Mechanism of the palladium-catalyzed homocoupling of arylboronic acids: Key involvement of a palladium peroxo complex

Article abstract of DOI:10.1021/ja0569959

The mechanism of the palladium-catalyzed homocoupling of arylboronic acids Arb(OH)₂ (Ar = 4-z-C₆H₄ with Z = MeO, H, CN) in the presence of dioxygen, leading to symmetrical biaryls, has been fully elucidated. The peroxo complex (η²-O₂)PdL₂ (L = PPh₃), generated in the reaction of dioxygen with the Pd(0) catalyst, was found to play a crucial role. Indeed, it reacts with the arylboronic acid to generatye an adduct (coordination of one oxygen atom of the peroxo complex to the oxophilic boron atom of the arylboronic acid) characterized by ²¹P NMR spectroscopy and ab initio calculations. This adducts reacts with a second molecule of arylboronic acid to generate trans-ArPd(OH)L₂ complexes. A transmetalation by the arylboronic acid gives trans-ArPdArL₂ complexes. The biaryl is then released in a reductive elimination. This reaction is at the origin of the formation of biaryls as byproducts in palladium-catalyzed Suzuki-Miyaura reactions when they are not conducted under oxygen-free atmosphere.

Full text of DOI:10.1021/ja0569959

Published on Web 05/05/2006
Mechanism of the Palladium-Catalyzed Homocoupling of
Arylboronic Acids: Key Involvement of a Palladium Peroxo
Complex
Carlo Adamo,† Christian Amatore,*,‡ Ilaria Ciofini,† Anny Jutand,*,‡ and
Hakim Lakmini‡
Contribution from the Ecole Nationale Supe´rieure de Chimie, Laboratoire d’Electrochimie et
Chimie Analytique, UMR CNRS 7575, 11 Rue Pierre et Marie Curie, F-75231 Paris Cedex 5,
France, and Ecole Normale Supe´rieure, De´partement de Chimie, UMR CNRS-ENS-UPMC 8640,
24 Rue Lhomond, F-75231 Paris Cedex 5, France
Received October 13, 2005; E-mail: Anny.Jutand@ens.fr; christian.amatore@ens.fr
Abstract: The mechanism of the palladium-catalyzed homocoupling of arylboronic acids ArB(OH)2 (Ar )
4-Z-C6H4 with Z ) MeO, H, CN) in the presence of dioxygen, leading to symmetrical biaryls, has been fully
elucidated. The peroxo complex (η2-O2)PdL2 (L ) PPh3), generated in the reaction of dioxygen with the
Pd(0) catalyst, was found to play a crucial role. Indeed, it reacts with the arylboronic acid to generate an
adduct (coordination of one oxygen atom of the peroxo complex to the oxophilic boron atom of the arylboronic
acid) characterized by 31P NMR spectroscopy and ab initio calculations. This adduct reacts with a second
molecule of arylboronic acid to generate trans-ArPd(OH)L2 complexes. A transmetalation by the arylboronic
acid gives trans-ArPdArL2 complexes. The biaryl is then released in a reductive elimination. This reaction
is at the origin of the formation of biaryls as byproducts in palladium-catalyzed Suzuki-Miyaura reactions
when they are not conducted under oxygen-free atmosphere.
Scheme 1
Introduction
The palladium-catalyzed cross-coupling reaction of nucleo-
philic aryl derivatives ArB(OR)2 (R ) H or Alkyl) with
electrophilic aryl derivatives, Ar′X (X ) I, Br, Cl, OTf) has
become one of the most widely used methods for the formation
of sp2-sp2 carbon-carbon bonds (Miyaura-Suzuki reaction,
Scheme 1),1 due to the high stability and low toxicity of
arylboronic acids This reaction enables the formation of
unsymmetrical biaryls, ArAr′.1,2
Scheme 2
Scheme 3
Symmetrical biaryls can be obtained by the palladium-
catalyzed homocoupling of electrophilic aryl derivatives, Ar′X
(X ) I, Br, Cl, OTf) in the presence of a reducing agent (Red
in Scheme 2).2,3 This reductiVe coupling has been widely
developed and its mechanism elucidated.3
derivatives ArB(OR)2 (R ) H or Alkyl) in the presence of an
oxidant4,5 (Ox in Scheme 3, including dioxygen) in an oxidatiVe
coupling.
This last oxidatiVe coupling was first observed under sto-
ichiometric conditions6 and then as a side-reaction in Pd-
catalyzed Miyaura-Suzuki cross-coupling reactions.7 Indeed,
the symmetrical biaryl ArAr generated from the arylboronic acid
ArB(OH)2 is often observed in Miyaura-Suzuki reactions
Conversely, symmetrical biaryls may also be synthesized by
the palladium-catalyzed homocoupling of nucleophilic aryl
† Ecole Nationale Supe´rieure de Chimie, Laboratoire d’Electrochimie et
Chimie Analytique.
‡ Ecole Normale Supe´rieure, De´partement de Chimie.
(1) (a) Miyaura, N.; Yanagi, T.; Suzuki, A. Synth. Commun. 1981, 11, 513-
519. For reviews, see: (b) Suzuki, A. Pure Appl. Chem. 1991, 63, 419-
422. (c) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457-2483. (d)
Suzuki, A. J. Organomet. Chem. 1999, 576, 147-168. (e) Miyaura, N.
Synthesis of biaryls via the cross-coupling reaction of arylboronic acids.
In AdVances in Metal-Organic Synthesis; JAI Press Inc.: Greenwich, CT,
1998; Vol. 6, pp 187-243.
(4) (a) Moreno-Man˜as, M.; Pe´rez, M.; Pleixats, R. J. Org. Chem. 1996, 61,
2346-2351. (b) Aramendia, M. A.; Lafont, M.; Moreno-Man˜as, M.; Pe´rez,
M.; Pleixats R. J. Org. Chem. 1999, 64, 3592-3594.
(2) For a recent review on the synthesis of biaryls, see: Hassan, J.; Sevignon,
M.; Gozzi, C.; Schulz, E.; Lemaire, M. Chem. ReV. 2002, 102, 1359-
1469.
(3) (a) Amatore, C.; Jutand, A.; Mottier, M. J. Electroanal. Chem. 306, 1991,
125-140. (b) A. Jutand, A. Mosleh, J. Org. Chem. 62, 1997, 261-274.
