Copper-free Sandmeyer cyanation of arenediazonium o-benzenedisulfonimides

Article abstract of DOI:10.1039/c5ob02321a

Arene and heteroarenediazonium o-benzenedisulfonimides can be used as efficient reagents in Sandmeyer cyanation. This work reports such reactions carried out by us under very mild conditions using tetrabutyl ammonium cyanide as a safe cyanide source and, interestingly, without the need for a Cu catalyst. The reactions have given rise to aryl nitriles in good yields (25 examples, average yield 75%). A good amount of o-benzenedisulfonimide was recovered from each reaction and then reused to prepare other salts. Mechanistic insights have allowed us to highlight the fundamental role of the o-benzenedisulfonimide anion as an electron transfer agent.

Full text of DOI:10.1039/c5ob02321a

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Copper-free Sandmeyer Cyanation of Arenediazonium o-
Benzenedisulfonimides
Margherita Barberoa, Silvano Cadamuroa and Stefano Dugheraa
Received 00th January 20xx,
Accepted 00th January 20xx
Arene and heteroarenediazonium o-benzenedisulfonimides can be used as efficient reagents in Sandmeyer cyanation. This
work report such reactions being carried out in very mild conditions using tetrabutyl ammonium cyanide as a safe cyanide
source and, interestingly, without the need for a Cu catalyst. The reaction have given rise to aryl nitriles in good yields (25
examples, average yield 75%). A good amount of o-benzenedisulfonimide was recovered from each reactions and then
reused to prepare other salts. Mechanistic insights have allowed us to highlight the fundamental role of the o-
benzenedisulfonimide anion as an electron transfer agent.
DOI: 10.1039/x0xx00000x
www.rsc.org/
On the other hand, few studies6 have been carried out (many by the
Beletskaya group6b,d
) on the Sandmeyer cyanation than the
Introduction
Rosenmund protocol. However, it must be stressed that there has
been a renewed interest in the Sandmeyer reaction very recently. In
fact, Li and coworkers have investigated other cyanide sources,
namely trimethylsilyl cyanide7a and acetonitrile7b (in the presence of
PdCl2 as a metal catalyst) as an alternative to CuCN in order to find
milder and safer reaction conditions.
Aryl nitriles are an important class of organic compounds that is
widely used in organic synthesis and a common structural motifs in
natural products.1 They are in fact versatile building blocks in the
synthesis of not only more complex natural products, but also
pharmaceuticals, agricultural chemicals, dyes and other
commercially important materials.2a,b Moreover, aryl nitriles are
easily transformed into a variety of functional groups, such as
amines, aldehydes, amides, ketones, carboxylic acids and many
others.2
Since 1998, our own researches has resulted in a new and large
family
of
dry
diazonium
salts:
arenediazonium
o-
benzenedisulfonimides 1 (Figure 1).8a
Although several protocols for aryl nitriles preparation have been
proposed,3 they have been conventionally synthesized via either
Sandmeyer4 or Rosenmund-von Braun5a,b reactions using
stoichiometric amounts of CuCN and diazonium salts4 or aryl
,b
halides.5a
Over the years, the cyanation of aryl halides has also been carried
out using various reagents, such as KCN, Zn(CN)2, NaCN and
cyanohydrins, always in the presence of Cu catalysts.1a Although
Figure 1 Arenediazonium o-benzenedisulfonimides 1
these protocols have found
a number of applications, they
Their properties lend them great potential in numerous synthetic
applications. These salts are, in fact, easy to prepare and isolate,
extremely stable, and can also be stored indefinetely. We have
therefore advantageously employed them in several classical
diazonium salts reactions.8b-h
sometimes suffer from various drawbacks including limited substrate
scope, harsh reaction conditions and the release of dangerous
gasses. A number of research groups have thus reported the
formation of aryl nitriles by means of other metal catalysts5c such as
nickel,5d rhodium,5e palladium5f-j and even using non-metallic CN
sources.1b-c,5k Finally, an interesting variant of the Rosenmund
protocol involves the use of boronic acids instead of aryl halides.5l
The paper reports the results of a detailed study on the Sandmeyer
reaction of arenediazonium o-benzenedisulfonimides 1, using
tetrabutylammonium cyanide (2a) or, alternatively, trimethylsilyl
cyanide (2b; Scheme 1) as cyanide sources.
a.Dipartimento di Chimica, Università di Torino, C.so Massimo d’Azeglio 48, 10125
Torino, Italy.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: 1. Synthesis of
heteroarenediazonium o-benzenedisulfonimides 1u–y; pag. 2; 2. Physical and
spectroscopic data of heteroarenediazonium o-benzenedisulfonimides 1u
–y ; pag. 3; 3. Physical and spectroscopic data of aryl cyanides 3 pag. 4-7 4. NMR
spectra of heteroarenediazonium o-benzenedisulfonimides 1u–y; pag. 8-12; 5.
