Catalytic oxidative cleavage of catechol by a non-heme iron(iii) complex as a green route to dimethyl adipate

Article abstract of DOI:10.1039/c3cc42423e

The catalyst system prepared in situ from iron(iii) salts, tris(2-pyridylmethyl)amine and a base readily catalyses the intradiol dioxygenation of pyrocatechol in methanol, to primarily afford the half-methyl ester of muconic acid. Dimethyl adipate is obtained by the subsequent, one-step catalytic hydrogenation/esterification, thus providing a green route to this important nylon precursor.

Full text of DOI:10.1039/c3cc42423e

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Catalytic oxidative cleavage of catechol by a non-heme
iron(III) complex as a green route to dimethyl adipate†
Cite this: Chem. Commun., 2013,
49, 6912
Robin Jastrzebski, Bert M. Weckhuysen and Pieter C. A. Bruijnincx*
Received 3rd April 2013,
Accepted 12th June 2013
DOI: 10.1039/c3cc42423e
www.rsc.org/chemcomm
The catalyst system prepared in situ from iron(III) salts, tris(2-pyridyl-
Reactions were performed for 6 h in methanol under controlled
methyl)amine and a base readily catalyses the intradiol dioxygenation air flow, at ambient pressure and at 50 1C. The half-methyl ester of
of pyrocatechol in methanol, to primarily afford the half-methyl ester cis,cis-muconic acid (3b) was found to be the major product (Fig. 1),
of muconic acid. Dimethyl adipate is obtained by the subsequent, one- while minor products formed included the free acid (3a), half-
step catalytic hydrogenation/esterification, thus providing a green catechol ester (3c) and cis,trans isomers of the above (4a–c), for a
route to this important nylon precursor.
typical mole balance of 90%. Notably, all these products derive
from the selective intradiol cleavage of catechol and may eventually
Intradiol catechol dioxygenases are a class of non-heme iron enzymes be converted to dimethyl adipate (2) (vide infra). Products resulting
that selectively cleave the C–C bond between the hydroxyl groups of a from the extradiol cleavage of catechol were not observed. Solvolysis
catecholic substrate using molecular oxygen and an Fe(III)-containing of the cyclic muconic anhydride, which is formed as the initial
active site, to afford cis,cis-muconic acid.1 Many synthetic non-heme product of the dioxygenation reaction,2 to obtain 3b has been
iron(III) complexes have been studied as functional models of these previously observed in methanol.3a,9 Catechol itself is apparently
enzymes, providing key insights into various mechanistic aspects of also a competent nucleophile for ring opening of the anhydride.
intradiol cleavage.2 Almost invariably, these studies were concerned Although catechol esters have not been previously reported as
with stoichiometric reactions of the activated and readily cleaved products of intradiol cleavage reactions, the previously reported
substrate 3,5-di-tert-butylcatechol. In contrast, the stoichiometric con- experiments typically used a stoichiometric amount of the catechol
version of the unactivated substrate pyrocatechol (1)3 has received at only 0.5 mM,8 whereas in the catalytic runs reported here a
little attention and no prior examples of its catalytic cleavage have concentration of 100 mM is used. The cis,trans isomers are not
been reported. Selective catalytic catechol cleavage would nevertheless expected to directly result from the oxidative cleavage cycle, but
be advantageous, as hydrogenation4 of the intradiol cleavage product, rather result from a subsequent isomerisation step, which may
muconic acid (3a), can provide a greener route to adipic acid or its proceed readily in solution and exclusively affords the isomer where
esters, which are key monomers, e.g., for nylon-6,6.5 Such a route the ester is in the trans position.10 The 10% of original catechol
would make use of non-toxic catalysts and oxidants, be highly atom- intake that cannot be accounted for is possibly lost to the
efficient and employ a renewable substrate, as catechol may be uncatalysed oxidation of catechol to 1,2-benzoquinone and
obtained sustainably from lignin6 or by fermentation of glucose.7
subsequent condensation with catechol.11
Que et al. have reported the most active system for stoichio-
UV-Vis-NIR spectra of diluted reaction mixtures show char-
metric intradiol cleavage of 3,5-di-tert-butylcatechol reported to acteristic LMCT bands at 518 nm and 812 nm, confirming that
date, using the ligand tris(2-pyridylmethyl)amine (tpa).8 We have [Fe(tpa)(catecholato)]+ complexes2b,3c,12 are formed and remain
found the catalytic intradiol dioxygenation of catechol to proceed present throughout the reaction. The reaction was found to
readily with oxygen as the oxidant using 5 mol% of a non-heme initially proceed by approximately 0th order kinetics in catechol,
iron(III) complex prepared in situ from iron(III) perchlorate, tpa consistent with a mechanism where reaction of oxygen with the
and two equivalents of base (with respect to iron) (Scheme 1).
