Chemical Upcycling of Waste Poly(bisphenol A carbonate) to 1,4,2-Dioxazol-5-ones and One-Pot C?H Amidation

Article abstract of DOI:10.1002/cssc.202100885

Chemical upcycling of poly(bisphenol A carbonate) (PC) was achieved in this study with hydroxamic acid nucleophiles, giving rise to synthetically valuable 1,4,2-dioxazol-5-ones and bisphenol A. Using 1,5,7-triazabicyclo[4.4.0]-dec-5-ene (TBD), non-green carbodiimidazole or phosgene carbonylation agents used in conventional dioxazolone synthesis were successfully replaced with PC, and environmentally harmful bisphenol A was simultaneously recovered. Assorted hydroxamic acids exhibited good-to-excellent efficiencies and green chemical features, promising broad synthetic application scope. In addition, a green aryl amide synthesis process was developed, involving one-pot depolymerization from polycarbonate to dioxazolone followed by rhodium-catalyzed C?H amidation, including gram-scale examples with used compact discs.

Full text of DOI:10.1002/cssc.202100885

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Chemical Upcycling of Waste Poly(bisphenol A carbonate) to
1,4,2-Dioxazol-5-ones and One-pot C−H Amidation
Authors: Hyun Jin Jung, Sora Park, Hyun Sub Lee, Hyun Gyu Shin,
Yeji Yoo, Ek Raj Baral, Jun Hee Lee, Jaesung Kwak, and
Jeung Gon Kim
This manuscript has been accepted after peer review and appears as an
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content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.202100885
Link to VoR: https://doi.org/10.1002/cssc.202100885
01/2020

10.1002/cssc.202100885
ChemSusChem
FULL PAPER
Chemical Upcycling of Waste Poly(bisphenol A carbonate) to
1,4,2-Dioxazol-5-ones and One-pot C−H Amidation
Hyun Jin Jung,[a] Sora Park,[a] Hyun Sub Lee,[a] Hyun Gyu Shin,[a] Yeji Yoo,[a] Ek Raj Baral,[a] Jun Hee
Lee,[b],* Jaesung Kwak[c],* and Jeung Gon Kim[a],*
[a]
H. J. Jung, S. Park, H. S. Lee, H. G. Shin, Y. Yoo, Dr. E. R. Baral, Prof. Dr. J. G. Kim
Department of Chemistry and Research Institute of Physics and Chemistry
Jeonbuk National University
Jeonju 54896, Republic of Korea
E-mail: jeunggonkim@jbnu.ac.kr
[b]
[c]
Prof. Dr. J. H. Lee
Department of Advanced Materials Chemistry, Dongguk University
Gyeongju 38066, Republic of Korea
E-mail: leejunhee@dongguk.ac.kr
Dr. Jaesung Kwak
Infectious Diseases Therapeutic Research Center
Korea Research Institute of Chemical Technology (KRICT)
Deajeon 34114, Republic of Korea
E-mail: jkwak@krict.re.kr
Supporting information for this article is given via a link at the end of the document.
Abstract: Chemical upcycling of poly(bisphenol A carbonate) (PC)
was achieved in this study with hydroxamic acid nucleophiles, giving
rise to synthetically valuable 1,4,2-dioxazol-5-ones and bisphenol A.
protective goods. Over 5 million tons of PC are produced, and a
similar amount of PC is wasted annually. It is classified under
code 7 (other plastics) according to the ASTM plastic recycling
system, and most used PCs end up in landfills.[8] As a result, a
large amount of bisphenol A (BPA) slowly enters the ecosystem.
Considering the health and environmental impact of BPA, efficient
recycling of PC is critical.[9,10] Therefore, chemical recycling of PC
is of interest.[11] The carbonate functionality can undergo various
chemical transformations, mostly toward nucleophilic additions.
