Silylpalladium Cations Enable the Oxidative Addition of C(sp3)-O Bonds

Article abstract of DOI:10.1021/jacs.9b08178

The synthesis and characterization of the room-temperature and solution-stable silylpalladium cations (PCy₃)₂Pd-SiR₃⁺(C₆F₅)₄B^- (SiR₃ = SiMe₂Et, SiHEt₂) and (Xantphos)Pd-SiR₃⁺(BAr^f₄) (SiR₃ = SiMe₂Et, SiHEt₂; Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene; BAr^f₄ = (3,5-(CF₃)₂C₆H₃)₄B^-) are reported. Spectroscopic and ligand addition experiments suggest that silylpalladium complexes of the type (PCy₃)₂Pd-SiR₃⁺ are three-coordinate and T-shaped. Addition of dialkyl ethers to both the PCy₃ and Xantphos-based silylpalladium cations resulted in the cleavage of C(sp³)-O bonds and the generation of cationic Pd-alkyl complexes. Mechanistically enabling is the ability of silylpalladium cations to behave as sources of both electrophilic silylium ions and nucleophilic L_nPd(0).

Full text of DOI:10.1021/jacs.9b08178

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Article
Silylpalladium Cations Enable the Oxidative Addition of C(sp3)-O Bonds
Andreas L. Wierschen, Neyen Romano, Stephen J. Lee, and Michel R Gagné
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b08178 • Publication Date (Web): 18 Sep 2019
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Journal of the American Chemical Society
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Silylpalladium Cations Enable the Oxidative Addition of C(sp3)−O
Bonds
Andreas L. Wierschen,† Neyen Romano,†Δ Stephen J. Lee,‡ and Michel R. Gagné†*
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†Caudill Laboratories, Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North
Carolina 27599-3290, United States
‡U.S. Army Research Office, P.O. Box 12211, Research Triangle Park, North Carolina 27709, United States
Supporting Information
ABSTRACT: The synthesis and characterization of the room temperature and solution stable silylpalladium cations
+
+
[(PCy3)2Pd−SiR3 (C6F5)4B− (SiR3 = SiMe2Et, SiHEt2)] and [(Xantphos)Pd−SiR3 (BArf4) (SiR3 = SiMe2Et, SiHEt2; Xantphos = 4,5-
bis(diphenylphosphino)-9,9-dimethylxanthene; BArf4 = (3,5-(CF3)2C6H3)4B−)] are reported. Spectroscopic and ligand addition
experiments suggest that silylpalladium complexes of the type (PCy3)2Pd−SiR3+ are 3-coordinate and T-shaped. Addition of dialkyl
ethers to both the PCy3 and Xantphos-based silylpalladium cations resulted in the cleavage of C(sp3)−O bonds and the generation of
cationic Pd−alkyl complexes. Mechanistically enabling is the ability of silylpalladium cations to behave as both sources of
electrophilic silylium ions and nucleophilic LnPd(0).
ethers with silylmetal complexes, respectively (Scheme 1b).
This type of silylmetal mediated C−O bond cleavage has also
INTRODUCTION
Since the first reported synthesis of a transition-metal−silyl
complex, Cp(CO)2FeSiMe3 by Wilkinson in 1953,1 the number
of compounds containing a formal metal−silicon bond have
grown to include nearly every transition metal.2–7 The synthesis
and study of silylmetal complexes has been driven, in part, by
the development of homogenous transition-metal catalysts for
olefin hydrosilylation.8,9 The most commonly proposed
mechanisms for this process, the Chalk-Harrod and modified
Chalk-Harrod mechanisms, invoke silylmetal complexes as key
catalytic intermediates. While the most familiar reactivity of
silylmetal complexes is the insertion of C=X (X = C, O, N)
bonds into the M−Si bond (to formally cleave the π-bond),
silylmetal complexes are also known to react with nucleophiles
to heterolytically cleave the M−Si bond and transfer the silyl
been exploited in catalytic applications by Murai and Sonoda,
who established the catalytic generation of α-siloxyaldehydes
from aldehydes, and silylated 1,n-diols from cyclic ethers, by
the application of an in situ generated silylcobalt catalyst in the
presence of CO and excess hydrosilane (Scheme 1c).22–24 The
mechanisms proposed by Gladysz, Murai, and Sonoda, all
involve the nucleophilic attack of oxygen at the electrophilic
silyl ligand. The resulting silyloxocarbenium(silyloxonium)
electrophiles and metal nucleophiles recombine to cleave the
C−O bond and forge a new M−C bond.
These examples illustrate the ability of silylmetal complexes
to cleave C−O bonds, particularly those that are unactivated,
aliphatic, unstrained, and thus typically described as inert. Their
Scheme 1. Examples of C−O bond cleavage by silylmetal
complexes.