[Fe(tpa)(catecholato)]+ complex is the rate-limiting step.13
Stoichiometric experiments have shown that addition of two
equivalents of base relative to iron is required to deprotonate the
catechol and form the [Fe(tpa)(catecholato)]+ complex.14 A systematic
screening of bases showed that two equivalents were again found to
be the optimum for the catalytic reaction (Table S1, ESI†). If more
than two equivalents of base were used, UV-Vis-NIR spectra of these
solutions initially showed a strong absorption at 565 nm, which may
Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science,
Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands.
E-mail: p.c.a.bruijnincx@uu.nl
† Electronic supplementary information (ESI) available: Experimental proce-
dures, additional catalysis data, UV-Vis-NIR traces, characterisation of isolated
m-oxo, m-carboxylato dibridged species and analytical data for dimethyl adipate.
See DOI: 10.1039/c3cc42423e
c
6912 Chem. Commun., 2013, 49, 6912--6914
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Scheme 1 The intradiol dioxygenation of catechol (1) in methanol exclusively affords intradiol cleavage products, which after hydrogenation/transesterification yield
dimethyl adipate (2).
well as two new products, which were identified as lactones of
muconic acid (5b, 5c). Both the isomerisation and lactonisation
have previously been shown to be base-catalysed.10c,16
Lower catalyst loading resulted in higher turnover frequencies,
although presumably this is because in those cases the 0th order
approximation is more valid. Running the reaction for 24 h at a
1.5% mol Fe loading gives full conversion with a TON of 51. The
use of ferric perchlorate or nitrate as the catalyst precursor expect-
edly gave similar results, as both salts have anions that are non-
coordinating. Remarkably, even iron(III) acetylacetonate (Fe(acac)3),
Fig. 1 Products observed in the catalytic dioxygenation of catechol are derivatives of
despite its strongly bound acac anions, is able to form the
catecholato complex and exhibit considerable activity (Table 1,
#8), with no additional base needed to be added to this reaction.
Several of the stoichiometric intradiol dioxygenation reactions of
cis,cis-muconic acid (3), cis,trans-muconic acid (4) and muconolactone (5) and include
free acids (a), methyl esters (b), catechol esters (c) and a piperidyl amide (d).
suggest competitive coordination by the base. In addition, a new (substituted) catechols use iron(III) chloride as the iron source,3a,b,17
product, identified as the muconamide of piperidine (3d), was but in our case low activity was observed with this precursor
formed at higher base loadings.
(Table 1, #7) with the UV-Vis spectra of this system pointing to
Even though an acidic product is formed (pKa1 3–4), the rapid catalyst deactivation (Fig. S3, ESI†).
[Fe(tpa)(catecholato)]+ complex remained present in the reac-
With the other iron precursors the LMCT bands of the
tion mixture even after multiple turnovers. As muconic acid is [Fe(tpa)(catecholato)]+ complex were present throughout the reaction,
certainly a sufficiently strong acid to protonate piperidine (pKa but their intensity did decrease as the reaction progressed. As catechol
11.2), it seems likely that once all base has been protonated, is still available in the reaction mixture, this points to a gradual
muconate anions become the base that deprotonates the deactivation of the catalyst. Remarkably, addition of fresh catechol
catechol, consistent with the remarkable stability of iron(III)- again led to an increase in intensity of the LMCT bands. This shows
catecholato complexes.15 This is further supported by the that while some decomposition of the active complex does seem to
observation that the reaction still proceeded rapidly even when take place during the reaction, it appears to be largely reversible. As
ammonium acetate is added as a base (Table 1, #2). Indeed, differric m-oxo, m-carboxylato complexes have been isolated as the
bases of different strengths (Table 1, #1–5) do not seem to products of the reaction in stoichiometric experiments with 3,5-di-tert-
significantly affect the activity. However, the use of diisopropy- butylcatechol,8 a similar complex was considered as the deactivation
lethylamine (DIPEA) or 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) product here. Indeed, stoichiometric cleavage of catechol with iron(III)
resulted in formation of considerably more cis,trans isomers as perchlorate, tpa and piperidine resulted in the precipitation of a
Table 1 Results of the catalytic dioxygenation of catechol, using a catalyst system prepared in situ from tpa and the listed iron precursors and basesa
Yield (%)
Precursor
Base
Conv.b (%)
3a
3b
4a + 4b
3c + 4c
5b + 5c
MBc (%)
TONd
TOFe (hÀ1
)
1
2
3
4
5
6
7
8
5% Fe(ClO4)3
5% Fe(ClO4)3
5% Fe(ClO4)3
5% Fe(ClO4)3
5% Fe(ClO4)3
5% Fe(NO3)3
5% FeCl3
Piperidine
NH4OAc
2,6-Lutidine
DIPEAf
92
94
90
86
88
88
30
73
50
100
56
51
5
6
6
4
4
6
2
4
3
2
4
4
48
48
49
35
40
49
12
21
27
64
31
30
2
4
4
8
7
13
11
10
9
12
11
2
5
7
8
7
—
—
—
3
88
87
88
81
90
91
92
84
93
88
93
94
16
15
16
14
16
14
4
11
20
51
19
18
2.7
2.5
2.7
2.4
2.6
2.3
0.7
1.8
3.4
2.1
3.2
2.9
TBDg
2
Piperidine
Piperidine
—
Piperidine
Piperidine
Piperidine
Piperidine
2
—
—
8
—
—
—
—
—
15
0
6
1
5% Fe(acac)3
2% Fe(ClO4)3
1.5% Fe(ClO4)3
1% 12i
9
h
10
11
12
1% m-oxoj
1
5
a
b
Conditions: 9.1 mmol catechol, 100 mL MeOH; iron precursor, tpa (1 eq.) and base (2 eq.); 6 h, 50 1C, air flow at 1 atm. Conversion of catechol.
