Photoredox/Cobalt Dual Catalysis for Visible-Light-Mediated Alkene-Alkyne Coupling

Article abstract of DOI:10.1021/acs.orglett.9b01201

Dual photoredox transition-metal catalysis has recently emerged as a powerful tool for making synthetically challenging carbon-carbon bonds under milder reaction conditions. Herein, we report on the visible-light-mediated controlled generation of low-valent cobalt catalyst without the need for a metallic reductant. It enabled C-C bond formation via ene-yne coupling at room temperature. The generality of this dual catalysis is demonstrated via the creation of sizable molecular diversity with the accommodation of several functional groups.

Full text of DOI:10.1021/acs.orglett.9b01201

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Photoredox/Cobalt Dual Catalysis for Visible-Light-Mediated
Alkene−Alkyne Coupling
Pramod Rai,‡ Kakoli Maji,‡ and Biplab Maji*
Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India
S
* Supporting Information
ABSTRACT: Dual photoredox transition-metal catalysis has recently
emerged as a powerful tool for making synthetically challenging
carbon−carbon bonds under milder reaction conditions. Herein, we
report on the visible-light-mediated controlled generation of low-valent
cobalt catalyst without the need for a metallic reductant. It enabled C−
C bond formation via ene-yne coupling at room temperature. The
generality of this dual catalysis is demonstrated via the creation of sizable molecular diversity with the accommodation of several
functional groups.
he carbon−carbon bond construction is one of the
T
central goals in organic synthesis.1 The intermolecular
cross-coupling of easily accessible, inexpensive, and bench-
stable π-component systems, specifically a CC triple bond
and a C=C double bond to form a linear carbon chain is an
attractive strategy.2 It is particularly intriguing transformation,
because of the possibility of the generation of molecular
complexity resulting from different regioisomers and stereo-
isomers with the retention of the entire atom economy.3 From
the pioneering work of Cheng,4 Montgomery,5 and others,6
several low-valent transition-metal catalysts were utilized for
the ene-yne coupling reactions (Figure 1a). However, in such a
thermal reaction, the low-valent metal catalysts were accessed
in situ using (super) stoichiometric heterogeneous metal,
Grignard reagent, triethylborane, etc. as reducing agents to
circumvent the limited accessibility and bench stability of these
low-valent metal complexes. Given the concerns associated
with the use of dissolving metal/Grignard as reducing agents,
an alternative catalytic activation mode under a benign
condition capable of accommodation of large functional
groups and, thus, the creation of vast molecular diversity is
highly desirable.
The visible light has been recognized as an environmentally
friendly and sustainable form of energy for triggering chemical
transformations and catalytic bond activation process.7
Notably, the use of a dual catalyst for the C−C bond
formation in the presence of visible light has drawn much
attention to the researchers over the past few years.7b,8 In this
regard, recently, Rovis and co-workers have utilized visible-
light-gated cobalt catalysis for [2 + 2 + 2] cycloaddition of
alkynes for the formation of arenes.9 Zhao, Wu, and co-
workers have developed a visible-light-triggered iridium/cobalt
catalyst system for the hydrocarboxylation and carbocarbox-
ylation of alkynes using CO2.10 Ritter et al. have demonstrated
an iridium/cobalt catalyst system for the dehydrogenative
decarboxyolefination of carboxylic acids.11 However, despite
such development, to best of our knowledge, visible-light-
triggered reductive coupling of an alkyne with an alkene to
Figure 1. Previous transition-metal-catalyzed C−C bond formation
via the ene-yne coupling and this work. Inset shows the proposed
mechanistic hypothesis. CoII= Co(dppe)2Cl2. IrIII= [Ir{dF(CF3)-
ppy2(dtbpy)]PF6. SET = single electron transfer.
form a trisubstituted alkene has not been developed thus far
(Figure 1b).
At the onset of our analysis, we have realized that a
reductively quenched iridium-based photoredox catalyst [Ir-
{dF(CF3)ppy}2(dtbpy)]PF6 (PC1, E1/2[IrIII/IrII] = −1.37 V vs
Received: April 5, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01201
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
SCE in MeCN),12 could reduce a high-valent transition-metal
catalyst (dppe)2CoCl2 (A, E1/2[CoII/CoI] = −0.88 V vs SCE in
MeCN) in a controlled manner using an organic sacrificial
reductant under visible-light irradiation (Figure 1, inset). The
low valent-metal complex B thus generated could be
intercepted by a proton donor to deliver a hydride complex
C.13 The latter could then undergo hydrometalation with an
olefin coupling partner 2 to produce the organometallic
intermediate D. The subsequent carbometalation and proto-
nation could deliver the ene-yne coupling product 3, where the
oxidized catalyst could then be reduced back to its original
state by the photocatalyst cycle. We envisioned that the key
advantage of such a photodriven reaction would be milder
reaction conditions and accommodation of a large number of
functionalities, including aryl halides and reducible groups. It
would open a new direction for visible-light-mediated low-
valent metal catalysis for the construction of C−C bonds.
