Posttreatment technique for SN2 alkylation of aromatics with alkyl halides: Aiming toward large-scale synthesis of building blocks for soft π-molecular materials

Article abstract of DOI:10.1246/bcsj.20180075

Unreacted alkyl halide and byproduct olefin contaminants in products of S_N2 alkylation reactions of aromatic compounds can be efficiently removed by sequential posttreatments with a base and a boron compound (sodium borohydride or 9- borabicyclo[3.3.1]nonane), followed by column chromatography on silica gel. These treatments permit large-scale purification of various alkylated aromatics, thereby assisting in the development of soft π-conjugated materials, such as monomers for semiconducting polymers or alkylated π-functional liquids.

Full text of DOI:10.1246/bcsj.20180075

Advance Publication Cover Page
Posttreatment Technique for SN2 Alkylation of Aromatics with Alkyl Halides: Aiming
Toward Large-Scale Synthesis of Building Blocks for Soft π-Molecular Materials
Ken Okamoto, Fengniu Lu, and Takashi Nakanishi*
Advance Publication on the web May 19, 2018
doi:10.1246/bcsj.20180075
© 2018 The Chemical Society of Japan
Advance Publication is a service for online publication of manuscripts prior to releasing fully edited, printed versions. Entire
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Publication manuscripts.

Posttreatment Technique for SN2 Alkylation of Aromatics with Alkyl Halides:
Aiming Toward Large-Scale Synthesis of Building Blocks for Soft -Molecular
Materials
Ken Okamoto,1,2 Fengniu Lu,1 and Takashi Nakanishi*1
1Frontier Molecules Group, International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science
(NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan
2Department of Engineering for Future Innovation, Division of Chemical Engineering and Biotechnology, National Institute of Technology,
Ichinoseki College, Takanashi, Hagisho, Ichinoseki 021-8511, Japan
E-mail: NAKANISHI.Takashi@nims.go.jp
Takashi Nakanishi
Takashi Nakanishi received PH.D. Degree in shorter period from Nagasaki University in 2000. Thereafter, he did JSPS
postdoctoral researches at Houston University (USA), and at Oxford University (UK) and joined the National Institute for
Materials Science (NIMS), Japan, in 2004. Since 2016, he is being the current position as a group leader at WPI-MANA,
NIMS. In 2007-2010, he was also a group leader at the Max Planck Institute of Colloids and Interfaces (Germany). He also has several
visiting professor experiences at Warsaw University of Technology (Poland), Institute for Molecular Science (Japan), Shenzhen University
(China), and Hokkaido University (Japan). His research focuses about functional soft materials with precisely controlled self-assembly, in
particular optoelectronically-functional molecular liquids.
Abstract
Unreacted alkyl halide and byproduct olefin contaminants
in products of SN2 alkylation reactions of aromatic compounds
can be efficiently removed by sequential posttreatments with a
the polymers on the relevant devices.23 In addition, several
recent perspective papers have emphasized that alkyl chains on
aromatics have a far greater role than merely increasing the
solubility and improving the wet-processability of these organic
materials.8,9,12 As other attractive and relatively new examples,
highly luminescent, light- and heat-stable, solvent-free,
-conjugated molecules that are liquid at room temperature
have been developed by attaching multiple long branched alkyl
chains to a -unit.24–33 To provide those versatile alkylated
-conjugated molecules, processes for the alkylation of the
corresponding aromatic substrates on a large scale are in high
demand.
base and
9-borabicyclo[3.3.1]nonane),
a
boron compound (sodium borohydride or
followed by column
chromatography on silica gel. These treatments permit
large-scale purification of various alkylated aromatics, thereby
assisting in the development of soft -conjugated materials,
such as monomers for semiconducting polymers or alkylated
-functional liquids.
In general, fluorenes, anilines, and phenols, which contain
highly acidic (or low-pKa) protons, are the most commonly
used substrates (Scheme 1a) to be functionalized to form
-conjugated molecules. To introduce the alkyl chains, a
bimolecular nucleophilic substitution (SN2) reaction is normally
employed for C-alkylation at cross-linking sites of fluorene,
N-alkylation of anilines, or Williamson ether synthesis from
phenols; these reactions will be referred to subsequently as C-,
N-, and O-alkylation, respectively. However, the SN2 reaction
is intrinsically accompanied by a bimolecular elimination (E2)
reaction of the starting alkyl halide. Consequently, the resulting
mixtures potentially include the target products, unreacted
chemicals, and olefins resulting from the E2 reaction (Scheme
1b). Although yields of alkylation with linear chains are
moderately good,34 the yields of alkylation with branched
chains tend to be lower,35 possibly due to more stabilized
intermediates of the E2 reaction by the branched alkyl chains.36
Because of the similar polarities of the target compound and
byproduct, the purification through conventional column
chromatography becomes bottleneck for the large-scale
synthesis. Furthermore, the difficulty in crystallization of
compounds with long linear or branched alkyl chains often
requires other more work-intensive purification processes.
Actually, with some of our previously reported liquid
-conjugated molecules, we had no choice but to use
1. Introduction
The self-assembly controlled spatial arrangement of
-conjugated moieties plays
a
decisive role in the
optoelectronic properties of organic soft materials such as
liquid crystals, gels, as well as nanoarchitectonics typified by
layer-by-layer (LbL).1–6 It has been recently found that the
installation of alkyl7–13 or siloxy groups14–18 onto -conjugated
molecular soft materials can often tailor their physical
properties such as physical state (e.g. solid-liquid or
crystal-liquid phase transformation), assembled morphology,
and molecular orientation, which significantly influence their
optoelectronic applications.7–9 In particular, the effects of alkyl
substituents on aromatics or rigid metal complexes, such as the
odd–even effect19,20 or the fastener effect,21,22 have become
widely known for liquid crystals. On the other hand, recent
insights into the role of alkyl chains attached to -conjugated
materials have resulted in the development of ‘side-chain
engineering’ or ‘alkyl-chain engineering’ strategies.7–12 There
are many reports in the literature that describe the significant
effects of alkyl chains on controlling the assembly and
orientation of optoelectronically active -conjugated molecules.
For instance, Takimiya and Osaka and their co-workers
reported that well-controlled substitution of branched alkyl
groups can modify the photovoltaic performance of organic
donor–acceptor-type polymers by changing the orientation of
high-performance liquid chromatography (HPLC) as
a
purification process, which does not allow large-scale

