Axially Chiral, Electrophilic Fluorophosphonium Cations: Synthesis, Lewis Acidity, and Reactivity in the Hydrosilylation of Ketones

Article abstract of DOI:10.1021/acs.organomet.8b00912

Axially chiral [(C₆F₅)₃PF][B(C₆F₅)₄] analogues based on dihydrophosphepines with a binaphthyl backbone were prepared and structurally characterized by X-ray diffraction analysis. Computational calculations of FIA and GEI values attest that these new fluorophosphonium cations have a higher Lewis acidity compared to the ubiquitous B(C₆F₅)₃. Furthermore, application of these highly electrophilic compounds in the catalytic hydrosilylation of ketones and an investigation of the mechanism lead to a refined picture of the role of highly electrophilic fluorophosphonium cations.

Full text of DOI:10.1021/acs.organomet.8b00912

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Axially Chiral, Electrophilic Fluorophosphonium Cations: Synthesis,
Lewis Acidity, and Reactivity in the Hydrosilylation of Ketones
Lars Susse, James H. W. LaFortune,‡ Douglas W. Stephan,*,†,‡ and Martin Oestreich*,†
†
̈
†Institut fu
̈
r Chemie, Technische Universitat Berlin, Strasse des 17. Juni 115, 10623 Berlin, Germany
̈
‡Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
S
* Supporting Information
ABSTRACT: Axially chiral [(C6F5)3PF][B(C6F5)4] analogues
based on dihydrophosphepines with a binaphthyl backbone were
prepared and structurally characterized by X-ray diffraction analysis.
Computational calculations of FIA and GEI values attest that these
new fluorophosphonium cations have a higher Lewis acidity
compared to the ubiquitous B(C6F5)3. Furthermore, application of
these highly electrophilic compounds in the catalytic hydrosilylation
of ketones and an investigation of the mechanism lead to a refined
picture of the role of highly electrophilic fluorophosphonium
cations.
their efficacy in enantioselective hydrosilylation of ketones.
INTRODUCTION
■
While silyl ethers are obtained in high yields, no
enantioselectivity is seen. This prompted a careful reinvesti-
gation of the mechanism. NMR spectroscopic and stoichio-
metric experiments reveal that EPCs do not serve as actual
catalysts but rather initiate the formation of achiral electro-
philic species that mediate the hydrosilylation.
A few years ago, the group of Stephan reported the synthesis of
the highly electrophilic, C6F5-substituted fluorophosphonium
salt [(C6F5)3PF][B(C6F5)4] (1, Scheme 1)1 and its application
in various Lewis acid-catalyzed reactions.2 In the presence of a
hydrosilane, transformations such as hydrodefluorination of
fluoroalkanes,1 dehydrocoupling reactions,3 hydrodeoxygena-
tion of ketones4 and phosphine oxides,5 as well as hydro-
silylation reactions of unsaturated hydrocarbons6 and ketones7
were accomplished. The mechanistic details for the latter
reduction reactions are of particular interest. For the B(C6F5)3-
catalyzed process developed by Piers,8a,b a counterintuitive
reaction pathway was proposed and has been supported
experimentally as well as computationally by Oestreich,8c
Sakata and Fujimoto,8d as well as Piers and Tuononen.8e The
proposed mechanism for the process catalyzed by electrophilic
phosphonium cations (EPCs) was thought to parallel the Piers
mechanism.7,9 Thus, activation of hydrosilane I by 1 via a η1
coordination of the hydridic Si−H bond prompts silyl transfer
to the carbonyl oxygen in III and hydride transfer to the
phosphorus atom, affording silylcarboxonium ion IV and
transient (C6F5)3P(F)H (2, Scheme 1). Subsequently, hydride
transfer affords silyl ether V and regenerates catalyst 1.
RESULTS AND DISCUSSION
■
Synthesis of Chiral EPCs with a Binaphthyl Backbone.
The phosphepines (S)-712a and (S)-812b (Scheme 2) were
prepared according to the method reported by Beller and co-
workers. Subsequently, using a slight modification of the
established protocol,1 the phosphines (S)-7 and (S)-8 were
oxidized with XeF2 in CH2Cl2 at −78 °C. The corresponding
difluoro-substituted phosphanes (S)-9 and (S)-10 were
obtained in quantitative yields. Subsequent fluoride abstraction
with freshly prepared silylium ion [Et3Si(C6D6)][B(C6F5)4]13
afforded (S)-5 and (S)-6 in 66% and 49% yield, respectively.
Both fluorophosphonium ions were characterized by multi-
nuclear NMR spectroscopy and showed typical P−F
resonances in the 31P{1H} NMR spectra as doublets with
1JP,F = 1058 Hz at 115.9 ppm for (S)-5 and 1JP,F = 1060 Hz at
123.2 ppm for (S)-6.
An asymmetric variant of the above EPC-catalyzed hydro-
silylation requires a chiral analogue of 1 that is sufficiently
electron deficient to promote the Si−H bond activation and
also provides a chiral environment that induces enantiose-
lectivity in the hydride transfer step.10 Inspired by the
It was further possible to secure the molecular structures of
both compounds by X-ray diffraction analysis (Figure 1),
confirming that the stereochemical integrity was retained. Both
cations showed typical P−F bond lengths for fluorophospho-
nium ions of 1.5430(19) Å for (S)-5 and 1.540(3) Å for (S)-
6.1 For (S)-5, the P−C(sp2) bond length is 1.770(4) Å, and
longer distances were obtained for the P−C(sp3) bonds with
successful application of the dihydroborepines (S)-3·
THF11a,b and (S)-4·SMe2
with binaphthyl backbones in
11c,d
the Piers hydrosilylation, we anticipated that a similar scaffold
in a fluorophosphonium cation could be promising (Scheme
1). Herein, we report the synthesis of axially chiral
fluorophosphonium ions (S)-5 as well as (S)-6 and assess
Received: December 17, 2018
© XXXX American Chemical Society
A
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Scheme 1. Proposed Mechanism for the EPC-Catalyzed
Hydrosilylation of Ketones and Chiral Congeners of 1
a
Inspired by the Corresponding Boranes
Figure 1. Molecular structure of (S)-5 (left) and (S)-6 (right).
Thermal ellipsoids represent 50% probability level. Hydrogen atoms
and the counteranion [B(C6F5)4]− are omitted for clarity. C: gray; P:
orange; F: green.
of (S)-5 in CD2Cl2 was treated with Et3PO (1.0 equiv), and
the resulting mixture was analyzed by means of 31P{1H} NMR
spectroscopy. Coordination of Et3PO to the phosphonium ion
was not evident and instead (S)-5 was transformed into the
corresponding phosphine oxide with formation of [Et3PF][B-
(C6F5)4] (see the Supporting Information). Similar fluoride-
oxide exchange reactions have been observed for other
fluorophosphonium ions,17 and this affirms that the
Gutmann−Beckett method is inappropriate in this case.
The FIA and GEI values for (S)-5 and (S)-6 (Table 1) were
computed at the MP2/def2-TZVPP//BP86/def2-TZVP level
Table 1. Computed FIA and GEI Values of (S)-5, (S)-6,
a
Achiral Fluorophosphonium Salts, and B(C6F5)3
entry
Lewis acid
FIA (kJ mol−1
)
GEI (eV)
1
2
(S)-5
(S)-6
688.6
649.6
779.3
742.0
700.1
656.3
452.6
2.555
2.080
3.622
3.187
2.820
2.455
1.408
b
3
[(C6F5)3PF][B(C6F5)4]
[Ph(C6F5)2PF][B(C6F5)4]
[Ph2(C6F5)PF][B(C6F5)4]
[Ph3PF][B(C6F5)4]
B(C6F5)3
b
4
a
Counteranion [B(C6F5)4]− is omitted. LB = Lewis base.
b
5
b
6
b
Scheme 2. Synthesis of Axially Chiral Fluorophosphonium
Ions
7
a
Computed with MP2/def2-TZVPP//BP86/def2-TZVP (see the
b
Supporting Information for details). FIA and GEI values from ref
16c.
of theory, using the established protocols.16c,17c,18 The FIA and
GEI values of (S)-5 are 688.6 kJ mol−1 and 2.555 eV, which are
slightly lower than those computed for [Ph2(C6F5)PF][B-
(C6F5)4] (entries 1 and 5). As expected, (S)-6 showed lower
FIA and GEI values of 649.6 kJ mol−1 and 2.080 eV,
respectively. These values are comparable with those reported
for [Ph3PF][B(C6F5)4] (entries 2 and 6). Nevertheless, both
new chiral phosphonium salts (S)-5 and (S)-6 showed higher
values than those obtained for B(C6F5)3 (entry 7). From this
point of view, both chiral EPCs seemed promising candidates
for the activation of hydrosilanes.
