Ortho-benzoxylation of N-alkyl benzamides with aromatic acids catalyzed by ruthenium(II) complex

Article abstract of DOI:10.1002/chem.201304646

A highly regioselective ortho-benzoxylation of N-alkyl benzamides with aromatic acids in the presence of [{RuCl₂(p-cymene)}₂], AgSbF₆, and (NH₄)₂S₂O₈ in 1,2-dichloroethane at 100°C for 24 h affording ortho-benzoxylated N-alkyl benzamides by C-H bond activation is described. Further, Ru-catalyzed alkenylation is done at the ortho C-H bond of benzoxylated N-alkyl benzamides with alkenes in water solvent. Subsequently, the benzoxyl moiety of N-alkyl benzamides was converted into a hydroxyl group in the presence of base or acid. A possible reaction mechanism was proposed to account for the present coupling reaction. C-H bond activation: A highly regioselective ortho-benzoxylation of benzamides with substituted benzoic acids in the presence of a ruthenium catalyst is described. Further, Ru-catalyzed alkenylation is carried out at the ortho C-H bond of benzoxylated N-alkyl benzamides with alkenes in water solvent. Subsequently, the benzoxyl moiety of N-alkyl benzamides was converted into the hydroxyl group in the presence of base or acid (see scheme).

Full text of DOI:10.1002/chem.201304646

DOI: 10.1002/chem.201304646
Full Paper
&
Organic Chemistry
ortho-Benzoxylation of N-Alkyl Benzamides with Aromatic Acids
Catalyzed by Ruthenium(II) Complex
Kishor Padala and Masilamani Jeganmohan*[a]
Abstract: A highly regioselective ortho-benzoxylation of N-
alkyl benzamides with aromatic acids in the presence of
[{RuCl2(p-cymene)}2], AgSbF6, and (NH4)2S2O8 in 1,2-dichloro-
ethane at 1008C for 24 h affording ortho-benzoxylated N-
alkyl benzamides by CÀH bond activation is described. Fur-
ther, Ru-catalyzed alkenylation is done at the ortho CÀH
bond of benzoxylated N-alkyl benzamides with alkenes in
water solvent. Subsequently, the benzoxyl moiety of N-alkyl
benzamides was converted into a hydroxyl group in the
presence of base or acid. A possible reaction mechanism
was proposed to account for the present coupling reaction.
Introduction
hydrides, have been widely used. Also, the known benzoxyla-
tion reaction is limited with 2-pyridyl, oxime, and NH-acetyl di-
recting-group-substituted aromatics.[6]
Metal-catalyzed chelation-assisted functionalization at the
ortho CÀH bond of aromatics with nucleophiles by CÀH bond
activation is an ideal method in organic synthesis.[1] By employ-
ing this method, various chemical bonds, such as CÀC, CÀN,
CÀX, and CÀO are efficiently constructed in a highly atom-eco-
nomical and environmentally friendly manner. Palladium, rho-
dium, and ruthenium complexes are widely used as catalysts
for these types of reactions. In the presence of these catalysts,
CÀC, CÀX, and CÀN bond formation have been extensively
studied in the literature. But, CÀO bond formation has not
been well explored. This is most probably due to the high elec-
tronegativity of the oxygen element and the strong metal–
oxygen bond strength.[2] Various oxygen sources, such as ace-
tates, alkoxides, alcohols, aldehydes, anhydrides, and carboxylic
acids, are used as nucleophiles in the reaction. Several direct-
ing groups, such as 2-pyridyl, carbonyl, ester, nitrile, carboxylic
acid, and N-acetyl, can be efficiently used for the reaction.
