Decelerating effect of alkenes in the oxidative addition of phenyl iodide to palladium(0) complexes in heck reactions

Article abstract of DOI:10.1021/om020439v

In DMF, the oxidative addition of PhI to Pd⁰(PPh₃)₄ or to the anionic Pd⁰(PPh₃)₃(OAc)^- is slower in the presence of an alkene (styrene, methyl acrylate). Indeed, the concentration of the reactive Pd⁰(PPh₃)₂ or Pd⁰(PPh₃)₂(OAc)^- complex decreases because of its coordination to the alkene to form the unreactive (η²-CH₂=CHR)Pd⁰(PPh₃)₂ (R = Ph, CO₂Me) or (η²-CH₂= CHPh)Pd⁰(PPh₃)₂(OAc)^-, respectively. As already evidenced in palladium-catalyzed Stille reactions, this work establishes that, in palladium-catalyzed Heck reactions as well, the nucleophile plays a role in the kinetics of the oxidative addition (decelerating effect), as soon as it may coordinate Pd⁰ complexes. This is an essential observation, in view of the general belief that the nucleophile enters the catalytic cycle only at the stage of the attack on the aryl-Pd^II complex formed in the oxidative addition. Whenever the oxidative addition is not rate determining, the decelerating effect of the alkene on this reaction is in favor of a higher efficiency of the catalytic cycle.

Full text of DOI:10.1021/om020439v

4540
Organometallics 2002, 21, 4540-4545
Deceler a tin g Effect of Alk en es in th e Oxid a tive Ad d ition
of P h en yl Iod id e to P a lla d iu m (0) Com p lexes in Heck
Rea ction s
Christian Amatore,* Emmanuelle Carre´, Anny J utand,* and Youcef Medjour
Ecole Normale Supe´rieure, De´partement de Chimie, UMR CNRS 8640, 24 Rue Lhomond,
F-75231 Paris Cedex 5, France
Received J une 3, 2002
In DMF, the oxidative addition of PhI to Pd0(PPh3)4 or to the anionic Pd0(PPh3)3(OAc)- is
slower in the presence of an alkene (styrene, methyl acrylate). Indeed, the concentration of
the reactive Pd0(PPh3)2 or Pd0(PPh3)2(OAc)- complex decreases because of its coordination
to the alkene to form the unreactive (η2-CH2dCHR)Pd0(PPh3)2 (R ) Ph, CO2Me) or (η2-CH2d
CHPh)Pd0(PPh3)2(OAc)-, respectively. As already evidenced in palladium-catalyzed Stille
reactions, this work establishes that, in palladium-catalyzed Heck reactions as well, the
nucleophile plays a role in the kinetics of the oxidative addition (decelerating effect), as
soon as it may coordinate Pd0 complexes. This is an essential observation, in view of the
general belief that the nucleophile enters the catalytic cycle only at the stage of the attack
on the aryl-PdII complex formed in the oxidative addition. Whenever the oxidative addition
is not rate determining, the decelerating effect of the alkene on this reaction is in favor of
a higher efficiency of the catalytic cycle.
In tr od u ction
catalyst.4 Indeed, the overall oxidative addition is slower
in the presence of the nucleophile because the concen-
tration of the active complex Pd0(AsPh3)2 decreases, due
to its reversible coordination by the CdC bond of the
nucleophile to form the unreactive complex: (η2-CH2d
CHSnBu3)Pd0(AsPh3)2 (Scheme 1).4
In palladium-catalyzed Stille reactions (eq 1)1 or Heck
reactions (eq 2),2 the first step of the catalytic cycle is
an oxidative addition of the aryl halide to a Pd0 complex,
which gives an aryl-PdII complex. The nucleophile is
In a Heck reaction between PhOTf and CH2dCH-
CO2Me (eq 2), catalyzed by Pd0(dba)2 associated with 1
equiv of dppf (dppf ) 1,1′-bis(diphenylphosphino)fer-
rocene), the complex (η2-CH2dCHCO2Me)Pd0(dppf) has
been isolated in the course of the reaction and found to
be the reactive species in the oxidative addition to aryl
triflates or iodides (Scheme 2).5The oxidative addition
was faster in the presence of Eu3+ cations, which
coordinate the methyl acrylate, thus increasing the
concentration of the more reactive Pd0(dppf) complex
(Scheme 2).5 Once again, the nucleophile plays a role
in the oxidative addition, leading to a decelerating effect
due to its complexation to the more reactive Pd0
complex.
