Photocatalytic Dehydrogenative Cross-Coupling of Alkenes with Alcohols or Azoles without External Oxidant

Article abstract of DOI:10.1002/anie.201609274

Direct cross-coupling between alkenes/R-H or alkenes/RXH is a dream reaction, especially without external oxidants. Inputting energy by photocatalysis and employing a cobalt catalyst as a two-electron acceptor, a direct C?H/X?H cross-coupling with H₂evolution has been achieved for C?O and C?N bond formation. A new radical alkenylation using alkene as the redox compound is presented. A wide range of aliphatic alcohols—even long chain alcohols—are tolerated well in this system, providing a new route to multi-substituted enol ether derivatives using simple alkenes. Additionally, this protocol can also be used for N-vinylazole synthesis. Mechanistic insights reveal that the cobalt catalyst oxidizes the photocatalyst to revive the photocatalytic cycle.

Full text of DOI:10.1002/anie.201609274

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International Edition: DOI: 10.1002/anie.201609274
German Edition: DOI: 10.1002/ange.201609274
Cross-Coupling Reactions
Photocatalytic Dehydrogenative Cross-Coupling of Alkenes with
Alcohols or Azoles without External Oxidant
Hong Yi, Linbin Niu, Chunlan Song, Yiying Li, Bowen Dou, Atul K. Singh, and Aiwen Lei*
Abstract: Direct cross-coupling between alkenes/R-H or
alkenes/RXH is a dream reaction, especially without external
oxidants. Inputting energy by photocatalysis and employing
À
À
a cobalt catalyst as a two-electron acceptor, a direct C H/X H
À
cross-coupling with H2 evolution has been achieved for C O
À
and C N bond formation. A new radical alkenylation using
alkene as the redox compound is presented. A wide range of
aliphatic alcohols—even long chain alcohols—are tolerated
well in this system, providing a new route to multi-substituted
enol ether derivatives using simple alkenes. Additionally, this
protocol can also be used for N-vinylazole synthesis. Mecha-
nistic insights reveal that the cobalt catalyst oxidizes the
photocatalyst to revive the photocatalytic cycle.
D
irectly utilizing wide-ranging alkene feedstocks is a con-
tinuing challenge in chemical synthesis. A particularly active
field of catalysis uses alkene reactants to realize alkenylation,
thereby constructing substituted alkenes. A well-known
process for alkenylation is the transition-metal-catalyzed
Heck reaction, which has become a fundamental synthetic
transformation in recent years.[1] Key steps involved in the
Heck reaction are alkene insertion and b-hydride elimination
in the presence of a transition metal (Scheme 1A part (a)).
However, the Heck reaction mainly deals with the alkenyla-
tion of RX (aryl or vinyl electrophiles). The radical-mediated
approach can be a complementary route to the functionaliza-
tion of alkenes because radicals have high activity and react
rapidly with olefinic acceptors.[2] This strategy of alkenylation
via a radical pathway has been widely researched over the
years, providing a novel choice of alkenylation products.[3]
The addition of in situ generated radical to the alkenes is
regarded as the key step (Scheme 1A part (b)). By virtue of
Scheme 1. Alkenylation using simple alkenes. A) Two alkenylation path-
ways: a) the Heck reaction and b) radical alkenylation. B) New route
for radical alkenylation using an alkene cation as a key intermediate.
C) Oxidant-free radical alkenylation merging photoredox catalysis and
cobalt catalysis.
alkenylation by a radical pathway will have broad prospects in
the synthesis of substituted alkenes.
In traditional radical alkenylation, alkenes are always
used as radical acceptors and react with various radicals to
form addition products.[5] Alkenes can also serve as redox
compounds, and the resulting radical cations can be generated
by removal of a single electron from a neutral parent
compound.[6] Visible-light-mediated photoredox catalysis
has captured extensive attention recently[7] and offers an
intriguing opportunity to explore synthetic chemistry.
