Co-catalysis over a tri-functional ligand modified Pd-catalyst for hydroxycarbonylation of terminal alkynes towards α,β-unsaturated carboxylic acids

Article abstract of DOI:10.1039/c9gc01887e

An amphiphilic tri-functional ligand (L1) containing a Lewis acidic phosphonium cation, a phosphino-fragment and a hydrophilic sulfonate anion (-SO₃^-) enabled Pd(OAc)₂ to efficiently co-catalyze the hydroxycarbonylation of terminal alkynes towards α,β-unsaturated carboxylic acids. These incorporated functional groups synergistically promoted the reaction, which proved more effective than the ligands lacking -SO₃^- and/or phosphonium and the mechanical mixtures of the individual functional groups independently. The molecular structure of Pd-L1 indicated that -SO₃^- in L1 served as a secondary O-donor ligand with reversible coordinating ability, cooperating with the phosphino-fragment to stabilize the Pd-catalyst. The in situ FT-IR analysis verified that the formation and stability of Pd-H active species in charge of hydroxycarbonylation were dramatically facilitated by the presence of L1. It was believed that, over the L1-based Pd-catalyst, H₂O was cooperatively activated by the Lewis acidic phosphonium via "acid-base pair" interaction (H₂O → P(v)⁺) and by the hydrophilic SO₃^-via hydrogen bonding (SO₃^-?H₂O), giving rise to the formation of dimeric and mono-nuclear Pd-H species driven by reversible SO₃^--coordination. In addition, the L1-based Pd-catalyst could be immobilized in the ionic liquid [Bmim]NTf₂ for six-run recycling uses without obvious activity loss and detectable metal leaching.

Full text of DOI:10.1039/c9gc01887e

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DOI: 10.1039/C9GC01887E
Graphical abstract
Pd(OAc)2-L1
R
No auxilary additive
OH
R
OH
R
+
+
CO H O
2
+
O
O
Me Me
O
+
SO3-
PPh2 Ph2 P
L1 (tri-functional ligand)

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DOI: 10.1039/C9GC01887E
ARTICLE
Co-catalysis over tri-functional ligand modified Pd-catalyst for
hydroxycarbonylation of terminal alkynes towards α,β-
unsaturated carboxylic acids
Received 00th January 20xx,
Accepted 00th January 20xx
Da Yang, Huan Liu, Lei Liu, Wen-Di Guo, Yong Lu and Ye Liu*
DOI: 10.1039/x0xx00000x
An amphiphilic tri-functional ligand (L1) containing a Lewis acidic phosphonium cation, a phosphino-fragment and a
-) enabled Pd(OAc)
hydrophilic sulfonate anion (-SO to efficiently co-catalyze the hydroxycarbonylation of terminal alkynes
3 2
towards α,β-unsaturated carboxylic acids. These incorporated functional groups synergistically promoted the reaction,
-
which proved more effective than the ligands lacking of -SO
3
or/and phosphonium as well as the mechanical mixtures of
in L1 severed as a
the individual functional groups independently. The molecular structure of Pd-L1 indicated that -SO -
3
secondary O-donor ligand with reversible coordinating ability, cooperating with the phosphino-fragment to stabilize Pd-
catalyst. The in situ FT-IR analysis verified that the formation and stability of Pd–H active species in charge of
hydroxycarbonylation were dramatically facilitated by the presence of L1. It was believed that, over L1-based Pd-catalyst,
+
2
H O was cooperatively activated by the Lewis acidic phosphonium via “acid-base pair” interaction (H
2
OP(V) ) and by the
-
-H
hydrophilic SO
3
via hydrogen bonding (SO
--coordination. In addition, L1-based Pd-catalyst could be immobilized in the ionic liquid [Bmim]NTf
for six-run recycling uses without obvious activity loss and detectable metal leaching.
3 2
O), giving rise to the formation of dimeric and mono-nuclear Pd-H species
driven by reversible SO
3
2
II
+
II
)11,17. On the other hand,
2
hydrogen gas (Pd -H + H  Pd + H
3
the phosphines such as PPh can be deactivated upon alkylation
Introduction
Pd-catalyzed hydroxycarbonylation of terminal alkynes with CO
over protic acids, necessitating the use of excess ligand to
warrantee Pd-catalyst stable as well as excess acid unavoidably
consumed in the reaction . Interestingly, some metal Lewis
acids were found to be effective additives to spur the catalysis
of phosphine-modified Pd-complexes for hydroxycarbonylation
2
and H O allows for one-step synthesis of ,-unsaturated
1
8
carboxylic acids (i.e. 2- or 3-substituted acrylic acid derivatives)
with 100% atom-economy
1
–4
.
This protocol is more
advantageous over the oxidative hydroxycarbonylation of
alkenes for the synthesis of acrylic acid due to the
disadvantages of low selectivity and risk of using dangerous
4
,6,19
of alkenes
. For example, FeCl
3
was able to significantly
-modified Pd-catalyst as
improve the performance of PPh
3
6
reported by Li , which demonstrated the benefit over
traditional Brønsted acids. It has been well known that
phosphonium salts are typical metal-free Lewis acids2 , which
have be applied as Lewis acid catalysts for isomerization and
hydrosilylation of alkenes
activations like silylcyanation of ketones , Baylis–Hillman
3
oxidants .