+
group as the Lewis acidic silylium ion (SiR3 ) to the Lewis
base.2,3,9
In the early 1970’s, MacDiarmid isolated the first reported
silylphosphonium salts with (CO)4Co− counteranions by adding
phosphines to (CO)4CoSiMe3 (Scheme 1a).10,11 Their seminal
work demonstrated the ability of certain silylmetal complexes
MacDiarmid
PMe3
Me3Si PMe3
Co(CO)4
(a)
(b)
(OC)4Co SiMe3
CH2Cl2
Gladysz
+
to behave as both sources of SiR3 and low-valent transition-
O
OSiMe3
Ph
(OC)5Mn SiMe3
(OC)5Mn
metal nucleophiles. Related studies by Corriu on the transfer of
SiR3+ to alcoholic and hydridic nucleophiles from neutral Mn−
and Co−SiR3 complexes bearing chiral silyl ligands,
neat, 5 °C
2 wk
Ph
O
H
+
(OC)5Mn SiMe3
+
(OC)5Mn
OSiMe3
demonstrated that SiR3 transfer to the nucleophile occurs
CD3CN, 2 °C
predominantly with inversion (i.e. SN2) at the stereogenic
silicon center.9,12–16 With the aim of generating M−C bonds
from C−O bonds, Gladysz exploited the ability of silylmetal
Murai & Sonoda
O
O
4% Co2(CO)8/PPh3
C6H6, 100 °C
C6H13
+ HSiEt2Me + CO
H
+
C6H13
H
complexes to be sources of SiR3 and metal nucleophiles to
OSiEt2Me
excess
(c)
oxidatively add to C−O bonds.17–21 They demonstrated that
metal−(α-silyloxy)alkyl and metal−alkyl compounds can be
generated from the reaction of carbonyl compounds and dialkyl
4% Co2(CO)8
n-hexane, 25 °C
O
OSiMeEt2
+ HSiEt Me +
EtMe2SiO
CO
2
excess
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ability to cleave C−O bonds is an important property as the
By 31P{1H} NMR spectroscopy, solutions of 1 and 2
oxidative addition of “inert” C(sp3)−O bonds by transition-
metals remains a challenge.25–30 The ability of silylmetal
complexes to simultaneously generate metal nucleophiles and
presented one major singlet for the pair of chemically
equivalent phosphorous atoms (34.8 and 32.7 ppm,
respectively). No clear coupling between the phosphorous
atoms and the protons of the silyl moiety of 1 was discernable
in the one-dimensional 1H NMR spectra. The P,H-HMBC
experiment, however, revealed weak 4JPH coupling between the
−SiMe2 and −SiCH2CH3 protons and the phosphorous atoms
(Figures 1 & 2). More obvious was JPH coupling between the
1
2
3
4
5
6
7
8
+
SiR3 , the latter being well established as electrophiles for C−X
(X = O, N) bond activation,31–34 demonstrate that silylmetal
complexes can serve as powerful but underutilized reagents for
organometallic synthesis.
The heterolytic cleavage of a M–Si bond to produce SiR3+ is
defined by Djukic as a complex’s intrinsic silylicity.35 Djukic
computed the silylicity of a number of complexes, and as
expected, this parameter depends on the nature of the
supporting ligands, metal, and oxidation state. In cases where
M–Si complexes exhibit silylicity, the silyl ligand is best
described as cationic, Lewis acidic, and of the Z-type.36,37
Conceptually related to the notion of a heterolyzed silylmetal
complex are Coates’ bimetallic catalysts for the carbonylation
1
Si−H proton and the phosphorous atoms of 2 in the H NMR
9
spectrum (tp, J = 16.2, 2.3 Hz); the triplet from coupling to the
ligand phosphorous atoms and the pentet from coupling to the
adjacent methylene protons of the ethyl substituents. The Si−H
proton of 2 also showed coupling to 29Si, discernible as satellites
(1JHSi = 190 Hz). The low natural abundance and low detection
sensitivity of 29Si hampered its direct detection, however, 29Si
chemical shifts could be obtained indirectly at even low
concentrations (~0.04 M) via the Si,H-HMBC experiment. The
extracted 29Si chemical shifts of 1 and 2 are 73.2 and 58.0 ppm,
respectively. In the 13C{1H} spectra, both 1 and 2 showed
coupling between the −SiMe2 and−SiCH2CH3 carbon atoms and
two chemically equivalent phosphorous atoms, splitting the
resonances into triplets. Finally, the methyne carbon atoms of
PCy3 for both 1 and 2 presented as 1:2:1 virtual triplets, which
is characteristic of mutually trans and chemically equivalent
phosphorus atoms.49–51
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of epoxides (e.g. [(salen)Al(THF)2]+[Co(CO)4]−).
In this
ion pair, the Lewis acidic aluminum cation activates the ethereal
oxygen atom for attack by the cobalt nucleophile/counteranion,
generating the Co–C intermediate.38,39
Literature cases of neutral M–Si complexes able to transfer a
+
metal bound SiR3
to exogenous nucleophiles are
predominantly limited to complexes bearing CO ligands. 9,12,21–
24,13–20
Metal−silyl complexes which exhibit silylicity but lack
π-acidic CO ligands are of interest as this obviates the
possibility of carbonyl insertion into any M−C bonds that may
form, while also enhancing the nucleophilicity of the metal
nucleofuge.9 The reactivity of the few cases of M–Si cations
(with the exception of metal silane adducts (M–H–Si), is mostly
limited to the insertion of unsaturated bonds into the M−Si
bond, silylative decyanation, or ligand displacement
reactions.2,3,40–47
Taken together, the NMR data for 1 and 2 indicated that the
silyl ligand is bound to Pd and is cis to two chemically
equivalent and mutually trans phosphine ligands. Along with
the lack of an obvious fourth ligand, the data suggested that 1
and 2 are 3-coordinate, formally 14 electron, and T-shaped with
an open coordination site trans to the silyl ligand. Several
examples of T-shaped, coordinatively unsaturated, group 10
cationic complexes with bulky phosphine ligands are known.
