Air-stable, recyclable, and regenerative phosphine sulfide palladium(0) catalysts for C-C coupling reaction

Article abstract of DOI:10.1021/om900588v

Mononuclear phosphine sulfide Pd(0) complexes and a polymer-supported triphenylphosphine sulfide Pd(0) complex were prepared as new air-stable Pd(0) catalysts for C-C coupling reactions. The phosphine sulfide Pd(0) complexes are not decomposed after compl

Full text of DOI:10.1021/om900588v

Organometallics 2009, 28, 6067–6072 6067
DOI: 10.1021/om900588v
Air-Stable, Recyclable, and Regenerative Phosphine Sulfide Palladium(0)
Catalysts for C-C Coupling Reaction
Sen-ichi Aizawa,*,† Arpi Majumder,† Yukihiro Yokoyama,‡ Mitsuyoshi Tamai,†
Daisuke Maeda,† and Akina Kitamura†
†Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555,
Japan, and ‡Research Facility Center for Science and Technology, University of Tsukuba,
Tsukuba, Ibaraki 305-8571, Japan
Received July 8, 2009
Mononuclear phosphine sulfide Pd(0) complexes and a polymer-supported triphenylphosphine
sulfide Pd(0) complex were prepared as new air-stable Pd(0) catalysts for C-C coupling reactions.
The phosphine sulfide Pd(0) complexes are not decomposed after completion of Suzuki-Miyaura
coupling, and the polymer-supported Pd(0) catalyst is practically recyclable, while phosphine Pd(0)
complexes are decomposed into inactive Pd(0) black after consuming the substrates. New catalytic
activity of Pd(0) that promotes chalcogen atom replacement of phosphine chalcogenides (R3PdX,
X = O, S, Se) is reported. A mechanistic study revealed that the new catalytic chalcogen replacement
results from activation of the PdX bond as well as promotion of the oxidative chalcogenide
formation. The intermediate phosphine was successfully trapped as a phosphine Pd(II) complex,
and the PdX bond activation is applicable to regeneration of phosphine or phosphine sulfide from
oxidized phosphine.
Introduction
reasons. The problem is that the catalytic cycles are blocked
by strongly bound ligands of stable phosphine-free com-
plexes to give the low catalytic activity while weakly ligated
complexes, which are likely to have high activity, are intrin-
sically unstable, and are liable to be decomposed into
inactive palladium black. In these circumstances, we need a
new ligand system that can thermodynamically stabilize the
Pd(0) center but does not deactivate the catalytic cycle
kinetically. It is also desirable that the new catalyst is
recoverable and reusable for industrial use.
Recently, some phosphine sulfides have been employed as
monodentate or bidentate ligands for some metal ions.5-10
The sulfur-phosphorus π-bonding orbital is not so stabi-
lized, and consequently the unoccupied π* orbital is moder-
ately low and can probably accept electrons from Pd(0) to
stabilize the low oxidation state. On the other hand, because
the phosphine sulfide group is not a strong σ-donor for
Pd(II), the formation of the substrate adduct and subsequent
catalytic reactions on Pd(II) are not likely to be blocked. In
this work, we have provided the isolable Pd(0) complex with
Palladium catalysts are now quite versatile and indispen-
sable in organic synthesis since they can catalyze the forma-
tion of various carbon-carbon and carbon-heteroatom
bonds. The catalytic activity in most coupling reactions
comes from the ability of Pd(0) species to activate car-
bon-halogen or pseudohalogen bonds by the oxidative
addition that initiates the catalytic cycle.1-4 Because phos-
phines have been regarded as effective ligands to stabilize the
zerovalent oxidation state of palladium, phosphine-assisted
reactions have been widely employed as the classical and
conservative method. However, while the substrate adducts
of Pd(II) formed during the catalytic cycle are relatively
stable, phosphines are usually susceptible to oxidation to
give inactive Pd sediment “palladium black” after the cata-
lytic reactions are completed.1-4 It is difficult to practically
regenerate phosphines from the phosphine oxides for recyc-
ling, so that even elaborate and valuable phosphines are
unavoidably discarded as phosphine oxides. Such air-sensi-
tivity is unfavorable for application of expensive and toxic
phosphines on industrial and semi-industrial scales. On the
other hand, phosphine-free catalytic systems have received
increasing interest for environmental and economical
(5) Wong, T. Y. H.; Rettig, S. J.; Jamws, B. R. Inorg. Chem. 1999, 38,
2143.
