Rate and mechanism of the reaction of alkenes with aryl palladium complexes ligated by a bidentate P,P ligand in heck reactions

Article abstract of DOI:10.1002/chem.200600153

The regioselectivity of the Heck reaction is supposed to be highly affected by the electronic properties of the alkene and the ionic or neutral character of the aryl palladium(II) complexes involved in the reaction with alkenes. In Heck reactions performed in dmf. [Pd(dppp)-(dppp(O))Ph]⁺ (dppp= 1,2-bis(diphenylphosphino)propane) is generated in the oxidative addition of Phi with [Pd⁰-(dppp)(OAc)] formed in situ from Pd(OAc): associated to two equivalents of dppp. [Pd(dppp)jdppp(O))Ph]⁺ is not very reactive with alkenes (styrene or methyl acrylate); however, it reacts with iodide ions (released in the catalytic reactions) to give [Pd(dppp)IPh) and with acetate ions (used as base) to give [Pd(dppp)(OAc)Ph]. [Pd(dppp)-(OAc)Ph] reacts with styrene and methyl acrylate exclusively by an ionic mechanism, that is. via the cationic complex [Pd(dppp)(dmf)Ph]⁺ formed by dissociation of the acetate ion. The reaction of [Pd(dppp)IPh] is more complex and substrate dependent. It reacts with styrene exclusively by the ionic mechanism via [Pd(dppp)-(dmf)Ph]⁺. [Pd(dppp)IPh] (neutral mechanism) and [Pd(dppp)(dmf)Ph]⁺ (ionic mechanism) react in parallel with methyl acrylate. [Pd(dppp)-(dmf)Ph]⁺ is more reactive than [Pd-(dppp)IPh) but is always generated at lower concentration.

Full text of DOI:10.1002/chem.200600153

DOI: 10.1002/chem.200600153
Rate and Mechanism of the Reaction of Alkenes with Aryl Palladium
Complexes Ligated bya Bidentate P,P Ligand in Heck Reactions
[a]
Christian Amatore,* BØatrice Godin, AnnyJutand,* and FrØdØric Lemaître
Abstract: The regioselectivity of the
Heck reaction is supposed to be highly
affected by the electronic properties of
the alkene and the ionic or neutral
character of the aryl palladium(II)
complexes involved in the reaction
with alkenes. In Heck reactions per-
not very reactive with alkenes (styrene
or methyl acrylate); however, it reacts
with iodide ions (released in the cata-
complex [Pd
by dissociation of the acetate ion. The
reaction of [Pd(dppp)IPh] is more
complex and substrate dependent. It
reacts with styrene exclusively by the
G
N
lytic reactions) to give [Pd
and with acetate ions (used as base) to
give [Pd(dppp)(OAc)Ph]. [Pd(dppp)-
(OAc)Ph] reacts with styrene and
(dppp)IPh]
A
G
A
ionic mechanism via [Pd
(dmf)Ph]+. [Pd
(dppp)IPh] (neutral
mechanism) and [Pd(dppp)
(dmf)Ph]+
(ionic mechanism) react in parallel
with methyl acrylate. [Pd(dppp)-
(dmf)Ph]+ is more reactive than [Pd-
(dppp)IPh] but is always generated at
lower concentration.
A
H
A
ACHTREUNG
formed
in
dmf,
[Pd
A
methyl acrylate exclusively by an ionic
mechanism, that is, via the cationic
A
ACHTREUNG
ACHTREUNG
nylphosphino)propane) is generated in
the oxidative addition of PhI with [Pd0-
ACHTREUNG
AHCTREUNG
Keywords: Heck reaction · kinetics ·
A
N
ACHTREUNG
P
ligands
·
palladium
·
ACHTREUNG
regioselectivity
A
ACHTREUNG
Introduction
ArX[1b,e,2a–d] or ArOTf (Tf=trifluoromethanesulfonyl) in the
presence of halide ions[3] (Scheme 2), or 2) an ionic mecha-
The catalytic precursor Pd
A
nism for cationic complexes [Pd
(P,P)S(Ar)]+ (S=solvent)
G
P,P ligands is used to catalyze Heck reactions.[1] Among
them, the ligand 1,2-bis(diphenylphosphino)propane (dppp)
proves to be very efficient (Scheme 1).[2] Two mechanisms
are proposed in the literature for the reaction of alkenes
with aryl palladium(II) complexes formed in the oxidative
generated in the oxidative addition of aryl triflates[1b,e,2,4] or
aryl halides in the presence of a halide scavenger (Ag+,[5a]
Tl+,[2b,5b] or K+ in aqueous dmf[2j]; Scheme 3).[6]
The ionic mechanism is supported by reactions of isolated
complexes [Pd
(P,P)S(Ar)]+ with alkenes.[7,8] The intermedi-
U
addition of [Pd0
(P,P)] with ArX (X=I, Br, Cl): 1) a neutral
mechanism for [Pd(P,P)X(Ar)] complexes generated from
ate complexes formed in the carbopalladation step have
been characterized at low temperatures (Schemes 3 and
4a,a’).[7,8] An investigation of the ionic mechanism involving
electron-deficient alkenes by Brown and Hii revealed that
N
the b-H elimination occurs from [Pd
CH2Ar)]+ leading to [Pd(dppf)(S)H]+, which cannot restore
a Pd0 complex in the absence of a base. Consequently, [Pd-
(dppf)(S)H]+ reacts with the initial alkene (Scheme 4a).[7a]
Škermark has also observed that [Pd(dppp)
(dmf)H]+ reacts
A
(thf)(CHR-
AHCTREUNG
Scheme 1.
