Organocatalytic oxidation of substituted anilines to azoxybenzenes and nitro compounds: Mechanistic studies excluding the involvement of a dioxirane intermediate

Article abstract of DOI:10.1039/c6gc03174a

An organocatalytic and environmentally friendly approach for the selective oxidation of substituted anilines was developed. Utilizing a 2,2,2-trifluoroacetophenone-mediated oxidation process, substituted anilines can be transformed into azoxybenzenes, while a simple treatment with MeCN and H₂O₂ leads to the corresponding nitro compounds, providing user-friendly protocols that can be easily scaled up. Various substitution patterns and functional groups were tolerated leading to products in high to excellent yields. Mechanistic studies utilizing HRMS provide clear evidence for the distinct mechanistic intermediates that are involved. This study constitutes an indirect proof excluding the involvement of a dioxirane intermediate in the green organocatalytic oxidation, utilizing 2,2,2-trifluoroacetophenone as the catalyst.

Full text of DOI:10.1039/c6gc03174a

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ARTICLE
Organocatalytic Oxidation of Substituted Anilines to
Azoxybenzenes and Nitro Compounds: Mechanistic Studies
Excluding the Involvement of a Dioxirane Intermediate
Received 00th January 20xx,
Accepted 00th January 20xx
Errika Voutyritsa, Alexis Theodorou, Maroula G. Kokotou and Christoforos G. Kokotos*
DOI: 10.1039/x0xx00000x
www.rsc.org/
An organocatalytic and environmentally friendly approach for the selective oxidation of substituted anilines was
developed. Utilizing a 2,2,2-trifluoroacetophenone-mediated oxidation process, substituted anilines can be transformed to
azoxybenzenes, while simple treatment with MeCN and H2O2 leads to the corresponding nitro compounds, providing user-
friendly protocols that can be easily scaled up. Various substitution patterns and functional groups were tolerated leading
to products in high to excellent yields. Mechanistic studies utilizing HRMS provide clear evidence for the distinct
mechanistic intermediates that are involved. This study constitutes an indirect proof excluding the involvement of a
dioxirane intermediate in the green organocatalytic oxidation, utilizing 2,2,2-trifluoroacetophenone as the catalyst.
Traditional methods for the preparation of aromatic nitro
compounds usually involve the direct nitration of aromatic
compounds, under harsh reaction conditions. Unfortunately, a
Introduction
The nitro functionality plays a focal role in modern organic
synthesis, since it can be transformed in numerous functionalities.1
Nitro compounds have always been considered as ideal
intermediates in organic synthesis.2 They have found numerous
industrial applications, such as raw materials for the preparation of
explosives, dyes, perfumes, pharmaceuticals and the plastic
industries.1 Aromatic nitro compounds also participate in numerous
studies, playing key role in the evaluation of mechanistic concepts.3
Except from nitro compounds, current interest in azoxy compounds
is increasing dramatically, because of their physiological activities
and applications in liquid crystals.4-6 They have also been utilized in
DNA cleavage, polymerization reactions, as inhibitors, stabilizers,
dyes and analytical reagents.7
number of drawbacks are usually encountered. Thus, alternative
methods, like the oxidation of aromatic amines can be envisaged as
useful alternatives for the synthesis of nitro compounds. In
literature, oxidation of anilines can be performed by
dimethyldioxirane,8 which converts primary amines and substituted
anilines to nitro compounds in a facile, mild and high-yielding
process (Scheme 1).
Furthermore, peracetic acid9 and trifluoroperacetic acid9 are
fairly good reagents for the oxidation of many anilines to nitroaryls
(Scheme 1). In addition, metal-based processes have been
developed.10 It is worth mentioning that a Ti-superoxide catalyst10c
has been reported as an effective catalyst for the selective
oxidation of amines, particularly of anilines and aliphatic primary
amines to the corresponding nitrobenzenes (Scheme 1). More
recently, a simple and convenient method has been developed for
the direct oxidation of aromatic primary amines to nitro
compounds in good yields, utilizing a catalytic amount of KI in the
presence of TBHP11 as the external oxidant.
