Copper catalyzed cross-coupling reactions of carboxylic acids: An expedient route to amides, 5-substituted γ-lactams and α-acyloxy esters

Article abstract of DOI:10.1039/c3ra41000e

A convenient and recyclable catalytic protocol for the synthesis of N,N-dimethyl substituted amides, 5-substituted γ-lactams and α-acyloxy ethers from carboxylic acids using CuO nanoparticles and TBHP is described.

Full text of DOI:10.1039/c3ra41000e

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Copper catalyzed crossꢀcoupling reactions of carboxylic acids: An
expedient route to amides, 5ꢀsubstituted γꢀlactams and αꢀacyloxy esters
S. Priyadarshini,a P. J. Amal Joseph,a and M. Lakshmi Kantam*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
using 70 wt% TBHP in water. The screening of catalyst revealed
50 Cu2O, Cu(OSO2CF3)2 and CuO np as the best catalysts (Table 1,
entries 1, 11 & 14). However, poor yields were obtained with
various copper salts like CuI, CuCl and CuBr2 (Table 1, entries 7ꢀ
9) whereas the reaction failed with ZnO np and TiO2 np (Table 1,
entries 16 & 17). Notably, CuO np catalyst can be easily
55 recovered from the reaction solution. Thus, among the various
optimization studies performed, the entry 1 in Table 1 represents
the most promising result. It is noteworthy, that the
corresponding reaction on a 10 mmol scale gave 78 % yield of
product (Table 1, entry 20). Moreover, ~93 % of CuO np were
60 recovered and was reused for the next cycle with good activity.
A convenient and recyclable catalytic protocol for the
synthesis of N,Nꢀdimethyl substituted amides, 5ꢀsubstituted γꢀ
lactams and αꢀacyloxy ethers from carboxylic acids using
CuO nanoparticles and TBHP is described.
10 Amides are important structural constituents in numerous
biomolecules, natural products, pharmaceuticals and
agrochemicals.1 The synthesis of amides by condensation of
carboxylic acids with amines are feasible only at high
temperature mainly due to the initial formation of stable
15 ammonium salts.2,3 N,Nꢀdimethyl substituted amides are routinely
synthesized by the reaction of preꢀactivated carboxylic acid
derivatives with dimethyl amine or by direct reaction of
carboxylic acids with amine in presence of suitable coupling
reagents.2,4 Transition metal catalyzed carbomylation of aryl
20 halides or direct cross coupling of aryl halides with DMF are
alternate direct methods for their synthesis.5,6 Catalytic oxidative
amidation of alcohols or aldehydes with N,Nꢀdimethylamine are
also promising approaches.7,8 Wan et al reported an important
method for their synthesis employing aldehyde and DMF in
25 presence of TBHP and catalytic amount of tetrabutylammonium
iodide.9 Recently, Wang et al demonstrated that these substrates
can also be synthesized by the reaction of benzyl alcohol with
DMF in presence of TBHP and catalytic amounts of iodine and
NaOH.10 All these processes represent important advancements
30 made in this field. On the other hand, it is important to develop
alternate protocol for their synthesis, owing to their growing
significance in synthetic chemistry. In this context, here we report
a complementary protocol for the synthesis of N,Nꢀdimethyl
substituted amides by crossꢀcoupling of carboxylic acids with
35 DMF using CuO nanoparticles and TBHP.11
Table 1. Optimization of reaction parameters for oxidative coupling of
benzoic acid with DMF
65
Entry
1
2
Catalyst
CuO np
CuO
Oxidant
TBHP
H2O2
Yield/%e
82
0
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
CuO
CuO
CuO
Cu powder
CuI
DTBP
TBPB
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
trace
12
79a
76
11
8
20
63
84
73
75
84
CuCl
CuBr2
Cu(OH)2
Cu2O
Cu(OAc)2
Cu(SO4)2
Cu(OSO2CF3)2
CuO
ZnO np
TiO2 np
NiO np
ꢀ
58
0
0
Recently, Mao et al reported that cinnamic acid in presence of
CuO and DTBP, undergoes decarboxylative C(sp2)ꢀC(sp3)
coupling with toluene.12ꢀ14 Inspired by this result, it was
envisioned that carboxylic acids could also be reacted with DMF
40 under oxidative conditions to yield N,Nꢀdimethyl substituted
amides. To verify this assumption, a model reaction was carried
out on a 0.5 mmol scale with benzoic acid employing 10 mol% of
CuO nanoparticles (np) and 3 equiv of TBHP in 1.5 mL DMF at
trace
0b
20
CuO np
TBHP
78c, 76c,d
a
b
TBHP (5ꢀ6 M in decane) was used. Reaction performed without
catalyst.c Reaction performed on 10 mmol scale using 25 mL DMF for
20h. d Yield for 2nd cycle. e Isolated Yield. np = nanoparticles.
