Catalyst-Free Decarboxylation of Carboxylic Acids and Deoxygenation of Alcohols by Electro-Induced Radical Formation

Article abstract of DOI:10.1002/chem.201905224

Electro-induced reduction of redox active esters and N-phthalimidoyl oxalates derived from naturally abundant carboxylic acids and alcohols provides a sustainable and inexpensive approach to radical formation via undivided electrochemical cells. The resulting radicals are trapped by an electron-poor olefin or hydrogen atom source to furnish the Giese reaction or reductive decarboxylation products, respectively. A broad range of carboxylic acid (1°, 2°, and 3°) and alcohol (2° and 3°) derivatives are applicable in this catalyst-free reaction, which tolerated a diverse range of functional groups. This method features simple operation, is a sustainable platform, and has broad application.

Full text of DOI:10.1002/chem.201905224

A Journal of
Accepted Article
Title: Catalyst-Free Decarboxylation of Carboxylic Acids and
Deoxygenation of Alcohols by Electro-Induced Radical Formation
Authors: Ping Wang, Xiaoping Chen, Xiao Peng, Jiaojiao Guo, Jiantao
Zai, and Xiaosheng Luo
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201905224
Link to VoR: http://dx.doi.org/10.1002/chem.201905224
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Catalyst-Free Decarboxylation of Carboxylic Acids and
Deoxygenation of Alcohols by Electro-Induced Radical Formation
Xiaoping Chen,†[a] Xiaosheng Luo,†[b] Xiao Peng,†[a] Jiaojiao Guo,[b] Jiantao Zai[b] and Ping Wang*[b]
Abstract: Electro-induced reduction of redox active esters and N-
phthalimidoyl oxalates derived from naturally abundant carboxylic
acids and alcohols provides a sustainable and inexpensive approach
to radical formation via undivided electrochemical cells. The resulting
radicals are trapped by an electron-poor olefin or hydrogen atom
source to furnish the Giese reaction or reductive decarboxylation
products, respectively. A broad range of carboxylic acid (1°, 2°, and
3°) and alcohol (2° and 3°) derivatives are applicable in this catalyst-
free reaction, which tolerated a diverse range of functional groups.
This method features simple operation, sustainable platform and
broad applications.
The discovery of Barton decarboxylation and Barton-McCombie
deoxygenation via radical processes has led to numerous
applications, albeit using toxic tin reagents under elevated
Figure 1. Decarboxylation and deoxygenation engaged Giese reactions under
temperature.[1] Recently, significant advances in radical
different conditions.
generation have been achieved, allowing the use of easily
accessible and abundant carboxylic acid or alcohol radical
precursors. For example, stabilized carbon-centered radicals
from α-hetero carboxylic acids can be directly generated under
robust synthetic methods to generate radicals for synthetic
application is still in high demand.
Meanwhile,
electrochemistry
has
experienced
a
Ir-catalyzed photoredox conditions (Figure 1A).[2] Furthermore,
decarboxylative fragmentation of redox active esters (RAEs)
enables the generation of alkyl radicals under photoredox
conditions using Ir, Ru, and other photocatalysts.[3] Meanwhile,
transition metals such as Ni,[4] Fe,[5] Cu,[6] Co,[7] Cr,[8] and other
metal species[9] have also been used to reduce RAEs via
thermal single-electron transfer (SET) to form alkyl radicals.
Through similar mechanisms, oxalate derivatives can produce
tertiary radicals through Ir-, Ru-catalyzed photoredox
conditions[10] or Zn metal and Ni with ligands.[11] However, these
approaches mainly rely on precious or toxic metal catalysts,
which is not ideal, especially during the preparation of medicinal
intermediates. Moreover, the needs for strict reaction conditions
such as anhydrous and anaerobic conditions are mandatory due
to the properties of reactive transition metals. Several studies
avoided the use of transition-metal catalysts and photo-
catalysts.[3f-h, 12] Therefore, the development of catalyst-free and
renaissance in recent years. Major advances on cross
coupling,[13] annulation,[14] dehydrogenation[15] and other diverse
reactions[16] have been accomplished by different groups.[17]
However, conjugate addition from RAEs and oxalates were rare
under catalyst-free and mild electrochemical conditions. Herein,
we report an electro-induced, catalyst-free method for Giese
reaction[18] and reductive decarboxylation. The features of this
method include: 1) unprecedented electro-induced Giese
reaction and reductive decarboxylation without toxic metal
species, 2) operational simplicity and water-tolerant condition, 3)
sustainable resources from carboxylic acid or alcohol derivatives
(about $ 0.4 per mmol for Giese reaction, see Supporting
Information), 4) robust and mild reaction condition with broad
functional group tolerance (Figure 1B).
