Simple kinetic method for distinguishing between homogeneous and heterogeneous mechanisms of catalysis, illustrated by the example of "ligand-free" suzuki and heck reactions of aryl iodides and aryl bromides

Article abstract of DOI:10.1134/S0023158412010107

Using a simple method based on an analysis of the phase trajectories of competing reactions of several substrates, it has been established that the selectivity of catalytically active species in the Suzuki reaction of aryl bromides depends on the nature of the catalyst precursor. This indicates that there is a considerable contribution from heterogeneous catalysis. At the same time, in the reaction involving aryl iodides, when the catalyst concentration in the solution is much higher, the selectivity of the catalyst is precursor-independent, suggesting that homogeneous catalysis is dominant. In the Heck reaction of both aryl bromides and aryl iodides, pure homogeneous catalysis takes place. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S0023158412010107

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2012, Vol. 53, No. 1, pp. 84–90. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © A.F. Schmidt, A.A. Kurokhtina, E.V. Larina, 2012, published in Kinetika i Kataliz, 2012, Vol. 53, No. 1, pp. 86–93.
Simple Kinetic Method for Distinguishing between Homogeneous
and Heterogeneous Mechanisms of Catalysis, Illustrated
by the Example of “LigandꢀFree” Suzuki and Heck Reactions
of Aryl Iodides and Aryl Bromides
A. F. Schmidt, A. A. Kurokhtina, and E. V. Larina
Faculty of Chemistry, Irkutsk State University, Irkutsk, 664033 Russia
eꢀmail: aschmidt@chem.isu.ru
Received May 11, 2011
Abstract—Using a simple method based on an analysis of the phase trajectories of competing reactions of
several substrates, it has been established that the selectivity of catalytically active species in the Suzuki reacꢀ
tion of aryl bromides depends on the nature of the catalyst precursor. This indicates that there is a considerꢀ
able contribution from heterogeneous catalysis. At the same time, in the reaction involving aryl iodides, when
the catalyst concentration in the solution is much higher, the selectivity of the catalyst is precursorꢀindepenꢀ
dent, suggesting that homogeneous catalysis is dominant. In the Heck reaction of both aryl bromides and aryl
iodides, pure homogeneous catalysis takes place.
DOI: 10.1134/S0023158412010107
Among the numerous coupling reactions of aryl tions ((I) and (II), respectively) are of greatest
halides with various compounds (alkenes, alkynes, importance and are the focus of researchers’ attenꢀ
amines, organometallics), the Heck and Suzuki reacꢀ tion [1, 2]:
Ph
Ph
[Pd]
NaOAc
R
Hal
+
,
.
(I)
R
R = H, COMe; Hal = Br, I
[Pd]
NaOAc
R
Hal + PhB(OH)2
R
Ph
(II)
R = H, COMe, Me; Hal = Br, I
The nature of catalysis in the Heck reaction has particles in solution and/or on the supports surface,
been keenly discussed in the literature since the late and larger particles of palladium metal. The heterogeꢀ
1
990s. The authors of some reviews [3–6] have arrived neous palladium species are only sources of catalytic,
at the conclusion that homogeneous catalysis in this dissolved Pd(0) and Pd(II) complexes, the species that
reaction is more likely, irrespective of the nature of the are directly involved in the catalytic cycle. It is believed
catalyst precursor generating active species. All that all active and inactive species generated by the
authors believe that the same catalytic species are catalyst precursor undergo interconversion during the
involved in the reaction, namely, dissolved molecular catalytic reaction and the most important role in these
complexes of Pd(0) and Pd(II), colloidal palladium conversions is played by the aryl halide (Scheme 1).
84

SIMPLE KINETIC METHOD FOR DISTINGUISHING BETWEEN HOMOGENEOUS
Palladium conversions outside the catalytic cycle of the Heck and Suzuki reactions
85
а
b
c
Pd(II)dissolv
Pd(0)dissolv
Pdcol
Pdhet
f
ArꢀHal
e
ArꢀHal
d
ArꢀHal
Scheme 1.
