Laccase-catalyzed synthesis of catechol thioethers by reaction of catechols with thiols using air as an oxidant

Article abstract of DOI:10.1039/c3gc41968a

The laccase-catalyzed reaction between catechols and thiols using aerial oxygen as the oxidant delivers the corresponding catechol thioethers with yields up to 96% under mild reaction conditions.

Full text of DOI:10.1039/c3gc41968a

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Laccase-catalyzed synthesis of catechol thioethers
by reaction of catechols with thiols using air as an
oxidant†
Cite this: Green Chem., 2014, 16, 90
Received 19th September 2013,
Accepted 30th October 2013
Heba T. Abdel-Mohsen, Jürgen Conrad and Uwe Beifuss*
DOI: 10.1039/c3gc41968a
www.rsc.org/greenchem
The laccase-catalyzed reaction between catechols and thiols using reaction between hydroquinones or catechols and thiols.8 So
aerial oxygen as the oxidant delivers the corresponding catechol far, it has been reported that 3-substituted 1,2,4-triazolo[4,3-b]-
thioethers with yields up to 96% under mild reaction conditions.
(4,1,2)benzothiadiazine-8-ones can be synthesized by laccase-
catalyzed in situ generation of p-benzoquinone followed by reac-
tion with 5-substituted 4-amino-3-mercapto-1,2,4-triazoles.8d
Wellington et al. have reported on the laccase-catalyzed reac-
tion between 1,4-dihydroxynaphthalenes and thiols to form
various naphthoquinone sulfides.8b,c Even less is known about
enzyme-catalyzed reactions between catechols and thiols.
The only exception is the laccase-initiated domino reaction
between catechols and 6-substituted 1,2,3,4-tetrahydro-4-oxo-2-
thioxo-5-pyrimidinecarbonitriles for the synthesis of pyrimido-
benzothiazole derivatives.8a The 1,4-addition of simple thiols
to o-benzoquinones generated by enzyme-catalyzed oxidation
of catechols to deliver the corresponding catechol thioethers is
still unexplored.
Catechol thioethers are known to exhibit various biological
(e.g., antibacterial and antioxidant) activities.9 That is why
their synthesis is of interest. Aromatic thioethers can be syn-
thesized by a number of methods, including the Pd-, Cu- and
Rh-catalyzed reaction of aryl halides with thiols under basic
conditions.10 Most catechol thioethers have been obtained by
oxidation of catechols in the presence of the corresponding
thiols. The oxidation of the catechols to o-benzoquinones has
been achieved either by using K3[Fe(CN)6] as the oxidant11 or
by anodic oxidation.12 Clearly, both methods face some pro-
blems, such as the use of at least stoichiometric amounts of
an inorganic oxidant or the use of special equipment not avail-
able in many laboratories.
The development of selective, efficient, economic and sustain-
able oxidations presents a major challenge to synthetic organic
chemistry.1 Against this background, enzyme-catalyzed oxi-
dations using aerial oxygen as the oxidant are receiving par-
ticular attention.2 Laccases, which belong to the class of
multicopper oxidases (benzenediol: O2 oxidoreductase E.C.
1.10.3.2.), are among the most attractive enzymes in this
respect2,3 since they are characterized by a number of features
and advantages: laccases are easily accessible; they are capable
of catalyzing the oxidation of numerous substrates in aqueous
solvent systems under mild reaction conditions (temperature,
pH, pressure) with oxygen; the oxidation of the substrates is
linked to the formation of water as the only byproduct; and the
substrate scope of laccase-catalyzed reactions can be broad-
ened by using laccase/mediator systems allowing for the oxi-
dation of substrates with higher oxidation potentials.
So far, laccase-catalyzed oxidations have mostly been used
for oxidizing several functional groups4 and for oxidative coup-
lings of electron rich phenolic substrates.5 One particular
promising aspect of the laccase-catalyzed transformations is
the potential for combining the laccase-catalyzed generation of
highly reactive molecules with subsequent non-oxidative
chemical transformations to new domino processes. This
approach has been used for the formation of quinoid systems
and their subsequent reactions with C-nucleophiles.6 The
nucleophilic 1,4-addition of different N-nucleophiles to o- and
p-quinones obtained by laccase-catalyzed oxidation of catechol
and hydroquinone, respectively, has also been explored.3b,7
However, little is known about the laccase-catalyzed
Against this background, we considered the laccase-
initiated domino reaction between catechols and thiols as a
highly attractive alternative for the synthesis of catechol
thioethers (Scheme 1).
