Efficient aerobic wacker oxidation of styrenes using palladium bis(isonitrile) catalysts

Article abstract of DOI:10.1002/chem.200901560

The palladium-catalyzed aerobic oxidation of alkenes and especially styrenes (Wacker oxidation) by using chiral pseudo C₂-symmetrical bis(isonitrile) ligands in the absence of further cocatalysts gives rise to methyl ketones in a highly chemoselective manner. The palladium bis(isonitrile) catalyst was characterized by NMR spectroscopy and X-ray structure analysis, revealing a dissymmetric coordination of palladium by the two isonitrile moieties.

Full text of DOI:10.1002/chem.200901560

DOI: 10.1002/chem.200901560
Efficient Aerobic Wacker Oxidation of Styrenes Using Palladium
Bis(isonitrile) Catalysts
Anu Naik,[a] Liu Meina,[a] Manfred Zabel,[b] and Oliver Reiser*[a]
Abstract: The palladium-catalyzed aerobic oxidation of alkenes and especially
styrenes (Wacker oxidation) by using chiral pseudo C2-symmetrical bis(isonitrile)
ligands in the absence of further cocatalysts gives rise to methyl ketones in a
highly chemoselective manner. The palladium bis(isonitrile) catalyst was character-
ized by NMR spectroscopy and X-ray structure analysis, revealing a dissymmetric
coordination of palladium by the two isonitrile moieties.
Keywords: isonitriles
ketones · palladium · styrenes ·
Wacker oxidation
·
methyl
Introduction
angles, a feature that has proved to be especially beneficial
for activity and selectivity in palladium-catalyzed cross-cou-
pling reactions.[6]
The palladium(II)-catalyzed oxidation of alkenes to methyl
ketones, known as the Wacker oxidation, is one of the most
important catalytic applications in industry.[1] The original
protocol calls for stoichiometric amounts of copper(II) chlo-
ride as cocatalyst, which has been recognized as a considera-
ble limitation for the overall process. Sustainable alterna-
tives have been developed, notably the application of tert-
butylhydroperoxide for the oxidation of styrenes[2] or molec-
ular oxygen for the oxidation of alkyl-substituted terminal
alkenes.[3] The coordination of palladium with strong s-
donor ligands, that is, N-heterocyclic carbenes or sparteine
proved to be crucial for these successful developments.
Isonitriles are recognized as valuable synthons in organic
synthesis,[4] but have been less frequently applied as ligands
for metal catalysts.[5] Owing to their electronic properties,
being also strong s-donor ligands like N-heterocyclic car-
benes, we thought that palladium isonitrile complexes might
be also candidates for aerobic Wacker oxidations. Moreover,
we envisioned that bidentate bis(isonitrile) ligands, being to
the best of our knowledge unexplored in catalysis, might
lead to chelated metal complexes with especially large bite
Results and Discussion
Oxazoles can be converted into isonitriles upon metalation
followed by trapping of the resulting anion with hard elec-
trophiles such as acetyl chloride or trimethylsilyl chloride.[7]
Following our interest in using oxazolines as building blocks
for multidentate ligand synthesis,[8] we applied this strategy
to chiral oxazolines 1 and phenylphosphonic dichloride,
from which bis(isonitriles) 2 were obtained (Scheme 1). The
[a] A. Naik, L. Meina, Prof. Dr. O. Reiser
Institut fꢀr Organische Chemie, Universitꢁt Regensburg
Universitꢁtsstrasse 31, 93053 Regensburg (Germany)
Fax : (+49)941-9434121
E-mail: Oliver.Reiser@chemie.uni-regensburg.de
[b] Dr. M. Zabel
Institut fꢀr Anorganische Chemie, Universitꢁt Regensburg
Universitꢁtsstrasse 31, 93053 Regensburg (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901560.
Scheme 1. Synthesis of BINC ligands 2, and X-ray structure of complex
2b.
