Cu(II)-Mediated keto C(sp3)-H bond α-acyloxylation of N, N-dialkylamides with aromatic carboxylic acids

Article abstract of DOI:10.1039/c7ob01190c

The selective oxidative coupling of aromatic carboxylic acids with the C(sp³)-H bond adjacent to the keto group of alkylamides has been developed by employing a low cost copper source. This provides an efficient approach for synthesis of O-benzoylglycolamides. The protocol displayed good functional group tolerance. A broad range of benzoic acids directly coupled with alkylamides to afford a variety of O-benzoylglycolamides in moderate to good yields. In addition, a reasonable radical mechanism was proposed based on EPR experiments.

Full text of DOI:10.1039/c7ob01190c

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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/C7OB01190C
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Cu(II)-mediated Keto C(sp3)-H Bond α-Acyloxylation of N,N-dialkyl
amides with Aromatic Carboxylic Acid
Received 00th January 20xx,
Accepted 00th January 20xx
Wenjing Li, Changzhen Yin, Xiao Yang, Hailong Liu, Xueli Zheng, Maolin Yuan, Ruixiang Li, Haiyan
Fu* and Hua Chen*
DOI: 10.1039/x0xx00000x
The selective oxidative coupling of the aromatic carboxylic acid with the C(sp3)-H bond adjacent to the keto group of the
alkylamides has been developed by employing the cheaper copper source. This provides an efficient approach for
synthesis of O-benzoylglycolamides. The protocol displayed good functional group tolerance. A broad range of the benzoic
acid directly coupled with alkylamides to afford a variety of O-benzoylglycolamides in moderate to good yields. In addition,
a reasonable radical mechanism was proposed based on the EPR experiments.
www.rsc.org/
Introduction
O-Benzoylglycolamides are highly biolabile prodrugs possess-
ing many biological activities such as antitumor, antidiabetic
and antiandrogen properties.1 In addition, the masked
ɑ-
hydroxyl amide moiety is a valuable synthon.2 Usually the
benzoylglycolamides are synthesized by the several routine
approaches including esterification benzoic acid with the
appropriate 2-haloacetamide3 or 2-hydroxy acetamide4 or via
a sequential chlorination of 2-(benzoyloxy)acetic acid and
subsequent amination. These methods often involve using
expensive reagents like 2-hydroxy acetamide or toxic, volatile
Scheme 1 Examples of prodrugs containing glycolamide ester
and corrosive chlorination reagents.5 Therefore,
a more
practical and efficient way to synthesize O-benzoylglycol-
amides is highly desirable. An alternative method for the
synthesis of O-benzoylglycolamides is direct coupling of
aromatic carboxylic acid with DMAc via a cross dehydro-
genation coupling (CDC), which undoubtedly is more straight-
forward and make the synthetic route shorter and atom-
economical. But, the functionalization of C(sp3)-H bond using
CDC strategy is quite challenging due to the strong C(sp3)-H
bond.6 In general, transition metals such as Rh,7 Ru,8 Pd,9 Ir10
have been widely used to implement this task.
In past two decades, the cheap and abundantly available
copper has been explored as an alternative catalyst for them.
Considerable efforts have been devoted into the development
of the copper-promoted CDC reactions. Diverse carbon-
carbon11 and carbon-heteroatom, including C-X,12 C-N,13 C-O,14
C-S15 and C-P16 bonds, can be feasibly constructed. Specifically,
the C-O bond formation via benzoxylation of C(sp3)-H bond has
a.Key Laboratory of Green Chemistry & Technology, Ministry of Education, College
of Chemistry, Sichuan University, Chengdu 610064, P. R. China. Fax: (+86)-28-
8541-2904; E-mail: scufhy@scu.edu.cn; scuhchen@scu.edu.cn.
