Transition-metal-free synthesis of thiocyanato- or nitro-arenes through diaryliodonium salts

Article abstract of DOI:10.1080/00397911.2016.1181764

A transition-metal-free approach to facile synthesis of thiocyanato- and nitro-arenes was developed from KSCN (potassiumthiocyanate) or NaNO₂with diaryliodonium salts in good yields under mild conditions. The reaction was compatible with a variety of sensitive functional substituents such as halides and nitro and ester groups. The usefulness of arylation products has been realized. (Formula Present).

Full text of DOI:10.1080/00397911.2016.1181764

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
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ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Transition Metal-Free Synthesis of Thiocyanato- or
Nitro-Arenes through Diaryliodonium Salts
Xiao-Hua Li, Liang-Gui Li, Xue-Ling Mo & Dong-Liang Mo
To cite this article: Xiao-Hua Li, Liang-Gui Li, Xue-Ling Mo & Dong-Liang Mo (2016): Transition
Metal-Free Synthesis of Thiocyanato- or Nitro-Arenes through Diaryliodonium Salts, Synthetic
Communications
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Date: 22 May 2016, At: 11:15

Transition Metal-Free Synthesis of Thiocyanato- or Nitro-arenes through
Diaryliodonium Salts
Xiao-Hua Li1, Liang-Gui Li1, Xue-Ling Mo1, Dong-Liang Mo1
1Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources,
Ministry of Education of China; School of Chemistry & Pharmaceutical Sciences,
Guangxi Normal University,Guilin, China.
Corresponding to author: Dong-Liang Mo Key Laboratory for the Chemistry and
Molecular Engineering of Medicinal Resources, Ministry of Education of China; School
of Chemistry & Pharmaceutical Sciences, Guangxi Normal University,Guilin, China.
E-mail:moeastlight@mailbox.gxnu.edu.cn
Abstract
A transition metal-free approach to facile synthesis of thiocyanato- and nitro-arenes was
developed from KSCN or NaNO2 with diaryliodonium salts in good yields under mild
conditions. The reaction was compatible with a variety of sensitive functional
substituents such as halides, nitro, and ester groups. The usefulness of arylation products
has been realized.
1

INTRODUCTION
Thiocyanato- and nitro arenes are versatile building blocks for synthesis of organic
molecules of higher complexity and these potential scaffolds are frequently found in
natural products, pharmaceuticals and commonly exhibit biological activities.1,2 Some of
which are listed in Figure 1.3 Thus, exploration of new methods for introducing
thiocyanato and nitro groups into organic molecules has been of broad interest in recent
years.4,5
The thiocyanation reaction is one of the most useful carbon−sulfur bond forming
reactions.6 Organic thiocyanates usually served as the synthetic precursors into various
sulfur-containing derivatives, such as sulfur heterocycles, thioethers, disulfides, thiols
and so on.7 Traditional methods to synthesize aryl thiocyanates were direct electrophilic
thiocyanation of arenes or by Sandmeyer-type reaction via nucleophilic substitution of
aryl diazonium salts (Scheme 1A).8 In the past few years, some approaches to synthesis
of aryl thiocyanates catalyzed by transition metal catalysts have been investigated, which
included the copper promoted or catalyzed nucleophilic replacement of aryl iodides with
thiocyanate salts (Scheme 1B),9 thiocyanation of aryl organometallic compounds
(aryllithium, arylzinc compounds and arylboronic acids) (Scheme 1C),10 and cyanation of
organosulfur compounds under transition metal or metal-free conditions (Scheme 1D).11
2

Although great efforts have been devoted to the improvement of these transformations,
most of them relied on the use of some special reagents and encountered some drawbacks,
such as harsh reaction conditions, limited substrate scope, poor sensitive functional group
compatibility, complicated ligands, and trace amount transition metal in the products.
In order to resolve the aforementioned drawbacks, the transition metal-free arylation
strategies have been attracted much attention in recent years.12 Diaryliodonium salts
possessing two aryl groups, have been extensively applied as versatile arylation agents
for a variety of nucleophiles in transition-metal free conditions due to its easy availability,
high reactivity and selectivity, and less toxic reagents.13,14 For examples, metal-free
arylations of α-carban of carbonyl, alcohol, oximes, hydroxylamines, and amides with
diaryliodonium salts have been developed.15 The transformations of diaryliodonium salts
with potassium thiocyanate and sodium nitrite have been studied before.16 In 2009,
Dimagno and coworkers reported a detailed effect of reductive-elimination state of
diaryliodonium thiocyanate based on computational and experimental studies (Scheme
1E).16b Very recently, Yusubov and Zhdankin not only reported a method to prepare
triisopropylsilyl-substituted aryliodonium salts but also studied the reactivity of the
diaryliodonium salts with KSCN (Scheme 1F).16c Although Dimagno and Zhdankin have
reported the studies of diaryliodonium salts with KSCN respectively, there were not a
general, practical and more detailed investigations of the substrate scope and
3

