Influence of the dba substitution on the reactivity of palladium(0) complexes generated from Pd02(dba-n,n′ -Z) 3 or Pd0(dba-n,n′-Z)2 and PPha 3 in oxidative addition with iodobenzene

Article abstract of DOI:10.1021/om0510876

The reactivity of Pd(0) complexes generated by addition of PPh₃ (PPh₃/Pd = 2, 4) to either Pd⁰₂(dban,n′- Z)₃ (n,n′-Z = 4,4′-F, 4,4′-H, 4,4′-MeO, 3,3′,4,4′,5,5′-OMe) or Pd⁰(dba-n,n′-Z) ₂ (n,n′-Z = 4,4′-Br, 4,4′-Cl, 4,4′-H, 4,4′-CH₃, 3,3′,5,5′-OMe) in DMF is affected by the electron-donating or -accepting properties of the groups Z substituted on the aromatic rings of dba. Whatever the nature of Z, the unreactive major complexes Pd⁰(η²-dba-n,n′-Z)(PPh₃)₂ are formed, which are in equilibrium with the common reactive complex Pd ⁰(PPh₃)₂ and dba-n,n′-Z. The latter controls the concentration of the reactive Pd⁰(PPh₃) ₂ and, consequently, also controls the rate of the overall oxidative addition with phenyl iodide. The more electron donating the Z group, the lower the affinity of dba-4,4′-Z for Pd⁰(PPh₃) ₂. As a result, the overall rate of the oxidative addition with Phi is faster when Z is an electron-donating group. For a given Z, the overall oxidative addition is faster when using Pd⁰2(dba-n,n′-Z) ₃ instead of Pd⁰(dba-n,n′-Z)₂. Therefore, the rate of the oxidative addition can be modulated by changing the electronic properties of the dba ligands determined by substituents on its phenyl groups and by changing the structure of the precursors: P⁰₂ (dba-n,n′-Z)₃ versus Pd⁰(dba-n,n′-Z) ₂.

Full text of DOI:10.1021/om0510876

Organometallics 2006, 25, 1795-1800
1795
Influence of the dba Substitution on the Reactivity of Palladium(0)
Complexes Generated from Pd0 (dba-n,n′-Z)3 or Pd0(dba-n,n′-Z)2
2
and PPh3 in Oxidative Addition with Iodobenzene
Yohan Mace´,† Anant R. Kapdi,‡ Ian J. S. Fairlamb,*,‡ 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 Department of Chemistry, UniVersity of York,
Heslington, York YO10 5DD, U.K.
ReceiVed December 22, 2005
The reactivity of Pd(0) complexes generated by addition of PPh3 (PPh3/Pd ) 2, 4) to either Pd0 (dba-
2
n,n′-Z)3 (n,n′-Z ) 4,4′-F, 4,4′-H, 4,4′-MeO, 3,3′,4,4′,5,5′-OMe) or Pd0(dba-n,n′-Z)2 (n,n′-Z ) 4,4′-Br,
4,4′-Cl, 4,4′-H, 4,4′-CH3, 3,3′,5,5′-OMe) in DMF is affected by the electron-donating or -accepting
properties of the groups Z substituted on the aromatic rings of dba. Whatever the nature of Z, the unreactive
major complexes Pd0(η2-dba-n,n′-Z)(PPh3)2 are formed, which are in equilibrium with the common reactive
complex Pd0(PPh3)2 and dba-n,n′-Z. The latter controls the concentration of the reactive Pd0(PPh3)2 and,
consequently, also controls the rate of the overall oxidative addition with phenyl iodide. The more electron
donating the Z group, the lower the affinity of dba-4,4′-Z for Pd0(PPh3)2. As a result, the overall rate of
the oxidative addition with PhI is faster when Z is an electron-donating group. For a given Z, the overall
oxidative addition is faster when using Pd0 (dba-n,n′-Z)3 instead of Pd0(dba-n,n′-Z)2. Therefore, the rate
2
of the oxidative addition can be modulated by changing the electronic properties of the dba ligands
determined by substituents on its phenyl groups and by changing the structure of the precursors: Pd0 -
2
(dba-n,n′-Z)3 versus Pd0(dba-n,n′-Z)2.
Scheme 1
Introduction
It has been established that the coordination of Pd0L2 (L )
phosphine, arsine) complexes by alkenes,1,2 alkynes,1,3 or
vinyltin derivatives,1,4 which are reagents in palladium-catalyzed
Heck, Sonogashira, and Stille coupling processes, respectively,
strongly affects the kinetics of the oxidative addition to aryl
halides, which is the first step of their catalytic cycles (Scheme
1). Indeed, the complexation of the active Pd0L2 complex to
form the unreactive complexes (η2-CH2dCHR)Pd0L2 or (η2-
CHtCR)Pd0L2 results in slower oxidative additions by decreas-
ing the concentration of the active Pd0L2 complex (Scheme 1).
