One stone two birds: Cobalt-catalyzed in situ generation of isocyanates and benzyl alcohols for the synthesis of N-aryl carbamates

Article abstract of DOI:10.1039/c9ob00924h

An efficient method for the synthesis of N-aryl carbamates from N-Boc-protected amines has been developed. The cobalt-catalyzed in situ generation of isocyanates from N-Boc-protected amines and benzyl alcohols from benzyl formates has been achieved for the first time, which in turn furnished the corresponding benzyl carbamates in moderate to high yields. The reaction was catalyzed by CoI₂ with tris-(4-dimethylaminophenyl)-phosphine as the ligand and zinc powder as the reductant. The developed reaction conditions were found to be compatible for aromatic amines with both electron-donating and -withdrawing substituents.

Full text of DOI:10.1039/c9ob00924h

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10.1039/C9OB00924H.
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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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DOI: 10.1039/C9OB00924H
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One Stone Two Birds: Cobalt Catalyzed in situ Generation of
Isocyanate and Benzyl alcohol for the Synthesis of N-aryl
Carbamates
Received 00th January 20xx,
Accepted 00th January 20xx
Sida Li,a,§ Ruhima Khan,a,§ Xia Zhang,a Yong Yang,*b Zheting Wang,a Yong Zhan,b Yuze Dai,a Yue-e
DOI: 10.1039/x0xx00000x
Liu,a and Baomin Fan*a
An efficient method for the synthesis of N-aryl carbamates from has been continuous efforts to improve the reaction pathway
N-Boc protected amines has been developed. Cobalt catalyzed in and for more environmentally benign route for the synthesis
situ generation of isocyanates from N-Boc protected amines and of carbamates and thiocarbamates.15
benzyl alcohols from benzyl formates has been achieved for the
One of the most widely used precursor for the synthesis of
first time which in turn furnished the corresponding benzyl carbamates is isocyanate group. There are various methods for
carbamates in moderate to high yields. The reaction was catalyzed the preparation of isocyanate intermediates such as Hoffmann
by CoI2 with tris-(4-dimethylaminophenyl)-phosphine as the ligand or Curtius rearrangements, Lossen rearrangement.16 The
and Zinc powder as the reductant. The developed reaction synthesis of carbamates via in situ generation of isocyanate
condition was found to be compatible for aromatic amines with intermediate has been furnished by different groups. Isobe
both electron donating and withdrawing substituents.
and his group has reported the transformation of
trichloroacetamides into the easily deprotectable carbamates
via in situ generated isocyanate in the presence of CuCl and n-
BuN4Cl.17 The synthesis of carbamates via in situ generation of
aryl ioscyanates has been furnished through the cross-coupling
of aryl boronic acids or aryl halides with potassium and sodium
cyanates catalyzed by palladium or copper.18 Kim and Lee have
developed the one-pot synthesis of carbamates and
thiocarbamates via in situ generated isocyanates which reacts
with alcohols and thiols to give the corresponding carbamates
and thiocarbamates.19 Kim and co-workers have achieved a
mild and efficient route for synthesis of N-arylcarbamates by
Copper-catalyzed Chan−Lam C−N cross-coupling reactions
under neutral conditions.20 We herein report the successful
synthesis of carbamates from N-protected anilines and benzyl
formates via in situ generated isocyanate and benzyl alcohol
intermediates. The reaction is catalyzed by inexpensive cobalt
(II) iodide in the presence of tris-(4-dimethylaminophenyl)-
phosphine ligand and zinc powder as reductant. Because of
simple operation, mild reaction conditions, fairly good yields
and use of easily available reagents, the developed protocol
will be a useful method for the synthesis of N-aryl carbamates.