NMR spectra of aryl nitriles 3, pag. 13-37. See DOI: 10.1039/x0xx00000x
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f
13
14
15
16
17
18
19
20
21
22
MeCN
THF
2a
2b
_
Cu2O
Cu2O
Cu2O
Cu2O
Cu
90
-
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DOI: 10.1039/C5OB02321A
60
60
10
10
10
30
90
90
60
-
DMF
2b
-
EtOH
DMSO
DMSO
MeCN
DMSO
EtOH
DMSO
2b
75
74
47
45
-
2b
Scheme 1 Sandmeyer reaction between arenediazonium o-
benzenedisulfonimides 1 and cyanides 2a or 2b.
2b
Results and discussion
2b
Cu2O
-
A
model reaction between 4-methoxybenzenediazonium o-
2b
benzenedisulfonimide (1a) and tetrabutylammonium cyanide (2a)
was studied under various conditions (Table 1), in order to optimize
the reaction conditions. Differing solvents (polar or slightly polar)
and various Cu catalysts were tested (Table 1). The optimal
combination was MeCN as solvent and Cu2O as catalyst (10 mol%.
Table 1; entry 10); 4-methoxybenzonitrile (3a) was obtained in an
excellent yield (95%) at room temperature and was easily purified
from the only by-product (anisole) using a short chromatographic
column.
2b
-
-
CuCN
-
62
a All the reactions were performed at r.t. with 10 mol% of appropriate
Cu catalyst. 1: 2a or 2b ratio was 1: 1.2 (2.5 and 3 mmol respectively).
b
Yields refer to pure and isolated 3a, purified from anisole, MS (EI)
m/z = 108 [M]+ (only by-product) on a silica gel chromatography
column. The eluent was petroleum ether/diethyl ether (9:1). c The
only detected products were iodobenzene, MS (EI) m/z = 204 [M]+
Table 1 Trial reactions: optimization of reaction conditions.
d
and anisole. The only detected products were chlorobenzene, MS
(EI) m/z = 112 [M]+ and anisole.e The reaction was carried out in the
presence of 1,3-dinitrobenzene (2.5 mmol, 0.41g). f The reaction was
performed using 4-methoxybenezenediazonium tetrafluoroborate
(2.5 mmol, 0.55 g).
The reaction (Table 1, entry 11) was subsequently performed
without Cu catalyst under optimized conditions. Interestingly and
gratifyingly, very good 3a yield was obtained (92%).
Entry
Solvent
Source
of CN
Catalyst
Time
(min)
Yield
of 3a
(%)a,b
In order to further exploit this reaction, we decided to change the
cyanide source to trimethylsilyl cyanide (TMSCN; 2b), which was
used in different solvents. Good results were also obtained here,
especially in DMSO and EtOH (Table 1, entries 16, 17): yields of 3a
were good (74% and 75% respectively). However, the Cu catalyst was
strictly required. In fact, no 3a was obtained without the Cu catalyst
(Table 1, entries 20,21)
1
2
THF
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
Cu2O
Cu
60
60
90
30
30
30
30
30
30
20
40
240
18
12
-
THF
3
DMF
Cu2O
Cu2O
Cu2O
Cu
4
EtOH
22
36
54
Encouraged by this excellent result and with optimized conditions in
hand, a number of other diazonium salts 1, bearing either electron
withdrawing or electron donating groups, were reacted. In light of
previous results, we decided to use 2a as cyanide source. In fact, solid
2a was easier to handle and safer than 2b while, most importantly,
allowed us to carry out the reaction without the Cu catalyst.
Moreover, it is important to note that 2a has never been used as a
cyanating agent in Sandmeyer protocol, to the best of our
knowledge.
5
DMSO
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
6
c
7
CuI
-
d
8
CuCl
Cu(OAc)2
Cu2O
-
-
9
-
10
11
12
95
92
44e
Good yields of target nitriles 3 were obtained (Table 2), although
even higher yields were obtained in the presence of electron
donating groups (Table 2, entries 1, 4, 5, 13). The influence of steric
hindrance was evident: when there were one (Table 2, entries 3, 9,
12) or two groups (Table 2, entries 18,19, 20) on the ortho positions
to the diazonium group, reaction times were significantly longer and
-
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smaller yields of 3 were obtained. The reaction was chemoselective
and no traces of terephthalonitrile (3p) were detected in the
reactions of diazonium salts containing a bromine or iodine atom,
which could potentially react in the same way as a diazonium group
(Table 2, entries 6, 8).