With the assistance of these directing groups, acetoxylation,[3]
alkoxylation,[4] hydroxylation,[5] and benzoxylation[6] at the
ortho CÀH bond of directing-group-substituted aromatics with
various oxygen nucleophiles have been studied. Among these
transformations, ortho-benzoxylation of substituted aromatics
is less focused in the literature.[6] This is mainly due to the
rapid complex formation of the carboxylic acids with the metal
complexes. To suppress the metal carboxylate complex forma-
tion, carboxylate sources, such as benzoate iodonium salts,
benzoyl chlorides, benzaldehydes, and aromatic carboxylic an-
Amide is a versatile functional group that has been widely
used for various organic transformations. Meanwhile, amide is
also a good directing group for metal-catalyzed CÀH function-
alization reactions.[1,7] Substituted amides, such as Ar-CONH2,
Ar-CONHR, and Ar-CONR2, are efficiently used for the CÀH acti-
vation reaction. By using amide directing groups, CÀC, CÀN,
and CÀX bonds can be formed at the ortho CÀH bond of aro-
matics.[1,7] But, the amide-directed CÀO bond is limited in the
literature except ortho hydroxylation at the ortho CÀH bond of
CONR2-substituted benzamides.[5e] Recently, we have reported
ortho-benzoxylation of acetanilides with benzoic acids in the
presence of ruthenium catalyst.[6f] Our continuous interest
in the ruthenium-catalyzed CÀH bond activation reaction
prompted us to explore the possibility of ortho-benzoxylation
of benzamides with aromatic acids in the presence of rutheni-
um catalyst.[8] Herein, we report a highly regioselective ortho-
benzoxylation at the ortho CÀH bond of substituted N-alkyl
benzamides with benzoic acids in the presence of [{RuCl2(p-
cymene)}2], AgSbF6, and (NH4)2S2O8. The catalytic reaction was
compatible with various sensitive functional-group-substituted
benzamides as well as aromatic acids. Further, a ruthenium-cat-
alyzed alkenylation at the ortho CÀH bond of benzoxylated N-
alkyl benzamides with alkenes in water solvent was conducted.
Subsequently, the benzoxyl moiety of N-alkyl benzamides was
converted into a hydroxyl group in the presence of base or
acid.
[a] K. Padala, Dr. M. Jeganmohan
Department of Chemistry
Results and Discussion
Indian Institution of Science Education and Research
Pune 411021, Maharashtra (India)
Fax: (+91)20-25865315
When N-methyl 3,4-dimethoxy benzamide (1a) was treated
with 4-chlorobenzoic acid (2a) in the presence of [{RuCl2(p-
cymene)}2] (4 mol%), AgSbF6 (20 mol%), and (NH4)2S2O8
(2.0 mmol) in 1,2-dichloroethane (DCE) at 1008C for 24 h, an
ortho-benzoxylated benzamide 3a was observed in 85% isolat-
E-mail: mjeganmohan@iiserpune.ac.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304646.
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in 62% yield (entry 5). Remaining
oxidants were totally ineffective
for the reaction. The catalytic re-
action was tested with bases,
such as K2CO3 and NaOAc. How-
ever, in the reaction, no product
3a was observed. Then, the cat-
Scheme 1. ortho-Benzoxylation of 1a with 2a.
alytic reaction was tested with
various solvents, such as tert-
ed yield (Scheme 1). In the substrate 1a, there are two ortho
CÀH bonds for the activation. The catalytic reaction is highly
regioselective; CÀH activation takes place selectively at a steri-
cally less hindered CÀH bond. It is important to point out that
CÀO bond formation at the ortho CÀH bond of N-alkyl benza-
mide is not known in the literature. This is the first report that
discloses CÀO bond formation at the ortho CÀH bond of N-
substituted benzamide in the presence of a less expensive and
easily affordable Ru catalyst[9] and (NH4)2S2O8 oxidant.
BuOH, THF, DMF, DMSO, dichloromethane, 1,4-dioxane, tolu-
ene, MeOH, AcOH, and 1,2-dichloroethane. Among them, 1,2-
dichloroethane was effective, providing 3a in 91% yield
(entry 4). The remaining solvents were totally ineffective. Based
on these optimization studies, we have concluded that
AgSbF6, (NH4)2S2O8, and 1,2-dichloroethane in the presence of
[{RuCl2(p-cymene)}2] at 1008C for 24 h are the best conditions
for the reaction. Under the optimized reaction conditions, the
present coupling reaction was tested with N-methoxy-3,4-di-
methoxybenzamide, N-N-diethyl 3,4-dimethoxybenzamide, and
3,4-dimethoxybenzamide [Eq. (1)]. In these reactions, no cou-
pling product was observed. These results clearly revealed that
COÀNHMe is the effective directing group for the reaction
compared with COÀNHOMe, COÀNEt2, and CONH2-substituted
benzamide.
