Highly Regio- And Enantioselective Reductive Coupling of Alkynes and Aldehydes via Photoredox Cobalt Dual Catalysis

Article abstract of DOI:10.1021/jacs.1c03527

A Co-catalyzed highly regio- and enantioselective reductive coupling of alkynes and aldehydes has been developed under visible light photoredox dual catalysis. A variety of enantioenriched allylic alcohols have been obtained by using unsymmetrical internal alkynes and commercially available catalyst, chiral ligand, and reagents. It is noteworthy that this approach has considerable advantages, such as excellent regio- (>95:5 for >40 examples), stereo- (up to >95:5 E/Z), and enantioselectivity (92-99% ee, >35 examples) control, mild reaction conditions, broad substrate scope, and good functional group compatibility, making it a great improvement to enantioselective alkyne-aldehyde reductive coupling reactions.

Full text of DOI:10.1021/jacs.1c03527

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Highly Regio- and Enantioselective Reductive Coupling of Alkynes
and Aldehydes via Photoredox Cobalt Dual Catalysis
Yan-Lin Li, Shi-Qi Zhang, Jie Chen, and Ji-Bao Xia*
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ABSTRACT: A Co-catalyzed highly regio- and enantioselective reductive coupling of alkynes and aldehydes has been developed
under visible light photoredox dual catalysis. A variety of enantioenriched allylic alcohols have been obtained by using unsymmetrical
internal alkynes and commercially available catalyst, chiral ligand, and reagents. It is noteworthy that this approach has considerable
advantages, such as excellent regio- (>95:5 for >40 examples), stereo- (up to >95:5 E/Z), and enantioselectivity (92−99% ee, >35
examples) control, mild reaction conditions, broad substrate scope, and good functional group compatibility, making it a great
improvement to enantioselective alkyne−aldehyde reductive coupling reactions.
Scheme 1. Enantioselective Reductive Couplings via Cobalt
Catalysis
ransition-metal-catalyzed reductive coupling reactions
provide a straightforward and modular approach for the
T
construction of complex targets from two pieces of electro-
philes in the presence of reductants.1 As asymmetric catalysis
constitutes a powerful tool toward optically pure compounds,
enantioselective catalytic reductive couplings have tremendous
potential in the preparation of enantioenriched molecules.2,3
The enantioselective reductive coupling of two π-components
has received substantial advances in recent decades.4 Readily
available alkynes have been applied to this strategy to couple
with another π-component to build a variety of substituted
chiral alkenes.5 Avoiding using vinyl halides or preformed vinyl
organometallic reagents,6 enantioselective alkyne−aldehyde
reductive coupling is a promising route toward chiral allylic
alcohols, which are one of the most versatile synthons in
organic synthesis and key chiral skeletons in numerous
pharmaceuticals and natural products. Significant advances
on these reactions have been achieved with 1,3-diynes, 1,3-
enynes, or alkynes as substrates by using Rh or Ni as a catalyst
and H2,7 Et3B,8 silane,9 or alcohol10 as a reductant.11 However,
obtaining highly regio- and enantioselectivity control with
simple unsymmetrical internal alkynes is still a big challenge.12
Therefore, the development of a new efficient catalytic system
for regio- and enantioselective alkyne−aldehyde reductive
coupling remains highly rewarding.
