Bifunctional ligands in combination with phosphines and Lewis acidic phospheniums for the carbonylative Sonogashira reaction

Article abstract of DOI:10.1039/c5cc03697f

The combination of phosphine-ligated Pd catalysis and phosphenium(v) Lewis acid catalysis has been developed for the carbonylative Sonogashira reaction using phosphino-phosphenium salts (L1-L4) as bifunctional ligands, in which the Lewis acidic phospheniu

Full text of DOI:10.1039/c5cc03697f

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Bifunctional Ligands in Combination with Phosphines and Lewis
Acidic Phospheniums for Carbonylative Sonogashira Reaction
Chen Tana, Peng Wanga, Huan Liua, Xiao-Li Zhaoa, Yong Lua and Ye Liua†
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The combination of phosphine-ligated Pd catalysis and modulating roles of phosphine [P(III)] ligands for transition metal
phosphenium(V) Lewis acid catalysis has been developed for catalysis for oxo-synthesis [12], herein, the
carbonylative Sonogashira reaction by using phosphino- diphenylphosphinoamino-phosphenium salts of L1~L4, in which the
phosphenium salts (L1-L4) as bifunctional ligands, in which the two P-atoms were closely bridged by one N(III)-atom with different
Lewis acidic phosphenium(V) cations can form secondary bonds substituents, were synthesized[13] and applied as the bifunctional
with
O atoms (in C=O) to cooperatively stabilize Pd-acyl ligands for Pd-catalyzed carbonylative Sonogashira reaction
intermediates.
(Scheme 1). The present protocol provides a synergetic
methodology to activate aryl halides and carbon monoxide (as –
C=O source) with terminal alkynes. Meanwhile, a stable Pd(II)-
complex of L4-Pd ligated by L4 was obtained and investigated as the
pre-catalyst in parallel.
The combination of metal catalysis and organocatalysis has been
emerged as a powerful strategy to promote new reactions and
improves the reactivity in recent years. Various organocatalysts
such as aminocatalysts, Lewis base catalysts, hydrogen-bonding
catalysts, Brønsted acid catalysts, Brønsted base catalysts, phase
transfer catalysts and N-heterocyclic carbene catalysts have been
exploited [1]. Bifunctional ligand has a unique role in this field and
combines activation modes of metal catalysts with organocatalysts
by stable chemical bonds [2]. However, despite the combination of
metal catalysis with B(III) Lewis acid catalysis has been developed
by using phosphine-borane ligands [3], the application of other
types of Lewis acidic organocatalysts has not been explored. In
practice, molecules containing carbenium, silyl or phosphenium
cations can function as Lewis acid catalysts like metal salts [4].
Markedly, many examples have proved that organophosphenium
cations [P(V)+] as typical Lewis acids [5] not only could catalyze
isomerization of olefins, cationic polymerization, and hydrosilation
of olefins and acetylene without involvement of any metal [6], but
also exhibit activities towards C=O bond activation, such as in
cyanosilylation of ketones [9], Baylis-Hillman reaction[10], and Aldol
and Micheal reactions of carbonyl compounds [11] due to the
secondary interaction of acidic P(V) centres with sp2 hybrid O-
atoms (in carbonyl compounds) to promote the reactions [4].
≠
(a)
P
P
P
P
P
P
Pd CO
Pd
Pd
O
O
P(V) Lewis acid
Ar
Ar
Ar
intermediate
transition state
(b)
PPh2
Ph
Ph
PPh2
Ph
MeO
TfO
PPh2
N
MeO
N
N
P
P
P
TfO Me
TfO Me
Me
Ph
Ph
Ph
Ph
L1
L2
L3
I
PdCl2
N
PPh2
N
P
PdCl2(MeCN)2
Ph
P
P
Ph
Ph
Ph
Ph
Ph
L4
L4-Pd
Scheme 1 (a) Concept of bifunctional ligands that contain phosphines
and Lewis acidic phosphenium [P(V)+] sites. (b) Structures of L1~L4 and
L4-Pd
.