(c) Moinet, C.; Hurvois, J. P.; Jutand, A. Organic and metal-catalyzed
electrosynthesis. In AdVances in Organic Chemistry; Jenner, G., Ed.;
Bentham Science Publishers: 2005; Vol. 1, pp 403-453.
(5) (a) Smith, C. A.; Campi, E. M.; Jackson, R.; Marcuccio, S.; Naeslund, C.
G. M.; Deacon, G. B. Synlett 1997, 131-132. (b) Wong, M. S.; Zhang, X.
L. Tetrahedron Lett. 2001, 42, 4087-4089. (c) Kabalka, G. W.; Wang, L.
Tetrahedron Lett. 2002, 43, 3067-3068. (d) Koza, D. J.; Carita, E. Synthesis
2002, 2183-2186. (e) Parrish, J. P.; Jung, Y. C.; Floyd, R. J.; Jung, K. W.
Tetrahedron Lett. 2002, 43, 7899-7902. (f) Yoshida, H.; Yamaryo, Y.;
Ohshita, J.; Kunai, A. Tetrahedron Lett. 2003, 44, 1541-1544.
(6) Davidson, J. M.; Triggs, C. J. Chem. Soc. A 1968, 1324-1330.
9
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J. AM. CHEM. SOC. 2006, 128, 6829-6836
6829

A R T I C L E S
Adamo et al.
(Scheme 1).7 This side-reaction was recently developed as a
main catalytic reaction in the presence of pure dioxygen or air
(Scheme 3).4,5a,e The homocoupling of arylboronic acids was
first reported by Moreno-Man˜as et al. in 1996, using as catalysts
Pd0 or PdII complexes associated with monodentate phosphines.4
This oxidatiVe coupling could be accelerated by an oxidant, e.g.,
Cu(NO3)2.5d The homocoupling was later extended to arylbo-
ronic esters by Yoshida et al. in 2003, using Pd(OAc)2 and dppp
(1,3-bis-(diphenylphosphino)propane) as catalyst and DMSO as
solvent.5f
However, very little is known about the reaction mechanism
of this oxidatiVe homocoupling. A first mechanism was proposed
involving the oxidative addition of ArB(OH)2 to Pd0 complexes
with the generation of ArPdII-[B(OH)2]L2 (L ) PPh3) com-
plexes.4 However, our attempts to observe this reaction in the
absence of dioxygen always failed, whereas a reaction was
observed between ArB(OH)2 and a peroxo complex (η2-O2)-
PdL2 (L ) PPh3), formed in the reaction of Pd0L4 with
dioxygen.8 Sheldon and Kochi have proposed the formation of
the intermediate complex Ar2PdL2 (L ) PPh3) by a double
transmetalation of ArB(OH)2 with (HO)Pd(OOH)L2 generated
by reaction of water with the peroxo complex (η2-O2)PdL2.9
Conversely, while we were performing this work, Yoshida et
al. proposed a reaction of arylboronic esters ArB(OR)2 with (η2-
O2)Pd(dppp), which would give ArPd-[OOB(OR)2](dppp)
complexes.5f A subsequent transmetalation of the latter complex
by ArB(OR)2 would give Ar2Pd(dppp) and henceforth the
homocoupling product ArAr by a reductive elimination.
We report herein evidences for the mechanism of the
palladium-catalyzed homocoupling of ArB(OH)2 in the presence
of dioxygen (Scheme 3). In this study, it is clearly mechanisti-
cally and kinetically established that a peroxo complex of
palladium, (η2-O2)PdL2 (L ) PPh3),10 plays a key role in the
catalytic homocoupling of arylboronic acids.
were carried out in a thermostated three-electrode cell connected to a
Schlenk line. The counterelectrode was a platinum wire of ca. 1 cm2
apparent surface area; the reference was a saturated calomel electrode
(Radiometer) separated from the solution by a bridge (3 mL) filled
with a 0.3 M n-Bu4NBF4 solution in chloroform (or DMF). Degassed
chloroform (or DMF) (15 mL) containing 0.3 M n-Bu4NBF4 was poured
into a cell. (η2-O2)Pd(PPh3)2 (20 mg, 30 µmol, 2 mM) was then
introduced into the cell. The kinetic measurements were performed at
a rotating gold disk electrode (diameter ) 2 mm, inserted into a Teflon
holder, EDI 65109, Radiometer) with an angular velocity of 105 rad‚s-1
(Radiometer controvit). The rotating electrode was polarized at +0.70
V (+0.48 in DMF) on the plateau of the oxidation wave of (η2-O2)-
Pd(PPh3)2. 4-MeO-C6H4-B(OH)2 (45 mg, 300 µmol, 20 mM) was then
added into the cell, and the decrease of the oxidation current was
recorded versus time up to 100% conversion.
Typical Procedure for NMR Experiments. All experiments were
performed under argon atmosphere. To a solution of (η2-O2)Pd(PPh3)2
(5 mg, 7.5 µmol) in 0.5 mL of degassed CDCl3 were added various
amounts of arylboronic acids 1a-c or esters 1′b (from 0.75 to 37.5
1
µmol). The 31P NMR and H NMR were then performed.
Characterization of [ArB(OH)2,(η2-O2)Pd(PPh3)2] 6a, 6b, 6c, and
6′b. All experiments were performed under argon atmosphere. The
complexes were generated by addition of 0.5 equiv of 1a, 1b, 1c, and
1′b, respectively, to (η2-O2)Pd(PPh3)2 in CDCl3. The 31P NMR data of
6a, 6b, 6c, and 6′b are collected in Table 2.
Characterization of trans-ArPd(OH)(PPh3)2 5a, 5b. All experi-
ments were performed under argon atmosphere. The complexes were
generated as above by addition of 5 equiv of 1a, 1b, respectively, to
(η2-O2)Pd(PPh3)2 in CDCl3. The 1H NMR and 31P NMR spectroscopies
and ESI MS spectrometry of 5a, 5b are collected in Table 1. The
characteristics of 5b were identical to those of an authentic sample
generated in situ by addition of 12 mg (46 µmol) of PPh3 to a solution
11a,b
of 3.4 mg (3.24 µmol) [PhPd(µ-OH)(PPh3)]2
in 0.5 mL of CDCl3.
Characterization of trans-ArPdAr′(PPh3)2 trans-8ac and trans-
8bc. All experiments were performed under argon atmosphere. The
complexes were generated in an NMR tube by addition of 1.1 equiv
of 1c to trans-ArPd(OH)(PPh3)2 5a or 5b generated as above in CDCl3.
