Synthetic protocols and building blocks for molecular electronics

Article abstract of DOI:10.1016/j.tet.2005.09.106

Simple and readily accessible aryl bromides are useful building blocks for thiol end-capped molecular wires. Thus, 4-bromophenyl tert-butyl sulfide and 1-bromo-4-(methoxymethyl)benzene serve as precursors for a variety of oligo(phenylenevinylene) and olig

Full text of DOI:10.1016/j.tet.2005.09.106

Tetrahedron 61 (2005) 12288–12295
Synthetic protocols and building blocks for molecular electronics
Nicolai Stuhr-Hansen, Jakob Kryger Sørensen, Kasper Moth-Poulsen, Jørn Bolstad Christensen,
*
Thomas Bjørnholm and Mogens Brøndsted Nielsen
Department of Chemistry, Nano-Science Center and Molecular Engineering Group, University of Copenhagen, Universitetsparken 5,
DK-2100 Copenhagen, Denmark
Received 29 June 2005; revised 6 September 2005; accepted 22 September 2005
Available online 19 October 2005
Abstract—Simple and readily accessible aryl bromides are useful building blocks for thiol end-capped molecular wires. Thus,
4-bromophenyl tert-butyl sulfide and 1-bromo-4-(methoxymethyl)benzene serve as precursors for a variety of oligo(phenylenevinylene) and
oligo(phenyleneethynylene) wires via efficient synthetic transformations as presented in this paper.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
found to exhibit some of the best conducting properties.6 In
order to utilize a molecular wire in a device, it has to be
placed between electrodes. One method is direct evapor-
ation of the molecular wire into a nanogap.2a However,
manufacture of stable devices with wires fixed between
electrodes requires ‘molecular alligator clips’. Adhesion to
gold electrodes can be accomplished with terminal end-
groups such as thiols.2b
The development of molecular wires for molecular
electronics has attracted immense interest in recent years.1
Thus, molecules are designed to interconnect molecular
devices, such as single electron transistors, electron turn-
stiles, molecular switches, and chemical sensors. Especially
sp2-carbon based molecular wires have been intensely
targeted due to their conducting properties. Compared to
semiconductors such as silicon, it is possible to fabricate
much smaller and better defined carbon based nano-
devices. Much work has focused on thiol-terminated
conjugated p-systems, such as oligo(phenylenevinylene)s
(OPVs),2 oligo(phenyleneethynylene)s (OPEs),3 and
oligothiophenes.4
It is important to establish how small changes in molecular
structure will affect the single molecule conductivity.
Parameters to vary are the nature of the wire (choice of
conjugated p-system, alternating p- and s-systems, non-
covalent junctions, etc.), the wire length, the number of
electrode attachment sites and the nature of these. Figure 1
shows schematically five important classes of molecular
wires (A–E) with protected thiol end-groups.7 The acetyl
group has found general applicability as a thiol protecting
Some of us have recently developed several new procedures
for the synthesis and applications of OPVs5 that have been
Figure 1. Schematic representation of five classes of molecular wires (A–E). PGZthiol protecting group.
Keywords: Molecular electronics; Oligo(phenylenevinylene)s; Oligo(phenyleneethynylene)s; Aryl thiols.
*
Corresponding author. Tel.: C45 3532 0210; fax: C45 3532 0212; e-mail: mbn@kiku.dk
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.09.106

N. Stuhr-Hansen et al. / Tetrahedron 61 (2005) 12288–12295
12289
group (PG), since it is readily cleaved in situ under mild
basic conditions.2b
Finally, we report the synthesis of unsymmetrical OPVs of
class B. Highly ordered self-assembled monolayers (SAMs)
of such molecules on electrode surfaces are interesting with
respect to mediating electron transfer across the interfacial
barrier represented by the monolayer.4a,14
In this paper we present synthetic protocols for wires of type
A, B, D, and E. Thus, we have developed: (i) a new
synthesis of OPE3 with terminal acetylthio groups (class A)
via a tert-butylthio precursor, a procedure that may be
generalized to related molecules, (ii) new synthetic
procedures for molecular wires where the conjugation is
disrupted by methylene bridges (classes D and E), and
(iii) efficient synthesis of unsymmetrical OPVs (class B).
