Oxidative addition of ligand-chelated palladium(0) to aryl halides: Comparison between 1,2-bisthioethers and 1,2-Bisphosphines

Article abstract of DOI:10.1021/om060846x

The kinetics of the oxidative addition of bidentate ligand-chelated Pd ⁰ complexes to phenyl iodide and bromide has been studied via cyclic voltammetry. The dibenzylideneacetone (dba) delivered by the palladium precursor Pd(dba)₂ was

Full text of DOI:10.1021/om060846x

Organometallics 2007, 26, 455-458
455
Notes
Oxidative Addition of Ligand-Chelated Palladium(0) to Aryl
Halides: Comparison between 1,2-Bisthioethers and
1,2-Bisphosphines
Giuseppe Paladino,† David Madec,‡ Guillaume Prestat,‡ Guillaume Maitro,‡
Giovanni Poli,*,‡ and Anny Jutand*,†
De´partement de Chimie, Ecole Normale Supe´rieure, UMR CNRS-ENS-UPMC 8640, 24 Rue Lhomond,
F-75231 Paris Cedex 5, France, and Laboratoire de Chimie Organique (UMR CNRS 7611), Institut de
Chimie Mole´culaire (FR2769), UniVersite´ Pierre et Marie Curie-Paris 6, Tour 44-45, Boˆıte 183,
4 Place Jussieu, F-75252 Paris Cedex 05, France
ReceiVed September 15, 2006
Scheme 1
Summary: The kinetics of the oxidatiVe addition of bidentate
ligand-chelated Pd0 complexes to phenyl iodide and bromide
has been studied Via cyclic Voltammetry. The dibenzylideneac-
etone (dba) deliVered by the palladium precursor Pd(dba)2 was
found to affect the concentration of the more reactiVe dba-free
Pd0 complex and consequently the kinetics of the oVerall
oxidatiVe addition. The complexes generated from Pd(dba)2 and
PhSCH2CH2SPh (pte) were found to be considerably more
reactiVe than those generated from Pd(dba)2 and Ph2PCH2CH2-
PPh2 (dppe). The former complexes can react with PhBr at low
temperatures.
Earlier works unambiguously established that interaction of
Pd0(dba)2 with 1 equiv of a bidentate P,P ligand such as diop,
binap, or dppf generates the two complexes Pd0(dba)(P,P) and
Pd0(P,P) (Scheme 1). Although both the complexes are reactive
toward PhI, the former and more abundant dba-ligated complex7
is intrinsically less reactive than the minor dba-free one (Scheme
1).8
Introduction
The field of catalysis is witnessing an ever-growing attention,
and several highly efficient methods are currently known in the
literature.1 Despite these achievements, knowledge in this field
is still partial and much effort will be needed to make this
methodology comprehensive and well-established. In this
context, the nature of the subsidiary ligands used for a given
metal-catalyzed process is central, P- and N-based ligands
occupying an unquestionable primary position.
On the other hand, despite the vast knowledge on sulfur-
metal interactions in coordination chemistry,2 the use of S-based
ligands in catalysis is still rather underdeveloped.3 Indeed, the
thioether function, known to be less coordinating than an amine,
an imine, or a phosphine, has been so far mainly exploited as
the hemilabile part of mixed (P,S)4 or (N,S) ligands.5 On the
other hand, ligands based exclusively on thioethers have been
only sporadically reported.6
(4) For (P,S) ligands see: (a) Khiar, N.; Sua´rez, B.; Valdivia, V.;
Ferna´ndez, I. Synlett 2005, 2963-2967. (b) Malacea, R; Manoury, E.;
Routaboul, L.; Daran, J.-C.; Poli, R.; Dunne, J. P.; Withwood, A. C.; Godard,
C.; Duckett, S. B. Eur. J. Inorg. Chem. 2006, 1803-1816. (c) Molander,
G. A.; Burke, J. P.; Carroll, P. J. J. Org. Chem. 2004, 69, 8062-8069. (d)
Die´guez, M.; Pa`mies, O.; Claver, C. J. Organomet. Chem. 2006, 691, 2257-
2262. (e) Guimet, E.; Die´guez, M.; Ruiz, A.; Claver, C. Tetrahedron:
Asymmetry 2005, 16, 959-963. (f) Lam, F. L.; Au-Yeung, T. T. L.; Cheung,
H. Y.; Kok, S. H. L.; Lam, W. S.; Wong, K. Y.; Chan, A. S. C.
Tetrahedron: Asymmetry 2006, 17, 497-499. (g) Evans, D. A.; Campos,
K. R.; Tedrow, J. S.; Michael, F. E.; Gagne´, M. R. J. Org. Chem. 1999,
64, 2994-2995. (h) Evans, D. A.; Campos, K. R.; Tedrow, J. S.; Michael,
F. E.; Gagne´, M. R. J. Am. Chem. Soc. 2000, 122, 7905-7920. (i) Faller,
J. W.; Wilt, J. C.; Parr, J. Org. Lett. 2004, 6, 1301-1304. (j) Faller, J. W.;
Wilt, J. C. Organometallics 2005, 24, 5076-5083. (k) Verdarguer, X.;
Perica`s, M. A.; Riera, A.; Maestro, M. A.; Mah´ıa, J. Organometallics 2003,
22, 1868-1877. (l) Manchen˜o, O. G.; Priego, J.; Cabrera, S.; Arraya´s, R.
G.; Llamas, T.; Carretero, J. C. J. Org. Chem. 2003, 68, 3679-3686. (m)
Cabrera, S.; Arraya´s, R. G.; Carretero, J. C. Angew. Chem., Int. Ed. 2004,
43, 3944-3949. (n) Cabrera, S.; Arraya´s, R. G.; Alonso, I.; Carretero, J.
C. J. Am. Chem. Soc. 2005, 127, 17938-17947. (o) Zhang, W.; Xu, Q.;
Shi, M. Tetrahedron: Asymmetry 2004, 15, 3161-3169.
(5) For (N,S) ligands see: (a) Chelucci, G.; Baldino, S. Tetrahedron:
Asymmetry 2006, 17, 1529-1536. (b) Ekegren, J. K.; Roth, P.; Ka¨lstro¨m,
K.; Tarnai, T.; Andersson, P. G. Org. Biomol. Chem. 2003, 1, 358-366.
(c) Koning, B.; Meetsma, A.; Kellog, R. M. J. Org. Chem. 1998, 63, 5533-
5540. (d) Voituriez, A.; Fiaud, J.-C.; Schulz, E. Tetrahedron Lett. 2002,
43, 4907-4909. (e) Chelucci, G.; Cabras, M. A. Tetrahedron: Asymmetry
1996, 7, 965-966. (f) Allen, J. V.; Bower, J. F.; Williams, J. M. J.
Tetrahedron: Asymetry 1994, 5, 1895-1898. (g) Bulman Page, P. C.;
Heaney, H.; Reignier, S.; Rassias, G. A. Synlett 2003, 22-28.
* To whom correspondence should be addressed. E-mail:
anny.jutand@ens.fr; giovanni.poli@upmc.fr.
† Ecole Normale Supe´rieure.
‡ Universite´ Pierre et Marie Curie.
(1) Handbook of Organopalladium Chemistry for Organic Synthesis;
Negishi, E.-I., Ed.; John Wiley & Sons: New York, 2002.
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(b) Schro¨der, M. In S-Donor Ligands, Encyclopedia of Inorganic Chemicals;
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10.1021/om060846x CCC: $37.00 © 2007 American Chemical Society
Publication on Web 12/13/2006

456 Organometallics, Vol. 26, No. 2, 2007
Notes
Figure 1. Cyclic voltammetry of Pd0(dba)(pte) generated from Pd0-
(dba)2 (2 mM) and pte (1.0 equiv) in DMF (containing nBu4NBF4,
0.3 M) at a stationary gold disk electrode (d ) 0.5 mm) with a
scan rate of 0.5 V s-1, at 20 °C.
Figure 2. Kinetics of the oxidative addition of PhI (2 mM) with
the palladium(0) complexes generated in situ from Pd0(dba)2 (2
mM) and pte (1.0 equiv) in DMF (containing nBu4NBF4, 0.3 M)
at 10 °C. Plot of 1/x (see text) versus time.