Cobalt, a low toxic, cost-effective, and earth-abundant first
row metal, has been established as an excellent option in
numerous transition-metal-catalyzed reactions,13 particularly
asymmetric catalysis.14,15 In comparison, Co-catalyzed reduc-
tive couplings have not been widely developed. In this respect,
couplings between two halides,16 two alkenes,17 alkynes and
alkenes,18 nitriles and alkenes,19 and related reactions20 have
been developed, but no alkyne−aldehyde reductive coupling
has been reported. These reactions require a stoichiometric
amount of metal (Zn or Mn powder) as the reductant, and the
asymmetric version is rare. One representative example is
enantioselective reductive coupling of alkynes and cyclic
enones to β-alkenyl ketones (Scheme 1A).21 Since 2014,
photoredox metal dual catalysis has emerged as a powerful tool
to forge chemical bonds under mild conditions.22 This
synergistic catalytic strategy can circumvent the use of
stoichiometric metal reductant.23 Dual photoredox cobalt
catalysis has also been applied in several types of reactions,
including dehydrogenative transformations,24 hydrofunctional-
ization of alkenes,25 cycloaddition of alkynes,26 allylation,27
etc.28 However, reports on the enantioselective transforma-
Received: April 2, 2021
Published: May 5, 2021
https://doi.org/10.1021/jacs.1c03527
J. Am. Chem. Soc. 2021, 143, 7306−7313
© 2021 American Chemical Society
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a
tions via synergistic photoredox cobalt catalysis have remained
scarce.29
Table 1. Reaction Development
Recently, we reported a regioselective reductive coupling of
1,3-dienes and aldehydes toward homoallyic alcohols via
photoredox nickel dual catalysis.30 We suspected that
application of this strategy to metal-catalyzed alkyne−aldehyde
reductive coupling would deliver enantioenriched allylic
alcohols in the presence of a suitable chiral ligand. Two
reaction pathways are initially considered for this reaction. A
metal-hydride species might be generated by oxidation of a
low-valent metal catalyst with Hantzsch ester (HE) under
photoredox conditions (Path a). Then, selective addition of
aldehyde with an alkenyl metal intermediate from hydro-
metalation of alkyne would deliver the desired product. For an
alternative pathway, metal mediated oxidative cyclometalation
of alkyne and aldehyde would provide a five-membered
metallacycle, which would provide the product after protolysis
(Path b). Herein, we report a novel and practical strategy for
the highly regio- and enantioselective reductive coupling of
alkynes and aldehydes via visible-light photoredox cobalt dual
catalysis (Scheme 1B).
b
c
metal
cat.
yield
(%)
ee
b
b
entry
ligand
rr
E/Z
(%)
1
2
3
NiCl2
CoBr2
CoCl2
CoBr2
CoBr2
CoBr2
CoBr2
CoBr2
CoBr2
CoBr2
2,2′-BPY
DPPP
DPPP
DPPP
(S)-BINAP
(S,S)-BDPP
Josiphos L1
Josiphos L2
Josiphos L3
(S,S)-BDPP
0
−
−
−
−
−
−
93
50
90
>95:5
>95:5
>95:5
−
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
1:2
d
4
5
6
7
8
9
0
−
−
80
20
18
31
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
98
97
96
97
98
98
e
f
10
11
80 (78)
75
g
CoBr2[(S,S)-BDPP]
We began our research by studying the reductive coupling of
benzaldehyde (1) and 1-phenyl-1-propyne (2) via visible light
photoredox metal dual catalysis. Initially, no desired product
was observed using nickel as the catalyst with various ligands.
After extensive investigations, it was gratifying that allylic
alcohol 4 was obtained in 93% yield with excellent regio- and
E/Z selectivity by using CoBr2 as the catalyst, DPPP as the
ligand, 4CzIPN as the organic photocatalyst, and Hantzsch
ester (HE) 3 as the reducing agent (Table 1, entry 2). A lower
yield was obtained with CoCl2 as the catalyst (Table 1, entry
3). We need to mention that the ligand plays a significant role
in the reaction (see Table S2 in Supporting Information).
Interestingly, the reversed E/Z selectivity was observed when
using an iridium photocatalyst (Table 1, entry 4). Then,
various chiral biphosphine ligands were investigated to achieve
the enantioselective reaction (Table 1, entries 5−9). No
reaction occurred with (S)-BINAP as the chiral ligand (Table
1, entry 5). To our delight, excellent yield, regio- (>95:5), E/Z-
(>95:5), and enantioselectivity (98% ee) were obtained by
using commercially available (S,S)-BDPP as the chiral ligand
(Table 1, entry 6). Several chiral Josiphos ligands (L1−3) were
also tested affording 4 with excellent ee but lower yields (Table
1, entries 7−9). When reducing the catalyst loading from 10
mol% to 5 mol%, the catalytic efficiency can also be maintained
(Table 1, entry 10, standard conditions). A similar result was
also obtained with the CoBr2[(S,S)-BDPP] complex as the
catalyst (Table 1, entry 11). Finally, control experiments
confirmed that 4 could not be obtained without any one of the
following elements: CoBr2, ligand, photocatalyst, Hantzsch
ester, and visible light. We want to comment that Ni-catalyzed
enantioselective reductive coupling of 1 and 2 toward 4 has
been studied via several systems.8,9 Up to 86% ee has been
obtained with a well-designed ferrocene-based planar chiral
NHC ligand.9c Obviously, the best result has been achieved
under our photoredox cobalt dual catalysis.
