One-pot formal synthesis of biorenewable terephthalic acid from methyl coumalate and methyl pyruvate

Article abstract of DOI:10.1039/c3gc42487a

Diverse functionalized aromatic compounds are constructed from captodative dienophiles with exclusive regioselectivity. 100% biorenewable dimethyl terephthalate (DMT) from methyl coumalate and methyl pyruvate is achieved in a one-pot, Diels-Alder/decarboxylation/elimination sequence in nearly quantitative yield. The DMT system is solvent-free and purification is accomplished through recrystallization. DMT hydrolysis reveals the co-monomer terephthalic acid (TPA) as a bio-based drop-in replacement for the polymer industry, avoiding harsh oxidation and petrochemicals. the Partner Organisations 2014.

Full text of DOI:10.1039/c3gc42487a

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One-pot formal synthesis of biorenewable
terephthalic acid from methyl coumalate and
methyl pyruvate†
Cite this: Green Chem., 2014, 16,
2111
Received 6th December 2013,
Accepted 23rd January 2014
Jennifer J. Lee and George A. Kraus*
DOI: 10.1039/c3gc42487a
www.rsc.org/greenchem
Diverse functionalized aromatic compounds are constructed from cycloadditions,18 and Diels–Alder transformations.19–21 In the
captodative dienophiles with exclusive regioselectivity. 100% bio- context of 2-pyrones, the Diels–Alder reaction has been exten-
renewable dimethyl terephthalate (DMT) from methyl coumalate sively explored to install consecutive stereogenic centers in the
and methyl pyruvate is achieved in
a
one-pot, Diels–Alder/ bicyclic adduct for further elaboration.22–24 Recently, aromatic
decarboxylation/elimination sequence in nearly quantitative yield. systems have been obtained with methyl coumalate (1) both
The DMT system is solvent-free and purification is accomplished with unactivated alkenes25 and electron-deficient olefins26
through recrystallization. DMT hydrolysis reveals the co-monomer (Scheme 1). In each instance, catalytic palladium was required
terephthalic acid (TPA) as a bio-based drop-in replacement for the to effect aromatization. One drawback is that the electron-
polymer industry, avoiding harsh oxidation and petrochemicals.
deficient alkenes or alkynes afforded mixtures of regioisomers.
Methyl coumalate as a diene represents a convenient bio-
based platform for diversification, since it arises from the
acid-catalyzed dimerization of naturally-occurring malic acid.27
The Kraus group recently reported metal-free thermal con-
ditions which convert methyl coumalate to aromatic systems
in a one-pot Diels–Alder/decarboxylation/elimination cascade
(Scheme 1).28 The regioselectively obtained aromatic products
reflect the compatibly matched electron-deficient methyl
coumalate diene with electron-rich dienophiles in an inverse
electron-demand Diels–Alder reaction (IEDDA).
While the literature describes the scope of electron-rich,
unactivated, and electron-poor dienophiles with methyl cou-
malate, dienophiles with both an electron-rich and an elec-
tron-poor moiety have remained largely uninvestigated. We
will present a methodology to couple methyl coumalate with
captodative dienophiles to regioselectively generate a broad
spectrum of aromatic compounds (Scheme 1), which culmi-
nates in an expedient formal synthesis of 100% biorenewable
terephthalic acid, a high-volume commodity chemical.
We initially began pairing substituted dienophiles with
methyl coumalate (Table 1) to determine whether the
additional electron-withdrawing group would mitigate the
regioselectivity previously observed with electron-rich vinyl
ethers.28 When methyl trans-3-methoxyacrylate (2a) is incorpor-
ated as the dienophile, only the meta-substituted dimethyl iso-
phthalate (3a) is produced in 83% yield. The singly obtained
regioisomer is attributed to the accompanying selectivity for
the bicyclo[2.2.2]octene adduct formed in situ. Earlier Diels–
Alder approaches with 2-pyrones and electron-deficient
alkenes have attempted to target 3a selectively; however, only
Functionalized aromatic systems are indispensable commodity
building blocks for industry and are embedded within elabo-
rate structures in natural products and unique materials. Aro-
matics permeate the chemical industry; unfortunately, they are
primarily derived from diminishing and volatilely priced pet-
roleum-based sources, with estimates that demand could
potentially exceed natural reserves as early as 2040.1 The rising
demand for aromatic compounds, especially polymerization
co-monomers like terephthalic acid (TPA) for consumer pro-
ducts, has attracted attention to the critical necessity of develo-
ping alternative feedstocks.2–5 Naturally-occurring materials
contain limited aromatic content outside of lignin; moreover,
the technology to isolate aromatics effectively from lignin is
still in its early stages.4,6,7 In contrast, upgrading malic acid
obtainable through glucose fermentation8,9 leads to a solution
for green aromatics through a key Diels–Alder reaction.
