Cationic cobalt-catalyzed [1,3]-rearrangement of N-alkoxycarbonyloxyanilines

Article abstract of DOI:10.3762/bjoc.14.172

A cationic cobalt catalyst efficiently promoted the reaction of N-alkoxycarbonyloxyanilines at 30 °C, affording the corresponding ortho-aminophenols in good to high yields. As reported previously, our mechanistic studies including oxygen-18 labelling expe

Full text of DOI:10.3762/bjoc.14.172

Cationic cobalt-catalyzed [1,3]-rearrangement of
N-alkoxycarbonyloxyanilines
Itaru Nakamura*1, Mao Owada2, Takeru Jo2 and Masahiro Terada1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 1972–1979.
1Research and Analytical Center for Giant Molecule, Graduate School
of Science, Tohoku University, 6-3 Aramaki Aza Aoba, Aoba-ku,
Sendai 980-8578 Japan and 2Department of Chemistry, Graduate
School of Science, Tohoku University, 6-3 Aramaki Aza Aoba,
Aoba-ku, Sendai 980-8578 Japan
doi:10.3762/bjoc.14.172
Received: 15 May 2018
Accepted: 17 July 2018
Published: 31 July 2018
Email:
This article is part of the thematic issue "Cobalt catalysis".
Guest Editor: S. Matsunaga
Itaru Nakamura* - itaru-n@tohoku.ac.jp
* Corresponding author
© 2018 Nakamura et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
anilines; cobalt catalyst; concerted reaction; N–O bond;
rearrangement
Abstract
A cationic cobalt catalyst efficiently promoted the reaction of N-alkoxycarbonyloxyanilines at 30 °C, affording the corresponding
ortho-aminophenols in good to high yields. As reported previously, our mechanistic studies including oxygen-18 labelling experi-
ments indicate that the rearrangement of the alkoxycarbonyloxy group proceeds in [1,3]-manner. In this article, we discuss the
overall picture of the cobalt-catalysed [1,3]-rearrangement reaction including details of the reaction conditions and substrate scope.
Introduction
The 2-aminophenol moiety is ubiquitously found as a core oxidation state during the transformation (Scheme 2a) [4-11].
structure of biologically active compounds, such as tigecycline However, there is a significant drawback, these [3,3]-rearrange-
[1], iguratimod [2], and phosalone (Scheme 1) [3]. The scaf- ments of carboxylic acyloxy and alkoxylcarbonyloxy groups
folds have also been frequently utilized as synthetic intermedi- generally require long heating times at elevated reaction tem-
ates not only in pharmaceutical chemistry but also in materials peratures (>140 °C) or microwave irradiation (Scheme 2a). In
science. Thus, it is of great importance to efficiently synthesize contrast, N-sulfonyloxyanilines are known to readily undergo
functionalized 2-aminophenols under mild reaction conditions the [3,3]-rearrangement during the preparation of the starting
in a regioselective manner. Among numerous methods, the material below −20 °C due to the strongly electron-with-
[3,3]-rearrangement of O-acyl-N-arylhydroxylamines 1 driven drawing nature of the sulfonyl group (Scheme 2b) [12]. Accord-
by cleavage of the N–O bond is an ideal approach to selectively ingly, we envisioned that appropriate Lewis acidic metal cata-
synthesize O-protected 2-aminophenols 2 while maintaining the lysts would promote the rearrangement reaction of stable
1972

