Ru-Photoredox-Catalyzed Decarboxylative Oxygenation of Aliphatic Carboxylic Acids through N-(acyloxy)phthalimide

Article abstract of DOI:10.1021/acs.orglett.8b01885

Decarboxylative aminoxylation of aliphatic carboxylic acid derivatives with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) in the presence of ruthenium photoredox catalysis is reported. The key transformation entails a highly efficient photoredox catalytic cycle using Hantzsch ester as a reductant. The ensuing alkoxyamine can be readily converted to the corresponding alcohol in one pot, representing an alternative approach to access aliphatic alcohols under photoredox conditions.

Full text of DOI:10.1021/acs.orglett.8b01885

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Ru-Photoredox-Catalyzed Decarboxylative Oxygenation of Aliphatic
Carboxylic Acids through N‑(acyloxy)phthalimide
Chao Zheng,*,†,‡ Yuting Wang,† Yangrui Xu,† Zhen Chen,‡ Guangying Chen,*,†
and Steven H. Liang*,‡
†Key Laboratory of Tropical Medicinal Plant Chemistry of Ministry of Education, Collaborative Innovation Center of Tropical
Biological Resources, Hainan Normal University, Hainan Haikou 571158, China
‡Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114, United
States
S
* Supporting Information
ABSTRACT: Decarboxylative aminoxylation of aliphatic
carboxylic acid derivatives with (2,2,6,6-tetramethylpiperidin-
1-yl)oxyl (TEMPO) in the presence of ruthenium photoredox
catalysis is reported. The key transformation entails a highly
efficient photoredox catalytic cycle using Hantzsch ester as a
reductant. The ensuing alkoxyamine can be readily converted
to the corresponding alcohol in one pot, representing an alternative approach to access aliphatic alcohols under photoredox
conditions.
ecarboxylative transformations that directly convert
undergo mesolytic cleavage17 by strong oxidative photoredox
Dreadily available and environmentally benign carboxylic catalysts, TEMPO is unable to be used in trapping alkyl
radicals generated in the reductive quenching process of
carboxylate anion with photoredox catalysts (path a in Figure
1). We next conceive that an oxidative quenching process using
acids to various functionalized organic molecules have gained
intensive attention in the synthetic community.1 In particular,
the rapid development of single-electron redox catalyzed
reactions enables a series of challenging and diverse
decarboxylative transformations2,3 of aliphatic carboxylates,
including decarboxylative arylation,4 alkynylation,5 alkenyla-
tion,6 alkylation,7 borylation,8 silylation,9 thiolation,10 seleny-
lation,11 and amination.12 However, a general and practical
decarboxylative oxygenation method to convert aliphatic
carboxylic acids to the corresponding alcohols has yet to be
developed. For example, Lu et al. reported a decarboxylative
hydroxylation method using an organophotoredox catalyst and
molecular oxygen under irradiation,13 which is most limited to
benzylic carboxylic acid substrates and requires an additional
functional group manipulation, i.e., reduction with NaBH4/
MeOH to obtain alcohol products due to overoxidation by
oxygen. Other literature methods to achieve decarboxylative
hydroxylation of aliphatic carboxylic acids include the use of
stoichiometric amounts of metal oxidants,14 and Barton esters
in the presence of a radical initiator and oxygen under UV light
irradiation.15 Furthermore, the use of oxygen under irradiation
(e.g., singlet oxygen generated in situ) substantially restricts
the practicality and substrate scope.
Figure 1. Decarboxylative hydroxylation of aliphatic carboxylic acid
through aminoxylation with TEMPO.
redox active esters to deliver an alkyl radical may circumvent
these problems and form a C−O bond between alkyl radicals
and TEMPO (path b in Figure 1). Importantly, a suitable
reductant to turn over the photoredox catalyst but not react
with TEMPO is crucial to achieve decarboxylative oxygenation
reactions. We herein report a ruthenium photoredox catalyst
and Hantzsch ester18 system to achieve decarboxylative
aminoxylation of aliphatic carboxylic acid-derived N-(acyloxy)-
phthalimide, followed by the formation of alcohols upon
treatment with Zn/HOAc.19 This approach provides a general
In this context, we consider TEMPO, which is a commonly
used reagent in mechanistic studies to probe the existence of
radical intermediate,16 but not widely used as coupling reagent
in photoredox catalysis, can be a suitable reagent to trap alkyl
radicals generated after radical decarboxylation to achieve
decarboxylative oxygenation reactions. However, because of
the reducing ability of TEMPO [E1/2(TEMPO+/TEMPO) =
+0.52 V]16d and the low stability of alkoxyamine products to
Received: June 24, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01885
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
and practical method to perform decarboxylative oxygenation/
hydroxylation with excellent chemoselectivity and broad
substrate scope using readily available reagents.
