Synthesis of a palladium(IV) complex by oxidative addition of an aryl halide to palladium(II) and its use as precatalyst in a C-C coupling reaction

Article abstract of DOI:10.1002/anie.201102214

Four's a charm: Complex 1·OAc (see scheme) reacts with 2-iodobenzoic acid to afford the stable Pd^IV complexes 2 a and 2 b. Complexes 2 are precatalysts for the orthovinylation of 2-iodobenzoic acid with CH ₂=CHCO₂Me and Ag

Full text of DOI:10.1002/anie.201102214

Communications
DOI: 10.1002/anie.201102214
Palladium Complexes
Synthesis of a Palladium(IV) Complex by Oxidative Addition of an
À
Aryl Halide to Palladium(II) and Its Use as Precatalyst in a C C
Coupling Reaction
Josꢀ Vicente,* Aurelia Arcas, Francisco Juliꢁ-Hernꢁndez, and Delia Bautista
The recent award of the 2010 Nobel Prize in Chemistry to
Heck, Negishi, and Suzuki for their studies on catalytic cross
couplings has recognized the important role of palladium in
organic synthesis.[1,2] Their methods were later improved by
the use of Herrmannꢀs palladacycles as precatalysts.[3] Most of
both types of reactions involve a palladium(0)/palladium(II)
catalytic cycle.[4,5] Palladium(0) species are the precatalysts in
the classical reactions[1] or form in situ from the PdII
precatalyst as a Pd0 complex or as Pd nanoparticles.[6–8]
However, based on various experimental data, PdII/PdIV
catalytic cycles have been proposed as an alternative.[9,10]
Some computational studies also support this proposal.[7,11,12]
However, since the oxidative addition of an aryl halide to a
PdII complex[13] and the existence of PdIV complexes in these
catalytic processes[14] have not yet been conclusively demon-
strated, the PdII/PdIV catalytic cycle has become one of the
most intriguing open problems in catalysis.[15] Herein, we
discuss the first of these two topics.
Recently, we reported the reaction of PdCl2 with 2,6-
diacetylpyridine in MeOH at reflux to give [Pd(O,N,C-L)Cl]
(1·Cl, Scheme 1)[20] which reacts with Cl2 or NaBr to give,
respectively, the stable PdIV complex [Pd(O,N,C-L)Cl3] or
[Pd(O,N,C-L)Br], which, in turn, reacts with Br2 to afford
A few aryl palladium(IV) complexes have been isolated
(or characterized in solution) from aryliodonium salts[16] or
from the oxidation of corresponding aryl palladium(II)
derivatives,[17,18] but never from an aryl halide as required in
the coupling reactions in which a PdII/PdIV catalytic cycle is
invoked, which is one of the weaknesses of the proposal. A
computational study has shown that oxidation of an N^C^N
pincer PdII complex with PhI is a highly endothermic
reaction,[19] while another study shows that not only the
oxidation of various P^C^P pincer PdII complexes with PhBr
but also a PdII/PdIV Heck-type catalytic cycle are viable.[11]
Similar conclusions were drawn from a previous investigation
based on a C^N chelating PdII complex, thus suggesting that
the presence of a weakly coordinating ligand would favor a
PdII/PdIV mechanism.[12] Therefore, the nature of the aryl
halide and the ligands around PdII seem to greatly influence
the viability of the crucial oxidative addition step in this
alternative catalytic cycle.
Scheme 1. Synthesis of complexes 1·OAc, AX, and 2 and a proposal for
the equilibrium between AI and the two isomers of complex 2.
[Pd(O,N,C-L)Br3].[21] The stability of these PdIV complexes
(which was attributed to the pincer ligand) and the weakly
coordinating ability of the MeO group in the PdII complexes
(which was expected to help its oxidation to PdIV (see above))
moved us to attempt the synthesis of a stable and unprece-
dented aryl PdIV complex from an aryl iodide and a [Pd-
(O,N,C-L)X] complex. Because PdIV complexes are thermally
unstable, and because we also set out to use such a PdIV
[*] Prof. Dr. J. Vicente, Prof. Dr. A. Arcas, M. Sc. F. Juliꢀ-Hernꢀndez
Grupo de Quꢁmica Organometꢀlica
Departamento de Quꢁmica Inorgꢀnica, Facultad de Quꢁmica
Universidad de Murcia, Aptdo. 4021, 30071 Murcia (Spain)
E-mail: jvs1@um.es
Homepage: http://www.um.es/gqo/
À
complex as a precatalyst in a C C coupling process, we
Dr. D. Bautista
decided to prepare and test its catalytic ability at room
temperature. We chose as reagents 2-iodobenzoic acid and the
new complex [Pd(O,N,C-L)(OAc)] (1·OAc, Scheme 1), pre-
SAI, Universidad de Murcia, Aptdo. 4021, 30071 Murcia (Spain)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102214.
6896
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Angew. Chem. Int. Ed. 2011, 50, 6896 –6899