It was recently reported that (η2-alkene)Pd0(iminophosphine)-
catalyzed Suzuki-Miyaura reactions resulted in higher reaction
rates when the alkene is a moderate π-accepting ligand.5 This
is explained by a stabilization of the active Pd0(iminophosphine)
complex by the alkene, thus preventing its decomposition to
palladium black.
Scheme 2
reactions (Heck, Stille, Tsuji-Trost, etc.). It has been established
by Amatore, Jutand, et al. that the presumably “innocent” dba
ligand delivered by the precursor Pd0(dba)2 in reality plays a
significant role in catalysis via alkene ligation to the active Pd0L2
complexes to generate Pd0(dba)L2 complexes (Scheme 2).6-9
The presence of the dba ligand has a great consequence on the
kinetics of oxidative additions with aryl halides, since dba
controls the concentration and then the reactivity of the active
species Pd0L2 (Scheme 3).6-8 Indeed, Pd0L2 is always generated
at low concentration, due to its unfavorable equilibrium with
Pd0(dba)L2, the major but unreactive species (Scheme 3).6-8
It has been reported by Fairlamb et al. that the efficiency of
palladium-catalyzed Suzuki-Miyaura cross-coupling of aryl
Pd0 (dba)3 or Pd0(dba)2 (dba ) trans,trans-dibenzylidene-
2
acetone), associated with ligands L (triarylphosphines), are often
employed as sources of Pd0 complexes in palladium-catalyzed
* To whom correspondence should be addressed. Fax: 33 1 4432 3325
(A.J.). E-mail: Anny.Jutand@ens.fr (A.J.); ijsf1@york.ac.uk (I.J.S.F.).
† Ecole Normale Supe´rieure.
‡ University of York.
(1) Jutand, A. Pure Appl. Chem. 2004, 76, 565-576.
(2) Amatore, C.; Carre´, E.; Jutand, A.; Medjour, Y. Organometallics
2002, 21, 4540-4545.
(3) Amatore, C.; Bensalem. S.; Ghalem, S.; Jutand, A.; Medjour, Y. Eur.
J. Org. Chem. 2004, 366-371.
(6) Amatore, C.; Jutand, A.; Khalil, F.; M’Barki, M. A.; L. Mottier, L.
Organometallics 1993, 12, 3168-3178.
(4) Amatore, C.; Bucaille, A.; Fuxa, A.; Jutand, A.; Meyer, G.; Ndedi
Ntepe, A. Chem. Eur. J. 2001, 7, 2134-2142.
(7) Amatore, C.; Jutand, A.; Meyer, G. Inorg. Chim. Acta 1998, 273,
76-84.
(5) (a) Scrivanti, A.; Beghetto, V.; Matteoli, U.; Antonaroli, S.; Marini,
A.; Mandoj, F.; Paolesse, R.; Crociani, B. Tetrahedron Lett. 2004, 45, 5861-
5864. (b) Scrivanti, A.; Beghetto, V.; Matteoli, U.; Antonaroli, S.; Marini,
A.; Crociani, B. Tetrahedron 2005, 61, 9752-9738.
(8) Amatore, C.; Jutand, A. Coord. Chem. ReV. 1998, 178-180, 511-
528.
(9) Herrmann, W. A.; Thiel, W. R.; Brossmer, C.; O¨ fele, K.; Priermeier,
T.; Scherer, W. J. Organomet. Chem. 1993, 461, 51-60.
10.1021/om0510876 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 03/03/2006

1796 Organometallics, Vol. 25, No. 7, 2006
Mace´ et al.
