Electrochemical Oxidative Phosphorylation of Aldehyde Hydrazones

Article abstract of DOI:10.1021/acs.orglett.0c01343

The electrochemical phosphorylation of aldehyde hydrazones has been developed under exogenous oxidant-free conditions. The strategy provides expedient access to highly functionalized α-iminophosphine oxides with ample scope and broad functional group tole

Full text of DOI:10.1021/acs.orglett.0c01343

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Letter
Electrochemical Oxidative Phosphorylation of Aldehyde Hydrazones
Zhongnan Xu,§ Yueheng Li,§ Guangquan Mo,§ Yucheng Zheng, Shaogao Zeng, Ping-Hua Sun,*
and Zhixiong Ruan*
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ABSTRACT: The electrochemical phosphorylation of aldehyde
hydrazones has been developed under exogenous oxidant-free
conditions. The strategy provides expedient access to highly function-
alized α-iminophosphine oxides with ample scope and broad functional
group tolerance by means of mild, user-friendly electrolysis, in an
undivided cell.
rganophosphorus compounds are of great importance
with transformative power for replacing sacrificial oxidizing or
Osince they are widely found in organic catalysis, reducing reagents.8 In this context, considerable progress has
medicinal chemistry, material science, and agriculture science.1
As a consequence, there is a continued strong demand for the
been accomplished in C−H functionalization by electro-
chemical anodic oxidation, such as C−H amination,9
alkylation,10 oxygenation,11 and thiolation.12 In spite of
development of phosphorus-containing compounds using
synthetic methods via step-economical C−P bond formation.2
undisputed advances, the electrochemical oxidative C−H
phosphorylation is still underdeveloped.13 Anodic oxidative
Although significant advances have been made in C−H
phosphorylation by employing transition metal catalysis3 and
phosphonation of arenes as an early example was reported by
Effenberger and Kottmann.14 However, for nearly three
decades, only a few elegant examples of electrochemical C−
H phosphorylation have been explored, including aryl C−H
phosphorylation by palladium15 or rhodium catalysis16 and
electron-deficient heteroaromatic C−H phosphonation reac-
tions.17 Herein, within our program on electrochemical
transformation,18 we have now uncovered an unprecedented
electrochemical phosphorylation of aldehyde hydrazones under
external oxidant-free conditions (Scheme 1b). Notable features
of our findings include (a) exceedingly mild reaction
conditions at 23 °C in an undivided cell under ambient air,
(b) avoiding the utilization of chemical oxidizing reagents, and
(c) ample scope toward α-iminophosphine oxides with broad
functional group tolerance.
Our studies were initiated by probing various reaction
conditions for the envisioned electrochemical phosphorylation
of aldehyde hydrazone 1a with diphenylphosphine oxide (2a)
under constant current electrolysis conditions (see Table 1).
After extensive optimization, the optimal results were obtained
when substrates 1a and 2a were electrolyzed at a constant
current in an electrolyte solution of Et4NClO4 in MeCN at
room temperature under atmospheric conditions with a
catalytic amount of MnBr2·H2O. Under these conditions, the
photoredox catalysis4 for the construction of various C−P
bonds, it is difficult to obviate the use of precious transition
metals or stoichiometric chemical redox reagents. As a result, a
direct C−H phosphorylation of quinoxalin-2(1H)-ones under
transition-metal-free conditions was developed by Cui and co-
workers,5 to access functionalized heterocycles bearing α-
iminophosphine oxide skeletons. Imine derivatives, especially
aldehyde hydrazones, are crucial intermediates in synthetic
organic chemistry.6 Recently, Zhu’s group reported a copper-
mediated oxidative C−H phosphorylation of aldehyde
hydrazones with diphenylphosphine oxide for the preparation
of α-iminophosphine oxide by using an excess amount of
K2S2O8 as a chemical oxidant inevitably (Scheme 1a).7
Additionally, organic electrochemistry has been established
as an increasingly powerful strategy for molecular synthesis,
Scheme 1. Conventional and Electrochemical
Phosphorylation of Aldehyde Hydrazones
Received: April 17, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01343
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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a
Table 1. Optimization of Reaction Conditions
Scheme 2. Electrochemical Phosphorylation of
a
Arylaldehyde Hydrazones with 2a
b
entry
deviation from standard conditions
none
no Et4NClO4
yield (%)
1
2
85
0
3
4
5
6
7
8
9
10
11
12
13
n-Bu4NBF4 instead of Et4NClO4
n-Bu4NPF6 instead of Et4NClO4
LiClO4 instead of Et4NClO4
no MnBr2·4H2O
Mn(OAc)3·3H2O instead of MnBr2·4H2O
Cu(OAc)2 instead of MnBr2·4H2O
Cp2Fe instead of MnBr2·4H2O
MeOH instead of MeCN
21
26
13
76
59
73
80
70
73
36
0
MeCN/MeOH (2/1) instead of MeCN
MeCN/H2O (2/1) instead of MeCN
no electricity
a
Reaction conditions: undivided cell, graphite anode, Pt cathode, 1a
(0.5 mmol), 2a (1.0 mmol), MnBr2·4H2O (10 mol %), Et4NClO4
(0.5 mmol), MeCN (5.0 mL), constant current = 4.0 mA, 8 h (2.6 F·
b
mol−1), under air, 23 °C. Yield of isolated products.
