Pyrazinium salts as efficient organocatalysts of mild oxidations with hydrogen peroxide

Article abstract of DOI:10.1002/adsc.201000906

A series of 3-substituted pyrazinium tetrafluoroborates was prepared as simple analogues of flavinium salts which are efficient organocatalysts for oxidations with hydrogen peroxide. It was shown that pyrazinium derivatives with an electron-withdrawing su

Full text of DOI:10.1002/adsc.201000906

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DOI: 10.1002/adsc.201000906
Pyrazinium Salts as Efficient Organocatalysts of Mild Oxidations
with Hydrogen Peroxide
a,b
a,b
c
a,d
ˇ
ˇ
Petra Mꢀnovꢁ, Frantisek Kafka, Hana Dvorꢁkovꢁ, Smita Gunnoo,
b
a,
ˇ
Miloslav Sanda, and Radek Cibulka *
a
Department of Organic Chemistry, Institute of Chemical Technology, Prague, Technickꢁ 5, 166 28 Prague 6, Czech
Republic
Fax : (+420)-220-444-288; phone: (+420)-220-444-279; e-mail: cibulkar@vscht.cz
Institute of Organic Chemistry and Biochemistry, Academy of Science of the Czech Republic, v.v.i., Flemingovo nꢁm. 2,
166 10, Prague 6, Czech Republic
Central Laboratories, Institute of Chemical Technology, Prague, Technickꢁ 5, 166 28 Prague 6, Czech Republic
Chemistry Research Laboratory, 12 Mansfield Road, Oxford, OX1 3TA, U.K.
b
c
d
Received: December 1, 2010; Revised: January 24, 2011; Published online: April 12, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000906.
Abstract: A series of 3-substituted pyrazinium tet-
rafluoroborates was prepared as simple analogues
of flavinium salts which are efficient organocata-
lysts for oxidations with hydrogen peroxide. It was
shown that pyrazinium derivatives with an electron-
withdrawing substituent catalyze mild oxidations of
sulfides to sulfoxides and Baeyer–Villiger oxida-
tions in a similar way to flavinium catalysts. The
most reactive catalyst, 3-cyanopyrazinium tetra-
fluoroborate, was efficiently employed in prepara-
tive sulfoxidations of aromatic and aliphatic sulfides
as well as in Baeyer–Villiger oxidations of cyclobu-
tanones. A proposed mechanism for the catalysis is
based on the formation of pyrazine hydroperoxide
which is the agent oxidizing the substrate.
Scheme 1. Catalytic function of heteroarenium salts in the
oxidation of a substrate S with hydrogen peroxide.
are the oxidizing agents in flavin- and pterin-depen-
dent monooxygenases, respectively.[3]
Although the formation and reactivity of various
heterocyclic peroxides have already been described,[4]
only flavinium salts[5] 1–3 (Scheme 2) have been sys-
tematically studied as catalysts for H2O2 oxidations.[2,6]
It was found that flavinium salts catalyze H2O2 oxida-
tions of sulfides to sulfoxides,[7] cyclobutanones to the
corresponding lactones (Baeyer–Villiger oxidations),[8]
secondary amines to nitrones[7a] and tertiary amines to
N-oxides.[9] These reactions proceed under mild con-
ditions with low catalyst loading (1–2 mol%). Several
chiral flavinium salts were efficiently employed in
Keywords: Baeyer–Villiger oxidation; organocataly-
sis; peroxides; pyrazinium salts; sulfoxidation
Over the last two decades, organocatalytic oxidations enantioselective oxidations of prochiral sulfides and
using hydrogen peroxide as a terminal oxidant have ketones.[10]
been intensively investigated due to their potential
Flavinium salts 1–3 participate in catalysis by the
applications in green chemistry.[1] One promising type reversible addition of hydrogen peroxide to either the
of organocatalytic system is based on heteroarenium azadiene group in isoalloxazine derivatives 1 or the
salts. They activate hydrogen peroxide via the forma- central pyrazinium ring in alloxazines 2 and 3
tion of heterocyclic hydroperoxides which oxidize a (Scheme 2). Thus, these substructures seem to be es-
substrate. After the oxygen transfer, heteroarenium sential for the catalytic function of flavinium salts.
salts are regenerated by the elimination of the hy- This idea led us to explore the catalytic ability of pyr-
droxy function (Scheme 1).[2] The use of heterocyclic azinium derivatives 4 as simpler structurally related
hydroperoxides for oxidations is inspired by nature; analogues of the alloxazinium skeletons 2 and 3. The
flavin-4a-hydroperoxide and pterin-4a-hydroperoxide advantage of pyrazinium salts is that they can be ob-
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Petra Mꢀnovꢁ et al.
