Copper-catalyzed pyrrole synthesis from 3,6-dihydro-1,2-oxazines

Article abstract of DOI:10.1039/c8gc01373j

Highly-functionalized pyrroles could be effectively synthesized from 3,6-dihydro-1,2-oxazines using a heterogeneous copper on carbon (Cu/C) under neat heating conditions. Furthermore, the in situ formation of 3,6-dihydro-1,2-oxazines via the hetero Diels-Alder reaction between nitroso dienophiles and 1,3-dienes and the following Cu/C-catalyzed pyrrole synthesis also provided the corresponding pyrrole derivatives in a one-pot manner.

Full text of DOI:10.1039/c8gc01373j

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Copper-catalyzed pyrrole synthesis from 3,6-dihydro-1,2-oxazines
Naoki Yasukawa, Marina Kuwata, Takuya Imai, Yasunari Monguchi, Hironao Sajiki* and Yoshinari
Sawama*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
o
Highly-functionalized pyrroles could be effectively synthesized although drastic reaction conditions (over 10 bar and 200 C)
from 3,6-dihydro-1,2-oxazines using a heterogeneous copper on were required.9 Recently, a cascade synthesis of pyrroles via
carbon (Cu/C) under neat heating conditions. Furthermore, the in the hetero Diels-Alder reaction between 1-boronodienes and
situ formation of 3,6-dihydro-1,2-oxazines via the hetero Diels- nitroso derivatives and the following ring contraction reaction
Alder reaction between nitroso dienophiles and 1,3-dienes and accompanied by the elimination of the boryl function were
the following Cu/C-catalyzed pyrrole synthesis also provided the also reported.10 However, to the best of our knowledge, the
corresponding pyrrole derivatives in a one-pot manner.
heterogeneously-catalyzed direct transformation of simple
3,6-dihydro-1,2-oxazine derivatives into pyrroles has not been
accomplished. The development of the environmentally-
benign and cost-efficient synthetic procedure has been
demanded from the viewpoints of green sustainable chemistry.
Heterogeneous transition metal catalysts are frequently used
in industrial synthetic processes because of their high stability,
easy removal from the reaction mixture and reuse. The
solvent-free reaction is also attractive to reduce waste and
costs, and it simplifies the synthetic process and handling.11
We now report an efficient heterogeneous copper on carbon
(Cu/C)-catalyzed synthesis of highly-functionalized pyrrole
The pyrrole nucleus is an important chemical scaffold of a wide
variety of functional materials, such as biologically-active
compounds, pharmaceuticals, electrical conductors, etc.1 In
particular, N-aryl pyrrole derivatives exhibit a variety of
interesting biological and biomedical propeties.1d-1g Therefore,
the development of efficient and novel synthetic methods to
construct highly-functionalized pyrrole derivatives is strongly
desired. 3,6-Dihydro-1,2-oxazines
prepared by [4+2]-cycloaddition (Diels-Alder reaction) using
nitroso dienophiles ( ) and 1,3-dienes ( ), can be transformed
into pyrroles ( ) via two-step sequences (reductive N-O bond
cleavage of followed by oxidative cyclization of the resulting
γ-amino alcohol).2 Although direct transformations of
to by
(3), which are easily
1
2
4
derivatives (
oxazines ( ) under neat heating conditions (Scheme 1). Nitroso
compounds ( ) and 1,3-dienes ( ) could be easily prepared
4) starting from easily-prepared 3,6-dihydro-1,2-
3
3
3
4
1
2
photolysis were accomplished,3 furan derivatives were also
produced as byproducts along with the pyrroles. 6-Silyl4 and 4
or 6-carbonyl-substituted5 3,6-dihydro-1,2-oxazines can be
directly converted into the corresponding pyrrole derivatives
via anionic intermediates under basic conditions. 3,6-Dihydro-
1,2-oxazines bearing the 6-hydroxyl or alkoxy group (acetal
derivatives) were directly transformed into pyrroles via cation
intermediates using Lewis acids,6 Brønsted acids7 and silica
gel.8 The transition metal-catalyzed reductive cycloaddition
between nitroarenes (Ar-NO2) and the 1,3-diene into 3,6-
dihydro-1,2-oxazines and the subsequent transformation to
the pyrrole have been accomplished in a one-pot manner,
from commercially available amines and aldehydes,
respectively (The detailed synthetic process is described in
ESI). Additionally, the one-pot synthesis via the hetero Diels-
Alder reaction between nitroso dienophiles (
1
) and 1,3-dienes
(
2) could also be accomplished.
