Rapid Metal-Free Formation of Free Phosphines from Phosphine Oxides

Article abstract of DOI:10.1002/adsc.201800723

A rapid method for the reduction of secondary phosphine oxides under mild conditions has been developed, allowing simple isolation of the corresponding free phosphines. The methodology involves the use of pinacol borane (HBpin) to effect the reduction while circumventing the formation of a phosphine borane adduct, as is usually the case with various other commonly used borane reducing agents such as borane tetrahydrofuran complex (BH₃?THF) and borane dimethyl sulfide complex (BH₃?SMe₂). In addition, this methodology requires only a small excess of reducing agent and therefore compares favourably not just with other borane reductants that do not require a metal co-catalyst, but also with silane and aluminium based reagents. (Figure presented.).

Full text of DOI:10.1002/adsc.201800723

Title: Rapid Metal-Free Formation of Free Phosphines from Phosphine
Oxides
Authors: Emma Emanuelsson, Ruth L Webster, and Cei Provis-Evans
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800723
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10.1002/adsc.201800723
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Rapid Metal-Free Formation of Free Phosphines from
Phosphine Oxides
Cei B. Provis-Evansa,b, Emma A. C. Emanuelssonc, Ruth L. Webstera*
a
Department of Chemistry, University of Bath, Bath, UK, BA2 7AY.
Centre for Sustainable Chemical Technologies, University of Bath, Bath, UK, BA2 7AY.
Department of Chemical Engineering, University of Bath, Bath, UK, BA2 7AY.
b
c
Email: r.l.webster@bath.ac.uk
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. A rapid method for the reduction of secondary and borane dimethyl sulfide complex (BH3·SMe2). In
phosphine oxides under mild conditions has been developed, addition, this methodology requires only a small excess of
allowing simple isolation of the corresponding free reducing agent and therefore compares favourably not just
phosphines. The methodology involves the use of pinacol with other borane reductants that do not require a metal co-
borane (HBpin) to effect the reduction while circumventing catalyst, but also with silane and aluminium based reagents.
the formation of a phosphine borane adduct, as is usually the
case with various other commonly used borane reducing
agents such as borane tetrahydrofuran complex (BH3·THF)
Keywords: Phosphine oxide; reduction; pinacol borane
phosphines are less stable to air than their tertiary
analogues, while primary phosphines are even less
stable; frequently demonstrating spontaneously
pyrophoric behaviour on exposure to air. Often
syntheses involving the use of secondary or primary
phosphines therefore require scrupulously air-free
techniques, which are difficult and expensive to
maintain. Conversely, the phosphine oxides which
these P(III) species so readily oxidise to are for the
most part stable to both air and heat.[7]
The availability of effective and mild synthetic
routes to P(V) phosphine oxides under air, and the
comparative difficulty of performing the same sort of
syntheses using less stable P(III) phosphines affords
an opportunity; to perform the synthesis of a desired
phosphine as the corresponding air-stable phosphine
oxide, and only once this is complete, to reduce the
P(V) centre to P(III) ready for whatever use is
envisaged.
The stoichiometric reduction of P(V) phosphine
oxides to P(III) phosphines was first discovered in the
1950s, and used relatively harsh reagents such as
LiAlH4 to reduce aryl and alkyl phosphine oxides,
with concurrent production of H2 at around 200 °C.[8]
A variety of less onerous methodologies have been
developed in the intervening years, notable among
which are reductions using silicon and aluminium
hydride sources, in addition to boranes.
