Combining the reactivity properties of pcy3 and P t Bu3 into a single ligand, P(i Pr)(t Bu)2. reaction via mono- or bisphosphine palladium(0) centers and palladium(i) dimer formation

Article abstract of DOI:10.1021/om5009605

Trialkylphosphine ligands are ubiquitous in catalysis. Via modulation of the steric bulk of these ligands, two central aspects that dictate reactivity and selectivity in catalysis can be controlled: i.e., the coordination sphere and favored oxidation stat

Full text of DOI:10.1021/om5009605

Article
pubs.acs.org/Organometallics
Combining the Reactivity Properties of PCy3 and PtBu3 into a Single
Ligand, P(iPr)(tBu)2. Reaction via Mono- or Bisphosphine
Palladium(0) Centers and Palladium(I) Dimer Formation
Fabien Proutiere,†,‡ Eirik Lyngvi,†,‡ Marialuisa Aufiero,†,‡ Italo A. Sanhueza,†,‡
and Franziska Schoenebeck*,†
†Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany
‡Laboratory for Organic Chemistry, ETH Zurich, Vladimir-Prelog-Weg 3, 8093 Zurich, Switzerland
̈
̈
S
* Supporting Information
ABSTRACT: Trialkylphosphine ligands are ubiquitous in catalysis. Via
modulation of the steric bulk of these ligands, two central aspects that dictate
reactivity and selectivity in catalysis can be controlled: i.e., the coordination
sphere and favored oxidation state of the reactive metal center. Within this
class of ligands, tricyclohexylphosphine (PCy3) and tri-tert-butylphosphine
(PtBu3) are most widely used in catalysis. While the smaller PCy3 favors
reactivity via Pd bisphosphine species with the test substrate 4-chlorophenyl
triflate 1 and does not form dinuclear Pd(I) complexes upon oxidation of
Pd(0), PtBu3 reacts via a monophosphine-ligated Pd complex with 1 and
forms dinuclear Pd(I) complexes on oxidation. We herein report that the
hybrid ligand P(iPr)(tBu)2, characterized by a cone angle that is between those
of PCy3 and PtBu3, features all of these reactivity properties in a single scaffold.
This is exemplified in chemoselectivity studies with test system 1, in which the
site selectivity could be readily modulated. The novel Pd(I) dimer {[P(iPr)(tBu)2]PdI}2 has also been synthesized.
nature of the reactive Pd species.2b In this context, the bulky
ver the past decades, the development of transition-
and widely used trialkylphosphine8 ligand PtBu3 (cone angle9
Ometal-catalyzed transformations has greatly enhanced the
synthetic chemist’s toolbox and revolutionized the way bond-
forming reactions are currently undertaken.1 A key challenge in
the development of organometallic reactivities is the control of
the metal complex to steer toward desired reactivities and
selectivities.1,2 To this end, the ligand sphere encompassing the
metal center is generally fine-tuned. While the modulation of
steric and electronic properties of ligands has undoubtedly had
a tremendous impact on organometallic catalysis, the precise
effects and characteristics of these ligands are not always well
understood.3 Clearly there is no ligand available yet that allows
for applications in a diverse set of transformations as an “all-
round” ligand, and continuous re-evaluation of the potential
utility of a given ligand in novel transformations has to be
undertaken. Accordingly, methodology developments fre-
quently rely on trial and error or screening approaches.4
Numerous organometallic transformations commence with
the oxidative addition step.1 The reactivity of this step is also
dependent on the ligation state of the metal.5 In general, with
increasing steric bulk of the phosphine, fewer ligands will be
coordinated. However, the overall mechanism of oxidative
addition may also depend on the substrate identity.6,7,10b,f,g,39
Given the diverse possibilities of pathways and intermediates,
subtle modifications in the ligand framework may have more or
less impact on the overall reaction speed. The study of site
selectivity may allow indirect deduction of information on the
182°) has been shown to involve a monophosphine metal
complex in oxidative addition to ArCl.5a,10 As a consequence of
its steric bulk, the bisphosphine counterpart Pd(PtBu3)2 is itself
unreactive and functions as a precursor for the monophosphine
Pd, following ligand dissociation.10 In competition experiments,
this allows for selective functionalization of a C−Br11 or C−
Cl12 bond over C−OTf, as shown by the groups of Hayashi,
Brown, and Fu (see Figure 1). Smaller ligands, such as the
ubiquitous tricyclohexylphosphine13 PCy3 (cone angle9 175°),
