Direct Electrochemical P(V) to P(III) Reduction of Phosphine Oxide Facilitated by Triaryl Borates

Article abstract of DOI:10.1021/jacs.8b07149

Triaryl borate Lewis acids facilitate the direct two-electron reduction of the P(V) center of triphenylphosphine oxide (TPPO) to the P(III) center of triphenylphosphine at faradaic efficiencies of 37%. Insight from direct P(V) to P(III) reduction is provided from cyclic voltammetry. The electrochemical reduction of TPPO proceeds through an unusual EC_rEC_i mechanism in which the breaking of the phosphoryl bond in a two-electron-reduced association complex with the triaryl borate is rate-determining. The rate and faradaic efficiency for TPPO reduction are tuned by judicious choice of substituents on triaryl borate, with tris(4-methoxyphenyl) borate demonstrating the highest for both. These results suggest that an attractive route toward the roomerature reduction of phosphate for phosphorus reclamation is greatly facilitated by the stabilization of reduced phosphate intermediates through their association with Lewis acids.

Full text of DOI:10.1021/jacs.8b07149

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Article
Direct electrochemical P(V) to P(III) reduction
of phosphine oxide facilitated by triaryl borates
Joseph S. Elias, Cyrille Costentin, and Daniel G. Nocera
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07149 • Publication Date (Web): 02 Oct 2018
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Direct electrochemical P(V) to P(III) reduction of phosphine oxide
facilitated by triaryl borates
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†
§ ,†,
†,
Joseph S. Elias, Cyrille Costentin, * and Daniel G. Nocera *
†Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138
§Laboratoire d’Electrochimie Moléculaire, Unité Mixte de Recherche Université – CNRS 7591, Université Paris Diderot, Sorbonne
Paris Cité, 75205 Paris Cedex 13, France
ABSTRACT: Triaryl borate Lewis acids facilitate the direct two-electron reduction of the P(V) center of triphenylphosphine oxide (TPPO)
to the P(III) center of triphenylphosphine at faradaic efficiencies of 37%. Insight from direct P(V) to P(III) reduction is provided from cyclic
voltammetry. The electrochemical reduction of TPPO proceeds through an unusual EC
r
EC mechanism in which the breaking of the
i
phosphoryl bond in a two-electron-reduced association complex with the triaryl borate is rate-determining. The rate and faradaic efficiency
for TPPO reduction are tuned by judicious choice of substituents on triaryl borate, with tris(4-methoxyphenyl) borate demonstrating the
highest for both. These results suggest that an attractive route towards the room-temperature reduction of phosphate for phosphorus
reclamation is greatly facilitated by the stabilization of reduced phosphate intermediates through their association with Lewis acids.
selectively reduce the phosphine oxide back to the parent
phosphine. The most successful strategies often involve the use of
an excess of silyl halide – most notably trimethylsilyl chloride
Introduction
1
3
Phosphorus is an essential element for life and holds tremendous
importance in the purview of commodity chemical synthesis. As of
(TMSCl) – which is reactive toward attack by a nucleophilic,
2018, the increase in the world consumption of phosphorus
activated phosphoryl bond to produce an equivalent of siloxane per
outpaces global population growth by nearly 23%, with the top two
producers of phosphate rock – China and the United States – on
track to deplete their reserves by the middle of the twenty-first
14,15
two equivalents of silyl halide under reducing potentials.
The
resulting siloxane is thermodynamically stable, and the silyl halide
can only be regenerated through the energy-intensive carbothermal
1
century. Mined phosphate rock is typically reduced to white
reduction of SiO
appropriate alkyl halide. Similarly, other reported strategies often
2
to elemental Si followed by reaction with the
phosphorus (P
subsequently oxidized to P(III) halide or sulfide intermediates
before further modification. Although the oxidation of P is
downhill (ΔH rxn = –304 kcal mol ), the reduction of phosphate
4
) used in fine chemical synthesis, where it is
16,17
18
involve the use of excess reagents (such as AlCl
3
,
or P(OPh) )
3
4
2–
0
–1
that serve as sinks for O after the cleavage of the phosphoryl bond,
0
–1
producing byproducts (Al
2
O
3
, O=P(OPh) ) that can only be
3
rock is considerably endothermic (ΔH rxn = 700 kcal mol ) making
the overall process both energy intensive and inefficient. The
direct, two-electron reduction of P(V) oxoacids to their
synthetically-useful P(III) counterparts, therefore, is of
considerable interest since this reaction is governed by a smaller
regenerated through high-energy routes.
Herein we report that triaryl borate Lewis acids facilitate the
electrochemical reduction of triphenylphosphine oxide (TPPO) to
PPh
3
at glassy carbon electrodes with faradaic efficiencies of 37%.