2. Results and discussion
The synthesis of SAc end-capped OPE3 proceeds according
to Scheme 1. First, bromide 1 was subjected to a Pd-
catalyzed cross-coupling with trimethylsilylacetylene,
employing the [Pd(PhCN)2Cl2]/P(t-Bu)3/CuI catalyst
system. Hundertmark et al.15 have shown that this system
allows room temperature Sonogashira coupling16 of aryl
bromides with a wide variety of terminal acetylenes. Indeed,
we managed to obtain 2 under these conditions in a yield of
75%.17 This same compound was previously prepared by
Mayor et al.18 from the more reactive, but less accessible,
4-iodophenyl tert-butyl sulfide. After silyl deprotection
using K2CO3 in THF/MeOH, 2 was subjected to a two-fold
cross-coupling with 1,4-diiodobenzene, which afforded the
OPE3 3 in 66% yield. The yield was increased significantly,
however, by employing microwave heating at 120 8C for
5 min, which gave 3 in 90%. This OPE was finally
converted quantitatively into the acetyl-protected wire 4
by means of AcCl/BBr3.
In our previous synthesis of thiol end-capped molecular
OPV wires,5 we adopted an approach where the aryl thiol
functionality was introduced as the t-Bu sulfide at the
beginning of the synthetic sequence via the building block
4-bromophenyl tert-butyl sulfide (1) and maintained
through the subsequent steps owing to the resistance of
t-Bu-S-Ar to both strongly basic and acidic conditions. In a
final step, the t-BuS group was converted into the AcS
moiety by means of AcCl/BBr3.8 We became interested to
employ the same approach to prepare acetyl-protected
OPEs, providing an alternative procedure to that of Tour and
co-workers.9 It deserves mention that the lability of the
acetyl protecting group has stimulated the exploitation of
other protecting groups as well. Thus, recently Bryce and
co-workers10 successfully utilized cyanoethyl as a thiolate
protecting group.
Disruption of the conjugated system or changes in the
conjugation pathway may lead to significant changes in the
tunnelling mechanism of the molecules inserted between
metal electrodes.2b,11 Several examples where the conju-
gation is broken within the wire (class C) have been
reported by Tour and co-workers.9 Synthetic methods for
two-terminal wires where the conjugation is instead broken
between the intact wire and the thiol end-caps are less
common. To our knowledge, only one example of a two-
terminal class D OPV3 (with –OBu substitution at the
central phenylene) has been reported in the literature with a
focus on the current-voltage characteristics rather than the
synthesis.12,13
An AcS-CH2 end-capped OPV3 was prepared according to
Scheme 2. The synthesis starts from 1-bromo-4-(methoxy-
methyl)benzene 519 (that we conveniently prepared from
4-bromobenzylbromide by treatment with sodium
methoxide). Compound 5 was treated with butyllithium
and DMF, which provided aldehyde 6. Several procedures
are available in the literature for synthesis of 6,20 but since
the existing procedures involve either carcinogenic com-
pounds,20a long reaction times (several days for one
reaction),20b or a starting material not readily available,20c
the present procedure is more convenient. The aldehyde was
subsequently coupled with tetraethyl 1,4-xylylenediphos-
phonate 721 in a typical Horner–Wadsworth–Emmons
(HWE) reaction22 upon treatment with potassium tert-
butoxide. Treatment of the product with iodine in toluene
provided the pure all-trans OPV3 8. Demasking this new
wire with bromotrimethylsilane gave the dibromide 9 that
was subsequently treated with potassium thioacetate to give
the protected thiomethyl wire 10.
The next logical step in synthetic manipulations towards
functional wires is further development of molecules having
isolated aromatic units (p-islands) between thiols, again
applying methylene spacers as isolating units between
individual islands (class E).
Scheme 1.

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Scheme 2.
Synthesis of bis[4-(S-acetylthiomethyl)phenyl]methane
(14) was carried out according to Scheme 3. First, Br/Li-
exchange on 5 with butyllithium followed by reaction of
the lithium reagent with ethyl N,N-diethylcarbamate23
gave the ketone 11. Subjecting 11 to a microwave-
assisted Huang–Minlon modification24 of the Wolff–
Kishner procedure produced 12 containing a central
methylene unit. Demasking the methoxymethyl func-
tionalities by means of bromotrimethylsilane gave the
dibromide 1325 that was treated with potassium thioacetate
to provide the acetyl-protected wire 14 with three methylene
bridges.
Scheme 3.
Scheme 4.

N. Stuhr-Hansen et al. / Tetrahedron 61 (2005) 12288–12295
12291
An unsymmetrical OPV wire was prepared in analogy to our
previous two-terminal OPV synthesis (Scheme 4).5a The
bromide 15,26 prepared from the corresponding alcohol,27
was first converted to the phosphonate 16. A HWE reaction
between 16 and aldehyde 17 (obtained from 15) gave, after
iodine-induced isomerization, the trans-stilbene 18. Finally,
the tert-butyl group was converted into an acetyl group to
provide 19.
0.1, 31.0, 46.4, 96.0, 104.4, 123.4, 131.8, 133.5, 137.1. MS
(EI) (%): m/z 262 (MC, 36%), 206 (60%), 191 (100%). HR-
MS (EI): m/z 262.1207 (MC, calcd for C15H22SSi
262.1211).
