The deprotection of either yields the amino thiols 14a,b in good
to excellent yields.
Conclusions
As could be demonstrated here, the careful choice of protective
group chemistry opens the door to the synthesis of arylalkanethi-
ols carrying Lewis-basic head groups, which can be used for the
formation of densely functionalized surfaces. The triisopropylsilyl
(TIPS) group turned out to be the protective group of choice for
the thiol groups, since the respective silylthioether is compatible
with the metal–organic reagents and the conditions under which
the molecular backbones are constructed. For the protection of
amino groups, the diBoc imide proved to be useful, since it is not
only stable under the coupling conditions but can be removed
using the same reagents as for the removal of the TIPS group
(that is, acidic methanol). By pre-forming the TIPS thioethers
with small 4-bromophenylalkyl residues, central building blocks
became accessible, which permit the fast and efficient synthesis
of all the Lewis-basic molecules in this study. We believe that
this divergent approach can easily be extended to many other
oligophenylalkanethiols bearing all kinds of head groups.
◦
(oil pump vacuum). Collection of the fraction boiling at 142 C
yielded the product as colourless liquid. 13.77 g (38.31 mmol),
1
91%. H NMR (CDCl3, 250 MHz): d = 7.45–7.37 (m, 2H, CH-
ar), 7.26–7.19 (m, 2H, CH-ar), 3.70 (s, 2H, CH2), 1.40–1.20 (m, 3H,
CH), 1.20–1.10 (m, 18H, CH3) ppm. 13C NMR (CDCl3, 63 MHz):
d = 139.8, 131.5, 130.1, 120.6, 29.5, 18.5, 12.7 ppm. IR (ATR):
˜
nmax = 2943 (CH-alk), 2889 (CH-alk), 2865 (CH-alk), 1486, 1461,
1071, 1012, 881 cm-1. MS (EI): m/z (%) = 358 (22) [M+], 315 (89),
273 (22), 90 (27). C16H27BrSSi (359.44): calcd. C 53.46, H 7.57, S
8.92; found C 53.31, H 7.47, S 9.10.
2-(4-Bromophenyl)ethyl-1-S-triisopropylsilylsulfide (1b) from
4-bromostyrene
A mixture of 4-bromostyrene (5.87 g, 32.0 mmol), 2 (12 mL,
11 g, 56 mmol), and some crystals of AIBN was heated to 120–
130 ◦C (no solvent) for 7 days with daily additions of AIBN. After
this time, no further conversion could be observed by GLC. The
crude mixture containing the product was separated by distillation
using a kugelrohr apparatus. First, volatile materials were distilled
off at 75 ◦C/10 mbar. After that, distillation was continued
in oil pump vacuum. Collection of the fraction boiling at an
Experimental section
General
All chemicals were purchased from commercial sources and used
without further purification except for TIPSCl and thioacetic
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oven temperature of 175 ◦C yielded the product as colourless
liquid. 5.97 g (16.0 mmol), 50%. The product still contained some
impurities but was of sufficient purity for further transformations.
3-(4-Bromophenyl)propane-1-S-triisopropylsilyl sulfide (1c),
deprotonation with NaH
Sodium hydride (2.33 g, 97.1 mmol) was suspended in 50 mL
of dry THF. To this, a solution of 6 (17.75 g, 76.79 mmol) in
50 mL of dry THF was added slowly. After the formation of
gas and heat ceased, TIPSCl (21.75 g, 112.8 mmol) was added
to the mixture. Stirring became impossible due to formation of
precipitates. Because of that, further 50 mL of THF were added.
The mixture was refluxed for 3 h and stirred at room temperature
for 14 h. Acetic acid (50 mL), water (200 mL), and light petroleum
(100 mL) were added. The organic phase was separated off and
washed with water. The organic solvents were evaporated and the
remaining liquid was distilled at 10-5 mbar. Some lower boiling
impurities had to be separated. Distillation of the fraction boiling
at a temperature of 140–145 ◦C yielded the product as colourless
liquid. 24.69 g (63.72 mmol), 83%. 1H NMR (CDCl3, 300 MHz):
d = 7.41–7.36 (m, 2H, CH-ar), 7.09–7.04 (m, 2H, CH-ar), 2.70 (t,
2H, 3J = 7.6 Hz, CH2), 2.52 (t, 2H, 3J = 7.1 Hz, CH2), 1.95–1.84
(m, 2H, CH2), 1.28–1.15 (m, 3H, CH), 1.12–1.06 (m, 18H, CH3)
ppm. 13C NMR (CDCl3, 63 MHz): d = 140.6, 131.3, 130.2, 119.6,
2-(4-Bromophenyl)ethyl-1-S-triisopropylsilylsulfide (1b) from
1-bromo-4-(2-chloroethyl)benzene (4)
KOtBu (8.45 g, 75.3 mmol) was dissolved in 50 mL of dry DMF. 2
(15.34 g, 80.69 mmol) was added under external cooling with an ice
bath. 1-Bromo-4-(2-chloroethyl)benzene 4 (14.85 g, 67.65 mmol)
was added directly to the cooled mixture. After being stirred for
48 h at room temperature, the solution was acidified by addition of
glacial acetic acid. The DMF and excess acetic acid were removed
in vacuum. The remainder was dissolved in a mixture of water and
light petroleum. The organic phase was separated and washed with
water once. After evaporation of the light petroleum, the remaining
liquid was distilled in oil pump vacuum. Collection of the fraction
boiling at 146 ◦C yielded the product as colourless liquid. 17.07 g
(45.71 mmol), 67%. 1H NMR (CDCl3, 300 MHz): d = 7.43–7.37
(m, 2H, CH-ar), 7.10–7.04 (m, 2H, CH-ar), 2.89–2.69 (m, 4H,
CH2), 1.32–1.17 (m, 3H, CH), 1.15–1.06 (m, 18H, CH3) ppm. 13
C
˜
34.3, 34.0, 25.1, 18.6, 12.7 ppm. IR (ATR): nmax = 2942 (CH-alk),
NMR (CDCl3, 75 MHz): d = 137.8, 129.6, 128.4, 118.3, 37.1, 25.3,
2864 (CH-alk), 2858 (CH-alk), 1488, 1462, 1072, 1012, 882 cm-1.
