Rates of the oxidative addition of benzyl halides to a metallacyclic palladium(II) complex and of the reductive elimination from a benzyl-palladium(IV) complex

Article abstract of DOI:10.1021/om800015x

Pd(II) metallacyclic complexes are key intermediates in sequential reactions in which three new C-C bonds are formed in an aromatic ring. The Pd(II) metallacyclic complex 10 with phenanthroline as ligand undergoes oxidative addition to benzyl bromide or chloride in DMF to generate benzyl-Pd(IV) complexes. The rate constant of the oxidative addition is determined by means of electrochemical techniques, with the expected reactivity order PhCH₂Br (k_1Br = 3.6 M^-1 s^-1) > PhCH₂Cl (k_1Cl = 6 × 10^-3 M ^-1 s^-1) at 29°C. When the benzyl-Pd(IV) complex is generated from PhCH₂Br, the oxidative addition is followed by a slow C-C reductive elimination, which gives a Pd(II) complex. The rate constant of the reductive elimination has been determined in DMF (k_2Br = 6 × 10^-4 s^-1) at 29°C.

Full text of DOI:10.1021/om800015x

Organometallics 2008, 27, 4549–4554
4549
Articles
Rates of the Oxidative Addition of Benzyl Halides to a Metallacyclic
Palladium(II) Complex and of the Reductive Elimination from a
Benzyl-Palladium(IV) Complex
Christian Amatore,*,† Marta Catellani,*,‡ Sara Deledda,† Anny Jutand,*,† and Elena Motti‡
De´partement de Chimie, Ecole Normale Supe´rieure, UMR ENS-CNRS-UPMC 8640, 24 Rue Lhomond,
F-75231 Paris Cedex 5, France, and Dipartimento di Chimica Organica e Industriale dell’UniVersita` degli
Studi di Parma, Viale G.P. Usberti, 17/A, I-43100, Parma, Italy
ReceiVed January 7, 2008
Pd(II) metallacyclic complexes are key intermediates in sequential reactions in which three new C-C
bonds are formed in an aromatic ring. The Pd(II) metallacyclic complex 10 with phenanthroline as ligand
undergoes oxidative addition to benzyl bromide or chloride in DMF to generate benzyl-Pd(IV) complexes.
The rate constant of the oxidative addition is determined by means of electrochemical techniques, with
the expected reactivity order PhCH2Br (k1Br ) 3.6 M-1 s-1) > PhCH2Cl (k1Cl ) 6 × 10-3 M-1 s-1) at
29 °C. When the benzyl-Pd(IV) complex is generated from PhCH2Br, the oxidative addition is followed
by a slow C-C reductive elimination, which gives a Pd(II) complex. The rate constant of the reductive
elimination has been determined in DMF (k2Br ) 6 × 10-4 s-1) at 29 °C.
Scheme 1
Introduction
Selective aromatic functionalization by means of transition
metal complexes is an important topic to which palladium
chemistry has contributed significantly. In particular, pallada-
cycles have been proved to be quite valuable to this goal in
providing a facile way for the activation of an aromatic ring.1
Some of us have intensively investigated this area and reported
a new methodology for selective aromatic alkylation and
arylation based on the catalytic activity of palladium and
norbornene.2
Scheme 2
Scheme 1 reports an example of a sequential one-pot reaction
in which three new C-C bonds are formed with selective
functionalization of two mutually meta positions of an aromatic
ring. As a result, an ortho,ortho′-disubstituted vinylarene is
formed.3
* To whom correspondence should be addressed. Fax: (+33) 1-44-32-
24-02. E-mail: Anny.Jutand@ens.fr; christian.amatore@ens.fr; marta.
catellani@unipr.it.
† Ecole Normale Supe´rieure.
‡ Universita` degli Studi di Parma.
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Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 3, pp 435-480. (b) Leong,
W. W.; Larock, R. C. In ComprehensiVe Organometallic Chemistry, II;
Abel, E. W., Stone, G., Wilkinson, G., Eds.; Pergamon: Oxford, 1995; Vol.