The spectra from 1H NMR, NMR 2D, and 31P NMR spectroscopies of
trans-8ac and trans-8bc are collected in Table 6.
Typical Procedure for (η2-O2)Pd(PPh3)2-Catalyzed Homocou-
pling of Arylboronic Acids (Scheme 5). To a solution of (η2-O2)Pd-
(PPh3)2 (3 mg, 4.6 µmol) in 0.5 mL of CDCl3 was added under dioxygen
500 µL (23 µmol) of the arylboronic acid 1b (or 1a) from a mother
solution of 1b (46 mM in CDCl3). After mixing for 10 min, the yield
of the homocoupling product PhPh 2b (or 4-MeO-C6H4-C6H4-OMe-4,
2a) was determined by 1H NMR spectroscopy after addition of a known
amount of toluene as internal standard. The 1H NMR spectrum of PhPh
(or 4-MeO-C6H4-C6H4-OMe-4) was identical to that of an authentic
commercial sample. The yield of the byproduct Ph-OH (or 4-MeO-
C6H4-OH) was determined in a similar way.
Experimental Section
General. 31P NMR spectra were recorded in CDCl3 or in DMF
containing 10% of acetone-d6 on a Bruker spectrometer (101 MHz)
1
with H3PO4 as an external reference. H NMR spectra were recorded
in CDCl3 on a Bruker spectrometer (250 MHz) with TMS as an internal
reference.
Chemicals. DMF was distilled from calcium hydride under vacuum
and kept under argon. PPh3, PhB(OH)2, 4-CN-C6H4-B(OH)2, 4-MeO-
C6H4-B(OH)2, and PhB(O-(CH2)3-O) were commercial and used as is.
8a
The peroxo complex (η2-O2)Pd(PPh3)2 and the dimeric complex
[PhPd(µ-OH)(PPh3)]211a,b were synthesized as reported in the literature.
Typical Procedure for the Kinetics of the Reaction of (η2-O2)-
Pd(PPh3)2 with Arylboronic Acids, As Monitored by Amperometry.
All experiments were performed under argon atmosphere. Experiments
Computational Methods. All calculations were carried out using
the Gaussian code.12 A hybrid Hartree-Fock/density functional model,
hereafter referred to as PBE0, was used throughout.13 In this functional,
derived from the PBE,14 the ratio of HF/DFT exchange is fixed a priori
to 1/4.15 A double ê quality LANL2 basis16 and corresponding
pseudopotential17 was used for all calculations. Such level of theory
was proven to provide reliable results both for thermochemical and
(7) (a) Campi, E. M.; Jackson, R.; Marcuccio, S.; Naeslund, C. G. M. J. Chem.
Soc. Chem. Commun. 1994, 2395-2395. (b) Gillmann, T.; Weeber, T.
Synlett 1994, 649-651. (c) Song, Z. Z.; Wong, H. N. C. J. Org. Chem.
1994, 59, 33-41.
(8) (a) Wilke, G.; Schott, H.; Heimbach, P. Angew. Chem., Int. Ed. 1967, 6,
92-93. (b) Amatore, C.; Aziz, S.; Jutand, A.; Meyer, G.; Cocolios, P. New
J. Chem. 1995, 19, 1047-1059.
(9) Sheldon, R. A.; Kochi, J. K. Activation of Molecular Oxygen by Metal
Complexes. In Metal-catalyzed Oxidations of Organic Compounds; Aca-
demic Press: New York, 1981; Chapter 4, pp 71-119.
(12) Frisch, M. J.; et al. Gaussian 03, Revision B5; Gaussian, Inc.: Pittsburgh,
PA, 2003.
(13) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158-6170.
(14) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865-
3868.
(15) Adamo, C.; Barone, V. Chem. Phys. Lett. 1997, 274, 242-250.
(16) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer,
H. F., III, Ed.; Plenum: New York, 1976; pp 1-28.
(17) Hay, J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310.
(10) For a review on palladium peroxo complexes as intermediates in catalytic
reactions, see: Stahl, S. S. Angew. Chem., Int. Ed. 2004, 43, 3400-3420.
(11) (a) Grushin, V. V.; Alper, H. Organometallics 1993, 12, 1890-1901. (b)
Grushin, V. V.; Alper, H. Organometallics 1996, 15, 5242-5245. (c)
Amatore, C.; Carre´, E.; Jutand, A.; M’Barki, M. A.; Meyer, G. Organo-
metallics 1995, 14, 5605-5614. (d) Matos, K.; Soderquist, J. A. J. Org.
Chem. 1998, 63, 461-470.
9
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Pd-Catalyzed Homocoupling of Arylboronic Acids
A R T I C L E S
Scheme 4
center, e.g. as observed in trans-ArPdX(PPh3)2 (X ) I, Br, Cl)
complexes.20 This suggests the formation of aryl-palladium(II)
complexes.
The reaction of the arylboronic acids 1a,b with isolated (η2-
O2)Pd(PPh3)2 4, synthesized as in Scheme 4,8 was also
investigated. When an excess of the arylboronic acids 1a or 1b
was added to a solution of the peroxo complex 4 in chloroform,
the aromatic 1H NMR signals of the aryl-palladium complexes
detected above were observed. Concomitantly, the 31P NMR
singlet of the peroxo complex 4 (33.2 ppm in CDCl3, 33.4 in
DMF) disappeared, and a new singlet was observed, charac-
teristic of a palladium ligated to two magnetically equivalent
PPh3 sitting in a trans position (Table 1). The structure of one
of the new aryl-palladium(II) complexes, trans-PhPd(OH)-
(PPh3)2 5b, formed in the reaction of (η2-O2)Pd(PPh3)2 with 1b
Scheme 5
spectroscopic properties.18 No symmetry constraints were imposed
during structural optimizations, and the nature of the optimized
structures and energy minima were defined by subsequent frequency
calculations. Basis set superposition errors were evaluated using the
standard Counterpoise correction, the largest being of the order of 6
kcal/mol. Additional calculations were performed using the larger
6-311G(d) basis set (hereafter BS2 basis) on O, P, and B atoms, to test
basis set dependence of energetic and structural features. The results
are collected in Table 4 and in the Supporting Information. Finally,
the effect of addition of a diffuse function on oxygen atoms (i.e.,
6-311+G(d)) was also tested. If not differently specified, all values
discussed in the following correspond to those obtained with the
LANL2DZ basis.