ArX + CH2dCHSnBu3 [Pd]8 ArCHdCH2 + XSnBu3
(1)
ArX + CH2dCHR + NEt3 [Pd]8
ArCHdCHR + Et3NH+X- (2)
supposed to enter the catalytic cycle only after this
reaction, by reacting with the aryl-PdII complex in a
transmetalation step in Stille reactions1,3 or a carbo-
palladation step2 in Heck reactions. Further evolution
of the ensuing intermediates gives the reaction product-
(s) and regenerates the active Pd0 catalyst.
In a Stille reaction between PhI and a nucleophile
such as tributyl(vinyl)tin (eq 1), catalyzed by Pd0(dba)2
(dba ) trans,trans-dibenzylideneacetone) associated
with 2 equiv of AsPh3, we previously established that
the nucleophile plays a role at the level of the first
step: i.e., in the oxidative addition of PhI to the Pd0
Such kinetics investigations4,5 could be successfully
achieved because they were carried out under experi-
mental conditions such as oxidative addition and the
transmetalation (Stille reaction) and carbopalladation
(Heck reaction) were disconnected. Indeed, the oxidative
addition, even if slowed by the presence of the nucleo-
phile, remained faster than the nucleophilic attack on
the aryl-PdII complex. Under such conditions, the Pd0
catalyst was recycled only after the slow nucleophilic
attack, viz., on a time scale considerably longer than
that required to complete the oxidative addition. Thus,
the kinetics of the oxidative addition could be investi-
(1) Farina, V.; Roth, G. P. In Advances in Metal-Organic Chemistry;
J AI Press: Greenwich, CT, 1996; p 1.
(2) (a) de Mejeire, A.; Meyer, F. E. Angew. Chem., Int. Ed. 1994,
106, 2473. (b) Crisp, G. T. Chem. Soc. Rev. 1998, 27, 427. (c) Beletskata,
I.; Cheprakov, A. V. Chem. Rev. 2000, 100, 3009. (d) Amatore, C.;
J utand, A. Acc. Chem. Res. 2000, 33, 314.
(3) (a) Farina, V.; Krishnan, B. J . Am. Chem. Soc. 1991, 113, 9585.
(b) Farina, V.; Krisnan, B.; Marshall, D. R.; Roth, G. P. J . Org. Chem.
1993, 58, 5434. (c) Casado, A. L.; Espinet, P. Organometallics 1998,
17, 954. (d) Casado, A. L.; Espinet, P. J . Am. Chem. Soc. 1998, 120,
8978. (e) Casado, A. L.; Espinet, P.; Gallego, A. J . Am. Chem. Soc. 2000,
122, 11771. (f) Casado, A. L.; Espinet, P.; Gallego, A. M.; Martinez-
Ilarduya, J . M. Chem. Commun. 2001, 120, 339.
(4) Amatore, C.; Bucaille, A.; Fuxa, A.; J utand, A.; Meyer, G.; Ndedi
Ntepe, A. Chem. Eur. J . 2001, 2134.
(5) J utand, A.; Hii, K. K.; Thornton-Pett, M.; Brown, J . M. Organo-
metallics 1999, 18, 536.
10.1021/om020439v CCC: $22.00 © 2002 American Chemical Society
Publication on Web 09/19/2002

Oxidative Addition of PhI to Pd(0) Complexes
Organometallics, Vol. 21, No. 21, 2002 4541
Sch em e 1
Sch em e 2
The kinetics of the oxidative addition of PhI (2 mM)
to Pd0(PPh3)4 (2 mM) in DMF (containing n-Bu4NBF4,
0.3 M) was first investigated in the absence of any
styrene (Scheme 3, eq 7). It was monitored analytically
using amperometry at a rotating disk electrode, polar-
ized at +0.2 V vs SCE, i.e., on the oxidation wave of
Pd0(PPh3)3, the major complex formed in solution from
Pd0(PPh3)4 (eqs 4 and 5).8 The decay of the oxidation
current (proportional to the Pd0 concentration) was
recorded as a function of time.
