Synthesis of Surface Organopalladium Intermediates in Coupling Reactions: The Mechanistic Insight

Article abstract of DOI:10.1021/ja038270f

The design and synthesis of surface transient organometallic intermediates on nanopalladium supported on layered double hydroxides is conceived and developed for the first time. The formation of only one STO intermediate in all the Heck-, Suzuki-, Sonogas

Full text of DOI:10.1021/ja038270f

Published on Web 02/10/2004
Synthesis of Surface Organopalladium Intermediates in Coupling Reactions:
The Mechanistic Insight
Boyapati M. Choudary,*,† Sateesh Madhi, Mannepalli L. Kantam, Bojja Sreedhar, and
†
†
†
‡
Yasuhiro Iwasawa
Inorganic Chemistry DiVision, Indian Institute of Chemical Technology, Hyderabad 500 007 India, and Department
of Chemistry, Graduate School of Science, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033 Japan
Received September 2, 2003; E-mail: choudary@iict.res.in
Palladium-catalyzed coupling of haloarenes by Heck-, Suzuki-,
Sonogashira-, and Stille-type reactions is a widely used methodol-
ogy in modern organic synthesis, because the coupling products
find good applications as intermediates in the preparation of
Scheme 1. Validated Mechanistic Cycle for the Coupling
Reactions
1
2
3
materials, natural products, and bioactive compounds. Although
4
5-7
kinetic, electrospray mass spectrometry,
NMR and cyclic
voltammetry,8 and IR studies suggest the plausible mechanistic
path based on the identification of some of the transient species
formed in the homogeneous catalytic reactions, no serious attempt
is made to isolate and characterize the intermediates to validate
the mechanism. Moreover, it is a very difficult task to isolate or
extract molecular organometallic intermediates from the homoge-
neous reaction, while the surface transient organometallic inter-
mediates formed in the heterogeneous catalysis can be isolated by
simple filtration. However, the greatest challenge lies in the
conceptual transfer of molecular organometallic chemistry to surface
organometallic chemistry to achieve single-site industrially desired
heterogeneous catalyst, and to realize high order of activity and
selectivity in the chemical reactions.11 In an effort to validate the
mechanism of the C-C coupling involving Heck-, Suzuki-,
Sonogashira-, and Stille-type reactions, and evolution of single site
catalysis, we disclose here the preparation of surface transient
organometallic (STO) intermediates sequentially for each of the
C-C coupling reactions of chloroarenes on layered double hy-
,9
10
hereafter for the sake of clarity.14 The XPS spectra of Cl 2p exhibits
three lines on deconvolution at 197.6, 198.4, and 199.6 eV, which
are assigned to the interlayer LDH-Cl, PdCl, and chloride present
as impurity, respectively. Deconvoluted C 1s XPS spectrum of the
(HK2) displays four lines at 284.2, 285.3, 288.8, and 292.0 eV
0
droxides (LDH)-Pd and characterization by XPS and TGA-MS
for the first time.
The oxidative addition of haloarenes to Pd is the first step in
0
(methoxy carbon), while (HK2′) displays three lines at 285.3, 287.1,
these coupling reactions (see the Supporting Information). LDH-
MeOC H PdCl (OA1) and LDH-PhPdCl (OA1′) freshly prepared
6 4
and 289.0 eV. As the specific assignment is difficult, these samples
were subjected to TGA-MS to detect the evolved gas fragments
as a function of temperature.15 The observed m/z values for (HK2)
are 15, 30, 36, 51, 90, 91, 103, 104, and 107 amu corresponding to
and well characterized by the reaction of 4-chloroanisole and
chlorobenzene, respectively, with the defect sites of the nanopal-
ladium particles (4-6 nm) immobilized on layered double hydrox-
0
12
2 4 3 7 6 7 7 8 7 8 8 6 4
Me, CH O, Cl, C H , C H , C H , C H , C H , and MeOC H ,
ides (LDH-Pd ) are used directly in the succeeding C-C coupling
reactions to isolate and elucidate the structures of sequentially
synthesized STO intermediates unambiguously in the reaction
path.13
respectively.16 The tropilium radical cation with one unit less is
confirmed, which corresponds to phenyl flanked to methoxy group.
On the other hand, the observed m/z values 51, 77, 91, and 104
The reaction of STO intermediates (OA1) and (OA1′) with excess
styrene gives rise to surface transient σ-complex (HK2) and (HK2′),
respectively, in quantitative yields, obtained from initially formed
π-complex via π-σ transition because of the faster π-σ transition,
which is in consonance with earlier observations (Scheme 1). High-
resolution XPS spectra of Pd 3d on deconvolution for both the
surface transient intermediates (HK2) and (HK2′) show a set of
two lines each at 336.4 eV (3d5/2), 342.2 eV (3d3/2) and 336.3 eV
4 3 7 7
amu for (HK2′), are attributed to C H (from phenyl), phenyl, C H
16
(tropilium), and C H , respectively. On heating, both the surface
8
8
transient species (HK2) and (HK2′) at 100 °C in N-methyl
pyrrolidinone (NMP) solvent give trans-4-methoxystilbene and
trans-stilbene, respectively (see the Supporting Information), which
reinforces the structure of STO intermediates as described.
Extended X-ray absorption fine structure (EXAFS) indicates (see
the Supporting Information) that the average coordination numbers
(
3d5/2), and 342.4 eV (3d3/2), respectively, in addition to the lines
0
of LDH-Pd fresh (CN ) 7.6) and used (CN ) 8.7) are much
0
for Pd {335.0 eV (3d5/2) and 340.2 eV (3d3/2)}, which are observed
