Redox active dendronized polystyrenes equipped with peripheral triarylamines http://www.beilstein-journals.org/bjoc/content/pdf/1860-5397-10-326.pdf

Article abstract of DOI:10.3762/bjoc.10.326

Dendronized polystyrene having peripheral bromo groups was prepared from the dendronization of unfunctionalized polystyrene with dendritic diarylcarbenium ions bearing peripheral bromo groups using the "cation pool" method. The palladium-catalyzed aminati

Full text of DOI:10.3762/bjoc.10.326

Redox active dendronized polystyrenes equipped with
peripheral triarylamines
Toshiki Nokami§, Naoki Musya, Tatsuya Morofuji, Keiji Takeda, Masahiro Takumi,
Akihiro Shimizu and Jun-ichi Yoshida*
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 3097–3103.
Department of Synthetic Chemistry and Biological Chemistry,
Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto
615-8510, Japan
doi:10.3762/bjoc.10.326
Received: 30 October 2014
Accepted: 10 December 2014
Published: 22 December 2014
Email:
Jun-ichi Yoshida* - yoshida@sbchem.kyoto-u.ac.jp
This article is part of the Thematic Series "Electrosynthesis".
Guest Editor: S. R. Waldvogel
* Corresponding author
§ Present address: Department of Chemistry and Biotechnology,
Graduate School of Engineering, Tottori University, 4-101 Koyama-
chominami, Tottori 680-8552, Japan
© 2014 Nokami et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
carbocation; cross-coupling; dendrimer; dendronized polymer; redox
Abstract
Dendronized polystyrene having peripheral bromo groups was prepared from the dendronization of unfunctionalized polystyrene
with dendritic diarylcarbenium ions bearing peripheral bromo groups using the “cation pool” method. The palladium-catalyzed
amination of the peripheral bromo groups with diarylamine gave dendronized polystyrene equipped with peripheral triarylamines,
which exhibited two sets of reversible redox peaks in the cyclic voltammetry curves.
Introduction
Assembling small functional molecules using dendrimers [1] ties to form functional organic materials, and therefore, the
and dendronized polymers [2-8] as scaffolds serves as a useful development of efficient methods for the synthesis of redox-
method for synthesizing organic functional materials having active dendronized polymers is highly desirable. Recently, we
nanosize three-dimensional structures. Although there are many have developed a new method [21-26] for the synthesis of
examples of redox-active dendrimers, including those equipped dendronized polymers [27] from the dendronization of unfunc-
with ferrocene [9,10], triarylamines [11-14], and tetrathiafulva- tionalized polystyrenes with electrogenerated dendritic diaryl-
lene (TTF) derivatives [15-19], the corresponding dendronized carbenium ions. The simplicity and step economy of this
polymers are rare [20]. One of the major reasons for this seems method prompted us to synthesize dendronized polymers
to be the difficulty in making such structures. However, redox- equipped with peripheral functional groups by the use of this
active dendronized polymers should provide more opportuni- method.
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In principle, there are two synthetic approaches for synthe-
sizing peripherally functionalized dendronized polystyrenes:
(a) the functionalization of dendronized polystyrene (the “graft
from” approach, Figure 1a); and (b) dendronization of poly-
styrenes with the dendritic carbocation equipped with func-
tional groups (the “graft to” approach, Figure 1b). Both
approaches have advantages and disadvantages. In the former
case, more dendritic scaffolds can be introduced by functional-
ization, but structural inhomogeneity can occur from incom-
plete peripheral functionalization. In the latter approach, direct
dendronization by the functionalized diarylcarbenium ions may
be difficult, although the dendritic substituents would have a
uniform structure. In this paper we report on the synthesis of
redox-active dendronized polystyrenes via the peripheral modi-
fication of dendronized polystyrene [28].
Figure 1: Two synthetic approaches toward the peripherally functional-
ized dendronized polystyrenes (blue dotted line = polystyrene, orange
sector = dendron of the 1st generation, dark orange sector = dendron
of the 2nd generation). (a) Peripheral functionalization of the
dendronized polystyrene (“graft from” approach). (b) Dendronization
with dendritic diarylcarbenium ions equipped with functional groups
(“graft to” approach).
Results and Discussion
Dendrimer 4, having a bromo functionality was prepared on (Eox = 1.25 V vs SCE) which was used as a precursor of the
a multigram scale, as shown in Figure 2. Di(p-bromo- dendritic cation in our previous work [25], indicating that
phenyl)carbinol (1) was treated with SOCl2 to obtain di(p- bromine analogue 4 can act as a precursor of the dendritic
bromophenyl)methyl chloride (2) in quantitative yield. A cation.
Friedel–Crafts-type alkylation [29] of diphenylsilane 3 with 2 in
the presence of boron trifluoride etherate as a Lewis acid gave 4 The functionalization of 4 was examined before studying the
in 86% yield. The oxidation potential of 4 (Eox = 1.17 V vs functionalization of dendronized polystyrene. Thus, di(p-
SCE) is slightly lower than that of the fluorine analogue methoxyphenyl)amino groups were introduced to 4 using a
Figure 2: Preparation of the dendrimer having peripheral bromo groups and their conversion to diarylamino groups. aOxdation potential (Eox). bRedox
potential (E1/2).