Introduction
Phosphines are
a
vitally important class of
compounds; found in the synthesis of a vast range of
metal complexes,[1] as organocatalysts in their own
right,[2] as auxiliaries in the ubiquitous Wittig
reaction,[3] and as chemical intermediates for P-C, P-
P and P-X bond formation to name but a few.[4]
Complexes using phosphine based ligands find
extensive use in homogeneous catalysis due to the
favourable solubility and stability which the
phosphine ligand can confer, and the tuneable
properties of the resulting complexes.[5] The
predictable and profound influence of ligand sterics
and electronics on the metal render phosphines one of
the most flexible ligand families available. Moreover,
chiral phosphines have demonstrated remarkable
efficacy for facilitating asymmetric induction in
catalytic reactions.[6]
Phosphines have also been used as nucleophilic
organocatalysts for a profusion of C-X (X: C, N, O,
S) bond forming reactions employing unsaturated
substrates, in addition to the formation of cyclic and
heterocyclic products.[2b] If the phosphines used are
themselves chiral, then these transformations can be
performed asymmetrically under the right conditions,
producing a myriad of invaluable enantiomerically
enriched products.[2c]
A range of reductions using silane and siloxane
based stoichiometric reductants have been
investigated. The first report from Fritzcsh et al. in
1964 used an excess of phenylsilane or
polymethylhydrosiloxane (PMHS) at elevated
temperatures and achieved yields of 33% to 98% for
a variety of alkyl and aryl tertiary phosphine
As a result of the diversity of uses for phosphines,
their preparation has been the subject of academic
and industrial interest for many decades. While
tertiary phosphines are generally air stable as ligands
in organometallic complexes, as free molecules they
often demonstrate a tendency to oxidise to the P(V)
phosphine oxides in the presence of air. Secondary
1
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10.1002/adsc.201800723
Advanced Synthesis & Catalysis
oxides.[9] Later work from the same group
documented improved yields using equimolar
chlorosilane in combination with triethylamine.[10]
More recently high yields were achieved without the
use of undesirable chlorosilanes by employing
centre, although its utility ultimately proved to be
limited to alkylphosphine oxides, with little activity
in those with aryl substituents.[14] This was later
mitigated using a Lewis acidic CeCl3 auxiliary, which
was theorised to activate the P-O bond and thereby
facilitate the reduction in a much wider variety of
substrates.[16] The addition of NaBH4 to this system
further improved the conditions required for the
reduction, and also afforded the protected BH3 adduct
(Scheme 1b).[17] More recently, and rather elegantly,
other aluminium based reductants have been shown
by Tyler and co-workers to be effective in the gram-
scale preparation of alkyl phosphines and primary
phosphines.[18] Busacca et al. have employed 5
equivalents of DIBAL-H active in reducing sterically
crowded secondary phosphine oxides in good to
excellent yields under mild conditions.[19]
catalytic quantities of Ti(OiPr)4,[11] InBr3,[12]
[14]
Cu(OTf)2,[13] B(C6F5)3
or acid additives[15] in
conjunction with much smaller excesses of an aryl
silane and/or siloxane (e.g. TMDS, PMHS, DPDS)
(Scheme 1a).
In addition to the myriad procedures for the
reduction of phosphine oxides using silicon and
aluminium based reagents, there are also ample
examples in the literature of simple boranes being
utilised for such reactions (Scheme 1c),[20] and these
invariably afford the borane adduct of the P(III)
species.[21] During recent studies into the
homodehydrocoupling of secondary phosphines[22]
we struggled to find
a
scalable reduction
methodology with a simple work-up; many of the
methods that employ aluminium reducing agents gel,
meaning filtration and isolation procedures are
laborious. We herein report a method for the
reduction of phosphine oxides to the free phosphines
using pinacol borane (HBpin) as the reducing agent
(Scheme 1d). The work-up procedure is
straightforward and rapid. While the free phosphine
is less stable than the analogous borane adduct, this
development allows the borane deprotection step to
be avoided (which requires a stoichiometric or excess
quantity of amine), and therefore synthetic
procedures are significantly simplified.
Results and Discussion
We rationalised that a simple, commercially available
borane such as HBpin would be the ideal reagent with
which to undertake the reduction of secondary
phosphines: formation of the insoluble side product
Scheme 1. Selected examples of phosphine reduction all of
which require harsh conditions (a), excess reductant and a
catalyst (a), a complex mixture of reagents in excess (b), or
result in adduct formation (c). This work uses a catalyst-
free method which generates the free phosphine (d).
HOBpin (or pinBOBpin for tertiary phosphines[23]
)
would be a driving force allowing for only a slight
excess of borane to be used and for facile work-up.
The steric bulk of the borane, even if used in excess,
should furnish the free phosphine on completion of
the reaction.
As previously mentioned, LiAlH4 was among the
first reagents to be used in the reduction of the P(V)
2
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10.1002/adsc.201800723
Advanced Synthesis & Catalysis
We quickly established that this is indeed the case
and work was undertaken to optimise the reduction of
diphenylphosphine oxide by varying the solvent,
temperature and molar ratio of HBpin (Table 1).