Highlighted by the promotion effect of phospheniums [P(V)+] as
Lewis acids to activate C=O bond [2d] and the significant
Herein, the carbonylative Sonogashira coupling reaction between
iodobenzene and phenylacetylene was selected as a model reaction
under copper-free conditions (Table 1). Dramatically, both resulting
yield and selectivity increased significantly with the increase of L/Pd
molar ratio (Entries 1~3). This relationship indicates excess ligands
play the essential role for carbonylation due to the well protection
of Pd-catalytic centres against deactivation. Structurally similar
ligands L2~L4 lead to close results under same conditions (Entries
6~8). In contrast, only 73% conversion of PhI was found when the
substantially electron-rich ligand PPh3 was used (Entry 10). While
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the reaction temperature was increased to 120 oC and time was in the reaction system. For comparison, the addition of TBA
decreased to 45 min, the highest TOF of 2270 h-1 and 99 % triflate
tetrabutylammonium trifluoromethanesulfonate) instead
(
selectivity were obtained under the up-scaled conditions (Entry 11). of TBAF led to the nearly unchanged PhI conversion (97%, entry 5),
Evidently, the decrease of CO pressure was seriously harmful to the indicating the negligible influence of TBA cation and triflate anion
transformation rate of PhI and the selectivity to 1,3-diphenylprop- (TfO-). The latter universally existed in the reaction system. Thus,
2-yn-1-one (Entry 12: Conv. 20%, Sel. 45%), which implied that we proposed that the Lewis acidic phosphenium cation in L could
the low CO pressure was unfavourable to the oxidative stabilize the Pd-acyl intermediates (Scheme 2-B and C) through
addition of the substrate and the subsequent carbonylation, forming the secondary bond between P(V) centre and O atom (in
corresponding to the poor carbonylative Sonogashira reaction. C=O) to accelerate the carbonylation (CO insertion) step (from
It was also found that when L4-Pd (Fig. 1) was used to replace
A to B), while the therein phosphine site was in charge of the
PdCl2(CH3CN)3 as the pre-catalyst, nearly the same conversions coordination to Pd-center for catalyst protection, in Pd/L-catalyzed
of phenylacetylene were observed at the same conditions carbonylative Sonogashira couplings. Certainly, besides of the
(Entries 9 vs 8), indicating the in situ formed L4-Pd exhibited effect of the phosphenium cation to accelerate the
the same activity as the as-synthesised one. However, when carbonylation step, the CO pressure was another important
Lewis basic F- (TBAF, tetrabutylammonium fluoride) was added to factor to influence the reversible steps of the oxidative
the reaction system, the conversion of PhI was declined to 84% addition (from Pd(0)-L to A) and the subsequent carbonylation.
(Entries 4 vs 3).
The increase of CO pressure reasonably propelled the
oxidative addition of PhI and CO insert proceeding forward to
favour the carbonylative Sonogashira reaction over
Sonogashira reaction, which was consistent to the results
observed in Table 1 (Entries 3 vs 12).
Table
1 Pd-catalyzed carbonylative Sonogashira coupling reaction of
iodobenzene with phenylacetylene in presence of the bifunctional ligands [a]
O
PdCl2(MeCN)2-L
(or L4-Pd/L4)
Ph
I
+
+
Ph
CO
Ph
base, solvent
Ph
Ph
Ph
by-product
Entry Ligand
L/Pd[b]
Conv. (%)[c] Sel. (%)[c]
TOF (h-1)
1
L1
L1
L1
L1
L1
L2
L3
L4
L4
PPh3
L1
L1
1
2
6
6
6
6
6
6
6
6
6
6
54
60
98
84
97
92
99
92
93
73
85
20
91
98
100
99
99
99
99
99
99
99
99
45
72
80
2[c]
3
131
112
128
123
132
123
124
97
4[d]
5[e]
6
7
8
Fig. 1 The single crystal structure of L4-Pd (all H-atoms have been
omitted for clarity). Selected bond distances (Å) and angles (o): Pd1–P1
2.240(3), Pd1–Cl1 2.487(2), Pd1-Cl1 2.382(3), Pd1-I1 2.5523(1); P1-Pd1-Cl1
177.8291), I1-Pd1-Cl2 176.28(9).