Introduction
Carbamates and thiocarbamates are present in various
bioactive compounds such as herbicides,1 pesticides,1a bac-
tericides,2 and antiviral agents.3 Because of their applications
in drug design and discovery, carbamates have received much
attention in recent years. Carbamates have gained more
importance in medicinal chemistry due to their chemical
stability and their potential to enhance permeability across
cellular membranes.4 In addition, carbamates have been used
as one of the protecting groups that enhance the chemical
stability of amines towards acids, bases and hydrogenation.4,5
There has been various methods reported for the preparation
of carbamates. The traditional method of carbamate synthesis
involves the use of highly toxic phosgene,6a phosgene
derivatives,6b-c and transition metal catalysts such as
palladium,7 nickel8 and rhodium,9 etc. Some other common
methods for the synthesis for carbamates are Hoffmann
rearrangement,10 the Curtius rearrangement,11 the reductive
carbonylation of aromatic nitro compounds,12 the
carbonylation of amines,13 and carbon dioxide alkylation.14
However, these methods suffer from certain limitations. There
Results and Discussion
Our investigation commenced with the reaction of N-Boc
protected aniline 1a with benzyl formate 2a using cobalt (II)
chloride as the catalyst and tris-(4-methoxyphenyl)-phosphine
as ligand in the presence of Zn powder in toluene. The reaction
mixture was heated at 100 oC for 96 h. To our delight, the
desired N-aryl carbamate 3aa was obtained in 74% yield. In
order to increase the yield of the product, various metal
a.
Key Laboratory of Chemistry in Ethnic Medicinal Resources, Yunnan Minzu
University, Kunming 650500, China. FanBM@ynni.edu.cn (Baomin Fan)
Chongqing Academy of Chinese Materia Medica, Chongqing, 400065, China.
neimen@163.com (Yong Yang)
b.
§
These authors contributed equally.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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catalysts were screened (Table 1). CoBr2 gave the product 3aa improvement in the reaction yield was observed Voienw sAcrtriceleeOnnilninge
DOI: 10.1039/C9OB00924H
in reduced yield (Table 1, entry 2). Gratifyingly, the desired different solvents such as 1,4-dioxane, DCE, THF, DMF, DMSO
carbamate was afforded in 80% yield when CoI2 was used and CH3CN indicating that toluene is the optimal solvent for
(Table 1, entry 3). Co(acac)2 resulted into diminished yield the present reaction. Surprisingly, on increasing the reaction
(Table 1, entry 4). The N-Boc protected aniline 1a was not temperature to 120 oC, the reaction was completed in just 24 h
completely consumed even after extending the reaction time with 92% yield (Table 2, entry 9). The reaction yield was
as indicated when CoCl2, CoBr2 and Co(acac)2 were used. Other reduced when the equivalent of zinc was lowered and no
cobalt catalysts such as CoF3, Co(OAc)2, CoCO3, Co(SCN)2 and reaction took place in the absence of zinc indicating the
Co3(PO4)2.8H2O were not suitable for the present reaction as importance of zinc in the present protocol. The reaction did
the reaction either did not occur or the product was formed not occur when Mn (3 equiv) was used instead of Zn under the
only in trace amount (Table 1, entries 5-9). This result shows same reaction conditions (See Table S1 in Supporting
that the most ideal metal catalyst for the present Information for detailed optimization study).
transformation is CoI2.
Table 1. Optimization of metal catalystsa
Table 2. Screening of different Ligands and solventsa
Entry
Ligand
Solvent
Temp
(oC)
Time
(h)
Yield
(%)b
Entry
Metal
Time (h)
96
Yield (%)b
74
1
2
3
4
5
-
Toluene
Toluene
Toluene
Toluene
Toluene
100
100
100
100
100
72
48
72
72
48
39
53
71
89
80
1
2
3
4
5
6
7
8
9
CoCl2
dppp
dppe
P(Cy)3
CoBr2
108
48
59
CoI2
80
Co(acac)2
CoF3
108
72
55
tris-(4-
methoxyphenyl)-
phosphine
NR
Co(OAc)2
CoCO3
72
Trace
NR
6
tris-(4-
dimethylaminop
henyl)-
phosphine
Toluene
DCE
100
100
100
120
120
72
72
48
24
48
93
72
Co(SCN)2
Co3(PO4)2.8H2O
72
NR
72
Trace
aReaction conditions: Cobalt salt (10 mol%), ligand (24 mol%) and Zn
powder (0.6 mmol) were stirred in toluene (1 mL) for 30 minutes at
room temperature (rt) under Ar. N-Boc protected aniline 1a (0.2
mmol), benzyl formate 2a (0.6 mmol) and toluene (1 mL) were added
to the above mixture. The reaction mixture was stirred under argon
atmosphere at 100 oC. The reaction was monitored by TLC. bIsolated
yields.