Despite there being a slightly higher isolated yield for
Entry 5, 1.1 equivalents of HBpin was chosen as the
standard conditions due to the difference in yield
between Entries 5 and 6 being negligible. MeCN was
chosen over toluene as it allows for a more
straightforward work-up procedure. It is worth noting
that catechol borane (HBcat) was tested but the
conversion was not as good as that obtained with
HBpin under identical conditions (compare Entry 1
and 3).
i
For the isolation of HPPh2 wet, degassed PrOH is
added to the reaction mixture under an inert
atmosphere, followed by filtration through a plug of
alumina to remove pinBOBpin, eluting with MeCN.
Removal of the solvent in vacuo furnishes the
product in 72% yield.
Table 1. Optimisation of reaction conditions.
Entry Solvent HBpin
(eq.)
Conversion
(%)a
Isolated
yield (%)
1b
MeCN
MeCN
Toluene
MeCN
MeCN
MeCN
1.0
1.0
1.0
1.0
1.2
1.1
84
95
90
40
100
100
-
-
63
-
75
72
Scheme 2. Products of HBpin-mediated reduction of
phosphine oxides. a)2h; b)60 °C; c)20 h; d)2 eq HBpin;
e)80 °C; f)100 °C, 5 days; g)100 °C, 5 days.
2c
3c
4b,d
5c
6b
The reduction of secondary aryl phosphine oxides
proceeds well with quantitative or near quantitative
spectroscopic yields obtained for all substrates tested.
A variety of aryl phosphine oxides with electron
withdrawing and donating substituents in the para-
position are tolerated (1b to 1f), with good isolated
yields achieved in all cases with the exception of 1e.
In addition to this phosphine oxides containing both
sterically and electronically challenging functionality
can be readily reduced: 3,5-di-trifluoromethyl and
ortho-methoxy substituted aryl phosphine oxides give
excellent yields of the respective products (1g and
1h). We also demonstrate that an unsymmetrical
secondary phosphine oxide can be readily reduced at
room temperature (1i) as can secondary alkyl
phosphine oxides (to generate 1j to 1l). Unfortunately,
no obvious reduction occurs with diethyl phosphite
(to give phosphonite 1n), but there is activity
observed for the reduction of tertiary phosphine
oxides (forming 1m and 1o), albeit under more
forcing conditions (5 to 10 days at 100 °C).[23]
a) 31P NMR data; b)RT 2h; c) RT, 20 h; Catecholborane
d)
used rather than HBpin. Reaction did not proceed cleanly.
With these optimised conditions in hand, the
substrate scope was explored. A variety of aryl
secondary phosphine oxides were tested, as well as a
tertiary phosphine oxide (Scheme 2).
The reaction is scalable with 4.61 mmol (933 mg)
HP(O)Ph2 furnishing 1a in 74% yield (636 mg). Even
on this scale, the same straightforward work-up
procedure can be followed and the resultant product
is analytically pure.
3
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10.1002/adsc.201800723
Advanced Synthesis & Catalysis
affords the free phosphine; a unique product for a
borane reduction. This reaction has been shown to be
fairly general and applicable to secondary phosphine
oxides and to proceed slowly but inexorably in
tertiary phosphine oxides. This allows the potential to
expand the flexibility of phosphine synthesis
generally, as it allows for phosphine oxides to be
used as protected analogues for phosphines with a
mild and efficient deprotection reaction.
We propose that the reaction proceeds through the
formation of a B–O adduct followed by a hydride
shift, affording the pentacoordinate trigonal
bipyramidal species 2 (Scheme 3). Proton abstraction
by the borolanolate can take place in an inter-
(depicted) or intramolecular fashion.[24] This matches
the generally accepted mechanism for borane-
mediated reduction of phosphine oxides,[20d. 24] whilst
avoiding phosphine-borane adduct formation that is
inherent to other methodologies. The steric bulk of
the pinacol and/or reduced Lewis acidity relative to
BH3 (due to hyperconjugation between the oxygen
lone pairs and the empty p-orbital on boron) is
hypothesised to disfavour the adduct formation,
therefore affording the free phosphine. The analogous
reaction performed using HBcat did not afford good
yields of the free phosphine, so it may be that the
HBpin has an optimal mix of Lewis acidic and steric
properties which lend itself to this sort of reactivity.