While pyrolysis yields a mixture of lightweight organic compounds,
chemical transformations that are more selective have shown
promise.[12,13] In particular, the carbonyl moiety could replace the
highly toxic phosgene and carbon monoxide in carbonylation
reactions.[14-16] Besides hydrolysis, which yields BPA and carbon
dioxide,[17,18] alcoholysis,[19-23] aminolysis,[24-26] and hydrogenation
Using
1,5,7-triazabicyclo[4.4.0]-dec-5-ene
(TBD),
non-green
carbodiimidazole or phosgene carbonylation agents used in
conventional dioxazolone synthesis were successfully replaced with
PC, and environmentally harmful bisphenol A was simultaneously
recovered. Assorted hydroxamic acids exhibited good to excellent
efficiencies and green chemical features, promising broad synthetic
application scope. In addition, a green aryl amide synthesis process
was developed, involving one-pot depolymerization from
polycarbonate to dioxazolone followed by rhodium-catalyzed C−H
amidation, including gram-scale examples with used compact discs.
[27-29]
yield value-added organic carbonates, carbamates, and
urea under mild conditions.
Introduction
In this regard, 1,4,2-dioxazol-5-one has recently attracted
research interest.[30] Though first reported in 1950, its synthetic
usage was seldom investigated until 2015.[31,32] It was recently
shown to be a safe and highly reactive acyl nitrene source for
various reactions, including C−H amidations.[33-36] However, the
synthesis of dioxazolones has received little attention. The
reaction between hydroxamic acid and highly reactive carbonyl
sources, such as phosgene, triphosgene, or 1,1’-
carbodiimidazole (CDI), has been carried out for a long time.[12,36]
However, considering the instability and toxicity of these reagents,
as well as the inconvenience of processing them, a better
synthetic process involving easy handling and safe carbonylation
reagents is necessary.[37] Usage of waste chemicals as starting
materials promotes greenness and economy. Based on our
experience with the chemical recycling of PC,[11,21] we envisioned
that PC could replace phosgene or CDI in dioxazolone synthesis
(Scheme 1). The details are discussed as follows.
Plastics are essential materials in the modern world. They are
lightweight, strong, durable, easy to process, and inexpensive,
and their benefits have been reaped over the last 100 years.
However, once plastics serve their designated purposed, they
pose a serious problem as they do not degrade.[1] A recent report
estimated that 4.9 billion tons of plastic waste ends up in landfill.
Only 0.8 billion tons are recycled, mostly mechanically. However,
even recycled plastics eventually end up in landfill or used for
energy recovery after a single recycle.[2] As an approach to
improve the processing of plastic waste, chemical recycling,
which is the recovery of chemical feedstocks through pyrolysis or
depolymerization of used plastics, has gained attention.[3-5] Waste
plastics are rich reservoirs in carbon, hydrogen, oxygen, and
nitrogen. Thus, chemical recycling could reduce the amount of
waste and diminish fossil fuel dependency.
Poly(bisphenol A carbonate) (polycarbonate or PC) is a leading
engineering plastic with excellent heat resistance, mechanical
strength, and transparency.[6,7] Thus, it is widely used in
electronics, automobiles, construction, optical parts, and
1
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10.1002/cssc.202100885
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FULL PAPER
THF) under mild conditions (30 oC, 2 h) (entry 2). DBU (2 mol%)
afforded a good yield (96%, entry 3). Subsequently, the selected
organic bases were further analyzed. Triethylamine exhibited
intermediate conversion to 2a (68%, entry 4). N-heterocyclic
bases, 4-dimethylaminopyridine (DMAP), pyridine, and imidazole
produced 2a in 51%, 17%, and 17% yields, respectively (entries
5−7). These results suggest that the basicity is closely related to
the reactivity. The order of basicity, pyridine (pKa (MeCN) =
12.53) < imidazole (pKa (MeCN) = 15.07) < DMAP (pKa (MeCN)
= 17.96) < triethylamine (pKa (MeCN) = 18.83) < DBU (pKa
(MeCN) = 24.31) < TBD (pKa (MeCN) = 26.02), is consistent with
the yields of dioxazolone 2a under the given conditions.[39] An
inorganic base NaOH also promoted the reaction efficiently under
standard conditions (91%, entry 8).
Scheme 1. Chemical recycling of PC: BPA recovery and green upcycling to
obtain value-added organic compounds.