c
d
e
f
Mole balance as moles of C6 units found divided by the starting amount of catechol. Turnover number. Turnover frequency. Diisopropyl-
g
h
i
ethylamine. 1,5,7-Triazabicyclo[4.4.0]dec-5-ene. 24 h reaction. Compound isolated from a stoichiometric reaction using Fe(ClO4)3, tpa (1 eq.)
and base (2 eq.). [Fe2(tpa)2(m-O)(m-OAc)](ClO4)3Á2H2O.
j
c
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hydrogenation/transesterification is not necessary, as the crude
reaction mixture, still containing the iron catalyst, could be likewise
converted to give dimethyl adipate in an isolated yield of 48%.
These results show the excellent potential for the two-step synthesis
of dimethyl adipate from catechol, a route that does not require any
intermediate purification steps.
Scheme 2 Reversible formation of the active [Fe(tpa)catecholato]+ complex
from a m-oxo, m-acetato dibridged ferric complex.
In conclusion, we have demonstrated the selective catalytic
intradiol dioxygenation of catechol under mild conditions, using a
catalyst prepared in situ from ferric perchlorate, tris(2-pyridylmethyl)-
amine and base. The formation of m-oxo, m-carboxylato iron
complexes was shown to be reversible under reaction conditions.
Subsequent hydrogenation and esterification of the dioxygena-
tion products using a Pd/C catalyst afforded pure dimethyl
adipate in 55% isolated yield. Thus, we have demonstrated a
potentially sustainable, new route towards synthesis of mono-
mers for nylon-6,6. Further studies will be concerned with
further improving the catalytic dioxygenation activity.
Notes and references
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Fig. 2 Spectrophotometric titration of a 0.2 mM solution of 12 in the presence
of 0.2 mM piperidine in CH3CN with addition of 0.5 eq. of catechol in between
spectra. The LMCT bands at 518 nm and 812 nm correspond to the formation of
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brown microcrystalline material from solution, which was identified
as a differric m-oxo, m-carboxylato complex based on its UV-Vis-NIR,
NMR and ATR-IR data.18 The bridging carboxylato group is presum-
ably provided by one of the muconate products formed. In order to
investigate the formation of the active catecholato catalyst from m-oxo,
m-carboxylato species (Scheme 2), which has previously been hypothe-
sised,19 [Fe2(tpa)2(m-O)(m-OAc)](ClO4)3Á2H2O (12) was prepared.
Spectrophotometric titration of 12 in acetonitrile with catechol
(Fig. 2) clearly shows the appearance of the catecholato LMCT
bands at the expense of the m-carboxylato absorptions. Indeed, use
of the differric m-oxo, m-carboxylato complexes as catalyst precursors
resulted in a catechol dioxygenation reaction of similar activity and
selectivity as with the in situ prepared catalysts (Table 1, #9, 11, 12).
To our knowledge, this is the first time that the formation of a
m-oxo, m-carboxylato complex is shown to be reversible under
reaction conditions in iron oxidation catalysis.
The product mixture of esters, acids and geometrical isomers
obtained after the catechol dioxygenation at first sight looks rather
complex, but this complexity can be readily resolved without any
required product separation steps by a simple hydrogenation/
transesterification reaction. Indeed, the product mixture could be
convergently converted to the single end product dimethyl adipate,
also a monomer for nylon-6,6, by performing a transesterification
of the product mixture simultaneously with hydrogenation. The
isolated product mixture was hydrogenated over a Pd/C catalyst
(1 atm H2, 50 1C) in the presence of 10 mol% p-toluenesulfonic acid
16 B. M. Trost and P. G. McDougal, J. Org. Chem., 1984, 49, 458–468.
17 T. Funabiki, T. Yamazaki, A. Fukui, T. Tanaka and S. Yoshida,
Angew. Chem., Int. Ed., 1998, 37, 513–515.
in methanol. Although the equilibrium position of adipic acid and 18 R. E. Norman, S. Yan, L. Que, G. Backes, J. Ling, J. Sanders-Loehr,
J. H. Zhang and C. J. O’Connor, J. Am. Chem. Soc., 1990, 112,
1554–1562.
19 W. O. Koch and H.-J. Kru¨ger, Angew. Chem., Int. Ed., 1995, 34,
its diester prohibits complete transesterification, dimethyl adipate
was still isolated in 55% yield (based on original catechol intake).
Most interestingly, isolation of the product mixture prior to
2671–2674.
c
This journal is The Royal Society of Chemistry 2013
6914 Chem. Commun., 2013, 49, 6912--6914