Beilstein J. Org. Chem. 2018, 14, 1972–1979.
N-acyloxyanilines to afford readily deprotectable 2-acyloxyani- Results and Discussion
lines under much milder reaction conditions with high func- At the beginning of this investigation, N,O-di(methoxy-
tional group tolerance. Based on this concept, we disclosed that carbonyl)hydroxylaniline (1a) was treated with catalytic
cationic cobalt catalysts efficiently promote the reaction of amounts of several copper salts in 1,2-dichloroethane (DCE) at
O-alkoxylcarbonyl-N-arylhydroxylamines 1 at 30 °C, affording 60 °C (Table 1, entries 1–7), according to our previous copper-
the corresponding 2-aminophenol derivatives 2 in good to high catalysed cascade reaction involving rearrangement via N–O
yields [13]. Our mechanistic studies revealed that the rearrange- bond cleavage [14]. While divalent copper acetate and copper
ment of the alkoxycarbonyloxy group proceeded in an unprece- chloride did not show any catalytic activities (Table 1, entries 1
dented [1,3]-manner (Scheme 2c). In this article, we describe and 2), more Lewis acidic copper complexes, such as
the overall picture of the intriguing [1,3]-rearrangement reac- [Cu(MeCN)4](PF6)2 and [Cu(OTf)]2·toluene, afforded the cor-
tion, particularly the detail of the reaction, which were not suffi- responding 2-aminophenol derivative 2a (Table 1, entries 3 and
ciently discussed in our preliminary communication.
4). Moreover, a cationic copper catalyst generated from CuCl2
and two equivalents of AgSbF6 was effective to afford 2a in
good yield (Table 1, entry 5), even at 30 °C (Table 1, entry 8).
The use of a ligand, such as 1,10-phenanthroline (phen) and
1,3-bis(diphenylphosphino)propane (dppp), totally diminished
the activity of cationic cobalt catalyst (Table 1, entries 6 and 7).
Among metal chlorides examined, CoCl2 exhibited the best cat-
alytic activity at 30 °C, affording the corresponding 2a in 68%
yield (Table 1, entry 9), as reported previously [13]. A divalent
cationic zinc catalyst also promoted the present reaction, albeit
with lower chemical yield than Co(II) (Table 1, entries 10 and
11), while the use of Fe(II) and Pd(II) resulted in low chemical
yield due to the formation of the para-isomer 3a (Table 1,
entries 12 and 13). Indeed the para-isomer 3a was obtained as a
major product when the reaction of 1a was conducted using
trivalent metal salts, such as FeCl3 and RuCl3, and tetravalent
salts, such as ZrCl4, as a catalyst (Table 1, entries 15–18). Al-
though we quite recently disclosed that cationic NHC-copper
Scheme 1: ortho-Aminophenol derivatives.
Scheme 2: Rearrangement of N-acyloxyanilines.
1973