(6) Liu, H.; Bandeira, N. A. G.; Calhorda, M. J.; Drew, M. G. B.;
ꢀ
Felix, V.; Novosad, J.; de Biani, F. F.; Zanwllo, P. J. Organomet. Chem.
2004, 689, 2808.
ꢀ~
ꢀ
(7) Zuniga-Villarreal, N.; German-Acacio, J. M.; Lemus-Santana,
A. A.; Reyes-Lezama, M.; Toscano, R. A. J. Organomet. Chem. 2004,
689, 2827.
*To whom correspondence should be addressed. E-mail: saizawa@
eng.u-toyama.ac.jp. Phone and fax: (þ81) 76 445 6980.
(1) Tsuji, J. In Palladium Reagents and Catalysts; Wiley: Chichester,
1995.
(2) Heck, R. F. In Palladium Reagents in Organic Synthesis; VCH:
Weinheim, 1996.
(8) Skvortsov, A. N.; Reznikov, A. N.; de Vekki, D. A.; Stash, A. I.;
Belsky, V. K.; Spevak, V. N.; Skvortsov, N. K. Inorg. Chim. Acta 2006,
359, 1031.
(9) Dutta, D. K.; Chutia, P.; Woollins, J. D.; Slawin, A. M. Z. Inorg.
(3) Suzuki, A. J. Organomet. Chem. 1999, 576, 147.
(4) Beletskaya, I. P.; Cheprakov, A. V. Chem. Rev. 2000, 100, 3009.
Chim. Acta 2006, 359, 877.
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2009 American Chemical Society
Published on Web 09/25/2009
pubs.acs.org/Organometallics

6068 Organometallics, Vol. 28, No. 20, 2009
Aizawa et al.
tris[2-(diphenylphosphino)ethyl]phosphine tetrasulfide
(pp3S4) as one of the most promising catalysts, where the
four phosphine sulfide groups linked by ethylene chains can
stabilize the Pd(0) complex entropically as well as electro-
nically.11 We also report practically recyclable polymer-
supported phosphine sulfide as a solid catalyst. In the course
of this study, we have found new catalytic activity of Pd(0)
that promotes chalcogen replacement of phosphine chalco-
genides (R3PdX, X = O, S, Se) by the PdX bond activation,
which is useful for regeneration of phosphine sulfides.12 We
have also attempted to separate the intermediate phosphine
as evidence of the dissociative mechanism and have applied
the catalytic chalcogen dissociation to the regeneration
of phosphines such as bidentate 1,2-bis(diphenylphos-
phino)ethane (p2) and optically active 2,20-bis(diphenyl-
phosphino)-1,10-binaphthalene (BINAP) from their oxides.
external D2O) 44.2 (d, terminal), 54.6 (q, center); JP-P = 56 Hz.
1H NMR (CDCl3): δ 1.21 (t, CH3- of diethyl ether), 2.11-2.17
and 2.55-2.65 (m, -CH2CH2- of pp3S4), 3.48 (q, -CH2- of
3
diethyl ether), 7.10 (d, Ph-CHdCH- of dba, JH-H=16 Hz),
7.26 (s, CHCl3), 7.41-7.54 and 7.81-7.87 (m, Ph of pp3S4),
=
7.61-7.64 (m, Ph of dba), 7.75 (d, Ph-CHdCH- of dba, 3JH-H
16 Hz).
[Pd(p2S2)(dba)] (2). A solution containing p2S2 (0.139 g, 0.300
mmol) and [Pd(dba)2] (0.174 g 0.299 mmol) in deoxygenated
chloroform was stirred for 30 min at room temperature under
nitrogen and then concentrated to a small volume. After the
addition of diethyl ether, the mixture was kept in a refrigerator
overnight. The resulting dark brown solid was filtered and
washed with diethyl ether and then air-dried. Yield: 0.125 g
(52%). Anal. Found: C, 57.30; H, 4.44; N, 0.00. Calcd for
C43H38OP2PdS2Μ CHCl3: C, 57.28; H, 4.26; N, 0.00. 31P{1H}
3
NMR (CHCl3): δ (relative to D3PO4 in external D2O) 44.2 (s).
