Phosphirenium ions as masked phosphenium Catalysts: Mechanistic evaluation and application in synthesis

Article abstract of DOI:10.1021/acscatal.1c01133

The utilization of phosphirenium ions is presented; optimized and broadened three-membered ring construction is described together with the use of these ions as efficient pre-catalysts for metal-free carbonyl reduction with silanes. Full characterization of the phosphirenium ions is presented, and initial experimental and computational mechanistic studies indicate that these act as a "masked phosphenium"source that is accessed via ring opening. Catalysis proceeds via associative transfer of {Ph2P+} to a carbonyl nucleophile, Ha'SiR3 bond addition over the C=O group, and associative displacement of the product by a further equivalent of the carbonyl substrate, which completes the catalytic cycle. A competing off-cycle process leading to vinyl phosphine formation is detailed for the hydrosilylation of benzophenone for which an inverse order in [silane] is observed. Experimentally, the formation of side products, including off-cycle vinyl phosphine, is favored by electrondonating substituents on the phosphirenium cation, while catalytic hydrosilylation is promoted by electron-withdrawing substituents. These observations are rationalized in parallel computational studies.

Full text of DOI:10.1021/acscatal.1c01133

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Research Article
Phosphirenium Ions as Masked Phosphenium Catalysts: Mechanistic
Evaluation and Application in Synthesis
Danila Gasperini, Samuel E. Neale, Mary F. Mahon, Stuart A. Macgregor,* and Ruth L. Webster*
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ABSTRACT: The utilization of phosphirenium ions is presented; optimized
and broadened three-membered ring construction is described together with
the use of these ions as efficient pre-catalysts for metal-free carbonyl
reduction with silanes. Full characterization of the phosphirenium ions is
presented, and initial experimental and computational mechanistic studies
indicate that these act as a “masked phosphenium” source that is accessed via
ring opening. Catalysis proceeds via associative transfer of {Ph2P+} to a
carbonyl nucleophile, H−SiR3 bond addition over the CO group, and
associative displacement of the product by a further equivalent of the
carbonyl substrate, which completes the catalytic cycle. A competing off-cycle
process leading to vinyl phosphine formation is detailed for the hydrosilylation of benzophenone for which an inverse order in
[silane] is observed. Experimentally, the formation of side products, including off-cycle vinyl phosphine, is favored by electron-
donating substituents on the phosphirenium cation, while catalytic hydrosilylation is promoted by electron-withdrawing substituents.
These observations are rationalized in parallel computational studies.
KEYWORDS: phosphenium ions, phosphirenium ions, organocatalysis, reaction mechanism studies, density functional theory,
hydrosilylation
as Lewis acidic sites for the activation of organic substrates and
so possess exciting opportunities for selective or divergent
reactivity. However, we herein report on the unexpected ability
of phosphirenium ions to undergo ring opening and
subsequent alkyne displacement via nucleophilic trapping of
a phosphenium ion {Ph2P+} by carbonyl reagents (Scheme
1d). Onward reaction with a silane and a further equivalent of
carbonyl releases the reduced carbonyl product and regener-
ates the highly active phosphenium adduct, which is the active
catalyst. Divalent phosphenium ions are highly elusive species,
relying on lone pair stabilization of the empty phosphorus p-
orbital by adjacent heteroatoms (e.g., N- or P-donors) without
which they are challenging to detect and characterize.13 It is
worth noting that divalent phosphenium species, in the form of
N-heterocyclic phosphenium ions, have recently started to
gather interest,14 not least due to their catalytic capabilities,
which are as yet unproven with a simple {Ar2P+} catalyst. Here,
our synergistic experimental and computational approach
strongly supports our assertion of easy-to-prepare and -handle
phosphirenium ions acting as a masked phosphenium catalyst
1. INTRODUCTION
The chemistry of the three-membered phosphacycle, the
phosphirenium ion, first appeared in the literature at the
beginning of the 1980s,1 along with a number of theoretical
studies that considered their stabilization by σ*-aromaticity.2
The chemistry to date is dominated by the use of these species
as reagents in synthesis; exploitation of their catalytic Lewis
acidic properties has yet to be reported. The first example of a
stabilized three-membered phosphirene−tungsten ring was
reported by Marinetti and Mathey in 19823 (Scheme 1a). A
year later, Hogeveen and Breslow reported and characterized
phosphirenium salts by formal addition of halophosphines to
alkynes,4 a method that was further improved using amine-
phosphenium ions [iPr2NPCl]+[AlCl4]− (Scheme 1b).5 Other
routes for their synthesis or methods for structural elaboration
have been developed,6−8 but it was only in 2017, when Hirano
and Miura reported the synthesis of highly functionalized
organophosphines postulated to proceed via a phosphirenium
intermediate, that the application in practical organic synthesis
was reported (Scheme 1c).9 They have since expanded their
methodology to the synthesis of dibenzophospholes,10,11 while
other groups have reported functionalization of phosphine
oxides in the presence of Tf2O.12
Received: March 11, 2021
Revised: April 2, 2021
Published: April 20, 2021
Phosphirenium ions pose an interesting mechanistic
challenge: they possess both electrophilic phosphorus and a
low-lying π*-acceptor centered on the highly activated
carbon−carbon double bond in the ring. Both could behave
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Scheme 1. (a,b) Different Approaches to the Synthesis of
Phosphirenium Ions; (c) Functionalization via a
Phosphirenium Ion; (d) This Work, where Mechanistic
Investigations Indicate that a Phosphirenium Pre-catalyst
Acts as a Masked Phosphenium Ion, Which is Itself the
Active Catalyst
Table 1. Optimization of the Synthesis of Phosphirenium
Ions
a
entry
activating agent
T (°C)
conversion to 3·OTf
1
2
3
4
5
6
7
8
Tf2O
Fe(OTf)2
AgOTf
HOTf
HOTf/PhSiH3
Ac2O
80
80
80
80
80
80
80
60
60%
0
0
0
Ph2PH (major)
several products
0
b
O(CO2tBu)2
Tf2O
c
d
>99% (98% )
a
Reaction conditions 1 (0.2 mmol), 2 (0.2 mmol), activating agent
b
(0.2 mmol), and CDCl3 (0.4 mM). See the Supporting Information.
