Visible-Light-Accelerated Copper(II)-Catalyzed Regio- and Chemoselective Oxo-Azidation of Vinyl Arenes

Article abstract of DOI:10.1002/anie.201801678

The visible-light-accelerated oxo-azidation of vinyl arenes with trimethylsilylazide and molecular oxygen as stoichiometric oxidant was achieved. In contrast to photocatalysts based on iridium, ruthenium, or organic dyes, [Cu(dap)₂]Cl or [Cu(dap)Cl₂] were found to be unique for this transformation, which is attributed to their ability to interact with the substrates through ligand exchange and rebound mechanisms. Cu^II is proposed as the catalytically active species, which upon coordinating azide will undergo light-accelerated homolysis to form Cu^I and azide radicals. This activation principle (Cu^II-X→Cu^I+X^.) opens up new avenues for copper-based photocatalysis.

Full text of DOI:10.1002/anie.201801678

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Visible-Light-Accelerated Copper(II) Catalyzed Regio- and
Chemo-selec¬tive Oxo-Azidation of Vinyl Arenes
Authors: Asik Hossain, Adiyala Vidyasagar, Christian Eichinger,
Christian Lankes, Jenny Phan, Julia Rehbein, and Oliver
Reiser
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201801678
Angew. Chem. 10.1002/ange.201801678
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10.1002/anie.201801678
Angewandte Chemie International Edition
COMMUNICATION
Visible-Light-Accelerated Copper(II) Catalyzed Regio- and Chemo-
selective Oxo-Azidation of Vinyl Arenes
Asik Hossain, Adiyala Vidyasagar, Christian Eichinger, Christian Lankes, Jenny Phan, Julia Rehbein*
and Oliver Reiser*[a]
General processes for the synthesis of alpha azido ketones:
Abstract: The visible-light-accelerated oxo-azidation of vinyl arenes
Previous works
is developed, operating at room temperature and utilizing molecular
O
oxygen as the stoichiometric oxidant. Contrasting iridium, ruthenium
or organic dye based photocatalysts, [Cu(dap)2]Cl or [Cu(dap)Cl2]
were found to be unique for this transformation, which is attributed to
their ability to interact with the substrates via ligand exchange and
rebound mechanisms. Cu(II) is proposed as the catalytically active
O
–
N3–, Oxidant
O2
N3
LG
N3
R
R
Nucleophilic
substitution
ref. 10
R
LG = Leaving
Group
R = Aryl
ref. 12
R = Aryl/Alkyl
PC
C
Cu(I/II)
hv
(2
species, which upon coordinating azide will undergo
a light
e
q
u
i
v)
accelerated homolysis, forming Cu(I) and azide radicals. To the best
of our knowledge, this represents the first visible light photocatalyzed
process triggered by copper in the oxidation state +2.
(This work)
ref. 16
ref. 17b
OH
OH
R
+
TMSN3, air
PC
hv
Mn(II)
N3
N3
R
R
PPh3
(1 equiv)
R = Aryl
PPh3
(1.2 equiv)
R = Aryl/Alkyl
The use of visible-light photoredox catalysis has regained
substantial attention during the last decade because of its
operational simplicity to carry out synthetic transformations in an
environmentally benign and economic way.[1,2] Photocatalysis, in
particular in combination with Cu(I)-based photoredox active
complexes,[3] has become a powerful tool for the difunctionaliza-
tion[4] of carbon-carbon multiple bonds, providing an attractive
alternative to their thermal activation by transition metal[5] or
organic oxidants.[6]
Organic azides[7] and especially azido ketones[8,9] are valuable
intermediates in organic synthesis, and consequently, various
methods are known for their synthesis. The most commonly used
method is via nucleophilic substitution[10] (Scheme 1), however,
requiring pre-functionalized starting materials.[11] Nair and co-
workers[12] reported a very efficient method for the azido-
ketonization of styrenes with sodium azide and molecular oxygen
in the presence of stoichiometric amounts of ceric ammonium
nitrate serving as a one electron oxidant. However, no catalytic
method for the oxo-azidation of vinyl arenes that does not require
superstoichiometric amount of strong oxidants is available.[12,13]
Recently, Greaney and co-workers[14] elegantly reported the
utilization of azido radicals generated from an azidoiodinane
(Zhdankin reagent) in the Cu(I)-catalyzed alkoxy- and di-azidation
of styrenes under photochemical and non-photochemical con-
ditions. In turn, we have investigated oxygen mediated photo-
catalytic transformations of carbon-carbon multiple bond
systems.[15] Based on these precedents, we envisioned that by a
three-component reaction between an azido radical source, an
olefin, and molecular oxygen the formation of a-azido ketones
should become possible.