9[f]
10
11[g]
12[h]
2270
16
1.5
[a]PdCl2(MeCN)2 0.5 mol% (0.025 mmol), PhI 5 mmol, phenylacetylene 6
mmol, Et3N 7.5 mmol, CO 1.0 MPa, DMF 3 mL, 90 ºC, 1.5 h; [b]Molar ratio;
[c]Determined by GC and GC-Mass analysis; [d]TBAF was added (10 mol%);
[e]TBA triflate was added (10 mol%); [f]L4-Pd was used instead of
PdCl2(MeCN)2; [g]PdCl2(MeCN)2 0.05 mol% (0.005 mmol), PhI 10 mmol,
phenylacetylene 12 mmol, Et3N 15 mmol, CO 1.0 MPa, DMF 6 mL, 120 ºC, 45
min; [h]0.1 MPa (balloon).
L1
1.0
L1 + TBAF (1:1)
0.5
0.0
270
280
290
300
310
320
Wavelength/nm
The UV-vis. spectra supportively indicated that, when L1 was
mixed with of F-, a significant decrease of absorbance was observed
(Fig. 2), indicating the quenching of the phosphenium site in L1 by F-
due to its strong fluorophilicity, which was a typical character for
Lewis acidic phosphononium cation [6,14]. Anyway, the
phosphenium site in L1 could not be quenched completely by
the equivalent amount of TBAF at the room temperature,
implying that the fluorophilic interaction of Lewis acidic
phosphenium site towards F- was a reversible process. Hence,
under the reaction temperature (90 oC), an excess of TBAF was
Fig. 2 The UV-vis. spectra of L1 and L1+TBAF (1:1 molar ratio) [2.0×
10-4
M in CH2Cl2].
In order to explore the existence of the secondary bonding
interaction between phosphenium and O-atom (acid-base pair
interaction in nature), the selected L1 (or L3) has to be firstly
coordinated to a metal centre to rule out the competitive
interference of the neighboured phosphine site as a soft base.
Then we had tried our best to prepare the Pd(II) complexes
ligated by L1 and L3 respectively. Unfortunately the obtained
Pd(II)-complexes were not stable enough under the ambient
conditions to afford the corresponding single crystals suitable
for the single crystal X-ray analysis, due to the steric hindrance
required to push the P(V)+
…
F- fluorophilic interaction. As a
result, only the limited influence of TBAF on L1 was observed
with a 14% decrease of the conversion (Entries 4 vs 3) because
of the always remaining of the Lewis acidic phosphenium sites
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for the square-planar Pd(II)-complexes coordinated by two
bulky ligands. Then, the typical linear configuration for Au(I)-
complexes with less steric hindrance highlighted us to prepare
L
the corresponding Au-complexes (L1-Au and L3-Au) and use
their single crystal structures to explore the P…O secondary
bonding interaction instead.
Fig. 3 The single crystal structures of Au(I)-complexes of L1-Au and L3-
Au (all H-atoms have been omitted for clarity).
Scheme
Sonogashira reaction with the involvement of phosphino-
phosphenium salts ( ).
2 Proposed mechanism for Pd-catalyzed carbonylative
L
Moreover, as the cationic ligands, L1 can be applied together
with room temperature ionic liquid (RTIL) solvents as the efficient
alternative to immobilize the Pd catalysts for recovery and recycling
[17]. Herein Pd(CH3CN)2Cl2 and L1 were dissolved in [Bmim]BF4 (1-
butyl-3-methylimidazolium tetrafloruoborate) for the recycling
uses. Under CO pressure of 1.0 MPa as applied to the
homogeneous reactions (in DMF, Table 1), over Pd(CH3CN)2Cl2-
L1 system, only 72% conversion of PhI was obtained with 93%
selectivity to 1,3-diphenylprop-2-yn-1-one due to the mass
transfer limitation in the IL-organic biphasic reaction system.
