Cerium(IV) Carboxylate Photocatalyst for Catalytic Radical Formation from Carboxylic Acids: Decarboxylative Oxygenation of Aliphatic Carboxylic Acids and Lactonization of Aromatic Carboxylic Acids

Article abstract of DOI:10.1021/jacs.9b12918

We found that in situ generated cerium(IV) carboxylate generated by mixing the precursor Ce(O^tBu)₄ with the corresponding carboxylic acids served as efficient photocatalysts for the direct formation of carboxyl radicals from carboxylic acids under blue light-emitting diodes (blue LEDs) irradiation and air, resulting in catalytic decarboxylative oxygenation of aliphatic carboxylic acids to give C-O bond-forming products such as aldehydes and ketones. Control experiments revealed that hexanuclear Ce(IV) carboxylate clusters initially formed in the reaction mixture and the ligand-to-metal charge transfer nature of the Ce(IV) carboxylate clusters was responsible for the high catalytic performance to transform the carboxylate ligands to the carboxyl radical. In addition, the Ce(IV) carboxylate cluster catalyzed direct lactonization of 2-isopropylbenzoic acid to produce the corresponding peroxy lactone and ?3-lactone via intramolecular 1,5-hydrogen atom transfer (1,5-HAT).

Full text of DOI:10.1021/jacs.9b12918

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Article
Cerium(IV) Carboxylate Photocatalyst for Catalytic Radical Formation
from Carboxylic Acids: Decarboxylative Oxygenation of Aliphatic
Carboxylic Acids and Lactonization of Aromatic Carboxylic Acids
Satoru Shirase, Sota Tamaki, Koichi Shinohara, Keishi
Hirosawa, Hayato Tsurugi, Tetsuya Satoh, and Kazushi Mashima
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b12918 • Publication Date (Web): 28 Feb 2020
Downloaded from pubs.acs.org on February 28, 2020
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Journal of the American Chemical Society
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Cerium(IV) Carboxylate Photocatalyst for Catalytic Radical
Formation from Carboxylic Acids: Decarboxylative Oxygena-
tion of Aliphatic Carboxylic Acids and Lactonization of Aro-
matic Carboxylic Acids
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†
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‡
,†
Satoru Shirase, Sota Tamaki, Koichi Shinohara, Keishi Hirosawa, Hayato Tsurugi,*
,
‡
,†
Tetsuya Satoh,* and Kazushi Mashima*
†
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan.
‡
Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka
5
58-8585, Japan.
t
ABSTRACT: We found that in situ-generated cerium(IV) carboxylate generated by mixing the precursor Ce(O Bu)
4
with the cor-
responding carboxylic acids served as efficient photocatalysts for the direct formation of carboxyl radicals from carboxylic acids
under blue light-emitting diodes (blue LEDs) irradiation and air, resulting in catalytic decarboxylative oxygenation of aliphatic
carboxylic acids to give C-O bond-forming products such as aldehydes and ketones. Control experiments revealed that hexanuclear
Ce(IV) carboxylate clusters initially formed in the reaction mixture and the ligand-to-metal charge transfer nature of the Ce(IV)
carboxylate clusters was responsible for the high catalytic performance to transform the carboxylate ligands to the carboxyl radical.
In addition, the Ce(IV) carboxylate cluster catalyzed direct lactonization of 2-isopropylbenzoic acid to produce the corresponding
peroxy lactone and γ-lactone via intramolecular 1,5-hydrogen atom transfer (1,5-HAT).