The catechols and the heterocyclic thiols used for the syn-
thesis of the catechol thioethers are depicted in Fig. 1. The
reaction between catechol (1a) and 2-mercaptobenzoxazole (2a)
on a 1 mmol scale was selected as a model reaction (Table 1).
When equimolar amounts of 1a and 2a were reacted in the
presence of 300 U laccase from Agaricus bisporus (6 U mg−1) in
Bioorganische Chemie, Institut für Chemie, Universität Hohenheim, Garbenstr. 30,
Stuttgart, D-70599, Germany. E-mail: ubeifuss@uni-hohenheim.de;
Fax: (+49)711 459 22951; Tel: (+49)711 459 22171
†Electronic supplementary information (ESI) available: Experimental data and
copies of the 1H NMR and 13C NMR spectra. See DOI: 10.1039/c3gc41968a
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was observed.4a This result confirms the observation that the
laccase-catalyzed generation of o-benzoquinones from cate-
chols does not require a mediator.6a–d,f,g,i–k It is also remark-
able that only the monothioether was formed; not a trace of
the corresponding bisthioether could be detected.
Scheme 1 Synthesis of catechol thioethers by laccase-catalyzed reac-
tion between catechols and thiols.
Further experiments devoted to the influence of the
amount of the laccase from A. bisporus clearly demonstrated
that the reaction of 1a with 2a tolerates a stepwise reduction of
the catalyst load from 300 U to 120 U without any loss of yield
(Table 1, entries 3 and 4). A further decrease did not pay off,
since with 100 or 75 U laccase the transformations did not run
to completion (Table 1, entries 7 and 8). The experiment with
120 U laccase was also performed at 50 °C; however, the yield
dropped from 93 to 84% (Table 1, entry 6). Notably, the reac-
tion could be run successfully on a 0.50 mmol scale with as
little as 60 U laccase (Table 1, entry 5). Therefore, all further
experiments were performed with 0.50 mmol thiol.‡ Finally, a
control experiment was conducted to show that the formation
of the thioether 3a does not proceed in the absence of the
laccase (Table 1, entry 9).
The reaction was not restricted to 2-mercaptobenzoxazole
(2a) as a thiol, but could also be performed with 2-mercapto-
benzothiazole (2b). When 2b was reacted with unsubstituted
catechol (1a) under the conditions developed for the trans-
formation with 2a (Table 1, entry 5), the catechol thioether 3b§
was obtained exclusively in 87% yield (Scheme 2).
It is assumed that the process starts with the laccase-cata-
lyzed oxidation of catechol (1a) to the corresponding o-benzo-
quinone (4a), followed by the intermolecular nucleophilic
1,4-addition of 2-mercaptobenzoxazole (2a) or 2-mercapto-
Fig. 1 Substrates for the laccase-catalyzed transformations.
Table 1 Optimization of the laccase-catalyzed reaction between cate- benzothiazole (2b) to produce the corresponding catechol
chol (1a) and 2-mercaptobenzoxazole (2a) under air
thioethers 3a, b (Scheme 3).
Then, we focused on the reactions between 2a, b and mono-
substituted catechols 1b–j. In a first set of experiments, 2a, b
were reacted with 3-substituted catechols 1b–d under the con-
ditions described in Table 1, entry 5. Depending on whether
the intermolecular nucleophilic attack of the heterocyclic
Entry
Laccase (U)
T
Time (h)
Yield of 3a (%)
thiols 2a, b occurs at C-4 or C-5 of the corresponding o-benzo-
quinones 4b–d (Scheme 4), either products of type 3 or type 5
were formed. The reaction between 3-methylcatechol (1b) and
2-mercaptobenzoxazole (2a) gave 90% of a 69 : 31 mixture of
the regioisomers 3c and 5c (Table 2, entry 3). A similar result
was observed with 2-mercaptobenzothiazole (2b) (Table 2,
entry 4). The isomers arising from the C-4 attack also preferen-
tially formed in the reactions of 3-fluorocatechol (1d): with 2a
as the nucleophile, a 63 : 37 mixture of 3g and 5g was obtained
(Table 2, entry 7); with 2b the isomers 3h and 5h were formed
in a 69 : 31 ratio (Table 2, entry 8). The only exceptions were
the transformations between 3-methoxycatechol (1c) and 2a, b.
