Highly efficient Sandmeyer reaction on immobilized CuI/CuII-based catalysts

Article abstract of DOI:10.1016/j.mencom.2018.05.010

Highly effective embodiment of Sandmeyer reaction has been revealed for Cu-based catalysts incorporating ionic liquid on Silochrom support. The most active catalyst (TOF = = 4000–8000 h^–1) contains comparable amounts of cuprous and cupric chloride anions. The reported method allows one to carry out the reaction for anilines in the one-pot mode.

Full text of DOI:10.1016/j.mencom.2018.05.010

Mendeleev
Communications
Mendeleev Commun., 2018, 28, 261–263
Highly efficient Sandmeyer reaction
on immobilized Cui/Cuii-based catalysts
Irina G. Tarkhanova,a Michail G. Gantman,b Alexander S. Sigeev,c
Konstantin I. Maslakov,a Vladimir M. Zelikmana and Irina P. Beletskaya*a
a Department of Chemistry, M. V. Lomonosov Moscow State University, 119991 Moscow,
Russian Federation. Fax: +7 495 932 8846; e-mail: beletska@org.chem.msu.ru
b Friedrich-Alexander-Universität Erlangen-Nürnberg Lehrstuhl für Chemische Reaktionstechnik,
91058 Erlangen, Germany
c A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russian Federation
DOI: 10.1016/j.mencom.2018.05.010
Nu
N2 BF4
Highly effective embodiment of Sandmeyer reaction has
been revealed for Cu-based catalysts incorporating ionic
liquid on Silochrom support. The most active catalyst (TOF =
= 4000–8000 h–1) contains comparable amounts of cuprous
and cupric chloride anions. The reported method allows one
to carry out the reaction for anilines in the one-pot mode.
Nu = Br–, Cl–, SCN–
F
F
TOF = 3000–8000 h–1
OMe
CuCl2–
MeO
O
Cu2Cl3–
Cu3Cl4–
Si
NEt3
Et3N
Si
O
O
O
SiO2
Sandmeyer reaction (S-reaction), i.e. conversion of anilines into
aryl halides or aryl pseudohalides, is a useful method for func-
tionalization of arenes. However, its initial embodiment required
a high amount of metallic copper or its salts.1
are known as a convenient medium for it.7 A combination of the
immobilization procedure16,17 and the methods for preparation
of CuCl-containing IL18 allowed us to synthesize several new
catalysts from alkylimidazolium (or alkylammonium) and CuCl
(or CuCl2) on Silochrom (see Online Supplementary Materials).
The copper (both Cui and Cuii) content in the samples was
determined by synthesis conditions. According to XPS data,
catalysts I (prepared from CuCl2 in air atmosphere) and III
(obtained from CuCl under argon) contained only CuCl2 or CuCl,
respectively (Table 1). Catalysts II and IV were prepared by
sustained reflux of modified supports and CuCl in isooctane in
air atmosphere and contained various Cui portions (see Table 1).
The reaction of 4-fluorophenyldiazonium tetrafluoroborate
with LiCl was used as a model for an initial evaluation of the
catalytic properties of the samples (Scheme 2, Table 1).
According to the mechanism suggested by Kochi2 and
Zollinger,3,4 copper salts act both as an electron-transfer agent
(SET mechanism) and as a ligand-transfer oxidant (the second
step in homolytic mechanism and the main step in heterolytic
mechanism) (Scheme 1).
ArX
CuX2 or
[CuXn]–(n – 2)
CuX or [CuXn]–(n – 1)
ArN2BF4 + MX
[ArN2]
Ar
solvent
CuX2
N2
ArH
Scheme 1
N
Cl
cat.
N
+
+
+
LiBF4
LiCl
N2
BF4
The recent reports disclose introduction of fluoro-containing
groups CF3, SCF3, and SeCF3 into the aromatic ring using
S-reaction,6–10 which allows one to affirm that the catalytic version
made this reaction popular in modern organic synthesis.5 The
initial procedure required at least 5–10 mol% of copper, some-
times one equivalent, e.g. in the ‘one-pot’ synthesis of fluorinated
compounds catalyzed by powdered copper.6 Recently we reported
a catalytic complex consisting of CuCl and CuCl2 with nitrogen-
containing ligands (TMEDA, phenanthroline, and bipyridine),
which provided a very fast conversion in S-reaction under mild
conditions.11,12 However, the described procedure required at
least 5 mol% of the catalyst loading because its lower amount
