Visible-light-activated copper(i) catalyzed oxidative Csp-Csp cross-coupling reaction: Efficient synthesis of unsymmetrical conjugated diynes without ligands and base

Article abstract of DOI:10.1039/c6gc01463a

A novel visible-light-promoted copper-catalysed process for the C_sp-C_sp cross-coupling reaction of terminal alkynes at room temperature is described. The current photochemical method is simple, highly functional group compatible, and more viable towards the construction of bio-active 1,3-unsymmetrical conjugated diynes without the need of bases/ligands, additives and expensive palladium/gold catalysts.

Full text of DOI:10.1039/c6gc01463a

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Visible-light-activated Copper(I) catalyzed Oxidative Csp-Csp Cross-
Coupling Reaction: Efficient Synthesis of Unsymmetrical
Conjugated Diynes without Ligands and Base
Received 00th January 20xx,
Accepted 00th January 20xx
Arunachalam Sagadevan,1 Ping-Chiang Lyu,2 and Kuo Chu Hwang1*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel visible-light-promoted copper-catalysed process for Csp-Csp Recently, it was reported that Csp-Csp cross-coupling reaction
cross-coupling reaction of terminal alkynes at room temperature can be achieved with 1-bromoalkyne using an expensive Cu/Pd
is described. The current photochemical method is simple, high bimetallic catalyst with or without air sensitive phosphorous
functional group compatible, and more viable towards the ligands (Scheme 2c).5 From an environmental and economic
construction of bio-active 1,3-unsymmetrical conjugated diynes viewpoint, direct coupling of two terminal alkynes is greener
without the need of bases/ligands, additives and expensive and more cost-effective.6 Cu/Ni bimetallic catalytic system
palladium/gold catalysts.
have shown to facilitate oxidative coupling of two terminal
alkynes in the presence of organic base and ligands with usage
of large excess of one alkynes (5.0 eqv)(see, Scheme 2a).6a
Very recently, gold catalysed selective heterocoupling of
terminal alkynes for the synthesis of 1,3-conjugated diynes in
presence of ligands and oxidants (see, Scheme 2d).6b Despite
their increasing attention, the method is still limited to cross-
coupling reaction of alkyl-aryl alkynes, moreover, this process
requires the use of expensive gold-catalyst along with excess
oxidants (PhI(OAc)2) and ligand (phenanthroline), thus make
the overall cost and is accompanied with the formation of
undesired side products/ wastes.
The development of new and mild synthetic approaches for
the construction of unsymmetrical 1,3-conjugated diynes and
polyynes is of great importance in synthetic chemistry. The
scaffold of 1,3-conjugated diynes is a valuable building block in
various natural products, pharmaceuticals (see, Scheme
1),
organic materials and supramolecular materials.1 Glaser-Hay
coupling2 and Cadiot-Chodkiewicz coupling3 are two of the
earliest and the most remarkable methods for preparation of
conjugated diynes. However, these processes require toxic
organic amine bases and excess of alkynes. In addition, Cadiot-
Chodkiewicz coupling (see, Scheme 2b
) involving pre-
Previous work
(thermal processes, do not work for e-deficient alkynes)
-one alkynes large excess
functionalized alkyne substrates (e.g., unstable 1-haloalkyne),
which is the leads to increase the overall cost for the synthesis
of unsymmetrical diynes.4
a)
R1
- bimetallic catalysts
- ligands and amine base
R1
b)
+
[
N
R2
H
]
,
[
C
+
i
l
i
g
e
s
)
d
n
a
g
Br
R2
u
s
a
n
d
,
b
]
a
b
,
O
C6H13
l
C
-usage of unstable
1-bromoalkyne
- ligands and amine
base
a
-
u
e
C
b
HO
i
l
CH3
(
-expensive Au-catalyst
-ligands is needed
R1
R2
OH
Oploxyne-A
capillene
[
P
d
]
,
,
)
r
2
B
-excess (PhI(OAc)2
s
t
a
[
C
u
s
n
A
u
]
e
OH
(
a
d
i
m
a
l
i
g
a
p
x
MeOOC
d)
p
o
n
d
,
R1
d
d
n
c)
R1
O
-expensive Pd-catalyst
-bimetallic catalyts
-usage of unstable
1-bromoalkyne
g
i
l
+
+
R2
H
R2
O
Br
Glaser-Hay coupling
Cadiot-Chodkiewicz coupling
matricaria-type ester
Falcarinol
e)
Present work
(photochemical process)
Scheme 1 Unsymmetrical 1,3-diynes in natural products.