INTRODUCTION
cerium(IV)-alkoxides were photochemically activated via
LMCT to form the corresponding alkoxide radicals as key
Photoredox catalysts (PCs) enhanced by visible light are ideal
synthetic tools to achieve organic transformations under mild
reaction conditions. Various types of transition metal PCs
have been developed, and mainly late transition metal com-
plexes such as ruthenium and iridium complexes are utilized
as photosensitizers of single electron transfer (SET) via metal-
to-ligand charge transfer (MLCT). Despite efficient intersys-
tem crossing of these heavy metal complexes for populating
emissive MLCT states, recent focus has been directed toward
intermediates for β-scission cleavage and hydrogen atom
n
transfer (HAT) using CeCl
3
·7H
2
O/ Bu
4
NCl as a catalyst sys-
5
tem (Scheme 1B). During our study of oxidative transfor-
mation of organic compounds catalyzed by cerium clusters
with dioxygen as the oxidant, we found that the catalyst sys-
8
t
tem of Ce(O Bu)
4
and carboxylic acids in toluene spontaneous-
1
ly produced cerium(IV) carboxylate compounds including
oxo-cerium(IV) carboxylate clusters, Ce (OH) (OCOR)12
6
O
4
4
,
as the main species, which worked as efficient photocatalysts
for decarboxylative oxygenation of aliphatic carboxylic acids
as well as lactonization of 2-isopropylbenzoic acid via 1,5-
HAT and cyclization under atmospheric pressure of air
2
the use of earth-abundant metals with simple ligands as PCs.
Among them, cerium compounds have attracted special inter-
est because of their potential application as PCs for generating
targeted radicals through homolysis of metal-ligand bonds
promoted by ligand-to-metal charge transfer (LMCT) from the
(
Scheme 1D): recently, König et al. reported photo-induced
decarboxylative hydrazination of carboxylic acids in the pres-
ence of CeCl ·7H O and inorganic base Cs CO , a similar
ligands to empty 4f orbital of cerium(IV) lying in the visible
3
2
2
3
3-7
region.
It is noteworthy that the LMCT of cerium com-
precursor to Zuo’s system, where in situ-generated decarbox-
ylated carbon radicals reacted with di-tert-butyl azodicarbox-
pounds favors the direct activation of organic ligands such as
alkoxides and carboxylates bound to the cerium center to give
the corresponding alkoxide and carboxyl radicals, respectively,
although MLCT of noble metal complexes assists an intermo-
lecular SET to generate alkoxide radicals from alcohols in the
presence of Brønsted bases as well as carboxyl radicals from
6
ylate to give hydrazines (Scheme 1C).
1
c,d
N‐(acyloxy)phthalimides (NHPI-esters).
Pioneering work demonstrating the photocatalytic behavior
of cerium compounds was reported by Sheldon and Kochi in
1
968, who found that cerium(IV) pivalate was photochemical-
ly activated to form the corresponding pivalate radical, result-
ing in a mixture of tert-butyl alcohol, tert-butyl hydroperoxide,
and acetone via decarboxylative oxygenation upon bubbling
4
dioxygen (Scheme 1A). Recently, Zuo et al. reported that
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Scheme 1. Cerium-catalyzed Transformations of Carbox-
ylic Acids and Alcohols via LMCT
as Ce(OH)
4
gave 4-fluorobenzyl hydroperoxide (4a) as anoth-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
er decarboxylative C-O bond-forming product in 6% yield
9
(entry 9). Cerium(III) compounds, including Ce(OAc)
and CeCl ·7H O, showed lower or no catalytic activity (entries
0 and 11). The addition of Cs CO (10 mol%) to
CeCl ·7H O improved the conversion of 1a to give 4a in 64%
yield (entry 12), and the addition of Bu
3 2
·H O
3
2
1
2
3
3
2
6
n
4
NCl (25 mol%) re-
5
sulted in a 44% yield of 2a and 20% yield of 3a (entry 13).
Photocatalytic activity of iron complexes for decarboxylation
was previously demonstrated by Sugimori et al. and Jin et al.,
respectively: we thus applied Fe(OAc)
tive oxygenation under the same reaction conditions, giving
lower yield of C-O bond-forming products (entry 14).
10
2
for this decarboxyla-
0
1
2
3
4
5
6
7
8
9
0
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2
3
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2
3
4
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6
7
8
9
0
Table 1. Optimization of Reaction Conditionsa,b,c
yield (%) of C-O bond
entry
catalyst
conditions
forming products
2a/3a)
(
t
1
2
3
Ce(O Bu)
4
4
4
-
>99 (86/14)
n.d.
t
Ce(O Bu)
light-shielded
under Ar
blue LEDs
(410-490 nm)
green LEDs
450-580 nm)
red LEDs
(570-670 nm)
anhydrous toluene
t
d
Ce(O Bu)
n.d.
e
t
4
5
6
Ce(O Bu)
4
99 (82/17)
74 (48/26)
n.d.
f
t
Ce(O Bu)
4
(
g
t
Ce(O Bu)
4
t
h
7
8
9
Ce(O Bu)
4
18 (13/5)
n.d.