o
85 C for 15 h (Table 1, entry 1). As anticipated, this reaction
45 gave 82
% yield of N,Nꢀdimethylbenzamide. Thereafter,
optimization studies were performed altering the catalysts and
oxidants. The results of these studies are tabulated in Table 1.
Among the oxidants screened, superior results were obtained
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Table 2. CuO catalyzed oxidative coupling of carboxylic acids with DMF
cinnamic
acid,
transꢀ3ꢀnitrocinnamic
acid,
transꢀ2,4ꢀ
O
CuO np (10 mol%)
difluorocinnamic acid, transꢀ3ꢀpyridyl acrylic acid and transꢀ2ꢀ
furylacrylic acids proceeded smoothly and afforded the
corresponding products in good yields with excellent
30 regioselectivity (3o, 3p, 3q, 3s, & 3t). Notably, the reaction of
sterically hindered (E)ꢀ2ꢀmethylꢀ3ꢀphenylacrylic acid furnished
the corresponding amide in good yield (3r). The protocol was
also successfully utilized for the amidation of arylꢀsubstituted
acetic acids (3u, 3v & 3w). It is significant to note that the scope
35 of the amidation reaction is not merely limited to DMF, as the
reaction of benzoic acid worked well with cylic formamide,
namely, Nꢀformyl piperidine (3aa).
Thereafter, the scope of this protocol was evaluated using NMP,
pyrrolidinꢀ2ꢀone and 1,4ꢀdioxane as the crossꢀcoupling partners
40 for carboxylic acids (Table 3). Remarkably, benzoic acid did not
react with NMP or pyrrolidinꢀ2ꢀone under the standard
conditions; however, cinnamic acid underwent decarboxylative
coupling at 70 oC to afford exclusively (E)ꢀ1ꢀmethylꢀ5ꢀ
O
RCOOH
+
H
N
R
N
TBHP (3 equiv)
(0.5 mmol)
(1.5 mL)
2
1aꢀw
3aꢀx
N
N
N
N
O
O
O
O
O
Cl
H2N
MeO
73 % (3d)
85 oC, 15 h
82 % (3a)
85 oC, 15 h
62 % (3b)
85 oC, 15 h
83 % (3c)
85 oC, 15 h
N
N
O
N
N
O
O2N
O
NC
O2N
I
80 % (3e)
85 oC, 15 h
53 % (3f)
52 % (3g)
85 oC, 24 h
82 % (3h)
85 oC, 18 h
85 oC, 18 h
N
N
N
N
F3C
Br
O
O
O
O
Ph
styrylpyrrolidinꢀ2ꢀone
and
(E)ꢀ5ꢀstyrylpyrrolidinꢀ2ꢀone,
CHO
81 % (3i)
84 % (3k)
3l
54 % ( )
85 oC, 18 h
81 % (3j)
45 respectively in good yields (5o & 5o’). In a similar way, transꢀ
2,4ꢀdimethoxyꢀcinnamic acid, transꢀ3ꢀpyridyl acrylic acid and
transꢀ2ꢀfurylacrylic acids also underwent decarboxylative crossꢀ
coupling with NMP to afford the corresponding products in good
yields and excellent regioselectivity (5y, 5s & 5t).14 Likewise, the
50 reaction of cinnamic acid with 1,4ꢀdioxane gave (E)ꢀ2ꢀstyrylꢀ1,4ꢀ
dioxane in 55 % yield (Scheme 1, 6). In contrast, benzoic acid
underwent crossꢀdehydrogenative coupling with 1,4ꢀdioxane
providing 1,4ꢀdioxanꢀ2ꢀyl benzoate in 91 % yield (Scheme 1,
85 oC, 15 h
85 oC, 15 h
85 oC, 18 h
N
O
N
N
O2N
N
O
O
O
O
S
83 % (3m)
80 oC, 12 h
82 % (3n)
80 oC, 12 h
85 % (3o)
80 oC, 15 h
65 % (3p)
80 oC, 15 h
N
N
N
N
O
O
O
O
O
8).15
F
F
N
68 % (3q)
80 oC, 24 h
78 % (3s)
80 oC, 15 h
82 % (3t)
80 oC, 12 h
70 % (3r)
90 oC, 24 h
55
Table 3. CuO catalyzed decarboxylative coupling of carboxylic acids
with NMP/pyrrolidinꢀ2ꢀone
O
N
N
N
N
O
O
O
HN
91 % (3aa)a
80 oC, 15 h
80 % (3v)
80 oC, 12 h
68 % (3w)
80 oC, 12 h
93 % (3u)
80 oC, 12 h
60
a Reaction performed with 1.5 mL of Nꢀformyl piperidine.
