Construction of an ortho-phenol polymer

Article abstract of DOI:10.1016/S0040-4039(01)01256-4

Synthesis and study of an ortho-phenol polymer, a highly functionalized polyphenylene, have been conducted. A dibromo ortho-biphenol monomer was synthesized and its homocoupling in the presence of Ni(1,5-cyclooctadiene)₂ followed by hydrolysis led to the formation of an ortho-phenol polymer. This polymer was soluble in common organic solvents. It was characterized by gel permeation chromatography, UV-vis, IR, ¹H and ¹³C NMR spectroscopic methods. The use of this polymer in the Lewis acid-catalyzed reaction of phenylacetylene with benzaldehyde in the presence of diethylzinc was studied. It was found that the polymer when treated with 1/4 equiv. (relative to the phenol unit of the polymer) of Ti(OⁱPr)₄ generated a much more active Lewis acid catalyst than when treated with excess Ti(OⁱPr)₄. This indicates that different types of catalytic sites in the polymer have been produced under these conditions.

Full text of DOI:10.1016/S0040-4039(01)01256-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 6235–6238
Construction of an ortho-phenol polymer
Ming-Hua Xu, Zhi-Ming Lin and Lin Pu*
Department of Chemistry, University of Virginia, Charlottesville, VA 22904-4319, USA
Received 6 June 2001; accepted 10 July 2001
Abstract—Synthesis and study of an ortho-phenol polymer, a highly functionalized polyphenylene, have been conducted. A
dibromo ortho-biphenol monomer was synthesized and its homocoupling in the presence of Ni(1,5-cyclooctadiene)2 followed by
hydrolysis led to the formation of an ortho-phenol polymer. This polymer was soluble in common organic solvents. It was
1
characterized by gel permeation chromatography, UV–vis, IR, H and 13C NMR spectroscopic methods. The use of this polymer
in the Lewis acid-catalyzed reaction of phenylacetylene with benzaldehyde in the presence of diethylzinc was studied. It was found
that the polymer when treated with 1/4 equiv. (relative to the phenol unit of the polymer) of Ti(OiPr)4 generated a much more
active Lewis acid catalyst than when treated with excess Ti(OiPr)4. This indicates that different types of catalytic sites in the
polymer have been produced under these conditions. © 2001 Elsevier Science Ltd. All rights reserved.
Since the discovery of the high doped conductivity of
polyacetylene by Shirakawa, MacDiarmid, Heeger and
co-workers in 1977,1 tremendous amounts of study
have been conducted on conjugated polymers.2 These
materials have exhibited many important and poten-
tially very useful electrical and optical properties.
Polyphenylenes are one class of conjugated polymers
whose polymer chains are made of all phenyl rings
linked at various positions, but in most cases at the
para positions. Extensive work on these polymers has
been reported.2,3 In our laboratory, we are particularly
interested in constructing functionalized polyphenyl-
enes. The functional groups of conjugated polymers may
allow them to interact with various organic molecules
for optical and electrical sensor development.4 Catalysts
based on functionalized rigid conjugated polymers may
have more easily accessible catalytic sites than flexible
polymer catalysts when used for organic synthesis.5
Properly designed functional groups may also allow the
polymers to undergo either intrachain self-organization
or interchain self-assembly for materials applications.
below. The adjacent biphenol dihedral angle is less than
90° for the cisoid conformation and larger than 90° for
the transoid conformation. In a chiral environment, e.g.
in the presence of optically active guest molecules or
substituents, the polymer chain could also achieve a
preferred one-handed helical sense because of the
induced chiral conformation at the biaryl units.6 How
factors such as temperature, solvent polarity, guest
molecules, metal ions, and substituents X on the phenol
rings influence the polymer conformation is of signifi-
cant fundamental interest. In addition, the conforma-
tion of the polymer might greatly influence its materials
properties. The conformation change of m-phenylene
ethynylene oligomers was investigated by Moore and
co-workers.7a,b
A
poly(m-phenylenevinylene) was
recently used to helically wrap around carbon nan-
otubes by Stoddart, Heath and co-workers.7c Macro-
cyclic oliogo-ortho-anisoles (spherands) were studied
extensively by Cram and co-workers.8 Stepwise cou-
plings of substituted phenols to generate ortho-phenol
oligomers were reported.9 Although enzymatic methods
have been developed for the polymerization of phenol
and its derivatives, the resulting polymers were often
found to contain a mixture of structural units due to
the formation of both carbonꢀcarbon bonds and car-
bonꢀoxygen bonds at different positions of the phenol
rings.10,11 In order to generate structurally well-defined
ortho-phenol polymers, we have studied transition
metal-mediated cross-coupling polymerizations of ortho
bromophenols. Herein, our preliminary results are
reported.