Sch em e 3. Mech a n ism of th e Oxid a tive Ad d ition
As reported, the oxidative addition proceeds from the
minor complex SPd0(PPh3)2 (Scheme 3, eq 6).8 Assuming
that the equilibrium (5) (K1 ) [SPd0L2][L]/[Pd0L3], L )
PPh3) is fast relative to the oxidative-addition step (6),
the kinetic law for the overall reaction (7) is given in eq
8. Under stoichiometric conditions, at any time t, one
of P h I to P d 0(P P h 3)4 (S ) Solven t)
kK1[PhI][Pd0]
d[Pd0]
dt
) -
) -kapp[Phl][Pd0] (8)
[L]
has [Pd0] ) xC0 (C0 ) initial concentration of Pd0(PPh3)4
and [PhI] ) xC0. The integration of eq 8 gives eq 9.
gated without kinetic interference, due to the follow-up
reactions of the catalytic cycle.
We report herein further investigations, which gen-
eralize the role played by the nucleophile in the kinetics
of oxidative additions, leading to a decelerating effect
as soon as the nucleophile can coordinate the active low
ligated Pd0 moiety. The effect of two representative
coordinating nucleophiles, styrene and methyl acrylate,
has been investigated in the oxidative addition of PhI
to Pd0(PPh3)4 or to Pd0(PPh3)3(OAc)-, the latter Pd0
precursor being an effective catalyst in Heck reactions
(eq 3).2d
kK1C0t
1
x
)
+ 1 ) kappC0t + 1
(9)
[L]
In agreement with the rate law in eq 9, the plot of
i0/i ) [Pd0]0/[Pd0] ) 1/x versus time was linear (Figure
1a, upper dashed line) (i ) intensity of the oxidation
current of Pd0(PPh3)3 at t, i0 ) initial intensity of the
oxidation current of Pd0(PPh3)3). The straight-line in-
tercept was unity, which is also in agreement with eq
9. The value of kapp, the apparent rate constant of the
overall reaction (7), was then determined from the slope
of the regression line (eq 9):9 kapp ) 24 M-1 s-1 (DMF,
25 °C).
PhI + CH2dCHPh + NEt3 [Pd]8
PhCHdCHPh + Et3NH+I- (3)
(8) (a) Fauvarque, J . F.; Pflu¨ger, F.; Troupel, M. J . Organomet.
Chem. 1981, 208, 419. (b) Amatore, C.; J utand, A.; Khalil, F.; M’Barki,
M. A.; Mottier, L. Organometallics 1993, 12, 3168.
Resu lts a n d Discu ssion
Oxid a tive Ad d ition of P h I to P d 0(P P h 3)4 in th e
P r esen ce of Styr en e, in DMF . The oxidative addition
of PhI to Pd0(PPh3)4 gives the trans-PhPdI(PPh3)2
complex6 (Scheme 3). This species does not react with
styrene in DMF at 25 °C to form stilbene, the would-be
final compound of a Heck reaction.7c,2d Consequently,
the Pd0 catalyst is not recycled and the kinetics of the
oxidative addition of PhI can be investigated in the
presence of styrene without any perturbation caused by
the recycling of the Pd0 complex.
(9) However, in the absence of an excess of PPh3, the concentration
of PPh3 (i.e., [L] in eqs 8 and 9) varies during the course of the oxidative
addition due to its continuous release in the fast equation (5).
Consequently, the variation of L concentration must be considered in
the kinetic law (8) with [L] ) (2 - x)C0. The integration of eq 8 gives
the new equation (9′): 1/x + 0.5 ln x ) 0.5kK1t + 1. The plot of i0/i +
0.5 ln(i/i0) versus time was linear (Figure 1c) (i ) intensity of the
oxidation current of Pd0(PPh3)3 at t, i0 ) initial intensity of the
oxidation current). The value of kK1 was determined from the slope of
the regression line (Figure 1c, eq 9′). kK1 ) 0.062 s-1. Since kapp
)
kK1/[L] ) 24 M-1 s-1 was obtained by application of the approximated
eq 9 (see text), it ensues that eq 9 is a good approximation for [L] )
2.6 mM. This latter value is not far from the average value of 3 mM
for PPh3 concentration, which varies from the initial concentration of
2 mM to 4 mM at 100% conversion. Consequently, the simplified kinetic
law in eq 9 can be used with a value of 2.6 mM for the average
concentration of PPh3. Since this simplifies greatly the presentation
of the kinetics analysis given here, without introducing any significant
bias in the kinetic treatment of the experimental data (see above), this
simplification was used everywhere in the presentation of this paper.