Alkenes can be transformed into their corresponding radical
cations using a recently developed photocatalysis strategy,
which can be used in cyclizations or alkene hydrofunctional-
ization reactions.[8] We hypothesize that an alkene radical
cation can be obtained by a single electron transfer (SET) and
that this radical cation can then be trapped by a nucleophile
because of the electrophilic reactivity of this type of species
(such as an alcohol or amine), giving a radical intermediate
(Scheme 1B). The formed radical intermediate may be
further oxidized to a cation intermediate, which then loses
one proton to afford the desired allkenylation product. This
new strategy employs alkene as the redox compound and the
À
this radical alkenylation strategy, several inert C H com-
pounds,[4] which are difficult to activate by the model Heck
reaction, have been successfully transformed into the corre-
sponding alkenes with high efficiency. Therefore, developing
[*] H. Yi, L. Niu, C. Song, Y. Li, B. Dou, A. K. Singh, Prof. A. Lei
College of Chemistry and Molecular Sciences
The Institute for Advanced Studies (IAS), Wuhan University
Wuhan 430072, Hubei (P.R. China)
E-mail: aiwenlei@whu.edu.cn
Homepage: http://aiwenlei.whu.edu.cn/Main Website/
Prof. A. Lei
State Key Laboratory and Institute of Elemento-Organic Chemistry
Nankai University
Tianjin 300071 (P.R. China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201609274.
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generated alkene radical cation is considered the key
intermediate.
Moreover, a screened hydrogen evolving catalyst, Co-
(dmgH)2Cl2, was less efficient than Co(dmgH)2PyCl. In the
control experiments, no desired product was observed with-
out photoredox catalyst, cobalt catalyst, or visible light,
indicating that the photoredox and cobalt catalysts are
essential in this transformation (Table 1, entries 5, 6, and 7).
With the optimal conditions established, we firstly exam-
ined the scope of different alkenes in this oxidant-free
alkenylation protocol (Table 2). The 1,1-disubstituted alkenes
with phenyl groups were well-tolerated under standard
conditions reacted with methanol (3a–3 f). Meanwhile, the
The above hypothesis in Scheme 1 involves loss of two
electrons and two protons; therefore, extra stoichiometric
oxidants and proton acceptors have to be applied as the
sacrificial reagents in cross-coupling reactions.[9] Using an
oxidant-free strategy to realize radical alkenylation is very
appealing. Inputting energy by photocatalysis and employing
a cobalt catalyst as an acceptor of two electrons,[10] we
attempted to apply the oxidant-free system to radical
alkenylation. Herein, we describe our efforts to develop
a new model of radical alkenylation under oxidant-free
conditions (Scheme 1C). Merging photocatalysis and cobalt
Table 2: Substrate scope for the oxidant-free coupling of alkenes 1 with
MeOH 2a.[a]
À
À
À
catalysis achieved direct C H/X H cross-coupling and C O
À
and C N bond formation with H2 evolution, affording enol
ether derivatives and N-vinylazoles.
Initially, 1,1-diphenylethylene (1a) and methanol (2a)
were chosen as the standard substrates for the desired
coupling reaction. The acridinium moiety has a strong
absorption band in the visible region (l = 430 nm) and high
excited state oxidizing power,[7a,11] which has already been
employed photocatalytically and has been successful in the
transformation of a diverse range of alkenes into cation
radicals. Thus, we consider Acr+-Mes ClO4 to be an ideal
À
photocatalyst for alkenylation. As shown in Table 1,
Acr+-Mes ClO4 (3 mol%) and Co(dmgH)2PyCl (3 mol%)
À
were added to a solution of 1a and 2a in an organic solvent
under an ambient nitrogen atmosphere. After 12 h of
irradiation with blue light-emitting diodes (LEDs), 79%
yield of the desired cross-coupling product 3a was obtained
(Table 1, entry 1). Other screening photocatalysts, such as
Eosin Y or Ru(bpy)3(PF6)2, did not promote this reaction and
did not afford the desired product (Table 1, entries 2 and 3).