MS (APCI-): m/z (%) = 229 (100) [BrPh(CH2)3S-]. C18H31BrSSi
(387.49): calcd. C 55.79, H 8.06, S, 8.27; found C 55.85, H 7.93, S
8.50.
˜
16.7, 10.8 ppm. IR (ATR): nmax = 2943 (CH-alk), 2889 (CH-alk),
2864 (CH-alk), 1488, 1462, 1072, 1012, 881 cm-1. MS (APCI-):
m/z (%) = 215 (100) [BrPh(CH2)2S-]. C17H29BrSSi (373.47): calcd.
C 54.67, H 7.83, S 8.59; found C 54.50, H 7.64, S 8.56.
2-(4-Bromophenyl)ethanol (3)
3-(4-Bromophenyl)propyl-1-S-triisopropylsilyl sulfide (1c),
Caution: The reaction is highly exothermic and might start
surprisingly after an induction period. During the addition of
the oxirane, the reaction mixture often solidifies. It is essential
to use a powerful mechanical stirrer and to have a cooling bath
of liquid nitrogen at hand. The mono-Grignard reagent of 1,4-
dibromobenzene was obtained by careful addition of a solution
of the dibromobenzene (140.33 g, 596.7 mmol) in 300 mL of
dry Et2O to magnesium turnings (17.78 g, 731.4 mmol) activated
with 1,2-dibromoethane. The reaction was completed by heating
under reflux for 1h. Then the solution of the Grignard reagent
was decanted from remaining magnesium. Upon addition of
CuBr (7.15 g, 49.8 mmol) the solution turned black. Oxirane
(~100 mL, ~90 g, ~2 mol) was dissolved in 150 mL of dry
THF. The reaction flask was cooled with ethanol/N2 and the
first 100 mL of the oxirane solution were added in such rate
that the internal temperature was maintained between –10 and
+ 5 ◦C. After that, the reaction mixture solidified and stirring
became impossible even if mechanical stirring was used. To regain
a stirrable reaction mixture, 100 mL of anhydrous THF were added
deprotonation with NEt3
3-(4-Bromophenyl)propane thiol (6) (55.66 g, 240.8 mmol) and
triisopropylsilyl chloride (54.62 g, 283.3 mmol) and triethylamine
(34.61 g, 342.0 mmol) were dissolved in 150 mL of THF. The
reaction mixture turned turbid, but no significant evolution of
heat was observed. After heating the mixture under reflux for 24 h,
only low conversion was observed. After refluxing for further 96 h
the solvent mixture was removed under reduced pressure. The
remainder was distributed between water and light petroleum.
The organic phase was separated and washed with water once.
After evaporation of the petrol ether, the remaining liquid was
fractionated in oil pump vacuum by means of a 40 cm vigreux
column. Collection of the fraction boiling at 140–160 ◦C yielded
the product as a slightly yellowish liquid. 43.22 g (111.5 mmol)
46% of 1c, with 24.83 g (107.4 mmol) of 6 being recovered.
3-(4-Bromophenyl)propyl-1-S-triisopropylsilyl sulfide (1c),
deprotonation with n-BuLi
◦
to the reaction mixture. At a temperature between 5 and 10 C,
To a solution of 6 (25.08 g, 108.5 mmol) in 170 mL of dry THF,
n-BuLi (104 mmol, 65 mL of a 1.6 mol L-1 solution in hexanes)
was added at an internal temperature between –60 and -50 ◦C. The
reaction mixture was allowed to warm to 0 ◦C. After that, TIPSCl
(22.93 g, 11.89 mmol) was added and the mixture was stirred for
24 h at room temperature. Addition of water and evaporation of
the THF followed. The aqueous suspension was extracted twice
with light petroleum. The organic extracts were evaporated under
reduced pressure. Final distillation via a 30 cm Vigreux column
yielded pure 1c as a colourless oil. 22.04 g (56.88 mmol), 52%.
the next 100 mL of the solution of ethylene oxide in THF were
slowly added. After that the cooling bath was removed and the
remaining 50 mL of the oxirane solution were added. Stirring
was continued for 30 min until an analytical sample taken from
the mixture and hydrolyzed did not show significant amounts of
bromobenzene by GC-MS analysis. Acetic acid (100 mL) was
added followed by addition of 200 mL of water. The organic
phase was separated off and ether (200 mL) was added to the
aqueous phase. The mixture was filtered through a pad of CeliteTM
and the ethereal phase was separated off. The combined organic
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phases were evaporated and the remaining liquid was distilled in
vacuum at 2 mbar using a 1.5 m spinning band column. First, some
more volatile impurities (e.g. b◦romobenzene) distilled. Collection
of the fraction boiling at 105 C/2 mbar yielded the product as
3-(4-Bromophenyl)propyl thiol (6)
To a solution of potassium hydroxide (32.06 g, 571.4 mmol)
in 300 mL of oxygen-free methanol, S-3-(4-bromophenyl)propyl
thioacetate (5) (64.49 g, 236.1 mmol) was added under nitrogen.
After the slightly exothermic reaction ceased, the reaction mixture
was heated under reflux for 1h, and stirred for 14 h at room
temperature. The mixture was acidified with 20% H2SO4 which
resulted in precipitation of copious amounts of salt. The mixture
was extracted twice with CH2Cl2 and once with light petroleum.
The organic extracts were washed with water once and evaporated.
Distillation of the remainder and collection of the fraction boiling
at 110 ◦C (3 mbar) yielded the product as a yellowish liquid.
