Selective photoredox decarboxylation of α-ketoacids to allylic ketones and 1,4-dicarbonyl compounds dependent on cobaloxime catalysis

Article abstract of DOI:10.1039/d0cc05580h

A photoredox/cobaloxime co-catalyzed coupling reaction of α-ketoacids and methacrylates to obtain allylic ketones is described. Without the cobaloxime catalyst, 1,4-dicarbonyl compounds are generated. The cobaloxime catalyst enables dehydrogenation to generate the formation of new olefins. The generality, good substrate scope and mild conditions are good features in the photoredox/cobaloxime catalysis protocol, and this method will provide new opportunities for the functionalization of more olefins.

Full text of DOI:10.1039/d0cc05580h

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DOI: 10.1039/D0CC05580H
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Selective Photoredox Decarboxylative of α-Ketoacids to Allylic
Ketones and 1,4-Dicarbonyl Compounds Depending on
Cobaloxime Catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Hong Zhang,a Qian Xiao,a Xu-Kuan Qi,a Xue-Wang Gao,b Qing-Xiao Tonga and Jian-Ji Zhong*a
DOI: 10.1039/x0xx00000x
A photoredox/cobaloxime cocatalyzed coupling of α-ketoacids and employed in path b of Scheme 1, the new alkyl radical could be
methacrylates to allylic ketones is described. Without cobaloxime captured by cobaloximes to generate the transient Co(III)-C
catalyst, 1,4-dicarbonyl compounds are generated. Cobaloxime intermediate, subsequent β-H elimination would result in the
catalyst enables a dehydrogenation to new olefins formation. The formation of new olefins.8
generality, good substrate scope and mild conditions features the
photoredox/cobaloxime catalysis protocol, which will provide new
opportunities for more olefins functionalization.
Hydrogen Abstraction
R1
(a)
R2
R2
or Reductants
R1
R1
+
R2
R2
Oxidants
R1
(b)
Scheme 1. Reaction types afterwards radical addition.
The addition of carbon radicals to alkenes is acknowledged as a
powerful and straightforward strategy for constructing C-C
bonds.1 In this respect, photoredox catalysis2 serving as an
environmental and efficient approach to initiate the generation
of radicals has witnessed a dramatic development in the last
decade.3 Conventionally, the radical generated in situ could be
trapped by olefins to give the new alkyl radical species (Scheme
1). Afterwards, two main reaction pathways might happen.
Hydrogen abstraction of the new alkyl radical or termination
with reductants will result in the generation of alkane products
(Scheme 1, a). And for the formation of new olefins, sacrificial
oxidants are usually required (Scheme 1, b). However, the use
of external oxidants would result in the stoichiometric amounts
of chemical waste. Recently, Wu and coworkers4 have
developed a photoredox/cobaloxime co-catalyzed strategy for
hydrogen evolution cross-coupling reactions.5 Cobaloximes as a
mimics of coenzyme B12 have been extensively investigated.6
Therein cobaloximes are used as catalysts to capture the
electrons and protons eliminated from the substrates to
generate H2 gas as the sole byproduct.7 It represents an ideal
atom-economic strategy for C-C bond formation from C-H/C-H
bonds. Inspired by this, we believe that if cobaloximes are
O
Photocatalysis &
O
(a)
(b)
R2
O
R1
Cobaloxime Catalysis
O
OH
O
+
R2
O
R1
O
O
O
Photocatalysis
R2
R1
O
Scheme 2. Cross-coupling of α-ketoacids and methacrylate derivatives
(this work).
