Ligand-assisted copper-catalyzed oxidative cross-coupling of simple phenols with formamides for the synthesis of carbamates

Article abstract of DOI:10.1055/s-0034-1378519

An oxidative approach for the synthesis of phenyl carbamates has been achieved by ligand-assisted copper-catalyzed cross-dehydrogenative coupling (CDC) of phenols with formamides. The direct coupling of simple phenols with mono- and dialkyl formamides pro

Full text of DOI:10.1055/s-0034-1378519

LETTER
▌2133
letter
Ligand-Assisted Copper-Catalyzed Oxidative Cross-Coupling of Simple
Phenols with Formamides for the Synthesis of Carbamates
Synthesis of Phenyl Carbamates
Nagireddy Veera Reddy, Gadde Sathish Kumar, Pailla Santhosh Kumar, M. Lakshmi Kantam, Kallu Rajender Reddy*
Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Tarnaka, Hyderabad – 500 607, India
Fax +91(40)27160921; E-mail: rajender@iict.res.in; E-mail: rajenderkallu@yahoo.com
Received: 22.04.2014; Accepted after revision: 20.06.2014
Unlike conventional couplings, where stoichiometric re-
Abstract: An oxidative approach for the synthesis of phenyl carba-
agents and prefunctionalized substrates have been em-
mates has been achieved by ligand-assisted copper-catalyzed cross-
ployed, an approach via C–H bond activation can provide
direct access for the construction of C–C and C–heteroat-
dehydrogenative coupling (CDC) of phenols with formamides. The
direct coupling of simple phenols with mono- and dialkyl form-
amides provides carbamate products in moderate to good yields om bonds and make synthetic routes shorter and more
atom-economical.5 The importance of approaches involv-
under mild reaction conditions.
ing both intermolecular and intramolecular couplings has
been summarized in several recent reviews.6 Ligands play
an important role in these coupling reactions.
Key words: oxidation, formamide, carbamate, copper acetate, cou-
pling reaction
For example, in several C–C and C–heteroatom bond for-
mations via C–H bond activations, participation of intra-
molecular directing functional groups through
coordination to the metal ion is proposed to be one of the
key steps for success.7 Equally, the influence of external
ligands on metal-catalyzed intermolecular oxidative cou-
plings has also been studied.8
Organic carbamates are synthetically useful chemicals
having diverse applications. For example carbamate units
are found in pharmaceuticals and agrochemicals (Figure
1).1 These units also play an important role in synthetic or-
ganic chemistry, primarily as ortho-directing and protect-
ing groups.2 A large number of reported synthetic
methods for organic carbamates rely on the utilization of
phosgene or phosgene derivatives such as chloroformates
or isocyanates.3 To avoid the use of toxic and harmful re-
agents, several phosgene-free routes have been reported in
recent years.1a,4 Among them, reductive carbonylation of
nitroaromatics,4a oxidative carbonylation of amines using
carbonates/bicarbonates,4c and utilization of carbon diox-
ide4d have received attention. However, certain drawbacks
such as the need to perform these reactions under high
pressures and temperatures and use of expensive metal
catalysts or hazardous reagents mean that there is still the
need for alternative synthetic approaches to these useful
products.
Over recent years, we have been investigating transition-
and nontransition-metal-catalyzed organic transforma-
tions under oxidative conditions.9 In one of these studies
we disclosed the application of copper-catalyzed oxida-
tive coupling of formamides with enols and 2-acetyl phe-
nols to generate the corresponding carbamates.10 During
these studies we observed that the coupling of simple phe-
nols with formamides was not successful. Based on these
observations, we searched for a methodology to permit di-
rect coupling of phenols with formamides in the presence
of an external ligand for the synthesis of phenyl carba-
mates via O–H bond activation (Scheme 1).