Uttinger showed (PCy3)2Pt(BCat)+ (BCat = catecholboryl) to be
T-shaped by X-ray crystallography.52 Low-temperature 1H and
31P{1H} NMR spectra additionally revealed features
characteristic of agostic interactions between a cyclohexyl C−H
bond
In this work, we report the synthesis and characterization of
two types of silylpalladium cations lacking CO ligands,
+
demonstrate that they are sources of SiR3 and nucleophilic
LnPd(0), detail their reactions with phosphino and ethereal
Lewis bases, and demonstrate that this new class of silylmetal
complex can enable the oxidative addition of typically inert
C(sp3)−O bonds to generate alkylpalladium cations.
RESULTS AND DISCUSSION
+
Preparation and Characterization of (PCy3)2Pd−SiR3 .
The general synthesis of (PCy3)2Pd−SiR3+ is presented in Eq. 1
and proceeds by the addition of a stabilized silylium source to
Pd(0). Smooth reactivity required pre-mixing the trityl
(C6F5)4B− salt, Ph2O, and hydrosilane to generate a solution
+
stable Ph2O−SiR3 salt.48 The mixture of o-difluorobenzene
(DFB) and toluene provided a good combination of solubility
and reactivity, and silylium stabilization by diphenyl ether was
essential for clean SiR3+ transfer to Pd(PCy3)2. Although 1 and
2 were observable in solution for several hours at room
temperature, both decomposed in solution and in the solid state,
hampering the crystallographic characterization of their
structures. Multinuclear NMR spectroscopy was therefore a key
characterization tool.
Figure 1: Selected spectroscopic data for 1 in 1:5 DFB:Toluene-d8.
B(C6F5)4
Cy3P
Pd SiR3
SiR3
O
1 eq Pd(PCy3)2
(1)
Ph
Ph 1:5 DFB:Tol-d8
Cy3P
-78 °C to RT
-Ph3CH, -Ph2O
in situ
1
2
SiR3 = SiMe2Et,
= SiHEt2
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Figure 2: Selected spectroscopic data for 2 in 1:5 DFB:Toluene-d8.
BArf4
1) 1.05 eq NaBArf4
1
2
3
4
5
6
7
8
PPh2
and the metal center. The agostic C–H breaks the symmetry and
generates a pair of strongly coupled doublets in the 31P{1H}
NMR spectra (2JPP = 258 Hz), consistent with the phosphines
being mutually trans. Interestingly, Uttinger noted a lack of
evidence for agostic interactions in the solid state or in solution
with complexes bearing boryl ligands more electron donating
than BCat, despite also adopting T-shaped geometries. They
attributed this difference to the greater trans influence of the
more donating boryl ligands.52,53 We have also shown that the
thermally unstable (PCy3)2PdMe+ complex displayed NMR
spectra indicative of a T-shaped geometry with stabilizing
agostic interactions trans to the methyl ligand (vide infra).
Although we propose that 1 and 2 are isostructural to
(PCy3)2PdMe+, we have observed no spectroscopic signatures
for agostic interactions in the former complexes. This
observation may be due to the higher trans influence of silyl
over methyl, in analogy to the (PCy3)2Pt−boryl+ systems
described by Uttinger.
2) 2.0 - 10 eq HSiR3
(3)
(Xantphos)PdMeCl
O
Pd SiR3
PPh2
CD2Cl2, RT
-NaCl, -MeSiR3, -H2
5
6
SiR3 = SiMe2Et, (57%)
= SiHEt2
(55%)
discussing them further.55,56 The xanthene carbon atoms directly
bound to phosphorous and the ipso carbon atoms of the −PPh2
moieties each present as 1:2:1 virtual triplets and suggest a
meridionally coordinated Xantphos ligand, as shown in the
crystallographically characterized structure of (Xantphos)Pd(4-
CN-C6H5)+ by van Leeuwen.57,58
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Addition of hydrosilane to 4 resulted in its conversion to
+
(Xantphos)Pd−SiR3 . The ambidentate nature of Xantphos was
hypothesized to facilitate the generation of 5 and 6 from
(Xantphos)PdMe+, as
a
putative κ3-P,O,P to κ2-P,P
isomerization of the ligand would open a coordination site cis
to the methyl ligand.41,43,44,59,60 While two equivalents of silane
were necessary to fully convert to the silylpalladium cation,
excess hydrosilane speeds the formation of 5 and 6. Both
products are stable in solution for days but decompose slowly
in the solid state. Once again, spectroscopic methods were
crucial to confirming their structure (Figures 3 & 4).
Particularly informative was the measurable coupling between
the proton and carbon atoms of the silyl ligand with the
phosphorous atoms, as this coupling confirms a P–Pd−Si array.
Additionally, splitting of the ligand P–C atoms into 1:2:1 virtual
triplets confirmed the mutually trans nature of P coordination.
The data thus suggests a square planar geometry with the
Xantphos ligand adopting the pincer form.