1H NMR (CDCl3): δ 2.72 (d, -CH2CH2- of p2S2), 7.10 (d, Ph-
CHdCH- of dba, 3JH-H=16 Hz), 7.26 (s, CHCl3), 7.40-7.43
and 7.61-7.65 (m, Ph of dba) 7.43-7.50 and 7.77-7.83 (m, Ph
of p2S2), 7.75 (d, Ph-CHdCH- of dba, 3JH-H=16 Hz).
Polymer-Supported PPh3 Sulfide Pd(0) Complex [-CH-
{C6H4P(S)Ph2}CH2-]2{Pd(dba)}0.42 (3). To a solution of sulfur
(0.100 g, 3.12 mmol) in toluene (10 cm3) was added 0.819 g (2.84
mmol of the phosphine unit) of polymer-supported triphenyl-
phosphine (Aldrich). The suspension was allowed to stand at
room temperature for 1 h, and the reacted polymer was collected
by filtration and air-dried. 31P{1H} NMR (solid): δ (relative to
external NH4H2PO4) 42.4.
Experimental Section
Reagents. Tris[2-(diphenylphosphino)ethyl]phosphine (pp3,
Aldrich), bis[2-(diphenylphosphino)ethyl]phenylphosphine
(p3, Aldrich), 1,2-bis(diphenylphosphino)ethane (p2, Kanto
Chemical), triphenylphosphine (Ph3P, Aldrich), (R)-2,20-bis-
(diphenylphosphino)-1,10-binaphthalene ((R)-BINAP, Aldrich),
sulfur (Wako), selenium (Wako), polymer-supported triphenyl-
phosphine (Aldrich), dibenzylideneacetone (dba, Aldrich),
tetrakis(triphenylphosphine)palladium(0) (Aldrich), bis(diben-
zylideneacetone)palladium ([Pd(dba)2], Kanto Chemical), trakis-
(acetonitrile)palladium(II) tetrafluoroborate ([Pd(CH3-
CN)4](BF4)2, Aldrich), tetra(n-butyl)ammonium iodide (Wako),
potassium tetrachloroplatinate(II) (K2[PtCl4], Aldrich), trans-bis-
(benzonitrile)dichloroplatinum(II) (trans-[PtCl2(NCC6H5)2],
Strem), iodobenzene (Kanto Chemical), phenylboronic acid
(Sigma-Aldrich), and bis(2-butoxyethyl) ether (Wako) were used
for preparation and catalytic reactions without further purifi-
cation.
Preparation. Tris[2-(diphenylphosphino)ethyl]phosphine Tet-
rasulfide (pp3S4). To a solution containing sulfur (0.101 g, 3.15
mmol) in deoxygenated chloroform was added tris-
[2-(diphenylphosphino)ethyl]phosphine, pp3 (0.520 g, 0.775
mmol). The solution was allowed to stand at room temperature
for 1 h followed by the addition of diethyl ether. The resultant
colorless crystals were collected by filtration and air-dried.
Yield: 0.58 g (94%). Anal. Found: C, 63.08; H, 5.30; N, 0.00.
Calcd for C42H42P4S4: C, 63.10; H, 5.30; N, 0.00. 31P{1H} NMR
(CHCl3): δ (relative to D3PO4 in external D2O) 44.1 (d,
terminal), 54.5 (q, center); JP-P = 56 Hz.
The obtained polymer-supported triphenylphosphine sulfide
was added to a solution of [Pd(dba)2] (0.408 g, 0.71 mmol) in a
mixture of toluene (10 cm3) and chloroform (3 cm3). The
suspension was stirred for 6 h under N2. The resultant dark
brown solid was collected by filtration and air-dried. Anal.
Found: C, 73.61; H, 5.35; N, 0.00; S, 6.80; Pd, 4.71. Calcd for
[(-CH{C6H4P(S)Ph2}CH2-)12:24 AM 9/22/20092{Pd-
(dba)}0.42(C7H8)1.5(CHCl3)&Qj;0.2]n: C, 73.44; H, 5.55; N,
0.00; S, 6.78; Pd, 4.72. 31P{1H} NMR (solid): δ (relative to
external NH4H2PO4) 42.0.
