Phenoxide-assisted P-C bond cleavage in PdCl2(PPh 3)2 under very mild conditions

Article abstract of DOI:10.1016/j.jorganchem.2005.11.055

PdCl₂(PPh₃)₂ reacted with NaOAr (Ar = Ph, p-tolyl) at 0 °C to afford PdCl(Ph)(PPh₃)₂, instead of PdCl(OAr)(PPh₃)₂, in 12-16% isolated yields based on Pd. The structure was confirmed by NMR and X-ray crystallography. GC-MS analysis of the reaction solution revealed that OPPh₂(OAr), OPPh(OAr) ₂, and OP(OAr)₃ are formed, while NMR studies indicated that PdCl(Ph)(PPh₃)₂ is produced when PdCl(OAr)(PPh ₃)₂ decomposes. The reaction of PdCl₂(PPh ₃)₂ with Bu₃Sn(OC₆H ₄-p-OMe) also gave PdCl(Ph)(PPh₃)₂ in 8% isolated yield. These results suggest that PdCl(OAr)(PPh₃) ₂ is highly labile and the aryloxy ligand exchanges with the phenyl groups in triphenylphosphine even under very mild conditions.

Full text of DOI:10.1016/j.jorganchem.2005.11.055

Journal of Organometallic Chemistry 691 (2006) 1307–1310
www.elsevier.com/locate/jorganchem
Note
Phenoxide-assisted P–C bond cleavage in PdCl2(PPh3)2
under very mild conditions
*
Hiroyuki Yasuda, Noriko Maki, Jun-Chul Choi, Mahmut Abla, Toshiyasu Sakakura
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Received 28 June 2005; received in revised form 21 November 2005; accepted 22 November 2005
Available online 27 December 2005
Abstract
PdCl2(PPh3)2 reacted with NaOAr (Ar = Ph, p-tolyl) at 0 ꢀC to afford PdCl(Ph)(PPh3)2, instead of PdCl(OAr)(PPh3)2, in 12–16% iso-
lated yields based on Pd. The structure was confirmed by NMR and X-ray crystallography. GC–MS analysis of the reaction solution
revealed that OPPh2(OAr), OPPh(OAr)2, and OP(OAr)3 are formed, while NMR studies indicated that PdCl(Ph)(PPh3)2 is produced
when PdCl(OAr)(PPh3)2 decomposes. The reaction of PdCl2(PPh3)2 with Bu3Sn(OC6H4-p-OMe) also gave PdCl(Ph)(PPh3)2 in 8%
isolated yield. These results suggest that PdCl(OAr)(PPh3)2 is highly labile and the aryloxy ligand exchanges with the phenyl groups
in triphenylphosphine even under very mild conditions.
ꢁ 2005 Elsevier B.V. All rights reserved.
Keywords: P–C bond cleavage; Triphenylphosphine; PdCl2(PPh3)2; PdCl(Ph)(PPh3)2; Phenoxide; Aryloxy-aryl exchange
1. Introduction
2. Results and discussion
Decomposition of tertiary phosphine ligands is an
important phenomenon relevant to catalyst degradation
or byproduct formation in homogeneous catalysis by phos-
phine-ligated transition metal complexes [1,2]. P–C bond
cleavage in Pd triphenylphosphine complexes such as
PdCl2(PPh3)2 and Pd(PPh3)4 is especially important since
these complexes are used in numerous reactions that require
Pd(II) and Pd(0)-catalyzed processes. Examples include C–
C bond forming reactions such as Stille coupling, Suzuki
coupling, Heck arylation, Sonogashira coupling, etc. [3].
During our attempts to synthesize PPh3-ligated Pd phenox-
ide complexes by reacting PdCl2(PPh3)2 with NaOPh, we
found that the P–C bond in PdCl2(PPh3)2 is easily cleaved
even at 0 ꢀC. Herein, we report an aryloxide-assisted trans-
fer of the phenyl group bound to P to Pd in PdCl2(PPh3)2
under very mild conditions.
Our interest in the mechanism of diphenyl carbonate
formation in the Pd-catalyzed oxidative carbonylation of
phenol led us to synthesize a Pd phenoxide complex,
PdX(OPh)L2 (X = halogen), and investigate its reactivity
[4]. Thus, we attempted to prepare a Pd chloro(phenoxy)
complex, PdCl(OPh)(PPh3)2, by reacting PdCl2(PPh3)2
with NaOPh.
When PdCl2(PPh3)2 reacted with 1 equiv. of NaOPh in
THF at 0 ꢀC, the solution color instantly changed from yel-
low to orange and then turned dark brown/black. After
stirring for 2 h, the work-up using the procedure described
in Section 3 provided a brown powder. This complex
exhibited a singlet at d 23.2 ppm in 31P NMR. Based on
1H NMR, the complex contained a phenyl group (d 6.2–
6.6 ppm) and two triphenylphosphines (d 7.2–7.5 ppm).