There is still no consensus to the nature of catalysis active species of the catalyst being tested (e.g., to withꢀ
in the Suzuki reaction, unlike the Heck reaction. drawal of the solid phase or introduction of a selective
Some researchers hold to the hypothesis that the poison). The most complicated case in which both
Suzuki reaction occurs via a homogeneous catalytic homogeneous and heterogeneous catalyses take place
mechanism [7–16], while others believe that this reacꢀ in the reaction also cannot be ruled out. This possibilꢀ
tion is heterogeneous catalytic and involves catalysis on ity for the Suzuki reaction was clearly formulated only
the surface of palladium nanoparticles as well [17–24]. in two reports [31, 32]. The relative contributions from
There is convincing evidence in favor of the homogeꢀ homogeneous and heterogeneous catalyses to the
neous mechanism. For example, there are numerous overall substrate conversion may vary during the reacꢀ
proofs of palladium dissolution from insoluble catalyst tion, so test data interpretation may depend on many
precursors, including palladium nanoparticles [8, 9, factors that are not usually controlled by researchers.
1
1, 14, 18, 25–27]. In addition, there is a correlation In addition, when deducing, from experimental data,
between catalytic activity and the amount of dissolved that catalysis is homogeneous in nature, researchers
palladium [14, 25, 27]. The homogeneous character of sometimes disregard the existence of “dissolved” palꢀ
catalysis in the Suzuki reaction is confirmed by the ladium nanoparticles, on which the catalytic reaction
specific dependence of the turnover frequency (TOF) can occur as well. The formation of nanoparticles in
of the catalyst on its amount [7], by the effects of catꢀ situ was observed not only when soluble precursors
alyst poisons [9], an by the changes in the particle size were used, but also in reactions involving common
of the nanosized catalyst precursor during the reaction heterogeneous catalysts [29, 33, 34]. Thus, although
[
15]. At the same time there is cogent evidence in favor there is extensive evidence in favor of the homogeꢀ
of heterogeneous catalysis in the Suzuki reaction neous catalytic mechanism of the Suzuki reaction, the
24, 28]. This evidence was obtained using Operando possibility of this reaction proceeding simultaneously
[
spectroscopy.
via the homogeneous and heterogeneous catalytic
pathways cannot be unequivocally ruled out.
The contradictoriness of the results obtained by
different authors and of the conclusions following
from these results, as well as the continuing discussion
of the mechanism of the Heck reaction [3–6], demonꢀ
strates how difficult it is to ascertain whether homogeꢀ
neous or heterogeneous catalysis takes place in couꢀ
pling reactions, including the Suzuki reaction. In our
opinion (see also [24, 28, 29]), this situation is mainly
due to the continuous interconversion of species genꢀ
erated by the catalyst precursor. This circumstance is
often disregarded in planning of experiments and in
data interpretation. The existence of this interconverꢀ
sion makes the results of all conventional homogeneꢀ
ity/heterogeneity tests ambiguous (for a review of
these tests, see [30]). This is mainly due to a “counterꢀ
Our previous modified tests for homogeneity/hetꢀ
erogeneity led us to conclude that heterogeneous
catalysis makes a large contribution to the Suzuki
reaction of aryl bromides [29]. In this study, in order to
verify this conclusion, we took another kinetic
approach: we measured the ratio of the conversion
rates of competing substrates in the Suzuki reaction
and performed a comparative investigation of catalyst
selectivities in the Suzuki reaction and in the wellꢀ
understood Heck reaction.
EXPERIMENTAL
All experiments were carried out without using an
balancing effect” occurring in the catalytic system inert atmosphere. The reaction mixture was sampled
like Le Chatelier’s principle in an equilibrium sysꢀ at intervals and was analyzed by GLC (HPꢀ4890 chroꢀ
tem): the system responds to removal of one of the matograph, flameꢀionization detector, 15ꢀmꢀlong
(
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86
SCHMIDT et al.
HPꢀ5 column, (5% phenyl)ꢀmethylpolysiloxane staꢀ five times with distilled water and acetone and was
tionary phase, oven temperature of 100–250 ). The then vacuumꢀdried at 110 for 3 h. The resulting catꢀ
°
С
°
С
bromobenzene conversion in the Suzuki reaction at alyst was reduced with flowing hydrogen at 250
°
С.