1
colourless liquid. 77.07 g (383.3 mmol), 64%. H NMR (CDCl3,
250 MHz): d = 7.45–7.39 (m, 2H, CH-ar), 7.12–7.06 (m, 2H, CH-
ar), 3.80 (t, 2H,3J = 6.6 Hz, CH2), 2.79 (t, 2H,3J = 6.5 Hz, CH2),
1.83 (s, 1H, OH) ppm. 13C NMR (CDCl3, 63 MHz): d = 137.5,
131.5, 130.7, 120.2, 63.2, 38.4 ppm. MS (EI): m/z (%) = 202 (39)
+
˜
[M ], 171 (100), 121 (15), 91 (94). IR (ATR): nmax = 3320 (OH),
2940 (CH-alk), 2874 (CH-alk), 1487, 1403, 1072, 1043, 1009, 833,
804 cm-1. C8H9BrO (201.06): calcd. C 47.79, H 4.51; found C 47.52,
H 4.39.
1
52.45 g (226.9 mmol), 96%. H NMR (CDCl3, 300 MHz): d =
7.42–7.36 (m, 2H, CH-ar), 7.07–7.01 (m, 2H, CH-ar), 2.70 (t, 2H,
3J = 7.5 Hz, CH2), 2.63 (dt, 2H, J = 7.4 Hz, CH2), 1.89 (q,
3
1-Bromo-4-(2-chloroethyl)benzene (4)
3
3
2H, J = 7.3 Hz, CH2), 1.35 (t, 1H, J = 7.8 Hz, SH) ppm. 13C
No Schlenk technique was used for this preparation. 3 (76.45 g,
380.2 mmol) was dissolved in 50 mL of CH2Cl2. To this solution,
thionyl chloride (60 mL, 98 g, 0.82 mol) was added. The mixture
was stirred at room temperature for 30 min and heated under reflux
for 1 h. The solvent and excess thionyl chloride was removed. The
intermediate chlorosulfinyl ester was transformed to the chloride
4 by pyrolysis. When heating to 140 ◦C evolution of gas was
observed. The mixture was further heated to 170 ◦C for 2 h. After
cooling to room temperature, the product was distilled through a
1.5 m spinning band column. Collection of the fraction boiling
at 85 ◦C at 2 mbar yielded the product as colourless liquid.
NMR (CDCl3, 75 MHz): d = 140.1, 131.4, 130.1, 119.6, 35.1, 33.6,
˜
23.8 ppm. IR (ATR): nmax = 3022 (CH-ar), 2931 (CH-alk), 2956
(CH-alk), 1486, 1072, 1010, 806 cm-1. MS (ESI-): m/z (%) = 263
(100), [M + CH3O-]. C9H11BrS (231.15): calcd. C 46.76, H 4.80, S
13.87; found C 46.49, H 4.63, S 14.04.
4-(4¢-((Triisopropylsilylthio)methyl)phenyl)pyridine (8a)
4-Bromopyridine hydrochloride (3.43 g, 17.6 mmol) was sus-
pended in 10 mL of THF. Ethylmagnesium chloride (18 mmol,
9.0 ml of a 2 mol L-1 solution in THF) was added, resulting in
warming of the solution and the appearance of a slight red colour.
Pd(dppf)Cl2 (0.27 g, 0.37 mmol) was added, followed by addition
of the Grignard reagent prepared from 1a. After stirring at room
temperature for 2 h, water was added to the deep violet solution.
Most of the THF was removed in vacuo and the remainder was
extracted with diethyl ether. The organic solvents were evaporated
and the residual oil was coevaporated with CH2Cl2 to remove
water. The product was column filtrated through a pad of silica
using ethyl acetate (some fast moving, non-polar impurities had to
be removed) to obtain a yellowish oil. 3.27 g (9.14 mmol), 52%. 1H
NMR (CDCl3, 300 MHz): d = 8.70–8.61 (m, 2H, CH-ar), 7.61–
7.55 (m, 2H, CH-ar), 7.54–7.44 (m, 4H, CH-ar), 3.80 (s, 2H, CH2),
1
56.54 g (257.6 mmol), 68%. H NMR (CDCl3, 400 MHz): d =
7.47–7.43 (m, 2H, CH-ar), 7.12–7.09 (m, 2H, CH-ar), 3.70 (t,
3
3
2H, J = 7.2 Hz, CH2), 3.02 (t, 2H, J = 7.2 Hz, CH2) ppm.
13C NMR (CDCl3, 100 MHz): d = 137.0, 131.6, 130.5, 120.8,
44.6, 38.4 ppm. MS (ESI-): m/z (%) = 219 (100), [M – H+]. IR
(ATR): nmax = 2957 (CH-alk), 1487, 1071, 1010, 901, 800, 739 cm-1.
˜
C8H8BrCl (219.51): calcd. C 43.77, H 3.67; found C 43.79,
H 3.64.
S-3-(4-Bromophenyl)propyl thioacetate (5)
No Schlenk technique was used for this preparation. Thioacetic
acid (44.84 g, 589.1 mmol) was added to 4-bromo(prop-2-
enyl)benzene (56.84 g, 296.0 mmol) under stirring. When about
half of the quantity of thioacetic acid was added, a vigorous
reaction started spontaneously, which generated copious amounts
of heat. After all of the thioacetic acid was added, a small amount
of AIBN was added to the still hot reaction mixture. Stirring
at room temperature was continued for further 14 h. The excess
thioacetic acid was evaporated at 2 mbar at room temperature. The
remaining product was distilled through a◦40 cm vigreux column.
Collection of the fraction boiling at 116 C at 0.4 mbar yielded
the product as a slightly yellowish liquid. 64.49 g (236.1 mmol),
˜
1.41–1.20 (m, 3H, CH), 1.19 (m, 18H, CH3) ppm. IR (ATR): nmax
= 2944 (CH-ar), 2863 (CH-alk), 1595, 1461, 1400, 1019, 986, 882,
812, 794 cm-1. MS (EI): m/z (%) = 357 (2) [M+], 336 (6), 314 (15),
169 (100).
Coupling of 1-(4-bromophenyl)alkane-x-S-triisopropylsilyl
sulfides with 4-bromophenylpyridine. Typical procedure
A solution of the Grignard reagent was prepared from 13.2 mmol
of the respective 1-(4-bromophenyl)alkyl-w-S-triisopropylsilyl
sulfide, 1.00 g (41.2 mmol) of magnesium and 25 mL of
THF. This solution was added to 2.50 g (10.5 mmol) of 4-(4-
bromophenyl)pyridine and 0.16 g (0.22 mmol) Pd(dppf)Cl2 in
50 mL of THF. After the reaction mixture was stirred for 16 h
at room temperature, water was added. The THF was removed
and the aqueous slurry was extracted with CH2Cl2. The CH2Cl2
was evaporated. The remaining material was dissolved in ethyl
acetate and filtered through a plug of alumina. After evaporation
of the ethyl acetate, the remaining solid was crystallised from
methylcyclohexane.
