Equilibrium study of Pd(dba)2 and P(OPh)3 in the Pd-catalyzed allylation of aniline by allyl alcohol

Article abstract of DOI:10.1021/om4009873

Reaction of Pd(dba)₂ and P(OPh)₃ shows a unique equilibrium where the Pd[P(OPh)₃]₃ complex is favored over both Pd(dba)[P(OPh)₃]₂ and Pd[P(OPh)₃] ₄ complexes at room temperature. At a lower temperature, Pd[P(OPh)₃]₄ becomes the most abundant complex in solution. X-ray studies of Pd[P(OPh)₃]₃ and Pd(dba)[P(OPh)₃]₂ complexes show that both complexes have a trigonal geometry with a Pd-P distance of 2.25 A due to the π-acidity of the phosphite ligand. In solution, pure Pd(dba)[P(OPh)₃] ₂ complex equilibrates to the favored Pd[P(OPh)₃] ₃ complex, which is the most stable complex of those studied, and also forms the most active catalytic species. This catalyst precursor dissociates one ligand to give the reactive Pd[P(OPh)₃]₂, which performs an oxidative addition of nonmanipulated allyl alcohol to generate the π-allyl-Pd[P(OPh)₃]₂ intermediate according to ESI-MS studies.

Full text of DOI:10.1021/om4009873

Article
pubs.acs.org/Organometallics
Equilibrium Study of Pd(dba)2 and P(OPh)3 in the Pd-Catalyzed
Allylation of Aniline by Allyl Alcohol
Supaporn Sawadjoon,† Andreas Orthaber,‡ Per J. R. Sjoberg,† Lars Eriksson,§ and Joseph S. M. Samec*,†
̈
†Department of Chemistry, BMC, Uppsala University, Box 576, 751 23 Uppsala, Sweden
‡Department of Chemistry, Ångstrom Laboratories, Uppsala University, Box 523, 751 20 Uppsala, Sweden
̈
§Department of Structural Chemistry, Stockholm University, 106 91, Stockholm, Sweden
S
* Supporting Information
ABSTRACT: Reaction of Pd(dba)2 and P(OPh)3 shows a unique
equilibrium where the Pd[P(OPh)3]3 complex is favored over
both Pd(dba)[P(OPh)3]2 and Pd[P(OPh)3]4 complexes at room
temperature. At a lower temperature, Pd[P(OPh)3]4 becomes the
most abundant complex in solution. X-ray studies of Pd[P(OPh)3]3
and Pd(dba)[P(OPh)3]2 complexes show that both complexes have
a trigonal geometry with a Pd−P distance of 2.25 Å due to the π-acidity of the phosphite ligand. In solution, pure Pd(dba)-
[P(OPh)3]2 complex equilibrates to the favored Pd[P(OPh)3]3 complex, which is the most stable complex of those studied, and also
forms the most active catalytic species. This catalyst precursor dissociates one ligand to give the reactive Pd[P(OPh)3]2, which
performs an oxidative addition of nonmanipulated allyl alcohol to generate the π-allyl-Pd[P(OPh)3]2 intermediate according to
ESI-MS studies.
Amatore and Jutand have in several studies shown that the dba
(E-dibenzylidene acetone) ligand is not as labile as expected in
a mixture of Pd(dba)2 and triphenylphosphine (PPh3) ligand
(eq 1).8 For a complete dissociation of dba, 50 equiv of
PPh3 to Pd was required according to UV spectroscopy.