On the other hand, azoxy compounds can be successfully
produced from the corresponding nitro compounds. For this
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transformations in an one-pot manner, and thus, N-allyl tertiary
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DOI: 10.1039/C6GC03174A
amines were oxidized, followed by a Meisenheimer rearrangement
leading to O-allyl hydroxyl amines,16d various ketones were
transformed to the corresponding α-hydroxylated carbonyl
compounds,16e and homoallylic alcohols to substituted
tetrahydrofurans.16f Herein, we challenged whether this green
oxidation protocol could be extended to the oxidation of
substituted anilines. Our target was to develop reaction conditions
that could lead to
a selective procedure that could be
environmentally friendly and could lead either to the nitro
compound, or the azoxy compound (Scheme 1).
Results and discussion
Scheme 1 Synthetic approaches for the oxidation of anilines to azoxy and
We began our investigations by employing aniline (1a) in the
presence of 10 mol% of 2,2,2-trifluoroacetophenone as the catalyst,
utilizing tBuOH and aqueous buffer as the solvent system and 2
equivalents of acetonitrile and H2O2 (entry 1, Table 1). Under these
reaction conditions, azoxybenzene 2a was identified as the main
product in excellent yield. After optimization of the reaction
conditions, ethanol was found to be the solvent of choice for the
isolation of compound 2a (entries 1-3, Table 1).17 While studying
this reaction, we realized that by increasing the amount of MeCN
and H2O2, not only the reaction time could be decreased, but also
nitro compounds
chemical transformation, sodium bis(trimethylsilyl)amide12 has
been successfully employed. Except from this indirect method, a
solvent-free process for the oxidation of anilines to the azoxy
products employing a ball mill has been developed.13 This selective
oxidation can be carried out, using oxone and Al2O3 as the oxidation
13
system (Scheme 1). Furthermore, metal-based processes14 and
zeolite-mediated15 oxidation of anilines to azoxybenzenes have
been reported.
Table 1. Optimization of the reaction conditionsa
However these oxidants are either expensive, or produce large
amounts of waste raising environmental issues. There are also
numerous reports that suffer from lack of selectivity, leading to
mixtures of various oxidation products. The development of mild
and efficient oxidation methods is an attractive research area in
organic chemistry, which is of high importance. Previously in our
laboratory, an oxidation method that involves H2O2 as the oxidant
has been developed, which is environmentally friendly, since its
only byproduct is water and it is generally recognized as a safe and
cheap reagent. Since hydrogen peroxide by itself is a poor oxidant
for organic oxidations, it has to be coupled with a catalyst or a
reagent to be activated into a reactive intermediate that will carry
out the oxidation. A couple of years ago, we introduced 2,2,2-
trifluoroacetophenone, among a number of activated ketones, as
an efficient organocatalyst for the oxidation of silanes to silanols,16a
tertiary amines and azines to N-oxides16b and alkenes to
epoxides.16c Also, this oxidative protocol was coupled with other
Entry
Catalyst
(x mol%)
MeCN/H2O2
(equiv.)
Solvent
(2M)
Yield 2a
(%)b
Yield 3a
(%)b
1
2
10
10
2
2
tBuOH
MeOH
94
90
traces
traces
3
10
10
-
2
EtOH
EtOH
95
74
4
traces
26
4c
5c
6c
7c
8c
6.5
6.5
6.5
6.5
6.5
EtOH
96
-
toluene
MeCN
MeCN
-
97
-
-
>99
25
10
75
a
Aniline (1.00 mmol) in solvent (0.5 mL). 2,2,2-trifluoroacetophenone (x
mol%), aqueous buffer solution (0.5 mL, 0.6 M K2CO3 – 4 x 10-4M EDTA
disodium salt), acetonitrile (y equiv.) and 30% aqueous H2O2 (y equiv.) were
added consecutively. The reaction mixture was left stirring for 18 h at room
temperature. b Yield determined by GC-MS. c Reaction time: 1 h.