4.1.2. 4.1.2. Desilylation of 2. To a solution of 2
(0.335 g, 1.28 mmol) in MeOH (40 mL) and THF (7 mL)
was added K2CO3 (0.190 g, 1.37 mmol), and the mixture
was stirred for 45 min at room temperature. Then it was
poured into water (200 mL) and extracted with diethyl
ether (3!100 mL). The combined organic phases were
dried (MgSO4), filtered, and concentrated to an orange
oil of 1-tert-butylthio-4-ethynylbenzene that can be cross-
coupled without further purification. A pure sample
(semi-crystalline oil) can be obtained by column
chromatography (SiO2, heptane/EtOAc gradual increase
3. Conclusion
In conclusion, we have devised efficient protocols for the
synthesis of a selection of wires for molecular electronics
applications. The protocols are both simple and reliable and
may allow future synthesis of more complex systems. The
successful development of molecular electronics and the
establishment of structure-property relationships is strongly
dependent on such synthetic advances.
1
from 1:0 to 50:1). H NMR (300 MHz, CDCl3): d 1.27
(s, 9H), 3.19 (s, 1H), 7.45 (m, 4H). 13C NMR (75 MHz,
CDCl3): d 30.9, 46.1, 78.6, 83.1, 123.4, 131.6, 131.8,
138.9. MS (GC): m/z 190 (MC).
4. Experimental
4.1.3. 1,4-Bis(4-tert-butylthiophenylethynyl)benzene (3).
Procedure (i). Compound 2 (0.307 g, 1.17 mmol) was
desilylated according to the procedure described above.
[Pd(PPh3)2Cl2] (0.020 g, 0.03 mmol) and CuI (0.010 g,
0.05 mmol) were dissolved in dry and argon-degassed
THF (3.5 mL), toluene (3.5 mL), and NEt3 (3 mL). The
mixture was stirred under a flow of argon for about
30 min, whereupon the desilylated 2 and 1,4-diiodoben-
zene (0.126 g, 0.383 mmol) were added. The solution
turned black immediately. After 30 min of stirring under
argon, a white precipitate had formed. After 2 h the
mixture was poured onto aqueous NH4Cl and extracted
with CH2Cl2 (3!100 mL). The combined organic phases
were dried (MgSO4), filtered, and concentrated in vacuo
until the precipitation of a white precipitate. Cooling on
ice provided further precipitation, and the solid was
filtered and washed with cold diethyl ether. The crude
product was recrystallized from hot diethyl ether to
afford 3 (115 mg, 66%) as a white powder. Procedure
(ii). Compound 2 (0.131 g, 0.50 mmol) was desilylated
according to the procedure described above. [Pd(PPh3)2-
Cl2] (9.5 mg, 0.014 mmol) and CuI (5 mg, 0.026 mmol)
were dissolved in dry and argon-degassed DMF (2 mL)
and NEt3 (1 mL). The mixture was stirred under a flow
of argon for about 10 min and then transferred to a
heavy-walled reaction vessel (sealed with a teflon
septum), containing the desilylated 2 and 1,4-diiodoben-
zene (0.072 g, 0.22 mmol). The solution turned black
immediately. The mixture was heated in a microwave
oven (ramp time: 2 min, temperature: 120 8C, hold time:
5 min, pressure: 1.5 bar). After cooling to room tempera-
ture, the mixture was evaporated to dryness and purified
by column chromatography (SiO2, heptane). After
recrystallization from hot heptane, the product 3
(90.5 mg, 90%) was obtained as a white powder. Mp
205–207 8C. Sublimation temperature 170 8C at 2!10K
2 mbar. IR(KBr): n (cmK1) 2951, 2927, 2860, 1920,
1799, 1736, 1670, 1586, 1506, 1466, 1397, 1366, 1300,
1263, 1160, 1101, 1015, 833, 730, 535. 1H NMR
(300 MHz, CDCl3): d 1.31 (s, 18H), 7.51 (m, 12H).
13C NMR (75 MHz, CDCl3): 31.3, 46.8, 90.8, 91.1,
4.1. General experimental procedures
Thin-layer chromatography (TLC) was carried out using
aluminium sheets pre-coated with silica gel 60F (Merck
5554). Column chromatography was carried out using
silica gel 60F (Merck 9385, 0.040–0.063 mm). For
microwave-assisted reactions, a CEM925110 Discover
microwave oven was employed. Melting points were
determined on a Bu¨chi melting point apparatus and are
uncorrected. 1H NMR (300 MHz) and 13C NMR
(75 MHz) spectra were recorded on a Varian instrument.
