A convenient and selective palladium-catalyzed aerobic oxidation of alcohols

Article abstract of DOI:10.1002/chem.201302526

An efficient procedure for the oxidation of primary and secondary alcohols to aldehydes and ketones, respectively, with molecular oxygen under ambient conditions has been achieved. By applying catalytic amounts of Pd(OAc) ₂ in the presence of tertiary phosphine oxides (O=PR₃) as ligands, a variety of substrates are selectively oxidized without formation of ester byproducts. Spectroscopic investigations and DFT calculations suggest stabilization of the active palladium(II) catalyst by phosphine oxide ligands.

Full text of DOI:10.1002/chem.201302526

FULL PAPER
DOI: 10.1002/chem.201302526
A Convenient and Selective Palladium-Catalyzed Aerobic Oxidation of
Alcohols
Saravanan Gowrisankar,[a] Helfried Neumann,[a] Dirk Gçrdes,[b] Kerstin Thurow,[c]
Haijun Jiao,[a] and Matthias Beller*[a]
Abstract: An efficient procedure for the oxidation of primary and secondary alco-
hols to aldehydes and ketones, respectively, with molecular oxygen under ambient
conditions has been achieved. By applying catalytic amounts of Pd
(OAc)2 in the
Keywords: alcohols · density func-
tional calculations · oxidation · pal-
ladium · phosphine oxides
presence of tertiary phosphine oxides (O=PR3) as ligands, a variety of substrates
are selectively oxidized without formation of ester byproducts. Spectroscopic in-
vestigations and DFT calculations suggest stabilization of the active palladium(II)
catalyst by phosphine oxide ligands.
Introduction
plications. We wondered whether the resulting tertiary phos-
phine oxide (TPO) might be the “real” stabilizing ligand in
such cases, although TPOs are not recognized as strong
binding ligands for late transition metals. In fact, the combi-
nation of palladium precursors and TPOs as a catalyst
system has only been described by Denmark and co-workers
for specific coupling reactions of potassium (4-methoxyphe-
nyl)dimethylsilanolate with aryl bromides.[8]
In organometallic chemistry, phosphine oxides have at-
tracted general attention with respect to their weak coordi-
nating abilities to various metal centers.[9] For example,
Grushin and co-workers reported firm evidence for the re-
versible Pd–O interaction of bidentate phosphine–phosphin-
oxide ligands.[10] In addition, the oxidized dppf(O)2 ligand
(dppf=1,1’-bis(diphenylphosphino)ferrocene) acts as an
[O,O]-chelating ligand and forms the stable palladium com-
The development of selective catalytic oxidations by apply-
ing molecular oxygen in organic synthesis remains a chal-
lenging task, which is of importance for both the chemical
industry and academic research.[1] In this regard, palladium-
catalyzed oxidations of alcohols to carbonyl compounds
have received significant attention in the past decade.[2] Ele-
gant catalyst developments for this reaction include the
combination of palladium(II) salts with heterocyclic N-
donor ligands[3] as well as carbene (NHC) ligands.[4] Notably,
most of the known catalyst systems have been mainly ap-
plied to the oxidation of reactive benzylic alcohols. Much
less work is known regarding the use of more challenging
aliphatic substrates.[5]
Apart from the well-investigated nitrogen-based ligands,
few organometallic complexes with phosphines have been
applied to the oxidation of alcohols.[6] For example, palladi-
um-catalyzed oxidations with tertiary phosphines, which
have been used extensively in palladium-catalyzed cross-
coupling reactions, are quite limited.[7] Clearly, the air sensi-
tivity of most phosphines stays in sharp contrast to such ap-
plex [PdCl2ACHTUNRGTNEUNG(dppfO2)] as shown by X-ray structure analy-
sis.[11] Moreover, TPOs were used as co-catalysts in Pauson–
Khand reactions,[12] and to facilitate crystallizations.[13]
Recently, we developed a novel palladium/cataCXium A-
catalyzed oxidative cross-esterification of alcohols in the
presence of molecular oxygen.[7a] During these investigations
we also observed partial oxidation of the respective diada-
mantyl-n-butylphosphine ligand. Based on this result, we
became interested in the use of TPOs as ligands in alcohol
oxidations and we report herein a general and highly selec-
tive catalytic oxidation of primary and secondary alcohols to
the corresponding carbonyl compounds by utilizing molecu-
lar oxygen under mild conditions (Scheme 1). To the best of
our knowledge no similar oxidation of alcohols with TPOs
as ligands is known.