c
d
CD3CN. Spectroscopic yield analyzed by inverse-gated 31P{1H}
NMR spectroscopy with PPh3 as an internal standard.
and 3), or by Brønsted acid activation with triflic acid16 (entry
4) or activated triflic acid (entry 5). Acetic anhydride does
activate 2, with the formation of Ph2P(OAc)17 and Ph2P(O)-
PPh2.18 However, these species do not react further with 1
(entry 6). Di-tert-butyl dicarbonate (Boc2O) is not effective for
this transformation, and even at higher temperatures, 1 and 2
are recovered (entry 7). Following a brief solvent and
temperature screening,19 the optimal conditions were
determined to be CH3CN at 60 °C for 30 min, which give
full conversion to 3·OTf (entry 8).
Adding 1 equiv of NaBArF to the optimized reaction mixture
of 1, 2, and Tf2O in CD3CN forms 3·BArF, which is isolated in
66% yield (Figure 1). A small downfield shift is observed in the
31P{1H} NMR spectrum from −108.3 ppm for 3·OTf to
−109.1 ppm for 3·BArF; the disappearance of the triflate peak
at −79.3 ppm is observed by 19F NMR spectroscopy and
−
F
substituted by a BAr counterion at −62.5 ppm. Counterion
exchange is not observed when using sodium or silver salts,
of the form {Ph2P+}, stabilized by a Lewis basic carbonyl
oxygen atom. An advantage of the work reported here is the
ease of synthesis, with a range of phosphirenium pre-catalysts
being readily prepared or formed in situ for use in catalysis.
2. RESULTS AND DISCUSSION
2.1. Synthesis and Characterization of Phosphire-
nium Ions. Due to the practicality of the reaction, Hirano and
Miura’s procedure (where the phosphirenium ion was detected
but not isolated)9,11,15 was employed for the synthesis of
phosphirenium ion 3·OTf (Table 1, entry 1). Formation of 3·
OTf is observed by following the reaction via 31P{1H} NMR
spectroscopy, where a characteristic peak at −108.3 ppm is
obtained.9 To optimize the protocol, different activating agents
were reacted with diphenyl acetylene 1 and secondary
phosphine oxide (SPO) 2 in CDCl3. 3·OTf does not form
when using triflate salts, such as silver or iron triflates (entries 2
Figure 1. Counterion exchange at the phosphirenium ion. Isolated
yield presented. ORTEP view of 3·BArF showing a thermal ellipsoid at
30% probability levels. H, F, and minor components of disordered
anion C atoms were omitted for clarity.23
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such as NaBF4 or AgNTf2, and 3·OTf is recovered; with
AgSbF6 and AgBF4, decomposition of the starting material is
20
1
observed via 31P{1H}, 19F, and H NMR spectroscopy. The
F
structure of 3·BAr was confirmed by single-crystal X-ray
diffraction analysis. The bond metrics associated with this
structure are in line with other examples of phosphirenium
cations, for example, the ring P−C distances are 1.748(2) and
1.746(3) Å, and the CC bond distance is 1.340(4) Å; both
are similar to reports from Mathey, Wild, and Cowley.3,6,7,21
The alternating single and double bonds show that the
structure depicted is the dominant resonance form of the
phosphirenium ion in the solid state. This species represents
the first isolated phosphirenium ion with a borate counter-
ion.22
With the optimized conditions in hand, the generality of the
methodology was explored (Figure 2). Some of the
phosphirenium ions were very sensitive and could only be
characterized in situ (5·OTf, 6·OTf, 7·OTf, 8·OTf, and 10·
OTf proved particularly challenging; see the Supporting
Information for spectral data). The use of different substituted
SPOs, bearing bearing aryl or alkyl groups, is well tolerated.
F
For example, using Cy2P(O)H, 4·OTf and 4·BAr are isolated
in high yields (97 and 98%, respectively). These species display
downfield 31P{1H} NMR signals compared to 3, with a singlet
at −85.4 and −86.5 ppm, respectively, due to the increased
electron donation of the alkyl substituents. There is only a very
modest difference in the 31P chemical shift observed between
the ions bearing different anions, 3·OTf versus 3·BArF Δδ = 0.8
F
and 4·OTf versus 4·BAr Δδ = 1.1 ppm, which is as expected
given the non-coordinating nature of the counterions.
Mixed SPOs bearing alkyl and phenyl groups, such as
tBuPhP(O)H and CyPhP(O)H, are also well tolerated with
full conversion after 1 h to form 5·OTf24 and 6·OTf, with
31P{1H} NMR signals at −97.1 and −93.3 ppm, respectively.