Hydrosilylation of Ketones. With the chiral phospho-
nium ions in hand, these species were tested for their ability to
mediate the hydrosilylation of ketones (Table 2). Using 1.0
mol % of (S)-5, ketone 11, and 1.1 equiv of Et3SiH in various
solvents (1,2-F2C6H4, C6H6, and CH2Cl2), moderate con-
version to the silyl ether 12 was achieved (entries 1−3). The
reaction required 70 h in CH2Cl2 for 91% conversion (entry
3). No enantioselectivity was observed. Using 1.2 equiv of the
hydrosilane and 1 mol % of (S)-5 or (S)-6, the hydrosilylations
were accelerated to completion in 6 h in CD2Cl2 (entries 4 and
5). Again, only racemic products were obtained. A higher
catalyst loading as well as a further increase of the amount of
1.782(3) Å and 1.791(3) Å as expected. A slightly longer bond
length of 1.759(5) Å for the P−C(sp2) of (S)-6 was observed,
whereas the P−C(sp3) bonds were similar to (S)-5 with
1.787(5) Å and 1.796(5) Å.
Determination of the Lewis Acidity. We next targeted
an assessment of the Lewis acidity of these new phosphonium
ions using the Gutmann−Beckett method14 and computation
of the fluoride ion affinity (FIA)15 as well as the global
electrophilicity index (GEI).16 For the first method, a solution
B
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a
Table 2. Screening of the EPC-Catalyzed Hydrosilylation
b
c
entry
Si−H (equiv)
EPC (mol %)
solvent
time (h)
silyl ether
conv. (%)
ee (%)
d
1
2
3
4
5
6
7
8
9
Et3SiH (1.1)
Et3SiH (1.1)
Et3SiH (1.1)
Et3SiH (1.2)
Et3SiH (1.2)
Et3SiH (7.0)
Me2PhSiH (1.2)
MePh2SiH (1.2)
Ph2SiH2 (1.2)
PhSiH3 (1.2)
(S)-5 (1.0)
(S)-5 (1.0)
(S)-5 (1.0)
(S)-5 (1.0)
(S)-6 (1.0)
(S)-5 (5.0)
(S)-5 (1.0)
(S)-5 (1.0)
(S)-5 (1.0)
(S)-5 (1.0)
1,2-F2C6H4
C6H6
70
70
70
6
6
5
4
2
2
24
12
12
12
12
12
12
13
14
15
70
85
91
0
0
0
0
0
0
0
0
0
0
d
d
CH2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
e
>95 (60)
e
>95 (73)
e
>95
e
>95 (92)
e
>95 (51)
e
>95 (12)
f
e
10
16
>95 (57)
a
b
All reactions were performed according to General Procedure GP1. Isolated yield after flash column chromatography in parentheses.
c
d
Determined by HPLC analysis using a chiral stationary phase. Conversion was determined by GLC analysis using mesitylene as internal standard.
e
f
Conversion was determined by NMR analysis. 16 was isolated as the free alcohol after hydrolysis of the silyl ether during flash column
chromatography.
hydrosilane had no effect on the outcome of the reaction.
Interestingly, deoxygenation and hydrodefluorination of 11
previously described with [(C6F5)3PF][B(C6F5)4] (1) under
the same setup were not observed (entry 6).4 The substitution
pattern at the silicon atom had no impact on the outcome of
the reaction (entries 7−10). The prochiral ketones 17−21
were also tested under the aforementioned conditions using
(S)-5 as catalyst. All the silyl ethers 22−26 were obtained as
racemic mixtures (Scheme 3). As previously reported for 1,
hydrosilylation was limited to alkyl-substituted ketones or
ketones with electron withdrawing groups as otherwise
deoxygenation of the ketone was observed.
Mechanistic Considerations. The observation of no
enantioinduction with either (S)-5 or (S)-6 prompted us to
further study the proposed reaction mechanism for the racemic
transformation using [(C6F5)3PF][B(C6F5)4] (1)4,6,7 as a
catalyst. 1 was found to be stable in the presence of the
ketone 11 as verified by 1H and heteronuclear NMR
spectroscopy.19 A 1:1 mixture of 1 and Et3SiH showed traces
of a decomposition product after 5 min (Scheme 4B), while
Scheme 4. Decomposition of 1 in the Presence of
a
Hydrosilane
Scheme 3. Representative Examples for the Hydrosilylation
a b c
, ,
of Ketones with (S)-5 as the Catalyst
a
Experiment was performed in a J. Young NMR tube. 31P{1H} NMR
spectra (283 MHz, CD2Cl2) of the reaction mixture over time: (A)
only 1; (B) 1 and Et3SiH (1:1 equiv) after 5 min; (C) 1 and Et3SiH
(1:10 equiv) after 1 h.
addition of excess hydrosilane (10 equiv) caused complete
decomposition of 1 after 1 h (Scheme 4C). NMR data revealed
that the resulting mixture consists of siloxyphosphonium ion
27, (C6F5)3P (28), Et3SiF (29), and traces of other unknown
fluorine-containing compounds. In a previous report, a
a
All reactions were performed according to General Procedure GP1.
Conversion was determined by NMR analysis. Enantiomeric
b
c
excesses were determined by HPLC analysis using a chiral stationary
phase or by optical rotation and all were found to be 0%. With 3.0
1
d
broadened H NMR resonance of the silicon-bound proton
mol % of (S)-5.
was attributed to coordination of the hydrosilane to the
C
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[(C6F5)3PF]+ cation.1,9 However, the 31P and 19F NMR
spectra presented herein indicate the reaction of 1 to the
mixture of products described above. Based on computations
by Kostenko and Dobrovetsky, the oxygen atom in 27 was
proposed to emerge from the degradation of (C6F5)3P(F)H
(2) through loss of HF along with (C6F5)3P (28).9 Reaction of
HF with the glass surface is thought to release H2O which then
serves as an oxidant for 1.
The species 27 was independently synthesized following a
reported procedure.20 Using 2 mol % of 27, the catalytic
hydrosilylation of 11 yielded the silyl ether 12 quantitatively
after 6 h (Scheme 5, top). It is proposed that 27 acts as a
Table 3. Deoxygenation of Acetophenone
entry
catalyst (mol %)
1 (1.5)
solvent
30:(31 + 32):33
a
1
CD2Cl2
CD2Cl2
toluene
toluene
toluene
1:1:0
1:1:0
1:1:0
1:1:0
0:0:1
a
2
(S)-6 (1.5)
b
3
[Ph3C][B(C6F5)4] (2.0)
[Et3Si][B(C6F5)4] (2.0)
B(C6F5)3 (2.0)
b
4
b
5
a
a
Scheme 5. Siloxyphosphonium-Catalyzed Hydrosilylation
Experiments were performed according to General Procedure GP1.
b
1
Ratio was determined by H NMR spectroscopy. Reference 8b.
The initially proposed mechanism suggested the neutral
(C6F5)3P(F)H (2) as the transient hydride source. Efforts to
independently prepare this species by the reaction of 1 with
traditional hydride sources were futile. Similar attempts to
prepare the phosphorane 2 by hydrosilane activation with 1 in
the presence of a Lewis base such as phosphines or pyridines
(targeting capture of the silyl group) were also unsuccessful.
