Electrooxidative CH/PH functionalization as a novel way to synthesize benzo[: B] phosphole oxides mediated by catalytic amounts of silver acetate

Article abstract of DOI:10.1039/c7nj03717a

Electrochemical joint oxidation of diphenylphosphine oxide with acetylenes in the presence of silver acetate (10%) as a catalyst yields benzo[b]phosphole oxide derivatives under mild conditions at room temperature without additional oxidants or initiators

Full text of DOI:10.1039/c7nj03717a

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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DOI: 10.1039/C7NJ03717A
Journal Name
ARTICLE
Electrooxidative CH/PH Functionalization as a Novel Way to
Benzo[b]phosphole Oxides Mediated by Catalytic Amounts of
Silver Acetate
Received 00th January 20xx,
Accepted 00th January 20xx
V.V. Khrizanforova,* K.V. Kholin, M.N. Khrizanforov, M.K. Kadirov, Yu.H. Budnikova
DOI: 10.1039/x0xx00000x
Electrochemical joint oxidation of diphenylphosphine oxide with acetylenes in presence of silver acetate (10%) as catalyst
yields to benzo[b]phosphole oxide derivatives in mild conditions at room temperature without additional oxidants or
initiators and with high yields up to 100%. Redox properties of key intermediate of catalytic cycle have been investigated
by cyclic voltammetry and EPR spectroscopy. The Ph2P(O)Ag intermediate oxidizes easier (+0.05 V vs Fc+/Fc) than all the
reaction precursors to form phosphinoyl radical Ph2P(=O)• fixed as a spin-adduct by ESR. Thus radical mechanism type
going throught Ag+/Ag0 couple was developed for electrocatalytic conditions.
www.rsc.org/
oxidants, such as Mn(OAc)3 or AgOAc (4 equiv) by Satoh, 15 Ag2O (2
equiv) or Pd(OAc)2 with threefold excess of Ag salts and equivalent
amount of metal nitrates by Duan,16 AgOAc (2 equiv) by
Introduction
18
Ackermann,17 TBHP (2 equiv) with CuSO4 or K2S2O8 (5 equiv) by
Phosphorus-containing heterocycles have become of great interest
to the synthetic chemists over the last few years because of their
wide application in organic synthesis, medicinal chemistry and
materials science.1-5 Among them, Benzo[b]phosphole oxide
Zhao.19 Catalytic amounts of metal derivatives (2% CuSO418, 5%
16
16
Ag2O16, 2.5% Cp*RhCl2 or Pd(OAc)2 perform well only in
conditions of excess of various oxidants, with the product yield
derivatives, the phosphorus-containing π-conjugated compounds, dependent on the nature of the latter (therefore, for each
have attracted significant attention as promising organic
optoelectronic materials due to their unique physical and
photoelectric properties.6-14 This explains urgent need in the new
effective, atom-economical catalytic routes of their synthesis.
condition,
a screening is required), as well as at elevated
temperatures and often in excess of diphenylphosphine
oxide..15,16,18 Low temperature for benzo[b]phosphole oxides
preparation was used only in some instances. Thus, only Lakhdar
synthesized benzo[b]phosphole oxides under photocatalytic
conditions by using eosin Y as the catalyst and N-ethoxy-2-
methylpyridinium tetrafluoroborate as the oxidant at lower
20
temperature, 35°C,
and Satoh used room temperature in the
presence of 4 equiv of Mn(OAc)315, but the yields were lower than
at 100°C. The use of additional reagents (oxidants, bases), typical
for the traditional reactions, leads to the increase of product’s
costs, complicates their separation and increases the wastes.