20
21
22
23
24
2,6-Br2C6H3
3-thienyl
150
60
3t
34
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DOI: 10.1039/C5OB02321A
3u
81
73
69
78
3-pyridyl
45
3v
2-pyrimidyl
60
3w
3x
We have not so far synthesized heteroarene diazonium o-
benzenedisulfonimides, with few exceptions.8g,h In order to address
this shortcoming, we decided to prepare stable salts 1u–y which
provided target heteroaryl nitriles 3u–y with satisfactory yields upon
reaction with 2a (Table 2, entries 21–25).
60
25
45
3y
80
It is worth nothing that is possible to recover o-benzenedisulfonimide
(6) in excess of 70% yield from all the reactions described above. 6
was recycled for the preparation of other salts 1.
a All the reactions were performed in MeCN at r.t.. 1: 2a ratio was 1:
1.2. b Yields refer to pure and isolated 3, purified from corresponding
arenes or heteroarenes, (only by-products) on
a silica gel
Table 2 Synthesis of aryl cyanides 3
chromatography column. The eluent was always petroleum
ether/diethyl ether (9:1).
O
O
S
The results obtained here have provided us with useful insights into
the mechanism of cyanodediazoniation without Cu catalyst. We
firstly carried out a reaction between 1a and 2a in the presence of
1,3-dinitrobenzene, an excellent electron acceptor.9 This reaction
was very much slower (4 h) than the reaction without 1,3-
dinitrobenzene and yielded much less 3a (Table 1, entry 12). We
therefore confirmed its homolytic course by means of this known
diagnostic test. We assumed that the anion in salts 1 would act as an
electron donor toward the arenediazonium cations, most likely
giving rise to the electron transfer complex 4, where the partner of
the aryldiazenyl radical is highly resonance stabilized (4a, 4b, etc).
Complex 4 would subsequently react with the CN- of 2a to create aryl
nitrile 3, as shown in Scheme 2. The role of the anion of 6 as an a
electron donor can be confirmed by the fact that 4-
methoxybenzenediazonium tetrafluoroborate did not react with 2a
in the absence of a Cu catalyst (Table 1, entry 13).
Bu4N+ CN
+
+
Ar N2
N
ArCN
S
O
3
1
2a
O
Entry
Ar in 1 and 3
Time
(min)
Product
Yield
(%)a,b
1
2
3
4
5
6
7
4-MeOC6H4
Ph
40
45
90
30
15
45
30
3a
3b
3c
3d
3e
3f
92
88
65
92
96
88
82
2-MeC6H4
3-MeC6H4
4-MeC6H4
4-BrC6H4
4-ClC6H4
3g
8
4-IC6H4
2-NO2C6H4
3-NO2C6H4
4-NO2C6H4
2-OHC6H4
30
100
45
3h
3i
79
62
87
84
54
90
82
74
78
82
45
37
9
10
11
12
13
14
15
16
17
18
19
3j
30
3k
3l
90
4-OHC6H4
30
3m
3n
3o
3p
3q
3r
Scheme 2 Reaction mechanism.
4-MeCOC6H4
4-MeOCOC6H4
4-CN
60
It is interesting to note that we have already described the
fascinating role as an electron transfer agent in bromodediazonation
of salts 1, played by the o-benzenedisulfonimide anion (5).10 Further
theoretical investigations into the role of 6 as an a electron donor are
currently underway.
60
30
2-naphthyl
2,6-Me2C6H3
2,6-Cl2C6H3
20
120
150
As mentioned above, 2b was did not react without Cu (Table 1;
entries 20, 21). The fact that 2b is a good source of CN in the presence
of a Cu catalyst has clearly demonstrated by Li and coworkers.7a
However, no mechanistic insights were reported. Moreover, the
3s
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literature shows that the Sandmeyer trifluoromethylation of All the reactions were carried out in open air glassware. Analytical
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DOI: 10.1039/C5OB02321A
arenediazonium tetrafluoroborates has been performed using grade reagents and solvents (purchased from Sigma Aldrich or Alfa-
TMSCF3 in the presence of Cu catalyst.11 The authors assumed that a Aesar) were used and reactions were monitored by GC, GC-MS and
trifluoromethyl copper (I) species, generated from Cu and TMSCF3 in TLC. Column chromatography and TLC were performed on Merck
the presence of a cesium base, was the real trifluoromethylating silica gel 60 (70-230 mesh ASTM) and GF 254, respectively.
agent. In light of this, we hypothesized that the cyanating agent is Petroleum ether refers to the fraction boiling in the range 40–70 °C.
most probably CuCN (7), which is derived from the interaction Room temperature is 22 °C. Mass spectra were recorded on an HP
between 2b and the Cu catalyst (Scheme 3).
5989B mass selective detector connected to an HP 5890 GC with a
methyl silicone capillary column. 1H NMR and 13C NMR spectra were
recorded on a Brucker Avance 200 spectrometer at 200 and 50 MHz
respectively. IR spectra were recorded on IR Perkin-Elmer UATR-two
spectrometer.