a
Reaction conditions: 1 (0.2 mmol), 2 (2 equiv), metal cat. (10 mol
i
%), ligand (10 mol%), 4CzIPN (2 mol%), Pr2NEt (12 mol%), 3 (2
equiv), THF (3 mL), 5 W blue LEDs, rt, if otherwise noted.
b
1
Determined by crude H NMR with 1,1,2,2-tetrachloroethane as an
c
d
internal standard. Determined by Chiral HPLC. With Ir[dF(CF3)-
ppy]2(dtbbpy)PF6 as the photocatalyst. With 5 mol% CoBr2 and 5
mol% (S,S)-BDPP. Isolated yield in the parentheses. With 5 mol%
CoBr2[(S,S)-BDPP].
e
f
g
OMe), electron-withdrawing (CF3), and halide (F, Cl)
substituents on the aryl ring are well tolerated. Trisubstituted
aryl alkynes were also proven successful (7, >95:5 rr, 94:6 E/Z,
99% ee). For heteroaryl alkynes, thiophene-containing alkyne
reacted smoothly affording 12 with excellent results (>95:5 rr,
>95:5 E/Z, 98% ee), while pyridyl alkyne gave 13 in lower
yield due to low conversion but with excellent regio- and
stereoselectivity (>95:5 rr, >95:5 E/Z, 98% ee). However,
relatively lower E/Z selectivities were observed when using
alkynes with electron-rich aryl substituents (6, 82:18 E/Z, and
8, 83:17 E/Z), which might be caused by corresponding E to Z
isomerization by the energy transfer process (see control
experiments on E/Z isomerization in the Supporting
Information). This low E/Z selectivity can be dramatically
improved by switching to iridium photocatalyst (6 and 8).
With internal dialkyl alkynes as substrates, the corresponding
allylic alcohol products (14−19) were obtained in moderate to
good yields with 96−99% ee. Remarkably, excellent
regioselectivity (>95:5) was obtained with these unsymmetric
internal dialkyl alkynes (15−19). Notably, 2-butyne, the
With the optimal reaction conditions in hand, we then
examined the generality of this enantioselective reductive
coupling (Figure 1). We first investigated the scope of the
alkynes. Using unsymmetrical aryl substituted internal alkynes,
excellent regio- (>95:5) and enantioselectivity (98−99% ee)
were obtained (5−13). Electronic properties have little
influence on the enantioselectivity, as electron-donating (Me,
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a
Figure 1. Reaction scope. All reactions were performed with 0.2 mmol of aldehyde; isolated yield of product with >95:5 rr and >95:5 E/Z if
1
b
otherwise noted; ee was determined by chiral HPLC; E/Z, rr, and dr were determined by crude H NMR. With Ir(ppy)2(dtbbpy)PF6 as the
c
photocatalyst. With 2 (3 equiv), 3 (3 equiv), 36 h.
smallest symmetrical internal alkyne, also reacted well affording
14 in 75% yield with 99% ee, which has a big improvement in
enantioselectivity control compared to the previous Ni-
catalyzed protocol (73% ee).10
Unfortunately, no desired product was obtained with
terminal alkynes and nonmethyl substituted alkynes in this
reaction (for details see Supporting Information).
efficient coupling components affording the chiral allylic
alcohols with good yield and excellent regio- and enantiose-
lectivities (20−37, >95:5 rr, 92−99% ee), exhibiting the best
regio- and enantioselectivity control with the same type of
substrates compared to the reported results.8−10 Here, the mild
reaction conditions are compatible with a wide range of
functional groups on the aryl ring at different sites, including
Me (20 and 21), OMe (22), SMe (23), OCF3 (24),
morpholino (25), phenol (26), electronically biased biphenyls
Next, the scope of the aldehyde coupling partner was also
explored. A variety of aromatic aldehydes were found to be
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(27 and 28), CF3 (33), ester (34), sulfanilamide (36), and
acetal (37). It is noteworthy that the halogen functionalities at
ortho-, meta-, and para-position (29−32) and boronic acid
pinacol ester (35) remain intact after the coupling, furnishing
additional reaction sites for further synthetic elaborations.
