Oxidative dimerization of anilines with heterogeneous sulfonic acid catalysts

Article abstract of DOI:10.1039/c7gc03060f

We report herein that suitably supported perfluorosulfonic acids can catalyze the oxidative dimerization of anilines using hydrogen peroxide as a clean oxidant. The reaction does not require the use of organic solvents and affords desired azobenzenes and water as products, minimizing the formation of wastes. The metal-free solid catalyst shows remarkable activity and selectivity for the reaction, which occurs under very mild conditions and with broad functional group tolerance.

Full text of DOI:10.1039/c7gc03060f

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DOI: 10.1039/C7GC03060F
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Oxidative dimerization of anilines with heterogeneous sulfonic
acid catalysts
Emanuele Paris,a Franca Bigi,a,b Daniele Cauzzi,a Raimondo Maggia and Giovanni Maestri*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report herein that suitably supported perfluorosulfonic acids organics as oxidant or solvent (Scheme 1).5 Through this
can catalyze the oxidative dimerization of anilines using hydrogen approach is therefore possible to use a sustainable oxidant and
peroxide as clean oxidant. The reaction does not require the use significantly reduce the amount of wastes. We are unaware of
of organic solvents and affords desired azobenzenes and water as previous uses of sulfonic acids for nitrogen oxidation.
products, minimizing the formation of wastes. The metal-free
Previous works
solid catalyst shows remarkable activity and selectivity for the
reaction, which occurs under very mild conditions and with broad
NH2
catayst (promoter)
functional group tolerance.
N
N
oxidant
organic additives
The development of organic frameworks able to mimic the
versatility and robustness of transition-metal catalysts has
been witnessed by a series of milestones in recent years.1 In
this area, while multi-electron redox processes still remain
challenging, several elegant mono-electronic reactions can be
efficiently catalyzed by organics,2 especially amines.3 On the
contrary, involvement of acids as catalysts to trigger redox
processes are scarce in organic synthesis.4
NaI
Mn2O3
CuBr
O
tBuOCl
air
N N
THF
tBu
tBu
toluene
(ref. 5a)
MeCN
(ref. 5b)
(ref. 5c)
We describe herein that sulfonic acids catalyze aniline
oxidation in the presence of hydrogen peroxide. This reactivity
can be exploited for the synthesis of trans-azobenzenes,
showing large functional group tolerance. The reaction uses 1
equiv. of oxidant and did not require any other organics,
minimizing wastes. The combination of an organic
perfluorinated polymer whose branches feature sulfonic acid
groups with an inorganic support, namely silica obtained via
sol-gel method, provided a material that is both a very efficient
and robust catalyst for this reaction. This method represents
therefore a valuable tool complementary to existing metal-
based ones, which are limited to few turnovers or require
This work
R
R'-SO3H
down to 0.26 mol%
NH2
N
N
1 equiv. H2O2
solvent-free,
40 °C
R
R
2, 45-89%,
15 examples
(R = Alk, OH, halides ... )
1, 10 mmol
F2
C
F2 F2
F
C
C
C
n
m
Aquivion on SiO2
R'-SO3H =
O
CF2
F2C
SO3H
Scheme 1. Alternative routes to diarylazobenzenes.
As part of our interest in the development of green catalytic
methods, we serendipitously observed that substituted
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J. Name., 2013, 00, 1-3 | 1
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anilines smoothly react with H2O2 to undergo oxidative 2a (6 and 12%, entries 5-6). On the contrary, a fluorinated
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dimerization with a sulfonic acid catalyst. Thus, in an initial organic tether proved beneficial (entry 6, 37% of
experiment, we observed formation of trans- and Aquivion allowed to retrieve 2a in higher yields compared
DOI: 10.1039/2Ca7G).CN03a0f6io0nF
diphenylazobenzene (2a, 15%) upon treatment of aniline (1a, 1 with their homogeneous peers (34 and 39%, entries 7-8).
