Design and synthesis of 2-(1,3-dialkoxy-2-methylpropan-2-yl)-1,3- diarylpropanes as tethering units for folded H-stacking polymers

Article abstract of DOI:10.1016/j.tetlet.2014.03.020

1,3-Diarylpropenes 9 having a 1,3-dialkoxy-2-methylpropan-2-yl group were designed as tethering monomers for folded H-stacking polymers, and were readily synthesized from 2-ethoxymethylidene malonate in four- or five-steps, including a facile sequential a

Full text of DOI:10.1016/j.tetlet.2014.03.020

Tetrahedron Letters 55 (2014) 2649–2653
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Design and synthesis of 2-(1,3-dialkoxy-2-methylpropan-2-yl)-1,
3-diarylpropanes as tethering units for folded H-stacking polymers
Ryota Moriai a, Yu-suke Naito a, Ryosuke Nomura a, Shigeaki Funyu b, Ken-ich Ishitsuka b, Naoki Asano b,
Sentaro Okamoto a,
⇑
a Department of Material and Life Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan
b Tsukuba Research Laboratory, Hitachi Chemical Co., Ltd, 48 Wadai, Tsukuba-shi, Ibaraki 300-4247, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
1,3-Diarylpropenes 9 having a 1,3-dialkoxy-2-methylpropan-2-yl group were designed as tethering
monomers for folded H-stacking polymers, and were readily synthesized from 2-ethoxymethylidene mal-
onate in four- or five-steps, including a facile sequential addition–elimination–addition reaction of benzyl
zinc reagents. The preference for the closed (stacked) conformation in the resulting 2-substituted 1,3-
diarylpropanes 9 was evaluated using MM2 calculations, 1H NMR analyses, and fluorescence measure-
Received 20 December 2013
Revised 27 January 2014
Accepted 5 March 2014
Available online 17 March 2014
ments. Copolymerization of the resulting monomers 9 with compounds containing
polymers with blue-shifted UV-absorptions both in solution and as films, compared with that of a model
p-units provided
Keywords:
Conformation analysis
H-Stacking
Folded polymers
Fluorescence
compound containing a single
p
-unit. This optical property is unique to H-aggregated
p-units.
Ó 2014 Elsevier Ltd. All rights reserved.
The two-dimensional orientation of
p
molecules, that is, J- and
H-aggregations, imparts distinct photophysical, electronic, and
photoelectronic properties. The ordered assembly of -moieties
p
in polymeric molecules is of interest because of low processing
costs and desirable mechanical properties for electronically active
polymers.1
In 1965, Hirayama reported the systematic investigation of the
efficiency of intramolecular excimer formation in 1,3-diarylalkanes
and found that 1,3-diarylpropanes and their 2-substituted deriva-
tives 1 dominantly exhibit excimer emission (Scheme 1).2
Recently, we revealed that the introduction of a bulky substitu-
ent at the 2-position of the propane can promote exclusive intra-
molecular excimer formation in the excited state as well as the
closed (stacked) conformation of the diaryl units with a parallel
orientation in the ground state, both in solution and as a solid
(Scheme 1).3,4 Specifically, the 2-substituted 1,3-diphenylpropanes
2 exhibited only excimer emission at 333 nm upon irradiation at
263 nm, and in their 1H NMR spectra, the chemical shifts of protons
at the 2-, 3-, and 4-positions of the benzene rings were largely
shifted upfield because of the shielding effect between each ben-
zene ring.5 These results combined with those of a conformational
analysis using MM2 calculations indicated that a closed (stacked)
conformation in which the two benzene rings of 2 face each other
predominates in both the ground and excited states. We then
⇑
Corresponding author. Tel.: +81 45 481 5661; fax: +81 45 413 9770.
E-mail address: okamos10@kanagawa-u.ac.jp (S. Okamoto).
Scheme 1. 1,3-Diarylpropanes and their application to folded H-stacking polymers.
http://dx.doi.org/10.1016/j.tetlet.2014.03.020
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