Samples were prepared using CDCl3 purchased from
Cambridge Isotope Labs. Electron impact ionisation mass
spectrometry (EI-MS) was performed on a Varian MAT
311A. Fast atom bombardment (FAB) spectra were
ontained on
a Jeol JMS-HX 110 Tandem Mass
Spectrometer in the positive ion mode using 3-nitro-
benzyl alcohol (NBA) as matrix. Gas chromatography–
Mass spectrometry (GC–MS) was performed on a
HP5890 Series II plus gas chromatograph coupled with
a HP5972 Series Mass analyzer. Microanalyses were
performed at the Microanalytical Laboratory at the
Department of Chemistry, University of Copenhagen.
4.1.1. 1-tert-Butylthio-4-trimethylsilylethynylbenzene
(2). [Pd(PhCN)2Cl2] (0.04 g, 0.104 mmol) and CuI
(0.005 g, 0.026 mmol) were dissolved in dry and argon-
degassed THF (2.5 mL) and toluene (2.5 mL) and stirred
under a flow of argon for 10 min. Then HN(i-Pr)2 (1 mL)
and P(t-Bu)3 (10% in hexane, 0.63 mL) were added
followed by 1-bromo-4-(tert-butylthio)benzene 1 (1.25 g,
5 mmol) and trimethylsilylacetylene (0.85 g, 8.7 mmol).
The dark solution was heated to 50 8C for 2 h. The reaction
was stopped by pouring the reaction mixture onto H2O
(100 mL) and extracting with diethyl ether (3!100 mL).
The combined organic phases were dried (MgSO4), filtered,
and concentrated to a yellow oil. Column chromatography
(SiO2, heptane) afforded 2 (994 mg, 75%) as a semi-
crystalline oil. 1H NMR (300 MHz, CDCl3): d 0.25 (s, 9H),
1.27 (s, 9H), 7.43 (m, 4H). 13C NMR (75 MHz, CDCl3): d

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N. Stuhr-Hansen et al. / Tetrahedron 61 (2005) 12288–12295
123.3, 123.6, 131.8, 131.8, 133.8, 137.5. HR-MS (FAB):
m/z 454.1793 (MC, calcd for C30H30S2 454.1789).
(80 mL) cooled in an ice bath was added t-BuOK (2.47 g,
22 mmol) in small portions during a period of 10 min. The
reaction mixture was further stirred at room temperature for
6 h under nitrogen and then poured into water (300 mL). A
yellow material was filtered off, washed with H2O, and dried
in a vacuum oven (120 8C, 1 mmHg). The product was
dissolved in the minimum amount of a boiling solution
containing iodine in toluene (0.1 mM). Reflux was
maintained for 12 h. By slow cooling at room temperature,
the pure trans-stilbene 8 (3.05 g, 82%) precipitated as
yellow crystals. Mp 259–260 8C. IR(KBr): n (cmK1) 3021,
2985, 2923, 2891, 2821, 2066, 1914, 1693, 1650, 1632,
1610, 1567, 1518, 1456, 1423, 1381, 1336, 1322, 1306,
1278, 1209, 1196, 1112, 1017, 970, 949, 927, 830, 803, 781,
713, 601, 549. Anal. Calcd for C26H26O2: C, 84.29; H, 7.07.
4.1.4. 1,4-Bis(4-acetylthiophenylethynyl)benzene (4).
OPE3 3 (0.100 g, 0.220 mmol) was dissolved in CH2Cl2
(10 mL) and toluene (10 mL). Then acetyl chloride (2 mL)
and boron tribromide (1 M in CH2Cl2, 4 mL, 4 mmol) were
added while stirring. The reaction was stirred for 3 h under
inert atmosphere and then poured onto ice and extracted
with CH2Cl2 (4!100 mL). The combined organic phases
were dried (MgSO4), filtered, concentrated, and purified by
column chromatography (SiO2, heptane/EtOAc gradual
increase from 10:1 to 0:1) to afford 4 as an off-white
powder (96 mg, 98%). Mp 188–189 8C. IR(KBr): n (cmK1
)
2922, 2852, 1916, 1696, 1591, 1513, 1480, 1422, 1407,
1396, 1354, 1304, 1269, 1117, 1013, 964, 873, 824, 695,
621, 542, 508. UV–vis (CHCl3): lmax (nm) (3 (MK1 cmK1))
1
Found: C, 84.45; H, 7.06. H NMR (300 MHz, CDCl3): d
3.40 (s, 6H), 4.47 (s, 4H), 7.12 (s, 4H), 7.33 (d, JZ8.1 Hz,
4H), 7.50 (s, 4H), 7.51 (d, JZ8.1 Hz, 4H). Crystals not
soluble enough for 13C NMR. MS (EI, 70 eV) (%): m/z 370
(MC, 100), 354 (10), 339 (17).