Table 1. Characterization of Pd0(η2-dba-n,n′-Z)(PPh3)2 Complexes by Cyclic Voltammetrya and NMR Spectroscopyb in DMF at
20 °Cc
Pd0(η2-dba-n,n′-Z)(PPh3)2
n,n′-Z
4,4′-CF3
4,4′-Br
4,4′-Cl
Epox (V vs SCE)
31P NMR: δ (ppm); ∆ν1/2 (Hz)
19F NMR: δ (ppm)
1
2
3
4
5
6
7
8
9
27.53 (s), 25.14 (s)
114.81 (s)
(+0.640)c
(+0.665)c
+0.630
26.82 (s), 24.73 (s); 14
26.91 (s), 24.83 (s); 14
26.93 (s), 24.65(s); 20
26.79 (s), 24.97 (s); 23
26.35 (s), 24.99 (s); 33
26.39 (s), 24.60 (s); 57
26.94 (s), 25.21 (s); 19
26.63 (s), 25.36 (s); 28
4,4′-F
64.34 (s), 58.29 (s)
4,4′-H
+0.580 (0.600)c
(0.570)c
4,4′-Me
4,4′-OMe
3,3′,5,5′-OMe
3,3′,4,4′,5,5′-OMe
+0.415
(+0.575)c
+0.765
a At a stationary-gold-disk electrode (diameter 2 mm) with a scan rate of 0.2 V s-1 in DMF (containing nBu4NBF4 0.3 M). b 31P NMR (101.3 MHz,
H3PO4) and 19F NMR (235.3 MHz, CFCl3) in DMF containing 10% acetone-d6 for the lock. c Pd0(η2-dba-n,n′-Z)(PPh3)2 complexes are generated from
Pd0 (dba-n,n′-Z)3 or Pd0(dba-n,n′-Z)2 associated with 2 equiv of PPh3/Pd. The oxidation potentials of Pd0(dba-n,n′-Z)(PPh3)2 generated from Pd0(dba-n,n′-
2
Z)2 are given in parentheses.
by 31P NMR spectroscopy and cyclic voltammetry (Scheme 4,
Scheme 3
Table 1). The η2-coordination of the substituted dba-n,n′-Z was
supported by the detection of two different 31P NMR signals,
as observed for nonsubstituted dba.6,8,9
Scheme 4
1/2Pd0(dba-n,n′-Z)3 + 2PPh3 f
chlorides was strongly affected by the electronic properties of
Pd0(η2-dba-n,n′-Z)(PPh3)2 + 1/2dba-n,n′-Z
substituted dba-n,n′-Z when the precursor was Pd0 (dba-n,n′-
2
Pd0(dba-n,n′-Z)2 + 2PPh3 f
Pd0(η2-dba-n,n′-Z)(PPh3)2 + dba-n,n′-Z
The characterization of complexes generated from Pd0 (dba-
2
4,4′-F)3 associated with PPh3 is reported here in detail and
compared to the case for Pd0 (dba)3. The behavior of the
2
precursor Pd0 (dba)3‚CHCl3 associated with phosphines has
Z)3 associated with electron-rich ligands.10 In the preliminary
catalysis studies, it was established that a more electron-rich
2
indeed never been reported. When PPh3 was added to Pd0 -
2
(dba-4,4′-F)3 (PPh3/Pd ) 2) in DMF (containing 10% acetone-
d6 for the lock), two broad singlets were observed in the 31P
NMR spectra, characterizing two different PPh3 groups and
attesting to the formation of Pd0(η2-dba-4,4′-F)(PPh3)2 in which
the dba-4,4′-F ligand was coordinated to the Pd0 center by one
of its CdC bonds (Scheme 5), as classically observed for the
simple nonsubstituted dba ligand.6-9 The monocoordination of
dba-4,4′-F was further confirmed by the 19F NMR spectrum
recorded from the same NMR tube, which exhibited three
singlets. One sharp singlet at 66.26 ppm characterizes the free
dba-4,4′-F (Figure 1, first equation in Scheme 5). The other
two broader singlets of equal magnitude at 64.34 and 58.29 ppm
characterized the unsymmetrical ligation of dba-4,4′-F, which
results in the two magnetically different fluorine atoms Fa and
Fb (Figure 1, Scheme 5).
dba (i.e., substituted by electron-donating groups Z) favors
higher yields of the cross-coupled product (an observation that
is not substrate dependent). It was then of considerable interest
to probe the influence of Z substitution of dba on the kinetics
of oxidative additions to determine whether the effect observed
in catalytic reactions took its origin in the oxidative-addition
step, which is probably rate determining when aryl chlorides
are considered.
We report herein that the kinetics of oxidative additions of
phenyl iodide with the palladium(0) complexes generated from
Pd0 (dba-n,n′-Z)3 or Pd0(dba-n,n′-Z)2 associated with PPh3
2
(taken as model ligand) are indeed affected by the electronic
properties of dba, which are modulated by substituents Z on its
phenyl groups.