5 mol% CuCl
R1
+
H
R2
R1
R2
Visible light
O2, RT
.
.
base and ligands not needed high selectivity towards e-deficient substrates
.
.
simple CuCl-catalyst under blue-LEDs irradiation O2 as a oxidant
Scheme 2 Transition metal-catalyzed oxidative cross-coupling
of alkynes.
Despite their own uniqueness and advantages, all reported
processes6 share common limitations, such as: (a) all processes
work well only for e-rich/neutral aryl/aliphatic alkynes; (b)
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require toxic amine base and ligands along with bimetallic coupling product 1a2y to 85%, and suppress the homo-
catalyst, which leads to environmental issues on the synthesis coupling product 2y2y to 11% (see entry 1, Table 1). However,
of 1,3-conjugated diynes (large scale). Therefore, it is highly cross-coupling reaction of 1:1.2 ratio of two different alkynes
desirable to develop simple, inexpensive catalytic system (e.g., provides mixture of hetero and homocoupling products (entry
Cu) for an effective Csp-Csp cross-coupling reaction of two
different terminal alkynes under low energy visible light eqv.) is necessary to achieve good selectivity (1a2y) (entry
irradiation with absence of expensive catalyst (Pd or Au), Table ) for the heterocoupling product. In the screening of
4, Table 1). Upon optimization, an excess one alkyne (up to 1.8
1,
1
ligands and base is great importance in organic synthesis.
Visible light-activated photoredox catalysis (Ru- or Ir-complexes)
catalysts, CuX (X: Cl, Br) affords the highest yield of the hetero
coupling product. However, CuCl2 fails to catalyze the
formation of the desired cross-coupling product (entries 1-3,
Table 1). It is worthy to note that the formation of the
homocoupling product (entry 4, Table 1) increases at higher
has emerged as a new reaction modality for organic reactions.7
Photoredox copper catalysis has been demonstrated to be
extremely successful in various coupling reactions, such as, alkyne-
azide cycloaddition (CuAAC) reactions,8 C-C cross-coupling,9 and
various C-N, C-S and C-O cross-coupling reactions.10 We have
previously developed a visible light-activated Cu-catalyzed process
for efficient C-C, C-N cross-coupling, and C-H annulation reactions.11
Herein, we report simple Cu-catalyzed direct Csp-Csp cross-coupling
reaction of terminal alkynes between e-rich and e-poor aryl alkynes
under blue-LEDs light irradiation at room temperature. The
reactions proceeded with high yields (64-95%) and good selectivity
without the needs of expensive Pd, ligands, and bases. The
significance of the present chemistry includes: 1) an unprecedented
visible-light-induced copper-catalysed Csp-Csp cross coupling
reaction of terminal alkynes for rapidly accessing a variety of
unsymmetrical 1,3-conjugated diynes at room temperature; 2) the
current photochemical method does not require the use of precious
expensive metal catalyst (e.g., Au-, Pd-catalyst), external
oxidants/additives, ligands and base (see, Scheme 2); 3) This is the
first literature example that direct aerobic oxidative coupling of two
different terminal alkynes for construction of unsymmetrical 1,3-
conjugated diynes under visible light induced copper-catalysed
process.
substrate (2y) concentrations.
Meanwhile, control
experiments also reveal that exclusion of either light, CuCl or
O2 results in no product formation (entries 10-12, Table 1).
Under the optimized condition (entry 1, Table 1),
phenylacetylene (1a) was used as one of the partner for
coupling with all substrates (see results in Table 2). In the
optimized condition, the reaction does not work as efficiently
for electron rich aromatic acetylenes (2a – 2e) (as compared to
electron deficient aryl alkynes) with lower yields of the desired
cross-coupled products (1a2a-1a2e). In the case of halo-
substituted aryl acetylenes, F, Cl, and I react smoothly to
generate the corresponding cross-coupling products in good to
excellent yield 73 to 88% (1a2f-1a2h). The presence of these
halogen functionalities is useful for further synthetic
modifications.12 Other aromatic alkynes (2i-2k), including
electron-donating group at the meta position, can undergo
cross-coupling reaction to generate the desired products in
good yields (1a2i-1a2k).
Table 1 Optimization of reaction condition.a
Our initial study was focused on coupling of phenylacetylene (1a)
with 4-OMe-phenylacetylene (2b) in acetonitrile in the presence of
O2 and the absence of any base under blue-LEDs irradiation. The
reaction yields 54% of the desired heterocoupling product along
with a mixture of homocoupling products [see eqn. (1)]. To our
delight, the Csp-Csp hetero-coupling products, 1a2m and 1a2y, were
obtained in good yields (76 and 70%, respectively) when electron-
deficient aliphatic alkynes (2m and 2y) were used as substrates [see
eqn.(1)].