These results are consistent with the formation of
PdCl(OPh)(PPh3)2. However, the X-ray structural analysis
of the single crystals obtained from the complex revealed
that it was not PdCl(OPh)(PPh3)2, but PdCl(Ph)(PPh3)2
[5]. The 1H and 31P NMR spectra were remeasured by dis-
solving the single crystals in THF-d8 and were in agreement
*
Corresponding author. Tel./fax: +81 29 861 4719.
E-mail address: t-sakakura@aist.go.jp (T. Sakakura).
0022-328X/$ - see front matter ꢁ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.11.055

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H. Yasuda et al. / Journal of Organometallic Chemistry 691 (2006) 1307–1310
with the spectra from the reaction mixture of PdCl2(PPh3)2
and NaOPh in THF-d8 at room temperature (vide infra).
The isolated yield of PdCl(Ph)(PPh3)2 based on Pd was
16% (Scheme 1).
probably PdCl(O(p-tolyl))(PPh3)2 formed by the reaction
of slightly dissolved PdCl2(PPh3)2 and NaO(p-tolyl). Upon
stepwise increasing the temperature every 2 h to 0 ꢀC and
20 ꢀC, the 31P NMR peak assigned to PdCl(O(p-tolyl))-
(PPh3)2 (d 18.0 ppm at 20 ꢀC) gradually diminished with
the concomitant appearance and growth of the peak due
to PdCl(Ph)(PPh3)2 (d 23.2 ppm at 20 ꢀC). During the reac-
tion several peaks due to P-containing species other than
PdCl2(PPh3)2 (d 22.7 ppm at 20 ꢀC) were also observed.
When the temperature was further increased to 40 ꢀC, the
peak assigned to PdCl(O(p-tolyl))(PPh3)2 completely disap-
peared and the peak due to PdCl(Ph)(PPh3)2 became the
major one. Other weak peaks due to several P-containing
species were also present. These results suggest that
PdCl(Ph)(PPh3)2 is produced during the decomposition of
PdCl(O(p-tolyl))(PPh3)2, the initial product in the reaction
of PdCl2(PPh3)2 and NaO(p-tolyl) (Scheme 2).
The reaction of PdCl2(PPh3)2 with 1 equiv. of Bu3Sn-
(OC6H4-p-OMe) instead of NaOAr was also attempted in
toluene at 80 ꢀC in order to obtain PdCl(OC6H4-p-
OMe)(PPh3)2. However, PdCl(Ph)(PPh3)2 was the only iso-
lated Pd complex in 8% yield (Scheme 3). The result also
indicates that PdCl(OC6H4-p-OMe)(PPh3)2 is labile and
the aryloxy–phenyl exchange readily occurs, even if
PdCl(OC6H4-p-OMe)(PPh3)2 is generated during the
reaction.
Although it is often reported that in Pd complexes the
aryl [8], alkyl [9], or acetoxy [10] ligand exchanges with
the aryl of the triarylphosphine ligand, to our best knowl-
edge, this is the first report on the aryloxy–aryl exchange
mediated by Pd [11,12]. It is noteworthy that the Pd-med-
iated aryloxy–aryl exchange reported here proceeds under
even milder conditions than the usual aryl–aryl or alkyl–
aryl exchange.
The three possible mechanisms for the aryloxy–aryl
exchange are roughly classified: oxidative addition of
arylphosphines involving ortho-metalation, nucleophilic
attack of aryloxy groups on coordinated phosphines, and
aryl transfer from the phosphine ligands via phosphonium
species (Scheme 4) [1a]. The reaction between PdCl2-
(P(p-tolyl)3)2 and NaOPh formed para-tolyl Pd complex,
An analogous reaction using NaO(p-tolyl) also resulted
in the formation of PdCl(Ph)(PPh3)2 in 12% isolated yield
based on Pd, confirming that the phenyl group bound to
Pd in PdCl(Ph)(PPh3)2 originated from the triphenylphos-
phine ligand and not from phenoxide (Scheme 1) [6]. The
GC–MS of the reaction solution of PdCl2(PPh3)2 and
NaO(p-tolyl) contained OPPh2(O-p-tolyl), OPPh(O-p-
tolyl)2, and OP(O-p-tolyl)3, in addition to PPh3 and OPPh3.
This suggests that the aryloxy ligand, which would have to
coordinate to Pd, exchanges with the phenyl groups in the
triphenylphosphine ligand under very mild conditions
(Scheme 2). The formations of OPPh(O-p-tolyl)2 and OP
(O-p-tolyl)3 indicate multiple aryloxy–phenyl exchanges.