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FULL PAPER
Table 1. Selected bond lengths [ꢃ] and angles [8] of 3b.[a]
Bond lengths [ꢃ] Bond angles [8]
C1-Pd-C8
best results for the valine- and tert-leucine-derived ligands
2a,b were observed when lithium diisopropylamide (LDA)
was used as base, while 2c could only be obtained in lower
yield by applying nBuLi for deprotonating 1c.
À
Pd Cl1
2.296
2.299
1.943
1.935
1.128
1.136
1.457
1.453
88.2
93.7
À
Pd Cl2
À
Pd C1
Cl1-Pd-Cl2
Pd-C1-N1
Pd-C8-N2
C1-N1-C2
C8-N2-C9
177.2
174.4
177.6
171.8
62.5
No erosion of stereochemistry was observed in the course
of the reaction, which gave rise to the new BINC ligands 2
as odorless compounds in enantiomerically pure form. The
structure of 2b was unambiguously established by X-ray
analysis,[9] which revealed a dissymmetric arrangement of
the two diastereotopic isonitrile arms in the solid state that
À
Pd C8
À
C1 N1
À
C8 N2
À
N1 C2
C1-N1-C2-C3
C8-N2-C9-C10
À
N2 C9
43.6
[a] For atom numbering see Scheme 2.
1
are also clearly distinguishable, in both the H and 13C NMR
spectra.
The BINC ligands 2 readily form metal complexes with a
broad variety of transition metals. Relevant for this study,
bered chelate ring. Nevertheless, the bite angle between the
isonitrile arms at the palladium center is with 888, quite
normal for a square-planar d8 complex, which is realized by
a dissymmetric arrangement of the two isonitrile moieties
around the metal center. This geometry is also reflected in
solution, as can be seen from the H NMR spectrum of 3b,
which displays one set of signals for each isonitrile arm (see
the Supporting Information).
complexation of 2a,b with [PdCl2ACHTNUTRGNEUNG(PhCN)2] gave rise to the
PdII complexes 3a,b, which were characterized by NMR
spectroscopy as well as by an X-ray structure analysis of 3b
(Scheme 2, Table 1). The latter revealed that 2b indeed acts
as a bidentate ligand forming with palladium a 12-mem-
1
Recently, Sigman and Cornell discovered the direct palla-
dium-catalyzed Wacker oxidation of terminal alkenes with-
out the need for employing copper cocatalysts.[3] Palladium
[(À)-sparteine] dichloride proved to be efficient for the con-
version of aliphatic alkenes to methyl ketones using molecu-
lar oxygen as the terminal oxidant. This palladium complex
proved also to be applicable for the oxidation of 4-methyl-
styrene to the corresponding methyl ketone when an excess
of tert-butylhydroperoxide (TBHP, 5.5 equiv) was employed.
Alternatively, a palladium(II)–NHC complex in the pres-
ence of catalytic amounts of AgOTf as cocatalyst using
again TBHP as the terminal oxidant was reported by the
same authors to be efficient for the generally more challeng-
ing oxidation of styrenes, a process that is often hampered
by competing C=C-bond cleavage.[2] Moreover, Kaneda and
co-workers disclosed that [PdACTHUNTRGNE(UNG CH3CN)2Cl2] is a Wacker cat-
alyst that can be used under 6 atm oxygen pressure.[10]
Inspired by these results, we tested 3a,b in the oxidation
of terminal alkenes in the absence of any further cocatalysts
using molecular oxygen at ambient pressure. An initial
screening revealed that both complexes give very similar
yields and selectivities in Wacker oxidations (Table 3, en-
tries 5 and 10). Therefore, we subsequently evaluated 3b,
which can be isolated in high puritiy by recrystallization and
stored without signs of decomposition. Compound 3b effec-
tively catalyzed the oxidation of aliphatic alkenes (Table 2)
by using dimethylacetamide (DMA)/water[3] as the solvent
system. Careful GC analyses revealed that no isomerization
or C=C-bond cleavage had occurred, and that the corre-
sponding methyl ketones were generated in high yields and
excellent purity. Terminal alkenes possessing oxygenated
functional groups were found to be suitable substrates as
well, and, notably, hydroxyl groups were not oxidized under
the reaction conditions.