result in typical reactivities of bisphosphine metal complexes,
preferentially functionalizing the C−OTf bond in 1.10a,12
In addition to the favored ligation state, the nature of the
ligand may also impact the overall stability of the oxidation
state of a metal complex. PtBu3 shows the special characteristics
of stabilizing palladium in the +1 oxidation state,14,15 resulting
in the dinculear PdI complex {LPdIX}2 (with X = Br, I and L =
PtBu3),16 which PCy3 does not.17 It has been shown that these
PdI dimer complexes may have a significant role in catalysis,
causing acceleration or inhibition,18 and they may function as
precatalysts19,20 or be involved directly in transformations,
displaying reactivities distinctly different from that of Pd0.21
Received: September 19, 2014
© XXXX American Chemical Society
A
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Article
the error margin of the calculations,22 this would suggest that
the favored ligation state might be changeable via alteration of
the number of equivalents of ligand. In contrast, the bulkier
analogue PtBu3 shows exclusive selectivity for C−Cl in THF,
also in the presence of excess ligand, which is supported by
computations that suggest a 11 kcal/mol preference for PdL
addition to C−Cl over PdL2 addition to C−OTf at the same
level of theory.28 PCy3, which is smaller in steric bulk, does not
access monoligated Pd and shows reactivity at C−OTf,
consistent with PdL2 as the active species.12,25,10 This suggested
that, depending on the number of equivalents of ligand,
P(iPr)(tBu)2 may feature the reactivity of either PCy3 or PtBu3.
To test for this as well as to explore other characteristic aspects,
such as the potential to stabilize PdI−PdI, we subsequently
undertook experiments.
Figure 1. Distinct effect of ligands on selectivity via ligation state
control and propensity to form PdI−PdI.
Di-tert-butylisopropylphosphine, P(iPr)(tBu)2,29 was pre-
pared according to Iwazaki’s procedure: i.e., di-tert-butylchlor-
ophosphine was reacted with isopropylmagnesium chloride.30,31
Purification was performed in a glovebox to avoid oxidation of
the ligand. With P(iPr)(tBu)2 in hand, the synthesis of the
bisphosphine Pd0 complex Pd[P(iPr)(tBu)2]2 (5) was sub-
sequently undertaken. This was accomplished via subjection of
the ligand to the PdII precursor complex [(tmeda)Pd0(Me)2].
Pd[P(iPr)(tBu)2]2 (5) was successfully obtained in good yield
(63%) and characterized by 1H and 31P{H} NMR spectroscopy
as well as X-ray diffraction. The X-ray crystallographic structure
of 5 is depicted in Figure 3. The P−Pd−P axis is linear, and the
Pd−P bond length was determined to be 2.265 Å. The C−P
bond is 1.820 Å for the iPr group and 1.913 and 1.881 Å for the
two tBu groups.
Moreover, the formation of +1 Pd species can also be a
pathway of catalyst deactivation.19a
While the ligands PtBu3 and PCy3 currently allow for either
reactivity paradigm, i.e. monoligated active species and Pd(I)
dimer for PtBu3 but bisligated Pd(0) reactive species and no
Pd(I) stabilization for PCy3, this report describes how a slight
modification of the steric nature may combine the properties of
these ligands in a single species: i.e., di-tert-butylisopropylphos-
phine, P(iPr)(tBu)2.
Stimulated by our mechanistic studies of the effects of ligands
on the ligation and favored oxidation state of a metal,25,19a,18,23
we have recently begun to conduct additional computational
studies.24 These analyses suggested that replacement of a single
methyl group by hydrogen in PtBu3 may allow for either a Pd−
bisphosphine or a Pd−monophosphine complex to be reactive
in catalysis. The reaction with 4-chlorophenyl triflate 1 (Figure
1) was computationally studied in this context. This substrate
constitutes an ideal mechanistic probe to test for the active
catalytic species, as a monoligated Pd species reacts at C−Cl,
while the bisligated counterpart reacts with C−OTf.25 The
computational prediction for C−Cl versus C−OTf site
selectivity for the reaction of PdL2 (L= P(iPr)(tBu)2) with 1
is illustrated in Figure 2. We applied the M06L/def2-QZVP
method and the solvation model COSMO-RS (for THF),
which have previously been shown to perform adequately in the
study of similar systems.26,27 A slight preference for C−Cl
addition by monoligated Pd0L was predicted in THF. The
activation free energy difference (ΔΔG⧧) for the competing
C−OTf addition by PdL2 is very small (0.3 kcal/mol). Within
Figure 3. X-ray structure of Pd0[P(iPr)(tBu)2]2 (5).