The reason that the aryloxyphosphines were detected on
the GC–MS as their oxides may be because the aryloxy-
phosphines were oxidized by air in the injector port
(200 ꢀC). In fact, when PPh2(O-p-tolyl) was synthesized
and analyzed by GC–MS, PPh2(O-p-tolyl) was partly
detected as OPPh2(O-p-tolyl). Another plausible reason is
the pyrolysis of the phosphonium salts in the GC–MS
injector port [7]. The intermediacy of a phosphonium salt
in the aryloxy–phenyl exchange will be discussed later.
To confirm that the Pd aryloxy complex formed in the
initial stage of the aryloxy–phenyl exchange, the reaction
of PdCl2(PPh3)2 and NaO(p-tolyl) in THF-d8 was moni-
tored by low temperature NMR spectroscopy. A single
peak immediately appeared at d 19.0 ppm in the 31P
NMR spectrum when the reaction was allowed to start at
À40 ꢀC by thawing the mixture. Note that this peak is
not attributed to PdCl2(PPh3)2. Since the solubility of
PdCl2(PPh3)2 in THF-d8 at À40 ꢀC is extremely poor, the
31P NMR peak due to PdCl2(PPh3)2 is, if any, very small.
In addition to triphenylphosphine (d 7.3–7.7 ppm) and
1
NaO(p-tolyl) (d 6.4–6.7 ppm), the H NMR spectrum had
a new p-tolyl group at d 6.2–6.3 ppm. Judging from the
integrated intensity ratio of the new p-tolyl group to the tri-
phenylphosphine, the species at d 19.0 ppm in 31P NMR is
PPh3
Cl Pd Cl
PPh3
PPh3
Cl Pd Ph
PPh3
NaOAr
THF
PPh3
Cl Pd Cl
PPh3
PPh3
Cl Pd Ph
PPh3
Bu3Sn(OAr)
Toluene
0 C, 2 h
˚
80 C, 24 h
˚
yield (%/Pd)
yield (%/Pd)
crude isolated
crude isolated
Ar = Ph
Ar = p-tolyl
40
31
16
12
Ar = C6H4-p-OMe
31
8
Scheme 1.
Scheme 3.
PPh3
Cl Pd OAr
PPh3
PPh2(OAr)
Cl Pd Ph
PPh3
PPh3
Cl Pd Cl
PPh3
PPh3
PPh2(OAr)
PPh3
Cl Pd Ph
PPh3
+ NaOAr
+ PPh3
Cl Pd OAr
PPh3
Cl Pd Ph
PPh3
[PPh3(OAr)]
[PdCl(PPh3)]
Scheme 2.
Scheme 4.

H. Yasuda et al. / Journal of Organometallic Chemistry 691 (2006) 1307–1310
1309
which is inconsistent with the ortho-metalation mechanism.
On the other hand, it is possible that the aryloxy–aryl
exchange proceeds via a phosphonium salt such as
[PPh3(OAr)][PdCl(PPh3)], since these phosphonium salts
are fairly stable [13].
0 ꢀC. After stirring the reaction mixture for 2 h at 0 ꢀC,
the solvent was evaporated. The residue was extracted with
toluene (5 mL) at 0 ꢀC, filtered, and fully evaporated to
dryness. The resulting product was washed with ether
(2 · 5 mL) and dried under a vacuum to give the crude
product, PdCl(Ph)(PPh3)2, as a brown powder (0.085 g,
40% yield based on Pd). Recrystallization from a THF–
hexane solution at room temperature gave a brown crystal-
line solid of PdCl(Ph)(PPh3)2 (0.034 g, 16% yield on Pd).
1H NMR (400 MHz, THF-d8, 25 ꢀC, ppm): d 6.18 (m,
2H, meta-H, Pd–Ph), 6.30 (m, 1H, para-H, Pd–Ph), 6.64
(d, 2H, ortho-H, Pd–Ph), 7.20–7.54 (m, 30H, PPh3).
31P{1H} NMR (161.7 MHz, THF-d8, 25 ꢀC, ppm): d 23.2
(s). The NMR data is consistent with the literature [5].
The reaction of PdCl2(PPh3)2 (0.20 g, 0.28 mmol) and
NaO(p-tolyl) (0.037 g, 0.28 mmol) was analogously con-
ducted to give the crude product, PdCl(Ph)(PPh3)2
(0.065 g, 31% yield on Pd). Recrystallization gave
PdCl(Ph)(PPh3)2 (0.025 g, 12% yield on Pd).