With both nucleophiles, the 5-substituted catechol thioethers 5e
and 5f were formed as the major isomers (Table 2, entries 5 and
6). It should be noted that the C-4 addition of 2a, b to 4b and 4d
(quinones) was unexpected, since laccase-catalyzed 1,4-additions
of C-nucleophiles are known to proceed at C-5.6a–d,f,g,i–k
However, the laccase-catalyzed 1,4-additions of thiols to the
o-quinone resulting from 3-methylcatechol (1b) show a close
1a
2b
3b
4b
5c
6b
7b
8b
9b
300
300
150
120
60
120
100
75
rt
rt
rt
rt
rt
50 °C
rt
rt
rt
20
15
16
16
16
12
17
24
24
77
93
95
93
93
84
d
—
d
—
e
—
—
a 1 mmol 1a and 1 mmol 2a were reacted. b 1.25 mmol 1a and 1 mmol
2a were reacted. c 0.63 mmol 1a and 0.50 mmol 2a were reacted.
d Incomplete conversion. e No reaction.
0.2 M phosphate buffer at pH 6.0/MeOH (9 : 1) for 20 h at rt,
the corresponding catechol thioether 3a was isolated in 77%
(Table 1, entry 1). The yield of 3a could be improved to 93%
when the reaction was performed with a slight excess (1.25
equiv.) of 1a (Table 1, entry 2). It should be emphasized that
the thioether 3a was formed exclusively; no disulfide formation
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Table 2 Laccase-catalyzed reaction between 3-substituted catechols
1a–d and thiols 2a, ba
Scheme 2 Laccase-catalyzed reaction between catechol (1a) and 2-
mercaptobenzothiazole (2b).
Entry
1
R
2
X
Time (h) Product 3 : 5b
Yield (%)
1
2
3
4
5
6
7
8
a
a
b
b
c
c
d
d
H
H
CH3
CH3
OCH3
OCH3
F
a
b
a
b
a
b
a
b
O
S
O
S
O
S
O
S
16
17
24
17
17
17
17
17
3a
3b
100 : 0
100 : 0
93
87
3c, 5c
3d, 5d
3e, 5e
3f, 5f
3g, 5g
3h, 5h
69 : 31 90
62 : 38 95
22 : 78 96
17 : 83 95
63 : 37 90
69 : 31 86
Scheme 3 Possible reaction mechanism for the laccase-catalyzed
reaction between 1a and 2a, b.
F
a 0.63 mmol 1 and 0.5 mmol 2 were reacted. b Ratio determined from
the 1H NMR spectrum of the crude product.
Finally, 2-mercaptothiazoline (2c) was reacted with un-
substituted catechol (1a) and the 4-substituted catechols 1h, i
under the conditions displayed in Table 1, entry 5. In accord-
ance with the results of 2a and 2b, the corresponding catechol
thioethers 8a–c were formed exclusively with yields ranging
from 74 to 83% (Table 4).
The newly developed laccase-catalyzed reaction between
catechols 1 and thiols 2 complies with the principles of green
chemistry13 for a number of reasons: the reaction is a highly
selective enzyme-catalyzed transformation that allows the
selective preparation of catechol thioethers in a single step
with yields up to 96%. No toxic byproducts were formed. The
reactions were performed with atmospheric oxygen as the
oxidant, avoiding any toxic reagents. Atmospheric oxygen is
Scheme 4 Regioselectivity of the laccase-catalyzed reactions between
3-substituted catechols 1b–d and thiols 2a, b.
resemblance to some of the results from reactions run under not only one of the cheapest oxidants, but it is also considered
electrochemical conditions.12b,c Thioethers were not formed to be environmentally benign. During the course of the reaction,
when 3-substituted catechols having an electron withdrawing oxygen is reduced to completely safe and nontoxic water,
group at the 3-position, i.e. 1e (R = CN), 1f (R = COOH) and 1g which is the only byproduct of the entire process. Using the
(R = COOMe), were used as substrates.