12, pp 131-160. (c) Dyker, G. Angew. Chem., Int. Ed. 1999, 38, 1698–
1712. (d) Luh, T.-Y.; Leung, M.-K.; Wong, K.-T. Chem. ReV. 2000, 100,
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Organomet. Chem. 2005, 14, 21–53. (c) Catellani, M.; Motti, E.; Faccini,
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M.; Motti, E. In Handbook of C-H Transformations; Dyker, G., Ed.; Wiley-
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Scheme 2 reports the proposed reaction pathway that shows
the important role played by palladium(II) and palladium(IV)
metallacycles 3, 4, 6, and 7.3 The cis,exo arylnorbornylpalla-
dium(II) species 2 is formed by oxidative addition of iodoben-
zene to palladium(0) and subsequent stereoselective norbornene
insertion into the resulting arylpalladium complex 1.4 Ring
10.1021/om800015x CCC: $40.75
2008 American Chemical Society
Publication on Web 08/20/2008

4550 Organometallics, Vol. 27, No. 18, 2008
Amatore et al.
closure through aromatic C-H activation5 leads to palladacycle
3.6 Oxidative addition of alkyl iodide to 3 gives the metallacycle
complex of palladium(IV), 4, which undergoes a reductive
elimination by selective migration of the R group to the aromatic
site of the palladacycle.3,7 The resulting arylnorbornylpalladium
species 5 behaves analogously to the precursor 2, undergoing
(a) ring closure to 6; (b) oxidative addition of a new molecule
of alkyl iodide to form the palladium(IV) species 7; and (c)
reductive elimination to give 8. Likely due to the steric effect
of norbornene, expulsion occurs at this stage, leading to the
o,o′-disubstituted arylpalladium complex 9, which undergoes
Heck reaction to give the final organic product. Among the many
interesting aspects of this catalytic cycle, the role played by
alkylaromatic metallacycles of palladium(II) and (IV) is worth
noting. The formation of palladacycles 3 and 6 allows C-H
bond activation, while through metallacycles of palladium(IV),
4 and 7, selective alkylation becomes possible. The reaction
scope has been largely explored using a variety of aromatic
compounds, organic halides, and terminating agents. Among
the latter arylboronic acids, hydrogen transfer agents and
acetylenic compounds have been used besides olefins.2,8
Lautens et al. also reported interesting extensions of this
sequential reaction. Using an alkyl halide containing an olefinic
group, they could obtain cyclic compounds through an intramo-
lecular Heck reaction and several other accomplishments
concerning sequential reactions involving C-C bond formation
and C-H activation.9
Figure 1. Cyclic voltammetry of complex 10 (3 mM) in DMF
(containing nBu4NBF4, 0.3 M) at a steady gold disk electrode (d
) 0.5 mm) and a scan rate of 0.5 V s-1 at 25 °C. (a) Reduction
first. (b) Oxidation first.
1a). The first reversible one, R1, at Ep ) -1.505 V vs SCE,
R1
In view of the potential of these metallacycle-driven reactions,
we have deemed useful to study the reactivity of the model
palladacycle 10, containing phenanthroline as ligand, in reactions
with benzyl halides, which readily give oxidative addition.7 The
kinetics of the C-C reductive elimination that follows the
oxidative addition has been investigated as well.
characterizes the monoelectronic reduction of the phenanthroline
ligated to the Pd(II) center. The second one, R2, at Ep
)
R2
-2.255 V vs SCE, was irreversible. It reduction peak current
was about twice that of R1, indicating an overall two-electron
reduction. It is followed by a third tiny reversible reduction peak
at Ep ) -2.430 V vs SCE.10
R3
More interestingly, the Pd(II) complex 10 exhibited an
irreversible oxidation peak O1 at Ep ) +0.795 V vs SCE
O1
preceded by a prewave (Figure 1b).11
When an excess of PhCH2Br was added to the Pd(II) complex
10, the oxidation peak O1 disappeared with time, suggesting
that a reaction proceeded between the two reagents, presumably
Results and Discussion
(9) (a) Lautens, M.; Piguel, S. Angew. Chem., Int. Ed. 2000, 39, 1045–
1046. (b) Alberico, D.; Scott, M. E.; Lautens, M. Chem. ReV. 2007, 107,
174–238. (c) Mariampillai, B.; Alliot, J.; Li, M.; Lautens, M. J. Am. Chem.
Soc. 2007, 129, 15372–15379.
