Visible-Light, Photoredox-Mediated Oxidative Tandem Nitroso-Diels–Alder Reaction of Arylhydroxylamines with Conjugated Dienes

Article abstract of DOI:10.1002/ejoc.201601492

Arylhydroxylamines were used in the nitroso-Diels–Alder reaction to generate in situ nitrosoarenes under visible-light, catalytic and aerobic conditions. Mixing a solution of aryl- or heteroarylhydroxylamines with conjuguated dienes in the presence of a catalytic amount of Ru(bpy)₃Cl₂ afforded 3,6-dihydro-1,2-oxazines in good yields under an oxygen atmosphere.

Full text of DOI:10.1002/ejoc.201601492

A Journal of
Accepted Article
Title: Visible Light Photoredox-Mediated Oxidative Tandem Nitroso
Diels-Alder Reaction of Arylhydroxylamines with Conjugated
Dienes
Authors: Géraldine Masson, Santacroce Veronica, MAESTRI Giovanni,
Raphael DUBOC, and Max MALACRIA
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201601492
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European Journal of Organic Chemistry
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Visible Light Photoredox-Mediated Oxidative Tandem Nitroso
Diels-Alder Reaction of Arylhydroxylamines with Conjugated
Dienes
Veronica Santacroce,[a,c] Raphael Duboc,[a] Max Malacria,[a,b], Giovanni Maestri[c] and Geraldine
Masson*[a]
Abstract: Arylhydroxylamines were used in the nitroso Diels-Alder
reaction to generate in-situ nitrosoarens under visible light catalytic
environmentally friendly than standard oxidation methods. Many
efficient catalytic systems have been developed using transition
metals or organic dyes as catalysts. Surprisingly, photoredox
catalysts have rarely been employed for direct oxidative
conversion of hydroxylamine into nitroso compounds. So far, only
one example using N-acylhydroxylamines was reported by Alaniz
et al.[8] In view of this and in continuation of our interest in the
development of oxidative tandem photoredox-catalyzed
reactions,[6a], [9] we describe herein a one-pot synthesis of 3,6-
dihydro-2H-1,2-oxazine using a catalytic amount of Ru(bpy)3Cl2
as photosensitizer (Scheme 1).
aerobic conditions. Mixing
a solution of aryl- or heteroaryl-
hydroxylamines with conjuguated dienes in the presence of a catalytic
amount of Ru(bpy)3Cl2 afforded, under an oxygen atmosphere, the
3,6-dihydro-1,2-oxazines in good yields.
Introduction
The [4 + 2] cycloaddition of dienes with nitrosoarenes, namely the
nitroso-Diels-Alder (NDA) reaction, is a versatile method for
constructing 3,6-dihydro-1,2-oxazines and has broad applications
in the synthesis of natural products and pharmaceuticals.[1] Since
some nitrosoarenes are quite unstable and highly reactive, their
generation by in situ oxidation of the corresponding N-
arylhydroxylamine derivatives increases the versatility and
efficiency of these reactions.[2] However, these tandem oxidative
processes with N-arylhydroxylamines have rarely been
documented[3] mainly because of the overoxidation of
arylhydroxylamines to arylnitro compounds and the competing
dimerization of the nitrosoarenes into azoxybenzene.[4] The first
example of oxidative tandem NDA reaction of N-
arylhydroxylamines was reported by Alaniz et al. in 2012 using
CuCl2 under an oxygen atmosphere.[2d] Two years later, Liu et al.
described an efficient NDA reaction of arylhydroxyanilines and
unactivated allenes catalyzed by a combination of AuCl3 and
CuCl2 under oxygen.[5] Very recently, we have developed an
efficient one-pot oxidation/enantioselective NDA sequence of N-
arylhydroxylamines with dienecarbamates by combining a chiral
phosphoric acid and mCPBA.[6] However, the above-reported
oxidative protocols remain associated with certain disadvantages
Scheme 1. Visible Light Photoredox-Mediated Oxidative Tandem Nitroso Diels-
Alder Reaction.
Results and Discussion
We commenced our studies by the reaction of phenyl
hydroxylamine (2a) with benzyl-(penta-1,3-dien-1-yl)carbamate
(1a) in DCM, at RT, under oxygen atmosphere and blue LED
irradiation, using 5 mol% of Ru(bpy)3(PF6) 3a as catalyst (Scheme
1). Under these conditions, NDA reaction took place smoothly,
and 44% yield could be reached. We also noted that significant
over-oxidation of the nitrosobenzene intermediate 4a to the
corresponding nitrobenzene occurred. [4] In order to improve the
yield of this oxidative tandem reaction, we investigated other
solvents and conditions (Table 1). Solvent screening revealed
protic solvents, such as MeOH (entry 3) and TFE (entry 6), were
the best choices and gave the expected 3-methyl-2-phenyl-3,6-
such as
a limited substrate scope and functional group
incompatibility. Thus, the development of convenient oxidative
tandem nitroso Diels-Alder reactions is still in great demand.
