Copper-Catalyzed Coupling of Amines with Carbazates: An Approach to Carbamates

Article abstract of DOI:10.1021/acs.joc.1c01031

A new approach for the preparation of carbamatesviathe copper-catalyzed cross-coupling reaction of amines with alkoxycarbonyl radicals generated from carbazates is described. This environmentally friendly protocol takes place under mild conditions and is compatible with a wide range of amines, including aromatic/aliphatic and primary/secondary substrates.

Full text of DOI:10.1021/acs.joc.1c01031

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Article
Copper-Catalyzed Coupling of Amines with Carbazates: An
Approach to Carbamates
Song-Ning Wang, Guo-Yu Zhang, Adedamola Shoberu, and Jian-Ping Zou*
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ABSTRACT: A new approach for the preparation of carbamates via the copper-catalyzed cross-coupling reaction of amines with
alkoxycarbonyl radicals generated from carbazates is described. This environmentally friendly protocol takes place under mild
conditions and is compatible with a wide range of amines, including aromatic/aliphatic and primary/secondary substrates.
sioned that alkoxycarbonyl radicals generated from carbazates
under suitable conditions could oxidatively couple with amines
to directly afford carbamates. Such a strategy would eliminate
the synthetic limitations, as well as toxicity issues commonly
encountered with traditional methods.
INTRODUCTION
■
The carbamate moiety plays an important role in organic
synthesis and modern drug discovery.1−3 In addition to being a
common feature in a number of approved drugs and prodrugs,
it is widely employed in industries such as agrochemicals and
polymers (Figure 1).4−6 Structurally, carbamates are like a
hybrid of amide and ester, and in general, exhibit very good
chemical stability. Hence, they are commonly employed as
protecting groups for amines7 as well as amino acids in peptide
chemistry.8,9 Owing to the enormous importance of the
carbamate motif, there is strong interest in the development of
sustainable preparatory methods.
Over the years, carbamates have been traditionally prepared
via the rearrangement of amides10−12 or acyl azides,13,14 the
reaction of phosgene or its derivative with amines,15−17 the
reductive carbonylation of nitroaromatics,18,19 the oxidative
carbonylation of amines,20−24 the carboxylation of
amines,25−27 and the reaction of alcohols with isocyanates28
(Scheme 1). In recent times, significant efforts have been
devoted to the development of modified reagents toward
optimizing the traditional procedures.29−34 Nevertheless, the
operational complexity, hazardous by-products, and the use of
toxic and/or specialized reagents are major limitations. In
recent times, oxidative carbonylation reactions utilizing
carbazates as a source of alkoxycarbonyl radicals have emerged
as a powerful means of synthesizing value-added products.35−40
For instance, Taniguchi et al.35 disclosed an iron-catalyzed
oxidative addition of alkoxycarbonyl radicals generated from
methyl carbazates, to alkenes to furnish β-hydroxyesters.
Furthermore, the radical alkoxycarbonylation of 2-isocyanobi-
phenyls with carbazates has been studied by other groups
toward the preparation of phenanthridine derivatives.37,38
However, these examples have largely focused on the addition
of multiple bonds. In recent times, our group has been active in
the development of novel oxidative coupling processes for the
construction of carbon−heteroatom bonds.41−44 We envi-
RESULTS AND DISCUSSION
■
Our initial investigations began by examining the oxidative
coupling reaction of aniline 1a and methyl carbazate 2 in
MeCN at 80 °C (Table 1). In the presence of an Fe(II)-
catalyst and tert-butyl hydroperoxide (TBHP), the reaction
could only afford trace amounts of the desired product 3a
(entry 1). Desirably, the use of CuCl2 in lieu of FeCl2 furnished
product 3a in 70% yield (entry 2). As a result of the efficacy
exhibited by CuCl2, a variety of copper salts were tested, and
CuBr2 was determined to be the most effective, affording
product 3a in 82% yield (entries 2−9). Notably, the reaction
efficiency was still maintained at ambient temperature (entries
4, 10, and 11). Finally, after a systematic screening of the ideal
amount of methyl carbazate, CuBr2 catalyst, and TBHP
(entries 12−22), the optimized conditions were determined to
be: aniline 1a (1 mmol), methyl carbazate 2 (5 equiv), CuBr2
(10 mol %), and TBHP (7.5 equiv) in CH3CN at room
temperature (rt) for 4 h to furnish 86% yield of 3a (entry 14).