It
is
widely
accepted
that
Pd-catalyzed
0–23
hydroxycarbonylation of alkynes occurs in the same mechanism
as for hydroxycarbonylation of alkenes. The latter has been
2
4,25
as well as electrophilic C=O bond
5
–10
more carefully studied in the literature . As review by van
2
6
5
Leeuwen , hydroxycarbonylation of alkenes involves
a
2
7
28
reaction , Aldol and Michael reactions , acetalization of
II
palladium hydride (Pd -H) mechanism, wherein the presence of
Brønsted/Lewis) acid additives and the properties of
2
9
aldehydes
etc. Inspiringly, the moisture-insensitive
(
phosphonium salts are suitable candidates in place of the metal
Lewis acid additives for hydroxycarbonylation.
In recent decade, co-catalysis in intramolecular
combination of transition metal catalysts and other kinds of
organocatalysts has emerged as a powerful strategy to promote
phosphine ligands dramatically influence the formation and
stability of Pd -H species. For example, Brønsted acid (such as
p-TsOH
II
2
,9,10
3
8
3 3
, CF SO H , or aqueous HCl ) can serve as a hydride
II
source through reactivating reductive Pd (0) to afford Pd -H
hydride species
Brønsted acid may consume Pd -H species with evolution of
1
1–16
. However, the high concentration of
(new or tandem) organic transformations that cannot be
II
achieved by each individual one independently or their
3
0–33
mechanical mixtures
. The uses of bi-functional ligands
containing phosphino-fragments and phosphoniums, which can
combine transition metal catalysts (upon phosphino-coordination)
with Lewis acid organocatalysts (phosphoniums) by stable chemical
a. Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry & Molecular Engineering, East China Normal University, Shanghai
2
00062, PR China. E-mail: yliu@chem.ecnu.edu.cn
†Electronic Supplementary Information (ESI) available: ESI and crystallographic data
for Pd-L1. See DOI: 10.1039/x0xx00000x
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ARTICLE
Journal Name
bonds, have become one of the most effective methods to fulfil co- phosphine-selenide detected by 31P NMR spectroscopy can be used
View Article Online
DOI: 13 07 ,. 31 80 39/C9GC01887E
catalysis with synergetic effect in this field as reported by us29,34
Based on the characteristics of hydroxycarbonylation of
hydrophobic alkynes with water, herein, a tri-functional ligand of
.
to evaluate the -acceptor ability of a phosphine
. An increase of
1
31 77
J
P- Se indicates an increase in the character of -acceptor ability
31
1
31
77
(
i.e., less -donor ability). The P NMR measurement for J P- Se of
L1 containing
phosphonium cation, and a hydrophilic sulfonate (SO
a
phosphino-fragment,
a
Lewis acidic
the obtained phosphine-selenides (Fig. 1) demonstrated that, the
phosphonium-based analogues of L1 and L2 possessed the increased
-donor ability with J P- Se of 741 Hz in compared to Xantphos with
P- Se of 758 Hz, indicating that, due to non-conjugated effect in
-
3
) anion
was designed with the following concerns. Firstly, due to the
involvement of water as the reactant in this reaction, L1 should be
conferred with amphiphilicity in order to facilitate the accessibility of
the reactants to L1-based Pd-catalytic center. Therefore, the
1
31 77

J
1
31 77
xanthene-skeleton, the resultant phosphonium exhibited the
-) was incorporated into the negligible electron-withdrawing effect on the remote PPh
-
hydrophilic sulfonate group (-SO
3
2
skeleton of Xantphos on purpose. Besides of the hydrophilicity, the fragements in L1 and L2.
-
introduced –SO
3
group was also expected to serve as an O-
containing ligand to work together with the phosphino-fragment to
coordinate to Pd-center with hemilabile and reversible ligating
Me Me
3
5
O
effect . Secondly, besides of Brønsted acid serving as the hydride-
source, Lewis acid can also activate ROH or H O via “acid-base pair”
Ph
P
Ph Ph
P
Ph
3
3.69
29.94
Se
Se
1
2
c
b
a
JSe-P = 758 Hz
Selenide of Xantphos
interaction to afford proton for the subsequent formation of Pd-H
active species, with advantages of avoiding the alkylation of the
involved phosphine, which occurred if excess Brønsted acid was
applied. Phosphonium cations are well-known metal-free organic
Me Me
O
3
3.44
29.77
Ph
P
P+Ph2
Me
Ph
OTf-
1
Se
J
Se-P = 741 Hz
Selenide of L2
Lewis acid, which are believed to be able to activate H
Hence, once one PPh -fragment in Xantphos was alkylated by 1,3-
propanesultone, the required functional moiety of the Lewis acidic
phosphonium cation as well as -SO group was introduced in the
designed ligand of L1. Thirdly, the rigidity and robustness of Xantphos
2
O effectively.