(6) Fitton, P.; Rick, E. A. J . Organomet. Chem. 1971, 28, 287.
(7) (a) Amatore, C.; J utand, A.; M’Barki, M. A. Organometallics
1992, 11, 3009. (b) Amatore, C.; Carre´, E.; J utand, A.; M’Barki, M. A.
Organometallics 1995, 14, 1818. (c) Amatore, C.; Carre´, E.; J utand,
A.; M’Barki, M. A.; Meyer, G. Organometallics 1995, 14, 5605. (d)
Amatore, C.; Carre´, E.; J utand, A. Acta Chem. Scand. 1998, 52, 100.

4542 Organometallics, Vol. 21, No. 21, 2002
Amatore et al.
Sch em e 4. Mech a n ism of th e Oxid a tive Ad d ition
of P h I to P d 0(P P h 3)4 in th e P r esen ce of a n Alk en e
(R ) P h , CO2Me)
Pd0(PPh3)2 moiety was decreased due to its increasing
coordination by the alkene. Consequently, a new mecha-
nistic scheme was considered with the overall reaction
(10) (Scheme 4; R ) Ph, K2 ) [//-Pd0L2]/[Pd0L2][//],
where the symbol // stands for the alkene). When any
reaction between PhI and (η2-CH2dCH-Ph)Pd0(PPh3)2
is neglected, the kinetic law becomes as expressed in
eq 11. Under stoichiometric conditions, its integration
kK1[PhI][Pd0]
[L] + K1K2[//]
d[Pd0]
dt
) -
(11)
(12)
kK1C0t
[L] + K1K2[//]
1
x
)
+ 1 ) kappC0t + 1
gives eq 12,10 now with 1/kapp ) [L]/kK1 + K2[//]/k. Note
that in the absence of styrene, K2[//]/k ) 0, so that one
obtains eq 9. The plot of 1/kapp versus the styrene
concentration (viz. constant [L]10) was linear (Figure 1b),
as predicted by the mechanism of Scheme 4: 1/kapp ) a
+ b[//]. The value b ) K2/k ) 0.077 s was obtained from
the slope, and that of a ) [L]/kK1 ) 0.041 M s was
calculated from the intercept. Using [L] ) 2.6 mM,10 one
obtains kK1 ) 0.063 s-1, which is identical with that
determined in the absence of styrene (kK1 ) 0.062 s-1).9
The product of those two values allows the calculation
of K0 ) K1K2 ) 4.8 × 10-3, which is the value of the
equilibrium constant K0 of the overall equilibrium (13)
F igu r e 1. Oxidative addition of PhI (2 mM) to Pd0(PPh3)4
(2 mM) in DMF (containing n-Bu4NBF4 0.3 M) at 25 °C,
monitored by amperometry at a rotating-gold-disk electrode
(diameter 2 mm, ω ) 105 rad s-1) polarized at +0.2 V vs
SCE in the absence or presence of styrene. (a) Plot of i0/i
) [Pd0]0/[Pd0] ) 1/x versus time (i ) intensity of the
oxidation current of Pd0(PPh3)3 at t, i0 ) initial intensity
of the oxidation current of Pd0(PPh3)3): 1/x ) kappC0t + 1
(eq 9 or 12). Oxidative addition was performed in the
absence of styrene (O, dashed line) and in the presence of
styrene at concentrations (M) of (b) 0.005, (2) 0.1, ([) 0.15,
and (9) 0.3. (b) Plot of 1/kapp versus the styrene concentra-
tion (eq 12). (c) Plot of i0/i + 0.5 ln(i/i0) versus time (eq 9′
in ref 9, i0/i ) [Pd0]0/[Pd0]) in the absence of styrene (see
Figure 1a caption for the definition of i and i0).