R2
6
R2
O
O
2
Cu/C
neat
R2
N
N
N
R1
4
R1
Hetero
Diels-Alder
reaction
R1
3
1
4
3
+) High atom economy
+) Solvent-free
+) Heterogeneous catalyst
+) One-pot synthesis from 1 and 2
Scheme 1. Copper on carbon-catalyzed and solvent-free synthesis of pyrrole
derivatives ( ) from 3,6-dihydro-1,2-oxazines ( ).
4
3
Our research group recently reported the palladium on carbon
(Pd/C)-catalyzed dehydrogenation of 1,4-cyclohexadienes or
cyclohexenes under aqueous conditions.12 Although the
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dehydrogenative oxidation of the 3,6-dihydro-1,2-oxazine (entries 1 and 9), while the reaction under air led the lower
derivatives into 1,2-oxazine derivatives was next examined, N- yield (entry 10). The scale-up reaction using 5 mmol of 3aa
phenyl-6-phenyl-3,6-dihydro-1,2-oxazine (3aa: 0.2 mmol) was could also be successfully performed to give 4aa in 72% yield
surprisingly transformed into the unexpected N-phenyl-2- (entry 11). 10% Cu/C could be reused at least fifth times
phenyl pyrrole (4aa) in 11% yield in water at 120 oC for 6 h without any significant loss of its catalytic activity (entries 12-
(Table 1, entry 1). Encouraged by this result, the catalyst 15).
efficiency among a wide variety of heterogeneous transition-
Table 2. Copper species and reaction temperature effects.
metal catalysts (10% Pt/C, Rh/C, Ru/C, Ni/C, Ir/C, Au/C, Ag/C
Ph
and Cu/C; each 5 mol%) was initially investigated (entries 2-9).
Ph
Consequently, 10% Cu/C and 10% Au/C indicated a relatively
good catalytic activity to produce 4aa, although 3aa was not
completely consumed (entries 7 and 9). 4aa was never
obtained without the catalyst, and 3aa was quantitatively
recovered (entry 10). On the other hand, the 10% Cu/C-
catalyzed reaction under solvent-free conditions efficiently
proceeded to provide 4aa in 84% isolated yield (entries 11),
while Au/C was inefficient under the neat conditions (entry
12). Additionally, the desired reaction could be hardly
accomplished in organic solvents (MeCN, DMF and toluene)
(entries 13-15).
catalyst (5 mol%)
temp., 6 h
O
N
N
Ph
Ph
3aa
(0.2 mmol)
4aa
entry catalyst
temp. [oC]
yield [%]a
recovered 4a
a
3a
0
a
1
10% Cu/C 120
Cu (0) powder 120
90 (84)b
45
2
39
0
3
CuBr
120
120
100
80
56
4
CuBr2
0
32
5
10% Cu/C
10% Cu/C
10% Cu/C
10% Cu/C
10% Cu/C
10% Cu/C
10% Cu/C
10% Cu/C
10% Cu/C
10% Cu/C
10% Cu/C
69
26
6
100
0
0
Table 1. Catalyst and solvent effects.
7c
120
120
120
120
120
120
120
120
120
79
8d
20
0
62
9e
85
10f
11g
12h,i
13h,j
14h,k
33
0
40
72b
83
yield [%]a
recovered 4a
0
entry catalyst
solvent
1
78
a
4
76
3a
39
86
86
81
91
90
44
78
54
99
0
a
15h,l
4
72
1
10% Pd/C
10% Pt/C
10% Rh/C
10% Ru/C
10% Ni/C
10% Ir/C
10% Au/C
10% Ag/C
10% Cu/C
—
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
—
11
4
Determined by 1H NMR using 1,4-dioxane as an internal standard. b Isolated yield. c
1
a
2
d
e
f
g
mol% of 10% Cu/C was used. For 3 h. Under Air. Under N2. 5 mmol of 3aa was
3
2
h
used. 10% Cu/C was reused after simple filtration, washed with H2O and MeOH, and
4
2
dried in vacuo. In each run, 99, 99, 91, 93 and 93% of the Cu/C could be recovered,
i
j
5
1
respectively. The second use of 10% Cu/C after the reaction in entry 1. The third use
k
of 10% Cu/C after the reaction in entry 12. The fourth use of 10% Cu/C after the
reaction in entry 13. l The fourth use of 10% Cu/C after the reaction in entry 14.
6
2
7
23
3
8
The scope of substrates was next investigated (Table 3).