Scheme 2. Synthesis of 3a,b and X-ray structure of the PdII complex 3b.
Abstract in German: Die aerobe Palladium-katalysierte Oxi-
dation von Alkenen zu Methylketonen (Wacker Oxidation),
insbesondere von Styrolen, gelingt mit chiralen Pseudo-C2-
symmetrischen Bisisonitrilliganden ohne weitere Cokatalysa-
toren mit hoher Chemoselektivitꢀt. Strukturelle Untersuchun-
gen der Katalysatoren mittels NMR und Rçntgenstrukturana-
lyse ergab, dass sowohl in Lçsung wie auch im Festkçrper
Palladium unsymmetrisch durch die zwei Isonitrileinheiten
koordiniert wird.
As a control experiment, we performed the oxidation of
1-octene also with the palladium(II) complex of the mono-
dentate isonitrile ligand 4,[7e] which also proceeded well but
Chem. Eur. J. 2010, 16, 1624 – 1628
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www.chemeurj.org
1625

O. Reiser et al.
Table 2. Wacker oxidation of aliphatic alkenes.
tions, we were delighted to find that also for these substrates
3b is effective at ambient oxygen pressure in the absence of
any further cocatalyst (Table 3). A reaction temperature of
708C and a DMA/water mixture of 6:1 was found to give
the highest ratios between methyl ketones and aldehydes
(Table 3, entry 5). As shown for the oxidation of 4-methyl-
styrene, the bidentate bis(isontrile) ligand 2b is clearly supe-
rior to the monodentate isonitrile ligand 4 (Table 3, entry 2)
or palladium chloride alone (Table 3, entry 1). Electron-rich
styrenes showed higher reactivity, but also better selectivity
towards methyl ketone formation than electron-poor deriva-
tives. Nevertheless, good yields and selectivities could be ob-
tained also for the latter (Table 3, entries 8–10).
Entry
Catalyst
Substrate
Reaction
time [h]
Conversion
[%][a]
1
2
3b
1-octene
1-octene
24
24
98 (75)
98[d]
ACHTUNGTRENNUNG
[PdCl2·(4)2][c]
3
3b
3b
48
97[e] (77)
Attempts to lower the catalyst concentration were not
successful: While the conversion of substrates still proceeds
well even at 1 mol% 3b, substantially higher amounts of al-
dehydes are observed due to carbon–carbon bond cleavage
(Table 1, entries 7–9), suggesting that the palladium isoni-
trile complexes are not stable under the reaction conditions
and that background reactions involving palladium(0) alone
occur over time. Control experiments showed that 2b is
stable in a 6:1 DMA/water mixture even at 1008C for elon-
gated times. However, 2b showed appreciable decomposi-
tion in a 6:1 DMA/water mixture at a reaction temperature
of 708C when palladium(II)chloride is present. In addition,
when 2b is employed in excess with respect to palladium,
complete decomposition of 2b is observed over time.
Judged by the disappearance of the isonitrile band in the IR
spectrum, we speculate that palladium(II) is capable of acti-
vating 2b towards the attack of nucleophiles such as water
present in the reaction, however, we did not observe the cor-
responding formamides that would result from addition of
water to 2b.
4
48
92 (71)
5
3b
48
75
6
7
3b
3b
48
48
84
98
[a] Determined by GC using decane as the internal standard; isolated
yields in parentheses. [b] 5 mol% PdCl2, 10 mol% 4. [c] 3% isomerized
alkene oxidation products. [d] Reaction conditions: 0.125m concentration
of substrate, 6:1 DMA/H2O.
Table 3. Wacker oxidation of aromatic alkenes.