would result in reduced product yields.
F
F
Scheme 2
Catalysts I and III had almost no activity. Catalyst II with
Cui/Cuii ratio of 10:1 demonstrated a moderate activity and good
selectivity. The full conversion of 4-fluorophenyldiazonium tetra-
fluoroborate was achieved in 1 h using catalyst iV with Cui/Cuii
Table 1 Composition of the immobilized ILs and their catalytic activity.a
Grafted
[Cui]/ Conversion of
Copper
chloride
Entry Catalyst
quaternary [Cu]S 4-FC6H4N+ºNBF4–
salt
(%)b
(%)
Since some obtained by S-reaction compounds are interesting
intermediates for pharmaceutical industry,13,14 there is a need for
an efficient heterogeneous catalyst that would contain a relatively
low amount of copper and, at the same time, be recyclable.
There are no reported examples of immobilized on a solid
support ionic liquids (ILs) applied in S-reaction,15 although ILs
1
2
3
4
I
CuCl2
CuCl
EtPrImCl
EtPrImCl
0
90
99
36
0.6
II
37
4
III
IV
CuCl (Ar) EtPrImCl
CuCl Et3PrNCl
100
a Conditions: Ar, [Cu] = 5 mol%, 20°C, 1 h. bAccording to XPS data.
© 2018 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
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Mendeleev Commun., 2018, 28, 261–263
Table 2 Catalysts II and IV recycling in the model reaction.a
was not effective for heterogeneous reaction under above condi-
tions, because it did not provide sufficient Br– concentration in
the solution, and resulted in a side reaction of the fluorophenyl
radical with the solvent, which led to the fluorobenzene formation
in significant amounts (Table 3, entries 1–3).
The best results were achieved for acetonitrile-soluble salts,
such as Et4NBr and LiBr, which provided 4-FC6H4Br in 94–96%
yields (Table 3, entries 4 and 5). The same condition allowed us
to obtain of 4-FC6H4Cl and 4-FC6H4SCN in high yields, using LiCl
or Et3BnNCl and KSCN, respectively (Scheme 4).
Number
of cycles
Time of full conversion
Yield of
Catalyst
II
of 4-FC6H4N+ºNBF4–/h 4-FC6H4Cl(%)
1
2
3
3
3
3
98
97
98
IV
1
2
3
1
1
1
98
96
90
a See Scheme 2, the same conditions as given in Table 1.
N
Nu
1 mol% [Cu], [MNu], MeCN
room temperature
ratio of 1:2. In S-reaction, both the Cui oxidation (electron
transfer from the Cui anionic complex to ArN2+, see Scheme 1,
the first step) and the reaction between anionic Cuii complex
and the radical Ar• that maintains the process selectivity (see
Scheme 1) play the crucial role. Therefore, the process requires
the simultaneous participation of two valence forms of copper.
The next step of our study was comparison between the two
most active catalysts in recycling experiments (Table 2).
The complete formation of 4-chlorofluorobenzene within 3 h
and in a similar yield for 3 runs was observed for catalyst II,
while catalyst IV provided the full conversion in 1 h but also with
a small decrease in the product yield (98, 96, and 90%). Since the
copper content remained unchanged (see Online Supplementary
Materials), the catalyst IV degradation occurred probably due to
its lower resistance to oxidation in comparison with catalyst II,
which resulted in a stepwise decrease of the copper(i) fraction.
The most active catalyst IV (1 mol% [Cu]) was selected for an
optimization of the preparative reaction with bromides (Scheme 3).
N
BF4
F
F
Nu = Br, Cl, SCN
Scheme 4
In all cases, catalyst IV provided the high yields within 1 h.
It was surprising that the decreased catalyst loading had no effect
on the yields in the reactions with LiBr, LiCl, Et3BnNCl, and
KSCN, which remained high even at the copper concentrations
being as low as 5×10–3 mol% (Table 4). Such concentrations in the
copper catalysis look like a ‘homeopathic’ amount.20 According
to the acquired data, the turnover number (TON) could achieve
up to 20000 for all substrates (see Table 4, entry 3) and TOF
value reached 3000–8000 h–1 (entries 1–3).
The reactions with LiBr, Et3BnNCl, and KSCN were completed