The reactivity of Pd0[P(iPr)(tBu)2]2 was subsequently
explored in the Suzuki cross-coupling of 4-chlorophenyl triflate
(1), employing 3 mol % catalyst loading, KF as base, and 2-
methylphenylboronic acid in THF. Reactivity was observed at
room temperature, albeit slow, resulting in 15% of the C−Cl
functionalization product 3, along with recovered starting
material (84%) (see Table 1). Modification to an overall 1:1 Pd
to ligand ratio led to faster reaction, consistent with the
reactivity previously seen with PtBu3.12 Using Pd2(dba)3 (1.5
mol %)/P(iPr)(tBu)2 (3 mol %) gave 86% of 3 (C−Cl
addition) in 24 h at room temperature (Table 1, entry 2).
The selectivity for C−Cl addition would be in agreement
with monoligated Pd[P(iPr)(tBu)2] as the active catalytic
speciesa reactivity previously observed for PtBu3.
To further test for the reactivity of a bisphosphine Pd0
complex, we repeated the experiments in the presence of excess
phosphine ligand (10 equiv relative to Pd) that is expected to
disfavor the required ligand dissociation for the monoligated
Figure 2. Calculated selectivity (ΔΔG⧧ in kcal/mol) for C−OTf vs
C−Cl addition for the reaction of Pd[P(iPr)(tBu)2]2 with 1 in THF at
the COSMO-RS (THF) M06L/def2-QZVP//ωB97XD/6-31G(d)
level.22
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Table 1. Selectivity Switch with Excess Ligand
yield (%)
entry
catalyst (amt (mol %))
Pd[P(iPr)(tBu)2]2 (3)
time (h)
2
3
1
2
3
4
125
24
15
86
Pd2(dba)3 (1.5)/P(iPr)(tBu)2 (3)
Pd[P(iPr)(tBu)2]2 (3)/P(iPr)(tBu)2 (30)
Pd2(dba)3 (1.5)/PtBu3 (30)
125
169
10
5
pathway, hence forcing the reaction to take place via a bisligated
pathway (=C−OTf addition). Indeed, this led to exclusive
functionalization of the C−OTf bond in 1, consistent with the
involvement of Pd0[P(iPr)(tBu)2]2 as an active species. As a
control, when we conducted the analogous experiment with
PtBu3 as ligand (also in excess), the reactivity was very low, and
exclusive functionalization of C−Cl was observed (entry 4),
consistent with the much greater energy differences predicted
between mono- and bisligated pathways for this ligand. This
suggests that P(iPr)(tBu)2 may adopt and be utilized for the
reactivity behavior of both very bulky ligands that typically favor
monophosphine Pd species and for less bulky ligands in higher
coordination spheres.
Figure 4. Formation of Pd(I) dimer {[PtBu2(iPr)]PdI}2 (6) from 5.
Another key difference between PCy3 and PtBu3 is the ability
to form dinuclear PdI complexes. We recently uncovered that
additives commonly used in cross coupling, such as CuI and
CuBr2, rapidly oxidize Pd0L2 complexes bearing electron-rich
phosphine ligands.18 In the case of L = PtBu3, the PdI dimer 4 is
obtained (Figure 1), the bridging halogens of which depend on
the Cu salt employed and whose reactivities are consistent with
several of the previously observed Cu effects in catalysis. On the
other hand, for tricyclohexylphosphine, no stable PdI dimer
results upon oxidation and instead PdIIL2 is formed, high-
lighting the different abilities of the ligands in stabilizing the +1
Pd oxidation state.17,18 Given that the hybrid ligand P(iPr)-
(tBu)2 was able to adopt and mimic the reactivities of both
PtBu3 and PCy3 in the Suzuki cross-coupling discussed above,
we were intrigued to explore the propensity of PdI dimer
formation for Pd0[P(iPr)(tBu)2]2 (5). In analogy to our
previous report on the redox reactions involving PCy3- and
PtBu3-derived Pd0 complexes,18 we subjected 1 equiv of CuI to
5 in THF at room temperature. Analysis of the reaction mixture
by 31P{1H} NMR after 1 h showed that a new species had
formed at 88.8 ppm in addition to Pd0[P(iPr)(tBu)2]2 (5) at
Figure 5. X-ray structure of Pd(I) dimer {[PtBu2(iPr)]PdI}2 (6).