Allylic ketones are versatile and widely applicable building
blocks for synthesis of complex pharmaceutical molecules and
natural products. In view of their importance, the
methodologies for the synthesis have attracted much attention
involving the reactions of various allylic organometallics with
acyl halides,9 or transition-metal catalyzed reactions of pre-
functionalized allylic precursors and acyl precursors.10 Despite
the considerable progress in this field, the pre-functionalization
of substrates has limited the application of these
methodologies. In our design, one simple and straightforward
approach to access allylic ketones is direct addition of acyl
radicals to allylic substrates. In recent years, the
decarboxylation of α-ketoacids by photoredox catalysis to
generate acyl radical for synthesis of alkyl ketones has been
extensively investigated.11 However, the photoredox acylation
of olefins to unsaturated ketones without adding any external
oxidants has rarely been reported. In this work, we would like
to report a photoredox/cobaloxime cocatalyzed strategy for the
synthesis of allylic ketones by direct benzoyl radical addition to
methacrylate derivatives (Scheme 2, a). In case that without
cobaloxime catalyst in the same reaction, the corresponding
a. Department of Chemistry and Key Laboratory for Preparation and Application of
Ordered Structural Materials of Guangdong Province, Shantou University, and
Chemistry and Chemical Engineering Laboratory of Guangdong Province,
Guangdong 515063, P. R. China. Email: jjzhong@stu.edu.cn
b. Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing
100190, P. R. China.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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alkyl ketone products of 1,4-dicarbonyl compounds were entry 11). t-BuOK replacing 2,6-lutidine VimewaAdrteicle Otnhlinies
generated (Scheme 2, b).
transformation more feasible with 89% yDieOldI: 1(0T.a10b3le9/1D,0eCnCt0r5y51820H).
We next investigated the generality and limitations of this
approach. Various α-ketoacids were examined, and the
corresponding allylic ketones were produced in moderate to
good yields (Table 2, c1-c20). The aromatic α-ketoacids with
electron-donating groups or electron-withdrawing groups at
different positions on the phenyl moiety (ortho, meta and para
positions) were found to be compatible substrates in this
reaction. While when strong electron-withdrawing groups such
as -CF3 and -NO2 were introduced, the reaction efficiency would
decrease sharply (Table 2, c18, c21). Meanwhile, steric effect
seemed not to be neglected since the reaction efficiency
decreased when the ortho-position of the substrates was
substituted (Table 2, c3 62%, c5 55%, c6 45%, c12 46% and c17
42%). It is noteworthy that halo-substituted substrates were
tolerated well, which provides opportunities for the products to
further transformations (Table 2, c10-c17). Naphthyl and
heterocyclic α-ketoacids could also undergo this reaction to give
moderate yields of 50% and 42% respectively (Table 2, c19,
c20). Unfortunately, no reaction was observed when the
aliphatic α-ketoacids were used (Table 2, c22, c23). Notably, a
10 mmol scale reaction could undergo smoothly to give c1 in
65% yield (SI, Scheme S1). For alkyl ketones synthesis, these
substrates were also examined to produce the corresponding
1,4-dicarbonyl compounds in moderate to good yields in the
absence of cobaloxime catalyst (Table 2, d1-d21). Nevertheless,
some substrates, e.g. d5, d12, d17, d18 and d21 were
ineffective in this transformation probably due to the steric
hindrance effect or electronic effect.
Table 1. Optimization of the reaction conditionsa
O
O
5% 4-CzIPN
20% Co(dmgBF2 2(H2O)2
O
OH
)
O
O
O
+
O
O
1.5 equiv. 2,6-lutidine
O
O
CH3CN, 450 nm LEDs, r.t., N2
a1
c1
b1
d1
H2
O
Cl
F
Me
N
Me
O
Me
F
B
O
O
H
N
N
N
N
Me
F
NC
N
CN
O
Co
O
O
N
N
Co
N
N
H
O
Me
Me
N
N
Me
O
B
N
Me
F
H2
O
Co(dmgBF2
)
2(H2O)2
Co(dmgH)2PyCl
4-CzIPN
Entr
y
Variation from the “standard reaction
Yield (%)b
conditions”
None
c1
d1
--
85
1
Ru(bpy)3(PF6)2 as a photocatalyst
Acr+-MesClO4- as a photocatalyst
CH2Cl2 as a solvent
0
--
--
2
0
3
0
--
4
DMF as a solvent
0
--
5
CH3OH as a solvent
trace
37
47
51
54
--
--
6
No 2,6-lutidine
--
7
t-BuOK as a base
trace
--
8
4-methylpyridine as a base
Co(dmgH)2PyCl as a catalyst
No Cobaloxime catalyst
No Cobaloxime catalyst, t-BuOK as a base
9
--
10
11
12c
17
89
--
aReaction conditions for allylic ketones synthesis: a1 (0.2 mmol), b1 (0.6
mmol), 4-CzIPN (5 mol%), Co(dmgBF2)2(H2O)2 (20 mol%), 2,6-lutidine (0.3
mmol), CH3CN (5 mL), under N2, room temperature, 450 nm LEDs irradiation
for 16~24 h. bIsolated yield. cReaction conditions for alkyl ketones synthesis:
a1 (0.2 mmol), b1 (0.4 mmol), 4-CzIPN (5 mol%), t-BuOK (0.2 mmol), CH3CN
(5 mL), under N2, room temperature, 450 nm LEDs irradiation for 8~12 h.