Kinetics of the Oxidative Addition of Benzyl Halides to
the Pd(II) Complex 10. The Pd(II) complex 10 (3 mM) was
characterized by cyclic voltammetry in DMF containing
nBu4NBF4 (0.3 M). It exhibited three reduction peaks (Figure
(10) Our purpose was not to investigate the mechanism of the reduction
of complex 10 and to determine the structure of the complexes formed
after the successive electron transfers. However, one may assume that the
first reversible reduction process at R1 involves the reduction of the
phenanthroline ligand and the resulting complex 10•-. After an overall
dielectronic process at R2, a cleavage of the two Pd-C bonds would occur
with formation of the dianionic phenylnorbornyl organic ligand together with
the unstable 15-electron Pd0(phenanthroline)•-. The latter was reduced in the
third reduction step at R3 into the 16-electron complex Pd0(phenanthroline)2-
,the two electrons being located on the phenanthroline ligand. Under similar
experimental conditions, the free phenanthroline exhibited two successive
reversible reduction peaks at-1.975 and-2.15 V vs SCE.
(3) (a) Catellani, M.; Frignani, F.; Rangoni, A. Angew. Chem., Int. Ed.
Engl. 1997, 36, 119–122, and references therein. (b) Catellani, M.; Fagnola,
M. C. Angew. Chem., Int. Ed. Engl. 1994, 33, 2421–2422.
(4) (a) Horino, H.; Arai, M.; Inoue, N. Tetrahedron Lett. 1974, 647–
650. (b) Portnoy, M.; Ben-David, Y.; Rousso, I.; Milstein, D. Organome-
tallics 1994, 13, 3465–3479. (c) Li, C.-S.; Cheng, C.-H.; Liao, F.-L.; Wang,
S.-L. Chem. Commun. 1991, 710–712. (d) Catellani, M.; Mealli, C.; Motti,
E.; Paoli, P.; Perez-Carreno, E.; Pregosin, P. S. J. Am. Chem. Soc. 2002,
124, 4336–4346.
(5) (a) Dyker, G. Handbook of CH Transformation; Wiley-VCH:
Weinheim, 2005. (b) Dyker, G. Angew. Chem., Int. Ed. 1999, 38, 1698–
1712.
(6) (a) Catellani, M.; Chiusoli, G. P. J. Organomet. Chem. 1988, 346,
C27-C30. (b) Catellani, M.; Marmiroli, B.; Fagnola, M. C.; Acquotti, D.
J. Organomet. Chem. 1996, 507, 157–162. (c) Dyker, G. Chem. Ber. 1997,
130, 1567–1578. (d) Ca´mpora, J.; Palma, P.; Carmona, E. Coord. Chem.
ReV. 1999, 193-195, 207–281.
(7) (a) Catellani, M.; Mann, B. E. J. Organomet. Chem. 1990, 390, 251–
255. (b) Bocelli, G.; Catellani, M.; Ghelli, S. J. Organomet. Chem. 1993,
458, C12-C15.
(8) (a) Catellani, M.; Cugini, F. Tetrahedron 1999, 55, 6595–6602. (b)
Catellani, M.; Motti, E.; Minari, M. Chem. Commun. 2000, 157–158. (c)
Catellani, M. Pure Appl. Chem. 2002, 74, 63–68.

OxidatiVe Addition of Benzyl Halides to a Pd(II) Complex
Organometallics, Vol. 27, No. 18, 2008 4551
Scheme 3
an oxidative addition that gave the Pd(IV) complex 11Br
(Scheme 3). Complexes of this type have been isolated and
characterized by NMR spectroscopy. Their structure in solution
is fully consistent with the ligand arrangement shown in formula
11X.7
The oxidative addition of PhCH2Br to the Pd(II) complex
PhCH2PdBr(PPh3)2 generates a Pd(IV) complex, (PhCH2)2Pd-
(Br)2(PPh3)2, as proposed by Stille et al.12 Oxidative additions
of alkyl halides to Pd(II) complexes ligated to diamine or
bipyridine ligands that generate Pd(IV) complexes are also
reported.13
Kinetics of the Oxidative Addition of PhCH2Br to the
Pd(II) Complex 10. The kinetics of the oxidative addition of
PhCH2Br to complex 10 (C0 ) 3 mM) (rate constant k1Br in
Scheme 3) was monitored by amperometry performed at a
rotating gold disk electrode (d ) 2 mm) polarized at +1.0 V,
on the oxidation wave O1 of 10. The decay of the oxidation
current of 10 (proportional to its concentration)14 was recorded
versus time in the presence of excess PhCH2Br until total
conversion. The plot of ln x against time was linear (x ) [10]/
[10]0 ) i/i0 with i ) oxidation current at wave O1 at t, i0 )
initial oxidation current), attesting to a first-order reaction for
Figure 2. Kinetics of the oxidative addition of PhCH2Br to the
Pd(II) complex 10 (3 mM) in DMF (containing nBu4NBF4, 0.3 M)
at 29 °C, as monitored by amperometry at a rotating gold disk
electrode (d ) 2 mm) polarized at +1 V, on the oxidation wave of
10. (a) Variations of ln x versus time (x ) [10]/[10]0 ) i/i0 with i
) oxidation current at wave O1 at t, i0 ) initial oxidation current).