Also, 2 could not be observed by NMR spectroscopy during
monitoring of the hydrosilylation catalysis at −60 °C. The
reported calculations indicate that 2 can decompose to 28 and
HF.9 A HF-mediated hydrosilylation, on the other hand, can
be excluded, because the corresponding chloro- and phenoxy-
substituted phosphonium ions ([(C6F5)3PCl][B(C6F5)4] and
[(C6F5)3POPh][B(C6F5)4]) afforded the silyl ether 12 under
the standard reaction conditions. Attempts to absorb HF with
added Lewis bases such as phosphines and pyridines were
unsuccessful as the EPCs were decomposed by these σ-donors;
the corresponding phosphines were seen as part of a complex
mixture.
a
b
Experiments were performed in a J. Young NMR tube. Ratios
determined by 19F{1H} NMR spectroscopy.
stabilized silylium ion. This view was corroborated in the
hydrosilylation catalysis of 11 using 20 mol % of 27 and excess
Me2PhSiH as the hydrosilane (Scheme 5, bottom). This
afforded the silyl ethers 12 and 13 in a 19:81 ratio, consistent
with the participation of 27 in transfer of the silyl group.
The aforementioned results summarize to (1) similar
substrate scope compared to the silylium ion-catalyzed
carbonyl reduction,25(2) decomposition of 1 in the presence
of hydrosilane to a catalytically active siloxyphosphonium ion,
(3) discrepancies in the behavior to the borane-catalyzed
hydrosilylation, and (4) absence of enantioinduction with
chiral EPCs.26 These observations call for two alternative
pathways in the catalytic cycle (Scheme 6). In the first case, 1
is not the actual catalyst but rather an initiator to generate the
siloxyphosphonium ion 27 via (C6F5)3P(F)H (2) (Scheme 6,
highlighted in cyan). 27 might then transfer the silyl group to
the carbonyl group in III and lead to a silylcarboxonium ion IV
and the phosphine oxide 34.27 The hydrosilane I can then act
as hydride transfer reagent. This transfer step produces the
desired silyl ether V and the stabilized silylium ion 27.
Alternatively, the silylcarboxonium ion IV generated through
the initially proposed mechanism (Scheme 6, highlighted in
gray) could directly react with hydrosilane I to give the silyl
ether V and another donor-stabilized silylium ion VI (Scheme
6, highlighted in orange). This, in turn, might react with ketone
III, thereby regenerating the silylcarboxonium ion IV.
However, based on the experimental data, both pathways are
possible and could be competing. Furthermore, the initially
proposed EPC-catalyzed variant with a hydride transfer from 2
to IV as initially proposed cannot be completely excluded.
Mu
̈
ller and co-workers studied the Lewis acidity of a library of
various siloxyphosphonium ions.21 Gagne and co-workers
described the use of arylphosphines in the stabilization of
silylium ions,22 wherein addition of (C6F5)3P (28) to a freshly
prepared solution of [Et3Si(C6D6)][B(C6F5)4] showed no
evidence of adduct formation. A complex mixture suggested
phosphine degradation.23
́
These results show that, even when employing 27, the
hydrosilane delivers its hydride to the silylcarboxonium ion.
This prompted us to question the role of the neutral
phosphorane in the proposed EPC-mediated hydrosilylation.
The hydrosilylation of acetophenone (30) using 1 or (S)-6 as
catalysts afforded only the products ethylbenzene 31 and
hexaethyldisiloxane 32 in a 1:1 ratio (Table 3, entries 1 and
2).4,9 Using 1.0 equiv of Et3SiH, the deoxygenation product
was formed in 50% yield. Similar results were reported by Piers
using [Ph3C][B(C6F5)4] or [Et3Si][B(C6F5)4] as catalysts
(entries 3 and 4), whereas B(C6F5)3 only produced the silyl
ether 33 (entry 5).8b In the first two cases, the hydride source
is the hydrosilane, whereas an in-situ-formed borohydride
transfers the hydride in the latter case. As the experimental
results obtained with EPCs strongly resemble silylium ion
catalysis and not borane catalysis, we concluded that the
hydrosilane could also act as hydride transfer reagent in this
case.24
D
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a
Scheme 6. Alternative Pathways for the Hydride Transfer by the Hydrosilane Itself
a
Counteranion [B(C6F5)4]− is omitted for clarity. Do = Donor.
Bruker AV500, or Bruker AV700 instruments. Chemical shifts are
reported in parts per million (ppm) and are referenced to the residual
solvent resonance as the internal standard (CHDCl2, δ 5.32 ppm for
1H NMR; CD2Cl2, δ 53.84 ppm for 13C NMR; C6D5H, δ 7.16 ppm
CONCLUSION
■
Herein, we have prepared axially chiral fluorophosphonium
salts with a binaphthyl backbone and determined their Lewis
acidity using FIA and GEI values. Both compounds (S)-5 and
(S)-6 were active in Si−H bond activation and efficiently
mediated the hydrosilylation of ketones, but no enantioinduc-
tion was seen. This prompted us to (re)examine the reported
mechanism for the hydrosilylation of ketones.7 The results of
our experimental investigations suggest the possibility that
these reactions may be initiated by fluorophosphonium ion but
catalyzed by an achiral silylium ion. Further applications of the
new phosphonium ions in reactions avoiding these side-
reactions are ongoing in our laboratories.
1
for H NMR; C6D6, δ 128.06 ppm for 13C NMR). 11B, 19F, 29Si, and
31P NMR were calibrated according to the IUPAC recommendation
using a unified chemical shift scale based on the proton resonance of
tetramethylsilane as primary reference.29 Data are reported as follows:
chemical shift, multiplicity (sbr = broad singlet, s = singlet, d =
doublet, t = triplet, q = quartet, m = multiplet, mc = centrosymmetric
multiplet), coupling constants (Hz), and integration. Infrared (IR)
spectra were recorded on an Agilent Technologies Cary 630 FTIR
spectrophotometer equipped with an ATR unit and are reported as
wavenumbers [cm−1]. Gas liquid chromatography (GLC) was
performed on an Agilent Technologies 7820A gas chromatograph
equipped with an HP-5 capillary column (30 m × 0.32 mm, 0.25 μm
film thickness) using the following program: N2 carrier gas, injection
temperature 240 °C, detector temperature 300 °C, flow rate: 1.74
mL/min; temperature program: start temperature 40 °C, heating rate
10 °C/min, end temperature 280 °C for 10 min. High-resolution mass
spectrometry (HRMS) was performed by the Analytical Facility at the
EXPERIMENTAL SECTION
■
General Information. All reactions were performed in flame-
dried glassware using an MBraun glovebox or conventional Schlenk
techniques under a static pressure of argon (glovebox) or nitrogen.
Liquids and solutions were transferred with syringes and cannulas.
Solvents were dried and purified using standard procedures. Technical
grade solvents for extraction or chromatography were distilled prior to
use. C6D6 and CD2Cl2 were degassed and stored over 4 Å molecular
sieves. Phosphepine (S)-7,12a phosphepine (S)-8,12b [(C6F5)3PF][B-
(C6F5)4] (1),1 [(C6F5)3PCl][B(C6F5)4],7 [(C6F5)3POPh][B-
(C6F5)4],18 and tris(pentafluorophenyl)phosphine oxide28 were
synthesized according to reported procedures. Et3SiH, Me2PhiH,
and MePh2SiH were distilled, degassed, and stored over 4 Å
molecular sieves. Ph2SiH2 and PhSiH3 were degassed and stored
over 4 Å molecular sieves. Ketones 11, 17−21, and 30 were purified
by flash chromatography or distillation prior to use. All other
chemicals were purchased from commercial suppliers and used
without further purification. Analytical thin layer chromatography
(TLC) was performed on silica gel 60 F254 aluminum sheets by
Merck using the indicated solvents. Flash column chromatography
Institut fur Chemie, Technische Universitat Berlin. Optical rotation
̈
̈
was measured on a Schmidt & Haensch Polatronic H532 polarimeter
with [α]2λ0 values reported in 10−1 (° cm2 g−1); concentration c is in
g/100 mL and as indicated. Enantiomeric excesses were determined
by analytical high performance liquid chromatography (HPLC)
analysis on an Agilent Technologies 1290 Infinity instrument with a
chiral stationary phase using a Daicel Chiral OJ-H, or a Daicel
Chiralcel AD-H column (n-heptane/i-PrOH mixture as solvent), or
on an Agilent Technologies 1200 Infinity instrument with a stationary
phase using a Daicel Chiralcel OJ-RH column (MeCN/H2O mixture
as solvent). The chemical purity of all products was only established
by 1H NMR spectroscopy. Data for the single crystal structure
determination were collected with an Agilent SuperNova diffrac-
tometer equipped with a CCD area Atlas detector and a mirror
monochromator by utilizing Cu−Kα radiation (λ = 1.5418 Å).