The optimized reaction conditions are demonstrated in the
equation in Table 1. A mixture of N-(cyclohexanecarboxyl)-
10−12). Besides Ru(bpy)3Cl2·6H2O,20 fac-Ir(ppy)3 and
organophotoredox catalyst, 4-CzIPN also acted as effective
catalysts, albeit to offer lower yields (Table 1, entries 13 and
14). Reductant selection is crucial for this transformation to
turn over the photoredox catalyst on the conditions that the
reductant should not participate in radical coupling with
TEMPO. Thus, various amines and anilines such as DIPEA,
Et3N, and N-methyl-N-phenylaniline were all ineffective, as
their TEMPO adducts were detectable by GC-MS analysis
(Table 1, entries 16−18).
Table 1. Optimized Reaction Conditions of Decarboxylative
Oxygenation Reaction
We next investigated the substrate scope with aliphatic
carboxylic acids (see Scheme 1). Compared with the literature
methods,14,15 our method showed broad and diverse substrate
compatibility. Benzylic carboxylates, primary, secondary and
tertiary aliphatic carboxylates, as well as α-hydroxy acid- and α-
amine acid-derived esters can be smoothly converted to
TEMPO-derived alkoxyamine products in moderate to
a
yield
entry
variations from standard conditions
(%)
b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
none
95 (91 )
d
without Ru(bpy)2Cl2·6H2O
without Hantzsch ester
under air
n.r.
n.r.
trace
n.r.
36
Scheme 1. Scope of Aliphatic Carboxylic Acids
without light
Hantzsch ester (0.25 equiv)
Hantzsch ester (0.5 equiv)
Hantzsch ester (0.75 equiv)
Hantzsch ester (1.2 equiv)
1b instead of 1a
1c instead of 1a
1d instead of 1a
fac-Ir(ppy)3 instead of Ru(bpy)2Cl2·6H2O
4CzIPN instead of Ru(bpy)2Cl2·6H2O
1-benzyl-4H-pyridine-3-carboxamide instead of Hantzsch
ester
71
84
92
n.r.
n.r.
n.r.
70
80
n.r.
c
16
17
18
DIPEA instead of Hantzsch ester
Et3N instead of Hantzsch ester
N-methyl-N-phenylaniline instead of Hantzsch ester
trace
trace
trace
a
Reaction conditions: redox-active ester (0.2 mmol), TEMPO (0.3
mmol), photocatalyst (2 mol %), reducing agent (100 mol %) in
solvent (2 mL), irradiation by 34 W blue LEDs at room temperature
for 24 h under argon atmosphere. Yield determined by 1H NMR using
b
c
diphenylmethane as an internal standard. Isolated yield. DIPEA =
diisopropylethylamine.
d
n.r. = no reaction.
phthalimide (0.20 mmol), TEMPO (0.30 mmol), Ru-
(bpy)3Cl2·6H2O (2.0 mol %), and Hantzsch ester (0.20
mmol) in DMF solvent under an inert atmosphere was
irradiated with 34 W blue light-emitting diodes (LEDs) at
room temperature.
A complete conversion of redox active ester was achieved
after 24 h, and the corresponding alkoxyamine product was
isolated in 95% yield (Table 1, entry 1). Photoredox catalyst
and Hantzsch were essential for this reaction (Table 1, entries
2 and 3). Exposure to air was detrimental (Table 1, entry 4).
The reaction did not proceed in the absence of light (Table 1,
entry 5). Because one Hantzsch molecule accepts two
electrons to turn over the photoredox cycle, the optimal yield
was generated using 1.0 equiv of Hantzsch ester (Table 1,
entries 6−9). Other types of redox esters were also tested, and
only N-(acyloxy)phthalimide was reactive (Table 1, entries
a
Reaction conditions: redox-active esters (0.20 mmol), TEMPO
(0.30 mmol), Ru(bpy)3Cl2·6H2O (2.0 mol %), Hantzsch ester (1.0
equiv) in DMF (2.0 mL), irradiation by 34 W blue LEDs at room
temperature for 24 h under Ar atmosphere. Yields of isolated
products.
B
DOI: 10.1021/acs.orglett.8b01885
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
excellent yields (52%−96%). Ketones (6, 21), ethers (5, 8),
internal alkenes (13), terminal alkenes (18), alkynes (20), and
alkyl chlorides (17) were all tolerated. In addition, a phosphine
functional group, which is very sensitive to oxidation, was
tolerated under our reaction conditions (22).
product can also be used as an alkyl electrophile to couple
with a variety of nucleophiles upon oxidative generation of an
alkoxyamine radical cation, followed by mesolytic cleavage.17
The proposed reaction mechanism is shown in Figure 5.