Me Me
2
O
Ph
P
Ph2+P
3
2.79
29.12
Ph
SO3
-
1
Se
J
Se-P = 741 Hz
3
Selenide of L1
can guarantee one PPh
center and another one was modified to be SO
2
-frangment remain intact to coordinate Pd-
40
20
Chemical shift (ppm)
0
-20
-
3
-tailed phosphonium
cation without potential “acid-base pair” incompatibility. Based
Fig. 1 The 31P NMR spectra (202 MHz) of the selenides of L1, L2 and Xantphos: (a)
on these three points, the tri-functional and zwitterionic ligand of reacting elemental selenium with L1 in CD OD at 70 °C for 10 h; (b) reacting
3
-
L1 containing phosphino-fragment, phosphonium cation, and SO
3
elemental selenium with L2 in CD
3
CN at 70 °C for 10 h; (c) reacting elemental
group was developed on purpose, and applied to the co-catalysis of selenium with Xantphos in CDCl
hydroxycarbonylation for the first time (Scheme 1)
comparison, the phosphines including Xantphos and its
3
at 70 °C for 10 h.
.
For
-
o
3
derivative of L2 without the dangling SO -tail were studied in
2
47( C)
parallel.
o
3
30 ( C)
Xantphos
Pd(OAc)2-Ligand
OH
R
OH
R
+
CO
+
2
H O
R
+
No auxilary additive
Me Me
O
O
Me Me
Me Me
o
3
80 ( C)
L1
OTf-
Me
O
O
O
+
SO3-
+
PPh2 Ph2 P
PPh2 Ph2 P
PPh2
PPh2
L2
L1
L2
Xantphos
100
200
300
400
500
600
700
800
Scheme 1 Tri-functional ligand (L1) containing phosphino-fragment, phosphonium
cation and hydrophilic -SO -
anion for Pd-catalyzed hydroxycarbonylation of
alkynes
o
Temperature ( C)
3
o
2
47( C)
Xantphos
Results and discussion
Synthesis and characterization
o
3
30 ( C)
L1
L2
o
The amphiphilic zwitterionic ligand of L1 as well as its analogue of L2
380 ( C)
was synthesized according to the procedures firstly reported by our
3
6
group . Although L1, L2 and Xantphos possessed the identical
xanthene-skeleton, the alkylation of one PPh -fragement in
2
Xantphos can change the π-accepting ability of the resultant mono-
phosphines (L1 and L2) due to the intensive electron-withdrawing
effect of the resultant positive-charged phosphonium. It has been
100
200
300
400
500
600
700
800
o
Temperature ( C)
1
31 77
widely accepted that the magnitude of J P- Se of the corresponding Fig. 2 TG/DTG analysis of the phosphine ligands of L1, L2 and Xantphos in air flow
2
| J. Name., 2012, 00, 1-3
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Journal Name
In addition, the TG/DTG analysis showed that the ionic
ARTICLE
Table 1 Pd-catalyzed hydroxycarbonylation of phenylacetylene wVi
i
e
th
w
i
A
n
r
v
ti
o
c
l
e
ve
O
m
nl
ent
in
e
mono-phosphines of L1 and L2 possessed the better thermal of different ligandsa
stability than the neutral Xantphos (Fig. 2). In air flow, L1
DOI: 10.1039/C9GC01887E
O
o
o
OH
decomposed ca. 330 C and L2 at 380 C whereas Xantphos
Pd-Ligand
OH
cinnamic acid
Me Me
+
CO
+
H2O
+
o
O
decomposed just at 247 C. Obviously, L1 and L2 tailed with
2
-phenylacrylic acid
phosphonium cations were of less oxygen- and moisture-
sensitivity, which facilitated their application in water-involved
reaction and manipulations in open air.
Me Me
Me Me
O
O
OTf-
Me
O
PPh2 Ph2+
P
SO3-
PPh2 Ph2+
P
PPh2
PPh2
L1
L2
Xantphos
Pd-catalyzed hydroxycarbonylation of terminal alkynes
Entry
Ligand
P/Pd
Conv.
Sel.acids (%)b
Yieldacids
The hydroxycarbonylation of phenylacetylene with H
selected as a model reaction to investigate co-catalysis over L1-
based Pd-catalysts (Table 1). Herein, Pd(OAc) was selected as
2
O was
(
molar ratio) (%)b
Branched
Linear
(%)
-
1
2
3
4
5
6
-
-
<5
72
92
33
15
33
<5
-
-
2
L1
L1
L2
1
3
3
3
3
3
90
90
87
71
88
-
10
10
13
29
12
-
72
92
33
15
33
-
the precursor instead of PdCl
potential hydrolysis of the latter for the release of H as the
hydride source for formation of Pd -H species. Under the
selected conditions (CO 1.0 MPa, 120 C, 6 h, NMP as solvent),
2
(MeCN)
2
, in order to rule out the
+
II
o
Xantphos
L2 + n-BuSO
L1
the mixing use of L1 and Pd(OAc)
2% conversion of phenylacetylene and 100% selectivity to the
target α,β-unsaturated acids with B/L of 9 (Entry 2). The further
2
with P/Pd ratio of 1 led to
3
Na
7
7
c
a
2 2
Pd(OAc) 0.02 mmol (1 mol%), phenylacetylene 2 mmol, CO 1.0 MPa, H O 4
o
b
screening of P/Pd ratio found that the P/Pd ratio of 3 in L1- mmol, NMP 3 mL, 120 C, 6 h; Determined by GC. Conv. represents the
invovled Pd(OAc) system corresponded to the highest activity
2
conversion of phenylacetylene. Sel.acids represents the total selectivities to
branched and linear α,β-unsaturated acids;
c
0.06 mmol of TBAF
with 92% yields to the target products with B/L of 9, whereas (tetrabutylammonium fluoride) was added additionally.