Varying the alkyne was also explored; phenyl acetylene and
Cy2P(O)H lead to full conversion to 7·OTf with a phosphorus
signal at −80.2 ppm and a downfield signal for the terminal
C−H at 8.4 ppm. For all phosphirenium species presented,
there are deshielded signals in the proton and carbon NMR
spectra, which can be explained by the resonance structure of
the phosphirenium ion.25,26 1-Phenyl-1-propyne reacts well
with 2 to form 8·OTf with a 31P signal at −103.6 ppm. Further
functionalization of the alkyne is tolerated with 9·OTf (38%)
and 10·OTf (64%) isolated using the optimized conditions.
The phosphorus signals for 9·OTf and 10·OTf appear at δ
−104.3 and −110.9 ppm, respectively, versus −108.3 ppm for
3·OTf; thus, by this measure, the alkyne substituents only have
a minor impact on P+ electronics, but as we show below, the
same substituents have a dramatic effect on catalytic reactivity.
Figure 2. Synthesis of phosphirenium ions; alkyne (0.2−0.5 mmol),
F
phosphine oxide (0.2−0.5 mmol), Tf2O (0.2−0.5 mmol), NaBAr
a
(0.2−0.5 mmol), CD3CN, 60 °C. Isolated yield presented; spectro-
scopic yield analyzed by inverse-gated 31P{1H} NMR spectroscopy
b
with PPh3 as an internal standard; CD3Cl. ORTEP view of 10·OTf
showing a thermal ellipsoid at 30% probability levels. H atoms were
omitted for clarity.27
because it would allow comprehensive mechanistic inves-
tigations using a range of experimental techniques.
Initial studies were conducted with 10 mol % phosphirenium
species 4·BArF, prepared and activated in situ prior to the
addition of the substrates, benzophenone 11 and dimethyl-
phenyl silane 12, in an equimolar ratio. The reaction was
monitored by 1H NMR spectroscopy and examined after 18 h
of reaction at r.t. (Table 2). Initial results were promising with
44% total conversion of 11 (entry 1) as 28% 13a and 16% 14a.
By increasing the temperature to 80 °C (entry 2), full
conversion of the starting material is observed with 84%
formation of 13a, while 16% 14a persists. Using 10 mol %
isolated 4·BArF, lower conversions (73%) are observed after 18
F
Crystallization is not limited to BAr counterions, and X-ray
single-crystal analysis of 10·OTf confirms the three-membered
structure of this phosphirenium species (Figure 2, bottom),
with similar metrics as observed for 3. Short contacts between
the counterion and the phosphirenium substituents are
observed, although no direct P···O interaction was found,
probably due to the steric effect of the two phenyl rings
compared to, for example, the mixed Ph and Me system
reported by Wild.7
2.2. Hydrosilylation Optimization. With these phos-
phirenium species in hand, we explored the use of
phosphirenium ions as pre-catalysts for the hydrosilylation of
aldehydes and ketones. We opted for this transformation
F
h (entry 3), with 58% 13a. Isolated pre-catalyst 3·BAr shows
higher reactivity, with increased conversion to 14a versus 13a
at 10 mol % loading (entry 5). When phosphirenium triflate
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a
conversion to 13a when using NEt3 and pyridine (entry 9 and
10) with 59 and 52%, respectively, and recovery of the starting
materials with K2CO3 (entry 11) after 18 h at 80 °C. These
results show that a slightly bulkier, aromatic organic base like
2,6-lutidine is most effective.29
Table 2. Optimization Reduction of Carbonyls with Silanes
2.3. Mechanistic Studies. We initiated our mechanistic
investigations by analyzing the catalytic reaction mixture, using
in situ-prepared 3·BArF, by NMR spectroscopy (Scheme 2a).
After 30 min, our standard reaction in CD3CN only shows the
starting material (H2SiMePh 12, −16.9 ppm) and product
(13a, 8.3 ppm) in the 29Si{1H} NMR spectrum. However, a
plethora of species are observed in the 31P{1H} NMR
F
spectrum, with the major species being 3·BAr at −108.5
ppm, along with P2Ph4 at −17.8 ppm, (E)-(1,2-diphenylvinyl)-
diphenylphosphine at 7.1 ppm,9 smaller amounts of (Z)-(1,2-
diphenylvinyl)diphenylphosphine at −7.4 ppm,30 and benz-
hydryl diphenylphosphinate at 36.5 ppm.31 While phosphines
are known to facilitate hydrofunctionalization of carbonyls,32
noteworthy is the lack of catalytic activity observed using 10
mol % HPPh2 or P2Ph4.
b
b
entry
catalyst (mol %)
temp.
13a
14a
F
1
2
3
4·BAr (10)
4·BAr (10)
4·BAr (10)
3·BAr (10)
3·BAr (10)
4·BAr (5) + 2,6-lutidine (5)
3·BAr (5) + 2,6-lutidine (5)
r.t.