R
mmol) with 1 equiv. of hydrogen peroxide in the presence of 1
NH2
Aquivion @SiO2
(0.26 mol% -SO3H)
mol% of trifluoromethanesulfonic acid at 60 °C for 24 hours.
Under these conditions, conversion of 1a did not reach
completion (90%) and azoxy product 3a was the main product
of the reaction (77%). Reasoning of the synthetic interest
towards products 25,6 we varied reaction parameters to
achieve a selective reaction. We noticed that presence of
oxygen favors formation of 3a and we thus performed
reactions in a Schlenk-type flask under N2 for further
optimization. All liquids were degassed by bubbling nitrogen
for 10 minutes prior to use. Upon preliminary screening,
reactions were performed on 10 mmol scale by gently
warming at 40 °C 1a and H2O2 in the presence of a sulfonic
acid. The effect of the catalyst on the formation of 2a is
presented in Table 1.
N
N
H2O2 (1 equiv.)
N2, 40 °C, 24 h
R
2b-o, 46-80%
(conv. of1 )
R
1b-o, 10 mmol
N
N
N
N
2b, 80% (86%)
2c, 74% (78%)
N
N
N
N
2d, 76% (81%)
2e, 51% (55%)
Table 1. Catalyst screening
NH2
O
N
2g, 60% (68%)
sulfonic acid
N
N
N
N
N
H2O2 (1 equiv.)
N2, 40 °C, 24 h
N
N
1a, 10 mmol
2a
3a
2f, 78% (82%)
O
OH
Conv. 1a
Yield 2a
Yield 3a
Entrya
Acid
(%)b
(%)c
(%)c
N
N
N
N
1
2
3
4
5
6
7
8
9
10
11d
12e
13f
14h
15f,h
H2SO4
CH3SO3H
CF3SO3H
91
88
90
85
66
78
89
93
97
98
98
98
96
33
93
11
8
20
6
23
74
66
77
50
35
50
47
78
18
61
15
4
O
HO
2h, 70% (75%)
2i, 77% (81%)
-(CH2)3-SO3H @SiO2
-(C6H4)-SO3H @SiO2
-(CF2)3-SO3H @SiO2
Nafion
I
I
12
37
34
39
15
72
28
81
89g
4
N
N
N
N
I
I
Aquivion
2j, 51% (57%)
2k, 46% (50%)
Aquivion @Al2O3
Aquivion @SiO2
Aquivion @SiO2
Aquivion @SiO2
Aquivion @SiO2
CH3SO3H
Cl
Br
N
N
N
N
Br
Cl
28
3
2l, 50% (56%)
2m, 49% (54%)
Aquivion @SiO2
86
CF3
a
Reaction conditions: aniline (10 mmol, 1 equiv.), 35% aqueous H2O2 (860 µL, 1
equiv.), acid catalyst (1 mol% for homogeneous acids, 250 mg for solid ones),
under N2 at 40 °C; b by GC using n-decane as external standard; c isolated yields; d
F3C
N
N
CF3
F3C
N
e
f
N
CF3
with 2 equiv. H2O2; with 100 mg cat.; with 50 mg cat. (0.26 mol% of -SO3H
groups); this entry gave a 91% GC-based yield of 2a; performed with freeze-
g
h
2n, 69% (76%)
thaw techniques instead of N2 bubbling to remove traces of O2.
2o, 63% (68%)
CF3
Aniline oxidation occurs smoothly with simple homogeneous
acids (entries 1-3). Sulfuric acid leads however to extensive
decomposition (entry 1). Methanesulfonic and triflic acid
ensured high conversion of 1a, although 3a was the main
product in these cases (74 and 66%, entries 2 and 3
respectively). We then reasoned to smooth the reactivity of
the acid catalyst by testing heterogeneous sulfonic acid
derivatives. Solid acid presenting either propylsulfonic or
phenylsulfonic groups did not increase the selectivity towards
Figure 1. Reaction scope, conditions as Table 1, entry 13; isolated yields.