1
318 (sh, 50400), 332 (61800), 350 (sh, 38300). H NMR
(300 MHz, CDCl3): d 2.44 (s, 6H), 7.41 (d, JZ7.8 Hz, 4H),
7.52 (s, 4H), 7.56 (d, JZ7.8 Hz, 4H). 13C NMR (75 MHz,
CDCl3): d 30.3, 90.6, 90.7, 123.0, 124.2, 128.3, 131.6,
132.2, 134.2, 199.4. HR-MS (FAB): m/z 426.0728 (MC,
calcd for C26H18O2S2 426.0748).
4.1.8. (E,E)-1,4-Bis[4-(bromomethyl)styryl]benzene (9).
To a slurry of 8 (0.74 g, 2 mmol) in chlorobenzene (15 mL)
was added bromotrimethylsilane (0.70 g, 4.6 mmol) and the
resulting slurry was stirred at 75 8C for 4 h under nitrogen.
The grey slurry was poured into MeOH (150 mL), and a
yellow powder was filtered off. The product was dissolved
in the minimum amount of a boiling solution containing
iodine in toluene (0.1 mM). Reflux was maintained for 12 h.
By slow cooling at room temperature, the product 9 (0.73 g,
78%) precipitated as yellow crystals. Mp O280 8C. Anal.
Calcd for C24H20Br2: C, 61.56; H, 4.31. Found: C, 61.20; H,
4.21. Crystals not soluble enough for NMR. MS (EI, 70 eV)
(%): m/z 468 (MC, 63), 389 (100), 308 (26).
4.1.5. 1-Bromo-4-(methoxymethyl)benzene (5). To a
slurry of 4-bromobenzylbromide (37.5 g, 150 mmol) in
MeOH (100 mL) was added NaOMe (25% in MeOH,
35.7 g, 0.165 mmol). The resulting reaction mixture was
refluxed for 2 h under nitrogen and poured into water
(350 mL) and extracted with pentane (3!50 mL). The
pooled organic extracts were filtered through basic alumina
(10 g) and evaporated. Bulb-to-bulb distillation (0.5 mmHg,
air bath 100 8C) gave 5 (25.1 g, 83%) as a colorless liquid.
1H NMR (300 MHz, CDCl3): d 3.38 (s, 3H), 4.40 (s, 2H),
7.20 (d, JZ8.2 Hz, 2H), 7.47 (d, JZ8.2 Hz, 2H). 13C NMR
(75 MHz, CDCl3): d 58.0, 73.7, 121.3, 129.1, 131.3, 137.1.
MS (EI, 70 eV) (%): m/z 202 (MC, 15), 171 (37), 157 (5),
121 (100).
4.1.9. (E,E)-1,4-Bis[4-(S-acetylthiomethyl)styryl]ben-
zene (10). A slurry of 9 (0.23 g, 0.5 mmol) and potassium
thioacetate (0.14 g, 1.2 mmol) in NMP (15 mL) was heated
at 60 8C for 2 h. The clear solution was poured into ice
(200 g). Filtration and separation on silica gel 60F (30 g) by
means of CH2Cl2 gave yellow crystalline material which
was dissolved in the minimum amount of a boiling solution
containing iodine in toluene (0.1 mM). Reflux was
maintained for 12 h. By slow cooling at room temperature,
the product 10 (0.13 g, 57%) precipitated as light-yellow
plates. Mp 257–258 8C. Anal. Calcd for C28H26O2S2: C,
73.33; H, 5.71; S, 13.98. Found: C, 73.32; H, 5.70; S, 13.84.
IR(KBr): n (cmK1) 3015, 2909, 1910, 1698, 1654, 1604,
1515, 1422, 1352, 1335, 1141, 1099, 1015, 961, 947, 889,
833, 779, 751, 701, 633, 570, 546, 540. UV–vis (CHCl3):
lmax (nm) (3 (MK1 cmK1)) 350 (sh, 61100), 362 (68700),
4.1.6. 4-(Methoxymethyl)benzaldehyde (6). Compound 5
(10.05 g, 50 mmol) was added dropwise under an argon
atmosphere to a solution of butyllithium (2.5 M in hexanes,
20 mL, 50 mmol) in THF (50 mL) cooled in a dry ice/
acetone bath. The reaction mixture was stirred at K78 8C
for 15 min, then DMF (10 mL) was added in one portion,
and stirring was maintained at room temperature for 30 min.