5 mol% CuCl
Ph
R2
+
Ph
1a
1.8 eqv
H +
R2
homo-pdts (1)
O2
CH3CN, 8 h,
2
1.0 eqv
blue-LEDs, RT
1a2b (54%)a, 2b2b (44%)a, 1a1a ( 47%)b
2b
R2=4-OMeC6H4
R2=4-AcC6H4 2m
2y
a
a
b
1a2m
2m2m
1a1a
( 34%)
(76%) ,
(7%) ,
1a2y (70%)a, 2y2y (12%)a, 1a1a ( 37%)b
a
b
OAc
R2=
2 1
, yield based on
yield based on
aUnless otherwise mentioned, reaction conditions are as follows; 0.6 M of
1a (1.2 mmol), 0.35 M of 2y (0.7 mmol), 5 mol% of [Cu] catalyst. The
mixture was irradiated with blue LEDs (power density: 40 mW/cm2 at 460
nm) for 8 h in an O2 atmosphere (1 atm.). bTotal conversion of the isolated
product 1a2y based on 2y. cYield of the isolated product 1a2y based on 2y. d
Yield of the homocoupling product 1a1a based on 1a. eReaction condition:
0.6 M (1.2 mmol) of 1a, 0.5 M (1 mmol) of 2a.firradiation with an ambient
white light bulb for 15 h (power density: 8 mW/cm2 at 460 nm). gReaction
After optimization (see Table 1), we found that acetonitrile is
the best solvent (85% yield) for the cross-coupling reactions;
other polar solvents, such as, MeOH, DMF, and THF, also work
well (entries 5-7, Table 1). Non-polar solvents, such as,
toluene, fail to produce the heterocoupling product (see entry
8, Table 1). In addition, we found that low substrate
concentrations can significantly improve the yield of hetero-
2 | J. Name., 2012, 00, 1-3
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was conducted in the dark at RT. hIn the absence of CuCl catalyst. In the
absence of O2.
i
accomplish, since these groups have less π–basicity on the C≡C
triple bond, and thus is less favorable towards soft Lewis acid
catalysis, such as, Cu(I) ion.13 However, the present catalysis system
is effective for cross-coupling to generate the unsymmetrical 1,3-
diynes in 83-95% yields (1a2l-1a2t). Thus, this visible light induced
cross coupling protocol is very important and complementary to the
literature reported thermal Csp-Csp coupling reaction.5,6 It could be
understood that the acetylene proton in terminal alkynes having
electron deficient substituent groups is more acidic, and can be
easily abstracted by photo-generated radical species, such as,
superoxide radical anion (for details, please see the proposed
mechanism below). Therefore, the Csp-Csp cross-coupling reaction
between electron-rich/neutral and electron-poor aryl alkynes
occurs upon photo-irradiation.
Table 2 Substrates scopea
5 mol% CuCl
R2
OMe
tBu
H
R2
+
CH3CN, O2, RT
blue LEDs
1a
2
N
1a2a (64%, 5h)b
1a2c (72%, 5h)b
1a2b (70%, 5h)b
OH
1a2d (67%, 5h)b
1a2f (73%, 5h)b
O
O
F
Moreover, aliphatic alkynes, such as, propargylamine
derivative and linear alkynes, can also be effectively coupled
with 1a to generate desired unsymmetrical 1,3-diynes (1a2u-
1a2y). In addition, electron rich aromatic groups (e.g., R= tBu
& Et) can also effectively couple with aliphatic alkynes to
generate the desired products in good yields (e.g., 2d2y &
2z2v). In contrast to reported thermal methods, the current
photochemical process works well for cross-coupling reaction
between electron-rich and electron-deficient aryl alkynes to
afford hetero-coupling 1,3-diynes products in good yields (e.g.,
1a2e (72%, 5h)b
Cl
I
1a2h (88%, 8h)c
c
1a2g
(77%, 8h)
HO
1a2j (82%, 8h)c
1a2i(81%, 8h)c
OMe
Ac
CN
CN
c
c
1a2l
(83%, 12h)
1a2k
1a2m
1a2o
(86%, 8h)
2b2m & 2d2m). Interestingly, an electron-poor aryl alkyne (2s
)
can also effectively couple with aliphatic alkynes to generate
1a2n (95%, 12h)d
d
the desired cross-coupling products in good yield (e.g., 2s2x).
