Mo(CO)6 as a Solid CO Source in the Synthesis of Aryl/Heteroaryl Weinreb Amides under Microwave-Enhanced Condition

Article abstract of DOI:10.1071/CH16213

The facile transformation of aryl/heteroaryl nonaflates into corresponding amides via Pd-catalyzed aminocarbonylation using Mo(CO)6 as a solid CO source under microwave-enhanced condition is reported. The method was found to be tolerant with respect to a

Full text of DOI:10.1071/CH16213

CSIRO PUBLISHING
Aust. J. Chem.
Full Paper
http://dx.doi.org/10.1071/CH16213
Mo(CO)6 as a Solid CO Source in the Synthesis
of Aryl/Heteroaryl Weinreb Amides under
Microwave-Enhanced Condition
A B
,
C
D E
,
Raghu Ningegowda,
Savitha Bhaskaran, Ayyiliath M. Sajith,
F
C
Chandrashekar Aswathanarayanappa, M. Syed Ali Padusha,
Nanjunda Swamy Shivananju
G H
,
A H
,
and Babu Shubha Priya
A
Department of Studies in Chemistry, University of Mysore, Mysuru-570006, India.
Biocon Ltd, Hosur road, Electronic city, Bengaluru- 560100, Karnataka, India.
B
C
Post Graduate and Research Department of Chemistry, Jamal Mohamed College,
Bharathidasan University, Tiruchirapalli, India.
D
E
Department of Chemistry, Nehru Arts and Science College, Kannur University,
Kannur, India.
Post Graduate and Research Department of Chemistry, Government College Kasargod,
Kannur University, Kannur, India.
F
Department of Industrial Chemistry, Jnanasahyadri, Kuvempu University,
Shimoga, India.
G
Department of Biotechnology, Sri Jayachamarajendra College of Engineering,
JSS Technical Institutions Campus, Mysuru –570006, Karnataka, India.
H
Corresponding authors: Email: priyabs_chem@yahoo.com; nanju_chem@yahoo.com
The facile transformation of aryl/heteroaryl nonaflates into corresponding amides via Pd-catalyzed aminocarbonylation
using Mo(CO)6 as a solid CO source under microwave-enhanced condition is reported. The method was found to be
tolerant with respect to a diverse range of electronically biased aryl/heteroaryl nonaflates, and exceptional yields were
obtained. The optimized protocol was further extended to a diverse range of amines.
Manuscript received: 2 April 2016.
Manuscript accepted: 17 May 2016.
Published online: 8 July 2016.
Introduction
fragment in plastics, detergents, lubricants, as well as in
many biologically relevant molecules; thus, robust synthetic
methods that would allow access to these analogues are highly
desirable.
The development of efficient synthetic methods that would
allow access to a wide range of synthetically viable inter-
mediates continues to be a key area of research in chemistry.
Among these methods, palladium-catalyzed cross-coupling
reactions have been considered as the most reliable synthetic
technique for achieving these objectives. Palladium-catalyzed
C–C and C–N bond-forming reactions have gained tremendous
potentials in both academia and industry.[1–9] Ever since the
discovery of Pd-catalyzed three-component reaction by Heck
and Schoenberg[10] in 1974, considerable efforts have been
made by various researchers across the globe in improving
its synthetic applicability by developing efficient catalytic
systems.[11–16] The amide functionality being an important
The use of aryl/heteroaryl halides (I, Br, Cl) in aminocar-
bonylation reactions has been well documented in litera-
ture.[17–33] Activated aryl/heteroaryl chlorides were also
found to be efficient synthons in this transformation.[34,35]
Efforts in using pseudo halide (e.g. triflate (Tf), tosylate (Ts),
mesylate (Ms)) have expanded the substrate scope for this
transformation.[36–39] As demonstrated in the literature, aryl/
heteroaryl triflates are reactive; however, the stability con-
cerns associated with this pseudo halide open the room for
developing a highly reactive and stable counterpart that can
Journal compilation ꢀ CSIRO 2016
www.publish.csiro.au/journals/ajc