[Pd((R)-binapS2)(dba)]. The (R)-BINAPS2 complex was pre-
pared by a procedure similar to that for 2 using (R)-BINAPS2
instead of S2P2. Anal. Found: C, 53.81; H, 3.92; N, 0.00. Calcd
for C61H46OP2PdS2 3.5CHCl3 C4H10O: C, 54.15; H, 3.95; N,
3
3
0.00. 31P{1H} NMR (CHCl3): δ (relative to D3PO4 in external
1
D2O) 42.5 (s). H NMR (CDCl3): δ 1.21 (t, CH3- of diethyl
ether), 3.48 (q, -CH2- of diethyl ether), 7.10 (d, Ph-
3
CHdCH- of dba, JH-H = 16 Hz), 7.26 (s, CHCl3), 7.20-
7.80 (m, binap), 7.40-7.43 and 7.61-7.64 (m, Ph of dba), 7.75
(d, Ph-CHdCH- of dba, 3JH-H=16 Hz).
Bis[2-(diphenylphosphino)ethyl]phenylphosphine Trisulfide
(p3S3), 1,2-Bis(diphenylphosphino)ethane Disulfide (p2S2), Tri-
phenylphosphine Sulfide (Ph3PS), (R)-2,20-Bis(diphenylphos-
phino)-1,10-binaphthalene Disulfide ((R)-BINAPS2), and Triphe-
nylphosphine Selenide (Ph3PSe). These phosphine chalcogenides
[PdI2(P2)] and [PdI2((R)-binap)]. These diiodo complexes were
prepared by a similar procedure to that described in the litera-
ture14 using [Pd(CH3CN)4](BF4)2, the bidentate phosphine, and
[N(n-Bu)4]I (Supporting Information).
13
General Procedure for the C-C Coupling Reaction. The
Suzuki-Miyaura coupling reaction of iodobenzene (5.7 g,
27 mmol) with phenylboronic acid (4.1 g, 27 mmol) in DMF
(2.5 cm3) was carried out in air and under N2 at 125 °C in the
presence of the Pd(0) catalyst (0.011 mmol) and K2CO3 (3.5 g,
25 mmol) as a base. The yields were calculated by the 1H NMR
intensity of the ortho protons of formed biphenyl on the basis of
the intensity of the ethylene protons of bis(2-butoxyethyl) ether
contained as an internal reference and followed as a function of
time. For the polymer-supported catalyst, Pd(0) content was
adjusted to 0.011 mmol. To check the recyclability of the
catalyst, the residue containing the polymer-supported catalyst
was filtered after completion of the reaction, washed with DMF,
were prepared by a procedure similar to that reported for p2S2
using the corresponding phosphine and chalcogen.
[Pd(pp3S4)(dba)] (1). The tetrasulfide ligand pp3S4 (0.158 g,
0.198 mmol) and [Pd(dba)2] (0.104 g, 0.181 mmol) were dis-
solved in chloroform (5 cm3). The reaction mixture was allowed
to stand at room temperature for 1 h. The dark brown solid was
precipitated by adding diethyl ether and then filtered and air-
dried. Yield: 0.143 g (44%). Anal. Found: C, 44.64; H, 3.80; N,
0.00. Calcd for C59H56OP4PdS4 5CHCl3 C4H10O: C, 45.11; H,
3
3
3.95; N, 0.00%. 31P{1H} NMR (CHCl3): δ (relative to D3PO4 in
(11) Aizawa, S.; Tamai, M.; Sakuma, M.; Kubo, A. Chem. Lett. 2007,
36, 130.
(12) Aizawa, S.; Majumder, A.; Maeda, D.; Kitamura, A. Chem.
Lett. 2009, 38, 18.
(13) Aizawa, S.; Kondo, M.; Miyatake, R.; Tamai, M. Inorg. Chim.
Acta 2007, 360, 2809.
€
(14) Oberhauser, W.; Bachmann, C.; Stampfl, T.; Haid, R.; Bruggeller,
P. Polyhedron 1997, 16, 2827.

Article
Organometallics, Vol. 28, No. 20, 2009 6069
and air-dried. The reaction was repeated under the same condi-
tions by using the residual catalyst.