Entry Catalyst
R
Reaction Conversion
Ketone/
aldehyde
time [h]
[%][a]
1
2
PdCl2
ACHTUNGTRENNUNG[PdCl2·(4)2] 4-Me
4-Me
70
70
70
40
40
70
40
40
40
40
70
70
>99
>99
91
>99
>99
81
98
96
90
98
4
6
Conclusions
3[b]
4[c]
5
3b
3b
3b
3b
3b
3b
3b
3a
3b
3b
4-Me
4-Me
4-Me
4-Me
4-Me
4-Me
4-Me
4-Me
H
2-vinyl-
naphthalene
3-Cl
4-Cl
4-Br
18
21
26
14
14
6
In conclusion, the novel palladium–bis(isonitrile) complex
3b was synthesized and fully characterized and was used for
the aerobic oxidation of terminal aliphatic and aromatic al-
kenes in good yields and selectivities. No cocatalysts had to
be employed, which is, to the best of our knowledge, unpre-
cedented for styrenes when molecular oxygen is used at am-
bient pressure as the terminal oxidant.[11] Further explora-
tion of the BINC ligands introduced here in other metal-cat-
alyzed reactions is currently ongoing in our laboratories.
6[d]
7[e]
8[f]
9[g]
10
11
12
8
23
17
11
84
88
13
14
15
16
3b
3b
3b
3b
96
96
96
48
72
98
50
7
7
9
4-OMe
>99(75)
21
Experimental Section
[a] Determined by GC using decane as internal standard; isolated yields
in parentheses. [b] DMA/water 2:1. [c] DMA/water 4:1. [d] Reaction
temperature 1008C. [e] 2.5 mol% 3b. [f] 1 mol% 3b. [g] 1 mol% 3b +
2 mol% 2b.
All reagents, unless otherwise specified were purchased from commercial
sources and used without further purification. THF, diethyl ether, and
toluene were distilled over sodium/benzophenone. DMA was dried over
vacuum activated 4 ꢃ molecular sieves. GC conversions for the reactions
were determined relative to decane as an internal standard.
gave around 3% of oxidation products stemming from
alkene isomerizations.
Synthesis of BINC ligands
Turning to the more challenging styrenes because of their
propensity for C=C-bond cleavage under oxidative condi-
2a: n-Butyllithium (15% in hexane, 0.5 mL, 1.15 mmol) was added to a
solution of diisopropylamine (0.2 mL, 1.42 mmol) in THF (4 mL) under
1626
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Chem. Eur. J. 2010, 16, 1624 – 1628

Aerobic Wacker Oxidation of Styrenes
FULL PAPER
nitrogen atmosphere at 08C. After the mixture had been stirred for
15 min it was cooled down to À788C and isopropyl-2-oxazoline (1a,
100 mg, 0.885 mmol) in THF (4 mL) was added. The mixture was stirred
for 30 min, and then phenylphosphonic dichloride (0.07 mL, 0.53 mmol)
was added followed by immediate removal of the cooling bath. Subse-
quently, the mixture was allowed to stir at room temperature for 2 h.
Aqueous NH4Cl solution was added followed by workup of the mixture
with ethyl acetate and brine. The organic layer was dried over Na2SO4
and concentrated under vacuum, and the residue was purified on silica
(hexanes/ethyl acetate 2:3, Rf =0.52). Yield: 113 mg, 61%, yellowish vis-
three times. The resulting solid was washed with diethyl ether (3ꢄ1 mL)
to give 3a (67 mg, 91% yield) as a white solid. Crystals suitable for X-
ray-crystallography were obtained by recrystallization of 3 from benzene.
IR (neat): n˜ =2237 (CN); 1250 (P=O); 343, 321 (Pd-Cl) cmÀ1 1H NMR
;
(CDCl3, 300 MHz): d=1.05 (s, 9H), 1.12 (s, 9H), 3.93 (dd, 1H, J=
10.3 Hz, 3.19 Hz), 3.98 (ddd, 1H, J=10.5 Hz, 11.08 Hz, 7.6 Hz), 4.16 (dd,
1H, J=7.6 Hz, 4.2 Hz), 4.18 (ddd, 1H, J=3.6 Hz, 10.9 Hz, 3.2 Hz), 4.29
(ddd, 1H, J=7.6 Hz, 11.08 Hz, 4.2 Hz), 4.49 (ddd, 1H, J=8.4 Hz,
10.9 Hz, 10.3 Hz), 7.55–7.64 (m, 2H), 7.65–7.74 (m, 1H), 7.77–7.88 ppm
(m, 2H); 13C NMR (CDCl3, 150 MHz): d=26.29, 33.95, 62.94, 64.20,
67.95, 68.72, 119.67 (NC), 120.94 (NC), 125.23 (d, J=187 Hz), 129.26,
131.81, 133.90 ppm; 31P NMR (CDCl3, 121.5 MHz): d=21.05 ppm (s); ele-
mental analysis calcd (%) for C20H29Cl2N2O3PPd: C 43.38, H 5.28, N
5.06; found: C 42.88, H 5.50, N 4.83.