Table 2. Scope of α-ketoacidsa
O
O
OH
O
standard conditions
O
+
R1
O
R1
O
O
O
O
b1
Our initial study focused on the cross-coupling of
phenylglyoxylic acid (a1) and ethyl methacrylate (b1). As shown
in Table 1, after examining a series of reaction parameters, the
standard reaction conditions were identified. Treatment of a1
with b1 in the presence of 5 mol% 4-CzIPN, 20 mol%
Co(dmgBF2)2(H2O)2 and 1.5 equivalents of 2,6-lutidine in
deaerated CH3CN under 450 nm LEDs irradiation for 16 h at
room temperature selectively gave the allylic ketone c1 in 85%
yield (Table 1, entry 1). Other photocatalysts such as
a
c or d
O
O
O
O
O
O
O
O
O
O
c1b: 85%, d1: 89%
c3: 62%, d3: 40%
c4: 82%, d4: 58%
c2: 77%, d2: 80%
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
c7: 73%, d7: 54%
c8: 70%, d8: 61%
c6: 45%, d6: 72%
c5: 55%, d5: trace
O
O
Cl
O
O
O
O
O
O
O
Ru(bpy)3(PF6)2 and Acr+-MesClO4 were investigated, they were
-
O
O
O
Ph
Cl
F
F
c9: 76%, d9: 41%
c11: 85%, d11: 45%
c12: 46%, d12: trace
c10: 83%, d10: 60%
found to be ineffective in this reaction (Table 1, entry 2 and 3).
The solvent test proved that CH3CN was the best choice (Table
1, entry 4-6). Two main factors we speculated might be critical
to the solvent effect: 1) the solubility of the substrate and the
cobaloxime catalyst, 2) the coordinated ability of the solvent.
No base or other bases addition resulted in less effective than
that of 2,6-lutidine (Table 1, entry 7-9). The reaction yield
O
O
O
O
O
O
Br
O
O
O
O
O
O
Br
Cl
c15: 65%, d15: 62%
c16: 77%, d16: 75%
c14: 86%, d14: 88%
c13: 63%, d13: 65%
O
O
I
O
O
O
O
O
O
O
F3C
c18: 50%, d18: trace
c19: 50%, d19: 43%
c17: 42%, d17: trace
O
O
O
O
O
decreased to 54
%
when Co(dmgH)2PyCl instead of
O
O
O
O
O
S
O
NO2
O
Co(dmgBF2)2(H2O)2 was used (Table 1, entry 10). Control
experiments indicated that photocatalyst, cobaloxime catalyst
and base were essential to the synthesis of allylic ketones (Table
S1). Further studies found that when cobaloxime catalyst was
removed in the reaction, the alkyl ketone 1,4-dicarbonyl
compound d1 was formed, but just with a 17% yield (Table 1,
c23: --, d23: --
c22: --, d22: --
c20: 42%, d20: 38%
c21: trace, d21: trace
aReaction conditions for allylic ketones synthesis c: a (0.2 mmol), b1 (0.6
mmol), 4-CzIPN (5 mol%), Co(dmgBF2)2(H2O)2 (20 mol%), 2,6-lutidine (0.3
mmol), CH3CN (5 mL), under N2, room temperature, 450 nm LEDs irradiation
for 16~24 h. For alkyl ketones synthesis d: a (0.2 mmol), b1 (0.4 mmol), 4-
2 | J. Name., 2012, 00, 1-3
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CzIPN (5 mol%), t-BuOK (0.2 mmol), CH3CN (5 mL), under N2, room inhibited the reaction and the TEMPO-trapped benzoyl
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temperature, 450 nm LEDs irradiation for 8~12 h. bIsolated yield.
product was generated in 69% yield, indicating that a benzoyl
radical is involved in this reaction (Scheme 3, a). When d1
instead of a1 and b1 was added into the solution in standard
conditions, no reaction was observed, suggesting that the allylic
ketone c1 was generated from β-H elimination of transient Co-
C intermediate rather than direct dehydrogenation of alkyl
ketone (Scheme 3, b). We further investigated the kinetic
isotope effect (KIE) of benzyl methacrylate (b34), the
intermolecular KIE value of 1.56 indicated that the methyl C-H
elimination was fast and did not involve in the rate-determining
step in this reaction (Scheme 3, c). Since for some substrates
which are not sufficient enough such as a5 (c5, 55%) and a6 (c6,
45%), the corresponding aldehydes were observed (Scheme S2),
we speculated that the acyl radical addition to alkenes might be
involved in the rate determining step.