may favor monoligated Pd0 as the active species in the absence
of excess ligand, paralleling the reactivity of PtBu3 in
preferentially reacting at the aromatic C−Cl over the generally
more reactive C−OTf bond. However, in contrast to the case
for PtBu3, for P(iPr)(tBu)2 modulation of the number of
equivalents of ligand allows steering of the reactivity via a Pd0
bisphosphine complex, in analogy to the case for PCy3. This
behavior is in line with the cone angles of the ligands, with the
hybrid ligand P(iPr)(tBu)2 being in between in steric bulk:
PtBu3 (182°) > P(iPr)(tBu)2 (175°) > PCy3 (170°).9 We
anticipate diverse applicability of this ligand on the basis of
these properties.
EXPERIMENTAL SECTION
■
Chemicals and Reagents. All reactions were performed under an
atmosphere of nitrogen unless otherwise stated. All reactions were
carried out in oven-dried glassware unless otherwise stated. THF and
hexane were purified by an SP-105 solvent drying system from LC
Technology Solutions Inc. under an atmosphere of nitrogen (H2O
content <10 ppm as determined by Karl Fischer titration). Anhydrous
solvents were purchased from Acros and were deoxygenated with a
flux of nitrogen. Reagents and solvents were purchased at reagent
grade from Acros, Aldrich, ABCR, and Fluka and were used as received
unless otherwise stated; boronic acids were recrystallized and
deoxygenated and aryl chlorides deoxygenated before their use.
72.0 ppm. Since only 1 equiv of CuI was employed, a maximum
1
of
/
equiv of PdI dimer could have potentially formed,
2
consistent with the fact that Pd0 was still detected by 31P NMR
spectroscopy. The same new species at 88.8 ppm was
exclusively formed when we subjected 5 to a comproportio-
nation with PdI2 (see Figure 4). Subsequent isolation and
characterization by X-ray crystallographic analysis confirmed
that this species was the novel PdI dimer {[P(iPr)(tBu)2]PdI}2
6 (see Figure 5).40
1
Physical Methods. The H NMR, 13C{1H} NMR, and 31P{1H}
NMR spectra were recorded at 293 K on a Bruker ARX 300 or Bruker
In conclusion, di-tert-butylisopropylphosphine, P(iPr)(tBu)2,
was shown to feature reactivity properties of two of the most
commonly used trialkylphosphine ligands (PtBu3 and PCy3). It
was demonstrated that P(iPr)(tBu)2 has the ability to stabilize
the rare +1 oxidation state at Pd centers, paralleling the
properties of PtBu3. Moreover, computational and experimental
reactivity data strongly support the notion that P(iPr)(tBu)2
1
DRX 400 spectrometer. Residual solvent signals in the H and 13C
NMR spectra were used as an internal reference, and trimethox-
yphosphine oxide was used as an internal standard for 31P NMR
spectra. Infrared (IR) spectra were recorded on a PerkinElmer
Spectrum One FTIR spectrometer. The spectra were measured neat,
and selected absorption bands are reported in wavenumbers (cm−1).
All cross-coupling reactions were monitored using an Agilent
C
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Technologies 5975 series MSD mass spectrometer coupled with an
Agilent Technologies 7820A gas chromatograph equipped with an
Agilent 19091s-433 HP-SMS column (30 m × 0.250 μm × 0.25 μm).
triethylamine (10.0 mL, 71.7 mmol, 3.8 equiv) was added via syringe.