The mechanism of the most Ag-catalyzed reactions is proposed as
radical type based on the absence of target product when some
equivalent of TEMPO presents in reaction mixture. However, the
failure of the reaction in the presence of TEMPO can be explained
by the fact that the latter is easily oxidized itself by the applied
oxidant К2S2O8, for example. Moreover, if TEMPO was taken in
equivalent amount or two-fold excess over the oxidant, the entire
oxidant was spent by the side reaction of TEMPO oxidation, which is
easily oxidized to about 0 V ref. Fc+/Fc or +0.42 V ref.Ag/AgCl.21-22
But in 2017 Li group23 described novel Mn(II)-promoted tandem
phosphorylation/cyclization reaction with phosphine oxides and
suggested that unlike all the previously discussed reactions, the
Scheme 1. Synthesis of benzophosphol oxide derivatives
Since 2013 a lot of various one-stage methods for the synthesis of
benzo[b]phosphole oxide derivatives 3 through dehydrogenative
annulation (Scheme 1) have been demonstrated.15-20 Disadvantages
of all described methods are high temperature and using excess of
Dr. V.V. Khrizanforova
A.E. Arbuzov Institute of Organic and Physical Chemistry Russian Academy of
Sciences, Kazan Scientific Center, Arbuzov str. 8, 420088 Kazan, Russian Federation,
Fax: (+7)8432752253
E-mail: khrizanforovavera@yandex.ru†
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formation of P-cycle is carried out by an ionic, not radical, path. As a
rule, proposed cycles are tentative and can’t explain the use of
silver salt excess for example.15,16
View Article Online
DOI: 10.1039/C7NJ03717A
In recent years the successful series of electrocatalytic reactions
with C-C, C-P bonds formation catalyzed by metal complexes in
reductive or oxidative conditions have been performed.24-33 In
present work the synthesis of benzo[b]phosphole oxides from
diphenylphosphine oxide and acetylenes catalyzed by Ag salt (0.1
equiv.) at room temperature in electrochemical conditions has been
demonstrated. In this case an excess of oxidants or Ag salt is not
required due to electrochemical regeneration of catalyst's active
form near electrode's surface.
Fig. 1 31P NMR monitoring of electrochemical oxidation of diphenylphosphine
oxide and diphenylacetylene mixture before and after 1 and 2 F electricity was
passed
Cyclic voltammetry
Results and Discussion
For the detailed investigations of reaction pathway the methods of
cyclic voltammetry (CV), electron-spin resonance spectroscopy
(ESR) were used. At first redox properties of AgOAc, Ph2P(O)Ag –
key intermediate of catalytic cycle (formed from AgOAc and
diphenylphosphine oxide), acetylenes and Ph2P(O)H have been
studied by cyclic voltammetry. Acetylenes and Ph2P(O)H are not
redox active.
Electrocatalytic oxidative synthesis
AgOAc is frequently used as an oxidant or catalyst for successful Ag-
mediated CH/PH functionalization of diphenylphosphine oxide by
acetylenes.15-17,34,35
It was found that joint electrochemical oxidation of
diphenylphosphine oxide and acetylene in the presence of 0.1
equiv. AgOAc at room temperature leads to the formation of
benzo[b]phosphole oxide 3 with mild to high yields (Table 1).
CVs of AgOAc contain one reduction and two oxidation peaks at -
0.48 V and -0.16, 1.43 V (vs. Fc+/Fc) for Ag(I/0) and Ag(0/I/II),
respectively in CH3CN (Fig. 2).There is little data on oxidation
potentials for Ag0/Ag+/Ag2+ couples in organic media in literature.
Silver (II) high standard potential in acidic water (E0 = 1.98 V vs.
normal hydrogen electrode, NHE)37 is found. Potentials for Ag+/Ag0
couple (strongly dependant on solvent and counter-ions) is 0.04 in
acetonitrile.38 AgBF4 reveals the following peaks at -0.28, 0.016,
1.40 V (vs. Fc+/Fc) for Ag(I/0) and Ag(0/I/II) in similar conditions.
Thus, the excess of Bu4NBF4 supporting electrolyte does not
substitute OAc- in silver salt AgOAc and is suitable for investigation.
We proposed that one key step of investigated catalytic cycle
should be AgP(O)Ph2 oxidation.16,35 Some contradictions should be
noted in the studies of Ag-catalyzed C-H phosphonations. Most of
the numerous publications postulate the instant formation of a
phosphinoyl radical in the reaction of Ag(I) with Ph2P(O)H. 39-41
However, at room temperature, when protected from visible light,
this compound is stable and does not change the properties when
31P NMR monitoring of electrochemical oxidation of reaction
mixture revealed the need for 2F electricity per 1 mole of
diphenylphosphine oxide
cyclization product 3 with yield up to 100% (Table 1, Fig. 1). The
electrolysis potential is 0.15-0.35
(vs. Fc+/Fc). Thus, the
1 for conversion of 1 into target
V
diphenylphosphine oxide 1 (commercially available, δ = 22.07)36
fully converts to benzo[b]phosphole oxide after passing of 2F
electricity per 1 mole of phosphine oxide with δ = 37.0-40.0.16 The
31P NMR spectra monitoring is demonstrated in Figure 1. The yield
of target benzo[b]phosphole oxides is 85-100% in case of symmetric
acetylenes and 40-50 % for asymmetric acetylenes. In case of 3d-f
only one cyclization product is formed.