[a] Conditions: 1 (0.2 mmol), Acr+-Mes ClO4À (3 mol%), Co-
(dmgH)2PyCl (3 mol%) in solvent (CH3CN/MeOH (2a)=1.5 mL/
0.5 mL) under a nitrogen atmosphere, irradiation using 3 W blue LEDs,
at room temperature for 12 h; yields of isolated products. The ratio of
isomers was determined by NMR spectroscopy. [b] 0.3 mmol scale.
[c] 0.5 mmol scale.
Table 1: Optimization of the reaction conditions.[a]
Entry
Photocatalyst
Co catalyst
Yield [%][b]
Acr+-Mes ClO4
Eosin Y
Co(dmgH)2PyCl
Co(dmgH)2PyCl
Co(dmgH)2PyCl
Co(dmgH)2Cl2
–
79
À
1
2
3
4
5
n.d.
n.d.
51
n.d.
n.d.
n.d.
alkene substituted with a Cl group was also smoothly
converted into ester product in high yield, which has the
potential for further transformation. Notably, cyclic alkenes
such as indene and 1,2-dihydronaphthalene derivatives could
also be employed to reach the target enol ether scaffolds
without any difficulty (3g–3i). a-methylstyrene was, likewise,
well-tolerated and afforded the product in 69% yield (3j).
Furthermore, internal olefin such as (E)-1-bromo-4-(prop-1-
en-1-yl)benzene furnished the enol ether product in moderate
yield as well (3k). This method provides an easy route to
multi-substituted enol ether derivatives, which are widely
used synthetic intermediates in organic synthesis.[12]
Ru(bpy)3(PF6)2
Acr+-Mes ClO4À
Acr+-Mes ClO4
–
À
6
Co(dmgH)2PyCl
Co(dmgH)2PyCl
7[c]
Acr+-Mes ClO4
À
Subsequently, we turned our attention to defining the
capacity of different alkyl alcohols using 1,1-diphenylethylene
(1a) as the model substrate under oxidant-free conditions.
Primary alcohols such as MeOH, EtOH, nPrOH, nBuOH,
and iBuOH (Table 3, entries 1–4 and 8) afforded the desired
[a] Conditions: 1a (0.2 mmol), photocatalyst (3 mol%), Co catalyst
(3 mol%) in solvent (CH3CN/MeOH=1.5 mL/0.5 mL) under a nitrogen
atmosphere, irradiation with 3 W blue LEDs, at room temperature for
12 h; [b] yields of isolated products; [c] without light.
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Table 3: Substrate scope for the oxidant-free coupling of 1,1-diphenyl-
ethylene 1a with alcohols 2.[a]
Table 4: Substrate scope for the oxidant-free coupling of alkenes 1 with
azoles 4.[a]
Entry
ROH
3
Yield [%][b]
1
2
3
MeOH
EtOH
3a
3l
79
80
89
3m
4
5
6
7
3n
3o
3p
3q
81
72
85
70
n-C6H13OH
n-C7H15OH
n-C11H23OH
8
9
3r
3s
3t
85
65
81
10
11
12
13
3u
3v
68
76
71
À
[a] Conditions: 1 (0.2 mmol), 4 (0.4 mmol), Acr+-Mes ClO4 (3 mol%),
Co(dmgH)2PyCl (3 mol%) in dichloroethane under an argon atmos-
phere, irradiation with 3 W blue LEDs, at room temperature for 24 h;
yields of isolated products. The ratio of isomers was determined by NMR
spectroscopy.