Scheme 1
Kinetics. Kinetic measurements for the chalcogen atom re-
placement reaction were carried out by monitoring the reaction
of triphenylphosphine selenide (0.0212 mol kg-1) with excess
sulfur (0.212-0.636 mol kg-1) in DMF in the presence (0.0106
mol kg-1) and absence of [Pd(dba)2]. Since the 31P NMR signal
intensities of phosphine selenide and sulfide were proportional
to their concentrations, the rate constants were obtained by the
initial slope method following change in the concentrations of
the phosphine chalcogenides, which were calculated from the
initial concentrations and the ratio of the 31P NMR signal
intensities. The reaction temperature was changed from 305 to
333 K for the catalytic reaction and from 333 to 373 K for the
noncatalytic reaction at fixed concentrations of phosphine
selenide (0.0212 mol kg-1) and sulfur (0.212 mol kg-1). Under
the same concentrations, the reaction rate was also checked in
chloroform at 333 K. The temperature dependence of the rate
constant was obtained under the same conditions as those for
the catalytic reaction using the corresponding Pt(0) complex
[Pt(dba)2].15
Conversion from Phosphine Oxide to Phosphine Sulfide. The
phosphine sulfide complex [Pd(pp3S4)(dba)] (0.029 mmol) was
heated in DMF (10 cm3) under reflux for 6 h, and the formation
of phosphine oxide groups was confirmed by 31P NMR mea-
surements. The solution was reacted with excess sulfur (29
mmol) at 125 °C for 2 h under N2, and conversion to phosphine
sulfide was checked by 31P NMR measurements. Similarly,
phosphine sulfide groups of p3S3 and p2S2 were also partially
converted to phosphine oxide ones, and phosphino groups of
(R)-BINAP were partially oxidized in the presence of 1/10 equiv
of Pd(0) complex [Pd(dba)2]. Conversion from phosphine oxide
groups to phosphine sulfide ones was carried out under the same
conditions as described for pp3S4.
polymer-supported PPh3, and (R)-BINAPS2, respectively.
The coordination of dba and phosphine sulfide ligands and
the compositions were confirmed by 31P and 1H NMR and
elementary analysis. All the diphenylphosphine sulfide
groups for each complex are regarded as equivalent from
the 31P NMR spectra, and the dba ligand is assumed to be
coordinated with the double bonds as proposed in the
literature.15 The schematic structures are shown in Scheme 1.
The catalytic activity for Suzuki-Miyaura coupling reaction
of 1 was compared with that of [Pd(PPh3)4] by using iodo-
benzene and phenylboronic acid as the substrates at high
temperature (125 °C) to confirm the stability of the catalyst.
The phosphine sulfide complex 1 exhibited almost the same
catalytic activity under N2 and in air, which was comparable
to that for [Pd(PPh3)4] under N2, while [Pd(PPh3)4] was
decomposed and deactivated during long-time reaction at
high temperature in air (Figure 1).
Furthermore, after keeping the completely reacted solu-
tion in air for several additional hours without residual
unreacted substrates, the brown solution for 1 maintained
catalytic activity for another addition of the substrates. To
the contrary, [Pd(PPh3)4] gradually decomposed into inac-
tive palladium black in air after consuming the substrates to
give the almost colorless supernatant. This fact indicates that
the Pd(0) in 1 is stabilized by the phosphine sulfide groups,
while [Pd(PPh3)4] itself is air-sensitive unless forming the
Pd(II) substrate adduct by oxidative addition. We have also
prepared the polymer-supported phosphine sulfide Pd(0)
complex 3 as a readily recyclable C-C coupling catalyst. 3
used in air was filtered from the completely reacted solution
and then reused in air again. The recycled 3 was not
deactivated by repetition of this procedure at least several
times (Figure 2).
Conversion from Phosphine Sulfide to Phosphine. The p2S2
complex [Pd(p2S2)(dba)] (0.0803 g, 0.10 mmol) was reacted with
an excess of iodobenzene (2.04 g, 10 mmol) in DMF (10 cm3) at
70 °C for 48 h under N2. The reaction solution was separated by
chromatography with an SiO2 column by elution with chloro-
form. The yellow eluate was concentrated to dryness, and the
yellow complex was recrystallized from chloroform and char-
acterized by 31P NMR. Only the yellow complex was formed by
the successive addition of 2 equiv of [Pd(dba)2] at 70 °C and
isolated by adding ethanol without column separation. Con-
version from BINAPS2 to BINAP was carried out by a
similar procedure using [Pd((R)-binapS2)(dba)] instead of
[Pd(p2S2)(dba)]. The purple complex was isolated by column
separation with SiO2 as described above and characterized by
31P NMR spectroscopy.
