Cis-Selective Decarboxylative Alkenylation of Aliphatic Carboxylic Acids with Vinyl Arenes Enabled by Photoredox/Palladium/Uphill Triple Catalysis

Article abstract of DOI:10.1021/acs.orglett.8b00712

An iridium photoredox catalyst in combination with phenanthroline-supported palladium catalyst catalyzes decarboxylative alkenylation of tertiary and secondary aliphatic carboxylic acids with vinyl arenes to deliver β-alkylated styrenes with Z-selectivity. A broad scope of aliphatic carboxylic acids, including amino acids, exhibit as amenable substrates, and external oxidant is not required. The reaction proceeds by synergistic utilization of both energy-transfer and electron-transfer reactivity of iridium photoredox catalyst merging with palladium-catalyzed hydride elimination and insertion.

Full text of DOI:10.1021/acs.orglett.8b00712

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cis-Selective Decarboxylative Alkenylation of Aliphatic Carboxylic
Acids with Vinyl Arenes Enabled by Photoredox/Palladium/Uphill
Triple Catalysis
Chao Zheng,†,‡,∥ Wan-Min Cheng,†,∥ Hong-Lian Li,† Ri-Song Na,*,† and Rui Shang*,§
†Collaborative Innovation Center of Henan Grain Crops, National Key Laboratory of Wheat and Maize Crop Science, College of
Plant Protection, Henan Agricultural University, Wenhua Road No. 95, Zhengzhou 450002, China
‡Key Laboratory of Tropical Medicinal Plant Chemistry of Ministry of Education, Collaborative Innovation Center of Tropical
Biological Resources, Hainan Normal University, Haikou, Hainan 571158, China
§Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113- 0033, Japan
S
* Supporting Information
ABSTRACT: An iridium photoredox catalyst in combination
with phenanthroline-supported palladium catalyst catalyzes
decarboxylative alkenylation of tertiary and secondary aliphatic
carboxylic acids with vinyl arenes to deliver β-alkylated styrenes
with Z-selectivity. A broad scope of aliphatic carboxylic acids,
including amino acids, exhibit as amenable substrates, and external
oxidant is not required. The reaction proceeds by synergistic
utilization of both energy-transfer and electron-transfer reactivity of iridium photoredox catalyst merging with palladium-
catalyzed hydride elimination and insertion.
hotoredox catalysis has experienced an increase in use in
Porganic synthesis by utilizing either its electron-transfer or
energy-transfer reactivity.1 The recent development of tran-
sition-metal-catalyzed cross-coupling reactions2 also signifi-
cantly benefited from merging transition-metal-catalyst with
photoredox catalyst. Among various types of cross-couplings,
the methodology of transition-metal-catalyzed decarboxylative
cross-couplings3 is arguably most significantly improved by
Figure 1. Hypothetical mechanism for Z-selective decarboxylative
judicious merging with photoredox catalyst to work synergisti-
cally.4 As elegantly developed by MacMillan and co-workers,5
nickel catalysis merging with photoredox catalysis by using
either its electron-transfer or energy-transfer properties enables
a series of unachievable cross-coupling reactions6 (e.g.,
decarboxylative alkylation,7 challenging nickel-catalyzed C−O,
C−N formation8) to take place under very mild conditions. As
part of our continuous interest in utilizing biomass-derived
carboxylic acids as feedstock in cross-coupling reactions,9 we
wondered whether free aliphatic carboxylic acids could be
utilized in a Heck process10 to deliver alkylated styrene
derivatives with Z-selectivity. This unprecedented process is
very challenging by using transition-metal catalysis in its ground
state, because of the high energy-barrier for redox neutral
decarboxylation11 and the thermodynamically favored E-
selectivity of olefin products. We conceived a palladium/
iridium dual catalytic system12 to address this challenging issue
as depicted in Figure 1.
Heck reaction of free aliphatic carboxylic acids with vinyl arenes by
merging Ir photocatalysis and palladium catalysis.
