Accessing nitrosocarbonyl compounds with temporal and spatial control via the photoredox oxidation of N-substituted hydroxylamines

Article abstract of DOI:10.1016/j.tetlet.2015.01.024

Photoredox catalysis is employed to generate highly reactive acylnitroso species from hydroxamic acid derivatives. The conditions are shown to be comparable to a previously developed transition metal aerobic oxidation and are amenable to a range of transf

Full text of DOI:10.1016/j.tetlet.2015.01.024

Tetrahedron Letters 56 (2015) 3353–3357
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Tetrahedron Letters
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Accessing nitrosocarbonyl compounds with temporal and spatial
control via the photoredox oxidation of N-substituted
hydroxylamines
⇑
Charles P. Frazier, Leoni I. Palmer, Andrey V. Samoshin, Javier Read de Alaniz
Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, CA 93106-9510, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Photoredox catalysis is employed to generate highly reactive acylnitroso species from hydroxamic acid
derivatives. The conditions are shown to be comparable to a previously developed transition metal aer-
obic oxidation and are amenable to a range of transformations including Diels–Alder and ene reactions.
This unique application of such an approach gives access to temporal and spatial control in nitroso
chemistry.
Received 2 December 2014
Revised 29 December 2014
Accepted 2 January 2015
Available online 13 January 2015
Published by Elsevier Ltd.
Keywords:
Hydroxylamine
Nitrosocarbonyl
Photoredox
Temporal and spatial control
Block co-polymer
Nitrosocarbonyl compounds are exceptionally reactive interme-
diates that have found widespread use in a number of classic C–N
or C–O bond-forming reactions, such as the ene,1 Diels–Alder,2 and
aldol reactions.3 In recent years, a number of research groups have
explored new methodologies to generate nitrosocarbonyl interme-
diates, such as oxidation under aerobic conditions,1–3 utilizing
transition metals with stoichiometric peroxide,4 manganese diox-
ide oxidations,5 and oxidations employing TEMPO and BPO,6
among others.7 While many of these new oxidation methodologies
have facilitated the expansion of nitrosocarbonyl chemistry, none
have explored the potential of generating nitrosocarbonyl interme-
diates with temporal and spatial control. This unique property,
often neglected in the context of small molecule organic synthesis,
is an important facet to consider when expanding nitrosocarbonyl
chemistry into biological and materials science applications.
Photoredox catalysis is an emerging tool for the synthesis of
complex molecules that has been shown to be effective in contexts
requiring temporal and spatial control. While the genesis of photo-
redox catalysis can be traced back to Kellog in the 1970’s, the field
has undergone a renaissance in recent years, based on the pioneer-
ing work of a number of research groups.8 In particular, the recent
work by the groups of Yoon,9 and Stephenson,10 and Macmillan11
highlights the advantageous usage of photoredox catalysis in the
oxidative generation of reactive intermediates. In 2012, Hawker
and co-workers translated photoredox catalysis into material sci-
ence, providing control of a living radical polymerization using vis-
ible light12 and subsequently demonstrated excellent spatial
arrangement of functional groups at surfaces.13 The key to this
methodology was visible-light mediated Ir-catalyzed atom-trans-
fer radical addition (ATRA) chemistry developed by Stephenson
and co-workers.14 Because nitrosocarbonyl compounds participate
in a variety of organic reactions, the in situ formation of this highly
reactive functional group using photoredox conditions would fur-
nish a general procedure for patterning surfaces bearing a range
of properties. Moreover, because nitrosocarbonyl compounds serve
as HNO donors,15 it could also provide a means to generate HNO
in situ using visible-light to control its release. To evaluate this
potential, we began by investigating the in situ generation of nitr-
osocarbonyl compounds using photoredox conditions. A recent
report by Tan inspired us to disclose our results in this area
(Fig. 1).16
Due to the transient nature of nitroso compounds and their
associated inherent instability,17 we selected the nitroso HDA reac-
tion to test the feasibility of a photoredox catalyst system. Nitroso
compounds directly attached to an electron withdrawing group
undergo rapid [4+2] cycloaddition reactions with conjugated
dienes, a reaction platform that has been explored extensively by
our research group2a and others.18 Cbz-protected hydroxylamine
1 and 1,3-dicyclohexadiene 2 were elected as candidates for
⇑
Corresponding author. Fax: +1 8058934120.