Based on the above hypothesis, we have started our
investigation by employing 3-hexyne (1a) and n-butyl acrylate
(2a) as a model substrate in the presence of a photocatalyst
and transition-metal catalyst under the irradiation of blue-light-
emitting diode LED (440 nm) at room temperature (Table 1).
result, providing the coupling product in 84% isolated yield
with perfect stereoselectivity (Table 1, entry 1). The more
strongly reducing photoredox catalysts, such as Ir-
(ppy)2(dtbbpy)PF6 (PC2) or Ir(ppy)2(bpy)PF6 (PC3) having
a potential of E1/2[IrIII/IrII] = −1.51 V and −1.38 vs SCE in
MeCN, respectively were less active than the fluorinated
catalyst PC1, which gave higher yields from increased
longevity (Table 1, entries 2 and 3). Among the phosphine
ligands, PPh3 displayed similar reactivity as that of dppe (Table
1, entry 4), whereas the use of other phosphine ligands was
found less useful (Table 1, entry 5). Furthermore, the
transition-metal precatalysts FeBr2 and NiCl2 with dppe as
ligand was found to be moderately active and low yields of the
products were obtained (Table 1, entries 6 and 7).
The choice and the stoichiometry of the sacrificial reducing
agent was found to be crucial, because i-Pr2NEt or Hantzsch
ester alone or in a different ratio were less effective (Table 1,
entries 8−11). Only a trace amount of product was detected
when pivalic acid was used as a proton source (Table 1, entry
12).14 No product was detected in the absence of the light,
photocatalyst, cobalt catalyst, or iPr2NEt/Hantzsch ester
(Table 1, entries 13−16). They are thus demonstrating the
requirement of all of the components. The coupling reaction
did not proceed under the thermal condition (80 °C, 20 h)
(Table 1, entry 17). Further optimization, in terms of solvents,
the wavelength of the employed light, the stoichiometry of the
reactants, and catalyst loading were also performed and are
detailed in the Supporting Information.
a
Table 1. Reaction Development: Key Optimizations
We have then explored the substrate scope of this
photodriven cobalt-catalyzed reductive coupling reactions
(Scheme 1). Initially, the scope of various alkenes was tested
using 1a as the coupling partner, and we are pleased to find
that the conditions for the model reaction could be applied for
a large number of alkenes furnishing the desired products in
moderate to good yields and all cases the (E)-stereoisomer was
formed selectively. We account the observed stereoselectivity
as the manifestation of syn-carboacobaltation as shown in
Figure 1. Under the standard reaction conditions, readily
available aliphatic acrylates like n-butyl (2a), ethyl (2b), and
methyl acrylate (2c) gave the corresponding products in 68%−
84% yields. Also, acrylates bearing benzyl group (2d) and
biphenyl methyl group (2e) were tolerated. To further extend
the scope of this reductive coupling reaction various function-
alized substrates were tested. Substrates bearing functional
groups such as aryl ether (2f), trifluoromethyl (2g), and nitrile
(2h) groups reacted efficiently to give the corresponding
coupling products (3af−3ah) in 74%−80% yields with
exclusive stereoselectivity. Importantly, the substrates contain-
ing aryl halide moieties such as fluoro (2i), chloro (2j), and
bromo (2k), which are less compatible with metal reductants,
were found to be viable coupling partners under the milder
photodriven conditions and the products were obtained in
good to excellent yields with the complete retention of the
halide functional groups. Substrates containing pyridine
heterocycle (2l) were also tested under the dual photoredox
conditions which provide the desired product in moderate 49%
yields.