cous liquid. [a]20 =+36 ( c=1.0, CHCl3); IR (neat): n˜ =2969, 2141,
D
1595, 1475, 1442, 1253 cmÀ1 1H NMR (CDCl3, 300 MHz): d=7.80–7.90
;
(m, 2H), 7.58–7.66 ( m, 1H), 7.48–7.55 (m, 2H), 4.03–4.28 (m, 4H), 3.63–
3.76 (m, 2H), 1.88–2.03 (m, 2H), 1.03 (d, 6H), 1.01 ppm (d, 6H);
13C NMR (CDCl3, 75.5 MHz): d=158.3 (CN), 133.37 (d, J=2.8 Hz),
131.88 (d, J=10.5 Hz), 128.85(d, J=15.4 Hz), 126.05 (d, J=190.9 Hz),
65.49, 60.80, 28.83, 28.78, 19.40, 19.34, 17.09, 16.97 ppm; 31P NMR
(CDCl3, 121.5 MHz): d=20.78 ppm (s); MS (EI-MS): m/z: [M+H+] 349,
[MNH4+] 366; HRMS: calcd for C18H25O3N2P [M+]: 348.160, found:
348.1603.
Representative procedure for the Wacker oxidation of alkenes: In a
flame-dried 10 mL Schlenck tube equipped with a sidearm and stir bar, a
mixture of [PdCl2ACHTUNTRGNEUNG(tBu-BINC)] (3; 14 mg, 5 mol%) and a 6:1 (v/v) solu-
tion of DMA:H2O (4 mL) were heated at 708C for 10 min to assure com-
plete solubility of the catalyst. The tube was allowed to cool to room
temperature and connected with a condenser and a one-way joint with a
balloon of O2. The tube was evacuated (50 mbar) and refilled with O2
three times. The reaction mixture was stirred vigorously for 10 min after
which the alkene (0.5 mmol) was added. The mixture was then heated at
708C for the indicated reaction time. After cooling to room temperature,
the reaction mixture was analyzed by GC using decane as the internal
standard.
2b: n-Butyllithium (15% in hexane, 0.22 mL, 0.512 mmol) was added to
a solution of diisopropylamine (0.09 mL, 0.630 mmol) in THF (2 mL)
under nitrogen atmosphere at 08C. After the mixture had been stirred
for 15 min it was cooled down to À788C and tert-butyl-2-oxazoline (1b,
50 mg, 0.394 mmol) in THF (2 mL) was added. The mixture was stirred
for 30 min, and then phenylphosphonic dichloride (0.03 mL, 0.236 mmol)
was added followed by immediate removal of the cooling bath. Subse-
quently, the mixture was allowed to stir at room temperature for 2 h.
Aqueous NH4Cl solution was added followed by workup of the mixture
with ethyl acetate and brine. The organic layer was dried over Na2SO4
and concentrated under vacuum, and the residue was purified on silica.
(hexanes/ethyl acetate 2:3, Rf =0.56). Yield: 52 mg, 59%, white solid.
[a]20D =+83 (c=1.0, CHCl3); IR (KBr): n˜ =2964, 2140, 1594, 1475, 1442,
For product isolation diethyl ether was added to the reaction mixture fol-
lowed by extraction twice with 1n HCl. The aqueous layers were com-
bined and extracted three times with diethyl ether. The organic layers
were combined and washed with brine, dried over Na2SO4, and concen-
trated. The residue was purified by flash silica chromatography.