(K0 ) [//-Pd0L2][L]/[Pd0L3][//]), in which a phosphine
ligand is substituted from Pd0(PPh3)3 by the styrene to
form the new unreactive 16-electron complex (η2-CH2d
CHPh)Pd0(PPh3)2. Under comparable PPh3 and styrene
concentrations, eq 13 would play a negligible role, since
it lies in favor of Pd0(PPh3)3; however, it is worth noting
that the effect would be reverse for higher styrene
concentrations, as occurs in catalytic reactions.
In the presence of increasing amounts of styrene, the
overall oxidative addition was found to be slower and
slower, as evidenced by the decrease in the slopes of the
plots of i0/i ) [Pd0]0/[Pd0] ) 1/x versus time, determined
for different styrene concentrations (Figure 1a). This
suggests that the concentration of the active low ligated
Oxid a tive Ad d ition of P h I to P d 0(P P h 3)4 in th e
P r esen ce of Meth yl Acr yla te, in DMF . When a very
electron deficient alkene such as methyl acrylate was
considered, it was observed that the oxidation wave of
(10) Due to the complexity of the kinetics laws in the presence of
styrene, we use eq 11 but we consider a constant value of [L] in its
integration (see note in ref 9). In eq 12, the value of [L] is thus
considered as constant and equal to its average value of 2.6 mM over
the reaction (see note in ref 9).

Oxidative Addition of PhI to Pd(0) Complexes
Organometallics, Vol. 21, No. 21, 2002 4543
Sch em e 5. Mech a n ism of th e Oxid a tive Ad d ition
of P h I to th e P d 0 Com p lex Gen er a ted fr om
P d (OAc)2 + n Equ iv of P P h 3 (n g 4)
constant K2 is then 1200-fold higher for methyl acrylate
than for styrene. This is consistent with the fact that
the electron-deficient methyl acrylate is a much better
ligand for the electron-rich Pd0(PPh3)2 complex than
styrene. The value of the equilibrium constant of eq 14
was calculated as above: K0 ) 7.5 (DMF, 25 °C). For
identical concentrations of styrene and methyl acrylate,
eq 14 is then considerably more shifted toward (η2-
CH2dCHCO2Me)Pd0(PPh3)2 than is the equilibrium (13)
toward (η2-CH2dCHPh)Pd0(PPh3)2.
F igu r e 2. Oxidative addition of PhI (2 mM) to Pd0(PPh3)4
(2 mM) in DMF (containing n-Bu4NBF4 0.3 M) at 25 °C,
monitored by amperometry at a rotating-gold-disk electrode
(diameter 2 mm, ω ) 105 rad s-1) polarized at +0.3 V vs
SCE in the presence of methyl acrylate. Plot of 1/kapp versus
the methyl acrylate concentration (eq 12).
Pd0(PPh3)3 (+0.14 V vs SCE) partially disappeared upon
addition of methyl acrylate and a new oxidation peak
appeared at a more positive potential (+0.48 V vs SCE).
The oxidation peak current of this second peak in-
creased at the expense of that of Pd0(PPh3)3 upon
increasing the methyl acrylate concentration. Investiga-
tion of a solution of Pd0(PPh3)4 (8 mM) and methyl
acrylate (0.8 M) in DMF by 31P NMR spectroscopy
showed a broad signal characteristic of Pd0(PPh3)3 at
-5.13 ppm and two thin doublets of similar integration
(22.27 ppm, J P-P ) 32 Hz; 31.48 ppm, J P-P ) 32 Hz)
characteristic of two chemically different PPh3 groups
ligated to one Pd atom, as expected for (η2-CH2dCHCO2-
Me)Pd0(PPh3)2 (eq 14).11 Consequently, the addition of
Oxid a tive Ad d ition of P h I to P d 0(P P h 3)3(OAc)-
(Gen er a ted fr om P d (OAc)2 a n d 5 Equ iv of P P h 3)
in th e P r esen ce of Styr en e, in DMF . The catalytic
precursor Pd(OAc)2 associated with PPh3 has been
selected because it is commonly used in Heck reactions
(eq 3).2 We previously established that the addition of
excess PPh3 to Pd(OAc)2 in DMF, leads to the formation
of an anionic Pd0 complex via the intramolecular reduc-
tion of Pd(OAc)2(PPh3)2 by PPh3, which is oxidized to
the phosphine oxide (eq 15).7 The oxidative addition of
Pd(OAc)2 + 4PPh3 + H2O f
Pd0(PPh3)3(OAc)- + H+ + AcOH + (O)PPh3 (15)
PhI proceeds as in Scheme 5: i.e., exclusively from the
complex Pd0(PPh3)2(OAc)-,7c which represents a very
minor fraction of the overall Pd0 (K′1 ) [Pd0L2(OAc)-]
× [L]/[Pd0L3(OAc)-]; L ) PPh3).