The photoredox catalytic oxidation[7] has attracted much
attention in recent years because this protocols is mild and more
[a]
V. Santacroce, R. Duboc, M. Malacria, G. Masson
Institut de Chimie des Substances Naturelles, CNRS UPR 2301,
Université Paris-Sud, Université Paris-Saclay, 1, av. de la Terrasse,
91198 Gif-sur-Yvette Cedex, France
dihydro-1,2-oxazin-6-yl)carbamate 5a as
a
single cis-
diastereomer in 57% and 63% yield, respectively (entries 3 and
6). Indeed, lower yields were achieved with less acid solvents
such as t-BuOH and n-BuOH (entries 4 and 5). This observation
seems to indicate that a stronger acid protic solvents can
participate in hydrogen bonding interactions with the nitrosoarene
intermediate 4a, further accelerating the cycloaddition by greatly
improving its conversion and hereby reducing its overoxidation.
On the other hand, replacing the oxygen atmosphere with air gas
(entry 7) diminished significantly the reaction efficiency. A similar
E-mail: Geraldine.MASSON@cnrs.fr
[b]
[c]
M. Malacria
UPMC Sorbonne Universités, IPCM (UMR CNRS 7201)
4 place Jussieu, C. 229, 75005 Paris, France
G. Maestri, V. Santacroce
Università degli Studi di Parma, Dipartimento di Chimica
17/A Parco Area delle Scienze, 43124 Parma, Italy
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Table 2. Substrate scope of the photocatalyzed tandem NDA reaction.[a]
result was observed when the BrCCl3 was used as oxidative
quencher (entry 8).[10] Further improvement of the product yield
can be achieved by the addition of a base such as 2,6-lutidine
(entries 10-13). Other photoredox catalysts were also examined.
With a stronger oxidant such as [Ru(phen)3]2+ 3b, the yield was
significantly reduced mainly because of the overoxidation of 2a.
The nature of the counterion of [Ru(bpy)3]2+ (3a, 3c, 3d) revealed
to be also important. An improvement of the yield was observed
with tighter ionic couples. The best catalytic system resulted to be
Ru(bpy)3Cl2 3d (entry 13), which provided 5a with 90% yield.
Control experiments indicated that the use of a photocatalyst, light
and oxygen was important for the production of 5a. In the absence
of one of them, the tandem process became much less efficient.
For example, in absence of Ru(bpy)3Cl2 only trace of 5a was
observed (entry 9). From these results we can conclude that O2
alone cannot ensure the complete oxidation of hydroxylamine.
R
5, yield (%)[b]
H
Me
F
5a, 90
5b, 95
5c, 86
5d, 71
5e, 89
5f, 74
Cl
Br
I
Table 1. Survey of reaction conditions.
H
Me
F
5g, 73
5h, 80
5i, 80
5j, 59
5k, 54
5l, 47
Entry[a]
P cat 3
Solvent
Yield of 5a (%)[b]
Cl
Br
I
1
2
Ru(bpy)3(PF6)2 (3a)
Ru(bpy)3(PF6)2 (3a)
Ru(bpy)3(PF6)2 (3a)
Ru(bpy)3(PF6)2 (3a)
Ru(bpy)3(PF6)2 (3a)
Ru(bpy)3(PF6)2 (3a)
Ru(bpy)3(PF6)2 (3a)
Ru(bpy)3(PF6)2 (3a)
_
DCM
MeCN
MeOH
t-BuOH
n-BuOH
TFE
44 [75][c]
51 [80][c]
57 [80][c]
37
3
4
H
5m, 81
5n, 85
5
34
Me
6
63
7[d]
8[e]
9
DCM
MeCN
TFE
[50][c]
30
5o, 71
trace
77
10[f]
11[f]
12[f]
13[f]
Ru(bpy)3(PF6)2 (3a)
Ru(bpz)3(PF6)2 (3b)
Ru(bpy)3(SbF6)2 (3c)
Ru(bpy)3Cl2*6H2O (3d)
TFE
TFE
75
TFE
83
OBn
OCH2C CH
SBn
5p, 59[c]
5q, 83[c]
5r, 51[c]
TFE
90
[a] Reaction performed on a 0.1 mmol scale; reaction conditions: 1.0 equiv. 1a,
3.0 equiv. 2a, 5 mol% 3, 0.1 mL solvent, O2 atm., 16 h, RT, under blue led
irradiation. [b] Isolated yield. . [c] Conversion from 1a. [d] Reaction conducted
under air [e] Reaction conducted in presence of BrCCl3. [f] 2.0 equiv. of 2,6-
lutidine were added.