[a] Dr. S. Gowrisankar, Dr. H. Neumann, Dr. H. Jiao,
Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
Fax : (+49)381-1281-5000
E-mail: matthias.beller@catalysis.de
[b] D. Gçrdes
Institut fꢀr Automatisierungstechnik, Universitꢁt Rostock
Richard-Wagner-Strasse 31, 18051 Rostock (Germany)
[c] K. Thurow
Center for Life Science Automation (CELISCA)
Universitꢁt Rostock
Results and Discussion
Friedrich-Barnewitz-Strasse 8, 18119 Rostock (Germany)
At the start of our investigations we took the oxidation of 1-
naphthylmethanol to 1-naphthylaldehyde as a model reac-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302526.
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Scheme 1. Palladium-catalyzed oxidation of alcohols to aldehydes and ke-
tones.
tion. Typically, catalytic experiments were performed in the
presence of 3 mol% PdACTHNUTRGNE(UNG OAc)2, 5 mol% ligand, and
30 mol% sodium acetate as base at 808C under 1 bar of O2
(balloon) as oxidant. To remove the formed water, molecu-
lar sieves were added.[14] Notably, even without ligand and/
or base low yields (30–48%) of the desired product were
obtained (Table 1, entries 1 and 2). Next, we explored the
Scheme 2. Phosphine oxide ligands (L) tested in the Pd-catalyzed oxida-
tion of 1-naphthylmethanol. Cy=cyclohexyl , Ad=adamantyl.
Table 1. Palladium-catalyzed oxidation of 1-naphthylmethanol: variation
of TPOs.[a–e]
byproduct formation. Although ligand L11 gave the best re-
sults, for our further studies we used L3 due to its commer-
cial availability and easy handling.
Entry
Ligand
Base
Yield [%][a,d]
1[b]
2[c]
3
4
5
6
7
8
9
10
11
12
13[e]
14[e]
15[e]
–
–
–
30
48
82
52
50
64
57
78
32
36
72
79
88
87
65
After having demonstrated the good performance of the
PdII/TPO catalyst in the model reaction, we investigated the
selective oxidation of a series of structurally diverse benzylic
alcohols and one allylic substrate (Table 2). Benzyl alcohols
with electron-donating groups gave high yields (76–79%,
Table 2, entries 2, 3, and 6), whereas benzyl alcohols with
electron-withdrawing groups were slightly less reactive (60–
64%, Table 2, entries 4 and 5). The synthetically more inter-
esting 3-phenylallyl alcohol gave cinnamyl aldehyde in ex-
cellent yield (98%) and selectivity (99%).
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
–
L3
L1
L2
L4
L5
L6
L7
L8
L9
L10
L11
L12
L13
–
–
Table 2. Palladium-catalyzed oxidation of primary benzylic and allylic al-
cohols.[a]
[a] Reaction conditions: 1-naphthylmethanol (0.63 mmol), 3 mol% Pd-
(OAc)2 (0.018 mmol), 5 mol% L1–L10 (0.032 mmol), 50% NaOAc,
100 mg molecular sieve (MS) 4 ꢃ, 1 mL toluene, 80–858C, 1 bar O2, 10–
20 h. [b] 3mol% Pd(OAc)2. [c] 100 mol% NaOAc. [d] GC yield. [e] No
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
base, 30 mol% L11–L13.
Entry
Product
Yield[b] [%]
reactivity of the palladium catalyst in the presence of a varie-
ty of TPO ligands (Scheme 2, L1–L13). To our delight, we
found that addition of simple tri-n-butylphosphine oxide L3
increased the catalyst performance and gave 82% yield
(Table 1, entry 3).
Application of phosphine sulfides (L7 and L8) had basi-
cally no influence on the reaction (Table 1, entries 9 and
10). However, phosphine oxides L1, L2, L4, L5, L6, L9, and
L10 had a positive effect on the reaction outcome. Interest-
ingly, the pyridine-modified phosphine oxide ligand L11 re-
sulted in the best product yield of 88% (Table 1,
entry 13).[15]
Notably, most of the experiments shown were performed
at least twice to ensure reproducibility. In addition to the
ligand screening the effects of different bases and solvents
were investigated, too. Apart from sodium acetate, pyridine
gave good results, whereas the addition of NaHCO3 and
K3PO4·H2O resulted in lower product yields and increased
1
2
3
4
5
6
7
8
R=H
81
79
76
64
60
77
80
78
R=CH3
R=OCH3
R=CF3
R=Cl
R=OCH2Ph
R=CH3
R=OCH3
9
80
98
10
[a] Reaction conditions: alcohol (200 mg), 3 mol% PdACTHUNRGTNEUNG(OAc)2, 5 mol%
L3, 50 mol% NaOAc, MS 4 ꢃ (100 mg), toluene (3 mL), 808C, O2
(1 bar). [b] Yield of isolated product. Complete consumption of starting
material was monitored by TLC and GC-MS analysis.