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Figure 1. Course of the oxidation of thioanisole with H2O2
Scheme 2. Structures of flavinium salts and the correspond-
ing flavin hydroperoxides.
~
!
*
catalyzed by pyrazinium salts 4a ( ), 4b (&), 4c ( ), 4d ( ),
&
4e (<), and 4f (3). The non-catalyzed reaction ( ) is shown
for comparison. Conditions: n(thioanisole)=0.255 mmol,
nACHTNUTGRNEUNG(H2O2)=0.392 mmol, n(4)=12.8 mmol, T=258C, solvent=
CD3OD/H2O (see Experimental Section for full details).
tained from commercially available starting materials
in only one step – quaternization.
effect on the rate of oxidation. We also tested the cat-
alytic activity of the 2,3-dicyano derivative 4f but the
effect of the second cyano group on the observed effi-
ciency was rather negative. At the beginning, the rate
of reaction catalyzed by 4f slightly exceeds the rate of
reaction catalyzed by 4a (Figure 1), but at conversions
higher than 50% it decelerates due to the decomposi-
tion of 4f in the catalytic system.
As a test to investigate whether both nitrogen
In the first series of experiments, the ability of pyr- atoms in the ring are necessary for catalytic activity of
azinium salts to catalyze oxidations with hydrogen heteroarenium salts, sulfoxidations were performed
peroxide was examined using the sulfoxidation of with analogous pyridinium derivatives 5. It was found
thioanisole as a model reaction. We have also ex- that the rate of thioanisole oxidation was not en-
plored the effect of substituents on the catalytic activ- hanced with these salts even if they are substituted
ity. Experiments were performed with 5 mol% of the with an electron-withdrawing cyano group (for details
catalyst under identical conditions to those used for see the Supporting Information). The inability of pyri-
oxidations catalyzed by flavinium salts, that is, in deu- dinium salts 5 (in comparison with pyrazinium deriva-
terated methanol at room temperature using tives 4) to catalyze sulfoxidations with hydrogen per-
1.5 equivalents of aqueous hydrogen peroxide.[7] Con- oxide is probably caused by the substantially lower
1
version was monitored by H NMR spectroscopy. In electrophilicity of the pyridinium vs. the pyrazinium
Figure 1, the kinetic profiles of sulfoxidations cata- nucleus which precludes the formation of the oxida-
lyzed by 4a–f are compared with that of the non-cata- tive species, pyridine hydroperoxide, under the reac-
lyzed process.
tion conditions. The lower ability of pyridinium salts
The results show that pyrazinium salts bearing an to react with nucleophiles can be easily demonstrated
electron-withdrawing group accelerate the sulfoxida- by their higher pKR+ values[11] representing the heter-
tion of thioanisole. Whilst the rate enhancement by 4c oarenium salt/pseudobase equilibrium (Table 1), that
and 4d is relatively low, derivatives 4a and 4b with is, the ability of a heteroarenium salt to add a hydrox-
cyano and ethoxycarbonyl substituents accelerate the ide ion. While the pKR+ value of the efficient pyrazi-
oxidation significantly. Interestingly, salt 4e bearing nium salt 4a (for the determination see the Support-
an electron-donating methyl group has a negligible ing Information) fits well to those known for flavini-
866
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Adv. Synth. Catal. 2011, 353, 865 – 870

Pyrazinium Salts as Efficient Organocatalysts of Mild Oxidations with Hydrogen Peroxide
Table 1. The pKR+ values for heteroarenium salt/pseudobase
equilibrium.
Heteroarenium salt
pKR+
1a
2a
4a
5b
4.1[a]
7.9[b]
4.5
12.2[c]
tive. The rate enhancement for dibutyl sulfide oxida-
tion by 4a is almost identical to that by 2a.
Ref.[12a]
.
.
[a]
The scope of application of the most active pyrazi-
nium catalyst 4a for H2O2 sulfoxidations was tested
on a variety of sulfides on a 100-mg scale (Table 3).
