Zr-promoted cyclization of diynes bearing C2-chirality: Synthesis and properties of new chiral conjugated molecules

Article abstract of DOI:10.1016/S0040-4039(02)00520-8

Conjugated oligomers bearing 4,5,6,7-tetrahydro-5S,6S-dioctyloxybenzothiophene as a central linkage were synthesized by Negishi's reagent (n-Bu₂ZrCp₂) promoted intramolecular cyclization of a diyne and subsequent Suzuki coupling reac

Full text of DOI:10.1016/S0040-4039(02)00520-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 3313–3317
Zr-promoted cyclization of diynes bearing C2-chirality: synthesis
and properties of new chiral conjugated molecules
Ken-Tsung Wong* and Ruei-Tang Chen
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
Received 10 January 2002; revised 28 February 2002; accepted 15 March 2002
Abstract—Conjugated oligomers bearing 4,5,6,7-tetrahydro-5S,6S-dioctyloxybenzothiophene as a central linkage were synthesized
by Negishi’s reagent (n-Bu2ZrCp2) promoted intramolecular cyclization of a diyne and subsequent Suzuki coupling reactions. The
chirality in the central linkage originated from tartaric acid, which induced the conjugated backbone of oligomers to exhibit
interesting optical activity. © 2002 Elsevier Science Ltd. All rights reserved.
Conjugated organic materials with well-defined struc-
tures and properties are of interest for electronic
devices. Over the last decade, intensive investigations
on p-conjugated systems have resulted in the successful
development of new synthetic methodologies.1 Among
these efficient strategies for constructing p-conjugated
oligomers with precise conjugation length, the zir-
conocene diyne-coupling pathway has been used to
synthesize conjugated systems with novel architecture.
Zirconocene-promoted intramolecular cyclization of
diynes generates synthetic intermediates containing zir-
conacyclopentadiene unit(s) which can be subsequently
transformed into a variety of structures, for example
thiophene,2 thiophene-1-oxide,3 thiophene-1,1-dioxide,3
phosphole,4 germole5 and highly functionalized benzene
derivatives.6 These subsequent chemical transforma-
tions can be used to tailor the structure of conjugated
backbones, giving new options for efficiently tuning the
electronic and optical properties. In addition, the phys-
ical properties of a p-conjugated system not only rely
on the main chain structure, but are also governed by
the attached side chains. The nature of the side chains
has a substantial impact on the physical properties of
the conjugated oligomer and polymers. For example,
the hexyl side chains of regioregular poly(3-hexylthio-
phene), P3HT, play an important role on the molecular
ordering, resulting in the highest field-effect mobility
reported for polymer transistors.7 In contrast, many
optically active polymers have been obtained by intro-
ducing side chains carrying chirality center(s). The
resulting polymers exhibit interesting optical behavior
induced by the chiral side chains.8 It is generally
accepted that studies on the fundamental properties of
oligomeric species often provide specific information
enabling prediction of the behavior of their correspond-
ing polymeric analogs.9 Along this line, we took advan-
tage of the Zr-promoted cyclization of diynes as a
versatile tool to introduce side chains with C2-chirality
originating from tartaric acid, giving a series of interest-
ing precursors for new conjugated systems.
Intramolecular zirconocene coupling of diynes for the
synthesis of new dibromide building blocks is outlined
in Scheme 1. The C2 chirality in the diyne precursor
used for zirconocene-promoted intramolecular cycliza-
tion originates from natural
L
-tartaric acid. Following a
-tartrate was converted
literature procedure,10 diethyl
L
to the bis-mesylate 1, which was further converted to
the diyne 2 with an excess amount of lithium acetylide–
ethylenediamine complex.11 Alkylation of the 1,2-dihy-
droxy group of 2 with 1-bromoctane was accomplished
by the modified Williamson’s ether synthesis,12 using
phase transfer catalysis to give the dioctyloxydiyne 3 in
62% yield. Further modification on the terminal acetyl-
ene is necessary, because the zirconocene coupling
cyclization is not applicable for unprotected terminal
diynes. The deprotonation of the terminal alkyne with
n-butyllithium followed by trapping of the lithium
acetylide with trimethylsilyl chloride cleanly afforded
the bistrimethylsilylated diyne compound 4 in 99%
yield. Treatment of 4 with Negishi’s reagent (n-
Bu2ZrCp2), generated in situ from zirconocene dichlo-
ride and n-butyllithium, gave the zirconocycle
intermediate which was subsequently quenched with
sulfur monochloride, S2Cl2, affording a new chiral
Keywords: zirconocene; C2-chirality; chiral conjugated oligomer.
* Corresponding author. Tel.: +886-2-2363-0231 ext. 3315; fax: 886-2-
2363-6359; e-mail: kenwong@ccms.ntu.edu.tw
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00520-8

