Redox-active catechol-functionalized molecular rods: Suitable protection groups and single-molecule transport investigations

Article abstract of DOI:10.1002/ejoc.200700810

The synthesis of molecular rod 1 comprising a central redox-active unit and two terminal acetyl-protected sulfur groups is reported. Various protecting groups for the catechol moiety were investigated and protected precursors 14-18 were synthesized. In pa

Full text of DOI:10.1002/ejoc.200700810

FULL PAPER
DOI: 10.1002/ejoc.200700810
Redox-Active Catechol-Functionalized Molecular Rods: Suitable Protection
Groups and Single-Molecule Transport Investigations
Nicolas Weibel,[a] Alfred Błaszczyk,[b] Carsten von Hänisch,[b] Marcel Mayor,*[a,b]
Ilya Pobelov,[c,d] Thomas Wandlowski,*[c,d] Fang Chen,[e] and Nongjian Tao*[e]
Keywords: Molecular electronics / Protecting groups / Electrochemistry / Molecular rods / Catechol / Single-molecule
conductance
The synthesis of molecular rod 1 comprising a central redox-
active unit and two terminal acetyl-protected sulfur groups
is reported. Various protecting groups for the catechol moiety
were investigated and protected precursors 14–18 were syn-
thesized. In particular, ethyl orthoformate 17 can be easily
and selectively deprotected in mild acidic conditions ruling
out the need for a strong Lewis acid like boron tribromide.
The redox activity of 1 was confirmed by cyclic voltammetric
investigations. Gold–molecule–gold junctions were formed
with the dimethyl-protected rod 18. Single-molecule trans-
port investigations using this molecular rod revealed a set of
three single-junction conductance values varying over two
orders of magnitude.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Devices comprising molecular switches are currently of
great interest. In order to alter the current through a molec-
Introduction
Molecular electronics, understood as the engineering of ular junction, compounds that possess structural bistability
electronic properties by the integration of tailor-made mole- controlled by an external stimulus are required. Potential
cules into electronic circuits, is of both fundamental and stimuli are electromagnetic radiation, electronic potential or
economic interest.[1–6] In particular, the combination of chemical gradients. So far, several devices comprising single
minute size and huge designable and synthesizable struc- molecules as switches have been reported, for example, pho-
tural variety renders molecules promising functional units toswitchable dithienylethene derivatives in a mechanically
in electronic devices. Driven by the prospect of tiny elec- controlled break-junction (MCB)[18] or electrochemically
tronically active building blocks at low cost, the field of addressable redox chromophores like viologens in an elec-
molecular electronics has even made its way into the re- trochemically addressable scanning tunnelling microscopy
search portfolios of microelectronic companies. In the past (STM) set-up.[19–21] A molecular rod comprising a central
decade, the investigation of single-molecule junctions has 6,6Ј-dinitro-2,2Ј-bipyridine subunit has been reported to
revealed not only interesting correlations between molecu- display hysteretic transport properties in a variety of set-
lar structure and observed transport properties,[7–16] but ups[22] ranging from a single-molecule MCB[23] to self-as-
also allowed the design and assembly of the first devices sembled monolayers (SAMs) in a crossed wire junction.[22]
with electronic properties arising from integrated molecules, Hysteretic transport properties have been reported for junc-
for example, a single-molecule rectifier.[17]
tions comprising SAMs of mechanically interlinked super-
molecules like catenanes[24] and rotaxanes.[25,26] The ob-
served bistability of these junctions presumably arises from
structural rearrangements of the integrated supermole-
cules.[27] Laterally limited SAMs of nitro-functionalized
molecular rods displayed a sudden collapse of the transport
current at a threshold voltage referred to as the negative
differential resistance (NDR).[28]
Under electrochemical control, junctions comprising sin-
gle viologen derivatives show redox-state-dependent trans-
port properties.[19–21] Furthermore, redox chromophores
comprising substructures like an extended tetrathiafulva-
lene[29] or a quinone[30] have been proposed as their redox
states are expected to control the extent of electronic delo-
calization and thus to strongly affect their electric trans-
[a] Department of Chemistry, University of Basel,
4056 Basel, Switzerland
E-mail: marcel.mayor@unibas.ch
[b] Institute for Nanotechnology, Research Center Karlsruhe
GmbH,
76021 Karlsruhe, Germany
[c] Institute of Bio- and Nanosystems IBN 3, Research Center
Jülich GmbH,
52425 Jülich, Germany
E-mail: th.wandlowski@fz-juelich.de
[d] Department of Chemistry and Biochemistry, University of
Berne,
3012 Berne, Switzerland
[e] Department of Electrical Engineering, Arizona State Univer-
sity,
Tempe, Arizona 85287-6206, USA
E-mail: nongjian.tao@asu.edu
136
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Redox-Active Catechol-Functionalized Molecular Rods
parency. Indeed, molecular rods comprising quinone chro-
As depicted in Scheme 1, a series of potential precursors
mophores have been reported to display NDR properties in of the target structures masked with different protecting
SAMs investigated by STM.[31,32]
groups have been synthesized to investigate their tolerance
Our current interest is focused on catechol subunits as towards the terminal acetylsulfanyl functions. Furthermore,
redox addressable chromophores in oligo(phenyleneethyn- the intrinsic lability of the target motif itself does not only
ylene) (OPE) rod systems. As depicted in Scheme 1, the ex- point towards the importance of suitable protecting groups,
tent of conjugation is not expected to be affected to a large but also further reduces the scope of applicable deprotec-
degree by the redox state of the catechol subunit as both tion conditions. As displayed in Scheme 2, the free phenolic
redox states provide a conjugated π system as alternating hydroxy groups ortho to the ethynyl groups tend to form
single and double bonds or as an aromatic unit. However, five-membered heterocyclic benzofuran rings by reacting
its ability to form hydrogen bonds will be altered consider- with the neighbouring alkyne. This intramolecular cycliza-
ably. Whereas the catechol subunit with its two hydroxy tion can be promoted by various reaction conditions, for
groups acts as a hydrogen donor, the quinone form after example, mild bases,[33–35] palladium/copper catalytic sys-
oxidation acts as a hydrogen acceptor as only the lone pairs tems,[36] TBAF[37] and UV irradiation.[38] The fact that the
of the two carbonyls are available to form hydrogen bonds. assembly of OPE rods depends on such conditions (e.g., in
To be able to integrate this electrochemically addressable Sonogashira cross-coupling or TMS deprotection reactions)
subunit into molecular architectures of increasing complex- further motivated the search for a suitable catechol-protect-
ity comprising suitable functional groups as binding part- ing group.
ners for hydrogen bonds, its chemical reactivity has to be
Typically, catechol derivatives can be masked as diethers
controlled by suitable protecting groups. To investigate the (including silyl ethers) or diesters similar to phenolic com-
feasibility of catechol-functionalized OPE rods, we targeted pounds. Cyclic acetals, ketals and esters allow selective pro-
molecular rod 1 comprising terminal acetyl-protected sulfur tection of two adjacent hydroxy groups, as found in cate-
groups with a view to immobilizing it between gold elec- chols, even in the presence of another isolated phenol func-
trodes.
tion.[39] Methylene acetals are usually obtained by reaction
Scheme 1. A) Target molecular rod 1 and its various protected precursors 14–18. B) Redox chemistry of the molecular rod 1.
Scheme 2. Intermolecular cyclization to benzofuran derivatives.
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M. Mayor, T. Wandlowski, N. Tao et al.
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with a methylene dihalide.[40,41] The resulting methylene- to the dimethoxy precursor 18, various protecting groups
dioxy group is relatively stable, but can be efficiently depro- for the catechol moiety, namely, benzyl ether 14, diphenyl-
tected by use of a suitable Lewis acid such as a boron[42,43] methylene ketal 15, methylene acetal 16 and ethyl orthofor-
or aluminium[44,45] halide. Acetone can be used to convert mate 17, have also been investigated (Scheme 1).
catechols into the corresponding acetonides[46] which are
hydrolyzed in acidic conditions.[47,48] Cyclohexylidene ketals
are prepared from cyclohexanone[49] and tolerate the use of
butyllithium for metallation.[50] The reaction of dichlorodi-
phenylmethane[51] or dimethoxydiphenylmethane[52] with
catechols leads to diphenylmethylene ketals. Deprotection
of the latter is promoted by catalytic hydrogenation[53] or
acidic hydrolysis.[54] The ethyl orthoformate protecting
group is formed with triethyl orthoformate[54] and hy-
drolyzed in the presence of a catalytic amount of an acid
such as TsOH.[55] Cyclic borates tolerate the basic condi-
tions required for alkylation or acylation of a phenolic
function, for example, and are easily removed in a dilute
acid medium.[56] Lastly, the use of cyclic carbonates turns
out to be rather limited owing to an enhanced sensitivity
towards hydrolysis.[57]
Results and Discussion
Suitable building blocks with a protected catechol sub-
unit were synthesized and the molecular rods comprising
these subunits assembled. Subsequently, the lability of the
different protecting groups was investigated by applying a
variety of deprotection protocols. Tentative cyclic voltam-
metry studies of these molecular rods have allowed us to
demonstrate the redox activity of the integrated and depro-
tected catechol subunit. Finally, immobilization of one of
these molecular rods in a molecular STM junction enabled
preliminary single-molecule transport investigations to be
undertaken.
