Electrochemical Reductive Smiles Rearrangement for C-N Bond Formation

Article abstract of DOI:10.1021/acs.orglett.8b03178

A conceptually new and synthetically valuable radical Smiles rearrangement reaction is reported under undivided electrolytic conditions. This protocol employs an entirely new strategy for the electrochemical radical Smiles rearrangement. Remarkably, an amidyl radical generated from the cleavage of the N-O bond under reductive electrolytic conditions plays a crucial role in this transformation. Various hydroxylamine derivatives bearing different substituents are suitable in this electrochemical transformation, furnishing the corresponding amides in up to 86% yield.

Full text of DOI:10.1021/acs.orglett.8b03178

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Electrochemical Reductive Smiles Rearrangement for C−N Bond
Formation
Xihao Chang, Qinglin Zhang, and Chang Guo*
Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026,
China
S
* Supporting Information
ABSTRACT: A conceptually new and synthetically valuable radical Smiles rearrangement reaction is reported under undivided
electrolytic conditions. This protocol employs an entirely new strategy for the electrochemical radical Smiles rearrangement.
Remarkably, an amidyl radical generated from the cleavage of the N−O bond under reductive electrolytic conditions plays a
crucial role in this transformation. Various hydroxylamine derivatives bearing different substituents are suitable in this
electrochemical transformation, furnishing the corresponding amides in up to 86% yield.
Scheme 1. Radical Smiles Rearrangement
he direct carbon−nitrogen (C−N) bond formation has
Temerged as one of the most efficient and frequently
employed methodologies, allowing access to a wide array of
heterocycles and bioactive compounds.1 While great achieve-
ments have been accomplished, the development of C−N bond
formation from common precursors under mild conditions
remains a formidable challenge. In particular, early reports on a
C−N bond-forming reaction via Smiles rearrangement under
base conditions introduce new methods and strategies for the
direct transformation of unactivated C−O bonds, and reshape
the field of retrosynthetic analysis.2 The classical Smiles
rearrangement is an intramolecular nucleophilic aromatic
substitution reaction featuring an anionic N-nucleophile
(Scheme 1A).3,4 Moreover, radical versions of the rearrange-
ment reaction have been demonstrated without the requirement
for a strong electron-withdrawing group adjacent to the
migrating aryl ring.5 In comparison, there are still challenges
for the Smiles rearrangement based on N-centered radicals,
probably due to the lack of versatile radical initiation methods to
achieve high reaction efficiency.6 We recognized that the
successful realization of these ideals would enable a new C−N
bond-forming reaction under mild conditions that would allow
efficient access of constructing aromatic C−N bonds. Perhaps
most important, to our knowledge this N-centered radical
Smiles rearrangement has not previously been accomplished in a
generic format.
voltammogram record of 1a shows an irreversible oxidation
wave at 2.25 V versus SCE, thus indicating that 1a would be
oxidized at a potential much higher than that of the
ox
3a
rearrangement product 3a (E = 1.50 V versus SCE). Therefore,
even if the radical Smiles rearrangement is successful, over-
oxidation is inevitable because of the lower oxidative potential of
3a (Scheme 1B).
Recently, electrochemistry has emerged as a powerful
technique through which single electron-transfer (SET)
reactions can be performed for numerous challenging
reactions.7−9 Oxidative N-center radicals, electrochemically
generated from N−H precursors,10 show their applications in
the C−N bond-forming reactions.11 Typically, the cyclic
Received: October 5, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03178
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
We set out to explore the application of an amidyl radical and
the development of novel, base-free electrosynthesis-mediated
radical Smiles rearrangement (Scheme 1B). However, there are
several challenges associated with the development of this
radical rearrangement, such as (1) finding an entirely new
strategy to generate amidyl radicals upon electrolytic conditions;
(2) avoiding the overoxidation; and (3) identification of
electrochemical reaction conditions which afford high yields.