To allow a meaningful comparison with the case of
Pd0(PPh3)4 investigated above (where the major complex
is Pd0L3 and 1 equiv of free L is present initially in
solution), the kinetics of the oxidative addition of PhI
(2 mM) was determined in DMF at 25 °C from the
anionic Pd0 complex generated from Pd(OAc)2 (2 mM)
and 5 equiv of PPh3 (major complex Pd0L3(OAc)-, 1
equiv of free L, one L being oxidized (eq 15)). The
oxidative addition was performed in the presence of 3
equiv of NEt3, to neutralize the protons generated
together with the Pd0 complex (eq 15), which may react
with the anionic Pd0(PPh3)2(OAc)- to generate Pd0-
(PPh3)2.2d Under stoichiometric conditions (1 equiv of
PhI relative to Pd0(PPh3)2(OAc)-), the kinetic law was
the same as that used for Pd0(PPh3)4 (eq 9) and the rate
constant of the overall reaction (18) was then deter-
mined: k′app ) 22 M-1 s-1 with k′K′1 ) k′app[L] ) 0.057
methyl acrylate to Pd0(PPh3)4 results in the formation
of a new Pd0 complex ligated by the methyl acrylate (eq
14).12
Whereas the complex (η2-CH2dCHPh)Pd0(PPh3)2 could
not be detected by 31P NMR spectroscopy due to its too
low concentration in eq 13, the detection and charac-
terization of (η2-CH2dCHCO2Me)Pd0(PPh3)2 show that
eq 14 is much more shifted toward its right-hand side
than eq 13. Consequently, for identical concentrations
of styrene and methyl acrylate a higher decelerating
effect is expected for the oxidative addition performed
in the presence of methyl acrylate. In agreement with
this expectation, the oxidative addition was considerably
slowed when performed in the presence of methyl
acrylate. The plot of 1/kapp versus the methyl acrylate
concentration was linear (Figure 2), in agreement with
the formulation in eq 12 and Scheme 4 (R ) CO2Me):
K2/k ) 92 s for methyl acrylate versus 0.077 s for styrene
(vide supra). Consequently, the value of the equilibrium
s-1 9 The value of k′ ) 65 M-1 s-1 was determined
.
independently by reacting PhI and Pd0(PPh3)2(OAc)-,
generated from Pd(OAc)2 and 3 equiv of PPh3.7c Con-
sequently, the value of K′1 was determined: K′1 ) 9 ×
10-4 M. The value of k′K′1 ) 0.057 s-1 (Scheme 5) is
very similar to that found for of Pd0(PPh3)4 (kK1 ) 0.062
s-1, Scheme 3). However, the values of k and K1 remain
unknown for Pd0(PPh3)4. This unfortunately precludes
any comparison of the equilibrium constants and rate
(11) This complexation is very reminiscent of that of Pd0(PPh3)2 by
dba to form unreactive (η2-dba)Pd0(PPh3)2, which has been character-
ized by its oxidation peak and by two 31P NMR signals.8b
(12) For Pd0 complexes ligated by methyl acrylate, see also ref 5.

4544 Organometallics, Vol. 21, No. 21, 2002
Amatore et al.
This latter value agrees completely with that found in
the absence of styrene (vide supra). The product of those
two values affords K′0 ) K′1K′2 ) 5.5 × 10-3, which is
the equilibrium constant of the overall eq 21, in which
one phosphine ligand is substituted from Pd0(PPh3)3-
(OAc)- by the styrene to form the new unreactive 18-
electron complex (η2-CH2dCH-Ph)Pd0(PPh3)2(OAc)-.
Since K′1 ) 9 × 10-4 M, then K′2 ) 61 M-1 (Scheme 6).