With the optimized reaction conditions in hand, the scope of
the reaction with respect to amines was examined. As
illustrated in Table 2, the reaction was amenable to a variety
of amines, including aromatic and aliphatic substrates. In the
case of anilines bearing meta- or para-electron-donating groups
Received: May 1, 2021
Published: June 18, 2021
https://doi.org/10.1021/acs.joc.1c01031
© 2021 American Chemical Society
J. Org. Chem. 2021, 86, 9067−9075
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Figure 1. Examples of important carbamate-containing compounds.
a
Scheme 1. Traditional Methods for the Synthesis of
Carbamates
Table 1. Optimization of the Reaction
b
entry
1a/2
catalyst (10 mol %)
temp. (°C)
yield (%)
1
2
3
4
5
6
7
8
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:1.5
1:3
1:5
1:6
1:5
1:5
1:5
1:5
1:5
1:5
1:5
FeCl2·4H2O
CuCl2
CuCl
CuBr2
CuBr
Cul
CuO
Cu(OAc)2
CuSO4·5H2O
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
80
80
80
80
80
80
80
80
80
25
0
25
25
25
25
25
25
25
25
25
25
25
trace
70
58
82
57
58
15
10
30
83
82
54
78
86
82
51
64
75
86
52
0
9
10
11
12
13
14
15
16
17
18
19
c
such as methyl and methoxy, the reaction gave the
corresponding carbamates in good yields (3b−3e). However,
the yields of anilines bearing ortho-substituents were notice-
ably affected by steric hindrance as exemplified by products 3f
and 3g. Moreover, this effect was more pronounced with the
bulky 2-tert-butyl group (3h). In addition, the reactions of
anilines bearing electron-withdrawing groups such as halogens
(3i−3l), ethoxycarbonyl (3m), formyl (3n), and acetyl (3o) all
afforded the desired carbamates in moderate to good yields.
However, 4-nitroaniline could afford the corresponding
product 3p only in trace amounts, probably due to the greater
electron-pulling effect of the −NO2 group. Meanwhile, the
reaction of 2-naphthylamine occurred smoothly to provide the
desired product 3q in 67% yield. Otherwise, the reaction took
place readily with aliphatic amines: product 3r was obtained in
d
e
f
CuBr2
CuBr2
g
20
21
22
h
CuBr2
82
a
Reaction conditions: 1a (1 mmol), 2a (1−6 equiv), metal catalyst
(10 mol %), and TBHP (70% in H2O, 7.5 equiv) in CH3CN (5 mL)
b
c
d
under air. Isolated yield. TBHP (70% in H2O, 3 equiv). TBHP
(70% in H2O, 4.5 equiv). TBHP (70% in H2O, 6 equiv). TBHP
(70% in H2O, 9 equiv). 5 mol % of CuBr2 used. 15 mol % of CuBr2
used.
e
f
g
h
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a
Table 2. Reaction Scope of Amines
a
Reactions were conducted using 1 (1 mmol), 2 (5 mmol), CuBr2 (10 mol %), and TBHP (70% in water, 7.5 mmol) in CH3CN (5 mL) at rt for 4
b
c
d
h. Isolated yield. N.D. = not detected. N.R. = no reaction.
79% yield from 1-hexylamine. Next, we examined the coupling
reaction of secondary amines with methyl carbazate. With the
exception of diphenylamine (3s), the coupling products
corresponding to N-methylaniline (3t), morpholine (3u),
piperidine (3v), N-methylbenzylamine (3w), and N-methyl-
cyclohexylamine (3y) were obtained in satisfactory yields.
Unfortunately, other than the reaction of 2-aminopyridine with
methyl carbazate that gave the expected product 3aa in 65%
yield, substrates both heteroaromatic amines like indole and
pyrrole, and in which the amine nitrogen atom is electron-
deficient such as benzamide and benzenesulfonamide were
completely inert under standard conditions (Table 2, 4−7).
Next, the scope of the reaction with respect to carbazates
was also examined under optimized reaction conditions. As
illustrated in Table 3, the reactions of carbazates like ethyl
carbazate, tert-butyl carbazate, phenyl carbazate, and benzyl
carbazate have been carried out, and the results indicated that
all of the reactions gave the expected products in satisfactory
yields (52−79%, see Table 3, 3ab−3ae).
To gain an insight into the reaction mechanism, some
control experiments were carried out (Scheme 2). First, well-
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a
Table 3. Reaction Scope of Carbazates
a
Reactions were conducted using 1a (1 mmol), 2 (5 mmol), CuBr2 (10 mol %), and TBHP (70% in water, 7.5 mmol) in CH3CN (5 mL) at rt for
4 h.