AHCTREUNG
G
with styrene in the absence of a base (Scheme 4a’).[8] In the
presence of a base, a Pd0 complex is generated directly due
to the easier deprotonation of the agostic hydrogen atom,
thus bypassing the b-H elimination, as proposed by Brown
et al. (Scheme 4b).[7c]
[a] Dr. C. Amatore, Dr. B. Godin, Dr. A. Jutand, Dr. F. Lemaître
Ecole Normale SupØrieure, DØpartement de Chimie
UMR CNRS-ENS-UPMC 8640
24 Rue Lhomond, 75231Paris Cedex 5 (France)
Fax : (+33)144-323-325
E-mail: christian.amatore@ens.fr
The regioselectivity of Heck reactions may be affected by
the mechanism: ionic versus neutral.[1b,e,2,9] Branched al-
kenes are usually produced from electron-rich alkenes
Anny.Jutand@ens.fr
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
2002
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 2002 – 2011

FULL PAPER
Scheme 2. Neutral mechanism.
associated with two equivalents of dppp.[10a] The complex
[Pd
N
N
intramolecular reaction which generates the hemioxide
[10a]
ꢀ
ꢀ
(O)PPh2
The existence and stability of the anionic complexes [Pd0-
(dppp)
(OAc)]ꢀ have recently
G
Scheme 3. Ionic mechanism (R=electron-donating group).
been supported by theoretical
calculations (DFT).[11] The main
complex formed in its oxidative
addition with iodobenzene is
[Pd
C
ACHTREUNG
A
ACHTREUNG
(2a) is formed when the oxida-
tive addition is performed in
the presence of excess acetate
ions (Scheme 5).[1,2] Conversely,
[Pd(dppp)IPh] (2b) is generat-
G
ed in the presence of excess
iodide ions.[10a] Consequently,
three complexes might poten-
tially react with the alkene in a
Heck reaction, depending on
the exact reaction conditions:
1) the primary complex [Pd-
Scheme 4. Ionic mechanism. a) Experimental work. a’) Experimental work; the branched complex was also
formed. b) Density functional theory (DFT) calculations. dppf=1,1’-bis(diphenylphosphino)ferrocene.
under the conditions of the ionic mechanism
(Scheme 3).[1e,2f–k] As far as electron-poor alkenes are con-
cerned, the origin of the regioselectivity is more tricky.[1,2,9]
C
ACHTREUNG
A
ACHTREUNG
The reaction of [Pd(dppp)XPh] (X=I, OAc, BF4) with sty-
U
rene has been investigated by Škermark et al., who focused
on the effect of X on the regioselectivity of the reaction.[8]
In dmf, the ratio PhCH=CHPh/CH2=CPh2 was found to de-
crease when going from X=I, OAc to BF4. However, no ki-
netic data were provided on the relative reactivity of such
neutral and cationic complexes.
Most mechanisms described above were postulated or es-
tablished from isolated complexes [Pd(P,P)X(Ar)] and do
G
not take into account the real conditions of catalytic reac-
tions. In particular, the role of acetate ions, delivered by Pd-
A
this precursor) or by the base, is bypassed. Contrastingly, we
have previously established that an anionic Pd0 complex
[Pd0
[Pd0
C
G
Scheme 5.
Chem. Eur. J. 2007, 13, 2002 – 2011
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www.chemeurj.org
2003

C. Amatore, A. Jutand et al.
A
tration of [Pd
G
N
tion due to the release of iodides from ArI.
species may be formed (chemical–electrochemical mecha-
The purpose of this work is to compare the reactivity of
the three aryl palladium(II) complexes mentioned above
with alkenes and to delineate the relative contribution of
the neutral and ionic mechanisms in the context of real cata-
lytic reactions (precursor, base, ions released in the reaction,
ionic strength, etc.).
nism).[14] This has been addressed in full detail in the case of
[Pd
A
of [Pd(dppp)IPh] (2 mm) was obtained in dmf containing
C
nBu4NBF4 (0.3m). It exhibited a major reduction peak R2 at
EpR2 =ꢀ2.10 V versus a saturated calomel electrode (SCE) at
the scan rate of 0.2 Vsꢀ1 (Figure 1a). Importantly, a minor
Results and Discussion
Characterization of the equilibrium between [Pd
and [Pd(dppp)(OAc)Ph] in dmf: As mentioned in the Intro-
duction, [Pd(dppp)IPh] (2b) and [Pd(dppp)(OAc)Ph] (2a)
may be generated in the oxidative addition of PhI with [Pd0-
(dppp)
(OAc)]ꢀ.[10a] As evidenced by 31P NMR spectroscopy,
addition of acetate ions (introduced as nBu4NOAc) to iso-
lated [Pd(dppp)IPh] resulted in the formation of [Pd(dppp)-
(OAc)Ph] in dmf, whereas [Pd(dppp)IPh] was generated by
addition of iodide ions (introduced as nBu4NI) to isolated
[Pd(dppp)(OAc)Ph]. Therefore, 2a and 2b are in equilibri-
(dppp)IPh]
A
ACHTREUNG
A
N
ACHTREUNG
A
ACHTREUNG
A
ACHTREUNG
A
ACHTREUNG
A
ACHTREUNG
um with the corresponding ions (Scheme 6).
Figure 1. Cyclic voltammetry of [Pd(dppp)IPh] (2 mm) in dmf containing
U
nBu4NBF4 (0.3m) at a steady gold-disk electrode (d=0.5 mm) with a
scan rate of 0.2 Vsꢀ1. a) Reduction first. b) Oxidation first.
Scheme 6.
reduction wave R1 was observed as a shoulder located at
less negative potential: EpR1 =ꢀ1.7 V (Figure 1a). The latter
disappeared upon addition of 10 equivalents of nBu4NI,
while peak R2 was conserved. This means that 2b was the
main species reduced at R2, but being in equilibrium with
the minor cationic complex 3+ reduced at R1 (Scheme 7).
Two oxidation peaks were also observed at +0.25 and
+0.7 V when an oxidative potential scan was performed in
The equilibrium constant K=([2a][Iꢀ]/
0.09 (dmf, 258C) was determined from 31P NMR spectro-
scopic data. [Pd(dppp)IPh] is thus thermodynamically more
stable than [Pd(dppp)
(OAc)Ph] at comparable Iꢀ and AcOꢀ
concentrations.[12] The Iꢀ/OAcꢀ exchange in [Pd
(dppp)IPh]
probably proceeds via the cationic complex [Pd(dppp)-
(dmf)Ph]+ (3+, Scheme 7), as proposed for the related com-
A
G
ACHTREUNG
E
ACHTREUNG
AHCTREUNG
AHCTREUNG
ACHTREUNG
plex [Pd
A
the range 0–1V (Figure 1b). Peak O characterizes the oxi-
2
dation of [Pd
(dppp)IPh][15] and peak O1 the oxidation of Iꢀ
G
released in the equilibrium of Scheme 7, when it is continu-
ously shifted by the electrochemical oxidation of Iꢀ. Thus,
even if [Pd
N
N
centration to be observed by 31P NMR spectroscopy, its
presence was established voltammetrically by the dual pres-
ence of waves O1 and R1.
Scheme 7.