RESULTS AND DISCUSSION
[NH
4
]
2
[Ce(NO
Ce(OH)
Ce(OAc) ·H
CeCl ·7H
CeCl ·7H O
3
)
6
]
-
-
-
-
Decarboxylative Oxygenation of Aliphatic Carboxylic
Acids
i
4
n.d.
i
1
0
3
2
O
n.d.
We started by searching for the best cerium catalyst for decar-
boxylative oxygenation of 4-fluorophenylacetic acid (1a) un-
der air with irradiation of 40W blue LEDs (380 – 530 nm) at
11
3
2
O
n.d.
j
i
12
13
3
2
Cs CO (10 mol%)
2
3
n.d.
j
n
CeCl ·7H O
3
2
Bu NCl (25 mol%)
4
-
-
64 (44/20)
33 (26/7)
room temperature in toluene for 3 h, and the results are sum-
14
Fe(OAc)2
t
marized in Table 1.
When Ce(O Bu)
4
was used, 4-
k
1
5
6
>99 (83/17)
a
b
fluorobenzaldehyde (2a) and 4-fluorobenzyl alcohol (3a) were
obtained in 86% yield and 14% yield, respectively, a totally
quantitative yield of C-O bond-forming products (entry 1).
This transformation required both blue LED-irradiation and
air: without the blue LED-irradiation, C-O bond forming
products were not obtained (entry 2), and under argon atmos-
phere, a decarboxylative homo-coupling product of 1a, 4,4’-
NMR Yield. Reaction conditions: in a test tube (φ = 13 mm), 1a (0.100
t
4
mmol), Ce(O Bu) (5 mol%), toluene (1.5 mL), under air, 40W blue LEDs
c
(380 – 530 nm), 1,3,5-trimethoxybenzene as an internal standard. Effect
of solvents, temperature, and reaction time as well as screening of other
metal sources, see Supporting Information. 4,4’-difluorobibenzyl was
formed as a major product. 1.2W blue LEDs (410-490 nm) instead of
d
e
f
40W blue LEDs. 1.2W green LEDs (450-580 nm) instead of 40W blue
g
h
LEDs. 1.2W red LEDs (570-670 nm) instead of 40W blue LEDs.
Commercially available dry toluene (H O, <1 ppm, Kanto Chemical,
difluorobibenzyl, was produced as the sole product in a half
2
2
t
toluene dehydrated -Super Plus-) was further purified by passing through
Grubbs Column system. 4-fluorobenzyl hydroperoxide (4a) was formed
as a major product. CH CN was used as a solvent. 6 h.