The structure of these two perfluorinated polymeric resins is
characterized by a linear backbone which contains short
branches terminating with sulfonic acid groups. This trend
(entries 6-8) could confirm a positive effect exerted by a
fluorinated organic backbone on the reaction (for a possible
rationalization vide infra). Combination of fluorinated
2 | J. Name., 2012, 00, 1-3
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commercial resins with a suitable inorganic support proved safe and minimizes the use of chemicals to recover products.
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decisive. While the use of alumina did not steer the outcome Chromatography on silica gel affords spDeOctI:ro10s.c10o3p9i/cCa7llGyCp0u30re602F
(15%, entry 9), product 2a become the major species using and
3. It is worth noting that the latter could in principle led to
silica (72%, entry 10). The yield sunk to 28% performing the
2
as well thanks to existing reduction procedures.7
reaction with 2 equiv. of oxidant (entry 11). Further increase of The recovered solid acid has become brown at the end of the
the yield was achieved reducing the amount of catalyst reaction and, disappointingly, it is no longer active. Washings
(entries 12-13, 81 and 89% respectively). Remarkably, the best with solvents of different polarity had no effect. Titration
result (entry 13) is obtained with as low as 0.26 mol% of showed however that used material did not present any acid
sulfonic acids groups. We then repeated the experiment of site. We reasoned that this could be due to chemisorption of
entry 2 and 13 to check whether traces of oxygen could be byproducts presenting basic nitrogen atoms. We thus treated
responsible for aniline oxidation, either to deliver desired the used material with an excess of aqueous acid solution. This
product 2a or overoxidized derivative 3a. It is worth noting replenished its original acidity (0.26 mmol –SO3H/g), removed
that oxygen is still present in liquids using typical bubbling traces of absorbed colored organics and, to our delight,
procedures, although this usually did not hamper the outcome delivered again 2a in 88% yield repeating our model reaction.
of experiments in most cases. A stricter control could be We rule out any significant leaching from the solid catalyst by
achieved through freeze-thaw technique. On the other hand, performing the Sheldon test and not observing any further
bare thermal decomposition of H2O2 triggers an increasing conversion of the filtrate.8 Try as we might, we were unable to
presence of O2 in these mixtures through time. Interestingly, detect any intermediate of aniline oxidation from these
the result using methanesulfonic acid (entry 14) showed a mixtures and we thus performed additional experiments to
significant difference. Conversion remained stuck at 33% and gain insights on the mechanism at work (Scheme 2).
the amount of 3a reduced sharply (28%). This procedure did
Aquivion-Na @SiO2
(0.26 mol% -SO3Na)
O
N
not change significantly the outcome using optimized
conditions instead (entry 15, 86% of 2a). Taken together these
results strongly suggest that the mechanism of formation of 2a
NH2
N
N
N
+
1 equiv. H2O2
N
2, 40°C, 24 h
2a, not detected
3a, 31%
is different from that of 3a
.
Without catalyst conversion of 1a remained below 10%,
delivering exclusively 3a. No reaction took place without H2O2.
Product 2a was not detected in the presence of bare silica.
Similarly, 2a did not form replacing H2O2 with stronger organic
peroxides (tert-butyl- and ditert-butylperoxide), 3a being the
major product. With best conditions in our hands (Table 1,
entry 13), we then studied the scope of this reaction (Figure 1).
The phenyl ring of aniline can be decorated with various alkyl
substituents in the ortho-, meta- and para- positions delivering
Aquivion-H @SiO2
(0.26 mol% -SO3H)
2% (X = 1)
4% (X = 0.5)
19% (X = 0.05)
27% (X = 0.003)
45% (X = 0.0003)
NH2
N
N
1 equiv. H2O2,
TEMPO (X equiv.)
2a
N
2, 40°C, 24 h
CF3
O
N
+
Aquivion-H @SiO2
(0.26 mol% -SO3H)
F3C
NH2
N
N
CF3
H2O,
CF3
the corresponding products 2b-f in good yields (74-80%).