Herein we report the synthesis of the molecular rod 1
and the investigation of the suitability of several different
protected precursors as potential building blocks not only
Synthesis of the Protected Molecular Rods
The central building block, 2,3-dihydroxy-1,4-diiodo-
for the target compound 1, but also for further function- benzene, was obtained in four steps according to a pre-
alized structures. In addition to the initial electrochemical viously reported procedure[58] starting with commercially
investigations carried out on these compounds, the di- available veratrole (1,2-dimethoxybenzene). A sequence of
methyl-protected rod 18 was immobilized in Au–18Ј–Au monolithiation followed by reaction with trimethylsilyl
junctions to demonstrate the suitability of the parent struc- chloride allowed the stepwise introduction of two TMS
ture as a functional unit in a molecular device. In addition groups onto the dimethoxybenzene ring. Iodo-desilylation
Scheme 3. Synthesis of protected diiodocatechol derivatives 2–5 using 2,3-dihydroxy-1,4-diiodobenzene as the central building block.
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Redox-Active Catechol-Functionalized Molecular Rods
with iodine monochloride led to the diiodo intermediate to give doubly TMS-protected diacetylenes 6–9 (97–100%
which was finally converted into the desired 2,3-dihydroxy- yield). Potassium carbonate in a mixture of dichlorometh-
1,4-diiodobenzene by demethylation with BBr3. The corre- ane and methanol was then used to cleave almost quantita-
sponding benzyl-protected diiodocatechol 2 was readily ob- tively the two TMS protecting groups at room temperature,
tained by alkylation of 2,3-dihydroxy-1,4-diiodobenzene leading to intermediates 10–13. The free diacetylenes 10–13
with benzyl bromide in the presence of potassium carbonate were subjected to a two-fold palladium- and copper-cata-
and a catalytic amount of potassium iodide in refluxing lyzed cross-coupling reaction with 2.2 equivalents of 1-ace-
acetone (Scheme 3). Heating 2,3-dihydroxy-1,4-diiodo- tylsulfanyl-4-iodobenzene,[59,60] leading to the desired pro-
benzene at 180 °C in neat dichlorodiphenylmethane gave tected molecular rods 14–17 in reasonable yields (54–64%).
the diphenylmethylene ketal 3 in 90% yield. The introduc-
In an alternative synthetic strategy, these molecular rods
tion of a methylene acetal as a catechol protecting group can also be assembled in a one-step Sonogashira reaction,
was achieved in hot DMSO containing dichloromethane as displayed in Scheme 5 for molecular rods 18 and 19. Rod
and sodium hydroxide in excess, affording methylenedioxy 18, comprising a dimethyl-protected catechol subunit, was
derivative 4 (56% yield). Finally, the ethyl orthoformate in- obtained from 1,4-diiodo-2,3-dimethoxybenzene[58] and 1-
termediate 5 was isolated in 62% yield after condensing the acetylsulfanyl-4-ethynylbenzene[59,60] in 46% yield. Interest-
free diiodocatechol with triethyl orthoformate in refluxing ingly, 1,4-diiodo-2,3-dimethoxybenzene was previously ob-
toluene in the presence of Amberlyst 15 resin as the acidic tained as an intermediate in the synthesis of the key build-
catalyst.
ing block 2,3-dihydroxy-1,4-diiodobenzene.[58] Finally, the
The four protected molecular rods 14–17 were assembled orthoformate-protected diiodocatechol 5 and 1-tert-butyl-
by comparable synthetic sequences (Scheme 4). Protected sulfanyl-4-ethynylbenzene[9,30] provided molecular rod 19
diiodocatechol derivatives 2–5 readily reacted with trimeth- with more stable terminal tert-butylsulfanyl groups in 64%
ylsilylacetylene in classical Sonogashira reaction conditions yield. The increased stability of the sulfur protecting groups
Scheme 4. Synthesis of protected molecular rods 14–17 from the corresponding protected diiodocatechol derivatives 2–5.
Scheme 5. Synthesis of protected molecular rods 18 and 19 equipped with acetyl- and tert-butyl-protected sulfur groups, respectively.
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M. Mayor, T. Wandlowski, N. Tao et al.
FULL PAPER
Figure 1. Molecular structure of 18 (ORTEP, thermal ellipsoids set at the 40% probability level). Selected bond lengths [Å] and angles
[°]: C(1)–C(2) 1.495(4), C(2)–O(1) 1.203(4), C(2)–S(1) 1.788(4), S(1)–C(3) 1.771(3), C(27)–C(28) 1.510(5), C(27)–O(2) 1.191(4), C(27)–
S(2) 1.770(4), S(2)–C(24) 1.775(3), O(3)–C(16) 1.361(4), O(3)–C(17) 1.405(5), O(4)–C(15) 1.377(3), O(4)–C(18) 1.349(5); C(2)–S(1)–C(3)
101.62(16), C(24)–S(2)–C(27) 103.16(15).
of 19 is of particular interest for electrochemical investi- the evolution of the reaction which was easily monitored by
gations.
TLC. The molecular rod 1 was also obtained from di-
All the new compounds were fully characterized by con- methyl-protected precursor 18 (58% yield). In this case,
ventional analytical and spectroscopic techniques, that is, only CH2Cl2 was used as the solvent and the reaction was
1H and 13C NMR spectroscopy and mass spectrometry. performed at a lower temperature (–7 °C). The slightly
Furthermore, the purity of the compounds was confirmed lower yield may partially arise from simultaneous deprotec-
by elemental analysis. In order to obtain additional struc- tion of the acetylsulfanyl groups with BBr3, highlighting the
tural details, numerous attempts to grow single crystals importance of using acetylating conditions (AcCl in tolu-
from these molecular rods were made. Finally, single crys- ene).
tals of compound 18 suitable for X-ray analysis were ob-
Precursor 15 could not be deprotected by reaction with
tained. Molecular rod 18 crystallizes in the triclinic space BBr3 as a Lewis acid or by standard catalytic hydro-
[61]
¯
group P1 (Figure 1). The intramolecular sulfur-to-sulfur genolysis (H2, 10% Pd/C, Table 1). Compound 15, compris-
distance is 20.02(25) Å and the average C–C bond length of ing a diphenylmethylene ketal, promptly reacted in the pres-
the acetylene bridges is 1.201 Å. The two lateral phenyl ence of BBr3, but the desired product 1 could neither be
rings are twisted with respect to the central one with torsion detected nor isolated after chromatography. Interestingly,
angles of 60.79(6)° [C(12)–C(11)–C(6)–C(5)] and 43.63(6)° the diphenyl ketal was hydrolyzed in acidic conditions
[C(13)–C(14)–C(21)–C(22)].
(AcOH/EtOH/H2O mixture at reflux).[54,62] However, as ex-
pected in such an acidic and aqueous medium, the thio-
acetyl functions were also cleaved. Generating the free cate-
chol rod 1 from the methylenedioxy precursor 16 by the use
Deprotection of the Catechol Core
Catechol deprotection was first attempted with boron tri- of BBr3 or in acidic conditions (HCOOH or TFA) failed as
bromide (1  in CH2Cl2) and was achieved with precursors well. It is worth mentioning that the dibenzyl rod 14 was
14, 17 and 18, leading to the formation of the acetyl- also found to be exceptionally inert towards catalytic hydro-
sulfanyl-functionalized molecular rod 1 (Scheme 6 and genation or treatment with TFA.
Table 1). With the dibenzyl- and orthoformate-protected
The main drawback of the deprotection strategies so far
rods 14 and 17, protecting-group cleavage was conducted discussed are the harsh reaction conditions, reducing the
at room temperature in toluene containing acetyl chloride, variety of potential functional groups in future macromole-
ensuring in situ trapping of the sulfur end groups if ever cules comprising catechol subunits. Of particular interest
cleaved (70 and 77% yields, respectively). Furthermore, the was the search for alternative routes to achieving orthogo-
excess BBr3 was added in several portions, depending on nal deprotection of the catechol core. Finding mild and se-
Scheme 6. Synthesis of target molecular rod 1 from protected precursors 14, 17 and 18.