3
equivalent to Ce(O Bu)
4
(entry 3). Notably, other light sources
i
with narrow emission bands such as 1.2W blue LEDs (410 –
90 nm), 1.2W green LEDs (450 – 580 nm), and 1.2W red
j
k
4
LEDs (570 – 670 nm) clarified that the light with shorter
wavelength was effective for the catalytic reaction; the blue
LEDs afforded 82% yield of 2a and 17% yield of 3a, while the
green LEDs and the red LEDs showed moderate or no catalyt-
ic activities (entries 4-6). When anhydrous toluene obtained
ESI-MS Analysis of Catalytic Reaction Mixtures
Under the optimized conditions, we conducted some control
experiments to gain information about any catalytically active
species responsible for the carboxyl radical generation. Reac-
tion of Ce(O Bu)
t
4
with an excess amount (10 equiv) of 1a in
2
by passing through Grubbs column (H O, <1 ppm, Kanto
toluene without photoirradiation under air smoothly produced
2
Chemical, toluene dehydrated -Super Plus- for second drying
process) was used, the yields of 2a and 3a were decreased
with initial formation of poorly soluble cerium species,
a
Ce
hexanuclear oxo-cerium(IV) carboxylate complex,
(OH) (OCOCH F)12 (5), whose reaction mixture
2808.8 for
– 2 H } ) as the major
6
O
4
4
2 6 4
C H
displayed its parent peak (m/z
[Ce (OH) (OCOCH
=
Ce(OCOCH
2
C
6
H
4
F)
4
(entry 7), indicating the importance of
+ 2-
{
6
O
4
4
2 6 4 2
C H F)12]
trace water in the reaction media for the formation of soluble
catalytically active species (vide infra). Typical cerium(IV)
compounds, [NH ] [Ce(NO ) ], was inactive (entry 8), where-
4 2 3 6
9
species by ESI-MS (eq 1, Figure 1). The reaction mixture
showed a broad absorption below 500 nm. Such broad and
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low-energy absorption is typical for cerium(IV) species due to
the charge transfer from the ligand to empty 4f orbital of ceri-
um(IV), suggesting that complex 5 is the blue light-responsive
coalesced at 203 K, and two broad signals were finally ob-
served as approximately a 3:1 ratio at 183 K. Activation pa-
9
1
2
3
4
5
6
7
8
9
rameters for the ligand exchange process between the carbox-
ylate and the coordinated carboxylic acid were estimated to be
3
species. On the other hands, in anhydrous toluene, mixture of
t
‡
Ce(O Bu)
4
and 1a afforded yellow precipitates of
F) , which caused lower catalytic activity
due to the low solubility in toluene (Table 1, entry 7). Further
addition of H O to the reaction mixture resulted in dissolving
ΔG183 = 8.5 kcal/mol at 183 K.
Ce(OCOCH
2
C
6
H
4
4
2
the yellow precipitates to give cluster 5, which was consistent
with the formation of cerium(IV) carboxylate cluster under the
9
optimized reaction condition.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
a)
observed
6 4 4 2 6 4 2
O (OH) (OCOCH C H F)12]
- 2 H+}2-
{
[Ce
Figure 2. Molecular structure of 6 with 30% probability. All
hydrogen atoms and Bu groups on carboxylates and carbox-
ylic acids are omitted for clarity. Wireframe style is applied
for carboxylates and carboxylic acids. Selected bond distanc-
es (Å) and angles (degree) of 6: Ce1—O1 2.277(6), Ce1—
O2 2.287(5), Ce1—O3 2.267(3), Ce1—O4 2.263(4), Ce2—O1
t
observed
(b)
2
2
2
1
1
.314(4), Ce2—O2 2.303(6), Ce3—O1 2.309(3), Ce3—O3
.307(4), Ce2—O5 2.358(3), Ce3—O6 2.397(3), Ce2—O16
.697(3); Ce1—O1—Ce2 109.93(14), Ce1—O2—Ce2
09.97(17), Ce1—O1—Ce3 110.66(19), Ce2—O1—Ce3
13.15(18), Ce1—O3—Ce3 111.12(16).
simulate pattern of {[Ce
6
O
4
(OH)
4
(OCOCH
2
C
6
H
4
F)12]
2
- 2 H+}2-
(c)
The spontaneous formation of hexanuclear oxo-cerium
clusters under the catalytic reaction conditions suggested that
the Ce(IV) clusters such as 5 and 6 were catalytically active
species for this decarboxylative oxygenation. In fact, the iso-
t
Figure 1. ESI-MS spectra of the mixture of Ce(O Bu)
equiv of 1a in toluene at negative mode (a) in the wide region,
(b) expanded spectrum for 2808, and (c) simulate pattern for
6 4 4 2 6 4 2
{[Ce O (OH) (OCOCH C H F)12] – 2 H } .
4
and 10
lated hexanuclear cerium cluster 6 exhibited almost the same
t
catalytic activity as Ce(O Bu)
4
under the optimized reaction
+
2-
conditions: in the presence of 6 (5 mol% on Ce) in toluene
under irradiation of blue LEDs in air for 6 h, 1a produced 2a
and 3a in 83% and 17% yields, respectively, along with the
formation of catalytic amounts of neopentyl alcohol and
pivalaldehyde derived from 6 (Table 1, entry 15).