1
Measurements. 31P and H NMR spectra for solutions were
recorded on a JEOL JNM-A400 FT-NMR spectrometer oper-
ating at 160.70 and 399.65 MHz, respectively. In order to
determine the chemical shifts of 31P NMR signals, a 3 mm o.d.
NMR tube containing the sample solution was coaxially
mounted in a 5 mm o.d. NMR tube containing deuterated water
as a lock solvent and phosphoric acid as a reference. Solid state
31P NMR spectra were recorded on a Bruker AVANCE-600
NMR spectrometer operating at 242.94 MHz by using ammo-
nium dihydrogen phosphate as a reference. Inductively coupled
plasma (ICP) analyses were carried out using a Perkin-Elmer
Optima 2000XL.
Results and Discussion
C-C Coupling Reaction. The air-stable Pd(0) complexes 1,
2, 3, and [Pd((R)-binapS2)(dba)] were prepared by the reac-
tions of [Pd(dba)2] with phosphine sulfides pp3S4, and p2S2,
Chalcogen-Replacement Reaction. While phosphines are
known to be susceptible to oxidation in air, we found in this
work that the oxidation is catalyzed by Pd(0). As shown in
Figure 3, PPh3 was completely oxidized in a DMF solution of
(15) Cherwinski, W. J.; Johnson, B. F. G.; Lewis, J. J. Chem. Soc.,
Dalton Trans. 1974, 1405.

6070 Organometallics, Vol. 28, No. 20, 2009
Aizawa et al.
Figure 1. Change in the yield of biphenyl with time in the
Suzuki coupling reaction catalyzed by 1 (9, in air; 0, under
N2) and [Pd(PPh3)4] (b in air; O, under N2).
Figure 4. 31P NMR spectra of 1 (0.029 mmol) heated in DMF
(10 cm3) under reflux for 6 h (a) and the subsequently reacted
solution with excess sulfur (29 mmol) at 125 °C for 2 h (b).
replacement reaction of phosphine sulfide groups is possible
during the prolonged or repeated catalytic reaction using 1 or
3. However, pp3S4 was readily regenerated from the partially
oxygenated ligand in 1 by the reaction with excess sulfur in
DMF at 125 °C for 2 h (Figure 4b), while the conversion from
phosphine oxide to phosphine sulfide does not proceed at all
without Pd(0) even by reflux in DMF. Similarly, p3S3 and
p2S2 were also regenerated from the partially oxygenated
ligands in the presence of Pd(0) (Figure S1), and this new
catalytic reaction is applicable to recycling phosphine sulfide
Pd(0) catalysts.
Figure 2. Change in the yield of biphenyl with time in the
Suzuki coupling reaction catalyzed by 3 (b, freshly prepared;
O, recycled once; 9, recycled twice; 0, recycled three times; 2,
recycled four times) in air.
Reaction Mechanism. So far, a few chalcogen transfer
reactions from phosphine chalcogenides to phosphines were
reported,16,17 and two associative mechanisms have been
proposed for the chalcogen transfer by means of theoretical
calculations, as shown in Scheme 2.18 Mechanism 1 involves
a nucleophilic attack of the phosphine phosphorus on the
phosphine-chalcogenide phosphorus via a three-membered
cyclic transition state, and mechanism 2 is an X-philic attack
via a linear transition state.
In the present system, however, the chalcogen replacement
proceeds without phosphine nucleophile. In order to eluci-
date the reaction mechanism of the replacement, we under-
took kinetic experiments. Kinetic measurements were
carried out by monitoring the reaction of triphenylpho-
sphine selenide with excess sulfur in DMF in the presence
and absence of [Pd(dba)2] (dba = dibenzylideneacetone). By
following an increase in the 31P NMR signal intensity of
phosphine sulfide formed, it was confirmed that the observed
rates are first-order with respect to the phosphine-selenide con-
centration. Since the rates were considerably slow especially in
Figure 3. 31P NMR spectra of DMF solutions of PPh3 (0.171
mol kg-1) (a) and [Pd(PPh3)4] (0.043 mol kg-1) (b) allowed to
stand at room temperature for 4 days and 2 h, respectively. ref
denotes the signal for D3PO4 in the outer D2O.
