Exploring the Reactivity of Donor-Stabilized Phosphenium Cations: Lewis Acid-Catalyzed Reduction of Chlorophosphanes by Silanes

Article abstract of DOI:10.1021/acs.inorgchem.8b01578

Phosphane-stabilized phosphenium cations react with silanes to effect either reduction to primary or secondary phosphanes, or formation of P-P bonded species depending upon counteranion. This operates for in situ generated phosphenium cations, allowing catalytic reduction of P(III)-Cl bonds in the absence of strong reducing agents. Anion and substituent dependence studies have allowed insight into the competing mechanisms involved.

Full text of DOI:10.1021/acs.inorgchem.8b01578

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Exploring the Reactivity of Donor-Stabilized Phosphenium Cations:
Lewis Acid-Catalyzed Reduction of Chlorophosphanes by Silanes
Kyle G. Pearce, Andryj M. Borys, Ewan R. Clark,* and Helena J. Shepherd
School of Physical Sciences, University of Kent, Ingram Building, Canterbury, Kent CT2 7NH, United Kingdom of Great Britain
and Northern Ireland
S
* Supporting Information
ABSTRACT: Phosphane-stabilized phosphenium cations react
with silanes to effect either reduction to primary or secondary
phosphanes, or formation of P−P bonded species depending
upon counteranion. This operates for in situ generated
phosphenium cations, allowing catalytic reduction of P(III)−
Cl bonds in the absence of strong reducing agents. Anion and
substituent dependence studies have allowed insight into the
competing mechanisms involved.
which may then be deprotected if required, but does not
reduce other PX bonds (X = OR, O).16 Borane itself, BH3,
does not reduce PCl bonds, instead forming chlorophos-
phane−borane adducts which may then be cleanly reduced to
PH species with the protecting group intact;17,18 a mixture
of LiAlH4 and NaBH4 may also be used to form phosphane−
borane adducts, generating the required BH3 in situ.19
INTRODUCTION
■
Organophosphorus species find use as optoelectronic materi-
als,1 pharmaceuticals,2 ligands,3 and many other applications,
and have historically been prepared primarily by reaction of
organometallic nucleophiles with chlorophosphane electro-
philes.4 Complementary routes have been developed exploiting
phosphane reaction with electrophiles,5 transition-metal-
catalyzed cross-coupling,6 or hydrophosphination of unsatu-
rated species.7 These processes all rely on the presence of a
PH bond for later functionalization, but PH species are
not generally commercially available for any but the simplest
derivatives. PH species are accessible by reductive cleavage
of PC bonds using alkali metals (Na/NH3 or Li/THF)
followed by aqueous workup.8 This approach shows poor
functional group tolerance and selectivity in heteroleptic
phosphanes, however, so it is typically used only with simple,
homoleptic phosphane precursors. It has been reported that
PCl bonds may be reduced under milder conditions using
Zn metal.9 PH species are instead typically synthesized by
the milder reduction of phosphorus−halogen, phosphorus−
oxygen, or phosphorus−nitrogen bonds using stoichiometric
reduction by main group metal hydrides, with a single
reference in the literature reporting Pd-catalyzed reduction of
PCl bonds under H2;10 PO moieties are resistant to Pd-
catalyzed hydrogenation.11 Aluminum hydride reducing agents
are effective at reducing a wide range of PX bonds (X =
halide, OR, O), with reductive coupling to form PP bonds a
common side reaction.12 The most common such reagent,
LiAlH4, is pyrophoric and its use is made hazardous by the
exothermic aqueous workup which releases dihydrogen as a
byproduct. Reductive coupling can be avoided by using the
milder reagent DIBAL, but with a significant increase in cost
and retention of the hazardous workup.13 NaBH4 has been
reported to reduce secondary chlorophosphanes to directly
form the protected secondary phosphane−borane adduct,14,15
Silanes have been extensively used as mild reducing agents,
with and without catalysts,20−22 for the reduction of PO
bonds to convert phosphane oxides to phosphanes, a reaction
driven by the formation of strong SiO bonds.23 This has
been used not just in phosphane synthesis, but to develop
variations of the Wittig,11 Mitsunobu,24 and Appel25 reactions
which are catalytic in phosphane. Investigation has shown at
least two competing mechanisms for this process,26,27 both of
which rely on the nucleophilicity of the terminal oxygen to
drive the reaction. For this reason, more Lewis acidic
halosilanes (e.g., HSiCl3, Si2Cl6, PhSi(Cl)H2) are in general
more effective reducing agents rather than the more hydridic
species as might be expected.28 Unhalogenated silanes
therefore require extended reaction times and higher temper-
atures for less nucleophilic phosphine oxides.25 These silanes
are insufficiently reducing, however, to directly reduce P−Cl
bonds due to the combined low nucleophilicity of the
unactivated Si−H moiety and the reduced thermodynamic
driving force of Si−Cl bond formation.