After addition of PhI (10 equiv), only the 19F NMR singlet
of the free 4,4′-F-dba was observed as well as the 31P NMR
singlet of trans-PhPdI(PPh3)2 at 22.98 ppm, similar to that of
Results and Discussion
Characterization of the Pd0 Complexes Generated from
Pd0 (dba-n,n′-Z)3 or Pd0(dba-n,n′-Z)2 and PPh3 (PPh3/Pd )
2
2, 4) in DMF. Two kinds of precursors are available, Pd0 -
Scheme 5
2
(dba-n,n′-Z)3 (n,n′-Z ) 4,4′-F, 4,4′-H, 4,4′-MeO, 3,3′,4,4′,5,5′-
OMe) and Pd0(dba-n,n′-Z)2 (n,n′-Z ) 4,4′-CF3, 4,4′-Br, 4,4′-
Cl, 4,4′-H, 3,3′,5,5′-OMe), according to their ability to
crystallize.10,11 All precursors have been reacted with PPh3 (2
equiv per Pd) in DMF, and the major complexes Pd0(η2-dba-
n,n′-Z)(PPh3)2, formed in all reactions, have been characterized
(10) Fairlamb, I. J. S.; Kapdi, A. R.; Lee, A. F. Org. Lett. 2004, 6, 4435-
4438.
(11) Fairlamb, I. J. S.; Kapdi, A. R.; Whitwood, A. C. Manuscript in
preparation.

Palladium(0) Complexes of Dibenzylideneacetone
Organometallics, Vol. 25, No. 7, 2006 1797
Figure 1. 19F NMR (235.3 MHz, CFCl3) spectrum of Pd0(η2-dba-4,4′-F)(PPh3)2 generated from Pd0 (dba-4,4′-F)3 associated with 2 equiv
2
of PPh3/Pd in DMF containing 10% acetone-d6.
Scheme 6
(Scheme 2).6,8,14 When PPh3 was added to Pd0 (dba-4,4′-F)3
2
(PPh3/Pd ) 2), two oxidation peaks were detected by cyclic
voltammetry (Figure 2c). The minor peak appeared at the same
potential O1 as for Pd0(PPh3)2 (Figure 2a); the major peak at
O′2 characterizes the major complex Pd0(dba-4,4′-F)(PPh3)2
Figure 2. Cyclic voltammetry at a stationary-gold-disk electrode
(Figure 2c, first equation in Scheme 6), which is less easily
(diameter 2 mm) performed in DMF (containing nBu4NBF4 0.3
oxidized than Pd0(dba)(PPh3)2 due to the electron-withdrawing
M) with a scan rate of 0.2 V s-1, at 20 °C: (a) Pd0 (dba)3 (1 mM)
2
effect of the two fluorine atoms (Table 1, entries 4 and 5). The
+ PPh3 (4 mM); (b) Pd0 (dba)3 (1 mM) + PPh3 (8 mM); (c) Pd0 -
2
2
oxidation peak current of Pd0(PPh3)2 at O1 increased at the
expense of O′2 upon addition of excess PPh3, resulting in
formation of Pd0(PPh3)3 (Figure 2d, second equation in Scheme
6), as also observed for the simple nonsubstituted dba ligand
(Figure 2b). Due to the fast equilibrium between Pd0(PPh3)3,
Pd0(PPh3)2, and PPh3 (Scheme 6), a single oxidation peak was
observed at O1.6
(dba-4,4′-F)3 (1 mM) + PPh3 (4 mM); (d) Pd0 (dba-4,4′-F)3 (1 mM)
2
+ PPh3 (8 mM).
an authentic sample. A second minor singlet was also observed
at 20.86 ppm, which characterizes the cationic complex trans-
[PhPd(PPh3)2(DMF)]+ in equilibrium with trans-PhPdI(PPh3)2
12
(last equation in Scheme 5). The spectroscopic data indicate
that a definite oxidative addition reaction with PhI took place.
After addition of PhI (10 equiv), the oxidation peaks of Pd0-
(η2-dba-4,4′-F)(PPh3)2 and Pd0(PPh3)2, disappeared, attesting to
When PPh3 was added to Pd0 (dba)3‚CHCl3 (PPh3/Pd ) 2)
2
in DMF containing nBu4NBF4 (0.3 M), two oxidation peaks
were observed by cyclic voltammetry (Figure 2a): the minor
peak at O1 (Ep ) +0.10 V vs SCE) characterizes Pd0(PPh3)2,6,13
and the major peak at O2 characterizes the major complex Pd0-
(dba)(PPh3)2,14 as already observed for the precursor Pd0(dba)2
(14) (a) The oxidation potential of Pd0(dba)(PPh3)2 generated from Pd0 -
2
(dba)3 is slightly less positive than when it is generated from Pd0(dba)2
(entry 5 in Table 1). This is due to their respective equilibria with Pd0-
(PPh3)2 and dba, which is affected by the dba concentration (CE mecha-
nism).15 (b) The oxidation peak currents of Pd0(dba)(PPh3)2 and Pd0(PPh3)2
do not reflect the thermodynamic concentrations of the two complexes but
their dynamic concentrations. Indeed, the oxidation of Pd0(PPh3)2 at the
electrode (at O1) causes a continuous shift of the equilibrium in Scheme 2
toward its right-hand side. Under these conditions, the concentration of Pd0-
(PPh3)2 measured by its oxidation peak current is higher than its thermo-
dynamic concentration in the equilibrium of Scheme 2 (CE mechanism).15
(12) Amatore, C.; Carre´, E.; Jutand, A. Acta Chem. Scand. 1998, 52,
100-106.
