Synthesis, characterization, and modification of hyperbranched poly(arylene oxindoles) with a degree of branching of 100%

Article abstract of DOI:10.1002/1521-3773(20021202)41:23<4547::AID-ANIE4547>3.0.CO;2-A

Excellent thermal stability and a relatively high glass-transition temperature are exhibited by the hyperbranched poly(arylene oxindoles), which were prepared by an acidcatalyzed polycondensation of the substituted isatine 1. The polymers thus obtained ca

Full text of DOI:10.1002/1521-3773(20021202)41:23<4547::AID-ANIE4547>3.0.CO;2-A

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Synthesis, Characterization,
and Modification of
Hyperbranched Poly(arylene
oxindoles) with a Degree of
Branching of 100%**
O
O
Br
+
O
N
O
R
2 R = Br
CH3
4
Bu6Sn2
[Pd(PPh3)4]
[Pd(PPh3)4]
3 R = Bu3Sn
Mario Smet,* Etienne Schacht,
and Wim Dehaen
O
O
Hyperbranched polymers are of high
importance as readily available substi-
tutes for dendrimers. Whereas the
O
O
1
N
CH3
preparation of the latter requires a
multistep synthesis, hyperbranched
polymers can be obtained in a single
step.[1] However, a degree of branching
CF3SO2OH
O
CH3
O
of 100%, which is a characteristic
property of dendrimers, can be ach-
N
CH3
ieved only by postsynthetic modifica-
O
O
tion[2] or when certain requirements for
N
O
O
the monomer are met.[3] To date, the
O
only example of a hyperbranched poly-
mer (which necessarily consists exclu-
sively of dendritic and terminal units)
O
O
N
O
was based on an AB2 monomer pos-
sessing an azine and a maleimide func-
tional group, which react in a ™criss-
cross∫ cycloaddition.[4] However, to
CH3
n
5a–d
Scheme 1. Synthesis of the polymers 5a d.
improve features such as accessibility,
convenience of functionalization, and
by NMR spectroscopy. As it is known that this type of
condensation of isatin and aromatic compounds exclusively
yields 3,3-diaryl oxindoles,[5] we can exclude the possibility of
the presence of linear units in the hyperbranched polymers
obtained from 1. The tin derivative 3 was obtained by
palladium-catalyzed transmetalation of the bromide 2 and
hexakis-n-butylditin. The bromide 2 was formed by Friedel
Crafts acylation of diphenyl ether (five-fold excess) with
4-bromobenzoyl chloride.
Monomer 1 was polymerized over 24 h using trifluorome-
thanesulfonic acid, to yield the polymers 5a d, which possess
3,3-diaryl oxindole groups as dendritic units and isatin
residues at the periphery. These polymers were soluble in
chloroform, dichloromethane, and toluene. They were poorly
soluble in THF and insoluble in acetone. These solubility
properties are in sharp contrast to the linear analogues which
were only soluble in highly polar solvents such as N,N-
dimethylacetamide and N-methylpyrrolidinone, or in the
presence of strong acids.[6] As can be seen from Table 1, the
obtained molecular weights are strongly dependent upon the
reaction temperature and the concentration. At room temper-
ature, only oligomers were obtained although, according to
the literature,[6] the linear polymerization occurred readily at
258C. Higher temperatures and higher concentrations favor
the formation of higher molecular-mass species. Higher
molecular masses result in higher polydispersities. It should
be emphasized that the molecular weights obtained by size-
exclusion chromatography (SEC) are likely to be under-
estimated because of the compact structure of our macro-
attractive physical properties, it is highly desirable to devise
new strategies towards hyperbranched polymers with a degree
of branching of 100%.