28
84
58
21
16
16
15
75
F
80 °C
80 °C
80 °C
80 °C
80 °C
60 °C
60 °C
80 °C
80 °C
80 °C
80 °C
F
b
F
A stoichiometric reaction to form 3·BAr with subsequent
F
4
bc
,
addition of 2,6-lutidine and 12 results in two new species by
29Si{1H} NMR spectroscopy. The major species is identified as
Me2SiHOTf at 23 ppm, formed by the facile cleavage of the
silane phenyl group.33 Analysis of the mixture by 31P{1H}
NMR spectroscopy shows 3·BArF, P2Ph4, and vinyl phosphine
as per the catalytic reaction, along with HPPh2 with its
characteristic peak at −40 ppm (Scheme 2b). Finally, when
performing the same stoichiometric study in the presence of
benzophenone 11 and analyzing the mixture after 10 min by
29Si{1H} NMR spectroscopy, we observe 13a at 8.3 ppm but
no traces of silyl triflate (Me2SiHOTf or Me2PhSiOTf).33 The
latter mixture was also analyzed by 31P{1H} NMR spectros-
copy, which shows the presence of 3·BArF, vinyl phosphine, and
P2Ph4, but no traces of HPPh2 are visible at any point of the
analysis (Scheme 2c).
F
5
5
6
7
8
9
>99
>99
>99
>99
c
F
c
F
d
e
F
9·BAr (5) + 2,6-lutidine (5)
F
10·BAr (5) + 2,6-lutidine (5)
F
3·BAr (10) + NEt3 (10)
3·BAr (10) + pyridine (10)
3·BAr (10) + K2CO3 (10)
59
52
F
10
11
F
a
Reaction condition: (i) activation step R2P(O)H (5−10 mol %),
F
Tf2O (5−10 mol %), NaBAr (5−10 mol %), 1 (5−10 mol %),
CD3CN (0.41 M), 60 °C, and 30 min; (ii) benzophenone 11 (0.25
mmol), dimethylphenyl silane 12 (0.25 mmol), 18 h, and conversion.
b
c
d
e
Isolated pre-catalyst. 5 h. 1 h. >99% after 58 h.
When performing hydrosilylation to form 13a using 10 mol
species are used, such as 3·OTf or 4·OTf, only deoxygenation
occurs with full conversion into 14a; this is due to hydrogen
release from silane reacting with co-catalytic TfOH (formed
from Tf2O).28 Thus, in line with Miura’s recent studies on
phosphirenium-derived heterocycles,9,11 we also note that the
use of a base is crucial to quench TfOH released during pre-
catalyst formation and therefore increase the selectivity toward
13a. See Table S2 in the Supporting Information where
comprehensive benchmarking and optimization were under-
taken demonstrating that not only does TfOH lead exclusively
to the deoxygenation product 14a but also that superior
hydrosilylation catalysis takes place using a phosphirenium pre-
catalyst compared to the equivalent reaction using TfOPPh2. A
total of 5 mol % 2,6-lutidine was added to 5 mol % in situ-
F
% 9·BAr pre-catalyst and 2,6-lutidine, we also observe the
formation of substituted vinyl phosphine [i.e., (E)-(1,2-bis(4-
(trifluoromethyl)phenyl)vinyl)diphenylphosphine] in situ. Hy-
F
drosilylation to form 13a using 10 mol % 10·BAr pre-catalyst
and 2,6-lutidine gives a complex array of peaks in the 31P NMR
spectrum, making identification difficult although likely
including vinyl phosphines.
The use of 20 mol % (E)-(1,2-diphenylvinyl)-
diphenylphosphine and 20 mol % 2,6-lutidine in a catalytic
reaction of 11 and 12 does not give an appreciable quantity of
13a after 18 h at 80 C, indicating that the vinyl phosphine is
not an active catalyst in the hydrosilylation reaction, and this is
further supported by computational studies.34
F
Next, we undertook kinetic studies using NMR spectro-
scopic monitoring. By pre-forming the catalyst in situ, we have
analyzed the order in substrates 11 and 12 by varying the
amount of the starting material using 10 mol % 3·BArF. The
data obtained suggest first-order dependence in 11, while an
inverse order is observed for 1235 (Figure 3). Product
inhibition is not observed, with similar reaction rates obtained
when adding 0.5 equiv of product 13a to the catalytic reaction
of 11 and 12 in a 1:1 mixture (5.32 × 10−4 vs 5.63 × 10−4
mmol/dm3 min). Therefore, the negative order in silane can be
attributed to silane involvement in side-product formation or
catalyst degradation. Comparable data are obtained when using
prepared pre-catalyst 4·BAr prior to addition of substrates 11
and 12, and the selective formation of 13a is observed (5 h, 80
°C, CD3CN, entry 5). Changing to 3·BArF, with 5 mol % 2,6-
lutidine, gives good reactivity at lower temperature (entry 6),
and further improvement is seen with the electron-poor
phosphirenium species 9·BArF, for which reaction with 5 mol %
2,6-lutidine leads to full conversion into 13a at 60 °C after only
F
1 h (entry 7). In contrast, electron-rich 10·BAr reacted at a
slower rate with full conversion to the hydrosilylation product
requiring 58 h at 80 °C (entry 8). Clearly the electronics of the
alkyne used to prepare the phosphirenium pre-catalyst play a
substantial role in catalytic competency. Screening of the base
F
F
with 10 mol % 3·BAr as the catalyst of choice shows lower
isolated pre-catalyst 3·BAr in the absence of 2,6-lutidine,
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Scheme 2. 29Si{1H} and 31P{1H} NMR Spectroscopy Speciation Studies of (a) Standard Catalytic Reaction of 3·BArF, 2,6-
Lutidine, 11, and 12; (b) Stoichiometric Reaction of 3·BArF, 2,6-Lutidine and 12; and (c) Stoichiometric Reaction of 3·BAr
2,6-Lutidine, 11, and 12
F
,
further supporting the assertion that the role of 2,6-lutidine is
(ii) The inverse order in silane indicates that there is a
catalytically inactive pathway dominating at higher silane
loadings.
simply to mop up TfOH released on formation of 3·BArF from
26
F
reaction of Tf2O with alkyne, SPO, and NaBAr
.