(13) Pd0(PPh3)2 is probably coordinated by the solvent (DMF), which is
omitted for clarity.

1798 Organometallics, Vol. 25, No. 7, 2006
Mace´ et al.
an oxidative addition leading to trans-PhPdI(PPh3)2. A new
oxidation peak appeared at +0.540 V, which characterized the
iodide ions released in the equilibrium between trans-PhPdI-
(PPh3)2 and the cationic complex trans-[PhPd(PPh3)2(DMF)]+
(Scheme 5).12
The cyclic voltammetric and 31P NMR spectroscopic data of
other complexes Pd0(η2-dba-n,n′-Z)(PPh3)2 are listed in Table
1. As expected, Pd0(η2-dba-n,n′-Z)(PPh3)2 complexes are less
and less easily oxidized when Z becomes more electron
accepting, an outcome explained by Pd(0) becoming less and
less electron rich. The cyclic voltammograms exhibited the
minor common complex Pd0(PPh3)2, which was generated in
an unfavorable equilibrium with Pd0(dba-n,n′-Z)(PPh3)2 and dba-
n,n′-Z, as observed for the nonsubstituted dba (Scheme 2).6
Role of dba-n,n′-Z in the Kinetics of the Oxidative
Addition of PhI with the Pd0 Complexes Generated from
Figure 3. (a) Kinetics of the oxidative addition of PhI (2 mM) to
the Pd0(PPh3)2 complex generated from (s) Pd0 (dba)3 (1 mM) +
2
PPh3 (4 mM); (‚‚‚) Pd0 (dba-4,4′-F)3 (1 mM) + PPh3 (4 mM), and
2
(-- -) Pd0 (dba-4,4′-F)3 (1 mM) + PPh3 (8 mM) in DMF at 20 °C.
2
Plot of the molar fraction x of Pd0(PPh3)2 (x ) i/i0, i ) oxidation
current of Pd0(PPh3)2 at t, i0 ) initial oxidation current of Pd0-
(PPh3)2). (b) Kinetics of the oxidative addition of PhI (2 mM) to
Pd0 (dba-n,n′-Z)3 and PPh3 in DMF. The oxidative addition
2
reactions were performed in DMF at 20 °C from PhI (2 mM)
the Pd0(PPh3)2 complex generated from Pd0 (dba-4,4′-F)3 (1 mM)
2
and Pd0 (dba-4,4′-F)3 (1 mM) associated with PPh3/Pd ) 2, 4.
2
+ PPh3 (4 mM) in DMF at 20 °C. Plot of lnx + 1.5/x versus time
(x was determined from Figure 3a).
Accurate kinetics were monitored by amperometry performed
at a rotating-gold-disk electrode, polarized on the oxidation wave
of Pd0(PPh3)2.6,16. After addition of PhI, the decrease of the
oxidation current of Pd0(PPh3)2 (proportional to is concentra-
tion)6 was recorded versus time until total conversion (Figure
3a for 4,4′-F). The kinetics were compared to those for the
Table 2. Comparative Reactivity of Pd0 Complexes
Generated from Pd0 (dba-4,4′-Z)3 (1 mM) Associated to 2
2
PPh3/Pd or 4 PPh3/Pd or from Pd0(dba-4,4′-Z)2 (2 mM)
Associated to 4 PPh3/Pd in the Oxidative Addition with PhI
(2 mM) in DMF (20 °C)a
reaction of PhI (2 mM) with Pd0 (dba)3 (1 mM) associated with
2
t
1/2 (s)
4 PPh3/Pd0
2 equiv of PPh3 per Pd (Figure 3a). The half-reaction times are
given in Table 2. By a comparison of the three kinetic curves
of Figure 3a (or the half-reaction times t1/2 given in entries 2
and 3 in Table 2), the following reactivity orders were
established:
n,n′-Z
4,4′-OMe
4,4′-H
4,4′-F
3,3′,4,4′,5,5′-OMe
2 PPh3/Pd0
103KZk (s-1
)
Pd0 (dba-4,4′-Z)3
2
1
2
3
4
40
115
150
65
80
130
180
125
20
7.0
5.1
12
1/2Pd0 (dba-4,4'-Z)3 + 2PPh3 < 1/2Pd0 (dba)3 + 2PPh3
Pd0(dba-4,4′-Z)2
2
2
5
6
7
8
9
4,4′-Me
4,4′-H
4,4′-Cl
4,4′-Br
3,3′,5,5′-OMe
125
150
345
412
198
1/2Pd0 (dba-4,4'-Z)3 + 4PPh3 <
2
1/2Pd0 (dba-4,4'-Z)3 + 2PPh3
2
This is in agreement with the mechanism of Scheme 7, already
established for Pd0(dba)2.6,8 The kinetic law was solved for the
a t1/2 ) half-reaction time. See Schemes 7 and 8 for the mechanism and
the definition of KZ and k.
