Visible-Light-Mediated Generation of Nitrogen-Centered Radicals: Metal-Free Hydroimination and Iminohydroxylation Cyclization Reactions

Article abstract of DOI:10.1002/anie.201507641

The formation and use of iminyl radicals in novel and divergent hydroimination and iminohydroxylation cyclization reactions has been accomplished through the design of a new class of reactive O-aryl oximes. Owing to their low reduction potentials, the inexpensive organic dye eosin Y could be used as the photocatalyst of the organocatalytic hydroimination reaction. Furthermore, reaction conditions for a unique iminohydroxylation were identified; visible-light-mediated electron transfer from novel electron donor-acceptor complexes of the oximes and Et₃N was proposed as a key step of this process.

Full text of DOI:10.1002/anie.201507641

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201507641
German Edition: DOI: 10.1002/ange.201507641
Photoredox Catalysis
Visible-Light-Mediated Generation of Nitrogen-Centered Radicals:
Metal-Free Hydroimination and Iminohydroxylation Cyclization
Reactions
Jacob Davies, Samuel G. Booth, Stephanie Essafi, Robert A. W. Dryfe, and Daniele Leonori*
Abstract: The formation and use of iminyl radicals in novel
and divergent hydroimination and iminohydroxylation cycli-
zation reactions has been accomplished through the design of
a new class of reactive O-aryl oximes. Owing to their low
reduction potentials, the inexpensive organic dye eosin Y could
be used as the photocatalyst of the organocatalytic hydro-
imination reaction. Furthermore, reaction conditions for
a unique iminohydroxylation were identified; visible-light-
mediated electron transfer from novel electron donor–acceptor
complexes of the oximes and Et3N was proposed as a key step
of this process.
reagents at elevated temperatures.[1a,2] The development of
a mild, selective, and general method to catalytically generate
NCRs from readily available precursors would enable the
facile construction of many N-heterocycles, which are priv-
ileged motifs in natural products and therapeutic agents.[3]
Photoredox catalysis has emerged as a powerful technique
through which single electron transfer (SET) reactions can be
performed under mild conditions.[4] MacMillan[5] and co-
workers have developed an asymmetric visible-light-medi-
ated amination of aldehydes by enamine catalysis, and the
groups of Sanford,[6] Lee,[7] Yu,[8] and Luo[9] have reported the
photoredox generation of phthalimidyl and saccharyl radicals
and their use in Minisci-type reactions. The groups of
Zheng[10] and Knowles[11] have developed a method for the
photoredox generation of diaryl and aryl alkyl aminium
N
itrogen-centered radicals (NCRs) are a versatile class of
intermediates that have wide applications in the synthesis of
N-containing molecules (Scheme 1A).[1] However, the diffi-
culties associated with their generation have significantly
thwarted their use in synthetic chemistry. In fact, established
methods often rely on the homolysis of difficult-to-construct
À
radical cations and employed them in C N bond-forming
reactions.
Drawing inspiration from the work of Forrester,[12]
Narasaka,[13] and Walton,[14] we speculated that appropriately
functionalized O-aryl oximes could serve as general, bench-
stable NCR precursors that could deliver iminyl radicals upon
photoredox activation under mild conditions.[15] Such an
approach would clearly benefit from the facile synthesis of
aryl oximes, and we hoped that the high structural modularity
of the O-aryl hydroxylamines would allow us to identify
substrates that do not require the use of transition-metal-
based photocatalysts.[16] Herein, we describe the successful
implementation of this approach and the development of
novel, transition-metal-free, visible-light-mediated hydroimi-
nation and iminohydroxylation cyclization reactions (Sche-
me 1B).
À
N X bonds and require the use of toxic and hazardous
Scheme 1. Nitrogen-centered radicals and divergent functionalization
The guiding principle of our photoredox NCR synthesis
capitalized on the evidence that electron-poor aromatic
compounds have reduction potentials compatible with SET
reduction by visible-light-excited photocatalysts,[17] as shown
by MacMillan and co-workers.[18] Our envisaged photoredox
iminyl NCR generation was initiated by the visible-light-
promoted excitation of a photocatalyst (PC!*PC)[19] fol-
lowed by SET reduction of the aryl unit of oxime A to give
radical anion B (Scheme 2A). A fragmentation leading to
phenoxide C and the desired NCR D was anticipated to occur
processes developed in this work.
[*] J. Davies, S. G. Booth, Prof. R. A. W. Dryfe, Dr. D. Leonori
School of Chemistry, University of Manchester
Oxford Road, Manchester, M13 9PL (UK)
E-mail: daniele.leonori@manchester.ac.uk
Dr. S. Essafi
School of Chemistry, University of Bristol
Cantock’s Close, Bristol, BS8 1TS (UK)
À
next owing to the low bond dissociation energy of the N O
bond.[20] At this stage, we decided to test the viability of this
activation mode by combining it with an intramolecular
cyclization to synthesize valuable five-membered N-hetero-
cycles.[21] After 5-exo-trig cyclization, the C-centered radical E
was expected to abstract a H atom from 1,4-cyclohexadiene
(CHD)[20b] to give the desired product F and radical G, which
regenerates the photocatalyst by SET, closing the catalytic
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201507641.