Beilstein J. Org. Chem. 2018, 14, 1972–1979.
Table 1: Catalytic activity.
entry
catalyst (mol %)
temp. (°C)
2a (%)a
3a (%)a
1a (%)a
1
CuCl2 (10)
60
60
60
60
60
60
60
30
30
30
30
30
30
30
30
30
30
30
30
60
30
30
30
30
30
30
30
30
30
<1
<1
5
<1
<1
<1
4
>99
>99
75
23
14
>99
80
<1
<1
<1
1
2
Cu(OAc)2 (10)
3
[Cu(MeCN)4](PF6) (10)
[Cu(OTf)]2·C6H5CH3 (10)
CuCl2 (10), AgSbF6 (20)
CuCl2 (10), AgSbF6 (20), phen (20)
CuCl2 (10), AgSbF6 (20), dppp (20)
CuCl2 (10), AgSbF6 (20)
CoCl2 (10), AgSbF6 (20)
ZnCl2 (10), AgSbF6 (20)
ZnCl2 (10), AgSbF6 (10)
PdCl2 (10), AgSbF6 (20)
FeCl2 (10), AgSbF6 (20)
FeCl3 (10), AgSbF6 (30)
RuCl3 (10), AgSbF6 (30)
RuCl3 (10), AgSbF6 (20)
IrCl3 (10), AgSbF6 (30)
ZrCl4 (10), AgSbF6 (40)
IPrCuBr (10), AgSbF6 (10)
AgSbF6 (10)
4
50
52
<1
<1
57
63 (68)
54
54
24
32
22
18
21
15
16
<1
53
<3
<1
5
5
<1
<1
<1
10
<3
8
6
7
8b
9b
10b
11
12b
13b
14b
15b
16
17
18
19
20
21b
22b
23
24
25b
26b
27b
28
29
8
7
<1
1
21
26
35
35
37
28
<1
17
<3
<1
6
<1
<1
<1
<1
<1
12
7
AgSbF6 (10)
80
90
74
90
3
CoCl2 (10)
TfOH (10)
(PhO)2P(O)OH
<1
50
<1
<1
53
59
<1
1
CoCl2 (10), AgNTf2 (20)
CoCl2 (10), AgPF6 (20)
CoCl2 (10), AgOTf (20)
CoCl2 (10), AgSbF6 (10)
CoCl2 (5), AgSbF6 (10)
<1
<1
3
98
97
<1
<1
2
aYields were determined by 1H NMR using CH2Br2 as an internal standard. Isolated yield in parenthesis. bReported in the Supporting Information of
our previous paper [13], except for yields of recovered 1a.
catalysts efficiently promoted the [1,3]-alkoxy rearrangement of hexafluoroantimonate and bis(trifluoromethanesulfonyl)imidate
N-alkoxyaniline [15], the cationic NHC-Cu catalyst generated were efficient (Table 1, entries 9 and 25), while the use of hexa-
from IPrCuBr and AgSbF6 was totally inefficient for the present fluorophosphate and trifluoromethanesulfonate did not promote
reaction; 1a was decomposed under the reaction conditions the reaction at all (Table 1, entries 26 and 27). The use of an
(Table 1, entry 19). Whereas AgSbF6 promoted the reaction at equal amount of AgSbF6 to CoCl2 resulted in slightly decreas-
60 °C (Table 1, entry 20), the catalytic activity was diminished ing the chemical yield (Table 1, entry 28).
at 30 °C (Table 1, entry 21). Neutral CoCl2 did not promote the
present reaction (Table 1, entry 22). Brønsted acids, such as tri- Next solvent and concentration effects were examined as sum-
fluoromethanesulfonic acid and diphenylphosphoric acid, were marized in Table 2. 1,2-Dichloroethane (DCE) gave the best
much less active (Table 1, entries 23 and 24). The kind of the result (Table 2, entry 1), as described previously [13]. Other
counteranion significantly affected the reaction efficiency; halogen solvents, such as CHCl3, CH2Cl2, and PhCl, ethereal
1974