It is known that, due to their acceptor character, phosphorus-
based ligands can efficiently stabilize the transiently generated
Pd(0) complex, whereas less acceptor ligands tend to bring about
premature Pd(0) precipitation.9 Furthermore, phosphorus-based
ligands are known to electronically activate η3-allyl Pd com-
plexes toward nucleophilic additions. Nevertheless, 13C spec-
troscopic measurements on η3-allyl Pd complexes indicate that
Ph2PCH2CH2PPh2 (dppe) and PhSCH2CH2SPh (pte) do not
differ substantially in their π-accepting properties.10
Scheme 2
The oxidation peak at +0.495 V was not modified upon
addition of excess dba. Consequently, it characterizes the
complex Pd0(dba)(pte) (Scheme 2). This result confirms that,
despite the negligible back-donation properties of sulfur with
respect to phosphorus, the bis-thioether ligand is able to stabilize
a Pd(0) complex.11
We report herein kinetic data on the reactivity of the Pd(0)
complexes, generated by reacting Pd0(dba)2 with 1 equiv of the
bidentate S,S ligand: pte, in oxidative addition with aryl halides
in DMF. The kinetic data are compared with those obtained
with the related P,P ligand dppe.
The oxidation peak of Pd0(dba)(pte) disappeared upon addi-
tion of PhI (10 equiv), thereby attesting to the formation of
reactive Pd(0) complexes generated from interaction between
Pd0(dba)2 and pte. These results are then very similar to those
obtained with P,P7 or P,N12 ligands. The major complex is still
ligated by dba delivered from the precursor Pd0(dba)2. No
oxidation peak due to the dba-free complex Pd0(pte) could be
detected due to its very low concentration (vide infra).
2. Rate and Mechanism of the Oxidative Addition of Aryl
Halides to the Pd(0) Complexes Generated from Pd0(dba)2
and pte in DMF. The reaction of PhI (2 mM) with the Pd(0)
complexes generated from Pd0(dba)2 (2 mM) and pte (2 mM)
in DMF was too fast to be followed by cyclic voltammetry at
room temperature. At the lower temperature of 10 °C, the
oxidative addition was still fast, but the kinetics could be
followed by performing cyclic voltammetry versus time,
exploiting the linear relation between the oxidation peak current
of Pd0(dba)(pte) and its concentration. The plot of 1/x (x )
[Pd0]t/[Pd0]0 ) it/i0; it, oxidation peak current of Pd0(dba)(pte)
at t; i0, initial oxidation peak current of Pd0(dba)(pte)) versus
time was linear (Figure 2), as expected for a reaction performed
under stoichiometric conditions (1/x ) kobst + 1) and attesting
to a first-order reaction for the Pd(0) complex and PhI as well.
However, the oxidative addition was too fast to get accurate
data (low correlation coefficient). The half-life time was
nevertheless estimated to be t1/2 ) 150 ( 10 s (DMF, [PhI]0 )
2 mM, 10 °C).
Results and Discussion
1. Characterization of the Complex Generated from Pd0-
(dba)2 and PhSCH2CH2SPh (pte) in DMF. The Pd(0) complex
generated by reacting Pd0(dba)2 (2 mM) with pte (1.0 equiv)
was characterized by cyclic voltammetry performed in DMF
containing nBu4NBF4 (0.3 M). A single oxidation peak was
observed at Eoxp ) +0.495 V vs SCE at a stationary gold disk
electrode with a scan rate of 0.5 V s-1 (Figure 1). The oxidation
peak of Pd0(dba)2 at +1.06 V (observed when alone) was no
longer detected. A reduction peak characteristic of the reduction
of 1 equiv of free dba was also observed.
(6) (a) Khiar, N.; Arau´jo, C. S.; Alvarez, E.; Ferna´ndez, I. Tetrahedron
Lett. 2003, 44, 3401-3404. (b) Khiar, N.; Arau´jo, C. S.; Sua´rez, B.; Alvarez
E.; Ferna´ndez, I. Chem. Commun. 2004, 714-715. (c) Khiar, N.; Sua´rez,
B.; Ferna´ndez, I. Inorg. Chim. Acta 2006, 359, 3048-3053. (d) Khiar, N.;
Arau´jo, C. S.; Sua´rez, B.; Ferna´ndez, I. Eur. J. Org. Chem. 2006, 1685-
1700. (e) Ferna´ndez, F.; Go´mez, M.; Jansat, S.; Muller, G.; Martin, E.;
Flores-Santos, L.; Garc´ıa, P. X.; Acosta, A.; Aghmiz, A.; Gime´nez-Pedro´s,
M.; Masdeu-Bulto´.; A. M.; Die´guez, M.; Claver, C.; Maestro, N. A.