The clear reaction mixture was poured into H2O (200 mL)
and extracted with pentane (3!50 mL). The combined
organic layers were washed with H2O (30 mL), dried
(MgSO4), filtered, and concentrated by rotary evaporation
(30 8C, 20 mmHg). Distillation at 77–79 8C (0.40–
0.45 mmHg) in a column-free standard Claisen equipment
gave 6 (6.28 g, 84%) as a colorless liquid. Purity O98%
(GC–MS). 1H NMR (300 MHz, CDCl3): d 3.40 (s, 3H), 4.50
(s, 2H), 7.46 (d, JZ7.9 Hz, 2H), 7.83 (d, JZ7.9 Hz, 2H),
9.97 (s, 1H). 13C NMR (75 MHz, CDCl3): d 58.3, 73.7,
127.5, 129.7, 135.6, 145.2, 191.8. MS (EI, 70 eV) (%): m/z
150 (MC, 40), 135 (100), 121 (71). HR-MS (EI): m/z
150.0699 (MC, calcd for C9H10O2 150.0681).
1
383 (sh, 43800). H NMR (300 MHz, CDCl3): d 2.36 (s,
6H), 4.13 (s, 4H), 7.08 (s, 4H), 7.28 (d, JZ8.2 Hz, 4H), 7.45
(d, JZ8.2 Hz, 4H), 7.49 (s, 4H). Crystals not soluble
enough for 13C NMR. MS (EI, 70 eV) (%): m/z 458 (MC,
100), 415 (11), 383 (72), 339 (14).
4.1.10. 4,40-Bis(methoxymethyl)benzophenone (11).
Compound 5 (10.05 g, 50 mmol) was added dropwise
under an argon atmosphere to a solution of butyllithium
(2.5 M in hexanes, 20 mL, 50 mmol) in THF (50 mL)
cooled in a dry ice/acetone bath. After stirring the reaction
mixture at K8 8C for 15 min, ethyl N,N-diethylcarbamate
4.1.7. (E,E)-1,4-Bis[4-(methoxymethyl)styryl]benzene
(8). To a solution of 6 (3.00 g, 20 mmol) and tetraethyl
1,4-xylylenediphosphonate 7 (3.78 g, 10 mmol) in THF

N. Stuhr-Hansen et al. / Tetrahedron 61 (2005) 12288–12295
12293
(3.63 g, 25 mmol) was added during a 10 min period in a
dropwise fashion. After stirring the reaction mixture at
K78 8C for an additional 15 min, H2O (10 mL) was added
dropwise, and the reaction mixture was poured into H2O
(300 mL) and extracted with pentane (3!50 mL). The
combined organic layers were washed with H2O (30 mL),
dried (MgSO4), filtered, and concentrated by rotary
evaporation (30 8C, 20 mmHg). Bulb-to-bulb distillation
(0.05 mmHg, air bath 200 8C) afforded 11 (5.14 g, 76%) as a
colorless liquid that crystallized into white crystals upon
standing. Mp 45–46 8C. Purity O98% (GC–MS). Anal.
Calcd for C17H18O3: C, 75.53; H, 6.71. Found: C, 75.95; H,
6.72. IR(KBr): n (cmK1) 3037, 3000, 2982, 2926, 2893,
2861, 2824, 2732, 2069, 1934, 1824, 1733, 1651, 1609,
1571, 1510, 1469, 1455, 1409, 1379, 1310, 1277, 1211,
1195, 1175, 1148, 1104, 1017, 972, 928, 854, 841, 819, 754,
141.0. MS (EI, 70 eV) (%): m/z 354 (MC, 5), 275 (100),
194 (100).
4.1.13. 4,40-Bis(S-acetylthiomethyl)diphenylmethane
(14). A slurry of dibromide 13 (0.35 g, 1 mmol) and
potassium thioacetate (0.25 g, 2.2 mmol) in NMP (10 mL)
was heated at 60 8C for 2 h. The clear colorless solution was
poured into ice (100 g) and extracted with Et2O (3!
20 mL). The combined organic layers were washed with
H2O (30 mL) and evaporated. Separation on silica gel 60F
(20 g) by means of CH2Cl2 gave a white crystalline material
upon evaporation. Recrystallization from heptane afforded
14 (0.27 g, 78%) as white needles. Mp 90–91 8C. Anal.
Calcd for C19H20O2S2: C, 66.25; H, 5.85; S, 18.65. Found:
C, 66.25; H, 5.84; S, 18.69. IR(KBr): n (cmK1) 3045, 3002,
2920, 2832, 1912, 1693, 1510, 1431, 1411, 1359, 1241,
1130, 1098, 1023, 1000, 964, 914, 866, 839, 823, 774, 728,
1
681, 629, 487, 474, 436. H NMR (300 MHz, CDCl3): d
1
3.38 (s, 6H), 4.48 (s, 4H), 7.40 (d, JZ8.2 Hz, 4H), 7.74 (d,
JZ8.2 Hz, 4H). 13C NMR (75 MHz, CDCl3): d 58.1, 73.7,
126.8, 129.8, 136.5, 142.7, 195.7. MS (EI, 70 eV) (%): m/z
270 (MC, 25), 255 (4), 225 (7), 210 (34), 195 (7), 180 (18),
165 (18), 149 (100).