Interestingly, the electronic property of the phosphorus-
based ligand has a profound effect on the equilibrium.9
When 1-phenyldibenzophosphole (DBP) was used as ligand,
the equilibrium is shifted opposite, in favor of Pd(DBP)4, which
was slower to initiate an oxidative addition reaction than
the Pd(dba)(DBP)2 complex.10 Several examples of Pd(dba)Ln
(n = 1, 2) complexes of dba and phosphorus ligands have been
isolated and characterized.11
he palladium-catalyzed allylic substitution via a π-allylpalladium
Tintermediate, known as the Tsuji−Trost reaction, is one
of the most powerful methods for constructing carbon−carbon
and carbon−heteroatom bonds in organic synthesis (Scheme 1).1
Scheme 1. Palladium-Catalyzed Allylic Amination with Allyl
Substrates
The reaction has been carried out using palladium and various
allyl sources such as halides,2a esters,2b,c ethers,2d and carbon-
ates.2e The direct catalytic substitution of allyl alcohol has
recently attracted considerable attention for its environmental
and economic advantages.3 However, it is generally difficult
to cleave the C−O bond because of the poor leaving group
ability of the hydroxyl group.4 The palladium-catalyzed direct
substitution of allyl alcohols generally requires activation by a
Lewis acid.4,5 Recently, palladium complexes bearing strong
π-acceptor ligands (bisphosphaalkene and triphenylphosphite)6
have been reported to achieve the transformation without the
use of activators. In 2004 Ikariya reported that Pd[P(OPh)3]4
catalyzed the direct allylic substitution of allyl alcohol by
various nucleophiles. The catalyst was generated either from
PdCl2(MeCN)2 in the presence of NEt3 and P(OPh)3 or in situ
from tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3]
and P(OPh)3 in a 1:4 molar ratio.6c
Pd(dba)(PPh3)2 + PPh3 ⥃ Pd(PPh3)3 + dba
(1)
The reactivity of pure and in situ generated complexes may
differ. For example, the oxidative addition of phenyl iodide by
a mixture of Pd(dba)2 and 4 equiv of PPh3 proceeded at an
overall rate 10 times lower than for isolated Pd(PPh3)4.8a In
addition, several research groups have observed that in situ
prepared Pd(dba)(PPh3)2 shows a higher reactivity in certain
reactions than both in situ prepared and isolated Pd(PPh3)4.8a,12
Mechanistic studies, including ESI-MS experiments, have
shown that the dba ligand may act as a ligand in the Heck
reaction.13 In the catalytic procedure developed by Ikariya using
in situ prepared Pd[P(OPh)3]4, different palladium species may
be present in the reaction mixture (eq 2). We were interested
in how the electron-deficient triphenylphosphite ligand would
affect the equilibrium as compared to PPh3 and DBP. We were
The air-stable palladium(0)complexes [Pd2(dba)3] and bis-
(dibenzylideneacetone)palladium(0) Pd(dba)2 have been used as
convenient sources of Pd(0) in palladium-catalyzed reactions.7
Received: October 10, 2013
© XXXX American Chemical Society
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also interested in investigating whether Pd(dba)[P(OPh)3]2
or Pd[P(OPh)3]4 is the most efficient precursor to generate
the active catalytic species in an allylation of unactivated allyl alcohol
and to determine whether Pd[P(OPh)3]2 or Pd(dba)P(OPh)3
is the active catalyst. To our knowledge, the corresponding
study performed by Jutand and Amatore has not been performed
on π-acidic triphenylphosphite ligands. Taking into account the
importance of such ligands in the allylation of unactivated allyl
alcohols motivated us to study this equilibrium.14
broadened signals could correspond to either Pd[P(OPh)3]3
(Figure 1b), Pd(dba)[P(OPh)3]2 (Figure 1c), free P(OPh)3
(Figure 1a), or a new complex and were therefore difficult to
interpret. Above 8 equiv of P(OPh)3 to Pd, only one broadened
signal in the 31P NMR spectrum at δ 130.0 ppm was observed
(Figure 1f). A similar spectrum was observed when isolated
Pd[P(OPh)3]3 and P(OPh)3 were mixed.
The broadened signals observed at a 1:4 ratio between
Pd(dba)2 and P(OPh)3 could have different explanations (vide
supra). Therefore, the probe was cooled and the temperature
effect of the equilibrium was studied by 31P NMR spectroscopy.
At −20 °C the two signals separated to give two chemical shifts
at δ 127.2 and 138.5 ppm (Figure 2). At this temperature, both
Pd(dba)2 + 4L ⇌ Pd(dba)L2 ⇌ PdL4 + 2dba
(2)
L = P(OPh)3
An interesting difference between the triphenylphosphine
and triphenylphosphite complexes of palladium was the num-
ber of ligands coordinated to the metal. In the phosphine palladium
complex either three or preferably four ligands are coordinated to
the palladium in Pd(PPh3)3 or Pd(PPh3)4, respectively. In contrast,
the triphenylphosphite palladium complex has only three ligands
coordinated to the palladium in Pd[P(OPh)3]3 (vide infra) at
room temperature. To study the equilibrium between Pd(dba)2
and triphenylphosphite P(OPh)3, Pd(dba)2 and the P(OPh)3
were mixed at different concentrations and analyzed by 31P
NMR spectroscopy (Figure 1). At a 1:2 ratio between Pd(dba)2
Figure 2. VT-31P{1H} NMR spectra (121 MHz) performed in 0.6 mL
of C7D8 Pd(dba)2 (15 mM) + P(OPh)3 (60 mM) with H3PO4 as an
external standard at temperatures between 25 and −60 °C.