purification because of the limited volume of HPLC systems.30
Step 1
Base treatment
R
Ar
(a) alkylation reactions
Target
R'
R
+
Target
KX
tBuOH
R
KOtBu
+
+
N
H
O
H
X
H
H
H
R'
R'
pKa in H2O
22
30.6
10
(18.0)
unreacted
alkyl halide
(DMSO) (22.6)
SN2
+
+
Step 2
Reduction
extraction
R
X
(i) Hydrogenation
H
R'
R' = H or alkyl chain
H
H
E2
R
NaBH4 / AcOH
with cat. Pd/C
H
Target
+
Base (NaOtBu, Na2CO3, KOtBu, K2CO3 etc.)
R'
R
alkane
+
C-alkylation
N-alkylation
O-alkylation
R'
(ii) Hydroboration
+
Target
(b)
H
R'
R
R
B
Target
+
R
BH
target
product
O
R
R
N
R'
9-BBN
(0.5 M in THF)
R'
R'
trialkyl borane
R'
R'
R
Step 3
Chromatography
Unreacted alkyl halide and byproduct via E2 reaction
R
Silica gel
Chromatography
Ar
Crude of step 2
H
R
R
R'
Base
X
X
R
R'
R'
R'
Target
alkyl halide
byproduct
(olefin)
Scheme 2. Proposed three-step treatment.
2-Bromo-9H-fluorene,
1-bromo-3,5-dimethoxybenzene,
1-bromododecane,
1-bromobutane,
Scheme 1. (a) Typical alkylation reactions of aromatics and (b)
possible mixtures resulting from alkylation reactions.
2-ethylhexylbromide, potassium borohydride (NaBH4) were
purchased from TCI Japan. Fluorene and
In this study, we describe a series of posttreatments for
the purification of alkylated aromatics by using common
chemicals to remove unreacted alkyl halides and byproduct
olefins from the reaction mixtures (Scheme 2). The first
posttreatment involves the transformation of unreacted alkyl
halides into olefins with potassium tert-butoxide (KOtBu). The
second posttreatment is either (a) a mild hydrogenation of the
olefins with sodium borohydride (NaBH4) in acetic acid in the
presence of Pd/C or (b) hydroboration of the olefins with a 0.5
2-dimethylaminoethanol were purchased from Wako chemicals.
Ammonium chloride (NH4Cl), dry THF, n-hexane, ethy acetate
(EtOAc), dichloromethane (DCM), I2 (for TLC stain) were
purchased from Kanto Chemical. All Chemical were used
without further purification. The branched alkylhalide
(so-called swallowtail-type alkyl chain): 2-hexyldecanbromide
and 5-bromobenzene-1,3-diol were synthesized according to
the literature procedures.37,38
2.2 General Procedure of Alkylation: Fluorenes. The general
procedure for the alkylation of fluorene (C-alkylation) is as
follows. A two-neck flask with a three-way stopcock and a
rubber septum was charged with fluorene (4 mmol, 0.664 g)
and dry THF (20 mL). KOtBu (12 mmol, 1.346 g) was added to
the flask at room temperature, then the mixture was heated at
reflux for 20 min. The flask was allowed to reach room
temperature with stirring for 30 min. 1-Bromododecane (8.8
mmol, 2.20 g) was slowly added to the reaction mixture at 0 °C,
then again the flask was allowed to reach room temperature
with stirring for overnight.
M
solution of 9-borabicyclo[3.3.1]nonane (9-BBN) in
tetrahydrofuran (THF). In case (a), the resulting compound is a
saturated alkane that is not as reactive as the original olefin. In
case (b), the olefin is converted into an alkyl boron compound
that can be readily removed by column chromatography on
silica gel. As a result of these treatments, the target alkylated
aromatic compounds could be collected as the first fraction
from column chromatography on silica gel (the third
posttreatment). Notably, even products produced on gram-scale
could be purified by one short-column after the first and second
posttreatments.
1
2.2.1 The First Posttreatment. After checking of TLC and H
2. Experimental
NMR spectrum, the reaction mixture refluxed for one hour. The
reaction was quenched by the addition of sat. NH4Cl aqueous
solution, extracted into n-hexane (50 × 3 mL), washed
successively with water, brine, dried (anhydrous Na2SO4) and
concentrated by rotary evaporator to afford the reaction mixture
(including the target dialkylated fluorene and olefin,).
2.2.2 The Second Posttreatment. 2-(a): To a mixture of the
crude mixture of the first posttreatment, 5wt% Pd/C (0.100 g)
and acetic acid (AcOH, 4 mmol) in THF (10 mL) were slowly
added portionwise NaBH4 (4 mmol, 0.151 g), and the resulting
mixture was stirred at room temperature for 12 hours.
Thereafter, the mixture was filtered and the precipitate was
washed with hexane and concentrated by rotary evaporator for
NMR measurement.
1
2.1 Instrumentation and Materials. H NMR and 13C NMR
spectra were obtained on JEOL ECS-400 spectrometers using
CDCl3 as a solvent (peak position δ1H = 7.26 ppm) and
tetramethylsilane was used as an internal standard for H NMR
spectra (0.00 ppm). For thin-layer chromatography (TLC)
analyses in this work, EMD/MerckTM KGaA precoated TLC
plates (silica-gel 60 Al F254) were used.
All reactions were carried out under an argon atmosphere
using a standard 2-neck flask with a three-way stopcock and a
rubber septum. Aniline, 2-hexyl-1-decanol, and KOtBu were
purchased form Sigma-Aldrich Company.
1
2-(b): Since the alkylation was an almost quantitative