Numerous Pd aryloxides with phosphine ligands, such
as monodentate PMe3, PCy3, PMePh2, and bidentate
DPPM (bis(diphenylphosphino)methane), have been syn-
thesized and characterized [14]. For instance, [Pd(H)-
(OPh)(PCy3)2] Æ PhOH is synthesized by reacting Pd(PCy3)2
with phenol at room temperature [14e], and Pd(OAr)2
(PMe3)2 (Ar = C6H4-p-NO2) is obtained from Pd(styrene)
(PMe3)2 and CF3CO2Ar at room temperature [14b]. There
are also a few reports on the synthesis of PPh3-ligated Pd
aryloxides [15,16], but the structural evidence is quite
weak [17]. Strengthening the coordination by electron-
donating alkylphosphines or bidentate phosphines, and
stabilizing the complexes by introducing electron-with-
drawing substituents onto the aryloxy groups may enable
these aryloxide complexes to be isolated. The reason
PdCl(OPh)(PPh3)2 is highly labile remains unknown. A
large P–O bond energy may be related to the facile aryl-
oxy–aryl exchange.
3.1.2. NMR tube reaction
In an NMR tube (5 mm in diameter), THF-d8 (0.5 mL)
was vacuum-transferred into a mixture of PdCl2(PPh3)2
(20 mg, 0.028 mmol) and 1 equiv. of NaO(p-tolyl)
(3.7 mg, 0.028 mmol). The NMR tube was sealed under a
vacuum while the mixture was frozen. When the mixture
was thawed at À40 ꢀC, the solution was orange. The
In summary, we have demonstrated that reacting
PdCl2(PPh3)2 with NaOAr under very mild conditions
yields PdCl(Ph)(PPh3)2. It is likely that the reaction pro-
ceeds via an intermediate, PdCl(OAr)(PPh3)2, and subse-
quent aryloxy–phenyl exchange that involves P–C bond
cleavage.
1
31P{1H} and H NMR spectra were monitored for 1 h at
À40 ꢀC, where the peaks assigned to PdCl(O(p-tolyl))-
1
(PPh3)2 appeared. H NMR (400 MHz, THF-d8, À40 ꢀC,
ppm): d 2.12 (s, 3H, Me), 6.21 (d, 2H, meta-H, p-tolyl),
6.33 (d, 2H, ortho-H, p-tolyl), 7.32–7.68 (m, 30H, PPh3).
31P{1H} NMR (161.7 MHz, THF-d8, À40 ꢀC, ppm): d
19.0 (s). Then the temperature was increased, stepwise,
every 2 h to 0 and 20 ꢀC. The temperature was finally
increased to 40 ꢀC. The 31P{1H} and 1H NMR spectra were
periodically recorded throughout the reaction.
3. Experimental
All manipulations were conducted in a purified Ar
atmosphere using standard Schlenk and glovebox tech-
niques. Solvents were dried in a typical manner and
distilled prior to use. All other reagents were used without
further purification. NaOAr (Ar = Ph, p-tolyl) was pre-
pared by reacting the corresponding phenol with sodium
hydride. PdCl2(PPh3)2 and PdCl2(P(p-tolyl)3)2 were pre-
pared by reacting PdCl2(PhCN)2 with PPh3 and P(p-tolyl)3,
respectively. PPh2(O-p-tolyl) was prepared by reacting
PPh2Cl with Na(O-p-tolyl). Bu3Sn(OC6H4-p-OMe) was
prepared according to the literature [18]. The 1H and
31P{1H} NMR spectra were recorded on a JEOL LA400
3.2. Reaction of PdCl2(PPh3)2 with Bu3Sn(OC6H4-p-
OMe)
Toluene (5 mL) and Bu3Sn(OC6H4-p-OMe) (0.12 g,
0.29 mmol) were added to a Schlenk tube containing
PdCl2(PPh3)2 (0.20 g, 0.28 mmol). After stirring for 24 h
at 80 ꢀC, the reaction mixture was fully evaporated to dry-
ness. The resulting product was washed with ether (2 ·
5 mL) and dried under a vacuum to give the crude product,
PdCl(Ph)(PPh3)2 (0.065 g, 31% yield on Pd). Recrystalliza-
tion from a THF–hexane solution at room temperature
gave PdCl(Ph)(PPh3)2 (0.017 g, 8% yield on Pd).
1
WB spectrometer (400 MHz for H). 31P{1H} NMR spec-
tra were referenced to external 85% phosphoric acid. The
reaction products were analyzed by a Shimadzu GC-17A
gas chromatograph connected to a QP-5000 mass spec-
trometer (70 eV EI) (GC–MS) using a capillary column
(J&W Scientific, DB-1, 30 m).
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˚
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˚
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