Researchers have efficiently generated aromatic systems
through variegated approaches,10,11 including the acclaimed
Diels–Alder reaction which has long been instrumental in
building complexity through domino reaction sequences.12–14
Furthermore, captodative olefins with stabilization arising
from an electron-rich and an electron-poor substituent
appended to an alkene15 have been successfully incorporated
in radical additions,16 Friedel–Crafts reactions,17 2,3-
Department of Chemistry and NSF Engineering Research Center for Biorenewable
Chemicals, Iowa State University, Ames, IA 50011, USA. E-mail: gakraus@iastate.edu
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3gc42487a
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Scheme 1 General reaction summary of literature precedent and current research.
Table 1 meta-Selective aromatics from 1,2-substituted dienophilesa
Table 2 para-Selective aromatics from captodative dienophilesa
Yieldb (%)
94
Entry
1
Dienophile
Aromatic product
Yieldb (%)
83
Entry
1
Captodative dienophile
Aromatic product
2
92
2
75
a Reaction conditions: 1 (1 mmol) and 2 (3 mmol) in 2.0 mL toluene at
200 °C for 16 h in a sealable tube. b Isolated yield.
a Reaction conditions: 1 (1 mmol) and 2 (3 mmol) in 2.0 mL toluene at
200 °C for 16 h in a sealable tube. b Isolated yield.
mixtures of regioisomers resulted.29 The newly introduced syn-
thetic strategy provides facile access to 3a for integration into
polymers.30,31 Consonantly, ketones can function as the elec- para-substituted aromatic compounds (Table 2). The explora-
tron-withdrawing component in the dienophile, which under- tion commences with 3,3-dimethoxy-2-butanone39 (4a) as a
goes similar reactivity (Table 1, entry 2). The resultant captodative dienophile equivalent since ketals are primed to
3-acetylbenzoic acid methyl ester (3b) is not readily available eliminate methanol under the thermal conditions to reveal the
through commercial suppliers, although it is a building block dienophile.28 As predicted, the Diels–Alder reaction sequence
for anti-obesity32,33 or cardiovascular drugs.34 Most approaches forms only para-substituted methyl 4-acetylbenzoate (5a). This
to 3b begin with a di-substituted aromatic system which can compound has been utilized as an advanced intermediate to
undergo further functionalization.35,36 Although a Diels–Alder synthesize alkaloids40 and enzyme modulators.41 Literature
sequence has been developed from a butadiene equivalent and precedent majorly focuses on manipulating commercially
an electron-deficient alkyne, two discrete steps are necessary available aromatic compounds;42 however, the posited meth-
and the Diels–Alder reaction is not completely regioselective.37 odology allows functionalized aromatics to be constructed
Dienophiles 2a and 2b do not conform to the traditional capto- from two non-aromatic substrates. While ketals are obtained
dative definition where the electron-rich and electron-donating in one step from the corresponding ketones, we envisioned
groups exist on the same carbon. However, they are syntheti- that the thermal conditions might induce a shift in the keto–
cally practical synthons since both substituents provide syner- enol equilibrium to allow the enol to react as a captodative di-
gistic electronic stabilization, analogous to the cooperativity of enophile. By employing 2,3-butanedione, the same regioselec-
enamine dienophiles.38
tive aromatic isomer 5a resulted, albeit in 18% unoptimized
The successful generation of meta-substituted aromatics yield. The yield likely was limited by the experimentally deter-
stimulated expansion of the captodative dienophiles to favor mined 1.1% enol content at equilibrium based on the
2112 | Green Chem., 2014, 16, 2111–2116
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entropically disfavored restricted rotation in the enol over the significance is reflected by its annual production of nearly
keto form, characteristic of acyclic systems.43 However, the 60 million tons and its incorporation in numerous consumer
1
yield is significantly greater than the 3% observed by crude H applications, with the highest volumes in polyester fibers then
NMR with acetophenone as the substrate, which lacks captoda- in bottles and packaging.46 While TPA has formidably
tive stabilization. Among the di-carbonyl systems, α-diones are impacted society and is projected to continue its ascent into
the most amenable to generate aromatics through the reaction the future,50 its synthesis relies on a harsh oxidation of pet-
sequence. Although the 1,3-dione acetylacetone (6) was calcu- roleum-based p-xylene,51 itself isolated from
a complex
lated to have a higher 7.6% enol content at equilibrium,43 it mixture of isomers. In contrast, bio-based captodative dieno-
was unreactive to the same reaction conditions (Scheme 2). philes in conjunction with methyl coumalate may create a sus-
The non-reactivity may imply a predilection for reaction at the tainable shift toward biorenewable feedstocks.