We applied the acid-catalyzed condensation of isatins[5]
with aromatic compounds for the synthesis of hyperbranched
poly(arylene oxindoles). The linear analogues have been
shown to display a high glass-transition temperature (Tg) and
excellent thermal stability.[6] The use of para-phenoxybenzo-
phenones as the aromatic reaction partner results exclusively
in para-substitution and hence in structural homogeneity of
the polymer.[6] Therefore, the AB2 monomer 1 was designed
and synthesized by Stille coupling of the tin derivative 3 and 5-
bromo-1-methylisatin (4)[7] (Scheme 1). The methyl group was
introduced to enhance the solubility of both the monomer and
the polymers and to facilitate the characterization of the latter
[*] Dr. M. Smet, Prof. W. Dehaen
Department of Chemistry
University of Leuven
Celestijnenlaan 200F, 3001 Heverlee, Leuven (Belgium)
Fax : (þ 32)16 327 990
E-mail: mario.smet@chem.kuleuven.ac.be
Prof. E. Schacht
Department of Chemistry
University of Ghent
Krijgslaan 281 (S4), 9000 Ghent (Belgium)
[**] This work was supported by the University of Leuven, the Ministerie
voor Wetenschapsbeleid and the FWO-Vlaanderen. M.S. thanks the
FWO Vlaanderen for a postdoctoral fellowship.
Supporting information for this article is available on the WWW under
http://www.angewandte.org or from the author.
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Table 1. Overview of the synthesized hyperbranched polymers.
Polymer
Temperature [8C]
Concentration [mgmLÀ1
]
Yield [%]
Mn[a] [gmolÀ1
]
Mw[a] [gmolÀ1]
D[a]
[b]
[b]
[b]
5a
5b
5c
5d
25
40
60
60
10
10
10
20
58
63
68
76
À
À
À
3900
4200
7100
4900
5700
20000
1.27
1.37
2.9
[a] GPC with chloroform as the eluent. [b] Only oligomers were obtained.
molecules. In the future, absolute molecular weights will be
determined, to provide evidence for this assumption.
In the FTIR spectrum of the monomer, a strong absorption
band at 1731 cmÀ1 is observed which can be attributed to both
the carbonyl groups of the five-membered ring. Another
absorption band of medium intensity at 1650 cmÀ1 is charac-
teristic for the carbonyl group of the benzophenone unit. In
the polymers, the latter absorption remains unaffected, yet
two signals are observed for the carbonyl groups of the indole
residues at 1742 and 1718 cmÀ1, respectively. The first one can
be assigned to both the carbonyl groups of the terminal isatin
units, whereas the lactam carbonyl group of the dendritic
diphenyloxindole residues gives rise to the latter. It is also
possible to monitor the polymerization by UV spectroscopy.
The monomer has an extinction coefficient of
789 LmolÀ1 cmÀ1 at 437 nm. However, for the polymers, an
apparent extinction coefficient of 427 LmolÀ1 cmÀ1 was found
at this wavelength. This is in agreement with the theoretical
expectations that the ratio of terminal units (which can
reasonably be expected to have an extinction coefficient close
to that of the monomer) and the dendritic units (showing no
absorbance in the visible region) should be close to (or over)
1,[8] which should give rise to an extinction coefficient of about
half the value of the monomer.
In the 1H NMR spectrum, extensive line broadening is
observed (Figure 1). Two main clusters of signals for the CH3
groups can be distinguished. The resonance signals at
3.37 ppm can be assigned to the internal (dendritic) groups,
whereas the cluster at 3.31 ppm can be assigned to peripheral
isatin residues. This assignment is supported by comparison of
the chemical shifts of the latter peak and that arising from the
methyl protons of the monomer. The observation that the
ratio of the integral of the latter peak to the first is near 0.9
reflects again the expected ratio of terminal and dendritic
units. The intensity of the peak at 7.80 ppm is lowered to the
expected intensity for one proton with respect to the intensity
of the signals of the monomer in this region. Indeed, the
signals arising from the two protons which are ortho and para
to the reactive isatin carbonyl group shift to ꢀ 7.5 and
ꢀ 7.6 ppm, respectively, for the dendritic residues, but hardly
move for the terminal units. Hence, the signal for the
equivalent of only one proton per repeat unit will shift to
the ensemble of signals between 7.49 and 7.6 ppm. Finally, it is
clear that the protons which are ortho to the spiro center
account for the resonance observed at 7.33 ppm.