3w
[a] Conditions: 1a (0.2 mmol), Acr+-Mes ClO4À (3 mol%), Co-
(dmgH)2PyCl (3 mol%) in solvent (CH3CN/ROH (2)=1.5 mL/0.5 mL)
under a nitrogen atmosphere, irradiation using 3 W blue LEDs, at room
temperature for 12 h; [b] yields of isolated products.
styrene derivatives (Table 4; 5d–5k). Other N-heterocyclic
nucleophiles, such as pyrrole 5l, substituted pyrazoles 5m–5p,
benzotriazole 5q, and indazole 5r, also produced the corre-
sponding amination products in good yields under the same
conditions. However, some other amines, such as aniline,
diphenylamine, and dibenzylamine did not afford the desired
products under this catalytic system.
enol ether products in moderate to high yields. Long chain
primary alcohols such as n-C6H13OH, n-C7H15OH, and
n-C11H23OH were suitable for this reaction and yielded the
corresponding products with good efficiency (Table 3, entries
5–7; 3o–3q). Phenylethyl alcohol could be transformed into
the desired product with up to 65% yield (Table 3, entry 9;
3s). Additionally, secondary alcohols were also tested for this
procedure under standard conditions. The alcohols iPrOH
and heptan-2-ol were appropriate substrates (Table 3, entries
10 and 11). To our surprise, sterically hindered tertiary
alcohols were smoothly and effectively accommodated into
this process (Table 3, entries 12 and 13). In general, primary,
secondary, and tertiary alcohols are all efficient coupling
partners in this oxidant-free alkenylation method.
With this dual-catalytic system, we further explored
another application; namely, the synthesis of N-vinylazoles,
which are an important class of building blocks in organic
synthesis and are also key structural motifs in many medica-
ments. It has been shown in the literature that palladium,
mercury, or copper complexes are often used to catalyze
vinylation of azoles with vinyl bromides.[13] To our delight, this
dual-catalytic system could also achieve the oxidant-free
Mechanistic studies were conducted to gain insights into
the aforementioned transformation. We initially detected the
H2 in the reaction system by GC-TCD. Under standard
conditions, 79% and 60% yield of the desired cross-coupling
product 3a and H2 were obtained, respectively (Scheme 2A).
Hydrogenation of 1,1-diphenylene was the main side-reaction
detected by GC-MS, which explains why the yield of H2 is
lower than our enol ether product. When using (2-methoxy-
ethane-1,1-diyl)dibenzene 6a as the starting material under
oxidant-free conditions, no desired product or H2 was
detected (Scheme 2B). It is implied that (2-methoxyethane-
1,1-diyl)dibenzene 6a is not the intermediate in the reaction.
Moreover, the time profile of the photocatalytic reaction
revealed that the reaction was totally inhibited in the absence
of light (Supporting Information, Figure S1), which suggests
that continuous visible-light irradiation is essential to this
photocatalytic transformation. As shown in Figure S2 (Sup-
porting Information), 1,1-diphenylethylene displayed lumi-
nescence quenching of activated acridinium salt. Further-
more, we found that Co(dmgH)2PyCl could quench the
photocatalyst (Supporting Information, Figure S3). That is,
À
alkene C H amination process when pyrazole was selected as
À
an N H nucleophile to couple with indene derivatives in
dichloroethane (Table 4; 5a–5c). The conditions were
successfully extended to the coupling of pyrazole with
a variety of substituted alkenes described above, including
1,2-dihydronaphthalene, 1,1-diphenylethene, and b-methyl-
À
the electron transfer of [Acr+-Mes ClO4 ]* proceeds with
1,1-diphenylethylene as well as Co(dmgH)2PyCl. We pre-
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À
À
Scheme 3. Proposed mechanism for oxidant-free C H/ O H cross-
coupling.
Overall, a conceptually new and synthetically valuable
À
À
À
À
oxidant-free C H/O H and C H/N H cross-coupling reac-
À
À
tion achieved C O or C N bond formation with H2 evolution,
using a photoredox catalytic system promoted by visible light.
A new radical alkenylation pathway using alkene as the redox
compound was presented and the alkene radical cation was
the key intermediate in this oxidant-free system. This method
provides a mild and robust synthesis of multisubstituted enol
ethers and N-vinylazoles from simple and easily available
alkenes, alcohols, and azoles. This oxidant-free system, using
a photocatalyst and cobalt catalyst, offers a new strategy
beyond traditional photoredox catalysis using a stoichiometric
oxidant. A detailed mechanistic investigation of this reaction
is under way in our laboratory.