It is
important to note that the kinetic data found for this
transformation are in contrast with Piers’ hydrosilylation-type
activation via Lewis acids [e.g., B(C6F5)3],36 where the increase
in ketone concentration slows down the reaction. Indeed,
interrogating the 11B and 19F NMR spectra from catalytic
reactions also shows that only borate is present in solution; no
borane signals are observed, and so we feel that borane-
mediated hydrosilylation is unlikely in this case.
(iii) Phosphirenium ions with different electronic substitu-
ents have very different capabilities in hydrosilylation (9·
F
F
BAr > 3·BAr ≫ 10·BArF), but these changes in
electronics only have a modest impact on the
phosphorus center (e.g., compare 3·OTf, 9·OTf, and
10·OTf 31P NMR chemical shift). We therefore believe
that the role of the phosphirenium pre-catalyst is not
merely to act as a simple Lewis acid.37
At this stage, we have three distinct pieces of experimental
data that necessitate further investigation:
With these experimental results in hand, we undertook a
computational study of the reactions of the phosphirenium
cation, 3+ (denoted as I in the computational study) with
acetone and Me2PhSiH. Initial studies considered the direct
nucleophilic attack of either the ketone or silane at the
phosphirenium ring;29 however, the characterization of these
processes indicated that a ring-opening step occurs first to
form an ethenylium intermediate II at +22.0 kcal/mol. From
(i) Observation of vinyl phosphine both during catalysis
and in stoichiometric studies indicates that phosphire-
nium ring opening could be involved, either as a catalyst
deactivation process or on-cycle ring-opening/ring-
closure sequence;
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principle, IVH,E could re-enter catalysis via hydride transfer to
the silylether cation, generating the hydrosilylation product
and intermediate II. However, this process has a computed
barrier of 48.3 kcal/mol, and so the formation of vinyl
phosphine represents an unproductive off-cycle process. The
second competing process (Figure 4d) involves the reaction of
VP with alkyne via TS(V−II)P at +3.3 kcal/mol with the
release of the hydrosilylation product and formation of II. II
can then rapidly undergo ring closure to reform I.
The significance of these two competing processes is
captured in the free energy differences (ΔΔG) between the
rate-limiting transition states. For vinyl phosphine formation,
these are TS(II−III)H and TS(II−III)P, where the latter is
more stable and ΔΔG1 = +2.1 kcal/mol. The productive
onward reaction to form IIIP should therefore dominate;
however, the small energy difference suggests that vinyl
phosphine formation may be significant at higher silane
concentrations, and this is consistent with the experimental
identification of an inverse first-order reaction with respect to
silane. For the competing reactions of VP with either the
ketone or the alkyne, the key transition states are TS(III−IV)P
and TS(V−II)P, respectively. In this case, TS(V−II)P is only
marginally disfavored with ΔΔG2 = +0.4 kcal/mol, and this
small energy difference suggests reaction with alkyne and
therefore the reformation of I will be competitive. However,
unlike the vinyl phosphine IVE, I remains a competent catalyst
precursor, albeit one that reacts with a relatively high overall
barrier of 28.6 kcal/mol. There is therefore an ensemble of
processes at play, in which efficient catalysis via IIIP is balanced
by catalyst loss through vinyl phosphine formation and a less
efficient cycle based on phosphirenium precursor I. The net
effect of these various processes is qualitatively consistent with
a reaction that requires several hours at 60 °C to reach full
conversion.
Figure 3. a) Graphical analysis showing inverse order relationship for
the formation of 13a at different phenyldimethylsilane 12 loadings
◆
◆
■
1.25, and green 1.5 equiv); (b)
●
(yellow
0.75, blue 1, red
plot showing first-order dependence for the formation of 13a at
different benzophenone 11 loadings (0.75, 1, 1.25, and 1.5 equiv);
and reaction conditions: (i) activation step Ph2P(O)H 2 (10 mol %),
F
Tf2O (10 mol %), NaBAr (10 mol %), diphenylacetylene 1 (10 mol
%), CD3CN (0.4 M), 60 °C, and 30 min and (ii) 2,6-lutidine (10 mol
%), ketone or aldehyde (0.20 mmol), silane (0.20 mmol), 80 °C, and
PhMe used as an internal standard.
2.3.1. Substituent Effects. Reaction profiles computed for
cations 9+ and 10+ were similar to those for the parent species
3+ (see the Supporting Information). However, subtle changes
in the relative energies of the key competing transition states
are computed that have a significant impact on the behavior of
these systems (see Figure 5). For 9+, the electron-withdrawing
p-CF3 substituents cause an increase in both ΔΔG1 and ΔΔG2.
As a result, catalysis becomes more efficient both in terms of a
reduced likelihood of vinyl phosphine formation and a lower
propensity to reform the phosphirenium cation. This is
reflected in the short (ca. 1 h) times to full conversion seen
experimentally with 9+. In contrast, the electron-donating p-
OMe substituents switch the preference toward vinyl
phosphine formation from II (ΔΔG1 = −1.8 kcal/mol) and
favoring the reaction of VP with alkyne that leads to the
formation of I over the more direct catalytic reaction with
ketone. Both factors should render catalysis less efficient,
consistent with the very sluggish catalytic reaction that is
observed with 10+ (58 h to full conversion). The calculations
indicate that while the p-OMe substituents in 10+ promote ring
opening [ΔGI→II = +17.6 kcal/mol (p-OMe), +22.0 kcal/mol
(p-H), and +24.1 kcal/mol (p-CF3)], they also disfavor the
subsequent {Ph2P+} transfer via TS(II−III)P, and this results
in hydride transfer via TS(II−III)H dominating.