E-mail address: javier@chem.ucsb.edu (J. Read de Alaniz).
http://dx.doi.org/10.1016/j.tetlet.2015.01.024
0040-4039/Published by Elsevier Ltd.

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C. P. Frazier et al. / Tetrahedron Letters 56 (2015) 3353–3357
indicated 2.0 equiv of 2,6-lutidine was optimal, and a brief solvent
O2
screen (entries 13 and 14) indicated DCE to indeed be our preferred
solvent. Finally, the reaction does not proceed in the dark under
the same conditions and duration (entry 15), nor is any conversion
observed in the absence of catalyst (entry 16). Based on these
results we elected to explore the reaction with 1 mol % Ru(bpy)3
Cl2ꢀ6H2O and 2 equiv 2,6-lutidine in DCE (entry 10).
RuII
CuIIO
CuI
RuI
RuII*
O
O
O
OH
OH
O
R
N
H
R
N
H
R
N
H2O
With optimized reaction conditions in hand, we elected to
briefly explore the scope of the Diels–Alder reaction to ensure gen-
erality of reaction conditions (Table 2). To our satisfaction, both the
5- and 6-membered ring dienes readily underwent the Diels–Alder
reaction with no competing production of the endoperoxide or
hydroxyenone (7 and 3). Additionally, 1,4-diphenylbutadiene also
underwent clean conversion to the desired oxazine product (8).
The reaction was not limited to Cbz-protected hydroxylamines,
as Boc-protected hydroxylamines were readily converted to the
corresponding oxazine products (9 and 10).
This Work: Photoredox Conditions
Advantage: temporal and
spatial control
Previous Work: Aerobic Conditions
Figure 1. Exploring photoredox conditions for the generation of nitrosocarbonyls.
standardizing reaction conditions (Table 1) and investigations
began with commercially available and widely employed Ru(bpy)3
Cl2ꢀ6H2O. At 1 mol % loading and in DCE at room temperature, we
were pleased to see a 35% yield of the desired oxazine product
(entry 1). Analysis of the reaction mixture revealed a large amount
of an undesired hetero Diels–Alder (HDA) product resulting from
reaction of the diene with diatomic oxygen in a [4+2] cycloaddition
and subsequent ring-opening reaction,19 suggesting the potential
interference of singlet oxygen on our desired reaction pathway.20
Pleasingly, the addition of pyridine (2.0 equiv) completely inhib-
ited this unproductive pathway (entry 2) and allowed us to achieve
an improved yield of 80%. Screening other Ru photocatalysts,
(entries 3–6) showed no improvement in yield or reaction duration
over Ru(bpy)3Cl2ꢀ6H2O. In considering which photoredox catalysts
to employ, Ru(bpy)23+⁄ is both a good oxidant and reductant.8
Switching to a photocatalyst like Ir(ppy)3, which is more reducing
in its excited state, had a negative impact on the yield (entries 7
and 8). Using a Eu-based catalyst (entry 9) satisfactorily aug-
mented the yield but has the added disadvantage of requiring UV
light for activation. Based on this, we elected to further optimize
the reaction with Ru(bpy)3Cl2ꢀ6H2O.