Organometallics 2005, 24, 3946-3956. (f) Bastero, A.; Claver, C.; Ruiz,
A. Catal. Lett. 2002, 82, 85-88. (g) Okuyama, Y.; Nakano, H.; Takahashi,
K.; Hongo, H.; Kabuto, C. Chem. Commun. 2003, 524-525. See also: (h)
Madec, D.; Mingoia, F.; Macovei, C.; Maitro, G.; Giambastiani, G.; Poli,
G. Eur. J. Org. Chem. 2005, 552-557.
(7) Amatore, C.; Broeker, G.; Jutand, A.; Khalil, F. J. Am. Chem. Soc.
1997, 119, 5176-5185.
(8) For a review, see: Amatore, C.; Jutand, A. Coord. Chem. ReV. 1998,
178-180, 511-528.
(9) Åkermark, B.; Zetterberg, K.; Hansson, S.; Krakenberger, B. J.
Organomet. Chem. 1987, 335, 133-142.
In order to get more information on the mechanism of the
oxidative addition, we then turned our attention to the reaction
(10) Chelated Pd(II) complexes involving pte as ligand: (a) Åkermark,
B.; Krakenberger, B.; Hansson, S.; Vitagliano, A. Organometallics 1987,
6, 620-628. (b) Bartolome´, C.; Espinet, P.; Mart´ın-Alvarez, J. M.; Villafan˜e,
F. Eur. J. Inorg. Chem. 2003, 3127-3138. (c) Bartolome´, C.; Espinet, P.;
Mart´ın-Alvarez, J. M.; Villafan˜e, F. Eur. J. Inorg. Chem. 2004, 2326-
2337.
1
(11) The complex could not be better characterized by H NMR due to
its limited stability in CDCl3.
(12) Amatore, C.; Fuxa, A.; Jutand, A. Chem.-Eur. J. 2000, 6, 1474-
1482.

Notes
Organometallics, Vol. 26, No. 2, 2007 457
Table 1. Comparative Reactivity of Aryl Halides in Their
Oxidative Addition to the Pd(0) Complexes Generated from
Pd0(dba)2 and pte or dppea in DMF at 10 °C
ligand
PhI (mM)
t
1/2 (s)
PhBr (mM)
t1/2 (s)
pte
dppe
2
200
150
8500
200
1830
n.d.b
n.d.b
b
a [Pd0(dba)2] ) 2 mM; [pte] ) 2 mM, [dppe] ) 2 mM. n.d. ) not
determined.
(dba)(pte). In the absence of spectroscopic characterization, this
is an a posteriori proof that the major Pd(0) complex is indeed
ligated by dba.
From the kinetic law, one has kobs ) k1 + k0[PhBr].7 This
gives k1 ) 3 × 10-4 s-1, k0 ) 3.7 × 10-4 M-1 s-1, and k2 .
3.7 × 10-4 M-1 s-1 (DMF, PhBr, 10 °C).
3. Comparative Reactivity of PhI and PhBr in Their
Oxidative Addition with the Pd(0) Complexes Generated
from Pd0(dba)2 and pte or dppe in DMF. From the half-life
times collected in Table 1, the expected reactivity order is
observed, PhI . PhBr, when pte is the ligand of the Pd(0)
complexes (first line in Table 1). The reaction of PhI (200 mM)
with the Pd(0) complexes generated from Pd0(dba)2 (2 mM)
and dppe (1.0 equiv) in DMF was very slow at 10 °C. The
value of the half-life time given in Table 1 is compared to that
obtained for pte. The Pd(0) complexes ligated by pte are thus
considerably more reactive than those ligated by dppe in DMF
at 10 °C.
Conclusion. The major Pd(0) complex formed in the reaction
of Pd(dba)2 with 1 equiv of PhSCH2CH2SPh (pte) in DMF is
still ligated by dba, Pd0(dba)(pte), and is in equilibrium with
Pd0(pte). Both complexes are reactive toward oxidative addition.
The oxidative addition of PhBr proceeds at 10 °C. Comparison
with the related ligand dppe shows that the oxidative addition
in the presence of pte is more rapid. This may be due either to
the higher intrinsic reactivity of the (pte)-ligated Pd(0) com-
plexes compared to (dppe)-ligated Pd(0) or/and to the fact that,
in the case of the pte ligand, the equilibrium lies more in favor
of the more reactive Pd(0)(pte) complex. In other words, the
concentration of the Pd(0)(pte) would be higher than that of
Pd(0)(dppe) in their respective equilibrium with the (dba)-ligated
Pd(0) complexes. Once again, the dba delivered by the precursor
appeared to be “noninnocent”, as it controls the concentration
and thus the reactivity of the more reactive complex Pd0(pte).