[PhCH2Br] ) 90 mM. ln x ) sk1Brobst. (b) Determination of the
complex 10 (Figure 2a).
The value of the observed rate constant k1Br
obs
obs
of the
reaction order for PhCH2Br: plot of k1Br
versus PhCH2Br
obs
oxidative addition was calculated from the slope of the
concentration. k1Br ) k1Br[PhCH2Br].
straight line: ln x ) -k1Brobst. It varied linearly with PhCH2Br
obs
concentration with a zero intercept (Figure 2b): k1Br
)
k1Br[PhCH2Br], attesting to a first-order reaction for PhCH2Br.
The rate constant k1Br was determined from the slope of the
straight line of Figure 2b.
k1Br ) 3.6 M-1 s-1 (DMF, 29 °C)
(1)
Kinetics of the Oxidative Addition of PhCH2Cl to the
Pd(II) Complex 10. The kinetics of the oxidative addition of
PhCH2Cl to the Pd(II) complex 10 was investigated under the
same experimental conditions as for PhCH2Br (rate constant
k1Cl in Scheme 3). The oxidation current of 10 dropped to zero
upon addition of excess PhCH2Cl (in the range 0.07-0.3 M).
The oxidative addition of PhCH2Cl was considerably slower
than that of PhCH2Br at identical concentrations. When the
PhCH2Cl concentration was varied, the plot of ln x against time
afforded a series of straight lines whose slopes (k1Clobs) varied
linearly with PhCH2Cl concentration, attesting to a first-order
reaction for PhCH2Cl (Figure 3).
Figure 3. Kinetics of the oxidative addition of PhCH2Cl to complex
10 (3 mM) in DMF (containing nBu4NBF4, 0.3 M) at 29 °C.
obs
Determination of the reaction order for PhCH2Cl: plot of k1Cl
obs
versus PhCH2Cl concentration. k1Cl ) k1Cl[PhCH2Cl].
The rate constant of the oxidative addition was determined
from the slope of the straight line of Figure 3.
(11) The mechanism of the oxidation of complex 10 has not been
investigated in full detail since this was not relevant to this study.
Nevertheless, we can assume that the oxidation peak O1 of 10 involved
one electron by comparison of its oxidation peak current (Figure 1b) with
its reduction peak current at R1 (reversible process, Figure 1a). O1 was
preceded by a prewave. This is characteristic of a CE mechanism in which
a species that was more easily oxidized than 10 but present at very low
thermodynamic concentration in an equilibrium with the main species was
however detected by its oxidation peak because the equilibrium was
continuously shifted toward its formation by its consumption in the diffusion
layer by the electron transfer during the cyclic voltammetry performed at
a low scan rate (long time scale).
k1Cl)6 × 10-3M-1 s-1 (DMF, 29 °C)
(2)
PhCH2Br is then 600 times more reactive than PhCH2Cl in the
oxidative addition to the Pd(II) complex 10. The reactivity order
PhCH2Br > PhCH2Cl is usual in oxidative additions to Pd(0)
complexes.12 It was also expected for oxidative additions to
Pd(II) complexes. This oxidative addition gives Pd(IV) com-
plexes 11X (X ) Br, Cl), which, according to the mechanism
proposed in Scheme 2, should undergo a C-C reductive
elimination, giving back a Pd(II) complex.
(12) (a) Milstein, D.; Stille, J. K. J. Am. Chem. Soc. 1979, 101, 4992–
4998. (b) Gillie, A.; Stille, J. K. J. Am. Chem. Soc. 1980, 102, 4933–4941.

4552 Organometallics, Vol. 27, No. 18, 2008
Amatore et al.
Scheme 4
characterized by 1H NMR as the final complex formed in a
stoichiometric reaction of PhCH2Br and the Pd(II) complex 10.