We began our exploration of the decarboxylation of RAE 1
by cathodic reduction, and using phenyl vinyl sulfone 2 as a
trapping reagent under electrochemical conditions. Conducting
the reaction under a constant voltage of 2.5 V with a graphite
anode and cathode using nBu4NBF4 as the electrolyte in DMF in
an undivided cell at room temperature provided product 3 in low
yield with most of RAE 1 and acceptor 2 remaining unreacted
(Table 1, entry 1). When the voltage was elevated to 5.0 V, 3
was produced in 69% yield (entry 2). Pleasingly, under 2.5 V
with Hantzsch ester (HE) (1.2 equiv.), 3 was produced in 87%
yield (entry 3). Here, HE could effectively lower the reaction
potential by 2.5 V, which improved the reaction efficiency and
minimized the side reactions caused by high voltage. Changing
the reductant from HE to PhSiH3 resulted in a low product yield
(entry 4). Several other observations are worth noting during the
reaction optimization: 1) DMF is much better when compared
[a]
[b]
Dr. X. Chen,† Dr. X. Peng†
Key Laboratory of Optoelectronic Devices and Systems of Ministry
of Education and Guangdong Province, and College of Physics and
Optoelectronic Engineering
Shenzhen University
Shenzhen 518060, China
X. Luo,† J. Guo, Pro. J. Zai and Pro. P. Wang
Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs,
School of Chemistry and Chemical Engineering
Shanghai Jiao Tong University
800 Dongchuan Road, Shanghai 200240, China
E-mail: wangp1@sjtu.edu.cn
† These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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Table 1: Optimization experiments.
Under open air conditions, the yield is slightly diminished (entry
12), indicating this reaction is insensitive to moisture and there is
no requirement for the strict exclusion of oxygen. 5) An electric
current is essential for this transformation, and visible light[12] is
not critical in the reaction (entries 13-15).
Under the optimized conditions, the reaction of 1 with
various Michael acceptors bearing different functional groups
was examined (Scheme 1A). Products containing sulfones (3),
esters (4), nitriles (5), amides (6), ketones (7) and α- or β-
substituted adducts (8 and 9) were produced in excellent yields,
demonstrating the broad functional tolerance. It was notable that
7 and 8 were obtained in near quantitative yield.
Non-stabilized primary alkyl radicals, which were less
explored in other studies, were also investigated, yielding
products 10–13 in good yields (Scheme 1B). Electron-rich olefin
11 was obtained in 53% yield with the internal alkene moieties in
linoleic acid left intact. For complex substrates 12 and 13, higher
equivalents of the olefin and HE were required to give the
desired products in good yields.
Excellent compatibility for a broad scope of secondary and
tertiary alkyl RAEs bearing different functional groups was
demonstrated (14–22) under standard conditions (Scheme 1C).
The presence of α-heteroatom in cyclic substrates 16 and 17
resulted in higher yields (89 and 94%, respectively).
Encouragingly, the formation of quaternary carbons (19 and 20)
was highly efficient under electrochemical conditions when
compared with photoredox and metal reagents.[19] Natural
product derivatives (21 and 22) from steroid oleanolic acid were
easily generated in excellent yields and high stereoselectivity.
Entry[a]
Deviation from standard conditions
Yield (%)[b]
1
2
3
4
5
6
7
8
9
10
No reductant
trace
69
No reductant at 5.0 V
None
87
PhSiH3 as reductant
trace
53
DME instead of DMF
ACN instead of DMF
34
Bu4NOTf instead of nBu4NBF4
Bu4NClO4 instead of nBu4NBF4
LiClO4·3H2O instead of nBu4NBF4
Ni electrodes instead of graphite electrodes
46
36
32
51
Glassy carbon electrodes instead of graphite
electrodes
11
44
12
Performed under an air atmosphere
No current
61
0
13
14
15[c]
In the dark
85
53
Under 450 nm irridiation, no current
[a] Standard conditions (0.15 mmol). 1 (2.0 equiv.), 2 (1.0 equiv.), HE (1.2
equiv.), nBu4NBF4 (0.25 M), DMF (1.5 mL), graphite electrodes, 2.5 V,
undivided cell, 20 °C , N2 atmosphere, 2 h. [b] Isolated yield. [c] Reaction
time: 12 h.