Table 2
Scope of the nitrosocarbonyl HDA under photoredox conditions
R3
R3
OH
NH
1 mol % Ru(bpy)3Cl2
O
R1O
R1O
N
6
R2
DCE, 2,6-lutidine, air
26W white light
O
4
O
R2
5
Ph
Ph
O
N
O
O
N
N
BnO
BnO
BnO
O
O
O
7
3
8
Next we looked at the dependence of the reaction on the chosen
additive; screening various bases (NEt3, iPr2NEt, etc.) led us to
determine that 2,6-lutidine was the additive of choice for our reac-
tion set up. Variations in the additive loading (entries 10–12)
93%
87%
54%
O
O
N
N
t-BuO
t-BuO
O
O
9
10
77%
Table 1
89%
Optimizing photoredox conditions for the generation of nitrosocarbonyls
OH
NH
conditions
O
BnO
N
BnO
1 atm air, 26W hν
Table 3
Scope of nitrosocarbonyl ene reaction under photoredox conditions
O
1
O
2
3
H
R4
R4
R3
OH
NH
OH
N
1 mol % Ru(bpy)3Cl2
BnO
R1
BnO
Entry
Catalyst (1 mol %)
Solvent
Additive (equiv)
Yielda (%)
R3
DCE, 2,6-lutidine, air
26W white light
R1 R2
12
O
1
R2
O
1
2
3
4
5
6
7
8
Ru(bpy)3Cl2ꢀ6H2O
Ru(bpy)3Cl2ꢀ6H2O
Ru(bpz)3Cl2
Ru(bpz)3Cl2
Ru(bpm)3BArF2
Ru(phen)3Cl2
DCE
DCE
—
—
35
80
49
73
30
24
34
63
95
87
74
60
63
55
<5%c
<5%c
Pyridine (2.0)
—
11
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
MeNO2
MeCN
DCE
DCE
Pyridine (2.0)
pyridine (2.0)
Pyridine (2.0)
Pyridine (2.0)
Pyridine (2.0)
Pyridine (2.0)
2,6-Lutidine (2.0)
2,6-Lutidine (0.5)
2,6-Lutidine (0.05)
2,6-Lutidine (2.0)
2,6-Lutidine (2.0)
2,6-Lutidine (2.0)
2,6-Lutidine (2.0)
OH
OH
OH
BnO
N
BnO
N
BnO
N
Ir-fac(ppy)3
Ir(BTP)3
Ph
O
O
O
9
Eu(dbm)3(phen)b
Ru(bpy)3Cl2ꢀ6H2O
Ru(bpy)3Cl2ꢀ6H2O
Ru(bpy)3Cl2ꢀ6H2O
Ru(bpy)3Cl2ꢀ6H2O
Ru(bpy)3Cl2ꢀ6H2O
Ru(bpy)3Cl2ꢀ6H2O
—
10
11
12
13
14
15
16
13
14
15
99%
BnO
68%
85%
OH
N
OH
N
Et
BnO
C5H11
O
nPr
O
a
b
c
16
78%
Isolated yields.
280 nm light was used.
Starting material was recovered.
17
34%

C. P. Frazier et al. / Tetrahedron Letters 56 (2015) 3353–3357
3355
While the Diels–Alder reaction with nitrosoformate intermedi-
ates is known to be a highly efficient reaction,18 the corresponding
ene reaction of nitrosoformate intermediates often suffers from
poor yields due to the over-oxidation of the allylic hydroxylamine
products.21 Recent efforts have been undertaken by a number of
research groups to circumvent this known decomposition reac-
tion,22 and thus the ene reaction has proven to be an effective tool
for evaluating the mildness of oxidation reaction conditions.7 In
this context, we elected to test our newly developed oxidation con-
ditions with a subset of olefin partners. We were delighted to find
that the ene adducts were readily formed under our optimized
conditions with no observable product decomposition (Table 3).
As with other studies, the efficiency of the ene reaction is directly
correlated to the substitution of the olefin; fully substituted olefins,
such as 2,3-dimethylbutene, form the corresponding allylic
hydroxylamine in quantitative yield (13). In contrast, mono-substi-
tuted olefins such as 1-octene react with nitrosocarbonyl com-
pounds with reduced yields (17). In each case, the photoredox
conditions compared favorably with previous oxidative nitrosocar-
bonyl ene methodologies (13–17).1 Based on these results, we
envision that these conditions can be utilized for a wide variety
of reaction platforms and will facilitate the expansion of nitroso-
carbonyl chemistry into previously unexplored areas of organic
synthesis.
the synthesis of a nitroso-Diels –Alder block copolymer 21.24
Hydroxamic acid terminated PEG polymer (2k mPEG-HA) 19 was
allowed to react under slightly modified reaction conditions with
cyclopentadiene end-capped polystyrene polymer (12k PS-Cp)
20. After 24 h, complete conversion of 20 was observed and block
copolymer 21 was successfully isolated by trituration in methanol–
water (1:1) (Scheme 1).