Figure 1. 1H NMR spectrum in CDCl3 of a) the monomer 1 and the
hyperbranched polymers b) 5d and c) 7 (see Scheme 2).
center. The cluster of signals at 177.3 ppm arises from the
amide carbonyl group of the dendritic units. All NMR
spectroscopic data are consistent with our assumption of a
degree of branching of 100% as no traces of linear units could
be observed.
The glass-transition temperature of the polymer 5d is
1908C, which is significantly lower than that of the linear
analogues in the literature.[6] The thermal stability was very
good; no decomposition could be observed by heating at
4208C under a nitrogen atmosphere (heating rate in both
cases ¼ 10 KminÀ1).
The isatin periphery of the hyperbranched polymers can
easily be functionalized with differently substituted arenes in
the presence of trifluoromethanesulfonic acid (Scheme 2). In
this way, bromophenyl and methoxyphenyl substituents could
readily be introduced, with complete conversion reached
within 1 h, to yield the derivatives 6 and 7 (see Scheme 2).
This conversion was confirmed by NMR spectroscopy. For
In the 13C NMR spectrum (see Supporting Information),
analogous observations can be made. Multiple splitting is
observed. Two clusters of almost equal intensity at 26.4 and
26.9 ppm account for the methyl groups. The signal at
61.6 ppm corresponds to the newly formed spiro carbon
1
instance, in the H NMR spectrum of the methoxy deriva-
tive 7, apart from the new signals from the methoxyphenyl
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was found to be slightly higher (ꢀ 10%) than that of the
starting material 5d. This clearly indicates that modification
of the periphery of the polymers in the described way does not
degrade the substrate.
R
T
R
T
T
O
T =
T =
T
T
O
O
In conclusion, we have shown that hyperbranched poly-
(arylene oxindoles) with a degree of branching of 100% can
be prepared in a convenient way. Moreover, these compounds
show a high glass-transition temperature and excellent
thermal stability. These polymers allow easy and extensive
functionalization at the periphery which opens up a number of
possibilities to modify their mechanical and physical proper-
ties. Further research towards the latter goal, as well as
towards modification of the internal skeleton, is currently
ongoing.
N
N
a)
CH3
CH3
T
A
5d
6 R = Br
b)
7 R = OCH3
8 R = OH
Scheme 2. a) Trifluoromethanesulfonic acid, bromobenzene (for the prep-
aration of 6) or anisole (for the preparation of 7); b) BBr3, CHCl3. A ¼
focal phenoxy residue.
substituents, another intensity shift of the equivalent of one
proton in intensity from the signal at 7.80 ppm to the ensemble
of signals between 7.50 and 7.65 ppm is observed, which
indicates the disappearance of all the isatin residues. The
functional groups introduced in this way allow further
functionalization. As an example, the methoxyphenyl deriv-
ative was deprotected using BBr3, which resulted in the
polyhydroxy derivative 8. These observations show that a
large variety of functional groups can be introduced at the
periphery of these hyperbranched macromolecules, which
allows a convenient modification of their physical properties.
As exchange between the phenoxy residues of the incorpor-
ated monomers and the added aromatic compound, which is
supposed only to react with the peripheral isatin moieties,
could reduce the molecular weight of the hyperbranched
poly(arylene oxindoles), we ran a control experiment to
provide evidence for the absence of such an exchange.