1
(Scheme 6), was determined by comparing its H, 31P NMR,
and ESI MS spectra to those of the authentic complex. The latter
was generated by reacting the dimeric µ-hydroxo complexes
cis- and trans-[PhPd(µ-OH)(PPh3)]2 (31P NMR: δ ) 33.66 and
32.96 ppm) with 10 equiv of PPh3, as reported by Grushin and
Alper (Scheme 7).11a,b The same complex 5b was also formed
by reacting trans-PhPdI(PPh3)2 with n-Bu4NOH in chloroform
(this work) or DMF11c or THF.11d
At this level, we have established that a fast reaction occurred
between the peroxo-palladium complex 4 and arylboronic acids
1a,b. Nevertheless, a crucial question arises about the mecha-
nism of this reaction. When 1a-c (0.5 equiv) were added to a
solution of 4 in chloroform, i.e., under substoichiometric
conditions, the 31P NMR spectrum exhibited, besides the singlet
of complex 4, two doublets of equal magnitude (Table 2).23 This
suggests the formation of new complexes 6a-c possessing two
magnetically nonequivalent phosphines (Scheme 8).
Results and Discussion
Homocoupling of Arylboronic Acids Catalyzed by (η2-O2)-
Pd(PPh3)2. Moreno-Man˜as et al. have observed that the
homocoupling of arylboronic acids catalyzed by Pd0(PPh3)4 in
toluene was more efficient in the presence of dioxygen.4
Interestingly, the complex Pd0(PPh3)4 is known to react with
dioxygen to generate the peroxo complex (η2-O2)Pd(PPh3)2 48a
via Pd0(PPh3)2 (Scheme 4).8b Complex 4 was isolated as a stable
greenish complex.
This prompted us to investigate the homocoupling of aryl-
boronic acids in the presence of O2Pd(PPh3)2 (20%) as a catalyst
(Scheme 5). In nonoptimized test reactions, yields of 59% and
40% were obtained for 2a and 2b, respectively, after 10 min in
chloroform. The corresponding phenols 3a and 3b were also
obtained as byproducts. The biaryl 2c was not formed in
chloroform due to the low solubility of 1c in this solvent,
whereas it was generated in DMF.
The peroxo complex 4 thus appears to be a true catalyst for
the homocoupling reaction of arylboronic acids. This prompted
us to fully investigate its reaction with arylboronic acids.
Rate and Mechanism of the Reaction of (η2-O2)Pd(PPh3)2
with Arylboronic Acids 1a-c. Characterization of the Aryl-
Palladium(II) Complexes Formed in the Reaction of (η2-
O2)Pd(PPh3)2 with Arylboronic Acids or Esters. As recalled
in the Introduction, we could not observe any reaction between
Pd0(PPh3)4 and the arylboronic acids, 1a,c, at room temperature,
in chloroform or DMF under argon. However, when the same
reaction was performed in the presence of dioxygen, i.e. from
(η2-O2)Pd(PPh3)2 generated in situ, new signals for aromatic
protons appeared in the 1H NMR spectra, in the range 5.9-6.6
ppm, which differed from those of the arylboronic acids 1a,b,
the biaryls 2a,b or the phenols 3a,b.19 These signals, located
upfield, characterized an aryl group attached to a palladium(II)
1
At this stage, the H NMR spectroscopy did not reveal the
presence of complexes of type 5, nor biaryls 2, nor phenols 3.
The two doublets of 6a-c and the singlet of the nonreacted
peroxo complex 4 disappeared with time, and only the phosphine
oxide (O)PPh3 was observed at long times. Similar sets of
doublets were also observed in DMF containing 10% acetone-
d6 as well as from the ester 1′b (Table 2).
When an excess of 1b was added to a solution in chloroform
exhibiting the two doublets of 6b, the latter disappeared, and
the singlet of complex trans-5b was observed, suggesting that
complex 6b might be an intermediate complex generated on
the way to complex 5b. When an excess of peroxo complex 4
was added to the previous solution, the two doublets of complex
6b were recovered. This establishes that complex 6b is only
observed under substoichiometric concentrations of ArB(OH)2
and that an excess of ArB(OH)2 is required to yield complex
5b. Similar reactions were observed by reacting 1a with the
peroxo complex 4. Complex trans-5c was not observed in
chloroform, whereas 6c was observed. Due to the low solubility
(19) No reaction occurs by bubbling dioxygen in a solution of 1a-c, in the
absence of palladium.
(20) Fitton, P.; Rick, E. A. J. Organomet. Chem. 1971, 28, 287-291.
(21) Other peaks at 753 for 5a or 723 for 5b were also observed. MS/MS
experiments on 5a and 5b showed that (O)PPh3 was released, suggesting
the formation of ArPd(PPh3)((O)PPh3)+. This is due to the ESI MS
technique in which the ionization also produces some triphenylphosphine
oxide which coordinates the cationic Pd complex.22
(22) (a) Aramendia M. A.; Lafont, F.; Moreno-Man˜as, M.; Pleixats, R.; Roglans,
A. J. Org. Chem. 1999, 64, 3592-3694. (b) Moreno-Man˜as, M.; Pleixats,
R.; Spengler, J.; Cherin, C.; Estrine, B.; Bouquillon, S.; He´nin, F.; Pla-
Quintina A.; Roglans, Eur. J. Org. Chem. 2003, 274-283.
(18) (a) Ciofini, I.; Laine´, P. P.; Bedioui, F.; Adamo, C. J. Am. Chem. Soc.
2004, 126, 10763-10777. (b) Ciofini, I.; Adamo, C. J. Phys. Chem. A
2001, 105, 1086-1092.
(23) Some phosphine oxide was also detected at 29.2 ppm.
9
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A R T I C L E S
Adamo et al.
Table 1. 1H NMR (250 MHz, TMS), 31P NMR (101.3 MHz, H3PO4) Shifts and ESI MS of Complexes 5a,b Formed by Reacting
(η2-O2)Pd(PPh3)2 4 with the Arylboronic Acids 1a,b in Chloroform (Scheme 6)a
trans-ArPd(OH)(PPh3)2
1H NMR
31P NMR
δ
ppm (CDCl3)
δ
ppm (CDCl3)
ESI MS21
Ar
5a MeO-C6H4
5b C6H5
6.42 (d, J ) 8.5 Hz, 2H, o-H)
5.93 (d, J ) 8.5 Hz, 2H, m-H)
3.5 (s, 3H, Me)
6.62 (d, J ) 7.3 Hz, 2H, o-H)
6.38 (t, J ) 7 Hz, 1H, p-H)
6.30 (t, J ) 7.3 Hz, 2H, m-H)
23.5 (s)
23.7 (s)
737 [M-OH]+
630 [Pd(PPh3)2]+
707 [M-OH]+
630 [Pd(PPh3)2]+
a The aromatic protons of the PPh3 ligand are omitted for clarity.