Scheme 1. Alpha azido ketone synthesis.
Our approach complements the hydroxyazidation of alkenes
being possible with 9-mesityl-10-methylacridiniumperchlorate
(Fukuzumi’s catalyst)[16] under photochemical conditions or under
thermal conditions by catalysis with manganese(II),[17b] followed
by reduction of the stoichiometrically generated hydroperoxides
with equimolar amounts of triphenylphosphine. Notably, in the
latter approach, copper-based catalysts proved to be
unsuccessful. Alternatively, iodine-[17c] or Sn(IV)-catalysis[17a]
using organic peroxides have been employed towards azido
alcohols, which, in turn, can then be oxidized to azidoketones with
pyridiniumchlorochromate (PCC).[16]
We started our investigation by screening various photo-
catalysts[1] (visible-light irradiation, λmax = 455 nm) using TMSN3
as an inexpensive azide source and oxygen (reaction was run
open to air) to achieve a coupling with styrene 1a. Assuming that
oxidation of the azide anion to its radical is required (E = +1.32 V
vs. SCE), it was not unexpected that neither iridium (EIr(III)*/Ir(II)
=
+0.31 V vs. SCE for [Ir(ppy)3]; ppy = 2-phenylpyridine) nor
ruthenium (ERu(II)*/Ru(I) = +0.77 V vs. SCE for [Ru(bpy)3]Cl2; bpy =
2,2’-bipyridine), nor eosin Y[18] (EEY*/EY.- = +0.83 V vs. SCE) would
promote a reaction, given their low oxidation potential (Table 1,
entries1-3). While azide radicals can be generated from TMSN3
(EN3•/EN3– = +1.32 V) with Fukuzumi’s catalyst (Edye*/dye.- = +2.06 V
vs. SCE; dye = 9-mesityl-10-methylacridiniumperchlorate) in the
presence of styrenes and air, azidoperoxides are obtained as
products.[16] To our delight, when the reaction was performed
instead with 1 mol% [Cu(dap)2]Cl[19] (ECu(II)/Cu(I)*= –1.43 V vs. SCE;
dap = 2,9-bis(p-anisyl)-1,10-phenanthroline), we were pleased to
observe full conversion of 1a after 12 h visible-light-irradiation
(λmax = 530 nm) and 2a was isolated in 76% yield along with 18%
(1H-NMR yield) of benzaldehyde 3a (Table 1, entry 4). Likewise,
the reaction proceeded well upon irradiation employing
[Cu(dap)Cl2] or CuCl2/dap as catalyst (Entry 6, 7), corroborating
the hypothesis that an oxidation of azide takes place, which is
triggered by Cu(II) (vide infra).
[a]
M.Sc. A. Hossain, Dr. A. Vidyasagar, M. Sc. C. Eichinger, Dr. C.
Lankes, M.Sc. J. Phan, Prof. Dr. J. Rehbein, Prof. Dr. O. Reiser
Institut für OrganischeChemie, Universität Regensburg
Universitätsstr. 31, 93053 Regensburg, Germany
E-mail: oliver.reiser@chemie.uni-regensburg.de
http://www-oc.chemie.uni-regensburg.de/reiser/index_e.html
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201801678
Angewandte Chemie International Edition
COMMUNICATION
observed, which allows in turn the chemoselective oxo-azidation
Table 1. Reaction Optimization[a]
of 1ae (Scheme 2b).
O
N3
Catalyst (mol%)
O
TMSN3
+
Table 2. Substrate Scope of the oxo-azidation reaction[a]
+
CH3CN, Air, rt
visible light
2a
3a
1a
entry
catalyst (mol%)
[Ir(ppy)3] (1)
visible light
455 nm
455 nm
530 nm
530 nm
No
yield[c] 2a
yield[c] 3a
1[b]
2[b]
3[b]
4[d]
5
2
-
[Ru(bpy)3]Cl2 (1)
Na2-Eosin Y (1)
[Cu(dap)2]Cl (1)
[Cu(dap)2]Cl (1)
[Cu(dap)Cl2] (1)
CuCl2 (1) + dap (2)
CuCl2 (1) + dap (2)
No
3
-
nr
-
79 (76)
11
72
62
12
nr
18
3
21
22
4
-
6
530 nm
530 nm
No
7
8
9
530 nm
530 nm
530 nm
10
11
CuCl (5)
8
-
dap(5)
3
-
Reaction conditions: [a] Styrene 1a (0.50 mmol, 1 equiv), TMSN3 (1.00 mmol, 2
equiv), catalyst (1 – 5 mol%) in CH3CN (0.25 M) under air at 25 ºC (maintained
by a temperature-controlled water bath) for 12 h unless otherwise noted. [b]
Reaction time was 20 h. [c] 1H-NMR yield, diphenylmethane as internal
standard. [d] Isolated yield in parenthesis. nr = no reaction.