Experimental Section
General method for reduction of phosphine oxides
Manipulations were carried out under an argon atmosphere
in an M-Braun glove box. Phosphine oxides (0.25 mmol)
and pinacolborane (1.1 - 2.0 equiv.) were added to a J.
Young Schlenk tube along with 1 ml of dry MeCN or THF.
The sealed tube was then maintained at the required
temperature with stirring for the time specified. 200 µl of
degassed isopropyl alcohol was then added to quench the
residual HBPin, after which all the solvent was removed
under vacuum. The residue was re-dissolved in MeCN or
toluene under argon and passed through a plug of alumina
into a pre-weighed vial inside a Schlenk tube. The solvent
was then removed under vacuum and the vial re-weighed
under argon to afford a yield.
Diphenylphosphane, 1a
1
Isolated yield: 72%. H NMR (CD3CN, 500 MHz): δ
7.53-7.49 (m, 4H), 7.35-7.34 (m, 6H), 5.26 (d, 1H, J =
219.9 Hz); 13C{1H} NMR (CD3CN, 126 MHz): δ 136.1 (d,
i-Ar, J = 10.0 Hz), 134.7 (d, o-Ar, J = 16.7 Hz), 129.7 (d,
m-Ar, J = 6.0 Hz), 129.6 (s, p-Ar); 31P NMR (CD3CN, 202
MHz): δ -39.7 (d, J = 220.0 Hz). Data comparable to
previous reports in the literature.[19]
Scheme 3. Postulated mechanism of phosphine reduction
with HBpin.
Di-p-tolylphosphane, 1b
1
Isolated yield: 85%. H NMR (CD3CN, 300 MHz): δ
7.39-7.36 (m, 4H), 7.16-7.14 (m, 4H), 5.15 (d, 1H, J =
218.7 Hz), 2.30 (s, 6H); 13C{1H} NMR (CD3CN, 126
MHz): δ 138.7 (s, p-Ar), 133.8 (d, o-Ar, J = 17.1 Hz),
131.7 (d, i-Ar, J = 9.0 Hz), 129.4 (d, m-Ar, J = 6.3 Hz),
20.3 (s, Ar-CH3); 31P NMR (CD3CN, 202 MHz): δ –41.8
(d, J = 218.9 Hz). Data comparable to previous reports in
the literature.[19]
A key transformation that uses secondary phosphines
of the form 1a is hydrophosphination. In order to
show how useable the reduction procedure is, we
employed a one pot procedure using first our
reduction method followed by Alonso’s catalyst-free
hydrophosphination methodology[25] (Scheme 4).
Hydrophosphination product
3 is obtained in
excellent spectroscopic yield (93%).
Bis(4-methoxyphenyl)phosphane, 1c
Isolated yield: 91%. H NMR (CD3CN, 500 MHz): δ
1
7.43-7.39 (m, 4H), 6.90-6.87 (m, 4H), 5.14 (d, 1H, J =
218.5 Hz), 3.95 (s, 6H); 13C{1H} NMR (CD3CN, 126
MHz): δ 161.2 (s, p-Ar), 136.3 (d, o-Ar, J = 18.4 Hz),
127.0 (d, i-Ar, J = 7.6 Hz), 115.3 (d, m-Ar, J = 6.7 Hz),
55.9 (s, O-CH3); 31P NMR (CD3CN, 202 MHz): δ –44.5
(d, J = 218.8 Hz). Data comparable to previous reports in
the literature.[19]
Scheme 4. One pot reduction/ hydrophosphination.