Using 5 mol% of 4a, complete conversion of all the
tested substrates was achieved within a relatively
short time (20 min–24 h). Work-up of the reaction
mixture was simple; the pure product without any
traces of the catalyst was obtained by simple extrac-
tion in high yields. Similarly, as in the sulfoxidations
catalyzed by flavinium salts[7], the reactions catalyzed
by 4a proceeded chemoselectively. Neither over-oxi-
dation to sulfones, nor oxidation of the double bond
(in allyl phenyl sulfide; Table 3, entry 4) was ob-
served. The significant effect of the catalyst 4a on the
oxidations is evident by comparing conversions of the
catalyzed and the non-catalyzed process after the
same reaction time. For instance, the catalyzed oxida-
tion of thioanisole (entry 1) and dihydrobenzothio-
phene (entry 5) was completed within 1 hour, the ali-
phatic dibutyl sulfide (entry 6) was oxidized within
20 min. After the same time, conversion of non-cata-
lyzed processes did not exceed 4% and 1%, respec-
tively. Conversely, the effect of the catalyst 4a on the
oxidation of the relatively electron-poor substrate p-
nitrothioanisole (Table 3, entry 2) is much smaller.
From the point of view of potential practical applica-
tions, an interesting result was obtained with the oxi-
dation of (diphenylmethylthio)acetamide (a precursor
of the psychostimulant Modafinil[14]; entry 7). After
[b]
[c]
Ref.[7h]
.
Ref.[12b]
Table 2. Comparison of the catalytic activity of the pyrazini-
um salt 4a with that of the alloxazine derivative 2a-H2 in
H2O2 sulfoxidations.[a]
Entry Catalyst Substrate
Rate enhancement[b]
À À
1
2
3
4
5
6
7
8
4a
2a-H2
4a
2a-H2
4a
2a-H2
4a
Ph S CH3
30:1
64:1
À À
Ph S CH3
À À
4-MeC6H4 S CH3
4-MeC6H4 S CH3
38:1
74:1[c,d]
À À
À À
4-MeOC6H4 S CH3 18:1
4-MeOC6H4 S CH3 36:1[c,e]
À À
À À
Bu S Bu
10.5:1
2a-H2
Bu S Bu
12.5:1[c,e]
À À
[a]
Reaction conditions: n(substrate)=0.255 mmol, nACTHNUTRGNE(UNG H2O2)
=0.392 mmol, 2 mol% catalyst, T=258C.
[b]
Rate enhancement for catalyzed vs. non-catalyzed reac-
tions calculated by dividing the rate of the catalyzed re-
action by that of the non-catalyzed reaction at low con-
version (up to 10%).[7]
[c]
[d]
[e]
Ref.[7b]
.
1.83 mol% catalyst.
1.60 mol% catalyst.
um catalysts 1a (R1 =R2 =R3 =R4 =CH3) and 2a 24 h, the desired product Modafinil was obtained in a
(R1 =R2 =CH3, R3 =R4 =H), the value for analogous quantitative yield with the catalyzed process, whilst
the 3-cyanopyridinium derivative 5b is substantially the yield of the non-catalyzed oxidation was only 4%.
higher. The lower electrophilicity of pyridinium salts
Having observed promising catalytic activity of 4a
in comparison with pyrazinium salts is also evident in sulfoxidations, we decided to test the catalyst also
from thereduction potentials which are less negative in Baeyer–Villiger oxidations of cyclobutanones in t-
for pyrazinium compounds (by about 550 mV).[13]
BuOH.[8a] The catalyst has a significant effect at load-
The catalytic activity of the pyrazinium salt 4a was ings of 2 mol% (Table 3, entries 8 and 9). For the oxi-
compared with that of an already known[7b–d] alloxazi- dation of bicyclo
AHCTUNGTRENNUNG
nium salt 2a with the same pyrazinium core. Since al- gioselectivity with an excess of the “normal” lac-
loxazinium salts 2 are not used directly as catalysts of tone[15] was achieved.
H2O2 oxidations but are generated in situ from the
In order to prove the existence and structure of
corresponding dihydro forms 2-H2,[7b–d] the data for pyrazine hydroperoxide 4a-OOH as the active oxidiz-
2a-H2 were taken into account for comparison. ing agent, a series of pyrazine adducts 4a-Nu was pre-
Table 2 shows that for aromatic substrates (entries 1– pared with hydrogen peroxide, methanol and water
6), the catalytic efficiency of 4a is approximately one (Scheme 3). Due to their instability, the adducts were
half of that of 2a generated from its dihydro deriva- prepared in situ following a previously reported pro-
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Table 3. Preparative H2O2 oxidations catalyzed by 5 mol% of pyrazinium salt 4a.[a]
Entry
Substrate
Product
Time [h][b]
Conv. [%][c]
Yield [%][d]
90
Blank conv. [%][e]
1
1
100
4
2
24
96
88
37
3
4
8
6
99
96
84
10
6
100
5
6
7
1
100
100
100
95
94
92
4
1
4
0.3
24
8[f]
1
100
100
75
91
17
20
9[f]
0.4
[a]
Conditions for the preparative oxidations: m(substrate)=100 mg, 1.5 equiv. H2O2, 5 mol% catalyst, T=258C.