The same sequence, i.e., oxidative addition followed by C-C
reductive elimination, had been observed in the reaction of 10
with p-nitrobenzyl bromide, leading to p-nitro-substituted 11Br,
then to p-nitro-substituted 12Br.7b The C-C reductive elimina-
tion from Pd(IV) complexes ligated to diamine or bipyridine
ligands has been reported by Canty et al.,13a van Koten et al.,13d
and Elsevier et al.13f
According to Scheme 4, the increase of the oxidation current
at long times observed in Figure 4a, assigned to the Pd(II)
complex 12Br, characterized the kinetics of the reductive
elimination (rate constant k2Br). d[12Br]/dt ) -d[11Br]/dt )
k2Br[11Br] with dx′/x′ ) -k2Brt.
The kinetic law for the reductive elimination is ln x′ ) -k2Brt
(x′ ) (ilim - i)/ilim; ilim ) oxidation current of the Pd(II) complex
12Br at the end of the reductive elimination; i ) oxidation current
of 12Br at t). The plot of ln x′ against time was indeed linear
(Figure 5). The value of k2Br was calculated from the slope of
the straight line.
A series of experiments was performed with three different
concentrations of PhCH2Br and the rate constants k2Br deter-
mined as described above. The three values of k2Br did not
depend on the PhCH2Br concentration within the accuracy of
their determination, attesting to a zero-order reaction for
PhCH2Br, as expected for a reductive elimination (Scheme 4).
An average value for the rate constant of the reductive
elimination was then determined.
Figure 4. (a) Decrease of the oxidation current of the Pd(II)
complex 10 (3 mM) in DMF (containing nBu4NBF4 0.3 M) versus
time, in its oxidative addition to PhCH2Br (0.57 M) (0 < t < 12s),
as monitored by amperometry at a rotating gold disk electrode (d
) 2 mm) polarized at +1 V, at 29 °C. It is followed by the increase
of the oxidation current of the Pd(II) complex 12Br generated by
reductive elimination from the Pd(IV) complex 11Br formed in the
previous oxidative addition. (b) Cyclic voltammetry of the Pd(II)
complex 12Br generated as in Figure 4a at a steady gold disk
electrode (d ) 2 mm) at the scan rate of 0.5 V s-1
.
Kinetics of the C-C Reductive Elimination from the
Benzyl-Pd(IV) Complex 11Br. When the oxidative addition of
the Pd(II) complex 10 (3 mM) was performed in the presence
of a large excess of PhCH2Br (0.57 M), the oxidation current of
complex 10 rapidly dropped to zero, attesting to a fast oxidative
addition (0 < t < 12 s, decreasing part of Figure 4a) with formation
of the Pd(IV) complex 11Br. An oxidation current was then
observed that slowly increased with times (t > 15 s in Figure 4a).
Consequently, once the fast oxidative addition was over, a new
complex was formed in a slower reaction from complex 11Br and
exhibited an oxidation current at +1 V vs SCE, the polarization
potential of the rotating disk electrode. A cyclic voltammetry was
performed once the formation of the new complex was over (t >
k2Br ) 6((1) × 10-4 s-1 (DMF, 29 °C)
(3)
The slow reductive elimination did not interfere in the kinetics
of the oxidative addition for the PhCH2Br concentration range
investigated here (0.03-0.12 M): k1Br[PhCH2Br] . k2Br
.
Moreover, the Pd(II) complex 12Br did not undergo any
oxidative addition to PhCH2Br (although present in excess
3000 s). An oxidation peak, O2, was indeed observed at Ep
)
O2
+0.830 V vs SCE (Figure 4b), i.e., with a peak potential slightly
more positive than that of the Pd(II) complex 10. This oxidation
peak is assigned to the Pd(II) complex 12Br7b generated by a C-C
reductive elimination taking place from the Pd(IV) complex 11Br
(Scheme 4).15a Complex 12Br has been isolated in 74% yield and
(13) (a) Byers, P. K.; Canty, A. J.; Crespo, M.; Puddephatt, R. J.; Scott,
J. D. Organometallics 1988, 7, 1363–1367. (b) Byers, P. K.; Canty, A. J.;
Skelton, B. W.; Traill, P. R.; Watson, A. A.; White, A. H. Organometallics
1992, 11, 3085–3088. (c) Byers, P. K.; Canty, A. J.; Honeyman, T.; Skelton,
B. W.; White, A. H. J. Organomet. Chem. 1992, 433, 223–229. (d) de Graaf,
W.; Boersma, J.; Smeets, W. J. J.; Spek, A. L.; van Koten, G. Organome-
tallics 1989, 8, 2907–2917. (e) van Asselt, R.; Rijnberg, E.; Elsevier, C. J.
