Pinacolophanes as versatile precursor for the practical synthesis of tolanophanes

Article abstract of DOI:10.1007/s00706-017-2125-3

Abstract: A new strategy for the synthesis of tolanophanes was investigated. The coupling of bridged dialdehydes gave selectively the corresponding monomer of pinacolophanes in quantitative yields under a simple and clean reaction condition. Without any f

Full text of DOI:10.1007/s00706-017-2125-3

Monatshefte für Chemie - Chemical Monthly
https://doi.org/10.1007/s00706-017-2125-3
ORIGINAL PAPER
Pinacolophanes as versatile precursor for the practical synthesis
of tolanophanes
Hossein Reza Darabi1 Saeed Rastgar1 Kioumars Aghapoor1 Farshid Mohsenzadeh1
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•
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Received: 30 October 2017 / Accepted: 3 December 2017
Ó Springer-Verlag GmbH Austria, part of Springer Nature 2017
Abstract
A new strategy for the synthesis of tolanophanes was investigated. The coupling of bridged dialdehydes gave selectively
the corresponding monomer of pinacolophanes in quantitative yields under a simple and clean reaction condition. Without
any further purification, pinacolophanes were quantitatively converted to tolanophanes using a two-step bromination–
dehydrobromination process. The overall yield of this practical protocol is upper than 90%. This precursor is highly
advantageous compared to the reported stilbenophane in terms of safety, operational simplicity, easy workup, and high
efficiency.
Graphical abstract
Keywords Cyclophanes Á Strained molecules Á Alkynes Á Methodology Á Bromination
Introduction
and have the potential to be incorporated into nonlinear
optical devices and chemical probes [1–3].
The diarylacetylenes are extensively utilized for the syn-
thesis of larger ethynylated aromatic systems. Many
Diarylacetylenes are also important building blocks for
the production of hexaarylbenzenes [4]. Hexaarylbenzene
derivatives are important organic precursors for fabrication
of semiconducting materials [5, 6], graphene fragments [7],
and advanced polymers [8], and have been found to be of
importance in crystal engineering [9–11] and photovoltaic
devices [12]. Despite various approaches to their synthesis,
the conventional method remains in catalytic cyclotrimer-
ization of 1,2-disubstituted alkynes in the presence of
activated transition metal complex [13]. However, this
methodology has the drawback of producing multiple
rotamers due to the restricted rotation around the C–C
bonds between the six peripheral aryl rings and central
benzene ring [14]. Recently, Rathore and coworkers
reported the synthesis of hexaarylbenzenes from bridged
diarylacetylenes (tolanophanes) leading to solely two iso-
mers [15].
ethynylated
aromatic
systems,
such
as
1,4-
bis(phenylethynyl)benzenes, 9,10-bis(phenylethynyl)an-
thracenes, 2,5-bis(phenylethynyl)thiophenes, and 2,5-
bis(phenylethynyl)metallacylopentadienes, exhibit inter-
esting structural, optoelectronic, luminescent properties,
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s00706-017-2125-3) contains supplementary
material, which is available to authorized users.
& Hossein Reza Darabi
darabi@ccerci.ac.ir; r_darabi@yahoo.com
1
Nano and Organic Synthesis Laboratory, Chemistry and
Chemical Engineering Research Center of Iran, Pajoohesh
Blvd., 17th Km of Tehran-Karaj Highway,
Tehran 14968-13151, Iran
123

H. R. Darabi et al.
Tolanophanes (cyclic diarylacetylenes) are cyclophanes
in which the rotation of tolan moiety around the alkynyl–
aryl single bond is restricted. In this regard, the small-sized
sterically constrained ortho-alkoxy cyclic diarylacetylenes,
namely tolanophanes 3, are promising targets applicable in
synthetic methodology [16–18], dynamic conformation
[18–20], building blocks for the production of hexaaryl-
benzenes [15], as well as host–guest chemistry [21].
Tolanophanes have been so far prepared by three
approaches, as shown in Scheme 1. Bunz and coworkers
recently reported the synthesis of some tolanophanes by
ring-closing metathesis of alkynes at 130 °C (method A)
[18]. Crisp and coworkers reported synthesis of tolano-
phanes by Heck–Cassar–Sonogashira–Hagihara coupling
of aryl halides with alkynes in the presence of palladium
salts (method B) [20]. As shown in Scheme 1, despite the
usefulness of these methods to prepare acyclic tolans, the
synthesis of the tolanophanes 3 from both procedures was
possible only in very poor yields. Therefore, we recently
developed a practical method as third route to access these
compounds in which bromination/dehydrobromination of
stilbenophanes led to the formation of 3 in high yields
(method C) [16]. However, the preparation of precursors,
i.e., stilbenophanes, suffers from some drawbacks, such as
using of bromine, harsh reaction condition, and cumber-
some isolation of monomers and dimers which their sep-
aration from the reaction mixture required time-consuming
column. Moreover, the formation of dimers led to the lower
formation of monomers 3.
pinacolophanes that, to the best of our knowledge, has not
yet been reported.
Results and discussion
Our new strategy to obtain tolanophanes 3 with the highest
efficiency and convenient reaction condition from
salicylaldehyde includes four steps. First, one-pot reaction of
salicylaldehyde with dihaloalkanes and potassium hydrox-
ide in THF/DMSO gave bridged dialdehydes 1a–1c as
crystalline solids in high yield (95%) (Scheme 2).
We recently achieved the synthesis of the pinacolo-
phanes 2 by the coupling of the corresponding dialdehydes
1 at room temperature under low-valent titanium reaction
condition (Scheme 3). The method gave; however, a mix-
ture of monomer and dimer in which the obtained isomers
were difficult to separate, leading to lower yield of the
desired monomers 2 [22].
To get a sole monomer, an intramolecular coupling of
compounds 1a–1c was carried out in the presence of Zn,
NaOH solution, and PEG-300 following a modified
methodology established by Tashiro and coworkers [23]. A
quantitative conversion of 1a–1c was observed (Scheme 2)
and it was found that the use of PEG-300 instead of ethanol
resulted in almost a sole monomer in excellent yields
(2a = 92%, 2b = 94%, and 2c = 95%).
A
literature survey revealed the conversion of
hydrobenzoin to 1,2-dibromo-1,2-diphenylethane in the
presence of phosphorous tribromide [24]. Considering its
effectiveness, it was thought that PBr3 might satisfactorily
convert pinacolophanes to their corresponding dibromo
compounds, and indeed, this was observed. The reaction
took place rapidly and was completed in 10 min. The
Taking into account the importance of tolanophanes as
potential precursors for hexaarylbenzene derivatives [15]
and our interest to their potential application [19, 21],
herein, we introduce a facile, practical, and convenient
two-step method for the preparation of tolanophanes from
Scheme 1
123