0
–1
Cyclic voltammetry reveals as a main pathway a unique EC
reaction mechanism in which the breaking of the phosphoryl bond
is rate-determining. As a result of the slow P=O bond-breaking
r
EC
i
thermodynamic penalty (ΔG rxn = 68 kcal mol at pH 7). A
fundamental challenge facing the reduction of P(V) compounds is
the selective cleavage of the phosphoryl (P=O) bond, which has
multiple-bond character and a high bond-dissociation energy (110
step, the two-electron-reduced association complex Ph
3
P–O–
2–
–
1
2
B(OAr)
3
disproportionates to give the four-electron product
kcal mol for H
3
PO). Thus, from both energy and environmental
3
PHPh
2
at nearly identical faradaic efficiencies at glassy carbon
standpoints of sustainability, it is critical to develop direct methods
that can reclaim, recycle, and repurpose phosphorus compounds
from phosphates.
electrodes, a reaction that may be suppressed at gold surfaces. The
overall faradaic efficiency for TPPO reduction can be tuned by
judicious choice of aryl substituents on the Lewis acid, with tris(4-
Phosphines make up an important class of ligands in
organometallic chemistry and they are ubiquitous reagents in the
methoxyphenyl) borate (B(OC
6
H
4
OMe) ) giving the highest
3
4
– 6
7,8
9,10
11,12
faradaic efficiency. The electrochemical reduction of TPPO with
such Lewis acids proceeds with the formation of coordinatively-
saturated aryl borate species, which can be regenerated to B(OAr)
through hydrolysis and subsequent condensation with ArOH and
appropriate volumes of toluene. The methodology reported herein
for the reduction of TPPO bypasses the use of high-energy density
Appel,
Mitsunobu, Staudinger,
and Wittig reactions,
where the thermodynamic preference to form the phosphoryl bond
drives these transformations. As the resulting P(V) phosphine
oxide byproduct is generally discarded after separation, recent
efforts have been directed toward developing strategies to
3
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Table 1. Electrochemical reduction of TPPO with B(OAr)
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B(OAr)
3
F.E. (PPh
3
)/% a
F.E. (PHPh
)/% a
2
none
0
0
B(OPh)
3
23(10)
37(5)
21(4)
40(7)
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B(OC
6
H
4
OMe)
3
a
Faradaic efficiencies were averaged over four points during 30 min
Figure 1. Cyclic voltammetry of triphenylphosphine oxide (1 mM)
with the titration of B(OC OMe) . Experiments were carried out in
0.1 M TBAClO /MeCN at a glassy carbon electrode at 0.25 V s .
potentiostatic bulk electrolysis. The standard deviations are given in the
parentheses.
6
H
4
3
–
1
4
PHPh
Figure S3, and Table S2). The appearance of the four-electron
reduced product PHPh cannot arise from PPh reduction because
the standard reduction potential for PPh is 155 mV more
negative than that for TPPO reduction. In support of this
contention, electrochemical measurements were carried out in 1.0
2
, is observed with comparable faradaic efficiencies (Table 1,
reagents and suggests a general strategy for the electrochemical
reduction of thermodynamically stable X=O (X = main group
element) bonds.
2
3
3
1
9
Results and Discussion
mM PPh
3
with 5 equiv of B(OAr) . The CVs (Figure S7) indicate
3
Bulk electrolyses of TPPO with B(OAr)
cyclic voltammograms (CVs) of a 1 mM acetonitrile solution of
TPPO in 0.1 M TBAClO upon the addition of the Lewis acid
B(OC OMe) . In the absence of any Lewis acid (black), the CV
3
. Figure 1 shows the
reductive waves with peak currents sufficiently negative of –3.0 V.
No reduced phosphorus species were detected by P NMR after
bulk electrolysis (Figures S8), even when carried out for 6 h.
Additionally, no organophosphinous acid esters, such as Ph POBu,
are detected as products in the bulk electrolysis. Such products
have been reported previously to arise from the reaction of
electrochemically-generated TPPO
ammonium cation in acetonitrile solutions. Along these lines,
bulk electrolysis in the absence of B(OAr)
at 14.2, 13.4 and 10.1 ppm that we attribute to organophosphinic
O=PR OH) and organophosphonic (O=PR(OH) ) acids and
their esters (Figure S4), which arise from the cleavage of the P–C
bond. Benzene, as a side product of P–C bond cleavage, is also
detected in the absence of B(OAr)
Lewis acid is required for phosphoryl bond cleavage as neither PPh
nor PHPh are detected without B(OAr)
31
4
6
H
4
3
2
°
+
of TPPO exhibits a single wave at E = –2.95V vs. Fc/Fc . The peak
separation (60 mV) indicates a reversible, one-electron process
that has previously been attributed to the generation of the TPPO
anion radical (TPPO ). The addition of substoichiometric
amounts of B(OC OMe) leads to the increase of the cathodic
•–
with the tetrabutyl-
•–
19
20
6
H
4
3
3
resulted in resonances
component with the concurrent decrease of the anodic component
until the wave becomes completely irreversible with excess
(
2
2
B(OC
with the titration of the Lewis acid. Further addition of
B(OC OMe) increases the cathodic peak current but to a less
dramatic extent and also induces an anodic shift of the wave
Figure S1). Plots of the cathodic peak current, normalized to the
peak current of pure TPPO, exhibit two regimes (Figure S2C).
Over the regime of 0 – 1 mM B(OC OMe) , addition of the
6
H
4
OMe)
3
. Notably, the cathodic peak potential is unshifted
6
H
4
3
3
(Figure S4C). Importantly, the
3
(
2
3
.
11
Analysis of the bulk electrolysis products by B NMR
6
H
4
3
spectroscopy (Figure S5) indicated the presence of
B(OC
appearance of sharp resonances at 1.5–0.4 ppm that we attribute to
tetrahedral borate species. Specifically, a set of peaks at 1.52, 0.53,
and 0.43 ppm appears after 5 min (0.5 C) at the expense of a pair of
peaks at 1.02 and 0.96 ppm. We assign the peaks at 1.02, 0.96, and
.43 ppm to reduced TPPO/B(OC
the growth of the features at 1.52 and 0.53 ppm correspond to the
accumulation of tetrahedral borate byproducts such as
Lewis acid leads to a ~2 electron reduction of TPPO that is highly
dependent on the concentration of the Lewis acid, whereas in the
6
H
4
OMe) (broad resonance at 16 ppm) in addition to the
3
regime >1 mM B(OC
6
H
4
OMe) , the cathodic current increases
3
with a lesser dependence on the concentration of the borate.