Synthesis and Reactivity of Oxo-Cerium(IV) Carbox-
ylate Cluster 6
Isolation of 5 in crystalline form was unsuccessful, thereby
hampering further structural and spectroscopic characteriza-
tion; however, we successfully isolated another hexanuclear
The isolated complex 6 had a broad and weak absorption
below 500 nm in toluene, which resulted in effective photo
t
t
complex, Ce
6 4 4 2 2 4
O (OH) (OCOCH Bu)12(HOCOCH Bu) (6) as
excitation of 6 by blue LEDs with emission bands from 380
yellow crystals in 56% yield, which was prepared by treating
Ce(O Bu)
butylacetic acid in toluene (eq 2). Figure 2 shows the molecu-
lar structure of 6, which has six cerium atoms bridged by four
9
nm to 530 nm. When the solution of 6 in toluene-d
8
was irra-
t
4
with an excess amount (10 equiv) of tert-
diated for 1 h under argon atmosphere in the presence of ex-
cess tert-butylacetic acid, one of two tert-butylacetate ligands
bound to each Ce(IV) ion of 6 was photochemically activated
to give 3 equiv of 2,2,5,5-tetramethylhexane (with respect to
the amount of 6) as a consecutive decarboxylation-radical
oxo and four hydroxo ligands, forming a “Ce
6
O
4
4
(OH) ” core,
1
1
surrounded by twelve bridging µ
2
-η :η -carboxylates and
1
11,12
1
four ҡ -O coordinated carboxylic acids.
spectrum of 6 in CD Cl displayed one broad signal centered
at δ 2.05 due to the methylene moieties of both µ
The H NMR
coupling product via an initial formation of the carboxyl radi-
2
2
t
cal, along with precipitation of [Ce(OCOCH
1; 7b: n = 2; 7c : n = 3) (eq 3).
2
3
Bu) ]
n
(7a: n =
1
1
2
-η :η -
9
1
carboxylates and ҡ -O coordinated carboxylic acids at room
temperature, indicating that carboxylate ligands and carbox-
ylic acids bound to the cerium center were rapidly exchanged
in solution at room temperature. The broad signal assignable
to twelve carboxylates and four carboxylic acids was de-
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Journal of the American Chemical Society
and the results are shown in Table 2. Decarboxylative oxy-
Page 4 of 9
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
genation reaction of 1 produced the corresponding arylalde-
hydes 2 as the major C-O bond-forming product and aryl-
methanols 3, except for NH -substituted substrate 1d (vide
2
infra). Arylacetic acids 1b,c with electron-donating substitu-
ents at the para-position of the aromatic ring were quantita-
tively converted to give arylaldehydes 2b,c in 81% and 84%
yields as well as alcohols 3b,c in 12% and 13% yields, respec-
tively. p-Aminophenylacetic acid (1d) selectively afforded 2d
in 42% yield, in which N,N-dimethylacetamide (DMA) was
used as a solvent to dissolve 1d. The reaction of p-
hydroxyphenylacetic acid (1e) in DMA gave both 2e (56%
yield) and 3e (26% yield) with a longer reaction time. p-
Trifluoromethylphenylacetic acid (1f) and p-nitrophenylacetic
acid (1g) exhibited high reactivity to afford 2f,g in 80% and
Next, we conducted ESI-MS analysis of the reaction mix-
ture under the optimized reaction conditions to identify the
fate of cerium intermediates derived from the hexanuclear
cerium(IV) cluster during the catalytic reaction. When a mix-
ture of 2-(trifluoromethyl)phenylacetic acid and 10 mol% of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
t
Ce(O Bu)
4
was irradiated with blue LEDs for 10 minutes in
toluene under air, we observed cerium(III) carboxylates simi-
lar to 7a-c, [Ce(OCOCH CF , as the main species
along with a small amount of cerium(IV) alkylperoxo complex,
Ce O(OCOCH CF (OOCH CF (8), indicating
2
C
6
H
4
3 3 n
) ]
7
4% yields, and 3f,g in 17% and 26% yields, respectively.