5
With the optimized reaction conditions in hand, various
structurally diverse carboxylic acids were tested for oxidative
coupling with DMF. As illustrated in Table 2, benzoic acids
containing both electronꢀrich as well as electronꢀdeficient groups
reacted well with DMF irrespective of their orientation in
65
10 aromatic ring (orthoꢀ or meta- or para-substituted) to afford the
corresponding N,Nꢀdimethyl substituted amides in good yields.
The reaction of halogenated carboxylic acids like 4ꢀchlorobenzoic
acid, 2ꢀiodobenzoic acid and 3ꢀbromobenzoic acids gave the
corresponding amides in good yields (3c, 3h & 3k). The reaction
15 of 4ꢀcyanobenzoic acid and 2ꢀformylbenzoic acid proceeded
smoothly without affecting the formyl and cyano groups
respectively (3e & 3i).9 In addition carboxylic acids containing
various sensitive groups like NH2, NO2 and CF3 were tolerated
(3d, 3f & 3j). On the other hand, amino acids like Lꢀ
20 phenylalanine and carboxylic acids containing OH group, for
example vanillic acid, did not survive the reaction conditions. It is
noteworthy that heterocyclic acids such as furylic acid and 2ꢀ
thiophenecarboxylic acid also reacted well with DMF to afford
the corresponding product in good yields (3m & 3n). In a similar
25 way, amidation of α,βꢀunsaturated carboxylic acids such as
Scheme 1. Decarboxylative vs dehydrogenative crossꢀcoupling
70 reaction
75
To get a preliminary idea about the mechanism of amidation, a
series of studies were performed. Firstly, the reaction of benzoic
acid performed under the standard conditions using dimethyl
amine instead of DMF afforded only an ammonium salt,
80 consequently excluding the chance of an acid base reaction step
in the mechanism. Moreover, the reaction of benzoic acid in
presence of 3 equiv of a free radical scavenger, namely, TEMPO
2
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failed to yield the normal amidation product as well as the radical
Scheme 3. Free radical initiation step
combination product of TEMPO with acyl radical reported in the
previous studies.9,10,16 These observations demonstrate the
absence of an acyl radical intermediacy in this mechanism and
5
also insinuate the likely operation of
a
free radical
60
mechanism.9,10,17,18 Notably, the reaction of (R)ꢀ2ꢀmethoxyꢀ2ꢀ
phenylacetic acid with DMF gave exclusively (R)ꢀ1ꢀmethoxyꢀ
N,Nꢀdimethylꢀ1ꢀphenylmethanamine which indicates that the
carbonyl group in product have originated from carboxylic acid
10 (Scheme 2, eqn 1). Next, to find out whether copper carboxylates
or peracids or peresters could be intermediates in amidation, a set
of experiments were performed using copper(
)benzoate, mꢀ
chloroperoxybenzoic acid and tertꢀbutylperoxybenzoate (TBPB)
respectively (Scheme 2, eqn 2ꢀ4). These reactions afforded the
15 corresponding amides in 25 %, 48 % and 26 % yields
respectively. Moreover, the reaction of 4ꢀcyanobenzoic acid
performed with 10 mol% of copper(II)benzoate catalyst furnished
4ꢀcyanoꢀN,Nꢀdimethylbenzamide in 76 % yield along with 11 %
of N,Nꢀdimethylbenzamide (Scheme 2, eqn 5). The formation of
20 later indicates that copper carboxylates are involved in the
mechanistic pathway.19
65
Scheme 4. Plausible mechanism for decarboxylative coupling of
carboxylic acids with DMF.