In previous studies of DPC and polycarbonate alcoholysis to
organic carbonates, the strong nucleophilic nature of TBD and
DBU played a principal role in activating the aromatic carbonate
group.[16,20,21] Thus, weak nucleophiles such as DMAP, pyridine,
imidazole, and triethylamine did not promote the reaction. In this
study, catalyst-free conditions produced conversion, and a more
basic catalyst exhibited better efficiency. These observations led
us to hypothesize that the reaction commences with the
deprotonation of hydroxamic acid, followed by the nucleophilic
addition of the conjugated anion. The plausible mechanism is
described in scheme 2. In the presence of a base, hydroxamic
acid 1a forms hydroxamates A. The nucleophilic attack of A to
DPC generates oxy-carbonyl-hydroxamate B via tetrahedral
intermediate and phenoxide departure. The second deprotonation
by either catalytic base or phenoxide gives anion C, which is
known to undergo Lossen rearrangement. However,
intramolecular nucleophilic addition of its resonance structure D
is favored over Lossen rearrangement and furnishes dioxazolone
2a.[40,41]
Table 1. Optimization results: phenyl dioxazolone from DPC.
Entry
1
Catalyst
None
Solvent
Yield of 2a a
8%
2-Me-THF
2-Me-THF
2-Me-THF
2-Me-THF
2-Me-THF
2-Me-THF
2-Me-THF
2-Me-THF
2-Me-THF
2-Me-THF
2-Me-THF
Acetone
2
TBD (2 mol%)
99%
3
DBU (2 mol%)
Triethylamine (2 mol%)
DMAP (2 mol%)
Pyridine (2 mol%)
Imidazole (2 mol%)
NaOH (2 mol%)
TBD (1 mol%)
TBD (0.5 mol%)
TBD (0.1 mol%)
TBD (2 mol%)
96%
4
68%
5
51%
6
17%
7
17%
8
91%
9
98%
97%
10
11
12
We further evaluated the most active TBD catalyst systems. The
synthetic efficiency remained high at lower catalyst loadings of 1.0
mol% and 0.5 mol% (entries 9 and 10). In case of a 0.1 mol%
loading, elevated temperature (50 oC) was required for high
86% (50 ℃)
98%
Dimethyl
Carbonate
13
14
15
TBD (2 mol%)
TBD (2 mol%)
TBD (2 mol %)
96%
92%
54%
conversion (86%, turn-over-number
=
860, entry 11).
Ethyl Acetate
Environmentally and process-friendly solvents were tested to
enhance the green chemistry metrics. Aprotic polar solvents such
as acetone, dimethyl carbonate, and ethyl acetates promoted
excellent DPC to dioxazolone conversion, with 98% (entry 12),
96% (entry 13), and 92% (entry 14) yields, respectively; polar
protic t-BuOH decreased the efficiency (54%, entry 15).
t-BuOH
a Yields were determined via 1H NMR using hexamethylbenzene as an internal
standard.
The optimized conditions were successfully applied for the
depolymerization of PC (Table 2). The complete depolymerization
to BPA and dioxazolone required a longer reaction time than the
DPC did. Under the TBD (2 mol%) condition, 4 h was required for
near-complete depolymerization, as after 2 h some BPA units
containing the carbonates BPAMC and BPADC remained (entries
1 and 2). The use of either TBD (1 mol%) (entry 3) or DBU as a
catalyst (2 mol%) (entry 4) could not achieve the same reactivity
as in the case of 2 mol% TBD. We monitored the catalytic
Results and Discussion
To optimize the dioxazolone formation conditions, we used a
model compound, diphenyl carbonate (DPC) (Table 1). The
aromatic carbonate, structurally similar to polycarbonate, has
exhibited similar chemical behavior in many studies.[38,16] The
formation of 1,4,2-dioxazol-5-one from highly reactive CDI and
phenyl hydroxamic acid (1a) does not require a catalyst,[36] but the
direct reaction between DPC and 1a afforded only 8% conversion
(entry 1). To compensate for such low reactivity, a series of
catalysts were applied. In our previous study, we achieved facile
alcoholysis of DPC or PC to organic carbonate with the organic
superbases, 1,5,7-triazabicyclo[4.4.0]-dec-5-ene (TBD), and 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU).[16,21] Likewise, excellent
catalytic efficiency was maintained with nucleophile 1a. TBD (2
mol%) was used to successfully produce the desired phenyl
dioxazolone (2a) quantitatively in 2-methyl tetrahydrofuran (2-Me-
1
depolymerization using H NMR (Figure 1A) and size-exclusion
chromatography (SEC) (Figure 1B). In 5 min, most of the
polymeric BPA was converted to small molecular structures such
as BPAMC and BPA. Only 22% of the polymeric carbonate unit,
BPADC, remained and no BPADC signal was observed after 30
min. These distributions were highly correlated with changes in
molecular weights. The high molecular weight portion of PC (blue
line) disappeared in 5 min (red line). The oligomeric peaks
2
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10.1002/cssc.202100885
ChemSusChem
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between 5 and 6.5 mL elution volume were completely gone after
30 min. All polycarbonates were depolymerized in 30 min to BPA
and BPAMC. These observations suggest that depolymerization
occurs through random chain scission rather than chain-end
degradation, which could result in gradual molecular weight
reduction and low BPAMC accumulation.