4,4'-Phosphanediylbis(N,N-dimethylaniline), 1d
1
Isolated yield: 69%. H NMR (C6D6, 500 MHz): δ 7.67-
Conclusion
7.65 (m, 4H), 6.63-6.61 (m, 4H), 5.61 (d, 1H, J = 212.2
Hz), 2.55 (s, 12H); 13C{1H} NMR (C6D6, 126 MHz): δ
150.9 (s, p-Ar), 135.7 (d, o-Ar, J = 16.2 Hz), 125.0 (app. s,
We have developed a rapid and simple reduction
method for secondary phosphine oxides which
4
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10.1002/adsc.201800723
Advanced Synthesis & Catalysis
i-Ar), 113.0 (d, m-Ar, J = 2.8 Hz), 40.0 (s, N-(CH3)2); 31P
NMR (C6D6, 202 MHz): δ -45.6 (d, J = 212.1 Hz). Data
comparable to previous reports in the literature.[19]
27.8 (d, J = 21.7 Hz), 27.1 (d, J = 0.7 Hz); 31P NMR
(CD3CN, 202 MHz): δ -28.1 (d, J = 198.1 Hz). Data
comparable to previous reports in the literature.[19]
Bis(4-chlorophenyl)phosphane, 1e
Isolated yield: 34%. H NMR (CD3CN, 500 MHz): δ
Trioctylphosphane, 1m
1
1
Isolated yield: 57%. H NMR (C6D6, 500 MHz): δ 1.36-
7.47-7.44 (m, 4H), 7.36-7.34 (m, 4H), 5.22 (d, 1H, J =
1.20 (m, 42H), 0.82 (t, 9H, J = 6.8 Hz); 13C{1H} NMR (C-
222.4 Hz); 13C{1H} NMR (CD3CN, 126 MHz): δ 135.4 (d, 6D6, 126 MHz): δ 32.1, 31.6 (d, J = 10.4 Hz), 29.5 (d, J =
o-Ar, J = 17.5 Hz), 134.6 (s, p-Ar), 133.5 (d, i-Ar, J = 11.3
Hz), 128.8 (d, m-Ar, J = 6.2 Hz); 31P NMR (CD3CN, 202
MHz): δ -42.8 (d, J = 222.3 Hz). Data comparable to
previous reports in the literature.[19]
12.0 Hz), 29.2 (d, J = 10.4 Hz), 27.6 (d, J = 13.2 Hz), 26.2
(d, J = 13.0 Hz), 22.8, 13.9; 31P NMR (C6D6, 202 MHz): δ
-26.7. Data comparable to previous reports in the
literature.[26]
Bis(4-fluorophenyl)phosphane, 1f
Isolated yield: 79%. H NMR (CD3CN, 500 MHz): δ
Method for sequential reduction/hydrophosphination
Manipulations were carried out under an argon atmosphere
in an M-Braun glove box. Diphenylphosphine oxide (0.25
1
7.56-7.51 (m, 4H), 7.14-7.09 (m, 4H), 5.3 (d, 1H, J =
221.1 Hz); 13C{1H} NMR (CD3CN, 126 MHz): δ 164.2 (d, mmol), pinacolborane (1.1 equiv.) and dichloroethane as
p-Ar, J = 246.5 Hz), 137.0 (dd, o-Ar, J = 18.3, 8.1 Hz),
131.6 (dd, i-Ar, J = 10.8, 3.8 Hz), 116.7 (dd, m-Ar, J =
21.2, 6.9 Hz ); 31P NMR (CD3CN, 202 MHz): δ -44.2 (d,
J = 221.0 Hz). Data comparable to previous reports in the
literature.[19]
an internal standard (1 equiv.) were added to a J. Young
NMR tube and allowed to react without solvent at room
temperature for 2 hours. Styrene (2 equiv.) was then added
and the resulting neat mixture was heated at 70 °C for 20
hours to effect the hydrophosphination.[25]
Bis(3,5-bis(trifluoromethyl)phenyl)phosphane, 1g
Isolated yield: 78%. H NMR (CD3CN, 500 MHz): δ
Phenethyldiphenylphosphane, 3
1
Spectroscopic yield: 93% (1,2-dichloroethane used as an
1
8.14-8.12 (m, o-Ar, 4H), 7.97 (m, p-Ar, 2H), 5.58 (d, 1H, J
= 230.1 Hz); 13C{1H} NMR (CD3CN, 126 MHz): δ 138.6
(d, i-Ar, J = 16.6 Hz), 135.3 (dq, o-Ar, J = 17.7, 4.2 Hz),
132.4 (qd, m-Ar, J = 33.2, 5.7 Hz), 124.4 (q, CF3, J =
272.2 Hz), 124.1 (app. hept, p-Ar, J = 3.8 Hz); 31P NMR
(CD3CN, 202 MHz): δ –41.7 (d, J = 229.9 Hz). Data
comparable to previous reports in the literature.[19]
analytic standard). H NMR (CD3CN, 500 MHz): δ 3.80
1
(s, 4H (DCE)), 2.73-1.68 (m, C-H2 1.76H), 2.42-2.38 (m,
2C-H2, 1.96H); 31P NMR (CD3CN, 202 MHz): δ -16.7.
