Zwitterionic phosphonium ligands: Synthesis, characterization and application in telomerization of 1,3-butadiene

Article abstract of DOI:10.1039/c6cc02747d

Novel zwitterionic phosphonium alkylsulfonate ligands are chemoselectively synthesized from N-heterocyclic phosphines and cyclic sulfones in one step in good to excellent yields. Their in situ generated palladium complexes showed high productivity in the

Full text of DOI:10.1039/c6cc02747d

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Zwitterionic Phosphonium Ligands: Synthesis, Characteri–
zation and Application in Telomerization of 1,3–Butadiene
Received 00th January 20xx,
Accepted 00th January 20xx
A. Pews–Davtyan, R. Jackstell, A. Spannenberg, and M. Beller*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Novel zwitterionic phosphonium alkylsulfonate ligands are At the start of this project, 10 different phosphonium salts were
chemoselectively synthesized from N–heterocyclic phosphines prepared by reaction of the corresponding phosphine and
3
b,5c
and cyclic sulfones in one step in good to excellent yields. Their sultone using modified literature procedure (Table 1).
in–situ generated palladium complexes showed high productivity Preliminary experiments with 2–((diphenylphosphino)methyl)–
in the industrial telomerization of 1,3–butadiene with methanol. 3–methylpyridine led to the formation of a new product with
Optimal results are obtained in the presence of cyclohexyl– the expected molecular weight (Scheme 1, B). However,
substituted ligands under mild conditions and at metal loadings contrary to our expectations alkylation proceeded cleanly at the
as low as 0.001 mol%.
phosphorous atom and not at the pyridinic nitrogen atom.
Hence, the P NMR spectra of the product L1 showed >40
3
1
The design and synthesis of new hydrophilic ligands is an
important task for organometallic catalysis. Apart from the
ppm downfield shift of the original phosphine signal.
1
steric and electronic control of the metal center, such ligands
allow their application in various polar solvents, including
aqueous and biphasic systems. Despite of literally thousands of
known phosphorus–based ligands, little is known on the
application of phosphonium zwitterions in catalysis. Indeed,
X
X
Y
R´
A)
R
P
(CH
I
)
SO
R
P
(CH
II
2 4 3
) SO H
R1
R1
3
2
4
3
3
PR
3
PR3
PR
2
X
X
X
V
(C
H
III
IV
2
4
)
not known SO
3
X, Y = N, CH
only few betainic compounds
which are applied as ionic liquids, are known (Scheme 1,
Furthermore, phosphonium salts of type III and phosphonium
ylides IV (Scheme 1, have been described. More
I and their corresponding salts II,
2
–5
A).
Me
O
O
Me
Me
Ph
Ph
S
B)
+
O
P
P
N
Ph
N
N
Ph
(
CH
2
)
4
SO
3
(
P
C
A
)
H
Ph
Ph
2
4
)
specifically, homo and heteronuclear transition metal
complexes of these latter ligands were synthesized and well–
L1
SO
not formed
3
6
characterized. In addition, phosphonium ylides IV were used
7
as Wittig reagents, and ligands in coupling reactions, acetylene
8
polymerization and olefin hydrogenation.
Recently, we published the synthesis and catalytic
alkoxycarbonylation of olefins in the presence of phosphine
ligands based on
pyrazine and quinoxaline. Based on this work, we were
interested in the preparation of novel hydrophilic derivatives of
N–heterocyclic backbones like lutidine,
9
Scheme 1. A) Zwitterionic phosphonium alkylsulfonates I, corresponding ionic liquids
II, quaternary phosphonium salts III, phosphorus ylides IV, and novel ligands V; B)
this class of compounds. In this respect, here we report for the Model ligand preparation and molecular structure of L1. Displacement ellipsoids are
drawn at the 30% probability level. Hydrogen atoms are omitted for clarity.
first time the synthesis of zwitterionic phosphonium
alkylsulfonate ligands
V and their catalytic applications.