718, 687, 626, 568, 541, 513, 475. H NMR (300 MHz,
CDCl3): d 2.33 (s, 6H), 3.90 (s, 2H), 4.08 (s, 4H), 7.09 (d,
JZ6.3 Hz, 4H), 7.19 (d, JZ6.3 Hz, 4H). 13C NMR
(75 MHz, CDCl3): d 30.2, 33.0, 41.1, 128.8, 129.0, 135.2,
139.9, 195.0. MS (EI, 70 eV) (%): m/z 344 (MC, 21), 301
(8), 269 (100).
4.1.11. 4,40-Bis(methoxymethyl)diphenylmethane (12). A
slurry of ketone 11 (1.08 g, 4 mmol), KOH (1.35 g,
24 mmol), and hydrazine monohydrate (0.30 g, 6 mmol)
in diethylene glycol (25 mL) was stirred at room tempera-
ture for 10 min in order to become homogeneous and then
heated in a microwave oven for 20 min at 130 8C under the
influence of 20 W microwaves. The clear colorless solution
was poured into H2O (200 mL) and extracted with pentane
(3!50 mL). The combined organic layers were washed
with H2O (30 mL), dried (MgSO4), filtered, and concen-
trated by rotary evaporation (30 8C, 20 mmHg). Bulb-to-
bulb distillation (0.05 mmHg, air bath 200 8C) afforded 12
(0.89 g, 87%) as a colorless liquid. Purity O98% (GC–MS).
Anal. Calcd for C17H20O2: C, 79.65; H, 7.86. Found: C,
79.78; H, 7.66. IR(KBr): n (cmK1) 3050, 3010, 2983, 2925,
2894, 2852, 2820, 2736, 1912, 1803, 1721, 1656, 1612,
1577, 1512, 1450, 1417, 1381, 1364, 1304, 1278, 1193,
1155, 1099, 1021, 967, 919, 857, 806, 754, 616, 579, 485.
1H NMR (300 MHz, CDCl3): d 3.38 (s, 6H), 3.98 (s, 2H),
4.43 (s, 4H), 7.17 (d, JZ8.1 Hz, 4H), 7.26 (d, JZ8.1 Hz,
4H). 13C NMR (75 MHz, CDCl3): d 41.2, 57.9, 74.4, 127.9,
128.8, 135.8, 140.4. MS (EI, 70 eV) (%): m/z 256 (MC, 47),
225 (19), 211 (26), 179 (42), 121 (100).
4.1.14. Diethyl 4-ethylbenzylphosphonate (16). A solution
of bromide 15 (4.36 g, 22 mmol) and triethylphosphite (5 g,
30 mmol) in dioxane (40 mL) was refluxed under nitrogen
for 12 h. The solvent was evaporated in vacuo, and bulb-to-
bulb distillation (0.1 mbar, air bath 180 8C) gave the
phosphonate 16 (3.8 g, 68%) as a colorless liquid. Anal.
Calcd for C13H21O3P: C, 60.93; H, 8.26. Found: C, 60.51;
H, 8.69. 1H NMR (400 MHz, CDCl3): d 1.22 (t, JZ7.2 Hz,
3H), 1.24 (t, JZ7.2 Hz, 6H), 2.62 (q, JZ7.2 Hz, 2H), 3.23
(d, JZ21 Hz, 2H), 4.01 (q, JZ7.2 Hz, 4H), 7.13 (d, JZ8.1
Hz, 2H), 7.22 (d, JZ8.2, 2H). 13C NMR (62.9 MHz,
CDCl3) d 15.4, 16.2 (d, JZ6 Hz), 28.3, 33.1 (d, JZ138 Hz),
61.7 (d, JZ7 Hz), 127.8 (d, JZ3 Hz), 128.0 (d, JZ3 Hz),
129.5 (d, JZ3 Hz), 142.6 (d, JZ3 Hz). MS–S (EI) (%): m/z
256 (MC, 100).
4.1.15. Trans-4-tert-butylthio-40-ethylstilbene (18). A
solution of phosphonate 16 (0.77 g, 3 mmol) and aldehyde
17 (0.58 g, 3 mmol) in freshly distilled THF (50 mL) was
cooled to 0 8C, and potassium tert-butoxide (0.37 g,
3.3 mmol) was added. The reaction mixture was stirred at
room temperature for 1 h and poured into water. The white
crystalline precipitate was filtered off and dried in vacuo.