1
80%. H NMR (CDCl3, 400 MHz): d = 7.45–7.35 (m, 2H, CH-
3
ar), 7.10–7.00 (m, 2H, CH-ar), 2.86 (t, 2H, J = 7.3 Hz, CH2),
3
2.63 (t, 2H, J = 7.6 Hz, CH2), 2.33 (s, 3H, CH3), 1.92–1.82
(m, 2H, CH2) ppm. 13C NMR (CDCl3, 100 MHz): d = 195.7,
140.1, 131.6, 130.2, 119.8, 34.2, 31.0, 30.7, 28.4 ppm. IR (ATR):
˜
nmax = 3023 (CH-ar), 2935 (CH-ar), 2858 (CH-alk), 1687 (C=O),
1487, 1133, 1011 cm-1. HRMS (C11H14BrOS, [HM]+): calcd. for
272.9943; found 272.9938. C11H13BrOS (273.19): calcd. C 48.36,
H 4.80, S 11.74; found C 48.25, H 4.72, S 12.00.
3558 | Org. Biomol. Chem., 2010, 8, 3552–3562
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4-(4¢-((Triisopropylsilylthio)methyl)biphenyl-4-yl)pyridine (8b)
light petroleum, and dried. The products could be further purified
by vacuum gradient sublimation at a pressure of about 10-3 Pa.
The amino-terminated thiols 14a,b were sublimed in the gradi-
ent apparatus without previous crystallisation.
◦
1
Colourless solid, yield 2.71 g, 47%, m.p. 118–121 C. H NMR
(C6D6, 400 MHz): d = 8.67–8.63 (m, 2H, CH-ar), 7.50–7.34 (m,
8H, CH-ar), 7.12–7.09 (m, 2H, CH-ar), 3.74 (s, 2H, CH2), 1.27–
1.03 (m, 21H, CH(CH3)2) ppm. 13C NMR (C6D6, 100 MHz): d =
150.9, 147.4, 141.8, 140.9, 139.3, 137.3, 129.5, 127.9, 127.6, 127.5,
121.4, 30.2, 18.8, 13.1 ppm. MS (APCI+): m/z (%) = 434 (100)
[HM+]. C27H35NSSi (433.72): calcd. C 74.77, H 8.13, N 3.23, S
7.39; found C 74.74, H 7.99, N 3.09, S 7.43.
(4-(Pyridin-4-yl)phenyl)methanethiol (9a)
◦
1
Colourless solid, yield 3.27 g (62%, m.p. 64–66 C. H NMR
(C6D6, 400 MHz): d = 8.63–8.58 (m, 2H, CH-ar), 7.24–7.18 (m,
2H, CH-ar), 7.07–7.04 (m, 2H, CH-ar), 7.04–7.01 (m, 2H, CH-
ar), 3.29 (d, 2H, 3J = 7.6 Hz, CH2), 1.51 (t, 1H, 3J = 7.6 Hz, SH)
ppm. 13C NMR (C6D6, 100 MHz): d = 150.9, 147.4, 142.3, 137.1,
4-(4¢-(2-(Triisopropylsilylthio)ethyl)biphenyl-4-yl)pyridine (8c)
˜
128.9, 127.3, 121.4, 28.5 ppm. IR (ATR): nmax = 3026 (CH-ar),
◦
1
2907 (CH-alk), 2568 (SH), 1594, 1485, 721 cm-1. MS (EI): m/z
(%) = 201 (27) [M+], 168 (100). C12H11NS (201.29): calcd. C 71.60,
H 5.51, N 6.96, S 15.93; found C 71.85, H 5.73, N 6.99, S 15.96.
Colourless solid, yield 2.03 g (34%), m.p. 108–109 C. H NMR
(C6D6, 400 MHz): d = 8.67–8.62 (m, 2H, CH-ar), 7.50–7.43 (m,
4H, CH-ar), 7.37–7.33 (m, 2H, CH-ar), 7.19–7.14 ((m, 2H, CH-
ar), 7.11–7.07 (m, 2H, CH-ar), 2.97–2.90 (m, 2H, CH2), 2.83–
2.75 (m, 2H, CH2), 1.22–1.03 (m, 21H, CH(CH3)2) ppm. 13C
NMR (C6D6, 100 MHz): d = 150.9, 147.4, 142.0, 140.8, 138.9,
137.3, 129.5, 127.8, 127.6, 127.5, 121.4, 39.7, 27.9, 18.8, 13.0 ppm.
HRMS (C28H38NSSi, [HM]+): calcd. 448.2489; found 438.2473.
C28H37NSSi (447.75): calcd. C 75.11, H 8.33, N 3.13, S 7.16; found
C 75.06, H 8.29, N 2.99, S 6.96.
(4¢-(Pyridin-4-yl)biphenyl-4-yl)methanethiol (9b)
◦
Colourless solid, yield 0.55 g (82%) m.p. 254–256 C (decomp.).
1H NMR (C6D6, 400 MHz): d = 8.67–8.62 (m, 2H, CH-ar), 7.48–
7.44 (m, 2H, CH-ar), 7.43–7.39 (m, 2H, CH-ar), 7.38–7.33 (m, 2H,
CH-ar), 7.15–7.11 (m, 2H, CH-ar), 7.11–7.08 (m, 2H, CH-ar), 3.34
3
3
(d, 2H, J = 7.5 Hz, CH2), 1.50 (t, 1H, J = 7.5 Hz, SH) ppm.