signals were sharper than at room temperature. The ratio of the
integral of the two chemical shifts shifted from 7:3 (at 20 °C in
favor of the signal at δ 138.5 ppm) to 1:1 at −20 °C. At −40 °C
the signal at δ 127.2 ppm split into two different signals at
δ 126.1 and 127.1 ppm. The ratio between the three signals at
δ 126.1, 127.1, and 139.2 ppm was 1:2:1. At −60 °C the signals
further separated to give new chemical shifts at δ 125.4, 127.4,
and 139.4 ppm. The ratio between these signals was 4:13:1.
We propose that the signal at δ 127.4 ppm (−60 °C) cor-
responds to the Pd[P(OPh)3]4 complex. At higher temperatures,
this complex is in an equilibrium with Pd[P(OPh)3]3 and
P(OPh)3, giving rise to a broadened signal. At low temperatures,
the tetracoordinated complex is more stable than the Pd[P(OPh)3]3
complex.
Figure 1. 31P{1H} NMR spectra (121 MHz) performed in 0.6 mL
of C6D6 with H3PO4 as an external standard: (a) P(OPh)3 (b)
Pd[P(OPh)3]3, (c) Pd(dba)[P(OPh)3]2, (d) Pd(dba)2 (15 mM) +
P(OPh)3 (30 mM), (e) Pd(dba)2 (15 mM) + P(OPh)3 (60 mM), (f)
Pd(dba)2 (15 mM) + P(OPh)3 (120 mM).
Pd(dba)[P(OPh)3]2 was prepared by addition of 2 equiv of
P(OPh)3 to Pd(dba)2 in dry, degassed CH2Cl2 at room tem-
perature under argon. The complex was purified by inert column
chromatography and recrystallized from a saturated solution of
pentane/CH2Cl2 at −20 °C, affording yellowish-green X-ray-quality
single crystals in a 74% yield. The compound was air sensitive
and decomposed after a few seconds in air and also in solution
under inert conditions. The formulation of Pd(dba)[P(OPh)3]2
was confirmed by single-crystal X-ray crystallography. The
molecular structure of the complex is shown in Figure 3. The
central Pd(0) atom is coordinated in a trigonal planar way with
respect to the two phosphorus atoms and the center of the two
carbon atoms (η2-coordination). The corresponding angles are
P−Pd−P 105.1(2)° and P−Pd−C 102.7(5)° and 115.4(5)°.
The arrangement has small deviations of the atoms from the
and P(OPh)3, a 1:3 ratio between Pd(dba)[P(OPh)3]2 and
Pd[P(OPh)3]3 was observed. The 31P NMR spectrum of the
mixture of Pd(dba)2 with 2 equiv of P(OPh)3 shows three signals
(Figure 1d): one signal at δ 138.8 ppm, which was characteristic
of Pd[P(OPh)3]3, as shown in Figure 1b, and two broad signals
(Δν1/2 = 61 Hz) of equal magnitude at δ 137.6 and 133.4 ppm,
which correspond to the two nonequivalent phosphorus
atoms in Pd(dba)[P(OPh)3]2, as shown in Figure 1c. This
was different compared to the corresponding study using PPh3,
in which only the corresponding Pd(dba)[PPh3]2 complex was
observed at the same ratio.8 At a 1:4 ratio between Pd(dba)2
and P(OPh)3, broadening of the signals in the 31P NMR
spectrum was observed (Figure 1e). Unfortunately, the
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hence very similar to the free ligand. The P(OPh)3 units are
arranged in a paddle wheel structure. As expected, the three
phosphorus atoms of Pd[P(OPh)3]3 are identical and imply a
high molecular symmetry (Figure 4); thus the 31P NMR spectrum
exhibits one signal for the complex (Figure 1) The steric
hindrance by the bulkier triphenylphosphite ligand may explain
why three and not four ligands are the preferred coordination
to the metal as compared to PPh3.