yield
entry
deviation
(%)
1
2
none
84
59
Ir(ppy)2(dtbpy)PF6 (PC2) instead of [Ir{dF(CF3)
ppy}2(dtbpy)]PF6 (PC1)
Ir(ppy)2(bpy)PF6 (PC3) instead of [Ir{dF(CF3)
ppy}2(dtbpy)]PF6 (PC1)
3
52
4
5
PPh3 instead of dppe
80
up to
56
41
38
60
dppp or dppm or dppf or xantphos or dppbz instead of
dppe
6
7
8
FeBr2 instead of CoCl2
NiCl2 instead of CoCl2
i-Pr2NEt (3 equiv) instead of HE (2 equiv), i-Pr2NEt
(1 equiv)
9
10
11
NEt3 (3 equiv) instead of HE (2 equiv), i-Pr2NEt (1 equiv)
HE (3 equiv) instead of HE (2 equiv), i-Pr2NEt (1 equiv)
HE (1 equiv), i-Pr2NEt (2 equiv) instead of HE (2 equiv),
i-Pr2NEt (1 equiv)
PivOH (1 equiv) instead of H2O (1 equiv)
no light
no [Ir{dF(CF3)ppy}2(dtbpy)]PF6
no CoCl2/dppe
40
50
65
12
13
14
15
16
17
nd
nd
nd
nd
trace
trace
no HE, i-Pr2NEt
no light at 80 °C
a
Reaction conditions: 1a (0.2 mmol), 2a (0.24 mmol), PC1 (1
mol %), CoCl2 (10 mol %), dppe (20 mol %), i-Pr2NEt:HE (1:2, 0.3
mmol), H2O (1 equiv), blue LED (440 nm) in MeCN (0.5 mL) at
room temperature under Ar. Yield of the isolated product was
given. HE = Hantzsch ester.
Interestingly, menthyl (2m) and bioactive molecules such as
estrone (2n) and cholesterol (2o) were also derivatized with
complete stereoselectivity using the reductive coupling
method, affording the desired products (3am−3ao) in good
yields (Scheme 1). Besides, acrylamide (2p) derived from
natural amino acids like (L)-phenyl alanine could also undergo
After sizable optimizations, we have found that the
combination of PC1 (1 mol %) as the photocatalyst, CoCl2
(10 mol %), in combination with dppe 1,2-bis-
(diphenylphosphino)ethane, (20 mol %) as the transition-
metal catalyst, water (1 equiv). as the proton source in the
i
presence of Pr2NEt/Hantzsch ester (1:2) furnished the best
B
DOI: 10.1021/acs.orglett.9b01201
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Organic Letters
Letter
a
Scheme 1. Visible-Light-Driven Cobalt-Catalyzed Ene-Yne Coupling
a
Reaction conditions: 1 (0.2 mmol), 2 (0.24 mmol), PC1 (1 mol %), CoCl2 (10 mol %), dppe (20 mol %), i-Pr2NEt:HE (1:2, 0.3 mmol), H2O (1
equiv), Blue LED (440 nm) in MeCN (0.5 mL) at room temperature under Ar. The yields of the isolated products are given. b77% yield in 1 mmol
scale 20 h. NMR yield. Ratios of (Z)-3:(E)-3 are given in parentheses. Ratios of (E)-3:(Z)-3:r-(E)-3 are given in parentheses.
c
d
e
this reaction affording the coupled product in moderate yield.
Phenyl vinyl sulfone (2q) which is known to be the Michael
acceptor also undergoes the reaction very smoothly. In all
these cases, the reducible functional groups such as an alkene,
keto, amide, sulfone groups were retained in the product.
Furthermore, we have investigated the scope concerning the
alkyne coupling partner (Scheme 1). In addition to 3-hexyne
(1a), the symmetrical aliphatic alkynes such as 4-octyne (1b)
was converted to the corresponding product 3ba in 91% yield
with complete stereoselectivity. However, in the case of
diarylacetylenes (1c−1e), a stereoisomeric mixture due to E/Z
isomerization via a triplet−triplet energy transfer process is
observed.15 In these cases, the products were obtained in
56%−90% yield with Z/E ratio up to 3:1.
generation of a broad molecular diversity resulting not only
from E/Z isomerization but also from different regioselectivity.
Gratifyingly, for the studied reactions, the regioselectivity is
found to be controlled by the cobalt catalyst, and, in all cases,
the r-(E)-3 product is obtained only in a small amount. In fact,
in some cases, they are not even observed. For example, ethyl
propiolate (1f) was found to couple with ethyl acrylate (2a)
and ethyl vinyl ketone (2r) in 53%−65% yield in exclusive
stereoselectivity.
In the case of prop-1-yn-1-ylbenzene (1g), minor E/Z-
isomerization product was observed. Interestingly, in addition
to acrylates (2a, 2b), enones (2r), and acrylonitrile (2s) are
also accommodated as a suitable substrate. Notably, boc-
protected acrylamide (2t) could also be very compatible with
this method. Similarly, a variety of unsymmetrical alkynes
(1h−1m) were tolerated with the reaction conditions,
providing the respective cross-coupling products in good to
A large variety of unsymmetrical alkynes were then tested to
further expand the scope of this photodriven ene-yne coupling
reaction (Scheme 1). In such cases, we have anticipated the
C
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Letter
excellent yields and selectivities. Thiophene containing hetero-
cycles (1n) was also tolerated, as demonstrated by the
generation of 3na.
