Electrosynthesis of Phosphacycles via Dehydrogenative C-P Bond Formation Using DABCO as a Mediator

Article abstract of DOI:10.1021/acs.orglett.1c00807

The first electrochemical synthesis of diarylphosphole oxides (DPOs) was achieved under mild conditions. The practical protocol employs commercially available and inexpensive DABCO as a hydrogen atom transfer (HAT) mediator, leading to various DPOs in moderate to good yields. This procedure can also be applied to the synthesis of six-membered phosphacycles, such as phenophosphazine derivatives. Mechanistic studies suggested that the reaction proceeds via an electro-generated phosphinyl radical.

Full text of DOI:10.1021/acs.orglett.1c00807

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Letter
Electrosynthesis of Phosphacycles via Dehydrogenative C−P Bond
Formation Using DABCO as a Mediator
Yuji Kurimoto, Jun Yamashita, Koichi Mitsudo,* Eisuke Sato, and Seiji Suga*
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ABSTRACT: The first electrochemical synthesis of diarylphosphole oxides (DPOs) was achieved under mild conditions. The
practical protocol employs commercially available and inexpensive DABCO as a hydrogen atom transfer (HAT) mediator, leading to
various DPOs in moderate to good yields. This procedure can also be applied to the synthesis of six-membered phosphacycles, such
as phenophosphazine derivatives. Mechanistic studies suggested that the reaction proceeds via an electro-generated phosphinyl
radical.
hosphole oxide-containing π-conjugated molecules have
been attracting attention because they have high electron
coupling using a secondary hydrophosphine oxide as a
substrate has been reported.6,7 In these methods, the target
products were synthesized by either direct electrolysis6 or
mediated electrolysis7 using a transition metal mediator, such
as Cu,7a Rh,7b Ag,7c Mn,7d,e or Ni.7f Mediated electrolysis is
usually more practical than direct electrolysis because it has a
wider substrate scope and facilitates reactions under milder
conditions. Furthermore, mediated electrolysis using a non-
metallic mediator is useful as a more attractive methodology
from the perspective of green chemistry. Thus, we aimed to
develop a method for the synthesis of DPOs from BPOs using
mediated electrolysis (Scheme 1B, red arrow). We report here
the first electrochemical synthesis of DPOs. This synthetic
method can also be applied to the synthesis of six-membered
phosphacycles, such as phenophosphazine derivatives.
P
acceptability and thermal and chemical stabilities and have
been used as basic skeletons for organic functional materials.1
Among them, diarylphosphole oxide (DPO) is a key motif in
the design of phosphole-containing organic functional
materials.2 DPOs are generally synthesized by the bislithiation
of 2,2′-dihalobiaryl derivatives, followed by trapping with
PhPCl2 (Scheme 1A);3 however, this conventional synthetic
procedure requires tedious and multistep sequences with
complicated and unstable starting substrates and reagents. As a
more straightforward route to DPOs, the intramolecular
cyclization of biarylphosphine oxides (BPOs) has recently
received attention. BPOs are readily available and easy-to-
handle starting substrates. Several efficient methods have been
reported for the synthesis of DPOs with this protocol (Scheme
1B, black arrows).4 However, these methods require transition
metal catalysts, strong acids, or excess amounts of acid.
Therefore, the development of a transition metal-free and
environmentally benign synthetic methodology to synthesize
DPOs is highly desirable.
We started the optimization studies with BPO (biphenyl-
phenylphosphine oxide, 1a) as a model substrate (Table 1).
The electrolysis was first conducted under a constant current
of 2.5 mA in an undivided cell equipped with a platinum (Pt)
anode and a Pt cathode, using Bu4NBF4 as a supporting
electrolyte in a mixed solvent of CH3CN/H2O (99:1). By
Meanwhile, electrochemistry offers a green and efficient
alternative to conventional chemical approaches.5 By employ-
ing only an electric current as an inexpensive and sustainable
reducing or oxidizing agent, the amount of waste is diminished
and toxic reagents can be avoided. To the best of our
knowledge, the synthesis of DPOs from BPOs via an
electrochemical method has never been reported. On a related
topic, electrochemical dehydrogenative C−H and P−H cross-
Received: March 8, 2021
Published: April 5, 2021
https://doi.org/10.1021/acs.orglett.1c00807
© 2021 American Chemical Society
Org. Lett. 2021, 23, 3120−3124
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(Table 1, entry 1). A 1.0 mmol scale reaction could also be
conducted to afford 2a in a moderate yield, indicating the
scalability of this protocol (Table 1, entry 2). The use of
quinuclidine and Et3N instead of DABCO resulted in inferior
efficiency (Table 1, entries 4 and 5, respectively). The
replacement of Bu4NBF4 by other supporting electrolytes,
such as LiBF4 and Bu4NClO4, was found to be inappropriate
for this reaction (Table 1, entries 6 and 7, respectively).