Scheme 2. Control Experiments
Scheme 3. Proposed Mechanism
known radical trapping agents such as 2,2,6,6-tetramethyl-1-
piperidinyloxy (TEMPO) and butylated hydroxytoluene
(BHT) were added to the coupling reaction of aniline 1a
and methyl carbazate 2; and as expected, the formation of 3a
was significantly inhibited (Scheme 2a). Moreover, in the
absence of aniline 1a, compound 8 presumably formed from
the capture of the alkoxycarbonyl radical by TEMPO was
isolated in 80% yield (Scheme 2b). This result suggests the
generation of alkoxycarbonyl radicals in the reaction medium
under the standard conditions.
On the basis of results obtained above and previous
literature reports,35,37−40,58−62 a plausible mechanism is
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4-methylaniline (107 mg, 1 mmol) as a white solid (145 mg, 88%),
mp = 99.9−100.4 °C. IR: ν 3345, 2953, 1707, 1616, 1555, 1493, 1438
cm−1. 1H NMR (CDCl3, 400 MHz): δ 7.29−7.22 (m, 2H), 7.14−7.08
(m, 2H), 6.55 (s, 1H), 3.77 (s, 3H), 2.30 (s, 3H); 13C{1H} NMR
(CDCl3, 100 MHz): δ 153.8, 134.8, 132.6, 129.0, 118.4, 51.8, 20.3.
HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C9H11NO2Na
188.0687, found 188.0693.
proposed as depicted in Scheme 3. Initially, methyl carbazate
2a was oxidized by the CuII/TBHP system to form a diazene
intermediate A. Further, the proton abstraction step generates
the diazenyl radical B, from which the alkoxycarbonyl radical C
is formed, alongside the release of molecular nitrogen.35,37−40
Radical C can be trapped by complex D formed in situ from
CuII species and amine 1 to produce complex E, followed by
reductive elimination to afford product 3 and release of CuI
species to participate in the next cycle reaction.58−62
Methyl (3-Methylphenyl)carbamate (3c).45 According to the
general procedure A, compound 3c was obtained from the reaction of
3-methylaniline (107 mg, 1 mmol) as a yellow solid (139 mg, 87%),
mp = 70.5−72.8 °C. IR: ν 3345, 2952, 1707, 1616, 1555, 1492, 1438
cm−1. 1H NMR (CDCl3, 400 MHz): δ 7.24−7.13 (m, 3H), 6.91−6.85
(m, 1H), 6.63 (s, 1H), 3.77 (s, 3H), 2.33 (s, 3H); 13C{1H} NMR
(CDCl3, 100 MHz): δ 154.1, 139.0, 137.8, 128.9, 124.3, 119.5, 116.0,
52.3, 21.5. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for
C9H11NO2Na 188.0687, found 188.0685.
CONCLUSIONS
■
In summary, we have developed a new approach for the
synthesis of carbamates via the copper-catalyzed cross-coupling
reaction of primary and secondary amines with alkoxycarbonyl
radicals generated from carbazates. Notably, the reaction
proceeds with high efficiency utilizing inexpensive and easy-to-
handle reagents. The benign protocol was shown to be
compatible with a wide range of amines including aromatic/
aliphatic and primary/secondary substrates.
Methyl (4-Methoxyphenyl)carbamate (3d).46 According to the
general procedure A, compound 3d was obtained from the reaction of
4-methoxyaniline (123 mg, 1 mmol) as a white solid (165 mg, 91%),
mp = 108.1−108.9 °C. IR: ν 3323, 2953, 2938, 1714, 1610, 1598,
1
1541, 1497, 1438 cm−1. H NMR (CDCl3, 400 MHz): δ 7.36−7.22
(m, 2H), 6.95−6.77 (m, 2H), 6.56 (s, 1H), 3.78 (s, 3H), 3.75 (s,
3H); 13C{1H} NMR (CDCl3, 100 MHz): δ 156.0, 154.5, 130.9,
120.7, 114.3, 55.5, 52.3. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for
C9H11NO3Na 204.0637, found 204.0632.
EXPERIMENTAL SECTION
■
1
General Remarks. H and 13C NMR spectra were recorded in
Methyl (3-Methoxyphenyl)carbamate (3e).45 According to the
general procedure A, compound 3e was obtained from the reaction of
3-methoxyaniline (123 mg, 1 mmol) as a yellow liquid (163 mg,
90%). IR: ν 3324, 2953, 2939, 1714, 1609, 1598, 1541, 1497, 1457
cm−1. 1H NMR (CDCl3, 400 MHz): δ 7.18 (t, J = 8.2 Hz, 1H), 7.12
(s, 1H), 6.89−6.85 (m, 1H), 6.77 (s, 1H), 6.64−6.59 (m, 1H), 3.79
(s, 3H), 3.77 (s, 3H); 13C{1H} NMR (CDCl3, 100 MHz): δ 159.8,
153.5, 138.7, 129.3, 110.4, 108.7, 103.9, 54.8, 51.8. HRMS (ESI-
TOF) m/z: [M + Na]+ calcd for C9H11NO3Na 204.0637, found
204.0640.