Scheme 6
The intermediate complexes 6 are stabilized through the direct
interaction between the boron atom and one of the two oxygen
atoms of the peroxo complex. It is worthwhile to note that in
our preliminary DFT calculations made using L ) PH3 as model
ligand for O2PdL2 intermediates of type 6 were found to be
unstable. Instead, the only adducts which were found to be stable
were complexes in which the O2PdL2 was hydrogen bonded
through one of its oxygen atoms to one proton of the arylboronic
acid. The possibility of having a direct interaction between the
boron atom and one oxygen atom of the peroxo was ruled out
using this simplified model. This highlights the crucial electronic
role of the ligand and the fact that DFT results obtained using
PH3 as model for the PPh3 ligand cannot be always straight-
forwardly extrapolated to fully describe the behavior of the real
systems.24 For the sake of clarity, it should also be noted that
hydrogen-bonded structures analogous to those found in the case
of PH3 were also found to be stable in the case of the systems
containing PPh3 in the gas phase. However, we expect solvent
effects to be more important in this latter case, so that the two
coordination modes of the arylboronic acid could become
competitive in polar or protic solvents, but in favor of complexes
6. To evaluate bulk solvent effects, single-point calculations on
the gas-phase optimized structures (using the BS2 basis) were
performed using the polarizable continuum model (Table 4).25
Inclusion of solvent, here chloroform, does not significantly
affect the relative stability of the complexes that are computed
to be likely to be formed also in solution. Finally, from a more
technical point of view we can notice that addition of a
polarization function on O, B, and P atoms does not strongly
change either structure (refer to Supporting Information) or
thermochemistry of the formation of complexes 6. In particular,
adding a diffuse function only on O atoms in the case of 6b
has a negligible effect on both structure (about 0.01Å) and
thermochemistry (about 3 kJ/mol).
Scheme 7
Table 2. 31P NMR Shifts (101.3 MHz, H3PO4) of the Intermediate
Complexes 6a-c Formed by Reacting (η2-O2)Pd(PPh3)2 4 with 0.5
equiv of Arylboronic Acids 1a-c in Chloroform or DMFa
6
6
1
δ
ppm (CDCl3)
δ
ppm (DMF
+
10% acetone-d6)
1a
31.1 (d, JPP ) 37 Hz, 1P)
27.3 (d, JPP ) 37 Hz, 1P)
31.2 (d, JPP ) 37 Hz, 1P)
27.3 (d, JPP ) 37 Hz, 1P)
31.6 (d, JPP ) 37 Hz, 1P)
27.6 (d, JPP ) 37 Hz, 1P)
31.2 (d, JPP ) 37 Hz, 1P)
27.3 (d, JPP ) 37 Hz, 1P)
31.2 (d, JPP ) 38 Hz, 1P)
27.3 (d, JPP ) 38 Hz, 1P)
31.1 (d, JPP ) 38 Hz, 1P)
27.1 (d, JPP ) 38 Hz, 1P)
31.3 (d, JPP ) 38 Hz, 1P)
27.4 (d, JPP ) 38 Hz, 1P)
31.2 (d, JPP ) 38 Hz, 1P)
27.1 (d, JPP ) 38 Hz, 1P)
1b
1c
1′b
a Same reaction with 1′b. a The 11B NMR broad signal (29 ppm) of 1b
in CDCl3 disappeared after addition of 4 in excess, but any other signal
was observed, suggesting the formation of 6 in equilibrium with 4.
Scheme 8
Kinetics of the Reaction of (η2-O2)Pd(PPh3)2 with Aryl-
boronic Acids. The overall reaction in Scheme 8 was monitored
by electrochemistry under an argon atmosphere, taking the
advantage that the peroxo complex (η2-O2)Pd(PPh3)2 4 (2 mM)
exhibited an oxidation peak at +0.63 V vs SCE in chloroform
containing n-Bu4NBF4 (0.3 M) (+0.43 V vs in DMF),8b at a
steady gold disk electrode and a scan rate of 0.2 V s-1. The
oxidation peak totally disappeared when an excess of 1a-c was
added to the peroxo complex 4. In the experiments performed
in chloroform, 1a-c were introduced as a concentrated solution
on DMF due to their low solubility in chloroform. The kinetics
of 1c in chloroform, it could not be added in large excess. Since
the 31P NMR shifts of the two doublets depend on the nature
of the aryl group, the intermediate complexes 6 necessarily
contain one aryl group. This rules out the formation of
complexes such as trans-(HO)Pd(OOH)L2 or [Pd2(µ-OH)(µ-
OO)L4]+ which might have been generated by the reaction of
water with (η2-O2)Pd(PPh3)2 4, as proposed by Sheldon and
Kochi.9 Intermediate complexes 6 are proposed on the basis of
kinetic data (see below), and their structures were confirmed
by DFT calculations (Scheme 8, Figure 1). Selected computed
structural parameters of the optimized complexes 6 are collected
in Table 3. Their computed formation energies and enthalpies
are gathered in Table 4.
(24) (a) Kozuch, S.; Amatore, C.; Jutand, A.; Shaik, S. Organometallics 2005,
24, 2319-2330. (b) Goossen, L. J.; Koley, D.; Herrmann, H. L.; Thiel, W.
J. Am. Chem. Soc. 2005, 127, 11102-11114.
(25) (a) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117-129.
(b) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003,
24, 669-681.
9
6832 J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006

Pd-Catalyzed Homocoupling of Arylboronic Acids
A R T I C L E S
Figure 1. Computed optimized structure for complex 6b (left): B atom (blue); O atoms (red); Pd atom (yellow); P atoms (purple); C atoms (green). The
H atoms are omitted for clarity.
