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but it can be rationalized if we assume that NO2 was
removed from the reaction medium at a higher tem-
perature. The change of solvent had an important
effect on the reaction outcome. For example, the re-
action in acetonitrile gave the target product in 85%
yield (Table 1, entry 4), whereas THF and DMF were
less effective (Table 1, entries 10 and 11). Notably, the
complete conversion of the substrate was observed if
toluene was used, but the thiocyanation product was
obtained in a very low yield (Table 1, entry 9). Among
the screened acids, trifluoroacetic acid was the most
effective. If we used acetic acid instead of trifluoro-
acetic acid, a poor conversion of the substrate was
observed (Table 1, entry 13). Although the reaction
with H2SO4 gave a complete conversion, the yield of
the target product was low (Table 1, entry 12). The
loading of the thiocyanation agent was optimized to
1.3 equivalents (Table 1, entries 7 and 8). By contrast,
the selectivity was lower in the presence of 1.5 or
2 equivalents of ammonium thiocyanate, which re-
sulted in a decreased yield of the target product. The
thiocyanation reaction proceeded efficiently with
5 mol% of NO2, but the experimental results were
not reproducible. We tried to reduce the catalyst
loading to 1 mol% but found that this was not possi-
ble without a sacrifice of the product yield even if
the reaction time was prolonged to 20 h.
Table 1. Oxidative thiocyanation of 1-methoxynaphthalene catalyzed by
NO2.[a]
Entry
Thiocyanate
salt
Solvent
Acid
T
[8C]
Conv.[b]
[%]
Yield[b]
[%]
1
2
3
4
5
6
7[c]
8[c]
9
10
11
12
13
14
15
16[d]
NH4SCN
NH4SCN
NH4SCN
NH4SCN
NH4SCN
NH4SCN
NH4SCN
NH4SCN
NH4SCN
NH4SCN
NH4SCN
NH4SCN
NH4SCN
NaSCN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
toluene
DMF
–
15
0
1
ꢀ100
ꢀ100
ꢀ100
93
54
96
ꢀ100
ꢀ100
1
27
ꢀ100
5
ꢀ100
ꢀ100
0
trace
88
86
85
92
51
90
94
17
trace
23
26
5
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
H2SO4
10
15
30
40
30
15
15
15
15
15
15
15
15
15
THF
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
85
17
0
CuSCN
NH4SCN
[a] Reaction conditions: 1-alkoxynaphthalene (0.5 mmol), thiocyanate salt
(1 mmol), acid (0.2 mL), solvent (2 mL), NO2 (0.046 mmol), the air in the
tube was not removed, 3 h. [b] Determined by using GC with an internal
standard. [c] 1.3 equiv. of NH4SCN. [d] No NO2 was added.
tigation. The catalytic ability of NO2 was very poor if 1-alkoxy-
naphthalene was stirred with ammonium thiocyanate in aceto-
nitrile in the presence of catalytic NO2 at 158C (Table 1,
entry 1). The presence of a Brønsted acid can often facilitate
the aerobic oxidative reactions catalyzed by oxynitride spe-
cies,[17] which prompted us to add a Brønsted acid into the re-
action system. To our delight, the addition of trifluoroacetic
acid allowed 1-alkoxynaphthalene to be thiocyanated effective-
ly to give the target product in 85% yield (Table 1, entry 4).
The only byproduct was 1-methoxy-4-nitronaphthalene, which
resulted from the nitration of the naphthalene ring,[18] and no
dithiocyanation byproducts were observed (Scheme 2).
Subsequently, preliminary studies were conducted with a va-
riety of representative aromatic and heterocyclic compounds
to explore the scope and generality of this reaction under the
optimal conditions (Table 2). A series of arenes, thiophenes,
and indoles was thiocyanated smoothly in moderate to high
yields. Moreover, the thiocyanation reaction was compatible
with various groups, for example, alkoxy, aryloxy, alkyl, substi-
tuted amino, ester, cyano, bromo, and benzyloxy groups.
Among the tested substituted arenes, N-substituted amino
benzenes were effective substrates for the thiocyanation reac-
tion (Table 2, entries 1 and 7–17). For example, the reaction
with N,N-dimethylaniline gave the target product in 95% GC
yield with an excellent selectivity (Table 2, entry 1), and a small
amount of nitration and dithiocyanation byproducts was ob-
served. Although the amino group is an ortho-para-directing
group, no ortho-thiocyanation relative to the N,N-dimethylami-
no group was observed, which could be attributed to the
larger steric hindrance of the position ortho to the amino
group. However, if the para CÀH bond of the amino group
was absent, the thiocyanation had to occur at the ortho posi-
tion to this directing group (Table 2, entry 17). Other suitable
substrates were alkoxynaphthalenes. Both 1-alkoxynaphthalene
and 2-alkoxynaphthalene were converted smoothly to the
target product (Table 2, entries 2 and 3). Different from aniline
derivatives, the alkoxynaphthalene substrates did not undergo
dithiocyanation. This phenomenon has been reported previ-
ously.[19] If 2,7-dimethoxynaphthalene was used as the sub-
strate, 2,7-dimethoxy-1-thiocyanatonaphthalene was obtained
as the major product in 93% GC yield. Among the test alkoxy-
benzenes, 1,2,3-trimethoxybenzene was thiocyanated smoothly
Scheme 2. NO2-catalyzed oxidative thiocyanation of 1-methoxynaphthalene.
Further studies were undertaken to obtain higher yields and
more generally practical reaction conditions. The reaction was
highly dependent on the reaction temperature. The reaction
with 2 equivalents of ammonium thiocyanate at 308C allowed
the desired product to be obtained in 92% yield with a high
selectivity (Table 1, entry 5), whereas 158C was the optimal
temperature in terms of yield with 1.3 equivalents of ammoni-
um thiocyanate. It is difficult to understand why the conver-
sion of the substrate became lower with the increase of the re-
action temperature from 30 to 408C (Table 1, entries 5 and 6),
&
ChemCatChem 2016, 8, 1 – 7
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