CDCl3 on a Varian-Inova 300 or 400 MHz spectrometer. Spectra
1
were referenced to residual chloroform (δ = 7.26 ppm, H; 77.16
ppm, 13C). Chemical shifts were reported in parts per million (ppm)
relative to tetramethylsilane (TMS) (δ). Multiplicities were indicated
by s (singlet), d (doublet), t (triplet), q (quartet), and m (multiplet),
and coupling constants J were reported in hertz (Hz). High-resolution
mass spectra were recorded on a MicroMass-TOF machine (ESI).
Column chromatography purifications were performed using 300−
400 mesh silica gel.
Starting Materials. Unless otherwise noted, all reagents were
purchased from commercial suppliers and used without further
purification.
Methyl (2-Methylphenyl)carbamate (3f).50 According to the
general procedure A, compound 3f was obtained from the reaction
of 2-methylaniline (107 mg, 1 mmol) as a yellow liquid (96 mg, 58%).
General Procedure A: Preparation of Carbamates 3. To a
Schlenk tube equipped with a rubber septum was successively added
amine 1 (1.0 mmol, 1.0 equiv), methyl carbazate 2a (450 mg, 5.0
mmol, 5.0 equiv), CuBr2 (22.3 mg, 0.1 mmol, 10 mol %), and
acetonitrile (5 mL). The tube was capped and TBHP (70% in H2O)
(0.7 mL, 7.5 mmol, 7.5 equiv) was added dropwise. Then, the mixture
was stirred for 4 h at room temperature. After the completion of the
reaction (indicated by thin-layer chromatography (TLC)), the
reaction mixture was concentrated under reduced pressure and the
crude mixture was purified by column chromatography using
petroleum ether/ethyl acetate 20:1 (v/v) as the eluent to obtain
the pure products 3.
1
IR: ν 3344, 2955, 1707, 1616, 1554, 1493, 1438 cm−1. H NMR
(CDCl3, 400 MHz): δ 7.86−7.61 (m, 1H), 7.24−7.18 (m, 1H),
7.18−7.13 (m, 1H), 7.07−7.00 (m, 1H), 6.42 (s, 1H), 3.78 (s, 3H),
2.25 (s, 3H); 13C{1H} NMR (CDCl3, 100 MHz): δ 153.9, 135.3,
132.5, 129.9, 129.3, 126.4, 123.8, 51.9, 17.2. HRMS (ESI-TOF) m/z:
[M + Na]+ calcd for C9H11NO2Na 188.0687, found 188.0684.
Methyl (2-Methoxyphenyl)carbamate (3g).48 According to the
general procedure A, compound 3g was obtained from the reaction of
2-methoxyaniline (123 mg, 1 mmol) as a yellow liquid (156 mg,
1
64%). H NMR (CDCl3, 400 MHz): δ 8.16−8.04 (m, 1H), 7.34−
7.23 (m, 1H), 7.02−6.93 (m, 2H), 6.86−6.81 (m, 1H), 3.93 (s, 3H),
3.77 (s, 3H); 13C{1H} NMR (CDCl3, 100 MHz): δ 153.5, 147.1,
127.1, 122.3, 120.6, 117.7, 109.5, 55.1, 51.7. HRMS (ESI-TOF) m/z:
[M + Na]+ calcd for C9H11NO3Na 204.0637, found 204.0634.
Methyl (4-Fluorophenyl)carbamate (3i).47 According to the
general procedure A, compound 3i was obtained from the reaction
of 4-fluoroaniline (111 mg, 1 mmol) as a white solid (142 mg, 84%),
mp = 91.1−92.5 °C. IR: ν 3317, 2937, 1713, 1598, 1534, 1491, 1443
cm−1. 1H NMR (CDCl3, 400 MHz): δ 7.38−7.29 (m, 2H), 7.07−6.91
(m, 2H), 6.67 (s, 1H), 3.77 (s, 3H); 13C{1H} NMR (CDCl3, 100
MHz): δ 158.5 (d, J = 242.1 Hz), 153.8, 133.3, 120.0, 115.2 (d, J =
22.6 Hz), 51.9; 19F NMR (376 MHz, CDCl3): δ −119.5. HRMS
(ESI-TOF) m/z: [M + H]+ calcd for C8H9FNO2 170.0617, found
170.0625.
Procedure for the Trapping of the Alkoxycarbonyl Radical.