Additional features in the CV appear at more positive potentials
after 1 equiv of B(OC
multiple electrochemical processes.
6
H
4
OMe) is added (Figure S1), suggesting
3
0
6
H
4
OMe) intermediates while
3
Bulk electrolyses were carried out potentiostatically at Eapp = –3.0
V in a three-compartment cell in order to determine the products
of the electrochemical reduction of TPPO in the presence of
–
2–
BOH(OC
6
H
4
OMe)
3
or [(MeOC
6
H
4
O)
3
B]
2
O . Bulk electrolysis
in the absence of TPPO (Figure S6) leads to the decomposition of
B(OC OMe) and the appearance of a single new resonance at
.89 ppm, thereby excluding the possibility of the direct reduction
of B(OC OMe) during TPPO reduction.
Decomposition products are also observed for B(OAr)
equivalencies < 5. Bulk electrolysis performed with 2 equiv of
B(OC
6
H
4
OMe) . For these experiments, a glassy carbon rod (6
3
6
H
4
3
mm Ø) working electrode was separated by a glass frit from a
0
platinum mesh counter electrode, which was separated by a Vycor
6
H
4
3
+
frit from a Ag/Ag pseudo-reference electrode. The volume of the
3
solution in the working electrode compartment was 5.0 mL. Full
conversion of 1 mM TPPO to PPh
3
corresponds to 0.965 C of
OMe) is required
to observe, by P NMR spectroscopy, reduced phosphorus species.
The two-electron P(III) product, PPh , is generated at a faradaic
efficiency of 37%; additionally, the four-electron reduced product,
B(OC
6
H
4
OMe)
3
did not afford any reduced phosphorus products
charge passed. An excess of 5 equiv of B(OC
6
H
4
3
11
(
Figure S9). Inspection of byproducts by B NMR reveals
evidence for the reductive decomposition of the triaryl borate
under these conditions, with a peak at 1.52 ppm growing in
3
1
3
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Scheme 1. The EC
borates.
r
EC mechanism for TPPO reduction with triaryl
i
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0
1
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0
1
2
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4
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8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. Scan rate dependence for cyclic voltammograms of TPPO (1
mM) with 0.75 mM B(OC OMe) . CVs were carried out in 0.1 M
TBAClO /MeCN at a glassy carbon electrode.
6
H
4
3
4
intensity at the expense of peaks at 0.96 and 0.43 ppm (Figure
S9C). A similar result is obtained with 5 equiv of aryl borate
(Figure S6). Importantly, no peaks are detected at 0.53 or 1.02
ppm, leading us to conclude that the peak that grows in at 1.52 ppm
corresponds to an off-pathway decomposition product of the aryl
borate. Similarly, we conclude that the resonances at 1.02, 0.96 and
transfer. Association of the radical anion with B(OAr) follows the
3
•–
electron transfer to give Ph P–O –B(OAr) ; the association of the
3
3
•–
0.43 ppm correspond to intermediates along the pathway for
Lewis acid to the TPPO anion (k /k ) is considerably larger than
1
-1
TPPO reduction while the peak at 0.53 ppm is likely the tetrahedral
borate byproduct of TPPO reduction. As indicated by Tables 1 and
S2, ~20% of the charge passed is unaccounted for in bulk
that to neutral TPPO (K
a
). As a result, the contribution of the
association of neutral TPPO to B(OAr)
3
to the mechanism of
TPPO reduction is minor, and only comes into play at slow scan
rates where we observe a current plateau feature at ca. –2.7 V that is
electrolysis with 5 equiv of B(OC
6
H
4
OMe)
3
that we attribute to
2
1
current consumed by B(OAr)
3
decomposition along the pathway
consistent with a CE mechanism (Figure S14). A second, outer
of TPPO reduction.
sphere electron transfer from the electrode gives the two-electron-
2
–
Mechanistic studies. In order to interrogate the mechanism of
TPPO reduction by B(OC OMe) , we examined the scan rate
reduced complex Ph
slowness of the following chemical step, has an appreciable lifetime
in solution. Formation of PPh occurs with the cleavage of the
3
P–O –B(OAr)
3
, which, owing to the
6
H
4
3
dependence of the cyclic voltammetric response at low
concentrations of Lewis acid (<2 mM) to focus on the two-electron
pathway. Shown in Figure 2 is the scan rate dependence using 0.75
3
2
–
phosphoryl bond in Ph
3
P–O –B(OAr)
borate complex B(O )(OAr) , which associates with a second
equivalent of B(OAr) to generate a dimeric (ArO) B–O–
byproduct. The ECEC mechanism in Scheme 1 is akin
to what has been reported for TPPO reduction with TMSCl for
3
to give the tetrahedral
2
–
3
mM B(OC
S14 show additional CVs for other concentrations of
B(OC OMe) and those using B(OPh) ). Under all of the
6
H
4
OMe)
3
as a representative example (Figures S10 –
3
3
2
–
B(OAr)
3
6
H
4
3
3
1
5
conditions investigated here, the scan rate dependence of the CVs
exhibit qualitatively similar salient features. At all scan rates, the
normalized cathodic peak current remains approximately constant,
consistent with a two-electron wave, whereas the anodic peak upon
reversal grows with scan rate. The half-wave potential does not vary
which the rate determining step was not identified. As ECEC
•
–
significantly from that of the TPPO/TPPO couple. These
features are consistent with an overall two-electron reversible
process followed by a rate-determining irreversible chemical step.