in concert with a palladium catalyst, which traps the
photoredox-generated alkyl radical and alkene to generate a
benzylic palladium species, followed by oxidative regeneration
of photoredox catalyst to complete a photoredox cycle. Then,
β-H elimination on benzylic palladium species may proceed to
deliver E-alkene and generates a palladium hydride species. If
the same photoredox catalyst can work as a triplet sensitizer to
sensitize E-alkene to the triplet state, inducing uphill catalysis to
isomerize E-alkene to Z-alkene,13 this process can be used to
directly achieve decarboxylative cis-alkyl Heck reaction in one
catalytic system. However, to balance all these processes
together to work synergistically in one catalytic system is of
considerable challenge. Here, we show that by judicious
selection of palladium catalyst, ligand, and photoredox catalyst,
Z-selective decarboxylative alkyl Heck reaction between free
aliphatic carboxylic acid and vinyl arene proceeds smoothly at
Figure 1 demonstrates our working hypothesis. A photo-
redox catalyst with suitable oxidation potential can catalyze
radical decarboxylation of aliphatic carboxylic acid to deliver
alkyl radical. The photoredox catalyst is hypothesized to work
Received: March 2, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00712
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Letter
room temperature. This reaction not only presents a novel
example of decarboxylative Heck reaction using free aliphatic
carboxylic acids as alkyl donor, demonstrating a general case of
intermolecular alkyl Heck process with high Z-selectivity, but
also demonstrates the potential of merging photoredox catalysis
with palladium catalysis for solving challenges in ground-state
palladium-catalyzed cross-coupling reactions.
We demonstrated the optimized reaction condition and
structures of the optimal catalysts in the top equation in
Scheme 1. A transparent Schlenk tube charged with Ir[dF-
yield with a high Z/E ratio of 96/4 (Scheme 1, entry 1). The
key parameters that control the reaction outcomes are listed in
Scheme 1.
The reaction did not proceed at all in the absence of
irradiation (Scheme 1, entry 2). Control experiments also
revealed that iridium photoredox catalyst, palladium salts, and
TMP ligand are all essential for the reaction to proceed (entries
3−5). Screening the palladium catalysts revealed that Pd(OAc)2
was the optimal catalyst and other palladium salts such as
PdCl2, Pd(OTFA)2, and Pd2(dba)3 were less effective,
delivering alkene product in lower Z/E ratio (entries 6−8),
probably due to the isomerization of Z- alkene to E- alkene
catalyzed by palladium catalyst.16 Exposure to oxygen entirely
killed this reaction, probably due the reaction of triplet oxygen
with the excited photoredox catalyst or a reactive intermediate
(such as alkylpalladium species) (entry 9). It should be noted
that this reaction requires oxidant, but there is no external
oxidant in the reaction mixture. It was revealed a 1 equiv excess
of vinyl arene worked as hydrogen acceptor to generate ethyl
arene. Testing other chemical oxidants together with a reduced
amount of vinyl arene either showed reduced efficiency or was
entirely unproductive (entries 10 and 11). Judicious selection of
supporting ligand for palladium catalyst and the selection of
photoredox catalyst are key for the successful development of
this reaction. The results of the ligand effect and screen of
different photoredox catalysts are demonstrated at the bottom
of Scheme 1. Among various ligands tested, phosphine ligands
were all much less effective compared with 1,10-phenanthro-
line. Testing other bidentate nitrogen ligands revealed the
backbone structure heavily affects the catalyst efficacy, as
evidenced by the entire ineffectiveness of bisoxazoline ligands
and bipyridine ligand. The substituent on 1,10-phenanthroline
also significantly influenced the catalyst efficacy. Substitution at
the 2,9-position to increase the steric hindrance on palladium
catalyst is not beneficial for this reaction. TMP was proved to
be the optimal ligand. As for the photoredox catalyst, based on
our working hypothesis in Figure 1, we believe that suitable
redox potential, singlet−triplet energy level (ET), and
excitation-state lifetime are all important factors to determine
its efficiency. Ir[dF(CF3)ppy]2(dtbbpy)PF6 (Ir-cat.) worked as
the best photocatalyst due to its suitable redox potential
Scheme 1. Parameters Controlling Z-Selective
Decarboxylative Heck Reaction
III/II
III/II
(E1/2*
= +1.21 V, E1/2
= −1.37 V),1b matched triplet
energy level with styrene (61 kcal/mol, and ET ∼ 60 kcal/mol
for styrene),14 and long excitation-state lifetime (τ = 2300
ns).1b Other photoredox catalysts such as Ir(ppy)2(dtbbpy)PF6
(a), fac-Ir(ppy)3 (b), and Ru(bpy)3(PF6)2 (c) were totally
ineffective to give the decarboxylative Heck type product,
probably because of their weak oxidation potentials that are
ineffective to oxidize carboxylate anion [E1/2(*M/M−) = +0.66
V, +0.31 V, and +0.77 V, respectively].1b An organophotoredox
catalyst, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4-
Cz-IPN), of suitable redox potential [E1/2(*4Cz-IPN/4Cz-
IPN−) = +1.35 V, E1/2(4CzIPN−/4Cz-IPN) = −1.21 V] and
excitation-state lifetime (τ = 5.1 μs),15 was, however, ineffective,
probably due to its incompatibility to merge with palladium
catalyst under the reaction conditions.