DOI: 10.1039/D0CC05580H
We further explored the variety of methacrylate derivatives.
To our delight, the methacrylate derivatives bearing different
kinds of groups tolerated well in this transformation, including
various linear or branch aliphatic chain groups, or benzyl groups
with electron-donating or electron-withdrawing substituents
(Table 3, c24-c39). Furan-2-ylmethyl methacrylate also
proceeded smoothly to give the corresponding allylic ketones in
53% yield (Table 3, c38). Note that when allyl methacrylate
underwent this reaction, the selective addition of benzoyl
radical to the double bond of methylallyl rather than that of allyl
led to the corresponding allylic ketone (Table 3, c39). Other
alkenes were also investigated in the reaction, however, we
found that the scope of alkenes was limited to methacrylate
derivatives (Table 3, b40-d45). Furthermore, the substrate
scope of alkyl ketones synthesis was also investigated without
use of cobaloxime catalyst, good to excellent yields could be
achieved as shown in Table 3 (d24-d39).
O
O
O
standard conditions
O
OH
O
N
(a)
+
O
+
2.0 equiv. TEMPO
O
O
O
trace
69%
O
O
5% 4-CzIPN
Table 3. Scope of methacrylate derivativesa
O
20% Co(dmgBF2
)
2(H2O)2
O
(b)
O
1.5 equiv. 2,6-lutidine, CH3CN
450 nm LEDs, 16 h, r.t., N2
O
O
O
c1
, not detected
d1
, starting material remaining
O
standard conditions
OH
O
R2
+
R2
O
O
O
b
c or d
a1
O
Ph
(H)D D(H)
O
5% 4-CzIPN
O
O
20% Co(dmgBF2)2(H2O)2
O
O
O
OH
O
1.5 equiv.
Ph
(c)
+
O
O
O
O
1.5 equiv. 2,6-lutidine, CH3CN
6 h, 32%
O
O
450 nm LEDs, r.t., N2
,
O
Ph
O
O
D3
C
1.0 equiv.
kH/kD = 1.56
c24b: 76%, d24: 85%
c25: 81%, d25: 85%
c26: 80%, d26: 78%
O
1.5 equiv.
O
O
O
Scheme 3. Reaction mechanism investigations.
O
O
O
O
O
O
O
O
O
base
c27: 81%, d27: 87%
c28: 78%, d28: 88%
c29: 68%, d29: 81%
CO2Et
radical addition
Co(II)
COO
-CO2
a1
CO2Et
A
O
O
O
O
O
O
O
O
O
O
Co(III)
CO2Et
4-CzIPN
c30: 62%, d30: 80%
c31: 71%, d31: 72%
c32: 82%, d32: 90%
B
O
R'
*4-CzIPN
O
R' = H, c34: 62%, d34: 67%
R' = Me, c35: 87%, d35: 84%
Cobaloxime
Catalysis
Photoredox
Catalysis
O
O
O
R' = OMe, c36: 86%, d36: 92%
R' = Cl, c37: 55%, d37: 65%
O
O
c33: 76%, d33: 67%
h
Co(III)
H2
H
O
O
O
CO2Et
O
reductive
byproduct
O
4-CzIPN
Co(III)-H
O
b1
O
c39: 72%, d39: 88%
c38: 53%, d38:70%
Scheme 4. Proposed reaction pathway for allylic ketones synthesis.
