Construction of a visible light-driven hydrocarboxylation cycle of alkenes by the combined use of Rh(i) and photoredox catalysts

Article abstract of DOI:10.1039/c7cc00678k

A visible light driven catalytic cycle for hydrocarboxylation of alkenes with CO₂ was established using a combination of a Rh(i) complex as a carboxylation catalyst and [Ru(bpy)₃]²⁺ (bpy = 2,2′- bipyridyl) as a photoredox catalyst. Two key steps, the generation of Rh(i) hydride species and nucleophilic addition of π-benzyl Rh(i) species to CO₂, were found to be mediated by light.

Full text of DOI:10.1039/c7cc00678k

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DOI: 10.1039/C7CC00678K
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Construction of a Visible Light-Driven Hydrocarboxylation Cycle of
Alkenes by the Combined Use of Rh(I) and Photoredox Catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
Kei Murata, Nobutsugu Numasawa, Katsuya Shimomaki, Jun Takaya and Nobuharu Iwasawa*
DOI: 10.1039/x0xx00000x
www.rsc.org/
photocatalytic cycle for hydrocarboxylation of alkenes using
CO2 by the combined use of catalytic amounts of a Rh(I)
hydrocarboxylation catalyst and a photoredox catalyst.
A visible light driven catalytic cycle for hydrocarboxylation of
alkenes with CO2 was established using a combination of Rh(I)
complex as a carboxylation catalyst and [Ru(bpy)3]2+ (bpy = 2,2’-
bipyridiyl) as a photoredox catalyst. Two key steps, the generation
of the Rh(I) hydride species and nucleophilic addition of the π-
benzyl Rh(I) species to CO2, were found to be mediated by light.
Catalytic hydrocarboxylation of unsaturated hydrocarbons
with CO2 has recently attracted considerable attention as a
method to utilize CO2 as a carbon source for preparation of
carboxylic acids from easily available materials.1,2 This type of
reaction usually necessitates the use of a stoichiometric
amount of organometallic reductant such as ZnEt2,1a-d,2a,b
AlEt3,2c or hydrosilanes2d,e to generate active metal hydride
species from the corresponding metal carboxylates. Therefore
it is highly desirable to develop a new system, which does not
necessitate the use of a strong reductant and enables
hydrocarboxylation reaction just using catalytic amounts of
organometallic reagents.
Scheme 1 Proposed reaction mechanism for the photocatalytic
hydrocarboxylation of alkenes with CO2 by Rh/photoredox
catalysis.
To realize such system, we thought of the possibility of
utilizing redox-photosensitized reaction with visible light. By
the appropriate combination of a hydrocarboxylation catalyst
and a photoredox catalyst in the presence of a sacrificial
electron donor, it is expected that the key metal hydride
species could be regenerated from the corresponding metal
carboxylate by the successive protonation and reduction
processes triggered by photoinduced electron transfer.3
Recently, the single-electron transfer event has been widely
used in organic synthesis,4 and some dual catalytic systems
combining transition metal and photoredox catalysts have also
been developed.5 However, there has been no report on their
application to carboxylation reactions with CO2, which could
be regarded as a kind of artificial photosynthesis for the
utilization of solar energy for preparation of organic molecules
Our proposed catalytic cycle for hydrocarboxylation
mediated by photoredox catalysis is shown in Scheme 1. We
have chosen Rh(I) hydride complex A as the key catalytic
species, as organorhodium(I) species are expected to have
7
sufficient nucleophilicity towards CO2 and some Rh(I)
complexes are known to be active in redox-photosensitized
reactions.3c,8 Hydrometallation of an alkene, followed by
nucleophilic carboxylation of the generated Rh(I) alkyl complex
B with CO2 would give Rh(I) carboxylate complex C. The next
step is the regeneration of the Rh(I) hydride complex A from C
with liberation of the carboxylated product by accepting two
electrons and two protons, which is the most challenging step
in the cycle. We expected that photoredox catalyst would
enable the electron and proton transfers to generate Rh(III)
dihydride D, which would be converted to Rh(I) monohydride
A by eliminating the carboxylic acid in the presence of a base.
using CO2.6 We herein describe
a novel cooperative
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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complex 8, formed by insertion of 4-cyanostyrene 7a to
(a)
[Ru(bpy)3](PF6)2 (30 mol%)
View Article Online
PCy3
iPr2NEt (60 equiv.)