3314
K.-T. Wong, R.-T. Chen / Tetrahedron Letters 43 (2002) 3313–3317
HO
HO
C8H17
O
O
HO
HO
b
OMs
OMs
a
62%
85%
C8H17
2
1
3
OC8H17
C8H17O
C8H17
O
O
c
d
SiMe3
SiMe3
49%
99%
C8H17
4
X
X
S
5
X = SiMe3
e 88%
6-L X= Br
C8H17
OC8H17
O
C8H17
O
O
Ar
Ar
d
f
Br
Br
3
S
53%
72%
C8H17
8-L
7
Ar = 4-bromo-Ph
C8H17
O
OC8H17
g
Br
7
Br
44%
MeO2C
CO2Me
9
Scheme 1. Reagents and conditions: (a) lithium acetylide ethylenediamine complex, DMSO/THF; (b) 1-bromooctane, NaOH,
CTAB, THF; (c) n-BuLi, (CH3)3SiCl; (d) (i) Cp2ZrCl2, n-BuLi, THF (ii) S2Cl2; (e) Br2, CCl4; (f) 1-bromo-4-iodobenzene,
PdCl2(PPh3)2, CuI, Et2NH; (g) (i) Cp2ZrCl2, n-BuLi (ii) CuCl, DMAD, THF.
molecule 5. The trimethylsilyl groups in 5 were dis-
placed by treating 5 with bromine under electrophilic
substitution conditions to give a 49% yield of 2,9-
dibromo-4,5,6,7-tetrahydro-5S,6S-dioctyloxylbenzoth-
iophene (6-L).13 To probe the influences of the side
chain chirality on the physical properties, 6-D, which is
an enantiomer of 6-L, was synthesized in a similar
The extension of the conjugation of the chiral core by
Suzuki coupling reactions of dibromides 6, 8, and 9
with arylboronic esters or various arylboronic acids is
outlined in Scheme 2. A catalytic amount of the bulky
phosphine ligand, PtBu3, was added to improve the
efficiency of the Suzuki coupling reaction. For dibro-
mide 6-L, a bulky aryl group, 9,9-diphenylfluorene, was
introduced to the C2 and C9 positions of the 4,5,6,7-
tetrahydrobenzothiophene by using 2-(9,9-diphenylfluo-
renyl) pinacol boronate as the coupling reagent.
Consequently, oligomer 10-L was isolated in 75% yield.
The enantiomeric oligomer 10-D was also isolated in a
similar manner from 6-D. Two different arylboronic
acids were tested for coupling with the chiral dibromide
8 to afford oligomers 11 and 12-L in 71 and 76% yields,
respectively. For a comparison, the enantiomer 12-D
was also synthesized. In addition, the conjugation of
manner but starting from diethyl
D-tartrate.
The terminal alkyne could be further modified via a
transition metal catalyzed reaction. The selective
PdCl2(PPh3)2-catalyzed Sonogashira coupling reaction
of 3 with 4-iodobromobenzene in the presence of a
co-catalyst, CuI, in diethylamine at room temperature
afforded the dibromide compound 7 in a yield of 72%.
Compound 7 was further cyclized in a similar manner
as 4 by treatment with Negishi’s reagent and followed
by trapping the zirconocycle intermediate with S2Cl2 to
give 53% of the dibromide compound 8-L, an interest-
ing chiral building block possessing a 4,5,6,7-tetra-
hydro-5S,6S-dioctyloxylbenzothiophene unit as the
central linkage. The enantiomer 8-D was isolated via a
similar synthetic pathway to 8-L. Another interesting
building block with a highly functionalized phenylene
core was obtained according to the Takahashi’s
method;6 the zirconacyclopentadiene intermediate
derived in situ from 7 with Negishi’s reagent was con-
verted to the diester 9 in an isolated yield of 44% in the
presence of CuCl and dimethyl acetylenedicarboxylate
(DMAD).
dibromide
9 was extended by reacting with 4-
butoxyphenyl boronic acid under standard Suzuki cou-
pling conditions, giving the oligomer 13 in an isolated
yield of 67%.
UV–vis absorption and photoluminescence (PL) of chi-
ral molecules 10, 12, and 13 in dilute CHCl3 solution
were investigated to probe their electronic properties.
The HOMO–LUMO energy band gap was estimated
according to the absorption onsets. Intense blue
fluorescence for 10–13 with quantum yields ranging
from 0.15 to 0.32 was detected upon irradiating at the
corresponding absorption maximum. The photophysi-
cal properties are summarized in Table 1. A compari-
son of the UV–vis and PL spectra of 10–13 is shown in
Fig. 1. As we can see from Fig. 1, 10 exhibits evident
The viability of the synthesis of conjugated molecules
based on these chiral dibromide cores was investigated.

