Acceptorless Dehydrogenation of Hydrocarbons by Noble-Metal-Free Hybrid Catalyst System

Article abstract of DOI:10.1021/acs.orglett.8b00583

A hybrid catalysis that comprises an acridinium photoredox catalyst, a thiophosphate organocatalyst, and a nickel catalyst-enabled acceptorless dehydrogenation of hydrocarbons is reported. The cationic nickel complex played a critical role in the reactivity. This is the first example of acceptorless dehydrogenation of hydrocarbons by base metal catalysis under mild reaction conditions of visible light irradiation at room temperature.

Full text of DOI:10.1021/acs.orglett.8b00583

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Acceptorless Dehydrogenation of Hydrocarbons by Noble-Metal-
Free Hybrid Catalyst System
Hiromu Fuse,† Masahiro Kojima,† Harunobu Mitsunuma,† and Motomu Kanai*,†
†Graduate School of Pharmaceutical Sciences, The University of Tokyo, Tokyo 113-0033, Japan
S
* Supporting Information
ABSTRACT: A hybrid catalysis that comprises an acridinium
photoredox catalyst, a thiophosphate organocatalyst, and a nickel
catalyst-enabled acceptorless dehydrogenation of hydrocarbons is
reported. The cationic nickel complex played a critical role in the
reactivity. This is the first example of acceptorless dehydrogenation
of hydrocarbons by base metal catalysis under mild reaction
conditions of visible light irradiation at room temperature.
atalytic acceptorless dehydrogenation (CAD) of saturated
Calkyl motifs in hydrocarbons to produce unsaturated C−
C bonds and hydrogen gas is an attractive transformation in
organic synthesis, and a technological foundation for a potential
future hydrogen society.1 However, because of the unfavorable
enthalpy factor, the CAD of hydrocarbons generally requires
harsh conditions, such as a high temperature (∼200 °C) or UV
light irradiation, and noble-metal catalysts, such as iridium2 or
rhodium3 complexes.
Based on previous reports of the combined use of a
photoredox catalyst and a nickel catalyst in C−C bond
formation,11,12 we envisioned that nickel would be a suitable
element to substitute for palladium. Our mechanistic blueprint
for CAD of tetrahydronaphthalene (1a) as a model substrate is
illustrated in Figure 1.8,13 Visible-light irradiation of a
Hybrid catalyst systems having a photoredox catalyst4 as a
component recently emerged to facilitate CAD of organic
compounds, including hydrocarbons and N-heterocycles, under
milder conditions.5 Sorensen’s group pioneered the base metal-
catalyzed acceptorless dehydrogenation of hydrocarbons at
room temperature using a combination of decatungstate and
cobaloxime catalysts.6 However, the disadvantages of this
catalytic system are its requirement for near-UV light
irradiation, low product yield, and insufficient substrate
generality. Li reported that merging Ru(bpy)3Cl2·6H2O and
cobaloxime catalysts promoted acceptorless dehydrogenation of
N-heterocycles at room temperature.7 We simultaneously
developed the first example of CAD of hydrocarbons that
proceeded under visible-light irradiation at room temperature,
devising a three-component hybrid catalysis that comprises an
organo-photoredox catalyst, a hydrogen atom transfer (HAT)
organocatalyst, and a palladium catalyst.8 Specifically, we
identified a unique combination of acridinium photoredox
catalyst 79 and thiophosphoric imide (TPI) to generate a HAT-
active thiyl radical. This reaction exhibited relatively broad
functional group tolerance, because of the mild reaction
conditions. Considering the chemical foundation underlying a
hydrogen society as a long-term goal, however, a critical
drawback of our reaction is the requirement for the rare and
noble-metal element palladium. Thus, we were motivated to
achieve CAD using a base metal catalyst10 in place of the
palladium catalyst. Herein, we report a nickel-catalyzed CAD of
hydrocarbons that proceeded under visible-light irradiation at
room temperature.
Figure 1. Working hypothesis for CAD of 1a.
photoredox catalyst (PC+) generates a long-lived photoexcited
state (*PC+). This excited photoredox catalyst oxidizes the
organocatalyst (RSH) to produce a HAT-active thiyl radical
(RS•)14,15 and a proton (H+). Then, the thiyl radical abstracts a
benzylic hydrogen atom of substrate 1a, affording benzyl radical
3. A nickel catalyst (NiII) would intercept radical 3 to generate
an organonickel species 4 bearing a high valent state (NiIII).11
The organonickel 4 is then reduced by a photoredox catalyst
acting as a reductant (PC), affording organonickel 5 bearing a
low oxidation state (NiII). This reduction would facilitate β-
hydride elimination from 5 to produce unsaturated dihydro-
naphthalene (6) and nickel hydride species NiII−H. Finally,
hydrogen gas evolves through the reaction between NiII−H and
a proton16 generated in the photooxidation step of the
Received: February 17, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00583
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Letter
Table 1. Optimization of CAD of Tetrahydronaphthalene: Effects of Metal Complex Catalyst, Photoredox Catalyst, and
Organocatalyst
a
entry
photoredox catalyst
organocatalyst
metal complex catalyst
2a (%)
1
2
7
TPA
TPA
NiCl2·6H2O
NiCl2·6H2O+dtbpy
NiBr2·dme
<1
<1
34
5
7
3
7
TPA
4
7
TPA
Nil2
5
7
TPA
Ni(OAc)2·4H2O
Ni(BF4)2·6H2O
Ni(NTf 2)2·xH2O
Fe(BF4)2·6H2O
Co(BF4)2·6H2O
Cu(BF4)2·xH2O
Ni(NTf2)2·xH2O
Ni(NTf2)2·xH2O
Ni(NTf2)2·xH2O
Ni(NTf2)2·xH2O
Ni(NTf2)2·xH2O