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a significant amount of nitrobenzene 3a was observed (entry 4, yield than m-aniside due to steric effects (2i Vievws Ar2ticj)le. OnOliuner
Table 1). Omitting the catalyst from the reaction mixture led to a experimental results showed that anilines wDitOhI:a10su.1b0s3t9it/Cut6iGoCn0a3t17th4Ae
significant change in the reaction outcome, constituting ortho position are difficult substrates and could not be oxidized to
nitrobenzene 3a the main product of the reaction. Again, thorough the corresponding azoxybenzenes in our reaction conditions.
reaction optimization led to the optimum reaction conditions However, other electron rich substituents like p-phenoxy-
(entries 5-7, Table 1).17 It has to be noted that acetonitrile was aniline, p-hexyloxy-aniline and p-ethoxy-aniline were tested
employed both as the solvent and as part of the oxidation system, providing azoxy benzenes in high yields (2h
while the addition of the catalyst led again to a switch in the Unfortunately, 1-naphthylamine and 2-naphthylamine
reaction outcome (entry 8 vs 7, Table 1). provided low yields (12% and traces, respectively), while 2-
Once the ideal conditions were recognized, the substrate scope aminopyridine afforded the corresponding N-oxide.
of the oxidation of anilines to azoxybenzenes was investigated We then turned our attention into the oxidation of
, 2k and 2l).
(Scheme 2). A variety of substituted anilines were employed using substituted anilines to nitro compounds (Scheme 3). Optimized
2,2,2-trifluoroacetophenone as the catalyst and EtOH as the reaction conditions in this case involved 6.5 equiv. of H2O2 in a
solvent. All substrates were converted to the corresponding mixture of MeCN (dual role, solvent and oxidation reagent)
products in high yields. Initially, the oxidation was performed with and an aqueous buffer solution (0.6 M K2CO3, 4 × 10-4M EDTA
aniline leading to the azoxybenzene 2a in high yield (94% isolated disodium salt, pH 11). The reaction mixture was allowed to stir
yield). Anilines substituted with electron-withdrawing groups, like for 1 h at room temperature, but for more demanding
p-fluoro-aniline, p-chloro-aniline and p-bromo-aniline led to slightly substrates, the reaction time could be expanded to 18 h.
lower yields of the isolated product (2b-2d). Various alkyl or Initially, aniline was oxidized providing nitrobenzene in high
aromatic groups at the para position led to products 2e-g in high yield (3a). When methoxy-substituted anilines were tested, the
yields. In addition p-anisidine formed the relevant product in better corresponding nitro compounds were isolated in high yields
(84-89%). More specifically, m-anisidine was converted to m-
nitroanisole in higher yield than o-anisidine (3b vs 3c).
Scheme 2 Substrate scope of the oxidation of substituted anilines to azoxy
Scheme 3 Substrate scope of the oxidation of substituted anilines to nitro
benzenes
benzenes
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However, in the case of p-anisidine, a separable mixture of
nitro- and nitroso-product was observed, where the
nitrobenzene 3d was the main product. In addition, o-
chloroaniline, m-methylaniline and p-fluoroaniline provided
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DOI: 10.1039/C6GC03174A
the nitro compounds in lower yields (3e, 3g and 3j), while 1,2-
diaminobenzene was converted to o-nitroaniline as only one of
the amino groups was oxidized (3f). Substitution at the meta-
or para-position had a minor effect in the case of anilines
which contained a carboxylic acid (3h, 3i and 3k). The
Scheme 4 Previous proposal of the catalytic mechanism for the epoxidation
utilizing 2,2,2-trilfuoroacetophenone as the catalyst16
oxidation of o-aminopyridine was an efficient reaction as the
nitro product was isolated in a considerable yield (3l). Finally,
nitro-substituted anilines afforded the dinitro compounds in
slightly lower yields (3m and 3n). In summary, anilines
containing electron-donating groups were transformed to the
relevant nitrobenzenes in good to high yields, while lower
yields were observed with electron-withdrawing groups.
Unfortunately, in both manifolds, the use of aliphatic amines
does not lead to the desired products. Unfortunately, 1-
naphthylamine and 2-naphthylamine provided low yields (12%
and 20%, respectively), while 4-aminopyridine and 2-
aminobenzothiazole did not afforded the desired product.
This intriguing switch in the selectivity depending on the
reaction conditions coupled with literature precedent that
dioxiranes oxidize anilines to nitro compounds,8 prompted us
to study in more detail the reaction mechanism and the
intermediates involved in these processes. Our previous
proposed reaction mechanism for alkene oxidation is
summarized in Scheme 4. Initially, under the aqueous
environment of the buffer, the activated ketone is
active oxidant, similarly to the elegant work of Adam, Shi,
Yang, Denmark and Miller,19 our experimental evidence were
not conclusive. It is known that dioxiranes can mediate
hydroxylation of alkanes (for example, adamantane).