The two pure complexes were evaluated in the allylation of
aniline by allyl alcohol under identical reaction conditions
(Figure 5). The measurements of the catalytic activity of pure
Figure 3. ORTEP representation of Pd(dba)[P(OPh)3]2 at thermal
ellipsoids of 50%. Hydrogen atoms and CH2Cl2 solvent molecules are
omitted for clarity. Selected bond lengths [Å] and angles [deg]: Pd1−
C97 2.126(15), Pd1−C98 2.159(15), Pd1−P1 2.247(5), Pd1−P2
2.254(5), C97−C98 1.348(19), C97−Pd1−C98 36.7(5), P1−Pd1−P2
105.14(17), C97−Pd1−P2 102.7(5), C98−Pd1−P1 115.4(5).
least-squares (l.s.) plane (0.05 Å rms). The Pd−P bond
distances are in the range 2.244(5)−2.251(5) Å, which is
considerably shorter than in analogous palladium phosphine
complexes (2.277−2.348 Å)15 and similar to an example of a
dinuclear palladium bistriphenylphosphite complex (2.251−
2.271 Å) in accordance with the π-acidity of the triphenyl-
phosphite ligand.16 The P−O distances are 1.585(4)−1.621(4) Å.
The P(OPh)3 units are arranged in a paddle wheel structure.
The bond distance of the coordinated olefin (C97C98 of
1.348(19) Å) is similar to the free olefin (C101C102 of
1.329(18) Å) but significantly shorter than in the related
Pd(dba)[PPh3]2 (CC of 1.395(7) Å).17 As expected, the two
phosphorus atoms of Pd(dba)[P(OPh)3]2 are crystallographi-
cally nonequivalent (Figure 3); thus the 31P NMR spectrum
exhibits two broad signals for the complex (Figure 1).
Pd[P(OPh)3]3 was prepared in accordance with a literature
procedure.6c Recrystallization of the complex from cold acetone
afforded white X-ray-quality single crystals in 55% yield. The
solid-state structure of Pd[P(OPh)3]3 was confirmed by a
single-crystal X-ray crystallographic study. The molecular
structure of the complex is shown in Figure 4. The central
Figure 5. Pd(dba)[P(OPh)3]2 and Pd[P(OPh)3]3 complexes in the
allylation of aniline by allyl alcohol. Reaction conditions: allyl alcohol
(0.840 M), aniline (0.209 M), Pd catalyst (2 mol %), C6D6, 60 °C.
Pd(dba)[P(OPh)3]2 and Pd[P(OPh)3]3 showed that the
Pd(dba)[P(OPh)3]2 complex exhibited a 70% lower reactivity
than the Pd[P(OPh)3]3 complex. This may suggest that a fast
equilibrium of the Pd(dba)[P(OPh)3]2 complex and Pd[P(OPh)3]3
was operating. In fact, this would correspond to the equilibrium
study where the observed ratio of Pd(dba)[P(OPh)3]2 and
Pd[P(OPh)3]3 was 1:3 at a Pd(dba)2:P(OPh)3 ratio of 1:2 (vide
supra, Figure 1d). Thereby, the reactivity of Pd(dba)[P(OPh)3]2
(70% of the reactivity for pure Pd[P(OPh)3]3) was within experi-
mental error of what was expected for the actual concentration
of Pd[P(OPh)3]3 (75%) at a Pd(dba)2:P(OPh)3 ratio of 1:2. It
should be noted that the pure complex and the in situ prepared
complex showed equal reactivity. Furthermore, excess triphenyl-
phosphite ligand (16 equiv) did not inhibit the reactivity as
compared to the DBP ligand.10
ESI-MS monitoring of the palladium-catalyzed allylic
substitution of allyl alcohol (Figures 6 and 7) was used to
Figure 4. ORTEP representation of Pd[P(OPh)3]3 at thermal
ellipsoids of 50%. Only one of the two independent units is displayed.
Hydrogen atoms and acetone solvent molecules are omitted for clarity.
