C-C bond formation through oxidatively induced elimination of platinum complexes - A novel approach towards conjugated macrocycles

Article abstract of DOI:10.1039/b300542a

A novel and effective method for the synthesis of conjugated macrocycles is described. By the self-assembly of conveniently accessible building blocks to a metalla-macrocycle, and subsequent C-C bond formation through elimination of transition metal units, the strained cyclodimeric terthiophene-diyne 4 -as a precursor for cyclo[8]thiophene 5- was synthesized in a good overall yield.

Full text of DOI:10.1039/b300542a

C–C bond formation through oxidatively induced elimination of
platinum complexes—A novel approach towards conjugated
macrocycles†
Gerda Fuhrmann,a Tony Debaerdemaekerb and Peter Bäuerle*a
a Department Organic Chemistry II (Organic Materials and Combinatorial Chemistry), Ulm University,
Albert-Einstein-Allee 11, 89081 Ulm, Germany. E-mail: peter.baeuerle@chemie.uni-ulm.de
b Division of X-ray and Electron Diffraction, Ulm University, Albert-Einstein-Allee 11, 89081 Ulm, Germany
Received (in Cambridge, UK) 15th January 2003, Accepted 21st February 2003
First published as an Advance Article on the web 14th March 2003
A novel and effective method for the synthesis of conjugated
macrocycles is described. By the self-assembly of conven-
iently accessible building blocks to a metalla-macrocycle,
and subsequent C–C bond formation through elimination of
transition metal units, the strained cyclodimeric terthio-
phene-diyne 4 -as a precursor for cyclo[8]thiophene 5- was
synthesized in a good overall yield.
single peak at 25.59 ppm with a set of 195Pt satellites (1JPt–P
=
2243 Hz). Ultimate proof of the cyclodimeric structure of 3
comes from MALDI-TOF mass spectrometry revealing molec-
ular ions at m/z = 2249.5. IR spectra corroborate the structure
of 3 by showing typical n(C·C–Pt) stretches at 2362 and 2106
cm21 and by the lack of n(C·C–H) stretches at 3308 cm21
which appear for precursor 2. These results are in full
accordance with other recently published platinum s-acetylide
macrocycles.6
The field of nano-sized and shape-persistent cycles is growing
rapidly and today various systems have become available due to
considerable efforts in developing new strategies for cyclization
reactions.1 Few recent examples, such as the zirconocene-
coupling2 or the supramolecular formation of macrocycles
containing transition metals as tectons,3 are reported to give
high yields. This is due to the ideal geometric factors of the
building blocks and the reversibility of the reactions under
thermodynamic equilibrium conditions. However, the facile
accessibility of rigid, all-carbon perimeter macrocycles on a
larger scale still remains a major challenge.
We recently obtained a set of 39- to 133-membered
conjugated thiophene-derived macrocycles which represent a
novel topology and class of compounds in the area of
conducting polymers and oligomers.4 Due to their highly
symmetrical circular structures completely new properties have
been discovered.5 Terthiophene-diyne 2 and a corresponding
pentamer were oxidatively coupled in a random cyclization
approach under pseudo high dilution conditions to give
mixtures of macrocyclic oligothiophene-diynes in expected
modest total yields (8.5%, 13.5%) which were then separated
into the individual macrocycles by preparative HPLC in yields
of 3.9 to < 0.5%. In a second step, by reacting the butadiyne
units in the cyclo(oligothiophene-diynes) with sulfide anions
the fully conjugated cyclo[n]thiophenes (n = 12,16,18) were
obtained in yields of 7–27% only on a milligram scale.
In this respect, we have now developed a novel and effective
protocol for the synthesis of conjugated macrocycles in which
conveniently accessible diyne building blocks and transition
metal precursor complexes self-assemble to form a stable,
coordinatively bound metalla-macrocycle. Subsequently and
most importantly, the transition metal units are expelled under
simultaneous formation of C–C bonds leading to the conjugated
macrocycle.
Reductive 1,1-eliminations for C–C coupling products from
Pd s-acetylide complexes are well known to effectively proceed
in the catalytic cycle of the Sonogashira-Hagihara reaction7 or
the Pd-catalyzed version of the Glaser coupling.8 While
stabilized cis-(RA3P)2PdR2 complexes allow the thermal elim-
ination of C–C coupling products to be controlled and followed9
the corresponding Pt complexes are very stable toward
reductive elimination and only few examples of cis-(RA3P)2-
PtAr2 are known where the pyrolitic formation of biaryls is
accounted.10 Generally, an increase in the oxidation state of a
complexed transition metal facilitates its reductive elimination.
An example for oxidatively induced reductive elimination of
Pt(II) complexes, but only promoted by a ferrocenyl group, has
been reported.11
Thus, by reaction of cis-Pt(dppp)Cl2 and terthiophene-diyne
2 in the presence of 10 mol% CuI and 2 eq. of NEt3 at room
temperature bisplatino-macrocycle 3 was isolated after chroma-
tography as a stable orange-brown powder in 91% yield
(Scheme 1). 1H and 31P NMR spectra suggest the formation of
a cyclic bisplatinum s-acetylide complex. In the 1H NMR
spectrum of 3, signals for terminal acetylenic protons are
completely lacking and only those belonging to the phenyl
protons of the dppp-ligand (7.90, 7.38 ppm) and to the b protons
of the terthiophene unit (6.98 ppm) are visible in the aromatic
part of the spectrum. The 31P NMR spectrum of 3 exhibits a
† Electronic supplementary information (ESI) available: experimental
section. See http://www.rsc.org/suppdata/cc/b3/b300542a/
Scheme 1 Synthesis of macrocycles 3–5.
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This journal is © The Royal Society of Chemistry 2003