system {1/2Pd0 (dba-4,4′-Z)3 + 2PPh3} (Scheme 8).17Taking into
2
Scheme 7
account the variation of dba-4,4′-Z concentration during the
reaction and the stoichiometry PhI/Pd0 ) 1, the kinetic law is
expressed as in eq 1, where x is the molar fraction of Pd0(PPh3)2
ln x + 1.5/x ) KZkt + 1.5
(1)
(x ) i/i0, i ) oxidation current of Pd0(PPh3)2 at t, i0 ) initial
oxidation current of Pd0(PPh3)2).17 The plot of ln x + 1.5/x
against time was linear (Figure 3b for n,n′-Z ) 4,4′-F) in
agreement with eq 1, confirming a first-order reaction for PhI
and the Pd0 complex. The value of KZk was determined from
the slope (Table 2).
Scheme 8
The kinetics of the oxidative addition were similarly inves-
tigated for dba-4,4′-OMe and dba-3,3′,4,4′,5,5′-OMe (entries
1-4 in Table 2). By comparing the values of KZk which
characterized the reactivity of the Pd0 complexes generated from
Pd0 (dba-4,4′-Z)3, one deduces the following order of reactivity:
2
(15) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.;
Wiley: New York, 2001.
4,4'-OMe > 4,4'-H > 4,4'-F
(16) Due to the partial release of iodide ions in the oxidative addition
which are oxidized at +0.54 V, it was not possible to investigate the kinetics
of the oxidative addition upon polarizing the electrode at the oxidation
potential of Pd0(dba-4,4′-Z)(PPh3)2.
When the Pd0 precursor Pd0 (dba-4,4′-Z)3 is ligated by a dba
2
substituted by two electron-donating groups Z in the para
position, the resulting system is more reactive in oxidative
addition than when it is substituted by electron-withdrawing
groups. Since the same reactive Pd0(PPh3)2 species is involved
(17) d([Pd0])/dt ) - k[PhI][Pd0]/([dba-4,4′-H]/KZ + 1); d([Pd0])/[Pd0]
) -k[PhI]dt/([dba-4,4′-H]/KZ + 1) with [Pd0] ) C0x; [PhI] ) C0x and
[dba-4,4′-H] ) C0(1.5 - x). Since C0/KZ . 1, one gets (1.5 - x)dx/x2 )
-kKZdt, which gives eq 1 after integration (see text).

Palladium(0) Complexes of Dibenzylideneacetone
Organometallics, Vol. 25, No. 7, 2006 1799
6, t1/2 ) 150 s) is less reactive than the Pd0 complex generated
from Pd0 (dba-4,4′-H)3 (Table 2, entry 2, t1/2 ) 130 s) in
2
oxidative additions performed at identical concentrations (Pd0/
PPh3/PhI ) 1/4/1, C0 ) 2 mM), due to the higher initial
concentration of the free dba in the former system.
Pd0(dba)2 + 4PPh3 < 1/2Pd0 (dba)3 + 4PPh3
2
By now considering Pd0(dba-n,n′-Z)2 precursors associated with
4 equiv of PPh3 and comparing the values of the half-reaction
times t1/2 (Table 2, entries 5-8), the following reactivity order
is observed:
Figure 4. (a) Hammett plot for the oxidative addition of PhI (2
mM) with the Pd0(PPh3)2 complex generated from (a) Pd0 (dba-
2
4,4′-Z)3 (1 mM) and PPh3 (4 mM) in DMF at 20 °C and (b) Pd0-
(dba-4,4′-Z)2 (2 mM) and PPh3 (8 mM) in DMF at 20 °C.
