Iridium(I)-catalyzed hydrogen peroxide oxidation of hydroxamic acids and hetero Diels-Alder reaction of the acyl nitroso intermediates with cyclopentadiene

Article abstract of DOI:10.1016/S0040-4039(02)01277-7

[Ir(I)(coe)₂Cl]₂ (2 mol%)-catalyzed hydrogen peroxide oxidation of hydroxamic acids in the presence of cyclopentadiene gave the corresponding cycloadducts up to 97% yield. With a catalyst loading of 0.05 mol%, a maximum TON was attai

Full text of DOI:10.1016/S0040-4039(02)01277-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 6159–6161
Iridium(I)-catalyzed hydrogen peroxide oxidation of hydroxamic
acids and hetero Diels–Alder reaction of the acyl nitroso
intermediates with cyclopentadiene
a,
a
a
b
Seiji Iwasa, * Ahmad Fakhruddin, Yasuyuki Tsukamoto, Masayuki Kameyama and
a
Hisao Nishiyama
a
School of Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan
b
School of Materials Science, Oyama National College of Technology, Nakakuki-cho, Oyama, Tochigi 323-0806, Japan
Received 7 June 2002; revised 27 June 2002; accepted 1 July 2002
Abstract—[Ir(I)(coe) Cl] (2 mol%)-catalyzed hydrogen peroxide oxidation of hydroxamic acids in the presence of cyclopentadiene
2
2
gave the corresponding cycloadducts up to 97% yield. With a catalyst loading of 0.05 mol%, a maximum TON was attained to
40. © 2002 Elsevier Science Ltd. All rights reserved.
7
4
The synthesis of nitroso intermediates is of vital interest
from both an organic synthesis and biochemistry point
of view. Because this can be widely applicable to
achieve many multi-functionalized molecules of natural
products and also the nitroso intermediate can be
ero Diels–Alder reaction with cyclopentadiene. During
the course of our continuous research, we were search-
ing to find out other new catalytic systems which can be
effective to the above mentioned type of oxidation
process (Scheme 1). Here, we report a novel Ir(I) based
method for the synthesis of nitroso intermediates via
hydrogen peroxide oxidation of hydroxamic acids and
its subsequent hetero Diels–Alder reaction with
cyclopentadiene.
1
formed in biological systems. Nitroso intermediates are
commonly obtained from the oxidation of hydroxy-
lamine derivatives using organic and inorganic oxidants
with the subsequent formation of an undesirable side
2
product of 1 equiv. In the choice of oxidant, hydrogen
peroxide gets the top priority since water is the only
side product after the oxidation process. In this regard,
we have previously reported one of the highly efficient
method for the synthesis of nitroso intermediates via
Ru(II)(pybox-dh)(pydic)-catalyzed hydrogen peroxide
oxidation of hydroxamic acids and its consequent het-
The results are summarized in Table 1. Initially we have
selected a number of metallic systems (Table 1, entries
3
1
–4). Among the tested metallic systems only
[Ir(cod)Cl]2 (cod=cyclooctadiene) and CuI have
5
responded moderately (entries 3 and 4). Owing to the
solubility of CuI, we focus on Ir(I) catalyst. Since
tautomerism of hydroxamic acids is well known, the
effects of bases were also examined for this catalytic
6
oxidation system. In the presence of catalytic or stoi-
chiometric amounts of some bases with [Ir(cod)Cl]2,
slight increases of cycloadduct were observed (entries
5
–10).
On the other hand, [Ir(coe) Cl]2 (coe=cyclooctene)
2
gave the highest yield without any additives (entry
5d,7
1
1).
The optimized condition of both solvent and
Scheme 1.
reaction temperature was briefly surveyed. The yield of
cycloadduct varied with different solvent systems.
Entries 11–16 shows the yields of the reaction of hydro-
gen peroxide oxidation of benzohydroxamic acid in the
presence of cyclopentadiene in various solvent medium.
Keywords: nitroso intermediate; hydrogen peroxide; iridium catalyst;
hetero Diels–Alder reaction; 1,2-oxazine.
*
Corresponding author.
0
040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01277-7

