Thiolated π-extended tetrathiafulvalenes: Versatile multifunctional π-systems

Article abstract of DOI:10.1021/jo062199p

(Chemical Equation Presented) Derivatives of 9,10-bis(1,3-dithiol-2- ylidene)-9,10-dihydroanthracene (ex-TTF) have been synthesized by a new synthetic methodology, viz., direct phosphite-mediated cross-couplings of anthraquinone with 1,3-dithiole-2-thione

Full text of DOI:10.1021/jo062199p

Thiolated π-Extended Tetrathiafulvalenes: Versatile Multifunctional
π-Systems
Christian A. Christensen, Andrei S. Batsanov, and Martin R. Bryce*
Department of Chemistry, Durham UniVersity, Durham DH1 3LE, United Kingdom
m.r.bryce@durham.ac.uk
ReceiVed October 23, 2006
Derivatives of 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (ex-TTF) have been synthesized
by a new synthetic methodology, viz., direct phosphite-mediated cross-couplings of anthraquinone with
1,3-dithiole-2-thione derivatives. These ex-TTFs bear one, two, or four cyanoethyl-protected thiol groups
on the dithiole rings. Deprotection (NaOMe, MeOH, DMF, 20 °C) and trapping of the transient thiolates
with electrophiles have afforded the new ex-TTF trimer 19, dimeric cyclophanes 22 and 25, the tetrakis-
(hydroxyethylthio) derivative 23, and the strained cyclophane 24. Solution redox properties have been
studied by cyclic voltammetry. For compounds 19, 22, and 25 each ex-TTF unit behaves as an independent
2-electron redox system giving rise to a single, quasi-reversible 6-, 4-, and 4-electron wave, respectively.
The Eox value for 24 (0 f 2+ wave) is positively shifted by 290 mV compared to that of its precursor
15 due to the short bridge in 24 obstructing the conformational change which accompanies oxidation.
X-ray crystal structures of 23‚2.5MeOH, 23‚1.5MeCN, 24‚CH2Cl2, and 24‚1.5CH2Cl2 show the saddle-
shape folding (typical of ex-TTF derivatives), which in 24 is enhanced by the pentamethylene chain
bridging the dithiole units. Both solvates of 23 show an unprecedented crystal packing motif due to
hydrogen bonding.
Introduction
solution electrochemical experiments, the ex-TTF system 1
(Chart 1) displays inverted potentials (E1ox > E2ox)4 manifested
as a single, 2-electron oxidation process,5 D0 f D2+, with Eox
) 0.30-0.60 V vs Ag/AgCl, depending upon the solvent and
the substituent R.6 This redox process, which is electrochemi-
cally quasi-reversible and chemically reversible, triggers a
dramatic conformational change in the molecule.7 X-ray crystal-
lographic studies established that the neutral molecule 1b is
Tetrathiafulvalene (TTF) derivatives which possess extended
π-electron conjugation between the two dithiole rings are
important as building blocks for supramolecular and organic
materials chemistry.1 The incorporation of a central vinylogous2
or p-quinodimethane spacer3 leads to reduced on-site Coulombic
repulsion and stabilization of the oxidized states, compared to
those of the parent TTF. As an example of the latter family,
9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (1a) (herein
abbreviated as “ex-TTF”) provides a unique combination of
redox, charge delocalization, and structural properties. Whereas
TTF undergoes two, reversible, 1-electron oxidation waves in
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10.1021/jo062199p CCC: $37.00 © 2007 American Chemical Society
Published on Web 01/24/2007
J. Org. Chem. 2007, 72, 1301-1308
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Christensen et al.
CHART 1
SCHEME 1a
butterfly shaped with the anthracenediylidene unit strongly
folded as a result of peri-interactions between the sulfur atoms
and the hydrogen atoms on the benzoannulated quinoid moiety.