Pd(OAc) itself didn’t exhibited any activity (Entry 3 vs 1).
2
Comparatively, the uses of L2 and Xantphos respectively in
place of L1 only resulted in 33% and 15% yields of the target
products (Entries 4 and 5). It was noticed that, the simple
mixtures of L2 (0.06 mmol) and n-BuSO Na (0.06 mmol), which
3
could be regarded as the mechanical adduct of L1, barely
Since hydroxycarbonylation of alkynes occurs on the basis
II
5
of a palladium hydride (Pd -H) mechanism , any effect that can
II
improve the formation and stability of Pd -H species will
facilitate this reaction. The evolving processes over L1-bases
Pd(OAc) system with presence of phenylacetylene and H O in
2 2
CO atmosphere was detected by the high pressure in situ FT-IR
spectroscopy (Fig. 3-A). For comparison, the evolving processes
over Xantphos-Pd(OAc) were monitored under the same
2
conditions (Fig. 3-B). Universally, the characteristic broad
double peaks for gaseous CO were clearly observed with the
exhibited the improving effect on the reaction rate (Entry 6).
The co-catalysis over the stoichiometric sum of two individual
-
functions (phosphonium-based phosphine of L2 and SO
salt of n-BuSO
catalyst (Entry 6 vs 3). In the mechanical mixtures of L2 (0.06
mmol) and n-BuSO Na (0.06 mmol), the Lewis acidic property of
3
-based
3
Na) didn’t equal to that over L1-based Pd-
3
-
1
vibrations of ~2170 and ~2100 cm (Fig. 3). Over L1-Pd(OAc)
2
phosphonium in L2 could possibly be quenched by the free
o
system (Fig. 3-A), when the temperature increased up to 100 C,
a weak absorption peak at 1936 cm attributed to Pd -H
appeared and then reached the maximum intensity
at 120 C with well longevity. Accompanied with the
appearance of the peak at 1936 cm- (Pd -H species), the
characteristic vibration at ca. 1706 cm- assigned to the
products of acids (the standard FT-IR spectrum of 2-
phenylacrylic acid was given as S. Fig. 1 in ESI) was concurrently
-
mobile electron-rich n-BuSO
3
anion through “acid-base pair”
-1
II
interaction, leading to mutual counteracted behaviour in the
involved catalytic system (Entry 6). Notably, when the strong
3
9,40
species
o
-
Lewis basic F , which could badly quench the acidity of
1
II
3
7,38
phosphonium due to the unique fluorophilicity
, was added
1
into L1-based Pd-system, the hydroxycarbonylation of
phenylacetylene was completely inhibited (Entry 7), indicating
that the phosphonium in L1 indeed cooperated with phosphino-
2
observed. In contrast, over Xantphos-Pd(OAc) system (Fig. 3-
B), due to the poor stability and short longevity of the resultant
Pd -H species, its corresponding vibration was unobservable
-
fragment as well as SO
3
to synergistically promote the
performance of L1-modified Pd-catalyst. The obtained results
indicated that in L1, co-existence of the three functional groups
II
totally in the overall monitoring. Consequently, only the
including phosphino-fragment, Lewis acidic phosphonium and
-1
relatively weak characteristic peak at 1706 cm assigned to 2-
phenylacrylic acid was observed, which implied the sluggish
-
SO
3
was essentially responsible for the improved performance
of Pd-catalyst due to the intramolecular synergetic catalytic
effect.
2
hydroxycarbonylation of phenylacetylene with H O and CO. The
high pressure in situ FT-IR analysis supportively showed that the
presence of L1 indeed facilitated the formation and stability of
II
-1
Pd -H species ( 1936 cm ) which inherently resulted in the
more efficient hydroxycarbonylation.