When CO pressure was increased to 2.0 MPa, the
carbonylative reaction was obviously improved along with 84%
conversion of PhI and 96% selectivity. Hence the recycling
experiments were performed at 2.0 MPa (see S. Fig. 1 in SI).
The single-crystal X-ray diffraction analysis (Fig. 3) shows that the
conformation of C(26)-C(27) bond in L1-Au (gauche) is completely
different to that in L3-Au (skew). Moreover, the O-atom in L1-Au is
projecting inward towards the P-atom, and the P(2)-O(1) distance
(3.19 Å) is significantly shorter than the sum of the van der Waals
radii of P and O (~3.34 Å) [15]. Such information indicates that the
strong secondary bond in P…O linkage [4,16] is formed due to the
available Lewis acidity-basicity interaction between the
phosphenium(V) centre and O-atom of ether group in L1-Au. The
absence of the Lewis basic counterpart in L3-Au led to the outward
projection of phenethyl group. The crystal structure of L1-Au
confirmed that the Lewis acidic phosphenium site [P(V)+] could
develop a secondary bonding interaction with a weak base of
O-atom in ether group at the ambient temperature. It was
then very rational to support the proposed mechanism that
such Lewis acidic phosphenium in L indeed could stabilize the Pd-
acyl intermediates (Scheme 2-B and C) through forming the
secondary bond between P(V)+ centre and O atom (in C=O) to
After
5 runs, the yields of 1,3-diphenylprop-2-yn-1-one
decreased from 81% to 60% mainly due to the encapsulation of the
Pd-catalyst in the formed slurry Et3N∙HI salt, along with the well
maintained selectivity from 96% to 100%. The ICP-AES analysis
indicated that the leaching of Pd in the combined organic phase was
non-detectable (below the detection limit of <0.1 ꢀg/g).
accelerate the carbonylation step (from
A to B) in
At last, the scope of the reaction was explored (see S. Table
2 in SI). The electron density and steric hindrance of the aryl
substituents have influence on the catalytic activity. In
addition, when an aliphatic alkyne was used, the yield of the
product was very low (See Supplementary Information). It was
suggested that the kinetic characteristic of this reaction varies
considerably among the substituents. The repetition of the
reaction with PhBr instead of PhI did not produce the desired
carbonylative product under the same conditions, which also
indicated the developed L1 was only responsible for the
carbonylative Sonogashira reaction. It should be noted that,
under the applied reaction conditions (90 oC, 1.0 MPa), the
intramolecular P…O secondary bonding interaction in L1 was
doomed to vanish due to thermodynamic instability at high
temperature and the completive coordination of a large
amount of CO molecules in the surroundings, and then the
intermolecular P
…O (in CO) secondary bonding interaction
was able to exhibit its function for stabilizing the Pd-acyl
intermediates, making L1 exhibit the same activity as L3 as
observed in Table 1 (Entries 3 vs 7).
stabilization of Pd-acyl intermediates (Scheme 2-B and C), but
had no positive effect on the oxidative addition step of aryl
halides.
In summary, a series of bifunctional ligands in combination with
the phosphines and the Lewis acidic phosphenium [P(V)+] cations
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were synthesized and characterized. It was found that in the
catalytic cycle the Lewis acidity of phosphenium site play an
important role in cooperatively activating CO through forming the
secondary bond between acidic P(V)+ and O-atom (in C=O) to
stabilize the Pd-acyl intermediates, while the phosphine sites were
in charge of the coordination to Pd-center for catalyst protection.
When strong Lewis base F- was added to the reaction system, the
catalytic activity was depressed due to the quenching of the P(V)+
acidic centres with strong fluorophilicities. The secondary P(V)+-O
bond with distance of 3.19 Å was observed in the L1-ligated Au(I)-
complex (L1-Au), in which even the weak basic O-atom in ether
group could develop the chemical interaction with the acidic
phosphenium site. Moreover, the PdCl2(CH3CN)-L1 system with the
wide substrate generality could be applied as a recoverable and
recyclable catalyst in RTIL of [Bmim]BF4.
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (Nos. 21473058, 21273077, and
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