Beilstein J. Org. Chem. 2018, 14, 1972–1979.
Table 2: Solvent and concentration effects.
entry
solvent
concentration (M)
2a (%)a
3a (%)a
1a (%)a
1b
2b
3b
4b
5b
6b
7
DCE
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1.0
0.25
0.05
63
49
40
39
43
38
49
<1
<1
<1
4
3
<1
19
<1
25
11
1
CHCl3
CH2Cl2
PhCl
7
2
2
toluene
Et2O
1
<1
1
MTBE
THF
4
8
<1
<1
<1
<1
2
>99
98
>99
81
<1
<1
11
9
CH3CN
DMF
10
11
12
13b
14b,c
MeOH
DCE
51
72
53
DCE
3
DCE
9
aYields were determined by 1H NMR using CH2Br2 as an internal standard. bReported in the Supporting Information of our previous paper (ref. [13]),
except for yields of recovered 1a. cFor 5 days.
solvent, such as Et2O and tert-butyl methyl ether (MTBE), and the oxygen atom was readily isomerized to the ortho-amino-
toluene were less efficient (Table 2, entries 2–7), while the use phenol derivative under its preparing conditions in the absence
of polar solvents, such as tetrahydrofuran (THF), acetonitrile, of the cationic cobalt catalyst. In sharp contrast to alkoxycar-
and N,N-dimethylformamide (DMF), resulted in quantitative bonyloxy groups, acyloxy groups, such as the benzoyloxy
recovery of the starting material 1a (Table 2, entries 8–10). A group, were not migrated to the ortho-position, resulting in
protic solvent, such as methanol, was ineffective (Table 2, entry decomposing the starting material (Table 3, entry 13), as
11). Slight dilution of the reaction solution (0.25 M) improved mentioned previously [13].
the chemical yield (Table 2, entry 12).
The reaction was applied to 1o–ab having various substituents
As mentioned previously [13], carbamate-type groups, such as at the para-position, as summarized in Table 4. As reported pre-
methoxycarbonyl, Alloc and Cbz were tolerated as a protective viously [13], reactive functional groups, such as bromo, iodo,
group on the nitrogen atom, affording the desired products 2 in and alkynyl groups were tolerated, affording the desired prod-
good yields (Table 3, entries 1–3). The reaction of 1e having a ucts in good to high yields (Table 4, entries 5–7). The substrate
2,2,2-trichloroethoxycarbonyl (Troc) group, however, resulted 1v having a methoxycarbonyl group afforded 2v in good yield,
in decomposing 1e (Table 3, entry 4). The use of aroyl groups while cyano and acetyl groups interrupted the present reaction
gave the desired product in good yields (Table 3, entries 5–7), presumably due to deactivation of the catalyst, recovering the
while the acetyl group required a prolonged reaction time starting materials quantitatively (Table 4, entries 9 and 10).
(Table 3, entry 8). Substrate 1j having a tosyl group on the Notably, our catalytic conditions successfully promoted the re-
nitrogen resulted in decomposition of 1j (Table 3, entry 9). The arrangement of 1y’, having a highly electron-deficient
alkoxycarbonyl groups, such as Cbz, methoxycarbonyl, and p-trifluromethylphenyl group, which have not been employed in
2-chloroethoxy groups, were employed as good migrating the thermal [3,3]-rearrangement reaction, when using a
groups (Table 3, entries 7–11), while 1m having a Boc group on p-nitrobenzyloxycarbonyl group in place of Cbz as the
the oxygen atom did not give the desired product, due to de- migrating group (Table 4, entry 11). In addition, compatibility
composition of 1m (Table 3, entry 12). It is noteworthy that the of the protective group on the oxygen atom was tested (Table 4,
substrate having a highly electron-withdrawing Troc group on entries 12–14), since it is expected that the cationic cobalt
1975

Beilstein J. Org. Chem. 2018, 14, 1972–1979.
Table 3: Substituent effect at the hydroxylamine moiety.a
entry
1
R1
R2
time (h)
2
yield (%)b
1c
2c
3c
4
1b
1c
1d
1e
1f
OMe
Cbz
5
2b
2c
2d
–
64
45
64
<1
60d
75
61
44
<1
82
56
<1
<1
OMe
Alloc
4
OMe
Boc
6
OMe
Troc
18
2
5c
6c
7c
8c
9
OMe
p-MeOC6H4C(O)
2f
2g
2h
2i
–
1g
1h
1i
OMe
Bz
3
OMe
p-F3CC6H4C(O)
24
120
11
2
OMe
Ac
Ts
Bz
Bz
Bz
Bz
1j
OMe
10c
11
12
13c
1k
1l
OBn
2k
2l
–
O-CH2CH2Cl
O-tBu
Ph
2
1m
1n
10
120
–
aThe reactions of 1 (0.4 mmol) were conducted in the presence of 10 mol % CoCl2 and 20 mol % of AgSbF6 in DCE (1.6 mL) at 30 °C. bIsolated yield.
cPreviously reported in [13]. d1H NMR yield using dibromomethane as an internal standard.
Table 4: Co-catalyzed reaction of N-alkoxycarbonyloxyanilines 1o–ab.a
entry
1
R
R1
time (h)
2
yield (%)b
1c
2c
3c
4c
5c
6c
7c
8c
9
1o
1p
1q
1r
Me
Bn
3
2o
2p
2q
2r
74
F
Bn
11
1
66d
88
Cl
Bn
Cl
Me
3
79
1s
1t
Br
Bn
1
2s
2t
86
I
Bn
1
77
1u
1v
1w
1x
1y’
1z
TMSC≡C
CO2Me
Ac
Bn
2
2u
2v
–
62
Bn
15
>120
>120
48
72
14
14
84d
<1e
<1e
76d,f
50d,g
64
Bn
10
11c
12
13
14
CN
Bn
–
CF3
p-O2NC6H4CH2
2y’
2z
2aa
2ab
BzO(CH2)2
TBSO(CH2)2
MOM(CH2)2
Bn
Bn
Bn
1aa
1ab
59
aThe reactions of 1 (0.4 mmol) were conducted in the presence of 10 mol % CoCl2 and 20 mol % of AgSbF6 in DCE (1.6 mL) at 30 °C. bIsolated yield.
cPreviously reported in [13]. d1H NMR yield using dibromomethane as an internal standard. See Supporting Information File 1 for details. eThe starting
material was quantitatively recovered. fYield brsm (28% of 1y’ was recovered). gIsolation of 2z was unsuccessful due to contamination by insepa-
rable byproducts (see Supporting Information File 1).
1976