II, catalysis is initiated via the nucleophilic attack of the ketone
at the P center (see Figure 4a). This proceeds through TS(II−
III)P at +28.6 kcal/mol and results in both P−O bond
formation and displacement of the alkyne to form a
phosphenium-ketone adduct IIIP. Catalysis now proceeds
(Figure 4b) via Si−H addition across the CO moiety
[TS(III−IV)P, +27.9 kcal/mol] to form IVP, in which partial
hydride transfer has occurred (C···H = 1.29 Å; H···Si = 1.71
Å). Silyl group migration then readily occurs to form VP at
−7.8 kcal/mol, featuring a silylalkyloxonium adduct of the
{Ph2P} moiety. Ketone attack at VP then displaces the
hydrosilylation product and generates IIIP that can then
enter a second cycle with further reaction with silane.38
Overall, the calculations imply a relatively slow initiation
process via TS(II−III)P at 28.6 kcal/mol to form IIIP, from
which catalysis can then proceed with an energy span of only
10.8 kcal/mol. Catalysis from IIIP should therefore be very
efficient; however, the situation is complicated by at least two
possible competing reactions. The first of these is vinyl
phosphine formation (Figure 4c). This involves silane attack at
the C1 position of intermediate II and proceeds via TS(II−
III)H at +30.7 kcal/mol with net insertion into the Si−H bond
to give silylalkylphosphonium ion IIIH,Z at −9.0 kcal/mol,
featuring a short P−C bond (1.76 Å). Assuming facile rotation
about the C−C bond leads to IIIH,E (−7.3 kcal/mol), which
can react with ketone with displacement of a silylether cation
to form E-vinyl phosphine IVH,E (−33.3 kcal/mol).39 In
These combined experimental and computational studies
allow us to propose a reaction mechanism for the hydro-
silylation of carbonyls catalyzed by phosphirenium cations.
These species act as a masked phosphenium source that is
accessed via ring opening. Productive turnover then involves
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Figure 4. Free energy reaction profiles (kcal/mol) for the reactions of Me2PhSiH with acetone mediated by the phosphirenium cation 3+ computed
at the M052X-D3(CH3CN)/def2-TZVP//TPSS/def2-SVP level of theory. (a) Initiation via ketone attack at P in the ring-opened ethenylium
cation, II; (b) catalytic cycle based on intermediate IIIP; (c) competing vinyl phosphine formation; and (d) competing phosphirenium reformation.
opened form in the catalytic reaction. Given the capricious
nature of unstabilized phosphenium ions, we do not observe
free {Ph2P+} in the catalytic reaction. Calculations indicate that
loss of {Ph2P+} from the parent phosphirenium is uphill by 34
kcal/mol; thus, observation of the free species is unlikely,
lending further support to it being transferred directly between
nucleophiles during the cycle. Catalytic reactions do show the
presence of a phosphinate peak in the 31P NMR spectrum
(−36.5 ppm, corresponding to benzhydryl diphenyl phosphi-
nate31), which gives tentative support for the presence of
Figure 5. Comparison of key transition states (free energies, kcal/
mol) for the competing processes outlined in Figure 4. ΔΔG1
=
phosphenium adducts.
TS(II−III)H − TS(II−III)P and ΔΔG2 = TS(V−II)P − TS(III−
2.4. Hydrosilylation Substrate Scope. With the optimal
conditions for catalytic hydrosilylation and choosing to use
commercially available diphenyl acetylene for in situ pre-
catalyst formation (Table 2, entry 6), we explored the
generality of the reaction. Compound 13a can be isolated in
73% yield, after 5 h at 60 °C using 2.5 mol % 3·BArF (Figure 6).
Aryl-substituted ketones react well under the reaction
conditions, giving 13b-13d in excellent 83−97% isolated
yields. Electron-withdrawing and electron-donating groups in
the para-position are well tolerated. These results are
interesting because diaryl ketones have been reported to
typically react to yield fully deoxygenated products when
subjected to higher temperatures or longer reaction times.40
In this procedure, the higher temperature does not affect the
reaction outcome with hydrosilylation of diaryl ketones
IV)P.
associative phosphenium transfer to the ketone, Si−H addition
over the CO bond, and associative displacement of the
hydrosilylation product with further ketone. Spectroscopic and
computational insights show that the formation of vinyl
phosphine is an irreversible off-cycle process that will sequester
the {Ph2P+} moiety through reaction with silane, in accordance
with inverse order kinetics. Para-substituents on the
phosphirenium aryl groups can significantly impact the
behavior; electron-donating p-OMe substituents undermine
catalysis by favoring the formation of side products.