Software packages used: CrysAlis PRO for data collection, cell
refinement, and data reduction,30 SHELXS-97 for structure
solution,31 SHELXL-97 for structure refinement,32 and Mercury
was performed on silica gel 60 (40−63 μm, 230−400 mesh, ASTM)
by Merck using the indicated solvents. H, 13C, 11B, 19F, 29Si, and 31
P
1
NMR spectra were recorded in CD2Cl2 or C6D6 on Bruker AV400,
E
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3.1.133 for graphics. Electronic structure calculations, including
geometry optimization, frequency calculations, and energy calcu-
lations, were performed using Gaussian 16.34 Geometry optimizations
and frequency calculations were performed using the BP86 functional
and def2-TZVP basis set.35 X-ray coordinates were used as the
starting geometries where available. Each geometry was confirmed to
be a minimum on its potential energy surface by confirming the
Hessian to be positive definite with a frequency calculation. The
Cartesian coordinates of the optimized structures are collected in
Tables S1−S6 (see the Supporting Information). Orbital and internal
energies needed to calculate global electrophilicity indices (GEIs) and
fluoride ion affinities (FIAs) were obtained from MP2/def2-TZVPP36
calculations at the BP86/def2-TZVP geometries. FIA and GEI were
calculated as previously described.17c,18
128.8 (s, 2C), 130.3−130.5 (m, 2C), 131.9 (s), 133.2 (s, 2C), 133.3−
133.5 (m, 4C), 133.8 (dt, 2JC,P = 12 Hz, 3JC,F = 4.8 Hz, 2C), 134.4 (d,
3JC,P = 5.5 Hz, 2C), 138.3 (dt, 1JC,P = 174 Hz, 2JC,F = 33 Hz). 19F{1H}
1
NMR (471 MHz, CD2Cl2): δ/ppm −32.0 (d, JF,P = 635 Hz, 2F).
31P{1H} NMR (202 MHz, CD2Cl2): δ/ppm −14.9 (t, 1JP,F = 635 Hz).
Fluorophosphonium Ion (S)-5. In a glovebox, difluorophos-
phorane (S)-9 (20 mg, 39 μmol, 1.0 equiv) was placed in a GLC vial,
dissolved in C6D6 (0.2 mL), and the solution was added to a freshly
prepared solution of [Et3Si(C6D6)][B(C6F5)4] (34 mg, 43 μmol, 1.1
equiv) in C6D6 (0.3 mL). The resulting two-phase system was stirred
for 1 h at room temperature before CH2Cl2 (3 drops) was added.
After 5 min, the supernatant was removed and the lower phase was
washed with C6D6 (3 0.1 mL). After removal of all volatiles in high
vacuum, the fluorophosphonium ion (S)-5 (30 mg, 26 μmol, 66%)
was obtained as a brown solid. Single crystals of (S)-5 suitable for X-
ray diffraction were obtained by slow evaporization of a CD2Cl2/C6D6
mixture. HRMS (APCI): calculated for C28H16F6P+ [(M −
B(C6F5)4)+]: 497.0888; found: 497.0879. 1H NMR (500 MHz,
CD2Cl2): δ/ppm 4.11−4.20 (m, 1H), 4.20−4.28 (m, 1H), 4.32−4.53
General Procedure for EPC-Catalyzed Hydrosilylation of
Ketones (GP1). In a glovebox, a GLC vial is charged with the
corresponding EPC (1.0−5.0 mol %), the hydrosilane (1.1−7.0
equiv), the indicated ketone (1.0 equiv), and solvent (0.6 mL). The
resulting mixture is stirred at room temperature for the indicated time.
1
The progress of the reaction is monitored by GLC analysis or H
3
3
(m, 2H), 7.20 (d, JH,H = 8.9 Hz, 1H), 7.25 (d, JH,H = 8.6 Hz, 1H),
NMR analysis. The volatiles are removed under reduced pressure.
Purification of the residue by flash chromatography on silica gel
(eluent: n-petane) affords the silyl ether.
General Procedure for Preparation of Racemic Silyl Ethers
(GP2). In a glovebox, a GLC vial is charged with B(C6F5)3 (3.0 mol
%), the hydrosilane (1.0−2.0 equiv), the indicated ketone (1.0 equiv),
and CH2Cl2 (0.6 mL). The resulting mixture is stirred at room
temperature overnight. The volatiles are removed under reduced
pressure. Purification of the residue by flash chromatography on silica
gel (eluent: n-petane) affords the racemic silyl ether.
7.38−7.46 (m, 3H), 7.63−7.74 (m, 3H), 8.03 (d, 3JH,H = 8.3 Hz, 1H),
8.07 (d, JH,H = 8.6 Hz, 1H), 8.12 (d, JH,H = 8.3 Hz, 1H), 8.26 (d,
3JH,H = 8.5 Hz, 1H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm
3
3
2
31.5−32.3 (m), 32.3−33.2 (m), 118.0 (d, JC,P = 8.4 Hz), 119.6 (d,
2JC,P = 11.3 Hz), 126.5 (d, JC,P = 5.6 Hz), 127.0 (s), 127.17 (s),
3
127.23 (s), 128.72 (s), 128.74 (s), 128.8 (s), 129.0 (s), 129.2 (s),
129.3 (s), 132.27 (s), 132.32 (s), 132.4 (s), 132.7 (s), 135.1−135.2
3
3
(m, 2C), 135.6 (d, JC,P = 4.4 Hz), 135.8 (d, JC,P = 5.7 Hz), 136.5
1
1
(dmc, JC,F = 250 Hz, 8C), 138.5 (dmc, JC,F = 242 Hz, 4C), 148.5
1
(S)-4,4-Difluoro-4-(pentafluorophenyl)-4,5-dihydro-3H-4λ5-
dinaphtho[2,1-c:1′,2′-e]phosphoran [(S)-9]. C6F5-substituted
phosphepine (S)-7 (60 mg, 0.13 mmol, 1.0 equiv) was placed in a
10 mL Schlenk tube, dissolved in CH2Cl2 (2 mL), and the resulting
solution was cooled to −78 °C under a nitrogen atmosphere. XeF2
(32 mg, 0.19 mmol, 1.5 equiv) in CH2Cl2 (0.6 mL) was added, and
the resulting mixture was stirred for 2 h at −78 °C. After removal of
all volatiles in high vacuum, difluorophosphoran (S)-9 (65 mg, 0.13
mmol, >99%) was obtained as a white solid. HRMS (EI): calculated
for C28H16F7P+ [M+]: 516.0872; found: 516.0878. 1H NMR (500
MHz, CD2Cl2): δ/ppm 3.19 (mc, 2H), 3.98−4.16 (m, 2H), 7.05 (d,
(dmc, JC,F = 237 Hz, 8C). Signals for the quaternary carbon atoms
B(ipso-C6F5) and P(ipso-C6F5) could not be detected. 11B{1H} NMR
(161 MHz, CD2Cl2): δ/ppm −16.7 (s). 19F{1H} NMR (471 MHz,
CD2Cl2): δ/ppm −123.4 (mc, 2F), −127.4 (mc, 1F), −129.0 (d, 1JF,P
= 1098 Hz, 1F), −133.3 (mc, 8F), −151.6 (mc, 2F), −163.6 (t, 3JF,F
=
21 Hz, 4F), −167.5 (mc, 8F). 19F/13C HMQC (659/176 MHz,
CD2Cl2): δ/ppm −123.4/148.6, −127.4/149.4, −133.3/148.0,
−151.6/139.1, −163.6/138.2, −167.5/136.3. 31P{1H} NMR (202
1
MHz, CD2Cl2): δ/ppm 115.9 (d, JP,F = 1098 Hz). The crystallo-
graphic data are available online in the CCDC database under number
CCDC 1882909.