different points in time was in good agreement with
sum of the biphenyl and benzene yields, indicating
that the contribution from side reactions such as the
Catalytic Tests
reductive homocoupling reaction of bromobenzene
A glass reactor fitted with a rubber septum and a
and the oxidative homocoupling of phenylboric acid is magnetic stirrer was charged with 4ꢀbromoacetopheꢀ
insignificant. In all runs, the product (ArH) yield in none and bromobenzene or, alternatively, 4ꢀiodotoluꢀ
the side reaction of aryl halide reduction was 3–10%. ene and iodobenzene (2.5 mmol of either reactant), a
PdCl2 or 4% Pd/support catalyst (0.08 mmol Pd),
5
mmol of PhB(OH)2 (Suzuki reaction) or 5 mmol of
Catalyst Preparation
styrene (Heck reaction), 1 mmol of naphthalene as the
internal standard for GLC, 6.5 mmol of NaOAc as the
The Pd/C catalyst was prepared using Pd(OAc)2
,
whose amount was set so that the catalyst contained base, and 5 ml of
wt % palladium. The carbon support Sibunit (1 g, tions were conducted at 140
particle size of 0.2–0.25 mm) was placed in toluene run was repeated three times to make sure that it is
20 ml). Thereafter, Pd(OAc)2 was added and the mixꢀ reproducible.
ture was stirred for 30 min at 95 . Next, formic acid
0.04 ml) was added and the mixture was stirred for
N,
N
ꢀdimethylformamide. The reacꢀ
4
°
С
under stirring. Each
(
°
C
(
RESULTS AND DISCUSSION
another 20 min, until the solution became colorless.
The resulting catalyst was collected on a filter, washed
When unequal differential selectivities are observed
with acetone, and vacuumꢀdried. The Pd/Al O3 and with different catalyst precursors, this is unambiguous
2
Pd/SiO2 catalysts were prepared in a similar way, and evidence that these precursors generate different
Pd/MgO was obtained using Pd(NO3)2. In this case, active species acting as true catalysts. This circumꢀ
an aqueous solution of NaOH (0.14 mol/l, 4 ml) was stance can be used to verify assumptions as to identical
added to a suspension of MgO (3 g) in water (20 ml). or different natures of catalytic species in reactions
The mixture was stirred for 10 min, and an aqueous involving different catalyst precursors. For measuring
solution of Pd(NO3)2 (0.028 mol/l, 40 ml) was then differential selectivity in the Suzuki and Heck reactions,
added at room temperature. After 20 min, the solution it is necessary to choose at least one pair of competing
was centrifuged and decanted. The solid was washed aryl halides with different substituents (Scheme 2).
Competing reactions used to estimate the selectivity of catalytic systems in the Suzuki and Heck reactions
(the neutral nucleophile NuX in the Suzuki reaction is PhB(OH)2, and that in the Heck reaction is styrene)
Hal
Nu
NuX
Pd], NaOAc
[
R
Hal
R
Nu
Scheme 2.
Note that a comparative study of catalytic activity respect to the substrates [36, 37]. This equation does
rather than selectivity under “noncompetitive” condiꢀ not take into account the conversions of the catalyst
tions (when a series of experiments is performed for outside the catalytic cycle. Taking into consideration
each aryl halide alone) may lead the researcher to the kinetics of these conversions complicates the reacꢀ
wrong conclusions, because different catalyst precurꢀ tion rate equations and makes it impossible to derive
sors may generate different quantities of active species. integral equations for selectivity. At the same time, in
Unlike catalytic activity, the ratio of the rates of comꢀ order to obtain a qualitative estimate of the ratio of the
peting reactions is independent of the quantity of rates of competing reactions and to see how this ratio
active species [35] and is determined only by their varies during the process, one can use integral kinetic
nature. (If there only two such reactions, the ratio of data on product accumulation. Variation of the rate
their rates is an unambiguous measure of differential ratio during the reaction can readily be detected by
selectivity.)
correlating the yield of the product of one competing
The relative reactivity of two substrates is convenꢀ reaction to the yield of the product of the other one at
tionally estimated using the simplest integral equation different points in time. The curves obtained in this
for competing reactions that are firstꢀorder with way without any processing of primary experimental
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SIMPLE KINETIC METHOD FOR DISTINGUISHING BETWEEN HOMOGENEOUS
87
data are soꢀcalled phase trajectories of the reaction,
since the coordinate system in which they are plotted
involves no time axis. (If there are only two products,
we have a phase surface.) The slope of the tangent to
the phase trajectory at any of its points is the ratio of
the product formation rates. The shape of the phase
trajectory determined by the accumulation kinetics of
the target products is a very specific and sensitive
parameter depending on the nature of the catalytically
active compounds.