A proper selection of oxidant, additive, and solvent is highly
important for the success of the catalytic reaction. Initially, the
coupling of 1a and 2a was tested with various additives
(20 mol%), such as AgBF4, AgOTf, KPF6, and AgSbF6 in the
presence of Ru catalyst and (NH4)2S2O8 oxidant in DCE at
1008C for 24 h (Table 1, entries 1–4). AgBF4 was not effective
Table 1. Optimization studies.[a]
Under the optimized reaction conditions, the coupling reac-
tion was examined with various ortho, para, and meta-substi-
tuted aromatics acids 2b–l with 1a (Table 2). The coupling re-
action was compatible with electron-donating, halogen and
electron-withdrawing functional groups, such as Me, H, I, Br, Cl,
F, CF3, and NO2-substituted aromatic acids 2a–l. Thus, 4-meth-
ylbenzoic acid (2b) and benzoic acid (2c) reacted with 1a
yielding ortho-benzoxylated products 3b and 3c in 69 and
68% yields, respectively (entries 1 and 2). Halogen groups,
such as 4-iodo 2d, 4-bromo- 2e, and 4-fluoro- 2 f substituted
benzoic acids were efficiently involved in the reaction, provid-
ing coupling products 3d–f in 65, 74, and 66% yields, respec-
tively (entries 3–5). Electron-withdrawing groups, such as 4-
CF3- 2g and 4-nitro- 2h substituted benzoic acids were also
nicely involved in the reaction, affording products 3g and 3h
in 76 and 64% yields, respectively (entries 6 and 7). Next, the
coupling reaction was tested with meta- and ortho-substituted
aromatic acids 2i–k. meta-Iodo 2i or chloro- 2j benzoic acids
reacted nicely with 1a giving the corresponding coupling
products 3i and 3j in 71 and 75% yields, respectively (en-
tries 8 and 9). Highly sensitive ortho iodobenzoic acid (2k) also
efficiently participated in the reaction, providing coupling
product 3k in 69% yield (entry 10). In the product 3k, the
iodo group was retained and not cleaved under the reaction
Entry
Solvent
Oxidant
Additive
Yield of 3a [%][b]
1
2
3
4
5
6
7
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
Ag2CO3
AgBF4
KPF6
NR
64
45
91
62
AgOTf
AgSbF6
AgSbF6
AgSbF6
–
–
trace
NR
(NH4)2S2O8
[a] All reactions were carried out under the following conditions: 1a
(1.0 mmol), 2a (1.0 mmol), [{RuCl2(p-cymene)}2] (4 mol%), additive
(20 mol%), and oxidant (2.0 mmol) in solvent (4.0 mL) at 1008C for 24 h
under the nitrogen atmosphere. [b] Yields were determined by the
1H NMR spectroscopic integration method, by using mesitylene as an in-
ternal standard.
for the reaction (entry 1). KPF6 and AgOTf were partially effec-
tive, yielding 3a in 64 and 45% yields, respectively (entries 2
and 3). AgSbF6 was very effective for the reaction affording
product 3a in 91% NMR spectroscopic yield (entry 4). Next,
the coupling reaction was examined with various oxidants,
such as oxone, K2S2O8, benzoquinone, Na2S2O8, Ag2CO3, Ag2O,
Cu(OAc)2, AgOAc, K2S2O8, PhI(OAc)2, and (NH4)2S2O8. Among
them, only (NH4)2S2O8 was very effective, affording product 3a
in 91% yield (entry 4). Ag2CO3 was slightly effective, giving 3a
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Table 2. Coupling of 1a with substituted benzoic acids 2b–l.[a]
Table 3. Coupling of benzamides 1b–k with 2a, 2c, and 2h.[a]
Entry
2
Product 3
Yield [%][b]
Entry
1b–k
Product 3
Yield [%][b]
1
2
3
4
5
6
7
2b: R1 =Me
2c: R1 =H
2d: R1 =I
3b: R1 =Me
3c: R1 =H
3d: R1 =I
69
68
65
74
66
76
64
1
2
3
1b
1b
1b
3m: R1 =Cl
3n: R1 =H
3o: R1 =NO2
75
66
56
2e: R1 =Br
2 f: R1 =F
3e: R1 =Br
3 f: R1 =F
2g: R1 =CF3
2h: R1 =NO2
3g: R1 =CF3
3h: R1 =NO2
4
5
6
7
8
9
10
1c: R2 =Me
1d: R2 =H
1e: R2 =I
1 f: R2 =Br
1g: R2 =Cl
1 h: R2 =F
1i: R2 =CF3
3p: R2 =Me
3q: R2 =H
3r: R2 =I
3s: R2 =Br
3t: R2 =Cl
3u: R2 =F
3v: R2 =CF3
65
69
72
69
67
65
57
8
9
2i: R1 =I
2j: R1 =Cl
3i: R1 =I
3j: R1 =Cl
71
75
10
11
69
73
11
12
66
63
[a] All reactions were carried out using 1b–k (1.0 mmol), 2a,c,h
(1.0 mmol), [{RuCl2(p-cymene)}2] (4 mol%), AgSbF6 (20 mol%), and
(NH4)2S2O8 (2.0 mmol) in ClCH2CH2Cl (3.0 mL) at 1008C for 24 h. [b] Isolat-
ed yield.