[b]
[c]
[d]
[e]
[f]
Reactions were performed until quantitative conversion was achieved (TLC monitoring).
1
Conversions are based on H NMR spectra.
Isolated yields.
Conversions of the non-catalyzed reactions after the same reaction time.
2 mol% of the catalyst.
cedure for adducts of flavinium salts,[7i,16] that is, by of the nucleophile to the solution of the pyrazinium
the addition of a nucleophile into the solution of the salt 4a in pyridine-d5, one signal of a hydrogen atom
pyrazinium salt 4a in pyridine (see the Supporting In- on the heterocyclic ring shifted from the aromatic
formation for details). The existence of 4a-Nu adducts region to 5.5 ppm, and the signals of the two remain-
was confirmed by MS and NMR. After the addition ing, originally heteroarenium, hydrogen atoms shifted
À
upfield by about 2 ppm. Furthermore, the N CH2
signal (originally a quartet) split into two multiplets,
which indicates the presence of a chiral centre close
to the CH2 group. A large upfield shift (from 143 to
57–80 ppm) of a carbon atom signal in 13C NMR was
also observed confirming the change of the carbon
hybridization from sp2 to sp3. The attack of the nucle-
ophile at position 6 of the pyrazinium ring (attack at
Scheme 3. Formation of pyrazine adducts with nucleophiles. position 2 could also be expected) was confirmed by
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Pyrazinium Salts as Efficient Organocatalysts of Mild Oxidations with Hydrogen Peroxide
heteroarenium salts as well as the improvement of
their activity are now under investigation.
Experimental Section
General Conditions
Mass spectra were recorded on a Thermo Scientific LXQ
linear trap mass spectrometer in tandem with a Janeiro LC
system consisting of two Rheos 2200 pumps and a CTC
PAL autosampler. High-resolution mass spectra were ob-
tained on a hybrid FT/MS mass spectrometer LTQ Orbitrap
XL by direct infusion of the sample solution into the mobile
phase (100% acetonitrile or methanol). The mass spectrom-
eter operated in ESI+ mode with a resolution of 100000
and mass range 100–1500. NMR spectra were recorded on a
Varian Mercury Plus 300 (299.97 MHz for 1H and
75.44 MHz for 13C), Bruker Avance DRX 500 (500.13 MHz
Scheme 4. Proposed mechanism of H2O2 oxidations cata-
lyzed by pyrazinium salts 4.
NOE experiments with the methoxy adduct 4a-OCH3
(see the Supporting Information).
The proposed mechanism of sulfoxidations cata-
lyzed by pyrazinium salts 4 is analogous to that in the
presence of flavinium catalysts: (i) formation of pyra-
zine hydroperoxide 4-OOH, (ii) oxidation of the sub-
strate accompanied by the formation of the pseudo-
1
for H and 125.77 MHz for 13C) and Bruker 600 Avance III
(600.13 MHz for 1H and 150.90 MHz for 13C). Chemical
shifts d are given in ppm, using residual solvent or tetrame-
thylsilane as an internal standard. Coupling constants are re-
ported in Hz. Experimental procedures and spectroscopic
data can be found in the Supporting Information.
base 4-OH, and (iii) regeneration of the catalyst by Synthesis of Pyrazinium and Pyridinium Salts
the elimination of a hydroxy function (see Scheme 4).
The pyrazine (or pyridine) compound (2.3 mmol) was dis-
solved in dry chloroform (8 mL). Triethyloxonium tetra-
fluoroborate (0.475 g, 2.5 mmol) was added and the mixture
Murahashi found[7a] that the rate-limiting step in N-
and S-oxidations catalyzed by isoalloxazinium salt 1a
is the elimination of hydroxy group (step iii). This
step is proposed to be much faster in oxidations cata-
lyzed by alloxazinium derivatives 2 (with a pyrazini-
um core) due to the formation of a fused aromatic
system.[7b] According to the values of pKR+ (see
Table 1) representing the third (equilibrium) step of
the catalytic cycle, the formation of a salt from its
pseudobase seems to be disfavoured for pyrazinium
salt 4a (and most likely for all other pyrazine deriva-
tives with an electron-withdrawing substituent) simi-
was stirred for 4–24 h under a nitrogen atmosphere. The
solid product was collected by filtration, washed with di-
chloromethane and dried under vacuum. The crude product
was dissolved in a minimum volume of acetonitrile and pre-
cipitated with diethyl ether. For details about the synthesis
and for the characterization, see Supporting Information.