Initial studies were carried out with phenol and dimethyl
formamide (DMF) as the reaction partners in the presence
Me
Me
N
Me
N
Me
Me
O
Me
N
N
O
O
H
Me
Me
Et
N
Me
N
O
Me
O
N
O
H
Me
physostigmine
rivastigmine
neostigmine
O
O
O
NH
Me
O
H
N
O
Me
O
bendiocarb
carbaryl
Figure 1 Representative examples of important pharmaceuticals and agrochemicals containing carbamate linkages
SYNLETT 2014, 25, 2133–2138
Advanced online publication: 31.07.2014
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0034-1378519; Art ID: st-2014-d0335-l
© Georg Thieme Verlag Stuttgart · New York

2134
N. V. Reddy et al.
LETTER
O
O
R3
O
O
O
R4
R3
R4
N
H
H
N
O
N
N
OH
H
R2
H
R2
Cu(OAc)2 (20 mol%)
Cu(OAc)2 (20 mol%)
1,10-phen (20 mol%)
TBHP (3 equiv), 80 °C, 8 h
R1
1,10-phen (20 mol%)
TBHP (3 equiv), r.t., 2 h
R1
R1
Scheme 1
of Cu(OAc)2 using various ligands (Table 1). To our sat- optimized procedure of 20 mol% of Cu(OAc)2, 2 mL of
isfaction, bidentate ligands such as 2,2′-bipyridine (bpy) formamide, three equivalents of TBHP in decane solution,
and 1,10-phenanthroline (phen) showed the highest activ- 20 mol% of 1,10-phenanthroline at 80 °C for eight hours,
ity among the different ligands tested. The absence of any and the results are summarized in Scheme 2. Phenol and
product in control experiments clearly indicates that the li- substrates having electron-withdrawing groups at the
gand is essential for the activation of the phenol by the para position provided the carbamate products in good
metal ion (Table 1, entry 3). In this context, using phenan- yields (Scheme 2, 3a, 3b, and 3c). Similarly, the yields
throline as the ligand, various copper salts were exam- obtained for ortho-substituted derivatives clearly indicate
ined.11 Although most of the copper-metal ions showed that there is a negligible steric influence (Scheme 2, 3d
some activity, Cu(OAc)2 proved to be the best. Regarding and 3e). As anticipated the meta-substituted 3-acetylphe-
the oxidants, we observed a drop in yield of the product, nol provided the product 3f in low yield. In the case of 2-
when TBHP in decane was replaced by 70% aqueous naphthols, moderate yields were obtained, along with
TBHP, whereas other oxidants such as H2O2, meta-chlo- some unidentified products (Scheme 2, 3g and 3h). A sim-
roperoxybenzoic acid (MCPBA) and urea hydrogen per- ilar reactivity pattern was observed for other formamide
oxide (UHP) were found to be inactive for this coupling derivatives (Scheme 2, 3i–o). To explore the generality of
reaction
this reaction further, heteroaromatic phenols such as 7-hy-
droxy-2H-chromen-2-one and 8-hydroxyquinoline were
tested. The chromenone derivative provided the corre-
sponding carbamate products 3p and 3q with DMF and
diethyl formamide in good yields; whereas relatively low
yields of carbamate products 3r and 3s were observed
with 8-hydroxyquinoline, which could be due to the
strong chelating property of 8-hydroxyquinoline for metal
ions. Finally, reaction with the readily enolizable 5,5-di-
methylcyclohexane-1,3-dione also provided the corre-
sponding carbamate product 3t (the insecticide dimetan12)
in low yield.
Table 1 Ligand Optimization Studies for the Synthesis of Phenyl
Carbamatesa
O
Me
OH
O
O
N
Cu(OAc)2, TBHP
ligand, 80 °C , 8 h
Me
Me
H
N
+
Me
Entry
Ligand
pyridine
Isolated yield (%)
1
2
3
4
5
6
7
8
9
10
7
We then turned our attention to N-methylformamide
(NMF) as the formamide source, as the resulting products
have much more importance; such substructures being
found in pesticides and insecticides. Moreover, it is inter-
esting to note that most recent reports on coupling chem-
istry involving formamides use N,N′-disubstituted
derivatives and rarely use monosubstituted forma-
mides.13,14 In this case, the reaction of phenol with NMF
under the standard reaction conditions provided only
<10% of the carbamate product. However, the product
yield was improved to 35% when the reaction was per-
formed at room temperature for two hours.