N2, 40°C, 6 h
Aniline 1e is the least reactive of the series, likely owing to the
presence of a bulky isopropyl group ortho to nitrogen, but
yield nonetheless 2e in 51% yield. The methylene group of
fluorenes remains untouched, as witnessed by the isolation of
2g in 60% yield. Unhindered hydroxy (vide infra) and methoxy
groups are similarly tolerated, delivering the corresponding
4a, 0.5 equiv. 1o, 0.5 equiv.
2p, 77% (no reaction without cat.)
Scheme 2. Mechanistic probes.
We ascertained that sulfonic acid groups act as catalysts at the
molecular level in analogy to their homogeneous peers (Table
1) by treating our material with an excess of aqueous NaOH.
The resulting solid did not exhibit anymore protic acid sites, as
determined by titration. It proved unable to induce formation
of 2a too. Conversion of 1a was 35% after 24 hours at 40 °C
and 3a became the major product (31%). Taken together,
these results strongly suggest that the oxidative dimerization
products 2h-i in 70 and 77% yield respectively. Switching to
electron withdrawing groups, halogenated anilines could be
coupled through this method, affording products in moderate
yields (2j-m, 49-51%). Products 2h-m offer convenient handles
for
further
functionalization
and
desimetrization.
Trifluoromethylated anilines deliver the corresponding
azobenzenes 2n and 2o in 69% and 63% respectively.
of anilines to
2 is triggered by sulfonic acid groups.
Conversion of 1a is 2% and 4% in the presence of 1 and 0.5
equiv. of TEMPO respectively. The use of 0.05 equiv. enabled
the formation of 2a in 19% yield. Further reduction to 0.003
equiv. allowed to retrieve 27% of 2a. The use of 0.0003 equiv.
(thus 10 times lower than sulfonic acid groups) still had a
negative effect on conversion and yield (49% and 45%
respectively). A comparable trend was observed replacing
TEMPO with 2,6-ditert-butylphenol (details in ESI). This is in
The solid acid is recovered by filtration at the end of the
reaction and the biphasic mixture is then separated in a
funnel. The organic phase is washed with a 1 M HCl solution (3
x 10 mL). This enables to recover technically pure products
2
(>90% by NMR) and the salt of unreacted anilines from
aqueous phases, which do not contain peroxides anymore,
likely owing to decomposition of unreacted H2O2 (procedure in
ESI). Taken together, these results show the practical and
environmental viability of present method, which is inherently
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striking contrast with the smooth coupling occurred using In this scenario, the sulfonic acid would play two distinct roles,
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unhindered (amino)phenols as substrates (2h, 70%, Figure 1).
triggering aniline oxidation to nitrosobeDnOzeI:n1e0.1i0n39th/Ce7GfiCrs0t30p6a0rFt
These results strongly suggest the involvement of and then acting as dehydration catalyst in the sequential
paramagnetic intermediates in these reactions. However, we redox-neutral cycle. Present sequence would thus represent a
did not observe any products of radical recombination in all cascade reaction.13 To the best of our knowledge, this
these cases. Similarly, no trapping occurred performing the behaviour has not been previously observed among organic
reaction with 1 equiv. of benzylidenemalononitrile as radical acids.
acceptor.
We could not rule out at present stage formation of 4a via
Nitrosobenzene 4a did not react with anilines upon 24 hours at chain reaction. In this case, the sulfonic acid would be the
40 °C. On the contrary, even employing the weakly initiator of the first part of the sequence. However, the
nucleophilic aniline 1o, the addition of the heterogeneized significant suppression of the yield of 2a in the presence of tiny
sulfonic acid enabled to retrieve product 2p in 77% (Scheme 2, molar amounts of radical quenchers (vide supra) seems at
bottom). Using aniline, 2a forms in 94% yield. These results are odds with a chain propagation process.14 This combines with
consistent with the intermediate formation of nitrosobenzenes the failure to trap any radical intermediate, which would be
in these sequences. The solid acid catalyzes the formation of surprising on the basis of concentration effects in these
2p likely via nucleophilic addition of 1p on 4a and subsequent solvent-free experiments. The low loading (0.26 mol%) and the
formal dehydration.9 Under these conditions, product 2p did stability of the polymeric sulfonic acid further suggest that the
not form replacing 4a with either nitrobenzene or latter plays the role of a catalyst on the basis of present
hydroxyaniline. These results reduce the likeliness that these preliminary results.9
species are intermediates in present catalytic oxidative
dimerization of anilines to azobenzenes 2.