However, the complex 3+ must be a transient intermedi-
Similarly, two reduction peaks were also observed at
ate present at very low concentration, as the 31P NMR spec-
EpR01 =ꢀ1.75 V (minor) and ERp 02 =ꢀ2.19 V (major) for the
trum of [Pd
(dppp)IPh] or [Pd
(dppp)
U
complex [Pd
N
N
only one set of two doublets at 258C (Supporting Informa-
current ratio iR’1/iR’2 was higher than iR1/iR2 obtained for [Pd-
tion). Nevertheless, despite such a low concentration, [Pd-
C
(dppp)
G
ly by its reaction with a competing reagent. If so, the same
continuous displacement may be performed electrochemi-
cally whenever the cationic palladium species is reducible.
A significant reduction wave may be observed, whose cur-
rent intensity will not represent the thermodynamic concen-
Scheme 8.
2004
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Chem. Eur. J. 2007, 13, 2002 – 2011

Reaction of Alkenes with Aryl Palladium Complexes
FULL PAPER
than the dissociation of Iꢀ from [Pd
under the same experimental conditions of concentration
and scan rate, in full agreement with the fact that the equi-
A
CO2Me). The rate of the b-H elimination is not relevant
when the reaction 4+!5+ is irreversible.
k1k2k3½2Š½CH2¼CHRŠ
librium in Scheme 6 lies in favor of [Pd(dppp)IPh] (K=
0.09=K1/K’1).
A
ð1Þ
rate ¼
kꢀ1kꢀ2½XꢀŠ þ kꢀ1k3½XꢀŠ þ k2k3½CH2¼CHRŠ
When kꢀ2 @k3 (fast 2nd equilibrium), Equation (1) can be
simplified into Equation (2):
Rate and mechanism of the reaction of [Pd
(2a) with alkenes (RCH=CH2, R=CO2Me, Ph) in dmf
N
A
k1k2k3½2Š½CH2¼CHRŠ
Reaction of [Pd
A
N
ð2Þ
rate ¼
kꢀ1kꢀ2½XꢀŠ þ k2k3½CH2¼CHRŠ
1
mechanism: The reaction was first monitored by H NMR
spectroscopy in [D6]acetone by using a slight excess of
methyl acrylate in the absence of any base. (E)-Methyl cin-
namate was generated in 75% yield as the single product
(Scheme 9). No branched product was detected, indicative
of a good regioselectivity in acetone.
Considering the intrinsic fast rate of the equilibrium, 2a
versus 3+, at low methyl acrylate concentrations kꢀ1[Xꢀ]@
[CH2=CHR]. This gives the simplified Equation (3):
k2
A
k1k2k3½2Š½CH2¼CHRŠ
kꢀ1kꢀ2½XꢀŠ
ð3Þ
rate ¼
The kinetic law is given in Equation (4) (x=[2a]/C0; C0 =
initial concentration of 2a), taking into account the varia-
tion of concentration of Xꢀ =AcOꢀ all along the reaction
course. Its integration affords Equation (5):
Scheme 9.
The kinetics of the reaction of 2a with methyl acrylate in
dmf (containing 10% [D6]acetone) in the absence of base
was followed by 31P NMR spectroscopy by using a capillary
of H3PO4 (80% in water) as internal standard. The two dou-
blets of 2a slowly disappeared after addition of excess
dx
dt
K1K2k3½CH2¼CHRŠx
C0ð1ꢀxÞ
ð4Þ
ð5Þ
¼ ꢀ
K1K2k3½CH2¼CHRŠt
lnxꢀx þ 1 ¼ ꢀ
¼ ꢀkobs
t
methyl acrylate. Since the cationic complex [Pd
A
C0
ACHTREUNG
Conversely, at higher methyl acrylate concentrations,
when kꢀ1kꢀ2[Xꢀ]!k2k3
[CH2=CHR], the simplified kinetic
mechanism of Scheme 10 may be proposed in a first ap-
AHCTREUNG
law in Equation (6) is obtained whose integration gives
Equation (7). The rate-limiting step will thus be the dissocia-
tion of 2a to 3+, independent of the methyl acrylate concen-
tration.
dx
dt
ð6Þ
ð7Þ
¼ ꢀk1x
lnx ¼ ꢀk1t
At low methyl acrylate concentrations (up to 1.5m), the
plot of lnxꢀx+1against time were linear (Figure S1in the
Supporting Information), as expressed in Equation (5). The
observed rate constant kobs, determined from the slope of
the straight line, varied linearly with methyl acrylate concen-
tration (Figure 2a), attesting to a first-order reaction for the
methyl acrylate as predicted in Equation (5). Conversely, at
higher methyl acrylate concentrations, lnx was found to vary
linearly with time (Figure 2a). The observed rate constant
kobs did not depend on the methyl acrylate concentration as
predicted in Equation (7). K1K2k3 was calculated from the
slope of the straight line obtained at low methyl acrylate
concentration and k1 from the limiting value of kobs in Fig-
ure 2a (Table 1).
Scheme 10.
proach. This considers a reversible complexation of the cat-
ionic complex 3+ by the alkene followed by an irreversible
carbopalladation step (rate constant k3). The ensuing com-
plex 5+ is expected to evolve towards the final products
after b-H elimination (as in Scheme 4a).
According to this mechanism, the kinetic law for the dis-
appearance of 2a is given by Equation (1) (X=OAc, R=
The kinetic data thus agree with the ionic mechanism pro-
posed in Scheme 10. For confirmation, the effects of AcOꢀ
Chem. Eur. J. 2007, 13, 2002 – 2011
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2005

C. Amatore, A. Jutand et al.
Figure 3. Kinetics of the reaction of [Pd
N
N
12.2 mm) with methyl acrylate in dmf at 258C. Effect of ionic strength
and acetate ions: plot of kobs versus methyl acrylate concentration in the
&
*
);
absence of salts ( ); in the presence of [nBu4NBF4]=244 mm
(
^
*
[nBu4NBF4]=122 mm
( ); [nBu4NOAc]=244 mm ( ); [nBu4NOAc]=
^
122 mm ( ).
kobs against methyl acrylate concentration was linear for
each concentration of nBu4NOAc (Figure 3b’’,c’’), but the
saturating effect could not be observed in the concentration
range investigated here. When comparing the slope of the
straight lines obtained in the presence of nBu4NOAc and
nBu4NBF4 at the same concentration, that is, at identical
ionic strength, one observes that the reaction was slower in
the presence of AcOꢀ (Figure 3). Consequently, a specific
effect of the acetate ions is observed, the reaction of [Pd-
Figure 2. Kinetics of the reaction of [Pd
258C with a) methyl acrylate and b) styrene. Determination of the reac-
tion order in alkenes: plot of kobs against alkene concentration.