B
R. Ningegowda et al.
F
N
N
H
OH OH
N
N
HOOC
O
N
N
N
HN
H
NH
COOH
O
O
Tamibarotene
Lipitor
HMG-CoA reductase inhibitor
Gleevec
Anti-tumour agent
Anticancer agent
Fig. 1. Biologically relevant amide containing pharmaceuticals.
Table 1. Optimization of the reaction conditionsA,B
F
CF3
F
F
O
F
F
O
N
S
O
ONf
O
F
O
O
Catalyst, ligand, base
·
ϩ HCl HN
Mo(CO)6, 1,4-dioxane
MW, 120ЊC, 20 min
O
1
R
1a
2a
O
Nf ϭ ϪSO2(CF2)3ϪCF3; R ϭ alkyl, alkoxy, halo, cyano, nitro, ester, etc.
Chart 1. Synthesis of aryl/heteroaryl nonaflates.
Entry
Catalyst [mol-%], ligand [mol-%]
Base
Yield [%]
1
2
Pd(OAc)2 (2), PPh3 (4)
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
20
55
50
65
82
61
59
87
65
30
Pd(OAc)2 (2), SPhos (4)
Pd(OAc)2 (2), (S)-BINAP (4)
Pd(OAc)2 (2), dppp (4)
3
4
influence these transformations more efficiently. Aryl tosy-
lates and mesylates, though found to be attractive, are often
problematic cross-coupling substrates in palladium-catalyzed
reactions owing to their high reluctance towards oxidative
addition. Fig. 1 shows biologically relevant amide-containing
pharmaceuticals.
5
Pd(OAc)2 (2), dppb (4)
6
Pd(OAc)2 (2), dppf (4)
7
Pd(OAc)2 (2), DPEphos (4)
Pd(OAc)2 (2), xantphos (4)
Pd(OAc)2 (2), xantphos (4)
Pd(OAc)2 (2), xantphos (4)
8
9
DABCO
TEA
10
AOptimized reaction conditions performed at 5-g scale (Table 1, entry 8):
nonaflate (250 mg, 1 equiv.), amine (1.0 equiv.), base (3 equiv.), Pd catalyst
(0.02 equiv.), ligand (0.04 equiv.), Mo(CO)6 (0.35 equiv.), 1,4-dioxane,
1208C, microwave (MW) irradiation for 20 min.
BAbbreviations: PPh3, triphenyl phosphine; SPhos, 2-dicyclohexylphosphino-
20,60-dimethoxybiphenyl; BINAP,(S)-(ꢀ)-(1,10-Binaphthalene-2,20-diyl)bis
(diphenylphosphine); dppb, 1,4-Bis(diphenylphosphino)butane; dppf, 1,10-Bis
(diphenylphosphino)ferrocene; DPEphos, (Oxydi-2,1-phenylene)bis(diphenyl-
phosphine); DBU, 1,8-diazabicycloundec-7-ene; DABCO, 1,4-diazabicyclo
[2.2.2]octane; TEA, triethylamine
Result and Discussion
As a part of our research program on palladium-mediated
cross-coupling reactions,[11–16] we were interested in the use
of aryl/heteroaryl nonaflate as an efficient and stable pseudo
halide in aminocarbonylation reaction. Beller et al. reported
the in situ generation of aryl nonaflates from corresponding
phenols and their subsequent conversion into benzamides
under palladium-catalyzed condition.[40] The use of this
pseudo halide in other cross-coupling reactions is also well
studied and documented in literature.[41–43] The excellent
stability and high reactivity of this electrophile compared with
that of other pseudo halides prompted us to explore its
synthetic utility in aminocarbonylation. In most of the proto-
cols, a high gaseous CO pressure, long reaction times, and
high temperatures are essential to drive the reaction to
success.[44–48] In this paper, we report the use of Mo(CO)6 as a
solid CO-releasing agent in the aminocarbonylation reactions
of aryl/heteroaryl nonaflates (Tables 2, 4) under microwave-
assisted condition.
Guided by our previous results,[49,50] we anticipated that the
types of catalyst, solvent, and ligand would have crucial influence
on the reactivity. In the present study, a wide range of activated
and deactivated aryl/heteroaryl nonaflates were synthesized from
corresponding phenols (Chart 1).
Theseelectrophilic intermediateswerefoundtohave excellent
shelf life and can be stored under usual laboratory conditions for
long periods of time. Literature studies indicate that electrophiles
containing an electron-donating group (deactivated system)