[Pd(PPh3)4] at room temperature in 2 h by dissolved oxygen,
although only ca. 20% of free PPh3 was oxidized by keeping
the solution for 4 days under the same conditions. This
catalysis leads to a serious problem in the phosphine-assisted
palladium-catalyzed reactions. Furthermore, chalcogen-re-
placement reactions of phosphine chalcogenides were also
revealed to be catalyzed by Pd(0). For example, the sulfur
atoms in free pp3S4 are hardly replaced by oxygen atoms, but
those in 1 were partially replaced to give some phos-
phine oxide groups at high temperature (Figure 4a). Such a
(16) Field, L. D.; Wilkinson, M. P. Tetrahedron Lett. 1997, 37, 2779.
(17) Bollmark, M.; Stawinski, J. J. Chem Soc., Chem. Commun. 2001,
771.
(18) Kullberg, M.; Stawinsky, J. J. Organomet. Chem. 2005, 690,
2571.

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Organometallics, Vol. 28, No. 20, 2009 6071
Scheme 2
Figure 5. Temperature dependence of the rate constants for the
chalcogen replacement reaction in the presence (9) and absence
(b) of Pd(0).
the absence of Pd(0), the rate constants were obtained by the
initial slope method. The observed rate constants were inde-
pendent of the sulfur concentration (Figure S2), and the
temperature dependence of the rate constants in Figure 5 gave
the activation parameters ΔH‡=110 ( 1 kJ mol-1 and ΔS‡ =
26 ( 2 J mol-1 K-1 in the presence of Pd(0) and ΔH‡=159 (
1 kJ mol-1 and ΔS‡=103 ( 2 J mol-1 K-1 in the absence of
Pd(0).
The sulfur-concentration independency of the observed
rate constants and the positive ΔS‡ values indicate dissocia-
tive activation. The smaller positive ΔS‡ value in the presence
of Pd(0) is attributed to the interaction between the phos-
phine chalcogenide and Pd(0) in the activation state in which
the PdX bond breaking is promoted enthalpically. The
catalytic activation may be caused by π back-donation from
Pd(0) to the π* orbital of the PdX bond,19 which is sup-
ported by the fact that the π back-donating Pt(0) center of
the corresponding Pt(0) complex, [Pt(dba)2],15 also showed
similar catalytic activity. By the kinetic experiments, the
sulfur-concentration independency of the observed rate con-
stant and the activation parameters, ΔH‡=116 ( 1 kJ mol-1
and ΔS‡ = 32 ( 2 J mol-1 K-1, similar to those for the Pd(0)
catalysis were obtained for the Pt(0) complex (Figure S3).
Because the catalytic reaction rate was unchanged in chloro-
form, it seems that DMF solvent does not participate in the
activation. Considering that oxidation of phosphine, that is,
phosphine chalcogenide formation, was also promoted by
Pd(0) as mentioned above, acceleration of the chalcogen-
atom replacement results from the catalytic PdX bond
cleavage and also formation by Pd(0). The π back-donation
from Pd(0) to the phosphine P atom can increase electron
density on the P atom to promote the oxidative chalcogenide
formation as well as the activation of the PdX bond.
Isolation of Phosphine Intermediate. When 1 was reacted
with relatively labile [PtCl2(NCC6H5)2] in DMF at 95 °C for
several hours, the 31P NMR signals for phosphino groups
coordinated to Pt(II) and Pd(II) formed by oxidation of
Pd(0) were observed (Figure S4).20 Such a PdS bond clea-
vage was not observed for a DMF solution containing pp3S4
and [PtCl2(NCC6H5)2] without Pd(0). The phosphine inter-
mediate 1,2-bis(diphenylphosphino)ethane (p2) can be
successfully separated as the chelate compound of Pd(II)
Figure 6. 31P NMR spectrum of the DMF solution of
[Pd(p2S2)(dba)] reacted with an excess of iodobenzene at 70 °C
for 48 h under N2 (a) and chloroform solution of isolated yellow
Pd(II) complex (b).
by employing the novel catalysis of Pd(0) as follows. The
p2S2 complex 2 was reacted with an excess of iodobenzene in
DMF at 70 °C for 48 h under N2 (see Experimental Section).