with the other solvents studied (entries 5, 6). 2) Low yields are
obtained when different electrolytes are used (entries 7-9). 3)
The material of electrode affects the yield (entries 10, 11). 4)
Furthermore, we extended this method to peptide
Unprotected peptide 23 was readily
accessed via an intramolecular Giese addition reaction in 39%
20]
macrocyclization.[2b,
Scheme 1. Scope of the electro-induced Giese reaction and reduction of RAEs. Reaction conditions for the Giese reaction: RAE (2.0 equiv.), Michael acceptor
(1.0 equiv.), HE (1.2 equiv.) nBu4NBF4 (0.25 M) in DMF (1.5 mL, 0.1 M for acceptor concentration), graphite electrodes, 20 °C , 2.5 V, undivided cell. Reaction
conditions for reduction of RAEs: RAE (1.0 equiv.), DTT (2.0 equiv.), nBu4NBF4 (0.25 M) in DME (1.5 mL, 0.1 M), graphite electrodes, 20 °C , 3.0 V, undivided cell.
aUsing 1.0 equiv. of RAE, 5.0 equiv. of Michael acceptor and 2.4 equiv. of HE. bUsing 1.0 equiv. of RAE and 2.0 equiv. of Michael acceptor. cUsing the peptide
derivative at 5 mM concentration. dUsing 2.0 equiv. HE.
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yield. Macrocyclic peptides are important therapeutics and this
transformation suggests its potential value in peptide
macrocyclization. Overall, the Giese reaction proceeded rapidly
at ambient temperature.
As with the Giese reaction, a scope of primary, secondary
and tertiary RAEs bearing a broad range of functional groups
[Fmoc (24 and 26), Boc (25, 27–28), olefins (29), ketone (30),
and hydroxyl (31)] can be employed in reductive decarboxylation
reaction in the presence of HE or DTT (1,4-dithiothrietol) as
reductive reagents, producing 24–31 in excellent yields (Scheme
1D). In the case of primary product 29, HE proved to be more
efficient than DTT. Different from other Giese reaction protocols
or decarboxylation conditions, a broad range of carboxylic acids
(1, 2, and 3) with no requisite for transition metal species
features this transformation.
Scheme 2. Scope of the electro-induced Giese reaction of oxalate derivatives.
Reaction conditions: Oxalate (2.0 equiv.), Michael acceptor (1.0 equiv.), HE
(2.0 equiv.), nBu4NBF4 (0.25 M) in DME, RVC electrodes, 20 °C , 2.5 V,
undivided cell. aReaction run at 50 °C and 5.0 V with 1.0 equiv. of oxalate, 5.0
equiv. of Michael acceptor and 2.5 equiv. of HE. bReaction run at 50 °C and
5.0 V.
Next, we proceeded to investigate the potential application
21]
of alcohol-derived N-phthalimidoyl oxalates[10b,
in the
electrochemical conditions. Despite an abundance of natural
alcohols, direct methods for C(sp3)-C(sp3) bond formation from
alcohols are rare due to the strong CO bond.
Pleasingly, the Giese reaction of N-phthalimidoyl oxalates
with a variety of Michael acceptors were also found to be
efficient when reticulated vitreous carbon (RVC) electrodes were
used in DME (Scheme 2). Quaternary carbons were formed to
give products 20, 32–36 in good yields. Primary benzylic and
secondary alkyl oxalates failed to undergo the conjugate
addition under the standard conditions. However, under an
elevated voltage and reaction temperature (5 V, 50 °C), products
37, 3, 4, 9 and 14 were obtained in moderate yield. This is an
unprecedented example of the Barton deoxygenation reaction
triggered by electrochemical process.