In contrast, the dication (1b2+) possesses a planar and aromatic
anthracene unit, with the heteroaromatic 1,3-dithiolium cations
almost orthogonal to this plane.8 These unusual redox and
structural properties have been explained by theoretical studies
of the minimum energy conformations and charge distribution
in the neutral, radical cation, and dication states of 1.9 The
transient radical cation of system 1 has been detected in flash
photolysis10 and pulse radiolysis11 experiments. This remarkable
interplay of electron donor properties and conformational change
has led to derivatives of 1 being used as building blocks for
charge-transfer salts,12 nonlinear optical materials,13 multistage
redox assemblies,14 dimeric ex-TTFs,15 dendrimers,16 cyclo-
a Reagents and conditions: (i) 3, LDA or BuLi, THF, -78 °C.
phanes,17 cation sensors,18 self-assembled monolayers,19 and
donor-acceptor arrays, some of which possess long-lived
charge-separated excited states.20
Given this wide range of applications, the development of
methodology leading to new derivatives of 1 is clearly important.
To date two complementary routes to derivatives of 1 func-
tionalized at the dithiole units have been reported: (1) Horner-
Wadsworth-Emmons olefination of anthraquinone 2 with
prefunctionalized (1,3-dithiol-2-yl)phosphonate ester reagents
3 (Scheme 1)21 or (2) lithiation of ex-TTF derivatives bearing
a hydrogen on one, or both, of the dithiole rings and trapping
with electrophiles, e.g., reactions of 7 to yield 8 (Scheme 2).22
(3) (a) Mart´ın, N.; Ort´ı, E. In Handbook of AdVanced Electronic Photonic
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1302 J. Org. Chem., Vol. 72, No. 4, 2007

Thiolated π-Extended TetrathiafulValenes
SCHEME 2a
comparable to the literature yield of 10 from anthrone and the
corresponding dithiolium salt.25 In the second cross-coupling
reaction 3.3 equiv of Becher’s reagent 1126 was slowly added
to 10 in neat P(OEt)3 at 130 °C. The slow addition minimized
self-coupling of 11, and the ex-TTF system 12 was obtained in
73% yield (Scheme 3).
Following this protocol, 2-fold reactions of anthraquinone (2)
and the thione reagents 11 and 14 gave the protected tetrathi-
olated and dithiolated ex-TTF derivatives 13 and 15 in 60%
and 47% yields, respectively (Scheme 4). The lower yield for
the latter compound was due to a recrystallization needed to
separate 15 from the self-coupled TTF derivative. The synthetic
versatility of these protected thiolate derivatives 12, 13, and 15
has been established in the series of reactions shown in Schemes
5-7.
a Reagents and conditions: (i) 5, pyridine, MeCO2H, reflux; (ii) 6, LDA
or BuLi, THF, -78 °C; (iii) LDA, THF, -78 °C, then electrophile+.
(Note that the preparation of compound 7 required a phospho-
nate reagent, 6, for the second step, like route 1.)
Following the precedents of TTF chemistry,26 the mono-
deprotection of 12 was readily achieved by using 1 equiv of
sodium methoxide: trapping of the intermediate thiolate anion
16 with methyl iodide gave compound 17 in 96% yield. The
analogous deprotection of 17 and trapping with the trifunctional
alkylating reagent 18 gave the ex-TTF trimer 19 in 97% yield.
Compound 12 served as the precursor to a new dimeric ex-
TTF cyclophane, 22, by using the difunctional alkylating reagent
20, which was chosen because of its very reactive iodides and
the tert-butyl groups, which should enhance the solubility of
the products (Scheme 5). (The dibromo analogue of 20 has been
used previously in the synthesis of TTF cyclophanes.27) Two,
stepwise deprotections and alkylations afforded 22 in 77% yield
from 12: the probable intermediate 21 was neither isolated nor
characterized. Although cyclophane 22 contains four tert-butyl
groups, it was only sparingly soluble in many solvents.
Elemental analysis and mass spectroscopy were consistent with
We now report alternative methodology which is both labor-
and cost-effective, avoiding the multistep syntheses of the
phosphonate ester reagents 3 and 6. We have found that the
direct phosphite-mediated cross-coupling of readily available
1,3-dithiole-2-thione units with anthraquinone is an efficient
route to symmetrical ex-TTF derivatives (i.e., 2-fold reaction
with both dithiole units the same) and unsymmetrical systems
(stepwise attachment of different dithiole units). Moreover, this
new protocol allows the formation of products bearing protected
thiol functionality, which will not survive the basic conditions
of either route 1 or 2 above. The value of these thiolated ex-
TTF derivatives has been established in a series of further
transformations.