ortho-Phenol polymers are one type of functionalized
polyphenylenes in which their phenol units are con-
nected at the ortho positions by forming carbonꢀcarbon
bonds. An ortho-phenol polymer is expected to have
two conformations, cisoid and transoid, as shown
Keywords:
ortho-phenol
polymer;
conjugated
polymer;
polyphenylene; polymer catalyst; alkynylzinc addition.
* Corresponding author. E-mail: lp6n@virginia.edu
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01256-4

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M.-H. Xu et al. / Tetrahedron Letters 42 (2001) 6235–6238
We first examined the direct polymerization of ortho-
dibromo phenyl acetate 1 in the presence of Ni(cyclooc-
tadiene)2 or NiCl2/Zn.12 However, only low molecular
weight materials were observed from these reactions and
no polymer was obtained. Another alkylated ortho-
dibromophenol 2 was prepared, which again could not
be polymerized. It seems that directly making two
phenyl–phenyl bonds on the phenyl ring of these
monomers might not be suitable for polymer production.
terization and spectroscopic study; (3) the triple bonds
extend the conjugation of the substrates and should also
allow further functionalization of the polymer. Unlike
compounds 1 and 2, the biphenyl monomer 5 underwent
the aryl–aryl cross-coupling polymerization smoothly in
the presence of Ni(cycloocatadiene)2.16 Therefore, mak-
ing two carbonꢀcarbon bonds on the two phenyl rings
of 5 might have greatly reduced the interference when the
coupling was conducted on the single phenyl ring of
monomer 1 or 2. After the Ni-mediated polymerization
of 5 followed by hydrolysis under basic conditions, the
desired ortho-phenol polymer 6 was obtained.16 This
polymer was a brown solid and soluble in common
organic solvents such as methylene chloride, chloroform
and THF. The molecular weight of the polymer was
analyzed by gel permeation chromatography (GPC)
relative to polystyrene standards, which showed Mw=
10,400 and Mn=4,500 (PDI=2.3). The UV spectrum of
6 in methylene chloride showed umax 302 (sh, m=4,100)
and 246 (m=15,100) nm. The longest wavelength absorp-
tion of the polymer matches with that calculated
for p-hydroxyphenylacetylene,17 consistent with the
expected absence of extended conjugation along the
polymer chain of 1,3-linked polyphenylenes. In the IR
spectrum of 6, a strong and broad OH stretch at 3393
cm−1 appeared with the disappearance of the original
ester stretch at 1762 cm−1. A broad absorption was
observed at ꢀ1700 cm−1 probably due to partial oxida-
A different approach was then taken which involved the
synthesis of a 3,3%-dibromo-1,1%-bi-2-phenol monomer
and its polymerization. By using such a biphenol
monomer, the cross-coupling reactions to make car-
bonꢀcarbon bonds for the polymer formation would be
conducted on two phenyl rings rather than on one phenyl
ring. Following the literature procedures, the tetrabromo
biphenyl acetate 4 was prepared in high yield from
ortho-biphenol (3) (Scheme 1).13 Go´mez-Lor et al found
that the 5,5%-bromine atoms of 4 were much more reactive
than the 3,3%-bromines in the Sonogashira coupling.13c,14
We thus reacted 1-octyne with 4 in the presence of
Pd(PPh3)2Cl2, which generated the 5,5%-dialkynyl substi-
tuted monomer 5.15 Introduction of the two alkynyl
groups in monomer 5 serves the following purposes: (1)
they block the 5,5%-positions and allow polymerization to
take place specifically at the 3,3%-positions; (2) these
substituents should increase the solubility of the resulting
polymer in organic solvents and make it easy for charac-
1
tion of the polymer. Both the H and 13C NMR spectra
of the polymer displayed well-defined alkyl signals.
However, the aromatic (l 110.0–155.0) and alkynyl (l
79.7 and 88.8) carbon signals in the 13C NMR spectrum
were broad and weak.
Scheme 1. Synthesis of an ortho-phenol polymer.

M.-H. Xu et al. / Tetrahedron Letters 42 (2001) 6235–6238
6237
Scheme 2. Reaction of phenylacetylene with benzaldehyde catalyzed by polymer 6 and Ti(OiPr)4.