Unsuccessful Substrates:
O
O
H
Based on these above results and literature reports, a
plausible reaction pathway is proposed for allylic ketones
synthesis (Scheme 4). The excited *4-CzIPN is reductively
quenched by the anion of a1 leading to benzoyl radical with CO2
extrusion, which was detected by gas chromatography, and the
reductive species 4-CzIPN-. The redox potentials from previous
reports12 also support the electron transfer from the anion of
a1 (0.98 V vs SCE) to the excited 4-CzIPN (1.35 V vs SCE). Benzoyl
radical addition to the double bond of ethyl methacrylate
generates a new alkyl radical species A, which could be captured
by Co(II) to form transient Co(III)-C intermediate. The cleavage
of Co-C and subsequent β-H elimination result in the final
product allylic ketone c1 and Co(III)-H. According to previous
reports,4-6 a plausible pathway of Co(III)-H reacting with a
proton or another Co(III)-H species to generate H2 and Co(III)
might happen. Indeed, detection of H2 by GC-TCD confirmed
this possibility. However, only 23% of H2 was detected,
N
N
Ph
B
Ph
O
CN
O
O
O
b40
b41
b42
b43
b44
b45
aReaction conditions for allylic ketones synthesis c: a1 (0.2 mmol), b (0.6
mmol), 4-CzIPN (5 mol%), Co(dmgBF2)2(H2O)2 (20 mol%), 2,6-lutidine (0.3
mmol), CH3CN (5 mL), under N2, room temperature, 450 nm LEDs irradiation
for 16~24 h. For alkyl ketones synthesis d: a1 (0.2 mmol), b (0.4 mmol), 4-
CzIPN (5 mol%), t-BuOK (0.2 mmol), CH3CN (5 mL), under N2, room
temperature, 450 nm LEDs irradiation for 8~12 h. bIsolated yield.
To gain insights into the reaction mechanism, more
experiments were carried out. Quenching experiments showed
that a1 just slightly quenched the emission of 4-CzIPN, however,
addition of a1 with base 2,6-lutidine could sharply decrease the
luminescence (Figure S1), revealing that the photoinduced
electron transfer from the anion of a1 to 4-CzIPN is more
feasible than a1. Furthermore, we found that a radical inhibitor
(2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)) completely
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J. Name., 2013, 00, 1-3 | 3
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revealing that herein other pathways to generate Co(III) could
also happen. Due to that the excess of methacrylate is
necessary in the reaction, we speculated b1 might compete
with proton to react with Co(III)-H to generate some reductive
byproduct and Co(III). Subsequent single electron transfer from
reductive 4-CzIPN- to Co(III) regenerates both photoredox
catalytic cycle and cobaloxime catalytic cycle. A plausible
mechanism for alkyl ketones synthesis is also described in
Scheme S3. Without cobaloxime catalyst, the new alkyl radical
species A is directly reduced by the reductive 4-CzIPN- to alkyl
anion species C. Protonation of C furnishes the alkyl ketones.
In summary, we have described a photoredox/cobaloxime
cocatalyzed dehydrogenative cross-coupling of α-ketoacids and
methacrylate derivatives for the synthesis of allylic ketones.
Interestingly, the products could be selectively achieved by
controlling the catalytic system. When the cobaloxime catalyst
was removed, the corresponding alkyl ketones rather than
allylic ketones were produced via photoredox catalysis. Herein,
cobaloxime catalyst enables a dehydrogenation to new olefins
formation. The generality, good substrate scope and mild
reaction conditions features the dual photoredox/cobaloxime
catalysis protocol. We believe that this protocol will provide
opportunities for more radical-involved olefin functionalization
reactions, which is undergoing in our laboratory.
4
5
6
7
The authors gratefully acknowledge for the financial support
from the National Natural Science Foundation of China
(21801163, 21702213), STU Scientific Research Foundation for
Talents (NTF18003) and the Guangdong Province Universities
and Colleges Pearl River Scholar Funded Scheme 2019
(GDUPS2019).
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Conflicts of interest
There are no conflicts to declare.
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O
OH
DOI: 10.1039/D0CC05580H
R1
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Photocatalysis
Photocatalysis &
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R2
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R1
R1
Cobaloxime Catalysis
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R2
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H2O
F
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Selective products formation
Non-noble metal catalysis
Mild reaction conditions
F
B
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N
N
Me
F
NC
N
CN
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N
Co
N
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H2O
Good substrate scope
Co(dmgBF2)2(H2O)2
4-CzIPN
A selective synthesis of allylic ketones and 1,4-dicarbonyl compounds by photoredox/cobaloxime
cocatalysis and photoredox catalysis respectively is described herein.