Silver- versus gold-catalyzed sequential oxidative cyclization of unprotected 2-alkynylanilines with oxone

Article abstract of DOI:10.1039/c5cc08543h

Unprecedented domino oxidative cyclization reactions of unprotected 2-alkynylanilines to give functionalized 4H-benzo[d][1,3]oxazin-4-one or benzisoxazole derivatives in moderate to good yields are achieved by silver vs. gold selective catalysis. The search for the optimal reaction conditions revealed the divergent catalytic activity of NaAuCl₄·H₂O and AgNO₃.

Full text of DOI:10.1039/c5cc08543h

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DOI: 10.1039/C5CC08543H
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COMMUNICATION
Silver- versus gold-catalyzed sequential oxidative cyclizations of
unprotected 2-alkynylanilines with oxone
A. Arcadi,a* M. Chiarini,b L. Del Vecchio,a, c F. Marinellia and V. Micheletc*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Unprecedented domino oxidative cyclization reactions of alkynylaniline derivatives would be suitable for oxidative
unprotected 2-alkynylanilines to give functionalized 4H- cyclization rearrangements to build-up valuable nitrogen
benzo[d][1,3]oxazin-4-one or benzisoxazole derivatives in containing heterocycle systems (Scheme 1c) and wish
moderate to good yields are achieved by silver vs gold selective therefore to report therein our preliminary results.
Ar
1. CuCl (20 mol%)
O
catalysis. The search for the optimal reaction conditions revealed
the divergent catalytic activity of NaAuCl4.H2O and AgNO3.
Na2CO3 (2 equiv)
1 atm O2
O
O
(a)
DMSO, 80°C, 24h
2. Ac2O (5 equiv)
N
H
CF3
Ar
N
Ar
The product selectivity control by modification of the reaction
conditions represents a significant challenge in organic
synthesis.1 In particular, selective catalysis that leads to
different products from the same starting materials is a
powerful tool for divergent synthesis.2 2-Alkynylaniline
derivatives are usually employed as versatile building blocks to
assemble indole scaffolds by means of transition metals
catalysis.3 Recently, copper mediated cyclization/oxidative
reactions of 2-alkynyltrifluoroacetanilides 2 with molecular
4 (28- 49% yield)
2
Pd(NO3)2 (10 mol%)
Oxone (1 equiv)
LiCO3 (2 equiv)
4 (35- 79% yield)
(b)
(c)
MeCN, 60°C, dark
N3
3
1
O
O
N
R
R
[Au] or [Ag]
O
or
O
oxidant
R
N
NH2
This work
oxygen achieved
a
one pot/2-step synthesis of 2-
Scheme 1 [M]-catalyzed synthesis of 4H-benzo[d][1,3]oxazin-4-ones
arylbenzoxazinones 4 in low to moderate yield (Scheme 1a).4
The presence of N-TFA-protecting group was crucial to
promote the cyclization process as no reactivity was observed
in the case of the free aniline. The palladium-catalyzed
reaction of 2-azidoalkynylbenzenes 3, which are prepared from
alkynylanilines 1, was disclosed to afford a variety of the 2-
arylbenzoxazinones 4 in better yields by means of a domino
First, the readily available 2-(phenylethynyl)aniline 1a was
chosen as a model substrate to carry out oxidative cyclization
sequences with a variety of oxidants in the presence of gold or
silver catalysts. The results of Table 1 show that only the
cycloisomerization reaction 1a was observed in high yield in
the presence of molecular oxygen as the oxidant and