Table 3. Selected Computed Structural Parameters of the Optimized Structures (distances d in Å and Angles θ in Degrees)a
6
d (Oa
−Ob)
d (Oa−B)
d (Pd−Oa)
d (Pd−Ob)
θ
(PPdP)
θ
(PdOaB)
θ
(OaObB)
6a
6b
6c
1.452
1.452
1.455
1.432
1.653
1.650
1.642
3.193
2.128
2.123
2.136
2.042
2.024
2.025
2.021
2.058
104.5
104.2
104.4
105.8
105.7
106.2
105.8
98.8
112.8
113.0
112.8
97.2
6′b
a See Figure 1 for nomenclature and labeling. See also Supporting Information for computed structural parameters of free (η2-O2)Pd(PPh3)2 4.
Table 4. Formation Energies (∆E), Enthalpies (∆H), BSSE Energy Correction (BSSE), and Formation Energies in Solution (∆ESolvent
)
Computed for Different Complexes 6 and 6′ as a Function of the Substituent (Z) on the Arylboronic Acida
6
∆E
∆H
BSSE
∆Esolvent
6a
6b
6c
-28.101 (-29.377)
-33.125 (-34.195)
-51.285 (-53.107)
-19.633 (-23.523)
-26.788 (-27.153)
-31.783 (-31.993)
-49.432 (-50.834)
-18.851 (-21.875)
29.015 (29.998)
29.025 (32.096)
29.090 (30.384)
14.879 (17.611)
(1.713)
(-2.545)
(-17.895)
(-9.309)
ester 6′b
a Values reported in parentheses correspond to the BS2 basis. All values are in kJ/mol.
Figure 2. Kinetics of the reaction of the arylboronic acid 1a with (η2-O2)Pd(PPh3)2 4 (C0 ) 2 mM) in chloroform (containing n-B4NBF4, 0.3 M) at 25 °C,
as monitored by amperometry at a rotating gold disk electrode. (a) [1a] ) 20 mM. Variation against time of ln x (x ) [4]t/[4]0 ) it/i0; it ) oxidation current
of 4 at t, i0 ) initial oxidation current of 4). ln x ) -kobs × t. (b) Determination of the reaction order in 1a: plot of ln(kobs) against ln([1a]): ln(kobs) ) 1.9
× ln([1a] + 6. (c) Determination of the reaction order in 1a: plot of kobs against [1a]2. kobs ) kapp × [1a]2.
of the reaction was followed by amperometry performed at a
rotating gold disk electrode polarized on the plateau of the
oxidation peak of 4 (+0.48 V in DMF, +0.70 V in chloroform).
After addition of 1a-c in excess, the decrease of the oxidation
current of 4 (proportional to its concentration) was recorded
with time, up to total conversion. A plot of ln x (x ) [4]t/[4]0
) it/i0; it ) oxidation current of 4 at t, i0 ) initial oxidation
current of 4) against time was linear (Figure 2a) indicating a
first-order reaction of the peroxo complex. The observed rate
constant kobs was determined from the slope of the straight line
in Figure 2a, viz. evidencing the rate law: ln x ) -kobst.
2b,c). This suggests the occurrence of a mechanism involving
sequentially two molecules of 1, as reported in Scheme 9.
The arylboronic acid is involved both in the reversible
formation of complex 6 and in the irreversible evolution of this
complex to complex cis-7. The latter may evolve to complex
trans-5 either by route A or B (Scheme 9). The water involved
in route A and B may result from the formation of a boroxine
from the arylboronic acid.26 Whatever the relative weight of
route A or B, neither route affects the overall rate of disap-
pearance of 4. This latter assumption was confirmed by the fact
that, in the reaction of 4 with 1 in excess, no sets of two doublets
The reaction order in 1a was found to be +2, as evidenced
by the observed dependence of kobs: kobs ) kapp[1a]2 (Figure
(26) Lappert, M. F. Chem. ReV. 1956, 56, 959-1064.
9
J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006 6833

A R T I C L E S
Scheme 9
Adamo et al.
Whatever the solvent, the following reactivity order was
observed: K1CNk2 > K1Hk2 > K1OMek2OMe. The reaction of
the second molecule of 1 may be considered as a transmetalation
of complexes 6 by the arylboronic acid (Scheme 9). This
suggests that k2CN < k2H < k2OMe, viz. because the arylboronic
acid is expected to be more nucleophilic when the aryl group
is substituted by an electron donor group. One then deduces
CN
H
CN
H
OMe
that K1 . K1 . K1
in agreement with the decreasing
Lewis acidity of the arylboronic acid when going from CN to
OMe (Table 4). The reaction was faster in chloroform than in
DMF, suggesting a competitive solvatation of ArB(OH)2 by
DMF.
Transmetalation on ArPd(OH)(PPh3)2 (trans-5a,b) Com-
plexes. Thus far we have identified and characterized the
formation of complexes ArPd(OH)(PPh3)2 (trans-5a,b, Scheme
9) through the fast reaction of the peroxo complex (η2-O2)Pd-
(PPh3)2 with 2 equiv of 1a,b, respectively. It was of interest to
investigate the role of such complexes in the context of the
palladium-catalyzed homocoupling of arylboronic acids. The
complex trans-C6H5-Pd(OH)(PPh3)2 (trans-5b), generated by
reaction of 1b with 4, was reacted with 1.5 equiv of 4-CN-
C6H4-B(OH)2 (1c) and the reaction monitored by 1H NMR. The
recorded spectrum exhibited two different aryl groups on a PdII
center, in the range 6.2-6.8 ppm. The 2D NMR spectrum
allowed the identification of the three different protons of the
C6H5 group associated with the two different protons of the
4-CN-C6H4 group (Table 6, Figure 3). The 31P NMR spectrum
exhibited one sharp singlet (Table 6), attesting a trans coordina-
tion of the two phosphines. The reaction gave the new complex
trans-8bc in a reaction which is a transmetalation step (Scheme
10). The same spectrum was observed when starting from trans-
5b synthesized in situ as in Scheme 7.
This reaction was confirmed by reacting trans-5a with 1c
leading to trans-8ac (Scheme 10, Table 6). The coupling
products C6H5-C6H4-CN-4 and 4-MeO-C6H4-C6H4-CN-4 were
also formed and characterized by 1H NMR and mass spectrom-
etry. They could only be formed by a reductive elimination from
cis-8bc and cis-8ac (Scheme 11). The detection of complexes
trans-8bc and trans-8ac indicates that they were rather stable
Vis-a´-Vis a reductive elimination in agreement with the fact that
this must proceed from the uphill cis-8bc and cis-8bc complexes,
respectively.