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
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Table 1: Optimization of the hydroimination cyclization.
Entry
Substrate[a]
Photocatalyst
Solvent
Yield [%][b]
1
2
3
2a
2a
2a
[Ir(ppy)3]
eosin Y
eosin Y
DMF
DMF
acetone
81
68
93[c]
4
5
2b
2b
[Ir(ppy)3]
eosin Y
DMF
DMF
53
15
6
7
2c
2c
[Ir(ppy)3]
eosin Y
DMF
DMF
91
7
[a] 2a: Ar=2,4-(NO2)2C6H3; 2b: Ar=4-(NO2)C6H4; 2a: Ar=
1
4-(CN)C6H4. [b] Determined by H NMR analysis; see the Supporting
Information for control experiments. [c] 2,4-Dinitrophenol (8-H) was also
isolated; see the Supporting Information.
A broad range of oximes with diverse electronic and steric
properties participated efficiently in the visible-light-pro-
moted process (Scheme 3). Bicyclic heterocycles were also
obtained in good yields as well as products arising from the
cyclization onto di- and trisubstituted olefins.
Scheme 2. Proposed photoredox cycle and electrochemical studies.
EY=eosin Y, ppy=2-phenylpyridine.
cycle. As 5-exo-trig cyclizations occur rapidly (kc ꢀ 9 ” 103 sÀ1
at RT),[22] we expected that the reaction yields would
correlate with the efficiency of the photoredox system.
The SET between the visible-light-excited photocatalyst
and the aryl oxime became a focal point. As the reduction
potentials of many photocatalysts are known,[4a] we started
our investigations by evaluating the redox profiles of various
aryl oximes with the goal of identifying the most suitable/
active substrates. Analysis of oximes 1a–1g by cyclic voltam-
metry revealed irreversible reduction profiles that are in
accordance with the expected fragmentation process. Accord-
ing to our electrochemical scale (Scheme 2B), almost all of
the examined oximes are expected to undergo SET reduction
by *IrIII; whereas only the nitro-substituted substrates 1a–1d
red
have E1/2 potentials suitable for SET with the excited state
of the organic dye eosin Y.[23]
Based on these results, we selected oximes 2a–2c as
representative substrates for the evaluation of the proposed
radical cyclization reaction (Table 1). To our delight, visible-
light irradiation of 2a–2c in the presence of [Ir(ppy)3],
cyclohexadiene, and K2CO3 gave pyrroline 3a in good to
excellent yields (entries 2, 4, and 6). As predicted by the
electrochemical studies, 2a and 2b furnished 3a when eosin Y
was used as the photocatalyst (entries 3 and 5), thus setting
the stage for a fully organocatalytic photoredox hydroimina-
tion cyclization.
The substrate scope was evaluated with a focus on 2,4-
dinitro-substituted aryl oximes owing to four favorable
aspects: 1) The required hydroxylamine is commercially
available, 2) these oximes are typically purified by crystal-
lization, 3) their photoredox reactions do not require a tran-
sition-metal catalyst, and 4) the products can be purified by
a simple acid–base wash (no chromatographic purification
needed on the way from the ketone to the final product).
Scheme 3. Reaction scope. Yields of isolated products after acid–base
wash are given. Yields determined by NMR spectroscopy are given in
parentheses. [a] 1 mmol scale. [b] Reaction run in DMF. Boc=tert-
butyloxycarbonyl.
Intrigued by the low reduction potential and LUMO
energy[24] of the 2,4-dinitro-substituted aryl oxime 1a, and
inspired by the reports of Kochi,[25] Cossy,[26] and Mel-
chiorre[27] on SET, we wondered whether a complementary
activation mode could be exploited for the generation of
NCRs by visible-light irradiation. As illustrated in Sche-
me 4A, we speculated that a simple tertiary amine would be
able to reversibly interact with 2a to give an electron donor–
acceptor complex H.[28] Visible-light irradiation should then
initiate a SET process to give the radical ion pair J.[29]
Fragmentation to give D, 5-exo-trig cyclization, and H atom
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Scheme 4. A) Proposed visible-light-mediated electron transfer via an electron donor–acceptor complex for the hydroimination of O-aryl oximes.