pared by treating 1·Cl[20] with AgOAc. Coordination of the
benzoate moiety, after deprotonation, would bring the iodine
atom close to the palladium atom, and this situation, along
with the presence of the electron-withdrawing ortho sub-
stituent,[4,22] would give the oxidative addition reaction a
chance to occur at room temperature. Additionally, the
resulting chelating phenyl benzoato ligand would increase the
stability of the PdIV complex. The product obtained was the
expected PdIV complex [Pd(O,N,C-L)(O,C-2-CO2C6H4)I] (2),
which is indefinitely stable in the solid state. This first PdII to
PdIV oxidative addition using an aryl halide does not contra-
dict studies demonstrating that other aryl halides do not
oxidatively add to other PdII complexes.[23]
coordination,[26] but nothing similar had been found in the
chemistry of Pd. The different nature of the assistant group in
our case (anionic) with respect to those present in PtII
(neutral) merits emphasis.
During this reaction, an intermediate was detected, which
remained in solution until the end. Its CH2 protons resonate
at a value (d = 3.69 ppm) similar to those found for the other
PdII(O,N,C-L) complexes (d = 3.73–3.27 ppm)[20,21] and lower
than those for 2 (dA = 4.95, dB = 4.55 ppm (2JHH = 12.4 Hz))
and its homologues (d = 6.06–6.04 ppm).[21] Therefore, we
propose that this intermediate is the PdII benzoato complex
[Pd(O,N,C-L)(O2C-2-IC6H4)] (AI, Scheme 1), which is also
detected when 2 is dissolved in CDCl3. Additional evidence
on the nature of AI was obtained when, while attempting to
prepare the Br homologue of 2 at room temperature by
treating 1·OAc with 2-bromobenzoic acid, we isolated only
the complex ABr (Scheme 1 and Figure 1), which we fully
characterized, including by X-ray crystallography.
The 1H NMR spectrum of a CDCl3 solution of 2 at room
temperature shows the presence of traces of two unidentified
decomposition products after 5 h and 24% decomposition
30 h later. Complexes 1·OAc and 2a were characterized by X-
ray diffraction (Figure 1), NMR spectroscopy, and elemental
analyses. The crystal structure of 1·OAc showed it to be a
The 1H NMR spectrum of 2 was studied in CDCl3 between
À55 and 358C, its upper limit of stability (see the Supporting
Information), and a slow 2QAI equilibrium on the NMR time
scale was detected. The 2:AI molar ratio decrease from
approximately 9:1 in the range À55 to À58C to about 2.3:1 at
258C and 1.9:1 at 358C. Furthermore, the AB system
corresponding to the CH2 protons in 2 at room temperature
coalesces at À208C and splits into two AB systems below
À358C, thus indicating the existence of the two possible PdIV
geometric isomers 2a and 2b in equilibrium (Scheme 1; 1.5:1
molar ratio; 2a: dA = 4.84, dB = 4.71 ppm (2JHH = 12 Hz); 2b:
dA = 5.08, dB = 4.38 ppm (2JHH = 13.2 Hz)), which probably
À
dimer with bridging acetato ligands (Figure 1); the Pd Pd
interconvert through the unobserved intermediate
B
(Scheme 1).[27] The isomer with the greater dAÀdB value is
assigned as that bearing the iodo ligand trans to the aryl group
(2b), which shields the nearest CH2 proton. A line-shape
analysis of the CH2 proton resonances of the equilibrium
2aQ2b did not allow the determination of its activation
parameters. Scheme 1 shows a proposal to account for the
formation of 2a and 2b and their equilibrium. Oxidative
addition reactions giving two PdIV geometrical isomers have
been reported,[18,28] but those species were not in equilibrium.
=
Complex 2 did not react with CH2 CHCO2Me in
[D6]acetone at room temperature, but when it was treated
=
with two equivalents CH2 CHCO2Me in [D6]acetone in the
presence of one equivalent AgClO4 to remove the iodo ligand
at room temperature, the desired (E)-methyl 2-carboxycin-
namate (3) was quantitatively obtained in less than 1 h
(Scheme 2). The Heck synthesis of 3 using the corresponding
diazonium salts instead of 2-iodobenzoic acid has been
reported.[29] Complex 2 is a precatalyst for this reaction
using 10% of the stoichiometric amount (83% yield of 3 after
Figure 1. X-ray crystal structures of complexes 1·OAc, 2a, and ABr.[31]
separation of 3.0315(5) ꢁ[24] is significantly longer than twice
1
the covalent radius of Pd (1.39 ꢁ).[25] However, its H and
13C NMR spectra (in particular, the MeO resonances) are
similar to those of other Pd(O,N,C-L) complexes and differ-
ent from those of complexes containing the chelating ligand
N,C-L,[20] thus suggesting that in solution, the structure of
1·OAc is that shown in Scheme 1. Oxidative addition of aryl
halides to PtII complexes has been reported to be assisted by
Scheme 2. Catalytic synthesis of 3 using complex 2 as precatalyst.
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Communications
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À
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À
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=
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À
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Received: March 30, 2011
Revised: May 12, 2011
Published online: June 8, 2011
À
Keywords: C C coupling · homogeneous catalysis ·
oxidative addition reactions · palladium · structure elucidation
.
À
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