Beilstein J. Org. Chem. 2018, 14, 1972–1979.
would make the protective group labile as well as the protec- tive cleavage of the Cbz group afforded the phenol 4h-18O, of
tive group would deactivate the cationic cobalt catalyst. As which the oxygen-18 content was 64% (Scheme 3b). The result
results, tert-butyldimethylsilyl (TBS) and methoxymethyl clearly indicates that the rearrangement of the CbzO group in
(MOM) groups were tolerated under the cationic cobalt-cata- the presence of cationic cobalt catalysts proceeds in a concerted
lyzed reaction conditions to afford the desired product in good [1,3]-manner [18-24]. In addition, the reaction of 1h-18O (23%
yields (Table 4, entries 13 and 14). The reaction using a benzoyl 18O) in the absence of the cationic cobalt catalyst at 140 °C fol-
group was sluggish, affording the desired product 2z in moder- lowed by hydrogenative deprotection afforded 4h, of which the
ate yield with formation of inseparable byproducts (Table 4, oxygen-18 content was less than 2% (Scheme 3c). Therefore,
entry 12). Thus, the use of silyl- and acetal-type protective we concluded that the cationic cobalt catalyst not only made the
groups is suitable for the present reaction.
reaction much milder than the thermally-induced reaction but
also changed the rearrangement mode to an unprecedented
As reported previously [13], the fact that the present rearrange- [1,3]-manner. In addition, intermolecular and intramolecular
ment reaction proceeds in a [1,3]-manner was confirmed by a competitive experiments using deuterium-labelled substrates
crossover experiment and oxygen-18 labeling experiments. That resulted in no kinetic effect (Scheme 4). These results suggest
is, the reaction of a 1:1 mixture of equally-reactive substrates that the C–O bond would form prior to cleavage of the C–H
1h and 1r under the standard reaction conditions afforded only bond in the [1,3]-rearrangement reaction.
the products 2h and 2r derived from the starting materials
(Scheme 3a). Thus, we confirmed that the present reaction Due to the fact that the reaction of 1a in the presence of tri- and
proceeds in an intramolecular manner. Next, 18-oxygen- tetravalent cationic metal catalysts afforded the para-isomer 3a
labelling experiments were conducted using substrate 1h-18O, as a major product (Table 1, entries 14–18), the reaction of
of which the oxygen-18 content at the hydroxylamine oxygen ortho-aminophenol derivative 2a in the presence of catalytic
atom was 62% [16,17]. The reaction of 1h-18O in the presence amounts of RuCl3 and AgSbF6 was conducted. However, the
of the cationic cobalt catalyst at 30 °C followed by hydrogena- para-isomer 3a was not afforded; 73% of 2a was recovered
Scheme 3: Mechanistic studies, reported in [13].
1977