Results and Discussion
Synthesis. The self-coupling and cross-coupling of 1,3-
dithiole-2-one/thione/selenone units in refluxing triethyl phos-
phite is a well-established route to symmetrical and unsym-
metrical TTF derivatives, respectively, and a good alternative
to the functionalization of TTF by lithiation and trapping with
electrophiles.23 Generally the phosphite coupling proceeds
efficiently with dithioles bearing electron-withdrawing groups,
and oxones and selenones tend to give higher yields than the
corresponding thiones.24 To extend this methodology to cross-
couplings with the less reactive anthraquinone, it was necessary
to limit the formation of the self-coupled TTF derivative. An
initial experiment established that heating a mixture of an-
thraquinone and 4,5-bis(methylthio)-1,3-dithiol-2-thione (9) (2
equiv) in neat triethyl phosphite at 125 °C for 3 h gave only
tetrakis(methylthio)-TTF (56% yield), unreacted anthraquinone,
and decomposed starting material. No cross-coupled product
was detected (TLC evidence). However, slow addition of thione
9 to an excess of anthraquinone (2) in triethyl phosphite-
chlorobenzene afforded the ketone 10 in 52% yield. This is
1
structure 22, but the H NMR spectrum was hard to assign as
the signals were weak and were broadened or split due to two
different conformations of 22 in which the ex-TTF moieties
were flipped up or down [i.e., conformation 1, (up, up) ≡ (down,
down); conformation 2, (up, down) ≡ (down, up)]. To circum-
vent this problem, 22 was oxidized with iodine to give the
tetracation 224+, which was soluble in DMSO-d6. The 1H NMR
spectrum (Figure S1 in the Supporting Information) reflected
the high symmetry of 224+, which comprises planar symmetric
moieties: none of the peaks are broadened or split. These data
further secured the structure of 22.
There is continued interest in supramolecular assemblies
which are ordered by hydrogen bonding.28 For example, in the
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J. Org. Chem, Vol. 72, No. 4, 2007 1303

Christensen et al.
SCHEME 3a
a Reagents and conditions: (i) 9, P(OEt)3, PhCl, 135 °C, 3 h (52% yield); (ii) 11, P(OEt)3, 130 °C, 3.5 h (73% yield).
SCHEME 4a
and the cyclic voltammogram was consistent with a nonstrained
ex-TTF derivative (see below). The structure is confidently
assigned as the macrocycle 25, presumably as a mixture of
isomers.
X-ray Crystal Structures of 23 and 24. Compound 23
crystallized from methanol and acetonitrile as two different
solvates (pseudopolymorphs). Both contain one molecule of 23
per asymmetric unit together with nonstoichiometric amounts
of solvent, tentatively 23‚2.5MeOH (23a) and 23‚1.5MeCN
(23b). The ex-TTF core of 23 adopts the usual saddle
conformation8,22b,25 (Table 1 and Figure 1). The dihydroan-
thracene system is folded along the C(9)‚‚‚C(10) vector, as
shown by the angle (æ) between the planar benzene rings. Both
dithiole rings are folded inward along the S(1)‚‚‚S(2) and
S(5)‚‚‚S(6) vectors (folding angles δ1 and δ2, respectively), so
that the planar S(1)C(16)C(17)S(2) and S(5)C(19)C(20)S(6)
moieties are tilted inward, toward one another. The angle (θ)
between these two planes and the transannular distances between
exocyclic sulfur atoms, S(3)‚‚‚S(7) and S(4)‚‚‚S(8), or the
adjacent dithiole carbons, C(16)‚‚‚C(19) and C(17)‚‚‚C(20), give
a measure of the overall U-bending of the ex-TTF. The bending
of 23 is comparable with that of other nonbridged ex-TTFs
(where θ varies from 73° to 101° and æ from 139° to 145°)
and somewhat weaker than in compound 1c25 (see Table 1).
a Reagents and conditions: (i) 11, P(OEt)3, 125 °C, 3 h (60% yield);
(ii) 14, P(OEt)3, 125 °C, 3 h (47% yield).