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Redox-Active Catechol-Functionalized Molecular Rods
Table 1. Attempts at deprotection of catechols 14–19.
Substrate
Protecting group
Bn
Conditions
Solvent
Temperature [°C]
Yield [%]
14
BBr3
H2, 10% Pd/C
TFA
BBr3
H2, 10% Pd/C
TFA
aqueous HCl
AcOH
BBr3
BBr3
HCOOH
TFA
BBr3
toluene, AcCl
CH2Cl2, MeOH
CH2Cl2, Ac2O
toluene, AcCl
THF
CH2Cl2
EtOH
EtOH, H2O
CH2Cl2
toluene, AcCl
CH2Cl2
r.t.
r.t.
r.t.
r.t.
r.t.
70
0[a]
0[a]
0[b]
0[a]
0[a]
0[a]
0[b]
0[c]
0[c]
0[a]
0[a]
77
15
–C(Ph)2–
r.t. to 45
100
115
–78 to r.t.
r.t.
r.t. to 45
r.t. to 45
r.t.
50
50
r.t. to 50
–7
16
17
–CH2–
CH2Cl2
–CH(OEt)–
toluene, AcCl
CH2Cl2, MeOH, H2O
CH2Cl2, MeOH, H2O
CH2Cl2, MeOH, H2O
CH2Cl2
AcOH
TFA
TsOH
BBr3
71
51
0[a]
58
18
19
Me
–CH(OEt)–
aqueous HCl
THF
r.t.
87
[a] No reaction. [b] Starting material consumed. [c] Starting material partially consumed.
lective conditions is crucial to expand this methodology to deprotection of the redox-active unit without affecting the
advanced architectures comprising a broader variety of acetylsulfanyl end groups. Finally, the tert-butylsulfanyl-ter-
chemical functions. Thus, 17 turned out to be a promising minated rod 19 permits the safe use of a stronger acid such
candidate owing to the more labile nature of the ethyl or- as aqueous HCl[54] to cleave the orthoformate moiety at
thoformate protecting group, as already highlighted by its room temp., leading to product 20 in 87% yield (Scheme 7
efficient cleavage with BBr3 in 77% yield (Table 1). Accord- and Table 1).
ing to the literature,[55,63,64] the orthoester was expected to
The BBr3/AcCl/toluene system previously used to cleave
be readily hydrolyzed at room temp. in the presence of a the dibenzyl rod 14 and orthoester 17 with efficiency (70–
catalytic amount of TsOH. Although the protected rod 17 77% yield) is also known to allow the trans-protection of
exhibited surprising inertness towards TsOH, free catechol S-tert-butyl groups into S-acetyl groups.[65–68] To our satis-
1 could be obtained in 71% yield by using acetic acid in a faction, concomitant removal of the ethyl orthoformate
boiling mixture of CH2Cl2, MeOH and H2O (Scheme 7 and protecting group and trans-protection of both terminal sul-
Table 1). TFA can also be used instead of AcOH (51% fur groups was successful with tert-butylsulfanyl orthoester
yield). Despite the presence of water in the reaction me- 19, giving the target acetylsulfanyl rod 1 in a good 79%
dium, these acidic conditions gently promoted the selective yield (Scheme 8).
Scheme 7. Acidic deprotection of acetylsulfanyl- and tert-butylsulfanyl-functionalized orthoesters 17 and 19, respectively.
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M. Mayor, T. Wandlowski, N. Tao et al.
FULL PAPER
Scheme 8. Simultaneous catechol deprotection and sulfur transprotection yielding acetylsulfanyl molecular rod 1 from tert-butylsulfanyl
orthoester 19.
Electrochemistry
In the case of rod 20 equipped with less labile tert-butyl-
sulfanyl end groups, the behaviour of the catechol subunit
is quite similar to that of 1, with an oxidation wave at
+0.19 V and a reduction wave at +0.09 V (Figure 2, B).
Furthermore, a secondary cathodic peak appears at around
–0.13 V. This feature is more pronounced at lower scan rates
and is also visible as a shoulder in the voltammogram of
catechol. This additional cathodic peak may be indicative
of the reduction of a newly formed species during the
course of the measurement. As already reported before,
electro-methoxylation of the catechol unit is prone to occur
by 1,4-addition of a solvent molecule to the ortho-quinone
form.[72–74] Orthoester 19 remains electrochemically inert in
the applied conditions.
In preliminary cyclic voltammetry experiments, the redox
activity of the catechol subunit integrated into the molecu-
lar rod was investigated. Thus, the electrochemical proper-
ties of the free and orthoformate-protected catechol rods
were investigated in methanol and in a 1:1 mixture of meth-
anol and dichloromethane, respectively, containing 0.15 
sodium acetate as the supporting electrolyte using a glassy
carbon electrode.
The acetylsulfanyl-terminated rod 1 reveals an anodic
wave at +0.23 V assigned to the oxidation of the catechol
core to the corresponding ortho-quinone (Figure 2, A). On
the reverse scan, a cathodic peak appears at +0.03 V upon
reduction of the newly formed ortho-quinone, leading to the
regeneration of the initial catechol. Thus, a quasi-reversible
redox process is found for the catechol/ortho-quinone cou-
Comparison of the electrochemical properties of the or-
ple. Interestingly, both the oxidation (0.21 V shift to nega- thoformate-protected precursors (17 and 19) with those of
tive potentials) and reduction (0.08 V shift to positive po- the corresponding free catechol rods (1 and 20, respectively)
tentials) of 1 are more favourable than for catechol. The demonstrates the deprotected catechol to be the origin of
electron-donating phenylethynyl groups decorating the cen- the observed redox activity (Figure 2A and B). Clearly, the
tral catechol unit in 1 most likely facilitate its oxi- integrated catechol subunit in these molecular rods provides
dation.[69–71] Owing to the orthoformate-protecting group a reversible two-electron two-proton process in a protic me-
masking the catechol moiety, 17 shows no redox activity in dium such as methanol even more easily than the parent
the same potential range.
unsubstituted catechol.
Figure 2. A) Cyclic voltammograms of 1 m solutions of catechol (black) and the acetylsulfanyl rod 1 (dashed) in MeOH, and of the
acetylsulfanyl orthoester 17 (grey) in MeOH/CH2Cl2 (1:1) at a glassy carbon electrode. B) Cyclic voltammograms of 1 m solutions of
catechol (black) and the tert-butylsulfanyl rod 20 (dashed) in MeOH, and of the tert-butylsulfanyl orthoester 19 (grey) in MeOH/CH2Cl2
(1:1) at a glassy carbon electrode. Supporting electrolyte: 0.15  NaOAc. Scan rate: 300 mVs–1.
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Redox-Active Catechol-Functionalized Molecular Rods
Molecular Conductance Measurements
To investigate the suitability of these molecular rods for
transport investigations, the molecular rod 18 comprising a
dimethyl-protected central catechol subunit was exposed to
preliminary conductance measurements. Therefore the rod
18 with terminal acetyl-protected sulfur anchor groups was
studied in gold–molecule–gold junctions by using the STM-
based stretching technique.[21,75,76] In conventional STM,
the electrical potential (bias) is applied between a conduc-
tive probe (tip) and a bare or modified substrate surface.
The tunnelling current, employed as a feedback signal, pro-
vides information on the surface topographic and electronic
properties. The stretching experiment involves the forma-
tion and breaking of metal–molecule–metal junctions be-
tween the STM tip and an adsorbate-covered substrate sur-
face.[19,20,75,76] The molecule under investigation bears thiol
anchor groups after in situ deprotection with a drop of
aqueous ammonia, enabling the formation of chemical
bonds to both metal electrodes. As the tip approaches the
adsorbate-modified substrate, molecular junctions are cre-
ated with a certain probability. Subsequently, the current of
the preformed metal–molecule–metal junctions is recorded
Figure 3. Current–distance retraction traces recorded for a 1 m
solution of 18 in mesitylene, Ebias = 0.1 V. The plateaux highlighted
by thick lines represent stable configurations of metal–molecule–
metal contacts. Plateaux corresponding to the highest conductance
regime (H) are shown in the main plot. The two insets illustrate
selected current traces in lower current ranges featuring plateaux
corresponding to configurations with middle (M) and low (L) con-
ductance junctions. Traces are displaced along the x axis for clarity.