Reducing and increasing the electric current resulted in poor
results (Table 1, entries 8 and 9, respectively). Increasing the
amount of charge decreased the yield (Table 1, entries 10 and
11, respectively). Changing the amount of DABCO and the
amount of H2O negatively affected the yield (Table 1, entries
12−17). Moreover, without the electric current, this electro-
chemical progression was mostly suppressed (Table 1, entries
18 and 19). These results suggest that both the electric current
and DABCO are essential for the progress of this reaction.
We next investigated the substrate scope for the synthesis of
DPOs 2 via electrochemical cyclization (Scheme 2). BPOs
bearing several substituents on the benzene ring, including Me,
Scheme 1. Synthesis of Diarylphosphole Oxides (DPOs)
a
Scheme 2. Substrate Scope
a
Table 1. Optimization for the Synthesis of DPO 2a
entry
deviation from standard conditions
none
yield (%)
b
1
71 (62)
2
3
1.0 mmol scale
no DABCO
48
8
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
quinuclidine instead of DABCO
Et3N instead of DABCO
LiBF4 instead of Bu4NBF4
Bu4NClO4 instead of Bu4NBF4
2.0 mA instead of 2.5 mA
3.0 mA instead of 2.5 mA
2.5 F mol−1 instead of 2.0 F mol−1
3.0 F mol−1 instead of 2.0 F mol−1
DABCO (0.5 equiv)
DABCO (1.0 equiv)
DABCO (2.5 equiv)
performed in CH3CN/H2O (99.5/0.5)
performed in CH3CN/H2O (98/2)
performed in CH3CN
27
16
61
49
64
54
62
59
29
43
58
67
66
50
11
N.D.
no electric current
no electric current, no DABCO
a
Reaction conditions are as follows: 1a (0.4 mmol) and DABCO (0.8
mmol) in CH3CN/H2O (3.96/0.04 mL) with 0.1 M Bu4NBF4 as a
supporting electrolyte were electrolyzed with 2.0 F mol−1 of charge
under a constant current of 2.5 mA at 25 °C. The yield was
determined by 1H NMR analysis using 1,1,2,2-tetrachloroethane as an
b
internal standard. N.D. stands for not detected. Isolated yield.
direct electrolysis at room temperature, the corresponding
product 2a was obtained in an 8% yield, and the starting
material 1a was mainly recovered (Table 1, entry 3). We next
examined mediated electrolysis using a tertiary amine as a
mediator.8 The reaction with DABCO gave 2a in a 71% yield
a
Reaction conditions are as follows: 1 or 3 (0.4 mmol) and DABCO
(0.8 mmol) in CH3CN/H2O (3.96/0.04 mL) with 0.1 M Bu4NBF4 as
a supporting electrolyte were electrolyzed with 2.5 F mol−1 of charge
under a constant current of 2.0 mA at 25 °C. Isolated yield.
b
c
Performed with 2.5 F mol−1 of charge. N.D. stands for not detected.
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OMe, F, CF3, and Cl, were applicable, and the corresponding
products 2b−k were obtained in moderate to high yields.
Furthermore, this reaction can be applied to the synthesis of
heteroring-fused DPOs 2i and 2i′. The reactions were slightly
inhibited by a substituent at the ortho-position, and the
corresponding DPOs 2g and 2h were isolated in respective
yields of 55% and 54%. Unfortunately, the reaction of
substrates 1l−n did not give the desired product (2l−n,
respectively). This synthetic method can also be applied to the
synthesis of six-membered phosphacycles (4a−4c). To the
best of our knowledge, this is the first electrochemical synthesis
of 4 from 3.9 In the case of substrate 3d, the corresponding
product 4d was not observed under the standard conditions,
and the oxidized six-membered phosphacycle 4e was
selectively obtained (Scheme 3i). After further investigation,
potential electrolysis at 1.5 V (3.6 h). From these results,
DABCO•+ generated by the electro-oxidation of DABCO
should serve as a key reactive intermediate.