2
3
4
80
1
5
6
0
0
4
Note that, even if the phase trajectories in a series
of runs involving different catalyst precursors coincide
totally, this is not ultimate evidence that the active speꢀ
cies forming in situ in these runs are identical. The
equality of the ratios of the rates of competing reacꢀ
tions may be due to the similarity of the rate constants
of the interactions of the substrate with different active
species. Another plausible explanation is that changꢀ
ing the catalyst precursor causes only insignificant
changes in the proportions of catalytic species. Thereꢀ
fore, additional experimental data are required here
for making better substantiated inferences. The nonꢀ
coincidence of phase trajectories alone provides good
reason to conclude that the species responsible for
catalysis differ in their nature.
2
0
0
2
4
6
8
10
Ph–Ph yield, %
Fig. 1. Phase trajectories (4ꢀphenylacetophenone yield
versus biphenyl yield) for the Suzuki reaction of the comꢀ
peting substrates 4ꢀbromoacetophenone and bromobenꢀ
zene with phenylboric acid in the presence of different catꢀ
alyst precursors: (
1
5
) Pd/C, (
) PdCl2.
2
) Pd/SiO , (
3
) Pd/MgO,
2
(
4
) Pd/Al O , and (
2
3
The competitive Suzuki reaction involving 4ꢀbroꢀ
moacetophenone and bromobenzene as the competꢀ
ing substrates and phenylboric acid as the common
reactant was carried out in the presence of different
insoluble catalyst precursors (Pd/C, Pd/MgO,
Pd/SiO , Pd/Al O3) and a soluble one (PdCl2). The
plot of the 4ꢀphenylacetophenone concentration verꢀ
sus the biphenyl concentration at different points in
time in a competition run is a phase trajectory (Fig. 1).
The noncoincidence of phase trajectories obtained
with different catalysts is unambiguous evidence that
the nature of the active species forming in the system
depends on the catalyst precursor. It is a commonly
accepted view that, in the “ligandꢀfree” Suzuki and
Heck reactions, molecular palladium complexes in
the solution are represented by Pd(0) complexes
1
00
80
2
2
6
4
2
0
0
0
1
2
3
0
2
4
6
8
PhCH=CHPh yield, %
(
“naked Pd”) [38] and Pd(II)
σ
ꢀaryl complexes,
which are likely stabilized by endogenic halide ions
resulting from aryl halide conversion [4, 5, 39]. Thereꢀ
fore, the phase trajectories in the case of the reaction
taking place only on dissolved palladium complexes
should remain invariable, because the nature of the
Pd(0) and Pd(II) complexes involved in the “homogeꢀ
neous” catalytic cycle remain the same, irrespective of
whether the catalyst precursor is soluble or insoluble in
Fig. 2. Phase trajectories (4ꢀacetylstilbene yield versus stilꢀ
bene yield) for the Heck reaction of the competing subꢀ
strates 4ꢀbromoacetophenone and bromobenzene with
styrene in the presence of different catalyst precursors:
(1) Pd/SiO , (2) Pd/Al O , and (3) PdCl2.
2 2 3
hypothesize that catalysis is purely heterogeneous.
the reaction medium. This view is proved by the absoꢀ Heterogeneous catalysis is also possible with dissolved
lute coincidence of the phase trajectories for the Heck PdCl2 as the precursor, since visually observable pallaꢀ
reaction in the presence of different catalysts (Fig. 2).
dium black precipitates instantaneously when this preꢀ
As was mentioned above, the hypothesis that catalysis
cursor is used in reactions involving aryl bromides.
in this reaction is purely homogeneous is universally
However, a more careful analysis of experimental data
accepted. The fact that phase trajectories for the
taking into account the interconversions of palladium
Suzuki reaction do not coincide with one another
species (Scheme 1) unambiguously indicates a signifiꢀ
(
Fig. 1) provides reason to revise the conclusion that
catalytic species in this reaction differ in nature and cant contribution from heterogeneous catalysis, it
KINETICS AND CATALYSIS Vol. 53
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88
SCHMIDT et al.
their size distribution [30], and, accordingly, their
60
50
40
1
2
3
4
capacity to dissolve under the action of an aryl halide.