To a Schlenk tube equipped with a rubber septum was successively
added methyl carbazate 2a (90 mg, 1.0 mmol, 1.0 equiv), CuBr2 (22.3
mg, 0.1 mmol, 10 mol %), 2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO, 187 mg, 1.2 mmol, 1.2 equiv), and acetonitrile (5 mL).
The tube was capped and TBHP (70% in H2O) (0.2 mL, 2.0 mmol,
2.0 equiv) was added dropwise. Then, the mixture was stirred for 4 h
at room temperature. Afterward, the reaction mixture was
concentrated under reduced pressure and the crude mixture was
purified by column chromatography using petroleum ether/ethyl
acetate 20:1 (v/v) as the eluent to obtain the pure compound 8.
Methyl Phenylcarbamate (3a).45 According to the general
procedure A, compound 3a was obtained from aniline (93 mg, 1
mmol) as a yellow solid (127 mg, 86%), mp = 42.3−43.8 °C. IR: ν
Methyl (4-Chlorophenyl)carbamate (3j).45 According to the
general procedure A, compound 3j was obtained from the reaction
of 4-chloroaniline (127 mg, 1 mmol) as a yellow solid (152 mg, 82%),
mp = 126.1−126.9 °C. IR: ν 3341, 2971, 2892, 1698, 1599, 1542,
1
3342, 2950, 1707, 1599, 1543, 1490, 1435 cm−1. H NMR (CDCl3,
400 MHz): δ 7.43−7.37 (m, 2H), 7.34−7.27 (m, 2H), 7.10−7.02 (m,
1H), 6.91 (s, 1H), 3.77 (s, 3H); 13C{1H} NMR (CDCl3, 100 MHz):
δ 153.7, 137.5, 128.5, 123.0, 118.3, 51.8. HRMS (ESI-TOF) m/z: [M
+ Na]+ calcd for C8H9NO2Na 174.0531, found 174.0524.
1
1489, 1435 cm−1. H NMR (CDCl3, 400 MHz): δ 7.35−7.30 (m,
Methyl (4-Methylphenyl)carbamate (3b).46 According to the
general procedure A, compound 3b was obtained from the reaction of
2H), 7.28−7.22 (m, 2H), 6.72 (s, 1H), 3.77 (s, 3H); 13C{1H} NMR
(CDCl3, 100 MHz): δ 153.5, 136.0, 128.6, 128.0, 119.4, 52.0. HRMS
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(ESI-TOF) m/z: [M + Na]+ calcd for C8H8ClNO2Na 208.0141,
found 208.0147.
Methyl Morpholine-4-carboxylate (3u).52 According to the
general procedure A, compound 3u was obtained from the reaction
of morpholine (87 mg, 1 mmol) as a clear liquid (93 mg, 64%). H
Methyl (4-Bromophenyl)carbamate (3k).45 According to the
general procedure A, compound 3k was obtained from the reaction of
4-bromoaniline (172 mg, 1 mmol) as a yellow solid (191 mg, 83%),
mp = 135.1−136.2 °C. IR: ν 3341, 2972, 1699, 1599, 1543, 1490,
1
NMR (CDCl3, 400 MHz): δ 3.70 (s, 3H), 3.68−3.61 (m, 4H), 3.49−
3.41 (m, 4H); 13C{1H} NMR (CDCl3, 100 MHz): δ 155.9, 66.6,
52.7, 44.1. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C6H12NO3
146.0817, found 146.0809.
1
1435 cm−1. H NMR (CDCl3, 400 MHz): δ 7.43−7.38 (m, 2H),
Methyl 4-Methylpiperidine-1-carboxylate (3v).62 According to
the general procedure A, compound 3v was obtained from the
reaction of piperidine (85 mg, 1 mmol) as a colorless liquid (104 mg,
7.32−7.25 (m, 2H), 6.69 (s, 1H), 3.77 (s, 3H); 13C{1H} NMR
(CDCl3, 100 MHz): δ 153.4, 136.5, 131.5, 119.8, 115.5, 52.0. HRMS
(ESI-TOF) m/z: [M + Na]+ calcd for C8H8BrNO2Na 251.9636,
found 251.9640.
1
73%). H NMR (CDCl3, 400 MHz): δ 3.66 (s, 3H), 3.59−3.34 (m,
Methyl (3-Bromophenyl)carbamate (3l).45 According to the
general procedure A, compound 3l was obtained from the reaction
of 3-bromoaniline (172 mg, 1 mmol) as a yellow solid (195 mg, 85%),
4H), 1.70−1.44 (m, 6H); 13C{1H} NMR (CDCl3, 100 MHz): δ
156.0, 52.4, 44.8, 25.7, 24.4. HRMS (ESI-TOF) m/z: [M + H]+ calcd
for C7H14NO2 144.1025, found 144.1029.