laccase-catalyzed reaction between catechol (1a) and 2-mercapto-
Next, the reactions between 2-mercaptobenozoxazole (2a) benzoxazole (2a) as an example, the E-factor (kg waste per kg
and 2-mercaptobenzothiazole (2b) with a number of 4-substi- product)1a,14 of the process is 7.88 kg kg−1. This value com-
tuted catechols were performed under the reaction conditions pares well with the E-factors of other synthetic methods for the
given in Table 1, entry 5. It was interesting to see that the reac- synthesis of catechol thioethers. The atom economy15 of the
tions with 4-methylcatechol (1h) and 4-ethylcatechol (1i) exclu- presented method is high (94%). A mixture of phosphate
sively delivered the catechol thioethers 6a–d resulting from the buffer (pH 6.0) and 10 vol% methanol, which is a safe and
nucleophilic attack of 2a, b at C-5 of the corresponding o-benzo- environmentally preferred solvent,1c,16 was used as the reaction
quinone intermediates 4h,i (Scheme 5) with yields ranging medium. The laccase-catalyzed domino reaction can be carried
from 83 to 90% (Table 3, entries 1–4). However, with 4-nitro- out under mild reaction conditions (room temperature, atmos-
catechol (1j) as the substrate, the 1,4-addition selectively took pheric pressure, pH 6.0). The laccase-catalyzed formation of
place at C-3 of the corresponding o-benzoquinone 4j to deliver catechol thioethers is characterized by high turnover numbers
the thioether 7a exclusively in 85% yield (Scheme 5, Table 3, (TON), e.g., in the reaction between 1a and 2a, it amounts to
entry 5).
4512. This value underlines the high catalytic efficiency of the
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Fig. 2 Structure of 3b and important HMBC correlations.
Structures of all compounds were unambiguously eluci-
dated by mass spectrometry and NMR spectroscopy including
2D NMR for the full assignment of the 1H and 13C chemical
shifts. For example, analysis of the gCOSY spectrum of com-
pound 3b (Fig. 2) revealed two scalar coupled 1H spin systems,
one consisting of protons 3-H, 5-H and 6-H (ring A) and the
other of aromatic protons 4′-H, 5′-H, 6′-H and 7′-H (ring B).
Their corresponding directly bonded carbons were identified
by gHSQC. The quaternary carbons C-1, C-2 and C-4 (ring A)
were assigned by gHMBC optimized for JCH = 8 Hz. A strong
3J-HMBC correlation between 3-H as well as 5-H and the
phenolic carbon at δ 148.66 ppm established the C-1 position
of the latter. The chemical shifts of the remaining A ring
carbons (C-2/C-4) at δ 146.61 and 116.66 ppm respectively were
Scheme 5 Regioselectivity of the laccase-catalyzed reactions between
4-substituted catechols 1h–j and thiols 2a, b.
Table 3 Laccase-catalyzed domino reaction between 4-substituted
catechols 1h–j and thiols 2a, b
3
deduced by the strong J-HMBC correlations to 6-H. However,
the unambiguous assignment of the C-2 or C-4 position to the
observed values and thus the substitution pattern at C-2 and
C-4 was not possible even with a gHMBC experiment opti-
mized for JCH = 1 Hz. Therefore, the 13C NMR chemical shifts
were computed by DFT quantum mechanical DFT calculations
at the DFT GIAO mPW1PW91/6-311+G(2d,p)//mPW1PW91/
6-31G(d) level of theory.17 On the basis of computationally calcu-
lated 13C chemical shifts, the quaternary carbons C-2 and C-4
were assigned to be at δ 146.61 (δ calcd 142.71 ppm) and
116.66 (δ calcd 119.25 ppm), respectively. Similarly, the qua-
ternary carbons C-3a′ and C-7a′ of ring B were assigned at δ
153.71 (δ calcd 153.07 ppm) and δ 134.75 (δ calcd 139.77 ppm),
Entry
1
R
2
X
Product
Yield (%)
1a
2a
3a
4a
5b
h
h
i
i
j
CH3
CH3
C2H5
C2H5
NO2
a
b
a
b
a
O
S
O
S
6a
6b
6c
6d
7a
85
90
83
83
85
O
a 0.63 mmol 1 and 0.5 mmol 2 were reacted. b 0.50 mmol 1j and
0.5 mmol 2a were reacted.
respectively. The most downfield shifted carbon at
δ
171.99 ppm was tentatively assigned to the C-2′ position.
Table 4 Laccase-catalyzed reaction between catechols 1a, h, i and 2-
mercapto-thiazoline (2c) under aira
Conclusion
To summarize, a simple-to-perform, efficient and sustainable
method for the synthesis of substituted catechol thioethers
has been developed. It relies on the laccase-catalyzed oxidation
of a catechol to the corresponding o-benzoquinone which in
turn undergoes an intermolecular 1,4-addition with a thiol as
a nucleophile. The reactions can be performed under mild
reaction conditions with air as the oxidant, and the products
are formed with yields ranging between 74 and 96%. Depend-
ing on the substituent on the catechol moiety, different regio-
isomers are formed.