3
3
3JH,H = 8.4 Hz, 2H), 7.21 (dd, JH,H = 7.4 Hz, JH,H = 7.3 Hz, 2H),
7.46 (dd, 2JH,H = 7.3 Hz, 2JH,H = 6.9 Hz, 2H), 7.53 (d, 3JH,H = 7.9 Hz,
2H), 7.88−8.02 (m, 4H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/
Fluorophosphonium Ion (S)-6. In a glovebox, difluorophos-
phorane (S)-10 (50 mg, 0.12 mmol, 1.0 equiv) was placed in a GLC
vial, dissolved in C6D6 (0.4 mL), and the solution was added to a
freshly prepared solution of [Et3Si(C6D6)][B(C6F5)4] (0.10 g, 0.13
mmol, 1.1 equiv) in C6D6 (0.3 mL). The resulting two-phase system
was stirred for 5 min at room temperature before CH2Cl2 (3 drops)
was added. After 5 min, the supernatant was removed and the lower
phase was washed with C6D6 (1 0.1 mL) and n-pentane (3 0.1 mL).
After removal of all volatiles in high vacuum, the fluorophosphonium
ion (S)-6 (62 mg, 53 μmol, 45%) was obtained as a brown solid along
with traces of an unknown phosphorus-containing species. Single
crystals of (S)-6 suitable for X-ray diffraction were obtained by slow
evaporization of a CD2Cl2/n-pentane mixture. HRMS (APCI):
calculated for C28H21FP+ [(M − B(C6F5)4)+]: 407.1359; found:
407.1356. 1H NMR (500 MHz, CD2Cl2): δ/ppm. 3.96−4.22 (m,
4H), 7.34 (d, 3JH,H = 8.5 Hz, 1H), 7.68−7.86 (m, 13H), 8.04 (t, 3JH,H
ppm 39.5 (dt, 1JC,P = 117 Hz, 2JC,F = 19.7 Hz, 2C), 115.2 (dm, 1JC,P
=
158 Hz), 126.3 (s, 2C), 126.7 (s, 2C), 127.2 (s, 2C), 128.6 (s, 2C),
129.2 (s, 2C), 130.1 (d, 3JC,P = 7.7 Hz, 2C), 132.0 (d, 2JC,P = 12.0 Hz,
2C), 133.2 (s, 2C), 133.5 (s, 2C), 134.9 (d, 3JC,P = 5.9 Hz, 2C), 138.0
1
1
(dmc, JC,F = 255 Hz, 2C), 142.5 (dmc, JC,F = 252 Hz), 144.8 (dmc,
1JC,F = 246 Hz, 2C). 19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm
1
−17.7 (d, JF,P = 650 Hz, 2F), −134.9 (mc, 2F), −152.5 (mc, 1F),
−160.9 (mc, 2F). 31P{1H} NMR (202 MHz, CD2Cl2): δ/ppm −11.3
1
(t, JP,F = 650 Hz).
(S)-4,4-Difluoro-4-phenyl-4,5-dihydro-3H-4λ5-dinaphtho-
[2,1-c:1′,2′-e]phosphoran [(S)-10]. Phenyl-substituted phosphe-
pine (S)-8 (100 mg, 0.263 mmol, 1.00 equiv) was placed in a 10 mL
Schlenk tube, dissolved in CH2Cl2 (2 mL), and the resulting solution
was cooled to −78 °C under a nitrogen atmosphere. XeF2 (65 mg,
0.38 mmol, 1.5 equiv) in CH2Cl2 (1 mL) was added, and the resulting
mixture was stirred for 1 h at −78 °C. After removal of all volatiles in
high vacuum, difluorophosphoran (S)-10 (112 mg, 0.263 mmol,
>99%) was obtained as a white solid. HRMS (APCI): calculated for
3
3
= 7.3 Hz, 1H), 8.08 (d, JH,H = 8.4 Hz, 1H), 8.24 (d, JH,H = 8.4 Hz,
1H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm 31.4 (dd, 1JC,P = 60
Hz, 2JC,F = 9.1 Hz), 31.9 (dd, 1JC,P = 60 Hz, 2JC,F = 9.0 Hz), 115.1 (dd,
1JC,P = 86 Hz, 2JC,F = 9.0 Hz, 2C), 119.4 (dd, 2JC,P = 7.8 Hz, 3JC,F = 2.8
2
3
Hz), 121.4 (d, JC,P = 11.9 Hz), 123.7−125.0 (m), 127.2 (d, JC,P
=
3
2
1
C28H21FP+ [(M − F)+]: 407.1359; found: 407.1353. H NMR (500
4.7 Hz), 127.3 (d, JC,P = 5.2 Hz), 131.5 (d, JC,P = 13 Hz, 2C),
131.8−131.9 (m, 10C), 132.3 (s), 132.7 (s), 134.7−134.8 (m, 2C),
MHz, CD2Cl2): δ/ppm 3.09−3.27 (m, 2H), 3.68−3.80 (m, 2H), 7.07
(d, 3JH,H = 8.7 Hz, 2H), 7.21 (dd, 3JH,H = 7.8 Hz, 3JH,H = 7.3 Hz, 2H),
7.43−7.51 (m, 4H), 7.51−7.54 (m, 1H), 7.56 (d, 3JH,H = 8.7 Hz, 2H),
134.9 (d, 3JC,P = 3.9 Hz), 135.8 (d, 3JC,P = 5.5 Hz), 136.6 (dmc, 1JC,F
=
245 Hz, 8C), 138.6 (dmc, 1JC,F = 244 Hz, 4C), 139.3 (s), 148.6 (dmc,
1JC,F = 245 Hz, 8C). A signal for the quaternary carbon atoms B(ipso-
C6F5) could not be detected. 11B{1H} NMR (161 MHz, CD2Cl2): δ/
ppm −16.7 (s). 19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm −133.1
3
3
7.91 (d, JH,H = 7.5 Hz, 1H), 7.94 (d, JH,H = 7.5 Hz, 1H), 7.98 (dd,
3JH,H = 8.3 Hz, JH,H = 8.2 Hz, 4H). 13C{1H} NMR (126 MHz,
3
CD2Cl2): δ/ppm 40.0 (dt, 1JC,P = 117 Hz, 2JC,F = 24.5 Hz, 2C), 125.9
1
3
(s, 2C), 126.4 (s, 2C), 127.2 (s, 2C), 128.5 (s, 2C), 128.7 (s, 2C),
(mc, 8F), −138.5 (d, JF,P = 1060 Hz, 1F), −163.5 (t, JF,F = 21 Hz,
F
DOI: 10.1021/acs.organomet.8b00912
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
4F), −167.4 (mc, 8F). 31P{1H} NMR (202 MHz, CD2Cl2): δ/ppm
silica gel (eluent: n-pentane) afforded 15 (5.0 mg, 14 μmol, 12%, 0%
123.2 (d, JP,F = 1060 Hz). The crystallographic data are available
ee) as a colorless liquid. IR (ATR): ν
̃
/cm−1 2137, 1428, 1366, 1268,
1
online in the CCDC database under number CCDC 1882910.
Triethyl(2,2,2-trifluoro-1-phenylethoxy)silane (12). Prepared
according to GP1 from 2,2,2-trifluoro-1-phenylethan-1-one (11, 30
mg, 0.17 mmol, 1.0 equiv), Et3SiH (24 mg, 0.21 mmol, 1.2 equiv),
and (S)-6 (2.0 mg, 1.7 μmol, 1.0 mol %) in CD2Cl2 (0.6 mL) within a
reaction time of 6 h. Purification of the crude product on silica gel
(eluent: n-pentane) afforded 12 (39 mg, 0.13 mmol, 73%, 0% ee) as a
1206, 1170, 1121, 1070, 1028, 849, 809, 733, 695. HRMS (EI):
calculated for C8H7F3O+ [(M + H − C12H11Si)+]: 176.0444; found:
176.0434. HRMS (EI): calculated for C12H11Si• [(M − C8H7F3O)•]:
1
183.0630; found: 183.0622. H NMR (500 MHz, CD2Cl2): δ/ppm
5.04 (q, 3JH,F = 6.7 Hz, 1H), 5.42 (s, 1H), 7.34−7.51 (m, 11H), 7.53−
7.57 (m, 2H), 7.58−7.62 (m, 2H). 13C{1H} NMR (126 MHz,
CD2Cl2): δ/ppm 75.0 (q, 2JC,F = 32.7 Hz), 124.6 (q, 1JC,F = 285 Hz),
128.2 (s, 2C), 128.5 (s, 2C), 128.6 (s, 2C), 128.8 (s, 2C), 129.8 (s),
131.2 (s), 131.3 (s), 132.67 (s), 132.71 (s), 134.5 (s), 135.0 (s, 2C),
135.1 (s, 2C). 19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm = −78.3
(s). 1H/29Si HMQC (500/99 MHz, CD2Cl2, optimized for J = 7 Hz):
δ/ppm 5.04/−8.1, 5.42/−8.1, (7.53−7.57)/−8.1, (7.58−7.62)/−8.1.