The substrate scope of this reaction is demonstrated in
Scheme 2. A variety of tertiary and secondary carboxylic acids
were amenable substrates to give Z-type alkene products in
good yield with high Z/E selectivity. α-Amino acids were also
tolerable to deliver Z-type allylic amine derivatives as valuable
synthetic intermediates. Various functional groups such as ether
(2, 8), amide (6), aryl halide (10, 12, 24), ester (7, 16),
a
The Z/E ratio determined by 1H NMR analysis. Isolated yield of Z-1
b
was 75%. 0.3 mmol of 4-fluorostyrene was used.
(CF3)ppy]2(dtbbpy)PF6 (Ir-cat.) (2 mol %), Pd(OAc)2 (5 mol
%), 3,4,7,8-tetramethyl-1,10-phenanthroline (TMP, 6 mol %),
1-methylcyclohexane-1-carboxylic acid (0.2 mmol), 4-fluoros-
tyrene (0.5 mmol, 2.5 equiv), and K2HPO4 (0.5 mmol) in
chlorobenzene solvent was exposed under irradiation of 36 W
blue LEDs at room temperature (1−2 cm distance from the
LED source). After irradiation for 24 h at room temperature,
the desired decarboxylative Heck product was detected in 80%
B
DOI: 10.1021/acs.orglett.8b00712
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Letter
crucial for the Z/E selectivity. Several unsuccessful examples are
also listed at the bottom of Scheme 2. A triarylamine moiety
was not tolerated in this reaction, due the high reducing ability
of triarylamine that may quench the excited triplet photoredox
catalyst (34). Vinyl mesitylene was unreactive due to the large
steric hindrance that prevents coordination with palladium
catalyst (35). Primary aliphatic carboxylic acids and α-alkoxy
aliphatic carboxylic acids were poorly reactive under the
optimized reaction conditions (36, 37), recovering large
amount of alkenes and carboxylic acids after the reaction,
probably due to the low efficiency of decarboxylation.
Scheme 2. Substrate Scope of Ir/Pd-Catalyzed Z-Selective
Decarboxylative Heck Reaction
a
Control experiments to confirm the origin of the Z-selectivity
are shown in Scheme 3. As demonstrated in Scheme 3, eq 1, the
Scheme 3. Control Experiments Confirming the Origin of Z-
Selectivity
E-styrene derivative successfully isomerized under the optimal
reaction conditions in the presence of iridium photoredox
catalyst and palladium catalyst. The iridium photocatalyst alone
can also catalyze the E to Z isomerization with comparable
efficiency13 (Scheme 3, eq 2). However, the palladium catalyst
alone showed no effect on isomerization of the E-alkene
(Scheme 3, eq 3). This set of control experiments
unambiguously proved the iridium photosensitizer is the key
catalyst to determine the Z/E selectivity. Although the
literature reported that a palladium(II) catalyst can catalyze
the Z to E isomerization of alkenes,16 the palladium catalyst
under our optimized condition did not cause any deterioration
of the Z/E selectivity under irradiation.
One of the intriguing factors of this reaction is that oxidant is
not required in this oxidative coupling type reaction and ethyl
arene was generated as side product, suggesting vinyl arene
works as both coupling partner and hydrogen acceptor. We
performed a deuterium-labeling experiment using 4-(vinyl-2,2-
d2)-1,1′-biphenyl as substrate (Scheme 4). An almost equally
molar amount of 4-ethylbiphenyl was generated with desired
coupling product. 1H NMR analysis of the isolated product and
byproduct revealed the deuterium distribution. In the alkene
a
Reaction conditions: aliphatic carboxylic acids (0.2 mmol), alkene
(0.5 mmol), Ir-cat. (2.0 mol %), Pd(OAc)2 (5.0 mol %), TMP (6.0
mol %), K2HPO4 (0.5 mmol), PhCl (2.0 mL), irradiated by 36 W blue
LEDs for 24 h under Ar. Yields of isolated Z/E mixtures, Z/E ratio
determined by 1H NMR analysis. Yields of isolated Z-isomer are
b
shown in parentheses. Reaction performed on a 1.0 mmol scale.
sulfonamide (11), trifluoromethyl (13), trifluoromethoxy (23),
cyano (26), and boronate (29) were all well tolerated. Testing
the scope of vinyl arenes revealed various electron-rich and
electron-deficient styrene substrates were all amenable to
generate alkene product in moderate to good yields with Z-
selectivity ranging from 92/8 to 99/1. Pentafluorostyrene was
also suitable substrate to deliver Z-product in high yield (18).