Electronic Supplementary Information (ESI) available: Experimental procedures,
mechanism studies, characterisation data and NMR spectra. See DOI: 10.1039/
x0xx00000x
Scheme 2 Strategies for C−O bond formation
aroused increasing interest. Various organic compounds such
as phenylglyoxylic acid,17 benzaldehyde,18 toluene,19
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benzylalcohol,20 or benzoic peroxyanhydride21 have been Initially, we chose o-methylbenzoic acid 1a as the model
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demonstrated as benzoxylation reagents. While the formation substrate and heated it in DMAc in the pDreOsIe: 1n0c.1e03o9f/cCo7OppB0e1r19s0aClt
of C(sp3)-O bond employing readily available, stable and easily under nitrogen at 150
℃ (Table 1). The desired product 3aa
handled benzoic acid through CDC reaction would be more was obtained in 25% yield together with a significant amount
attractive. The C(sp3)-H bonds at benzylic22a and allylic of byproduct 2-(dimethylamino)-2-oxoethyl acetate when 1.0
position22b or α-position of carbonyl group23 have been equivalent of CuCl was used (Table 1, entry 1). No reaction was
demonstrated being able to couple with benzoic acid (Scheme observed in the absence of it, indicating the essential
2a and 2b). The benzoxylation of C(sp3)-H bond adjacent to requirement of copper salt. Among the tested copper sources,
oxygen24 or nitrogen25 using benzoic acid have been also the CuCl2 proved to be the best, affording the desired product
reported (Scheme 2c and 2d). Even the unactivated C(sp3)-H in 30% yield (entry 6). Cu(OAc)2, CuSO4 and Cu2O only
bond could be benzoxylated by applying Cu/Selectfluor system delivered a trace amount of products. Different bases were
(Scheme 2e).26 However, Cu-catalyzed/mediated the benzoxyl- further screened and KOAc showed the best efficiency. Varying
ation between benzoic acid with DMAc is quite challenging the ratios of CuCl2, KOAc and the o-methylbenzoic acid
especially for the benzoic acid bearing electron-withdrawing influenced the outcome considerably (entries 7, 13-19). The
group at ortho-position, probably due to the tendency for 75% yield could be achieved with the o-methylbenzoic
decarboxylation of benzoic acid in amide solvent.27 In addition, acid/CuCl2/KOAc ratio at 1/3/3 (entry 17), and further
there are two kinds of C(sp3)-H bonds in the DMAc molecule, increased the amount of KOAc led to a gradually decreased
which may also pose a problem on the selectivity. Recently, yield (entry 18 and 19), which might be ascribed to the
Paster has developed a method for selective coupling of competition reaction between KOAc and benzoic acid with
benzoic acid with N,N-dialkylamides at the C(sp3)-H bond DMAc. The yield could be further increased to 87% under N2
adjacent to nitrogen employing Cu-MOFs and TBHP (tertiary- balloon (entry 20). It should be noted that the reaction showed
butyl hydroperoxide) (Scheme 2f).28 Herein, we would like to high selectivity. In all the cases, the oxidative coupling
report the first example of Cu(II)-mediated oxidative coupling occurred exclusively at the C(sp3)-H bond adjacent to the keto
of aromatic carboxylic acid and alkylamides via a keto sp3 C- group of the DMAc, and no reaction proceeded at the position
H/O-H cross coupling process (Scheme 2g). Furthermore, the alpha to the nitrogen.
reaction proceeded in absence of any peroxide oxidant, which
is potentially hazardous in large-scale experiment.
With the optimized conditions in hand, we set out to
explore the substrate scope. Firstly, a wide array of diversely
substituted benzoic acids were tested for coupling with DMAc.
As shown in Scheme 3, benzoic acid bearing either electron-
Results and Discussion
Table 1 Optimization of the reaction conditionsa
Entry
Cu-salt
Base
Yield of 3aa [%]b
1
2
CuCl
Cu2O
NaOAc
NaOAc
25
5
3
4
5
6
7
8
Cu(OAc)2
CuSO4
CuBr
CuCl2
CuCl2
NaOAc
NaOAc
NaOAc
NaOAc
KOAc
K2CO3
Trace
Trace
10
30
33
6
CuCl2
9
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2 (2 eq)
CuCl2 (3 eq)
CuCl2 (4 eq)
CuCl2 (3 eq)
CuCl2 (3 eq)
CuCl2 (3 eq)
CuCl2 (3 eq)
CuCl2 (3 eq)
K3PO4
Cs2CO3
CsOAc
LiOAc
KOAc (1 eq)
KOAc (1 eq)
KOAc (1 eq)
KOAc (2 eq)
KOAc (3 eq)
KOAc (4 eq)
KOAc (5 eq)
KOAc (3 eq)
21
5
10
11
12
13
14
15
16
17
18
19
20
15
21
41
50
43
62
75
42
25
87c
aReaction conditions: o-methylbenzoic acid 1a (0.25 mmol, 34 mg), Cu-salt (0.25 mmol), base (0.25 mmol), DMAc (1 mL), under
nitrogen, 150 oC, 24 h. bIsolated yields. cUnder nitrogen balloon.