NaAuCl4·2H2O as the catalyst (entry 1). The expected gold-
catalyzed intramolecular hydroamination of 1a/gold-catalyzed
oxidative hydroxylation6a or C-H bond9/Bayer-Villiger4
sequence failed to occur under the present reaction
conditions. Noteworthy that the reported gold-catalyzed
domino cycloisomerization/C-H oxidative homocoupling
process of alkynylanilines 1 to give 3,3’-biindoles under
heterogeneous conditions was also precluded.10 The unique
formation of 2-substituted indole 5a with 2,6-lutidine N-oxide
as the oxidant (entry 2) was in agreement with the recent
results which highlighted that the intramolecular
hydroamination was prevalent over the intermolecular gold-
catalyzed oxidation with internal alkynes.11 Sequential
cycloisomerization/oxidation reactions were not observed
even in the presence of m-CPBA and H2O2 (entries 3, 4), which
aminopalladation/oxidative
rearrangement
under
the
presence of oxone as the oxidant (Scheme 1b).5 We
investigated the sequential gold-catalyzed cycloisomerization /
fluorination reactions of unprotected 2-alkynylanilines to
access to 3,3-difluoro-2-aryl-3H-indoles, 3-fluoro-2-aryl-indoles
and indolin-3-ones in the presence of selectfluor.6,7 As part of
our continued interest in the synthesis of heterocycles based
on atom-economical methodologies and easily accessible
starting materials,6,8 we wondered if unprotected 2-
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Chem. Commun., 2015, 00, 1-3 | 1
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are known to promote peroxidation reaction of indoles.12 (entries 11, 12). Considering that the oxidation ofV2ie-waAryrtlicinledOonllienes
Similarly, the use of AgNO3 catalyst resulted in the same to the corresponding 2-arylbenzoxazinoDnOeIs: 1w0.i1t0h39o/xCo5nCeC0a8s54t3hHe
observations and the cyclization of 1a to form 5a was favored sole oxidant was carried out in CH3NO2,13 we also examined
albeit in a lower efficiency (entries 5, 6, 7). Interestingly, the our domino process in this reaction medium without any
reaction of 1a in the presence of H2O2 as the oxidant and improvement (entry 13). The AgNO3-catalyzed reaction of 1a
AgNO3 as the catalyst led to the formation of a new (1 equiv) with oxone as the oxidant (1 equiv) in CH3CN/H2O
compound,
identified
as
the
(benzo[c]isoxazol-3- (1:1) at 80°C led to the formation of the (benzo[c]isoxazol-3-
yl)(phenyl)methanone 6a (entry 8). The formation of this yl)(phenyl)methanone 6a as the sole product (entry 14).
adduct in a moderate yield could come from a divergent Among the different silver catalysts tested, AgNO3 gave the
oxidation/annulation process. Further screening of oxidants best result (entries 15, 16). The reaction temperature was also
revealed that oxone showed higher efficiency in both the gold- decreased to 60°C, 40°C and room temperature, the best
catalyzed cycloisomerization/oxidative hydroxylation / Bayer- result being observed at 60°C (entry 17). Finally, a control
Villiger sequence of 1a and in its divergent silver-catalyzed experiment demonstrated that 6a was isolated only in low
oxidation/annulation cascade process to give 6a. Indeed, the yield without the silver catalyst (entry 18).