Pinacolophanes as versatile precursor for the practical synthesis of tolanophanes
Scheme 2
Scheme 3
conversion was quantitative and no byproduct was
observed. These dibromo compounds were subjected to
dehydrobromination to produce tolanophanes 3a–3c
quantitatively (Scheme 2). NMR spectroscopy data con-
firmed the formation of tolanophanes 3a–3c [16, 18].
present protocol are currently underway in our lab to elu-
cidate the reaction pathway.
Experimental
2-Hydroxybenzaldehyde (99%, Merck-800640), potassium
carbonate (99.5%, Merck-104924), zinc (95%, Merck-
108789), phosphorous tribromide (98%, Merck-822321),
PEG-300 (Merck-807484), potassium tert-butylate (Merck-
804918), silica gel 60 (230–400 mesh ASTM, Merck-
109385), 1,2-dibromoethane (Merck-800952), 1,3-dibro-
mopropane (Merck-803279), and 1,4-dibromobutane
(Merck-803275) were used as received. All other chemi-
cals (KOH, NaOH, and organic solvents) were of analytical
quality, and water was purified in an SG Water purification
Conclusion
A simple, clean, efficient, and improved protocol is intro-
duced for the preparation of pinacolophanes. These pre-
cursors were quantitatively converted to tolanophanes
using a two-step dibromination–dehydrobromination pro-
cess. The overall yield of this protocol is upper than 90%.
The important features of this efficient protocol are the
operational simplicity and easy product isolation and
purification. This procedure towards tolanophanes is highly
advantageous compared to the reported methods in terms
of operational simplicity, easy workup, and high efficiency.
Further investigations on various aromatic diols, the
substrate generality, and the scope and limitations of the
1
system (Germany). H and 13C NMR were recorded on a
Bruker 500 MHz spectrometer using TMS as internal
standard and CDCl3 as solvent.
123

H. R. Darabi et al.
Evaporation of the solvent afforded a quantitative yield of
the corresponding dibromo compounds, which was used in
the next step without further purification.
General procedure for synthesis of compounds
1a–1c
Dibromo compounds (10 mmol), 40 cm3 THF, and
potassium tert-butoxide (50 mmol) were placed in a round-
bottom flask and stirred at room temperature for 15 min.
After the completion of the reaction, the obtained solution
poured into 200 cm3 water. The resulting mixture was
extracted with dichloromethane (4 9 30 cm3). The organic
medium was removed with rotary evaporator to afford pure
tolanophanes 3a–3c in quantitative yields [16, 18].
The crude reaction mixture was purified by flash column
chromatography on dry silica gel using a short column
(eluent: hexane) to afford pure products 3a–3c (3a: 2.21 g,
94%; 3b: 2.34 g, 94%; 2c: 2.5 g, 95%).
Salicylaldehyde (200 mmol), KOH (240 mmol), 50 cm3
THF, and 10 cm3 DMSO were placed in a 250 cm3 two-
necked round-bottom flask provided with a reflux con-
denser. The obtained mixture was refluxed for 30 min
under continuous stirring. A solution of 100 mmol of 1,2-
dibromoethane, 1,3-dibromopropane, or 1,4-dibromobu-
tane was dissolved in 10 cm3 of THF/DMSO (9:1) and
added slowly to the reaction mixture using a dropping
funnel. After the addition, the mixture was kept under
reflux for 16 h. After completion of the reaction [monitored
by TLC using ethyl acetate/hexane (1:3 v/v)], the resulting
mixture was cooled to room temperature and washed with
250 cm3 water. The pale yellow precipitate was filtered off
and recrystallized in ethanol to afford pure products 1a–1b
as colorless crystalline solids. (1a: 25 g, yield: 92%; 1b:
27 g, yield: 95%; 1c: 28.8 g, yield: 97%) [22, 23].
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Phosphorous tribromide (15 mmol) was added dropwise to
a solution of pinacolophanes 2a–2c (10 mmol) in 50 cm3
dichloromethane at 0 °C. The reaction mixture was stirred
for 15 min at 0 °C. After the completion of the reaction,
the obtained solution was washed with 10% aqueous
sodium carbonate (2 9 20 cm3) and poured into 200 cm3
water. The resulting mixture was extracted with dichlor-
omethane (3 9 50 cm3) and dried over anhydrous Na2SO4.
123