B) UV/Vis studies. C) Initial findings. D) Proposed reaction mechanism for the iminohydroxylation cyclization.
abstraction would deliver pyrroline 3a. By using the Rehm–
Weller equation for electron transfer [DGET
rigorously moisture- and oxygen-free conditions.[24] In con-
=
trast to the hydroimination cyclization, 2,4-dinitrophenol
(8-H) was not formed, but we obtained 2-NO-4-NO2-
C6H3OH (10-H). This observation indicates a unique trifunc-
tional role of the aromatic unit of the O-aryl oximes, which
sequentially serves as a sensitizer, an electron acceptor, and
an oxidant. Initial rate kinetics revealed the reaction to be
first order in 2a and to display saturation behavior in Et3N
(1st order at 0 < [Et3N] < 1 equiv and zero order at [Et3N] >
1 equiv). Based on these findings, we propose the following
mechanism: Fast and reversible binding of Et3N and 2a gives
intermediate 5, which undergoes SET upon visible-light
excitation to give the dipolar species 6. Fragmentation and
5-exo-trig cyclization give the C-centered radical 7 and the
stable phenoxide 8 (pKa ꢀ 4). Subsequent oxidation by attack
of the radical onto the NO2 group[32] leads to 9, and successive
0.24F(E1/2 ÀE1/22a)ÀDEexcit+DEcoul],[30] the process was cal-
culated to be exergonic (DG ꢀ À30 kcalmolÀ1), which indi-
cates a very favorable SET. UV/Vis spectroscopy data further
corroborated this proposal. When a CH3CN[31] solution of 2a
was treated with Et3N, a bathochromic shift was observed,
which indicates the formation of a donor–acceptor complex
(Scheme 4B). The formation of such complexes has not been
studied extensively, prompting us to evaluate the strength of
this key interaction. By using Jobꢀs method, the 2a/Et3N
stoichiometry in the complex was confirmed to be 1:1, and
titration experiments gave an association constant of K
ꢀ 22mÀ1 (Scheme 4B). TD-DFT calculations [CAM-B3LYP/
6-311 ++ G(d,p) in CH3CN] confirmed that absorption at
approximately 440 nm is due to a transition from the nitrogen
lone pair to the p* orbital of the aromatic unit of the oxime.[24]
Exposure of 2b and 2c to Et3N (up to 10 equiv) did not lead to
significant bathochromic shifts, which suggests that there is
limited or no donor–acceptor complex formation.[24]
Et3N
À
N O bond homolysis furnishes 10 and the O-centered radical
11, which undergoes a fast hydrogen atom abstraction.[24]
With this very simple optimized procedure in hand, the
scope of the iminohydroxylation was evaluated with the aryl
oximes 2a and 2e–2y. All examined substrates reacted well
and provided the desired iminoalcohols 4a–4u in good to high
yields (Scheme 5). Bicyclic products could be obtained, and
substrates containing di- and trisubstituted olefins also
reacted well, giving access to products containing up to
three contiguous stereogenic centers.
Encouraged by the UV/Vis studies, we decided to
evaluate the ability of 2a to undergo the proposed visible-
light- and Et3N-mediated SET process. Irradiation of a solu-
tion of 2a, Et3N, and cyclohexadiene in CH3CN furnished the
desired product 3a (47%) together with iminoalcohol 4a
(41%; Scheme 4C). The unforeseen formation of 4a opened
the way to the development of the first visible-light-mediated
iminohydroxylation cyclization reaction. By simply excluding
cyclohexadiene from the reaction mixture, the yield of 4a was
increased to 85%. Other amines were evaluated, and they
also selectively provided 4a, albeit in lower yields. As
suggested by the UV/Vis studies, substrate 2b gave the
desired product in low yield whereas 2c did not react.[24]
The formation of 4a raised additional questions about the
underlying mechanism and the origin of the oxygen atom in
the final product (Scheme 4D). The involvement of adven-
titious O2 or H2O was excluded by running the reaction under
In conclusion, we have developed a divergent strategy for
the hydroimination and iminohydroxylation cyclization of
unactivated olefins. Electrochemical studies facilitated the
identification of a very reactive class of O-aryl oximes that
obviate the need for a transition-metal photocatalyst and
undergo organocatalytic hydroimination cyclizations. The
unprecedented ability of the aryl unit to sequentially act as
a sensitizer, electron acceptor, and oxidant enabled the
development of a unique Et3N- and visible-light-mediated
iminohydroxylation cyclization. Future studies will focus on
applying this method to other nitrogen-centered radicals and
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Acknowledgements
We thank Dr. H. Burton (nØe Scott), Dr. X. Couso Cambeiro,
Dr. M. De Poli, Dr. G. Gil-Ramirez, Dr. P. Harvey, Dr. T.
Poisson, Dr. A. Pulis, and Dr. I. Vilotijevic for helpful
discussions. D.L. thanks the European Union for a Marie
Curie Career Integration Grant (PCIG13-GA-2013-631556)
and the School of Chemistry at the University of Manchester
for generous support.
Keywords: electron transfer · hydroimination ·
iminohydroxylation · photoredox catalysis · visible light
How to cite: Angew. Chem. Int. Ed. 2015, 54, 14017–14021
Angew. Chem. 2015, 127, 14223–14227
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Received: August 15, 2015
Published online: September 28, 2015
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