N
G
A
N
Table 1. Rate constants of the reaction of [Pd(dppp)XPh] (X=I, OAc)
with alkenes (see Schemes 10 and 11 for the definitions of k1 and
K1K2k3).
A
presence of AcOꢀ, in agreement with the ionic mechanism
of Scheme 10.[16]
Methyl acrylate Styrene
k1 [sꢀ1
(OAc)Ph] 1.6
(ꢁ0.1)”10ꢀ4 1.5
(dppp)IPh] 1.1
(ꢁ0.1)”10ꢀ4 1.0(ꢁ0.1)”10ꢀ6 3.8
]
K1K2k3 [sꢀ1
]
K1K2k3 [sꢀ1
]
Reaction of [Pd
N
U
nism: This reaction has been investigated by Škermark
et al. in dmf but without any kinetic data.[8] It gives a mix-
ture of the branched CH2=CPh2 and linear PhCH=CHPh
products in the ratio 18:82. The kinetics of the reaction of
[Pd
[Pd
U
R
G
(ꢁ0.1)”10ꢀ6 6.6
(ꢁ0.1)”10ꢀ8
G
C
(ꢁ0.1)”10ꢀ8
and ionic strength were probed. The effect of the ionic
strength was first investigated by monitoring the reaction of
[Pd
A
G
was monitored by 31P NMR spectroscopy, as for methyl ac-
rylate. The reaction was considerably slower than that in-
volving methyl acrylate at identical concentrations. The re-
action was first order in styrene (Figure 2b) and did not dis-
play any saturating effect all along the investigated styrene
concentration range (up to 2.3m). The reaction was thus
never limited by the dissociation of 2a to 3+.[17] An acceler-
ating effect was observed when performing the reaction at
high ionic strength.[18] No reaction was observed in the pres-
ence of 10 equivalents of nBu4NOAc, which means that the
decelerating effect of AcOꢀ was now considerably higher
than the accelerating effect due to its ionic strength. All
these results again support the involvement of the ionic
[Pd
N
N
methyl acrylate, in the presence of increasing amounts of
nBu4NBF4. The reaction went faster upon increasing the
ionic strength (Figure 3). The values of K1K2k3 increased
with increasing ionic strength (Figure 3a–c), as did those of
k1 (Figure 3a’–c’). As expected, the dissociation of 2a to 3+
was favored at high ionic strength.
According to the ionic mechanism proposed in
Scheme 10, a decelerating effect of AcOꢀ (introduced as
nBu4NOAc) on the reactivity of [Pd
N
N
be observed [Eq. (3), with X=OAc]. However, this effect
should be partly compensated by the accelerating effect of
the ionic strength due to the addition of ionic species
nBu4N+ and AcOꢀ (free ions in dmf). The value of kobs was
determined for different methyl acrylate concentrations in
the presence of various amounts of nBu4NOAc. The plot of
mechanism (Scheme 10) for the reaction of [Pd
(OAc)Ph] with styrene, which proceeds via [Pd
(dmf)Ph]+. K1K2k3 was determined from the slope of the
straight line in Figure 2b (Equation (5), Table 1).
ACHTREUNG
A
ACHTREUNG
AHCTREUNG
2006
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Reaction of Alkenes with Aryl Palladium Complexes
FULL PAPER
Rate and mechanism of the reaction of [Pd
with alkenes (RCH=CH2; R=CO2Me, Ph) in dmf
A
late concentration is shown in Figure 4a. A straight line (A)
was obtained at low methyl acrylate concentrations
(<1.5m). At higher methyl acrylate concentrations, a pro-
gressive saturation was observed (line B in Figure 4a), al-
Reaction of [Pd(dppp)IPh] with styrene; ionic mechanism:
A
This reaction has also been investigated by Škermark et al.
but without any kinetic data.[8] The branched CH2=CPh2 and
linear PhCH=CHPh products were formed in the ratio
20:80. The kinetics of the reaction of 2b with styrene were
monitored by 31P NMR spectroscopy. The reaction was first
order in styrene (Figure 4b) in agreement with Equation (5).
though less sharp than in the case of [Pd
(compare Figure 2a). However, the saturation limit that
would deliver the value of k1 (the rate constant of the disso-
A
G
ciation of [Pd
A
G
N
the pure ionic mechanism of Scheme 10) has to be larger
than 2.3”10ꢀ4 sꢀ1 (Figure 4a). Such a value would be higher
than the corresponding one, k1 =1.6”10ꢀ4 sꢀ1, determined
for the dissociation of [Pd
result is inconsistent, as [Pd
ated than [Pd(dppp)(OAc)Ph] (vide supra). Accordingly, the
A
N
AHCTREUNG
A
ACHTREUNG
hypothesis concerning a saturating effect limited by k1
cannot be retained. We thus need to consider that the
would-be saturating effect is unobservable because another
mechanism overtakes it. In other words, we need to consider
two competing mechanisms occurring at low methyl acrylate
concentrations, one being the ionic one. When the ionic
mechanism reaches its saturation limit, the second slower
one shows up. Within this framework, the rate law of the
second mechanism is given by the straight line observed at
high methyl acrylate concentrations (line B in Figure 4a). It
ensues that the slope of line A in Figure 4a represents the
sum of the rates of the two competing mechanisms.
To test for this rationalization, the reaction of 2b
(12.2 mm) with methyl acrylate was monitored in the pres-
ence of excess Iꢀ (nBu4NI, 122 mm). A first-order reaction
for methyl acrylate was found all along the concentration
range investigated here (0–4m, Figure S3a in the Supporting
Information). At identical methyl acrylate concentrations,
the reaction was slower (kapp =4.3”10ꢀ5 mꢀ1 sꢀ1) than that
performed in the presence of nBu4NBF4 (122 mm) at the
same ionic strength (kapp =2.5”10ꢀ4 mꢀ1 sꢀ1, Figure S3b in
the Supporting Information), and also slower than the one
performed in the absence of Iꢀ(kapp=9.3”1ꢀ5 mꢀ1 sꢀ1, line A
in Figure 4a). These results attest to a specific effect of Iꢀ,
leading to a deceleration much more important than the ac-
celeration due to its ionic strength.