Therefore, compounds 9 and 10 were prepared by treatment
of isatin and bromobenzene or anisole with trifluoromethane-
sulfonic acid (Scheme 3). These compounds were then left
Experimental Section
Synthesis of 4-bromo-4’-phenoxybenzophenone (2): Thionyl chloride
(20 mL) and one drop of DMF was added to 4-bromobenzoic acid (2 g,
10 mmol) and the mixture was heated under reflux for 2 h. The solvent was
evaporated in vacuum, the residue dissolved in (CH2Cl)2 (10 mL) and
added dropwise to a suspension of diphenyl ether (8.5 g, 50 mmol) and
AlCl3 (3.2 g, 24 mmol) in (CH2Cl)2 (150 mL) at room temperature. The
mixture was heated under reflux for 10 min, cooled to room temperature,
and poured onto crushed ice (ca. 200 g). The organic phase was separated
and the solvent evaporated in vacuum. The bromide 2 was obtained as a
white solid (3.2 g, 91%) by crystallization from petroleum ether. 1H NMR
3
(300 MHz, CDCl3, 258C, TMS): d ¼ 7.03 (d, J (H-H) ¼ 9 Hz, 2H), 7.10 (d,
3J(H-H) ¼ 7 Hz, 2H), 7.21 (t, 3 J(H-H) ¼ 7 Hz, 1H), 7.41 (d, 3J(H-H) ¼ 7 Hz,
2H), 7.63 (AB system, 4H), 7.79 ppm (d, J (H-H) ¼ 9 Hz, 2H); 13C NMR
3
(75 MHz, CDCl3, 258C, TMS): d ¼ 117.2, 120.2, 124.7, 127.1, 130.1, 131.3,
131.4, 131.5, 132.3, 136.7, 155.4, 161.9, 194.3 ppm; MS (EI, 70 eV): m/z 352
[Mþ].
Synthesis of 4-(tris-n-butyltin)-4’-phenoxybenzophenone (3): [Pd(PPh3)4]
(3 mol%) was added to a solution of 2 (0.20 g, 0.57 mmol) and hexabu-
tylditin (0.66 g, 1.14 mmol) in toluene under argon. The mixture was heated
at 808C for 15 h, cooled to room temperature, and the solvent evaporated
in vacuum. The tin derivative 3 was obtained as a clear oil (0.13 g, 40%)
after column chromatography (SiO2, CH2Cl2). 1H NMR (300 MHz, CDCl3,
258C, TMS): d ¼ 0.89 (t, 3J(H-H) ¼ 8 Hz, 9H), 1.10 (m, 6H), 1.35 (m, 6H),
3
3
1.56 (m, 6H), 7.04 (d, J(H-H) ¼ 8 Hz, 2H), 7.09 (d, J(H-H) ¼ 7 Hz, 2H),
7.19 (t, 3J(H-H) ¼ 7 Hz, 1H), 7.22 (t, 3J(H-H) ¼ 7 Hz, 2H), 7.37 (d, 3J(H-
H) ¼ 7 Hz, 2H), 7.42 (d, 3J(H-H) ¼ 7 Hz, 2H), 7.61 ppm (d, 3J(H-H) ¼ 8 Hz,
2H); 13C NMR (75MHz, CDCl3, 258C, TMS): d ¼ 9.7, 13.7, 27.3, 29.0, 117.1,
120.1, 124.5, 128.7, 130.0, 132.1, 132.5, 136.2, 137.3, 148.8, 155.6, 161.5,
195.8 ppm; MS (ES): m/z 564.3 [Mþ].
Synthesis of the monomer 1: 1-Methyl-5-bromoisatin (4; 97 mg, 0.40 mmol)
and the tin derivative 3 (0.19 g, 0.34 mmol) were dissolved in toluene
(4 mL). The solution was placed under argon and [Pd(PPh3)4] (3 mol%)
was added. The mixture was heated under reflux for 16 h. After cooling to
room temperature, the solvent was evaporated in vacuum and 1 was
obtained as an orange solid (106 mg, 72%) after column chromatography
(silica, CH2Cl2). 1H NMR (300MHz, CDCl3, 258C, TMS): d ¼ 3.31 (s, 3H;
CH3), 7.02 (d, 3J(H-H) ¼ 9 Hz, 1H; 7-H isatin), 7.05 (d, 3J(H-H) ¼ 9 Hz, 2H;
o-H to OPh), 7.11 (d, 3J(H-H) ¼ 7 Hz, 2H; o-H to COPhO), 7.18 (d, 3J(H-
H) ¼ 7 Hz, 1H; p-H to COPhO), 7.42 (t, 3J(H-H) ¼ 7 Hz, 2H; m-H to
COPhO), 7.64 (d, 3J(H-H) ¼ 8 Hz, 2H; o-H to isatin), 7.84 (d, 3J(H-H) ¼
Scheme 3. a) Trifluoromethanesulfonic acid and bromobenzene (for the
preparation of 9) or anisole (for the preparation of 10); b) Trifluorome-
thanesulfonic acid, anisole (in combination with 9) or bromobenzene (in
combination with 10).