Histograms constructed from experimental traces re-
upon retraction of the tip. Such current–distance traces are corded in the current range up to 100 nA reveal two pro-
not exponential; they often exhibit characteristic plateaux nounced and equally separated peaks with maxima around
and steps which are attributed to the breaking of individual 30 and 65 nA (Figure 4, A). These peaks are attributed to
or small groups of junctions. The statistical analysis of a the single and double junctions of a high conductance state.
large number of these retraction transients allows the evalu- The experimental data are represented by two Gaussian
ation of electrical properties of single-molecule junctions in curves. Their centres were taken as the positions of the
a certain environment.
corresponding peaks and full-width-at-half-maximum
Figure 3 shows a set of individual current–distance re- (FWHM) values were taken as their errors giving 30Ϯ5
traction curves for the dimethyl-protected rod 18 in dif- and 66Ϯ12 nA. The dependence of the peak current on the
ferent current ranges recorded at Ebias = 0.1 V. The main peak number is plotted in the inset. The average current
panel reveals features within the high current range. The through a high-conductance gold–molecule–gold junction
two insets illustrate details of the low current limits by em- was estimated as the slope of the linear fit with the intercept
ploying a magnified y axis. Three sets of well-resolved pla- fixed to zero and values weighted by error. The correspond-
teaux are observed in the current range studied ing current through the single junction and its conductance
(0ϽIՅ100 nA). The values of the corresponding junction were found to be 31Ϯ2 nA and 310Ϯ20 nS (here and else-
conductances are referred to as high (H), mid (M) and low where, the error in the slope corresponds to the confidence
(L) conductances. They vary over two orders of magnitude. limit of 95%, respectively). The initial part of the histogram
The experimentally measured currents approach zero in part A of Figure 4 exhibits a pronounced distribution tail
(Ͻ10 pA) after the low-current conductance plateau L.
if the bin width is chosen to be 2 nA. Magnification of this
A statistical analysis of the experimental current–dis- region (IϽ3 nA) and reduction of the bin width to 0.1 nA
tance traces was carried out to extract reliable values of the reveals an additional set of conductance peaks (Figure 4,
respective junction conductances. Plateaux in the current– B). These data were analyzed in the same way as described
distance traces were chosen as the criteria for the existence above for high-conductance junctions. The corresponding
of a molecular junction. Plateaux were selected manually mid-conductance junction is estimated to be 12Ϯ1 nS. Fur-
assuming a minimum length of 0.03 nm. The average length ther magnification of the current range (IϽ0.8 nA) and re-
was 0.14 nm. The probability of forming a molecular junc- duction of the bin width to 0.01 nA still reveals structured
tion is represented by 0.1–0.2 plateaux per curve.
histograms (Figure 4, C). However, the small number of
The extracted vertical positions of plateaux (e.g., cur- low-conductance junctions does not lead to well-developed
rents) were used to construct histograms (Figure 4). The x Gaussian-type peaks. A low-junction conductance of
axis represents the plateau currents grouped in equidistant 1.4Ϯ0.1 nS is obtained by grouping the maximum currents
intervals (bins). The y axis represents the number of pla- of the observed discrete features.
teaux weighted with respect to their length in order to em-
Three conductance values (1.4, 12 and 310 nS) were as-
phasize the contribution of the mechanically stable pla- signed to the gold–molecule–gold junctions formed in the
teaux. The histograms constructed without weighting have present experiments. The ratio between the numbers of
less pronounced but essentially the same features.
junctions exhibiting low (L), mid (M) and high (H) conduc-
Eur. J. Org. Chem. 2008, 136–149
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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143

M. Mayor, T. Wandlowski, N. Tao et al.
FULL PAPER
high-level quantum chemical simulations of the junction
conductance.[76] The gold–molecule–gold junction conduc-
tance of the dithiolated OPE lacking the two methoxy
groups was found to be 13 nS.[78] Values of the single-junc-
tion conductances for various derivatives were of the same
order of magnitude.[78,79] This value is close to the mid-
conductance M found in our experiments and reflects the
general trend that substituents only slightly affect the junc-
tion conductance as long as they do not possess low-lying
energy levels. Interestingly, high and low conductances of
rigid rod-like molecules have not been described in the lit-
erature before. STM-based conductance measurement of
single-molecule junctions of compound 18 and related
molecules are currently in progress in order to systemati-
cally study the effect of substituents on the conductance of
molecular junctions and to evaluate the electrochemically
triggered switching effect in OPEs with redox-active substit-
uents such as the free catechol rod 1.
Conclusions
The synthesis of a novel redox-active molecular rod de-
rived from an OPE skeleton comprising a catechol subunit
has been achieved. An ensemble of five protecting groups
for the catechol unit was investigated; three of them af-
forded the target acetylsulfanyl-functionalized rod 1. No-
tably, the ethyl orthoformate catechol protecting group
turned out to be cleavable by BBr3 as well as by mild acids
like AcOH. These mild deprotection conditions, which are
selective and take place readily, are paving the way to more
complex molecular architectures comprising the redox-
active catechol subunit. The residual redox activity of the
catechol subunit integrated into the molecular rod has been
demonstrated by cyclic voltammetry investigations.
Furthermore, preliminary single-molecule transport investi-
gations with the dimethyl-protected rod 18 have been pre-
sented. The molecular junctions were created mechanically
in a STM-based set-up and analyzed statistically. Prelimi-
nary measurements revealed a set of three conductance val-
ues varying over two orders of magnitude. The current hy-
pothesis is to attribute them to different binding geometries
of the anchoring groups to the electrodes, providing an un-
precedented insight into the molecule/electrode interface.
Currently we are synthesizing larger macrocyclic structures
of increasing complexity based on the catechol subunit
that will undergo redox-state-dependent conformational
changes.
Figure 4. Length-weighted histograms of the plateau currents (Ebias
= 0.1 V) for different current ranges and with sequentially decreas-
ing bin widths. The insets illustrate plots of the peak position vs.
peak number and the corresponding linear fits. A) 0–90 nA, bin
width 2 nA; B) 0–3 nA, bin width 0.1 nA; C) 0–0.75 nA, bin width
0.01 nA.
tance values for the presented data is 9:38:53. The H and
M junctions have rather similar formation probabilities. The
number of low-conductance junctions is significantly
smaller. Other experimental data support these observa-
tions as well. Metal–molecule–metal junctions exhibiting
multiple values of single-junction conductance have already
been found for a number of molecular systems.[23,76,77] Our
current hypothesis is to assign these three different conduc-
tance values to different contact geometries between the
molecule and the electrode. The estimated experimental ra-
tio for the probability of plateau formation may give a clue
Experimental Section
General Methods: Toluene and THF were dried by distillation over
sodium/benzophenone. CH2Cl2 was dried by distillation over
CaH2. iPr2NH and iPr2NEt were dried by distillation over KOH.
All other solvents and chemicals were of analytical grade and were
used as supplied without further purification. Flash chromatog-
1
raphy was performed on Fluka silica gel 60 (0.040–0.063 mm). H
(300 and 400 MHz) and 13C (75 and 100 MHz) NMR spectra were
to this point. However, this is a separate subject requiring recorded at room temperature with the residual proton resonances
144 Eur. J. Org. Chem. 2008, 136–149
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Redox-Active Catechol-Functionalized Molecular Rods
in deuteriated solvents used as the internal references. Chemical
shifts (δ) are reported in ppm. Mass spectra were obtained by elec-
tron-impact (EI) mass spectrometry. All mixtures of solvents are
given in v/v ratio. The STM stretching experiments were carried
out with a Molecular Imaging PicoSPM 2000 instrument. A thin
gold film prepared by vapour deposition on a glass sheet followed
by annealing in a hydrogen flame was used as the substrate. A cut
and uncoated gold wire (0.25 mm diameter) served as the tip. All
measurements were carried out on molecule 18 in a 1 mm solution
of mesitylene after deprotection of the thiol groups. The bias volt-
age Ebias = Etip – Esample = 0.1 V was used throughout the experi-
ment. The vertical movement of the tip was controlled by a dedi-
cated LabView program. The current–distance traces were recorded
by using a digital oscilloscope Yokogawa DL720 apparatus. The
approach was stopped after reaching physical contact between gold
tip and gold sample corresponding to a junction conductance of
several G0. The retraction rate of the tip varied between 25–
30 nms–1.
Amberlyst 15 (12 mg) was refluxed for 18 h. After cooling to room
temp., the solution was filtered through Celite and the solvents
evaporated. Flash chromatography (SiO2, hexane/CH2Cl2, 80:20)
afforded 5 (610 mg, 62%) as a white solid, m.p. 58–59 °C. Rf = 0.52
(SiO2, hexane/CH2Cl2, 50:50). 1H NMR (400 MHz, CDCl3): δ =
1.29 (t, JH,H = 7.0 Hz, 3 H), 3.76 (q, JH,H = 7.1 Hz, 2 H), 6.93
(s, 2 H), 6.95 (s, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 14.9,
60.1, 70.4, 117.8, 131.8, 146.9 ppm. C9H8I2O3 (417.97): calcd. C
25.86, H 1.93; found C 26.02, H 1.86. MS (EI): m/z (%) = 417.8
(92) [M]+, 372.8 (47) [M – C2H5O]+, 361.8 (100) [M – C3H6O +
2H]+.