The enantiomeric excess was determined by HPLC analysis on a
chiral stationary phase (Daicel Chiracel OJ-RH column, column
temperature 20 °C, solvent MeCN/H2O 50/50, flow rate 0.4 mL/
min, λ 210 nm): tR,1 = 46.9 min, tR,2 = 80.1 min.
colorless liquid. IR (ATR): ν/
̃
cm−1 2957, 2879, 1456, 1365, 1269,
1240, 1205, 1166, 1126, 1072, 1004, 974, 853, 830, 728, 697, 671.
HRMS (EI): calculated for C12H16F3OSi+ [(M − C2H5)+]: 261.0917;
1
found: 261.0923. H NMR (500 MHz, CD2Cl2): δ/ppm 0.55−0.66
3
3
(m, 6H), 0.90 (t, JH,H = 7.9 Hz, 9H), 4.97 (q, JH,F = 6.7 Hz, 1H),
7.37−7.42 (m, 3H), 7.44−7.79 (m, 2H). 13C{1H} NMR (126 MHz,
CD2Cl2): δ/ppm 4.8 (s, 3C), 6.6 (s, 3C), 73.7 (q, JC,F = 31.8 Hz),
2
1
124.8 (q, JC,F = 284 Hz), 128.0 (s, 2C), 128.7 (s, 2C), 129.5 (s),
136.1 (s). 19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm −78.7 (s).
1H/29Si HMQC (500/99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/
ppm 0.55−0.66/24.4, 0.90/24.4. The enantiomeric excess was
determined by HPLC analysis on a chiral stationary phase (Daicel
Chiracel OJ-RH column, column temperature 20 °C, solvent MeCN/
2,2,2-Trifluoro-1-phenylethanol (16). Prepared according to
GP1 from 2,2,2-trifluoro-1-phenylethan-1-one (11, 20 mg, 0.12
mmol, 1.0 equiv), PhSiH3 (14 mg, 0.13 mmol, 1.1 equiv), and (S)-
5 (1.4 mg, 1.1 μmol, 1.0 mol %) in CD2Cl2 (0.6 mL) within a reaction
time of 24 h. Purification of the crude product on silica gel (eluent: n-
pentane/tert-butyl methyl ether 100/0 → 6/1) afforded 16 (12 mg,
H2O 50/50, flow rate 0.4 mL/min, λ 210 nm): tR,1 = 59.7 min, tR,2
=
65.2 min. The analytical data are in accordance with those reported.4
(2,2,2-Trifluoro-1-phenylethoxy)dimethyl(phenyl)silane
(13). Prepared according to GP1 from 2,2,2-trifluoro-1-phenylethan-
1-one (11, 20 mg, 0.12 mmol, 1.0 equiv), Me2PhSiH (18 mg, 0.13
mmol, 1.1 equiv), and (S)-5 (1.4 mg, 1.1 μmol, 1.0 mol %) in CD2Cl2
(0.6 mL) within a reaction time of 4 h. Purification of the crude
product on silica gel (eluent: n-pentane) afforded 13 (34 mg, 0.11
68 μmol, 57%, 0% ee) as a yellowish liquid. IR (ATR): ν
̃
/cm−1 3403,
1456, 1429, 1356, 1262, 1165, 1121, 1058, 922, 865, 832, 758, 700.
HRMS (EI): calculated for C8H7F3O+ [M+]: 176.0444; found:
176.0445. 1H NMR (500 MHz, CD2Cl2): δ/ppm 2.75 (d, 3JH,H = 4.7
Hz, 1H), 5.06 (mc, 1H), 7.41−7.45 (m, 3H), 7.45−7.51 (m, 2H).
13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm 72.9 (q, 2JC,F = 34.4 Hz),
1
mmol, 95%, 0% ee) as a colorless liquid. IR (ATR): ν
̃
124.8 (q, JC,F = 285 Hz), 127.8 (s, 2C), 129.0 (s, 2C), 129.9 (s),
1428, 1367, 1267, 1206, 1167, 1117, 1072, 1029, 999, 860, 828, 786,
738, 697. HRMS (EI): calculated for C16H17F3OSi+ [M+]: 310.0995;
found: 310.0981. 1H NMR (500 MHz, CD2Cl2): δ/ppm 0.33 (s, 3H),
0.38 (s, 3H), 4.90 (q, 3JH,F = 6.7 Hz, 1H), 7.32−7.43 (m, 8H), 7.49−
7.53 (m, 2H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm −1.5 (s),
−1.4 (s), 73.8 (q, 2JC,F = 32.6 Hz), 124.7 (q, 1JC,F = 281 Hz), 128.1 (s,
2C), 128.3 (s, 2C), 128.7 (s, 2C), 129.6 (s), 130.4 (s), 133.9 (s, 2C),
135.5 (s), 136.4 (s). 19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm
−78.6 (s). 29Si{1H} DEPT (99 MHz, CD2Cl2, optimized for J = 7
Hz): δ/ppm 13.1 (s). The enantiomeric excess was determined by
HPLC analysis on a chiral stationary phase (Daicel Chiracel OJ-H
column, column temperature 20 °C, solvent n-heptane/i-PrOH 95/5,
flow rate 0.8 mL/min, λ 210 nm): tR,1 = 6.2 min, tR,2 = 8.2 min.
(2,2,2-Trifluoro-1-phenylethoxy)methyl(diphenyl)silane
(14). Prepared according to GP1 from 2,2,2-trifluoro-1-phenylethan-
1-one (11, 20 mg, 0.12 mmol, 1.0 equiv), MePh2SiH (25 mg, 0.13
mmol, 1.1 equiv), and (S)-5 (1.4 mg, 1.1 μmol, 1.0 mol %) in CD2Cl2
(0.6 mL) with a reaction time of 2 h. Purification of the crude product
on silica gel (eluent: n-pentane) afforded 14 (22 mg, 0.059 mmol,
134.6 (s). 19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm = −78.7 (s).
The enantiomeric excess was determined by HPLC analysis on a
chiral stationary phase (Daicel Chiracel AD-H column, column
temperature 20 °C, solvent n-heptane/i-PrOH 98/2, flow rate 0.8
mL/min, λ 210 nm): tR,1 = 22.5 min, tR,2 = 24.1 min. The analytical
data are in accordance with those reported.37
(1-Cyclohexylethoxy)triethylsilane (22). Prepared according to
GP1 from 1-cyclohexylethan-1-one (17, 15 mg, 0.12 mmol, 1.0
equiv), Et3SiH (29 mg, 0.25 mmol, 2.1 equiv), and (S)-5 (1.4 mg, 12
μmol, 1.0 mol %) in CD2Cl2 (0.6 mL) within a reaction time of 1 h.
Purification of the crude product on silica gel (eluent: n-pentane)
afforded 22 (>95% conversion, 0% ee) in a mixture with disiloxane as
a colorless liquid. IR (ATR): ν/
̃
cm−1 2923, 2875, 1450, 1413, 1372,
1298, 1236, 1188, 1139, 1069, 1003, 967, 910, 840, 775, 722, 672.