As an example to explore the application of the ortho-
phenol polymer in catalysis, we have studied the titani-
um(IV)-based Lewis acid-catalyzed alkynylzinc addition
to aldehydes.18 This reaction is very useful for the
synthesis of propargyl alcohols that are versatile precur-
sors to many organic compounds of biological signifi-
cance. In this reaction, an alkynylzinc intermediate is
generated from the reaction of a terminal alkyne with
diethylzinc, which then undergoes nucleophilic addition
to aldehydes in the presence of Lewis acids.18a,b In our
experiment, a polymeric titanium complex based on 6
was prepared which was used to catalyze the reaction of
phenylacetylene with benzaldehyde in the presence of
diethylzinc to generate 1,3-diphenyl-2-propynol (7)
(Scheme 2). Polymer 6 was first treated with Ti(OiPr)4 in
THF, and then combined with phenylacetylene, diethyl-
zinc, and benzaldehyde at room temperature. It was
found that the catalytic activity of the polymer–Ti com-
plex generated from the reaction of 6 with 1/4 equivalent
(relative to the polymer phenol unit) of Ti(OiPr)4 is much
higher than the polymer–Ti complex generated from the
reaction with excess Ti(OiPr)4.19 In the presence of 20
mol% (the phenol unit of the polymer relative to ben-
zaldehyde) of the polymer and 5 mol% of Ti(OiPr)4, 7 was
obtained in 71% isolated yield after 80 h at room
temperature. However, under the same conditions, in the
presence of 20 mol% of polymer 6 and 100 mol% of
Ti(OiPr)4, the yield of 7 was only 16%. Without Ti(OiPr)4
and/or without the polymer, similar low yields of 7 were
observed.
Acknowledgements
We thank the support for this research from the US
Office of Naval Research (N00014-99-1-0531).
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In order to explain the observed dramatic reactivity
difference between the use of small and excess amounts
of Ti(OiPr)4, we propose that when polymer 6 was treated
with 1/4 equiv. of Ti(OiPr)4, the polymeric complex 8 that
contains tetradentate aryloxy coordinated Ti sites might
have been generated. Such tetradentate complex may
have higher catalytic activity than the monodentate Ti
complex 9 generated from the reaction of the polymer
with excess Ti(OiPr)4. A similar but chiral tetradentate
aryloxy coordinated Ti complex was found by
Yamamoto and co-workers to be a very efficient Lewis
acid catalyst.20 Besides as a very interesting polymeric
ligand for catalysis, the highly functionalized ortho-phe-
nol polymer is also potentially useful for electrical and
optical applications.

6238
M.-H. Xu et al. / Tetrahedron Letters 42 (2001) 6235–6238
11. Chemical- and electro-polymerization of phenols to gen-
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14. Sonogashira, K. In Comprehensive Organic Synthesis;
Trost, B. M.; Fleming, I., Eds.; Pergamon: Oxford, 1991;
Vol. 3, Chapter 2.4.
with CH2Cl2 at room temperature, washed with 1N HCl
and brine and dried over Na2SO4. The solution was
concentrated and precipitated with MeOH. Centrifuga-
tion and filtration gave a solid, which was re-dissolved in
CH2Cl2 and precipitated with MeOH again. The resulting
solid was dried under vacuum to give a polymer as a
brown powder (166 mg). GPC showed Mw=18,000 and
Mn=8,700 (PDI=2.1). This polymer was then dissolved
in THF, combined with aqueous KOH (2 M), and heated
at reflux for 24 h. The resulting mixture was acidified
with 1N HCl, extracted with CH2Cl2, washed with water
and dried over Na2SO4. The solution was concentrated
under vacuum, and precipitated with MeOH. After cen-
trifugation and filtration, polymer 6 was obtained as a
brown solid in 62% yield. IR (KBr, cm−1) 3393 (br s),
2954 (s), 2929 (s), 2857 (s), 2229 (w), 2190 (w), ꢀ1700
(br, m), 1596 (m), 1465 (s), 1233 (br, s). 1H NMR (300
MHz, CDCl3) l 0.88 (br, 6H), 0.96–1.70 (m, br, 16H),
2.37 (br, 4H), 3.45 (br, 2H), 6.82–7.82 (m, br, 4H). 13C
NMR (75 MHz, CDCl3) l 13.8, 19.2, 22.4, 28.6, 29.5,
31.3, 79.7 (br), 88.8 (br), 110.0–155.0 (br, weak peaks).
17. Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectro-
metric Identification of Organic Compounds, 5th ed.; John
Wiley & Sons: New York, 1991; pp. 308–309.
15. Preparation and characterization of monomer 5: Under
nitrogen, a solution of 1-octyne (1.525 mL, 10.32 mmol)
in THF (10 mL) and freshly distilled triethylamine (20
mL) were syringed into a 50 mL dried Schlenk flask
charged with Pd(PPh3)2Cl2 (604 mg, 0.86 mmol), cop-
per(I) iodide (164 mg, 0.86 mmol) and tetrabromide 4
(2.520 g, 4.30 mmol). After the reaction mixture was
heated at reflux for 20 h, it was diluted with THF at
room temperature, filtered through a pad of Celite, con-
centrated and dissolved in CH2Cl2. The CH2Cl2 solution
was then washed with 1N HCl and brine, dried over
Na2SO4 and concentrated under vacuum. Flash chro-
matography of the residue on silica gel using hexane/ethyl
acetate as eluent gave 5 as a white solid in 66% yield
(1.820 g). Mp 85–86°C; IR (KBr, cm−1) 2951 (s), 2926 (s),
2857 (s), 2228 (w), 1762 (s), 1445 (s), 1367 (s), 1187 (s),
18. (a) Ishizaki, M.; Hoshino, O. Tetrahedron: Asymmetry
1994, 5, 1901–1904; (b) Li, Z.; Upadhyay, V.; DeCamp,
A. E.; DiMichele, L.; Reider, P. Synthesis 1999, 1453–
1458; (c) Frantz, D. E.; Fa¨ssler, R.; Carreira, E. M. J.
Am. Chem. Soc. 2000, 122, 1806–1807.
19. Experimental procedure for the catalysis: Under nitrogen,
Ti(OiPr)4 (4 mL, 5 mol%) was added via a syringe to a
solution of polymer 6 (10 mg, 20 mol%) in freshly
distilled THF (1.0 mL) at room temperature. The mixture
was stirred for about 10 min and was then slowly added
into a 10 mL Schlenk flask containing a solution of
phenylacetylene (34 mL, 0.31 mmol), diethylzinc (32 mL,
0.31 mmol) in THF (2.0 mL). After the homogenous
solution was stirred at room temperature for 30 min,
benzaldehyde (26 mL, 0.25 mmol) was added via a
syringe. The stirring continued and the reaction progress
was monitored by TLC. After 80 h, the reaction was
quenched with 1N HCl (1.5 mL). The resulting mixture
was then diluted with CH2Cl2, washed with water and
dried over Na2SO4. The solvent was removed under
vacuum, and the residue was purified by column chro-
matography on silica gel using hexane/ethyl acetate as
eluent to give pure 7 in 71% yield (37 mg). When 1.0
equiv. (relative to benzaldehyde) of Ti(OiPr)4 was used,
the reaction gave only 16% yield of 7 under the above
conditions.
1
1017 (m), 905 (m), 876 (m). H NMR (300 MHz, CDCl3)
l 0.90 (t, J=6.6 Hz, 6H), 1.23–1.48 (m, 12H), 1.53–1.62
(m, 4H), 2.10 (s, 6H), 2.38 (t, J=6.6 Hz, 4H), 7.22 (d,
J=1.8 Hz, 2H), 7.65 (d, J=1.8 Hz, 2H). 13C NMR (75
MHz, CDCl3) l 14.27, 19.53, 20.60, 22.73, 28.68, 28.77,
31.52, 78.30, 92.87, 117.14, 123.78, 131.75, 133.37, 136.23,
145.44, 167.84. APCI-MS m/z: M+H+ peaks at 643, 645
and 647 with a 1:2:1 ratio indicate a dibromide.
16. Preparation and characterization of polymer 6: Under
nitrogen, a mixture of 5 (322 mg, 0.50 mmol), 2,2%-
bipyridine (117 mg, 0.75 mmol), bis(1,5-cyclooctadi-
ene)nickel(0) (172 mg, 0.63 mmol), 1,5-cyclooctadiene
(212 mL, 1.73 mmol) was added DMF (8 mL). After the
mixture was stirred at 70–75°C for 36 h, it was diluted
20. Maruoka, K.; Murase, N.; Yamamoto, H. J. Org. Chem.
1993, 58, 2938–2939.
.