2
2
C
6
H
4
3
)
5
2
C
6
H
4
3
)
Halogen-substituted arylacetic acids such as p-chloro-, p-
bromo-, and p-iodophenylacetic acid (1h-1j) were applicable
to this reaction to produce 2h-2j and 3h-3j in totally quantita-
tive yield without any loss of the halide substituents. No sig-
nificant electronic effects were observed with meta-substituted
arylacetic acids 1k-1n: all compounds were decarboxylated
followed by oxygenation to produce the corresponding alde-
hydes 2k-2n and alcohols 3k-3n in totally excellent yields
with an approximately 8:2 ratio of 2:3. On the other hand,
ortho-substituted arylacetic acids 1o-1r required a longer reac-
tion time (24 h) to consume the substrates, and the correspond-
ing aldehydes 2o-2r were obtained in 67%-78% yields with
alcohols 3o-3r in 21%-31% yields.
that photo-reduced cerium(III) species was oxidized by in situ-
9
generated alkylperoxo radical under the reaction conditions.
However, isolation of the alkylperoxo cerium(IV) from the
reaction mixture was unsuccessful.
Reaction Mechanism
On the basis of these observations, we propose a plausible
mechanism as outlined in Scheme 2. Photolysis of Ce(IV)
carboxylate I, including a hexanuclear oxo-cerium(IV) com-
t
pound in situ-derived by the reaction of Ce(O Bu)
4
with a sub-
strate carboxylic acid, RCH
of the corresponding carboxyl radical, RCH
reduces the Ce(IV) carboxylate to Ce(III) species
II. Subsequently, A proceeds in decarboxylation to afford
the corresponding alkyl radical, RCH · (B), which readily
reacts with dioxygen to produce an alkyl peroxyl radical,
2
COOH (1), leads to the formation
2
COO· (A), and
I
4
,6
Table 2. Substrate Scope of 2-Arylacetic Acids for Decar-
2
a, b
boxylative Oxygenation
4
,13
RCH
2
OO· (C).
Then, alkyl peroxy radical C oxidizes
Ce(III) species II to generate the corresponding alkyl peroxo
4
adduct of Ce(IV) species III. The alkyl peroxo ligand in III
is replaced by a substrate carboxylic acid 1 to give the Ce(IV)
carboxylate I and liberate alkyl hydroperoxides, RCH
4). Cerium-catalyzed dehydration of 4 yields the correspond-
ing aldehyde 2 as a terminal oxidized product along with H
as well as alcohol 3 as a minor product, in accordance with the
2
OOH
(
2
O
4
,14
metal-catalyzed decomposition of organic hydroperoxides.
Catalytic oxidation of alcohols to carbonyl compounds by
Ce(IV) species is excluded, because no conversion of 3 to 2
9
was observed under the catalytic conditions.
Scheme 2. A Plausible Mechanism of Decarboxylative Ox-
ygenation of Carboxylic Acids Catalyzed by Cerium(IV)
Carboxylate
a
b
NMR Yield. Reaction conditions: in a test tube (φ = 15 mm), substrate
t
(
0.100 mmol), Ce(O Bu)
4
(5 mol%), toluene (1.5 mL), under air, 40W blue
c
d
LEDs, 1,3,5-trimethoxybenzene as an internal standard. DMA, 26 h.
DMA, 60 h. CH CN, 24 h. 24 h.
3
e
f
In addition to a series of arylacetic acids, various aliphatic
carboxylic acids were applicable to this transformation (Table
3
5
): stearic acid (1s) was transformed into heptadecanal (2s) in
7% yield and 1-heptadecanol (3s) in 23% yield, respectively.