desired α-iminophosphine oxide 3aa was achieved in 85%
isolated yield (Table 1, entry 1). The Et4NClO4 salt was found
to be essential for the phosphorylation reaction (entry 2),
while other electrolytes such as n-Bu4NBF4, n-Bu4NPF6, and
LiClO4 showed poor performance (entries 3−5). Control
experiment in the absence of manganese salt resulted in a
slightly lower yield of 3aa, demonstrating that the additive of
manganese salt might probably promote the C−H trans-
formation. In contrast, other transition metal mediators were
found to be less efficient (entries 7−9). Additionally, protic
solvents, such as MeOH and other mixed solvents, were less
efficient than the aprotic solvent (MeCN) (entries 10−12).
Further control experiments verified the essential nature of the
external electricity (entry 13).
With the optimal reaction conditions in hand, we next
explored its versatility with a set of representative aldehyde
hydrazones 1 (Scheme 2). Thereby, the electrochemical
phosphorylation manifold proved amenable to both electron-
rich and electron-deficient substituents. Thus, the robust
nature of the electrooxidative transformation was reflected by
fully tolerating a wealth of synthetically useful functionalities,
including chloro, bromo, phenolic hydroxyl, ester, and cyano,
as well as terminal alkynyl groups, which could serve as a
handle for further late-stage modifications. The electro-
chemical C−H phosphorylation smoothly proceeded with
various sterically hindered substituted substrates (1m−1p),
albeit 1o was in lower yield. Besides, the naphthalene
substrates (1q and 1r) could well convert to the desired
products in good yields. Notably, the reaction could also be
performed with heterocyclic aldehyde-derived hydrazones (1t
and 1u).
a
Reaction conditions: undivided cell, graphite anode, Pt cathode, 1
(0.5 mmol), 2a (1.0 mmol), MnBr2·4H2O (10 mol %), Et4NClO4
(0.5 mmol), MeCN (5.0 mL), constant current = 4.0 mA, 8 h, under
b
air, 23 °C. Isolated yields were given. 50 °C.
tolerated a diverse array of diphenylphosphine oxides bearing
two functionalized aryl groups at the meta- or para-position,
such as methyl, methoxyl, tert-butyl, fluoro, and chloro
substituents (2b−2f). It is noteworthy that the aliphatic
aldehyde-derived hydrazones also proved to be suitable
substrates for the electrochemical phosphorylation and
furnished the desired products with moderate yields (3va−
3ya). The structure of compound 3af was further definitely
determined by X-ray single-crystal diffraction. To our delight,
the electrochemical transformation was not just restricted to
morpholine- and piperidinyl-derived hydrazones; the other
N,N-disubstituted hydrazones with acyclic amino groups, such
as dibenzylhydrazone and 1-methyl-1-phenylhydrazone, were
also tolerated for this electrooxidative phosphorylation (1x−
1a′). Notably, the phenylamine-derived hydrazone 1b′ with
free NH was also suitable for the C−H transformation, albeit
in lower conversion.