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ARTICLE
Journal Name
of A. In A, SO -
-arm as a hard O-donor with reversible
3
View Article Online
DOI: 10.1039/C9GC01887E
A: L1-Pd(OAc)2
coordinating ability was able to temporarily occupy a
coordination site on Pd-center for stabilization and then
dissociated to provide unsaturation vacancy for the alkyne
coordination timely, resulting in the formation of mono-nuclear
o
3
0 C
o
1
00 C
(-acetenyl)palladium intermediate (B). The reversible
o
1
20 C, 1 min
dimerization not only gave rise to the stability of A but also
guaranteed the timely release of active intermediate B whose
formation was the driving force of the reaction. Subsequently,
upon intramolecular addition, B transformed to the dimeric
1
936
o
1
20 C, 60 min
vinylpalladium intermediates (C and C’) through the timely re-
1
706
-
coordination of -SO
3
to Pd-center. The migration and insertion
2
200
2000
1800
1600
1400
of CO to Pd-C bonds in C afforded the acylpalladium
intermediates (D and D’) which proceeded hydrolysis to yield
the branched ,-unsaturated acids (E and E’) accompanied by
-1
Wavenumber (cm )
o
B: Xantphos-Pd(OAc)2
30 C
II
the regeneration of dimeric Pd -H species (A). Rationally, the
steric hindrance between -R and bulky xanthene-skeleton goes
against the formation of Pd-intermediates complexes of C’ and
D’, corresponding to the unfavorable generation of the linear
unsaturated carboxylic acid. In addition, when -R is aryl group,
the formation of the thermodynamically stable benzylic Pd-
o
1
20 C, 1 min
o
1
20 C, 45 min
3
species induced by the ƞ -electron donation from aryl ring
o
1
20 C, 68 min
further give rise to the formation gem-vinylpalladium
intermediate of C. In such circumstances, the branched
unsaturated carboxylic acids were predominantly formed as
observed in this work. In this proposed mechanism, it is
believed that the formation of the intermediates of A, C/C’,
1
706
2
200
2000
1800
1600
1
1400
-
Wavenumber (cm )
D/D’ in the dimeric form play indispensable role in driving up
Fig. 3 The high-pressure in situ stacked FT-IR spectra recorded respectively from 30 to
20 °C after mixing Pd(OAc) , L1 (or Xantphos), phenylacetylene, H O and DMSO under
CO atmosphere (1.0 MPa)
-
this catalytic cycle, wherein -SO
3
group in L1 serves as the
-fragment as well
1
2
2
effective ligand to form such dimers with PPh
2
as CO supported by the crystal structure of Pd-L1. Instead, the
-
formation of the corresponding SO
3
-coordinated monomeric
Based on the obtained results, a mechanism for
Pd-complexes is absolutely unfeasible due to the steric
constrains. On the other hand, once the monomeric
intermediate complexes were formed with superiority as in the
case of L2-based Pd-catalytic system wherein SO
unavailable and the two phosphine ligands of L2 has to be
occupying for the need of coordination number of four, the
poor transformation efficiency was obtained (Conv. 33%, Entry
4 of Table 1). Based on these facts, the dimeric form of SO
coordinated Pd-complexes (A, C/C’, D/D’) is rational in this
hydroxycarbonylation over L1-based Pd(OAc)
proposed in Scheme 2. In the presence of L1, H
2
system was
O as a proton
2
reservoir was synergistically activated by the Lewis acidic
-
+
3
-ligand was
phosphonium via “acid-base pair” interaction (H
2
OP(V) ) as
-
-
well as hydrophilic SO
3
via hydrogen bonding (SO
3
H
2
O) to
II
II
facilitate the formation of Pd -H species (A). The available Pd -
H species (A) was supposed to be a dimeric Pd-complex
-
-
3
-
intermediate, wherein PPh
2
-fragment and SO
3
coming from
two L1 served as a P,O-hybrid ligand to guarantee the stability
mechanism.
4
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Journal Name
ARTICLE
H
View Article Online
DOI: 10.1039/C9GC01887E
H
O
R
P
C
O
O
Ph
2
O-
+ P
Pd
CO
S
Ph2
Me
Me
Me
Me Me
O
O
R
CO
Pd
C
Me
HC
Ph2
O-
+
HO
P
S
P
O
L1
Ph2
PPh2 Ph2+
P
O
O
O
O
O
O
(
E', unfavored)
S
(
D', unfavored)
R
H
O
O-
CH2
H
H
H
H
HO
R
R
Pd0
H
O
O
(
E, favored)
C
O
O
H
H
O
OC
Pd
Ph2
+ P
O
O
O-
S
Ph2
+ P
P
Pd
O-
Ph2
P
S
Ph2
CO
Me
Me
Me
Me
O
O
Me
Me
H
Me
Me
CO
O
O
H
Ph2
O-
+
S
P
Pd
P
Ph2
+
S
O-
Pd
P
O
O
P
Ph2
O
Ph2
O
C
O
CO
H
dimeric palladium hydride
A)
(
D, favored)
R
(
H
R
CO
H
H
R
R
OC
Pd
O
O
Ph2
+ P
Pd
O-
S
P
P
Ph2
Ph2
CO
H
Me
Me
Me
Me
Me
2
Me
O
O
O
CO
O
O
Ph2
P
Ph2
P
+
O- Pd
+
S
S
O-
P
Ph2
O
O
R
H
(
C, favored)
(B)
H
H
R
H
O
O
Ph2
O-
S
+ P
P
Pd
Ph2
CO
Me
Me
Me
Me
O
O
CO
Pd
Ph2
P
S
O-
+
P
Ph2
O
O
H
R
(
C', unfavored)
H
2
Scheme 2 The proposed catalytic mechanism over L1-Pd(OAc) for hydroxycarbonylation of terminal alkynes
In order to explore the role of L1 as a P,O-hybrid ligand in fragment from another L1 to guarantee the stability of the resultant
-
facilitating the stability of Pd-complex and the related intermediates Pd-complex in a dimeric form. Without the presence of –SO
3
like L2,
proposed in Scheme 2, we made our effort to prepare the Pd(II)- the corresponding complex was not as stable as Pd-L1 as a result of
+
complexes ligated by L1 and L2 respectively. Considering the unavailability. In addition, in Pd-L1, the P(III)…P(V) distance between
-
interference from OAc as an O-containing ligand as well as its bulky the phosphino-fragment and the phosphonium was 4.9596 Å, which
steric hindrance, PdCl
2
(MeCN)
2
was applied to coordinate with L1 (or was beyond the sum of the van der Waals radii of two P-atoms (~3.80
-frangment
L2) in CH Cl at RT under N
2
2
2
atmosphere. As a result, a stable complex Å). Such information indicated that the electron-rich PPh
2
of Pd-L1 was obtained as the orange solid with yield of 67% whereas (as the Lewis base) and the electron-deficient phosphonium (as the
L2-ligated Pd-complex was not stable enough to be isolated during Lewis acid) were indeed compatible in L1 without “acid-base pair”
the preparation and crystal growing. The single crystals of Pd-L1 quenching problem due to the separation by the rigid and bulky
suitable for X-ray diffraction analysis were obtained by re- xanthene-skeleton, which guaranteed their individual functions
crystallization from CH Cl / diethyl ether (anhydrous). The molecular unchanged.