We reasoned that unactivated hydrosilanes should never-
theless react with a sufficiently electrophilic P(III) center;
29
Vidovic and Stephan30 recently reported analogous reaction
́
of P(III) dications with silanes. While ligand exchange
reactions about donor-stabilized phosphenium cations have
been studied in the past,31,32 and they have been investigated
Received: June 7, 2018
© XXXX American Chemical Society
A
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as ligands in transition metal complexes,33,34 comparatively
little is known about their other reactivity, in part due to their
perceived instability and high Lewis acidity.35 The principal
exceptions to this are the N-heterocyclic phosphenium cations
where chelation and nitrogen π-donor ligands stabilize the
resultant cations to give catalytically useful species.36−38 For
phosphane-stabilized phosphenium cations, the empty p
orbital on phosphorus is quenched by donation of a lone
pair from a second phosphane. These species may also be
regarded as phosphino-phosphonium species39 but as they
remain electrophilic at the three-coordinate phosphorus center
due to the low-lying and minimally hindered P−P σ* orbital,
the phosphane-phosphenium nomenclature is used herein as a
better representation of the observed reactivity.
Table 1. Screening Lewis Acids for Catalytic Efficacy
a
conversion (%)
Lewis acid
GaCl3
loading (%)
time (days)
4
5
5
10
25
7
5
1
>99
>99
>99
−
−
−
AlCl3
25
100
7
1
61
92
−
8
FeCl3
5
7
−
7
RESULTS AND DISCUSSION
■
To our delight, on reaction of the known adduct of the weak
donor ligand 1, [Ph2(Cl)P-PPh2]GaCl4, [2]GaCl4,39 with 1
equiv of Et3SiH in PhCl (Scheme 1), the 31P NMR showed
TMSOTf
25
100
5
7
−
−
82
>99
b
NaBArF
25
25
1
1
68
26
38
Scheme 1. Successive Hydride Transfer from Silane to
Phosphorus Center
b
NaBArCl
62
a
NMR conversion by relative 31P NMR intensity (see Supporting
b
Information for full details). Anion decomposition observed.
1
the H NMR corresponding to the formation of H2, for an
effective dehydrocoupling reaction (Scheme 2).
Scheme 2. Reductive Coupling in the Presence of TMSOTf,
Leading to Overall Dehydrocoupling
immediate PH bond formation and, after heating at 60 °C
for 1 h, clean conversion to [Ph2(H)P-PPh2]GaCl4, [3]GaCl4,
with transformation of Et3SiH to Et3SiCl. No immediate
reaction was observed on addition of a second equivalent of
Et3SiH, but further heating at 60 °C overnight lead to almost
complete conversion to Ph2PH, 4, with trace formation of
Ph2P-PPh2, 5; all Et3SiH was converted to Et3SiCl with GaCl3
liberated overall. We subsequently tested the more stable
In comparison, at elevated temperatures both NaBArF and
NaBArCl (BArF = tetrakis(3,5-trifluoromethylphenyl)borate,
BArCl= tetrakis(3,5-dichlorophenyl)borate) give simultaneous
dehydrocoupling and PH bond formation, in direct contrast
to the behavior of OTf, coupled with anion decomposition,
either by hydro-dehalogenation(BArF) or protodeboronation
(BArCl). On heating at 100 °C, the BArF anion undergoes
fluoride abstraction, leading to the formation of partially
fluorinated phosphane centers and Et3SiF, confirmed by 11B,
19F and 29Si NMR, but no change in the final 11B NMR
spectrum is observed on addition of excess pyridine, indicating
an absence of free 3° boron species. In contrast, the BArCl
system showed almost complete loss of signal intensity in the
11B NMR, indicating protodeboronation and formation of BAr3
species. Together, these confirm the presence of anion-
dependent reaction mechanisms.