Experimentally, the observation of HPPh2 and P2Ph4 by in
situ reaction monitoring also supports the presence of the ring-
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F
Figure 6. Reaction conditions: (i) activation step Ph2P(O)H 2 (5 mol %), Tf2O (5 mol %), NaBAr (5 mol %), diphenylacetylene 1 (5 mol %),
CD3CN (0.41 M), 60 °C, and 30 min and (ii) 2,6-lutidine (5 mol %), ketone or aldehyde (0.25 mmol), silane (0.25 mmol), 80 °C, and 5 h,
a
b
c
reported as an average of two runs; conversion; 10 mol %, 18 h; spectroscopic yield.
occurring selectively in 5 h. Unsymmetrical ketones with an
acetone react well using 10 mol % 3·BArF, giving 13g and 13h
3
2
aryl substituent and Csp and Csp substituents yield 13e and
13f in full conversion (77 and 80% isolated yields,
respectively). Alkyne-containing carbonyl substrates are not
suitable for this transformation, possibly due to alkyne
scrambling of the propargyl group.41 Cyclohexanone and
in 87 and 62% yields, respectively, while acetophenone reacts
sluggishly, and even with 10 mol % catalyst loading, only 23%
of 13i is obtained. The use of other tertiary silanes was found
to be suitable, and 13j is obtained in full conversion and high
yield (85%). However, triethoxy silane and triphenyl silane do
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not convert, but the phosphirenium pre-catalyst is unaffected
and is still observed in the reaction mixture. Secondary or
primary silanes do not react with benzophenone 11 under the
reaction conditions, and decomposition of the phosphirenium
is observed by NMR spectroscopy.
and NaBArF (1 equiv.) were stirred in CD3CN or CDCl3 at 60
°C for 30 min. The reaction was monitored by 31P{1H} NMR
spectroscopy until complete disappearance of the phosphine
oxide peak. After the reaction was completed, the mixture was
concentrated under vacuum. The product was washed with
pentane and recrystallized at −20 °C.
4.2. General Procedure for the Hydrosilylation of
Ketones and Aldehydes. To a J-Young NMR tube,
dicyclohexyl phosphine oxide (5 mol %, 12.5 × 10−3 mmol,
2.7 mg) or diphenyl phosphine oxide (5 mol %, 12.5 × 10−3 or
25 × 10−3 mmol, 2.5 mg), diphenylacetylene (5 mol %, 12.5 ×
10−3 mmol, 2.3 mg), triflic anhydride (5 mol %, 12.5 × 10−3
mmol, 2.1 μL), and NaBArF (5 mol %, 12.5 × 10−3 mmol, 11.1
mg) were stirred in CH3CN (3 mL) for 30 min at 60 °C. After
formation of pre-catalyst 3·BArF, the reaction mixture was
cooled down, and 2,6-lutidine (5 mol %, 12.5 × 10−3 mmol,
1.5 μL), benzophenone (0.25 mmol, 45.5 mg), and
dimethylphenylsilane (0.25 mmol, 38.3 μL) were added, and
the reaction was heated at 80 or 60 °C for 18 and 5 h,
respectively. After drying the volatiles, the pure product was
obtained by purification by flash chromatography (9:1
petroleum ether/EtOAc) or vacuum distillation.
4.3. DFT Calculations. Gaussian 09 (Revision D.01) was
used with geometry optimizations carried out at the TPSS/
def2-SVP level and all stationary points characterized with
analytical frequency calculations. Electronic energies were
recomputed at the M052X-D3(CH3CN)/def2-TZVP level,
which has been shown to be a reliable approach for main group
kinetics, thermodynamics, and non-covalent interactions.42
These corrected energies were then combined with the
thermodynamic corrections from the gas-phase calculations
to give the free energies quoted in the text. Full details and
references are supplied in the Supporting Information.
Several aldehydes were subjected to the optimized reaction
conditions, and the results are good to excellent when using
electron-poor aryl aldehydes, with 60−98% isolated yields for
13k−13p. Cinnamaldehyde reacts smoothly to yield 13q in
75% yield. The reaction also works well with 4-methylbenzal-
dehyde, furnishing 13r in 89% isolated yield, but is quite
limited with other electron-rich aldehydes. 4-Dimethylamino
benzaldehyde shows low conversion into 13s (22%) after 18h
with 10 mol % 3·BArF; 4-methoxybenzaldehyde does not show
any conversion after 24 h, which suggests a potential
deactivation of the carbonyl or catalyst poisoning. Electron-
withdrawing groups in the ortho- and meta-position are
detrimental, giving low conversion and yield of 13t, 13u, and
13v. Tiglic aldehyde reacts using 10 mol % catalyst at 80 °C
with full conversion of silane to 13w in only 40% spectroscopic
yield, with traces of the product derived from the reduction of
the double bond observed in situ. Alkyl aldehydes such as
pentanal, heptanal, and dodecanal give 13x, 13y, and 13z in
72−88% isolated yields, respectively, and cyclopropane
aldehyde reacts to give 13aa in 77% isolated yield with no
ring opening observed, highlighting that radicals are not
involved in the reaction.
3. CONCLUSIONS
The ease of synthesis and the newly discovered reactivity of
phosphirenium ions allow us to conclude that they are effective
organocatalysts for the hydrosilylation of ketones and
aldehydes; to the best of our knowledge, this is the first report
of a phosphirenium ion being used as a pre-catalyst. Full
characterization of the phosphirenium ions bearing triflate or
borate counterions is reported, and further details on their
synthesis were explored. The practical pre-catalyst synthesis
from typically cheap and readily available starting materials
such as phosphine oxides and alkynes allows a straightforward
catalytic procedure that is easily scalable. Our mechanistic
investigations give insights into the principles governing their
reactivity; this is underpinned by DFT studies and
experimentally determined kinetics and stoichiometric studies.