K.-T. Wong, R.-T. Chen / Tetrahedron Letters 43 (2002) 3313–3317
3315
tions between the ortho-hydrogens and the ester groups
as well as the methylene (CH2) groups are expected,
which would prevent the formation of a coplanar con-
formation of the conjugated backbone in the ground
state. The highly twisted conformation completely
blocks the extension of the p-conjugation along the
molecular axis, resulting in an increased band gap and
the blue-shifts, both in absorption and emission max-
ima. The distinct Stokes shift of 13 further confirms the
existence of a highly twisted ground state. The redox
behavior of molecules 10–13 was investigated by cyclic
voltammetry in CH2Cl2. These chiral molecules, except
for 13, exhibit a reversible cathodic oxidation redox
couple. The radical cation is believed to be generated
first on the thiophene ring and subsequently delocalized
over the whole molecule. The lower oxidation potential
of 10 compared to that of 11 and 12 is consistent with
the assumption of a more coplanar ground state con-
formation in 10. The thermal properties of chiral
molecules 10–13 were investigated by means of ther-
mogravimetric analysis (TGA). The temperatures corre-
sponding to 5% weight loss ranged from 340 to 417°C
were observed upon heating 10–13 under a nitrogen
atmosphere. The morphological properties were studied
by differential scanning calorimetry (DSC) analysis.
The glass transition temperature (Tg) of 10 was deter-
mined to be 69°C by heating the liquid nitrogen
quenched melt sample. The rigidity of 9,9-diphenyl-
fluorene makes a significant contribution to the high
morphological stability of 10. In contrast to 10,
molecules 12 and 13 exhibit lower Tg values, which
could be due to the introduction of two morphological
flexible long alkyl chains. The redox and thermal prop-
erties are also summarized in Table 1.
OC8H17
C8H17
O
a
6-L
S
75%
10-L
C8H17
O
OC8H17
b
Ar
Ar
8-L
S
11
Ar = phenyl
12-L Ar = 2-biphenyl
71%
76%
C8H17
O
OC8H17
c
9
Ar
Ar
67%
MeO2C
CO2Me
13 Ar = 4-butoxyphenyl
Scheme 2. Reagents and conditions: (a) 2-(9,9-diphenylfluo-
renyl) pinacol boronate, Pd(PPh3)4, K2CO3, PtBu3, DME; (b)
arylboronic acids, Pd(PPh3)4, K2CO3, PtBu3, toluene; (c) 4-
butoxyphenyl boronic acid, Pd(PPh3)4, K2CO3, PtBu3, tolu-
ene.
red-shifts both in absorption and emission maxima
when compared to those of 11, which is expected to
have a similar effective conjugation length as 10 based
on the structural analysis. The bathochromic shifts in
10 could be ascribed to the coplanar structure in the
rigid 9,9-diphenylfluorene moieties. It is intriguing that
11 and 12 exhibit very similar absorption and emission
behaviors, although 12 is expected to have a shorter
conjugation length due to the twisted conformation of
the 2-biphenyl moiety. However, the absorption maxi-
mum of 11 is red-shifted only by 10 nm when compared
with that of 2,5-diphenylthiophene.14 In other aspects,
the UV–vis and PL spectra of 13 are strongly blue-
shifted when compared to those of 10 and 11. In
molecule 13, the significant ortho–ortho steric interac-
To probe the influence of the chirality on the photo-
physical behavior of 10 and 12, the two pairs of enan-
tiomers were subjected to circular dichroism (CD)
analysis (Fig. 2). There is no CD effect associated with
the backbone chromophore absorption of the enan-
tiomeric pair of 10. In contrast to 10, the enantiomeric
pair of 12 demonstrated a weak CD effect, which could
be due to the exciton coupling of the conjugated main
chain and the twisted biphenyl subunit. The appearance
of a mirror image CD spectra of 10-L versus 10-D and
12-L vs 12-D indicates that the optical behavior of the
conjugated backbone is strongly reliant on the chirality
Table 1. The physical properties of chiral conjugated molecules 10–13
c
Compound
umax (nm)a
uem (nm)b
bFL
Band gap (eV)d
CV (V)e
Tg (°C)f
TGAg
10
11
12
13
354
336
335
283
424
418
415
406
0.27
0.32
0.31
0.15
3.1
3.2
3.2
3.6
0.69
0.76
0.75
–
69
−6
21
417
340
370
375
12
a 1.5×10−5 M in CHCl3.
b 1.5×10−5 M in CHCl3, excitation at absorption maximum.
c In EtOAc with Coumarin I as a standard.
d Determined by the absorption onset.
e In anhydrous CH2Cl2, TBAPF6 was used as a supporting electrolyte, potential was recorded relative to ferrocene/ferrocenium oxidation.
f By DSC analysis of the liquid nitrogen quenched melt sample.
g Temperature corresponding to 5% weight loss upon heating under nitrogen.