reaction, the expected amount of olefin in the mixture was
equal to the excess amount of starting alkyl halide (0.8 mmol in
the above case). It could be assumed that 0.8 mmol of olefin
should be treated with the small excess of 9-BBN (1.2 mmol,
1.5 eq vs olefin). A two neck flask with a three-way stopcock
and a rubber septum was charged with the above reaction
mixture and dry THF (20 mL). 0.5 M 9-BBN in THF (1.2
mmol, 2.4 mL) was added to the flask at 0 °C and then the flask
was allowed to warm to room temperature and stirred for one
hour. To quench the remaining 9-BBN in the reaction,
aminoalchol (2-dimethylaminoethanol, 0.5 mmol, 0.045 g) was
added to the flask and stirred for another one hour.39 The final
reaction mixture was quenched by the addition of water (5 mL),
extracted into EtOAc (50 × 3 mL), washed successively with
water, brine, dried (anhydrous Na2SO4) and concentrated by
rotary evaporator to afford the crude mixture.
the presence of dec-1-ene as the sole byproduct, and no signals
for unreacted 1-bromodecane were observed (Figure 2).
1
Figure 2. Expanded H NMR spectra (400 MHz, CDCl3, room
2.2.3 The Third Posttreatment: The crude product was
purified on short silica-gel [5 cm (diameter of column) × 10 cm
(length of deposition of silica-gel) was enough for this scale]
with n-hexane as eluent to give the dialkylated fluorene.
temperature) of crude 9,9-didodecyl-9H-fluorene (a) before and
(b) after refluxing for one hour with residual KOtBu, without
purification by silica-gel column chromatography. Notations:
▲: 1-bromodecane, ●: dec-1-ene, **: THF.
3. Results and Discussion
Removal of the resulting olefin is highly dependent on its
chain length. If the alkyl halide has six carbon atoms or less,
the resultant olefins (e.g., hex-1-ene) can be removed by rotary
evaporation, because the boiling points of short-chain alkyl
halides and their corresponding olefins are sufficiently low,
even under atmospheric pressure (Table 1). For instance, when
the alkylation of fluorene with 1-bromobutane was examined,
1-bromobutane and but-1-ene were not observed after
evaporation (in practice, by using a rotary evaporator) and
subsequent filtration through a short column of silica gel,
without any other posttreatment (see Figure S1 in supporting
information). In the case of alkylation of fluorene with
2-ethylhexyl bromide, neither the alkyl bromide nor its
corresponding olefin (2-ethylhex-1-ene) were detected after the
first posttreatment (see Figure S2 in supporting information).
Therefore, depending on the boiling points of short-chain alkyl
bromides and their corresponding olefins, the first
posttreatment plus evaporation using a conventional rotary
evaporator might be sufficient to eliminate these impurities.
However, for long-chain linear or branched olefins, which
have much higher boiling points, evaporative removal is an
uncertain method unless it is checked by a qualitative test. To
Our investigation began with the alkylation of
9H-fluorene with 1-bromododecane as a representative linear
1
long-chain alkyl halide. The crude H NMR spectrum showed
an almost quantitative reaction, with no residual fluorene
(Figure 1; the peak at  = 3.89 ppm corresponding to protons in
the 9-position of fluorene is absent). However, the signals of
small amounts of unreacted 1-bromodecane and byproduct
dec-1-ene were observed in the 1H NMR spectrum.
establish
a
versatile and universal method for the
transformation of impurities by using common chemicals, we
attempted to convert the byproduct olefins into alkanes or alkyl
boranes, as shown in Scheme 2.
Figure 1. 1H NMR spectrum (400 MHz, CDCl3, room
temperature) of crude 9,9-didodecyl-9H-fluorene without
First, we attempted to treat the crude product in a second
posttreatment procedure by using the conditions reported by
Cordes and co-workers.44,45 A mixture of 9,9-didodecylfluorene
and dodec-1-ene was treated with NaBH4 in AcOH in the
presence of Pd/C at room temperature. After 12 hours, no
dodec-1-ene was observed in the 1H NMR spectrum (Figure 3a;
compare with Figure 2). As an alternative second posttreatment
step, we treated the reaction mixture with a 0.5 M THF solution
of 9-BBN, a highly reactive hydroboration agent. After
hydroboration at 0 °C for 30 minutes then at room temperature
purification or further treatment. Notations:
○ :
9,9-didodecyl-9H-fluorene, ▲: 1-bromodecane, ●: dec-1-ene,
*: CHCl3, **: THF.
Despite the quantitative nature of this reaction, previously
reported yields of this type of alkylation of a fluorene unit are
about 80–85%,32 possibly as a result of unavoidable losses
during purification. Both the unreacted alkyl halides and olefins
can be active partners for subsequent cross-coupling reactions,
such as the Kumada–Tamao–Corriu coupling40,41 or Mizoroki–
Heck reaction,42,43 or further polymerizations to give
π-conjugated polymers. For reliability and ease of purification,
the unreacted alkyl halide was transformed into the
corresponding olefin by refluxing the reaction mixture with
residual KOtBu in the same flask for one hour, as the E2
reaction does not proceed smoothly at room temperature. After
this first treatment, the 1H NMR of the reaction mixture showed
1
for one hour, none of the olefin was detected in the H NMR
spectrum (Figure 3b). We then subjected the mixture to
chromatography on a short silica gel column, which gave the
target 9,9-didodecyl-9H-fluorene in an excellent 95% yield
(Figure 3c).