The spectroscopic data for 1 and 2 combined with the
reactivity described in the following sections thus indicate that
T-shaped, 14 electron, (PCy3)2Pd−SiR3+ cations can be formed
by oxidative addition of a suitably stabilized silylium ion to
Pd(PCy3)2.
Coordination of MeCN-d3 to 1. To probe the suggestion that
1 was T-shaped with a vacant site trans to SiR3, we investigated
whether added nitrile would coordinate to this site. As expected,
adding one equivalent of acetonitrile-d3 to 1 rapidly generated
a new complex 3 with a 31P{1H} NMR chemical shift of 37.3
2
ppm (Eq. 2). The H NMR spectrum for this new complex
indicates a CD3CN downfield chemical shift change of 1.48
ppm relative to uncomplex CD3CN, characteristic of nitrile
binding to an electrophilic center.43,44 By 13C{1H} NMR
spectroscopy, the carbon resonances for the −SiMe2 and
−SiCH2CH3 carbon atoms have lost their coupling to
phosphorous. Weak cross peaks between the −SiMe2 protons
and the phosphorous resonances in the P,H-HMBC spectrum
indicated that the complex still contained a bound silylium. The
extracted 29Si NMR resonance of 3 was shifted 60 ppm upfield
of 1 to 13.2 ppm. The attenuation of 4JHSi coupling is consistent
with nitrile coordination, as Kubas has shown that for trans-
L(iPr3P)2PtH+ complexes, the donating ability of ligand L
influences JHPt, with stronger binding ligands reducing this
coupling.54 All evidence thus indicates that the nitrile
coordinates trans to silicon.
In situ spectroscopic analysis of the formation of 5 and 6
revealed that the extra equivalent of silane forms Me3SiEt and
HSiEt2Me, respectively (see Supporting Information Figure
S1). Dihydrogen was also spectroscopically observed and
visibly bubbles from the solution. The organosilane and H2 by-
products suggested that Pd–Si formation involves a putative
Pd–H intermediate. We hypothesize that oxidative addition of
the Si−H bond to 4, followed by Si–C reductive elimination
forms the observed organosilanes and (Xantphos)PdH+ (7).
While the reaction is discussed as proceeding via oxidative
Cy3P
Cy3P
Pd SiMe2Et
(2)
D3CCN Pd SiMe2Et
Cy3P
+
CD3CN
1:5 DFB:Tol-d8
Cy3P
1
3
Preparation
and
Characterization
of
+
(Xantphos)Pd−SiR3 . Optimal for the synthesis of
(Xantphos)Pd−SiR3 was the hydrosilylative approach
+
presented in Eq. 3. Combining NaBArf4 and (Xantphos)PdMeCl
generated (Xantphos)PdMe+ (4) in situ, as evidenced by a
singlet in the 31P{1H} NMR spectrum (19.1 ppm). Since BArf4–
and (C6F5)4B− are known to be inert counteranions in reactions
involving silylium when Lewis bases are present, we will not
be
Figure 3: Selected spectroscopic data for 5 in CD2Cl2.
3
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be able to cleave the methyl C−O bond of
hexyl(methyl)(silyl)oxonium (9) by oxidative addition to
furnish an alkoxysilane and a Pd−Me cation.
1
2
3
4
5
In a glovebox, a solution of Pd(PCy3)2 was added to a solution
of in situ generated 9 frozen in a liquid nitrogen cooled cold
well. The reaction was mixed and warmed to -78 °C before
6
Scheme 3: Stoichiometric silyloxonium methyl C−O bond
7
cleavage by Pd(PCy3)2
8
9
SiMe2Et
Pd(PCy3)2
MePd(PCy3)2
+
OSiMe2Et
O
-78 °C
CH3
4
4
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57
58
59
60
10
fast
9
> -5 °C
[Pd]
Figure 4: Selected spectroscopic data for 6 in CD2Cl2.
MePCy3
addition of Si–H, a σ-bond metathesis-type transformation can’t
be ruled out.4 The reactive hydride 7, which was not
spectroscopically observed, is surmised to rapidly react with
hydrosilane to evolve hydrogen gas and produce the
silylpalladium cation (Scheme 2). Schubert has shown that a
similar mechanism is operative in the generation of neutral
silylplatinum complexes from the reaction of neutral
platinum−methyl complexes with hydrosilanes.60 This picture
is additionally supported by experiments showing that in situ
generated 7 rapidly reacts with hydrosilane to generate
(Xantphos)Pd−SiR3+ (vide infra).
being placed into a NMR spectrometer probe pre-cooled to -80
°C. Low-temperature NMR spectral data of the reaction mixture
revealed that even at -70 °C, complete and rapid demethylation
of 9 occurred to form silylether and a Pd−Me cation (10)
(Scheme 3). Diagnostic of methyl C−O bond cleavage was the
appearance of the silylether R3SiOCH2− resonance and the
disappearance of the silyloxonium methyl resonance. Up to -5
°C, both the 1H and 31P{1H} NMR spectra indicated that
MePCy3+ was not formed, clearly demonstrating that the methyl
C−O bond of 9 was not cleaved by dissociated phosphine. Only
above -5 °C, was the gradual formation of MePCy3+ observed.