Finally, the structure of L1 was confirmed by crystallographic
analysis (Scheme 1, and supporting information). Notably,
B
the protons of the activated benzylic group are so agile, that in
deuterated solvents they are completely exchanged and even the
1
3
corresponding C signal disappeared. For this reason, most of
the samples were additionally measured in water or methanol as
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solvents at elevated temperatures, because of their low zwitterion L11 was synthesized from fresh prepared 2–
solubility (see NMR scans in supporting information). bis((dicyclohexylphosphanyl)methyl)benzene (see supporting
Next, further analogous ligands with electronic and steric information).
variations at the phosphorous were prepared in
a
straightforward manner under optimized conditions (Table 1).
Simply heating the corresponding phosphine and sultone (1.1
equiv. for each phosphorus atom) in dimethylformamide (140
°
C, for 20 h), filtration and subsequent crystallization gave the
pure products in good to excellent yields (Table 1, L1 L3 L5
L8 L9 L11).
,
,
,
,
,
Table 1 General synthesis of zwitterionic alkylsulfonate phosphonium salts and related
products.
Figure 1. Molecular structure of ligands L3 (left) and L9 (right). Displacement
ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted for
clarity.
Having all these phosphonium salts in hand, we were interested
in the catalytic performance of the resulting palladium
complexes. Interestingly, most of these derivatives possess at
least three possible coordination sites for the metal center:
heterocyclic nitrogen, sulfonate anion and after deprotonation
a
Products structure, ligand, yield
the carbanionic moiety in
group.
Based on our long lasting interest in understanding and
α–position to the phosphonium
L1, 68 %
L4, 68 %
L7, 77 %
L2, 89%
L5, 55 %
L8, 80 %
L11, 94 %
L3, 73 %
L6, 74 %
L9, 84 %
1
0
optimizing telomerization reactions, we investigated the
catalytic dimerization of 1,3–butadiene with methanol
1
1,12
(
telomerization reaction).
process 1–methoxy–2,7–octadiene
precursor for 1–octene, which is made on >100.000 ton–scale/a
The resulting product of this
1
(Table 2) is a key
1
3
as a co–monomer for polyethylene. As a result of the
increasing industrial interest in 1–octene, telomerization
processes have been intensively studied by many industrial and
1
4
15
academic
laboratories.
Indeed,
Dow
Chemical
commercialized this 100% atom–efficient process in the last
decade: Here, the initial telomerization product, 1–methoxy–
2
,7–octadiene, 1 is hydrogenated to 1–methoxyoctane, which is
subsequently cracked to give 1–octene and methanol and in part
dimethylether. As byproducts of the first reaction 3–
L10, 75 %
methoxyocta–1,7–diene
vinylcyclohexene are observed (Table 2).
Until to date, only one report on the use zwitterionic ligands in
2
,
1,3,7–octatriene
3
,
and 4–
a
Isolated yields.
4
For the synthesis of bulky tert.–butyl–substituted ligands L6,
L7 and for L4 more sultone (1,5 eq.), higher reaction
temperature (160 °C) and longer time (up to 36 h) were
necessary for complete conversion.
All products are colorless or off–white air stable solids thus
their handling require no special precautions or dried solvents.
In the course of recrystallizations several single crystals were
formed (from water or methanol) and examined by
crystallographic analysis (Figure 1 and supporting information).
For comparison phosphonium salts L2 and L10, along with
phosphonium zwitterion L11 were synthesized, too (Table 1).
Hence, L2 was obtained from 2–((diphenylphosphino)methyl)–
1
6
telomerization reactions has been reported. In addition, a
palladium complex with carbon coordinated alpha–keto–
1
7
stabilized ylide was applied. They both showed low activity
and low chemoselectivities towards the desired telomer. To the
best of our knowledge, no reports exist on the application of
phosphonium–based zwitterionic ligands in the telomerization
of butadiene.
Our initial experiments were performed with L1 under standard
conditions previously developed in our group using palladium
acetate or palladium acetylacetonate as pre–catalyst and sodium
1
0b
methanolate as base.