Figure 4. Kinetics of the reaction of [Pd(dppp)IPh] (2b) in dmf at 258C
with a) methyl acrylate and b) styrene. Determination of the reaction
A
The reaction of 2b (C0 =12.2 mm) with methyl acrylate
(2.44m) was then monitored in the presence of various
amounts of Iꢀ (up to 0.3m). The higher the Iꢀ concentration,
the slower the reaction. The plot of kobs versus the reciprocal
of the Iꢀ concentration was linear but with a positive inter-
cept (Figure 5). A purely ionic mechanism would have given
a zero intercept [Eq. (3)]. This result confirms that the
mechanism is a mixture of a neutral mechanism (Iꢀ inde-
pendent) and an ionic mechanism (Iꢀ dependent;
order in alkenes: plot of kobs against alkene concentration.
The reaction was highly accelerated at high ionic strength
(by a factor of 100 in the presence of 0.3m nBu4NBF4) but
slower in the presence of Iꢀ (10 equiv introduced as nBu4NI;
Figure S2 in Supporting Information), attesting to the in-
volvement of [Pd
N
N
termediate (Scheme 10). K1K2k3 was determined from the
slope of the straight line of Figure 4b (Table 1).
Scheme 11). Both [Pd
A
G
N
react competitively, although the cationic complex is more
reactive than the neutral one when its concentration is suffi-
cient, namely, at lower methyl acrylate and Iꢀ concentra-
tions.
Reaction of [Pd
neutralmechanisms : The reaction of [Pd
with methyl acrylate in the absence of base yielded (E)-
methyl cinnamate in 89% yield in acetone. The kinetics of
the reaction were investigated in dmf without any base by
31P NMR spectroscopy. The plot of kobs versus methyl acry-
ACHTREUNG
AHCTREUNG
Considering that the two h2-CH=CHR ligated compounds
4b and 4+ are in a steady state, the rate law corresponding
to the mixed mechanism of Scheme 11 is given by Equa-
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2007

C. Amatore, A. Jutand et al.
gives Equation (10) upon considering that kꢀ1kꢀ2[Iꢀ]!
k2k3[CH2=CHCO2Me].
k4k5½CH2¼CHRŠ
ð10Þ
kobs
¼
þ k1
kꢀ4 þ k5
This corresponds to a saturation limit for the ionic mecha-
nism, while the neutral one proceeds. The value of k1 =6.5”
10ꢀ3 minꢀ1 (dmf, 258C) is calculated from the intercept of
line B of Figure 4a, while the slope gives k4k5/(kꢀ4+k5)=
G
2.3”10ꢀ5 mꢀ1 sꢀ1 (dmf, 258C), a value which is close to that
determined above from Figure 5 (2.5”10ꢀ5 mꢀ1 sꢀ1).[19]
Therefore, albeit the results obtained under the different
conditions of Figures 4a and 5 are independent, they are
fully consistent. Furthermore, within this dual-mechanism
Figure 5. Effect of iodide ions on the kinetics of the reaction of [Pd-
(dppp)IPh] (2b) (C0 =12.2 mm) with methyl acrylate (2.44m) in dmf at
258C. Plot of kobs versus 1/[Iꢀ].
framework, k1 =1.1”10ꢀ4 sꢀ1 is found for [Pd
ACHTREUNG
ACHTREUNG
AHCTREUNG
AHCTREUNG
date the formulation in Scheme 11, that is, a parallel in-
volvement of the ionic and neutral mechanisms in the reac-
tion of methyl acrylate. The mechanism of the reaction of
[Pd(dppp)IPh] with alkenes is then substrate dependent.
U
Reactivityof [Pd
in dmf: The complex [Pd
erated in situ in the reaction of PhI (1equiv) with the Pd 0
complex formed from Pd(OAc)2, dppp (2 equiv), H2O
A
ACHTREUNG
A
ACHTREUNG
AHCTREUNG
(20 equiv), and NEt3 (30 equiv), as reported in our previous
work.[10a] Its reaction with methyl acrylate was monitored by
31P NMR spectroscopy (Supporting Information) and was
extremely slow, even in the presence of a large excess of
alkene (500 equiv). The signals of [Pd(dppp)IPh] were then
G
observed at long time intervals due to the slow reaction of
Scheme 11.
Iꢀ released during the oxidative addition of PhI with [Pd-
A
N
tion (8) (R=CO2Me, X=I), in which the first term is the
contribution of the neutral mechanism and the second one
that of the ionic mechanism.
ics of the reaction of 1 with methyl acrylate or styrene were
too slow to be monitored precisely.
Nevertheless, such results emphasize the role of Iꢀ or
AcOꢀ ions. By their parallel reaction with [Pd
ACHTREUNG
k4k5½CH2¼CHRŠ
kꢀ4 þ k5
k1k2k3½CH2¼CHRŠ
A
G
ACHTREUNG
kobs
¼
þ
kꢀ1kꢀ2½XꢀŠ þ k2k3½CH2¼CHRŠ
AHCTREUNG
ð8Þ
A
ACHTREUNG
as its lifetime is expected to be short in the presence of a
large number of Iꢀ or AcOꢀ ions (as in catalytic reactions).
Yet, it cannot be considered as truly reactive in the carbo-
palladation step.
At high Iꢀ concentrations, kꢀ1kꢀ2[Iꢀ]@k2k3[CH2=
CHCO2Et], Equation (8) simplifies into Equation (9):
k4k5½CH2¼CHRŠ k1k2k3½CH2¼CHRŠ
ð9Þ
kobs
¼
þ
kꢀ1kꢀ2½XꢀŠ
kꢀ4 þ k5
Conclusion
The intercept of the straight line of Figure 5 gives the
value of k4k5/
(kꢀ4+k5)=2.5”10ꢀ5 mꢀ1 sꢀ1 (dmf, 258C), where-
E
In Heck reactions performed from iodobenzene and cata-
as the slope gives the value of K1K2k3 =1”10ꢀ6 sꢀ1 (dmf,
258C).
lyzed by Pd
the complex [Pd
oxidative addition of PhI with [Pd0
[Pd(dppp)
{dppp(O)}Ph]+ is not very reactive with alkenes,
ACHTREUNG
A
ACHTREUNG
Coming back to Figure 4a, in which no Iꢀ had been added
and at high methyl acrylate concentrations, Equation (8)
A
ACHTREUNG
A
ACHTREUNG
2008
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Chem. Eur. J. 2007, 13, 2002 – 2011

Reaction of Alkenes with Aryl Palladium Complexes
FULL PAPER
such as those investigated here (styrene or methyl acrylate).