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Buchwald–Hartwig amination [11]. The choice of a base is ized dendronized polystyrene (Figure 1b) employing 5 as a
crucial for this transformation, and the transformation was precursor of the dendritic carbocation was unsuccessful. Low-
successfully carried out using Cs2CO3 as a base and temperature electrochemical oxidation of dendrimer 5 using the
Pd[P(t-Bu)3]2 as a catalyst [30] to obtain the dendrimer 5 “cation pool” method [31-47] did not give the corresponding
having peripheral diarylamino groups in a yield of 77%. Com- dendritic diarylcarbenium ion even, when subjected to excess
pound 5 can serve as the precursor of dendritic diarylcarbenium capacitance (up to 5.0 F/mol).
ions having peripheral triarylamine structures. However, its
redox potential (E1/2 = 0.79 V vs SCE) is much lower than the Next, we examined the “graft-from” approach (Figure 3). The
oxidation potential of dendrimer 4, indicating that the triaryl- low-temperature electrochemical oxidation of dendrimer 4
amine moiety is oxidized before the benzylsilane moiety. Using using the “cation pool” method was performed in CD2Cl2, and
the “graft to” approach to synthesize peripherally functional- the resulting anodic solution was transferred to NMR tubes.
Figure 3: Preparation of dendronized polystyrenes having peripheral diarylamino groups.
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Beilstein J. Org. Chem. 2014, 10, 3097–3103.
Low-temperature NMR analysis indicated that an accumulation implemented rate (77%) calculated from the increase in weight
of dendritic diarylcarbenium ion 6 in the solution had occurred. of the polystyrene (22 mg to 150 mg).
The chemical shift of the cationic carbon (13C NMR δ 194.8)
and that of the proton attached to the cationic carbon (1H NMR In a similar manner, dendronized polystyrene 8b was synthe-
δ 9.93) indicated that there was no interaction between the sized from polystyrene having a longer chain length 7b
cationic carbon and peripheral bromo groups, because the (Mn = 9,300, PDI = 1.02). Using GPC analysis to estimate the
chemical shifts were almost identical to those of the fluorine molecular weight of the dendronized polymer was not appro-
analogue (13C NMR δ 194.4, 1H NMR δ 9.92) [25].
priate, because both the Mn and Mw values of the obtained
dendronized polystyrene 8b were only twice that of the starting
The reaction of 6 (generated on a preparative scale in CH2Cl2) polystyrene 7b (Table 1). SEC–MALLS analysis indicated that
with a low-molecular weight polystyrene 7a (Mn = 1,580, poly- 6 reacted with about 70% of the phenyl groups on polystyrene
dispersity index (PDI) = 1.04, 22 mg) was performed at 0 °C 7b. This ratio is slightly larger than the implemented rate (55%)
(Figure 3). The resulting dendronized polystyrene 8a was char- calculated from the increase in weight of the polystyrene
acterized using MALDI–TOF MS analysis. Six peak groups (156 mg to 820 mg).
were observed, as shown in Figure 4. The peak occurring at
10,573 Da [M + Ag+] is derived from 11 dendritic substituents The peripheral bromo groups of 8b were converted to diaryl-
(815 Da × 11), 14 styrene units (103 Da × 11 + 104 Da × 3), amino groups using a Hartwig–Buchwald amination employing
and a butyl group (58 Da), which was derived from the initiator Pd[P(t-Bu)3]2 (10 mol %) and Cs2CO3 (12 equiv) to obtain 9.
at the end of the polystyrene. The broader peaks seem to be Dendronized polystyrene 9 was also analyzed using GPC and
attributable to two isotopes of the bromo groups (79 and 81 Da), SEC–MALLS measurements employing DMF as an eluent. The
and the small peak separation 101–103 Da is consistent with the results are summarized in Table 1. GPC analysis indicated
molecular weight of styrene monomer (104 Da). The MS either a slight increase or no increase in molecular weight from
analysis indicated that 6 reacted with about 80% of the phenyl the peripheral functionalization, but SEC–MALLS measure-
groups on polystyrene 7a. This ratio is consistent with the ments showed an identifiable increase in the molecular weight
(Mw = 62,000 to 226,000), which is more than double the theo-
retical value (Mw = 100,000 after 100% conversion of the
peripheral bromo groups to diarylamino groups). Although the
PDI did not change appreciably after dendronization (7b to 8b),
the PDI increased slightly after peripheral functionalization (8b
to 9). Presently, the ratio of methine protons in the focal points
of the dendritic structure (δ 5.38–5.00, broad singlet, 3H) and
the methyl protons of the peripheral methoxy groups
(δ 3.80–3.40, broad singlet, 24H) in the 1H NMR spectra is the
only evidence for the conversion of the peripheral bromo
groups to diarylamino groups. The observed ratio was 1:8.4
(methane/methyl), which indicates a quantitative conversion
(theoretical ratio is 1:8).