overnight in the presence of anisole (for 9) or bromobenzene
(for 10), respectively, in the same acid. In neither case was
evidence for exchange found. However, in the case of
compound 10, some decomposition occurred after long reac-
tion times (> 24 h). As the hyperbranched polymers con-
densed within a short time (< 1 h) with the large excess of
aromatic compound added, and as the electron-withdrawing
benzophenone unit can be expected to stabilize the phenoxy-
phenyloxindole unit, we did not anticipate any problems
arising from decomposition upon functionalization of the
isatin periphery. By SEC, the molecular mass of the polymer 7
3
9 Hz, 2H; m-H to OPh), 7.87 (d, J(H-H) ¼ 8 Hz, 2H; m-H to isatin), 7.89
(d, 4J(H-H) ¼ 2 Hz, 1H; 4-H isatin), 7.90 ppm (dd, 3J(H-H) ¼ 9 Hz, 4J(H-
H) ¼ 2 Hz, 1H; 5-H isatin); 13C NMR (100mHz, CDCl3, 258C, TMS): d ¼
26.4, 194.6, 183.1, 161.7, 158.2, 155.4, 151.0, 142.5, 137.1, 136.9, 136.0, 132.3,
131.7, 130.6, 130.0, 126.3, 124.6, 123.7, 120.1, 117.9, 117.1, 110.5 ppm; MS (EI,
70 eV): m/z 433 [Mþ].
Preparation of polymer 5d: The monomer (20 mg) was dissolved in
trifluoromethanesulfonic acid (1 mL) and placed under argon. The mixture
was stirred at 608C for 24 h. After cooling to room temperature, the
solution was added dropwise to vigorously stirred water (50 mL). After
stirring for 30 min, the suspension was centrifuged (3600 rminÀ1; 5 min)
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and the supernatant decanted. Water (10 mL) was added to resuspend the
precipitate, which was shaken vigorously and again centrifuged. After
decanting, acetone was added to the precipitate which was again shaken
vigorously and centrifuged. This was repeated once more, after which the
precipitate was dried in vacuum. The polymer was obtained as an orange-
brown solid (15 mg, 76%).
[4þ2] cycloadditions of dienes and p systems. More recently,
the groups of Gilbertson,[4e] Trost,[5] and Zhang[6] have
reported other catalysts that are also effective in certain
types of [5þ2] cycloadditions.
Recently, Chung and co-workers have reported a readily
prepared, air-stable naphthalene complex of rhodium
Received: August 20, 2002 [Z50010]
À
([(C10H8)Rh(cod)]þBF4 ) which is an efficient catalyst for
[4þ2] cycloaddition reactions of dienes and alkynes.[7] We
have now examined the effectiveness of this and several
related complexes in [5þ2] cycloaddition reactions and have
found them to be exceptionally effective in all cases studied,
often providing cycloadducts in excellent yields in minutes at
room temperature. The details of this study and X-ray
crystallographic data on selected complexes are described
herein.
[1] a) Y. H. Kim, J. Polym. Sci. Polym. Chem. Ed. 1998, 36, 1685 1698;
b) B. I. Voit, J. Polym. Sci. Part A: Polym. Chem. 2000, 38, 2505; M.
Jikei, M. Kakimoto, Progr. Polym. Sci. 2001, 26, 1233 1285.
[2] a) K. Yamanaka, M. Jikei, M. Kakimoto, Macromolecules 2001, 34,
3910 3915; b) R. Haag, A. Sunder, J. F. Stumbÿ, J. Am. Chem. Soc.
2000, 122, 2954 2955; c) C. Lach, H. Frey, Macromolecules 1998, 31,
2381 2383.
[3] J.L. Hobson, W. J. Feast, Polymer 1999, 40, 1279 1297.
[4] a) G. Maier, C. Zech, B. Voit, H. Komber, Macromol. Chem. Phys.