However, it rapidly becomes a transient species under effec-
tive catalytic conditions. Indeed, its reaction with Iꢀ ions
which are released in the catalytic reaction, or with AcOꢀ
All these kinetic results, associated with a previous work
on the kinetics of the oxidative addition to PhI,[10a] establish-
ed that for identical concentrations of PhI and alkenes (sty-
rene, methyl acrylate), the oxidative addition is much faster
than the so-called carbopalladation step (reaction of alkenes
ions used as a base, generates [Pd
(dppp)(OAc)Ph], respectively. [Pd(dppp)
with styrene and methyl acrylate exclusively by an ionic
mechanism, that is, via the cationic complex [Pd(dppp)-
(dmf)Ph]+ formed by dissociation of AcOꢀ (Scheme 10).
The reaction of [Pd(dppp)IPh] is more complex and sub-
ACHTREUNG
A
N
G
N
with [Pd(dppp)X(Ar)] complexes), which is the rate-deter-
N
mining step of the catalytic reaction and introduces the
main mechanistic complexities of the overall Heck reaction.
AHCTREUNG
ACHTREUNG
AHCTREUNG
strate dependent, due to the larger stability of this species
vis-à-vis its ionic dissociation. It reacts with styrene exclu-
Experimental Section
sively by the ionic mechanism via [Pd
(dppp)
(dmf)Ph]+
N
General: 31P NMR spectra were recorded on a Bruker spectrometer
(101.3 MHz) in dmf containing 10% [D6]acetone. A capillary tube con-
taining H3PO4 (85% in water) was introduced into the NMR tube to
serve as an internal standard for the kinetic studies. 1H NMR spectra
were recorded on a Bruker spectrometer (250 MHz). Cyclic voltammetry
was performed at gold disk electrodes with a homemade potentiostat and
a waveform generator (Tacussel GSTP4). The cyclic voltammograms
were recorded on a Nicolet 301oscilloscope.
(Scheme 10). As far as methyl acrylate is concerned, the
neutral and ionic mechanisms compete in parallel
(Scheme 11). Clearly, [Pd
G
G
than [Pd(dppp)IPh], but its intrinsic higher energy maintains
ACHTREUNG
it at concentrations that are too low to allow it to compete
efficiently at high Iꢀ or methyl acrylate concentrations.
The following reactivity orders have been established:-
Chemicals: dmf was distilled from calcium hydride under vacuum and
kept under argon. Pd
nBu4NOAc, nBu4NBF4, and the ligand dppp were commercial.
Synthesis of [Pd(dppp)(OAc)Ph] (2a): This compound was synthesized
from [Pd
(dppp)IPh] according to the literature.[8] 31P NMR (101.3 MHz,
dmf+[D6]acetone, H3PO4): d=18.91 (d, J
2J(P,P)=48.5 Hz); 31P NMR (101.3 MHz, [D6]acetone, H3PO4): d=19.40
(d, 2J
(OAc)2, styrene, methyl acrylate, PhI, nBu4NI,
whatever the complex [Pd
CHCO2Me>CH2=CHPh; whatever the alkene: [Pd
(dmf)Ph]+ >[Pd
(dppp)XPh] (X=OAc, I)@[Pd
{dppp(O)}Ph]+
All these results differ from the generally postulated
mechanism, which mostly proposes a neutral mechanism
from [Pd(dppp)XPh] (X=I). Moreover, it has been estab-
lished in this work that [Pd(dppp)IPh] and [Pd(dppp)-
ACHTREUNG
AHCTREUNG
G
ACHTREUNG
A
N
ACHTREUNG
AHCTREUNG
2
AHCTREUNG
ACHTREUNG
AHCTREUNG
(P,P)=48.5 Hz), ꢀ3.70 ppm (d, 2J
ACHTREUNG
595 [M+ꢀOAc], 518 [M+ꢀOAcꢀPh].
ACHTREUNG
Synthesis of [Pd(dppp)IPh] (2b): This compound was synthesized ac-
A
A
ACHTREUNG
cording to a published procedure.[8] A slow crystallization from acetone
gave orange crystals (yield: 75%). 31P NMR (101.3 MHz, [D6]acetone,
A
anions Iꢀ and AcOꢀ. In a real Heck reaction, the respective
anion concentrations will vary in the course of the catalytic
reaction and consequently the concentration of the reactive
H3PO4): d=11.94 (d, 2J
A
ACHTREUNG
(P,P)=53.6 Hz), ꢀ8.88 ppm (d, 2J
N
complex [Pd
N
N
Reaction of [Pd
N
U
along the catalytic reaction. However, an antagonistic effect
is then observed between the specific inhibiting effect of Iꢀ
and AcOꢀ and the accelerating effect due to increasing ionic
strength.
was performed in a Schlenk tube under argon. Methyl acrylate (1 mL,
11.2 mmol) was added at room temperature to a solution of 2a (2.3 mg,
3.4 mmol) in [D6]acetone (400 mL). The yellow solution turned progres-
1
sively red. The H NMR spectra showed the progressive disappearance of
the signal of the Ph group linked to the Pd(II) center of 2a and those of
the methyl acrylate to afford the signals of (E)-methyl cinnamate similar
to those of an authentic sample. After one month, a black solution was
obtained, 2a was no longer observed, and (E)-methyl cinnamate was ex-
clusively formed without any (Z)-methyl cinnamate or branched isometh-
yl cinnamate. The 31P NMR spectrum revealed the signal of the hemiox-
ide dppp(O) at 29.9 ppm. The excess methyl acrylate was eliminated
under vacuum. 1,1,2,2-Tetrachloroethane (1mL, 9.3 mmol) was then added
as an internal standard to determine the amount of (E)-methyl cinna-
mate formed in the reaction (75% yield).
As evidenced by Škermark et al., the solvent may also
play an important role through its dissociation properties
which favor the ionic process.[8] Heck reactions performed
from an electron-rich alkene usually give branched alkenes
under the conditions of the ionic mechanism, that is, starting
from aryl triflates or aryl iodides in the presence of Ag+ or
Tl+ salts, which both generate [Pd
titatively.[1e,2f–k,4] Xiao et al. have reported that a Heck reac-
tion catalyzed by Pd(OAc)2 associated with dppp and per-
A
N
H
Reaction of [Pd(dppp)IPh] (2b) with methyl acrylate: The reaction was
A
performed as above starting from a solution of 2b (5 mg, 6.9 mmol).