1
In a newly developed reaction, we could achieve 1,1-re-
ductive elimination of the Pt(dppp) “corners” from bismetalla-
cycle 3 under simultaneous C–C bond formation and preserva-
tion of the cyclic structure by treatment of 3 with 2 eq. of iodine.
100% conversion of the starting material took place at an
optimum temperature of 60 °C and a reaction time of 24 hours
in THF. After chromatographic separation from polymeric by-
products and Pt(dppp)I2 the cyclodimeric terthiophene-diyne 4
was isolated in 54% yield as a stable red microcrystalline solid
(Scheme 1). It represents the smallest (26-memb.) macrocycle
in the homologous series which, however, could never be
detected in the previous random cyclooligomerization reac-
tions. Due to the highly symmetrical structure, 1H and 13C NMR
spectra consist of only few signals which are directly compara-
ble to those of the corresponding higher members in the series.
The cyclic structure of 4 is finally, unambiguously proven by a
molecular mass of m/z = 1036.7 in the MALDI-TOF mass
spectra.
conjugated cyclo[n]thiophenes came from H and 13C NMR
spectra, which are very simple due to the high symmetry of the
molecule, and MALDI-TOF mass spectra exhibiting molecular
ions at m/z = 1104.5. Very recently, the synthesis and structural
characterization of a comparable cyclo[8]pyrrole have been
reported.15
In summary, we have developed a very effective method for
the synthesis of conjugated macrocycles involving supramo-
lecular formation of platino-macrocycles and subsequent C–C
bond formation through elimination of the transition metal units
by means of an oxidant. In the meanwhile, we successfully used
this protocol for the synthesis of other macrocyclic systems,
indicating a more general applicability of the method.
We are grateful to the German Research Foundation (SFB
569) and the Fonds der Chemischen Industrie for their financial
support of this work, Dr. E. Mena-Osteritz and Dr. P. Kilickiran
(Dept OCII, Ulm) for valuable discussions and Dr. M.
Wunderlin (Division of Mass Spectrometry, Ulm) for various
MALDI-TOF mass spectra.
A more detailed characterization of 4 was possible by X-ray
structure analysis of single crystals which could be obtained by
slow evaporation from chloroform solutions.12 The top view of
an individual molecule (including atomic labelling) shows a
nearly perfect circular shape comprising all-syn-oriented ter-
thiophene moieties which are connected by concavely bowed
butadiyne units indicating considerable ring strain (Fig. 1). The
bond angles of the butadiyne fragments (C1A–C19–C17 163.2°,
C19–C17–C15 167.7°, C17–C15–C13 167.4° and C15–C13–
C11 160.3°) are severely distorted and largely deviate from the
normal linear geometry. They are under the smallest values
reported for strained dehydroannulenes13 or cyclophanes.14 The
alternating carbon–carbon bond lengths are quite typical for
strained macrocycles (C1A–C19 1.416 Å, C19–C17 1.205 Å,
C17–C15 1.368 Å, C15–C13 1.203 Å, C13–C11 1.417 Å). Due
to the steric interactions of the butyl side chains adjacent
thiophene rings are distorted by 13.80° (T1, T2) and 229.09°
(T2, T3), a phenomenon which was also detected in the X-ray
structure analyses of the less strained next larger members in the
homologous series, the cyclotrimeric5b and cyclotetrameric
terthiophene diynes.4a The nonbonding distances S1…S1A and
S1A…S3 are 11.04 Å (largest) and 8.06 Å (smallest), re-
spectively. The space filling model of the 26-membered ring
suggests an interior cavity of about 35 Å2. Despite the distortion
of the thiophene rings and the butadiyne units, the 32p-electron
perimeter shows remarkable conjugation. In the 1H-NMR
spectrum a substantial up-field shift of the thiophene b-protons
in comparison to those of the next larger members in the series
(Dd = 40.2 ppm) shows antiaromatic behaviour.
Notes and references
‡ CCDC 201462. See http://www.rsc.org/suppdata/cc/b3/b300542a/ for
crystallographic data in .cif or other electronic format.
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Cyclodimeric terthiophene-diyne 4 was finally reacted with
disodium sulfide in xylene/2-methoxyethanol at 140 °C to give
octabutyl-cyclo[8]thiophene 5 which after chromatography was
isolated in 19% yield as a stable red microcrystalline solid. The
modest yield in this reaction might be due to the ring strain in 4
and the high reaction temperature which cause partial destruc-
tion of the cycle. Structural proof of this smallest member of
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monoclinic, space group P21/a, a = 9.073(1), b = 20.222(2), c =
16.503(2) Å, b = 104.87(2)°, V = 2926.6(6) Å3, Z = 2, rcalcd = 1.177
g cm23, m(MoKa) = 0.272. Least square refinement based on 3949
reflections with I42s(I) and 468 parameters led to convergence, with a
final R1 = 0.0331, wR2 = 0.0755, and GOF = 0.945. ‡.
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Fig. 1 POVRAY top view of cyclodimeric terthiophenediyne 4.
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