(95%, 12h)
CHO
Table 3 Evaluation of green chemistry metrics for the synthesis of 1,3 diynes
CO2Me
d
(1a2m)
1a2p
d
(95%, 12h)
MeO2C
(94%, 12h)
standard
Ph
Ac
Ac
(2)
Ph
+
COPh
condition
16 h
1a
9.3 mmol
1a2m
2m
1a2q (90%, 12h)d
1a2r (90%, 12h)d
84% yield (4.3 mmol)
5.25 mmol
NO2
NC
d
d
1a2s
(91%, 12h)
1a2t
(92%, 12h)
O
HN
OH
d
d
1a2v
(78%, 8h)
1a2u
(84%, 12h)
OH
Cl
2
2
d
d
1a2w
1a2x
2d2y
(80%, 8h)
78%, 8h)
OAc
tBu
OAc
d
1a2y
e
(85%, 8h)
HO
(88%, 8h)
MeO
Ac
Et
2z2v (80%, 8h)e
e
2b2m
2s2x
(86%, 12h)
tBu
Cl
2
f
e
(84%, 12h)
NC
2d2s
74%, 8h)
CN
In addition, the current Csp-Csp cross coupling process can be readily
scaled up to a gram scale (0.75 g, 5.25 mmol); 1.06 grams of 1a2m
(84% yield) can be obtained after 16 hours of irradiation with blue
LEDs at room temperature (see eqn (2)). We have evaluated the
green chemistry metrics11c for the synthesis of the unsymmetrical
conjugated 1,3-diynes (1a2m) on a preparative scale (see Table 3).
Overall, our green process can enable the 1,3-diynes (1a2m) on the
preparative scale with an E factor of 11.7, 93% atom economy, 78%
atom efficiency, 100% carbon efficiency, and 63% reaction mass
aStandard condition. Isolated yield based on 2 after purification by column
chromatography on silica gel, breaction condition: 1.0 mmol of 1a (0.5 M),
0.8 mmol of 2 (0.4 M), creaction condition: 1.2 mmol of 1a (0.6 M), 0.7 mmol
d
of 2g-2l (0.35 M). reaction condition: 1.3 mmol of 1a (0.65 M), 0.7 mmol of
e
2m-2y (0.35 M). reaction condition: 1.3 mmol of 2b, 2d & 2z (0.65 M), 0.7
mmol of 2y, 2v, 2m & 2s (0.35 M). freaction condition: 0.7 mmol of 2s (0.65
M), 1.0 mmol of 2x (0. 5 M).
In general, the Csp-Csp cross coupling between electron deficient
aromatic terminal alkynes and phenylacetylene is very difficult to
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efficiency, which are far better than previously reported process6a
(E factor= 27, atom economy= 56%, atom efficiency= 48%, reaction
mass efficiency: 27%, see, details in S.I.), since the current
photochemical process does not require the any base/ligands and
expensive metal catalyst and thus leads to a sustainable and green
process for the synthesis of various bio-active unsymmetrical 1,3-
diynes (especially in large scale).
Conclusions
We have developed a novel visible-light initiated CuCl
catalyzed Csp-Csp cross-coupling of terminal alkynes for the
facial synthesis of conjugated diynes without need for a base,
ligands, and additives. This protocol can be used to prepare a
variety of unsymmetrical conjugated diynes from readily
available terminal alkynes without the need of pre-
functionalized substrates. Meanwhile, the current process uses
an excess one alkyne (up to 1.8 eqv.) to accomplish a good
selectivity for the heterocoupling product. The key step toward
the success of heterocoupling reaction is the formation of a
stable ground state bipolar hetero-dimeric CuI-acetylide
complex in polar organic solvents. Photo excitation provides
sufficient energy for the ground state hetero-dimeric CuI-
acetylide complex to overcome an energy barrier, to donate an
electron to molecular oxygen, and to initiate the intra-complex
hetero-ligands cross-coupling reaction. From synthetic point of
view, exclusion of expensive Pd/Au catalysts, ligands, oxidants
and amine base makes this method extremely simple and eco-
friendly process for synthesis of a variety of bio-active
unsymmetrical conjugated diynes.
A possible reaction mechanism for the visible light-initiated, CuCl
catalysed Csp-Csp cross coupling reaction is proposed in Scheme 2.