1394, 1370, 1346, 1246, 926 cmÀ1 1H NMR (CDCl3, 300 MHz): d=7.87
;
Acknowledgements
(dd, 2H, J=7 Hz, 13.8 Hz), 7.61(t, 1H, J=7.5 Hz), 7.45–7.54 (m, 2H),
4.28- 4.36 (m, 1H), 4.16–4.27 (m, 2H), 4.01 (q, 1H, J=9.2 Hz), 3.68 (dd,
1H, J=3.1, 9.1 Hz), 3.54 (dd, 1H, J=3.8, 8.9 Hz), 1.03 ppm (s, 18H);
13C NMR (CDCl3, 75.5 MHz): d=158.37 (NC), 133.45 (d, CArÀp, J=
3.05 Hz)), 132.06 (d, CArÀo, J=10.3 Hz), 128.96 (d, J=15.5 Hz), 126.4 (d,
J=191 Hz), 65.07, 64.77, 33.56 , 33.50, 26.37 ppm; 31P NMR (CDCl3,
This work was supported by the DAAD (fellowship for AN) and the
Fonds der Chemischen Industrie. Helpful comments of the referees in
the reviewing process of this manuscript are gratefully acknowledged.
+
121.5 MHz): d=20.96 ppm (s); MS (EI-MS), m/z: [M+H+] 377, [MNH4
] 394. HRMS: calcd for C20H29O3N2P [M+]: 376.190, found: 376.191.
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2c: n-Butyllithium (15% in hexane, 1.7 mL, 4.04 mmol) was added to a
solution of benzyl-2-oxazoline (1c, 500 mg, 3.11 mmol) in THF (40 mL)
under nitrogen atmosphere at À788C. After the mixture had been stirred
for 30 min, and then phenylphosphonic dichloride (0.3 mL, 1.9 mmol)
was added followed by immediate removal of the cooling bath. Subse-
quently, the solution was allowed to stir at room temperature for 2 h.
Aqueous NH4Cl solution was added, followed by workup of the mixture
with ethyl acetate. The organic layer was dried over Na2SO4 and concen-
trated under vacuum, and the residue was purified on silica. (hexanes:
ethyl acetate=2:3, Rf =0.47). Yield: 276 mg, 33%, yellowish viscous
liquid. [a]20D =+3.4 ( c=1.5, CHCl3); IR (KBr): n˜ =2140, 1593, 1495,
1253, 961, 746 cmÀ1 1H NMR (CDCl3, 300 MHz): d=7.83–7.91 (m, 2H),
;
7.61–7.66 (m, 1H), 7.49–7.56 (m, 2H), 7.17–7.36 (m, 10H), 4.11–4.28 (m,
3H), 3.97–4.06 (m, 3H), 2.97 ppm (t, 4H); 13C NMR (CDCl3, 75.5 MHz):
d=159.01 (NC), 134.82, 133.60 (d, J=3.05 Hz), 132.06 (d, J=10 Hz),
129.38, 129.04 (d, J=15 Hz), 129.02, 127.76, 126.06 (d, J=191 Hz), 65.80,
56.09, 37.64 ppm; 31P NMR (CDCl3, 121.5 MHz): d=20.77 ppm (s); MS
(ES-MS): m/z: [M+H+] 455, [MH++H2O] 463; HRMS: calcd for
C26H26O3N2P [MH+]: 445.168, found: 445.167.
A
(tBu-BINC)] (3b): A mixture of diisonitrile ligand (2b, 50 mg,
2A(PhCN)2] (50 mg, 0.131 mmol) in dichlorome-
CTHUNGTRENNUNG
thane (2 mL) was stirred at room temperature for 16 h. The mixture was
filtered through a small pad of celite and washed with dichloromethane.
The filtrate wasconcentrated to a volume of approximately 0.5 mL. The
crude product was precipitated by addition of hexane (1 mL) and the sol-
vent was decanted after stirring for 10 min. This procedure was repeated
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[11] While this manuscript was under revision, a report on the aerobic
oxidation of styrene using palladium chloride without cocatalyst ap-
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Received: June 9, 2009
Revised: October 1, 2009
Published online: December 10, 2009
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