Owing to the slow rate of the ensuing chemical step, increased scan
rates allow for the overall process to become more reversible, hence
the growth of the anodic peak with scan rate. At the very lowest
scan rates (Figure S14), a pre-wave current plateau can be observed
at –2.7 V. The plateau is consistent with the reduction of a
TPPO:B(OAr)
equilibrium. Evaluation of K
yields very weak binding constants for B(OC
3
host:guest association complex that is in minor
for these complexes (Figure S16)
OMe) and
= 19 ± 5 M and 25 ± 8 M , respectively).
The CV results are consistent with the primary reaction pathway
corresponding to the EC EC mechanism (E = electrode electron
transfer; C = reversible chemical step; C = irreversible chemical
a
6
H
4
3
–1
–1
B(OPh)
3
(K
a
r
i
Figure 3. Zone diagram for the EC
r
EC
i
mechanism with λ → 0 and no
2
r
i
consumption of the aryl borate species. Characteristic CVs are
0
1
step) presented in Scheme 1. The pathway comprises the reduction
included and the potential where ξ = –F/RT(E – E ) = 0 are indicated
•–
of TPPO to TPPO at the electrode by outer-sphere electron
by the dashed gray lines. See Supporting Information for the definition
of the zones.
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mechanisms reported to date typically have the first chemical step constant k . For small values of λ (i.e. high scan rates) and under
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rate-determining and irreversible, the EC
EC
mechanism
the assumption that the concentration of the added Lewis acid
remains constant within the diffusion-reaction layer, the
electrochemical response can be summarized in a zone diagram
(Figure 3, see Table S1 for the governing parameters for each
r
i
introduced here is unique in that the final chemical step is rate-
determining and irreversible.
Three other commonly encountered mechanisms were
considered for TPPO reduction with B(OAr)
3
. The mechanisms
zone), which describes the several limiting behaviors in the EC E
r
are introduced in Figure S15A and vary from each other with
respect to the sequence of electrochemical (E) and chemical (C)
system as function of the remaining governing parameters κγ and ρ.
Comparison of the experimental electrochemical behavior with the
expected CV response for each of these zones allows us to impose
steps and whether the chemical steps are irreversible (C
equilibrium (C ). All were rejected on the basis of their expected
i
) or under
r
reasonable limits to κγ and ρ. Since the potential of the partially
0
0
1
2
3
4
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6
7
8
9
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2
3
4
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
scan rate behavior in cyclic voltammetry. Simulated CVs for each
mechanism are presented in Figure S15B as a function of the
dimensionless kinetic constant λ = k/(Fν/RT), where k is the rate
reversible wave at the highest scan rates is close to E
DI
waves far from E
1
, zones DO ,
∞
∞
, and DE may be disregarded because CVs are expected to have
0
1
, (ξ1/2 << 0 in the dimensionless formulation, see
–1
0
constant of the irreversible step, ν is the scan rate in V s , F is the
SI and Figure 3) and thus impose limiting values on κγ = k
< 10 (from DO and DI
and DO (as well as DI
1
C
B
/k–1
Faraday constant, R is the universal gas constant, and T is the
∞
∞
) and ρ/κγ > 0.1 (from DE). Zones DO
again), which are characterized by two
0
2
1
temperature in Kelvin. In the case of the common EC
i
EC
r
∞
mechanism, decreasing λ (i.e., increasing the scan rate) results in
the CV collapsing into a reversible one-electron wave as the first
chemical step is rate-determining. Since the cyclic voltammograms
of TPPO with B(OAr)
3
retain their two-electron stoichiometry
even at 25 V s we thus reject the EC EC mechanism. Similarly,
the C EC E mechanism may be rejected because the CVs approach
a reversible, one-electron wave at low λ (for a large value of k
–
1
i
r
r
i
1
/k–1
;
2
1
i.e. the initial C
r
E sequence is fast and at equilibrium), and at
increasing λ the cathodic component grows while the anodic
component remains approximately constant. Finally, in the case of
C
i
EC
determining, and the anodic and cathodic components of the scan
rate normalized CVs increase with λ while their ratio (ia,p/ic,p
remains constant, which is inconsistent with our experimental data.
Only the EC EC mechanism successfully reproduces the salient
r
E, the initial association of TPPO with the aryl borate is rate-
)
r
i
features of our experimental data, namely the persistence of a two-
electron wave with enhanced reversibility upon scan reversal with
decreasing λ.