Having identified a suitable Lewis acid and loading, several
commercially available silanes were screened as hydride donors
(Table 2). At a 5% catalyst loading, Et3SiH proved the most
effective donor, but increasing the catalyst loading could be
used to improve yield of 4 with other, cheaper silanes. For
Et3SiH to PHMS, the trend in reactivity follows that predicted
by Mayr’s nucleophilicity index,40,41 but this trend is reversed
for Ph3SiH to PhSiH3. This may indicate that the steric
hindrance about Si is such that the assumptions about rate of
reaction in Mayr’s scale are not valid for the very hindered
phosphenium electrophiles, as seen for other bulky electro-
philes.42
39
[Ph3P-PPh2]GaCl4 with Et3SiH and, while this required 2 h
at 60 °C to go to completion, 4 and 5 were formed in 49:1
ratio. When 1 and Et3SiH were reacted with 25 mol% GaCl3
(i.e., a catalytic loading) in PhCl, immediate formation of
[3]GaCl4 was evident, and heating overnight at 100 °C gave
complete conversion to 4, implying catalytic behavior.
A range of Lewis acids, silanes, and halophosphanes were
screened to probe the scope of this potentially useful catalytic
reactivity. The Lewis acids were screened by reaction of 1 and
Et3SiH with an initial 25% loading of Lewis acid and heated for
up to 7 days at 100 °C, with daily monitoring (Table 1). The
exception to this was FeCl3, for which a 5% loading was
initially tested to avoid issues with paramagnetic broadening in
the NMR. Of these results, GaCl3 was found to be the optimal
Lewis acid for PH bond formation, giving essentially
quantitative yields even at 5% catalyst loading. The use of
weakly coordinating anions (WCAs) resulted in very different
reactivity from that observed for GaCl3 and AlCl3. Me3SiOTf is
an insufficiently strong halide abstraction agent to form
[2]OTf but, reasoning that a small thermal population may
be formed on heating, was nevertheless tested as a potential
Lewis acid. Prolonged heating at 100 °C lead to clean
conversion to a sharp singlet at δ −15.3 ppm, indicating the
formation of Ph2P-PPh2, 5, and growth of a peak at 4.5 ppm in
B
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hydride to form the protio-phosphane-stabilized phosphenium
is easily achieved, the transfer of the second is not. Repetition
at 50% GaCl3 loading (i.e., preforming the halophosphane-
phosphenium) did give a small yield of Ph(nBu)PH (see
Supporting Information) but the dimers remained the
dominant product. To probe the influence of anion, the
reduction of both Ph(tBu)PCl and Ph(nBu)PCl was repeated
in the presence of 25% TMSOTf, and the same phenomena
were observed. The reduction of P−N and P−O bonds using a
5% loading of GaCl3 does proceed but much more slowly and
with unwanted side reactions. For the reduction of Ph2PNiPr2,
the consumption of Et3SiH is coupled with the formation of
Et3SiCl rather than Et3SiNiPr2, indicating consumption of the
GaCl3 and the formation of less Lewis acidic gallium amido
speciesthis ultimately is not therefore a catalytic reaction,
and a different mechanism may be in play, driven by the
relative difference in Si−N vs Ga−N bond strengths. On
reaction of Ph2POEt with Et3SiH in the presence of 5% GaCl3,
slow formation of 4 and 5 is observed, along with Ph2P(O)Et
and Ph2PEt and a number of unknown byproducts, indicating
simultaneous competing Arbuzov-type reactivity.