The complete demethylation of 9 at low temperatures thus
indicated that oxidative addition of the silyloxonium methyl
C−O bond to Pd(PCy3)2 was rapid and kinetically formed 10.
The conclusion is therefore that Pd(PCy3)2 is a suitable
nucleophile for silyloxonium electrophiles and that the reaction
between 9 and Pd(PCy3)2 is fast.
Despite not being stable at room temperature (PCy3)2Pd−Me+
(10) could be characterized by low-temperature NMR
spectroscopy (Figure 5; see Supporting Information Figures S4
and S5). At -60 °C, three signals were observed in the 31P{1H}
NMR spectrum; two coupled doublets at 29.0 and 47.0 ppm
(2JPP = 318 Hz), and a singlet at 24.0 ppm. Warming to 0 ° C
resulted in signal broadening and coalescence into a broad
singlet at 30.2 ppm. The signals in the variable temperature
31P{1H} NMR
+
Scheme 2: Proposed mechanism for (Xantphos)Pd−SiR3
generation involving a palladium−hydride intermediate.
PPh2
PPh2
PPh2
HSiR3
HSiR3
O
Pd SiR3
PPh2
O
Pd Me
O
Pd H
-H2
fast
-MeSiR3
PPh2
PPh2
7
4
proposed
intermediate
SiMe2Et; Gcalc. = -25.8 kcal/mol*
SiHEt2; Gcalc. = -27.1 kcal/mol*
*calculated free energies; see Supporting Information
Silylium Transfer From 1 to PCy3. Evidence that 1 is a
+
source of SiR3 was obtained from its reaction with PCy3, in
analogy to MacDiarmid’s silylphosphonium synthesis with
(CO)4CoSiMe3.10,11 Addition of PCy3 to in situ generated 1 at
room temperature resulted in the formation of the
silylphosphonium salt 8 and the Pd(PCy3)2 nucleofuge (Eq. 4;
see Supporting Information Figure S2). The assignment of the
signals to 8 were confirmed by its independent synthesis (see
Supporting Information). The quantitative transfer of SiMe2Et+
PCy2
Cy3P
Pd Me
H
Pd CH3
< -5 °C
H
Cy3P
10a
PCy3
10b
+
from 1 to PCy3 confirmed that 1 is a source of SiR3 and a
neutral and zero-valent Pd complex. The analogous reaction
between PCy3 and 3 also provided 8 and Pd(PCy3)2, revealing
that the nitrile adduct 3 is also a source of SiR3+ (see Supporting
Information Figure S3).
Cy3P
EtMe2Si PCy3
Pd SiMe2Et + PCy3
Pd(PCy3)2
(4)
+
1:5 DFB:Tol-d8
Cy3P
8
1
Stoichiometric Silyloxonium C−O Cleavage by
Pd(PCy3)2. To confirm that Pd(PCy3)2, the leaving group of
silylium transfer from 1 to a Lewis base, is a suitable
nucleophile for silylium-activated oxygenates, we next looked
to react a stable dialkylsilyloxonium ion (R2O−SiR3 ) with
Pd(PCy3)2. We surmised that the nucleophilic Pd(PCy3)2 would
+
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+
eventually leads to MePCy3 . Although the slow decomposition
of 10 complicates a kinetic analysis, one can glean informative
data from the timescale of the ether demethylation. Since
demethylation of 9 is rapid (Scheme 3), the observed rate of
hexyl methyl ether demethylation (~ 4 h) likely reflects the
kinetics of silylium transfer from 1 to ether.
1
2
3
4
5
6
7
Scheme 4: Stoichiometric demethylation of hexyl methyl
ether by 1.
8
9
Pd(PCy3)2
MeO
Cy3P
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
SiR3
O
4
OSiR3
+
MePd(PCy3)2
Pd SiR3
Cy3P
4
10
Me
76%*
(77%* conversion)
4
9
1
[Pd]
proposed
intermediates
SiR3 = SiMe2Et
*NMR yields
MePCy3
78%*
Stoichiometric Ether Demethylation by 3. Since CD3CN
coordinates to 1 to generate the nitrile ligated silylpalladium
cation 3, we sought to determine if 3 can similarly demethylate
hexyl methyl ether. Addition of the hexyl methyl ether to in situ
generated 3 gave partial demethylation (60% NMR yield) over
18 h at room temperature (Eq. 5; see Supporting Information
Figure S7). Under these conditions the stable palladium−methyl
cation (11) forms. Repeating this reaction with excess nitrile
slowed the demethylation and suggested that the mechanism
1) n eq CD3CN
MeO
Cy3P
Cy3P
D3CCN Pd SiR3
Cy3P
2)
+
OSiR3
D3CCN Pd Me (5)
Cy3P
4
1
Figure 5. Low-temperature 31P{1H} and H NMR spectra of the
4
RT, 18 h
reaction depicted in Scheme 3, highlighting the diagnostic
phosphorous and agostic proton resonances of in situ generated 10.
Spectrometer frequency: 202 MHz (31P{1H}), 500 MHz (1H).
Solvent: 1:5 DFB:Toluene-d8.