Encouragingly, coupling with natural products vanillin (26)
and aldehyde containing drug molecule derivatives (36 and
37) afforded the desired products in high yields with
significant enantioenrichment (97−98% ee) or complete
diastereocontrol (>95:5 dr), indicating the potential applic-
ability of this reaction in late-stage functionalization or
complex targets synthesis. Moreover, coupling with alkyl
aldehydes, including long-chain decanal, isobutyraldehyde,
and natural citronellal, also occurred smoothly leading to the
products (38−42) in good yield with excellent regioselectivity
(>95:5 rr), E/Z control (93:7−>95:5), and good enantio-
(82−87% ee) or diastereoselectivity (10:1 dr).
Scheme 3. Control Experiments
To further demonstrate the synthetic utility of the present
method, diastereoselective reductive coupling of alkyne and
chiral amido aldehyde (R)-43 toward chiral 1,2-amino alcohol
was carried out. Enantioenriched 1,2-amino alcohol (2R,3R)-
44 or (2R,3S)-44 was easily obtained under the standard
conditions with good yield and excellent regio-, E/Z-, and
diastereoselectivity by using commercially available (S,S)-
BDPP or (R,R)-BDPP as the chiral ligand (Scheme 2A).
Next, the 1,2-amino alcohol product 44 was directly converted
into 1,2-diamine 45 and 2-azide amine 46 with good
stereoselectivity through Mitsunobu reaction (Scheme 2B).
Scheme 2. Synthetic Applications
Next, control experiments were conducted to understand the
mechanism of this reaction. First, radical capture studies
showed that addition of 2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO) as the radical scavenger inhibits the reaction while
addition of ethene-1,1-diyldibenzene does not inhibit the
reaction (Scheme 3A). However, we did not detect any radical
capturing adduct in both cases. Considering that TEMPO may
not solely act as a radical scavenger and may inhibit the
reaction through another avenue, these result suggested that
the reaction may not involve formation of free radical
intermediate. We detected trace amounts of benzyl alcohol
and alkyne semihydrogenation product during screening of the
reaction conditions (Scheme 3B). Semihydrogenation of
alkynes 47 was achieved in the absence of aldehyde with the
same catalytic system (Scheme 3C). These results suggested
that cobalt hydride species might be generated to a certain
extent in the reaction. No reductive coupling product was
detected with allene 49 as the coupling component, which
ruled out the process in which alkyne participates in the
reaction as an allene precursor (Scheme 3D).31 In addition, a
variety of deuterium scrambling experiments have been carried
out with heavy water (D2O), deuterated HE d1-3, d2-3, d3-3,
d8-THF, or deuterated alkyne substrate d3-2 (Scheme 3E−J).
The results clearly indicate the transfer of hydrogen at both the
1- and 4-postion of HE into the alkenyl hydrogen of product
and suggest that an intermediate proton might exist in this
reaction. Moreover, a series of Stern−Volmer fluorescence
quenching experiments were performed to further understand
the mechanism. We found that the emission intensity of the
excited 4CzIPN* was dramatically diminished in the presence
of HE 3 rather than other components, thereby indicating that
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Scheme 4. Plausible Reaction Pathways
electron transfer between 4CzIPN* and HE was more favored
(for details see Supporting Information).
lective synthesis of 1,2-amino alcohols. Further application and
mechanistic clarification of this strategy are currently ongoing
in our laboratory.