Conclusions
Nitrosobenzenes are often prepared in-situ from the
corresponding hydroxylamines.10 Present findings could
represent therefore a complementary tool to access a valuable
synthetic motif from simpler reagents, namely anilines.
Nitrosobenzenes are labile species in oxidizing environments
and over oxidation to nitrobenzenes is indeed a recurrent side-
reaction that impose the use of molar excess of precursors.10
This could correlate with the reactivity trend observed varying
the nature of the sulfonic acid derivative (Table 1).
Heterogeneization and a low molar loading might be indeed
the key to slowly generate nitrosobenzene preventing
undesired oxidations. On the basis of these results, we
propose the rationale presented in Scheme 3 for the formation
of trans-azobenzene 2a in these sequences.
We reported that sulfonic acids can catalyze oxidation of
anilines under mild conditions. Suitable solid acids steer this
reactivity towards the selective formation of trans-
diarylazanes. The method is complementary to existing
strategies that require the consumption of transition metals,
organic oxidants and co-solvents. The use of stoichiometric
hydrogen peroxide combined with broad functional group
tolerance and simple work-up/purifications portrays the
potential of these findings for future developments.
Conflicts of interest
There are no conflicts to declare.
The sulfonic acid
mechanism11 or via electrophilic oxygen transfer.12 These steps
liberate nitrosobenzene 4a and regenerate sulfonic acid
I could oxidize aniline either through a radical
I
,
Acknowledgements
We thank MIUR and UniPR for the financial support. GM
warmly acknowledges support from SIR project AROMA-TriP
(Grant No.: RBSI14NKFL).
which could thus act catalytically. Nucleophilic attack of the
second molecule of 1a on 4a and sequential dehydration by
sulfonic acid
I
eventually delivers desired product 2a in its
trans- form.9
R'-SO3H I
(Aquivion on SiO2)
+ 2H2O2
O
N
Notes and references
NH2
oxidation
cycle
- 2H2O
1
(a) D. W. C. MacMillan, Nature, 2008, 455, 304–308; (b) C. M.
R. Volla, I. Atodiresei and M. Rueping, Chem. Rev., 2014, 114
,
,
2390–2431; (c) C.-L. Sun and Z.-J. Shi, Chem. Rev., 2014, 114
9219–9280.
Reviews: (a) K. L. Skubi, T. R. Blum and T. P. Yoon, Chem.
1a
4a
4a
2
O
N
Rev., 2016, 116, 10035–10074; (b) D. Ravelli, S. Protti and M.
Fagnoni, Chem. Rev., 2016, 116, 9850–9913; (c) C. K. Prier, D.
A. Rankic and D. W. C. MacMillan, Chem. Rev., 2013, 113
N
I, 1a
Ph
dehydration
cycle
N
,
- H2O
5322–5363; (d) J. M. R. Narayanam and C. R. J. Stephenson,
Chem. Soc. Rev., 2011, 40, 102–113; for selected examples
involving enzimes: (e) H. B. Gray and J. R. Wrinkler, Isr. J.
Chem., 2016, 56, 640–648; (f) L. Betancor, G. R. Johnson and
2a
Scheme 3. Possible reaction rationale.