Triflic anhydride (4.0 mL, 23.8 mmol, 1.3 equiv) was then added
dropwise, and the reaction mixture turned brown. The reaction
mixture was warmed to room temperature and stirred for 4 h. Water
(50 mL) was then added and the phases were separated. The aqueous
phase was extracted with dichloromethane (4 × 40 mL). The
combined organic phases were washed successively with an aqueous
solution of HCl (2 N, 2 × 50 mL), water (2 × 50 mL), and brine (50
mL), dried over Na2SO4, filtered, concentrated under reduced
pressure, and purified by flash column chromatography (100%
hexane) to yield a colorless oil (3.75 g, 80%): νmax 1484 (C−H),
1424 (SO2), 1205 (SO2), 1135 (C−F), 1089 (C−Cl) cm−1; δH
(CDCl3, 400 MHz) 7.20−7.26 (2H, m, ArH), 7.40−7.46 (2H, m,
ArH); δC (CDCl3, 100 MHz) 118.7 (C−F, q, JCF 320.9), 122.7 (CH),
130.4 (CH), 134.3 (C), 147.9 (C); m/z (EI) 262 ([M]+, 20%, 37Cl),
260 ([M]+, 49%, 35Cl), 129 (33%, 37Cl), 127 (100%, 35Cl), 101 (16%,
37Cl) 99 (52%, 35Cl). These data are consistent with those reported
previously.10
General Procedure for the Suzuki Cross-Coupling Reactions.
Inside a glovebox, 4-chlorophenyl trifluoromethanesulfonate (1; 276.3
mg, 1.06 mmol, 1.0 equiv), 2-methylphenylboronic acid (145.5 mg,
1.07 mmol, 1.1 equiv), and KF (185.6 mg, 3.2 mmol, 3.0 equiv) were
placed in an oven-dried vessel. THF (1 mL) was added, and the
catalyst system in THF (1 mL) was then added to the reaction
mixture. The reaction mixture was stirred in the glovebox for the
indicated time at room temperature. At the end of the reaction, the
reaction mixture was diluted with Et2O (50 mL), filtered through a
short pad of silica gel, concentrated under reduced pressure, and
purified by flash column chromatography. Ratios were then
determined by calibrated GC-MS analysis, using mesitylene as an
internal standard.10
4′-Chloro-2-methyl-1,1′-biphenyl (2). 4′-Chloro-2-methyl-1,1′-bi-
phenyl (2) was isolated by flash column chromatography (95/5,
hexane/EtOAc) as a colorless oil: νmax 3020 (Ar−H), 1604 (Ar) cm−1;
δH (CDCl3, 400 MHz) 2.26 (3H, s, CH3), 7.16−7.28 (6H, m, ArH),
7.35−7.41 (2H, m, ArH); δC (CDCl3, 100 MHz) 20.4 (CH3), 126.0
(CH), 127.6 (CH), 128.3 (CH), 129.7 (CH), 130.5 (CH), 130.6
(CH), 132.9 (C), 135.3 (C), 140.4 (C), 140.7 (C); m/z (EI) 204
([M]+, 31%, 37Cl), 202 ([M]+, 100%, 35Cl), 167 (70%, 37Cl), 165
(58%, 35Cl), 152 (20%). These data are consistent with those reported
previously.10
4′-Trifluoromethanesulfonyloxy-2-methyl-1,1′-biphenyl (3). 4′-
Trifluoromethanesulfonyloxy-2-methyl-1,1′-biphenyl (3) was isolated
by flash column chromatography (95/5, hexane/EtOAc) as a colorless
oil: νmax 1422 (SO2), 1205 (SO2), 1135 (C−F) cm−1; δH (CDCl3, 400
MHz) 2.26 (3H, s, CH3), 7.16−7.35 (6H, m, ArH), 7.37−7.45 (2H,
m, ArH); δC (CDCl3, 100 MHz) 20.4 (CH3), 118.8 (C−F, q, JCF
320.7), 121.0 (CH), 126.0 (CH), 128.0 (CH), 129.7 (CH), 130.6
(CH), 131.0 (CH), 135.2 (C), 139.9 (C), 142.4 (C), 148.5 (C); m/z
(EI) 316 ([M]+, 45%), 183 (100%). These data are consistent with
those reported previously.10
Melting points were obtained on a Buchi B540 and a Buchi M-560
̈
̈
apparatus in open capillary tubes. X-ray crystallographic data were
obtained with a Bruker ApexIID8-Cu or Bruker APEX-II CCD
diffractometer. CH elemental analysis was performed on a Elementar
Vario EL analyzer.
GC-MS conditions: front inlet mode, split; temperature, 250 °C;
pressure, 1.1066 psi; total flow, 50.474 mL/min; split ratio, 100:1; split
flow, 50 mL/min; run time, 18.667 min; oven program, 50 °C for 2
min then 15 °C/min to 300 °C for 0 min; flow, 0.5 mL/min.