7
tris-(4-
dimethylaminop
henyl)-
trace
62
phosphine
8
tris-(4-
dimethylaminop
henyl)-
DMF
phosphine
Further optimization of the reaction conditions was performed
by screening various ligands and solvents (Table 2). When the
reaction was carried out in the absence of ligand, the product
was obtained in poor yield indicating that a ligand is required
for the synthesis of N-aryl carbamate under the present
catalytic system (Table 2, entry 1). Diphosphine ligands such as
dppp, dppe could not increase the reaction yield (Table 2,
entries 2 and 3). Improvement in the yield to 89% was
observed on using monophosphine ligand tricyclohexyl
phosphine (Table 2, entry 4). Other ligands such as dppm,
dppb, PPh3, 1,10-Phenanthroline, (±)-BINAP, tris-(3,5-
dimethylphenyl)-phosphine and tri-(furan-2-yl)-phosphine
were found unsuitable for the present transformation. There
was either no reaction or the product was formed in low yield
or trace amount. Gratifyingly, highest yield was observed
when the more electron rich tris-(4-dimethylaminophenyl)-
phosphine was used as the ligand (Table 2, entry 6). No
9
tris-(4-
dimethylaminop
henyl)-
phosphine
Toluene
Toluene
92
10c
tris-(4-
dimethylaminop
henyl)-
phosphine
NR
aReaction conditions: CoI2 (10 mol%), monophosphine ligands (24
mol%) or diphosphine ligands (12 mol%) and Zn powder were
stirred in solvent (1 mL) for 30 minutes at rt under Ar. N-Boc
protected aniline 1a (0.2 mmol), benzyl formate 2a (0.6 mmol) and 1
mL solvent were added. The reaction mixture was stirred under
o
o
argon atmosphere at 100 C or 120 C. The reaction was monitored
by TLC. bIsolated yields. cReaction was carried out in absence of Zn.
After obtaining the optimized reaction conditions, the
substrate scope of the developed catalytic system was
analyzed with respect to the aromatic amines (Table 3). The
substrates gave the desired products in different time
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intervals. Aromatic amines with electron donating substituents
1b performed well under the present reaction conditions and
afforded the corresponding benzyl carbamates 3ba fa in 82-
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DOI: 10.1039/C9OB00924H
8
38
48
48
72
48
94
89
60
80
72
-
f
-
90% yields (Table 3, entries 2-6). The results indicate that the
size or the position of the substituents does not have much
influence on the reactivity of the substrates. Unfortunately,
Boc-protected 4-aminophenol 1g resulted into diminished
yield (Table 3, entry 7). The present catalytic system were
found to be compatible for electron withdrawing halogen
substituted aromatic amines as they participated well in the
reaction. The desired products were obtained in 60-94% yields
(Table 3, entries 8-10). Nitro substituted amine 1k also gave
the corresponding carbamate 3ka in 80% (Table 3, entry 11).
The developed protocol was also found to be applicable to
naphthalen-1-amine 1l and the product 3la was obtained in
72% yield (Table 4, entry 12). Boc-protected aliphatic substrate
1m delivered the desired carbamate in 82% (Table 3, entry 13).
Boc-protected benzylamine 1n also participated in the reaction
and provided the expected product 3na in 54% yield (Table 3,
entry 14).
