1. SOP TRANSACTIONS ON ORGANIC CHEMISTRY
Volume 1, Number 1, August 2014
SOP TRANSACTIONS ON ORGANIC CHEMISTRY
Model Experiments Implicate a
Benzoquinoneketene Intermediate in
Poly-2-hydroxybenzoic Acid Synthesis
Matthew Hettinger, H. K. Hall, Jr, Robert Bates*
Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ, USA
*Corresponding author: batesr@email.arizona.edu
Abstract:
To show that the polyester formation from 2-hydroxybenzoic acid (2-HBA) with base and heat
proceeds partly via the ketoketene 2-oxo-3,5-cyclohexadienylideneketene, the polymer was
first formed from a dimer of 2-HBA. Then a related dimer which could not form this ketoketene
was shown to yield no polymer. When secondary amines were added to the original dimer,
again no polymer was formed, this time because the ketoketene was trapped as monomeric
amides. These results indicate that this ketoketene is an intermediate in base-catalyzed 2-HBA
homo-polymerization, paralleling what happens with 4-HBA homopolymers (LCPs).
Keywords:
Ketoketene; Polyester; Poly-2-hydroxybenzoic Acid
1. INTRODUCTION
In 2011 we reported that ketoketene 1, derived by the fragmentation of 4-hydroxybenzoic acid deriva-
tives 2, plays a role in the synthesis of liquid crystal polymers (LCPs) 3 [1]. We now report the results of
a parallel investigation of the role ketoketene 4, derived from a 2-hydroxybenzoic acid derivative 5, plays
in the homo-polymerization of 2-hydroxybenzoic acid 6. Pure ketoketene 4 is stable as a gas2 and has
been proposed as an intermediate in many reactions [2, 3].
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2. SOP TRANSACTIONS ON ORGANIC CHEMISTRY
2. RESULTS AND DISCUSSION
Synthesis and polymerization of dimer 7
Dimer 7 (Figure 1) was chosen for polymerization, following the work of Robertson[1] but with
ortho-dimer 7 instead of the corresponding para-dimer. When synthesis of dimer 7 via acetylsalicylic acid
was unsuccessful, the hydroxyl group was protected with a benzyl group which could be easily removed
by hydrogenolysis (Figure 1). 2-Benzyloxybenzoyl chloride (8 4)was coupled to methyl salicylate to
produce dimer 9. A two-phase reaction was chosen as the best method for coupling when a homogeneous
reaction with p-toluenesulfonyl chloride in pyridine gave a lower yield. Benzyl dimer 9 was deprotected
by hydrogenolysis to give hydroxy dimer 7.
Figure 1. Synthesis and polymerization of hydroxy dimer 7.
To show that 2-HBA dimer 7 polymerizes as does the 4-HBA dimer,1 dimer 7 was heated with catalytic
KOBut to give poly-2-HBA (10, Figure 1). The strongest peaks in a high-resolution MALDI mass
spectrum were due to linear oligomers of 10 of n = 3-11 units with a methyl ester at one end, a phenol at
the other, and an attached potassium cation.
Dimer 11 as a control to support polymerization via ketoketene 4
2’-(Methoxycarbonyl)phenyl 2-ethoxybenzoate (11, Figure 2) was chosen as a control; since its
hydroxyl group is protected it cannot form a ketoketene intermediate to polymerize. Dimer 11 was made
by reacting 2-ethoxybenzoyl chloride (12)4 with methyl salicylate in pyridine. There was no evidence of
polymer formation when dimer 11 was heated with KOBut.
Figure 2. Synthesis and lack of polymerization of ethyl ether dimer 11.
Trapping of ketoketene 4 with secondary amines
To further prove the intermediacy of ketoketene 4 in these reactions, it was trapped by morpholine and
diisopropylamine when dimer 7 was heated with these amines, producing amides 13 and 14 as shown in
Figure 3.
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3. Model Experiments Implicate a Benzoquinoneketene Intermediate in Poly-2-hydroxybenzoic Acid Synthesis
Figure 3. Morpholine and diisopropylamine trapping of ketoketene 4.
3. CONCLUSIONS
2-HBA dimer 7 was synthesized and polymerized to poly-2-HBA (10). A control reaction showed this
polymerization did not go via direct aminolysis. The proposed intermediate ketoketene 4 was trapped
with secondary amines. These results indicate that just as LCP synthesis of 4-HBA occurs partly via
ketoketene 1,1 synthesis of poly-2-HBA occurs partly via ketoketene 4.
4. EXPERIMENTAL
NMR spectra were obtained at 500 MHz in CDCl3 on a Bruker DRX 500 spectrometer. ESI mass
spectra were obtained on a Bruker 9.4T Apex Qh FT-ICR instrument.
2-(Methoxycarbonyl)phenyl 2-hydroxybenzoate (7).