Table 1 Yields of isolated benzo[b]phosphole oxides and conversion based on 31P NMR
stored at least for several months. Sometimes the participation of
monitoring (R1, R2 from Scheme 1).
16,35
AgP(O)Ph2 was assumed, but was not proved, for example in
.
R1
R2
Spectral yield, % Isolated yield, %
We obtained Ph2(O)PAg through previously reported method47-49
and characterized spectrally. The investigation of redox properties
of key Ph2(O)PAg intermediate by cyclic voltammetry was not
carried out yet. CVs of Ph2P(O)Ag contain only two oxidation peaks
at 0.05 and 1.85 V (Fig. 2). The first oxidation peak of Ph2P(O)Ag is
close to potential of electrolysis which involves electrochemical
phosphonation/cyclization. Thus it was confirmed that the
preparative electrochemical oxidation of diphenylphosphine oxide
with acetylenes proceeds at the potential of the first oxidation peak
of Ph2P(O)Ag.
Compound
3a
3b
3c
3d
3e
Ph
Ph
100
90
85
50
40
90
85
80
40
35
p-NO2-Ph p-NO2-Ph
Pr
Ph
Ph
Pr
Et
Pr
3f
Ph
Bu
40
35
3g
55
35
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(form Ph radical),45 camphorquinone (CQ)/diphenyl
View Article(2O,4nl,in6e-
DOI: 10.1039/C7NJ03717A
trimethylbenzoyl)-phosphine oxide (TPO)/amine photoinitiating
systems
isopropylthioxanthone,
(form
benzoyl
radical),44
camphorquinone,
benzophenone,
2-
2,4,6-(4-
methoxyphenyl)thiopyrylium tetrafluoroborate, and Eosin-Y42 are
used as oxidants and photosensibilisators. Thus, on the way to
phosphinoyl radical different types of radicals (phenyl, benzoyl,
etc.) have been observed. This fact complicates the estimation of
true PBN/phosphinoyl radical adduct parameters. The data closest
to our parameters were described by Buettner.52 Thus
electrooxidative conditions (without additional oxidants) for
Ph2P(O)Ag oxidation are the most correct for the detection of
phosphinoyl radical.
Thus we can conclude that Ph2P(=O)• reacts readily with the spin
trap, and the spin adduct is quite stable.
Scheme 2 Trapping of phosphorus-centered radical by PBN.
Fig. 2 CVs of AgOAc and Ph2(O)PAg in CH3CN. Conditions: 0.1 V/s scan rate, 0.1 M
Bu4NBF4, WE:GC, RE:Pt.
ESR study and mechanistic considerations
Thus the oxidation of Ph2(O)PAg salt at 0.05 V (vs. Fc+/Fc) is the key
step of electrochemical reaction. To establish the character of the
catalytic cycle and the intermediate P(O)-Ag bond scission (ionic or
radical type), we carried out a number of ESR experiments under
anaerobic conditions. Joint oxidation of Ph2P(O)Ag and spin trap
PBN (PBN = N-tert-butyl-α-phenylnitrone) mixture at 0.05 V (vs.
Fc+/Fc) in ESR-spectroelectrochemical cell is characterized by
appearance and increase in intensity of ESR signal from the PBN-
bound radical species of Ph2P(O) adduct (Fig.3, Scheme2).
Fig 3. ESR spectra obtained during anodic joint electrolysis of Ph2P(O)Ag and spin trap
PBN solution (left) at 0.05V (vs. Fc+/Fc) in CH3CN, recorded at 293 K with simulations.
The result suggested that this reaction follows a radical pathway. It
should be noted that in addition to this spectrum (Fig. 3), a very
wide line with g = 2.34 and ΔH peak-peak = 1200 G (the black line in
Fig. 4) was observed. This signal can be apparently assigned to the
"metallic" Ag (0),42 the particles of which precipitate on the bottom
of the cell.
Interestingly, the addition of diphenylacetylene does not
qualitatively change the type of the ESR spectra recorded during
electrolysis of this mixture at potential of 0.15 V. However, intensity
of the spectrum of Ph2P(O)•-PBN decreases, but the intensity of the
Ag(0) signal increases (Fig. 4) approximately four times. This
circumstance well confirms the possible mechanism of the catalytic
cycle, including the observed intermediates Ph2P(O)Ag, Ph2P(O)•
and Ag(0).