2650
R. Moriai et al. / Tetrahedron Letters 55 (2014) 2649–2653
Table 1
Results of conformation analyses for 2, 8, and 9
Z
Z
Z
Open
Half-Open
Calcd
Closed
Run
Z
Ratio of
dc
O:HAO:Ca Intensity of FL
at 285 and
(ppm)
333 nmb
(excited at, nm)
EtO2C
EtO2C
1
2
48:51:1
1:9:90
72:28 (259)d
1:99 (263)
7.17
6.98
2a
Et3SiO
HO
2a
8a
3
4
5
8a
2:21:77
1:13:86
1:11:88
29:71 (263)
30:70 (263)
6:94 (263)e
7.10
7.01
6.99
HO
Et3SiO
9a-Si
Et3SiO
9a-
Si
n-C6H13
9a-Hex
n-C6H13
O
9a-
Hex
O
a
Based on the energy differences calculated using MM2 (Quantum CAChe 4.9)
and estimated according to Boltzmann’s distribution at 300 K.
b
Fluorescence (FL) of a 10ꢀ4 M solution in CH2Cl2 at room temperature.
c
1H NMR (CDCl3, 500 MHz, 20 °C) chemical shift of the ortho-protons of the Ph
moieties.
d
Ratio of FL intensity at 282 and 333 nm.
Ratio of the FL intensity at 285 and 312 nm.
e
9b and its analogues 9c and 9d were synthesized as new folded
tethering units (Scheme 2). Diol compounds 8, which are
precursors of 9 and may be easily functionalized, were readily
synthesized in three steps from commercially available diethyl
2-ethoxymethylidene malonate. First, the tandem addition–elimi-
nation–addition reaction7 of an excess of (substituted) benzyl zinc
bromides 6, which were prepared from the corresponding benzyl
bromides and zinc powder in THF, with 2-ethoxymethylidene mal-
onate smoothly proceeded to afford 2-(1,3-diarylpropan-2-yl)mal-
Scheme 2. Synthesis of 8 and 9.
applied these monomers to the synthesis of polymers 5 that pre-
dominantly exist in a folded H-stacking conformation, via the cou-
pling polymerization of dibromo derivative 3 with bis-boronic
onates 7 in good yields. Next,
a-methylation of 7 followed by
reduction with LiAlH4 yielded diols 8 in high yields. This three-step
synthesis of 8 is concise and provides 8 in good overall yields
(>60%); therefore, it may be applicable for the large-scale synthesis
of these compounds.
The resulting intermediates 8 could readily be functionalized by
taking advantage of the high reactivity of the primary alcohol moi-
eties. Thus, Williamson etheration or silylation of the resulting
diols 8a and 8b produced 9a-Hex and 9b-Hex or 9a-Si and 9b-Si,
respectively. In addition, after etheration of 8c, the diallyl moieties
of the resulting dihexyl ether were easily cleaved via reaction with
the low-valent titanium reagent [Ti(O-i-Pr)4/Me3SiCl/Mg],8 which
was developed by our group, to provide diphenol derivative 9c in
79% isolated yield.8a,e
esters 4 containing various p-units. The folded H-stacking confor-
mation of the polymers 5 was evidenced by their blue-shifted UV
absorptions compared with that of model compounds,6 and they
predominantly exhibited excimer-like emission. The results of
AFM and XRD analyses also supported their folded H-stacking
conformation.
However, the tethering moieties bearing a tertiary alcohol that
were used for the synthesis of the polymers 5 are thermally unsta-
ble and decompose under acidic conditions, and it is not easy to
functionalize them for modifying structures and properties. There-
fore, we report, herein, a new tethering structure that guarantees
the folded H-stacking conformation, and it is more stable and read-
ily functionalized.
Monoalkylated monomer 9d was also readily synthesized in
79% yield from diol intermediate 8b via conversion to the acetal
followed by reduction with DIBAL-H.
The 1,3-dialkoxy-2-methylpropan-2-yl group was designed as
the bulky 2-substituent, and the corresponding 1,3-diarylpropane