HRMS (APCI): calculated for C14H31OSi+ [(M + H)+]: 243.2139;
1
found: 243.2140. H NMR (400 MHz, CD2Cl2): δ/ppm 0.58 (q,
3
3JH,H = 7.8 Hz, 6H), 0.89−0.98 (m, 11H), 1.07 (d, JH,H = 6.6 Hz,
3
3H), 1.11−1.27 (m, 4H), 1.59−1.83 (m, 5H), 3.55 (qd, JH,H = 6.1
Hz, 3JH,H = 6.1 Hz, 1H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm
5.4 (s, 3C), 7.1 (s, 3C), 21.0 (s), 26.86 (s), 26.88 (s), 27.2 (s), 29.1
(s), 29.2 (s), 46.2 (s), 72.9 (s). 1H/29Si HMQC (500/99 MHz,
CD2Cl2, optimized for J = 7 Hz): δ/ppm (0.55−0.62)/15.5, (0.89−
0.98)/15.5, 3.55/15.5. The enantiomeric excess was determined by
HPLC analysis of the corresponding 4-nitrobenzoyl ester38 on a chiral
stationary phase (Daicel Chiracel AD-H column, column temperature
20 °C, solvent n-heptane/i-PrOH 100/0, flow rate 0.8 mL/min, λ 210
nm): tR,1 = 35.2 min, tR,2 = 37.6 min. The analytical data are in
accordance with those reported.7
51%, 0% ee) as a colorless liquid. IR (ATR): ν
̃
/cm−1 3048, 1454,
1428, 1366, 1267, 1206, 1168, 1115, 1072, 1029, 998, 843, 791, 725,
696, 663. HRMS (EI): calculated for C21H19F3OSi+ [M+]: 372.1152;
found: 372.1140. 1H NMR (500 MHz, CD2Cl2): δ/ppm 0.61 (s, 3H),
3
5.00 (q, JH,F = 6.7 Hz, 1H), 7.35−7.49 (m, 11H), 7.54−7.59 (m,
4H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm −2.7 (s), 73.6 (q,
1
2JC,F = 32.9 Hz), 124.8 (q, JC,F = 281 Hz), 128.2 (s, 2C), 128.3 (s,
2C), 128.4 (s, 2C), 128.7 (s, 2C), 129.7 (s), 130.6 (s), 130.7 (s),
134.7 (s, 2C), 134.8 (s, 2C), 134.9 (s), 135.1 (s), 135.3 (s). 19F{1H}
NMR (471 MHz, CD2Cl2): δ/ppm −78.3 (s). 1H/29Si HMQC (500/
99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/ppm 0.61/1.8, 5.00/1.8.
The enantiomeric excess was determined by HPLC analysis on a
chiral stationary phase (Daicel Chiracel OJ-RH column, column
temperature 20 °C, solvent MeCN/H2O 60/40, flow rate 0.4 mL/
min, λ 210 nm): tR,1 = 36.3 min, tR,2 = 41.7 min.
(2,2,2-Trifluoro-1-phenylethoxy)diphenylsilane (15). Pre-
pared according to GP1 from 2,2,2-trifluoro-1-phenylethan-1-one
(11, 20 mg, 0.12 mmol, 1.0 equiv), Ph2SiH2 (23 mg, 0.13 mmol, 1.1
equiv), and (S)-5 (1.4 mg, 1.1 μmol, 1.0 mol %) in CD2Cl2 (0.6 mL)
within a reaction time of 2 h. Purification of the crude product on
Dimethyl[(4-methylpentan-2-yl)oxy](phenyl)silane (23).
Prepared according to GP1 from isobutyl methyl ketone (18, 15
mg, 0.15 mmol, 1.0 equiv), Me2PhSiH (28 mg, 0.21 mmol, 1.4 equiv),
and (S)-5 (1.8 mg, 15 μmol, 1.0 mol %) in CD2Cl2 (0.6 mL) within a
reaction time of 19 h. Purification of the crude product on silica gel
(eluent: n-pentane) afforded 23 (>95% conversion, 0% ee) as a
colorless liquid. HRMS (APCI): calculated for C13H21OSi+ [(M −
1
CH3)+]: 221.1362; found: 220.9857 (no better value obtained). H
NMR (500 MHz, CD2Cl2): δ/ppm 0.33 (mc, 6H), 0.90 (mc, 3H),
3
0.91 (mc, 3H), 1.15 (d, JH,H = 6.1 Hz, 3H), 1.17−1.24 (m, 1H),
1.33−1.40 (m, 1H), 1.67−1.76 (m, 1H), 3.80−3.88 (m, 1H), 7.32−
G
DOI: 10.1021/acs.organomet.8b00912
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
7.38 (m, 3H), 7.53−7.56 (m, 2H). 13C{1H} NMR (126 MHz,
CD2Cl2): δ/ppm −1.0 (s), −0.9 (s), 22.6 (s), 23.3 (s), 24.3 (s), 25.0
(s), 49.5 (s), 67.5 (s), 128.1 (s, 2C), 129.8 (s), 133.9 (s, 2C), 139.2
(s). 1H/29Si HMQC (500/99 MHz, CD2Cl2, optimized for J = 7 Hz):
δ/ppm 0.33/−0.9. The enantiomeric excess was determined by
HPLC analysis on a chiral stationary phase (Daicel Chiracel OJ-RH
column, column temperature 20 °C, solvent MeCN/H2O 50/50, flow
rate 0.4 mL/min, λ 210 nm): tR,1 = 40.3 min, tR,2 = 43.3 min.
[1-(4-Cyanophenyl)ethoxy]dimethyl(phenyl)silane (24). Pre-
pared according to GP1 from 4-acetylbenzonitrile (19, 20 mg, 0.14
mmol, 1.0 equiv), Me2PhSiH (20 mg, 0.15 mmol, 1.1 equiv), and (S)-
5 (4.9 mg, 4.2 μmol, 3.0 mol %) in CD2Cl2 (0.6 mL) within a reaction
time of 5 d. Purification of the crude product on silica gel (eluent: n-
pentane) afforded 24 (>95% conversion, 0% ee) as a colorless liquid.
optimized for J = 7 Hz): δ/ppm (0.51−0.61)/20.8, 0.89/20.8,
5.26/20.8. The enantiomeric excess was determined by optical
rotation: = 0° (c = 1.18, CHCl3, 0% ee).
Siloxyphosphonium Ion 27. In a glovebox, tris(pentafluoro-
phenyl)phosphine oxide (34, 10 mg, 18 μmol, 1.0 equiv) was placed
in a GLC vial, dissolved in C6D6 (0.4 mL), and was added to a freshly
prepared solution of [Et3Si(C6D6)][B(C6F5)4] (17 mg, 22 μmol, 1.2
equiv) in C6D6 (0.3 mL). The resulting two-phase system was stirred
for 5 min at room temperature before CH2Cl2 (3 drops) was added.
After 5 min, the supernatant was removed and the lower phase was
washed with C6D6 (3 0.1 mL). After removal of all volatiles in high
vacuum, the siloxyphosphonium ion 27 (17 mg, 13 μmol, 72%) was
1
obtained as a white solid. Selected analytical data: H NMR (500
MHz, CD2Cl2): δ/ppm 0.87−1.05 (m, 18H). 13C{1H} NMR (176
MHz, CD2Cl2): δ/ppm 5.6 (s, 3C), 5.9 (s, 3C), 98.0 (dm 1JC,P = 135
Hz, 3C). 11B{1H} NMR (161 MHz, CD2Cl2): δ/ppm −16.6 (s).
19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm −128.4 (sbr), −129.1
IR (ATR): ν/
̃
cm−1 2958, 2228, 1608, 1427, 1406, 1252, 1206, 1117,
1088, 1027, 956, 826, 785, 727, 698. HRMS (EI): calculated for
1
C16H16NOSi+ [(M − CH3)+]: 266.0996; found: 266.0994. H NMR
(500 MHz, CD2Cl2): δ/ppm 0.32 (s, 3H), 0.36 (s, 3H), 1.39 (d, 3JH,H
3
(sbr), −133.2 (sbr), −152.0 (sbr), −163.9 (t, JF,F = 20 Hz), −167.8
3
(sbr). 19F/13C HMQC (659/176 MHz, CD2Cl2): δ/ppm −128.4/
148.4, −129.1/149.4, −133.2/148.6, −152.0/139.5, −163.9/138.7,
−167.8/136.8. 31P{1H} NMR (202 MHz, CD2Cl2): δ/ppm 17.7 (sbr).