13C NMR (C6D6, 75 MHz): d = 150.9, 147.4, 141.7, 141.2, 139.4,
4-(4¢-(3-(Triisopropylsilylthio)propyl)biphenyl-4-yl)pyridine (8d)
˜
137.5, 130.0, 128.9, 127.9, 127.5, 121.4, 28.6 ppm. IR (ATR): nmax
◦
1
= 3035 (CH-ar), 2924 (CH-alk), 1591, 1562, 1399, cm-1. HRMS
(C18H16NS, [HM]+): calcd. 278.0998; found 278.0992. C18H15NS,
(277.38): calcd. C 77.94, H 5.45, N 5.05, S 11.56; found C 77.80,
H 5.60, N 4.89, S 11.83.
Brownish solid, yield 3.63 g (74%) m.p. 119–120 C. H NMR
(C6D6, 300 MHz): d = 8.67–8.60 (m, 2H, CH-ar), 7.53–7.49 (m,
2H, CH-ar), 7.49–7.45 (m, 2H, CH-ar), 7.39–7.33 (m, 2H, CH-
ar), 7.20–7.15 (m, 2H, CH-ar), 7.12–7.07 (m, 2H, CH-ar), 2.73
(mt, 2H, 3J = 7.5 Hz, CH2), 2.51 (t, 2H, 3J = 7.0 Hz, CH2), 1.95
(m, 2H, CH2), 1.22–1.04 (m, 21H, CH(CH3)2) ppm. 13C NMR
(C6D6, 75 MHz): d = 150.9, 147.5, 142.1, 141.6, 138.4, 137.2,
129.5, 127.8, 127.6, 127.4, 121.4, 34.9, 34.5, 25.5, 18.8, 13.1, ppm.
HRMS (C29H40NSSi, [HM]+): calcd. 462.2645; found 462.2628.
C29H39NSSi (461.78): calcd. C 75.43, H 8.51, N 3.03, S 6.94; found
C 75.61, H 8.66, N 2.80, S 6.44.
2-(4¢-(Pyridin-4-yl)biphenyl-4-yl)ethane-1-thiol (9c)
◦
1
Colourless solid, yield 0.90 g (72%) m.p. 162–164 C. H NMR
(C6D6, 300 MHz): d = 8.66–8.62 (m, 2H, CH-ar), 7.52–7.47 (m,
2H, CH-ar), 7.47–7.42 (m, 2H, CH-ar), 7.38–7.34 (m, 2H, CH-ar),
7.11–7.07 (m, 2H, CH-ar), 7.02–6.98 (m, 2H, CH-ar), 2.62 (t, 2H,
3
3J = 7.3 Hz, CH2), 2.58–2.49 (m, 2H, CH2), 1.17 (t, 1H, J =
7.9 Hz, SH) ppm. 13C NMR (C6D6, 75 MHz): d = 150.9, 147.4,
Deprotection of the oligoarylalkanethiols. Typical procedure
141.9, 139.9, 138.9, 137.4, 129.5, 127.8, 127.6, 127.4, 121.4, 40.1,
˜
The silyl sulfide (8a–d or 12/13a,b, 2.40 mmol) was dissolved in
a boiling mixture of 50 mL of oxygen-free methanol and 10 mL
of oxygen-free 20% HCl. The mixture was heated under reflux
for 20 h and allowed to cool to room temperature. In the case of
the pyridine thiols, 50 mL of a mixture of water and methanol
1 : 1vol was added resulting in clear solutions of the pyridinium
salts. This mixture was washed three times with light petroleum to
remove non-polar impurities before the methanol was removed.
This procedure could not be applied for the amino thiols because
of the lower solubility of their ammonium salts compared to the
pyridinium salts.
The following steps had to be performed under strict exclusion
of oxygen: The pH of the remaining aqueous phase was adjusted to
about 8 by addition of trisodium citrate. The slurry was extracted
with CH2Cl2 immediately. The extract was evaporated to dryness.
In case of the pyridine-terminated thiols 9a–d, the remaining
solid was dissolved in 250 mL of boiling, oxygen-free methyl-
cyclohexane, filtered hot through a Schlenk frit, and left for
crystallisation. The crystals were filtered by suction, washed with
26.0 ppm. IR (ATR): nmax = 3031 (CH-ar), 2929 (CH-alk), 2900
(CH-alk), 2837 (CH-alk), 1590, 1398 cm-1. MS (ESI+): m/z (%) =
292 (100), [HM]+. C19H17NS (291.41): calcd. C 78.31, H 5.88, N
4.81, S 11.00; found C 78.08, H 6.15, N 4.78, S 10.79.
3-(4¢-(Pyridin-4-yl)biphenyl-4-yl)propane-1-thiol (9d)
◦
1
Colourless solid, yield 0.86 g (55%), m.p. 159–161 C. H NMR
(C6D6, 300 MHz): d = 8.66–8.62 (m, 2H, CH-ar), 7.54–7.50 (m,
2H, CH-ar), 7.50–7.45 (m, 2H, CH-ar), 7.39–7.34 (m, 2H, CH-
ar), 7.11–7.08 (m, 2H, CH-ar), 7.08–7.04 (m, 2H, CH-ar), 2.47 (t,
3
3
3
2H, J = 7.5 Hz, CH2), 2.15 (dt, 2H, J = 7.0 Hz, J = 7.9 Hz,
3
CH2), 2.59–2.48 (m, 2H, CH2), 0.97 (t, 1H, J = 7.9 Hz, SH)
ppm. 13C NMR (C6D6, 75 MHz): d = 150.9, 147.4, 142.0, 141.3,
138.5, 137.3, 129.3, 127.8, 127.7, 127.4, 121.4, 35.7, 34.1, 26.0 ppm.
˜
IR (ATR): nmax = 3029 (CH-ar), 2934 (CH-alk), 2848 (CH-alk),
1587, 1484, 1399, 1002 cm-1. HRMS (C20H20NS, [HM]+): calcd.
306.1311; found 306.1303. C20H19NS (305.44): calcd. C 78.65, H
6.27, N 4.59, S 10.50; found C 78.94, H 6.47, N 4.50, S 10.76.