TTF series the crystal structure of tetrakis[(2-hydroxyethyl)-
thio]-TTF shows the coexistence of uniform π-π stacking and
hydrogen-bonded O‚‚‚H‚‚‚O networks.29 There are, however,
very few examples of ex-TTF derivatives bearing hydroxyalkyl
substituents.30 As stated by Mart´ın et al.,30b this is due to a lack
of available procedures for their synthesis. We recognized the
potential of compound 13 in this regard. Deprotection of 13
using sodium methoxide (5 equiv), followed by addition of
bromoethanol (5 equiv), gave the tetrakis[(hydroxyethyl)thio]
derivative 23 in 65% yield (Scheme 6), characterized by X-ray
crystallography (vide infra). The structure is organized by a
system of intra- and intermolecular hydrogen bonds in the
absence of effective π-π stacking.
To develop further the reactivity of these thiolated ex-TTF
reagents, the diprotected derivative 15 was used to synthesize
the ex-TTF cyclophanes 24 and 25. A solution of 15 and 1,5-
diiodopentane (1 equiv) was mixed with sodium methoxide (3
equiv) under high-dilution conditions to afford compounds 24
and 25 in 47% and 16% yield, respectively (Scheme 7).
The usual packing motif of ex-TTFs is a pair of “mutually
engulfing” molecules, so that one dithiole end of each molecule
fills the intramolecular cavity of the other.8,22b Neither pseudopoly-
morph of 23 shows such packing: in 23a the intramolecular
cavity is occupied by a guest methanol molecule (Figure 2), in
23b by a benzene ring of an adjacent (host) molecule. The
remaining methanol in 23a and (all) acetonitrile in 23b are
chaotically disordered along the infinite channels, which run
between the host molecules (see ESI). In both phases the host
molecule is also disordered. In 23a the entire bis[(2-hydroxy-
ethyl)thio]dithiolylidene substituent at C(10) is distributed
between two positions in a 84:16 ratio. This can be interpreted
as a libration of the entire molecule 23 around the opposite
dithiole end, the alternative positions of other atoms being too
close to resolve. In 23b, the ex-TTF core is ordered, but all
four (hydroxyethyl)thio chains are intensely disordered. Nev-
ertheless, the major conformation in each case suggests the
existence of intramolecular hydrogen bonds between adjacent
(hydroxyethyl)thio substituents.
The major compound was very soluble and analyzed correctly
for the bridged cyclophane (1H NMR spectra showed that it
was not a cis/trans mixture). An X-ray crystal structure (vide
infra) established that the structure was the cis isomer 24. This
route to 24 is considerably more efficient than the previous
synthesis of the dimethyl analogue 26 (Chart 2), which was
obtained in 10% yield by a 2-fold Horner-Wadsworth-
Emmons reaction of the corresponding bis[(1,3-dithiol-2-yl)-
phosphonate ester] reagent with anthraquinone.17a The minor
product (Scheme 7) was only sparingly soluble in a range of
solvents, including carbon disulfide, toluene, chlorofom, and
Different crystallizations of 24 from DCM/hexane also
yielded two pseudopolymorphs, 24‚CH2Cl2 (24a) and 24‚
1.5CH2Cl2 (24b). The asymmetric unit of 24a comprises one
molecule of 24 with a partially disordered DCM molecule inside
the cavity (Figure 3). In 24b there are two independent host
molecules, each with a similarly disordered DCM in the cavity,
and two symmetrically nonequivalent extraannular DCM mol-
ecules, each of them disordered between two overlapping
positions related by an inversion center (which gives two
additional half-molecules per asymmetric unit and one per
1
dimethyl sulfoxide. Detailed H NMR analysis was, therefore,
very difficult. Mass spectra showed that it was a 2 + 2 product,
(29) Batsanov, A. S.; Svenstrup, N.; Lau, J.; Becher, J.; Bryce, M. R.;
Howard, J. A. K. J. Chem. Soc., Chem. Commun. 1995, 1201-1202.
(30) (a) Godbert, N.; Bryce, M. R.; Dahaoui, S.; Batsanov, A. S.; Howard,
J. A. K.; Hazendonk, P. Eur. J. Org. Chem. 2001, 749-757. (b) D´ıaz, M.
C.; Illescas, B.; Mart´ın, N. Tetrahedron Lett. 2003, 44, 945-948.