However, it can be claimed with confidence that, as
was noted above, the nature of the homogeneous
active species accelerating the Heck and Suzuki reacꢀ
tions is precursorꢀindependent.
Thus, the dependence of the phase trajectories of
the Suzuki reaction on the nature of the catalyst preꢀ
cursor is evidence of a contribution made by heterogeꢀ
neous catalysis, which can at least affect the observed
ratio of the rates of the competing reactions. The operꢀ
ating heterogeneous catalyst in this case may be “disꢀ
solved” palladium particles of various sizes forming in
the solution and palladium on the support surface.
3
2
1
0
0
0
0
10
20
30
40
50
60
In order to demonstrate the sensitivity of our
approach to fine details of the mechanisms of compliꢀ
cated catalytic processes, we carried out similar experꢀ
iments for the Heck reaction. In these experiments,
phenylboric acid as a coupling partner was replaced
with styrene (Scheme 2). The catalyst precursors
included Pd/SiO2 and PdCl2, which differ most
greatly in their selectivity in the Suzuki reaction, was
used. The absolute coincidence of phase trajectories
Iodobenzene conversion, %
Fig. 3. Phase trajectories (4ꢀiodotolune conversion versus
iodobenzene conversion) for the Suzuki reaction involving
phenylboric acid and for the Heck reaction involving styꢀ
rene in the presence of different catalyst precursors:
(1
) Pd/C (Suzuki reaction), (
tion), ( ) PdCl (Suzuki reaction), and (
reaction).
2) Pd/Al O (Suzuki reacꢀ
2 3
3
4) PdCl (Heck
2
2
(
Fig. 2) indicates that the ratio of the rates of the comꢀ
peting reactions is the sme at all stages of the process.
Therefore, in the Heck reaction involving aryl broꢀ
mides, different catalyst precursors yield the same type
cannot be ruled out that the Suzuki reaction proceeds
partially via a homogeneous catalytic mechanism.
The point is that the noncoincidence of phase traꢀ of catalytic species. As was mentioned above, the
jectories can theoretically be due not only to different nature of dissolved palladium complexes should not
natures of the catalysts, but also to the difference depend on the precursor, so it can be concluded that
between the catalytic contributions from few actives only homogeneous catalysis takes place in this case, in
species differing in their selectivity toward the comꢀ full agreement with the universally accepted view.
peting substrates. It is for the Suzuki and Heck reacꢀ
The proportions of homogeneous and heterogeꢀ
tions that palladium distribution between the homogeꢀ neous catalytic species in the Heck reaction depend
neous species (molecular complexes) and the heteroꢀ significantly on the nature of aryl halide, because the
geneous species (palladium on the surface of a support latter is responsible for the oxidation and subsequent
and “dissolved” palladium nanoparticles) can depend dissolution of palladium metal (Scheme 1, converꢀ
on the nature of the catalyst precursor (Scheme 1). For sions
d, e). In particular, on replacing aryl bromides
example, the variation of the palladium dissolution with aryl iodides, which are more reactive compounds,
rate from one support to another (Scheme 1, converꢀ no palladium black precipitate is visually observable in
sions d, e) has an effect on the proportions of pallaꢀ the Suzuki and Heck reactions occurring in the presꢀ
dium as molecular complexes, palladium as “disꢀ ence of a soluble catalyst precursor. Thus, use of aryl
solved” nanoparticles, and supported palladium, iodides should increase the contribution from the
because, kinetically, palladium dissolution is conꢀ “homogeneous mechanism” to the overall substrate
nected with aggregation (Scheme 1, conversions
oxidation (Scheme 1, conversion ), and palladium case, the dependence of the ratio of the rates of comꢀ
reduction (Scheme 1, conversion ). Thus, if all of the peting reactions on the nature of the catalyst precursor
b, c), conversion in the Suzuki reaction. If this were the
f
a
palladium species or some of them are catalytically would weaken or disappear entirely. In order to verify
active, palladium redistribution among them can this assumption, we conducted the Suzuki reaction
affect the selectivity and, accordingly, the phase traꢀ involving 4ꢀiodotoluene and iodobenzene as the comꢀ
jectory of the reaction (Fig. 1). Passing from insoluble peting substrates. The phase trajectories obtained for
catalyst precursors to soluble ones (e.g., PdCl2) also this reaction in the presence of different catalyst preꢀ
changes palladium distribution owing to the mutual cursors coincide totally. Therefore, a single type of
effects of the processes presented in Scheme 1. In parꢀ catalytic species contributing most greatly to catalysis
ticular, use of soluble precursors sharply increases the forms in the Suzuki reaction of highly reactive aryl
proportion of palladium in the form of soluble molecꢀ iodides, irrespective of what is the catalyst precursor.