Scheme 2. Mechanistic insights. [a] Conditions: 1a (0.2 mmol), Acr+-
Mes ClO4À (3 mol%), TEMPO (0.4 mmol) in solvent (CH3CN/
ROH=1.5 mL/0.5 mL) under a nitrogen atmosphere, irradiation with
3 W blue LEDs, at room temperature for 12 h. Yields of isolated
products.
ferred a single-electron transfer between [Acr+-Mes ClO4 ]*
À
and alkene as the key initial step.[14]
In this oxidant-free system, a cobalt catalyst serves solely
as an oxidant of the photocatalyst, thereby reviving the
catalytic cycle. A variety of oxidants were examined to probe
the role of the cobalt catalyst. We were pleased to find that
radical alkenylation could also be achieved utilizing Acr+-
Acknowledgements
This work was supported by the 973 Program (2011CB808600,
2012CB725302, 2013CB834804), the National Natural Sci-
ence Foundation of China (21390400, 21272180, 21302148,
2109343, and 21402217), the Research Fund for the Doctoral
Program of Higher Education of China (20120141130002),
the Ministry of Science and Technology of China
(2012YQ120060), and the Program for Changjiang Scholars
and Innovative Research Team in University (IRT1030). The
Program of Introducing Talents of Discipline to Universities
of China (111 Program) is also appreciated.
À
Mes ClO4 as the catalyst and 2,2,6,6-tetramethyl-1-piperidi-
nyloxy (TEMPO) as the terminal oxidant, which is also used
as an oxidant in several photocatalytic reactions.[15] As shown
in Scheme 2C, primary alcohols (especially long chain
aliphatic alcohols), secondary alcohols, or tertiary alcohols
could all be transformed into the desired products in high
yields using a photocatalyst/TEMPO system.
A detailed description of our proposed mechanistic cycle
for the photoredox/cobalt-catalyzed radical alkenylation is
outlined in Scheme 3. After irradiating with low-energy
visible light (3 W blue LEDs), the photocatalyst Acr+-Mes
À
À
Keywords: C N bond formation · C O bond formation ·
ClO4À 7 was converted into the excited species [Acr+-Mes
oxidant-free · photocatalysis · radical alkenylation
À
+
ClO4 ]*
8
(E1/2red[AcrC-MesC /AcrC-Mes] =+ 2.06 V vs.
SCE),[16] which underwent a SET process with an alkene to
generate alkene radical cation 10 and Acr+-Mes ClO4À radical
anion 9. Subsequently, the alkene radical cation 10 reacted
with alcohol nucleophile to generate the radical intermediate
11. The carbon-centered radical was further oxidized by CoIII
(E1/2red[CoIII/CoII] = À0.67 V vs. SCE)[17] to generate CoII
species and a cation intermediate 12, which loses a proton
to produce ether enol product 3. This Acr+-Mes ClO4À radical
anion 9 may be oxidized by CoII to reinitiate the photo-
catalytic reaction and generate a CoI species. Protonation of
the formed CoI species provides a CoIII-H hydride,[18] which
can react with a proton to release H2 eventually.
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Manuscript received: September 21, 2016
Revised: November 6, 2016
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Cross-Coupling Reactions
H. Yi, L. Niu, C. Song, Y. Li, B. Dou,
A. K. Singh, A. Lei*
&&&& — &&&&
Photocatalytic Dehydrogenative Cross-
Coupling of Alkenes with Alcohols or
Azoles without External Oxidant
À
À
Footloose and oxidant-free: A photo-
catalytic process that employs a cobalt
catalyst as an electron acceptor mediates
direct C H/X H cross-coupling and H2
À
À
evolution, achieving C O and C N bond
formation.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!