The value of K′0 is rather low, which means that the
substitution in eq 21 is not favorable for comparable con-
centrations of PPh3 and styrene. However, in catalytic
Heck reactions where the styrene concentration might
be, for example, 100 times higher than that of the cata-
lytic Pd0, the concentration of (η2-CH2dCHPh)Pd0-
(PPh3)2(OAc)- would be half of that of Pd0(PPh3)3(OAc)-
and the rate of the overall oxidative addition would thus
be divided by 1.4. The decelerating effect will be more
important in the absence of any free PPh3: i.e., when
Pd0(PPh3)2(OAc)- is generated quantitatively from Pd-
(OAc)2 and 3 equiv of PPh3.7c Indeed, in the absence of
any free PPh3, only the right part of Scheme 6 must be
considered with the equilibrium constant K′2 and k′app
) k′/(1 + K′2[//]). From the value of K′2 determined
above, one predicts that, for a styrene concentration 100
times higher than that of the catalytic Pd0, the rate of
the oxidative addition would be divided by ca. 13.
Consequently, the mechanistic scheme of the Heck
reaction, when catalyzed by Pd(OAc)2 associated with
3 equiv of PPh3, established in previous works2d must
be improved by the introduction of the alkene at the
level of the oxidative addition (Scheme 7), including the
value of K2′ determined in the present work (the values
of k0,7b k′,7c k′′,7c k′′′,7c and KOAc7d have been determined
in previous works).
F igu r e 3. Oxidative addition of PhI (2 mM) to the
Pd0(PPh3)3(OAc)- generated from Pd(OAc)2 (2 mM) and
PPh3 (10 mM) in DMF (containing 0.3 M n-Bu4NBF4) at
25 °C, monitored by amperometry at a rotating-gold-disk
electrode (diameter 2 mm, ω ) 105 rad s-1) polarized at
+0.2 V vs SCE. The oxidative addition was performed in
the absence or in the presence of styrene. Plot of 1/k′app
versus the styrene concentration (eq 20).
Sch em e 6. Mech a n ism of th e Oxid a tive Ad d ition
of P h I to P d 0(P P h 3)3(OAc)- in th e P r esen ce of
Styr en e
constants of the intrinsic oxidative additions of Pd0-
(PPh3)2 and Pd0(PPh3)2(OAc)-.
In the presence of styrene, the oxidative addition was
slower. We previously established that the complex
PhPd(OAc)(PPh3)2, formed in the oxidative addition (eq
18), reacts with styrene to form stilbene and the Pd0
catalyst, but this reaction is considerably slower than
the oxidative addition.7c Nevertheless, it has been
checked that no stilbene was formed (eq 3) over the time
scale of the overall oxidative addition (20 min). Stilbene
production required more longer times (hours)7c as well
as the recycling of the Pd0 catalyst. Consequently the
investigated oxidative addition was not perturbed by
any fortuitous recycling of the Pd0 catalyst. Since
styrene was present at the very first time of the
oxidative addition, its coordination to the active Pd0
complex occurs concurrently with the oxidative addition,
as established above for Pd0(PPh3)4, to form the unre-
active 18-electron complex (η2-CH2dCHPh)Pd0(PPh3)2-
(OAc)- (Scheme 6, K′2 ) [//-Pd0L2(OAc)-]/[Pd0L2(OAc)-]-
[//]). The kinetic law (20) is then identical with eq 12
Con clu sion
In DMF, the oxidative addition of PhI to Pd0(PPh3)4
or to the anionic Pd0(PPh3)3(OAc)- (generated from Pd-
(OAc)2 and 5 equiv of PPh3) is slower in the presence of
an alkene such as styrene or methyl acrylate. The
concentration of the reactive Pd0 complex decreases due
to its coordination to the alkene to form the unreactive
complexes: (η2-CH2dCHPh)Pd0(PPh3)2, (η2-CH2dCHCO2-
Me)Pd0(PPh3)2, or (η2-CH2dCHPh)Pd0(PPh3)2(OAc)-,
accordingly.