A formal kinetic treatment of the EC
r
EC
i
mechanism shown in
Scheme 1 is presented in the Supporting Information (SI). Under
the simplifying assumptions that the first chemical step is under
equilibrium and that diffusion coefficients of all species are equal,
the system is governed by four dimensionless parameters – two
thermodynamic (κ and ρ), one kinetic (λ
2
), and an excess factor (γ)
that are defined as follows:
0
ꢁ1
ꢂP
(1)
ꢀ
=
ꢁ−1
ꢄ
0
0
2
ꢃ = exp � (ꢇ ꢈ ꢇ )ꢉ
(2)
(3)
1
ꢅꢆ
ꢁ2
ꢊ2 =
ꢄꢋ/ꢅꢆ
0
ꢂ
(4)
B
ꢌ =
0
P
ꢂ
0
0
where k
1
, k–1, and k
2
, are defined in Scheme 1, E
1
and E
2
are the
•–
•–
reduction potentials for the TPPO/TPPO and PPh
3
–O –
2
–
0
0
B(OAr)
3
/PPh
3
–O –B(OAr)
3
couples, and C
B
and C
P
are the
initial concentrations of B(OAr)
3
and TPPO in solution. Since the
cyclic voltammetric response is governed by four independent
parameters, an analytical determination of all parameters defined
by eqs (1) – (4) is untenable. Nevertheless, a formal kinetic
analysis proves to be helpful to circumscribe the behavior of the
system, and to provide an estimate of the key chemical rate
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1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
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4
4
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0
1
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9
0
1
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9
0
1
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 4. Experimental rate constants (k
reduction of TPPO as a function of triaryl borate concentration. Values
for k were derived from the peak current ratio (ip,a/ip,c) from scan rate
dependence of the cyclic voltammograms of TPPO with the use of a
working curve.
2
) for the electrochemical
Figure 5. The experimental peak ratios (open circles) determined
from CVs carried out over range of concentrations for
a
B(OC
6
H
4
OMe)
3
and the theoretical scan rate dependence (solid lines)
2
–
1
for k = 584 s .
2
aid of B(OAr)
3
that can be regenerated from the four-coordinate
borate final product of Scheme 1 under mild reflux conditions in a
Dean-Stark trap as follows,
electrolysis (Figure S3) suggests that one possible side reaction is
2
–
the disproportionation of TPPO . As faster scan rates minimize
the residence time of electroactive species at the electrode that may
lead to off-pathway reactions, experimental data recorded at high
–1
scan rates (25 and 10 V s ) were chosen for the determination of
. Figure 4 presents the averaged values of k obtained from fitting
working curves at both scan rates for varying concentrations of
B(OAr) ([B(OAr) ] = 0.25, 0.5, 0.75, and 1.0 mM). As expected
for a unimolecular process (Scheme 1), the concentration
dependence of k is minor and consequently the k value was
averaged over all concentrations of B(OC OMe) to give a value
. Using the same procedure for experiments
k
2
2
with very little energy input as compared to the silyl halides and
3
3
2
5
other previously-studied additives.
recyclability of B(OAr) , after an electrolysis of TPPO with 10
, the electrolyte was treated with TBAOH
To demonstrate the
2
2
3
6
H
4
3
equiv of B(OC
6
11
H
4
OMe)
3
–
1
of 584 ± 123 s for k
using B(OPh) , a value of 355 ± 161 s was obtained. These
averaged values for k were used as inputs to generate the
theoretical ip,a/ip,c vs ν curves presented in Figure 5 for
B(OC OMe) and Figure S19 for B(OPh) . The curves agree
with experimental data at the high scan rates owing to minimized
side reactions indicated by the formation of PHPh , as discussed
2
(Figure S20). B NMR spectroscopy confirms that all borate
species are converted to B(OH)
–1
–
4
as evidenced by the appearance
3
of a single peak at 1.9 ppm. Subsequent neutralization with HCl
gave a single broad resonance at 17.7 ppm which is consistent with
2
B(OH)
phenol in toluene will regenerate B(OAr)
3
. The condensation of B(OH) with the appropriate
3
6
H
4
3
3
2
6
3
, as has been reported,
though for the laboratory scale experiment reported here, the
volume of toluene (85 μL) is impractical for refluxing with a Dean
Stark trap. Thus, borate regeneration is best accomplished under
large-scale electrolysis conditions.
2
above. This disproportionation appears to be suppressed by
association of TPPO as the fitting, even at low scan rates, improves
as the concentration of B(OAr) is increased. Simulation of the
3
CVs with the derived k2's and diffusion coefficients nicely
reproduce the observed scan rate dependences (Figures S10 and
S12, right panels).
The P=O bond-breaking rate constant k
on the strength of the Lewis acid. As shown in Figure 4, the less
acidic B(OC OMe) promotes phosphoryl bond cleavage at
rates on average 65% faster than B(OPh) . This is also reflected in
the lower faradaic efficiency measured for B(OPh) (Table 1,
Figure S3, and Table S2). The greater Lewis acidity of B(OPh)
results in greater stabilization of the two-electron reduced Ph P–
intermediate and thus leads to a slower phosphoryl
bond breaking step. For both B(OC OMe) and B(OPh) , the
sluggish bond-breaking step results in the appreciable
2
is inversely dependent
6
H
4
3
The k rate constants associated with phosphoryl bond breaking
2
3
are sluggish. This low rate is intriguing inasmuch as the phosphoryl
bond has already been reduced by two electrons prior to bond-
breaking. These data reinforce the difficulty in breaking the P=O
bond and provides a compelling reason for the paucity of reports
on the catalytic reduction of phosphine oxides and phosphates. The
reduction of TPPO with general reductants, such as potassium,
often proceed with the cleavage of the P–C bond over the P=O
3
3
3
2
–
O –B(OAr)
3
6
H
4
3
3
2
–
concentration of the two-electron-reduced species Ph
B(OAr) . As a result, the four-electron-reduced PHPh product,
directly produced from the disproportionation of Ph
B(OAr) , is generated in appreciable amounts. Stabilization of the
long-lived Ph
off-pathway routes towards B(OAr)
3
P–O –
2
3
3
2
bond. The methodology developed here―where the P=O bond
2
–
3
P–O –
is activated by two outer-sphere electron transfers aided by
3
B(OAr) —thus provides a compelling strategy for the selective
3
2
–
3
P–O –B(OAr)
3
intermediate may be responsible for
decomposition (vide supra)
cleavage of phosphorus and other main group element-oxygen
multiple bonds. In order to drive the uphill cleavage of the
phosphoryl bond, strategies have historically resorted to employing
high energy-density additives such as silyl chlorides to force the
3
that amounts to 20% of charge lost during bulk electrolysis. The
presence of features at –2.1 and –2.5 V in CVs of TPPO with excess
2
4
B(OPh)
Figure S2). Inspection of CVs with B(OPh)
3
supports the presence of such off-pathway reductions
in the absence of
overall thermodynamic preference of the reaction more negative.