4,4'-Me > 4,4'-H > 4,4'-Cl > 4,4'-Br
This establishes that, in association with PPh3, the Pd0 precursor
Pd0(dba-4,4′-Z)2 ligated by a dba substituted by two electron-
donating groups Z in the para position gives rise to a more
reactive system in oxidative addition as compared to the case
when it is substituted by two electron-withdrawing groups, as
(Scheme 8), this means that its concentration decreases when
going from Z ) OMe to Z ) F. In other words, KZ must
decrease when going from Z ) OMe to Z ) F. The rate constant
k does not depend on Z; only the equilibrium constant KZ is
affected by Z. From the values of KZk given in Table 2 (entries
1-3), one concludes that indeed
observed above starting from Pd0 (dba-4,4′-Z)3. The equilibrium
2
constant K′ and the rate constant k (Scheme 7) are not dependent
on the structure of Z. Consequently, one finds
K4,4'-OMe > K4,4'-H > K4,4'-F
K4,4'-Me > K4,4'-H > K4,4'-Cl > K4,4'-Br
The Pd0(PPh3)2 concentration decreases when going from Z )
OMe to Z ) F because dba-4,4′-Z becomes an increasingly
better ligand (more electron deficient) for the electron-rich Pd0-
(PPh3)2 species. Only three substituents in the para position have
been investigated here, although their electronic properties can
be considered distinct. A Hammett plot was nevertheless tested
(Figure 4a). The kinetics of the overall oxidative addition obey
a Hammett plot with a negative slope (F ) -1.7). The reactivity
The overall oxidative addition process follows a Hammett plot
with a negative slope (F ) -1.3) (Figure 4b), as found for the
precursor Pd0 (dba-4,4′-Z)3 (Figure 4a).18
2
Conclusion
The ligand dba-n,n′-Z plays a crucial role in the kinetics of
oxidative additions by controlling the concentration of the active
Pd0(PPh3)2 via its equilibrium with Pd0(dba-4,4′-Z)(PPh3)2
(equilibrium constant KZ). As a result, the more electron
donating the Z group, the higher the KZ value and consequently
the faster the rate of the overall oxidative addition. The complex
Pd0(dba-4,4′-Z)(PPh3)2 is more dissociated to Pd0(PPh3)2 when
Z is an electron-donating group, because the affinity of dba
substituted by an electron-donating group, for the electron-rich
Pd0(PPh3)2 species, is less than that of dba substituted by an
electron-accepting group. It is then possible to increase the
reactivity of the palladium(0) complexes in the oxidative
addition process by changing the dba structure (substitution by
an electron-donating group Z) of the catalytic precursor and
of the Pd0 complex generated from Pd0 (dba-4,4′-CF3)3 associ-
2
ated with 2 or 4 equiv of PPh3 per Pd center could not be
investigated, because the oxidation peak of Pd0(PPh3)2 was not
detected by cyclic voltammetry, due its overly low concentration
in its equilibrium with Pd0(dba-4,4′-CF3)(PPh3)2.16
One also observes the following reactivity order (Table 2,
entries 1 and 4):
4,4'-OMe > 3,3',4,4',5,5'-OMe
This means that Pd0(dba-3,3′,4,4′,5,5′-OMe)(PPh3)2 is less
dissociated to Pd0(PPh3)2 than Pd0(dba-4,4′-OMe)(PPh3)2, which
could be explained by competing donating/withdrawing effects
in the para and meta positions, in the case of the dba-
3,3′,4,4′,5,5′-OMe ligand, for which the individual electronic
contributions cannot be easily deconstructed.
also by using Pd0 (dba)3 instead of Pd0(dba)2. The increased
2
catalytic activity observed by Fairlamb et al.10 with Z ) OMe
in Suzuki-Miyaura reactions involving poorly reactive aryl
chlorides is then rationalized by a faster oxidative addition step.
Role of dba-n,n′-Z in the Kinetics of the Oxidative
Addition of PhI with the Pd0 Complexes Generated from
Pd0(dba-n,n′-Z)2 and PPh3 in DMF. The Pd0 complexes
ligated to dba-4,4′-Z (Z ) Me, Br, Cl) were more stable when
isolated as Pd0(Z-dba)2.11 The oxidative addition of PhI (2 mM)
to Pd0(dba-4,4′-Z)2 (2 mM) associated to PPh3 (8 mM) (PPh3/
Pd0 ) 4) was investigated as above in DMF at 20 °C. For the
same initial concentration C0 of Pd0(dba-4,4′-Z)(PPh3)2, the
amount of Pd0(PPh3)2 is lower when starting from Pd0(dba-4,4′-
Experimental Section
General Methods. All experiments were performed using
standard Schlenk techniques under an argon atmosphere. The 31P
NMR spectra were recorded on a Bruker spectrometer (101 MHz)
using H3PO4 as an external reference; the 19F NMR spectra were
recorded on a Bruker spectrometer (235 MHz) with CFCl3 as an
Z)2 than from Pd0 (dba-4,4′-Z)3, due to the higher initial dba-
(18) Despite the accelerating effect found for the Pd(0) complex generated
from Pd0(dba-4,4′-Br)2 when compared to the case for Pd0(dba)2, no
competitive oxidative addition proceeded with the 4-bromobenzylidene
ligand. Indeed, the 31P NMR spectrum of Pd0(dba-4,4′-Br)(PPh3)2 generated
after addition of 2 PPh3 to Pd0(dba-4,4′-Br)2 in DMF or toluene displayed
the two doublets of Pd0(dba-4,4′-Br)(PPh3)2 without any additional signals
which would have characterized an (aryl)PdIIBr(PPh3)2 complex formed in
the oxidative addition of the 4-bromobenzylidene ligand with the Pd(0)
complex. Moreover, the free dba-4,4′-Br ligand was fully recovered by
chromatography after the NMR sample had decomposed (to give Pd black).