1H/29Si HMQC (500/99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/
ppm (0.87−1.05)/53.6. The analytical data are in accordance with
those reported.20
= 6.5 Hz, 3H), 4.90 (q, JH,H = 6.2 Hz, 1H), 7.32−7.40 (m, 3H),
7.40−7.44 (m, 2H), 7.51−7.56 (m, 2H), 7.58−7.62 (m, 2H).
13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm −1.3 (s), −1.1 (s), 26.8
(s), 70.9 (s), 111.1 (s), 119.3 (s), 126.5 (s, 2C), 128.2 (s, 2C), 130.1
(s), 132.5 (s, 2C), 133.9 (s, 2C), 138.0 (s), 152.2 (s). 1H/29Si HMQC
(500/99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/ppm 0.32/7.9,
0.36/7.9. The enantiomeric excess was determined by HPLC analysis
on a chiral stationary phase (Daicel Chiracel OJ-H column, column
temperature 20 °C, solvent n-heptane/i-PrOH 95/5, flow rate 0.5
mL/min, λ 210 nm): tR,1 = 20.3 min, tR,2 = 31.6 min. The analytical
data are in accordance with those reported.39
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00912.
■
S
[1-(2-Bromophenyl)ethoxy]triethylsilane (25). Prepared ac-
cording to GP1 from 1-(2-bromophenyl)ethan-1-one (20, 35 mg,
0.18 mmol, 1.0 equiv), Et3SiH (23 mg, 0.20 mmol, 1.1 equiv), and
(S)-5 (2.3 mg, 2.5 μmol, 1.0 mol %) in CD2Cl2 (0.6 mL) within a
reaction time of 2 h. Purification of the crude product on silica gel
(eluent: n-pentane) afforded 25 (>95% conversion, 0% ee) as a
Overview of screened catalysts and figures giving NMR
spectra of compounds synthesized in this paper (PDF)
Cartesian coordinates (S)-5 (XYZ)
Cartesian coordinates (S)-6 (XYZ)
Cartesian coordinates (S)-9 (XYZ)
colorless liquid. IR (ATR): ν/
̃
cm−1 2953, 2785, 1466, 1438, 1413,
1368, 1237, 1237, 1200, 1130, 1094, 1018, 950, 794, 722, 665. HRMS
(EI): calculated for C14H23BrOSi+ [(M − C2H5)+]: 285.0305; found:
285.0308. 1H NMR (500 MHz, CD2Cl2): δ/ppm 0.51−0.64 (m, 6H),
Cartesian coordinates (S)-10 (XYZ)
3
3
Accession Codes
0.91 (t, 9H), 1.38 (d, JH,H = 6.3 Hz, 3H), 5.18 (q, JH,H = 6.2 Hz,
3
3
4
1H), 7.10 (ddd, JH,H = 7.6 Hz, JH,H = 7.6 Hz, JH,H = 1.7 Hz, 1H),
7.33 (ddd, JH,H = 7.6 Hz, JH,H = 7.6 Hz, JH,H = 1.2 Hz, 1H), 7.47
CCDC 1882909−1882910 contain the supplementary crys-
tallographic 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
3
3
4
3
4
3
(dd, JH,H = 7.8 Hz, JH,H = 1.2 Hz, 1H), 7.63 (dd, JH,H = 7.8 Hz,
4JH,H = 1.8 Hz, 1H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm 5.1
(s, 3C), 6.9 (s, 3C), 25.8 (s), 69.9 (s), 121.1 (s), 127.8 (s), 128.0 (s),
128.7 (s), 132.5 (s), 146.5 (s). 1H/29Si HMQC (500/99 MHz,
CD2Cl2, optimized for J = 7 Hz): δ/ppm (0.51−0.64)/19.0, 0.91/
19.0, 5.18/19.0. The enantiomeric excess was determined by HPLC
analysis on a chiral stationary phase (Daicel Chiracel OJ-RH column,
column temperature 20 °C, solvent MeCN/H2O 50/50, flow rate 0.5
mL/min, λ 210 nm): tR,1 = 71.0 min, tR,2 = 76.6 min. The analytical
data are in accordance with those reported.40
AUTHOR INFORMATION
Corresponding Authors
*E-mail: douglas.stephan@utoronto.ca (D.W.S.).
*E-mail: martin.oestreich@tu-berlin.de (M.O.).
■
Triethyl[1-(pentafluorophenyl)ethoxy]silane (26). Prepared
according to GP1 from 1-(pentafluorophenyl)ethan-1-one (21, 36
mg, 0.17 mmol, 1.0 equiv), Et3SiH (23 mg, 0.20 mmol, 1.1 equiv),
and (S)-5 (2.0 mg, 2.2 μmol, 1.0 mol %) in CD2Cl2 (0.6 mL) with a
reaction time of 2 h. Purification of the crude product on silica gel
(eluent: n-pentane) afforded 26 (>95% conversion, 0% ee) as a
ORCID
Lars Susse: 0000-0002-4894-1198
̈
James H. W. LaFortune: 0000-0003-4217-1038
Douglas W. Stephan: 0000-0001-8140-8355
Martin Oestreich: 0000-0002-1487-9218
colorless liquid. IR (ATR): ν/
̃
cm−1 2957, 2879, 1650, 1499, 1458,
Notes
1415, 1374, 1302, 1238, 1151, 1103, 1055, 963, 890, 800, 727, 671.
HRMS (EI): calculated for C12H14F5OSi+ [(M − C2H5)+]: 297.0729;
The authors declare no competing financial interest.
1
found: 297.0730. H NMR (700 MHz, CD2Cl2): δ/ppm 0.51−0.61
3
3
(m, 6H), 0.89 (t, JH,H = 8.0 Hz, 9H), 1.56 (d, JH,H = 6.5 Hz, 3H),
5.26 (q, 3JH,H = 6.6 Hz, 1H). 13C{1H} NMR (176 MHz, CD2Cl2): δ/
ppm 4.8 (s, 3C), 6.7 (s, 3C), 24.0 (s), 62.5 (s). 1H/13C HMQC (700/
176 MHz, CD2Cl2): δ/ppm 1.56/119.6, 5.26/119.6. 19F NMR (659
ACKNOWLEDGMENTS
■
L.S. was supported through an Einstein Visiting Fellowship of
the Einstein Foundation Berlin to D.W.S. (2016−2019).
D.W.S. and M.O. thank the Einstein Foundation Berlin for
generous funding. D.W.S acknowledges the support of NSERC
of Canada and is grateful for the award of a Canada Research
3
MHz, CD2Cl2): δ/ppm −142.2 (dd, JF,F = 21.5 Hz, JF,F = 8.3 Hz,
3
2F), −155.6 (t, JF,F = 21.0 Hz, 1F), −161.5 (mc, 2F). 19F/13C
HMQC (659/176 MHz, CD2Cl2): δ/ppm −142.2/146.6, −155.6/
142.1, −161.5/139.3. 1H/29Si HMQC (500/99 MHz, CD2Cl2,
Chair. We are grateful to Dr. Susanne Bahr for fruitful
̈
H
DOI: 10.1021/acs.organomet.8b00912
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Asymmetric Piers Hydrosilylation. J. Am. Chem. Soc. 2016, 138,
6940−6493.
discussions and to Vittorio Bonetti for his experimental
contributions (both TU Berlin). We thank Dr. Elisabeth
Irran (TU Berlin) for the X-ray analysis.
(12) (a) Hagemann, B.; Junge, K.; Enthaler, S.; Michalik, M.;
Riermeier, T.; Monsees, A.; Beller, M. A General Method for the
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Synthesis of chiral monodentate binaphthophosphepine ligands and
their application in asymmetric hydrogenations. Tetrahedron:
Asymmetry 2004, 15, 2621−2631.
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I
DOI: 10.1021/acs.organomet.8b00912
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DOI: 10.1021/acs.organomet.8b00912
Organometallics XXXX, XXX, XXX−XXX