Here, we demonstrate that the P=O bond can be cleaved with the
(
3
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3
TPPO (Figures S21A, 21B and 21C) suggests that the feature at –
2.1 V may be due to the direct reduction of B(OPh) at the
electrode. Noticeably, this feature is markedly suppressed when
using B(OC OMe) (Figure S21D and E), which is in line with
the better faradaic efficiencies for B(OC OMe) . In addition, the
CD CN) 25 °C δ/ppm: 15.6 (bs). C NMR (101 MHz, CD3CN)
3
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
25 °C δ/ppm: 155.81, 146.80, 120.96, 114.34, 55.24.
Cyclic voltammetry. Measurements were performed in a three-
electrode cell assembled in an N -filled glovebox using a
potentiostat with positive-feedback ohmic drop compensation
(CH Instruments). Measurements were performed in 0.1 M
3
6
H
4
3
2
6
H
4
3
feature at –2.5 V, which is not present in the CVs in the absence of
TPPO, is consistent with the reductive decomposition of the
TBAClO
4
/MeCN with a Pt mesh counter electrode separated from
+
B(OPh)
Interestingly, gold surfaces were found to suppress the formation
of PHPh at the cost of lower overall faradaic efficiencies for PPh
3
in association with TPPO.
the working compartment with a standard glass frit, and a Ag/Ag
pseudo-reference electrode separated from the working
compartment with a Vycor frit. Potentials are referenced to the
2
3
+
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(20%, Figure S22). This is supported by a lower overall peak
current found during titration studies compared to measurements
performed on glassy carbon electrodes (Figure S23). The origin of
this phenomenon remains unclear and warrants further study but
suggests that the reduction of TPPO may occur by a different
mechanism on gold. Indeed, the CVs (Figure S23) indicate
multiple faradaic processes in support of a more complicated
mechanism than encompassed by Scheme 1.
ferrocene/ferrocenium (Fc/Fc ) couple, which was determined
after each experiment by the addition of a solution of ferrocene
(total [Fc] ≈ 1 mM). The volume of the solution in the working
compartment for each experiment was 5.0 mL. The ohmic drop
was carefully determined and corrected for using the positive
feedback feature of the potentiostat.
Before each set of CV experiments, the working electrodes – 3
mm diameter glassy carbon and 2 mm diameter Au buttons – were
carefully polished with diamond paste (1 μm) then Al
2
3
O (0.3 and
Conclusions
0.05 μm) polishing media before being briefly sonicated and dried
in vacuo. The working electrodes were carefully polished with lens
paper and washed with fresh MeCN between individual CV runs.
Bulk electrolyses and product quantification. Bulk
In summary, aryl borate Lewis acids facilitate the
electrochemical reduction of triphenylphosphine oxide to triphenyl
phosphine with faradaic efficiencies as high as 37%. Cyclic
+
voltammetry suggests that TPPO reduction with B(OAr)
via a unique EC EC mechanism in which the final, irreversible
cleavage of the phosphoryl bond is rate-determining and governed
3
occurs
electrolyses were carried out potentiostatically at –3.0 V vs Fc/Fc
r
i
in the same divided cell as described above with 5.0 mL of solution
in the working electrode compartment. The working electrodes
were either a glassy carbon rod (6 mm diameter, type 2, Alfa Aesar)
that was mechanically polished with 4000 grit (5 μm) SiC
sandpaper, or an Au foil (6×6 mm , 99.999%, Alfa Aesar) working
electrode, which was polished as described above. During bulk
electrolysis experiments, full conversion of 5.0 mL of 1 mM TPPO
–1
by a slow rate constant (~600 s ). Since the borate byproduct can
potentially be converted back to the active triaryl borate under
appropriate dehydration conditions, this method more generally
affords a practical strategy for selectively cleaving element-oxygen
multiple bonds in coordinatively-saturated phosphoryl complexes
2
and possibly other oxoacids pertinent to industrial pollution (NO
x
)
to PPh corresponds to 0.965 C of charge passed.
3
3
–
–
and wastewater treatment (PO
4
and ClO
4
).
Product quantification was carried out by taking 700 μL aliquots
during the course of each bulk electrolysis and adding 35 μL of a
105 mM CD CN solution of bis(triphenylphosphine)-iminium
Experimental Section
General. All reagents were purchased from commercial vendors
without further purification, unless noted otherwise. Synthetic
3
chloride (PPNCl) in a quartz NMR tube as an internal reference.
3
1
1
P{ H} spectra, averaged over 256 scans, were acquired with a
1
11
1
relaxation delay of 15 s, while H (32 scans) and B{ H} (2048
scans) spectra were acquired with a relaxation delay of 1 s.