Table 2. Screening Silanes as Hydride Donors
a
conversion
(%)
silane
catalyst
time (days)
7
4
5
Et3SiH
GaCl3 (5%)
>99
−
PhMe2SiH
GaCl3 (5%)
AlCl3 (100%)
7
5
40
87
34
13
Me2Si(H)-O-Si(H)Me2
Me2Si(H)-O-Si(H)Me2
PHMS
GaCl3 (5%)
7
7
31
97
42
GaCl3 (25%)
−
GaCl3 (5%)
AlCl3 (100%)
7
7
20
85
32
12
Ph3SiH
Ph2SiH2
PhSiH3
GaCl3 (5%)
GaCl3 (5%)
GaCl3 (5%)
7
7
7
19
51
82
18
26
18
Reduction of primary dichlorophosphanes by Et3SiH in the
presence of catalytic Lewis acid is also achievable, with cyclic
species and the rac- and meso-R(H)P-P(H)R products of
incomplete reduction as side products; no R(Cl)P-P(Cl)R
were observed (Table 4). Use of a higher loading of Lewis acid
a
NMR conversion by relative 31P NMR intensity.
Screening some simple aryl−alkyl and alkyl−alkyl chlor-
ophosphanes revealed significant influence of steric bulk and
Table 4. Reduction of Primary Chlorophosphanes and PCl3
electron-donating substituents on reduction (Table 3).
Table 3. Reduction of Secondary P−Cl, P−O, and P−N
Bonds
a
conversion (%)
R(H)P-
P(H)R
b
substrate
catalyst
time
RPH2
(RP)n
PhPCl2
PhPCl2
PhPCl2
GaCl3 (5%)
7 days
7 days
∼5 min
7 days
65
−
42
18
30
23
23
−
13
−
8
10
86
45
44
62
77
TMSOTf (25%)
NaBArCl (100%)
GaCl3 (5%)
a
conversion
c
tBuPCl2
tBuPCl2
substrate
catalyst
time (days)
R2PH
R2P-PR2
Ph2PCl
Ph2PCl
none
3
7
7
7
7
7
9
7
7
7
7
−
>99
−
26
3
84
23
66
8
trace
−
GaCl3 (25%)
GaCl3 (5%)
7 days
c
GaCl3 (5%)
PCl3
∼5 min
−
b
Ph(nBu)PCl
Ph(nBu)PCl
Ph(nBu)PCl
Ph(tBu)PCl
Ph(tBu)PCl
tBu2PCl
GaCl3 (25%)
GaCl3 (50%)
TMSOTf (25%)
GaCl3 (5%)
TMSOTf (100%)
GaCl3 (5%)
>99
74
95
16
34
−
−
10
5
a
NMR conversions by relative 31P NMR intensity. Cyclic systems
including (RP)4 and larger rings. See Supporting Information for
c
details Proceeds at ambient temperature.
leads to improved yield of primary phosphane and, as before,
use of TMSOTf provides only reductive coupling products.
PCl3 reacts rapidly with effervescence and a marked exotherm
on introduction of the GaCl3 to produce a red precipitate, with
PH3 and P4 the sole observable species in solution; the red
solid was confirmed as a polymeric phosphorus species by
chemical testing (see Supporting Information for details).
Given the stability of P−N bonds under these reaction
conditions, we attempted the reduction of the heteroleptic
species Ph(iPr2N)PCl. Slow cyclization was observed with a
5% GaCl3 loading, but increasing the loading to 50% (i.e.,
preforming the phosphane-phosphenium) lead to rapid
reaction at room temperature to form [(iPr2N)Ph(H)-
PPh(NiPr2)]+ as the dominant species with small quantities
of rac- and meso-(iPr2N(Ph)P)2, and cyclic byproducts. On
heating to 100 °C, however, the protio-phosphane-stabilized
phosphenium decomposed into a complex mixture and no
Ph(iPr2N)PH was observed.
tBu2PCl
TMSOTf (25%)
GaCl3 (5%)
GaCl3 (5%)
b
Ph2POEt
Ph2PNiPr2
3
15
a
b
NMR conversions by relative 31P NMR intensity. Other products
observed. See Supporting Information for details.