Drawing inspiration from the generation of the amidyl radical,
which results in hydroamination12 or cross-dehydrogenative
coupling processes,13 we speculated that appropriately function-
alized hydroxylamine derivatives could serve as general, bench-
stable N-centered radical precursors. Herein, we report the
successful implementation of an amidyl radical and the
development of novel, base-free electrosynthesis-mediated
radical Smiles rearrangement.
evaluated their ability to form amide 3a under electrolytic
conditions. To our delight, we observed the formation of amide
3a in 77% yield when 2a was employed (entry 2). Further
exploration showed that the 2,4-di-NO2-phenyl 2b was superior
to 2a and could be converted into 3a in 83% yield (see the
Supporting Information for details of further optimization of the
reaction conditions). The starting materials were fully recovered
when 2c−2e were employed (entries 4−6). The reaction could
be conducted under an argon atmosphere without any difference
(entry 7). Importantly, no conversion of 3a was observed
without the electric current (entry 8), which suggests the
rationality of the proposed electrolysis-induced SET mecha-
nism.
Under the optimized conditions, the substituents on the
migrating aryl ring (Ar1) were first investigated (Scheme 2).
a b
,
To evaluate the feasibility of such a radical rearrangement
protocol, we subjected 1a to oxidative electrolysis with Pt as
electrodes in an undivided cell under 10 mA constant current
with NH4Cl as the electrolyte (Table 1, entry 1). As we have
Scheme 2. Substrate Scope
Table 1. Optimization of the Smiles Rearrangement
a
Reaction
b
entry
substrate
yield [%]
1
2
3
4
5
6
1a
2a
2b
2c
2d
2e
2b
2b
0
77
83
0
0
0
a
Unless otherwise specified, all of the reactions were performed using
2 (0.2 mmol) with NH4Cl (0.25 M) and DMSO (4 mL) in an
undivided at 50 °C. Pt plate cathodes (1 cm × 1 cm), constant
b
c
current 10.0 mA, 3 h (5.6 F/mol). Isolated yield. 2.5 mmol scale
reaction.
c
7
d
80
0
8
a
Generally, the desired amide derivative was isolated in high to
excellent yields (3a, 3f−3w, 68−84%) irrespective of the
position and electric nature of substituents on the migrating aryl
moiety. Notably, the scalability of this electroreductive
rearrangement was evaluated by performing on a 2.5 mmol
scale reaction. Under atmospheric conditions, the gram scale
reaction of 2a afforded the corresponding product 3a in high
reaction efficiency with 78% yield. Furthermore, the observed
electronic effect of substituents in this radical aryl migration is
contrary to that observed in the classical anionic Smiles
rearrangement, in which strong electron withdrawing groups
are required.3b Typically, electron-donating groups in the para
position with a variety of functional groups performed well in
this radical reaction (3a, 3f−3h). Electron-withdrawing groups,
such as halogens, were well-tolerated under the optimized
conditions (3i−3k). Moreover, the influence of substituents in
the meta and ortho positions was also investigated (3m−3s).
Similar behavior of methyl and bromo substituents was
Unless otherwise specified, all of the reactions were performed using
2 (0.2 mmol) with NH4Cl (0.25 M) and DMSO (4 mL) in an
undivided cell at 50 °C. Pt plate cathodes (1 cm × 1 cm), constant
current 10.0 mA, 3 h (5.6 F/mol). Isolated yield. Under argon
b
c
d
atmosphere. Without the electric current.
predicted (Scheme 1B), the reaction failed because of the
competitive electron-transfer processes between 1a and 3a
under electrolytic conditions. Therefore, alternative reductive
electrosynthesis for the radical smiles rearrangement was
considered. For a study of the radical Smiles rearrangement
reaction, a series of hydroxylamine derivatives 2a−2e with
different functional groups were first investigated by performing
cyclic voltammetry studies on model precursors. All of these
substrates we tested displayed reduction profiles, while 2a and
2b fell well inside the SET reduction range of 3a (2a, E
−1.48 V versus SCE; 2b, E = −1.52 V versus SCE; 3a, E
−2.21 V versus SCE). Keeping this information in mind, we
red
=
=
2a
red
2b
red
3a
B
DOI: 10.1021/acs.orglett.8b03178
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
observed. Notably, the migration of aryl rings (Ar1) with two or
three substituents also occurred in high yields (3v and 3w).