To gain mechanistic understanding of this electrochemical
reaction, control reactions were performed. First, no desired
product was observed in the presence of TEMPO (2.0 equiv.)
under the standard reaction conditions, and the TEMPO adduct
was isolated with 24% yield, suggesting the involvement of
radicals (Figure 2A). Next, reaction conducted with deuterated
HE 38 gave product 3 in 71% yield with no formation of
deuterated product 3′ (Figure 2B). However, when 10% D2O was
used as a co-solvent, deuterated product 3′ was produced in
83% yield with 89% deuterium incorporation. Furthermore, the
Figure 2. A) Giese reaction performed in the presence of 2.0 equiv. of TEMPO. B) Deuterium incorporation experiments. C) Giese reaction performed with
acceptor 39 under the standard conditions. D) Cyclic voltammograms of different substrates at 10 mV/s in DMF, nBu4NBF4 (0.1 M). E) Cyclic voltammograms of
different mixtures.
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Figure 3. Proposed mechanisms for electrochemical C-C bond formation under the standard conditions using A) RAE 1 and B) oxalate 43 as a radical source.
reaction of RAE 1 with 39 bearing a leaving group (OBz) at
allylic α′ position was performed using a method developed by
Overman (Figure 2C).[22] Interestingly, product 41 was formed
exclusively, indicating radical A was involved via single-electron
reduction to α-cyanocarbanion intermediate B, followed by
intramolecular elimination to give 41. These results indicated
that HE acted as an electron-donor, but not a H-donor in this
reaction.
of natural products and peptides has demonstrated its potential
utility in the late-stage functionalization of complex natural
products. Finally, the combination of ubiquitous carboxylic acids
and alcohols with electrochemistry will facilitate their
decarboxylation and deoxygenation in a sustainable and easily
accessible way.
Subsequently, cyclic voltammetry (CV) experiments were
examined to explore the redox behavior of these substrates. As
shown in Figure 2, individual substrates 1 (curve b, −1.15 V), 2
(curve c, -1.95 V) and HE (curve d, +0.89 V) were recorded
respectively (Figure 2D). The reduction peak of 1 (−1.15 V)
indicates a reductive process at the cathode for the generation
of alkyl radical. Furthermore, the mixtures of these substrates
did not exhibit any significant change compared with each
substrate alone (Figure 2E).
Acknowledgements
Financial support for this work from National Natural Science
Foundation of China (21672146, 91753102, 21907064 and
31771584), Natural Science Foundation of Shanghai
(17JC1405300), China Postdoctoral Science Foundation
(2017M622747) and Science Foundation of SZU (000193).
Based on these experiments, a plausible mechanism for
this transformation is outlined in Figure 3. This process starts
with the cathodic reduction of 1 to generate radical C, CO2 and
phthalimide (Figure 3A). Nucleophilic radical C couples with the
terminal carbon of 2, furnishing radical D. Meanwhile, anodic
oxidation of HE produces radical cation E, followed by
subsequent deprotonation to generate strongly reducing radical
F. SET between radical D and intermediate F affords anion G,
followed by protonation to give 3. The plausible mechanism for
the electro-induced Giese reaction under 5.0 V conditions is
similar with that under 2.5 V conditions. In the absence of HE,
radical D will be reduced at the cathode to provide anion G (see
Supporting Information). Interestingly, the mechanism for N-
phthalimidoyl oxalate 43 is different to the RAE precursor
(Figure 3B). After radical I is generated via cathodic reduction, it
undergoes the Giese addition reaction with 2 to provide radical J.
Instead of SET, hydrogen-atom abstraction (HAT) between J
and HE generated the final adduct 32 together with intermediate
F (see Supporting Information).
Conflict of interest
The authors declare no conflict of interest.
Keywords: electrochemistry • cathodic reduction •
decarboxylation • deoxygenation • radical reaction
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Entry for the Table of Contents
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X.Chen, X. Luo, X. Peng, J. Guo, J. Zai
and P. Wang*
Page No. – Page No.
Catalyst-Free
Decarboxylation
of
Carboxylic Acids and Deoxygenation of
Alcohols by Electro-Induced Radical
Formation
An electrochemical strategy produces alkyl radicals from readily accessible redox
active esters and N-phthalimidoyl oxalates under catalyst-free and sustainable
conditions within an undivided cell. The resulting carbon-centred radical was
trapped by an electron-poor olefin or hydrogen atom source to furnish the new
formed C(sp3)(sp3) bond or reductive decarboxylation products, respectively.
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