2
4,4′-Z concentration in the former precursor (C0) than in the
latter one (C0/2). The ratio PPh3/Pd0 ) 4 was thus selected to
facilitate the detection of Pd0(PPh3)2 by amperometry at the
rotating disk electrode. The mechanistic scheme is given in
Scheme 7. The values of the half-reaction times, t1/2, are given
in Table 2 (entries 5-9). As predicted above, we observe that
the Pd0 complex generated from Pd0(dba-4,4′-H)2 (Table 2, entry

1800 Organometallics, Vol. 25, No. 7, 2006
Mace´ et al.
external reference. Cyclic voltammetry and amperometry were
performed using an in-house-constructed potentiostat and a GSTP4
waveform generator (Radiometer Analytical). The current was
recorded on a Nicolet 301 oscilloscope.
) 4,4′-F, 4,4′-H, 4,4′-MeO, 3,3′,4,4′,5,5′-OMe) or 30 µmol (2 mM)
of Pd0(dba-n,n′-Z)2 (n,n′-Z ) 4,4′-Me; 4,4′-Br;, 4,4′-Cl;, 4,4′-H,
3,3′,5,5′-OMe) was then introduced into the cell followed by 15.7
mg (60 µmol, 4 mM) or 31.4 mg (120 µmol, 8 mM) of PPh3. The
cyclic voltammetry was performed at a stationary-gold-disk elec-
Chemicals. DMF was distilled from CaH2 and kept under argon.
PPh3 and phenyl iodide were obtained from a commercial source
trode (diameter 2 mm) at a scan rate of 0.5 V s-1
.
(Aldrich). The precursors Pd0 (dba-n,n′-Z)3 (n,n′-Z ) 4,4′-F, 4,4′-
2
The kinetic measurements were performed at a rotating-gold-
disk electrode (diameter 2 mm, inserted into a Teflon holder, EDI
65109, Radiometer) with an angular velocity of 105 rad s-1
(Radiometer). The rotating electrode was polarized at +0.2 V on
the plateau of the oxidation wave of Pd0(PPh3)2. A 3.3 µL portion
(30 µmol, 2 mM) of phenyl iodide was then added to the cell and
the decay of the oxidation current recorded versus time up to 100%
conversion at 20 °C.
H, 4,4′-MeO, 3,3′,4,4′,5,5′-OMe) and Pd0(dba-n,n′-Z)2 (n,n′-Z )
4,4′-CF3, 4,4′-Me, 4,4′-Br, 4,4′-Cl, 4,4′-H, 3,3′,5,5′-OMe) were
synthesized according to published procedures.10,11
General Procedure for 31P and 19F NMR Experiments. To
0.75 mL of degassed DMF was introduced 13 µmol of Pd0(dba-
n,n′-Z)2 or 6.5 µmol of Pd0 (dba-n,n′-Z)3 followed by 6.8 mg (26
2
µmol) of PPh3. A 75 µL portion of degassed acetone-d6 was then
added for the lock.
General Procedure for the Cyclic Voltammetry and for the
Kinetics of the Oxidative Addition Followed by Amperometry.
Experiments were carried out in a three-electrode thermostated cell
connected to a Schlenk line. The reference was a saturated calomel
electrode (Radiometer) separated from the solution by a bridge filled
with 3 mL of a 0.3 M nBu4NBF4 solution in DMF. The counter
electrode was a platinum wire of ca. 1 cm2 apparent surface area.
A 15 mL portion of DMF containing nBu4NBF4 (0.3 M) was poured
Acknowledgment. This work was supported in part by the
Centre National de la Recherche Scientifique (UMR CNRS-
ENS-UPMC 8640) and the Ministe`re de la Recherche (Ecole
Normale Supe´rieure). We thank Johnson Matthey for a generous
loan of palladium salts (A.J. and I.J.S.F.). We are also grateful
to the Royal Society for funding a university research fellowship
and for providing a generous equipment grant (to I.J.S.F.).
into a cell. A 15 µmol portion (1 mM) of Pd0 (dba-n,n′-Z)3 (n,n′-Z
OM0510876
2