Substrate Scope
With the best cerium-photocatalyst system in hand, we
screened the substrate scope using various arylacetic acids 1,
Decarboxylation of unsaturated fatty acids such as 9-decenoic
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Journal of the American Chemical Society
acid (1t), 6-heptenoic acid (1u), and oleic acid (1v) afforded
Table 4. Lactonization of Aromatic Carboxylic Acidsa,b
1
2
3
4
5
6
7
8
9
the corresponding aldehydes 2t-2v and alcohols 3t-3v in mod-
erate yield without any contamination by oxidation at unsatu-
rated C=C bond linkages as well as intramolecular or intermo-
lecular radical addition to the C=C bond. -Branched aliphat-
ic carboxylic acids were converted to ketones in good yield: 2-
phenylpropanoic acid (1w) and diphenylacetic acid (1x) gave
the corresponding ketones 2w,x in 72% and 77% yields, re-
spectively, and alcohols 3w,x in 8% and 8% yields, respective-
ly. N-Protected proline 1y was converted to give the corre-
sponding amide 2y in 65% yield.
yield (%)
yield (%)
entry
cat. (xx mol%)
conditions
, 24 h
of 10
of 11
t
1
2
3
4
Ce(O Bu)
4
4
4
(1)
(1)
(5)
O
2
98
90
69
21
trace
10
t
Ce(O Bu)
air, 24 h
air, 6 h
air, 6 h
t
Ce(O Bu)
27
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
t
Ce(O Bu)
4
(20)
73
Table 3. Substrate Scope of Aliphatic Carboxylic Acids
and Secondary Carboxylic Acids for Decarboxylative Oxy-
6
(5 mol% based on
Ce atom)
5
air, 6 h
air, 48 h
air, 24 h
79
n.d.
9
21
28
91
a, b
c
genation
6
Fe(OAc) (5)
2
t
Ce(O Bu)
2
(5) /
d
7
Co(acac)
2
·2H O (5)
2
a
b
NMR Yield. Reaction conditions: in a test tube (φ = 13 mm), substrate
0.100 mmol), Ce(O Bu) (x mol%), toluene (1.5 mL), under air, 40W blue
LEDs, 1,3,5-trimethoxybenzene as an internal standard. oxidized prod-
ucts of toluene were observed. in a 5 mL Schlenk (φ = 7.4 mm).
t
(
4
c
d
Scheme 3 shows possible reaction pathways for the for-
mation of peroxy lactone 10 and -lactone 11 starting from 9
according to the previously explored Mn(III)-mediated cyclic
peroxide and cyclic ether formation. Photo-induced LMCT
of cerium(IV) benzoate produces a benzoate radical D, which
is subsequently transformed into an alkyl radical E via 1,5-
17
a
b
14
NMR Yield. Reaction conditions: in a test tube (φ = 15 mm), substrate
HAT. Peroxy radical F is formed from E and O , and 10 is
2
t
(
0.100 mmol), Ce(O Bu)
4
(5 mol%), toluene (1.5 mL), under air, 40W blue
produced by cyclization of hydroperoxide G. In contrast, fur-
ther one electron oxidation of E by Ce(IV) affords a carbo-
cation H that is spontaneously cyclized to give 11, which is
promoted by the presence of a large amount of Ce(IV) species
or other metal oxidants such as Co and Fe species as observed
for entries 4, 6, and 7 in Table 4. In addition to the direct cy-
clization of G to 10, there is another scenario starting from
hydroperoxide G to 11: decomposition of G by cerium cata-
lyst gives alcohol I, and subsequent intramolecular lactoniza-
tion to form 11.
c
d
LEDs, 1,3,5-trimethoxybenzene as an internal standard. 18 h. 16 h.
Lactonization of Aromatic Carboxylic Acids
In contrast to aliphatic carboxylic acids, no decarboxylation of
benzoic acid and its derivatives was observed under the opti-
mized reaction conditions due to the strong bond dissociation
2
2
energy for the C(sp )-C(sp ) bond in a benzoate radical. Such
stability of the benzoate radical was applicable for direct trans-
formation from aromatic carboxyl radicals to alkyl radicals via
intramolecular HAT, potentially leading to lactonization
1
5,16
through 1,5-HAT and cyclization.