To further demonstrate the practical utility of this
electrochemical protocol, a gram-scale synthesis of α-
iminophosphine oxides 3aa was carried out under standard
conditions (Scheme 4). This electrolysis was performed with
graphite as the anode and platinum as the cathode, at a higher
constant current (20 mA), and obtained in 80% isolated yield,
without appreciable loss in efficacy.
Furthermore, encouraged by these exciting results, we next
investigated the compatibility with various substituted
phosphorus coupling partners in the electrochemical C−H
phosphorylation approach (Scheme 3). Hence, with p-
tolualdehyde hydrazone 1a as the C−H donor, the reaction
In consideration of the remarkable performance of the
electrooxidative phosphorylation, we became intrigued with
delineating its mode of action. To this end, intermolecular
competition experiment between electronically discriminated
B
https://dx.doi.org/10.1021/acs.orglett.0c01343
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Letter
Scheme 3. Electrochemical Phosphorylation of Aldehyde
Hydrazones with Diarylphosphine Oxide 2
Scheme 5. Summary of Key Mechanistic Findings
a
cyclopropylvinyl)benzene (5) with 2a under the standard
electrochemical conditions (Scheme 5c).
The cyclic voltammetric (CV) analysis was also performed
to reveal key mechanistic insights into the electrochemical
functionalization (Figure 1 and Figures S1−S4). It was found
a
Reaction conditions: undivided cell, graphite anode, Pt cathode, 1
(0.5 mmol), 2a (1.0 mmol), MnBr2·4H2O (10 mol %), Et4NClO4
(0.5 mmol), MeCN (5.0 mL), constant current = 4.0 mA, 8 h, under
b
c
d
e
air, 23 °C. Isolated yields were given. 8.0 mA. 10 h. 11 h. 7 h,
f
under argon. 50 °C.
Scheme 4. Gram-Scale Reaction
Figure 1. Cyclic voltammograms of 1a, 2a, and MnBr2·4H2O.
Conditions: a glassy carbon working electrode, a Ag/AgCl (3 M KCl)
reference electrode, and a platinum wire counter electrode, Et4NClO4
(0.1 M in MeCN), 100 mV/s scan rate with (a) background; (b) 1a
(1 mM); (c) 1a (1 mM) + 2a (1 mM); (d) 1a (1 mM) + 2a (1 mM)
+ MnBr2·4H2O (1 mM); (e) 1a (1 mM) + MnBr2·4H2O (1 mM); (f)
2a (1 mM); (g) 2a (1 mM) + MnBr2·4H2O (1 mM); and (h) MnBr2·
4H2O (1 mM).
substrates 1 revealed that electron-rich aldehyde hydrazones
were inherently more reactive (Scheme 5a). In addition, 2,4-di-
tert-butyl-4-methylphenol (BHT) was selected as a typical
radical trapping reagent since its oxidative onset potential was
higher than that of substrate 1a (see Figure S2). Thus, the
phosphorylation of substrate 1a was fully suppressed upon the
addition of the radical scavenger BHT with a trace of the
desired product formed, being suggestive of involving a radical
process (Scheme 5b). Gratifyingly, when 1,1-diphenylethene
(4) was subjected to electrolysis with diphenylphosphine oxide
(2a) under the standard conditions, the addition products 4a
and 4b were isolated separately. This result indicated the
involvement of a radical pathway, which was further
demonstrated by the detection of adduct 5a and ring-opening
product 5b from the radical clock reaction of 1-(1-
that 1a could be oxidized at a low potential (Ep/2 = 0.89 V vs
Ag/AgCl), which was even lower than the oxidation potential
of phosphine oxide 2a (Ep/2 > 1 V vs Ag/AgCl). This
observation was different from the previous report.7 Notably,
the additive of MnBr2 could somewhat promote the electro-
oxidation of 1a or the mixture of 1a and 2a with a lower onset
potential and high oxidation current response. However, it is
currently unclear whether the catalysis of MnBr2 is from single
Br− or the synergistic effect of Mn2+ and Br−.19
Based on related literature reports6c,16,17,20 and our studies, a
plausible mechanistic scenario was proposed for the electro-
chemical oxidative phosphorylation of aldehyde hydrazones in
C
https://dx.doi.org/10.1021/acs.orglett.0c01343