1304 J. Org. Chem., Vol. 72, No. 4, 2007

Thiolated π-Extended TetrathiafulValenes
SCHEME 5a
a Reagents and conditions: (i) MeONa-MeOH (1 equiv), DMF, 20 °C, 1.5 h; (ii) MeI (excess), 20 °C, 30 min (96% yield from 12); (iii) MeONa-MeOH
(1 equiv), DMF, 20 °C, 30 min, then 18 (0.3 equiv), DMF, 20 °C, 18 h (97% yield from 18); (iv) 20 (0.5 equiv), DMF, 20 °C, 12 h; (v) 20 (1 equiv),
MeONa-MeOH (2.7 equiv), DMF, 20 °C, 20 h, high dilution (77% yield from 12); (vi) I2, DMSO-d6, 20 °C, 1 h (100% yield by NMR).
SCHEME 6a
cally enhances the U-bend of the ex-TTF core, as indicated by
the increase of both δ angles and a decrease of the θ angle
(Table 1 and Figure 1), whereas the æ angle is little affected.
Interestingly, the θ angle in 24 is ca. 10° wider than in molecule
26 containing the same bridge. The difference is probably due
to a wedging effect of the intraannular solvent molecule (not
present in the crystal of 26). Note that the transannular distances
in 26 are symmetrical, whereas in 24 they are longer on the
solvent side than on the bridge side.
Solution Electrochemical and Optical Absorption Proper-
ties. Solution electrochemical data, obtained by cyclic voltam-
metry (CV), are collated in Table 2. Data for compound 1c are
included for comparison.
a Reagents and conditions: (i) MeONa-MeOH (5 equiv), DMF, 20 °C,
5 min, then BrCH2CH2OH (5 equiv), DMF, 20 °C, 16 h (65% yield).
formula unit). The pentamethylenedithia bridge of 24 adopts a
nearly planar all-trans conformation, with S-C-C-C and
C-C-C-C torsion angles in the range 160-180°, and drasti-
With the exception of the strained cyclophane 24, which is
discussed below, all the oxidation potentials fall in the range
expected for tetrakis(alkylthio)-substituted ex-TTF derivatives.6
J. Org. Chem, Vol. 72, No. 4, 2007 1305

Christensen et al.
SCHEME 7a
a Reagents and conditions: (i) MeONa-MeOH (3 equiv), DMF, 20 °C,
16 h, 1,5-diiodopentane, high dilution (24, 47% yield; 25, 16% yield).
CHART 2
FIGURE 2. X-ray structure of 23‚2.5MeOH (23a), showing 30%
thermal ellipsoids and probable hydrogen bonds. Minor positions of
disordered atoms are omitted, and inversion-related atoms are primed.
TABLE 1. Transannular Distances (Å) and Interplanar Angles
(deg) in ex-TTF Derivatives
23a
23b
24a 24ba 24ba 26b
1cc
C(16)‚‚‚C(19)/Å 8.81
C(17)‚‚‚C(20)/Å 8.83
8.83
8.76
7.27 7.27 7.32 7.06 8.45
7.60 7.59 7.63 7.01 8.59
S(3)‚‚‚S(7)/Å
S(4)‚‚‚S(8)/Å
δ1/deg
10.99 10.92 8.13 8.13 8.16 8.09 10.24
11.08 10.86 9.24 9.33 9.32 10.60
11.4
14.3
86.0
38.8
16.0
11.1
83.8
38.2
23.3 22.0 26.2 29.4 17.4
28.6 27.2 23.8 22.6 8.0
44.1 45.4 45.2 34.7 77.4
41.7 40.3 39.8 43.4 37.8
δ2/deg
θ/deg
180-æ/deg
a Two independent molecules. b Reference 17a (the ordered molecule).
c Reference 25.
FIGURE 3. X-ray molecular structure of 24‚CH2Cl2 (24a), showing
50% thermal ellipsoids and the major (89%) orientation of the
disordered DCM molecule.
increased reversibility of the oxidation process is consistent with
the dication 242+ adopting a conformation closer to the neutral
saddle shape than is the case for nonbridged dications.
The CV of the dimeric cyclophane 25 was similar to those
of nonbridged tetrakis(alkylthio)-ex-TTFs, e.g., its precursor 12,
as expected for a system where the bridges are too long to
impose any strain on the π-system. The dimer 22 and the trimer
19 also showed a single, quasi-reversible, oxidation wave at
typical oxidation potentials. No intramolecular electronic in-
teraction was observed in the CVs of the multi-exTTFs 19, 22,
and 25: each ex-TTF unit behaves independently, and the
oxidation waves represent 6-, 4-, and 4-electron processes,
respectively.