Although L1 showed high n/iso
selectivity, nevertheless conversions were low, especially at
higher temperature (Table 2, entries 3 – 6). Notably, under
these conditions the benchmark ligand triphenylphosphine gave
high yield of telomers but lower n/iso selectivity (Table 2, entry
3
–methylpyridine and methyl iodide in 89 % yield, whereas the
reaction of 1–(chloromethyl)–2–methylbenzene with
tricyclohexylphosphane led to L10 in 75 % yield. Finally, the
2
| J. Name., 2012, 00, 1–3
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7
). To our delight, bis–zwitterionic ligands showed better formation was observed, which also demonstrates the
performance depending on the substitution pattern at the stabilization effect of L8 (Table 2, entries 15 and 16). Applying
phosphorus atom, their amount relative to palladium, reaction L8 in the presence of Pd(acac) instead of Pd(OAc) turned out
time and temperature. Thus, L3 enabled the formation of to be more active and gave up to 78 % yield, which is
methyl ethers and with 68 % yield in the presence of only comparable to Pd(acac) /tricyclohexylphosphine (Table 2,
,002 mol% of Pd. Doubling the ligand amount to 0.032 mol% entries 15 and 18 – 20). Inspired by these results, we were
2
2
1
2
2
0
led to an increase of the reaction rate (Table 2, entries 8 and 9). interested to study the catalytic performance of the cyclohexyl–
The monocationic phosphonium salt L2 was completely substituted phosphonium compounds L10 and L11. However,
inactive, while related ligands L4 with propylsulfonate group, as shown in Table 2 (entries 21 – 22) only low conversion
quinoxaline–based ligand L5, as well as tert.–butyl substituted occurred. Notably, under these conditions the parent
ligands L6 and L7 showed moderate performance (Table 2, diphosphine of L11 (1,2–bis((dicyclohexylphosphanyl)
entries 10 – 13).
methyl)benzene) was totally inactive. Finally, best results were
obtained with the quinoxaline–derived ligand L9, which gave
a
Table 2 Telomerization of 1,3–butadiene with methanol. Ligand testing.
8
–
7 % yield of the desired telomers at 60 °C (Table 2, entries 23
26). Again the parent diphosphine ligand of L9, namely 2,3–
bis((dicyclohexylphosphanyl)methyl)quinoxaline as well as
,3–dimethylquinoxaline turned to be completely inactive,
2
demonstrating the advantage of these
phosphonium alkylsulfonate ligands.
Interestingly, the in situ catalyst solution of L9/Pd(acac)
MeOH/NaOMe (intensive orange coloured) is stable for
months. In principle, the active ligand can be also extracted
from the reaction mixture with water and reused with fresh
N
–heterocyclic
b
Entry
Ligand /
Pd source
Ligand / Pd
(mol%)
T
1+2 (%) /
n:iso
(1:2) %
–
c
°C Chemosel.
2
in
d
1
– / Pd(OAc)
2
– / 0.002
50
50
70
90
70
50
50
50
50
50
50
50
50
50
50
50
50
50
50
6 / 67
7 / 66
d
2
– / Pd(acac)
2
– / 0.002
–
3
L1/Pd(OAc)
L1/Pd(OAc)
L1/Pd(OAc)
2
2
2
0.008 / 0.001
0.008 / 0.001
0.016 / 0.002
0.032 / 0.002
0.008 / 0.002
0.016 / 0.002
0.032 / 0.002
0.032 / 0.002
0.016 / 0.002
0.016 / 0.002
0.016 / 0.002
0.008 / 0.001
0.016 / 0.002
0.032 / 0.002
0.032 / 0.004
0.016 / 0.002
0.016 / 0.002
0.016 / 0.002
0.016 / 0.002
0.016 / 0.002
0.016 / 0.002
0.016 / 0.001
0.032 / 0.002
0.032 / 0.002
recycled L9 /
0.002
10 / 83
0.3 / 3
14 / 69
16 / 84
82 / 98
68 / 94
40 / 94
30 / 91
10 / 74
21 / 90
7 / 66
96 : 1
–
d
4
Pd(acac)
It is worth mentioning that in all experiments several hours (2–
h) of incubation period were observed, in which no
2
(Table 2, entry 27).