Surprisingly, Ph(tBu)PCl was reduced more efficiently than
the less bulky Ph(nBu)PCl, which even after 7 days at 100 °C
with an increased 25% catalyst loading showed only a complex,
unresolved dynamic mixture in the 31P NMR which is
attributed to free and rapid exchange between the many
diastereomeric possibilities of [Ph(nBu)(H)P-P(nBu)Ph]+ and
[Ph(nBu)P-P(nBu)Ph-P(nBu)Ph]+.
Addition of excess base at this point caused the spectra to
resolve to cleanly give exclusive formation of the rac- and meso-
P−P-coupled species, indicating that while transfer of the first
C
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Figure 1. Variable-temperature NMR Studies. (a) VT studies on reaction mixture showing de-coalescence on cooling. (b) Comparison of 31P and
31P{1H} NMR at −60 °C showing clean formation of [3]+ and [6]+.
The potential utility of silane/Lewis acid reduction of
chlorophosphanes to practical synthesis was explored by the
reduction of 1 on a 2 mmol scale using AlCl3 and PMHS as the
reductive system. Following workup with Me2S·BH3, the
desired product, Ph2P(BH3)H, was isolated in un-optimized
59% yield. This augurs well for the potential future application
of this reactivity, given the comparatively mild reaction
conditions and cheap, easily handled reagents, but the yield
remains low and reaction times long compared to other
reductive approaches to this compound (e.g., LiAlH4, NaBH4).
Additional work on optimization of reaction conditions and
Lewis acid are required before this can be considered a
generally useful method for primary and secondary phosphane
synthesis.
catalytic loadings of NaBArF and NaBArCl were used to initiate
reduction, we synthesized [2]BArF and [2]BArCl to preform
the phosphenium cation with a WCA. In both cases, on
addition of Et3SiH, formation of [3]+ occurred rapidly at
ambient temperature. Reaction stopped at that stage for the
BArF salt, and [3]BArF was isolated in 60% yield as a colorless
crystalline solid. Although [3][B(C6F5)4] is known in the
literature,44 this is the first crystallographically characterized
salt of this cation. The cation is disordered about an inversion
center, and the proton could not be located in the difference
map, but the proton position can be assigned by comparison to
calculated geometry (see Supporting Information for details).
The P−P bond length is short at 2.176 (3) Å, compared to
those of [2]GaCl4 and [Ph3P-PPh2]OTf (2.205(4) Å31 and
2.230 (1) Å39 respectively) as expected with the reduction in
steric demand. In contrast, the BArCl salt continued to react,
with slow formation of [6]BArCl seen over 19 days. This was
accompanied by loss of intensity in the 11B spectrum. On
addition of excess pyridine, a new signal formed at δ0.5 ppm in
the 11B NMR, indicating the formation of a four-coordinate
boron species and thus that the protio-phosphane-stabilized
phosphenium is sufficiently acidic to cause protodeboronation
of the BArCl anion even at ambient temperature.
MECHANISTIC CONCERNS
■
Given the dramatic influence on anion and substituents on
reaction products, we sought a deeper understanding of the
mechanisms involved. During reductions with substoichio-
metric Lewis acid, the 31P NMR shows a number of broad
product signals, indicative of multiple exchanging species.
Reductions were performed with a 25% Lewis acid loading
(GaCl3 and TMSOTf) and heated at 60 °C to allow the
reactions to proceed, after which variable temperature NMR
studies were used to freeze out the exchange processes and
identify the intermediates.