Consequently, a more elaborate mechanistic scheme
is proposed for the Heck reaction, when catalyzed by
Pd(OAc)2 associated with 3 equiv of PPh3 (Scheme 7),
in which the alkene plays a role in the oxidative
addition, by stocking part of the active Pd0 under the
unreactive complex (η2-CH2dCHR)Pd0(PPh3)2(OAc)-.
The oxidative addition is usually a fast reaction (espe-
cially with aryl iodides). It is noteworthy that the
nucleophile interference by slowing down the oxidative
addition is then in favor of a higher efficiency for the
catalytic cycle, by bringing the rate of the oxidative
addition closer to that of the carbopalladation, i.e., the
1/k′app ) [L]/k′K′1 + K′2[//]/k′
(20)
except for the exact meaning of the individual rate
constants (Scheme 6). The plot of 1/k′app versus the
styrene concentration was linear (Figure 3), thus es-
tablishing the validity of Scheme 6 and eq 20: 1/k′app
)
a + b[//]. The value of K′2/k′ ) 0.097 s was determined
from the slope and k′K′1 ) 0.057 s-1 from the intercept.

Oxidative Addition of PhI to Pd(0) Complexes
Organometallics, Vol. 21, No. 21, 2002 4545
Sch em e 7. Mech a n ism of th e Heck Rea ction Ca ta lyzed by P d (OAc)2 Associa ted w ith 3 Equ iv of P P h 3
nucleophilic attack on the aryl-PdII complexes formed
out in a three-electrode cell connected to a Schlenk line. The
counter electrode was a platinum wire of ca. 1 cm2 apparent
in the oxidative addition, which is usually rate deter-
mining.13,14
surface area; the reference was a saturated calomel electrode
(Radiometer Analytical) separated from the solution by a
bridge filled with 3 mL of DMF containing n-Bu4NBF4 (0.3
M). A 15 mL portion of DMF containing n-Bu4NBF4 (0.3 M)
Exp er im en ta l Section
31P NMR spectra were recorded in DMF containing 10%
CD3COCD3, on a Bruker spectrometer (101 MHz) using H3-
PO4 as an external reference. Voltammetry at a rotating-disk
electrode was performed with a homemade potentiostat and
a Radiometer Analytical GSTP4 waveform generator and
recorded on a Nicolet 301 oscilloscope.
DMF was distilled from calcium hydride under vacuum and
kept under argon. Phenyl iodide, styrene, methyl acrylate, and
triethylamine were obtained commercially (Acros) and used
after filtration on alumina. Pd(OAc)2 and PPh3 were obtained
commercially (Acros) and used without purification. Pd0(PPh3)4
was prepared according to a described procedure.15 Pd0(PPh3)3-
(OAc)- was synthesized from Pd(OAc)2 and 5 equiv of PPh3 in
the presence of NEt3 (3 equiv).7c
was poured into the cell followed by Pd0(PPh3)4 (34 mg, 0.03
mmol). The kinetics of the oxidative addition of PhI (6.12 mg,
0.03 mmol) was monitored at a rotating gold-disk electrode
(i.d. 2 mm, inserted into a Teflon holder, Radiometer Analytical
EDI 65109) polarized at +0.2 V, with an angular velocity of
105 rad s-1 (Radiometer Analytical Controvit) in the presence
of an alkene added before PhI (styrene in the range 0-0.3 M
or methyl acrylate in the range 0-0.02 M).
The same procedure was adopted for the investigation of
the reactivity of PhI with Pd0(PPh3)2(OAc)- in the presence of
styrene (in the range 0-0.2 M).
Ack n ow led gm en t. This work was supported by the
Centre National de la Recherche Scientifique (CNRS,
UMR 8640), the Ministe`re de la Recherche (Ecole
Normale Supe´rieure), and the University Paris VI. We
thank J ohnson Matthey for a generous loan of sodium
tetrachloropalladate.
Electr och em ica l Setu p a n d Electr och em ica l P r oce-
d u r e for Kin etic Mea su r em en ts. Experiments were carried
(13) Amatore, C.; J utand, A. J . Organomet. Chem. 1999, 576, 254.
(14) The base NEt3, used in Heck reactions, also plays a similar role.
The oxidative addition is slower in the presence of NEt3 and the
carbopalladation faster.2d
(15) Rosevear, D. T.; Stone, F. G. J . Chem. Soc. A 1968, 164.
OM020439V