-isopropylbenzoic acid (9) with 1 mol% of Ce(O Bu)
blue LEDs irradiation and 1 atm of O for 24 h, a 1,5-HAT-
In fact, when we treated
Scheme 3. Possible Reaction Pathways for Formation of
Lactones 10 and 11 from 2-Isopropylbenzoic Acid (9)
t
2
4
under
2
oxygenation-cyclization product, peroxy lactone 10, was ob-
tained in 98% along with trace amounts of -lactone, 3,3-
dimethylphthalide (11) (Table 4, entry 1). Under air, amount
of 11 was slightly increased (entry 2). Selectivity of 10 and 11
largely depended on the concentration of the cerium(IV) cata-
t
lyst; by using 5 mol% of Ce(O Bu)
4
, the amount of 11 was
further increased, and 11 was obtained as the major product by
20 mol% of the catalyst loading (entries 3 and 4). When pre-
6
formed Ce cluster 6 was used, 10 and 11 were obtained in
almost the same yields and selectivity as in the case of using
t
4 2
Ce(O Bu) (entry 5). Fe(OAc) also served as the photo-
catalyst for the cyclization to selectively afford 11 in 28%
yield (entry 6). Noteworthy was that the addition of
t
Co(acac)
2
·2H
2
O to Ce(O Bu)
4
(both in 5 mol%) increased the
t
amount of 11 in 91% yield compared with Ce(O Bu)
entry 3 vs 7), though no catalytic activity was observed for
Co(acac) ·2H O (5 mol%) under the same reaction conditions.
Such the drastic change of the product selectivity by the addi-
tion of Co(acac) ·2H O was due to rapid re-oxidation of in
situ-formed cerium(III) species, forming cerium(IV) clusters
4
system
(
2
2
Above-mentioned two reaction pathways for 11 were con-
firmed by the catalytic reaction of 9 with 20 mol% of
2
2
t
18
18
Ce(O Bu)
4
18
under
O
2
atmosphere (1.5 atm), in which [ O
2
]-
9
18
such as Ce1+nCoO
n
(OCOC
9
H
11
)6+2n (n = 1-3).
10, 11, [ O]-11, and [ O
2
]-11 were observed by the HRMS
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Journal of the American Chemical Society
Page 6 of 9
1
8
analysis (Scheme 4). Formation of [ O
2
]-10 clearly indicated
Corresponding Author
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
the involvement of F prior to the peroxy lactone formation,
*
*
*
Email: tsurugi@chem.es.osaka-u.ac.jp
Email: satoh@sci.osaka-cu.ac.jp
Email: mashima@chem.es.osaka-u.ac.jp
1
8
while detection of the non- O-incorporated 11 was assumed
to be the pathway through the carbocation H before the lac-
1
8
tone formation. In addition, [ O
2
]-11 implied the presence of
1
1
8
8
ORCID
[
[
O]-I before the lactone formation as well as hydrolysis of
O]-I with in situ-generated H O (eq 6).
2
18
9,18
Satoru Shirase: 0000-0002-6932-085X
Sota Tamaki: 0000-0003-2158-2604
Koichi Shinohara: 0000-0003-1622-0756
Keishi Hirosawa: 0000-0002-0818-6175
Hayato Tsurugi: 0000-0003-2492-7705
Tetsuya Satoh: 0000-0002-1792-140X
Kazushi Mashima: 0000-0002-1316-2995
Scheme 4. 18O Labeling Experiment
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Funding Sources
JSPS KAKENHI Grant Nos. JP19H04578 (Coordination
Asymmetry), JP18H04267 and JP15H05808 (Precisely De-
signed Catalysis with Customized Scaffolding).
ACKNOWLEDGEMENT
S. S. thanks the financial support by the JSPS Research Fel-
low-ships for Young Scientists. This work was supported by
JSPS KAKENHI Grant Nos. JP19H04578 (Coordination
Asymmetry) to H.T., JP18H04267 to T.S. and JP15H05808
(Precisely Designed Catalysis with Customized Scaffolding)
to K.M.
Furthermore, direct 6-endo-trig cyclization of 2-
phenylbenzoic acid (12) was carried out under the optimized
reaction conditions. In situ-generated radical derived from 12
only gave a six-membered lactone, biphenyl-2,2'-carbolactone
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ASSOCIATED CONTENT
The Supporting Information is available free of charge on the
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