FIGURE 1. Diagram of the ex-TTF framework, defining the angles
used in Table 1.
Each cyanoethyl group raises Eox by ca. 30 mV, and this effect
is additive. Thus, there is an incremental increase along the series
1c f 17 f 12/15 f 13, with the value for 13 raised by 110
mV compared to that for 1c.
The CV of 24 showed the 2-electron oxidation wave to be
positively shifted by 290 mV compared to that of its precursor
15 (Figure 4). This is a shift similar to that observed for other
ex-TTF cyclophanes which possess a single short bridge
between the two dithiole rings.17a,b An explanation is that the
bridge obstructs the marked conformational change which
accompanies oxidation, and thereby the dication is destabilized.
Consistent with this, ∆E for 24 is reduced from the typical value
for nonbridged derivatives of >100 mV to only 60 mV. This
With the exception of cyclophane 24, the solution UV-vis
absorption spectra of the new ex-TTF derivatives displayed
strong bands at λmax ca. 435 and 365 nm which are characteristic
of the tetrakis(alkylthio)-substituted ex-TTF framework.10 For
24 there is a blue shift of the longer wavelength band to λmax
1306 J. Org. Chem., Vol. 72, No. 4, 2007

Thiolated π-Extended TetrathiafulValenes
TABLE 2. Cyclic Voltammetric Dataa
both solvates of 23 the usual packing motif of ex-TTFs was
overruled by an unprecedented hydrogen-bonded self-assembly.
This methodology opens the way to a wide range of
derivatives of this fascinating redox system, with chemical and
physical functionality tailored for new applications in supramo-
lecular and materials chemistry, especially processes involving
inter- and intramolecular electron transfer.
compd
Eoxpa/V
Eoxpc/V
∆Eb/V
1c
12
13
15
17
19
22
23
24
25
0.55
0.60
0.66
0.61
0.57
0.55
0.60
0.54
0.90
0.59
0.43
0.50
0.54
0.45
0.44
0.45
0.42
0.43
0.84
0.41
0.12
0.10
0.12
0.16
0.13
0.10
0.18
0.11
0.06
0.18
Experimental Section
10-[4,5-Bis(methylthio)-1,3-dithiol-2-ylidene]anthracene-9(10H)-
one (10). A degassed mixture of triethyl phosphite (50 mL) and
chlorobenzene (150 mL) was stirred under argon and heated to 135
°C whereupon anthraquinone (2) (8.00 g, 38.4 mmol) was added
in one portion. Over the next 1.5 h thione 926c (4.53 g, 20.0 mmol)
was added as a solid in small portions, against a positive pressure
of argon, affording a dark red solution. A second batch of 2 (4.00
g, 19.2 mmol) was added in one portion, and addition of more thione
9 (4.53 g, 20.0 mmol) in small portions was continued over the
next 1.5 h. The reaction mixture was cooled to room temperature.
The excess anthraquinone which crystallized was removed by
filtration, and the remaining reaction mixture was concentrated
under reduced pressure to a volume of 50 mL, whereupon methanol
(300 mL) was added to give a red precipitate. The precipitate was
filtered, washed with methanol (3 × 15 mL), dried in vacuo, and
column chromatographed (silica gel, dichloromethane) to give 10
as an orange powder (7.97 g, 52%). Mp: 204-205 °C (lit.25
185 °C). 1H NMR (CDCl3): δ 8.27 (2H, d, J ) 7.8 Hz), 7.76 (2H,
d, J ) 7.8 Hz), 7.66 (2H, t, J ) 7.8 Hz), 7.44 (2H, t, J ) 7.8 Hz),
2.42 (6H, s).
a Data were obtained using a one-compartment cell with a Pt disk working
electrode and a Pt counter electrode vs Ag/AgCl. Conditions: compound
ca. 1 × 10-3 M, electrolyte 0.1 M Bu4NPF6 in dichloromethane, 20 °C,
scan rate 100 mV s-1 b ∆E ) Eox - Eox (Eox is the oxidation peak
.
pa pc pa
potential on the first anodic scan; Eox is the coupled reduction peak
pc
potential on the cathodic scan).