Calibrations to obtain the relative ratios of products were achieved
using a calibration curve generated using the supplied GC-MS software
(“ChemStation”). Mesitylene was used as an internal standard.
Preparation of Di-tert-butylisopropylphosphine.27 To a
solution of di-tert-butylchlorophosphine (5.0 g, 27.7 mmol, 1.0
equiv) and copper(I) chloride (27.1 mg, 0.28 mmol, 0.1 equiv) in
THF (20 mL) at 0 °C was added a solution of isopropylmagnesium
chloride (2 M in THF, 21 mL, 41.5 mmol, 1.5 equiv) dropwise over 1
h. After the dropwise addition was completed, stirring was conducted
at the same temperature for 3 h. The reaction mixture was warmed to
room temperature and stirred overnight. A saturated solution of
NH4Cl (20 mL, previously degassed) and Et2O (20 mL, previously
degassed) were added. The aqueous phase was extracted with Et2O
(20 mL, previously degassed). The organic layers were combined,
washed with degassed water (20 mL), dried over MgSO4, and
concentrated under reduced pressure to afford a colorless oil. The
residue was transferred into the glovebox, dissolved in hexane, and
filtered through a short pad of silica gel, and the compound was dried
in vacuo to give the product (4.5 g, 88%) as a colorless oil: δH (C6D6,
300 MHz) 1.05−1.33 (25H, m, CH); δP (C6D6, 126 MHz) 46.2.
These data are consistent with those reported previously.29
Preparation of (N,N,N′,N′-Tetramethylethylenediamine)-
dichloropalladium(II).30 Palladium dichloride (2.53 g, 14.3 mmol,
1.0 equiv) was dissolved in MeCN (150 mL) at reflux for 1 h. The
solution was cooled to room temperature, and N,N,N′,N′-
tetramethylethylenediamine (15.5 mL, 104 mmol, 7.3 equiv) was
added. The reaction mixture was stirred for 1 h at room temperature.
The yellow precipitate that formed was filtered, washed with diethyl
ether, and dried in vacuo to give the product (4.1 g, 98%), which was
used without further purification.
Preparation of (N,N,N′,N′-tetramethylethylenediamine)-
dimethylpalladium(II).31 (N,N,N′,N′-Tetramethylethylenediamine)-
dichloropalladium(II) (1.66 g, 5.64 mmol, 1.0 equiv) was dissolved in
dry methyl tert-butyl ether (50 mL), and the resulting mixture was
cooled to 0 °C. A solution of MeLi (1.6 M in THF, 10.5 mL, 16.9
mmol, 3.0 equiv) was added dropwise via syringe over 10 min. The
solution was stirred at 0 °C for 30 min and then at room temperature
for 1 h. The reaction mixture was cooled to 0 °C, and water (20 mL)
was added to quench the excess MeLi. The resulting black solution
was extracted with THF (5 × 30 mL). The combined organic phases
were dried over MgSO4 and concentrated under reduced pressure to
give the product (1.2 g, 84%) as an off-white solid: δH (C6D6, 400
MHz) 0.45 (6H, s, Pd−CH3), 1.69 (4H, s, CH2), 2.05 (12H, s, N−
CH3); δC (C6D6, 100 MHz) −8.4, 47.9, 59.2. These data are consistent
with those reported previously.31
Computational Details. All structures have been optimized with
Gaussian 09 Revision A.0222 in the gas phase using ωB97XD32 with
the basis sets 6-31G(d) and an effective core potential (ECP) for Pd
(LANL2DZ33). Frequency calculations were performed on the same
level of theory and were used to verify the nature of the stationary
point as either a transition state or a minimum. Single-point energy
corrections were then calculated with Gaussian 09 Revision D.0122 at
the M06L34 level of theory with the basis set def2-QZVP. The solvent
model COSMO-RS35−38 has been used to describe the solvation
influence of THF. The standard state was converted to 1 M in solution
(+1.89 kcal/mol).
Preparation of Bis(di-tert-butylisopropylphosphine)-
palladium(0) (5). (N,N,N′,N′-Tetramethylethylenediamine)-
dimethylpalladium(II) (0.61 g, 2.4 mmol, 1.0 equiv) and di-tert-
butylisopropylphosphine (1.0 g, 5.3 mmol, 2.2 equiv) in THF (8 mL)
were stirred at room temperature for 2 h. The solvent was removed in
vacuo, and DMF (5 mL) was added. The precipitate that formed was
collected by filtration, washed with hexane, and dried in vacuo to give
the product (0.73 g, 63%) as white crystals: δH (C6D6, 300 MHz)
1.38−1.59 (50H, m, CH); δP (C6D6, 126 MHz) 71.3. Anal. Calcd for
C22H55P2Pd: C, 54.71; H, 10.43. Found: C, 54.28; H: 10.31.