Phosphorus-containing products were quantified with the use of a
standard curve, normalized to the internal PPN standard (Figure
S24).
Determination of Ka from P{ H} NMR. Equilibrium
constants (K ) for the association of B(OAr) with TPPO were
determined by P{1H} NMR spectroscopy by monitoring the
chemical shift of the TPPO resonance of acetonitrile solutions of
TPPO (1.0 mM) upon the addition of varying concentrations of
manipulations were carried out under an N atmosphere using
2
standard Schlenk technique. Anhydrous solvents used for synthetic
manipulations and electrochemical measurements were dried by
passing through a two-column solvent purification system (Pure
+
1
Process Technologies) and were stored on molecular sieves. H,
3
1
1
1
1
31
B, and P NMR spectra were collected on a 400 MHz instrument
(
JEOL) at 25 °C in CD
Tris(4-methoxyphenyl) borate. Tris(4-methoxyphenyl)
borate was prepared as outlined previously with minor
3
CN unless noted otherwise.
a
3
3
1
2
7,28
modifications.
A solution of Me
2
S•BH
3
in THF (52.0 mL of a
B(OAr) (0 – 0.6 M). For the 1:1 host:guest interaction we
3
2
.0 M solution in THF, 104 mmol) was transferred to a three-neck
propose here, the concentration of the association complex [HG] is
given by,
round-bottom flask equipped with a reflux condenser and addition
funnel. To the stirred solution at 0 °C was added, dropwise, a 100
mL solution of 4-methoxyphenol (37.24 g, 312.0 mmol) in
anhydrous THF over the course of an hour. Upon addition, the
1
1
[
HG] = ꢍ[H] + [G] +
ꢐ
0
0
2
ꢎꢏ
colorless solution began to evolve H . The reaction mixture was
2
2
1
2
1
allowed to warm to room temperature, was stirred for 18 h at room
temperature, and then at a gentle reflux (60 °C) for 4 h. After
ꢈ
ꢑꢍ[H] + [G] + ꢐ ꢈ 4[H] ∙ [G]
0 0 0 0
ꢎꢏ
quantitative H evolution, solvent was removed from the colorless
2
where [H]
0
and [G] are the initial concentrations of the host and
0
solution in vacuo to give a white powder (39.194 g, 103.09 mmol,
1
guest species. The change observed in the chemical shift (Δδ) of a
host species upon titration of a guest is given by,
9
2
9.1%). H NMR (400 MHz, CD
3
CN) 25 °C δ/ppm: 7.02 (m,
1
1
1
H), 6.84 (m, 2H), 3.72 (s, 3H). B{ H} NMR (128 MHz,
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4
Cummins, C. C. Phosphorus: From the Stars to Land & Sea. Daedalus
2014, 143, 9–20.
[
HG]
1
2
3
4
5
6
7
8
9
∆ꢒ = ꢒ∆HG
ꢍ
ꢐ
[
H]0
Appel, R. Tertiäres Phosphan/Tetrachlormethan, Ein Vielseitiges
Reagens Zur Chlorierung, Dehydratisierung Und PN‐Verknüpfung.
Angew. Chem. 1975, 87, 863–874.
Appel, R. Tertiary Phosphane/Tetrachloromethane, a Versatile
Reagent for Chlorination, Dehydration, and P-N Linkage. Angew.
Chem. Int. Ed. Engl. 1975, 14, 801–811.
2
9
where δΔHG is a constant for the studied system. Thus K
can be determined from the non-linear regression of the chemical
shift as a function of guest concentration, which was performed
using the BindFit web application.
Determination of diffusion coefficients by DOSY. Diffusion
coefficients (D) for TPPO, B(OC OMe) , and B(OPh) were
determined by diffusion ordered spectroscopy (DOSY) using a 600
MHz NMR spectrometer (Varian) operating at 21 °C. Samples
were prepared from 0.1 M solutions of each of the species in 0.1 M
/CD CN and the DOSY gradient compensated
stimulated echo with spin lock and convection compensation
DgcsteSL_cc) pulse sequence was used. Twenty H spectra (32
scans with 1 s relaxation delays) were collected along a magnetic
a
and δΔHG
5
3
0
6
7
8
Calzada, J. G.; Hooz, J. Geranyl Chloride. In Organic Syntheses; Wiley
6
H
4
3
3
&
Sons: New York, 1988; Collect. Vol. No. VI, p 634.
Hughes, D.L. The Mitsunobu Reaction. In Organic Reactions, Paquette,
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
L. A., Ed.; Wiley New York, 1992; Vol. 42, pp 335–656.
Swamy, K. C. K.; Kumar, N. N. B.; Balaraman, E.; Kumar, K. V. P. P.
Mitsunobu and Related Reactions: Advances and Applications. Chem.
Rev. 2009, 109, 2551–2651.
TBAClO
4
3
3
1
1
9
1
Staudinger H.; Meyer Jules. Über Neue Organische
(
Phosphorverbindungen
III.
Phosphinmethylenderivate
Und
Phosphinimine. Helv. Chim. Acta 1919, 2, 635–646.
0 Gololobov, Y. G.; Kasukhin, L. F. Recent Advances in the Staudinger
Reaction. Tetrahedron 1992, 48, 1353–1406.
–
1
gradient that typically spanned 2.4 – 40 G cm with a diffusion
delay of 50 ms and a diffusion gradient length of 2 ms. The phenyl
protons were used for peak integration, and the conversion factor
11 Maercker, A. The Wittig Reaction. In Organic Reactions; Wiley: New
York, 1965; Vol. 14, pp 270–490.