After addition of methyl acrylate (1 mL, 11.5 mmol), the yellow solution
of 2b turned progressively red. After one month and workup, (E)-methyl
cinnamate was formed in 89% yield.
formed from an electron-rich alkene and ArI (without any
iodide scavenger, that is, under the conditions of the neutral
“textbook” mechanism of Scheme 2 gave a mixture of
branched and linear products in dmf, whereas the branched
product was exclusively produced in ionic liquids.[20] This
result emphasizes concretely the synthetic significance of
the results established in this work: the cationic complex
Electrochemical setup and procedure for cyclic voltammetry: Experi-
ments were carried out in a three-electrode thermostated cell connected
to a Schlenk line. The counter electrode was a platinum wire of approxi-
mately 1cm 2 apparent surface area. The reference was a SCE separated
from the solution by
a bridge filled with dmf (3 mL) containing
[Pd
(dppp)(S)Ph]+ is the most intrinsically reactive complex,
A
nBu4NBF4 (0.3m). The working electrode was a gold-disk electrode (d=
and its overall role is larger at high ionic strength (as in
ionic liquids) and affords the regioselectivity of the ionic
mechanism observed with electron-rich alkenes.[1e,2f–k]
0.5 mm).
Kinetics of the reaction of [Pd
(dppp)
(OAc)Ph] (2a) with methyl acrylate
as monitored by 31P NMR spectroscopy: Complex 2a (4.8 mg, 7.3 mmol)
Chem. Eur. J. 2007, 13, 2002 – 2011
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2009

C. Amatore, A. Jutand et al.
was introduced into an NMR tube containing dmf (400 mL) and
[D6]acetone (200 mL). A capillary of H3PO4 was then introduced as an in-
ternal standard. A known amount of methyl acrylate was added and the
kinetics were followed by 31P NMR spectroscopy, until total conversion,
by the decrease of the two doublets of 2a at ꢀ4.18 and 18.91 ppm whose
total integration was compared to that of H3PO4. In other experiments,
known amounts of nBu4NBF4 were added before the methyl acrylate to
probe the effect of the ionic strength on the kinetics. In addition, known
amounts of nBu4NOAc were added before the methyl acrylate to probe
the effect of acetate ions on the kinetics.
hedron 2000, 56, 7975–7979; j) K. S. A. Vallin, M. Larhed, A. Hall-
berg, J. Org. Chem. 2001, 66, 4340–4343; k) K. S. A. Vallin, Q. S.
Zhang, M. Larhed, D. P. Curran, A. Hallberg, J. Org. Chem. 2003,
68, 6639–6645.
[3] a) L. E. Overman, D. J. Poon, Angew. Chem. 1997, 109, 536–538;
Angew. Chem. Int. Ed. Engl. 1997, 36, 518–521; b) A. Ashimori,
M. A. Calter, S. P. Govek, L. E. Overman, D. J. Poon, J. Am. Chem.
Soc. 1998, 120, 6488–6499.
[4] a) F. Ozawa, A. Kubo, T. Hayashi, J. Am. Chem. Soc. 1991, 113,
1417–1419; b) T. Hayashi, A. Kubo, F. Ozawa, Pure Appl. Chem.
1992, 64, 421–427; c) F. Ozawa, A. Kubo, Y. Matsumoto, T. Hayashi,
Organometallics 1993, 12, 4188–4196.
Kinetics of the reaction of [Pd(dppp)IPh] (2b) with methyl acrylate as
A
monitored by 31P NMR spectroscopy: The reaction was performed as
above by starting from 2b (5 mg, 6.9 mmol) and various amounts of
methyl acrylate. nBu4NBF4 was added before the methyl acrylate to
probe the effect of the ionic strength on the kinetics. In other experi-
ments, known amounts nBu4NI were added before the methyl acrylate to
probe the effect of iodide ions on the kinetics.
[5] a) K. Karabelas, C. Westerlund, A. Hallberg, J. Org. Chem. 1985, 50,
3896–3900; b) R. Grigg, V. Loganathan, V. Santhakumar, A. Teas-
dale, Tetrahedron Lett. 1991, 32, 687–690.
[6] Milstein et al. have established that the cationic complex was the re-
active species generated from a neutral [Pd(dippp)ClPh] complex
G
(dippp=1,3-bis(diisopropylphosphino)propane). M. Portnoy, Y.
Ben-David, I. Rousso, D. Milstein, Organometallics 1994, 13, 3465–
3479.
Kinetics of the reaction of [Pd
N
A
monitored by 31P NMR spectroscopy: The reaction was performed as
above by starting from 2a (4.5 mg, 6.9 mmol) and various amounts of sty-
rene. nBu4NBF4 was added before the styrene to probe the effect of the
ionic strength on the kinetics. In other experiments, known amounts
nBu4NOAc were added before the styrene to probe the effect of acetate
ions on the kinetics.
[7] a) J. M. Brown, K. K. Hii, Angew. Chem. 1996, 108, 679–682;
Angew. Chem. Int. Ed. Engl. 1996, 35, 657–659; b) K. K. Hii,
T. D. W. Claridge, J. M. Brown, Angew. Chem. 1997, 109, 1033–
1036; Angew. Chem. Int. Ed. Engl. 1997, 36, 984–987; c) R. J.
Deeth, A. Smith, K. K. Hii, J. M. Brown, Tetrahedron Lett. 1998, 39,
3229–3232; d) K. K. Hii, T. D. W. Claridge, R. Giernoth, J. M.
Brown, Adv. Synth. Catal. 2004, 346, 983–988.
Kinetics of the reaction of [Pd
A
G
methyl acrylate as monitored by 31P NMR spectroscopy: NEt3 (50 mL,
0.36 mmol), water (5 mL, 0.24 mmol), dppp (10 mg, 24 mmol), and Pd-
[8] M. Ludwig, S. Strçmberg, M. Svensson, B. Škermark, Organometal-
lics 1999, 18, 970–975.
ACHTREUNG
AHCTREUNG
[9] For a theoretical approach to the regiochemistry of palladium-phos-
phine-catalyzed Heck reactions see: R. J. Deeth, A. Smith, J. M.
Brown, J. Am. Chem. Soc. 2004, 126, 7144–7151.
[10] a) C. Amatore, A. Jutand, A. Thuilliez, Organometallics 2001, 20,
3241–3249; For similar reactions involving the binap ligand see:
b) F. Ozawa, A. Kubo, T. Hayashi, Chem. Lett. 1992, 2177–2180.
[11] S. Kozuch, S. Shaik, A. Jutand, C. Amatore, Chem. Eur. J. 2004, 10,
3072–3080.