Methyl Benzyl(methyl)carbamate (3w).51 According to the
general procedure A, compound 3w was obtained from the reaction
of N-methyl-1-phenylmethanamine (121 mg, 1 mmol) as a colorless
liquid (111 mg, 62%). IR: ν 2957, 2887, 1706, 1598, 1541, 1498,
1
mp = 87.5−88.6 °C. H NMR (CDCl3, 400 MHz): δ 7.64 (s, 1H),
7.31−7.25 (m, 1H), 7.21−7.12 (m, 2H), 6.68 (s, 1H), 3.78 (s, 3H);
13C{1H} NMR (CDCl3, 100 MHz): δ 153.4, 138.7, 129.8, 125.9,
122.2, 121.1, 116.7, 52.1. HRMS (ESI-TOF) m/z: [M + Na]+ calcd
for C8H8BrNO2Na 251.9636, found 251.9631.
1
1448 cm−1. H NMR (CDCl3, 400 MHz): δ 7.40−7.32 (m, 2H),
Ethyl 4-((Methoxycarbonyl)amino)benzoate (3m).48 According
to the general procedure A, compound 3m was obtained from the
reaction of ethyl 4-aminobenzoate (165 mg, 1 mmol) as a yellow solid
(156 mg, 70%), mp = 100.5−101.8 °C. 1H NMR (CDCl3, 400 MHz):
δ 8.06−7.94 (m, 2H), 7.50−7.42 (m, 2H), 6.87 (s, 1H), 4.35 (q, J =
7.1 Hz, 2H), 3.79 (s, 3H), 1.38 (t, J = 7.1 Hz, 3H); 13C{1H} NMR
(CDCl3, 100 MHz): δ 166.2, 153.5, 142.0, 130.9, 125.2, 117.5, 80.8,
52.6, 14.4. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for
C11H13NO4Na 246.0742, found 246.0738.
7.31−7.16 (m, 3H), 4.60−4.40 (m, 2H), 3.77 (s, 3H), 3.00−2.70 (m,
3H),; 13C{1H} NMR (CDCl3, 100 MHz): δ 157.4, 157.1, 137.5,
128.6, 127.8, 127.4, 127.2, 52.8, 52.6, 52.3, 34.4, 33.6. HRMS (ESI-
TOF) m/z: [M + H]+ calcd for C10H14NO2 180.1025, found
180.1021.
Methyl Cyclohexyl(methyl)carbamate (3y).51 According to the
general procedure A, compound 3y was obtained from the reaction of
N-methylcyclohexanamine (113 mg, 1 mmol) as a colorless liquid
(133 mg, 78%). 1H NMR (CDCl3, 400 MHz): δ 4.01−3.71 (m, 1H),
3.63 (s, 3H), 2.70 (s, 3H), 1.77−1.55 (m, 5H), 1.39−1.22 (m, 4H),
1.08−0.96 (m, 1H); 13C{1H} NMR (CDCl3, 100 MHz): δ 156.8,
54.8, 52.4, 30.2, 28.1, 25.7, 25.5. HRMS (ESI-TOF) m/z: [M + H]+
calcd for C9H18NO2 172.1338, found 172.1341.
Methyl (4-Formylphenyl)carbamate (3n).49 According to the
general procedure A, compound 3n was obtained from the reaction of
4-aminobenzaldehyde (149 mg, 1 mmol) as a yellow solid (111 mg,
1
62%), mp = 155.1−156.2 °C. H NMR (CDCl3, 400 MHz): δ 9.91
(s, 1H), 7.90−7.78 (m, 2H), 7.61−7.52 (m, 2H), 6.95 (s, 1H), 3.81
(s, 3H); 13C{1H} NMR (CDCl3, 100 MHz): δ 190.5, 152.9, 143.1,
131.2, 130.8, 117.5, 52.3. HRMS (ESI-TOF) m/z: [M + Na]+ calcd
for C9H9NO3Na 202.0480, found 202.0474.
Methyl Pyridin-2-ylcarbamate (3aa).53 According to the general
procedure A, compound 3aa was obtained from the reaction of 2-
aminopyridine (94 mg, 1 mmol) as a white solid (98 mg, 65%), mp =
129.8−130.2 °C. IR: ν 3260, 2979, 1731, 1657, 1588, 1540, 1438
cm−1. 1H NMR (CDCl3, 400 MHz): δ 10.55 (s, 1H), 8.34 (d, J = 4.4
Hz, 1H), 8.05 (d, J = 8.4 Hz, 1H), 7.68 (t, J = 7.8 Hz, 1H), 7.02−6.92
(m, 1H), 3.82 (s, 3H); 13C{1H} NMR (CDCl3, 100 MHz): δ 154.2,
152.6, 147.5, 138.6, 118.4, 112.6, 52.3. HRMS (ESI-TOF) m/z: [M +
Na]+ calcd for C7H8N2O2Na 175.0483, found 175.0490.