less-substituted olefin 8 through isomerization, which was
observed when dienophile 7 was isolated and subjected to the volume commodity chemical, the reaction conditions were
standard reaction conditions.28
specifically optimized for DMT using the enol silyl ether of
With potential to profoundly impact the process for a high-
Finally, cyclic methoxy enone 4b cleanly furnishes substi- methyl pyruvate (9a) as delineated in Table 3. Methyl pyruvate
tuted tetralone 5b which is not readily available through com- was identified as an exemplary dienophile since its preparation
mercial suppliers. The complementary 1,2-cyclohexanedione involves an esterification of pyruvic acid, the major natural
was subjected to similar reaction conditions and provided 5b product from the glycolysis cycle. In general, the effectiveness
in a reasonable 36% unoptimized yield likely through an of the systematic changes in reaction conditions was analyzed
increased thermal enolization from the ambient 4.0% enol by comparing the ratio of the characteristic DMT methyl ester
content.43 Captodative dienophiles in the IEDDA with methyl peak at 3.95 ppm to the methyl ester peak of the limiting
1
coumalate maintained complete regioselectivity despite their reagent methyl coumalate at 3.88 ppm in the crude H NMR
electron-poor constituent, which suggests that the electron- spectra. The reaction sequence is concentration-dependent
donor is the major contributor to predict regiochemistry. In (Table 3, entries 2 and 11) and allowing the Diels–Alder/aroma-
addition to conserving regioselectivity, captodative olefins tization sequence to proceed without solvent allows the reac-
have proven their merit as entities that introduce electron- tion to occur with fewer equivalents of the dienophile but with
deficient functionality adjoined to the resulting aromatic similar completion. Along the parameter of the dienophile :
system.
diene ratio (Table 3, entries 3–5 and 10–11), an excess of 3.0
We then turned to terephthalic acid (TPA) with its para-sub- equivalents of the dienophile 9a was necessary for complete
stituted carboxylic acids, which was ultimately the high-value consumption of methyl coumalate (1) at 0.5 M, but an excess
target for our promising technology capitalizing on captodative of 1.5 equivalents was sufficient under neat conditions. The
dienophiles. TPA as a commodity chemical commands a domi- effect of temperature was next investigated (Table 3, entries
nant presence since it has been ranked within the six highest 2–6 and 11), and decreasing the temperature to 150 °C was
domestically produced organic commodity chemicals in 2001 effectual in the absence of solvent. Finally, the optimal
by the US International Trade Commission.44 On a global
scale, production reached 50.7 million tons which translates to
a $58 billion market within the last year.45 The industrial sig-
Table 3 Reaction optimization trials
nificance of TPA and its ester dimethyl terephthalate (DMT)
lies in their ability to act as condensation co-monomers for
poly(ethylene terephthalate) (PET).46 Various companies
specifically favor DMT as the co-monomer with ethylene glycol
due to its preferential properties.46–49 PET’s societal
Conc.
(M)a
Temp.
(°C)
Time
(h)
Equiv.
of 9a
Ratio of
Entry
DMT : 1b
1
2
3
4
5
6
7
8
0.5
0.5
0.5
0.5
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
200
150
100
200
150
125
150
150
150
150
150
200
16
18
17
17
16
18
6
3
1
17
16
16
3.0
3.0
3.0
1.5
1.5
1.5
1.5
3.0
1.5
1.0
3.0
1.5
1 : 0
1 : 0.48
1 : 4.00
1 : 0.53
1 : 0.12
1 : 0.72
1 : 0.50
1 : 0.49
1 : 1.31
1 : 1.12
1 : 0
9
10
11
12
1 : 0
a Entries 1–4 were run with toluene as the solvent. b Ratios determined
Scheme 2 Enol content and reactivity of acetylacetone.
by integration of crude 1H NMR.