3
3
2,3-Bis(benzyloxy)-1,4-bis(trimethylsilylethynyl)benzene (6): A solu-
tion of compound 2 (226 mg, 0.42 mmol), Pd(PPh3)2Cl2 (30 mg,
0.04 mmol) and CuI (10 mg, 0.05 mmol) in a mixture of dry THF
(10 mL) and dry iPr2NH (2 mL) was degassed with argon for
30 min. Trimethylsilylacetylene (135 µL, 0.93 mmol) was added and
the solution was stirred at room temp. for 65 h. The solution was
evaporated, CH2Cl2 and H2O were added, the organic phase was
separated and the aqueous phase was further extracted with
CH2Cl2. The combined organic layers were dried with MgSO4, fil-
tered and the solvents evaporated to dryness. Flash chromatog-
raphy (SiO2, hexane/CH2Cl2, 80:20) afforded 6 (195 mg, 97%) as
a yellow oil. Rf = 0.57 (SiO2, hexane/CH2Cl2, 50:50). 1H NMR
(400 MHz, CDCl3): δ = 0.27 (s, 18 H), 5.13 (s, 4 H), 7.17 (s, 2 H),
7.41–7.50 (m, 6 H), 7.59–7.64 (m, 4 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 0.0, 75.8, 100.7, 100.9, 119.7, 128.3, 128.5, 128.6(9),
128.7(1), 137.3, 153.7 ppm. C30H34O2Si2·0.3H2O (488.17): calcd. C
73.81, H 7.14; found C 73.94, H 7.06. MS (EI): m/z (%) = 482.3
(42) [M]+, 391.2 (9) [M – C7H7]+, 376.2 (9) [M – C7H7 – CH3]+,
361.1 (9) [M – C7H7 – 2CH3]+, 91.1 (100) [C7H7]+.
2,3-Bis(benzyloxy)-1,4-diiodobenzene (2): A mixture of 2,3-dihy-
droxy-1,4-diiodobenzene (670 mg, 1.85 mmol), K2CO3 (769 mg,
5.56 mmol), KI (62 mg, 0.37 mmol) and benzyl bromide (485 µL,
4.08 mmol) in acetone (30 mL) was refluxed for 16 h. Solvents were
evaporated, CH2Cl2 and H2O were added, the organic phase was
separated and the aqueous phase was further extracted with
CH2Cl2. The combined organic layers were dried with MgSO4, fil-
tered and the solvents evaporated to dryness. Flash chromatog-
raphy (SiO2, hexane/CH2Cl2, 90:10 to 80:20) afforded 2 (969 mg,
97%) as a white solid, m.p. 98–103 °C. Rf = 0.53 (SiO2, hexane/
CH2Cl2, 50:50). 1H NMR (400 MHz, CDCl3): δ = 5.07 (s, 4 H),
7.33 (s, 2 H), 7.37–7.46 (m, 6 H), 7.52–7.59 (m, 4 H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 75.3, 93.6, 128.5 (1), 128.5 (4), 129.0,
135.9, 136.3, 152.3 ppm. C20H16I2O2 (542.15): calcd. C 44.31, H
2.97; found C 43.39, H 3.00. MS (EI): m/z (%) = 541.9 (0.4) [M]+,
450.8 (4) [M – C7H7]+, 288.1 (3) [M – 2I]+, 181.1 (12) [M – 2I –
C7H7]+, 91.1 (100) [C7H7]+.
2,3-(Diphenylmethylenedioxy)-1,4-bis(trimethylsilylethynyl)benzene
(7): A solution of compound 3 (262 mg, 0.33 mmol), Pd(PPh3)2Cl2
(17 mg, 0.02 mmol) and CuI (9 mg, 0.05 mmol) in a mixture of dry
THF (12 mL) and dry iPr2NH (2 mL) was degassed with argon for
30 min. Trimethylsilylacetylene (160 µL, 1.10 mmol) was added and
the solution was stirred at room temp. for 3 h. The solution was
evaporated, CH2Cl2 and H2O were added, the organic phase was
separated and the aqueous phase was further extracted with
CH2Cl2. The combined organic layers were dried with MgSO4, fil-
tered and the solvents evaporated to dryness. Flash chromatog-
raphy (SiO2, hexane/CH2Cl2, 90:10) afforded 7 (225 mg, 97%) as a
white solid, m.p. 159–160 °C. Rf = 0.68 (SiO2, hexane/CH2Cl2,
1,4-Diiodo-2,3-(diphenylmethylenedioxy)benzene (3): A mixture of
2,3-dihydroxy-1,4-diiodobenzene (100 mg, 0.28 mmol) and di-
chlorodiphenylmethane (60 µL, 0.30 mmol) was heated at 180 °C
for 10 min. Flash chromatography (SiO2, hexane) afforded 3
(131 mg, 90%) as a white solid, m.p. 143–147 °C. Rf = 0.60 (SiO2,
1
hexane/CH2Cl2, 50:50). H NMR (400 MHz, CDCl3): δ = 6.91 (s,
2 H), 7.39–7.47 (m, 6 H), 7.63–7.68 (m, 4 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 71.1, 116.4, 126.5, 128.5, 129.6, 131.7,
139.2, 148.1 ppm. C19H12I2O2 (526.11): calcd. C 43.38, H 2.30;
found C 43.51, H 2.28. MS (EI): m/z (%) = 525.8 (83) [M]+, 448.8
(100) [M – C6H5]+, 398.9 (12) [M – I]+.
1
50:50). H NMR (400 MHz, CDCl3): δ = 0.31 (s, 18 H), 6.86 (s, 2
H), 7.39–7.47 (m, 6 H), 7.62–7.67 (m, 4 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 0.1, 98.9, 100.5, 105.2, 118.1, 125.1, 126.7,
128.4, 129.5, 139.7, 148.5 ppm. C29H30O2Si2 (466.72): calcd. C
74.63, H 6.48; found C 74.33, H 6.80. MS (EI): m/z (%) = 466.1
(89) [M]+, 451.0 (21) [M – CH3]+, 389.2 (100) [M – C6H5]+.
1,4-Diiodo-2,3-(methylenedioxy)benzene (4): A solution of 2,3-dihy-
droxy-1,4-diiodobenzene (923 mg, 2.55 mmol) and NaOH (675 mg,
16.9 mmol) in a mixture of dry CH2Cl2 (12 mL) and dry DMSO
(34 mL) was heated at 120 °C for 19 h. After cooling to room
temp., the solution was diluted with H2O and extracted with Ac-
OEt. The combined organic layers were dried with MgSO4, filtered
and the solvents evaporated to dryness. Flash chromatography
(SiO2, hexane/CH2Cl2, 100:0 to 95:5) afforded 4 (538 mg, 56%) as
a white solid, m.p. 137–139 °C. Rf = 0.54 (SiO2, hexane/CH2Cl2,
50:50). 1H NMR (400 MHz, CDCl3): δ = 6.07 (s, 2 H), 6.89 (s,
2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 70.8, 100.1, 131.8,
148.4 ppm. C7H4I2O2 (373.91): calcd. C 22.49, H 1.08; found C
22.75, H 1.17. MS (EI): m/z (%) = 373.8 (100) [M]+.
2,3-(Methylenedioxy)-1,4-bis(trimethylsilylethynyl)benzene (8): A
solution of compound 4 (506 mg, 1.35 mmol), Pd(PPh3)2Cl2
(47 mg, 0.07 mmol) and CuI (27 mg, 0.14 mmol) in a mixture of
dry THF (32 mL) and dry iPr2NH (6 mL) was degassed with argon
for 30 min. Trimethylsilylacetylene (435 µL, 2.99 mmol) was added
and the solution was stirred at room temp. for 17 h. The solution
was evaporated, CH2Cl2 and H2O were added, the organic phase
was separated and the aqueous phase was further extracted with
CH2Cl2. The combined organic layers were dried with MgSO4, fil-
tered and the solvents evaporated to dryness. Flash chromatog-
raphy (SiO2, hexane/CH2Cl2, 100:0 to 80:20) afforded 8 (412 mg,
97%) as a white solid, m.p. 137–139 °C. Rf = 0.61 (SiO2, hexane/
2,3-(Ethoxymethylenedioxy)-1,4-diiodobenzene (5): A solution of
2,3-dihydroxy-1,4-diiodobenzene (858 mg, 2.37 mmol) and triethyl
orthoformate (1.21 mL, 7.13 mmol) in toluene (25 mL) containing
1
CH2Cl2, 50:50). H NMR (400 MHz, CDCl3): δ = 0.25 (s, 18 H),
6.08 (s, 2 H), 6.82 (s, 2 H) ppm. 13C NMR (100 MHz, CDCl3):
Eur. J. Org. Chem. 2008, 136–149
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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145

M. Mayor, T. Wandlowski, N. Tao et al.
FULL PAPER
δ = 0.0, 98.3, 100.8, 102.1, 105.2, 125.1, 148.8 ppm. C17H22O2Si2
(314.53): calcd. C 64.92, H 7.05; found C 64.80, H 6.98. MS (EI):
m/z (%) = 314.1 (100) [M]+, 299.1 (25) [M – CH3]+.
raphy (SiO2, hexane/CH2Cl2, 80:20) afforded 12 (209 mg, 97%) as
a light-brown solid, m.p. 101–103 °C. Rf = 0.41 (SiO2, hexane/
CH2Cl2, 50:50). 1H NMR (400 MHz, CDCl3): δ = 3.34 (s, 2 H),
6.11 (s, 2 H), 6.86 (s, 2 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 77.3, 83.0, 102.3, 104.4, 125.1, 149.3 ppm. C11H6O2·0.1H2O
(171.97): calcd. C 76.83, H 3.63; found C 76.75, H 3.58. MS (EI):
m/z (%) = 170.1 (100) [M]+.