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4¢-Bromo-4-(di-N-Boc)-aminobiphenyl (11) and
N,N¢-di-Boc-N,N¢-bis(4¢-bromobiphenyl-4-yl)urea (11b)
dryness. Polar impurities were removed by filtration through a
plug of silica using CH2Cl2 as eluent. Final purification and
separation of the products was done by column chromatography
over silica with light petroleum/CH2Cl2 gradient from 1 : 1 as
eluent. Evaporation of the solvent yielded the products as solids.
Analytical samples could be obtained by crystallisation from
methylcyclohexane.
Sodium hydride (1.86 g, 77.5 mmol) was suspended in THF
(20 mL) and di-tert-butyldicarbonate (6.75 g, 30.9 mmol) was
added. To this, a solution of 4¢-bromo-4-aminobiphenyl (2.92 g,
11.8 mmol) in THF (40 mL) was slowly added, but no formation
of gas or heat was observed. Stirring was continued for 100 h
at room temperature. After that, TLC indicated that the free
amine was consumed. A catalytic amount of DMAP was added
and stirring was continued for further 70 h. The sodium hydride
was filtered from the solution and the solvent was removed. The
remaining oil was filtered over silica using dichloromethane as
eluent. The solvent of the eluate was removed and the remaining
oil was chromatographed over silica using CH2Cl2. First, the di-
Boc-urea was eluted. Elution of the di-Boc-derivative followed.
Evaporation of the solvent afforded the still impure products
which were crystallised from methylcyclohexane to yield the pure
products as colourless crystals.
4-(2-(Triisopropylsilylthio)ethyl)-4¢¢-aminoterphenyl (12a)
Yellowish solid, yield 1.46 g (27%), slightly impure, m.p. 125–
◦
129 C.1H-NMR (C6D6, 250 MHz): d = 7.55 (s, 4H, CH-ar),
7.55–7.50 (m, 2H, CH-ar), 7.45–7.39 (m, 2H, CH-ar), 7.20–7.10
(m, 2H, CH-ar), 6.43–6.35 (m, 2H, CH-ar), 2.89–2.99 (m, 2H,
CH2), 2.84 (brd. s, 2H, NH2),2.85–2.73 (m, 2H, CH2), 1.22–1.05
(m, 21H, -CH(CH3)2) ppm. 13C-NMR (C6D6, 63 MHz): 146.7,
140.7, 140.0, 139.6, 139.2, 129.3, 128.2, 127.4, 127.1, 115.4, 39.9,
28.0, 18.8, 13.0 ppm. IR (ATR): nmax = 3451, 3358, 2942 (CH-
alk), 2889 (CH-alk), 2864 (CH-alk), 1618, 1492, 1462, 1277, 881,
804 cm-1. MS (ESI+): m/z (%) = 462 (100) [HM]+. C29H39NSSi
(461.78): calcd. C 75.43, H 8.51, N 3.03, S 6.94; found C 74.01, H
7.86, N 3.04, S 6.64.
4¢-Bromo-4-(N,N-di-Boc-imido)biphenyl (11)
◦
1
Colourless solid, yield 2.69 g (51%), m.p. 150–152 C. H-NMR
(CDCl3, 250 MHz): d = 7.60–7.50 (m, 4H, CH-ar), 7.49–7.41
(m, 2H, CH-ar), 7.26–7.18 (m, 2H, CH-ar), 1.45 (s, 18H, CH3)
ppm. 13C-NMR (CDCl3, 63 MHz): 152.0, 139.3, 139.0, 138.9,
4-(3-(Triisopropylsilylthio)propyl)-4¢¢-aminoterphenyl (12b)
Slightly yellowish solid, yield 0.65 g (28%), m.p. 138 ◦C. 1H-NMR
(CDCl3, 300 MHz): d = 7.62–7.55 (m, 4H, CH-ar), 7.55–7.50 (m,
2H, CH-ar), 7.45–7.39 (m, 2H, CH-ar), 7.27–7.21 (m, 2H, CH-ar),
6.72–6.66 (m, 2H, CH-ar), 3.63 (brd. s, 2H, NH2), 2.81–2.72 (m,
2H, CH2), 2.56 (t, 2H, 3J = 7.9 Hz, CH2), 2.02–1.89 (m, 2H, CH2),
1.36–1.15 (m, 3H, CH), 1.15–1.00 (m, 18H, CH3) ppm. 13C-NMR
(CDCl3, 75 MHz): 145.8, 140.5, 139.7, 138.7, 138.3, 130.8, 128.8,
127.7, 127.1, 126.7, 126.5, 115.3, 34.4, 34.2, 25.2, 18.5, 12.6 ppm.
IR (ATR): nmax = 3479, 3370, 2940 (CH-alk), 2889 (CH-alk), 2863
(CH-alk), 1618, 1491, 1460, 1283, 881, 812, 678 cm-1. MS (ESI+):
m/z (%) = 477 (100) [HM]+. C30H41NSSi (475.27): calcd. C 75.73,
H 8.69, N 2.94, S 6.74; found C 75.85, H 8.76, N 2.69, S 6.96.
131.9, 128.7, 128.3, 127.2, 121.7, 82.9, 27.9 ppm. IR (ATR): nmax
=
3093 (CH-ar), 3041 (CH-ar), 2971 (CH-alk), 2929 (CH-alk), 1781
(C=O), 1703, 1486, 1367, 1282, 1246, 1147, 1096, 1004, 856, 817,
777 cm-1. HRMS (C22H26NO4, [HM]+): calcd. 448.1118; found
448.1113. C22H26NO4 (448.35): calcd. C 58.94, H 5.85, N 3.12;
found C 59.16, H 6.08, N 3.01.