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Aerobic Oxidation of Alcohols
FULL PAPER
With respect to reproducibility, the palladium-catalyzed
oxidation of secondary alcohols led to better results if pyri-
dine was used as base instead of NaOAc. As shown in
Table 3, a variety of secondary benzylic and aliphatic sub-
strates performed well under these conditions. For example,
1-arylethanol derivatives, 1-cyclohexylethanol, and 1-naph-
Table 3. (Continued)
Entry
Product
Yield [%][c]
85
15
16
80[d]
Table 3. Palladium-catalyzed oxidation of secondary alcohols.[a,b]
[a] Reaction conditions: 200 mg alcohol, 3 mol% PdACTHNUGTRENUNG(OAc)2, 5 mol% L3,
50 mol% pyridine, 100 mg MS 4 ꢃ, 3 mL toluene, 808C, 1 bar O2.
[b] Complete consumption of starting material was monitored by TLC
and GC-MS analysis. [c] Yield of isolated product. [d] GC yield.
Entry
Product
Yield [%][c]
thylethanol (Table 3, entries 1–6) reacted smoothly in the
presence of 1 bar of oxygen to give the corresponding ke-
tones in 81–88% yield. Similar good results were observed
on applying secondary allylic alcohols (Table 3, entry 16)
and cyclic secondary alcohols (Table 3, entries 7–11), thus
demonstrating the versatility of the reaction.
1
2
3
R=H
R=CF3
R=OCH3
81
83[d]
85[d]
Finally, we were interested in the selective oxidation of
demanding longer-chain aliphatic primary and secondary al-
cohols, which are of interest as detergents and plasticizer in-
termediates. Gratifyingly, less reactive linear alcohols, such
as 1- and 2-octanol, 1-decanol, and 1-hexadecanol, can be
easily oxidized to the corresponding aldehydes and ketones
in high selectivity and good yields, albeit a higher loading of
4
5
87
88[d]
PdACTHNUGTRENNG(U OAc)2 was required for these substrates (Table 4). Nota-
6
7
8
9
84
90
55
78
Table 4. Palladium-catalyzed oxidation of aliphatic alcohols.[a–b]
Entry
1
Product
Yield [%][a,b]
73
2
3
79
65
10
79[d]
4
68
11
12
77[d]
[a] Reaction conditions: 200 mg alcohol, 5 mol% PdACTHNUGTRENUNG(OAc)2, 5 mol% L3,
50 mol% pyridine, 100 mg MS 4 ꢃ, 3 mL toluene, 808C, 1 bar O2. [b] GC
yield.
90
bly, in all cases the corresponding carboxylic acids and
esters were not observed as byproducts, thereby revealing
the high chemoselectivity of this oxidation process.
13
14
93
83
To understand the influence of the phosphine oxide li-
gands on the palladium center, NMR and in situ mass spec-
troscopic investigations were performed. The resulting
mechanistic proposal is shown in Figure 1. We believe the
phosphine oxide ligand binds to the starting palladium ace-
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15981

M. Beller et al.
ligand is coordinating to the
metal center through the
oxygen of the phosphine oxide.
A similar trend was observed in
rhodium complexes with phos-
phine oxide.[17] Due to the
broad signal at room tempera-
ture a fast ligand exchange is
likely under the reaction condi-
tions.
To provide information about
the structure and stability of
the PdII/TPO complexes in
more detail, we carried out
density
functional
theory
(DFT) calculations on the com-
plexation of L10 to give the
corresponding palladium com-
plex (see Figure 4 and the Sup-
Figure 1. Proposed mechanism for PdII/TPO-catalyzed oxidation of alcohols.
tate species in a reversible manner. Either the phosphine
oxide-ligated complex or the nonligated palladium(II) spe-
cies reacts with the primary or secondary alcohol to form
the corresponding palladium(II)–alkoxide complex I. Subse-
quent b-hydride elimination gives the product and the hydri-
dopalladium acetate complex. After reductive elimination
electron-rich palladium(0) species are formed, which are
known to agglomerate easily to palladium black. Apparent-
ly, this deactivation process is avoided in the presence of
phosphine oxide ligands. Instead, reoxidation with molecular
oxygen takes place, which is likely to proceed via the cyclic
peroxy complex.[16]
Investigations on the complexation of phosphine oxides,
such as tri-n-butylphosphine oxide (L3), triphenylphosphine
oxide (L10), and diadamantyl-n-butylphosphine oxide (L6),
Figure 2. HRMS of the complexes [PdACHTUNGTRNEUNG
(O=PBu3)3Cl]+.
with different palladium precursors Pd
(CH3CN)2Cl2, and Pd(PhCN)2Cl2 revealed the formation of
stable PdII/TPO complexes. In fact, the combination of L3
and [Pd(PhCN)2Cl2] in toluene at 808C led to the formation
ACHTUNGRTENN(UNG OAc)2, Pd-
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
of stable ligated phosphine oxide complexes. High-resolu-
tion mass spectrometry (HRMS) of the reaction mixture
clearly indicates that three ligands are involved in the coor-
dination sphere of PdII. The isotopic pattern and mass prove
that [Pd
(Figure 2).