Scheme 2. Plausible reaction mechanism of 2a formation from 1a and DPC
Table 2. PC depolymerization by hydroxamic acid 1a and TBD.
BPA unit distribution
Entry
Catalyst
Conditions
2a (%)
BPA (%)
BPAMC (%)
BPADC (%)
1
2
3
4
TBD (2 mol%)
TBD (2 mol%)
TBD (1 mol%)
DBU (2 mol%)
2-Me-THF, 2h, 30℃
2-Me-THF, 4h, 30℃
2-Me-THF, 4h, 30℃
2-Me-THF, 4h, 30℃
89
95
93
91
80
98
94
83
17
2
2
0
0
0
5
17
a Yields were determined by 1H NMR using hexamethylbenzene as an internal standard.
3
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10.1002/cssc.202100885
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Figure 1. A) 1H NMR spectral changes in dimethyl protons in a BPA unit and B) molecular weight changes by SEC as a function of
depolymerization time.
Scheme 3. Scope of dioxazolone synthesis beginning with PC.a
a Reaction conditions: PC (0.11 mmol, 1.0 equiv), 1 (0.12 mmol, 1.1 equiv), TBD (2 mol %), and 2-methyltetrahydrofuran (0.3 mL).
Using the optimized conditions, we explored the scope of PC
depolymerization to dioxazolones and BPA with representative
hydroxamic acids (Scheme 2). The reaction with phenyl
hydroxamic acid (1a) afforded the corresponding phenyl
dioxazolone (2a) in 96% yield via 1H NMR and 80% isolation yield.
BPA recovery was recorded at 93% simultaneously. The
positioning of electron-donating groups, such as methyl and
methoxy, afforded good conversion to the corresponding
dioxazolones 2b and 2c. Both compounds achieved 90% yield as
per 1H NMR but had different isolation efficiencies 60% and 81%,
respectively. The BPA isolation rate were 89% and 69%,
respectively. The effect of electron-withdrawing groups was
prominent. Weakly withdrawing chlorine substitution exhibited
minimal influence, as demonstrated by the 70% isolation yield of
4
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10.1002/cssc.202100885
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2d. Stronger substituents such as 4-nitro (2e) and 4-cyano (2f)
groups resulted in reduced reactivity, 63% and 57% 1H NMR
yields, and 54% and 48% isolation yields, respectively. Electron-
withdrawing substituents reduced the nucleophilicity of the
conjugated anion of hydroxamic acids, explaining their low
efficiencies. The long conjugated moieties, namely the 2-naphthyl
(2g) and cinnamic (2h) units, gave high yields. The aliphatic
substituents produced mixed effects. Steric influence was
insignificant. Both primary alkyl and bulky adamantyl hydroxamic
acids exhibited good conversions to the corresponding
dioxazolones 99% (2i) and 89% (2j) respectively with good
isolation yields. In contrast, the benzylic-type groups exhibited low
conversions and isolation yields (2k and 2l). Hydroxamic acids
with heterocycles, herein furan and thiophene, converted BPA-PC
into dioxazolones (2m and 2n) smoothly.
Scheme 4. One-pot PC depolymerization to dioxazolone and rhodium-catalyzed and C−H amidation.
a Reaction conditions: step 1) PC (0.22 mmol, 1.0 equiv), 1 (0.24 mmol, 1.1 equiv), TBD (2 mol %), and 2-methyltetrahydrofuran (0.4 mL); step 2) [Cp*RhCl2]2 (1
mol%), AgNTf2 (4 mol%), and 2-phenyl pyridine (0.20 mmol, 1.0 equiv).
Scheme 5. Gram-scale upcycling of used PC to aryl amides.