Since ambient temperature reaction of the BArCl salts with
silanes leads to dehydrocoupling while 4 is observed when the
reaction mixture is heated, a different mechanism is implicated.
When a reaction in which BArCl had thermally degraded was
recharged with 1 and Et3SiH and further heated at 100 °C,
preferential reduction to form 4 was observed (see Supporting
Information for details). Ingleson has shown that tris(3,5-
dichlorophenyl)borane is a competent Lewis acid for activating
silanes via FLP chemistry,45 and it is therefore plausible that a
borohydride intermediate is involved in this process. Similar
behavior is implicated in the NaBArCl-induced reduction of
PhPCl2. When PhPCl2 and Et3SiH are premixed before
NaBArCl addition, rapid reaction ensues giving PhPH2 as the
For the GaCl3-catalyzed reaction, on cooling to −30 °C the
31P spectra resolve to show [3]GaCl4 and the known adduct
43
[Ph2P-P(Ph2)-PPh2]GaCl4, [6]GaCl4, as the exchanging
species (see Figure 1); the TMSOTf reaction mixture does
not fully resolve to show 1JP−P coupling even down to −70 °C
but the unresolved peaks do correspond to those seen for
GaCl3, confirming the formation of phosphenium intermedi-
ates in this reaction, and that transient M-H bond formation is
not required for Si-to-P hydride transfer.
Given the difference in reactivity observed for the [GaCl4]−
and TfO− salts, and the anion degradation observed when
D
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major product; addition of pyridine confirmed anion
degradation and the formation of a py-Ar3 species in situ.
The source of the hydride was confirmed to be the silane by
isotopic labeling. Reaction of 1 with catalytic (25%) GaCl3 in
the presence of Et3SiD gave clean formation of Ph2PD and a
1:1:1 triplet in the 31P NMR, ruling out solvent activation. An
analogous experiment combining 1, 25% Me3SiOTf and 1:1
mixture of Et3SiH and Et3SiD on heating at 100 °C cleanly
formed 5 in addition to H2, HD and D2 as seen in the 1H and
2H NMR, confirming hydrogen formation and overall
dehydrocoupling. The necessity for phosphenium formation
for reduction was confirmed by heating 1 with Et3SiH at 100
°C in the absence of Lewis acidno reduction was observed
after 3 days. To rule out the possibility that [GaCl4]− might be
acting as a soluble Cl− source interacting with Et3SiH to form a
5-coordinate activated silane, the combination of 1 and Et3SiH
were heated at 100 °C with 10% [BnNEt3]Clafter 7 days,
3% of the 1 had reacted to form 5 as the sole product. The
observation of 6[GaCl4] in the variable-temperature studies
indicated the formation of 5 as an intermediate, but this is not
seen under equivalent catalyst loadings at higher temperatures,
implying that reduction of the P−P bond may also occur under
these conditions. A control reaction of 5 with 25% GaCl3 and
Et3SiH showed that Lewis acid mediated cleavage of the P−P
bond to form 4 does occur, but that it is slow (47% conversion
after 3 days at 100 °C) relative to the formation of 4 from 1
(near quantitative conversion after 1 day at 100 °C), indicating
that it is a minor pathway.
Table 5. Relative Donor Strengths of Phosphanes As
Evaluated by Donor Exchange
donor
relative stability/kcal mol−1
Ph2PCl
Ph2PH
Ph2P-PPh2
Ph3P
0.00
−5.95
−10.45
−11.83
−0.74
−8.50
−1.80
−8.18
−0.51
−10.91
Ph(nBu)PCl
Ph(nBu)PH
Ph(tBu)PCl
Ph(tBu)PH
Ph(iPr2N)PCl
Ph(iPr2N)PH
a
All calculations were performed at the M06-2X/6-311g(d,p) level
with dichloromethane solvent model. See Supporting Information for
details.
important. We therefore propose a Piers−Oestreich-type
transfer mechanism,47,48 whereby the Si−H bond coordinates
to the Lewis acidic site at phosphorus followed by attack of a
donor center (solvent, anion, or one of the many phosphanes
in solution) generating a transient silylium cation intermediate.