N,N¢-Di-Boc-N,N¢-bis(4¢-bromobiphenyl-4-yl)urea (11b)
Colourless solid, yield 1.45 g (2.00 mmol) (34%), m.p. 175 ◦C
(dec). 1H-NMR (CDCl3, 250 MHz): d = 7.61–7.51 (m, 4H, CH-
ar), 7.50–7.42 (m, 2H, CH-ar), 7.40–7.33 (m, 2H, CH-ar), 1.48 (s,
9H, CH3) ppm. 13C-NMR (CDCl3, 63 MHz): 154.6, 152.0, 139.4,
139.3, 138.4, 131.9, 128.7, 128.3, 127.3, 121.8, 83.6, 28.0 ppm. IR
(ATR): nmax = 3085 (CH-ar), 3041 (CH-ar), 2980 (CH-alk), 2943
(CH-alk), 1736 (C=O), 1708 (C=O), 1480, 1320, 1296, 1248, 1138,
1075, 1003, 837, 810, 797 cm-1. MS (APCI+): m/z (%) = 524 (100)
[M – 2Boc]+, 567 (37). C35H34Br2N2O5 (722.46): calcd. C 58.19, H
4.74, N 3.88; found C 58.28, H 4.62, N 3.74.
4-(2-(Triisopropylsilylthio)ethyl-4¢¢-N-Boc-aminoterphenyl (13a)
Slightly yellowish solid, yield 2.67 g (41%), m.p. 134–135 ◦C. 1H-
NMR (CDCl3, 400 MHz): d = 7.64 (s, 4H, CH-ar), 7.60–7.57 (m,
2H, CH-ar), 7.57–7.55 (m, 2H, CH-ar), 7.48–7.43 (m, 2H, CH-
ar), 7.31–7.27 (m, 2H, CH-ar), 6.54 (brd. s, 1H, NH), 2.99–2.92
(m, 2H, CH2), 2.85–2.78 (m, 2H, CH2), 1.55 (s, 9H, CH3) 1.35–
1.21 (m, 3H, -CH), 1.15–1.10 (m, 18H, -CH3) ppm. 13C-NMR
(CDCl3, 100 MHz): 152.7, 140.0, 139.6, 139.4, 138.9, 137.8, 135.4,
129.0, 127.5, 127.4, 127.1, 127.1, 118.9, 80.7, 39.5, 28.4, 27.5, 18.6,
12.8 ppm. IR (ATR): nmax = 3423 (NH), 2942 (CH–Al), 2865
(CH–Al), 1727 (C=O), 1504, 1156, 808 cm-1. MS (APCI+): m/z
(%) = 463 (100) [H(H2N-(C6H4)3-(CH2)2-STIPS]+, 503 (85), 567
(65). C34H47NO2SSi (561.89): calcd. C 72.68, H 8.43, N 2.49, S
5.69; found C 72.52, H 8.26, N 2.45, S 5.65.
4-(x-(Triisopropylsilylthio)alkyl)-4¢¢-N-Boc-aminoterphenyls
(12a,b) and 4-(x-(triisopropylsilylthio)alkyl)-4¢¢-aminoterphenyls
(13a,b), typical procedure
A solution of the Grignard compound prepared from 1b,c
(6 mmol) and magnesium (0.30 g, 13 mmol) in THF (15 mL)
was added to ZnCl2·2thf (3.62 g, 12.9 mmol). Further 40 mL of
THF were added, followed by the addition of Pd(dppf)Cl2 (0.10 g
0.14 mmol) and 11 (2.21 g, 4.93 mmol). After the mixture was
stirred at room temperature for 16 h, the THF was removed and
water was added. During addition the formation of a gas was
observed. The suspension was acidified by addition of glacial
acetic acid and extracted with CH2Cl2. The organic phase was
washed with water and saturated NaHCO3 and evaporated to
4-(3-(Triisopropylsilylthio)propyl-4¢¢-N-Boc-aminoterphenyl (13b)
Slightly yellowish solid, yield 1.09 g (38%), m.p. 154–155 ◦C. 1H-
NMR (CDCl3, 300 MHz): d = 7.64–7.56 (m, 4H, CH-ar), 7.53–
7.46 (m, 2H, CH-ar), 7.46–7.37 (m, 2H, CH-ar), 7.46–7.39 (m,
2H, CH-ar), 7.28–7.22 (m, 2H, CH-ar), 6.65 (brd. s, 1H, NH),
3560 | Org. Biomol. Chem., 2010, 8, 3552–3562
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3
2.82–2.78 (m, 2H, CH2), 2.56 (t, 2H, J = 7.1 Hz, CH2), 2.20–
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Zacher, R. Fischer and C. Wo¨ll, Langmuir, 2007, 23, 7440–7442; (c) J.
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5 (a) R. G. Nuzzo and D. L. Allara, J. Am. Chem. Soc., 1983, 105, 4481–
4483; (b) E. B. Troughton, C. D. Bain, G. M. Whitesides, R. G. Nuzzo,
D. L. Allara and M. D. Porter, Langmuir, 1988, 4, 365–385; (c) C. D.
Bain, E. B. Troughton, Y. T. Tao, J. Evall, G. M. Whitesides and R. G.
Nuzzo, J. Am. Chem. Soc., 1989, 111, 321–335; (d) C. D. Bain, J. Evall
and G. M. Whitesides, J. Am. Chem. Soc., 1989, 111, 7155–7164.
6 (a) C. D. Chidsey and D. N. Loiacono, Langmuir, 1990, 6, 682–691;
(b) H.-J. Himmel, K. Weiss, B. Ja¨ger, O. Dannenberger, M. Grunze
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3949–3954;(b) J. Liu, B. Schu¨pbach, A. Bashir, O. Shekhah, A. Nefedov,
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4459–4472.
10 (a) M. Zharnikov, S. Frey, H. Rong, Y.-J. Yang, K. Heister, M.
Buck and M. Grunze, Phys. Chem. Chem. Phys., 2000, 2, 3359–3362;
(b) A. Shaporenko, M. Brunnbauer, A. Terfort, M. Grunze and M.
Zharnikov, J. Phys. Chem. B, 2004, 108, 14462–14469; (c) H.-T. Rong,
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11 J. Liu, L. Stratmann, S. Krakert, M. Kind, F. Olbrich, A. Terfort and
1.90 (m, 2H, CH2), 1.53 (s, 9H, -C(CH3)3) 1.33–1.15 (m, 3H, -
CH), 1.14–1.00 (m, 18H, -CH3) ppm. 13C-NMR (CDCl3, 75 MHz):
152.7, 140.7, 139.5, 139.1, 138.2, 137.7, 135.3, 128.9, 127.3, 127.2,
126.9, 126.8, 118.8, 80.5, 34.4, 34.2, 28.3, 25.2, 18.5, 12.7 ppm.