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Scheme 3 Scope of benzoic acida,b
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DOI: 10.1039/C7OB01190C
withdrawing or electron-donating substituents all reacted
smoothly with DMAc, regardless of the ortho-, meta- or para-
substitution patterns. Para- or meta-substituted benzoic
acids coupled with DMAc in 62-94% yields (3ja-3ta), whereas
ortho-substituted benzoic acids (3ca-3ia) gave relatively
lower yield probably due to the steric hindrance.
Nevertheless, the presence of the ortho-alkyl did not
significantly affect the reaction yield. 2, 4, 6-Trimethyl
benzoic acid (3va) gave a comparable yield with both the
para- and the meta-methylbenzoic acids (3na and 3la). Even
the bulkier ortho-phenyl benzoic acid (3ea) was successfully
converted into the corresponding product in 80% yield. The
reaction was highly compatible with various functional
groups. Importantly, the halogen substituents, including F, Cl,
Br and I were all well tolerated. These provided a diversified
handle for further transformation. Other synthetically useful
functional groups, such as -NO2, -OMe, -CN also survived
after the reaction (3fa-3ha, 3ma, 3sa). It should be noted that
the o-nitrobenzoic acid 1f which was very prone to undergo
the decarboxylation,27 also underwent this transformation to
give the corresponding product in 63% yield. The
functionality -OH group (3ia) was also compatible with the
reaction conditions, albeit in a relatively lower yield (24%). 2-
Oxo-2-phenylacetic acid (3wa) and 2-naphthoic acid (3xa
)
were also suitable substrates. 2-formylbenzoic acid (1y),
however, failed to undergo the desired coupling, and instead,
a
transamidation reaction took place between 2-
formylbenzoic acid and DMAc to give 2-formyl-N,N-dimethyl
benzamide. In case of phenyl propiolic acid, only the
decarboxylative coupling products was formed. The reaction
was also uncompatible with heterocyclic such as 1H-Indole-2-
carboxylic acid, thiophene-2-carboxylic acid and picolinic acid,
leading to a complicated mixture.
It is well known that cinnamic acid can be easily decarboxy-
lated under copper catalysis.29 Interestingly, under our
developed conditions (See the ESI, Table 2)，the cinnamic
acid could also react smoothly with DMAc to afford the
desired coupling products in a moderate yield, accompanied
with a significant amount of transamidation by-products
(Scheme 4). In the latter case, the DMAc served as a N,N-di-
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Scheme 4 Scope of cinnamic acida
aliphatic acids with higher pKa value such as nitroacetic acid,
trifluoroacetic acid and the cyanoacetic acid failed to couple
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DOI: 10.1039/C7OB01190C
with DMAc.
Then, we further explored the scope of N,N-dialkyl
acetamides using m-bromobenzoic acid 1k as the coupling
partner. A series of N,N-disubstituted acetamides 2b
examined (Scheme 5). As illustrated, the aliphatic amides
reacted smoothly with m-bromobenzoic acid, affording 4kb
-2i were
-
4kg in generally good yields (51-78%), irrespective of bearing
cyclic and acyclic carbon chain. The N-acetyl-morpholine (4ke
and the Weinreb amide (4kg), containing oxygen atom, were
also tolerant. But this approach was not so efficient for the N-
methyl-2-pyrrolidone (4kh) and N,N-dimethylpropionamide
)
(4ki), only around 20% yields of desired products were
obtained. The relative lower reactivity of the methylene C-H
bond as well as the steric hindrance might be the reason for
the obtained lower yields.
Subsequently, we ran a series of control experiments to
gain insight into the reaction mechanism. The reaction of o-
methylbenzoic acid 1a with DMAc in O2 atmosphere
substantially decreased the yield compared with that under
N2 (See the ESI, Table 5). Moreover, the reaction was almost
halted in the presence of commonly used radical quencher 2,
2, 6, 6-tetramethyl-piperidine-N-oxyl (TEMPO) (See the ESI,
Table 6).32 These observations suggested the reaction might
go through a radical process. An EPR signal trapped by free-
radical spin-trapping agent DMPO (5, 5-dimethyl-1-pyrroline-
N-oxide) exhibited in the EPR (electron paramagnetic
resonance) spectra,33 which consisted of six peaks with
calculated hyperfine splittings (g0 = 2.0061, αN = 14.3 G, αH =
Scheme 5 Scope of N,N-disubstituted acetamidesa
(a)
methylamine precursor, transferring the N,N-dimethylamino
group to the cinnamic acid. This is very similar to N,N-
dimethylformamide (DMF), which is frequently reported as
dimethylamine surrogate in the literature.5, 30 At this point, it
is not clear why the cinnamic acid was relatively easier to
undergo the transamidation reaction than the benzoic acid
under the developed condition.