in less than 1 h at the catalyst concentration of 0.050 mol%. The
reason, why heterogeneous catalysts demonstrated a higher activity
in the same reaction in comparison with the homogeneous one,
remains unclear. A catalyst activity examination revealed that the
reaction of aryldiazonium salts with lithium bromide could also
proceed in the absence of the catalyst, although at a much lower
rate. In the presence of 0.005 mol% of copper, the reaction was
completed within 6 h, while the full conversion without a catalyst
took about 40 h (see Table 4, entry 4). Anyhow, the yield of aryl
bromide remained quite high (89%). The non-catalytic reaction
with thiocyanate proceeded for 30 h and resulted in 20% conver-
sion. For chloride ions, the product was not detected even after 90 h.
Since LiCl is substantially less soluble in acetonitrile than LiBr,
the reaction was carried out at noticeably higher dilution and
required a longer time to be completed (Table 4, entry 3). Used as
sources of the chloride ion Et3BnNCl and Et4NCl substantially
increased the reaction rate.
N
Br
H
1 mol% [Cu], [Br–]
N
+
MeCN,
room temperature
BF4
F
F
F
Scheme 3
Phase-transfer catalysts, such as crown ethers or tetraalkyl-
ammonium salts,19 can be very helpful for this reaction in case of
homogeneous catalysis. However, the phase-transfer catalysis
Table 3 Reaction of 4-fluorophenyldiazonium tetrafluoroborate with bromides a
.
Yieldb of Yieldb of
Entry [Br–]
4-FC6H4Br (%) C6H5F (%)
40
Taking 0.025 mol% of copper allowed the reactions to proceed
quite rapidly with high product yields, which revealed the sub-
stituent effect in initial diazonium salts in the reaction with KBr
and KSCN (Scheme 5). In all cases, high product yields were
obtained for both electron-donating and electron-withdrawing
substituents.
1
2
3
4
5
KBr+10% dibenzo-18-crown-6 55
KBr+30% dibenzo-18-crown-6 70
27
47
5
KBr+10% Et3BnNClc
50
94
96
Et4NBr
LiBr
2
Since diazonium salts are not optimal reactants for preparative
synthesis, the in situ diazotization of anilines was performed using
alkyl nitrites with the following halodediazotization. The one-pot
reaction with LiBr or Et4NCl resulted in a series of aryl halides
aReagents and conditions: 4-FC6H4N2[BF4] (1 mmol), nucleophile (source
of [Br–], 2 mmol), catalyst IV (1 mol% [Cu]), MeCN (dry), Ar, room
temperature, stirring, 30 min. bAccording to 19FNMR. c 4-FC6H4Cl was not
detected.
Table 4 Dependence of the substitution product yield on the catalyst IV amount.a
LiBr
LiCl
Et3BnNCl
KSCN
Yieldb of
[Cu]
(mol%)
Entry
Yieldb of
4-FC6H4Br (%)
Yieldb of
4-FC6H4Cl (%)
Yieldb of
4-FC6H4Cl (%)
t/h
0.25
t/h
t/h
0.5
t/h
0.5
4-FC6H4SCN (%)
1
2
3
4
0.050
0.025
0.005
–
95
94
93
89
1
3
95
95
96
–
96
94
96
–
95
94
94
20
1
6
2
8
1
5
10
90
40
90
30
a See Scheme 4. b According to 19F NMR.
– 262 –

Mendeleev Commun., 2018, 28, 261–263
[Cu], [MNu], MeCN
room temperature
provides the opportunity to carry out reactions for anilines in the
one-pot mode.
–
ArN NBF4
ArNu
Nu = Br, SCN
This work was supported by the Russian Foundation for
Basic Research (grant no. 15-03-01995) and the Russian Science
Foundation (grant no. 14-23-001866P). The XPS study was
supported by the M. V. Lomonosov Moscow State University
Development Program.
Isolated yield (%)
Ar
ArBr
ArSCN
94
95
87
97
95
88
89
82
96
92
3-Me-4-BrC6H3
3,5-Cl2C6H3
4-MeOC6H4
3-Me-4-NO2C6H3
4-IC6H4
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2018.05.010.
Scheme 5 Reagents and conditions: [MNu] (6 mmol), catalyst IV (0.025 mol
%
of copper), MeCN (20 ml), Ar, stirring for 5 min (in case of ArSCN, the
mixture was cooled to 0°C); then diazonium salt (3 mmol) in MeCN (10 ml)
and stirring for 1 h.
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Scheme 6 Reagents and conditions: BF3·Et2O (3 mmol), aniline (3 mmol),
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a
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Received: 6th December 2017; Com. 17/5426
– 263 –