3
11
in situ
n = 0
60%*
11%*
trace*
66%*
17%*
8%*
SiR3 = SiMe2Et
*NMR yields
1
4
spectra were consistent with two equilibrating species, only one
of which had equivalent phosphine ligands.50,53 At temperatures
below -40 °C, the singlet at 24.0 ppm was assigned to the
symmetric Pd−Me complex 10a, and the pair of coupled
doublets to the asymmetric Pd−Me complex 10b. The large 2JPP
coupling constant for 10b indicated that the phosphine ligands
likely involves dissociation of nitrile from 3 (to form 1) prior to
silylium transfer.
Silylium Transfer from 5 to PCy3. Reacting 5 with PCy3
yields silylphosphonium 8 in analogy to Eq. 4 and demonstrates
that 5 is also a viable source of SiR3 . In this case, full
+
conversion required two equivalents of PCy3 to yield 8 and the
stable zero-valent palladium complex 12 (Eq. 6; see Supporting
Information Figure S8). The expected nucleofuge of this
reaction, (Xantphos)Pd0, sequesters an equivalent of phosphine
to form 12, which presented NMR spectra identical to
authentically prepared material (see Supporting Information).
The 31P{1H} NMR spectrum for 12 displayed a coupled triplet
and doublet at 35.0 and 1.8 ppm (2JPP = 107 Hz) in a 1:2 ratio,
consistent with the expected trigonal geometry.50
remain mutually trans,50
a scenario accommodated by
cyclohexyl C−H agostic interactions that rendered the
phosphorous nuclei chemically inequivalent.52,61 At
temperatures below -40 °C, a broad signal was observed at -
1.13 ppm in the 1H NMR spectrum, consistent with the
proposed agostic C−H.52,61 As the temperature increased, the
rate of equilibration between 10a and 10b also increased,
leading to signal broadening in the 31P{1H} NMR spectra and
the eventual coalescence into a peak at the weighted average of
the three resonances observed below -40 °C. Above -5 °C,
MePCy3+ begins to form and is observed as a sharp singlet at
31.8 ppm in the 31P{1H} NMR spectrum.
2 eq
O
O
PCy3
EtMe2Si PCy3
+
(6)
Ph2P
PPh2
Ph2P
Pd
PPh2
Stoichiometric Ether Demethylation by 1. Having
demonstrated that Pd(PCy3)2 is a suitable nucleophile for a
model silyloxonium salt, we tested whether 1 and hexyl methyl
ether could transiently generate 9 and Pd(PCy3)2 for subsequent
methyl C−O bond cleavage. Gratifyingly, addition of hexyl
methyl ether to in situ generated 1 formed silylether and
methylphosphonium over 4 hours (Scheme 4; see Supporting
Information Figure S6). During the room temperature
demethylation reaction, 10 could be observed by 31P{1H} NMR
spectroscopy, confirming the viability of silylium transfer from
1 to hexyl methyl ether (the reverse of Scheme 3) to form the
reactive pair highlighted in Scheme 4. The instability of 10
Pd
9
SiMe2Et
PCy3
5
12
Stoichiometric Benzyl Ether C−O Bond Cleavage by 5.
Unlike silylpalladium complex 1 which reacted to cleave the
methyl C(sp3)−O bond of hexyl methyl ether (Scheme 4),
complex 5 didn’t react with this ether, even at elevated
temperatures. The secondary benzylic C−O bond of (1-
methoxyethyl)benzene, however, was more prone to oxidative
addition by 5 and yielded the palladium−(η3-phenethyl) cation
13 (Eq. 7). Interestingly, no evidence for methyl C−O bond
cleavage was observed, pointing to a higher reactivity of the
5
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benzylic C−O bond towards oxidative addition. Complex 13,
Page 6 of 10
MeO
1
2
3
4
5
6
7
8
which is surprisingly stable to air, moisture, and silica gel, was
isolated as needle-like crystals. It’s 31P{1H} NMR spectrum
showed a single broad resonance at 9.0 ppm and the benzylic
and methyl
H
O
(8)
+
Ph2P
Pd
PPh2
H
PdL2
-MeOSiMe2Et
SiMe2Et
5
14
36% yield
OMe
L2 = Xantphos
O
Pd
(7)
H
Me
Ph2P
PPh2
-Me2EtSiOMe
2 days
PdL2
SiMe2Et
9
5
13
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
62% yield
L2 = Xantphos
Figure 7. ORTEP plot of 14 at 50% probability level (hydrogen
atoms, pentane, and BArf4 counteranion omitted for clarity; ligand
phenyl groups represented by the ipso carbon atom for clarity).
Selected bond lengths and angles: Pd1−P1 = 2.331(1) Å, Pd1−P2 =
2.412(1) Å, Pd1−O1 = 3.383(3) Å, Pd1−C40 = 2.114(6) Å,
Pd1−C41 = 2.239(5) Å, Pd1−C42 = 2.448(6) Å, Pd1−C46 =
3.150(4) Å, P1−Pd1−P2 = 108.82(4)°.
Figure 6. ORTEP plot of 13 at 30% probability level (hydrogen
atoms, CH2Cl2, and BArf4 counteranion, omitted for clarity; ligand
phenyl groups represented by the ipso carbon atoms for clarity).
The view along the benzallyl−palladium interaction shown in the
inset. Selected bond lengths and angles: Pd1−P1 = 2.315(1) Å,
Pd1−P2 = 2.409(1) Å, Pd1−O1 = 3.490(4) Å, Pd1−C41 = 2.109(8)
Å, Pd1−C42 = 2.228(9) Å, Pd1−C43 = 2.81(1) Å, Pd1−C47 =
2.85(1) Å, P1−Pd1−P2 = 107.76(5)°.