O
H
O
O
N
O
OH
+
n
O
m
O
Ph
19
20
2k mPEG-HA
12k PS-Cp
8 mol% Ru(bpy)3Cl2,
6 mol eq 2,6-lutidine
DCE, air, rt, hν, 24 h
O
O
O
N
O
n
O
O
m
O
Ph
21
14k mPEG-b-PS
Scheme 1. Photoredox catalyzed synthesis of nitroso-Diels–Alder block copolymer
21.
Encouraged by the mildness of our reaction conditions, we next
elected to explore the efficiency of temporal control for our opti-
mized conditions. While on–off studies are not normally per-
formed for photoredox reactions in small molecule chemistry,
this type of experiment is regularly conducted with photoredox
polymerizations,12,23 and is a prerequisite for transitioning a meth-
odology into a number of materials science applications such as
the patterning of functionalized surfaces.13 Therefore, we were
delighted to find that the reaction could be effectively turned on
and off based on the presence or absence of light. The reaction
showed no reduction in efficiency even after iteratively turning
the light on and off. Moreover, the reaction could even be shut
off for extended periods of time (e.g., 18 h) and started again once
the light was reintroduced, highlighting a unique element of con-
trol never exploited for the generation of nitrosocarbonyl interme-
diates (Fig. 2).
We next explored the distance dependence of our newly devel-
oped reaction conditions. For the design of this experiment, the
lights were placed at a variable distance away from the sidewall
of the reaction flask and the reaction was monitored periodically
for conversion. As expected, when the lights were touching the
sidewall of the flask, the reaction proceeded the fastest, with a rate
paralleling our on–off study. As the lights were moved further from
the flask, the reaction rate dropped precipitously. Based on these
results, there is a clear spatial dependence of the reaction rate:
the intensity of the light has a direct effect on the reaction
(Fig. 3). Based on the distance dependence and the on–off studies,
we believe that this reaction has the potential to find widespread
use in applications where temporal and spatial control are desired,
and our studies in this area will be reported in due time.
Eager to demonstrate application of these conditions in a mate-
rials setting, we decided to employ our photoredox conditions for
Based on the trends we observed during the reaction develop-
ment, we were interested in further exploring the mechanism. In
particular, we wanted to distinguish between a photoredox mech-
anism and a mechanism involving singlet oxygen. In order to probe
this, we turned to Stern–Volmer studies to confirm the direct inter-
action between the catalyst and the Cbz-protected hydroxyl-
amine.25 As expected, the magnitude of fluorescence of the
catalyst had a linear dependence on the concentration of the
Cbz-protected hydroxylamine,26 indicating an interaction between
the excited state of the catalyst and the Cbz-protected hydroxyl-
amine (Fig. 4). In contrast, no fluorescence quenching was
observed in the presence of varying concentrations of the diene
(See SI for more information). Based on these results, we propose
that the excited state of the catalyst (Ru(bpy)3Cl⁄2) is directly oxi-
dizing the Cbz-protected hydroxylamine to the corresponding rad-
ical cation, consistent with the known redox potentials of the
catalyst excited state (E1/2 = 1.08 V vs Ag/AgNO3)8 and the Cbz-pro-
tected hydroxylamine (Ep/2 = 0.95 V vs Ag/AgNO3, see Supporting
information). The subsequent reduction of molecular oxygen forms
the corresponding superoxide radical, which facilitates the conver-
sion of the Cbz-hydroxylamine radical cation to the nitrosocarbon-
yl species (Fig. 5A).