9
27
0
90 (84)b
Various N-phenyl-3-aryl-3,6-dihydro-1,2-oxazines
(3ab-3ah)
10
11
12
13
14
15
bearing electron-donating or electron-withdrawing groups at
10% Cu/C
10% Au/C
10% Cu/C
10% Cu/C
10% Cu/C
the para-, meta-, or ortho-position of the aromatic nucleus
—
40
99
66
62
22
0
were efficiently transformed into the desired pyrroles (4ab
4ah) in good to excellent yields (entries 1-7). The furyl, thienyl
or benzyl (Bn)-substituted 3,6-dihydro-1,2-oxazines (3ai 3ak
could be also converted into the corresponding pyrroles (4ai
-
MeCN
DMF
toluene
9
-
)
38
a Determined by 1H NMR using 1,4-dioxane as an internal standard. b Isolated yield.
-
4ak) in good to excellent yields (entries 8-10). The reaction
conditions also permitted to synthesize the trisubstituted
While N-penyl-2-phenyl pyrrole (4aa) was effectively obtained
by the use of 10% Cu/C (5 mol%) under neat conditions at 120
oC (Table 1, entry 11: Table 2, entry 1), other copper species
(Cu powder, CuBr and CuBr2) indicted lower catalyst activities
(entries 2-4). Lowering the reaction temperature to 100 oC and
pyrroles (4al
4be) derived from nitrosoarene derivatives as pre-substrates
: Scheme 1) were also effectively constructed in good yields
(entries 15-18). Furthermore, the N-benzyl-3,6-dihydro-1,2-
oxazine derivative 3fa was transformed into the
-4ao) (entries 11-14). Various N-aryl pyrroles (4ba-
(
1
o
(
)
80 C significantly decreased the reaction efficiency (entries 1
corresponding N-benzyl pyrrole (4fa) in a moderate yield
vs. 5 and 6). Furthermore, the yield of 4aa obviously decreased
when using 1 mmol of 10% Cu/C (entries 1 vs. 7) or shortening
(entry 19).
the reaction time (3 h) (entries
1 vs. 8) The present
transformation could proceed under Ar and N2 atmosphere
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Cu/C (5 mol%) at 120 oC for 6 h (The detailed optimization
process is described in Table S1 in ESI). Consequently, various
Table 3. Scope of substrate.
highly-functionalized pyrroles (4aa, 4ab, 4ac, 4ba and 4ca)
could be efficiently prepared in moderate to good yields.14, 15
entry
Substrate (
3
)
product (4)
3
3
3
3
3
3
3
3
3
3
a
a
a
a
a
a
a
b
: R1 = 4-MeO-C6H4
4
a
b
: 72%
4ac: 77%
1
2a
3
c
: R1 = 4-NO2-C6H4
d
e
: R1 = 4-Br-C6H4
4a
d
: 79%
: 67%
: R1 = 3-MeO-C6H4
4
a
e
4
f
: R1 = 3-Br-C6H4
4af: 80%
: 61%
: 76%
5
g
: R1 = 2-MeO-C6H4
4
a
g
6
h
: R1 = 2-Br-C6H4
4a
h
7
: R1 = 2-furyl
4
a
i
: 65%
: 88%
Scheme 2. One-pot synthesis.
ai
8
aj
: R1 = 2-thienyl
4
a
j
9
: R1 = Bn
4
a
k: 58%
The N-O bond-cleaved substrate (5), prepared by the reduction
ak
10
of 3aa, was never transformed into the corresponding pyrrole
product (4aa) under the present reaction conditions and 87%
11
12
of
5 was recovered (eq. 1). Moreover, the radical-trapping
experiments were conducted. When using 2,2,6,6-
tetramethylpiperidine 1-oxyl (TEMPO), galvinoxyl or 1,1-
diphenylethene as a radical scavenger, the Cu/C-catalyzed
transformation of 3aa to 4aa proceeded and the desired 4aa
was obtained in a good yield (eq. 2).16 Therefore, the single
3
al
4al
: 77%
3a
m
4am: 78%
electron transfer (radical) pathway can likely be ruled out.