Other control experiments were carried out to gain
additional insights. When 2,2,6,6-tetramethyl-1-piperidinyloxyl
(TEMPO) was added to the system, the target product 2a was
not obtained and the TEMPO-trapped product 5 was detected
by ESI-HRMS (Scheme 5).11,12 This result strongly suggests
that a phosphinyl radical was generated in situ by the reaction
of 1a with DABCO•+ and that the reaction should proceed via
a radical pathway.
Scheme 5. Radical Trapping Experiment
Scheme 3. Switching Synthesis of Six-Membered
Phosphacycles
On the basis of CVs and the control experiments described
above as well as related references,6,8 a plausible mechanism for
the electrochemical synthesis of DPO is suggested in Figure 1.
we were delighted to find that 4d was selectively obtained
when MeOH was used instead of H2O (Scheme 3ii). These
results suggest that the oxygen source for 4e would be H2O.8
To gain further insight into the reaction mechanism, cyclic
voltammetry (CV) was performed.10 The CV of DABCO
exhibits a quasi-reversible redox couple (EOx1 = 0.32 V vs Fc/
Fc+) and an irreversible wave (EOx2 = 1.50 V), which
correspond to the oxidation of DABCO to form DABCO•+
and that of DABCO•+ to the DABCO biradical cation,
respectively. Model substrate 1a showed an irreversible
oxidation wave around 1.83 V. A catalytic current was not
observed in the mixture of 1a and DABCO. We next examined
constant potential electrolysis (Scheme 4i). The electrolysis
was first conducted under a constant potential at 0.9 V vs Ag/
Ag+ (22.8 h), and the target product 2a was obtained in a 28%
yield. Similarly, 2a was obtained in a 14% yield by the constant
Figure 1. Plausible reaction mechanism for the electrochemical
synthesis of DPO.
First, DABCO is oxidized into DABCO•+ by anodic oxidation.
Subsequently, DABCO•+ abstracts a hydrogen atom from the
P−H bond of BPO to generate intermediate A and
DABCOH+. Finally, the intramolecular cyclization of inter-
mediate A, followed by hydrogen elimination, would then give
DPO.13 Two reaction pathways can be considered in this
hydrogen elimination step. One possibility is that DPO is
produced by the anodic oxidation of intermediate B (path A).
The other possibility is that the DABCO•+ generated by
anodic oxidation acts as a HAT mediator and abstracts a
hydrogen atom from intermediate B (path B).4c The generated
DABCOH+ or H2O would be reduced at the cathode.
DABCOH+ would be relatively unstable under the reaction
conditions, and the reduction of added H2O would promote
the efficiently electrochemical process.
Scheme 4. Mechanistic Studies
The experimental results shown in Scheme 5 support the
formation of this intermediate A. As mentioned above, no
catalytic current was observed in the CV measurement of the
mixture of 1a and DABCO. This is probably due to the slow
reaction rate between 1a and DABCO•+. The CV measure-
ment also revealed the instability of DABCO•+, which could be
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reaction.
In conclusion, we achieved the first electrochemical
synthesis of DPOs from BPOs under mild conditions. The
protocol does not use a transition metal mediator and instead
uses readily available and inexpensive DABCO as an organic
HAT mediator. A variety of BPOs could be obtained by the
electrochemical method. This method can also be applied to
the synthesis of six-membered phosphacycles. The control
experiments suggest that a phosphinyl radical was generated in
situ, and the reaction would proceed via a radical pathway.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00807.
Experimental details and spectra data for all new
compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Koichi Mitsudo − Division of Applied Chemistry, Graduate
School of Natural Science and Technology, Okayama
University, Okayama 700-8530, Japan; orcid.org/0000-
0002-6744-7136; Email: mitsudo@okayama-u.ac.jp
Seiji Suga − Division of Applied Chemistry, Graduate School
of Natural Science and Technology, Okayama University,
Okayama 700-8530, Japan; orcid.org/0000-0003-0635-
2077; Email: suga@cc.okayama-u.ac.jp
̋
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Authors
Yuji Kurimoto − Division of Applied Chemistry, Graduate
School of Natural Science and Technology, Okayama
University, Okayama 700-8530, Japan
Jun Yamashita − Department of Applied Chemistry and
Biotechnology, Faculty of Engineering, Okayama University,
Okayama 700-8530, Japan
Eisuke Sato − Division of Applied Chemistry, Graduate School
of Natural Science and Technology, Okayama University,
Okayama 700-8530, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00807
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported in part by JSPS KAKENHI Grants
JP18J21360, JP19K05477, JP19K05478, and JP18H04455 in
Middle Molecular Strategy.
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