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Letter
Scheme 6 based on the direct electrochemical initiation
Pharmacy, Jinan University, Guangzhou 510632, P.R. China;
Email: pinghuasunny@163.com
model,8r omitting the effect of MnBr2. Due to the much lower
Authors
Scheme 6. Proposed Mechanism
Zhongnan Xu − Key Laboratory of Molecular Target & Clinical
Pharmacology and the State Key Laboratory of Respiratory
Disease, School of Pharmaceutical Sciences & the Fifth Affiliated
Hospital, Guangzhou Medical University, Guangzhou 511436,
P.R. China
Yueheng Li − Key Laboratory of Molecular Target & Clinical
Pharmacology and the State Key Laboratory of Respiratory
Disease, School of Pharmaceutical Sciences & the Fifth Affiliated
Hospital, Guangzhou Medical University, Guangzhou 511436,
P.R. China
Guangquan Mo − Key Laboratory of Molecular Target &
Clinical Pharmacology and the State Key Laboratory of
Respiratory Disease, School of Pharmaceutical Sciences & the
Fifth Affiliated Hospital, Guangzhou Medical University,
Guangzhou 511436, P.R. China
Yucheng Zheng − Key Laboratory of Molecular Target &
Clinical Pharmacology and the State Key Laboratory of
Respiratory Disease, School of Pharmaceutical Sciences & the
Fifth Affiliated Hospital, Guangzhou Medical University,
Guangzhou 511436, P.R. China
Shaogao Zeng − International Cooperative Laboratory of
Traditional Chinese Medicine Modernization and Innovative
Drug Development of Chinese Ministry of Education, College of
Pharmacy, Jinan University, Guangzhou 510632, P.R. China
oxidative potential of aldehyde hydrazone 1a than 2a, the
substrate 1a is initially oxidized to carbocation intermediate 6,
which then reacts with 2a′ to form the C−N bond, generating
the aminyl radical intermediate 7. Further anodic oxidation of
7 produces the cation 8, followed by deprotonation, giving the
desired product 3aa. At the cathode, protons are concurrently
reduced to release molecular hydrogen.
In summary, we have developed a mild and efficient method
for the preparation of functionalized α-iminophosphine oxides
with ample scope and high functional group tolerance, by user-
friendly, electrochemical oxidative phosphorylation of aldehyde
hydrazones under external oxidant-free conditions. Impor-
tantly, this protocol could be easily conducted on a gram-scale
reaction. Detailed mechanistic studies provided strong support
for a diverse radical process compared with the previous
report.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01343
Author Contributions
§Z.X., Y.L., and G.M. contributed equally.
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01343.
■
ACKNOWLEDGMENTS
■
sı
Support by the National Natural Science Foundation of China
(21901052), the Guangdong Province Universities and
Colleges Pearl River Scholar Funded Scheme (2019), the
Guangzhou Education Bureau University Scientific Research
Project (201831845), Guangdong Basic and Applied Basic
Research Foundation (2020A1515010722), the National
Natural Science Fundation of China (81872759 to P.-H. S.),
and Guangzhou City Key Laboratory of Precision Chemical
Drug Development (201805010007 to P.-H. S.) is most
gratefully acknowledged.
1
Experimental procedures, characterization data, and H,
13C, 19F and 31P NMR spectra for all products (PDF)
Accession Codes
CCDC 1996719 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
■
Zhixiong Ruan − Key Laboratory of Molecular Target &
Clinical Pharmacology and the State Key Laboratory of
Respiratory Disease, School of Pharmaceutical Sciences & the
Fifth Affiliated Hospital, Guangzhou Medical University,
Guangzhou 511436, P.R. China; orcid.org/0000-0002-
5433-2460; Email: zruan@gzhmu.edu.cn
Ping-Hua Sun − International Cooperative Laboratory of
Traditional Chinese Medicine Modernization and Innovative
Drug Development of Chinese Ministry of Education, College of
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