The primary benzylic C−O bond of isochroman is also
cleaved by 5 with mild heating (Eq. 9). The organometallic
product isolated from this reaction was characterized by NMR
spectroscopy and determined to be the Pd−(π-benzyl) complex
(15) by its spectroscopic similarities to 14 (see Supporting
Information).
OSiMe2Et
proton and carbon atoms of the phenethyl ligand showed
discernable coupling to two chemically equivalent phosphorous
O
O
(9)
Ph2P
Pd
PPh2
atoms in both the H and 13C{1H} NMR spectra. The ipso
1
70 °C
SiMe2Et
PdL2
carbon atom of the phenethyl ligand is split into a triplet in the
13C{1H} NMR spectrum due to JCP coupling, consistent with
2
5
15
75% yield
L2 = Xantphos
metal-ring association. The carbon atoms of the xanthene
backbone directly attached to the phosphorous atoms are split
into doublets, indicating that unlike complexes 4-6, the
phosphorous atoms of 13 are not mutually trans. The crystal
structure of 13 confirmed the η3 binding mode and the metrical
parameters were similar to the related [(Xantphos)Pd(η3-
benzyl)][OTf] complex reported by Hartwig (Figure 6; see
Supporting Information).62
Cleavage of the primary C(sp3)−O bond of (1-
methoxymethyl)naphthalene also occurs on addition to 5 at
room temperature. The resulting Pd−(π-naphthalen-1-ylmethyl)
complex 14 was also stable to air, moisture, and silica gel (Eq.
8). While the 31P{1H} NMR spectrum of 14 exhibits significant
broadening due a fluxional process, acquiring spectra at 40 °C
sharpens the resonances into a singlet. Single crystals of 14
enabled X-ray crystallography to confirm that this complex
adopts a geometry analogous to that of 13 (Figure 7; see
Supporting Information).
Evidence for Palladium−Hydride 7. Addition of excess
ethyldimethylsilane to isolated 13 resulted in the slow
generation of 5 and styrene (Scheme 5; see Supporting
Information Figure S9). This is conveniently accounted for by
a mechanism where 13 undergoes net β-hydride elimination to
produce styrene and palladium−hydride (7). The metal−hydride
then subsequently reacts with hydrosilane to generate 5 and
hydrogen gas. It has been established that palladium−(η3-
benzyl) type complexes can undergo net β-hydride elimination
from
a
σ-benzylic intermediate.63 The observation of
ethylbenzene by NMR spectroscopy suggested that hydrosilane
may also react directly with 13, however, the transfer of hydride
from 7 to 13 to produce ethylbenzene by can’t be ruled out.
Addition of excess hydrosilane to (Xantphos)Pd(η3-
(CHOSiMe2Et)C6H5)+ (16), a Pd−(π-benzyl) complex lacking
β-hydrogens, provided only trace amounts of 5 and silyl
protected benzyl alcohol over three days (see Supporting
Information Figure S10). The lower reactivity of 16 towards
hydrosilane compared to 13 supports the notion that β-hydride
elimination is the initiating step in Scheme 5.
Scheme 5. Proposed mechanism for the generation of 5 from
the reaction of hydrosilane and 13.
6
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Journal of the American Chemical Society
+
(Xantphos)Pd−SiR3 , and the silyloxonium cations synthesized
and generated in this study (Table 1; see Supporting
Information Table S1). The optimized geometries agree with
those proposed by experimental spectroscopic observations.
Additionally, the method used by Djukic to compute the
“relative intrinsic silylicity” (Π) of Lewis base-silylium
adducts, was used to compute Π for several of the compounds
generated in this study.35 Silylicities >1 reflect a silicon that is
more electrophilic than it is in R3SiOTf; all compounds in Table
1 have silylicities >1. The relative ordering of Π also agrees
with experimentally observed silylicities. First, the silylium ion
in Ph2O−SiMe2Et+ stoichiometrically transfers to hexyl methyl
ether (by NMR)), and second, the Π of Ph2O−SiMe2Et+ is
higher than 1, befitting its role as stoichiometric silylium source
1
2
3
4
5
6
7
8
HSiR3
-H2
-styrene
+styrene
O
O
Ph2P
Pd
PPh2
Ph2P
Pd
PPh2
Me
PdL2
H
SiMe2Et
13
7
5
SiR3 = SiMe2Et
L2 = Xanthpos
To buttress the mechanism presented in Scheme 5, complex
17, a fluorinated congener of 13, was synthesized and treated
with excess styrene. After 1 day, all of 17 was converted to 13
and p-fluorostyrene (Eq. 10; see Supporting Information Figure
S11). This exchange of phenethyl ligands supported the
intermediacy of 7 and demonstrated that it can be intercepted
by styrene.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
for (PCy3)2Pd (eq 1). Although
a direct experimental
comparisons of the silylicities of 1 and 5 were not possible, we
note that 5 does not demethylate hexyl methyl ether, while 1 is
capable of this reactivity.
excess
F
Me
(10)
+
Table 1. DFT calculated silylicities of Lewis base silylium
adducts.a
Me
PdL2
PdL2
F
17
13
Compound
SiR3
Π
L2 = Xantphos
SiR3
Both the mechanism for the synthesis of 5 and 6, as well as
the reactivity of 13 with hydrosilane and 17 with styrene,
implied the existence of reactive hydride 7. We surmised that
the dehydrogenative silylation of an alcohol would also involve
the intermediacy of 7, which could be generated by
deprotonation of the silyloxonium ion by the LnPd(0) that would
form from silylium transfer from 5 to an alcohol. To test this
hypothesis, we carried out the catalytic dehydrogenative
silylation of 1-hexanol with excess hydrosilane at a 1 mol%
loading of 5 (Eq. 11; see Supporting Information Figure S12).