Finally, to further validate the proposed mechanistic
hypothesis, some control experiments were conducted.
Initially, a light-dark experiment (up to 12 h) was performed
to examine the role of visible light (Figure 2). Following our
under standard reaction condition for 4 h. As expected, the E/
Z ratio was changed from 12:1 to 5:1, indicating that the
isomerization is occurring after the C−C bond formation
(Scheme 2b). Meanwhile, to understand the role of water,
deuterium labeling studies were performed using D2O as the
1
external proton source. The H NMR analysis of the product
showed that one of the α-methylene proton and the vinyl
proton were deuterated in 44% and 52%, respectively (Scheme
2c). This suggests that (i) both the oxidized HE and i-Pr2NEt
might act as the proton source, along with added H2O, and i-
Pr2NEt might additionally facilitate the formation of pyridine
from HE by abstracting the H+, (ii) the proton source is
involved not only in the formation of the cobalt hydride C but
also for the release of product from the intermediate E, and
(iii) the cobalt hydride C underwent anti-Markovnikov
hydrometalation for the formation of D (see Figure 1, inset).
Along this direction, no deuterium incorporation occurred
when the reaction is performed using CD3CN as the solvent
(Scheme 2d). Furthermore, premature quenching of the
reaction revealed the formation of n-butyl propionate,
confirming the intermediacy of the cobaltacycle D. The
oxidized HE can be observed as the byproduct of this reaction.
In conclusion, we have demonstrated a visible-light-driven
dual Ir/Co-catalysis for the C−C bond formation via reductive
coupling of an alkene with alkynes at room temperature. The
catalysis bypasses the use of stoichiometric organometallic
reductants. It exhibits a wide range of functional groups
tolerance and generates a sizable molecular library from simple
starting materials. Detailed mechanistic studies including
deuterium labeling experiments, radical quenching experi-
ments, quantum yield measurements, and Stern−Volmer
quenching experiments have been presented to validate the
proposed mechanistic pathway.
Figure 2. Reaction profile with the light on/off over time.
mechanistic hypothesis, the cobalt-catalyst system is found to
be activated under the irradiation and deactivated in darkness.
The quantum-yield measurement (Φ = 0.91) by chemical
actinometry at 440 nm under the reaction conditions further
supports the temporal control of the cobalt catalyst under
visible light irradiation (see the Supporting Information for
details). Moreover, Stern−Volmer quenching experiments
were performed to determine the quenching efficiency of the
reagents.16 As anticipated, we have found that 1a and 2a does
not quench the luminescence of PC1. Whereas both HE and i-
Pr2NEt quenches the luminescence of PC1 at very high rates
(kq(HE) = 2.69 × 108 M−1 s−1 and kq(i-Pr2NEt) = 1.49 × 108
M−1 s−1) and HE was found to be only slightly faster. This
indicated that both HE and i-Pr2NEt could act as the terminal
reductant. In addition, we have determined the quenching
fraction Q, following the method described by Yoon,16b and it
reveals that 99% of the photon absorbed by the PC1
participates in a productive electron transfer process.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01201.
Under the standard reaction condition, the presence of 1
equiv of (2,2,6,6-tetramethylpiperidine-1-yl)oxyl (TEMPO)
does not suppress the product formation (83% isolated yield),
further indicating free radicals do not involve in the reaction
pathway (Scheme 2a). Furthermore, to validate that the
isomerization is occurring off-catalytic cycle via a triplet−triplet
energy transfer,15 the isolated product 3ga was again treated
Experimental details, compound characterizations, and
NMR spectra of the products (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: bm@iiserkol.ac.in.
ORCID
Scheme 2. Control Experiments
Biplab Maji: 0000-0001-5034-423X
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The author thanks DST INSPIRE Faculty Award [DST/
INSPIRE/04/2015/002518] for financial support. P.R. thanks
INSPIRE and K.M. thanks IISER K for a Ph.D. fellowship.
Helpful discussions with Dr. P. K. Mondal (Indian Institute of
Science Education and Research Kolkata), Dr. M. Baidya
(Indian Institute of Technology Madras), Dr. A. Jana (Tata
D
DOI: 10.1021/acs.orglett.9b01201
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Organic Letters
Letter
(15) Metternich, J. B.; Artiukhin, D. G.; Holland, M. C.; von
Institute of Fundamental Research Hyderabad), and Dr. S.
Lakhdar (CNRS−ENSI Caen) were gratefully acknowledged.
Bremen-Kuhne, M.; Neugebauer, J.; Gilmour, R. J. Org. Chem. 2017,
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Chem. Soc. 2016, 138, 16200−16203. (b) Cismesia, M. A.; Yoon, T.
P. Chem. Sci. 2015, 6, 5426−5434.
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