9-[4,5-Bis[(2-cyanoethyl)thio]-1,3-dithiol-2-ylidene]-10-[4,5-
bis(methylthio)-1,3-dithiol-2-ylidene]-9,10-dihydroanthracene (12).
Triethyl phosphite (10 mL) was degassed, heated to 130 °C, and
stirred under argon, and then 10 (0.317 g, 0.82 mmol) was added
in one portion. Over the next 3 h thione 1126b (0.830 g, 2.73 mmol)
was added as a solid in small portions against a positive pressure
of argon. The reaction mixture turned dark red-brown, and the
formation of 12 was monitored using TLC (silica, dichloromethane
as eluent). After addition of one-third of 11 a yellow solid began
to precipitate. The mixture was stirred for another 30 min at 130 °C
and then cooled to 0 °C, and methanol (70 mL) was added. The
precipitate was filtered, washed with methanol (3 × 5 mL), dried
in vacuo, and column chromatographed (silica gel, dichloromethane)
to give 12 as a yellow foam (0.386 g, 73%). Yellow prisms grew
by slow diffusion of hexane into a solution of 12 in dichlo-
FIGURE 4. Cyclic voltammograms of 24 and its precursor 15.
Experimental conditions are given in Table 2.
421 nm and a decrease of the extinction coefficient, indicating
loss of π-conjugation. We have noted recently that this trend is
consistent with increased folding of the ex-TTF moieties.17e
Qualitatively similar behavior is observed for strained TTF
cyclophanes.31
Conclusions
The availability of ex-TTF derivatives with protected thiolate
functionality (compounds 12, 13, and 15) is a significant mile-
stone in the synthetic chemistry of this redox system. The phos-
phite-mediated cross-coupling methodology by which they have
been obtained avoids the multistep synthesis of (1,3-dithiol-2-
yl)phosphonate ester reagents. The versatility of these new ex-
TTF building blocks has been demonstrated in the efficient
syntheses of a trimer, two dimeric cyclophanes, a hydrogen-
bonded self-assembly, and a strained cyclophane. In the trimer
19 and the dimers 22 and 25, each ex-TTF unit behaves inde-
pendently and the oxidation waves represent 6-, 4-, and 4-elec-
tron processes, respectively. For the cyclophane 24 the confor-
mational changes and gain in aromaticity which drive the second
oxidation process in ex-TTFs have been restricted by the
conformational constraints of the pentamethylene chain bridging
the dithiole units. This results in a positive shift of the oxidation
potential and increased reversibility of this 2-electron wave. In
1
romethane. Mp: 236-237 °C. H NMR (CDCl3): δ 7.62-7.60
(2H, m), 7.52-7.49 (2H, m), 7.37-7.31 (4H, m), 3.14-3.07 (2H,
m), 3.03-2.95 (2H, m), 2.72-2.68 (4H, m), 2.40 (6H, s). 13C NMR
(CDCl3): δ 134.6, 134.1, 131.7, 128.3, 126.8, 126.4, 126.2, 125.5,
125.4, 125.2, 124.9, 122.9, 117.4, 31.1, 18.92, 18.90. UV-vis (CH2-
Cl2): λmax (log ꢀ) 362 (4.21), 431 (4.44) nm. MS (EI): m/z (rel
intens) 642 (93, M+), 264 (100). Anal. Calcd for C28H22N2S8 (MW
643.02): C, 52.30; H, 3.45; N, 4.36. Found: C, 52.24; H, 3.42; N,
4.39.
9-[4-[(2-Cyanoethyl)thio]-5-(methylthio)-1,3-dithiol-2-ylidene]-
10-[4,5-bis(methylthio)-1,3-dithiol-2-ylidene]-9,10-dihydroan-
thracene (17). To a stirred solution of 12 (1.00 g, 1.56 mmol) in
dry degassed N,N-dimethylformamide (100 mL) under argon at
room temperature was added dropwise a solution of sodium
methoxide (3.11 mL of a 0.5 M solution in methanol, 1.56 mmol)
over 30 min. The color changed from yellow to red. The solution
was stirred for a further 1 h, and then methyl iodide (1.0 mL, 2.28
g, 16.1 mmol) was added in one portion. The yellow reaction
mixture was stirred for 30 min and concentrated in vacuo and the
residue purified by column chromatography (silica gel, dichlo-
romethane) to afford 17 as a yellow foam (0.907 g, 96%). Yellow
crystals grew by slow diffusion of hexane into a solution of 17 in
(31) (a) Lau, J.; Blanchard, P.; Riou, A.; Jubault, M.; Cava, M. P.; Becher,
J. J. Org. Chem. 1997, 62, 4936-4942. (b) Wang, C.; Bryce, M. R.;
Batsanov, A. S.; Howard, J. A. K. Chem.sEur. J. 1997, 3, 1679-1690.