IR (ATR): nmax = 3421 (NH), 3021 (CH-Ar), 2929 (CH–Al), 2865
(CH–Al), 1723 (C=O), 1505, 1156, 810 cm-1. MS (APCI+): m/z
(%) = 478 (100) [H(H2N-(C6H4)3-(CH2)3-STIPS+], 517 (90), 557
(25). C35H49NO2SSi (575.33): calcd. C 72.99, H 8.58, N 2.43, S
5.57; found C 73.09, H 8.77, N 2.27, S 5.71.
(4-Aminoterphenyl-4¢¢yl)ethane-2-thiol (14a)
◦
1
colourless solid, yield 0.39 g (65%), m.p. 227–230 C. H-NMR
(CDCl3, 250 MHz): d = 7.54 (s, 4H, CH-ar), 7.53–7.47 (m,
2H, CH-ar), 7.43–7.34 (m, 2H, CH-ar), 7.25–7.16 (m, 2H, CH-
ar), 6.75–6.65 (m, 2H, CH-ar), 3.67 (brd. s, 2H, NH2), 2.97–
3
2.83 (m, 2H, CH2), 2.83–2.69 (m, 2H, CH2), 1.36 (t, 1H, J =
7.6 Hz, SH) ppm. 13C-NMR (CDCl3, 63 MHz): 145.9, 140.0,
139.2, 138.8, 138.7, 131.0, 129.1, 127.9, 127.2, 127.0, 126.7, 115.4,
39.9, 26.0 ppm. IR (ATR): nmax = 3397, 3320, 3208, 3030 (CH-
ar), 2930 (CH-alk), 2849 (CH-alk), 1601, 1491, 1442, 1400, 1257,
1149, 804 cm-1. MS (ESI+): m/z (%) = 306 (100) [HM]+. C20H19NS
(305.44): calcd. C 78.65, H 6.27, N 4.59, S 10.50; found C 78.42,
H 6.50, N 4.40, S 10.42.
(4-Aminoterphenyl-4¢¢yl)propane-3-thiol (14b)
colourless solid, from 12b: yield 0.18 g (76%), from 13b: yield
0.17 g (100%), m.p. 240 ◦C (dec). 1H-NMR (CDCl3, 250 MHz): d
= 7.54 (s, 4H, CH-ar), 7.51–7.45 (m, 2H, CH-ar), 7.42–7.36 (m,
2H, CH-ar), 7.22–7.15 (m, 2H, CH-ar), 6.74–6.66 (m, 2H, CH-
ar), 3.67 (brd. s, 2H, NH2), 2.81–2.72 (t, 2H,3J = 7.5 Hz, CH2),
Ch. Woll, J. Electron Spectrosc. Relat. Phenom., 2009, 172, 120–127.
12 K. Nishiyama, M. Tsuchiyama, A. Kubo, H. Seriu, S. Miyazaki, S.
Yoshimoto and I. Taniguchi, Phys. Chem. Chem. Phys., 2008, 10, 6935–
6939.
¨
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J. Nucl. Sci. Technol., 2008, 45, 251–256; (b) I. Turyan and D. Mandler,
Anal. Chem., 1997, 69, 894–897.
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M. Buettner, A. Romanyuk and P. Oelhafen, Surf. Sci., 2005, 590, 146–
153; (b) V. Ivanova, T. Baunach and D. Kolb, Electrochim. Acta, 2005,
50, 4283–4288.
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Prinz, J. Electrochem. Soc., 2008, 155, B1008–B1012; (b) S. Monari,
G. Battistuzzi, M. Borsari, D. Millo, C. Gooijer, G. Zwan, A. Ranieri
and M. Sola, J. Appl. Electrochem., 2008, 38, 885–891.
16 S. Silien, M. Buck, G. Goretzki, D. Lahaye, M. R. Champness, T.
Weidner and M. Zharnikov, Langmuir, 2009, 25, 959–967.
17 J. F. Kang, A. Ulman, S. Liao, R. Jordan, G. Yang and G.-Y. Liu,
Langmuir, 2001, 17, 95–106.
2.51 (dt, 2H, 3J1= J2=7.3 Hz, CH2), 1.91 (tt, 2H,3J1= J2=7.3 Hz,
CH2), 1.31 (t, 1H,3J = 7.9 Hz, SH) ppm. 13C-NMR (CDCl3, 63
MHz): 145.9, 140.3, 139.9, 138.8, 138.6, 131.0, 128.9, 127.9, 127.2,
126.9, 126.6, 115.4, 35.4, 34.0, 24.0 ppm. IR (ATR): nmax = 3325
(NH), 3029 (CH-ar), 2919 (CH-al), 1605, 1491, 1399, 1256, 1144,
807 cm-1. MS (ESI+): m/z (%) = 320 (100) [HM]+. C21H21NS
(319.46): calcd. C 78.95, H 6.63, N 4.38, S 10.04; found C 78.95,
H 6.77, N 4.28, S 10.11.
3
3
Acknowledgements
18 J. F. Kang, J. Zaccaro, A. Ulman and A. Myerson, Langmuir, 2000, 16,
3791–3796.
The authors acknowledge the generous gift of several silanes by the
Wacker AG. Financial support by the DFG through the graduate
school 611 (“Functional materials”) is appreciated.
19 (a) K. P. Fears and R. A. Latour, Langmuir, 2009, 25, 13926–
13933; (b) S. Todorovic, C. Jung, P. Hildebrandt and D. H. Murgida,
JBIC, J. Biol. Inorg. Chem., 2006, 11, 119–127; (c) M. Kondo, Y.
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Gardiner, R. J. Cogdell and M. Nango, Biomacromolecules, 2007, 8,
2457–2463; (d) Y. Arima and H. Iwata, J. Mater. Chem., 2007, 17,
4079–4087; (e) X. Han, A. S. Achalkumar, R. J. Bushby and S. D.
Evans, Chem.–Eur. J., 2009, 15, 6363–6370.
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