2,3-(Ethoxymethylenedioxy)-1,4-bis(trimethylsilylethynyl)benzene
(9): A solution of compound 5 (485 mg, 1.16 mmol), Pd(PPh3)2Cl2
(41 mg, 0.06 mmol) and CuI (22 mg, 0.12 mmol) in a mixture of
dry THF (28 mL) and dry iPr2NH (5 mL) was degassed with argon
for 30 min. Trimethylsilylacetylene (370 µL, 2.55 mmol) was added
and the solution was stirred at room temp. for 3 h. The solution
was evaporated, CH2Cl2 and H2O were added, the organic phase
was separated and the aqueous phase was further extracted with
CH2Cl2. The combined organic layers were dried with MgSO4, fil-
tered and the solvents evaporated to dryness. Flash chromatog-
raphy (SiO2, hexane/AcOEt, 95:5) afforded 9 (415 mg, 100%) as a
yellowish solid, m.p. 118–121 °C. Rf = 0.58 (SiO2, hexane/CH2Cl2,
50:50). 1H NMR (400 MHz, CDCl3): δ = 0.25 (s, 18 H), 1.29 (t,
2,3-(Ethoxymethylenedioxy)-1,4-diethynylbenzene (13): K2CO3
(969 mg, 6.94 mmol) was added to a solution of compound 9
(415 mg, 1.16 mmol) in a degassed mixture of CH2Cl2 (25 mL) and
MeOH (25 mL). After stirring at room temp. for 30 min, the sol-
vents were evaporated. CH2Cl2 and H2O were added, the organic
phase was separated and the aqueous phase was further extracted
with CH2Cl2. The combined organic layers were dried with MgSO4,
filtered and the solvents evaporated to dryness. Flash chromatog-
raphy (SiO2, hexane/AcOEt, 90:10) afforded 13 (240 mg, 97%) as
3
3JH,H = 7.0 Hz, 3 H), 3.76 (q, JH,H = 7.2 Hz, 2 H), 6.85 (s, 2 H),
1
a yellowish oil. Rf = 0.53 (SiO2, hexane/CH2Cl2, 50:50). H NMR
6.97 (s, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 0.0, 14.9,
59.8, 98.2, 100.9, 104.9, 119.8, 125.2, 147.1 ppm. C19H26O3Si2
(358.58): calcd. C 63.64, H 7.31; found C 63.64, H 7.37. MS (EI):
m/z (%) = 358.1 (81) [M]+, 313.1 (25) [M – C2H5O]+, 287.1 (28)
[M – C3H9Si + 2H]+, 73.3 (100) [C3H9Si]+.
(400 MHz, CDCl3): δ = 1.28 (t, 3JH,H = 7.2 Hz, 3 H), 3.35 (s, 2 H),
3
3.77 (q, JH,H = 7.1 Hz, 2 H), 6.90 (s, 2 H), 6.99 (s, 1 H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 14.8, 59.9, 77.16, 83.2, 104.1, 120.0,
125.2, 147.6 ppm. C13H10O3 (214.22): calcd. C 72.89, H 4.70; found
C 72.79, H 4.74. MS (EI): m/z (%) = 214.1 (75) [M]+, 169.1 (100)
[M – C2H5O]+, 158.1 (51) [M – C3H6O + 2H]+.
2,3-Bis(benzyloxy)-1,4-diethynylbenzene (10): K2CO3 (333 mg,
2.41 mmol) was added to a solution of compound 6 (194 mg,
0.40 mmol) in a degassed mixture of CH2Cl2 (30 mL) and MeOH
(30 mL). After stirring at room temp. for 17 h, the solvents were
evaporated. CH2Cl2 and H2O were added, the organic phase was
separated and the aqueous phase was further extracted with
CH2Cl2. The combined organic layers were dried with MgSO4, fil-
tered and the solvents evaporated to dryness. Flash chromatog-
raphy (SiO2, hexane/CH2Cl2, 50:50) afforded 10 (133 mg, 98%) as
a yellow oil. Rf = 0.38 (SiO2, hexane/CH2Cl2, 50:50). 1H NMR
(400 MHz, CDCl3): δ = 3.42 (s, 2 H), 5.18 (s, 4 H), 7.22 (s, 2 H),
7.34–7.44 (m, 6 H), 7.50–7.57 (m, 4 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 77.7, 83.0, 104.4, 118.6, 125.2, 126.6, 128.4, 129.6,
139.3, 149.0 ppm. C24H18O2·0.2H2O (342.00): calcd. C 84.29, H
5.42; found C 84.46, H 5.69. MS (EI): m/z (%) = 338.1 (3) [M]+,
247.1 (13) [M – C7H7]+, 91.1 (100) [C7H7]+.
1,4-Bis[(4-acetylsulfanylphenyl)ethynyl]-2,3-bis(benzyloxy)benzene
(14): A solution of compound 10 (133 mg, 0.39 mmol) and 1-ace-
tylsulfanyl-4-iodobenzene (240 mg, 0.86 mmol) in a mixture of dry
THF (12 mL) and dry iPr2NH (2 mL) was degassed with argon for
30 min. Pd(PPh3)2Cl2 (28 mg, 0.04 mmol) and CuI (8 mg,
0.04 mmol) were added and the solution was stirred at room temp.
for 21 h. The solution was evaporated, CH2Cl2 and H2O were
added, the organic phase was separated and the aqueous phase was
further extracted with CH2Cl2. The combined organic layers were
dried with MgSO4, filtered and the solvents evaporated to dryness.
Flash chromatography (SiO2, hexane/CH2Cl2, 50:50 to 30:70) af-
forded 14 (141 mg, 56%) as a yellow solid, m.p. 148–150 °C. Rf =
1
0.54 (SiO2, CH2Cl2). H NMR (400 MHz, CDCl3): δ = 2.45 (s, 6
3
H), 5.22 (s, 4 H), 7.27 (s, 2 H), 7.33–7.39 (m, 6 H), 7.40 (d, JH,H
3
= 8.4 Hz, 4 H), 7.49 (d, JH,H = 8.4 Hz, 4 H), 7.51–7.57 (m, 4
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 30.4, 76.1, 87.4, 94.7,
119.6, 124.4, 128.4, 128.5, 128.5(8), 128.6(4), 128.9, 132.3, 134.3,
137.1, 153.5, 193.5 ppm. C40H30O4S2 (638.79): calcd. C 75.21, H
4.73; found C 75.23, H 4.72. MS (EI): m/z (%) = 638.2 (5) [M]+,
596.2 (18) [M – C2H3O + H]+, 554.1 (58) [M – 2C2H3O + 2H]+,
505.1 (9) [M – C2H3O – C7H7 + H]+, 463.1 (23) [M – 2C2H3O –
C7H7 + 2H]+, 91.1 (100) [C7H7]+.
1,4-Diethynyl-2,3-(diphenylmethylenedioxy)benzene (11): K2CO3
(407 mg, 2.92 mmol) was added to a solution of compound 7
(227 mg, 0.49 mmol) in a degassed mixture of CH2Cl2 (10 mL) and
MeOH (10 mL). After stirring at room temp. for 30 min, the sol-
vents were evaporated. CH2Cl2 and H2O were added, the organic
phase was separated and the aqueous phase was further extracted
with CH2Cl2. The combined organic layers were dried with MgSO4,
filtered and the solvents evaporated to dryness. Flash chromatog-
raphy (SiO2, hexane/CH2Cl2, 80:20) afforded 11 (150 mg, 96%) as
a white solid, m.p. 134–135 °C. Rf = 0.50 (SiO2, hexane/CH2Cl2,
50:50). 1H NMR (400 MHz, CDCl3): δ = 3.38 (s, 2 H), 6.90 (s, 2 H),
7.36–7.46 (m, 6 H), 7.60–7.69 (m, 4 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 77.7, 83.0, 104.4, 118.6, 125.2, 126.6, 128.4, 129.6,
139.3, 149.0 ppm. C23H14O2·0.1H2O (324.16): calcd. C 85.22, H
4.42; found C 85.23, H 4.51. MS (EI): m/z (%) = 322.0 (48) [M]+,
245.0 (100) [M – C6H5]+.