The ESR spectrum of the PBN adduct includes the splitting on the
nuclei of nitrogen and hydrogen (in the α-position) and a significa
nt splitting on the nucleus with nuclear spin I = ½, which is the
nucleus of phosphorus in the β-position. The magnetic resonance
parameters were obtained as a result of the simulation: g = 2.0059,
aN= 14.5 G, aH=4.3 G, aP = 14.9 G. The observed g values at about g =
2.006 indicate at the formation of PBN bound radical species and
other magnetic resonance parameters are characteristic of
phosphorus-containing PBN spin adducts.50-51These parameters are
close to those reported for the phosphinoyl Ph2P(=O)• radicals, but
are slightly different from them since they were obtained elsewhere
in other conditions.42-45
For example, in56 the presence of PBN/phosphinoyl radical adduct
Ph2P(O)-PBN was revealed, but the presence of the benzoyl radical
45
was also detected and spectra are complicated. In they observed
the spectrum of phenyl-PBN radical adduct, formed after phenyl –
radical generation, as a precursor to phosphinoyl radical. Ph2I+OTf
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DOI: 10.1039/C7NJ03717A
0
1000 2000 3000 4000 5000 6000
Magnetic field / G
with R-C
C-R
4
3
2
1
0
Scheme 3 Proposed catalytic cycle
Experimental
Solvents were purified, dried and distilled before use. The
supporting electrolyte, Bu4NBF4, used for electrochemical studies
was purchased from Sigma-Aldrich and dried overnight at 100 °C
under vacuum prior to use. Anhydrous acetonitrile (>99.8%) used
for electrochemistry was purchased from Sigma-Aldrich.
without R-C
C-R
Cyclic voltammetry measurements were performed with Е2Р
potentiostat of BASi Epsilon (USA) composed of measuring block,
Dell Optiplex 320 computer with installed Epsilon ES-USB-V200
program, and С3 electrochemical cell. Stationary glassy-carbon
electrode (3.0 mm dia.) was used as working electrode. Platinum
wire of 0.5 mm diameter was used as auxiliary electrode. The
Ag/AgCl was used as reference electrode. Ferrocenium is used as
internal standard. The potential of Fc+/Fc is 0.45 V vs. Ag/AgCl.
Measurements were performed under argon.
0
1000 2000 3000 4000 5000 6000
Magnetic field / G
Fig.
4 Comparison of Ag(0) signals in the presence (red) or absence(black) of
diphenylacetylene (upper), and the intensities of these signals (lower).
A mixture of Ph2P(O)H and PBN or Ph2P(O)Ag and PBN in the
absence of applied potential at room temperature does not give
noticeable ESR spectra.
Thus, catalytic cycle on Scheme 3 was suggested based on ESR and
CV experiments. The proposed catalytic pathway includes the
formation of the Ph2P(O)Ag at the first step; its oxidation leads to
Ph2P(O)• radical and Ag+ regeneration. Then the radical addition to
the alkyne affords an alkenyl radical 4. Next, the intramolecular
addition of the alkenyl radical 4 to the aryl ring at the ortho position
of the phosphorus atom, followed by the oxidation with Ag+ and the
elimination of a proton, affords the phosphorus heterocycle 3. Ag0-
intermediate is formed in the presence of acetylene at the transient
step, as it was detected by ESR. Keeping in mind these details, it is
All NMR experiments were performed with Bruker AVANCE-400
1
spectrometer. Frequencies 400.13 MHz in Н NMR, 161.90 MHz in
31Р NMR were referenced to residual signal of solvent. 31P chemical
shifts were referenced to the signals of 85% H3PO4 (0.00 ppm).
The ESR spectra were registered on a Brucker Elexsys E500 X-range
spectrometer with an ESR electrolysis cell operating in the three
electrode scheme placed into the resonator. Platinum spiral was
used as working electrode, platinum filament was used as auxiliary
electrode, and Ag/AgCl was used as reference electrode. Prior to
each experiment, helium was passed for 5 min through solution in
the cell. When passing was complete, the cell was filled with
helium. The spectra were registered in the potentiostatic mode.
The spectra were simulated by WinSim.
•
possible to explain the decrease of P(O)Ph2 ESR signal intensity in
the presence of alkyne (concurrent radical trapping with this
substrate) and subsequent increase of ESR signal of silver
aggregates (due to oxidation of resulting cyclization product). In
contrast to methods from the literature, for example, the Duan
protocol,16 Ag(I) completely regenerates during catalytic cycle, since
it is easily oxidized at anode rather than precipitates.