(b)
In general, no desired coupling products were observed for
aliphatic acids (See the ESI, Table 4), which was perhaps
because of the relatively lower acidity of the common
aliphatic acid in comparison with acetic acid. In sharp
contrast, the aromatic and cinnamic acids were acid enough
to be deprotonated by KOAc (pKa=11.1 for benzoic acid
versus pKa=12.3 for acetic acid in DMSO),31 thus the
produced carboxylic anion could successfully attack the
DMAc cation to give the product (vide infra, scheme 7). The
propensity of decarboxylation could be the reason why the
Fig. 1 Electron paramagnetic resonance (EPR) spectra (X band, 9.4 GHz, room
temperature). (a) o-methylbenzoic acid 1a (0.2 mmol), CuCl2 (0.1 mmol), KOAc
(0.2 mmol) in DMAc (1 mL) heated at 150 oC for 2 h under N2 balloon; (b) CuCl2
(0.1 mmol) in DMAc (1 mL) and CuCl2 (0.1 mmol), KOAc (0.2 mmol) in DMAc
heated at 150 oC for a given time under N2 balloon.
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20.3 G) (Fig. 1a), it might be attributed to the DMAc radical
species.34 As is shown in Fig. 1b, Cu (II) signal decreased
gradually with the reaction proceeded, which was probably
due to the reduction of Cu (II) to the EPR silent Cu (I) species
by DMAc. In addition, the EPR signal decreased further with
the addition of KOAc to the solution, suggesting that KOAc
could promote the reduction of the CuCl2.35
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DOI: 10.1039/C7OB01190C
When the DMAc was treated with CuCl2 in the absence or
presence of KOAc at 150
℃ for 24 h. The 2-chloro-N,N-di
methylacetamide and 2-(dimethylamino)-2-oxoethyl acetate
were detected, respectively (See the ESI, Table 7). In
comparison, no 2-(dimethylamino)-2-oxoethyl acetate was
observed when DMAc was treated with KOAc in the absence
of CuCl2. Both these control and the EPR experiments
suggested that the CuCl2 served an important role to oxidize
Scheme 7 The proposed mechanism
The positive Hammett correlation with substituent constants
suggested that at least a partially negative charge was
builded up on benzoic acid in the transitional state.36 In
addition, the relatively low ρ value shows the reaction is less
sensitive to substituents on the benzoic acid.
the DMAc, which possibly produced
a carbon cation
intermediate, and subsequently reacted with anions Cl- and
AcO-.
Scheme 6 Kinetic isotope effect
Based on the above experimental results and our
observations, a plausible mechanism is illustrated in Scheme
7. Firstly, DMAc was oxidized via a single-electron transfer
(SET) pathway by Cu (II) from Cu (I) to afford a free-radical
species A 37
Finally, the benzoate
acid with the aid of KOAc, reacted with
product 3ba
.
which was further oxidized to a carbon cation
which was formed from the benzoic
to give the target
B.
C
B
.
We also measured the kinetic isotope effect (KIE) (Scheme
6). The observed significant KIE value of 2.0 (See the ESI) for
the reaction between 4-methylbenzoic acid (1n) and a
mixture of DMAc and DMAc-d9, suggests that C(sp3)-H bond
cleavage of the DMAc may be related to the rate-determining
step.
A Hammett plot was constructed via a competitive
experiment between the para-substituted benzoic acids and
the benzoic acid in DMAc in the presence of CuCl2 and KOAc
(Fig. 2). The obtained Hammett plot gave a ρ value of +0.2392.
Conclusion
In summary, we described a rare example of oxidative C–O
coupling by direct C(sp3)-H bond activation of N,N-dimethyl
acetamides, using commercially available and inexpensive
copper source. This protocol proceeds through the cleavage
of C-H and O-H bond and the formation of a new C-O bond,
which provided a convenient approach to synthesize O-
benzoylglycolamides and possessed moderate functional
group tolerance. Furthermore, a plausible mechanism has
been proposed to account for the coupling reaction.
Acknowledgements
We thank the National Natural Science Foundation of China
(No. 21572137) for their financial surpport. We are grateful
for Comprehensive Training Platform of Specialized Labora-
tory, College of Chemistry, Sichuan University.
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