Organometallics 1994, 13, 706–720. (f) van Belzen, R.; Hoffmann, H.;
Elsevier, C. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 1743–1745.
(14) (a) Bard, A. J.; FaulknerL. R. In Electrochemical Methods, 2nd
ed.; John Wiley & Sons: New York, 2001; pp 516-521. (b) Fauvarque,
J. F.; Pflu¨ger, F.; Troupel, M. Organomet. Chem. 1981, 208, 419–427.
Figure 5. Kinetics of the formation of the Pd(II) complex 12Br
generated by a reductive elimination from the Pd(IV) complex 11Br
(formed by oxidative addition of PhCH2Br (0.57 M) to the Pd(II)
complex 10 (3 mM): decreasing part of Figure 4a), as monitored
by amperometry at a rotating gold disk electrode polarized at +1
V, in DMF at 29 °C (increasing part of Figure 4a). Variation of ln
x′ against time (x′ ) (ilim - i)/ilim; ilim ) oxidation current of the
Pd(II) complex 12Br at the end of the reductive elimination; i )
oxidation current of 12Br at t).

OxidatiVe Addition of Benzyl Halides to a Pd(II) Complex
Organometallics, Vol. 27, No. 18, 2008 4553
and used after filtration on alumina. Complex 10 was synthesized
during its formation) within the time scale of the reductive
elimination. This means that the Pd(II) complex 10 was
considerably more reactive with PhCH2Br than the Pd(II)
complex 12Br,15b thus pointing out the key importance of the
metallacyclic aryl and norbornyl C-Pd bonds in Pd(II) complex
10 to induce the oxidative addition.
according to a published procedure.6a
Electrochemical Setup and Electrochemical Procedure for
Cyclic Voltammetry. Experiments were carried out in a three-
electrode cell connected to a Schlenk line. The cell was equipped
with a double envelope to have a constant temperature (Lauda RC20
thermostat). The working electrode consisted of a gold disk (d )
0.5 mm). The counter electrode was a platinum wire of ca. 1 cm2
apparent surface area. The reference was a saturated calomel
electrode separated from the solution by a bridge filled with a
solution of nBu4NBF4 (0.3 M) in 3 mL of DMF. Then 8 mL of
DMF containing the same concentration of supporting electrolyte
was poured into the cell. A 10.9 mg amount of complex 10 (0.024
mmol, 3 mM) was then added. Cyclic voltammetry was performed
Analogously to the oxidative addition of PhCH2Br to 10, that
of PhCH2Cl gave the Pd(IV) complex 11Cl, which should
undergo a C-C reductive elimination to give back a Pd(II)
complex. The Pd(IV) complex 11Cl formed in the oxidative
1
addition was characterized by H NMR at -5 °C (Scheme 3)
because it underwent reductive elimination at room temperature.
Once the Pd(IV) complex 11Cl was generated in the oxidative
addition of PhCH2Cl to the Pd(II) complex 10 at 29 °C (as
followed by amperometry at the rotating disk electrode polarized
at +1 V), no oxidation current was detected at longer times.
The cyclic voltammogram did not exhibit any oxidation wave
in the range of potentials investigated here (less positive than
+1.0 V), suggesting that the oxidation wave of the expected
Pd(II) complex 12Cl should be located at more positive potential
than +1.0 V and consequently could not have been detected
during a kinetic experiment similar to that of Figure 4.
at a scan rate of 0.5 V s-1
.
General Procedure for the Kinetics of the Oxidative Addi-
tion As Monitored by Amperometry. Experiments were carried
out in the same cell as above at 29 °C. A 15 mL amount of
DMF containing 0.3 M nBu4NBF4 was poured into the cell. Then
20.5 mg (0.045 mmol, 3 mM) of complex 10 was introduced
into the cell. A rotating gold disk electrode (d ) 2 mm, EDI
65109 (Radiometer Analytical) with an angular velocity of 105
rad s-1 (Radiometer Analytical controvit)) was polarized at +1
V on the plateau of the oxidation wave of complex 10 at O1
(Figure 1b). It was checked that the oxidation current was
constant within at least 5 min. The appropriate amount of the
benzyl bromide or chloride was then added into the cell, and
the decrease of the oxidation current was recorded versus time
up to 100% conversion.