2 2
structure of Pd-L1 was depicted in Fig. 4. Pd-L1 was a zwitterionic
loop-like dimeric complex, in which each of Pd-center was four-
-
coordinated in the typical square-planar geometry by two Cl in trans-
-
position as well as one phosphino and one SO
3
anion. The
-
incorporated SO
3
truly served as an O-containing ligand to
coordinate to Pd-center together with the available phosphino-
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View Article Online
DOI: 10.1039/C9GC01887E
Ph Cl
Table 2 The recycling uses of L1-based Pd(OAc) catalytic system in [Bmim]NTf for
hydroxycarbonylation of phenylacetylene
O
Ph
P+
2
2
O
Me Me
Pd O-
a
P
S
PdCl2(MeCN)2
CH Cl , RT
Me
Me
Ph Cl
Ph
Me
Me
O
O
O
O
Ph
P+
Ph
Cl
O- Pd
Ph
2
2
PPh2 Ph2+
SO3-
Pd(OAc) -L1
OH
P
S
2
P
OH
+
CO
+
H2O
+
O
O
L1
Cl Ph
[Bmim]NTf2
O
Pd-L1
Run
1
Conv. (%)b
Sel. Branched acid (%)b
Sel. Linear acid (%)b
92
92
90
89
90
87
90
90
91
90
91
90
10
10
9
2
3
4
10
9
5
6
10
a
Pd(OAc)
2
0.02 mmol (Pd 1 mol%), L1 0.06 mmol (P/Pd=3), phenylacetylene 2.0
b
mmol, CO 1.0 MPa, H
2
O 4.0 mmol, [Bmim]NTf
2
3.0 mL; Determined by GC, Conv.
represents the conversation of phenylacetylene, Sel.Branched acid represents
selectivity of 2-phenylacrylic acid, Sel.Linear acid represents selectivity of cinnamic
acid.
The scope of hydroxycarbonylation of different alkynes
2
catalyzed by L1-Pd(OAc) was explored in Table 3. Universally,
Fig. 4 The single crystal structure of Pd-L1 (H-atoms and the solvent molecules
were omitted for clarity)
when phenylacetylene derivatives was applied as the
substrates, the branched unsaturated carboxylic acids were
predominantly formed due to the formation of the kinetic
Evidently, the three functional groups in L1 plays synergetic
roles in promoting the reaction rate of hydroxycarbonylation of
alkynes. The retained mono-phosphino linked with bulky
xanthene-skeleton undoubtedly coordinates to Pd-center to
behave as a typical catalyst to activate CO and the alkyne. The
3
stable benzylic Pd-species induced by the  -electron donation
from the benzene ring (Entries 1-9). The selectivities to the
unsaturated carboxylic acids including branched and linear
were all 100% without exception. The substituent of methyl at
para- or ortho-position had negligible influence on the reaction
rate along with 90% yields of the unsaturated carboxylic acids
involved Lewis acidic phosphonium can cooperatively activate
+
H
2
O via “acid-base pair” interaction (H
2
OP(V) ). The tailed-
(Entries 2 and 3 vs 1). Comparatively, methyl at meta-position
-
hydrophilic –SO
the accessibility of H
3
group renders L1 amphiphilicity to guarantee
O (via hydrogen bonding, SO
hydrophobic alkynes to Pd -H active species. In addition, the
obviously decelerated the reaction rate with 79% yield of the
target acids (Entry 4). Comparatively, the substituent of t-butyl
and methoxyl corresponded to the similar product yields as –
methyl (Entries 5, 6 vs 2). In contrast, the substituent of halide
with electron-withdrawing effect at para-position of
phenylacetylene had negative influence on the reaction rate
with the yields in the range of 69~74% (Entries 7-9). When the
linear aliphatic of 1-hexyne was applied as the substrate, the
target acids were obtained also in good yield of 82% (Entry 10).
And 1-octyne with the increased carbon-chain corresponded to
the relatively lower yield of ,-unsaturated acids (Entry 11). In
the cases of linear aliphatic of 1-hexyne or 1-octyne as the
substrates, the B/L ratio of the products obviously decreased
due to the lack of the stabilized benzylic Pd-species.