About one-third of p2S2 was converted to p2, which is
coordinated to Pd(II), showing a 31P NMR singlet at 65.9
ppm (Figure 6a). The yellow Pd(II) complex was separated
by chromatography with an SiO2 column. The 31P NMR
spectrum of the isolated Pd(II) complex in chloroform was in
agreement with that of [PdI2(p2)]12 exactly, showing the
singlet at 61.9 ppm (Figure 6b). The formation of the iodo
Pd(II) complex is attributed to oxidative addition of the
Pd(0) complex to iodobenzene followed by disproportiona-
tion (Scheme 3a). The phosphine oxide was mainly formed
without iodobenzene because oxidative addition of Pd(0) did
not proceed, and consequently p2 was not trapped by Pd(II)
and finally oxidized (Scheme 3b). When a Pd(II) complex
such as [Pd(CH3CN)4](BF4)2 was used to trap the phosphine
(19) Dobado, J. A.; Martınez-Garcıa, H.; Molina, J. M.; Sundberg,
´ ´
M. R. J. Am. Chem. Soc. 1998, 120, 8461.
(20) Aizawa, S.; Saito, K.; Kawamoto, T.; Matsumoto, E. Inorg.
Chem. 2006, 45, 4859.

6072 Organometallics, Vol. 28, No. 20, 2009
Aizawa et al.
Scheme 3
(R)-BINAP was easily converted to (R)-BINAPS2 in a
similar manner to that described for the other oxygenated
phosphine sulfides (see Experimental Section). Regeneration
of (R)-BINAP was successful by the reaction of [Pd((R)-
binapS2)(dba)] with excess iodobenzene, as in the case of
regeneration of p2. The 31P NMR spectrum of the isolated
purple complex is in good agreement with that of
[PdI2(binap)], showing a singlet at 17.8 ppm. It is confirmed
that the optical purity of (R)-BINAP was completely re-
tained during this regeneration procedure, because the CD
intensity of the isolated complex normalized by the absor-
bance was in good agreement with that for [PdI2((R)-binap)]
separately prepared from (R)-BINAP ligand (Figure S5).
These facts suggest that the new palladium(0)-promoted
PdX bond activation is widely applicable to regeneration
of phosphine or phosphine sulfide ligands.
Conclusions
Air-stable phosphine sulfide palladium(0) complexes 1-3
and [Pd((R)-binapS2)(dba)] were prepared. High thermal
stability of 1 and good recyclability of 3 were shown for
Suzuki-Miyaura coupling in air. A mechanistic study re-
vealed that the new catalytic chalcogen-atom replacement
results from activation of the PdX bond as well as promo-
tion of the oxidative chalcogenide formation. The intermedi-
ate phosphine was successfully trapped as a phosphine Pd(II)
complex. The palladium(0)-promoted PdX bond activation
is applicable to regeneration of phosphine or phosphine
sulfide from oxidized phosphine.
instead of adding iodobenzene, the conversion from p2S2 to
p2 was hardly observed due to the formation of the Pd(II)
13
complex with p2S2 prior to the catalytic reaction of p2S2
with Pd(0) (Scheme 3c). Almost quantitative formation of p2
from p2S2 and isolation as the Pd(II) complex was achieved
without chromatographic separation by the subsequent
addition of [Pd(dba)2], which acts as a catalyst and a source
of [PdI2(p2)] (Scheme 3d). This result gave us evidence of the
dissociation of chalcogen from the phosphine chalcogenide
and revealed that the phosphine can be regenerated easily
from the phosphine sulfide by PdS bond activation by Pd(0)
and its oxidative addition.
Supporting Information Available: Preparation procedures,
31P NMR for chalcogen-replacement reactions, kinetic data,
and CD spectrum of the isolated binap complex. This material is
available free of charge via the Internet at http://pubs.acs.org.
Regeneration Reaction. As an example of application of
this palladium-assisted PdX bond activation, regeneration
of optically active BINAP was performed. Partially oxidized