Synthesis of 4-phenylethynylphthalic anhydride, a useful end-capping agent for polyimides

Article abstract of DOI:10.3184/174751912X13366326240908

Hydrolysis and bromination of phthalic anhydride in water gave a mixture of the monosodium salt of 4-bromophthalic acid. These were reacted with phenylacetylene under Sonogashira coupling reaction conditions in an aqueous medium. Acidification afforded 4-phenylethynylphthalic acid and then dehydration of this gave 4-phenylethynylphthalic anhydride in 76% overall yield. Compared with traditional synthetic methods, this method has the advantage of low cost, convenient manipulation and is environmentally friendly.

Full text of DOI:10.3184/174751912X13366326240908

JOURNAL OF CHEMICAL RESEARCH 2012
RESEARCH PAPER 379
JUNE, 379–380
Synthesis of 4-phenylethynylphthalic anhydride, a useful end-capping
agent for polyimides
Mei-Jia Yang, Tao Shao, Jian Men, Guo-wei Gao* and Hua Chen
College of Chemistry, Sichuan University, Chengdu 610064, P. R. China
Hydrolysis and bromination of phthalic anhydride in water gave a mixture of the monosodium salt of 4-bromo-
phthalic acid. These were reacted with phenylacetylene under Sonogashira coupling reaction conditions in an aqueous
medium. Acidification afforded 4-phenylethynylphthalic acid and then dehydration of this gave 4-phenylethy-
nylphthalic anhydride in 76% overall yield. Compared with traditional synthetic methods, this method has the advan-
tage of low cost, convenient manipulation and is environmentally friendly.
Keywords: polyimides, 4-phenylethynylphthalic anhydride, Sonogashira coupling reaction, aqueous medium, synthesis
starting material 4-BPAc was expensive. We now report a more
facile method for the synthesis of 4-PEPA with a good yield
(Scheme 2).
Polyimides are well known for possessing excellent thermal
and oxidative stability, as well as excellent mechanical pro-
perties.1,2 4-Phenylethynylphthalic anhydride (4-PEPA) is a
commonly used end-capping agent, which can improve the
thermal stability, solvent resistance and processibility of
polyimides.3–5
What is believed to be a mixture of monosodium salt of
4-BPAc (2) was prepared from commercially available phthalic
anhydride (1) by hydrolysis and bromination in an aqueous
medium with 85% yield. To the best of our knowledge, there
has not been any report using 2 as the substrate in a Sono-
gashira coupling reaction. As 2 could be dissolved in water and
the Sonogashira coupling reaction was carried out under basic
conditions, 2 was employed as the substrate directly without
acidification and organic solvent extraction to afford 4-BPAc,13
which simplified the experimental operation. 4-BPAc is
believed to be mainly present as its dibasic anion in the basic
solution.
The Sonogashira coupling reaction is the key step in the
synthesis of 4-PEPA. Generally, Pd(PPh3)2Cl2-CuI/PPh3 was
chosen as the catalyst system, when organic solvents were used
as the reaction media. The water soluble palladium catalyst
prepared in situ from Pd(AcO)2 and TPPTS (tris-(3-sulfonatop
henyl)phosphine, sodium salt) is a useful and piratical cata-
lytic system for coupling reaction.15 In our revised method,
Pd(AcO)2/TPPTS was employed as catalyst, with water as the
reaction solvent. Compared with organic solvents, water is
non-toxic, non-flammable, inexpensive and environmentally
friendly. After optimisation of the conditions, e.g. co-catalysts,
phase transfer catalysts (PTC), bases, temperatures, catalyst
mole ratios and reaction times, the coupling reaction was cata-
lysed by Pd(AcO)2 (0.25 mol%), TPPTS (0.5 mol%) in the
presence of NaOH (2.0 equiv.) and TBAB (tetrabutyl ammo-
nium bromide) (5.0 mol%) at 100 °C for 3h without using CuI
The most convenient way of preparing 4-PEPA employs
4-bromophthalic anhydride (4-BPA) as starting material,
which is reacted with phenylacetylene under Sonogashira
coupling reaction conditions (Scheme 1, route 1).3–7 The start-
ing material 4-BPA was prepared from 4-bromophthalic acid
(4-BPAc) by a process involving fractional distillation8 or by
dehydration with acetic anhydride.9 However, since 4-PEPA
produced by this way was impure, it had to be purified by
hydrolysis in basic solution, neutralisation with concentrated
HCl and dehydration in acetic anhydride,5 or by vacuum
sublimation.7 In order to prepare high purity 4-PEPA, Urazoe10
used dimethyl 4-bromophthalate (4-BDMP) obtained from
either 4-BPA11 or 4-BPAc12,13 as the starting material which
was reacted with phenylacetylene to afford dimethyl 4-phenyl-
ethynylphthalate (4-PEDMP). 4-PEPA was then obtained after
complete hydrolysis in basic solution and dehydration with
acetic anhydride (Scheme 1, route 2).
Application of these methods has been restricted by their
tedious manipulations and low atom economy. Recently, we
developed a convenient procedure with a high yield for the
synthesis of 4-PEPA from 4-BPAc. Thus, 4-BPAc reacted with
phenylacetylene by a Sonogashira coupling reaction in THF/
EtN314 (Scheme 1, route 3). However, this method was carried
out in organic solvents which could pose recyclability prob-
lems and add to the environmental burden. In addition, the
Scheme 1
* Correspondent. E-mail: gaoguowei@scu.edu.cn