Based on the control experiments, a preliminary mechanism
is proposed in Scheme 4. Upon irradiation with visible light,
ox
photoexcited 4CzIPN* accepts one electron from HE [E1/2
ASSOCIATED CONTENT
■
(HE/HE•+) = 0.51 V vs Fc+/Fc in MeCN]32 to afford
sı
* Supporting Information
4CzIPN•− [E1/2 (4CzIPN*/4CzIPN•−) = −1.21 V vs SCE
red
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c03527.
in MeCN],33 which can reduce ligand coordinated Co(II)
complex [E1/2red (CoII/CoI)= −0.75 V vs SCE in MeCN]26a to
low valent Co(I) species via single electron transfer (SET).25b
Then, the coordination of alkyne at the equatorial position and
aldehyde with its Re face at the axial position of the Co(I)
center followed by oxidative cyclization gives oxacobalta-
cyclopentene intermediate INT1 (Scheme 4, Path A).34 The
following protonation by HE•+ produces the allylic cobalt
alkoxide INT2 and neutral HEH radical.26a Subsequently, the
strong electron donor HEH radical (Eox = −1.13 V versus
SCE) can donate one electron to photoexcited 4CzIPN* to
afford pyridinium ion PyH+ and 4CzIPN•−,35 which reduce
Co(III)-alkoxide to Co(II)-alkoxide to achieve the second
photocatalytic cycle.36 The final protolysis by PyH+ would
deliver the corresponding allylic alcohol product. The
corresponding pyridine compound (HP) has been isolated as
a byproduct. Alternatively, we cannot exclude another reaction
pathway involving addition of aldehyde with alkenyl cobalt
species from hydrometalation of alkyne with cobalt(III)
hydride, which is generated from oxidation of Co(I) species
via proton reduction of HE•+ under photoredox conditions
(Scheme 4, Path B).25b
Experimental procedures, characterizations and analyt-
ical data of new compounds; and spectra of NMR and
HPLC for new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Ji-Bao Xia − State Key Laboratory for Oxo Synthesis and
Selective Oxidation, Center for Excellence in Molecular
Synthesis, Suzhou Research Institute of LICP, Lanzhou
Institute of Chemical Physics (LICP), Chinese Academy of
Sciences, Lanzhou 730000, China; orcid.org/0000-0002-
2262-5488; Email: jibaoxia@licp.cas.cn
Authors
Yan-Lin Li − State Key Laboratory for Oxo Synthesis and
Selective Oxidation, Center for Excellence in Molecular
Synthesis, Suzhou Research Institute of LICP, Lanzhou
Institute of Chemical Physics (LICP), Chinese Academy of
Sciences, Lanzhou 730000, China; University of Chinese
Academy of Sciences, Beijing 100049, China
In summary, we have disclosed the first synergistic visible
light photoredox Co-catalyzed highly regio- and enantiose-
lective reductive coupling of alkynes and aldehydes toward
chiral allylic alcohols under mild conditions. This convenient
method uses commercially available catalyst, chiral ligand, and
reagents, delivering enantioenriched allylic alcohols with
excellent regio- (>95:5), E/Z- (up to >95:5), and enantiose-
lectivity (up to 99% ee). Remarkably, excellent regioselectivity
has been achieved with unsymmetical internal dialkylalkynes as
substrates. The broad scope and good functionality tolerance
of this novel method have been applied in the diastereose-
Shi-Qi Zhang − State Key Laboratory for Oxo Synthesis and
Selective Oxidation, Center for Excellence in Molecular
Synthesis, Suzhou Research Institute of LICP, Lanzhou
Institute of Chemical Physics (LICP), Chinese Academy of
Sciences, Lanzhou 730000, China; University of Chinese
Academy of Sciences, Beijing 100049, China
Jie Chen − State Key Laboratory for Oxo Synthesis and
Selective Oxidation, Center for Excellence in Molecular
Synthesis, Suzhou Research Institute of LICP, Lanzhou
Institute of Chemical Physics (LICP), Chinese Academy of
Sciences, Lanzhou 730000, China
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Communication
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Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c03527
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support from the
National Natural Science Foundation of China (21772208,
21702212, 21633013), Natural Science Foundation of Jiangsu
Province (BK20201183), and the Key Research Program of
Frontier Sciences of CAS (QYZDJSSW-SLH051).
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NOTE ADDED AFTER ASAP PUBLICATION
The figure of Ligand, Photocat. and reductant was added to
Table 1 on May 6, 2021.
■
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J. Am. Chem. Soc. 2021, 143, 7306−7313