Employing our procedure to adamantane, does not lead to any
product. Also, 1 equivalent of dioxirane is known to oxidize a
sulfide to the corresponding sulfoxide, while employing our
protocol (1 equivalent of oxidant) to the oxidation of a sulfide
led to a mixture of sulfoxide:sulfone:sulfide 1:1:1. Finally, in a
recent report,20 Yoshida and coworkers reported that only
dioxiranes can oxidize aryl iodides to hypervalent iodine (V)
compounds, and again utilizing our protocol, we cannot
perform this oxidation. We became interested in providing an
answer to this dilemma (dioxirane or not) by studying the
reaction mechanism of these two novel protocols.
In this work, the mechanistic studies were performed
employing High Resolution Mass Spectrometry (HRMS) under
positive and negative mode. HRMS has been previously
utilized to provide mechanistic insights.21 Initially, the
PhCOCF3-mediated reaction of aniline 1a to azoxybenzene 2a
was studied.17 In accordance with our previous observations
transformed to the corresponding diol I. At the optimum pH
provided by the aqueous buffer, MeCN reacts with H2O2 to
afford peroxycarboximidic acid II, an intermediate that has
been postulated by Payne to be involved in oxidative
processes, but due to its highly reactive character has never
utilizing different mechanistic tools,16a-c diol
I and perhydrate
IV were readily identified (Scheme 5, ). As discussed earlier,
D
been isolated or identified.18 This intermediate II
, in
peroxycarboximidic acid II has been implied in literature as a
reactive intermediate, but has never been identified due to its
highly reactive nature and short life. In our case, careful
experimentation allowed us to identify a peak that could be
attributed to the elusive intermediate II (Scheme 5, H). This is the
first time that such an elusive intermediate is detected and
identified proving its existence. In our current study, a peak that
could correspond to compound V is identified (Scheme 5, E, F and
conjunction with H2O2 oxidizes diol to perhydrate IV. Then,
I
this reacts with another molecule of II to afford the active
oxidant, which performs the oxidation (here an epoxidation
reaction) and regenerates the active form of the catalyst.16 In
our previous studies, we employed 19F-NMR, IR and MS, but
we were unable to conclude in the structure of the active
oxidant. Although one could expect that a dioxirane is the
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Scheme 5 HRMS studies of the oxidation of aniline 1a to azoxybenzene 2a. A) Full scan spectrum of the reaction mixture after 60 min. B) Spectrum of
nitrosobenzene in the reaction mixture after 60 min. C) Spectrum of phenylhydroxyamine in the reaction mixture after 60 min. D) Spectrum of diol I in the
reaction mixture after 60 min. E) Spectrum of intermediate V in the reaction mixture after 60 min. F) Spectra of intermediates from the reaction mixture
after 60 min utilizing aniline-d5. G) Spectra of intermediates from the reaction mixture after 60 min utilizing MeCN-d3. H) Payne’s intermediate from the
reaction mixture.
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G), which is a step closer to the active oxidant of our catalytic
system. In order to prove the existence of the proposed structures
and to cross out the possibility of artifacts, the same experiments
were carried out with aniline-d6 and/or acetonitrile-d3 (Scheme 5, F
and G).17 In both cases, the corresponding structures for the
proposed intermediates carrying the appropriate amount of d-
atoms were identified, verifying our proposal. Also, peaks that
correspond to phenylhydroxylamine (Scheme 5, C, F and G),
nitrosobenzene (Scheme 5, B, F and G) and azoxybenzene 2a
(Scheme 5, A) were identified in the reaction mixture. Thus, our
proposed mechanism involves an active oxidant, which could be
compound V or some other structure (Scheme 6). This active
species consumes 0.5 equiv. of the oxidant for the mono-oxidation
of aniline 1a to phenylhydroxylamine. Also, 1 equiv. of oxidant is
used for the double oxidation of the other half equivalent of aniline
1a to nitrosobenzene. The reaction of phenyl hydroxylamine with
nitrosobenzene is known to afford azoxybenzene 2a.22 If the active
oxidant of our catalytic system was a dioxirane, then literature
precedent suggests that it would oxidize aniline 1a to nitrobenzene
3a and not azoxybenzene 2a.8 Thus, the outcome of this reaction,
coupled with the previsouly-mentioned experiments (sulfide, iodide
and adamantane), is an indirect proof that a dioxirane is NOT the
active oxidant of our catalytic protocol.