Methyl (4-Acetylphenyl)carbamate (3o).48 According to the
general procedure A, compound 3o was obtained from the reaction
of 4-acetylaniline (135 mg, 1 mmol) as a yellow solid (139 mg, 72%),
mp = 165.1−166.5 °C. IR: ν 3264, 2914, 1732, 1667, 1588, 1539,
1438, 1414 cm−1. 1H NMR (CDCl3, 400 MHz): δ 7.92 (d, J = 8.7 Hz,
2H), 7.49 (d, J = 8.6 Hz, 2H), 7.13 (s, 1H), 3.79 (s, 3H), 2.56 (s,
3H); 13C{1H} NMR (CDCl3, 100 MHz): δ 196.5, 153.1, 142.0,
131.7, 129.4, 117.1, 52.1, 25.9. HRMS (ESI-TOF) m/z: [M + Na]+
calcd for C10H11NO3Na 216.0637, found 216.0629.
Ethyl Phenylcarbamate (3ab).54 According to the general
procedure A, compound 3ab was obtained from the reaction of
ethyl carbazate (520 mg, 5 mmol) as a yellow liquid (130 mg, 79%).
IR: ν 3317, 2987, 2968, 1716, 1599, 1540, 1489, 1444 cm−1. 1H NMR
(CDCl3, 400 MHz): δ 7.38 (d, J = 7.7 Hz, 2H), 7.27 (t, J = 7.6 Hz,
2H), 7.03 (t, J = 7.3 Hz, 1H), 6.93 (s, 1H), 4.21 (q, J = 7.1 Hz, 2H),
1.28 (t, J = 7.1 Hz, 3H); 13C{1H} NMR (CDCl3, 100 MHz): δ 153.9,
138.1, 129.0, 123.3, 118.8, 61.2, 14.6. HRMS (ESI-TOF) m/z: [M +
Na]+ calcd for C9H11NO2Na 188.0687, found 188.0692.
Methyl Naphthalen-2-ylcarbamate (3q).49 According to the
general procedure A, compound 3q was obtained from the reaction
of naphthalen-2-amine (143 mg, 1 mmol) as a white solid (135 mg,
1
67%), mp = 112.5−113.8 °C. H NMR (CDCl3, 400 MHz): δ 7.98
(s, 1H), 7.81−7.74 (m, 3H), 7.49−7.42 (m, 1H), 7.42−7.35 (m, 2H),
6.83 (s, 1H), 3.82 (s, 3H); 13C{1H} NMR (CDCl3, 100 MHz): δ
153.7, 134.8, 133.5, 129.7, 128.4, 127.1, 126.9, 126.1, 124.2, 118.7,
114.4, 52.0. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for
C12H11NO2Na 224.0687, found 224.0683.
tert-Butyl Phenylcarbamate (3ac).55 According to the general
procedure A, compound 3ac was obtained from the reaction of tert-
butyl carbazate (660 mg, 5 mmol) as a white solid (130 mg, 68%), mp
= 135.2−136.6 °C. IR: ν 3311, 2985, 2963, 1687, 1597, 1528, 1488,
1440 cm−1. 1H NMR (CDCl3, 400 MHz): δ 7.40−7.23 (m, 4H), 7.02
(t, J = 7.3 Hz, 1H), 6.56 (bs, 1H), 1.51 (s, 9H); 13C{1H} NMR
(CDCl3, 100 MHz): δ 152.8, 138.4, 129.0, 123.0, 118.6, 80.5, 28.4.
HRMS (ESI-TOF) m/z: [M + H]+ calcd for C11H16NO2 194.1181,
found 194.1189.
Methyl Hexylcarbamate (3r).51 According to the general
procedure A, compound 3r was obtained from the reaction of 1-
hexylaniline (101 mg, 1 mmol) as a yellow liquid (126 mg, 79%). IR:
1
ν 3348, 2959, 2932, 1700, 1557, 1457 cm−1. H NMR (CDCl3, 400
MHz): δ 4.75 (s, 1H), 3.63 (s, 3H), 3.14−2.98 (m, 2H), 1.51−1.33
(m, 2H), 1.30−1.11 (m, 6H), 0.88−0.70 (m, 3H); 13C{1H} NMR
(CDCl3, 100 MHz): δ 156.6, 51.3, 40.5, 30.9, 29.4, 25.8, 22.0, 13.4.
HRMS (ESI-TOF) m/z: [M + H]+ calcd for C8H18NO2 160.1338,
found 160.1344.