To explore the mechanism further, we tested the reaction effi-
ciency in the presence of a known singlet oxygen sensitizer, tetra-
phenylporphyrin (TPP).27 To our surprise, upon irradiation of the
Figure 2. On–off studies for the developed photoredox oxidation. aReactions
conducted using dimethyl terephthalate as the internal standard. bAliquots were
removed at the indicated times and conversion was determined by 1H NMR
analysis.
reaction,
a small amount of hetero-Diels–Alder adduct was

3356
C. P. Frazier et al. / Tetrahedron Letters 56 (2015) 3353–3357
conditions, direct energy transfer from Ru(bpy)3Cl⁄2 to molecular
oxygen will produce singlet oxygen, which can potentially oxidize
the Cbz-protected hydroxylamine directly and form the corre-
sponding nitrosocarbonyl compound (Fig. 5B).
OH
NH
1 mol % TPP
O
BnO
N
BnO
2 equiv 2,6-lutidine
ð1Þ
O
1
1 atm air, 26W hν
O
2
3
(3 x 3 equiv)
37%
Despite the results using TPP, we believe that the reaction pro-
ceeds predominantly through a photoredox mechanism (Fig. 5A),
based on the compatibility of redox potentials between the
hydroxylamine and Ru(bpy)3Cl⁄2, Stern–Volmer studies, and on
the lack of formation of products derived from singlet oxygen
under the optimized conditions. However, at this time a singlet
oxygen-mediated mechanism cannot be ruled out (Fig. 5B).
In conclusion, the developed photoredox conditions provide a
unique pathway to generate transient nitrosocarbonyl interme-
diates with both temporal and spatial control, demonstrated
by the capacity to turn the reaction on and off and the distance
dependence. Additionally, the reaction conditions are sufficiently
mild so as to not decompose the ene products, suggesting the
oxidation methodology can be used to explore new frontiers
of nitrosocarbonyl chemistry. The reaction is operationally sim-
ple to perform, using lights purchased from a local hardware
store, reagent grade solvents, air as the terminal oxidant and
a readily available photoredox catalyst. We envision, based on
the results, that this newly developed methodology has signifi-
cant potential utility both in the context of small molecule
chemistry and in the applied context of biology or materials
science.
Figure 3. Distance dependence of photoredox oxidation. aReactions conducted
using dimethyl terephthalate as the internal standard. bAliquots were removed at
the indicated times and conversion was determined by 1H NMR analysis. cThe 26 W
lights were placed at the indicated distances relative to the sidewall of the reaction
vessel.
Acknowledgments
We thank UCSB and MRSEC program (UCSB MRL) of the
National Science Foundation (DMR 1121053) for financial support,
and Profs. B. Fors and R.D. Little for helpful discussions.
Figure 4. Fluorescence quenching experiment with Cbz-protected hydroxylamine.
aperformed in a sealed cuvette under an atmosphere of argon using 1 mg/mL of
Ru(bpy)3Cl2ꢀ6H2O in 1,2-dichloroethane.
Supplementary data
Material associated with this publication is available free of
charge in the online version.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.01.
024. These data include MOL files and InChiKeys of the most
important compounds described in this article.
observed, although additional aliquots of cyclohexadiene were
added periodically (3 equiv every 2 h), due to competing consump-
tion of the diene by singlet oxygen.20 Under these conditions, a
significant amount of both the endoperoxide and ring-opened 4-
hydroxy-2-cyclohexenone were isolated,19 along with the desired
nitrosocarbonyl HDA adduct (Eq. 1). Under the optimized reaction
hν
Ru(II)*
Ru(II)
OH
RO
NH
Ru(II)*
1O2 3O2
O
O2
hν
OH
NH
O
N
OH
RO
RO
HN
OR
Ru(II)
Ru(I)
O
O
O
H2O2
H2O2
O2
O2
A
B
Figure 5. Proposed mechanisms for the photoredox generation of nitrosocarbonyls. (A) Photoredox catalyzed oxidation; (B) singlet oxygen mediated oxidation.

C. P. Frazier et al. / Tetrahedron Letters 56 (2015) 3353–3357
3357
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