lower than that of palladium foil (CN ) 12), and we may assume
that palladium is in nanoparticle form, which is reconfirmed by
TEM.12 Pd-Pd bond distances obtained from the EXAFS analysis
in all the STO intermediates which we will not be describing
†
Indian Institute of Chemical Technology.
The University of Tokyo.
‡
0
(2.739 Å for LDH-Pd fresh; 2.76 Å for used) are slightly shorter
2292
9
J. AM. CHEM. SOC. 2004, 126, 2292-2293
10.1021/ja038270f CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
than or similar to the usual value for palladium foil (2.76 Å). This
bond feature is also compatible with the formation of nanopar-
ticles.1
Next, the palladium(0)-catalyzed cross-coupling reaction of aryl
halides and arylboronic acids (Suzuki coupling), an important
synthetic method, is studied. The reaction of STO species (OA1)
and (OA1′) with potassium fluoride in more than equivalent
amounts (see the Supporting Information) to the STO intermediate
addition to the 284.6 eV as in LDH-Cl. The m/z values of (ST2)
are observed at 43 and 44 amu, which correspond to OAc and CO
respectively. The reaction of STO intermediate (ST2) with tribu-
tyltin hydride provides STO intermediate PhPdSnBu (ST3), which
2
,
7,18
3
is isolated and well characterized by XPS and TGA-MS. The high-
resolution narrow scans of STO species (ST3) for Sn and Pd (II)
show 3d5/2 lines at 485.7 and 336.9 eV and 3d3/2 lines at 494.1 and
339.4 eV, respectively, which indicate the presence of a Pd-Sn
bond. No residuum of the earlier STO intermediate (ST2) is
observed, which clearly indicates the complete conversion. The
deconvoluted C 1s XPS spectrum of (ST3) displays four lines at
284.2, 285.0, 286.3, and 288.7 eV. The m/z values observed in the
TGA-MS are 15, 26, 29, 43, 51, 54, 56, 57, and 77 amu, which
6 4
gives only the surface transient species MeOC H PdF (SZ2) and
PhPdF (SZ2′), respectively (Scheme 1). When potassium fluoride
is used in 0.5 M equivalent, the unreacted STO intermediates (OA1)
and (OA1′) are also detected.
The XPS narrow scans of Pd in (SZ2) and (SZ2′) show a shift
in the binding energy of ∼3 eV, which is attributed to the highly
electronegative fluoride (F 1s line at 685.3 eV) and clearly indicates
the presence of a Pd-F bond.19 The m/z values for (SZ2) and (SZ2′)
in the TGA-MS are 15, 19, 31, 76, 107 amu and 19, 51, 77 amu,
correspond to Me, C
2
H
2
6
, C
2
H
5
, C
3
H
7
, C
4
H
3
, C
4
H
6
, C
4
H
8
, C
4
H
9
,
and C
H
6 5
, respectively.1 On heating the STO species (ST3) at 50
°C in NMP solvent gives tributylarylstannane (Supporting Informa-
tion), which reinforces the structure of STO intermediate as
described.
which correspond to Me, F, OMe, C
6 4 6 4 4 3
H , MeOC H and F, C H ,
, respectively.16 Further, the reaction of STO intermediate
The formation of only one STO intermediate in all the coupling
reactions during their reaction sequences and the excellent isolated
yields of each of the coupling product (isolated yields 85-95% of
the theoretical yield, see the Supporting Information) from the
corresponding supported complexes, HK2, SZ3, SG2, and ST3
establish that these supported complexes are the key intermediates
of the coupling reactions. This demonstrates the evolution of the
single-site heterogeneous catalyst by transfer of molecular orga-
nometallic chemistry to surface organometallic chemistry that is
responsible for high order of activity.12
C H
6 5
(
SZ2) and (SZ2′) with phenylboronic acid affords tricoordinated
surface transient organometallic species MeOC PhPdB(OH)
SZ3) and PhPhPdB(OH) (SZ3′), respectively, which are isolated
H
6 4
2
(
2
and characterized by XPS and TGA-MS. Along with the charac-
teristic XPS binding energy peaks for Pd (II) (337.5 and 342.3 eV),
both the STO intermediates (SZ3) and (SZ3′) exhibit B 1s peak at
92.6 eV, which is assigned to Pd-B.20 The deconvoluted O 1s
1
XPS spectrum of (SZ3) and (SZ3′) displays one extra line at 532.6
21
eV, which is attributed to B(OH)
2
.
There is no remnant of the
Acknowledgment. S.M. thanks the CSIR, India, for the award
of SRF. We also thank Prof. M. Beller and Prof. K. Kaneda for
prereviewing our manuscript.
Supporting Information Available: Full characterization of all
surface transition organometallic intermediates with detailed experi-
mental procedures (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
earlier STO intermediates detected, which indicates complete con-
version. The observed evolved gas fragments for (SZ3) and (SZ3′),
when subjected to TGA-MS, are 15, 17, 19, 26, 31, 45, 51 and
4
5, 51, 77 amu, respectively, and correspond to Me, OH, F, C
, respectively.16 On
OMe, B(OH) , C and B(OH) , C , C
heating, both the transient species (SZ3) and (SZ3′) at 100 °C in
,4-dioxane/water solvent system give 4-methoxybiphenyl and bi-
2 2
H ,
2
4
H
3
2
H
4 3
6 5
H
1
phenyl, respectively (see the Supporting Information), which
reinforces the structure of transient intermediate species as indicated
above.
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(
(
(
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(
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4
H
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,
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STO intermediate as described above.
(
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(
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(
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