However, for 2-vinyl naphthalene, the Z/E ratio of the product
was much deteriorated (31). This low Z/E selectivity (46:54)
of 2-vinyl naphthalene can be explained by the triplet energy
(ET) of naphthalene (60.5 kcal/mol), which is almost the same
compared with 2-alkyl styrene.14 Thus, the naphthalene ring
will affect the triplet sensitization step, which is proposed to be
Scheme 4. Isotope-Labeling Experiment Revealing Pd−H
Insertion
C
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Letter
product, the deuterium was not incorporated in the α-position
of the double bond, while only 63% of deuterium remained on
the β-position. The reduced deuterium percentage on the β-
position of alkene product may be related to palladium-
catalyzed C−H activation of terminal olefin that leads to H/D
scrambling.17 However, we found deuterium was incorporated
into the benzylic position of the 4-ethylbiphenyl side product
(23% D incorporation), while only 36% of D remained on the
terminal methyl. The deuterium incorporation suggests that a
process of Pd−H insertion to styrene generating phenethyl
palladium species followed by protonation to deliver ethyl-
benzene is involved in the use of styrene as H-acceptor to
regenerate palladium(II) catalyst.
in the catalytic cycle in Figure 2 that recent studies by
Gevorgyan and our group revealed that palladium species can
also be excited by irradiation of blue LEDs,18 so that we should
notice the palladium complexes mentioned in this mechanism
are excited by irradiation to be distinct in reactivity from its
ground state under thermal conditions.18b,e
In summary, unprecedented Z-selective decarboxylative Heck
reactions of secondary and tertiary aliphatic carboxylic acids
with vinyl arenes are achieved by merging palladium/
phenathroline catalyst with iridium photocatalyst under
irradiation. The amenable substrates of this reaction encompass
a broad scope of tertiary and secondary aliphatic carboxylic
acids as well as natural and unnatural α-amino acids. The
catalyst system enabling this unprecedented transformation
features its unique property that merges a palladium catalyst
with iridium photoredox catalyst, where both electron-transfer
and energy-transfer properties of the photoredox catalyst are
utilized synergistically.
On the basis of the above studies, a proposed mechanism is
demonstrated in Figure 2 to rationalize this novel reaction.
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.8b00712.
Experimental procedures and spectroscopic data (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: four_leafclover@163.com.
*E-mail: rui@chem.s.u-tokyo.ac.jp.
Figure 2. Proposed mechanism.
ORCID
First, the Ir-photoredox catalyst in its excited triplet state is
reductively quenched by carboxylate anion to generate alkyl
radical.4a The alkyl radical is intercepted by styrene and
phenathroline-supported Pd(II) catalyst to generate a benzylic
Pd(III) species (B). The benzylic Pd(III) species oxidizes the
Ir(II) to regenerate Ir(III) and to complete the photoredox
cycle.12a β-H elimination proceeds on the benzylic Pd(II)
species (C) to deliver E-olefin product and Pd(II)−H species
(D). The Pd−H species inserts into styrene to generate
phenethyl Pd(II) (E) followed by protonation to give
ethylbenzene and regenerated Pd(II) catalyst (A). Simulta-
neously, the excited Ir-photoredox catalyst in its triplet state
sensitizes E-olefin product to its triplet state through energy
transfer. This uphill catalysis isomerizes E-olefin to its
thermodynamically less favored Z-isomer. We mention in the
catalytic cycle that Pd−H species (D) reacts with styrene to
generate phenethyl Pd(II) intermediate (E) followed by
protonation. This step is supported by the substantial
deuterium incorporation into the benzylic position of the
generated ethyl arene byproduct (Scheme 4). We cannot rule
out the possibility of Pd−H insertion to form a more stable
benzyl-Pd species (E′, arrow in gray). However, Pd−H
insertion steps (from D to E and E′) are reversible. Phenethyl
Pd(II) intermediate (E) of lower stability undergoes irreversible
protonation to deliver ethyl arene to complete the catalytic
cycle. We did not detect a significant amount of alkyl arene
generated from protonation of intermediate C, suggesting
benzylic palladium intermediate of higher stability does not
undergo protonation. This further supports the fact that ethyl
arene byproduct is formed through protonation of phenethyl
Pd(II) intermediate (E). It is notable although not mentioned
Rui Shang: 0000-0002-2513-2064
Author Contributions
∥C.Z. and W.-M.C. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Special Fund for Agro-scientific Research in the
Public Interest (No.201503112), the National Natural Science
Foundation of China (Nos. 21602043, 21702048, and
21702039), and Program for Innovative Research Team in
University (IRT-16R19).
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DOI: 10.1021/acs.orglett.8b00712
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