2-phenylbenzoxazinone 4a was isolated in satisfactory yield by The substrate scope of the new approach to the synthesis of
reacting 1a (1 equiv) with oxone as the oxidant (1 equiv) in the 4H-benzo[d][1,3]oxazin-4-ones 4 was briefly evaluated for
presence of NaAuCl4·2H2O in dry CH3CN at 80 °C (entry 9). The anilines bearing electron-donating groups on the aromatic ring
amount of water on the reaction medium played a critical role, of the alkyne (Scheme 2). The domino gold-catalyzed
as the 2-phenylbenzoxazinone 4a was prone to undergo hydroamination/oxidation sequence cleanly resulted in the
hydrolysis to give the N-benzoyl anthranilic acid 7a.4
formation of 4b and 4c in 73% and 72% isolated yields
respectively. Very likely, the presence of the CF3 substituent on
the aryl group slowed down the intramolecular gold-catalyzed
hydroamination process as a consequence of the poorer π-
coordination of the gold catalyst at the alkyne and the
competitive oxidation/annulation cascade could occur to some
extent. The heterocycle 6d was isolated in 48% yield. This is in
full agreement with some observations in the case of electron-
withdrawing-substituted alkynes.14
Table 1 Optimization of the reaction conditionsa
O
N
O
Ph
O
Ph
or
OH
H
[catalyst]
oxidant
O
Ph
or
or
O
N
NH2
N
N
4a
Ph
H
1a
5a
6a
7a
O
Ph
R1
O
Entry Catalyst (mol%)
Solvent
t (h) Oxidant
Product
b
NaAuCl4.2H2O
(5 mol%)
Yield (%)
O
c
Oxone (1 equiv)
MeCN, 80°C
1
2
3
4
5
6
7
8
9
NaAuCl4·2H2O
NaAuCl4·2H2O
NaAuCl4·2H2O
NaAuCl4·2H2O
AgNO3 (10)
AgNO3 (10)
AgNO3 (10)
AgNO3 (10)
NaAuCl4·2H2O
NaAuCl4·2H2O
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
7
7
7
7
7
7
7
7
2
2
8
O2
5a (95)
5a (94)
5a (39)
5a (63)
5a (20)d
5a (15)d
NH2
N
R1
N-oxide
m-CPBA
H2O2
1a-c
4a-c
O
N
O
O
N
O
O
O
c
O2
N
Ph
N-oxide
m-CPBA
H2O2
oxone
oxone
oxone
Me
OMe
e
4a, 3h, 63%
4b, 6h, 73%
4c, 6h, 72%
-
CF3
6a (21)
4a (63)
CF3
O
NaAuCl4.2H2O
(5 mol%)
10
11
12
CH3CN/H2O
7a (67)f
4a (36)g,h
O
Oxone (2 equiv)
[(PPh3)Au(NTf2)] CH3CN/H2O
N
NH2
MeCN/H2O (10/1)
80°C, 1h, 48%
1d
6d
CH3CN/H2O
8
4
oxone
oxone
oxone
oxone
oxone
oxone
oxone
7a (44)g,i
7a (40)j
6a (49)f
6a (50)g
6a (26)
Scheme 2 [Au]-catalyzed oxidative cyclization
13
14
15
16
17
18
NaAuCl4·2H2O
AgNO3 (10)
Ag2CO3 (10)
AgOTf (10)
AgNO3 (10)
/
CH3NO2
CH3CN/H2O
CH3CN/H2O
CH3CN
CH3CN/H2O
CH3CN/H2O
Mechanistically, according to previous results,15 the Au-
catalyzed process presumably occurs via the fast
hydroamination reaction (Scheme 3, intermediates A and B)
leading to 2-phenylindole 5a, which then undergoes C-H
bond/Bayer-Villiger oxidation13 with a potential initiation by
the gold catalyst considering the recent study in the presence
of copper chloride.16 This was confirmed, when the cyclization
of 2-alkynylanilines 1a was combined in a one-pot sequential
procedure with the oxidation of the in situ formed 2-
substituted indole 5a by oxone, the formation of 4H-
benzo[d][1,3]oxazin-4-ones 4a was observed in similar yield
(Scheme 3). The silver-catalyzed domino process is more
intricate as the obtained anthranyl skeleton may come from
different pathways.
0.2
24
24
1.5
1.5
6a (72)f,k,l
6a (29)f,l
a
Unless otherwise noted, all reaction were performed with 0.6 mmoles of 1a, 1
equivalents of oxidant and 5 mol% catalyst at 80 °C. b Isolated yield. c 1 atm O2. d
e
f
1a was recovered in 75% yield. 1a was recovered in 70% yield. CH3CN:H2O =
1:1. g CH3CN:H2O = 10:1. h 7a was isolated in 32 % yield. i 4a was isolated in 33 %
yield. j 10 °C, 4a was isolated in 31 % yield. k 60 °C. l 2 equivalents of oxone.