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Scheme 7 HRMS studies of the oxidation of aniline 1a to nitrobenzene 3a: A)
Full scan spectrum of the reaction mixture after 60 min. B) Full scan
spectrum of the reaction mixture after 60 min utilizing aniline-d5. C) Spectra
of intermediates of the reaction mixture after 60 min of aniline with MeCN-
d3. D) Payne’s intermediate from the reaction mixture. E) Payne’s
intermediate from the reaction mixture utilizing aniline-d5. F) Payne’s
intermediate from the reaction mixture utilizing aniline-d3
Scheme
6 Proposed mechanism for the oxidation of aniline 1a to
azoxybenzene 2a utilizing 2,2,2-trifluoroacetophenone as the catalyst
We then turned our attention in the oxidation of aniline 1a to
nitrobenzene 3a. As previously, HRMS spectra were recorded both
in positive and negative modes (Scheme 7). In accordance with
previous observations, in the reaction conditions, aniline 1a reacts
with MeCN to form benzimidine 4 (Scheme 7, A, B and C). This
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compound in the absence of the catalyst is oxidized either by
H2O2or peroxycarboximidic acid II to oxaziridine 5 (Scheme 7, A, B
and C). Again, experiments employing aniline-d6 and MeCN-d3
confirmed the presence of these structures. Also, Payne’s
intermediate was again identified in all experiments (Scheme 7, D, E
and F). In order to avoid the formation of azoxybenzene 2a, aniline
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Scheme 9 Gram scale oxidations of aniline 1a to azoxybenene 2a and
nitrobenzene 3a
has
to
produce
nitrosobenzene
without
forming
phenylhydroxylamine. This can happen by two different pathways
(Scheme 8). Either oxaziridine 5 is the active oxidant that oxidizes
aniline 1a to nitrosobenzene (Scheme 8). However, this seems
unlikely to happen, since there is no literature precedent that an
oxaziridine can oxidize anilines to nitrosobenzenes. The other
alternative involves the ring expansion of 5 to the cyclic four-
membered ring compound 6 (same molecular formula as 5). A retro
[2+2]-cycloaddition of compound 6 would give nitrosobenzene. This
retro [2+2] reaction is similar to the oxaphosphetane
decomposition in a Wittig reaction.23 Finally, another equivalent of
the oxidant oxidizes nitrosobenzene to nitrobenzene 3a. The
different nature of the oxidant in these processes was further
verified by kinetic studies (Hammett plot), where different ρ values
were found.17
Conclusions
Overall, two simple, cheap and efficient organocatalytic
protocols for the oxidation of anilines to either azoxy compounds,
or nitro compounds were developed. In both cases, substituted
anilines could be employed successfully leading to the desired
product from high to excellent yields. Mechanistic investigations
were carried out employing HRMS to understand the different
mechanistic pathways. We concluded that in the case where we
employ 2,2,2-trifluoroacetophenone as the catalyst, an active
oxidant species deriving from
V oxidizes aniline 1a to
phenylhydroxylamine and nitrosobenzene, which react to form
azoxybenzene 2a. This constitutes an indirect evidence that our
catalytic protocol does not involve a dioxirane intermediate, since
dioxiranes are known to oxidize anilines to nitro compounds.
Furthemore, Payne’s intermediate, which has been postulated in
literature but its existence has never been proven, was for the first
time identified employing HRMS techniques. Also, a novel, green
and cheap process for the oxidation of anilines to nitro compounds
was studied by HRMS and an interesting mechanism was identified.
Experimental
General Procedure for the synthesis of azoxy benzenes
Scheme
8 Proposed mechanism for the oxidation of aniline 1a to
nitrobenzene 3a
Substituted aniline (1.00 mmol) was placed in a round bottom flask
and dissolved in ethanol (0.5 mL), followed by 2,2,2-trifluoro-1-
phenylethanone (17.4 mg, 0.10 mmol). Aqueous buffer solution (0.5
mL, 0.6 M K2CO3 – 4 x 10-4M EDTA disodium salt), acetonitrile (0.075
mL, 1.50 mmol) and 30% aqueous H2O2 (0.18 mL, 1.50 mmol) were
added consecutively. The reaction mixture was left stirring for 18
hours at room temperature. The crude product was purified using
flash column chromatography (5% EtOAc in Pet. Ether) to afford the
desired product.