Figure 4: MALDI–TOF MS analysis of the dendronized polystyrene
with peripheral bromo groups.
The redox behavior of 9 was studied by means of cyclic voltam-
metry (CV, Figure 5). Compound 9 showed two sets of revers-
Table 1: Molecular weight analyses of the dendronized polystyrenes.
polymer
GPC
SEC-MALLS
Mw
Mn
Mw
PDI
Mn
PDI
7ba
8b
9
9,300
18,800
18,200
9,490
1.02
1.06
1.21
–
59,400
–
62,000
–
1.04
1.29
19,900
22,100
175,800
226,000
aPolymer 7b was analyzed only by GPC.
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Beilstein J. Org. Chem. 2014, 10, 3097–3103.
Figure 5: Cyclic voltammograms of dendronized polystyrene 9 (black line), model compound 10 (blue line), and the film of dendronized polystyrene 9
(red line). See the Experimental section for the details.
ible redox peaks (E1/2 = 0.80 and 1.04 V vs SCE), although it diarylamines. Thus, dendronized polystyrene having peripheral
has been reported that di(p-methoxyphenyl)phenylamine shows bromo groups may serve as a versatile precursor of dendronized
a single redox peak in its CV curves [48]. Therefore, the two polystyrenes equipped with various functional groups.
sets of redox peaks of 9 seem to be ascribable to the interaction Dendronized polystyrene having peripheral diarylamino groups
of the initially formed radical cation from the triarylamine showed reversible redox behavior in both solution and as a film
moiety with a neighboring neutral triarylamine moiety, which deposited on an electrode. Further optimization of the present
disfavors the second electron transfer from the latter, although method and applications of peripheral functionalization of
the details of this reaction are not clear as yet [49,50]. This is dendronized polystyrenes are under investigation in our labora-
consistent with the observation that dendrimer 10 prepared by tory.
desilylation of 5 followed by amination (see Supporting Infor-
Experimental
mation File 1 for preparative details) showed two reversible
waves occurring at similar potentials (E1/2 = 0.81 and 1.10 V vs Electrochemical analyses
SCE) (Figure 5, blue line).
Electrochemical analysis was performed using an ALS/
Chi700DS electrochemical analyzer. A saturated calomel elec-
A film of dendronized polystyrene 9 was prepared on a Pt plate trode (SCE) (RE-2B, ALS Co. Ltd.) was used as the reference
electrode as a film by drop casting of a CH2Cl2 solution of 9. electrode.
The redox behavior of the film of 9 on a Pt electrode was
analyzed using CV (Figure 5, red line). Reversible CV cycles The oxidation potential (Eox) of dendrimer 4 was measured
(E1/2 = 0.76 and 1.01 V vs SCE) were obtained in a mixed using linear sweep voltammetry employing a gassy carbon (GC)
solvent (CH3CN/toluene 1:3) using Bu4NB(C6F5)4 as the rotating disk electrode (diameter = 3.0 mm, ALS Co. Ltd.) and
electrolyte. The small shifts in the redox peaks from those a Pt wire counter electrode at room temperature. CH2Cl2 was
obtained from solution may be attributable to the changes in used as a solvent and Bu4NBF4 (0.1 M) was used as a support-
solvent and electrolyte. The peak separations for a film of 9 ing electrolyte. The scan rate was 100 mV/s.
(ΔE (Eox – Ered) = 52 and 27 mV) are significantly smaller than
those observed for a solution of 9 (ΔE = 103 and 87 mV), indi- The redox potentials (E1/2) of 5 and 10, and that of dendronized
cating immobilization of 9 occurred on the surface of the elec- polystyrene 9 were measured by cyclic voltammetry (CV) using
trode.
a GC working electrode (diameter = 3.0 mm, ALS Co. Ltd.) and
a Pt wire counter electrode at room temperature. CH2Cl2 was
used as a solvent and Bu4NBF4 (0.1 M) was used as a support-
Conclusion
In conclusion, redox-active dendronized polystyrene was ing electrolyte. The scan rate was 100 mV/s. The GC electrode
successfully synthesized by peripheral functionalization of was polished with alumina powder (0.05 μm) in water using a
dendronized polystyrene having peripheral bromo groups with polishing pad, water-washed, and air-dried before use.
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Acknowledgements
The authors gratefully acknowledge partial financial supports
from Grants-in-Aid for Scientific Research on Innovative Areas
(No. 2105) from the MEXT and Young Scientists (B) (No.
23750127) from the JSPS. T. N. thanks the Asahi Glass Foun-
dation for financial support. The authors thank Prof. Mitsuo
Sawamoto and Dr. Takaya Terashima of Kyoto University for
SCE-MALLS measurements and Dr. Keiko Kuwata of Kyoto
University for MS analyses.
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