Recrystallization and isomerization to the trans compound
was achieved by refluxing the compound for 6 h in toluene
(5 mL) containing iodine (0.1 mM), thus affording 18
(0.65 g, 73%). Mp 90.3–91.6 8C. Anal. Calcd for C20H24S:
C, 81.02; H, 8.16; S, 10.82. Found: C, 80.55; H, 8.40; S,
4.1.12. 4,40-Bis(bromomethyl)diphenylmethane (13). To
a mixture of 12 (0.51 g, 2 mmol) and chlorobenzene
(10 mL) was added bromotrimethylsilane (0.67 g,
4.4 mmol). The resulting yellow slurry was stirred at
75 8C for 4 h under nitrogen and poured into H2O
(100 mL). The phases were separated, and the aqueous
phase was further extracted with toluene (2!20 mL). The
combined organic extracts were filtered through silica gel
60F (10 g) by means of CH2Cl2–pentane (1/9). Evaporation
afforded 13 (0.89 g, 87%) as white crystals. Mp 146–147 8C
(lit.:25 151.5–153.5 8C). Anal. Calcd for C15H14Br2: C,
50.88; H, 3.99. Found: C, 51.18; H, 3.92. 1H NMR
(300 MHz, CDCl3): d 3.96 (s, 2H), 4.48 (s, 4H), 7.15 (d,
JZ8.1 Hz, 4H), 7.32 (d, JZ8.1 Hz, 4H). 13C NMR
(75 MHz, CDCl3): d 33.3, 41.2, 129.1, 129.2, 135.6,
1
10.51. H NMR (400 MHz, CDCl3): d 1.25 (t, JZ7.2 Hz,
3H), 1.30 (s, 9H), 2.62 (q, JZ7.2 Hz, 2H), 7.04 (d, JZ
16 Hz, 1H), 7.14 (d, JZ16 Hz, 1H), 7.20 (d, JZ7.7 Hz,
2H), 7.42–7.53 (m, 6H). 13C (62.9 MHz, CDCl3) d 15.4,
28.6, 30.9, 46.1, 126.2, 126.5, 126.9, 128.1, 129.5, 131.6,
134.5, 137.6, 137.9, 144.1. MS (FAB): m/z 296 (MC).
4.1.16. Trans-4-acetylthio-40-ethylstilbene (19). To a
solution of 18 (0.33 g, 1.1 mmol) and acetyl chloride
(1 mL) in CH2Cl2 (20 mL) was added BBr3 (1.5 mmol,
1 M in CH2Cl2). The black solution was stirred under

12294
N. Stuhr-Hansen et al. / Tetrahedron 61 (2005) 12288–12295
D. W., Jr.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss,
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nitrogen for 2 h and poured into ice/water (100 mL). The
precipitate was purified using coloumn chromatography
(SiO2, CH2Cl2). The product containing fraction was
recrystallized and isomerized to the trans compound by
refluxing the compound for 6 h in toluene (5 mL) containing
iodine (0.1 mM). The product 19 precipitated as white
crystals (170 mg, 53%). Mp 98.5–101.3 8C. Anal. Calcd for
C15H18OS: C, 76.56; H, 6.42; S, 11.35. Found: C, 76.44; H,
6.26; S, 11.24. IR(KBr): n (cmK1) 3019, 2961, 2928, 2870,
1911, 1704, 1587, 1508, 1455, 1411, 1350, 1179, 1121,
1008, 963, 831, 681, 617, 546. 1H NMR (400 MHz, CDCl3):
d 1.26 (t, JZ7.2 Hz, 3H), 2.42 (s, 3H) 2.62 (q, JZ7.2 Hz,
2H), 7.05 (d, JZ17 Hz, 1H), 7.14 (d, JZ17 Hz, 1H), 7.21
(d, JZ8.7 Hz, 2H), 7.39 (d, JZ8.7 Hz, 2H) 7.45 (d, JZ
8.7 Hz, 2H), 7.54 (d, JZ8.7 Hz, 2H). 13C NMR (62.9 MHz,
CDCl3): d 15.4, 28.6, 30.1, 126.4, 127.6, 126.9, 128.2,
130.1, 134.3, 134.6, 138.7, 144.3, 194.1; One signal
overlapping. MS (EI) (%): m/z 282 (MC, 55), 240 (100),
225 (25).
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Acknowledgements
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and selenium anchoring groups for molecules adsorbed onto
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This research was supported by EC FP6 funding (contract
no. FP6-2004-IST-003673, CANEL). This publication
reflects the views of the authors and not necessarily those
of the EC. The Community is not liable for any use that may
be made of the information contained herein. Moreover, the
Danish Technical Research Council (STVF, grant # 26-01-
0033) is gratefully acknowledged. We thank Niels V. Holst
for recording UV–vis spectra.
8. For a recent, alternative bromine catalyzed S(t-Bu) to SAc
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