This silylium intermediate can then abstract a halide from
another equivalent of chlorophosphane or tetrahalogallate,
closing the catalytic cycle (Scheme 3). After formation of
Both radical and Lewis acid hydride abstraction were
considered as potential mechanisms of hydride transfer.
t
However, as Bu2P-PtBu2 is a known species accessible via
Scheme 3. Proposed Catalytic Cycle for Halophosphane
Reduction
single electron reduction and is not formed under these
reaction conditions, this argues strongly against a radical
mechanism for P−P coupling in these species.46 For all
stoichiometric reactions, formation of protio-phosphane-
stabilized phosphenium cations is rapid (minutes to hours at
ambient temperature) whereas subsequent hydride transfer
requires extended heating. Furthermore, the observation of
[2]+ and [3]+, and [3]+ and [6]+ simultaneously, in
conjunction with free 1, giving well-resolved signals in the
31P NMR indicates that there is a significant increase in donor
strength at each stage of the process. A quantitative assessment
of relative donor strength was obtained by DFT evaluation of
ligand exchange about [2]+ for a selection of relevant donors
involved in the reactions observed, as shown in Table 5, where
a more negative value indicates an increasingly stable adduct
relative to [2]+. The results are in agreement with the
experimental observation that Ph3P displaces Ph2PCl from
[2]+,39 and the qualitative observation that the rate of hydride
transfer to phosphenium (of the order [2]+ > [3]+ > [6]+ ≈
[Ph3P-PPh2]+) correlates well with the calculated donor
strengths of the phosphanes. Furthermore, it can be seen
that in each instance the secondary phosphane is a stronger
donor than the corresponding secondary chlorophosphane and
that the alkyl-substituted phosphanes are stronger donors than
diphenylphosphane derivatives. This may in part explain the
increased formation of P−P coupled products in these cases as
reduced electrophilicity at phosphorus favors competitive
deprotonation instead.
protio-phosphane-phosphenium, subsequent reaction could
then either proceed via a second equivalent of silane reacting
at P, forming a second PH bond, or reaction with a base to
deprotonate the intermediate, forming a diphosphane. This
step would appear to be strongly anion dependent. The
proposed mechanism is likewise consistent with the less
successful reduction of P−N and P−O bonds in the presence
of Lewis acid and silane due to the reduced lability of these
bonds relative to PCl systems, and with the decomposition
of the BArF anion as ArCF3 groups are known to react with
silylium species.49
From these results, it can be seen that anion participation is
not required for the transfer of hydride from silane to
phosphorus for the halo-phosphenium but that phosphenium
stabilization (or lack thereof) and thus Lewis acidity is
E
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CONCLUSIONS
■
We have investigated the reactivity of phosphane-stabilized
phosphenium cations with hydrosilanes and shown that they
undergo facile hydride transfer to form protio-phosphane-
stabilized phosphenium species. These can then further react
through two, anion-dependent reaction pathways; deprotona-
tion effects reductive concatenation with the formation of P−P
bonds, while hydride transfer to the less electrophilic, protio-
phosphane-stabilized phosphenium center leads to primary or
secondary phosphanes. These transformations can be made
catalytic in Lewis acid, and can be extended using cheap Lewis
acids and silanes potentially offering a mild, operationally
simple reduction protocol without reactive M−H bonds.
Further catalytic applications of these donor-stabilized
phosphenium cations are currently under investigation.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01578.
Complete synthetic details, multinuclear NMR data,
computational results, and Cartesian coordinates of all
optimized species (PDF)
Accession Codes
CCDC 1846580 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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Synthesis and reactions of phosphine-boranes. Synthesis of new
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: e.r.clark@kent.ac.uk.
ORCID
Andryj M. Borys: 0000-0001-8437-2562
Ewan R. Clark: 0000-0001-7287-2631
Helena J. Shepherd: 0000-0003-0832-4475
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors wish to thank the University of Kent for funding.
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