5
6
7
98 : 1
99 : 1
93 : 1
97 : 1
97 : 1
97 : 1
–
L1/Pd(acac)
PPh /Pd(OAc)
L3/Pd(OAc)
2
7
3
2
e
8
2
measurable gas consumption was detected. We assume the
formation of the active palladium(0) species from the
palladium(II) pre–catalyst is responsible for this delay. For
better understanding of this active catalyst formation, a number
of NMR and electrospray mass spectrometry (ESI–MS)
measurements of ligand/base, ligand/base/palladium systems
were performed. Unfortunately, based on these results no
specific palladium species could be assigned. Deprotonation of
the ligand with sodium methanolate caused the change of
original phosphorus signal to a very broad singlet in P NMR
spectra, which indicated the dynamic processes in the reaction
mixture. Similarly, a complex mixture consisting of the
corresponding ylide, different phosphine derivatives including
phosphineoxide was observed by ESI–MS in the presence of
ligand/base/palladium. Hence, the “real” active catalyst in the
present system is not clear and different N-, P- or even C-
coordinating ligands might be involved. Notably, complexation
9
L3/Pd(acac)
L4/Pd(acac)
L5/Pd(acac)
2
2
2
1
0
d
1
1
1
2
L6/Pd(OAc)
L7/Pd(acac)
2
2
97 : 1
–
d
1
3
4
e
1
L8/Pd(OAc)
L8/Pd(OAc)
L8/Pd(OAc)
L8/Pd(OAc)
2
2
2
2
72 / 95
39 / 93
60 / 95
61 / 93
54 / 94
78 / 94
78 / 96
8 / 69
17 / 81
59 / 96
46 / 97
78 / 98
87 / 96
46 / 91
95 : 1
95 : 1
96 : 1
96 : 1
95 : 1
95 : 1
93 : 1
–
97 : 1
95 : 1
95 : 1
95 : 1
95 : 1
95 : 1
1
5
1
1
1
6
7
8
3
1
L8/Pd(acac)
L8/Pd(acac)
2
f
1
9
2
2
0
3
PCy /Pd(acac)
2
50
50
50
50
50
50
d
2
1
L10/Pd(acac)
L11/Pd(acac)
2
2
2
2
2
2
3
4
5
2
L9/Pd(acac)
L9/Pd(acac)
L9/Pd(acac)
L9/Pd(acac)
From entry
2
2
2
2
g
2
6
60
60
experiments of L1 and PPh
3
Au(acac) proved the formation of
2
7
mono– and binuclear gold (I) complexes, which are according
2
6/Pd(acac)
2
6
c,f
to the literature C-coordinated complexes. In order to gain
more insights on the coordination behaviour of these
alkylphosphonium pre–ligands, we performed complexation
a
Reaction conditions: 1,3–Butadiene (BD) (15 gr, 277 mmol), Pd source, ligand,
0 ml solution of MeONa (1 mol% relative to BD) in MeOH (BD/MeOH ~1:2),
8 h. GC yield. Chemoselectivity: (1+2/1+2+3+4)x100. Product 2 could not be
detected. Reaction time 60 h. Reaction time 48 h. TON 23 000, TOF 1280 h
2
1
b
c
d
e
f
g
–1
experiments using different palladium precursors with L1, L3,
.
and L9. In most cases we observed formation of complex
mixtures. Nevertheless, we succeeded to isolate single crystals
Better catalyst performance was observed in the presence of the
cyclohexyl–substituted ligand L8. In this case, a high catalyst
productivity (TON up to 37 000) is achieved with an 8–fold
excess of ligand (Table 2, entry 14). In general, the
telomerisation process is accelerated by increasing the amount
of ligand relative to metal or the metal itself. Using a large
excess of ligand (e.g. sixteen–fold) no palladium black
of
a
palladium complex formed from ligand L1 and
PdCl (NCPh)
bis(benzonitrile)palladium(II)chloride
2
2
.
According to crystallographic analysis, in the resulting complex
two ligand molecules are coordinated to the palladium centre
via pyridinic nitrogen atoms (Figure
2 and supporting
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accessible
N–heterocyclic phosphines and commercially
1
available cyclic sulfones in up to 94 % yield. These zwitterionic
phosphonium compounds constitute stable hydrophilic pre–
ligands. Their catalytic potential is demonstrated in the
industrially important telomerization of 1,3–butadiene with
methanol. Under optimal conditions in the presence of only
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telomers were obtained in up to 87 yield with
%
chemoselectivities up to 98 %. We believe these novel pre–
ligands have a broad potential for a variety of other catalytic
applications, too.
This work has been supported by the State of Mecklenburg–
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