R. Moriai et al. / Tetrahedron Letters 55 (2014) 2649–2653
2651
Fluorescence (FL) spectra for the 1,3-diphenylpropanes in
CH2Cl2 solution showed emission peaks at 285 and 333 nm, which
may be considered as the emissions from the individual benzene
rings and benzene excimers,2 respectively. The ratios of the inten-
sities of these peaks, which were calculated using the height of
each peak, are also listed in Table 1, and the representative FL spec-
tra are shown in Figure 1. Compounds 8a and 9a-Si predominantly
exhibited excimer emissions (333 nm), suggesting the dominant
formation of excimers, although a small emission peak from the
individual benzene ring remained. Therefore, these data strongly
support the predominant existence of the closed conformation
for 8a and 9a in both the ground and excited states.
Figure 2 presents the 1H NMR spectra for the aromatic protons
of the tethering monomer 9b-Si, the corresponding diester inter-
mediate, and 4-bromotoluene. Compared with the peaks for 4-bro-
motolueme, those of 9b-Si are shifted upfield 0.12ꢁ0.21 ppm
because of the shielding effect of the stacked benzene rings. Thus,
these results suggest that the dibromo compound 9b-Si also exists
preferentially in a closed (stacked) conformation.
Figure 1. Fluorescence spectra of 1,3-diphenylpropanes in
a CH2Cl2 solution:
Excited at 262 nm for toluene (2 ꢂ 10ꢀ4 M), 263 nm for 2a (10ꢀ4 M), 263 nm for 8a
(10ꢀ4 M), and 263 nm for 9a-Si (10ꢀ4 M).
Next, the preference of the closed conformation in 9 in the
ground and excited states was evaluated using MM2 calculations
and 1H NMR and fluorescence analyses of 8a and 9a. The results,
along with those for 2a (2: R = SiEt3, R0 = Me), are summarized in
Table 1. Three conformations [open (O), half-open (HAO) and
closed (C)] are listed and simplified for each 2-substituted 1,
3-diphenylpropane derivative based on the calculated energy dif-
ferences (the effect of solvent was not considered).9 The popula-
tions of these conformations (O:HAO:C) at 300 K were estimated
according to Boltzmann’s distribution. These results suggest that
compounds 8a and 9a preferentially exist in a closed (stacked) con-
With the new tethering units (monomers) 9 in hand, the
synthesis of polymers was then performed via A2-B2 type
formation, although 8a shows a somewhat lower ratio than 2a. 1
H
NMR analyses (500 MHz, CDCl3) also supported the stability of the
closed conformation for compounds 2a, 8a, and 9a. In comparison
to toluene (7.18 ppm for the ortho-protons), these compounds
displayed upfield shifts of the proton peaks at the o-position of
Ph moieties because of the shielding effect resulting from the
stacked structure: The compounds 9a-Si and 9a-Hex showed shifts
of 0.17 and 0.19 ppm, respectively. The order of the values of the
upfield shifts was in good agreement with those of the calculated
distributions of the closed conformers.
Figure 3. Thermo gravimetric analysis (TGA) of polymer P1.
Figure 2. Comparison of the 1H NMR spectra of 9b-Si, the corresponding diester, and 4-bromotoluene.

2652
R. Moriai et al. / Tetrahedron Letters 55 (2014) 2649–2653
copolymerization with representative
p-group-containing com-
pounds (Scheme 3). Thus, in the presence of a Pd-catalyst and base,
Suzuki–Miyaura coupling-polymerization10 of bis-boronic esters
10a and 10b with the dibromide 9b-Si proceeded to afford the
polymers P1 (Mn = 3.33 ꢂ 103, PDI = 1.51 determined by GPC with
a
polystyrene standard, PDI: polydispersity index) and P2
(Mn = 6.69 ꢂ 103, PDI = 1.90) in 79% and 64% yields, respectively.
Model compound 11 was prepared as a reference compound. The
ether-containing polymer P3 ((Mn = 7.46 ꢂ 103, PDI = 2.95) was
also synthesized in 36% yield via the polymerization of diphenol
derivative 9c with dibromo compound 10c in the presence of a
base (also see Scheme 3).
Thermo gravimetric analysis (TGA) of P1 revealed that the 5%
and 10% weight loss temperatures (Td5 and Td10) were 317 and
370 °C, respectively (Fig. 3), which were much greater than those
of the previously prepared polymer 5 [p = A(2,4-thiophenylene)2A]
(Td5 = 261.0 °C, Td10 = 268.6 °C).3 Therefore, the thermal stability
of polymer P1 derived from 9b-Si was greatly increased
(Scheme 3).
Figure 4. (a) UV–vis absorption spectra of P2 (in 10ꢀ6 M CH2Cl2 and film) and 11 (in
10ꢀ6 M CH2Cl2). (b) Fluorescence and excited spectra of P2 (in 10ꢀ7 M CH2Cl2 and
film) and 11 (in 10ꢀ7 M CH2Cl2).
Figure 4 represents the UV–vis absorption, fluorescence, and
excited spectra for P2 and reference compound 11. Compared
with the model compound 11, polymer P2 exhibited 13 and
27 nm blue-shifted UV absorption peaks in CH2Cl2 solution
(10ꢀ6 M) and as a solid film, respectively, which is characteristic
behavior
for
an
H-aggregated
(H-stacked)
structure
(Fig. 4(a)).3,6,11 On the other hand, the fluorescence emissions
for P2 in solution and as a film were largely red-shifted region,
compared with those of 11, particularly for the film (Fig. 4(b)). It
is noted that the maximum excited spectra (kex max) was blue-
shifted by nearly 40–50 nm from those of the model compound
11. These data strongly suggest that polymer P2 preferentially
exists in a stacked conformation, both in a solution and as a so-
lid film.12
In summary, the new monomer units 9 were designed for use in
the preparation of folded H-stacking polymers. Intermediates 8,
synthesized via addition–elimination–addition reaction of 2-eth-
oxymethylidene malonate with benzyl zinc reagents, were readily
functionalized to 9. MM2 calculations, 1H NMR and fluorescent
analyses of 9 indicated that these monomers preferentially exist
in a closed (stacked) conformation. Suzuki–Miyaura coupling poly-
merization of silylated monomer 9b-Si with bis-boronic esters of
p-units afforded skipped p polymers P1 and P2. The blue-shifted
UV-absorption and excited spectra of P2, which strongly indicate
an H-stacking structure. The ether tethered polymer P3 was also
synthesized, and its properties will be reported elsewhere in the
future.
Further investigation of the synthesis of polymers comprised of
other p-units, and their application as materials for devices such as
photovoltaic cells and field effect transistors is underway in these
Scheme 3. Synthesis of polymers P1 and P2.
laboratories.