70
Therefore, from these studies a plausible mechanism of amidation
is postulated in Schemes 3 & 4. Initially, tertꢀbutoxy radical is
formed by the reaction of copper catalyst with TBHP in presence
25 of carboxylic acid (Scheme 3, eqn 1ꢀ3).20 This radical abstracts
hydrogen from DMF/dioxane/NMP, thereby generating
corresponding radicals (Scheme 3, eqn 4ꢀ6). Notably, amidyl
radical can react with copper(II) carboxylates through either
single electron transfer reaction (SET) or ligand transfer reaction
30 (LT) affording the corresponding amides, through a carbamic
anhydride intermediacy (Scheme 4).21ꢀ23 The reaction of benzoic
acid with 1,4ꢀdioxane is also assumed to follow a similar
mechanistic pathway; albeit, without decarboxylation (Scheme
5). On the other hand, the reaction of cinnamic acid with 1,4ꢀ
35 dioxane or NMP, occurs through a different mechanistic pathway
involving the reaction of respective radicals with the styryl
double bond followed by concomitant decarboxylation (Scheme
6). Importantly, these results clearly demonstrate that the
electronic and steric factors of the substrates are crucial in
40 directing the mechanistic pathway.
Scheme 5. Plausible mechanism for crossꢀdehydrogenative coupling
of benzoic acid with 1,4ꢀdioxane.
75
Scheme 6. Plausible mechanism for decarboxylative coupling of
cinnamic acids with NMP or 1,4ꢀdioxane.
O
O
Cu(II)
Cu(II)
+
R
Ph
O
Ph
O
R
80
Ph
+
+
Cu(I)
CO2
R
O
O
Scheme 2. Mechanistic study of CuO catalyzed oxidative coupling of
R
O
=
N
carboxylic acids with DMF under oxidative conditions.
O
O
CuO np (10 mol %)
TBHP (3 equiv)
In summary, an efficient and recyclable catalytic protocol for the
synthesis of N,Nꢀdimethyl substituted amides, 5ꢀsubstituted γꢀ
N
OH
45
50
55
+
2
(1)
O
O
(91 %)
3x
1x
O
85 lactams, 2ꢀstyrylꢀ1,4ꢀdioxane and αꢀacyloxy ethers from
carboxylic acids using CuO np catalyst under oxidative
conditions is described. The protocol tolerates a wide variety of
functional groups and several structurally diverse carboxylic
acids were reacted with DMF to afford the corresponding amides
90 in good yields and excellent regioselectivity. The mechanistic
studies revealed that the carbonyl carbon of amide is derived
from carboxylic acid. The reaction of cinnamic acids with NMP/
pyrrolidinꢀ2ꢀone afforded novel 5ꢀsubstituted γꢀlactams through
decarboxylative coupling. The analogous reaction of benzoic acid
95 with 1,4ꢀdioxane proceeded through crossꢀdehydrogenative
coupling affording an αꢀacyloxy ether.
O
O
TBHP (3 equiv)
Ph
Ph
Cu
2
+
3a (25 %)
(2)
(3)
O
1a,
O
O
CuO np (10 mol %)
TBHP (3 equiv)
Cl
OH
Cl
O
N
2
2
+
+
1ab
(48 %)
3ab
CuO np (10 mol %)
TBHP (3 equiv)
O
O
(4)
(5)
3a (26 %)
Ph
O
1a''
O
1a, (10 mol %)
TBHP (3 equiv)
OH
2
+
(76 %)
3e
+ 3a (11 %)
NC
1e
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Notes and references
a
Inorganic and Physical Chemistry Division, CSIR-Indian Institute of
Chemical Technology, Hyderabad, 500607, India. Fax: +91-40-
27160921; Tel: 91-40-27193510 E-mail: mlakshmi@iict.res.in
† Electronic Supplementary Information (ESI) available: General
procedure for synthesis, characterization data and copies of 1H and 13C
NMR, HRMS and FTꢀIR spectra of the compounds, and XRD pattern of
catalyst. See DOI: 10.1039/b000000x/
;
75
5
10
15
20
25
30
35
40
45
50
16 Small amount of amidation product was obtained when 1.5 equiv of
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