-
2
3
2
H O) and
II
-
introduced –SO
3
group, with hemilabile and reversible ligating
effect, also serves as an O-containing ligand to coordinate to Pd-
center together with the phosphino-fragment located in
another L1 to enable the formation of the dimeric complexes
with
further
improving
activity
towards
the
hydroxycarbonylation.
On the other hand, as for L1 as a zwitterionic compound, it
could be used with the room temperature ionic liquid (RTIL) to
fulfil the recovery and recycling of the homogeneous Pd-
catalyst, which has been regard as the effective alternative to
4
1,42
immobilize homogenous catalysts in green chemistry
summarized in Table 2, L1-based Pd(OAc) system could be
recycled at least runs in [Bmim]NTf (1-butyl-3-
. As
2
6
2
methylimidazolium bis((trifluoromethyl)sulfonyl)imide). The
ICP-OES analysis indicated that the leaching of Pd in the
combined organic phase was non-detectable (below the
detection limit of 0.1 g/g).
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2
recycled for 6 runs in the ionic liquid of [Bmim]NTf without
obvious activity loss and detectable metal leaching.
Table 3 Generality of L1-Pd(OAc)
2
system for hydroxycarbonylation of the
different alkynes with watera
DOI: 10.1039/C9GC01887E
L1-Pd(OAc)2
OH
R
OH
R
+
CO
+
2
H O
R
+
O
O
Experimental
Reagents and analysis
Entry
1
Alkynes
Branched acid
Linear acid
Conv.
B/Lb
(
%)b
O
90
89
91
90/10
90/10
90/10
The chemical reagents were purchased from Shanghai Aladdin
Chemical Reagent Co. Ltd. and Alfa Aesar China, which were
used as received. NMP and DMSO were dried from anhydrous
sodium sulfate before use. The H/ P NMR spectra of ligands
and complexes were recorded on a Bruker Avance 500
OH
O
OH
O
2
3
OH
O
1
31
OH
Me
Me
Me
tBu
Me
O
OH
spectrometer at ambient temperature. To analyze the
OH
Me
O
31
phosphine–selenides, the P NMR spectra (85% H
PO
3 4
sealed in
Me
Me
O
O
a capillary tube as an internal standard) were recorded on a
4
5
Me
79
88
89/19
90/10
OH
O
Me
OH
OH
1
13
Bruker Avance 500 spectrometer. The H/ C NMR spectra of
the obtained products were recorded on a Bruker Avance 400
spectrometer at ambient temperature. Gas chromatography
OH
O
tBu
tBu
(GC) was performed on a SHIMADZU-2014 equipped with a DM-
O
6
7
79
69
90/10
91/9
OH
O
Wax capillary column (30 m × 0.25 mm × 0.25 μm). GC-mass
spectrometry (GC–Mass) was recorded on an Agilent 6890
instrument equipped with an Agilent 5973 mass selective
detector. The FT-IR spectra were recorded on a Nicolet NEXUS
OH
MeO
MeO
F
MeO
O
OH
O
OH
F
F
6
70 spectrometer. The amount of Pd and P in the sample was
8
9
O
74
73
91/9
OH
O
quantified by using an inductively coupled plasma optical
emission spectrometer (ICP-OES).
OH
Cl
Br
Cl
Br
Cl
Br
O
90/10
OH
O
OH
Synthesis
OH
OH
82
68
68/32
73/27
Synthesis of L1 and L2
1
1
0
1
OH
5
3
5
O
O
O
L1 and L2 were prepared according to the literature
methods36
31P NMR of L1 (202 MHz, CD OD): δ = 25.55 (s,
.
3
OH
3
5
3
+
31
O
P Ph
2
+
), -21.59 (s, PPh
), -21.10 (s, PPh ).
2 2
2
). P NMR L2 (202 MHz, CDCl
3
): δ = 22.2
(
s, P Ph
Synthesis of Pd-L1 complex.
In N atmosphere, L1 (70 mg, 0.1 mmol) was added to a solution of
PdCl (MeCN) (0.18 g, 0.55 mmol) in dry dichloromethane (10 mL).
a
Pd(OAc)
CO 1.0 MPa, H
of the branched acid and the linear acid (The isolated branched products were
2
0.02 mmol (Pd 1 mol%), P/Pd = 3 (L1 0.06 mmol), phenylacetylene 2 mmol,
O 4 mmol, NMP 3 mL; b Determined by GC and GC-Mass, B/L= the ratio
2
2
1
13
characterized by H/ C NMR spectra provided in ESI).