Phenyl Phenylcarbamate (3ad).56 According to the general
procedure A, compound 3ad was obtained from the reaction of
phenyl hydrazinecarboxylate (760 mg, 5 mmol) as a white solid (129
1
mg, 57%), mp = 77.2−78.5 °C. H NMR (CDCl3, 400 MHz): δ
Methyl N-Methyl(phenyl)carbamate (3t).45 According to the
general procedure A, compound 3t was obtained from the reaction of
N-methylaniline (107 mg, 1 mmol) as a yellow liquid (132 mg, 80%).
1H NMR (CDCl3, 400 MHz): δ 7.39−7.31 (m, 2H), 7.28−7.14 (m,
3H), 3.70 (s, 3H), 3.30 (s, 3H); 13C{1H} NMR (CDCl3, 100 MHz):
δ 156.1, 143.3, 128.9, 126.1, 125.8, 52.8, 37.8. HRMS (ESI-TOF) m/
z: [M + Na]+ calcd for C9H11NO2Na 188.0687, found 188.0689.
7.48−7.37 (m, 4H), 7.36−7.29 (m, 2H), 7.28−7.17 (m, 3H), 7.16−
7.00 (m, 2H); 13C{1H} NMR (CDCl3, 100 MHz): δ 151.7, 150.6,
137.4, 129.5, 129.2, 125.8, 123.9, 121.7, 118.8. HRMS (ESI-TOF) m/
z: [M + Na]+ calcd for C13H11NO2Na 236.0687, found 236.0686.
Benzyl Phenylcarbamate (3ae).57 According to the general
procedure A, compound 3ae was obtained from the reaction of
benzyl hydrazinecarboxylate (830 mg, 5 mmol) as a white solid (110
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mg, 52%), mp = 135.4−136.5 °C. IR: ν 3271, 2987, 2900, 1688, 1599,
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1965, 65, 567−602.
1
1545, 1493, 1444 cm−1. H NMR (CDCl3, 400 MHz): δ 7.43−7.22
(m, 9H), 7.04 (t, J = 7.3 Hz, 1H), 6.82 (bs, 1H), 5.17 (s, 2H);
13C{1H} NMR (CDCl3, 100 MHz): δ 153.5, 137.9, 136.1, 129.1,
128.7, 128.4, 128.4, 123.6, 118.8, 67.0. HRMS (ESI-TOF) m/z: [M +
Na]+ calcd for C14H13NO2Na 250.0844, found 250.0847.
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Methyl (2,2,6,6-Tetramethylpiperidin-1-yl) Carbonate (8).35
According to the procedure described for the radical trapping
experiment, compound 8 was obtained as a yellow liquid (172 mg,
1
80%). H NMR (CDCl3, 400 MHz): δ 3.79 (s, 3H), 1.71−1.48 (m,
5H), 1.42−1.35 (m, 1H), 1.15 (s, 6H), 1.11 (s, 6H); 13C{1H} NMR
(CDCl3, 100 MHz): δ 157.4, 60.5, 55.0, 39.2, 31.5, 20.3, 16.9. HRMS
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216.1605.
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ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.joc.1c01031.
1H and 13C{1H} NMR spectra of all compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
Jian-Ping Zou − Key Laboratory of Organic Synthesis of
Jiangsu Province, College of Chemistry and Chemical
Engineering, Soochow University, Suzhou, Jiangsu 215123,
China; orcid.org/0000-0002-8092-9527;
Email: jpzou@suda.edu.cn
■
(12) Gogoi, P.; Konwar, D. An Efficient Modification of the
Hofmann Rearrangement: Synthesis of Methyl Carbamates. Tetrahe-
dron Lett. 2007, 48, 531−533.
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Synthetic Uses. Chem. Rev. 1988, 88, 297−368.
Authors
Song-Ning Wang − Key Laboratory of Organic Synthesis of
Jiangsu Province, College of Chemistry and Chemical
Engineering, Soochow University, Suzhou, Jiangsu 215123,
China
Guo-Yu Zhang − Key Laboratory of Organic Synthesis of
Jiangsu Province, College of Chemistry and Chemical
Engineering, Soochow University, Suzhou, Jiangsu 215123,
China
Adedamola Shoberu − Key Laboratory of Organic Synthesis
of Jiangsu Province, College of Chemistry and Chemical
Engineering, Soochow University, Suzhou, Jiangsu 215123,
China
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.joc.1c01031
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Author Contributions
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.-P.Z. acknowledges financial support from the National
Natural Science Foundation of China (No. 21472133), the
Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD), and Key Laboratory of
Organic Synthesis of Jiangsu Province (KJS1749).
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