1,4-Bis[(4-acetylsulfanylphenyl)ethynyl]-2,3-(diphenylmethylenedi-
oxy)benzene (15): A solution of compound 11 (66 mg, 0.20 mmol)
and 1-acetylsulfanyl-4-iodobenzene (125 mg, 0.45 mmol) in a mix-
ture of dry THF (5 mL) and dry iPr2NH (1 mL) was degassed with
argon for 30 min. Pd(PPh3)2Cl2 (7 mg, 0.01 mmol) and CuI (4 mg,
0.02 mmol) were added and the solution was stirred at room temp.
for 68 h. The solution was evaporated, CH2Cl2 and H2O were
added, the organic phase was separated and the aqueous phase was
further extracted with CH2Cl2. The combined organic layers were
1,4-Diethynyl-2,3-(methylenedioxy)benzene (12): K2CO3 (1.07 g, dried with MgSO4, filtered and the solvents evaporated to dryness.
7.63 mmol) was added to a solution of compound 8 (400 mg,
Flash chromatography (SiO2, hexane/CH2Cl2, 50:50 to 40:60) af-
1.27 mmol) in a degassed mixture of CH2Cl2 (90 mL) and MeOH
(90 mL). After stirring at room temp. for 1.5 h, the solvents were 0.64 (SiO2, CH2Cl2). H NMR (400 MHz, CDCl3): δ = 2.45 (s, 6
evaporated. CH2Cl2 and H2O were added, the organic phase was
separated and the aqueous phase was further extracted with
CH2Cl2. The combined organic layers were dried with MgSO4, fil-
tered and the solvents evaporated to dryness. Flash chromatog-
forded 15 (73 mg, 57%) as a yellow solid, m.p. 166–168 °C. Rf =
1
3
H), 6.96 (s, 2 H), 7.38–7.47 (m, 10 H), 7.62 (d, JH,H = 7.6 Hz, 4
H), 7.65–7.71 (m, 4 H) ppm. 13C NMR (100 MHz, CDCl3): δ =
30.4, 85.3, 94.2, 105.1, 118.4, 124.3, 124.8, 126.6, 128.4, 128.5,
129.5, 132.4, 134.3, 139.5, 148.3, 193.5 ppm. C39H26O4S2 (622.75):
146
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 136–149

Redox-Active Catechol-Functionalized Molecular Rods
calcd. C 75.22, H 4.21; found C 75.38, H 4.17. MS (EI): m/z (%)
1,4-Bis[(4-tert-butylsulfanylphenyl)ethynyl]-2,3-(ethoxymethylene-
= 622.2 (28) [M]+, 580.1 (47) [M – C2H3O]+, 538.1 (100) [M – dioxy)benzene (19): A solution of compound 5 (578 mg, 1.38 mmol)
2C2H3O]+, 461.1 (33) [M – 2C2H3O – C6H5]+.
and 1-tert-butylsulfanyl-4-ethynylbenzene (581 mg, 3.05 mmol) in a
mixture of dry THF (33 mL) and dry iPr2NH (6 mL) was degassed
with argon for 30 min. Pd(PPh3)2Cl2 (48 mg, 0.07 mmol) and CuI
(26 mg, 0.14 mmol) were added and the solution was stirred at
room temp. for 19 h. The solution was evaporated, CH2Cl2 and
H2O were added, the organic phase was separated and the aqueous
phase was further extracted with CH2Cl2. The combined organic
layers were dried with MgSO4, filtered and the solvents evaporated
to dryness. Flash chromatography (SiO2, hexane/CH2Cl2, 100:0 to
60:40) afforded 19 (367 mg, 64 %) as a yellow solid, m.p. 131–
1,4-Bis[(4-acetylsulfanylphenyl)ethynyl]-2,3-(methylenedioxy)-
benzene (16): A solution of compound 12 (103 mg, 0.61 mmol) and
1-acetylsulfanyl-4-iodobenzene (370 mg, 1.33 mmol) in a mixture
of dry THF (7 mL) and dry iPr2NH (1 mL) was degassed with
argon for 30 min. Pd(PPh3)2Cl2 (28 mg, 0.02 mmol) and CuI (9 mg,
0.05 mmol) were added and the solution was stirred at room temp.
for 19 h. The solution was evaporated, CH2Cl2 and H2O were
added, the organic phase was separated and the aqueous phase was
further extracted with CH2Cl2. The combined organic layers were
dried with MgSO4, filtered and the solvents evaporated to dryness.
The crude product was purified by flash chromatography (SiO2,
hexane/CH2Cl2, 50:50 to 40:60). Crystallization from a CHCl3/hex-
ane mixture afforded 16 (155 mg, 54%) as a yellow solid, m.p. 171–
1
134 °C. Rf = 0.75 (SiO2, CH2Cl2). H NMR (400 MHz, CDCl3): δ
3
= 1.26–1.33 (m, 21 H), 3.81 (q, JH,H = 7.1 Hz, 2 H), 6.98 (s, 2 H),
7.03 (s, 1 H), 7.51 (s, 8 H) ppm. 13C NMR (100 MHz, CDCl3): δ
= 14.9, 31.1, 46.6, 59.8, 84.6, 94.7, 104.8, 119.8, 123.1, 124.8, 131.7,
133.9, 137.3, 146.9 ppm. C33H34O3S2 (542.75): calcd. C 73.03, H
6.31; found C 72.76, H 6.41. MS (EI): m/z (%) = 542.1 (59) [M]+,
497.1 (9) [M – C2H5O]+, 486.0 (13) [M – C4H9 + H]+, 430.0 (100)
[M – 2C4H9 + 2H]+.
1
172 °C. Rf = 0.50 (SiO2, CH2Cl2). H NMR (400 MHz, CDCl3): δ
3
= 2.43 (s, 6 H), 6.15 (s, 2 H), 6.95 (s, 2 H), 7.40 (d, JH,H = 8.8 Hz,
3
4 H), 7.57 (d, JH,H = 8.4 Hz, 4 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 30.4, 84.4, 94.3, 102.2, 105.1, 124.0, 124.9, 128.7,
132.4, 134.3, 148.6, 193.5 ppm. C27H18O4S2 (470.56): calcd. C
68.92, H 3.86; found C 68.71, H 3.97. MS (EI): m/z (%) = 470.0
(55) [M]+, 427.9 (39) [M – C2H3O + H]+, 386.1 (100) [M – 2C2H3O
+ 2H]+.
1,4-Bis[(4-acetylsulfanylphenyl)ethynyl]-2,3-dihydroxybenzene (1):
M.p. 193–196 °C. Rf = 0.25 (SiO2, CH2Cl2/MeOH, 99:1). 1H NMR
(400 MHz, CDCl3): δ = 2.44 (s, 6 H), 5.80 (br. s, 2 H), 7.00 (s, 2
3
3
H), 7.42 (d, JH,H = 8.8 Hz, 4 H), 7.58 (d, JH,H = 8.4 Hz, 4
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 30.4, 85.3, 96.1, 110.7,
1 2 3 . 7 , 1 2 3 . 8 , 1 2 8 . 9 , 1 3 2 . 3 , 1 3 4 . 4 , 1 4 4 . 6 , 1 9 3 . 5 pp m.
1,4-Bis[(4-acetylsulfanylphenyl)ethynyl]-2,3-(ethoxymethylenedi-
oxy)benzene (17): A solution of compound 13 (240 mg, 1.12 mmol)
and 1-acetylsulfanyl-4-iodobenzene (685 mg, 2.46 mmol) in a mix-
ture of dry THF (28 mL) and dry iPr2NH (5 mL) was degassed
with argon for 30 min. Pd(PPh3)2Cl2 (39 mg, 0.06 mmol) and CuI
(21 mg, 0.11 mmol) were added and the solution was stirred at
room temp. for 5 h. The solution was evaporated, CH2Cl2 and H2O
were added, the organic phase was separated and the aqueous
phase was further extracted with CH2Cl2. The combined organic
layers were dried with MgSO4, filtered and the solvents evaporated
to dryness. Flash chromatography (SiO2, hexane/CH2Cl2, 50:50 to
20:80) afforded 17 (367 mg, 64 %) as a yellow solid, m.p. 169–
C
26H18O4S2·0.6H2O (469.36): calcd. C 66.53, H 4.12; found C
66.44, H 3.99. MS (EI): m/z (%) = 458.1 (25) [M]+, 416.1 (41) [M –
C2H3O + H]+, 374.1 (100) [M – 2C2H3O + 2H]+, 342.1 (25) [M –
C2H3O – C2H3OS + 2H]+.