Table 1. Summary of alkylation reactions and retention factors
in TLC
Rf1
=
Rf1
RBr
(Rf1/bp)a,b
–
Rf3
Rf2
=
Rf2
–
Target
alkylated
compound
(Rf3)a
Aromatic
Entry
(Rf0)a
Olefin
(Rf2/bp)a,b
Rf3
nC12H25Br:
R1Br
0.13
(0.77/276)
nC10H21
R1 R1
1
(0.64)
: O1
0.19
0.01
(0.83c/21347)
Bu
Br
Et
: R2Br
(0.42)
(0.68/93‒98)
Bu
Et
: O2
2
R2
R2
(0.76c/120,48
66‒70 at 120
Torr49)
0.09
(0.67)
R1Br (0.77)
0.10
0.16
Br
R1 R1
3
4
O1 (0.83c)
(0.67)
C8H17
Br: R3Br
C6H13
Br
(0.78/114‒118
0.01
0.07
(0.51)
Br
at 0.5 Torr50)
C8H17
R3 R3
C6H13
: O3
(0.77)
(0.84c/165
at 9 Torr51)
R1Br (0.77)
0.05
0.01
0.09
0.15
1
Figure 3. H NMR (400 MHz, CDCl3, room temperature) of
R1
N
NH2
9,9-didodecyl-9H-fluorene (a) after the treatment with NaBH4
without purification by silica-gel column chromatography, (b)
after the treatment with 9-BBN without purification by
silica-gel column chromatography, and (c) after silica-gel
column chromatography of the crude product treated with
9-BBN.
R1
5
6
O1 (0.83c)
R3Br (0.78)
O3 (0.84c)
(<0.1)c
(0.82)
R3
O
OH
R3
Br
OH
Br
O
(0.0)
(0.69)
To examine the scope of application of the treatment,
reaction mixtures from other aromatic building blocks for
a TLC silica gel 60 Al plates (5 cm × 1.5 cm) were used in determining
the retention factors (Rf), The values of Rf1 and Rf2 were determined by
using pure chemicals and Rf3 was determined by using the crude
mixture. The Rf values were calculated from the averages of two or
three TLC tests. All TLC tests were carried out by using hexane as an
eluent at room temperature. b Boiling point/°C at 760 Torr are reported
unless otherwise noted. c With short tailing.
developing
optoelectronically
active
materials
[2-bromo-9H-fluorene, aniline, and 5-bromobenzene-1,3-diol]
were subjected to our three-step posttreatment, mainly using
9-BBN (Scheme 2 and Table 1), because the target alkylated
aromatics can be collected as first fractions from column
chromatography. As shown in Table 1, even when we used
hexane, a nonpolar solvent, as the eluent for thin-layer