Figure 3. Kinetics of the oxidative addition of PhBr with the
palladium(0) complexes generated in situ from Pd0(dba)2 (2 mM)
and pte (1.0 equiv) in DMF (containing nBu4NBF4, 0.3 M) at 10
°C. (a) [PhBr] ) 0.8 M. Plot of ln x (see text) versus time. ln x )
-kobst. (b) Determination of the reaction order of PhBr. Plot of
kobs versus PhBr concentration: (b) in absence of dba; (O) in
presence of dba (20 mM).
Scheme 3
Experimental Section
Chemicals. DMF was distilled from calcium hydride under
vacuum and kept under argon. Dba and dppe were commercially
13
available. Pd(dba)2 and pte14 were synthesized according to
reported procedures.
with bromobenzene, expected to be less reactive. The kinetics
of the oxidative addition was still followed by cyclic voltam-
metry at 10 °C in the presence of excess PhBr. The plot of ln
x (x ) [Pd0]t/[Pd0]0 ) it/i0; it, oxidation peak current of Pd0-
(dba)(pte) at t; i0, initial oxidation peak current of Pd0(dba)-
(pte)) against time was linear (Figure 3a) with ln x ) -kobst,
attesting to a first-order reaction for the Pd(0) complex.
The kinetics of the oxidative addition was investigated at
different PhBr concentrations, in the range 0.2-0.8 M. The plot
of kobs versus PhBr concentration gave a straight line with a
positive intercept (Figure 3b): kobs ) 0.00029422 + 0.00037281-
[PhBr]. This means that two different Pd(0) species react
concomitantly with PhBr (Scheme 3), as already observed for
bidentate P,P ligands.7 Moreover, the oxidative addition was
slower in the presence of added dba (Figure 3b), attesting to
the fact that Pd0(pte) is intrinsically more reactive than Pd0-
Electrochemical Setup for Cyclic Voltammetry. Experiments
were carried out in a three-electrode thermostated cell connected
to a Schlenk line. The counter electrode was a platinum wire of
ca. 1 cm2 apparent surface area. The reference was a saturated
calomel electrode separated from the solution by a bridge filled
with 3 mL of DMF containing nBu4NBF4 (0.3 M). The working
electrode was a gold disk electrode (d ) 0.5 mm).
Typical Procedure for the Characterization of the Pd(0)
Complexes Generated from Pd(dba)2 and pte and the Investiga-
tion of the Kinetics of the Oxidative Addition of Aryl Halides.
(13) (a) Takahashi, Y.; Ito, T.; Sakai, S.; Ishii, Y. J. Chem. Soc., Chem.
Commun. 1970, 1065-1066. (b) Rettig, M. F.; Maitlis, P. M. Inorg. Synth.
1977, 17, 134-137.
(14) Miranda-Soto, V.; Parra-Hake, M.; Morales-Morales, D.; Toscano,
R. A.; Boldt, G.; Koch, A.; Grotjahn, D. B. Organometallics 2005, 24,
5569-5575.

458 Organometallics, Vol. 26, No. 2, 2007
Notes
Degassed DMF (15 mL) containing nBu4NBF4 (0.3 M) was charged
into the electrochemical cell followed by 17.2 mg (30 µmol) of
Pd(dba)2 and 7.4 mg (30 µmol) of pte. Cyclic voltammetry was
performed at a scan rate of 0.5 V s-1 to characterize Pd0(dba)(pte).
Then 320 µL (3.03 mmol) of PhBr was then introduced, and the
reaction was monitored by performing cyclic voltammetry from
time to time at the same scan rate of 0.5 V s-1, until total
conversion. In other experiments, the amount of PhBr was varied
and 70.3 mg (300 µmol) of dba was added before the introduction
of PhBr. In another experiment, 3.4 µL (30 µmol) of PhI was added
and the kinetics was monitored by cyclic voltammetry. A similar
reaction was performed with the ligand dppe (11.94 mg, 30 µmol)
and 674 µL (3 mmol) of PhI.
Acknowledgment. The support and sponsorship concerted
by CNRS, ENS, and COST Action D24 “Sustainable Chemical
Processes: Stereoselective Transition Metal-Catalysed Reac-
tions” are kindly acknowledged.
OM060846X