Accordingly compound 7a was isolated as the main product
when CH3CN/H2O (1:1) was used as the reaction medium
(entry 10). Mixtures of 4a and 7a derivatives were isolated
when other gold(I) complexes were tested as the catalysts
2 | Chem. Commun., 2015, 00, 1-3
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COMMUNICATION
Gold catalysis
With the optimized reaction conditions established, the
sequential divergent silver-catalyzed oxidative reaction of
View Article Online
DOI: 10.1039/C5CC08543H
Ph
NaAuCl4.2H2O
O
N
Oxone
(2 equiv)
(5 mol %)
various 2-alkynylanilines 1 with oxone was investigated.
Substrates bearing both electron-rich and electron-poor
aromatic groups on the terminal triple bond could be
efficiently converted to the corresponding benzisoxazoles 6a-o
in moderate to good yields (32-88%, Scheme 4).
O
5a
1.5 h, 63%
MeCN, 80 °C
1.5 h
NH2
Ph
1a
4a
[Au]
[Au]
Ph
Ph
N
H+
NH2
Pleasingly, halo substituents, such as -Cl and –Br were well
tolerated, which could provide the possibility for further
functionalization. Functional groups such as acetyl and acetate
moieties were also preserved during the domino process,
leading to functionalized adducts 6h and 6i in 75% and 56%
isolated yields respectively. In addition to aryl-substituted
anilines, substrates bearing heteroaryl moieties as well as alkyl
group (R1 = heteroaryl, alkyl) were found to be suitable
substrates for this transformation. Methyl group on the aniline
moiety (R2) was also tolerated and the corresponding product
was isolated in moderate yields. It is worth noting that the
previous reported AuBr3-catalyzed cyclizazion of 2-
(arylalkynyl)nitrobenzenes led to a mixture of isatogens (main
product) and anthranils (minor product) and the selective
formation of anthranils was limited only to the gold-catalyzed
H
A
B
Silver catalysis
O
Ph
Ph
AgNO3 (10 mol%)
CH3CN/H2O (1/1)
80°C, 24h
O
R
N
R = NO2, 8a
R = NO, 9a
6a (0%)
Ph
AgNO3/Oxone
TEMPO (1 equiv)
6a (48% yield)
CH3CN/H2O (1/1)
80°C, 2h
NH2
1a
HO
Ph
HO
Ph
path a
- H2O
OH
6a
O
N
O
N
OH
D
Oxone
C
1a
H+
Ph
O3NAg
cyclization of 2-(alkylalkynyl)nitrobenzenes.18
O3NAg
Ph
O
O
OH
R1
R1
6a
AgNO3
O
path b
O
N
N
(10 mol%)
R2
R2
O
O
E
F
Oxone (2 equiv)
MeCN/H2O (1/1)
60°C
N
NH2
Scheme 3 [Ag]- and [Au]-mechanistic investigations and proposed mechanisms
1a-o
6a-o
OCH3
CH3
O
O
The competitive oxidation of the amino group of the 2-
alkynylaniline 1a seems to override the hydroamination step
under the silver catalysis. Based on recent “green” method for
the oxidation of nitrogen-rich heterocyclic amines with
oxone,17 we anticipated that either the nitro or nitroso
compounds derived from 1a could be intermediates in our
domino process. Anthranyls 6 were obtained selectively
O
O
N
O
N
N
6a, 1 h, 88%
6b, 2 h, 66%
6c, 2 h, 60%
CH3
CF3
Cl
O
O
O
O
N
O
O
through
Au-
or
Ir-catalyzed
or
reaction
of
2-
N
N
(alkylakynyl)nitrobenzenes18
iodine-mediated
6d, 2 h, 80%
6e, 2 h, 54%
6f, 2 h, 60%
Br
cycloisomerization of nitroheterocyclic adducts,19 and as by-
products in the specific case of Hg-catalyzed or oxone-
mediated oxidation of nitrobenzenyl-substituted alkynes.14a,b
However, our attempts to employ 1-(2-(2-nitrophenyl)ethynyl)
benzene 8a and the nitrosoadduct 9a as the precursors of the
corresponding anthranyl derivative 6a turned out to be
unfruitful in the presence of the silver catalyst (Scheme 3). The
addition of TEMPO had a slight influence but did not interrupt