Ni(NTf2)2·xH2O
Ni(NTf2)2·xH2O
Ni(NTf2)2·xH2O
−
5
6
7
TPA
52
88
3
7
7
TPA
8
7
TPA
9
7
TPA
3
10
11
12
13
14
15
16
17
18
19
20
7
TPA
4
Ru(bpy)3(PF6)2
TPA
N/D
<1
21
1
(Ir[dF(CF3)ppy]2(dtbpy))PF6
TPA
7
7
7
7
7
−
7
7
7
TPl
quinuclidine
Bu4N+PhCO2
−
6
SA
PA
3
3
TPA
TPA
−
2
<1
3
Ni(NTf2)2·xH2O
Ni(NTf2)2·xH2O
b
21
TPA
N/D
a
b
Yield is based on GC analysis. Without photoirradiation.
organocatalyst RSH. Repeating this cycle from 6 produces 2a
with a net generation of 2 M equivalents of hydrogen gas from
1a.
under milder conditions than nickel halide catalysts.18
Computational studies indicate that β-hydride elimination is
accelerated by using a cationic nickel complex.17a In the present
reaction, the use of a Ni(BF4)2 catalyst led to a moderate yield
of 2a (52%; see entry 6 in Table 1). The yield was further
improved to 88% when Ni(NTf2)2 was used (Table 1, entry 7).
Other first-row transition-metal catalysts did not promote
efficient dehydrogenation (see Table 1, entries 8−10).
A survey of photoredox catalysts revealed that only
acridinium salt 7 promoted the reaction (Table 1; entry 7 vs
entries 11 and 12), indicating that the high oxidation potential
of the excited state of 7 is critical in this reaction. We next
studied the effects of organocatalysts for HAT (Table 1, entries
13−17). Our previous study using a palladium catalyst as a
component revealed that TPI was the most effective organo-
Based on this hypothesis, we conducted optimization studies
of the reaction conditions using 5 mol % 7, 5 mol %
thiophosphoric acid (TPA), and 2.5 mol % metal complex
catalyst with 430 nm visible-light irradiation from a light-
emitting diode (LED) at room temperature (Table 1). The
reaction did not proceed to a significant extent when nickel
halides and acetate were used as the metal complex catalyst
(Table 1, entries 1−5). In our efforts to improve the efficiency,
we considered the following previous observations. In nickel-
catalyzed Heck reactions, β-hydride elimination is assumed to
be slower than palladium catalysis.17 Jamison and Skrydstrup
found that cationic nickel catalysts promote the Heck reaction
B
DOI: 10.1021/acs.orglett.8b00583
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Letter
catalyst.8 In the nickel-catalyzed system, however, the yield
diminished when TPI was used (Table 1, entry 13).
Quinuclidine,13,14e tetrabutylammonium benzoate,14f,g a sulfo-
namide (SA),14h and a phosphate (PA) were also ineffective
(see Table 1, entries 14−17).
To confirm that the ternary hybrid catalytic system is
essential for the CAD process, we performed several control
experiments (Table 1, entries 18−21). Almost no reactions
proceeded when the catalytic system lacked any one of the
three catalyst components: the photoredox catalyst (entry 18 in
Table 1), the metal complex catalyst (entry 19 in Table 1), and
the organocatalyst (entry 20 in Table 1). Furthermore, no
reaction proceeded without photoirradiation (entry 21 in Table
1). Thus, all three types of catalysts and photoirradiation are
indispensable for this CAD. We also confirmed a 2 M
equivalent evolution of hydrogen gas relative to 2a, based on
a two-pot transfer hydrogenation experiment.19
pinacolatoboryl group afforded the corresponding naphthylbor-
onic acids in high yield without C−B bond cleavage. CAD of
dihydronaphthalene (6) proceeded in excellent yield, giving 2a.
This result indicates that 6 is the intermediate of this reaction,
and the release of two-molar hydrogen gas from 1a proceeds in
a stepwise fashion (Figure 1). Dehydrogenation of 3-
methylcyclohexene (1o) proceeded in 50% yield, which is a
significantly higher yield than that under the previous
conditions using a palladium catalyst (17%).8 In addition,
dehydrogenation of 4-methylcyclohexene (1p) and phenyl-
cyclohexane (1q) proceeded in moderate yield, suggesting that
the current conditions will be applicable to substrates
containing molecular skeletons with a higher hydrogen content.
However, the present conditions did not promote the CAD of
alkanes, such as cyclohexane and methylcyclohexane.
In summary, we developed a ternary hybrid catalysis that
comprises an acridinium photoredox catalyst, a thiophosphoric
acid organocatalyst, and a cationic nickel catalyst for accept-
orless dehydrogenation of hydrocarbons. This is the first
example of base metal-catalyzed acceptorless dehydrogenation
of hydrocarbons under mild conditions of visible-light
irradiation at room temperature. Further improvements in the
efficiency and substrate scope are ongoing.
Under the optimized conditions, we examined the substrate
scope (Scheme 1). Tetrahydronaphthalenes 1b−1e bearing a
methyl substituent at different positions produced an excellent
yield. A phenyl, a halogen atom, and an ester substituent were
tolerated (1f−1l). Interestingly, substrates 1m and 1n with a
a
Scheme 1. Substrate Scope of CAD of Hydrocarbons
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.8b00583.
Experimental details and characterization data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kanai@mol.f.u-tokyo.ac.jp.
ORCID
Motomu Kanai: 0000-0003-1977-7648
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by the JSPS KAKENHI Grant
No. JP17H06442 (M. Kanai) (Hybrid Catalysis). M. Kojima
thanks the Graduate Program for Leaders in Life Innovation
(GPLLI) and JSPS for the fellowships.
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D
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