ACHTREUNG
(100 mL). The complex [Pd
N
A
tive addition was identified by 31P NMR spectroscopy as a unique com-
plex (Supporting Information).[10a] A capillary of H3PO4 was introduced
into the NMR tube followed by methyl acrylate (76.8 mL, 0.42 mmol).
The reaction was monitored by 31P NMR spectroscopy.
[12] a) K was smaller than K’=0.30 (dmf, 258C), which characterizes the
equilibrium between [Pd
The iodide is thus less easily exchanged by AcOꢀ in [Pd
than in [Pd(PPh3)2IPh]; b) C. Amatore, E. CarrØ, A. Jutand, M. A.
A
(PPh3)2
ACHTREUNG
ACHTREUNG
Acknowledgements
AHCTREUNG
MꢁBarki, G. Meyer, Organometallics 1995, 14, 5605–5614.
[13] C. Amatore, E. CarrØ, A. Jutand, Acta Chem. Scand. B 1998, 52,
100–106.
[14] A. J. Bard, L. R. Faulkner, Electrochemical Methods, Wiley, New
York, 1980, p. 136.
This work was supported in part by the Centre National de la Recherche
Scientifique (UMR CNRS-ENS-UPMC 8640) and the Minist›re de la
Recherche (Ecole Normale SupØrieure). Johnson Matthey is thanked for
a generous loan of palladium salts. AFIRST (Association Franco-IsraØli-
enne pour la Recherche Scientifique et la Technologie) is thanked for
supporting F.L. with a postdoctoral grant (991MAE CCF5).
[15] Contrarily to [Pd
ACHTREUNG
cally oxidized, the complexes [Pd
AHCTREUNG
phine ligand) are indeed oxidized. See: C. Amatore, G. Broeker, A.
Jutand, F. Khalil, J. Am. Chem. Soc. 1997, 119, 5176–5185.
[1] For reviews on Heck reactions see: a) A. de Mejeire, F. E. Meyer,
Angew. Chem. 1994, 106, 2473–2506; Angew. Chem. Int. Ed. Engl.
1994, 33, 2379–2411; b) W. Cabri, I. Candiani, Acc. Chem. Res.
1995, 28, 2–7; c) M. Shibasaki, E. M. Vogl, J. Organomet. Chem.
1999, 576, 1–15; d) I. P. Beletskata, A. V. Cheprakov, Chem. Rev.
2000, 100, 3009–3066; e) M. Larhed, A. Hallberg, Handbook of Or-
ganopalladium Chemistry for Organic Synthesis, Vol. I (Ed.: E. Ne-
gishi), Wiley-Interscience, New York, 2002, pp. 1133–1178.
[16] A direct reciprocal dependence on the acetate ion concentration
could not be established due to the compensating effect of ionic
strength. At identical ionic strength, the decelerating effect of ace-
tate ions is higher than the accelerating effect induced by the ionic
strength. However, the reaction performed in the presence of 20
equivalents of nBu4NOAc (Figure 3c’’) was faster than that per-
formed in the presence of 10 equivalents of nBu4NOAc (Fig-
ure 3b’’). This finding shows that the antagonistic effect between the
acceleration due to the ionic strength and the specific deceleration
due to the acetate ions is in this case in favor of the ionic strength.
The saturating effect was not observed at high AcOꢀ concentrations
[2] For Heck reactions catalyzed by Pd(OAc)2 and dppp see: a) W.
A
Cabri, I. Candiani, S. DeBernardinis, F. Francalanci, S. Penco, R.
Santi, J. Org. Chem. 1991, 56, 5796–5800; b) W. Cabri, I. Candiani,
A. Bedeshi, R. Santi, Tetrahedron Lett. 1991, 32, 1753–1756; c) W.
Cabri, I. Candiani, A. Bedeshi, S. Penco, R. Santi, J. Org. Chem.
1992, 57, 1481–1486; d) W. Cabri, I. Candiani, A. Bedeshi, R. Santi,
J. Org. Chem. 1992, 57, 3558–3563; e) W. A. Herrmann, C. Brossm-
er, K. Öfele, M. Beller, H. Fisher, J. Mol. Catal. A 1995, 103, 133–
146; f) M. Larhed, A. Hallberg, J. Org. Chem. 1996, 61, 9582–9584;
g) M. Larhed, A. Hallberg, J. Org. Chem. 1997, 62, 7858–7862;
h) K. S. A. Vallin, M. Larhed, K. Johansson, A. Hallberg, J. Org.
Chem. 2000, 65, 4537–4542; i) M. Qadir, T. Mçchel, K. K. Hii, Tetra-
because the condition kꢀ1kꢀ2
[AcOꢀ]!k2k3[CH2=CHCO2Me] was
G
not fulfilled over the range of available methyl acrylate concentra-
tions.
[17] The minimum concentration of styrene beyond which the reaction
would be limited by the dissociation of [Pd
estimated from the transition between the limits in Equations (4)
and (6): [styrene]/
[AcOꢀ]=k1/K1K2k3. From the values of k1
A
G
AHCTREUNG
(Table 1) and K1K2k3 (see below), and the fact that [AcOꢀ] cannot
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2007, 13, 2002 – 2011
2010
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Reaction of Alkenes with Aryl Palladium Complexes
FULL PAPER
exceed C0, this equality imposes the condition that at least 2500
equivalents of styrene (ꢂ30m) would be required to observe the
saturation effect.
order of C0 =12.2 mm in the experimental conditions of Figure 4a.
Thus, by using the values of k4k5/
above, it follows that the mean value of kobs at low methyl acrylate
concentrations is kobs
(minꢀ1)=5”10ꢀ3 ”[CH2=CHR], namely, exact-
A
1
[18] The half reaction time varied from t = =1030 to 417 min when the re-
ACHTREUNG
2
action of [Pd
G
(OAc)Ph] (1.15 mm) with styrene (1.15m) was
ly in the range observed for line A in Figure 4a.
performed in the presence of nBu4NBF4 (0.26m).
[20] a) L. J. Xu, W. P. Chen, J. Ross, J. L. Xiao, Org. Lett. 2001, 3, 295–
297; b) J. Mo, L. Xu, J. Xiao, J. Am. Chem. Soc. 2005, 127, 751–760.
[19] Conversely, when the methyl acrylate concentration is lower under
the conditions of Figure 4a (line A), the rate law is given by Equa-
tion (9). It predicts a nonlinear dependence of kobs versus [CH2=
CHR], because the term [Xꢀ] varies. However, this term is of the
Received: February 3, 2006
Published online: November 28, 2006
Chem. Eur. J. 2007, 13, 2002 – 2011
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2011