ular complexes (at least at the early stages of the reacꢀ As in the case of the Heck reaction of aryl bromides,
tion). In turn, this exerts an effect on the formation these species may be dissolved molecular complexes of
rate and subsequent aggregation of nanoparticles, palladium.
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SIMPLE KINETIC METHOD FOR DISTINGUISHING BETWEEN HOMOGENEOUS
89
Note that, in the commonly accepted mechanisms were actually found to coincide closely (Fig. 3), unlike
of the Suzuki and Heck reactions, the selectivity of the those of the same reactions involving competing aryl
catalyst toward the competing aryl halides is deterꢀ bromides.
mined by the same reaction stage, specifically, aryl
halide oxidative addition to the Pd(0) compound.
Therefore, if a catalytic system shows different selecꢀ
tivities toward the same pair of aryl halides in the
Suzuki and Heck reactions, this seems to be evidence
that the catalysts involved in these reactions are of difꢀ
ferent natures. Since the Heck reaction is known to
occur via a homogeneous catalytic mechanism, the
difference between the rate ratios observed in Heck
and Suzuki reactions proves the above assumption that
heterogeneous catalysis makes a considerable contriꢀ
bution to the Suzuki reaction of aryl bromides. The
phase trajectories of the Heck and Suzuki reactions
involving the same catalyst precursor and the same pair
of aryl bromides indeed do not coincide (Figs. 1, 2).
Significant deviations were also observed for pairs of
aryl halides of which at least one is a bromide (e.g.,
Thus, both of the conceptions discussed in the litꢀ
erature—purely homogeneous catalysis and purely
heterogeneous catalysis in the Suzuki reaction—are
not consistent with the observed dependence of the
phase trajectories of competing reactions on the
nature of the catalyst precursor. Most likely, the Suzuki
reaction involving aryl bromides proceeds simultaꢀ
neously via homogeneous catalytic and heterogeneous
catalytic mechanisms. There is direct evidence that
the contribution from heterogeneous catalysis is suffiꢀ
ciently large to affect at least the selectivity of the catꢀ
alyst toward the competing substrates. At the same
time, the reaction involving aryl iodides most likely
occurs via pure homogeneous catalytic mechanism.
Our results point to the incorrectness of the wideꢀ
spread view that catalysis in the Suzuki reaction of aryl
bromides is purely homogeneous. This should be
taken into account in development of new catalytic
systems for the Suzuki reaction. In particular, as disꢀ
tinct from the Heck reaction, which cannot be
effected over a heterogeneous catalyst, for Suzuki
reactions can be found a truly heterogeneous catalyst
systems.
4ꢀbromoacetophenone–iodobenzene and bromobenꢀ
zene and iodobenzene). Note, however, that the selecꢀ
tivity of the catalyst in the Heck and Suzuki reactions
may be determined not only by the oxidative addition
stage, but also by other stages of the catalytic cycle,
which are quite different in these reactions. This situꢀ
ation is possible if the oxidative addition stage is essenꢀ
tially reversible. It cannot also be ruled out that the
components specific to these two reactions (phenylboꢀ
ric acid in the Suzuki reactions and styrene in the
check reaction) exert a direct effect on the nature of
the forming catalytic species. For this reason, we carꢀ
ried out additional experiments in which we fixed a
pair of competing aryl bromides and varied the nature
and concentrations of the other components of the
Heck and Suzuki reactions (base, alkene, and arylboꢀ
ric acid). The results of these experiments led us to the
unambiguous conclusion that the phase trajectory is
independent of these parameters for both reactions.
Thus, the different selectivities of catalytic systems in
these reactions can indeed be due to the different
natures of the active species involved in oxidative addiꢀ
tion to the competing aryl bromides.
ACKNOWLEDGMENTS
This study was supported by the Ministry of Educaꢀ
tion and Science of the Russian Federation (state conꢀ
tract no. 14.740.11.1180).
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2012