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Scheme 3 Captodative dienophiles to generate dimethyl terephthalate.
reaction conditions are collated in Table 3, entries 11–12, to possible with the fewest modifications of bio-based methyl
eliminate the dependence on aromatic solvents. Both sets of pyruvate, which successively led to using the crude ketal
conditions completely consume the limiting reagent, depend- directly. Strikingly, methyl 2,2-dimethoxypropanoate56 (9d),
ing on the preference to run the reaction at a lower tempera- which is only a single step from methyl pyruvate, equally
ture or utilize higher equivalents of the dienophile.
affords high yields of DMT. We postulated that the <10% enol
With the optimal reaction conditions in hand, five captoda- content of pyruvic acid in CCl4, stabilized by intramolecular
tive dienophiles were explored to flexibly generate DMT hydrogen bonding,57 would allow even more direct access to
(Scheme 3), which can undergo a facile hydrolysis to TPA.52 At DMT with methyl pyruvate (9e). The hypothesis was corrobo-
the outset, the enol silyl ether of methyl pyruvate53 (9a) was rated by the 59% yield which may be elevated if left for a
isolated through recrystallization after subjection to the reac- longer period of time since methyl coumalate was not comple-
tion conditions in Table 3, entry 12, which resulted in 85% tely consumed during the standard 16 hours. Utilizing a
yield. Although recrystallization is a convenient purification ketone adjacent to the ester functions as a better captodative
method conducive for industrial scale, pure DMT spon- dienophile assumedly due to the placating electron-withdraw-
taneously sublimes on the walls of the sealed flask during the ing effect of the ester compared to a ketone as observed in pre-
reaction as captured in Fig. 1. Recognizing that the enol silane vious cases with an adjacent ketone. Since unaltered methyl
would not be feasible for industrial-scale reactions, 2-acetoxy- pyruvate presents itself as a captodative dienophile equivalent
acrylate54,55 (9b) became the next dienophilic partner, with a to successfully achieve DMT, it adds another dimension of
straightforward acid-catalyzed preparation from acetic anhy- flexibility and convenience to the biorenewable methodology.
dride and methyl pyruvate. The desired para-substituted DMT
In summary, captodative dienophiles coupled with the
was the major product, along with 3% dimethyl isophthalate methyl coumalate platform distinctively provide a regioselec-
(DMI), presumably due to the heightened electron-withdraw- tive route to functionalized aromatic systems through a
ing nature of the dienophile as
a
whole. Conversely, one-pot
domino
Diels–Alder/decarboxylation/elimination
2-methoxyacrylate56 (9c) generated DMT regioselectively in sequence. Previously, only electron-neutral or electron-rich
95% yield without formation of DMI. While advantageous, groups were plausible dienophile substituents to promote the
generating 9c from methyl pyruvate involved isolating the ketal electron-matched components under IEDDA conditions;
then eliminating one equivalent of methanol under acidic con- however, captodative dienophiles allow access to electron-with-
ditions. We were interested in assembling DMT as rapidly as drawing substituents directly on the resultant aromatic struc-
ture. Flexibility is inherent in designing captodative
dienophiles to generate either para- or meta-substituted aro-
matic compounds, depending on the relative placement of the
electron-withdrawing and electron-donating group across the
olefin. Furthermore, diversified aromatic systems can be
accessed through the combination of non-aromatic precursors
without being limited by the availability of aromatic com-
pounds for incremental functional group manipulation. Imple-
menting the strategy in the case of methyl coumalate and
methyl pyruvate leads to a 100% biorenewable formal syn-
thesis to the mass market commodity chemical terephthalic
acid, although dimethyl terephthalate itself is preferred as a
co-monomer in industry. Advantageously, an additional pet-
roleum-based solvent is avoided for DMT and a versatile
approach to accommodate varying dienophiles provides high
yields and rapid assembly of DMT. From an industrial
Fig. 1 DMT sublimation during the reaction.
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