Light proved to be beneficial for the transformation: while starting
either from Cu(I) or Cu(II) and running the reaction in the dark
resulted in significantly lower yields of 2a (entries 5 and 8
respectively).
Aiming to reduce the byproduct 3a, several more parameters
were screened (see SI for details), however, the conditions
established in Table 1, entry 4, were found to be best and were
subsequently applied to explore the scope of the reaction. In the
following, the bench stable copper(I) complex [Cu(dap)2]Cl was
used as the precatalyst since it can be prepared on gram scale
and conveniently handled to set up reactions.
Table 2: Reaction Conditions: [a] Olefin (0.50 mmol), TMSN3 (1.00 mmol),
[Cu(dap)2]Cl (1 mol%) in CH3CN (2 mL) under air at 25 ºC with 530 nm visible-
light irradiation for the indicated time. [b] 1H-NMR yield. Bn = Benzyl, Ac = Acetyl
and tBu = tert-butyl.
A broad variety of vinyl arenes could be converted to the
ketoazides (Table 2). Alkyl-, alkoxy- and acetoxy, halo-, amino-,
nitro- and cyano- substituents in meta or para position of the
arene ring were well tolerated. A decline in yield was observed for
mono-ortho-substituted styrenes, which is attributed to the dis-
ordered conjugation between the arene and the alkene moiety,
being further accentuated in case of the di-ortho-substituted
styrene 1s, which gave poor conversion even after 72 h of irradia-
tion. Thiophene and benzofuran substitution was also possible
(2y, 2z, 2ab). Internal aryl substituted alkenes can be also
converted to the corresponding ketoazides, as demonstrated with
2aa and 2ab.
a)
O
N3
[Cu(dap)2]Cl (1 mol%)
+ TMSN3
+ TMSN3
Cl
CH3CN (0.25 M), 24 h
25 ºC, air, Green LED
Cl
2ad (62%)
1ad
1ae
O
b)
N3
[Cu(dap)2]Cl (1 mol%)
CH3CN (0.25 M), 18 h
25 ºC, air, Green LED
(71%)
2ae
Scheme 2. Chemoselective oxo-azidation.
Trimethylsilylazide is known to readily react with organic halides
such as benzyl chloride to the corresponding azides.[20]
Nevertheless, under the photochemical protocol developed here,
1-(chloromethyl)-4-vinylbenzene (1ad) cleanly underwent oxo-
azidation to give rise to 2ad in 62% yield (Scheme 2a). In the case
of the unactivated olefin 1ac (Table 1), no conversion was
The great synthetic utility of a-azido ketones is amply
documented in the literature. Representative transformations of
a-azido ketones 2a[21], 2g, 2w and 2aa synthesized in this study
are shown in Scheme 3.
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10.1002/anie.201801678
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Scheme 3. Transformations of azidoketones 2
Especially, 2g serves as a key intermediate in the synthesis of the
antihyperlipidemic and antihyperglycemic Aegiline[22] and the
antiviral natural product (+) Tembamide.[23] 2w is an intermediate
for the synthesis of BMS-337197, being a potent uncompetitive
inhibitor of IMPDH II enzyme,[9] while 2aa has been used for the
synthesis of Cathinone,[24] which has similar biological activity like
amphetamines, especially on the cardiovascular system[25] and
Scheme 4. Proposed reaction mechanism
Elemental steps from III onwards are proposed to consist of a
homolytic dissociation of III to a Cu(I) species IV and azide radical.