The preparative electrolysis was performed using a direct current
source B5-49 in a three-electrode cell with 40 ml volume with a
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separation of the anode and cathode compartments. The potential JHH=7.3, 3H), 0.97-0.85 (m, 2H). 31P NMR (161.9 MHz, CDCl3) δ 38.7
View Article Online
DOI: 10.1039/C7NJ03717A
value of the working electrode was recorded using a direct-current (s).
V7-27 voltmeter in relation to the Ag/AgCl (0.01 M, NaCl) reference
electrode that had two sections separated with Vycor, the second
of which contained a saturated solution of the background salt in
3g: 1H NMR (400 MHz, CDCl3) δ 7.73-7.68 (m, 6H), 7.51-7.50 (m,
4H), 7.15-7.12 (m, 6H), 7.07-7.02 (m, 6H). 31P NMR (161.9 MHz,
CDCl3) δ 37.1 (s), 37.5 (s).
CH3CN. The surface area of the working platinum (Pt) U-shaped
electrode amounted to 48.00 cm2. A ceramic plate with the pore
size of 900 nm was used as a membrane. During the preparative
synthesis, the electrolyte was continuously stirred using a magnetic
stirrer with a continuous inflow of inert gas that was run through a
purification system in order to remove any traces of oxygen and
other gaseous impurities. PyHBF4 was used as supporting
electrolyte.
AgP(O)Ph2 was obtained by mixing and stirring of 1 equiv. of AgOAc
and 1 equiv. of diphenylphosphine oxide at room temperature
1
overnight in CH3CN. H NMR (400 MHz, CDCl3)δ 7.74-7.69 (m, 4H),
7.46-7.42 (t, 2H), 7.36-7.32 (m, 4H). 31P NMR (161.9 MHz, CDCl3) δ
32.9(s).
Electrochemical cell contained 0.4 mmol of diphenylphosphine
oxide (CAS 4559-70-0), 0.4 mmol of appropriate acetylene and
0.04mmol of AgOAc in 20 ml of CH3CN with 0.1 M of PyHBF4, the
solution was agitated with a magnetic stirrer. 2 F of electricity per
one mole of diphenylphosphine oxide (22 mA for 0.4 mmol) was
passed through the solution. The electrolysis potential is 0.15-0.35
V vs. Fc+/Fc. The electrolysis time is just over an hour. All syntheses
were carried out at ambient temperature. After electrolysis time is
over the reaction mixture was filtered and concentrated under
vacuum. Then dry residue was dissolved in CHCl3 and PyHBF4
(poorly soluble in CHCl3) was filtered off. The reaction mixture in
CHCl3 was concentrated under vacuum again and the residue was
purified by silica gel chromatography with hexane/ethyl acetate =
1/1 to afford the product as a white to yellow color solid. The
spectra of cyclization products are consistent with the literature
data.16,53 PyHBF4 can be further purified by recrystallization from
alcohol and used again.
Conclusions
Thus, the electrochemical oxidation of diphenylphosphine oxide
with acetylene in equal amounts was performed without chemical
oxidants and additives at room temperature with 0.1 equivalent of
Ag salt as catalyst. The yield of target benzo[b]phosphole oxide is
up to 100%. The redox properties of key intermediate were
investigated. The oxidation of Ph2P(O)Ag with the formation of
phosphinoyl radical is a key catalytic step for electrochemical
reaction. The total regeneration of Ag(I) during electrolysis leads to
the use of catalytic amounts of Ag salt.
Acknowledgements
The work is supported by Russian Science Foundation (grant № 14-
23-00016).
1
3a: H NMR (400 MHz, CDCl3) δ 7.90-7.80 (m, 2H), 7.75-7.70 (m,
5H), 7.55-7.60 (m, 1H), 7.55-7.50 (m, 5H), 7.40-7.35 (m, 6H). 31P
Conflicts of interest
NMR (161.9 MHz, CDCl3) δ 38.0 (s).
There are no conflicts to declare.
3b: 1H NMR (400 MHz, CDCl3) δ 7.80 (m, 3H), 7.65 (m, 2H), 7.50 (m,
3H), 7.35 (m, 7H), 7.00 (m, 2H). 31P NMR (161.9 MHz, CDCl3) δ 38.0
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New Journal of Chemistry
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DOI: 10.1039/C7NJ03717A
Electrochemical oxidation synthesis of benzo[b]phosphole oxides has been performed at room
temperature with catalytic amounts of AgOAc.