In order to confirm this hypothesis, we carried out the same reaction
using CuCN (normally used as a cyanating agent in the classical
Sandmeyer protocol)4c instead of 2b (Table 1; entry 22). 3a was
obtained under reaction conditions and yield similar to those
reported using 2b (Table1; entry 17)
Dry arene and heteroarenediazonium o-benzenedisulfonimides 1
were prepared as described previously by us.8 Crude salts 1 were
virtually pure (by 1H NMR spectroscopy) and were used in
subsequent reactions with 2 without further crystallization. Physical
data, spectroscopic data and NMR spectra of heteroarenediazonium
o-benzenedisulfonimides 1u, 1v, 1w, 1x and 1y are reported in
Electronic Supplementary Information (ESI). Satisfactory
microanalyses were obtained for all new compound..4-
Methoxybenzenediazonium tetrafluoroborate was prepared as
reported in the literature.12
Structures and purity of aryl nitriles 3 were confirmed by their
spectral (NMR, MS, IR) and physical data, substantially identical to
those of commercial (Sigma-Aldrich) samples. Yields of pure (GC, GC-
MS, TLC and NMR) isolated aryl cyanides 3 are collected in Table 2;
their physical data, spectroscopic data and NMR spectra are reported
in ESI.
Scheme 3 Cu catalyzed reaction between 1 and 2b.
4-Methoxybenzonitrile (3a); typical procedure
Conclusions
Tetrabutylammonium cyanide (2a, 0.81 g, 3 mmol) was added to a
We have proposed a mild, easy, safe and efficient method for the
synthesis of aryl and heteroaryl nitriles according to Sandmeyer
protocol. Target products are generally obtained in satisfactory
yields (25 examples, average yield 75%).
stirred
solution
of
4-methoxybenzenediazonium
o-
benzenedisulfonimide (1a, 0.88 g, 2.5 mmol) in anhydrous MeCN (10
ml). Immediately, the colour of the solution turned black and
evolution of nitrogen was observed.The mixture was stirred at room
temperature for 40 min; the completion of the reaction was
confirmed by the absence of azo coupling with 2-naphthol. Then, the
reaction mixture was poured into diethyl ether/water (100 mL, 1:1).
The aqueous layer was separated and extracted with diethyl ether
(50 mL). The combined organic extracts were washed with water (50
mL), dried with Na2SO4 and evaporated under reduced pressure.
Arenediazonium o-benzenedisulfonimides
1
have several
advantages over other diazonium salts. These include high stability,
ease of preparation and o-benzenedisulfonimide recovery at the end
of the reaction. Unlike other acids (e.g. tetrafluoroboric acid), o-
benzenedisulfonimide is a non-risk acid that can be reused to
prepare other salts, with ecological and economic advantages.
GC-MS analyses of the crude residue showed 4-methoxybenzonitrile
(3a, MS (EI): m/z = 133 [M]+) as the major product, besides traces of
anisole, MS (EI) m/z = 108 [M]+. The crude residue was purified on a
short column, eluting with petroleum ether/diethyl ether (9:1). The
only isolated product was title compound (3a, 0.31 g, 93% yield).
It is noteworthy that no copper or other metal catalysts are needed,
confirming the interesting role that the anion of o-
benzenedisulfonimide plays as an electron transfer agent. To the
best of our knowledge, this is the first example of Sandmeyer
cyanation being carried out without Cu catalyst. Moreover,
tetrabutylammonium cyanide (2a) has been used as a safe cyanating
agent for the first time.
The aqueous layer and aqueous washings were collected and
evaporated under reduced pressure. The tarry residue was passed
through a column of Dowex HCR-W2 ion exchange resin (1.6 g/1 g
of product), eluting with water (about 50 mL). After removal of water
under reduced pressure, virtually pure (1H NMR) o-
benzenedisulfonimide was recovered (0.42 g, 76% yield; mp 191–193
°C. Lit. 8a 191–193 °C).
Experimental Section
General
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All other aryl nitriles 3 were prepared according to this procedure.
and references cited therein; (h) S. Ding andVNie.wJAiartoic,leJ.OAnlmine.
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Trial reactions
The reactions between 1a and 2a or 2b in the presence of Cu catalyst
(Table 1) were performed as described above. 1a ( 0.88 g, 2.5 mmol)
was dissolved in the appropriate solvent (10 ml) and appropriate Cu
catalyst was added (10 mol%). Then cyanating agent 2a (0.81 g, 3
mmol) or 2b (0.30 g, 3 mmol) was added. The completion of the
reactions was confirmed by the absence of azo coupling with 2-
naphthol and the crude residues were purified on short columns
(eluent: petroleum ether/diethyl ether 9:1).
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This work has been supported by the University of Torino and by
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