The ability of these phosphirenium ions to undergo ring
opening paves the way for several reaction pathways, with the
dominant and most favorable pathway being the release of the
active catalyst, which is a phosphenium ion adduct. Increasing
the electron-donating ability of the alkyne used to prepare the
pre-catalyst slows catalysis, allowing an off-cycle deactivation
process to dominate. These elusive phosphenium species are
highly reactive and challenging to isolate and characterize;
therefore, by disguising a phosphenium catalyst as a more
stable and manageable phosphirenium adduct, we open up a
multitude of opportunities in organic synthesis. Work is still
ongoing in our laboratory to shed light on diverse reactivity
and the activation modes of phosphirenium rings and the
broader applicability of masked phosphenium catalysts.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c01133.
■
sı
Analysis data and NMR spectra for all products and
computational supporting information which contains all
data relevant to the computational studies reported here,
including methodology, references, benchmarking stud-
ies, energetics, and coordinates of characterized sta-
tionary points (CIF)
General considerations, synthesis and characterization of
the starting material, synthesis of phosphirenium ions,
substrate scope hydrosilylation, and optimization of the
catalytic reaction (PDF)
Computed geometries for all stationary points (XYZ)
AUTHOR INFORMATION
Corresponding Authors
■
Stuart A. Macgregor − Institute of Chemical Sciences, Heriot-
Watt University, Edinburgh EH14 4AS, U.K.; orcid.org/
0000-0003-3454-6776; Email: S.A.Macgregor@hw.ac.uk
Ruth L. Webster − Department of Chemistry, University of
Bath, Bath BA2 7AY, U.K.; orcid.org/0000-0001-9199-
7579; Email: R.L.Webster@bath.ac.uk
4. EXPERIMENTAL SECTION
Authors
4.1. General Procedure for Synthesis of Phosphire-
nium Ions. In a J-Young NMR tube or a Schlenk tube, alkyne
(0.25 mmol), SPO (0.25 mmol), triflic anhydride (1 equiv.),
Danila Gasperini − Department of Chemistry, University of
Bath, Bath BA2 7AY, U.K.
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(b) Nishimura, K.; Hirano, K.; Miura, M. Direct Synthesis of
Dibenzophospholes from Biaryls by Double C−P Bond Formation
Via Phosphenium Dication Equivalents. Org. Lett. 2020, 22, 3185−
3189.
Samuel E. Neale − Institute of Chemical Sciences, Heriot-Watt
University, Edinburgh EH14 4AS, U.K.
Mary F. Mahon − Department of Chemistry, University of
Bath, Bath BA2 7AY, U.K.
(11) Nishimura, K.; Unoh, Y.; Hirano, K.; Miura, M. Phosphenium-
Cation-Mediated Formal Cycloaddition Approach to Benzophosp-
holes. Chem.Eur. J. 2018, 24, 13089−13092.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c01133
(12) (a) Huang, H.; Ash, J.; Kang, J. Y. Tf2O-Promoted Activating
Strategy of Phosphate Analogues: Synthesis of Mixed Phosphates and
Phosphinate. Org. Lett. 2018, 20, 4938−4941. (b) Yuan, T.; Huang,
S.; Cai, C.; Lu, G.-p. Metal-Free Electrophilic Phosphination of
Electron-Rich Arenes, Arenols and Aromatic Thiols with Diary-
lphosphine Oxides. Org. Biomol. Chem. 2018, 16, 30−33.
(13) (a) Cowley, A. H.; Kemp, R. A. Synthesis and Reaction
Chemistry of Stable Two-Coordinate Phosphorus Cations (Phosphe-
nium Ions). Chem. Rev. 1985, 85, 367−382. (b) Rosenberg, L. Metal
Complexes of Planar PR2 Ligands: Examining the Carbene Analogy.
Coord. Chem. Rev. 2012, 256, 606−626.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The EPSRC is thanked for funding. S.E.N. thanks Heriot Watt
University for the award of a James Watt scholarship.
(14) (a) Nesterov, V.; Reiter, D.; Bag, P.; Frisch, P.; Holzner, R.;
Porzelt, A.; Inoue, S. NHCs in Main Group Chemistry. Chem. Rev.
2018, 118, 9678−9842. (b) Speed, A. W. H. Applications of
Diazaphospholene Hydrides in Chemical Catalysis. Chem. Soc. Rev.
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Oxides Via a Metal Free Coupling of Propargylic Substrates with
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Deng, Y.-J.; Yan, X.-X.; Liu, Y.-J.; Ge, C.-P.; Yan, Y.; Chao, S.; Zhou,
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10+ this alternative pathway becomes more competitive (ΔG‡ = 28.6
kcal/mol cf. +28.3 kcal/mol) and becomes the favored catalytic
process due to a destabilization of TS(III−IV)P in that case. Hydride
transfer from silane to phosphorus in II was also characterised and
involves a transition state at +31.1 kcal/mol. This may account for the
observation of HPPh2 experimentally. See Supporting Information.
(39) Direct reaction of IIIH,Z with ketone via TS(III−IV)H,Z at −2.8
kcal/mol results in the formation of IVH,Z at −28.7 kcal/mol and this
accounts for the formation of IVH,Z seen experimentally. Hydride
transfer from IVH,Z to the silylether cation has a barrier of 40.6 kcal/
mol.
(40) (a) Hatakeyama, S.; Mori, H.; Kitano, K.; Yamada, H.;
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ACS Catal. 2021, 11, 5452−5462