[a] All reactions were carried out by using 1a (1.0 mmol), 2b–
(1.0 mmol), [{RuCl2(p-cymene)}2] (4 mol%), AgSbF6 (20 mol%), and
(NH4)2S2O8 (2.0 mmol) in ClCH2CH2Cl (3.0 mL) at 1008C for 24 h. [b] Isolat-
l
ed yield.
volved in the reaction, yielding product 3w in 66% yield
(entry 11). N-Methyl 1-napthyl benzamide (1k) nicely under-
went coupling with 2a, affording product 3x in 63% yield
(entry 12).
conditions which is unusual in the metal-catalyzed reactions.
Sterically hindered 2-napthoic acid (2l) nicely coupled with 1a,
yielding product 3l in 73% yield (entry 11). All the catalytic re-
actions are completely regioselective and only CÀH activation
takes place at the sterically less hindered CÀH bond.
Next, the scope of regioselectivity of the unsymmetrical ben-
zamides 1l–o with 2a was examined (Scheme 2). The reaction
of N-methyl 3-methoxy- 1l and 3-chloro- 1m substituted ben-
zamides reacted with 2a providing products 4a and 4b in 65
and 62% yields, respectively, in a highly regioselective manner.
In the substrates 1l–m, ortho CÀH bond activation takes place
at a less hindered side. Similarly, substituted benzamides 1n
and 1o also efficiently coupled with 2a at a less hindered side,
yielding products 4c and 4d in 69 and 63% yields, respective-
ly, in a highly regioselective manner.
The scope of the catalytic reaction was further examined
with various sensitive functional-group-substituted benzamides
1b–j (Table 3). Treatment of N-methyl 4-methoxybenzamide
(1b) with substituted benzoic acids 2a, 2c, and 2h provided
coupling products 3m–o in 75, 66, and 56% yields, respective-
ly (entries 1–3). N-Methyl 4-methyl benzamide (1c) and N-
methyl benzamide (1d) reacted with 2a yielding ortho-benz-
oxylated products 3p and 3q in 65 and 69% yields, respective-
ly (entries 4 and 5). Halogen groups, such as I, Br, Cl, and F-sub-
stituted benzamides 1e–h provided the corresponding cou-
pling products 3r–u in 72, 69, 67, and 65% yields, respectively
(entries 6–9). Electron-withdrawing group CF3-substituted ben-
zamide 1i afforded the coupling product 3v in 57% yield
(entry 10). ortho-Methyl benzamide 1j was also efficiently in-
The catalytic reaction was also examined with other N-sub-
stituted, such as iso-propyl and benzyl, benzamides 1p and 1q
with 2a (Scheme 3). In the reaction of 1p, coupling product
4e was observed only in a 40% yield due to the steric hin-
drance of iso-propyl group. Nicely, 1q provided coupling prod-
uct 4 f in 79% yield.
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Scheme 2. Regioselective studies.
ing products 6a and 6b in 73
and 71% yields, respectively
(Scheme 4).[9g]
Subsequently,
benzoxyl moiety of benzamides
3a and 3t were converted into
the hydroxyl group, in which
products 6c and 6d were ob-
served in 92 and 81% yields
(Scheme 5), respectively, in the
presence of NaOH or H2SO4 solu-
tion.[10] It is important to men-
tion that this is one of the most
efficient methods to synthesize
ortho-hydroxy N-alkyl
benz-
amides.
A
possible reaction mecha-
nism is proposed for the present
reaction in Scheme 6. Silver salt
likely removes the chloride
ligand from the [{RuCl2(p-
cymene)}2] complex, providing
a cationic ruthenium carboxylate
Scheme 3. Reaction of N-substituted benzamides 1p and 1q with 4-chlorobenoic acid (2a).