In a two-phase reaction, a solution of acid chloride 8 (7.2 g) in dichloromethane (30 mL), a solution of
methyl salicylate (4.18 mL) and KOH (4.9 g) in water (30 mL), and a catalytic amount of phase transfer
catalyst benzyltriethylammonium chloride was stirred rapidly for 1 h. The DCM layer was washed with
water (3⇥30 mL) and evaporated. The resulting oil was vacuum-distilled at 250 oC and 0.1 tor to remove
methyl salicylate, leaving dimer 9 (3.2 g, 31%); 1H NMR d 3.78 (3H, s), 5.24 (2H, s), 7.09 (2H, m), 7.20
(1H, dd, J=8.0,1.1 Hz), 7.29 (1H, tt, J=7.0, 1.5 Hz), 7.34 (3H, m), 7.52 (3H, m), 7.57 (1H, ddd, J=7.5,
7.5, 1.7 Hz), 8.07 (1H, dd, J=7.9, 1.7 Hz), 8.16 (1H, dd, J=8.0, 1.9 Hz). MS: m/z calcd for C22H18O5Na
385.105; obsd 385.105. Dimer 9 (2.8 g) was dissolved in a minimal amount of ethanol, and Pd/C catalyst
(200 mg) was added to the solution in a Pyrex bottle. The bottle was pressurized to 60 psi with hydrogen
and shaken overnight. The solution was filtered over Celite to remove Pd/C catalyst. The ethanol was
evaporated, leaving 7 (1.54 g, 73%), mp 62-68 oC; 1H NMR: d 3.77 (3H, s), 7.00 (1H, ddd, J=8.2, 8.1, 1.1
Hz), 7.06 (1H, dd, J=8.4, 1.1 Hz), 7.26 (1H, dd, J=8.0, 1.2 Hz), 7.40 (1H, ddd, J=7.6, 7.6, 1.2 Hz), 7.55
(1H, ddd, J=7.2, 7.2, 1.8 Hz), 7.64 (1H, ddd, J=7.5, 7.5, 1.8 Hz), 8.10 (1H, dd, J=7.9, 1.7 Hz), 8.11(1H,
dd, J=7.9, 1.7 Hz); MS: m/z calcd for C15H11O5 271.061; obsd 271.061.
2’-(Methoxycarbonyl)-phenyl 2-ethoxybenzoate (11)
A mixture of methyl salicylate (4.8 mL), pyridine (10 mL) and 2-ethoxybenzoic acid chloride 12 (6.97
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4. SOP TRANSACTIONS ON ORGANIC CHEMISTRY
g) was stirred with ice for 1 h and ice water (50 mL) was added. The precipitate was recrystallized from
1:1 ethyl acetate-hexanes, yielding dimer 11 (5.9 g, 60%), mp 67-71 oC, 1H NMR d 1.48 (3H, t, J=6.5
Hz), 3.79 (3H, s), 4.16 (2H, q, J=6.5 Hz), 7.02 (2H, dd, J=8.3, 1.1 Hz), 7.05 (1H, ddd, J=7.5, 7.5, 1.0 Hz),
7.23 (1H, dd, J=8.0, 1.2 Hz), 7.33 (1H, ddd, J=7.4, 7.5, 1.2 Hz), 7.51 (1H, ddd, J=7.5, 7.4, 1.8 Hz), 7.59
(1H, ddd, J=7.4, 7.4, 1.8 Hz), 8.04 (1H, dd, J=7.9, 2.0 Hz), 8.10 (1H, dd, J=7.7, 1.8 Hz). MS: m/z calcd
for C17H16O5Na 323.089; obsd 323.089.
Poly-2-HBA synthesis from dimer 7
A mixture of dimer 7 (1 g), KOBut (60 mg) and 18-crown-6 ether (60 mg) was heated to 130 ˚ C for 3
h. The cooled mixture was washed with ether to remove methyl salicylate and analyzed by high-resolution
ESI+ MALDI mass spectrometry with a dithranol matrix.
Trapping ketoketene 4 with secondary amines to give amides 13 and 14
Dimer 7 (1 g) in morpholine (5 mL) was heated to 120 oC for 3 h. The cooled solution was added to
ether (50 mL) and water (50 mL). The insoluble solid between the organic and aqueous layers was filtered
off and dried to give amide 13 (232 mg, 30%; NMR spectrum as reported5).
Dimer 7 (0.5 g) in diisopropylamine (5 mL) was heated to 120 oC for 3 h. Excess diisopropylamine
was evaporated under vacuum. The residue was washed with water and dried, leaving amide 14 (75 mg,
19%; NMR spectrum as reported [4].
ACKNOWLEDGMENTS
We thank the Dreyfus Foundation (Grant to H. K. H., Jr.) and Solvay Advanced Polymers, Alpharetta,
GA for financial support.
References
[1] J. M. Robertson, G. C. Contreras, R. B. Bates, and H. H. K. Jr., “Model Experiments Implicate a
Benzoquinoneketene Intermediate in LCP Synthesis,” Macromolecules, vol. 44, no. 14, pp. 5586–5589,
2011.
[2] P. S. de Carvalho, F. M. Nachtigall, M. N. Eberlin, and L. A. B. Moraes, “Intrinsic gas-phase reactivity
of ionized 6-(Oxomethylene) cyclohexa-2, 4-dienone: Evidence pointing to its neutral -oxoketene
counterpart as a proper precursor of various benzopyran-4-ones and analogues,” The Journal of
Organic Chemistry, vol. 72, no. 16, pp. 5986–5993, 2007.
[3] S. P. Kamat and S. K. Paknikar, “A convenient one-pot synthesis of 4-methyl-3-phenyl-, 3-aryl-and
3-aryl-4-phenylcoumarins,” Journal of Chemical Research, vol. 24, no. B, pp. 38–41, 1985.
[4] L. Ackermann and A. V. Lygin, “Cationic Ruthenium (II) Catalysts for Oxidative C–H/N–H Bond
Functionalizations of Anilines with Removable Directing Group: Synthesis of Indoles in Water,”
Organic letters, vol. 14, no. 3, pp. 764–767, 2012.
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