Next, the influence of substituents on the aryl ring (Ar2) was
investigated (Scheme 2). Electron-donating and electron-
withdrawing groups which were located on the para to the
amide enabled the transformation to proceed in good to
excellent yield (3x−3aa). It was shown that functional groups,
such as methoxy, methyl, chloro, and bromo, attached on the
aryl ring (Ar2) in 2 were well-tolerated. A bromo group which
was located on the para to the phenol group resulted in a good
yield (3ab, 77% yield). Furthermore, N-alkyl substituents could
be varied to generate corresponding amides in moderate yields
(3ac−3ae). We next sought to examine the applicability of this
strategy for the late-stage modification. The application of this
rearrangement was further extended to the introduction of the
nitrogen atom at position C-3 of estrone in high yield (3af, 80%
yield), which has been identified as a versatile precursor of
biologically active compounds.14
O-aryl amides serving as a sensitizer of the radical Smiles
rearrangement (Scheme 4). Since N-centered radicals are a
versatile class of synthetic intermediates,17 we envisaged that an
electrolysis-mediated approach for amidyl radical generation
would be compatible with hydroamination−cyclization
(Scheme 4B).12 Under our optimized electrolytic conditions,
the hydroamination ring-opening product 10 was generated.
These results indicate that the Smiles rearrangement might go
through a radical mechanism which would start with SET
reduction of the 2,4-di-NO2-phenyl unit.
Then, we conducted a series of control experiments under the
optimized reaction conditions to provide insight into the
reaction mechanism (Scheme 5). The profile of the yield of 3a
Scheme 5. Electric Current Turned On and Off Experiments
Notably, axially chiral anilines have emerged as versatile
building blocks for catalysts, functional materials, and natural
products.15 Development of more general strategies for
construction of axially chiral aniline derivatives under mild
conditions is highly desirable. Under our optimized electro-
chemical conditions, 6a and 6b were obtained in moderate
yields without racemization (Scheme 3).
Scheme 3. Subsequent Transformation
was monitored with the electric current turned on and off at
intervals. We found that the Smiles rearrangement occurred
smoothly under electric current, and the further conversion
stopped in the absence of current source. Therefore, the amidyl
radical intermediate generated by electrolysis plays a crucial role
in the Smiles rearrangement.
On the basis of the above experimental results and previous
reports,12,13 a plausible reaction mechanism of the radical Smiles
rearrangement is presented in Scheme 6. A single-electron-
Intrigued by the positive reduction potential of the unique
role of the 2,4-dinitro-substituted O-aryl amide group (Table 1),
we wondered whether a complementary activation mode could
be exploited by the generation of amidyl radical by electrolysis
(Scheme 4A).16 Cyclic voltammetry experiments were con-
ducted to study the reductive potential of substrates. No obvious
Scheme 6. Proposed Reaction Mechanism
difference in the reduction peaks of 2b, 7, and 8 was observed
red
(2b, E = −1.52 V versus SCE; 7, Er7ed = −1.52 V versus SCE; 8,
2b
Er8ed = −1.52 V versus SCE), indicating the aromatic unit of the
Scheme 4. Mechanistic Studies
transfer reduction of 2b by cathodic reduction generates the
radical intermediate I. The following N−O bond fragmentation
leads to amidyl radical II and subsequently undergoes radical
Smiles rearrangement to form an O-centered radical inter-
mediate III. Then cathodic reduction and protonation of III
generates the reductive rearrangement product 3a.
In summary, we envisioned an entirely new reductive
activation mode to generate the amidyl radical for the
electrolytic Smiles rearrangement. External chemical reductant
and base are avoided in this protocol. Under our optimized
undivided electrolytic conditions, a series of amides can be
obtained in good to excellent yields under atmospheric
conditions. We anticipate that this radical strategy will have
C
DOI: 10.1021/acs.orglett.8b03178
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b03178.
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Experimental procedures and spectral data including
1
cyclic voltammograms, H and 13C NMR spectra, and
HPLC traces (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: guochang@ustc.edu.cn.
ORCID
Chang Guo: 0000-0003-4022-9582
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge financial support from the National
Natural Science Foundation of China (Grant 21702198), and
the Anhui Provincial Natural Science Foundation (Grant
1808085MB30).
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