The change of the Cu(II) concentration during the reaction
sequence was followed by quantitative EPR experiments and are
in line with this mechanistic postulate (see SI). Consecutively, the
azido radical can add to the substrate 1 forming a stabilized
radical V, confirmed by TEMPO trapping (Scheme 5a), which
further reacts with oxygen[16] to form VI. The now O-centered
radical can now bind to IV forming a Cu(II) species VII. Formation
of product 2 with concurrent elimination of II from VII closes the
catalytic cycle. In agreement with this proposal, no conversion is
observed if the reaction is carried out under a nitrogen
atmosphere, neither starting with Cu(I) nor with Cu(II) (Scheme
5b). Moreover, no azidoperoxides, being the products in related
processes[16b,17b] (cf. Scheme 1) were detected. The overall
mechanistic proposal is in line with recent reports[31] where Ni(II)
intermediates were excited by using an external Ir-based photo-
catalyst to facilitate a reductive elimination.
metabolism of dopamine[26]
.
A plausible mechanism (Scheme 4) for the azidoketonization
features a Cu(II)-complex like II as the catalytically active species.
If [Cu(dap)2]Cl is used as precatalyst, the corresponding Cu(II)
species should be easily obtainable by oxidation with dioxygen
(ECu(II)/Cu(I)*= –1.43 V vs. SCE; Eox = +0.33 V for molecular oxygen)
and indeed, the formation of an EPR-active copper species was
observed during the reaction sequence starting with Cu(I) (see SI
for details). In addition, formation of a Cu(II) species from
[Cu(dap)2]Cl should go hand in hand with the release of one
equivalent of dap ligand. This dissociation was confirmed by NMR
spectroscopy and identification of unbound dap ligand.[27],[28] The
stoichiometry of the Cu(II) dap complex was also confirmed by an
independent synthesis of the [Cu(dap)Cl2] and X-ray analysis (see
SI for details). Interestingly, even with 2 equivalents of dap ligand
[Cu(dap)Cl2] is formed selectively (5, Scheme 5c).
The formation of an azide bridged dimer[30] III was confirmed by
ATR-IR spectroscopy and rapidly occurs upon mixing of
Cu(dap)Cl2 already without irradiation.[30b] The formation and
photophysical properties of III also explain the wavelength
dependence of the overall reaction. III has a λmax of 525 nm (see
SI) contrary to the [Cu(dap2)]Cl or [Cu(dap)Cl2] complexes, and
consequently, irradiation of III with green LEDs (530 nm) leads to
a faster onward reaction then with blue LEDs (455 nm).
In conclusion we have disclosed a highly versatile visible-light
photocatalytic strategy for the step-economical synthesis of alpha
azido ketones from vinyl arenes and readily available TMSN3
under aerobic condition without the need for employing additional
stoichiometric oxidants. Moreover, to the best of our knowledge
this is the first example for a visible light accelerated Cu(II)-
catalyzed process, which might open up new opportunities to
develop radical based redox transformations based on a light
induced homolysis of Cu(II)-X species to give rise to Cu(I) and X•.
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10.1002/anie.201801678
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TMSN3 (2 equiv)
TEMPO (1.2 equiv)
N O
N3
[Cu(dap)2]Cl (1 mol%)
Ph
Ph
2a'
only product
in HRMS
CH CN (0.25 M),
Green LED
3
1a
12 h, 25 ºC, air
b) Reaction under nitrogen atmosphere:
CuCl2:dap (1:2) (1 mol%)
or
[Cu(dap)2]Cl (1 mol%)
No reaction
[6]
[7]
+ TMSN3 (2 equiv)
Ph
1a
CH3CN (0.25 M),Green LED
12 h, 25 ºC, N2
c) Synthesis of [Cu(dap)Cl2]:
dap (2 equiv)
CuCl2
CHCl3, 55 ºC, 1 h
[8]
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This work was supported by the German Science Foundation
(DFG)(GRK 1626, Chemical Photocatalysis) and Alexander von
Humboldt Foundation (fellowship to A.V.). The authors thank Mrs.
Sabine Stempfhuber and Dr. Uttam Chakraborty (UR) for X-Ray
crystal measurements. We also thank Prof. Samir Z. Zard (Ecole
Polytechnique, France) for helpful discussion on the reaction
mechanism. Ms. Julia Zach (UR) is further acknowledged for
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Keywords: Homogeneous Catalysis•Photocatalysis•Oxo-
azidation•Oxygen•Copper
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10.1002/anie.201801678
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Asik Hossain, AdiyalaVidyasagar,
Christian Eichinger, Christian Lankes,
Jenny Phan, Julia Rehbein,* Oliver
Reiser*
Page No. – Page No.
Visible-Light-Accelerated Copper(II)
Catalyzed Regio- and Chemoselective
Oxo-Azidation of Vinyl Arenes
Replacing traditional methods with simple and mild catalytic method.
This article is protected by copyright. All rights reserved.