After analysis of the redox potential of Ru-photoredox catalyst
It is worth noting that a few previously inaccessible
substrates,13 including primary/secondary aliphatic carboxylic
acids, α-hydroxy acids, and α-amino acids, were successfully
synthesized by our method. All of the N-acyloxyphthalimides
used in Scheme 1 can be easily prepared from corresponding
alkyl carboxylic acids in high yield (75%−96%; see the
Supporting Information for details).
The synthetic utility of our decarboxylative aminoxylation
approach was further demonstrated in gram-scale reactions.
The scale-up reaction was robust and performed at equal
efficiency in a large reaction flask on gram scale (Figure 2).
Figure 5. Plausible mechanism.
[redox potential of Ru(bpy)3Cl2·6H2O: E*1/I2I/I = 0.77 V vs SCE,
III/II
III/
EI1I//2I = −1.33 V vs SCE; E
= 1.29 V vs SCE, E *II = −0.81
Figure 2. Gram-scale reactions under optimal conditions.
1/2
1/2
V vs SCE] and redox active ester (E1/2 = −1.26 to −1.37 V vs
SCE), we postulate that the mechanism based on the Ru(III)/
Ru(II) species is less probable, because the redox potential of
The N−O bonds of the alkoxyamine products were readily
cleaved upon treatment with zinc powder and acetic acid under
mild conditions to produce aliphatic alcohols (see Figure 3).19
photoexcited *Ru(II) catalyst is insufficient to reduce N-
III/
(acyloxy)phthalimide (E *II = −0.81 V, compared with E1/2
1/2
= −1.26 to −1.37 V vs SCE). In contrast, Ru(I) is suitable to
reduce N-(acyloxy)phthalimide due to its strong redox
II/I
potential (E = −1.33 V vs SCE), and photoexcited *Ru(II)
1/2
red
1/2
is strong enough to oxidize Hantzsch esters (E = −2.3 V vs
SCE).18 Thus, a RuII/RuI redox mechanism is feasible, during
which the RuI reduces N-(acyloxy)phthalimide to generate an
alkyl radical and a Hantzsch ester reduces the excited *RuII to
RuI. The measured quantum yield of the model reaction is 6.7.
Since TEMPO trapping alkyl radical is a chain termination
process, it is unlikely that radical chain process is involved. The
value of quantum yield suggested the concomitance of other
energy transfer processes.21
In summary, we have developed a ruthenium photoredox-
catalyzed decarboxylative aminoxylation reaction of aliphatic
N-(acyloxy)phthalimide with TEMPO using Hantzsch ester as
a reducing agent to turn over photoredox cycle. A broad scope
of aliphatic carboxylates, including benzylic, tertiary, secon-
dary, primary, α-amino, and α-hydroxy aliphatic carboxylates,
are compatible to provide the corresponding alkoxyamines.
The alkoxyamine product can be easily reduced to aliphatic
alcohols, thus providing a convenient and practical method for
decarboxylative hydroxylation. Attributed to the diverse
reactivity of the TEMPO-derived alkoxyamine product as
alkyl electrophiles, our method also provides a platform to
utilize aliphatic carboxylic acids in organic synthesis.
Figure 3. Reduction of alkoxyamine products to alcohols.
Moreover, it is worth mentioning that the decarboxylative
hydroxylation could be performed in one pot in high efficiency
without isolating the TEMPO-derived aminoxylation product
(Figure 4). Besides, the TEMPO-derived aminoxylation
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.8b01885.
Figure 4. One-pot decarboxylative hydroxylation of N-(acyloxy)-
phthalimide.
C
DOI: 10.1021/acs.orglett.8b01885
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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AUTHOR INFORMATION
Corresponding Authors
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*E-mail: zhengchao@hainnu.edu.cn
*E-mail: chgying123@163.com
*E-mail: liang.steven@mgh.harvard.edu
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ORCID
Chao Zheng: 0000-0002-2179-8096
Zhen Chen: 0000-0002-6289-4332
Steven H. Liang: 0000-0003-1413-6315
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was funded by the National Natural Science
Foundation of China (NSFC) (Grant No. 21702039), the
Program for Innovative Research Team in University (No.
IRT-16R19), and Hainan Province Natural Science Founda-
tion of Innovative Research Team Project (No.
2016CXTD007). S.H.L. is thankful for the general support
from the Department of Radiology, MGH and Harvard
Medical School.
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