5s, 58
With the best conditions in hands, we investigated the
applicability of the optimized photocatalyzed oxidative tandem
process. As shown in Table 2, various substituted phenyl
hydroxylamine were appropriate substrates providing diverse cis-
3,6-dihydro-1,2-oxazines 5a-5f in good yields (71-95%) and
[a] General conditions: 1.0 equiv. 1, 3.0 equiv. 2, 2.0 equiv. lutidine, 5 mol% 3,
0.1 mL TFE, O2 atm., 16 h, RT, under blue led irradiation. [b] Isolated yield. [c]
An extra excess of hydroxylamine was used (6 equiv.).
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10.1002/ejoc.201601492
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excellent diastereoselectivity. Most remarkably, the pyrimidine
hydroxylamine, which was totally unstable in our previous
oxidative nitroso-Diels-Alder reaction,[6a] worked well when an
increased amount of hydroxylamine was used. This method led to
an efficient access to more complex heterocycles (products 5p-
5r, 51-83% yield). A wide range of diversely functionalized
dienecarbamates and thioenecarbamates were effectively
employed to afford the corresponding adducts 5g-5o in moderate
to high yields (47-85%). The photocatalytic system also proved
to be compatible with various functional groups such as ether,
thiothers alkenes and alkynes. Remarkably, this photocatalyzed
oxidative NDA reaction was successful with less reactive dienes.
For instance, when the bulky pentamethylcyclopentadiene was
used, the corresponding NDA products 5s was formed in 58%
yield as a mixture of two diastereoisomers in 2:1 ratio (calculated
by 1H-NMR, see supporting information).
Conclusions
In summary, we have developed an efficient photoredox-
catalyzed tandem oxidative NDA reaction for the synthesis of 3,6-
dihydro-1,2-oxazines from readily available N-aryl and N-
heteroaryl-hydroxylamines. The main attributes of the present
protocol are experimental simplicity, relatively broad scope and
good yields. Further investigations to probe the mechanism and
the application of this oxidative system to the synthesis of
pharmaceuticals is currently underway in our laboratories.
Experimental Section
Based on other reports[11] and our previous study,[9] we
propose a plausible catalytic cycle for the present oxidative
tandem NDA reaction as depicted in Scheme 2. First,
hydroxylamine 2 (Eox = +0.50 to +0.80 V vs. SCE)[12] is oxidized
General procedure for nitroso Dies-Alder reactions. In a flask were
successively added the diene carbamate (1.0 equiv.), the hydroxylamine
(3.0 equiv.), the ruthenium catalyst (0.05 equiv.), the solvent (0.1 mL) and
2,6-lutidine (2 equiv.). The reaction mixture was then stirred overnight at
RT under oxygen atmosphere and blue LED irradiation. The crude was
then purified on prep TLC to afford the desired compound.
2+
by Ru(bpy)3 in its triplet state (+0.77 V vs. SCE),[7] generating
ammonium-hydroxyl radical cation 6. The Ru(bpy)3+ is reoxidated
2+
by O2 to regenerate the catalyst Ru(bpy)3
and produce
superoxide radical anion (O2●−). Subsequently, deprotonation of
the ammonium-hydroxyl radical 6 by lutidine results in the
formation of radical 7. Single electron transfer between O2●−and 7
produce nitroso compound 4 and the hydroperoxide anion that
eventually deprotonates lutidinium to form H2O2. However, an
alternative pathway could involve the oxidation of
arylhydroxylamine 2 into nitrosoarene 4 by singlet oxygen. The
latter can be generated via direct energy transfer from the excited
state of Ru(bpy)32+ to molecular oxygen and this manifold cannot
be excluded at the current stage. According to the previous
studies,[13] while both mechanisms may operate, the
photocatalytic process is more likely.
Acknowledgements ((optional))
We thank CNRS, UniPR and MIUR for funding. VS and GM
warmly aknowledge SIR project RBSI14NKFL for support.
Keywords: Photocatalysis • Oxidative tandem • Nitroso-Diels-
Alder reaction • Radical • Nitrogen heterocycles
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Photoredox Catalysis*
Veronica Santacroce, Raphael Duboc,
Max Malacria, Giovanni Maestri and
Geraldine Masson*
Page No. – Page No.
Visible Light Photoredox-Mediated
Oxidative Tandem Nitroso Diels-Alder
Reaction of Arylhydroxylamines with
Conjugated Dienes
*one or two words that highlight the emphasis of the paper or the field of the study
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