Rh(PPh3)3H 5, was subjected to CO2 atmosphere,
DOI: 10.1039/C7CC006t7h8eK
Cy3P
Cy3P
O
O
H
H
O
Rh
carboxylation reaction did not proceed in the dark even upon
Rh
O
o
heating at 60 C.§§ However, when 8 was exposed to visible
DMA, hv (425 nm)
PCy3
2 (94%)
1
r.t., 12 h
light in the presence of [Ru(bpy)3](PF6)2, clean formation of
Rh(I) carboxylate 9 was observed (Figure S3), which was
confirmed by alternative synthesis of 9. This phenomenon
could be regarded as a light-promoted nucleophilic addition to
CO2, which is a quite rare process.10 Control experiments
revealed that [Ru(bpy)3]2+ and visible light were essential but
tertiary amine was not for this carboxylation reaction. The
luminescence quenching study demonstrated that the
emission from the triplet excited state of [Ru(bpy)3]2+ was
quenched by 8 (Figure S5). As the electron transfer pathway is
not likely in this reaction,§§§ the triplet-triplet energy transfer
from the photoexcited [Ru(bpy)3]2+ is considered to be a
trigger for the carboxylation with CO2.11
(b)
[Ru(bpy)3](PF6)2
(30 mol%)
iPr2NEt (60 equiv.)
PPh3
PPh3 (1.5 equiv.)
Ph3P
Ph3P
O
O
H
H
O
Rh
(PPh3)3Rh
H
Rh
O
DMA, hv (425 nm)
!CH3COOH
PPh3
iPr2NEt
4
5 (57%)
3
r.t., 6 h
Scheme 2 Photochemical generation of the rhodium hydride
complexes.
We first examined transformation of Rh(I) carboxylate to
Rh(I) hydride by photoredox catalysis, which is the key to
construct the proposed catalytic cycle. Through screening of
reaction conditions using Rh(PCy3)2(OAc) 1 as a model complex
of Rh(I) carboxylate, 1 was found to be transformed to
To estimate the effect of the energy transfer in the
carboxylation reaction, theoretical analysis was conducted in
terms of the ground state (S0) and lowest triplet excited state
(T1) of 8 which would be generated in the photochemical
process. The TD-DFT result indicated that metal-to-(metal-
ligand) charge transfer from the Rh center to the Rh-benzyl
unit has major contribution to the T1 state (Table S4). This
transition caused (i) the change in coordination mode of the
benzyl ligand from η3-benzyl to η1-benzyl form as indicated in
the optimized structure of the T1 state, and (ii) increase in the
electron density of the benzylic position (Table S3). These
changes would promote the subsequent nucleophilic addition
to CO2.
1
Rh(PCy3)2(OAc)(H)2 2 (94% yield based on H NMR using an
internal standard) in the presence of a catalytic amount of
[Ru(bpy)3](PF6)2 as a photoredox catalyst, an excess amount of
iPr2NEt as a sacrificial electron donor under visible light
irradiation (λirr = 425 nm) at room temperature (Scheme 2
(a)).‡ Control experiments indicated that [Ru(bpy)3](PF6)2,
tertiary amine, and visible light were all essential for the
transformation. Furthermore, when Rh(PPh3)2(OAc) 3 was
employed instead of 1 under the same reaction conditions in
the presence of PPh3 as an additional ligand, 3 was successfully
transformed to the desired Rh(PPh3)3H 5 (57% yield based on
1H NMR using an internal standard and ca 30% of 3 recovered),
accompanied by liberation of acetic acid as its ammonium salt
(Scheme 2 (b)). Rh(PPh3)2(OAc)(H)2 4 was thought to be a
possible intermediate in this transformation,§ and it is likely
that due to lower electron-donating ability of PPh3 compared
to PCy3, elimination of acetic acid from 4 was facilitated by the
action of iPr2NEt. Although detailed mechanism for the
formation of Rh(III) dihydride complex has not been clarified, it
would be generated by the two sets of protonation and single
electron transfer to Rh(I) carboxylate complex mediated by the
reductive quenching cycle of [Ru(bpy)3]2+ with iPr2NEt (Scheme
S6).9
As all the elementary steps of the catalytic cycle were
found to be feasible, we then examined the catalytic reaction.