R. Moriai et al. / Tetrahedron Letters 55 (2014) 2649–2653
2653
G.; VanLare, E. J. J. Photogr. Sci. 1953, 1, 173–183; (d) McRae, E. G.; Kasha, M. J.
Chem. Phys. 1958, 28, 721–722; (e) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A.
Pure Appl. Chem. 1965, 11, 371–392.
References and notes
1. (a) Smaldone, R. A.; Moore, J. S. Chem.-Eur. J. 2008, 14, 2650–2657;
(b)Foldamers: structure, properties and applications; Hecht, S., Huc, I., Eds.;
Wiley-VCH: Weinheim, 2007; (c) Cheng, P. Curr. Opin. Struct. Biol. 2004, 14,
512–520; (d) Scmuck, C. Angew. Chem., Int. Ed. 2003, 42, 2448–2452; (e) Hill, D.
J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem. Rev. 2001, 101, 3893–
4012; (f) Cheng, R. P.; Gellman, S. H.; DeGrado, W. F. Chem. Rev. 2001, 101,
3219–3232; (g) Nakano, T.; Okamoto, Y. Chem. Rev. 2001, 101, 4013–4038; (h)
Gellman, S. H. Acc. Chem. Res. 1998, 31, 173–180.
2. Hirayama, S. J. Chem. Phys. 1965, 42, 3163–3171.
3. Watanabe, J.; Hoshino, T.; Nakamura, Y.; Sakai, E.; Okamoto, S. Macromolecules
2010, 43, 6562–6569.
4. Nomura, R.; Moriai, R.; Kudo, M.; Hoshino, T.; Watanabe, J.; Funyu, S.; Ishitsuka,
K.; Okamoto, S. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3412–3419.
5. (a) Kato, S.; Takahashi, N.; Tanaka, H.; Kobayashi, A.; Yoshihara, T.; Tobita, S.;
Yamanobe, T.; Uehara, H.; Nakamura, Y. Chem. Eur. J. 2013, 19, 12138–12151;
(b) Majumder, M.; Sathyamurthy, N. Theor. Chem. Acc. 2012, 131, 1–11.
6. (a) Jelly, E. E. Nature 1936, 138, 1009–1010; (b) Scheibe, G. Angew. Chem. 1936,
49, 563; (c) Brooker, L. G. S.; White, F. L.; Heseltine, D. W.; Keyes, G. H.; Dent, S.
7. Von Doering, W.; Keliher, E. J. J. Am. Chem. Soc. 2007, 129, 2488–2495.
8. (a) Ohkubo, M.; Mochizuki, S.; Sano, T.; Kawaguchi, Y.; Okamoto, S. Org. Lett.
2007, 9, 773–776; (b) Okamoto, S.; He, J.-Q.; Ohno, C.; Oh-iwa, Y.; Kawaguchi,
Y. Tetrahedron Lett. 2010, 51, 387–390; (c) Kawaji, T.; Shohji, N.; Miyashita, K.;
Okamoto, S. Chem. Commun. 2011, 7857–7859; (d) Shohji, N.; Kawaji, T.;
Okamoto, S. Org. Lett. 2011, 13, 2626–2629; (e) Takekoshi, N.; Miyashita, K.;
Shoji, N.; Okamoto, S. Adv. Synth. Catal. 2013, 355, 2151–2157.
9. Molecular mechanics calculations (MM2) were carried out using a Quantum
CAChe 4.9 system.
10. (a) Koch, F.; Heitz, W. Macromol. Chem. Phys. 1997, 198, 1531–1544; (b)
Schlüter, A. D. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 1533–1556.
11. Hu, Z.; Fu, B.; Aiyar, A.; Reichmanis, E. J. Polym. Sci., Part A: Polym. Chem. 2012,
50, 199–206.
12. The behavior of these shifts of spectra was similar to that of the previously
prepared polymer 5 [p = ACH@CHA(1,4-C6H4)ACH@CHA]. See, Ref. 3