Preparation of 4-Chlorophenyl Trifluoromethanesulfonate
(1). 4-Chlorophenol (2.4 g, 18.7 mmol, 1.0 equiv) was dissolved in
dichloromethane (15 mL). The solution was cooled to 0 °C, and
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF and xyz files giving details on
experimental procedures, spectroscopic and X-ray crystallo-
graphic data, computational information and Cartesian
coordinates of calculated species, and the full ref 24. This
D
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail for F.S.: franziska.schoenebeck@rwth-aachen.de.
Author Contributions
E.L. and I.A.S. performed the calculations. M.A. provided the
data for Figures 4 and 5. Except for entry 2 of Table 1 (done by
E.L.), F.P. did all remaining experiments. F.S. wrote the
manuscript.
(12) Littke, A. F.; Dai, C. Y.; Fu, G. C. J. Am. Chem. Soc. 2000, 122,
4020.
Notes
(13) Issleb, K.; Brack, A. Z. Anorg. Allg. Chem. 1954, 277, 258.
(14) (a) Paton, R. S.; Brown, J. M. Angew. Chem., Int. Ed. 2012, 51,
10448. For reviews on dinuclear Pd complexes, see: (b) Mirica, L. M.;
Khusnutdinova, J. R. Coord. Chem. Rev. 2013, 257, 299. (c) Colacot, T.
J. Platinum Met. Rev. 2009, 53, 183. (d) Murahashi, T.; Kurosawa, H.
Coord. Chem. Rev. 2002, 231, 207.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the SNSF, ETH Zurich, MIWF NRW, and RWTH
Aachen for financial support of this research. We thank the
̈
(15) For detection of PdI−PdI under catalytically relevant conditions:
(a) Markert, C.; Neuburger, M.; Kulicke, K.; Meuwly, M.; Pfaltz, A.
Angew. Chem., Int. Ed. 2007, 46, 5892. (b) Hill, L. L.; Crowell, J. L.;
Tutwiler, S. L.; Massie, N. L.; Hines, C.; Griffin, S. T.; Rogers, R. D.;
Shaughnessy, K. H.; Grasa, G. A.; Seechurn, C. C. C. J.; Li, H.;
Colacot, T. J.; Chou, J.; Woltermann, C. J. J. Org. Chem. 2010, 75,
6477. (c) Bedford, R. B.; Haddow, M. F.; Mitchell, C. J.; Webster, R.
L. Angew. Chem., Int. Ed. 2011, 50, 5524. (d) Seechurn, C. C. C. J.;
Parisel, S. L.; Colacot, T. J. J. Org. Chem. 2011, 76, 7918.
(e) Hruszkewycz, D. P.; Balcells, D.; Guard, L. M.; Hazari, N.;
Tilset, M. J. Am. Chem. Soc. 2014, 136, 7300. (f) Borjian, S.; Baird, M.
C. Organometallics 2014, 33, 3936.
“Small Molecule Crystallography Center” at ETH Zurich for X-
̈
ray crystallographic analyses of 5 and 6.
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dx.doi.org/10.1021/om5009605 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
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(39) The mechanism of oxidative addition may proceed via an
associative (substrate-assisted ligand loss of PdL2) or dissociative
mechanism (involving ligand dissociation to a monoligated PdL prior
to oxidative addition). The provided references give more information
on this topic.
(40) In analogy to {P(tBu)3PdI}2 the new Pd(I) dimer complex
{[P(iPr)(tBu)2]PdI}2 was found to also be ineffective as a precatalyst
under the general Suzuki reaction conditions presented in Table 1
(KF, THF, room temperature). No conversion was observed after 1 or
24 h in reactions with 4-chloroacetophenone and 4-methoxyphenyl-
boronic acid. Compare this result with that of ref 18. The bromine-
bridged Pd(I) dimer {P(tBu)3PdBr}2 functions as effective precatalyst
under these conditions.
F
dx.doi.org/10.1021/om5009605 | Organometallics XXXX, XXX, XXX−XXX