Using 4-cyanostyrene 7a as substrate, the combination of
Rh(PPh3)3H 5 and [Ru(bpy)3](PF6)2 was found to give the
desired hydrocarboxylated product 11a in 33% yield upon
visible light irradiation (λirr. = 425 nm) in the presence of an
i
excess amount of Pr2NEt under atmospheric pressure of CO2
at room temperature (Table 1, entry 1). In this reaction,
significant amounts of hydrogenated product 12a and
polymers were also produced. When the catalytic reaction was
followed by 31P{1H} NMR spectroscopy, the accumulation of 9
was observed, suggesting the resting-state of the catalytic
reaction under visible light irradiation was the Rh(I)
carboxylate complex (Figure S4).
A series of control
The Rh(I) monohydride formation was similarly observed
experiments confirmed that the rhodium catalyst, photoredox
catalyst, tertiary amine and visible light were all essential for
the reaction progress (entries 2 – 5). Wilkinson’s complex 6
and chloro-bridged bis(phosphine) dimer 10a were found to be
applicable as a precatalyst (entries 6, 7).
when Wilkinson’s complex Rh(PPh3)3Cl 6 was irradiated with
visible light in the presence of [Ru(bpy)3](PF6)2 and Pr2NEt,
indicating Rh(I) hydride active species was also available from
a chloro Rh(I) complex as a catalyst precursor (Scheme S3).
i
CO2 (1 atm)
Table
1
Screening of catalytic conditions for the
[Ru(bpy)3](PF6)2
(30 mol%)
NC
(5 equiv.)
7a
hydrocarboxylation of 4-Cyanostyrene by Rh/photoredox
catalysis.
Ar
(PPh3)3Rh
H
(PPh3)3Rh
O
(PPh3)2Rh
O
DMA, dark
r.t., < 10 min
! PPh3
hv (425 nm)
5
8
9
r.t., 30 min
CN
+ PPh3
Scheme 3 Photochemical carboxylation of the π-benzyl
rhodium complex.
Next, the nucleophilic addition of Rh(I) alkyl complex,
which is another important step in the hydrocarboxylation
cycle, was investigated (Scheme 3). When π-benzyl Rh(I)
2 | J. Name., 2012, 00, 1-3
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Entry Rh catalyst
COMMUNICATION
mixed gas of 13C-labelled CO2 and non-labelled CO (Scheme S5)
View Article Online
Conv. Yield /%
/%
confirmed that this reaction did not proceed via carbonylation
DOI: 10.1039/C7CC00678K
11aa 12ab
reaction associated with release of CO by reduction of CO2,
which could also give the same carboxylation product.12 This
hydrocarboxylation reaction was also applicable to other
styrenes possessing an electron-withdrawing group. Although
the yields became considerably lower as the substrate became
less electron-deficient, the conversion yields were generally
1
Rh(PPh3)3H (5)
60
8
33
n.d. n.d.
n.d.
18
2c
3d
4e
5
Rh(PPh3)3H (5)
Rh(PPh3)3H (5)
1
1
good.
Other
electron-deficient
alkenes
such
as
methacrylonitrile and an alkyl acrylate also gave the
corresponding hydrocarboxylated product (Table 2).
Rh(PPh3)3H (5)
<1
12
56
60
95
70
80
34
n.d. n.d.
n.d. n.d.
none
Table 2 Photocatalytic hydrocarboxylation of other electron-
6
Rh(PPh3)3Cl (6)
31
21
54
67g
45
11
13
16
25
1
deficient alkenes by Rh/photoredox catalysis.a
7
[Rh(PPh3)2Cl]2 (10a)
[Rh(P(4-CF3C6H4)3)2Cl]2 (10b)
[Rh(P(4-CF3C6H4)3)2Cl]2 (10b)
[Rh(P(4-CNC6H4)3)2Cl]2 (10c)
CO2 (1 atm, closed)
[Rh(4-CF3C6H4)2Cl]2 10b (3.5 mol%)
8
[Ru(bpy)3](PF6)2 (2 mol%)
iPr2NEt (4 equiv.)
R'
COOH
R'
9f
10
11
H+
Cs2CO3 (1.2 equiv.)