The primary use of dioxazolone is direct C−H amidation.[42] With
an appropriate catalyst, amides can be introduced in various
transformations. To maximize the synthetic value of the chemical
upcycling of PC, we evaluated tandem PC-depolymerization to
dioxazolone and C−H amidations (Scheme 3). The original C−H
amidation system developed by Chang was applied to a mixture
of BPA and dioxazolone.[35,36] BPA and TBD did not significantly
affect the Cp*Rh(III) catalyst reactivity. Temperature adjustment
transformation was applied on a 2-gram scale, and produced 3a
in 60% and BPA in 79% isolation yields, respectively.
Conclusion
In this study, PC was chemically recycled to dioxazolones and
BPA in the presence of a TBD catalyst and hydroxamic acid. The
highly reactive organocatalytic system was applied to a broad
range of hydroxamic acids and produced a range of synthetically
valuable dioxazolones. In addition to the recovery of
environmentally harmful BPA, PC served as a green carbonyl
source in place of CDI or phosgene. The resulting dioxazolones
were subjected to one-pot C−H amidations. The Cp*Rh catalyst
maintained good efficiency under one-pot conditions. Real PC
upcycling from used compact discs demonstrated the practicality
o
to 60 C was required while the original reaction was performed
at room temperatue. Combinations of 2-phenyl pyridine and
hydroxamic acid derivatives exhibited promise in a one-pot
recycling and amidation sequence. Yields in the range of 69-79%
amide (3a – 3e) were recorded with high BPA isolation yields.
Only furanoic acid derived dioxazolone afforded low yields of
amide (3f) and BPA. The upcycling of waste polycarbonates was
tested using compact discs (Scheme 4). The one-pot
5
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General experimental procedure for dioxazolone synthesis
beginning with BPA-PC. PC (0.028 g, 0.11 mmol of carbonate
functionality), N-hydroxybenzamide (0.016 g, 0.12mmol), TBD
(0.30 mg, 0.0022 mmol, 2 mol%), 2-methyl THF (0.3 ml), and
dibromomethane (7.7 μl, 0.22 mmol, internal standard) were
added to a 4 mL glass vial. The vial was placed in a preheated
reaction block at 30 °C. After stirring for 4 h, a few drops of acetic
acid were added to quench the reaction. An aliquot of the reaction
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1
mixture was used for H NMR analysis. The crude mixture was
directly purified by preparative thin layer chromatography with
RP18F plates (hexane:acetone = 60:1) to afford BPA (23 mg,
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General experimental procedure for one-pot C−H amidation.
PC (0.056 g, 0.22 mmol of carbonate functionality), N-
hydroxybenzamide (0.033 g, 0.24mmol), TBD (0.60 mg, 0.0044
mmol, 2 mol%), and 2-methyl THF (0.4 ml) were added to a 4 mL
glass vial. The vial was placed in a preheated reaction block at
30 °C. After stirring for 4 h, [Cp*RhCl2]2 (1.2 mg, 1.0 mol%),
AgNTf2 (3.1mg, 4.0 mol%), and 2-phenylpyridine (31mg, 0.20
mmol) were added to the mixture. The reaction mixture was
stirred in a heating block at 60 °C for 15 h. The reaction was
cooled to room temperature, filtered through a pad of celite, and
then washed with CHCl3. The solvent was then removed under
reduced pressure. The resulting mixture was purified by column
chromatography (hexane:EtOAc from 99:1 to 70:30) to afford
BPA (47 mg, 94%) and N-(2-pyridin-2-yl)phenyl)benzamide (48
mg, 78%).
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Acknowledgements
This research was supported by the National Research
[42] Y. Park, Y. Kim, S. Chang, Chem. Rev. 2017, 117, 9247−9310.
Foundation
of
Korea
(NRF-2012M3A7B4049677
and
2018R1D1A1B07048318) and the selection of research-oriented
professor of Jeonbuk National University in 2020.
Keywords: Chemical recycling • Upcycling • Polycarbonate •
Dioxazolone • C-H amidation
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Entry for the Table of Contents
Chemical upcycling of poly(bisphenol A carbonate) to synthetically valuable 1,4,2-dioxazol-5-one is realized. In addition to toxic
bisphenol A recovery, waste PC replaces phosgene or its derivatives in a carbonylation reaction, boosting the metrics of green
chemistry.
Institute and/or researcher Twitter usernames: @ jeunggonkim
7
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