compatibility of the functional groups in the diaryliodonium reagents. Recently, we not
only reported the synthesis of α,β-unstratured N-aryl ketonitrones by the N-arylation of
oximes with diaryliodonium salts, but also developed a tandem C-O and C-N bonds
formation through O-arylation and [3,3]-rearrangement by diaryliodonium salts to
synthesize N-aryl benzotriazin-4(1H)-one derivarives.17 Continuing to investigate
arylation for new nucleophiles, we surmised that to explore the substrate scope and
functional group compatibility of the diaryliodonoium salts with KSCN and NaNO2
would provide an efficient, practical and useful method to access these organic sulfur- or
nitrogen-containing molecules.
DISCUSSION
Initially, we ran the reaction of diphenyliodonium triflate 1a and KSCN in DMSO at 80
˚C for 24 h. To our surprise, Phenyl thiocyanate 2a was observed in 14% yield (Table 1,
entry 1). As illustrated in Table 1, the solvents have a great effect on the yield of 2a,
which showed DCE (1,2-dichloroethane) was the best solvent for this transformation
(Table 1, entries 1-8). However, the yield of 2a decreased obviously whatever the
temperature was lower or higher (Table 1, entries 8-11). Moreover, when the amount of
KSCN increased to 4.0 equiv, the yield of 2a was improved to 83% (Table 1, entry 13).
The by-products of this arylation are PhI and KOTf, which make product 2a was easily to
be purified. In order to improve the solubility of KSCN, N(n-Bu)4F was added into the
4

reaction (Table 1, entries 14-15), we found that when N(n-Bu)4F was added only 0.2
equiv, the product 2a was obtained in 81% yield (Table 1, entry 14). However, increasing
N(n-Bu)4F would decrease the yield of 2a (Table 1, entry 15). When KSCN was dropped
to 1.0 equiv, the yield of 2a was afforded in 63% (Table 1, entry 16). These results
revealed that the phase-transfer catalyst could decrease the amounts of KSCN and
promote the reaction.
To test the scope of the present protocols, a variety of diaryliodonium salts 1 were
subjected to the optimal conditions (Table 1, entry 13). As shown in Table 2, a series of
desired product 2 were obtained from moderate to good yields. This method was suitable
for both electron-rich and electron-deficient either para-, meta-, or ortho- substituents on
diaryliodonium salts 1. Interestingly, when unsymmetric diaryliodonium salts were used,
the reaction proceeded with good chemo-selectivity and the more deficient aryl moieties
could be more preferably transferred to the desired products (Table 2, entries 9-16).18
However, for diaryliodonium salt 1i, both 4-PhO and 4-MeO substituted aryl partners
were transferred to the desired products and 4-PhO substituted aryl moiety was
transferred as a major product (Table 2, entry 10). The electron-rich substituents on aryl
rings are preferable to get high yields than the electron-deficient substituents (Table 2,
entries 1-8). This method was compatible with some important functional groups on the
aryl ring of the diaryliodonium salts, such as fluorine, bromine, chlorine, methoxy, nitro,
5

and ester substituents, which could be used to further potential synthetic transformations.
The reaction was easily scaled up to 30 mmol and analogue yield was obtained as 0.5
mmol scale (Table 2, entries 1 and 15), which provided a practical method to access these
compounds easily.
To be better application of this arylation process, NaNO2 was used as nucleophile in the
optimal condition to synthesize nitroarenes. Various diaryliodonium salts 1 was suitable
for both electron-rich and electron-deficient groups with either meta, para or ortho
substituents in this transformation and the desired products 3 was obtained from moderate
and good yields (Table 3). Especially, this method can be used to synthesize
m-dinitrobenzene (3g) easily in 60% yields which was obtained in lower yield for
copper-catalyzed strategies (Table 3, entry 7). This method was also compatible with
some useful substituents such as halides, nitro, ester groups. Moreover, the reaction can
also be easily scaled up to 30 mmol (Table 3, entry 10).
With a variety of thiocyanatoarenes 2 in hand, we decided to investigate the synthetic
utility of these compounds as useful intermediates in organic synthesis (Scheme 2-1). As
shown in Scheme 2-1, oxidation of 2n with m-chlorobenzoperoxoic acid (m-CPBA) at
room temperature proceeded to give product 4 in 60% yield. Treatment of 2n with
PhMgBr targeted the cyanide group afford sulphide product 5 in 74% yield, which has
6

been further applied into synthesis of dibenzothiophene scaffolds.[19] Nitroarenes are
extensively utilized in organic synthesis, which inspired us to investigate the further
application of 3n (Scheme 2-2). When 3n was treated with 2-methylindole proceeded
substitution to afford 2,3-disubstituted indole 6 in 65% yield, which has been successfully
used to prepare densely functionalized biiodoles, indoloindoles.[20]
CONCLUSIONS
We have developed a transition-metal free transformation of readily available
diaryliodonium salts into the corresponding thiocyanato- or nitroarenes in good yields
under mild conditions. This method features broad substrate scopes, less-toxic
hypervalent iodine reagents, easily operation, gram scalability and excellent functional
group compatibilities. This process is not only complementary to existing methods of
thiocyanate and nitro group formation, but also extends the applications of
diaryliodonium salts with diverse nucleophiles in organic synthesis.
EXPERIMENTAL
General procedure for synthesis of aryl thiocyanates 2: In Schlenk tube was charged
with diaryliodonium salts 1 (0.5 mmol) and KSCN (2.0 mmol, 4.0 equiv) in air
atmosphere, dissolved in DCE (5 mL). The mixture was stirred vigorously at RT for 5
min. Then, the reaction was stirred at 80 °C and monitored by TLC until 1 was consumed
completely (18-24 h). At this time, the DCE was removed under reduced pressure and the
7