(c) Otsubo, T.; Aso, Y.; Takimiya, K. AdV. Mater. 1996, 8, 203-211.
1
dichloromethane. Mp: 207-208 °C. H NMR (CDCl3): δ 7.60-
J. Org. Chem, Vol. 72, No. 4, 2007 1307

Christensen et al.
7.51 (4H, m), 7.34-7.31 (4H, m), 3.08-3.01 (1H, m), 2.96-2.88
(1H, m), 2.70-2.66 (2H, m), 2.43 (3H, s), 2.40 (3H, s), 2.39 (3H,
s). 13C NMR (CDCl3): δ 134.6, 134.5, 134.4, 134.3, 133.0, 131.3,
129.7, 126.5 (2C), 126.34, 126.33, 125.7, 125.5, 125.41, 125.36,
125.32, 125.28, 124.2, 123.2, 118.4, 117.6, 31.1, 19.00, 18.96,
18.93, 18.8. UV-vis (CH2Cl2): λmax (log ꢀ) 272 (4.34), 365 (4.20),
434 (4.44) nm. MS (EI): m/z (rel intens) 603 (100, M+). Anal.
Calcd for C26H21NS8 (MW 603.98): C, 51.70; H, 3.50; N, 2.32.
Found: C, 51.63; H, 3.45; N, 2.34.
by column chromatography (silica gel, dichloromethane-carbon
disulfide, 1:9, v/v) to give 19 as a yellow powder (0.175 g, 97%).
Mp: 225 °C dec. 1H NMR (CDCl3): δ 7.59-7.55 (12H, m), 7.34-
7.30 (12H, m), 4.19-4.16 (3H, m), 4.03-3.99 (3H, m), 2.53 (9H,
s) 2.39-2.36 (27H, m). UV-vis (CH2Cl2): λmax (log ꢀ) 367 (4.68),
436 (4.91) nm. MS (MALDI): m/z 1808.8 (calcd 1809.0). Anal.
Calcd for C81H66S24 (MW 1808.97): C, 53.78; H, 3.68. Found: C,
53.90; H, 3.79.
Trimer 19. To a stirred solution of 17 (0.199 g, 0.33 mmol) in
dry degassed N,N-dimethylformamide (10 mL) under argon at room
temperature was added a solution of sodium methoxide (0.66 mL
of a 0.5 M solution in methanol, 0.33 mmol) in one portion. The
color changed from yellow to red. The solution was stirred for 30
min, and then a solution of tris(bromomethyl)mesitylene (18)32
(0.040 g, 0.10 mmol) in dry degassed N,N-dimethylformamide (2
mL) was added dropwise over 15 min. A yellow precipitate formed.
The mixture was stirred overnight, and then excess methyl iodide
(0.1 mL, 1.6 mmol) was added to quench the excess thiolate. The
reaction mixture was concentrated in vacuo and the residue purified
Acknowledgment. We thank EPSRC and The Danish
Research Academy for funding to C.A.C. We thank Professor
J. A. K. Howard for the use of X-ray facilities and EPSRC for
funding the improvement to the X-ray instrumentation.
Supporting Information Available: Synthetic details and
characterization data for compounds 13, 15, 20, 22, 224+, and 23-
25, 1H NMR spectrum of 224+, X-ray crystallographic file for 23‚
2.5MeOH, 23‚1.5MeCN, 24‚CH2Cl2, and 24‚1.5CH2Cl2 in CIF
format, and diagrams of the crystal structures of 23a and 24b. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(32) Van der Made, A. W.; Van der Made, R. H. J. Org. Chem. 1993,
58, 1262-1263.
JO062199P
1308 J. Org. Chem., Vol. 72, No. 4, 2007