Conclusion
Summing up, it can be stated that the kinetic pathway of the
palladium-catalyzed alkylation of aryl with benzyl halides is in
agreement with the key steps of the mechanism of the catalytic
reactions (3 f 4 f 5 in Scheme 2), proposed on the basis of
the isolation of a Pd(IV) complex with phenanthroline as ligand.
It has been established that the Pd(II) metallacyclic complex
10 undergoes oxidative addition to benzyl bromide or chloride
in DMF to generate Pd(IV) complexes with the reactivity order
PhCH2Br > PhCH2Cl. The oxidative addition of PhCH2Br is
followed by a slow C-C reductive elimination from the benzyl-
Pd(IV) complex, which gives back a Pd(II) complex. The rate
constants of the oxidative additions and the reductive elimination
have been determined in DMF. These results are of general
interest because they add important information to the ongoing
debate on the competition of the mechanism involving a Pd(IV)
complex and the one based on transmetalation between Pd(II)
complexes.16 Whether the preference for the Pd(IV) complex
reported here also holds for aromatic arylation, it requires further
study, however, with a suitable model.
General Procedure for the Kinetics of the Reductive Elimina-
tion As Monitored by Amperometry. The kinetics of the
reductive elimination was followed under the same experimental
conditions reported above for the oxidative addition using the
same rotating gold disk electrode polarized at +1 V. Once the
oxidative addition of PhCH2Br (1.022 mL, 8.55 mmol, 0.57 M)
to 20.5 mg (0.045 mmol, 3 mM) of complex 10 was over, the
increase of the oxidation current of the Pd(II) complex 12Br was
recorded versus time up to 100% formation (increasing part of
Figure 4a).
Characterization of Complex 11Cl. By adding benzyl chloride
(3.6 µL, 0.031 mmol) to complex 10 (9 mg, 0.019 mmol) in 0.6
mL of CDCl3 at room temperature, a new compound, 11Cl, was
slowly formed. Its formation was followed by 1H NMR
spectroscopy, which showed its presence in solution in ca. 1:0.8
molar ratio with the starting complex 10 after 60 min. After
this time complex 11Cl was characterized by NMR spectroscopy
via COSY, NOESY, and TOCSY experiments at -5 °C using a
Varian INOVA 600 spectrometer in no spinning mode (see the
1H NMR spectrum in the Supporting Information). After longer
times, 11Cl began to decompose, making the spectrum more
Experimental Section
Chemicals. DMF was distilled from calcium hydride and kept
under argon. The benzyl chloride and bromide were commercial
1
(15) (a) At the end of the reductive elimination, the oxidation current
of the Pd(II) complex 12Br reached a value of ilim ) 2.2 µA, which was
about half of the initial oxidation current of the Pd(II) complex 10 (Figure
4a). This might be due to a lower diffusion coefficient D for 12Br compared
to that of 10 because the latter is more condensed (the current at a rotating
complex. H NMR (600 MHz, CDCl3, 268 K): δ 8.68 (H2′, d,
J ) 4.4 Hz), 8.43 (H11, d, J ) 7.7 Hz), 8.38 (H4′, d, J ) 7.9
Hz), 8.32 (H7′, d, J ) 8.1 Hz), 7.93-7.86 (H5′, H6′, H2′′, H6′′,
m), 7.67 (H3′, dd, J ) 7.9, 4.9 Hz), 7.44-7.31 (H8′, H3′′, H4′′,
H5′′, m), 7.22 (H10, t, J ) 7.3 Hz), 7.18 (H9, t, J ) 7.3 Hz),
7.07 (H8, d, J ) 7.3 Hz), 6.54 (H9′, d, J ) 4.8 Hz), 4.62
(benzylic-H, d, J ) 7.8 Hz), 3.98 (H2, d, J ) 7.0 Hz), 3.43
(H3, d, J ) 7.0 Hz), 3.31 (benzylic-H, d, J ) 7.9 Hz), 2.40-2.35
(H4, m), 1.51-1.40 (H5 exo, m), 1.32-1.20 (H5 endo, m), 1.13
(H7 syn, d, J ) 9.3 Hz), 1.00-0.83 (H6 exo, H1, H6 endo, m),
0.49 (H7 anti, d, J ) 9.3 Hz).