Data comparable to previous reports in the literature.[27]
Acknowledgements
This research has been funded through an EPSRC Early Career
Fellowship (EP/P024254/1) awarded to RLW. A PhD studentship
has been awarded to CBPE as part of the Centre for Doctoral
Training in Sustainable Chemical Technologies at the University
of Bath (EP/L016354/1). Andrew King is acknowledged for
provision of some phosphine oxide substrates that were used in
this work.
Bis(2-methoxyphenyl)phosphane, 1h
Isolated yield: 81%. 1H NMR (CD3CN, 500 MHz): δ 7.34
(ddd, 2H, 4Ar, J = 8.5, 8.1, 1.4 Hz), 7.19 (ddd, 2H, 3Ar, J =
5
8.4, 7.4, 1.6 Hz), 6.96 (dd, 2H, Ar, J = 8.2, 3.3 Hz), 6.89
2
(ddd, 2H, Ar, J = 7.4, 1.3 Hz), 5.1 (d, 1H, J = 226.3 Hz),
3.80 (s, 6H); 13C{1H} NMR (CD3CN, 126 MHz): δ 161.8
1
5
(d, Ar, J = 9.6 Hz), 135.9 (d, Ar, J = 9.5 Hz), 131.3 (s,
3Ar), 123.1 (d, 6Ar, J = 13.3 Hz), 121.9 (d, 4Ar, J = 3.4 Hz),
References
111.5 (s, Ar), 56.3 (s, O-CH3); 31P NMR (CD3CN, 202
2
MHz): δ –73.2 (d, J = 226.3 Hz). Data comparable to
[1] a) J. Hartwig, Organotransition Metal Chemistry:
From Bonding to Catalysis, University Science
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previous reports in the literature.[19]
Phenyl(p-tolyl)phosphane, 1i
1
Isolated yield: 68%. H NMR (CD3CN, 500 MHz): δ
7.54-7.45 (m, 2H), 7.42-7.37 (m, 2H), 7.34-7.31 (m, 3H),
7.18-7.14 (m, 2H), 5.19 (d, 1H, J = 219.3 Hz), 2.31 (s,
3H); 13C{1H} NMR (CD3CN, 126 MHz): δ 139.8 (s, 8Ar),
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3
136.6 (d, Ar, J = 10.4 Hz), 135.0 (d, Ar, J = 17.2 Hz),
6
4
134.5 (d, Ar, J = 16.7 Hz), 132.2 (d, Ar, J = 8.7 Hz),
130.4 (d, 2Ar, J = 6.8 Hz), 129.6 (d, 7Ar, J = 6.2 Hz) 129.4
(s, 1Ar), 21.3 (s, Ar-CH3); 31P NMR (CD3CN, 202 MHz):
δ –41.5 (d, J = 219.3 Hz). Data comparable to previous
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Dicyclohexylphosphane, 1k
Isolated yield: 88%. 1H NMR (CD3CN, 500 MHz): δ 2.77
(dt, 1H, J = 195.0, 5.6 Hz), 1.88-1.13 (m, 27H); 13C{1H}
NMR (CD3CN, 126 MHz): δ 34.0 (d, J = 4.2 Hz), 33.1 (d,
J = 18.9 Hz), 30.4 (d, J = 8.6 Hz), 27.8 (d, J = 3.1 Hz),
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10.1002/adsc.201800723
Advanced Synthesis & Catalysis
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10.1002/adsc.201800723
Advanced Synthesis & Catalysis
FULL PAPER
Rapid Metal-Free Formation of Free Phosphines
from Phosphine Oxides
Adv. Synth. Catal. Year, Volume, Page – Page
Cei B. Provis-Evans, Emma A. C. Emanuelsson,
Ruth L. Webster*
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