Ph
10% Cu/C (5 mol%)
120 oC, 6 h
HO
N
No Reaction
(recovered 5 : 87%)
(eq. 1)
(eq. 2)
13b
Ph
H
5
3
a
n
4
a
n
o
: 49%
: 35%
Ph
10% Cu/C (5 mol%)
additive (10 mol%)
Ph
O
N
14b
120 oC, 6 h
N
Ph
Ph
3aa
4aa
: 66%
: 57%
3a
o
4a
additive = TEMPO
galvinoxyl
1,1-diphenylethene
: 73%
A suggested reaction mechanism is shown in Scheme 3. The
oxidative addition of the copper species to the N-O bond of
3ba: R2 = 4-Me-C6H4
3ca: R2 = 4-Br-C6H4
3da: R2 = 3-Br-C6H4
3ea: R2 = 2-Br-C6H4
4ba: 71%
4ca: 66%
4da: 66%
4ea: 34%
3aa forms intermediate A 17,18
,
which undergoes the
elimination to give the intermediate B. The subsequent
reductive elimination to the intermediate , dehydrative ring
β-hydride
15
16
17c
18
C
closure and following isomerization produced 4aa
.
19b,d
3fa
4fa: 46%
a For 12 h. b 15 mol% of 10% Cu/C was used. c 10 mol% of 10% Cu/C was used. d For 48
h.
The one-pot synthesis of the target molecules is an important
approach to shorten and simplify the synthetic route.13 The
two-step one-pot reaction via the hetero Diels-Alder reaction
and the following above-mentioned pyrrole synthesis were
Scheme 3. Proposed reaction mechanism.
next examined (Scheme 2).
1
and
2
were directly transformed
) in the presence of 10%
into the desirable pyrrole derivative (
4
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9
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We have developed an efficient and direct transformation of
3,6-dihydro-1,2-oxazines into the corresponding pyrrole
derivatives in the presence of only heterogeneous Cu/C.
Furthermore, the one-pot pyrrole synthesis could also be
accomplished via the hetero Diels-Alder reaction between
nitroso dienophiles and 1,3-dienes and the subsequent Cu/C-
catalyzed pyrrole synthesis. The present method under neutral
conditions using an easily-removable and reusable
heterogeneous Cu/C catalyst without an additive and solvent is
valuable from the viewpoint of green sustainable chemistry
and provides a novel synthetic process of pyrrole derivatives.
10 (a) F. Tripoteau, L. Eberlin, M. A. Fox, B. Carboni, A. Whiting,
Chem. Commun., 2013, 49, 5414–5416; (b) L. Eberlin, B.
Carboni, A. Whiting, J. Org. Chem., 2015, 80, 6574–6583.
11 (a) Solvent-free Organic Synthesis, ed. K. Tanaka, 2nd edn,
Wiley-VCH, Weinheim, 2008; (b) Topics Current Chemistry
254, Organic Solid State Reactions, ed. F. Toda, Springer,
Berlin, 2005; (c) M. A. P. Martins, C. P. Frizzo, D. N. Moreira,
L. Buriol, P. Machado, Chem. Rev., 2009, 109, 4140–4182; (d)
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,
,
,
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Sajiki, Y. Sawama, Green Chem., 2018, 20, 1213–1217.
Acknowledgements
13 (a) Y. Hayashi, Chem. Sci., 2016,
7, 866–880; (b) T.
We thank the N. E. Chemcat Corporation for the kind gift of
the various metal on carbon catalysts. This study was
supported by a Grant-in-Aid for JSPS Research Fellows from
the Japan Society for the Promotion of Science (JSPS, Number:
18J14010 for N. Y.) and Grant-in-Aid for Scientific Research (C)
from the Japan Society for the Promotion of Science (JSPS:
16K08169 for Y. S.).
Newhouse, P. S. Baran, R. W. Hoffmann, Chem. Soc. Rev.,
2009, 48, 3010–3021; (c) P. T. Anastas, M. M. Kirchhochhoff,
Acc. Chem. Res., 2002, 35, 686–694.
14 Hetero Diels-Alder between nitrosobenzene (1a) and 1-
phenyl-1,3-butadiene (2a) gave 77% of 3aa (the detailed
procedure and result are described in ESI), which was
transformed to 4aa in 84% yield under the optimal reaction
conditions (Table 1, entry 1). 4aa could be synthesized in
65% yield in 2 steps. The yield of the stepwise synthesis is
similar to that of the one-pot procedure.
Conflicts of interest
15 Although three steps one-pot reaction via the oxidation of
amine using oxone, the hetero Diels-Alder reaction and the
following Cu/C-catalyzed pyrrole synthesis was also
examined, the desirable pyrrole product (4aa) was never
obtained.
16 The use of 1.5 equiv. of TEMPO or galvinoxyl as a radical
scavenger resulted in low yield (21% or 26%) of 4aa, but the
reaction was not completely inhibited. Meanwhile, the
present Cu/C-catalyzed reaction using 1.1-diphenylethene
(1.5 equiv.) could effectively proceed to provide 4aa in 73%
yield.
There are no conflicts to declare.
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