Finally, in order to probe the applicability of these reactions for
scale-up, the gram scale oxidation of aniline was performed
(Scheme 9). The trifluoroacetophenone-mediated oxidation to
azoxybenzene 2a worked as efficiently as in small scale.
Furthermore, the simple oxidation of aniline 1a to nitrobenzene 3a
was performed equally successfully. In this case, the product could
be isolated either by silica plug filtration (82% yield), or by
distillation (azeotrope with water), followed by extractions.
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General Procedure for the synthesis of nitro benzenes
10 For selected examples, see: a) G. Barak, Y. Sasson,VJie.wOArgrti.clCehOenmlin.e,
DOI: 10.1039/C6GC03174A
Substituted aniline (1.00 mmol) was placed in a round bottom flask
and dissolved in acetonitrile (2 mL). Aqueous buffer solution (1 mL,
0.6 M K2CO3 – 4 x 10-4M EDTA disodium salt), acetonitrile (MS
grade, 0.33 mL, 6.50 mmol) and 30% aqueous H2O2 (0.75 mL, 6.50
1989, 54, 3484; b) S. Tollari, D. Vergani, S. Banfi, F. Porta, J.
Chem. Soc. Chem. Commun., 1993, 442; c) G. K. Dewkar, M. D.
Nikalje, I. S. Ali, A. S. Paraskar, H. S. Jagtap, A. Sudalai, Angew.
Chem. Int. Ed., 2001, 40, 405.
mmol) were added consecutively. The reaction mixture was left 11 K. R. Reddy, C. U. Maheswari, M. Venkateshwar, M. L. Kantama,
stirring for 1 hour at room temperature. The mixture was extracted Adv. Synth. Catal., 2009, 351, 93.
with EtOAc (3 x 7 mL). All the organic layers were combined and 12 J. R. Hwu, A. R. Das, C. W. Yang, J.-J. Huang, M.-H. Hsu, Org.
dried over Na2SO4. The solvents were removed under vacuum and Lett., 2005, 15, 3212.
the product was isolated with enough purity (>95%) after filtration 13 R. Thorwirth, F. Bernhardt, A. Stolle, B. Ondruschka, J. Asghari,
through a short silica plug (10% EtOAc in Pet. Ether). In case the Chem. Eur. J., 2010, 16, 13236.
product was not of sufficient purity, the crude product was purified 14 J. H. Kim, J. H. Park, Y. K. Chung, K. H. Park, Adv. Synth. Catal.,
using flash column chromatography to afford the desired product.
2012, 354, 2412.
15 S. Suresh, J. B. Jayachandran, A. V. Pol, M. P. Vinod, A. Sudalai,
H. R. Sonawane, T. Ravindranathan, Tetrahedron, 1995, 51,
11305.
Acknowledgements
The authors gratefully acknowledge the Operational Program
“Education and Lifelong Learning” for financial support
through the NSRF program “ΕΝΙΣΧΥΣΗ ΜΕΤΑΔΙΔΑΚΤΟΡΩΝ
ΕΡΕΥΝΗΤΩΝ” (PE 2431)” co-financed by ESF and the Greek
State. The authors would like to thank Assist. Professor I.
Lykakis from the Aristotle University of Thessaloniki for helpful
discussions.
16 a) D. Limnios, C. G. Kokotos, ACS Catal., 2013, 3, 2239; b) D.
Limnios, C. G. Kokotos, Chem. Eur. J., 2014, 20, 559; c) D.
Limnios, C. G. Kokotos, J. Org. Chem., 2014, 79, 4270; d) A.
Theodorou, D. Limnios, C. G. Kokotos, Chem. Eur. J., 2015, 21,
5238; e) E. Voutyritsa, A. Theodorou, C. G. Kokotos, Org.
Biomol. Chem., 2016, 16, 5708; f) A. Theodorou, C. G. Kokotos,
Green Chem., 2017, DOI: 10.1039/C6GC02580C.
17 For full reaction optimization and mechanistic studies, see
supporting information.
Notes and references
1
N. Ono, The Nitro Group in Organic Synthesis, Wiley, VCH, New 18
a) G. B. Payne, P. H. Deming, P. H. Williams, J. Org. Chem.,
1961, 26, 659; b) Peroxyacetimidic Acid in e-EROS
Encyclopedia of Reagents for Organic Synthesis, 2001.
York, 2001.
2
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