Beilstein J. Org. Chem. 2018, 14, 1972–1979.
(Scheme 5a). The result indicates that the para-isomer 3a was Ru(III) as a much stronger Lewis acid, while it is also possible
not formed through the ortho-isomer 2a. It is assumed that 3a that the second migration of the alkoxycarbonyloxy group from
was furnished through direct C–O bond formation at the para- ortho to para occurs prior to proton transfer (Scheme 5b) [25].
position through ionic cleavage of the N–O bond by cationic Further mechanistic studies are underway in our laboratory.
Scheme 4: Competitive experiments, reported in [13].
Scheme 5: Mechanism for rearrangement to the para-position.
1978

Beilstein J. Org. Chem. 2018, 14, 1972–1979.
8. Ohta, T.; Shudo, K.; Okamoto, T. Tetrahedron Lett. 1978, 19, 1983.
doi:10.1016/S0040-4039(01)94727-6
Conclusion
The cationic cobalt catalysts enabled the rearrangement reac-
tion of N-alkoxycarbonyloxyanilines to proceed under much
milder reaction conditions, expanding the substrate scope to
more electron-deficient anilines. More importantly, the cobalt
catalyst changes the mode of the rearrangement to an unprece-
dented [1,3]-manner.
9. Porzelle, A.; Woodrow, M. D.; Tomkinson, N. C. O. Eur. J. Org. Chem.
2008, 5135. doi:10.1002/ejoc.200800672
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Experimental
To a mixture of 1k (138.9 mg, 0.4 mmol), CoCl2 (5.2 mg,
0.04 mmol), and AgSbF6 (27.5 mg, 0.08 mmol) under an argon
atmosphere in a pressure vial was added 1,2-dichloroethane
(1.6 mL). Then, the mixture was stirred at 30 °C for 2 hours.
After complete consumption of the starting material 1k, the
mixture was passed through a small pad of silica gel with ethyl
acetate. After removing the solvents in vacuo, the residue was
purified by flash silica gel column chromatography using
hexane/ethyl acetate (3:1) as eluent to obtain 2k (113.9 mg,
82%).
14.Nakamura, I.; Kudo, Y.; Terada, M. Angew. Chem., Int. Ed. 2013, 52,
7536. doi:10.1002/anie.201302751
15.Nakamura, I.; Jo, T.; Ishida, Y.; Tashiro, H.; Terada, M. Org. Lett. 2017,
19, 3059. doi:10.1021/acs.orglett.7b01110
16.The percentage of oxygen-18 content was determined by intensity of
the mass spectrum.
17.Rajendran, G.; Santini, R. E.; Van Etten, R. L. J. Am. Chem. Soc. 1987,
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Supporting Information
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440. doi:10.1246/cl.2011.440
Supporting Information File 1
General procedure and analytic data for obtained products.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-14-172-S1.pdf]
22.Xu, J. Curr. Org. Synth. 2017, 14, 511.
doi:10.2174/1570179413666161021103952
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Acknowledgements
This work was supported by JSPS KAKENHI Grant Number
JP16H00996 in Precisely Designed Catalysts with Customized
Scaffolding.
25.The reaction of a mixture of 1a and 1h in the presence of RuCl3 (10
mol %) and AgSbF6 (30 mol %) gave only the products 2a and 2h
derived from the starting materials; crossover products were not
detected (<3%) by HRMS. This result indicates that the rearrangement
to the para-position proceeds in an intramolecular manner.
ORCID® iDs
Itaru Nakamura - https://orcid.org/0000-0002-1452-5989
Masahiro Terada - https://orcid.org/0000-0002-0554-8652
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