electrode is proportional to D2/3 14a
associated with an oxidation for 12Br
)
involving fewer electrons (e.g., n ) 0.5, if a dimerization process occurs
during the electron transfer) than the one-electron oxidation observed for
10.11. (b) The peak potentials Ep of 10 and 12Br look close to each other (∆
) 35 mV at a scan rate of 0.5 V s-1) but cannot be compared to evaluate
the relative ability of complexes 10 and 12Br to undergo oxidative addition
(whenever it is indicative) because peak potentials Ep might be very different
from their respective standard potentials E0. The oxidation peaks of
complexes 10 and 12Br were fully irreversible at the scan rate of 0.5 V s-1
.
Characterization of Complex 12Br. To 3 mL of a CH2Cl2
solution of 10 (50 mg, 0.11 mmol) at 0 °C was added benzyl
bromide (21 mg, 0.12 mmol) dissolved in 1 mL of CH2Cl2 under
nitrogen. The reaction mixture was allowed to reach room
temperature under stirring and was maintained at room temper-
ature for an additional 3 h. Removal of most of the solvent and
No attempt was made to increase the scan rate to determine their respective
standard potentials E0, which could be compared and might be very different
from each other. See: Bard, A. J.; Faulkner L. R. In Electrochemical
Methods, 2nd ed.; John Wiley & Sons: New York, 2001; pp 234-236.
(16) (a) Ca´rdenas, D. J.; Mart´ın-Matute, B.; Echavarren, A. M. J. Am.
Chem. Soc. 2006, 128, 5033–5040. (b) Mota, A. J.; Dedieu, A. Organo-
metallics 2006, 25, 3130–3142.

4554 Organometallics, Vol. 27, No. 18, 2008
Amatore et al.
1H, H11), 6.84-6.71 centered at 6.81 (m, 2H, H10, H9), 4.14,
4.05 (AB system, J ) 16.1 Hz, benzylic-H, 2H), 3.32 (d further
split, J ) 7.6 Hz, 1H, H7 syn), 3.24 (br d, J ) 7.1 Hz, 1H, H2),
3.09 (br d, J ) 7.1 Hz, 1H, H3), 2.50-2.43 (m, 1H, H1),
1.87-1.81 (m, 1H, H4), 1.64-1.45 (m, 1H, H6 exo), 1.32-1.15
(m, 2H, H7 anti, H5 exo), 0.98-0.81 (m, 1H, H6 endo),
0.55-0.42 (m, 1H, H5 endo). Anal. Calcd for C32H29BrN2Pd:
C, 61.21; H, 4.66; N, 4.46. Found: C, 61.12; H, 4.50; N, 4.38.
Acknowledgment. This work has been supported in part
by the Centre National de la Recherche Scientifique (UMR
CNRS-ENS-UPMC 8640), the Ministe`re de la Recherche
(Ecole Normale Supe´rieure), and the Ministero dell’Universita`
e della Ricerca Scientifica e Tecnologica (Cofinanziamento
MIUR, 2006031888).
slow addition of diethyl ether led to the formation of a yellow-
orange powder. The mixture was cooled at ca. -15 °C to favor
precipitation of complex 12Br (51 mg, 0.08 mmol, 74%), which
was filtered and dried under vacuum. 1H NMR (300 MHz,
CDCl3, 293 K): δ 9.70 (dd, J ) 4.8, 1.5 Hz, 1H, H2′), 9.66 (dd,
J ) 7.7, 1.0 Hz, 1H, H12), 9.05 (dd, J ) 4.8, 1.3 Hz, 1H, H9′),
8.40 (dd, J ) 8.1, 1.5 Hz, 1H, H4′), 8.32 (dd, J ) 8.1, 1.5 Hz,
1H, H7′), 7.92, 7.89 (AB system, J ) 8.0 Hz, 2H, H5′, H6′),
7.73 (dd, J ) 8.1, 4.8 Hz, 1H, H8′), 7.38-7.16 (m, 4H, H3′,
H2′′, H4′′, H6′′), 7.16-7.08 (m, 2H, H3′′, H5′′), 7.05-6.95 (m,
Supporting Information Available: 1H NMR spectrum of 11Cl.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM800015X