1998, 199, 2655 2664; b) G. Maier, C. Zech, B. Voit, H. Komber,
Macromol. Symp. 2001, 163, 75 86.
[5] D. A. Klumpp, K. Y. Yeung, G. K. Surya Prakash, G. A. Olah, J. Org.
Chem. 1998, 63, 4481 4484.
[6] H. M. Colquhoun, M. G. Zolotukhin, Macromolecules 2001, 34, 1122
1124.
[7] a) W. Borsche, W. Jacobs, Chem. Ber. 1914, 47, 354 363; b) A.
Stankevicius, L. Mazilis, V. Garaliene, S. Riselis, M. Sapragoniene, L.
Dzekciorius, Khim.-Farm. Zh. 1981, 15, 31 34.
Complex [(C10H8)Rh(cod)]þSbF6 (1) is prepared in a
À
single step from commercially available [{RhCl(cod)}2][8] by
treatment with AgSbF6, followed by introduction of the
naphthalene ligand.[9] The complex is air-stable at room
temperature and retains its catalytic activity even after several
months of storage.
Although complexes differing from 1 in only the counter
ion have been reported,[7,9] characterization data available for
these complexes do not directly establish the hapticity of the
naphthalene ligand. Fortunately, with some effort, we were
able to crystallize complex 1 from a concentrated dichloro-
methane solution under an anhydrous atmosphere. X-ray
crystallographic analysis of these crystals led to the ORTEP
representation of this complex given in Figure 1. This is
apparently the first X-ray structure of a simple h6 naphtha-
lene rhodium complex,[10] and is comparable to crystal
structures that have been recorded for some similar ruthe-
nium and rhodium naphthalene complexes.[11]
T
D
nþ1
[8]
¼
where T is the number of terminal units, D the number of
nÀ1
dendritic units, and n the degree of polymerization. This equation is
readily derived by observing the obvious facts that T¼ D þ 1 and n ¼
Tþ D.
[(arene)Rh(cod)]þ Complexes as Catalysts for
[5þ2] Cycloaddition Reactions**
Paul A. Wender* and Travis J. Williams
As part of our studies on the design and development of
new metal-catalyzed [mþn] cycloaddition reactions, we
previously reported the first examples of metal-catalyzed
[5þ2] cycloadditions of vinylcyclopropanes and tethered p
systems.[1] In our initial and ongoing screening of catalysts for
these reactions, rhodium(i) complexes have proven to be the
most general. These complexes are active in both inter- and
intramolecular cycloadditions. The latter reactions involve
alkynes, alkenes, and allenes as the 2-p component and
provide cycloadducts in high yields often at 25 408C.[2] This
finding is consistent with earlier observations by our group[3]
and others[4] on the utility of rhodium(i) in metal-catalyzed
[*] Prof. P. A. Wender, T. J. Williams
Department of Chemistry, Stanford University
Stanford, CA 94305-5080 (USA)
Fax : (þ 1)650-725-0259
Figure 1. ORTEP diagram of [(C10H8)Rh(cod)]þSbF6 (1). Ellipsoids
drawn at 50% probability level.
À
E-mail: wenderp@leland.stanford.edu
[**] This research was supported from the National Science Foundation
(CHE-0131944). Mass spectra were provided by the Mass Spectrom-
etry Facility, University of California, San Francisco. Elemental
analysis was conducted by the Desert Analytics Laboratory of Tucson,
Arizona. The authors extend utmost gratitude to Mr. Adam Cole for
analysis of crystallographic data and Dr. Fred Hollander (University
of California, Berkeley) for X-ray data collection. (cod ¼ cycloocta-
1,5-diene.)
Complex 1 was tested in a variety of [5þ2] cycloaddition
reactions and compared, where relevant, with some other
effective catalysts (Table 1). The initial series was selected to
explore the utility of the catalyst with a diverse variety of
substitution patterns. Excellent results were obtained with
vinylcyclopropanes tethered to terminal and internal alkynes,
alkynoates, and alkenes. Substitution of the vinylcyclopro-
pane is tolerated both on and adjacent to the cyclopropane
Supporting information for this article is available on the WWW under
http://www.angewandte.org or from the author.
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