Entry
1
R
Time (h)
8
Yield (%)
1
2
3
a
h
i
H
CH3
C2H5
20
17
17
a
b
c
83
74
82
a 0.63 mmol 1 and 0.5 mmol 2c were reacted.
Acknowledgements
enzyme-catalyzed reaction. The turnover frequencies of the
process are good; in the example mentioned above the TOF
We thank Ms S. Mika for recording NMR spectra and
Dr. A. Baskakova as well as Dipl.-Ing. J. Trinkner and
amounts to 282 h−1
.
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Ms K. Wohlbold (Institut für Organische Chemie, Universität
Stuttgart) for recording mass spectra. H. T. A.-M. is grateful to
Deutscher Akademischer Austauschdienst (DAAD) for financial
support.
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Notes and references
‡General procedure for the laccase-catalyzed synthesis of catechol thioethers: a
100 mL round bottomed flask with a magnetic stir bar was charged with a solu-
tion or suspension of the catechol 1 (0.63 mmol) and the thiol 2 (0.5 mmol) in
methanol (3 mL). Phosphate buffer (0.2 M, pH 6, 27 mL) and laccase from A. bis-
porus (10 mg, 6 U mg−1) were added and the mixture was stirred under air at rt
for the time given (see Tables 1–4). The reaction mixture was acidified with 2 M
HCl to pH ∼ 4 and saturated with solid NaCl. The precipitated product was fil-
tered with suction using a Buchner funnel. The filter cake was washed with aq.
NaCl (15%, 25 mL) and water. The crude products obtained after drying exhibit
a purity of 90–95% (NMR). Analytically pure products were obtained by column
filtration (CH2Cl2–EtOAc 5 : 1 to 1 : 1) of the crude products.
§Selected analytical data of 4-(benzo[d]thiazol-2′-ylthio)benzene-1,2-diol (3b):
pale yellow solid; mp 198–200 °C; Rf = 0.53 (CH2Cl2–EtOAc = 5 : 1); λmax(MeCN)/
nm 284 (log ε, 4.60) and 206 (4.20); ˜νmax (atr)/cm−1 3426 (br), 3054 (C–H), 1593,
3
1415, 1270 and 1030; δH (300 MHz; DMSO-d6) 6.90 (1H, d, J5-H,6-H 8.1 Hz, 6-H),
3
4
4
7.07 (1H, dd, J5-H,6-H 8.4 Hz, J3-H,5-H 2.1 Hz, 5-H), 7.13 (1H, d, J3-H,5-H 2.1 Hz,
3
3
4
3-H), 7.29 (1H, ddd, J5′-H,6′-H 7.2 or 7.9 Hz, J6′-H,7′-H 7.2 or 7.9 Hz, J4′-H,6′-H 1.1
3
3
4
Hz, 6′-H), 7.42 (1H, ddd, J4′-H,5′-H 7.3 or 8.2 Hz, J5′-H,6′-H 7.3 or 8.2 Hz, J5′-H,7′-H
3
4
5
1.3 Hz, 5′-H), 7.79 (1H, ddd, J4′-H,5′-H 8.1 Hz, J4′-H,6′-H 1.2 or 0.6 Hz, J4′-H,7′-H 1.2
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3
4
5
or 0.6 Hz, 4′-H), 7.89 (1H, ddd, J6′-H,7′-H 8.0 Hz, J5′-H,7′-H 1.3 or 0.7 Hz, J4′-H,7′-H
1.3 or 0.7 Hz, 7′-H) and 9.63 (2H, br, 1-OH and 2-OH); δC (75 MHz; DMSO-d6)
116.66 (C-4), 117.01 (C-6), 121.12 (C-4′), 121.64 (C-7′), 122.45 (C-3), 124.13 (C-6′),
126.27 (C-5′), 127.66 (C-5), 134.75 (C-7a′), 146.61 (C-2), 148.66 (C-1), 153.71
(C-3a′) and 171.99 (C-2′); m/z (EI, 70 eV) 275 (M+, 100%), 242 (M+ − SH, 5), 167
(C7H5S2N+, 8) and 115 (15); HRMS (EI, M+) found 275.0051 calcd for
C
13H9S2NO2: 275.0075.
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