triethylamine
–
7
n.r.b
15
15
17
22
23
47
65
ethylenediamine
DMEDA
TMEDA
DMAP
2-aminomethylpyridine
2,2′-bipyridine
1,10-phenanthroline
Reaction with other substrates also resulted in carbamate
products in moderate yields (Scheme 3). Further improve-
ments in yields were achieved when the reaction was per-
formed in the presence of 4 Å molecular sieves under an
inert atmosphere. For example, the yield of 5a increased
from 35% to 60% and similarly for 5b and 5c from 40%
and 30% to 55% and 45%, respectively. Among these
compounds 5c is the pesticide carbaryl.15
a Reaction conditions: phenol (1 mmol), Cu(OAc)2 (20 mol%), DMF
(2 mL, 27 equiv), TBHP in decane (3 equiv), 1,10-phenanthroline (20
mol%).
b n.r. = no reaction.
Following these studies, the scope of the coupling proto-
col was examined for different phenol substrates using the
Synlett 2014, 25, 2133–2138
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Phenyl Carbamates
2135
O
R3
OH
O
N
O
Cu(OAc)2, TBHP
R2
R3
O
+
H
N
1,10-phen, 80 °C, 8 h
R1
R2
R1
O
O
O
O
O
O
NMe2
O
NMe2
O
NMe2
O
NMe2
O
NMe2
O
NMe2
F
Br
O
Cl
NO2
3c (61%)
3a (65%)
3b (81%)
3d (70%)
3e (72%)
3f (30%)
O
O
O
O
NEt2
O
NEt2
O
O
NMe2
Br
NMe2
O
NEt2
O
O
NO2
Cl
3k (72%)
3i (70%)
3j (65%)
3g (45%)
3h (53%)
O
O
O
N
O
NEt2
O
N
O
N
O
O
Me2N
O
O
O
O
Br
NO2
Br
3l (74%)
3m (63%)
3n (48%)
3o (45%)
3p (60%)
O
O
NEt2
O
O
NMe2
O
NEt2
O
O
O
O
N
N
O
O
NMe2
3q (66%)
3r (32%)
3s (50%)
3t (30%)
Scheme 2 Synthesis of carbamates by the coupling of phenols with N,N′-dialkylformamides. Reagents and conditions: phenol (1 mmol), cat-
alyst (20 mol%), formamide (2 mL), TBHP in decane (3 equiv), 1,10-phenanthroline (20 mol%), 80 °C, 8 h. Isolated yields.
As can be seen from the above results, the ligand plays an However, we have observed a small amount of carbamate
important role in the activation of the phenol via forma- product even in the absence of ligand, which supports an
tion of the copper–ligand complex which subsequently alternative pathway, possibly via an isocyanate intermedi-
couples with formamide. The reaction between phenol ate (Scheme 5, B). Here again the role of ligand and metal
and DMF under metal-to-ligand ratios of 1:0; 1:1; 1:2, and is clearly evident from the very low yield or absence of
1:3 resulted in 0%, 65%, 40%, and 20% yields of the car- product in the control experiments.
bamate product, respectively, which clearly confirms the
In summary, we have developed a ligand-assisted copper-
participation of the ligand with the metal ion in activation
catalyzed oxidative C–O coupling reaction for the synthe-
of the phenol.
sis of phenyl carbamates. We have shown the direct cou-
The fact that no product formation was observed in pres- pling of simple phenols with both mono- and disubstituted
ence of the radical scavenger TEMPO indicates that the formamides under relatively mild reaction conditions pro-
reaction occurs via a radical mechanism (Scheme 4).
viding carbamates including dimetan and carbaryl, which
are useful agrochemicals. Though the yields are moderate,
the new approach may provide an opportunity to develop
nonphosgene-based routes for the carbamate synthesis via
oxidative cross-coupling chemistry involving C–H bond
activation.
Based on this evidence, we tentatively propose a reaction
mechanism involving the direct coupling of a phenoxy
radical with the dialkylformamide radical (Scheme 5, A).