3316
K.-T. Wong, R.-T. Chen / Tetrahedron Letters 43 (2002) 3313–3317
Figure 1. UV–vis absorption spectra and emission spectra of 10, 12, and 13.
Figure 2. A comparison of the circular dichroism spectra of 10-L, 10-D, 12-L, and 12-D (2 mg in 5 mL CHCl3).
of the side chain. We suggest at this point that the
chiral information of the conjugated chromophore
could be induced by the stereogenic centers in the side
chain. The chiral molecules synthesized thus exhibit
interesting optical activity.
reactions. The chiral information originated from the
natural tartaric acid is effectively transferred to the
conjugated chromophore, resulting in interesting opti-
cal activities.
In summary, we have successfully synthesized a new
class of chiral conjugated molecules based on 4,5,6,7-
tetrahydro-5S,6S-dioctyloxylbenzothiophene as a cen-
tral linkage by zirconocene promoted intramolecular
cyclization of diynes and subsequent Suzuki coupling
Acknowledgements
We thank the National Science Council and the Min-
istry of Education of Taiwan for the financial support.

K.-T. Wong, R.-T. Chen / Tetrahedron Letters 43 (2002) 3313–3317
3317
We also thank Professor T. C. Chang for providing the
CD/ORD spectropolarimeter.
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