Addition of the alcohol to a mixture of 5 and hydrosilane
resulted in immediate and vigorous gas evolution. NMR
spectroscopy confirmed the generation of the expected
silylether and H2, however, the only phosphine containing
species observable by 31P{1H} NMR spectroscopy was 5.
SiMe2Et
2.68
O
Ph
Ph
SiR3
O
SiMe2Et (9)
2.56
CH3
4
Cy3P
SiMe2Et (1)
SiHEt2 (2)
1.42
1.36
Pd SiR3
Cy3P
SiMe2Et (5)
SiHEt2 (6)
1.29
1.28
O
Ph2P
Pd
PPh2
SiR3
a
Following Djukic’s procedure, silylicities were calculated
according to the equation
Π
=
+
+
[ΔEint(TfO−SiR3 )]/[ΔEint(LB−SiR3 )], where ΔEint is the basis set
superposition error corrected interaction energy and LB is the
Lewis base.
5
1 mol%
+
HSiMe2Et
excess
OSiMe2Et
OH
(11)
4
4
-H2
The palladium−hydride 7 could be directly generated by
stoichiometrically reacting 5 with 1-hexanol, resulting in a new
broad phosphorous resonance at 25.3 ppm (Eq. 12; see
Supporting Information Figure S13). Diagnostic for the
generation of 7 was the new upfield resonance at -14.40 ppm in
the H NMR spectrum, which we attribute to the hydridic
proton. Addition of excess hydrosilane to the reaction mixture
containing 7 resulted in its rapid conversion to 5, demonstrating
that 7 rapidly reacts with hydrosilane to generate
silylpalladium cation. Similarly, adding styrene to in situ
generated 7 led to the rapid formation of 13, proving that
styrene readily inserts into the Pd−H bond (see Supporting
Information Figure S14). The instability of (Xantphos)PdH+,
determined by its decomposition in solution over several hours,
prevented its isolation and independent characterization.
CONCLUSIONS
Herein, we present the synthesis of two different types of
+
silylpalladium
cations,
(PCy3)2Pd−SiR3
and
+
(Xantphos)Pd−SiR3 , both of which are sources of silylium
cations and LnPd(0) nucleophiles. Silylium transfer to
phosphines and aliphatic ethereal oxygen atoms occurs to
generate stable silylphosphonium ions or reactive silyloxonium
ions, respectively. In the case of silyloxonium generation,
C(sp3)−O bonds are activated towards oxidative addition by the
concomitantly generated LnPd(0) and thereby provides
synthetically versatile palladium−alkyl cations. When the
resulting Xantphos-based palladium−alkyl cations bear β-
hydrogens, reversible β-hydride elimination to a cationic
1
a
palladium−hydride
intermediate
was
noted.
This
palladium−hydride could be independently generated and it
rapidly reacts with excess hydrosilane to regenerate a
silylpalladium complex or insert styrene. In summary, cationic
LnPd−SiR3 complexes can be heterolytically cleaved to
generate a synergistic combination of silylium ions (for O
activation) and nucleophilic LnPd(0) complexes. Together,
these components enable the oxidative addition/activation of
otherwise difficult to react C(sp3)–O bonds.
O
O
(12)
OH
+
4
Ph2P
Pd
PPh2
Ph2P
Pd
PPh2
-ROSi
H
SiMe2Et
5
7
DFT Calculated Geometries and Silylicities (Π). Density
functional theory (DFT) was used to obtain gas-phase
ASSOCIATED CONTENT
+
geometry-optimized
structures
of
(PCy3)2Pd−SiR3 ,
7
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Page 8 of 10
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The Supporting Information is available free of charge on the ACS
Publications website.
Experimental details, computational details, and NMR
spectra used for characterization. (PDF)
Crystallographic data for 13 (CIF)
Crystallographic data for 14 (CIF)
AUTHOR INFORMATION
Corresponding Author
*mgagne@unc.edu
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Notes
The authors declare no competing financial interest.
Δ Author to whom queries about the X-ray structural work should
be directed: neyen@live.unc.edu.
ACKNOWLEDGMENT
This research was supported by the U.S. Army Research Office
(ARO) (W911NF-15-2-0119). The mass spectrometer used in these
studies was purchased with the support of the National Science
Foundation under Grant No. CHE-1726291.
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[LnPdIISiR3]
SiR3 +
LnPd0
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OMe
+
LnPdSiR3
:
SiR3+ electrophile
LnPd0 nucleophile
OMe
PR'3
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LnPdIIMe
LnPd0
+
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H
H
[R3SiPR'3]
PdL2
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