In the case of N-methylformamide, the absence of any car-
bamate product in the presence of radical scavenger
(TEMPO) strongly supports a similar radical pathway.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 2133–2138

2136
N. V. Reddy et al.
LETTER
O
Me
OH
O
N
O
H
Cu(OAc)2, TBHP
1,10-phen, r.t., 2 h
Me
+
N
H
H
R1
R1
O
1
4
5
O
Me
O
O
N
H
Me
Me
O
N
O
N
H
H
Cl
5a (35%)b (60%)c
5b (40%)b (55%)c
5c (30%)b (45%)c
O
Me
O
O
N
Me
H
H
O
N
H
O
N
O
Me
O
5d (20%)
5e (35%)
5f (40%)
Scheme 3 Synthesis of NMF-derived phenyl carbamates. Reagents and conditions: phenol (1 mmol), catalyst (20 mol%), NMF (2 mL), TBHP
in decane (3 equiv), 1,10-phenanthroline (20 mol%), r.t., 2 h. Isolated yields. a Reaction performed in the presence of 4 Å MS under nitrogen
atmosphere.
5a
3a
35%
65%
General Procedure for the Synthesis of Phenyl Carbamates
Cu(OAc)2 (20 mol%) and 1,10-phenanthroline (20 mol%) were dis-
solved in the requisite formamide (2 mL). The reaction mixture was
stirred for 5 min, and phenol (1.0 mmol) was added. TBHP (5–6 M
in decane, 3 mmol) was added dropwise, with stirring over a period
of 5 min, the reaction temperature was increased to 80 °C, and the
mixture stirred for 8 h. After cooling to r.t., the reaction mixture was
directly subjected to purification by column chromatography on sil-
ica gel, eluting with 5–10% EtOAc–hexane to afford the required
product.
no ligand
no catalyst
with TEMPO
OH
no ligand
NMF
(2 mL)
DMF
(2 mL)
10%
NR
NR
NR
no catalyst
r.t., 2 h
80 °C, 8 h
NR
NR
1a
with TEMPO
Scheme 4
Phenyl Dimethylcarbamate (3a)
1
IR: 2937, 1718, 1479, 1389, 1206, 1168, 842, 751, 691 cm–1. H
NMR (300 MHz, CDCl3): δ = 7.35 (t, J = 7.5 Hz, 2 H), 7.18 (t, J =
6.7 Hz, 1 H), 7.11 (d, J = 7.5 Hz, 2 H), 3.10 (s, 3 H), 3.01 (s, 3 H).
13C NMR (75 MHz, CDCl3): δ = 154.8, 151.3, 129.1, 125.0, 121.6,
115.2, 36.5, 36.3. GC–MS: m/z = 165 [M+]. ESI-MS: m/z = 188 [M
+ Na]+. ESI-HRMS: m/z calcd for C9H12NO2 [M + H]+: 166.08626;
found: 166.08582.
O
O
OH
•
Me
(A)
O
O
O
N
Cu cat, L
TBHP
Me
Me
Me
N
Me
+
•
+
Me
N
H
Acknowledgment
O
O.
Me
N.V.R and G.S.K thank the Council of Scientific and Industrial Re-
search (CSIR), and P.S.K. thanks the University Grants Commissi-
on (UGC) India, for Fellowships. K.R.R. thanks the CSIR for
financial support under network project CSC-0123. We thank Dr.
M. Srinivas Reddy for helpful discussions and LC–MS mass analy-
sis.
O
N
H
H
Me
O
OH
+
+
•
N
H
Me
O
N
Cu cat, L
TBHP
H
(B)
OH
O
O
C
Me
Supporting Information for this article is available online
N
N
H
at
http://www.thieme-connect.com/products/ejournals/journal/
H
Me
10.1055/s-00000083.SunogIpimrfiantoSuIpg
n
fonirtat
ori
Scheme 5 Plausible mechanism for carbamate formation
Synlett 2014, 25, 2133–2138
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Phenyl Carbamates
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