9
10
11
12
13
14
48
72
82
54
Table 3. Substrate scope of aromatic aminesa
aReaction conditions: CoI2 (10 mol%) , ligand (24 mol%) and Zn
powder (0.6 mmol) were stirred in toluene (1 mL) for 30 minutes at rt
under Ar. N-Boc protected amines 1a-1n (0.2 mmol), benzyl formate
2a (0.6 mmol) and 1 mL toluene were added. The reaction mixture
was stirred under argon atmosphere at 120 oC. The reaction was
monitored by TLC. bIsolated yields.
Entry Substrate
Product
Time
(h)
Yield
(%)b
The present investigation was further extended by examining
the scope of different benzyl formates 2b-j under the
1
24
92
developed reaction condition (Table 4). The efficiency of
benzyl formate with electron withdrawing chloro group at
meta-position was found to be less as compared to those at
ortho- and para-positions (Table 4, entries 1-3). On the other
hand, benzyl formate with bromo-group at para-position 2e
displayed good reactivity furnishing the expected product 3ae
in 79% yield (Table 4, entry 4). Electron withdrawing nitro
group substituted benzyl formate 2f also performed well in the
present protocol giving the product 3af with 87% yield (Table
4, entry 5). The present protocol was found to be amenable
for substrate with electron donating methyl group 2g with fair
yield (Table 4, entry 6). Phenethyl formate 2h also participated
under the present protocol and gave the desired product 3ah
in 62% yield (Table 4, entry 7). Unfortunately, the product 3ai
was obtained only in trace amount along with complex mixture
when m-methyl benzyl formate 2i was used (Table 4, entry 8).
The reason for this observation is obscure. When m-methoxy
benzyl formate 2j was used, the desired product 3aj was
obtained in 87% yield (Table 4, entry 9).
2
48
90
3
4
24
40
86
86
5
6
48
24
82
83
7
48
41
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our delight, aliphatic alcohols cyclohexanol 4c anVdiewetAhrtaicnleoOlnl4inde
also participated in the present reactiDoOnI:g10iv.1in03g9/tCh9eOBd0e0s9i2re4Hd
carbamates 5ac and 5ad in 98% and 82% yields respectively.
This observation furthers broadens the substrate scope of the
present catalytic system.
Table 4. Synthesis of carbamate using different Benzyl formatesa
Table 5. Synthesis of carbamate using alcohola,b
Entry
Benzyl formate
Product
Time
(h)
Yield
(%)b
1
12
24
64
38
2
3
24
65
aReaction conditions: CoI2 (10 mol%) , ligand (24 mol%) and Zn powder
(0.6 mmol) were stirred in toluene (1 mL) for 30 minutes at rt under Ar. N-
Boc protected aniline 1a (0.2 mmol), alcohols 4a-4d (0.6 mmol) and 1 mL
toluene were added. The reaction mixture was stirred under argon
atmosphere at 120 oC for the indicated time. The reaction was monitored
by TLC. bIsolated yields.
4
5
40
40
79
87
Control experiments
In order to get more information about the reaction
intermediates for the developed reaction protocol, we carried
out some control experiments (Scheme 1). No reaction took
place when N-Boc protected aryl amine 1e was subjected to
the catalytic system in the absence of Zn (equation a).
Generation of isocyanate intermediate was observed from the
GC-MS analysis when N-Boc protected amine 1e was subjected
to the reaction using Co(I) catalyst Co(CO)2(Cp) with the
6
7
8
36
24
62
62
o
phosphine ligand in toluene at 120 C for 48 h (equation b).