This confirms the observation made by Aliprantis and
Canary,28 when monitoring Pd-catalyzed Miyaura-Suzuki
cross-coupling reactions by ESI MS. Indeed, complexes ArPd-
Ar′(PPh3)2 formed in the transmetalation step were clearly
observed in their work and assumed to adopt trans configura-
tion.28 It is worthwhile to note that the reaction of ArPd(OH)-
(PPh3)2 with Ar′B(OH)2 observed here may also occur in the
Miyaura-Suzuki cross-coupling reaction performed in the
presence of hydroxyl bases which were found to react with
ArPdX(PPh3)2 (X ) I,11c Br11d) to form ArPd(OH)(PPh3)2.
When trans-5b PhPd(OH)(PPh3)2 was reacted with 1a
4-MeO-C6H4-B(OH)2 (1.3 equiv), the mixed complex trans-
8ba was not observed (Scheme 10), but the coupling biaryl
C6H5-C4H5-OMe-4 was produced together with 4-MeO-C6H4-
Pd(OH)(PPh3)2. The catalytic cycle was completed via the
successive transmetalation and reductive elimination to generate
Table 5. Rate Constants K1k2 for the Reaction of the Peroxo
Complex 4 with Arylboronic Acids 1a-c in Chloroformaand DMF at
25 °C (Scheme 9)
1
K1k2 (M-2 s-
)
solvent
1a (Z
)
OMe)
1b (Z
)
H)
1c (Z ) CN)
CDCl3
DMF
600
5.8
2500
1025
n.d.
46000
a Containing 1% DMF.
were observed which would have characterized cis-7 or cis-5,
whenever either of these species would accumulate.
Therefore, the peroxo complex must be activated by the
arylboronic acid before it undergoes a transmetalation with
another molecule of ArB(OH)2 (Scheme 9). We indeed observe
from the calculations that one Pd-O bond has been elongated
(by ca. 10 pm, Table 3) due to the coordination of the boron
atom. The kinetic law corresponding to the formation of cis-7
is complex and highly dependent on the excess of 1 (see
Supporting Information). However, for large excesses of 1, the
formation of 6 is fast and equilibrated and continuously
displaced by the formation of cis-7. Under those conditions,
the rate-determining kinetic law for Scheme 9 is given in eqs
a,b under specific conditions which apply here. Our experimental
results are in total agreement with eqs a,b. The value of K1k2
was determined from the slope of the straight line in Figure 2c
(Table 5).27
k1k2[4][1]2
rate )
) K1k2[4][1]2
(a)
(b)
k-1
ln x ) -K1k2[4][1]2t ) -kapp[1]2t ) -kobs
t
The kinetics of the reaction of the peroxo complex 4 with 1a-c
was similarly investigated in chloroform or DMF. The values
of K1k2 are collected in Table 5 (Figures S2-5 in Supporting
Information).
(27) In the time scale investigated here, the two doublets of the intermediate
complexes 6a-c were observed in chloroform and DMF (without the singlet
of complex trans-5a-c) when less than one equiv of 1a-c was added to
the peroxo complex 4. This supposes that k1[4] . k2[6]. Since [4] and [6]
are then very similar at low substoichiometric conditions, one must have
k1 . k2. with k-1 . k2[1]. See also Supporting Information.
(28) Aliprantis, A. O.; Canary, J. W. J. Am. Chem. Soc. 1994, 116, 6, 6985-
6986.
9
6834 J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006

Pd-Catalyzed Homocoupling of Arylboronic Acids
A R T I C L E S
Table 6. 1H (250 MHz, TMS) and 31P NMR (101.3 MHz, H3PO4) Shifts (δ ppm) of Complexes 8 Formed by Reacting Complexes 5 with
Arylboronic Acids 1 in CDCl3 (Scheme 10)a
4-Y-C6H4-Pd-C6H4-Z-4(L)2
PPh3
1H NMR
1H NMR
L
)
4-Y-C6H4-Pd
Pd-C6H4-Z-4
31P NMR
8bc (Y ) H, Z ) CN)
6.61 (d, J ) 7 Hz, 2H, o-H)
6.37 (t, J ) 7.5 Hz, 1H, p-H)
6.22 (t, J ) 7.3 Hz, 2H, m-H)
6.41 (d, J ) 7.4 Hz, 2H, o-H)
6.14 (m, 2H, m-H)
6.61 (d, J ) 6.7 Hz, 2H, m-H)
6.44 (d, J ) 6.7 Hz, 2H, o-H)
22.40
8ac (Y ) OMe, Z ) CN)
6.8 (d, 2H, o-H)
6.27 (d, J ) 8.8 Hz, 2H, m-H)
n.d.
3.56 (s, 3H, Me)
a The aromatic protons of the PPh3 ligand are omitted for more clarity. The terms: o, m, and p are defined relative to the Pd atom.
Scheme 12
the catalytic cycle was closed, the excess of 1b reacted with
the peroxo complex O2Pd(PPh3)2 to form trans-PhPd(OH)-
(PPh3)2.
All these results suggest that the overall reductive elimination
from complexes trans-8 is slower when one aryl is substituted
by a CN group. However, we could not establish whether this
slow kinetics was due to a more endergonic trans/cis isomer-
ization from trans-8 (Scheme 11) or/and to intrinsic effects
related to the reductive elimination from the cis-8 complexes.
We now have experimental evidence for the formation of
trans-ArPdAr′(PPh3)2 complexes in a transmetalation step
performed from trans-ArPd(OH)(PPh3)2 and Ar′B(OH)2. How-
ever, because the transmetalation and reductive elimination
might be simultaneous steps, the rate and mechanism of the
transmetalation could not be investigated in detail. A mechanism
is proposed (Scheme 12) on the basis of Miyaura’s proposal
for the reaction of Ar′B(OH)2 with ArPd(OAc)(PPh3)2.29 In a
theoretical work (DFT calculations), Ujaque, Maseras et al.30
gave further evidence of the involvement of a transmetalation
step between vinyl-B(OH)2 and trans-vinyl-Pd(OH)(PH3)2
complexes.
Figure 3. 1H-1H (250 MHz, TMS) of complex 8bc formed by reacting
complex 5b with 1c in CDCl3 (Scheme 10).
Scheme 10
Catalytic Cycle for the Pd-Catalyzed Homocoupling of
Arylboronic Acids in the Presence of Dioxygen. The reaction
of ArB(OH)2 with the peroxo complex followed by transmeta-
lation and reductive elimination steps is summarized in Scheme
13. However, such mechanism was established step by step,
and the intermediate complexes, trans-5 and trans-8, were
characterized at long times, which excluded the observation of
the intermediate cis-5 and cis-8 complexes under our experi-
mental conditions.