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S. Iwasa et al. / Tetrahedron Letters 43 (2002) 6159–6161
a
Table 1. Transition metal catalyzed hydrogen peroxide oxidation of hydroxamic acids and its hetero Diels-Alder reactions
b
Entry
Hydroxamic acid
Cat.
Solvent
Additive (equiv.)
2a yield (%)
1
2
3
4
1a
1a
1a
1a
Co(acac)2
[Rh(OAc)2]2
CuI
THF
THF
THF
THF
–
–
–
–
18
19
60
51
[Ir(cod)Cl]2
5
6
7
8
9
1a
1a
1a
1a
1a
1a
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
THF
THF
THF
THF
THF
THF
KOH (0.3)
74
70
53
64
13
25
Triethylamine (1.0)
Diisopropylamine (1.0)
N-Methylmorphorine (1.0)
Morphorine (1.0)
Pyridine (1.0)
1
0
11
12
13
14
15
16
1a
1a
1a
1a
1a
1a
[Ir(coe) Cl]
THF
–
–
–
–
–
–
97
60
86
82
90
19
2
2
2
2
2
2
2
[Ir(coe) Cl]
CH OH
2
3
[Ir(coe) Cl]
CH Cl
2
2
2
[Ir(coe) Cl]
1,4-Dioxane
DMF
2
[Ir(coe) Cl]
2
[Ir(coe) Cl]
H O
2
2
17
18
19
20
1b
1c
1d
1e
[Ir(coe) Cl]
THF
THF
THF
THF
–
–
–
–
94
80
60
92
2
2
2
2
2
[Ir(coe) Cl]
2
[Ir(coe) Cl]
2
[Ir(coe) Cl]
2
a
Hydroxamic acid (0.15 mmol), catalyst (0.003 mmol), H O (0.60 mmol), and cyclopentadiene (0.75 mmol) at 0°C to room temperature.
Isolated yields.
2
2
b
Uniquely, benzohydroxamic acid 1a gave the corre-
for the synthesis of highly functionalized molecules
since the hydrogen peroxide is environmentally sound
and a cost effective oxidant.
sponding cycloadduct with a high yield in THF solvent
system rather than CH OH and water, and it was
3
found that THF solvent and 0°C to room temperature
was the best condition for this reaction. As [Ir(coe) Cl]2
General procedure is as follows (Table 1, entry 11): To
a solution of hydroxamic acid (20.6 mg, 0.15 mmol) in
THF (1.0 mL) was added a solid of [Ir(coe) Cl] (2.7
2
is very soluble in DMF, CH Cl and 1,4-dioxane, these
2
2
solvents can also be used for the oxidation system.
2
2
mg, 0.003 mmol) at 0°C followed by addition of freshly
distilled cyclopentadiene (61 mL, 0.75 mmol) and
hydrogen peroxide (31%, 68 mL, 0.60 mmol). The
resulting yellowish green mixture was stirred for 20 h at
room temperature. The organic phase was extracted
with CH Cl , and dried over Na SO . The solvent was
Entries 17–20 summarize the products and yields of the
reactions of hydrogen peroxide oxidation of various
hydroxamic acids in presence of cyclopentadiene. The
catalyst loading was up to 0.05 mol% to give the
corresponding cycloadduct in 37% yield with TON
2
2
2
4
[
turnover number=product(mol)/catalyst(mol)]=740.
removed under reduced pressure and the residue was
purified by flash column chromatography on silica gel
to give hetero Diels–Alder cycloadduct (29.4 mg) in
Acyl nitroso intermediates can be efficiently generated
from hydroxamic acids with [Ir(coe) Cl] and hydrogen
2
2
8
peroxide in THF. The nitroso intermediates formed by
the iridium catalyst and hydrogen peroxide smoothly
reacted with the diene to produce the corresponding
cycloadducts, 1,2-oxazines in high yield. Furthermore,
other dienes such as cyclohexadiene and a-terpinene
were also found to react with 1a to give the correspond-
ing cycloadducts in 90 and 87% yield respectively along
with 9:1 regio isomer for the later.
97% isolated yield.
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