12
18
Trace
87
These observations indicate that in situ generation of
isocyanate is involved in the reaction pathway and it is
catalyzed by Co(I). Formation of the isocyanate was observed
in GC-MS under the optimized reaction conditions after 24 h of
reaction time which was converted into the corresponding
amine after 72 h which may have been caused by the
generated ZnI2 (equation c). This further supports the
involvement of in situ generation of isocyanate intermediate in
the reaction. Conversion of benzyl formate 2a to benzyl
alcohol was obtained under the standard reaction conditions
and also by using Co(I) catalyst (equations d and e). The benzyl
alcohol was afforded only in trace amount on using only CoI2
or ZnI2 or in the absence of catalyst. No reaction took place
when the unprotected aniline was subjected to react with
benzyl formate under the developed reaction conditions. This
observation shows that the carbonyl in the carbamate is from
the Boc-protecting group. No reaction occurred when the
reaction between 1a and benzyl alcohol was carried out in
absence of cobalt catalyst (equation f). The overall results led
9
aReaction conditions: CoI2 (10 mol%) , ligand (24 mol%) and Zn powder
(0.6 mmol) were stirred in toluene (1 mL) for 30 minutes at rt under Ar.
N-Boc protected aniline 1a (0.2 mmol), benzyl formate 2b-2j (0.6 mmol)
and 1 mL toluene were added. The reaction mixture was stirred under
argon atmosphere at 120 oC. The reaction was monitored by TLC.
bIsolated yields.
To further investigate the scope of the developed catalytic
system, alcohols were used instead of benzyl formate as
reaction partner under the present reaction conditions (Table
5). Benzyl alcohol 4a gave the desired carbamate 3aa in good
yield. Gratifyingly, when p-fluorobenzyl alcohol 4b was used,
the desired carbamate 5ab was furnished in excellent yield. To
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us to conclude that there is reduction of Co(II) to Co(I) which
catalyzes the in situ generation of both isocyanate and benzyl
alcohol in the catalytic cycle. The application of less expensive
alcohols for the transcarbamation of N-Boc protected amines
via in situ generated isocyanate has been reported earlier.19, 22
Although alcohols could also react under the developed
reaction conditions, we used benzyl formates because our
main focus was to investigate the application of the catalytic
system in the in situ generation of isocyanate from N-Boc
protected amines and benzyl alcohol from benzyl formate and
their subsequent reactions (See Scheme S1 in Supporting
Information for detailed control experiments).
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DOI: 10.1039/C9OB00924H
Scheme 2. Plausible mechanism for the synthesis of benzyl carbamate 3ea
Conclusions
In summary, we have developed a low valent cobalt catalyst
generated from CoI2, a monophosphine ligand and zinc
powder for the preparation of N-aryl carbamates from N-
protected amines and benzyl formates via in situ generation of
isocyanate and benzyl alcohol as intermediates. For the first
time, benzyl formates were successfully used for the synthesis
of carbamates (up to 92% yield). The developed catalytic
system was compatible for N-Boc protected amines with both
electron donating and withdrawing groups giving the
corresponding N-aryl carbamates in up to 94% yield.
We are grateful to the National Natural Science Foundation of
China (21572198), the Applied Basic Research Project of
Yunnan Province (2017FA004, 2018FB021), the Chongqing
Science & Technology Commission (cstc2017zdyfx0013) and
Yunnan Provincial Key Laboratory Construction Plan Funding of
Universities for their financial support.
Scheme 1. Control experiments to identify the reaction intermediates
Based on the results of the control experiments, we have
hypothesized a plausible reaction pathway for the present
reaction under the newly developed protocol (Scheme 2). The
catalytic cycle commenced with the combination of CoI2, tris-
(4-dimethylaminophenyl)-phosphine and Zinc forming the
Conflicts of interest
There are no conflicts to declare.
reduced cobalt complex
benzyl formate 2a to give intermediate
H activation to give complex C 21
Decarbonylation of
into that gives rise to benzyl alcohol by releasing a
molecule of CO. Then, benzyl alcohol combines with the
isocyanate generated in situ from 1e under the catalytic
system to give the final product 3ea
A
. Next, complex
which underwent C-
resulted
A combines with
B
.
C
D
E
E
Notes and references
F
1
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