chromatography (TLC) tests, the retention factors (Rf) of the
byproduct olefins were only slightly higher than those of the
target alkylated aromatics. For this reason, it is difficult to
separate the target from the unreacted alkyl halide and
byproduct olefin through chromatography on a short column of
silica gel. The values of the differences in retention factor
between the alkyl halide and the target alkylated compound
(Rf1) and between the alkene and the target alkylated
compound (Rf2) tended to be small for fluorenes bearing halo
groups and/or alkyl chains (Table 1, entries 1 and 2, 3 and 4).
Alternatively, other alkylation reagent such as alkyl tosylates
that have higher polarities than alkyl halides can be used.
However, the risk of the E2 reaction with alkyl tosylates is
similar to that with alkyl halides.46 Although linear alkyl
halides rarely undergo E2 reactions with weak bases such as
potassium carbonate (K2CO3), small amounts of olefin were
Scientific Research (JSPS KAKENHI Grant Number
JP25104011, JP15H03801, JP18H03922) and NIMS Molecule
& Material Synthesis Platform in “Nanotechnology Platform
Project” from MEXT, Japan.
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Figure 4. Flow chart of the three-step posttreatment method.
4. Conclusion
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unreacted alkyl halides and the corresponding olefin
byproducts from the reaction mixture. This posttreatment
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Acknowledgement
We gratefully acknowledge David C. Bowes in NIMBUS
Technologies LCC for help with preparation of the manuscript.
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