H. R. Luckarift, ChemCatChem, 2013, 5, 46–60; (g) R. A.
4 | J. Name., 2012, 00, 1-3
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Sheldon and I. W. C. E. Arends, Adv. Synth. Catal., 2004, 346
COMMUNICATION
,
9
(a) K. Krageloh, G. H. Anderson and P. J. Stang, J. Am. Chem.
Soc., 1984, 106, 6015–6021. (b) M. H. Davey, V.VYie.wLeAreti,clRe .ODnli.ne
1051–1071; (h) K. P. Bhabak and G. Mugesh, Acc. Chem. Res.,
2010, 43, 1408–1419.
For selected examples, see (a) G. Lelais and D. W. C.
MacMillan, Aldrichimica ACTA, 2006, 39, 79−87; (b) M. P. Sibi
and M. Hasegawa, J. Am. Chem. Soc., 2007, 129, 4124−4125.
For uses in fuel cells: A. Kusoglu and A. Z. Weber, Chem. Rev.,
2017, 117, 987–1104.
Recent examples: (a) B. Dutta, S. Biswas, V. Sharma, N. O.
Savage, P. S. Alpay and S. L. Suib, Angew. Chem. Int. Ed.,
2016, 55, 2171−2175; (b) Y. Takeda, S. Okumura and S.
Minakata, Angew. Chem. Int. Ed., 2012, 51, 7804−7808; (c) Y.
Zhu and Y. Shi, Org. Lett., 2013, 15, 1942−1945; (d) A.
Miller and T. J. Marks, J. Org. Chem., 1999,
DOI: 1604.1,04399/7C67–G4C907390.60F
3
10 (a) G. A. Molander and L. N. Cavalcanti, J. Org. Chem., 2012,
77, 4402–4413; (b) D. Beaudoin and J. D. Wuest, Chem. Rev.,
2016, 116, 258−286; (c) V. Santacroce, R. Duboc, M.
Malacria, G. Maestri and G. Masson, Eur. J. Org. Chem., 2017,
15, 2095–2098.
4
5
11 (a) W. Frank and B. Bertsch-Frank, Angew. Chem. Int. Ed.,
1992, 31, 436−437; (b) L. Ghassemzadeh and S. Holdcroft, J.
Am. Chem. Soc., 2013, 135, 8181–8184; (c) S. Z. Zard, Chem.
Soc. Rev., 2008, 37, 1603–1618; (d) O. M. Musa, J. H. Horner,
H. Shahin and M. Newcomb, J. Am. Chem. Soc., 1996, 118
,
Grirrane, A. Corma and H. Garcia, Science, 2008, 322
,
3862–3868.
1661−1664.
12 (a) B. Prieswisch and K. Rück-Braun, J. Org. Chem., 2005, 70,
6
Reviews: (a) E. Merino, Chem. Soc. Rev., 2011, 40
,
2350–2352; (b) W. R. Thiel, Coord. Chem. Rev., 2003, 245,
3835−3853; (b) A. A. Beharry and G. A. Woolley, Chem. Soc.
Rev., 2011, 40, 4422−4437; (c) Molecular Switches, 2nd ed.;
B. L. Feringa and W. R. Browne, Eds.; Wiley-VCH Verlag
GmbH: Weinheim, Germany, 2011.
G. A. Olah, B. G. Balaram Gupta and S. C. Narang, J. Org.
Chem., 1978, 43, 4503−4505.
95–106.
13 L. F. Tietze and U. Beifuss, Angew. Chem. Int. Ed., 1993, 32
,
131–163.
14 A. Studer and D. P. Curran, Angew. Chem. Int. Ed., 2016, 55
,
7
8
58–102.
H. E. B. Lempers and R. A. Sheldon, J. Catal., 1998, 175, 62–
69.
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DOI: 10.1039/C7GC03060F
R
Sulfonic acid
(0.26 mol%)
NH2
N
R
N
sequential
catalysis
H2O2
(1 equiv.)
R
no metal
no solvent
low wastes
Sulfonic acids selectively catalyze the oxidative dimerization of anilines to azobenzenes without
any unnecessary organics