Scheme 11
The (η2-O2)Pd(PPh3)2-catalyzed homocoupling of ArB(OH)2
1a,b under dioxygen worked readily, and the complexes
observed at the end of the reaction were trans-ArPd(OH)(PPh3)2
(trans-5a,b) which accumulated due to lack of reagent. The fact
that trans-ArPdAr′(PPh3)2 (trans-8ba) could not be observed
when the substituent on one aryl group was MeO, whereas ArAr′
was obtained, suggests that the reductive elimination from trans-
ArPdAr′(PPh3)2 8 via cis-ArPdAr′(PPh3)2 was fast in this case
and that the mechanism of the catalytic cycle may thus involve
the biaryl together with a Pd0 complex (Scheme 11). The peroxo
complex O2Pd(PPh3)2, formed by reaction of Pd0(PPh3)2 with
dioxygen, reacted again with excess 1a to generate 4-MeO-
C6H4-Pd(OH)(PPh3)2, as established in the first part of this work.
Then, the intermediate complex trans-8ba could not be observed
because it was involved in a fast reductive elimination via the
cis-8ba and consequently could not accumulate, as complexes
trans-8bc and trans-8ac did. Similarly, when trans-5a 4-MeO-
C6H4-Pd(OH)(PPh3)2 was reacted with 1b PhB(OH)2 (1.3 equiv),
the mixed complex trans-8ab was not observed (Scheme 10)
but the coupling biaryl 4-MeO-C6H4-C6H5 was produced
together with trans-PhPd(OH)(PPh3)2. This indicates that once
(29) (a) Ishiyama, T.; Murata, M.; Miyaura, N. J. Org. Chem. 1995, 60, 7508-
7510. (b) Miyaura, N. J. Organomet. Chem. 2002, 653, 54-57.
(30) Braga, A. A. C.; Morgon, N. H.; Ujaque, G.; Maseras, F. J. Am. Chem.
Soc. 2005, 127, 9298-9307.
9
J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006 6835

A R T I C L E S
Adamo et al.
Scheme 13. Mechanism Based on Observed 4, 6, trans-5, and
trans-8 Complexes
Scheme 15
of ArB(OH)2 over 3 days. It has been suggested that phenols
could be produced by reacting hydrogen peroxide with arylbo-
ronic acids.9,5f PhOH was indeed generated upon addition of
H2O2 to a solution of PhB(OH)2 in chloroform. This suggests
that H2O2 is produced in the Pd-catalyzed homocoupling of
arylboronic acids in the presence of dioxygen. A reasonable
route for H2O2 production would consist in the subsequent
hydrolysis of HOOB(OH)2 (Schemes 9 and 15).
Conclusions
The palladium-catalyzed homocoupling of arylboronic acids
(4-Z-C6H4-B(OH)2, Z ) MeO, H, CN) in the presence of
dioxygen proceeds via the peroxo complex (η2-O2)PdL2 (L )
PPh3), generated in the reaction of dioxygen with the Pd(0)
catalyst. The peroxo complex is indeed at the origin of the
formation of trans-Ar-Pd(OH)L2 via an activation of one of its
Pd-O bond by the arylboronic acid, followed by a transmeta-
lation step by a second arylboronic acid. trans-Ar-Pd(OH)L2
complexes react with the arylboronic acid to give trans-Ar-
PdArL2 complexes in a second transmetalation step. The biaryl
is formed in a reductive elimination. Because of the common
intermediate Pd0L2, the palladium-catalyzed homocoupling
reaction may compete with the Miyaura-Suzuki cross-coupling
if the work atmosphere is not inert. Work is in progress to extend
this oxidatiVe coupling to other nucleophiles, i.e., organostan-
nanes derivatives. Farina et al. have indeed observed as a side
reaction the homocoupling of arylstannanes in palladium-
catalyzed Stille reactions performed in the presence of dioxy-
gen.31 The palladium-catalyzed homocoupling of organostan-
nanes under oxygen (or air) has been reported later on.32
Scheme 14
trans-5 and trans-8 complexes as intermediate complexes
(Scheme 13).
However, in the real catalytic reactions, the catalytic cycle
may involve only reactive cis complexes. Indeed, at high ArB-
(OH)2 concentrations, the cis-5/trans-5 isomerization (route a
in Scheme 13) may be much slower than the transmetalation
on cis-5 (route b in Scheme 14). The latter reaction would
directly generate the reactive cis-8 complex, prone to undergo
reductive elimination (last step in Scheme 13). An even shorter
pathway to cis-8 in which the intermediate complex cis-7 would
be trapped by ArB(OH)2 cannot be excluded (route c, in Scheme
14).
Mechanism of the Formation of Phenol as Byproduct.
ArOH is often a byproduct in the Pd-catalyzed homocoupling
of arylboronic acids in the presence of dioxygen (Scheme 4).4
We observed that (i) phenol was formed together with the biaryl,
(ii) phenol was not formed from complexes 6a or 6b when they
were observed in absence of complexes 5a and 5b respectively,
(iii) phenol was not formed from trans-PhPd(OH)(PPh3)2 5b,
(iv) phenol was not formed upon bubbling dioxygen in a solution
Acknowledgment. This work has been supported in part by
the Centre National de la Recherche Scientifique (UMR CNRS-
ENS-UPMC 8640, UMR CNRS-ENSCP-UMPC 7575) and the
Ministe`re de la Recherche (Ecole Normale Supe´rieure). We
thank Prof. Rene´ Thouvenot for performing 11B NMR spec-
troscopy at the University Paris VI. Johnson Matthey is thanked
for a generous loan of palladium salt.
Supporting Information Available: Detailed kinetic analysis
of the reaction of O2Pd(PPh3)2 with arylboronic acids 1a-c in
chloroform or DMF, complementary structural information from
DFT calculations and complete ref 12. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA0569959
(31) Farina, R.; Krishnan, B.; Marshall, D. R.; Roth, G. P. J. Org. Chem. 1993,
58, 5434-5444.
(32) (a) Alcaraz, L.; Taylor, R. J. K. Synlett 1997, 791-792. (b) Shirakawa, E.;
Nakao, Y.; Murota, Y.; Hiyama T. J. Organomet. Chem. 2003, 670, 132-
136.
9
6836 J. AM. CHEM. SOC. VOL. 128, NO. 21, 2006