12 Boutagy, J.; Thomas, R. Olefin Synthesis with Organic Phosphonate
Carbanions. Chem. Rev. 1974, 74, 87–99.
–
1
from digital-to-analogue to G cm was determined by fitting a
DOSY spectrum of 99.9% D O to a diffusion coefficient of 1.902 ×
cm s . The decay of the integrated intensity (I) along a
magnetic gradient of strength G is given by the Stejskal-Tanner
2
–
5
2
–1 32
10
1
3 Hérault, D.; Nguyen, D. H.; Nuel, D.; Buono, G. Reduction of
Secondary and Tertiary Phosphine Oxides to Phosphines. Chem. Soc.
Rev. 2015, 44, 2508–2528.
3
3
formula,
−ꢕꢖꢗꢖꢘꢖ(∆−ꢗ/3)ꢙ
ꢓ = ꢓ0ꢔ
14 Kawakubo, H.; Kuroboshi, M.; Yano, T.; Kobayashi, K.; Kamenoue, S.;
Akagi, T.; Tanaka, H. Electroreduction of Triphenylphosphine Oxide
to Triphenylphosphine in the Presence of Chlorotrimethylsilane.
Synthesis 2011, 2011, 4091–4098.
where I
0
is the integrated intensity at G = 0, γ is the gyromagnetic
ratio of the nucleus, Δ is the diffusion delay and δ is the diffusion
gradient length. Diffusion coefficients were thus determined from
fitting the I vs γ δ G (Δ–δ/3) curves with a single exponential.
1
5 Tanaka, H.; Yano, T.; Kobayashi, K.; Kamenoue, S.; Kuroboshi, M.;
Kawakubo, H. TMSCl-Promoted Electroreduction of
Triphenylphosphine Oxide to Triphenylphosphine. Synlett 2011,
011, 582–584.
2
2
2
2
ASSOCIATED CONTENT
Supporting Information. The Supporting Information is available
free of charge on the ACS Publications website. Formal kinetic
analysis; additional electrochemical characterization; H, B, and
NMR spectra of bulk electrolysis byproducts; determination of
association constants by P NMR; determination of diffusion
coefficients by H DOSY (PDF)
1
6 Yanilkin, V. V.; Gromakov, Y. S.; Nigmadzyanov, F. F. Electrochemical
Deoxygenation of Triphenylphosphine Oxide. Russ. Chem. Bull. 1996,
45, 1257–1258.
1
11
31
P
17 Yano, T.; Kuroboshi, M.; Tanaka, H. Electroreduction of
Triphenylphosphine Dichloride and the Efficient One-Pot Reductive
Conversion of Phosphine Oxide to Triphenylphosphine. Tetrahedron
Lett. 2010, 51, 698–701.
3
1
1
1
8 Li, P.; Wischert, R.; Métivier, P. Mild Reduction of Phosphine Oxides
with Phosphites To Access Phosphines. Angew. Chem. Int. Ed. 2017,
AUTHOR INFORMATION
5
6, 15989–15992.
9 Santhanam, K. S. V.; Bard, A. J. Electrochemistry of
Organophosphorus Compounds. II. Electroreduction of
Triphenylphosphine and Triphenylphosphine Oxide. J. Am. Chem. Soc.
968, 90, 1118–1122.
Corresponding Author
* (D.G.N.) E-mail: dnocera@fas.harvard.edu.
(C.C.) E-mail: cyrille.costentin@univ-paris-diderot.fr.
1
*
1
Notes
2
2
0 Savéant, J. M.; Su, K. B. Catalytic Phenomena in the Electrochemical
Reduction of Triphenylphosphine and Triphenylphosphine Oxide in
Non-Aqueous Solvents with Tetraalkylammonium Supporting
Electrolytes. J. Electroanal. Chem. Inter. Electrochem. 1978, 88, 27–41.
1 Savéant, J-M. Elements of Molecular and Biomolecular
Electrochemistry: An Electrochemical Approach to Electron Transfer
Chemistry; John Wiley: Hoboken, NJ, 2006, Ch. 2.
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We gratefully acknowledge the National Science Foundation for
funding from grant CHE–1464232.
22 Rudolph, M. Digital Simulations on Unequally Spaced Grids.: Part 2.
Using the Box Method by Discretisation on a Transformed Equally
Spaced Grid. J. Electroanal. Chem. 2003, 543, 23–39.
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3 Evans, A. G.; Evans, J. C.; Sheppard, D. Reactions of radical anions Part
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4 Geeson, M. B.; Cummins, C. C. Phosphoric Acid as a Precursor to
Chemicals Traditionally Synthesized from White Phosphorus. Science
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1
U.S. Geological Survey. Phosphate Rock. In Mineral Commodity
Summaries 2018; Washington, D.C., 2018.
Chesnut, D. B. Atoms-in-Molecules and Electron Localization
Function Study of the Phosphoryl Bond. J. Phys. Chem. A 2003, 107,
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307–4313.
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5 Steinberg, H.; Hunter, D. L. Preparation and Rate of Hydrolysis of
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1
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2
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26 Malkowsky, I. M.; Fröhlich, R.; Griesbach, U.; Pütter, H.; Waldvogel,
S. R. Facile and Reliable Synthesis of Tetraphenoxyborates and Their
Properties. Eur. J. Inorg. Chem. 2006, 2006, 1690–1697.
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