crude product was purified by flash chromatography (dry loading with SiO2) using
eluents (1:50 ethyl acetate:petroleum ether to 10:1) to provide product 2 as oil or solid.
Thiocyanatobenzene (2a),[5g] colorless oil, 56.0 mg, 83% yield. 1H NMR (400 MHz,
CDCl3): δ (ppm) 7.44-7.42 (m, 2H), 7.36-7.32 (m, 3H); 13C NMR (100 MHz, CDCl3): δ
(ppm)130.1, 129.9, 129.4, 124.3, 110.4.
FUNDING
This work was supported by the “Oversea 100 Talents Program” of Guangxi Higher
Education, Basic Research Program of Guangxi Normal University (2014ZD005),
National Natural Science Foundation of China (21562005), and National Natural Science
Foundation of Guangxi (2015GXNSFCA139001).
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13

Table 1. Optimization of reaction.[a]
Entry
1
solvent
DMSO
DMF
T (˚C)
80
2a %[b]
14
2
80
63
3
MeCN
MeOH
THF
80
68
4
80
20
5
80
34
6
dioxane
toluene
DCE
80
31
7
80
64
8
80
74
9
DCE
60
51
10
11
12
DCE
100
120
80
57
DCE
52
DCE
80[c]
14

13
14
15
16
DCE
DCE
DCE
DCE
80
80
80
80
83[d]
81[e]
57[f]
63[g]
[a] Reaction conditions: 1a (0.5 mmol), KSCN (1.0 mmol, 2.0 equiv), solvent (5 mL),
18-24 h; [b] Isolated yields; [c] KSCN (1.5 mmol, 3.0 equiv); [d] KSCN (2.0 mmol, 4.0
equiv); [e] KSCN (1.0 mmol, 2.0 equiv), N(n-Bu)4F (0.2 equiv); [f] KSCN (1.0 mmol,
2.0 equiv), N(n-Bu)4F (1.0 equiv); [g] KSCN (0.5 mmol, 1.0 equiv), N(n-Bu)4F (0.2
equiv).
15

Table 2. The scope of diaryliodonium salts 1 with KSCN[a]
entry
1
R1
R2
Yield %[b]
1
2
H
H
83 (80)[c]
95
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
2
4-MeO
4-t-Bu
4-Cl
4-Br
4-F
4-MeO
4-t-Bu
4-Cl
3
78
4
78
5
4-Br
60
6
4-F
67
7
3-NO2
2-Me
H
3-NO2
2-Me
4-MeO
4-MeO
4-MeO
60
1g
1h
1aa
1i
2g
2h
2a
2i
8
73
9
65
10
11
12
4-PhO
4-Ph
4-i-Pr
56[d]
70
1j
2j
4-MeO
70
1k
2k
16

13
14
15
16
4-CF3
3-Br
4-MeO
4-MeO
4-MeO
H
69
1l
2l
65
1m
1n
1o
2m
2n
2o
2-Br
86 (75)[e]
65
4-CO2Me
[a] Reaction conditions: 1 (0.5 mmol), KSCN (2.0 mmol, 4.0 equiv), DCE (5 mL),
18-24 h; [b] Isolated yields; [c] 1a (30 mmol) was used; [d] 2b was also isolated with 25%
yield; [e] 1n (30 mmol) was used.
17

[a]
Table 3. The scope of diaryliodonium salts 1 with NaNO2
Entry
R1
R2
Yield %[b]
1
3
1
H
H
73
1a
1b
1c
1d
1e
1f
3a
3b
3c
3d
3e
3f
2
4-MeO
4-t-Bu
4-Cl
4-MeO
4-t-Bu
4-Cl
68
3
69
4
76
5
4-Br
4-Br
88
6
4-F
4-F
54
7
3-NO2
4-Ph
4-CF3
2-Br
3-NO2
4-MeO
4-MeO
4-MeO
H
60
1g
1j
3g
3j
8
68
9
63
1l
3l
10
11
85 (78)[c]
74
1n
1o
3n
3o
4-CO2Me
18

[a] Reaction conditions: 1 (0.5 mmol), NaNO2 (2.0 mmol, 4.0 equiv), DCE (5 mL),
18-24 h; [b] Isolated yields; [c] 1n (30 mmol) was used.
19

Scheme 1. Previous strategies for synthesis of thiocyanates.
20

Scheme 2. Transformation of 2n and 3n.
21

Figure 1. Selected examples of bioactive compounds with sulfides and nitro arenes.
22