the reaction, thus suggesting that radical intermediates were
not involved in the process (Scheme 3).20 Even though it’s
difficult to conclude on the mechanism implying silver
complex, the sequential oxidation of 1a would produce the
nitrosobenzene intermediate C, which upon intramolecular
nucleophilic addition of the enol oxygen (intermediate D)
followed by dehydration reaction would give anthranil 6a.18a
Similarly to Jung’s proposal for the oxidation of alkynes by Hg
salts,14a an alternative mechanism involving the π-
activation/hydration of the alkyne by silver salts (intermediate
E), followed by a pericyclic cyclization of intermediate F cannot
be ruled out.21
AcO
COCH3
O
O
CH3
O
O
N
O
N
O
N
6g, 6 h, 74%
6h, 1 h, 75%
6i, 1 h, 56%
NH
O
O
O
(CH2)5CH3
N
N
O
N
O
O
N
N
6j, 1 h, 48%
6k, 0.25 h, 40%
6l, 1 h, 80%
CN
O
O
O
H3C
O
O
O
N
N
F3C
F3C
N
6m, 1 h, 52%
6n, 1.5 h, 50%
6o, 26 h, 32%
Scheme 4 [Ag]-catalyzed oxidative cyclization – Substrate scope
The aniline moiety could also be functionalized by a CF3 group.
Interestingly, the presence of the phenylalkynyl group afforded
the corresponding anthranyl derivative 6n in a higher yield
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Pietropaolo, A. Alvino and V. Michelet, Org. Lett. 2013, 15,
than in the case of a 4-cyano-substituted phenylalkynyl
moiety. The presence of two electron-withdrawing groups
therefore hampered the domino oxidation/cyclization process
and gave rise to a mixture of the desired compound 6o and the
corresponding non-cyclized nitro derivative 4-{[2-nitro-4-
(trifluoromethyl)phenyl]ethynyl}benzonitrile 8o in 32% and
29% isolated yields respectively. The outcome of this latter
reaction also supports our proposed mechanism (Scheme 4),
excluding the 2-nitro-substituted derivative as a potential
intermediate.
In conclusion, we have developed an unprecedented Ag-
catalyzed domino oxidative cycloisomerization reaction of
unprotected 2-alkynylanilines, leading to benzisoxazole
derivatives in moderate to good yields. The divergent
efficiency of silver and gold catalysts was demonstrated, as the
gold-catalyzed process gave access to functionalized 4H-
benzo[d][1,3]oxazin-4-one. The challenging and rewarding
silver-catalyzed process was then further developed. The
methodology implying oxone as oxidant without any acids nor
bases, was compatible with several groups such as ketone and
halogen groups as well as heterocycles. The alkynyl moiety
could be substituted independently by aryl or alkyl groups,
which opens new opportunities for the synthesis of
biologically-active targets.
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2766.
DOI: 10.1039/C5CC08543H
7
8
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9
This work was supported by the Ministère des affaires
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dell'Aquila and the Centre National de la Recherche
Scientifique (CNRS).
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4
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4 | Chem. Commun., 2015, 00, 1-3
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DOI: 10.1039/C5CC08543H
O
R1
[Ag]
R1
O
C
R2
R2
R2
[Au]
O
O
Oxone
Oxone
NH2
N
N
R1
63-73%
32-88%
R1 = (CH2)5CH3, Ph, 3- and 4-MeC6H4, 4-CF3C6H4, 4-MeOC6H4,
4-Cl-C6H4, 3-Br-C6H4, 4-MeCOC6H4, 4-CNC6H4, 2-AcO-4-Me-C6H3,
NH
R2 = H, Me, CF3
,
N
N
Divergent catalytic activity of gold and silver complexes towards domino
oxidative cyclization reactions of unprotected 2-alkynylanilines