Kinetic Experiments
The catalyst (5.1 mmol) and the substrate (0.255 mmol) were
dissolved in MeOH-d4 (600 mL). The solution was trans-
larly as for isoalloxazinium salts 1. Therefore, a slow ferred to an NMR tube and thermostatted for 15 min at
25.0Æ0.58C. Hydrogen peroxide (30% aqueous solution,
elimination step (iii) is expected in sulfoxidations cat-
40 mL, 0.392 mmol) was added, and the reaction was moni-
alyzed by 4a.
1
tored by H NMR. The resulting spectra of the products cor-
In conclusion, it has been shown that pyrazinium
responded to the spectra of the authentic samples of sulfox-
salts substituted with an electron-withdrawing group
ides.
catalyze the oxidation of sulfides to sulfoxides and
the Baeyer–Villiger oxidation of cyclobutanones with
hydrogen peroxide under mild conditions. The cata-
Preparative Sulfoxidations
lytic activity of the most efficient 3-cyanopyrazinium The substrate (100 mg) and 4a (5 mol%) were dissolved in
methanol (2 mL). The solution was thermostatted for 20 min
at 25.0Æ0.58C. The reaction was initiated by the addition of
hydrogen peroxide (30% aqueous solution, 1.5 equiv.). The
reaction mixture was stirred at 25.0Æ0.58C and the course
of the reaction was monitored by TLC using toluene as a
mobile phase. After complete conversion had been
achieved, sodium sulfite (100 mg) was added to quench the
excess of hydrogen peroxide. The solid was filtered off and
the solution was evaporated under reduced pressure. The
residue was partitioned between water (4 mL) and dichloro-
derivative 4a is almost comparable with that of the al-
loxazinium salts 2, which are ranked amongst the
most efficient organocatalysts of H2O2 sulfoxida-
ACHTUNGTRENNUNG
tions.[1a,4,7b] To the best of our knowledge, this is the
first example of the application of structurally simple
heteroarenium salts to the activation of hydrogen per-
oxide and for the catalysis of oxidations. Being easily
available and environmentally benign, pyrazinium
salts seem to be promising also from the point of view
of practical applications. Studies on the use of other methane (4 mL). The phases were separated and the aque-
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Petra Mꢀnovꢁ et al.
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ous phase was extracted with dichloromethane (6ꢃ3 mL).
The combined organic phases were washed with brine
(5 mL) and dried over magnesium sulfate. The solvent was
evaporated under reduced pressure and the product was an-
[5] Isoalloxazinium salts 1 represent derivatives of natural
flavins. Alloxazine derivatives 2 and 3 are isomeric to
1, nevertheless, they are called flavins in the literature
as well.
1
alyzed by H NMR.
[6] I. Yasushi, N. Takeshi, Chem. Rec. 2007, 7, 354–361.
[7] a) S.-I. Murahashi, T. Oda, Y. Masui, J. Am. Chem. Soc.
1989, 111, 5002–5003; b) A. B. E. Minidis, J.-E. Bꢄck-
vall, Chem. Eur. J. 2001, 7, 297–302; c) A. A. Lindꢀn,
L. Krꢅger, J.-E. Bꢄckvall, J. Org. Chem. 2003, 68,
5890–5896; d) A. A. Lindꢀn, N. Hermanns, S. Ott, L.
Krꢅger, J.-E. Bꢄckvall, Chem. Eur. J. 2005, 11, 112–
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Preparative Baeyer–Villiger Oxidations
The substrate (0.599 mmol) and 4a (2 mol%, 6.6 mg,
0.030 mmol) were dissolved in t-BuOH (1 mL). Hydrogen
peroxide (30% aqueous solution, 0.1 mL, 0.980 mmol) was
added and the mixture was stirred at room temperature
until complete conversion was achieved. The course of the
reaction was monitored by TLC. The solvent was evaporat-
ed under reduced pressure and the crude product was puri-
fied by column chromatography (silica gel, hexane/ethyl ace-
tate 4:1).
ˇ
H. Dvorꢁkovꢁ, F. Hampl, P. Mꢀnovꢁ, V. Mojr, B.
Plancq, S. Sayin, Collect. Czech. Chem. Commun. 2009,
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The authors wish to thank the Czech Science Foundation
(Grant No. 203/07/1246) and Ministry of Education, Youth
and Sports (Grant No. LC03070) for financial support.
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Adv. Synth. Catal. 2011, 353, 865 – 870