The ortho CÀH bond of benzoxylated N-alkyl benzamides
3m and 3p were efficiently alkenylated with methyl acrylate
(5) in the presence of [{RuCl2(p-cymene)}2], KPF6 and Cu-
(OAc)2·H2O (2.0 mmol) in water solvent at 1008C for 24 h, yield-
complex 7.[11a] Coordination of the carbonyl oxygen of the ben-
zamide 1 to the cationic species 7 followed by ortho-metala-
tion affords a five-membered metalacycle 8. Coupling of 2 into
the ruthenacycle 8 affords an intermediate 9. Reductive elimi-
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Scheme 4. Ruthenium-catalyzed alkenylation reaction.
nation of intermediate 9 gives the final product 3 and a Ru0
species.[11b] Later, (NH4)2S2O8 oxidizes Ru0 to active RuII species
7 in the presence of carboxylic acid for the next catalytic cycle.
Conclusion
We have demonstrated a highly regioselective ortho-benzoxy-
lation of benzamides with substituted benzoic acids in the
presence of ruthenium catalyst. The catalytic reaction worked
very efficiently with various functional-group-substituted ben-
zamides and benzoic acids yielding ortho-benzoxylated benza-
mides in good to excellent yields. Further, a Heck-type alkeny-
lation reaction was done at the ortho CÀH bond of benzoxylat-
ed N-alkyl benzamides with alkenes in water solvent in the
presence of ruthenium catalyst. ortho-Hydroxy N-alkyl benza-
mides were prepared from benzoxylated N-alkyl benzamides in
the presence of base or acid. A possible reaction mechanism
was proposed to account for the coupling reaction.
Scheme 5. Preparation of ortho-hydroxy N-alkyl benzamides.
Experimental Section
General procedure for the
ortho-benzoxylation of N-alkyl
benzamides with aromatic
acids catalyzed by ruthenium
complex
[RuCl2(p-cymene)]2
4 mol%), AgSbF6
(0.04 mmol,
(0.20 mmol,
20 mol%), (NH4)2S2O8 (2.0 mmol),
benzamides 1 (1.00 mmol), and ar-
omatic acid 2 (1.00 mmol) were
taken in a 15 mL pressure tube,
which was equipped with a mag-
netic stirrer and septum. The pres-
sure tube was evacuated and
purged with nitrogen gas three
times (note: AgSbF6 is moisture
sensitive, thus AgSbF6 was taken
inside the nitrogen glove box). To
the tube was then added 1,2-di-
Scheme 6. Proposed mechanism.
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chloroethane (3.0 mL) as the solvent by syringe and again the tube
was evacuated and purged with nitrogen gas three times. Then,
the septum was taken out and immediately a screw cap was used
to cover the tube. The reaction mixture was allowed to stir at
1008C for 24 h. After cooling to ambient temperature, the reaction
mixture was diluted with CH2Cl2 and then filtered through Celite
and silica gel. The filtrate was concentrated, and the crude residue
was purified through a silica gel column (hexanes and ethyl ace-
tate) to give pure 3 and 4.
[5] Selected hydroxylations: a) X. Yang, G. Shan, Y. Rao, Org. Lett. 2013, 15,
2334; b) W. Liu, L. Ackermann, Org. Lett. 2013, 15, 3484; c) G. Shan, X.
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Spectral data and copies of 1H and 13C NMR spectra of all com-
pounds are listed below.
[6] a) A. R. Dick, J. W. Kampf, M. S. Sanford, J. Am. Chem. Soc. 2005, 127,
12790; b) Z. S. Ye, W. H. Wang, F. Luo, S. H. Zhang, Cheng, J. Org. Lett.
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Acknowledgements
We thank the DST (SR/S1/OC-26/2011), India for the support of
this research. K.P. thanks the CSIR for a fellowship.
[7] a) K. M. Engle, T.-S. Mei, M. Wasa, J.-Q. Yu, Acc. Chem. Res. 2012, 45, 788,
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Keywords: alkenylation
activation · hydroxylation · ruthenium
·
amides
·
benzoxylation
·
CÀH
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Received: November 27, 2013
Published online on February 26, 2014
Chem. Eur. J. 2014, 20, 4092 – 4097
www.chemeurj.org
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