380 JOURNAL OF CHEMICAL RESEARCH 2012
Scheme 2
1
3012–2525 (COO-H), 2208 (C≡C), 1708 (C=O), 1280 (C–O). H
NMR (DMSO-d6, 400 MHz) δ (ppm): 13.40 (s, 2H), 7.81 (s, 1H), 7.75
(s, 2H), 7.62 (dd, J = 3.0, 6.6 Hz, 2H), 7.47 (m, 3H). 13C NMR
(DMSO-d6, 100 MHz) δ (ppm): 168.40, 168.38, 134.2, 133.7, 132.6,
132.1, 131.4, 129.8, 129.6, 129.3, 125.3, 122.1, 92.3, 88.2.
4-Phenylethynylphthalic anhydride (4): 3 (20.0 g, 75.2 mmol) and
acetic anhydride (100 mL) were placed in a three necked flask. After
stirred at 80 °C for 1h, the solvent was removed and the residue was
recrystallised from toluene/n-hexane (V:V=1:1) to afford a pale yellow
crystals (17.6 g, 94%). m.p. 152–153 °C (lit.6 150–152 °C). IR (KBr,
cm−1): 3063 (aromatic C–H), 2205 (C≡C), 1843, 1772 (C=O), 1245
(C–O). 1H NMR (CDCl3, 400 MHz) δ (ppm): 8.11(s, 1H), 7.99(s, 2H),
7.58(m, 2H), 7.40(m, 3H). 13C NMR (CDCl3, 100 MHz) δ (ppm):162.2,
138.7, 132.04, 131.99, 131.7, 129.7, 129.6, 128.6, 128.2, 125.6, 121.6,
96.0, 86.9.
as co-catalyst, to afford 4-phenylethynylphthalic acid (3) in a
yield of 95%. The coupling reaction time in water was short-
ened from 12h (traditional methods) to 3h and the yield was
15%, 22%, and 9% higher than that in organic solvent starting
from 4-BPAc,14 4-BPA3 and 4-BDMP,10 respectively. Diacid 3
was then dehydrated in acetic anhydride at 80 °C for 1h to
afford 4-PEPA with a yield of 94%.
In conclusion, a facile synthesis of 4-PEPA in three steps is
described in this paper. The new method overcomes some of
the shortcomings of traditional synthetic routes to 4-PEPA
such as high cost, complex operation, more pollution, etc.
Experimental
Melting points were determined on an XRC melting point apparatus
and are uncorrected. IR spectra were recorded on a PK1600 FTIR-
Received 7 January 2012; accepted 7 April 2012
Paper 1201088 doi: 10.3184/174751912X13366326240908
Published online: 4 June 2012
1
type spectrophotometer using KBr pellets. H-NMR (400 MHz) and
13C-NMR (100 MHz) spectra were measured on a Bruker AV II-400
instrument spectrometer with TMS as an internal standard. TPPTS
was synthesised as described in the literature.16 All the other reagents
and chemicals were commercially available and were used without
purification.
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Monosodium salt mixture of 4-bromophthalic acid (2): To NaOH
(120.0 g, 3.0 mol) dissolved in water (1000 mL) was added 1 (222.0 g,
1.5 mol). When the complete solution was obtained, bromine (124.0 g,
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(lit.10 211.0–211.6 °C). IR (KBr, cm−1): 3074 (aromatic C–H)
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