R
R
12
3
DMA, hv (425 nm), r.t., 24 h
7
11
COOH
COOH
[Rh(P(3,5-(CF3)2C6H3)3)2Cl]2
F3C
COOH
COOH
(10d)
R''
(n-hexyl)O2C
NC
12
13
[Rh(P(p-tolyl)3)2Cl]2 (10e)
[Rh(PCy3)2Cl]2 (10f)
32
22
14
6
3
7
CF3
11d
11e
11f
R'' = COPh (11b) : 37% (37%)b
CO2Me (11c) : 26% (23%)b
50% (34%)b
46% (36%)c
40%
aNMR yield. GC yields of the hydrogenated byproduct were
aNMR yield. GC yield. without [Ru(bpy)3](PF6)2. without
b
c
d
b
<1% for all entries. isolated yield after methyl esterification.
iPr2NEt. under dark. 1.2 equiv. of Cs2CO3 was added.
gIsolated yield (62%) was obtained after methyl
esterification in a larger scale experiment, see the
supporting Information for detail.
e
f
cisolated yield after acid-base extraction.
The redox-photosensitized reaction with multi-electron
transfer processes has been used for CO2 reduction to organic
feedstock such as CO, formic acid and methanol as well as
hydrogen generation reactions.13 Although single electron
transfer by photoredox catalyst has been used actively in
organic synthesis, application of such multielectron transfer
processes to other C-C bond forming reactions has been quite
limited, and the reaction with CO2 has not been reported yet.
In addition to the photoinduced electron transfer, activation of
the Rh(I) benzyl complex by photoinduced energy transfer
enabled the carboxylation reaction. The combination of
different types of activation of the Rh catalyst by light energy
via photoredox catalyst is the key to construct such a novel
photocatalytic cycle.
Then various combinations of rhodium and photoredox
catalysts were examined to improve the yield of the desired
product 11a. With regard to its ancillary ligand,
triarylphosphines possessing electron-withdrawing groups
significantly improved the yield (entries 8, 10). This reactivity
enhancement was likely due to the acceleration of the
reduction process of the Rh(I) carboxylate, which was the
resting-state of the catalytic cycle. When PCy3 was introduced
to the rhodium center, only a small amount of the
carboxylated product 11a was obtained (entry 13), which
coincided with the result shown in Scheme 2(a). Thus, fine-
tuning of the electronic nature of the rhodium complex by
ancillary ligands was quite important for the catalytic efficiency.
Choice of photoredox catalyst, solvent, and ratio of the
rhodium/photoredox catalyst also affected the product
distribution as well as the reaction rate (Tables S1, S2). Very
importantly, addition of a small excess amount of Cs2CO3
almost completely suppressed the hydrogenation and
polymerization reactions and significantly improved the yield
of the hydrocarboxylated product (entry 9). Through these
optimizations, employment of 2.0 mol% of [Ru(bpy)3](PF6)2, 3.5
mol% of [Rh(P(4-CF3C6H4)3)2Cl]2 10b (7.0 mol% as Rh metal), 4
equiv. of iPr2NEt and 1.2 equiv. of Cs2CO3 in DMA gave the best
yield of 11a (67%, TON = 9.6 based on Rh). The quantum yield
of this reaction under the optimized conditions was 0.0036
(Figure S6). The catalytic reaction upon on-and-off irradiation
demonstrated the repeatable switching of the catalytic activity
with the high photoresponsivity (Figure S7). Selective
formation of the 13C-labelled product in the reaction using a
In conclusion, the visible light-driven catalytic
hydrocarboxylation of alkenes with CO2 was achieved for the
first time by using the Rh(I) hydride complex generated by
photoredox catalysis. Each elementary step of the proposed
catalytic cycle was found to be feasible and visible light energy
was required for the multiple steps. In contrast to the previous
related systems which required more than stoichiometric
amounts of highly reactive metallic species, this strategy
realized a reaction which employs only catalytic amounts of
two metal complexes. These results would open up a new
possibility in the field of CO2 fixation chemistry.
This research was supported by an ACT-C program from JST
and JSPS KAKENHI Grant Number JP15H05800, JP24245019.
Notes and references
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‡ Gaseous H2 was not detected by GC-TCD during the reaction.
§ 5 was found to be in equilibrium with 4 in the presence of
AcOH-iPr2NEt in C6D6 (see Scheme S1, S2). This equilibrium
shifted completely to 5 when the reaction was performed in
DMA.
§§ Decomposition of 8 occurred at higher temperature.
§§§ In the cyclic voltammetry measurement of 8 (generated in
situ by the reaction of 5 with 5 equiv. of 7a), no reduction wave
was observed in the range of -2.0 – 0 V (0.1 M [Bu4N][PF6] / DMF
vs. E (Fc/Fc+)).
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