The first step involves the formation of CuI-π–alkyne complex (A), of
which the alkynes terminal proton becomes more acidic13 and thus
favors the in-situ formation of copper(I) phenylacetylide (B).14 In
the next step, addition of electron-deficient alkynes to the solution
results in formation of a stable, bipolar ground state hetero-dimeric
CuI complex (C) in polar organic solvents, as evidenced by
isothermal titration calorimetry (ITC) measurements of heat release
upon titration of free e-deficient terminal alkynes into a solution
containing Cu(I) phenylacetylide (CuI-1a).(see more details in Figure
S3). From the titration curve, a complexation constant, Ka, of ~ 199
µM-1 was obtained for the formation of a ground state bipolar
hetero-dimeric Cu(I)-phenylacetylide complex. Such a hetero-
dimeric Cu(I) complex contains an electron-rich/neutral and
another electron-deficient alkyne, of which the dipolar character is
stabilized by a polar solvent. ITC measurements also showed that
such a bipolar hetero-dimeic Cu(I) complex was not formed in
Cu-phenylacetylide (B) in ACN
0.4
0.3
0.2
0.1
0.0
Cu-phenylacetylide (B) +2m in ACN (5min.)
Cu-phenylacetylide (B) +2m in ACN (10min.)
Cu-phenylacetylide (B) +2m in ACN (20min.)
nonpolar organic solvents (such as, toluene).
Similarly,
homodimeric Cu(I) complexes do not form, even in polar organic
solvents. The formation of the hetero-dimeric Cu(I)-1a complex is
supported by the observation of a red shift in the absorption
maximum of Cu(I)-1a from 479 nm to 490 nm in ACN upon addition
of equal amount of electron deficient (4-ethynylphenyl) ethanone
(2m) (see Figure 1). It is reasonable that formation of the ground
state bipolar hetero-dimeric complex is more favourable as
compared to homo-dimeric complex in polar organic solvents, since
the hetero-dimeric CuI complex is of dipolar nature (having both
electron rich and electron poor moieties in same complex). Direct
photo excitation of the complex (C) by blue light (λmax= 479 nm)
leads to the formation of a long-lived triplet excited hetero-dimeric
479 nm
490 nm
400
440
480
520
560
600
Wavelength (nm)
CuI complex (D) via ligand to metal charge transfer (LMCT).11c,d&15 Figure 1 UV-visible spectra of 0.05 M Cu-phenylacetylide (B) alone in
The photoexcited CuI-1a complex (D) could transfer a single
acetonitrile (ACN) (black curve) and spectra obtained at different time delay
electron to the molecular oxygen to generate CuII–phenylacetylide
after addition of 0.055 M 1-(4-ethynylphenyl) ethanone (2m) into the
complex (E) and superoxide,11c as evidenced by EPR detection of
solution.
superoxide radical anion (see Figures S1-S2 in supporting
information). Indeed, superoxide radical anion has a tendency to
abstract an acidic proton from terminal alkynes (favored by electron
withdrawing groups) to form the carbon centered alkynyl
radical.7a,16 Subsequent intra complex ligand-ligand cross coupling
between the electron deficient alkynyl radical and electron
rich/neutral copper(II) acetylide leads to formation of the desired
cross-coupling product,10a,11d in accompany with the formation of
hydrogen peroxide (as a by-product) and regeneration of CuCl
catalyst. Such an intra-complex ligand-ligand cross coupling process
dominates over bi-molecular coupling between the alkynyl radical
and free alkynes in the solution, especially under low substrate
concentration conditions.
Scheme 3 Proposed mechanism.
4 | J. Name., 2012, 00, 1-3
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Green Chemistry
Page 6 of 6
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DOI: 10.1039/C6GC01463A
COMMUNICATION
Green Chemistry
Graphical Abstract
Visible-light-activated Copper(I) catalyzed Oxidative Csp-Csp Cross-Coupling Reaction:
Efficient Synthesis of Unsymmetrical Conjugated Diynes without Ligands and Base
Arunachalam Sagadevan,1 Ping-Chiang Lyu,2 and Kuo Chu Hwang1*
2.
1.Department of Chemistry, Institute of Bioinformatics and Structural Biology and College of Life Sciences,
National Tsing Hua University, Hsinchu 30013, Taiwan. E-mail: kchwang@mx.nthu.edu.tw; Fax: (+886) 35711082.
An efficient and eco-friendly approach to Csp-Csp cross-coupling of terminal alkynes for construction of
unsymmetrical conjugated diynes via visible-light-induced CuCl catalysed process at room temperature.
6 | J. Name., 2012, 00, 1-3
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