Synthesis of 1 from 14: A solution of compound 14 (92 mg,
0.14 mmol) in a mixture of dry toluene (24 mL) and acetyl chloride
(6 mL) was stirred at room temp. and then BBr3 (1  in CH2Cl2,
290 µL, 0.29 mmol) was added. More BBr3 (1  in CH2Cl2, 290 µL,
0.29 mmol) was added after 30 min and 1 h. The solution was co-
oled to 0 °C, H2O was added, the organic phase was separated
and the aqueous phase was further extracted with CH2Cl2. The
combined organic layers were dried with MgSO4, filtered and the
solvents evaporated to dryness. Flash chromatography (SiO2,
CH2Cl2/MeOH, 100:0 to 99:1) followed by filtration through SiO2
with CHCl3 afforded 1 (46 mg, 70%) as a yellow solid.
1
171 °C. Rf = 0.55 (SiO2, CH2Cl2). H NMR (400 MHz, CDCl3): δ
3
3
= 1.32 (t, JH,H = 7.2 Hz, 3 H), 2.43 (s, 6 H), 3.83 (q, JH,H
=
3
7.1 Hz, 2 H), 6.99 (s, 2 H), 7.04 (s, 1 H), 7.40 (d, JH,H = 7.6 Hz,
4 H), 7.58 (d, JH,H = 8.4 Hz, 4 H) ppm. 13C NMR (100 MHz,
3
CDCl3): δ = 14.9, 30.4, 59.9, 84.8, 94.5, 104.8, 119.9, 124.0, 124.9,
128.7, 132.4, 134.3, 146.9, 193.3 ppm. C29H22O5S2 (514.61): calcd.
C 67.68, H 4.31; found C 67.53, H 4.47. MS (EI): m/z (%) = 514.1
(100) [M]+, 472.1 (83) [M – C2H3O + H]+, 430.1 (66) [M – 2C2H3O
+ 2H]+, 374.0 (34) [M – 2C2H3O – C3H6O + 4H]+.
Synthesis of 1 from 17 (BBr3 Deprotection): A solution of com-
pound 17 (66 mg, 0.13 mmol) in a mixture of dry toluene (21 mL)
and acetyl chloride (5 mL) was stirred at room temp. and then BBr3
(1  in CH2Cl2, 290 µL, 0.29 mmol) was added. More BBr3 (1  in
CH2Cl2, 290 µL, 0.29 mmol) was added after 30 min. The solution
was cooled to 0 °C, H2O was added, the organic phase was sepa-
rated and the aqueous phase was further extracted with CH2Cl2.
The combined organic layers were dried with MgSO4, filtered and
the solvents evaporated to dryness. Flash chromatography (SiO2,
CH2Cl2/MeOH, 100:0 to 99:1) afforded 1 (45 mg, 77%) as a yellow
solid.
1,4-Bis[(4-acetylsulfanylphenyl)ethynyl]-2,3-dimethoxybenzene (18):
1,4-Diiodo-2,3-dimethoxybenzene (800 mg, 2.05 mmol), Pd-
(PPh3)2Cl2 (88 mg, 0.13 mmol) and CuI (48 mg, 0.25 mmol) were
dissolved in dry and degassed THF (12 mL). Dry and degassed
iPr2NEt (6 mL) was added, followed by addition of 1-acetylsul-
fanyl-4-ethynylbenzene (796 mg, 4.52 mmol). The reaction mixture
was stirred at room temp. for 16 h, then H2O was added and the
product was extracted with CH2Cl2. The combined organic layers
were dried with MgSO4, filtered and the solvents evaporated to
dryness. Flash chromatography (SiO2, toluene/AcOEt, 95:5) af-
forded 18 (460 mg, 46%) as a yellow solid, m.p. 128–129 °C. Rf =
Synthesis of 1 from 17 (AcOH Deprotection): A solution of com-
pound 17 (44 mg, 0.09 mmol) and AcOH (1 mL) in a mixture of
CH2Cl2 (8 mL), MeOH (10 mL) and H2O (0.25 mL) was refluxed
for a week. After evaporation of the solvents, flash chromatography
(SiO2, CH2Cl2/MeOH, 100:0 to 99:1) afforded 1 (28 mg, 71%) as
a yellow solid.
1
0.50 (SiO2, CH2Cl2). H NMR (300 MHz, CDCl3): δ = 2.45 (s, 6
3
H), 4.04 (s, 6 H), 7.21 (s, 2 H), 7.43 (d, JH,H = 8.1 Hz, 4 H), 7.58
3
(d, JH,H = 8.1 Hz, 4 H) ppm. 13C NMR (75 MHz, CDCl3): δ =
30.4, 61.5, 86.9, 94.5, 119.0, 124.4, 128.1, 128.5, 132.3, 134.3, 154.3,
193.5 ppm. C28H22O4S2 (486.60): calcd. C 69.11, H 4.56; found C
68.81, H 4.39. MS (EI): m/z (%) = 486.1 (70) [M]+, 444.0 (53) [M –
C2H3O + H]+, 402.0 (100) [M – 2C2H3O + 2H]+.
Synthesis of 1 from 17 (TFA Deprotection): A solution of com-
pound 17 (94 mg, 0.18 mmol) and TFA (14 µL, 0.19 mmol) in a
mixture of CH2Cl2 (10 mL), MeOH (9.5 mL) and H2O (0.5 mL)
was refluxed for a week. After evaporation of the solvents, flash
Eur. J. Org. Chem. 2008, 136–149
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
147

M. Mayor, T. Wandlowski, N. Tao et al.
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Synthesis of 1 from 18: Compound 18 (206 mg, 0.42 mmol) was
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oled to 0 °C, H2O was added, the organic phase was separated
and the aqueous phase was further extracted with CH2Cl2. The
combined organic layers were dried with MgSO4, filtered and the
solvents evaporated to dryness. Flash chromatography (SiO2,
CH2Cl2/MeOH, 100:0 to 99:1) followed by filtration through SiO2
with CHCl3 afforded 1 (134 mg, 79%) as a yellow solid.
1,4-Bis[(4-tert-butylsulfanylphenyl)ethynyl]-2,3-dihydroxybenzene
(20): A solution of compound 19 (106 mg, 0.20 mmol) in a mixture
of THF (9.5 mL) and aqueous 37% HCl (0.5 mL) was stirred at
room temp. for 19 h. The solution was evaporated, CH2Cl2 and
H2O were added, the organic phase was separated and the aqueous
phase was further extracted with CH2Cl2. The combined organic
layers were dried with MgSO4, filtered and the solvents evaporated
to dryness. Flash chromatography (SiO2, CH2Cl2) afforded 20
(83 mg, 87%) as a yellow solid, m.p. 198–201 °C. Rf = 0.31 (SiO2,
CH2Cl2). 1H NMR (400 MHz, CDCl3): δ = 1.31 (s, 18 H), 5.78 (br.
s, 2 H), 7.00 (s, 2 H), 7.48–7.56 (m, 8 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 31.1, 46.8, 85.0, 96.4, 110.8, 122.9, 123.6, 131.7, 134.2,
137.4, 144.4 ppm. C30H30O2S2·0.3H2O (492.09): calcd. C 73.22, H
6.27; found C 73.20, H 6.28. MS (EI): m/z (%) = 486.1 (39) [M]+,
430.1 (7) [M – C4H9 + H]+, 374.0 (100) [M – 2C4H9 + 2H]+.
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[61] Crystal data for 18: C28H22O4S2, a = 915.15(18), b = 1266.3(3),
c = 1315.2(3) pm, α = 108.78(3), β = 106.98(3), γ = 105.74(3)°,
¯
V
=
1262.7(4)ϫ106 pm3; triclinic P1,
Z
=
2, ρcalcd.
=
1.2803 gcm–1, µ(Mo-Kα) = 0.242 mm–1, STOE IPDS2, Mo-Kα
radiation, λ = 0.71073 Å, T = 200 K, 2θmax = 52°, 5573 reflec-
tions measured, 3988 independent reflections (Rint = 0.0323),
2268 independent reflections with Fo Ͼ 4σ(Fo). The structure
Received: August 31, 2007
Published Online: November 21, 2007
Eur. J. Org. Chem. 2008, 136–149
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
149