2
2
The mixture was stirred vigorously at room temperature for 8 h. Then
diethyl ether was added to the solution to precipitate the orange
solids, which were washed by diethyl ether for three times. The solids
were dried under vacuum to get the product of Pd-L1 (66 mg, yield
Conclusions
7%). 1H NMR (500 MHz, CD
6
3
OD): δ = 8.15 (d, J = 10 Hz, 2H,
+
Co-catalysis over L1-based Pd catalyst was successfully 2P Ph CCH), 7.80-7.82 (m, 4H, phenyl), 7.72-7.76 (m, 8H, phenyl),
2
achieved for hydroxycarbonylation of terminal alkynes towards 7.64-7.68 (m, 10H, phenyl), 7.31-7.40 (m, 14H, phenyl), 7.12 (t, J = 10
α,β-unsaturated carboxylic acids, which proved more effective Hz, 2H, phenyl), 6.93 (t, J = 10 Hz, 10H, phenyl), 6.55-6.58 (m, 2H,
than the mechanical mixtures of the individual functional
groups independently. The incorporated three functional
+
-), 2.02-
phenyl), 3.70-3.77 (m, 4H, 2P CH
2
), 2.43-2.46 (m, 4H, 2CH
), 1.77 (s, 12H, 4CH
), 28.59 (s, PdPPh ).
2 3
SO
). 31P NMR (202 MHz,
3
2
CD
.10 (m, 4H, 2CH
2
CH
2
CH
2
groups in L1 containing a phosphino-fragment, a phosphonium
+
3
OD): δ = 26.60 (s, P Ph
2
2
-
cation, and a sulfonate anion (SO
3
) synergistically promoted the
reaction. With the evidences from in situ FT-IR spectra and the
single crystal structure of Pd-L1 complex, it was proposed that
over L1-based Pd-catalyst, H O was cooperatively activated by
2
Single Crystal X-ray Crystallography
Intensity data for Pd-L1 was collected on a Bruker SMARTAPEX
II diffractometer using graphite monochromated Mo-K
radiation ( = 0.71073 Å). Data reduction included absorption
corrections by the multi-scan method. The structures were
solved by direct methods and refined by full matrix least-
the Lewis acidic phosphonium via “acid-base pair” interaction
+
-

(
H
2
OP(V) ) and the hydrophilic SO
3
via hydrogen bonding
-
(SO
3
H
2
O), giving rise to the formation of dimeric/mono-
nuclear Pd-H species (A/B) driven by reversible SO -
3
-
coordination. In addition, L1-based Pd-catalyst could be
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ARTICLE
Journal Name
squares using SHELXS-97 (Sheldrick, 1990), with all non- directly used without further treatment for the next run (if
View Article Online
DOI: 10.1039/C9GC01887E
2
hydrogen atoms refined anisotropically. Hydrogen atoms were required H O was added additionally).
added at their geometrically ideal positions and refined
isotropically. The crystal data and refinement details were given In situ high-pressure FT-IR spectral characterization
in Table 4.
In situ high-pressure FT-IR spectra were obtained using a Nicolet
NEXUS 670 spectrometer. A mixture of phenylacetylene (0.5
mmol) and water (1.0 mmol) was added to a solution of L1 (0.06
Table 4. The crystal data and structure refinement for Pd-L1
Pd-L1
2
mmol, or Xantphos 0.03 mmol), Pd(OAc) (0.02 mmol) in DMSO
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
84 4 8 4 2 2
C H76Cl O P Pd S
(0.1 mL). Then the mixture was dropwise on a specially designed
1756.05
cylindric CaF , which used as sealing sheets for a high-pressure
2
monoclinic
P21/n
IR cell. Then, 1.0 MPa CO was inflated into the sealed cell, and
real-time monitoring was performed at different reaction
16.7161(2)
temperatures. The mixture compositions including Pd(OAc)
or Xantphos), phenylacetylene, water and CO were completely
the same as those for the real reaction in Table 1, except for the
2
, L1
(
b (Å)
13.98050(10)
19.1927(2)
90
c (Å)
2
much higher concentrations of Pd(OAc) and the ligand required
o



( )
for FT-IR spectral detection.
o
( )
105.6360(10)
90
o
( )
V (Å3)
4319.34(8)
4
Conflicts of interest
There are no conflicts of interest to declare.
Z
-3
d
calc (g cm )
1.350
-
1

(Mo-K
T (K)
(A)

) (mm )
6.059
100.00(10)
1.544184
117643
8798(0.0962)
0.0469
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21673077) and the Science and

Total reflections
Unique reflections (Rint
)
Technology
18JC1412100).
Commission
of
Shanghai
Municipality
R
1
[I>2(I)]
(
2
wR (all data)
0.1177
F(000)
1792
Goodness-of-fit on F2
Notes and references
1.044
‡
CCDC-1920683 (Pd-L1) contains the supplementary
General procedures for hydroxycarbonylation of alkynes with crystallographic data in this paper. These data can be obtained
water
free of charge from the Cambridge Crystallographic Data Centre
In a typical experiment, Pd(OAc)
2
(0.02 mmol) and L1 (0.06 via www.ccdc.cam.ac.uk/data_request/cif.
mmol, or the other ligand) were added into the mixture of
phenylacetylene (2 mmol, or the other alkynes), water (4 mmol)
and N-methyl pyrrolidone (NMP, 3 mL) in a 50 mL sealed Teflon-
lined stainless steel autoclave. Then CO was pressured to 1.0
MPa in the autoclave. The obtained mixture was stirred
vigorously at the reaction temperature for some time. Upon
completion, the reaction solution was cooled down to room
temperature and then extracted with diethyl ether (3 mL × 3).
The solution was analyzed by GC to determine the conversion
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