Aryl/Heteroaryl Nonafluorosulfonates
C
Table 2. Conversion of aryl/heteroaryl nonaflates to weinreb amide
via Pd-catalyzed aminocarbonylationA
PPh2
PPh2
Fe
O
O
PPh2
PPh2
PPh2
PPh2
0.02 equiv. Pd(OAc)2
0.04 equiv. xantphos
3.0 equiv. DBU
O
N
ONf
O
dppf
Xantphos
DPEphos
·
ϩ HCl HN
R
R
0.35 equiv. Mo(CO)6, dioxane
MW, 120ЊC, 20 min
O
1
2
PCy2
OMe
PPh2
PPh2
MeO
Ph2P
PPh2
n
Entry
Ar-ONf
Product
YieldB
[%]
dppp: n ϭ 1
dppb: n ϭ 2
SPhos
(S)-BINAP
N
O
ONf
O
Fig. 2. Ligands used for optimization.
1
2
3
87
91
92
O
O
2a
are often problematic coupling partners in palladium-catalyzed
reactions. Accordingly, 4-methoxy phenyl 1,1,2,2,3,3,4,4-
nonafluorobutane-1-sulfonate was first examined as the model
substrate. Speculating the high reactivity of the electrophile, we
explored a completely phosphine-free strategy on our model
substrate. Unfortunately, a product could not form under this
condition. Palladium catalysts along with bidentate ligands
were found to provide true catalytic species, which was found
to be the driving factor for the aminocarbonylation of triflates
(Table 1). Therefore, a diverse set of bidentate ligands (Fig. 2)
were screened in our model reaction. Among the bidentate
ligands examined, the combination of palladium(II) acetate
(Pd(OAc)2) and xantphos was found to give superior results
(Table 1, entry 8).
O
N
O
ONf
2b
O
N
ONf
NO2
O
NO2
The optimized condition (Table 1, entry 8) was subsequ-
ently applied to a wide range of aryl/heteroaryl nonaflates
bearing electron-donating or electron-withdrawing groups.
All substrates studied produced exceptional results. The
results are depicted as shown in Table 2, Notably, 4-nitro
and 4-cyano nonaflates (activated systems) reacted much
faster without any deleterious side reactions[51] (Table 2,
entries 3 and 20). A great deal of generality was observed
with nonaflates. Even sterically crowded aryl nonaflates were
found to give excellent results under this reaction condition.
(Table 2, entries 18 and 26).
The importance of hetero aromatic systems in the field of
material science and medicinal chemistry prompted us to further
expand the scope of this protocol on heterocyclic systems. This
methodology was found to give good results, and reasonable
yields of the amino-carbonylated products were obtained
(Table 2, entries 7, 10, 13, and 28).
2c
O
N
ONf
O
F
F
4
90
NC
NC
2d
O
N
O
ONf
5
90
2e
Chemoselective studies were also performed to check the
reactivity of aryl nonaflates with halides (Table 3, entries 1 and
2). A mixture of products were obtained using 4-iodo and
4-bromo nonaflates, as anticipated. However, the 4-chloro
nonaflate was found to be intact under the conditions explored
(Table 3, entry 3).
Encouraged by these exciting results, we then explored this
optimized reaction condition with respect to other amines.
Excellent yields of the amino-carbonylated products were
obtained (Table 4).
O
N
ONf
O
F
F
6
79
F
F
2f
(Continued)