Selected bond lengths [Å] and angles [deg]: P1−Pd1 2.2498(14),
P2−Pd1 2.2525(14), P3−Pd1 2.2573(14), P1−Pd1−P2 116.03(5),
P1−Pd1−P3 120.93(5), P2−Pd1−P3 122.94(5).
Figure 6. ESI mass spectrum of reaction of Pd(dba)[P(OPh)3]2 + allyl
alcohol after 5 min at 60 °C.
Pd(0) atom is coordinated in a trigonal planar fashion with only
small deviations of the atoms from the l.s. plane (0.001 Å for Pd).
The Pd−P bond distances are in the range 2.2431(3)−
2.2572(3) Å, which is considerably shorter than in analogous
palladium phosphine complexes15 and similar to an example
of a dinuclear palladium bistriphenylphosphite complex in
accordance with the π-acidity of the triphenylphosphite
ligand.16 The P−O distances are 1.6087(2)−1.6373(2) Å and
determine whether Pd[P(OPh)3]3 and Pd(dba)[P(OPh)3]2
generated the same reactive catalyst.13,18 Samples were
withdrawn from the reaction mixture of either Pd(dba)-
[P(OPh)3]2 or Pd[P(OPh)3]3 together with allyl alcohol after
5 min reaction time. The ESI(+)-MS spectrum shows the same
C
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ASSOCIATED CONTENT
* Supporting Information
■
S
Full experimental section, including characterization of com-
pounds as well as CIF files giving X-ray data. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: joseph.samec@kemi.uu.se.
■
Notes
The authors declare no competing financial interest.
Figure 7. ESI mass spectrum of reaction of Pd[P(OPh)3]3 + allyl
alcohol after 5 min at 60 °C.
ACKNOWLEDGMENTS
Financial support from the Swedish Research Council and Stiftelsen
■
Olle Engkvists Byggmastare is gratefully acknowledged. We thank
̈
Prof. Adolf Gogoll for fruitful discussions. A.O. is grateful to
signals that correspond to π-allylpalladium intermediates
for both catalyst precursors but at different intensities (all
m/z values are given for the most abundant 106Pd complex).
These species were characterized by collision-induced
dissociation via ESI(+)-MS/MS experiments (SI). The
identical ESI-MS patterns (SI) of allylic substitution with
both Pd(dba)[P(OPh)3]2 (Figure 6) and Pd[P(OPh)3]3
(Figure 7) indicate that Pd[P(OPh)3]2 is generated from
Pd(dba)[P(OPh)3]2 by loss of dba to generate Pd[P(OPh)3]2
as the active species for the allylation reaction. Interestingly, the
reaction mixture initially containing Pd[P(OPh)3]3 gives higher
intensities of the active π-allyl-Pd[P(OPh)3]2 complex and less
of the decomposed π-allyl-PdP(OPh)3P(OH)(OPh)2 complex
compared to the reaction mixture initially containing Pd(dba)-
[P(OPh)3]2, and this may reflect their relative stabilities in
solution.
In conclusion we have isolated pure Pd(dba)[P(OPh)3]2 and
Pd[P(OPh)3]3 complexes. The equilibrium between Pd(dba)2
and P(OPh)3 has been studied. Pd[P(OPh)3]3 is the favored
complex in solution at room temperature. At lower temperatures,
the Pd[P(OPh)3]4 is the favored complex. Both Pd(dba)-
[P(OPh)3]2 and Pd[P(OPh)3]3 complexes generate Pd[P-
(OPh)3]2, which is the active species in catalysis; however
Pd[P(OPh)3]3 is more stable.
the Austrian Science Fund (FWF) for an Erwin-Schrodinger
fellowship (Proj. No. J3193-N17).
̈
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EXPERIMENTAL SECTION
■
Preparation of Pd(dba)[P(OPh)3]2. A flame-dried Schlenk tube
was charged with Pd(dba)2 (40 mg, 0.0696 mmol), dissolved in
0.4 mL of CH2Cl2, and P(OPh)3 (36 μL, 0.139 mmol) was added via
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−20 °C, affording yellowish-green air-sensitive crystals. 1H NMR
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dx.doi.org/10.1021/om4009873 | Organometallics XXXX, XXX, XXX−XXX