D
R. Ningegowda et al.
Table 2. (Continued)
Product
Table 2. (Continued)
Entry
Ar-ONf
YieldB
[%]
Entry
Ar-ONf
Product
YieldB
[%]
O
N
N
O
N
ONf
ONf
N
O
O
7
83
15
87
NO2
NO2
2g
2o
O
N
O
N
O
ONf
ONf
O
8
87
16
88
NO2
NO2
2h
Cl
Cl
2p
O
N
ONf
O
CF3
O
N
CF3
O
ONf
9
89
71
89
F
F
2i
17
91
O
N
ONf
O
10
2q
N
Cl
N
2j
Cl
O
N
O
ONf
O
F
O
F
18
79
O
N
ONf
O
F
F
11
2r
2k
2l
O
N
O
ONf
O
O
ONf
26C,D
N
O
19
12
13
93
65
Br
Br
2s
O
ONf
N
N
N
N
O
N
ONf
O
N
2m
20
90
CN
CN
O
N
O
ONf
2t
14
89
O
N
Cl
Cl
ONf
O
Cl
Cl
NO2
NO2
21
79
2n
2u
(Continued)
(Continued)

Aryl/Heteroaryl Nonafluorosulfonates
E
Table 2. (Continued)
Product
Table 2. (Continued)
Product
Entry
Ar-ONf
YieldB
[%]
Entry
Ar-ONf
YieldB
[%]
ONf
ONf
O
N
O
ONf
COOMe
COOMe
22
83
31
80
COOMe
COOMe
ONf
2v
2ze
O
N
O
AReaction conditions: Pd(OAc)2 (0.02 equiv.), xantphos (0.04 equiv.), aryl
nonaflate (250 mg, 1.0 equiv.), amine (1.0 equiv.), Mo(CO)6 (0.35 equiv.),
DBU (3.0 equiv.) in dioxane (2 mL) at 1208C, 20 min, MW
BIsolated yields.
23
24
88
CA mixture of products were observed.
2w
DConversion evaluated by gas chromatography–mass spectrometry
(GC-MS).
O
ONf
O
N
27C,D
Br
Br
2x
Table 3. Chemoselective studies of aryl nonaflatesA
O
N
ONf
O
O
N
O
N
0.02 equiv. Pd(OAc)2
0.04 equiv. xantphos
3.0 equiv. DBU
O
ONf
O
25
85
·
HCl HN
ϩ
ϩ
ϩ 1
O
0.35 equiv. Mo(CO)6, dioxane
MW, 120ЊC, 20 min
X
X
2y
2z
N
O
O
1
3
4
O
O
ONf
N
26
27
77
84
4
B [%]
Recovery of 1B
Cl
Cl
Entry
X
3
B [%]
Cl
Cl
1
2
3
4-Iodo (1l)
28
26
90
24
30
16
13
7
4-Bromo (1k)
4-Chloro (1j)
O
O
Traces
N
N
AReaction conditions: Pd(OAc)2 (0.02 equiv.), xantphos (0.04 equiv.), aryl
nonaflate (250 mg, 1.0 equiv.), amine (1.0 equiv.), Mo(CO)6 (0.35 equiv.),
DBU (3.0 equiv.) in dioxane (2 mL), 1208C, 20 min, MW
BConversion evaluated by GC-MS.
N
O
NfO
O
2za
N
N
O
28
29
67
N
ONf
O
2zb
After demonstrating the synthetic utility of Mo(CO)6-
mediated palladium-catalyzed aminocarbonylation reaction
of aryl/heteroaryl nonaflates, we studied the reactivity
of other pseudo halides (OTs, OMs, OTf) and halides (I, Br,
Cl) under similar conditions. The results are summarized in
Table 5.
O
F
F
ONf
F
F
O
N
26C,D
Br
Br
2zc
In Scheme 1, the general reaction mechanism for the sequen-
tial activation and palladium-catalyzed aminocarbonylation of
phenyl nonaflate is shown. Phenyl nonaflate undergoes oxida-
tive addition to the active Pd0 phosphine species to form the
corresponding arylpalladium(II) complex. After coordination
and insertion of CO followed by the nucleophile attack of
methoxy methyl amine, the phenyl weinreb amide is formed.
Finally, under the assistance of base, the active Pd0 species is
regenerated.
O
N
ONf
O
30
86
CF3
CF3
2zd
(Continued)

F
R. Ningegowda et al.
Table 4. Aminocarbonylation of aryl nonaflates using primary and secondary aminesA
R1
O
N
R2
0.02 equiv. Pd(OAc)2
0.04 equiv. xantphos
3.0 equiv. DBU
ONf
R2
R1
ϩ
N
H
0.35 equiv. Mo(CO)6, dioxane
MW, 120ЊC, 20 min
O
O
5
1
Entry
Ar-ONf
Amine
Product
YieldB [%]
ONf
O
NH
NH2
1
91
O
5a
O
F
O
NH
2
92
ONf
NH2
O
O
5b
F
O
ONf
O
NH2
NH
O
3
90
O
O
5c
O
O
N
ONf
4
91
HN
O
O
5d
O
O
N
ONf
5
89
HN
O
O
5e
(Continued)

Aryl/Heteroaryl Nonafluorosulfonates
G
Table 4. (Continued)
Entry
Ar-ONf
Amine
Product
YieldB [%]
O
N
ONf
6
90
HN
O
5f
O
AReaction conditions: Pd(OAc)2 (0.02 equiv.), xantphos (0.04 equiv.), aryl nonaflate (250 mg, 1.0 equiv.), amine
(1.0 equiv.), Mo(CO)6 (0.35 equiv.), DBU (3.0 equiv.) in dioxane (2 mL), 1208C, 20 min, MW
BIsolated yields.
Table 5. Screening of different leaving groups under the optimized
conditionsA
Conclusion
A mild and efficient synthetic protocol for the aminocarbony-
lation of aryl/heteroaryl nonaflates using Mo(CO)6 as a solid CO
source under microwave-enhanced condition was developed
(Tables 2, 4). The methodology was found to be tolerant to a
wide range of functional groups. Comparative studies with
respect to other pseudo halides revealed the superior reactivity
and stability of nonaflates in aminocarbonylation reaction.
O
N
0.02 equiv. Pd(OAc)2
0.04 equiv. xantphos
3.0 equiv. DBU
X
O
·
ϩ HCl HN
ϩ 6
0.35 equiv. Mo(CO)6,
1,4-dioxane
O
O
O
MW, 120ЊC, 20 min
6
2a
Supplementary Material
Experimental details, characterization and spectral data are
available on the Journal’s website.
Entry
X
2aB [%]
YieldC [%]
1
2
3
4
5
6
OTs (6a)
50
6
35
Traces
44
Acknowledgements
OMs (6b)
OTf (6c)
4-Iodo
R.N. is thankful to Biocon Ltd, Bangalore, India for providing access to
laboratory facilities and analytical support for conducting the present study.
This work was financially supported by the Science and Engineering
Research Board (New Delhi, Government of India) through a Start-Up
Research Grant (Young Scientists) and Life Sciences Grant No. SB/FT/LS-
297/2012 (B.S.P.) and Department of Science and Technology (New Delhi,
Government of India) under a DST/INDO/SOUTH AFRICA/P-12/2014
grant (S.N.S.).
64
90
85
5
83
4-Bromo
4-Chloro
79
Traces
AReaction conditions: Pd(OAc)2 (0.02 equiv.), xantphos (0.04 equiv.), aryl
pseudo halide (250 mg, 1 equiv.), amine (1.0 equiv.), Mo(CO)6 (0.35 equiv.),
DBU (3.0 equiv.) in dioxane (2 mL), 1208C, 20 min, MW
BConversion evaluated by GC-MS.
CIsolated yields.
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