Ligands L10 and L6 lead to the corresponding [Pd
PPh3)3Cl]+ and [Pd(O=PBuAd2)3Cl]+ complexes (see the
ACHTUNGTRENNUNG
(O=PBu3)3Cl]+ species have been formed
AHCTUNGERTG(NNUN O=
AHCTUNGTRENNUNG
Supporting Information). The 31P NMR spectra of the differ-
ent palladium complexes in CDCl3 exhibit a broad signal,
which is shifted downfield relative to the phosphorus signal
of the free phosphine oxide ligand. Although at room tem-
perature the 31P chemical shift of the free ligand L3 in
CDCl3 occurs at d=49.0 ppm, a broad singlet is observed at
d=56.3 ppm in the presence of PdII. This signal splits into
two singlets at d=51.7 and 70.2 ppm when the spectrum is
recorded at ꢀ608C (Figure 3). This indicates a dynamic pro-
cess in which scrambling among the ligands occurred. Fur-
thermore, low-temperature 31P NMR results suggest that the
Figure 3. 31P NMR studies of the complexes [Pd
ACHTNUTRGNE(UGN O=PBu3)3Cl2].
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Aerobic Oxidation of Alcohols
FULL PAPER
Experimental Section
General procedure for oxidation of primary alcohols: An oven-dried
10 mL vial tube was charged with Pd(OAc)2 (3 mol%), ligand L3
AHCTUNGTRENNUNG
(5 mol%), NaOAc (50 mol%), primary alcohols (200 mg, 1.0 equiv), mo-
lecular sieve (MS) 4 ꢃ (100 mg), and toluene (3 mL). The reaction mix-
ture was stirred at 808C for 20–40 h in the presence of an oxygen balloon,
and then cooled to room temperature, diluted with water, extracted with
ethyl acetate (50 mL), and concentrated. The crude product was purified
by flash chromatography.
General procedure for oxidation of secondary alcohols: An oven-dried
10 mL vial tube was charged with PdACHTNUTRGNEUNG(OAc)2 (3 mol%), ligand L3
(5 mol%), pyridine (50 mol%), secondary alcohols (200 mg, 1.0 equiv),
MS 4 ꢃ (100 mg), and toluene (3 mL). The reaction mixture was stirred
at 808C for 20–40 h in the presence of an oxygen balloon, and then
cooled to room temperature, diluted with water, extracted with ethyl ace-
tate (50 mL), and concentrated. The crude product was purified by flash
chromatography.
General procedure for synthesis of complex [Pd
ACHTUNGTRENNUNG
dried 10 mL Schlenk tube was charged with PdACHTUNGTRENNNUG
Figure 4. BP86/TZVP-optimized [PdACTHNUTRGNEUNG
(O=PPh3)3Cl]+ structure (bond
length in ꢃ, and hydrogen atoms are omitted for clarity).
ligand L3 (3.0 equiv), and toluene (1–2 mL). The reaction mixture was
stirred at 858C for 16 h under an argon atmosphere, and then cooled to
room temperature. After the toluene was removed, a portion of crude
product was subjected to NMR spectroscopy. 1H NMR (300 MHz,
CDCl3): d=1.89 (td, J=11.3, 6.0 Hz, 6H), 1.64–1.31 (m, 12H), 0.90 ppm
(t, J=7.1 Hz, 9H); 13C NMR (75 MHz, CDCl3): d=27.04, 26.19, 24.26,
24.07, 23.59, 23.53, 13.75 ppm; 31P NMR (121 MHz, CDCl3): d=
56.35 ppm.
porting Information). We found that the assumed complexes
[Pd
ligand O=PPh3 constitute energy minimum structures. In the
case of [Pd
(O=PPh3)3Cl]+ a nearly planar structure was cal-
ACHTUNGTRENNUNG ACHTUNGTRENNUGN
(O=PPh3)3Cl]+ and [Pd(O=PPh3)2Cl]+ as well as the free
General procedure for synthesis of 2-(dialkylphosphoryl)pyridine (L11–
L13): 2-Bromopyridine (1.0 equiv), PdACHTNUTRGNEUNG(OAc)2 (1 mol%), dppf (2 mol%),
ACHTUNGTRENNUNG
ꢀ
culated. In the parent ligand O=PPh3 the P O distance is
and NaOtBu (1.5 equiv) were placed in a Schlenk tube. The Schlenk tube
was closed and placed under argon. The mixture was dissolved in dry tol-
uene. Dialkyl phosphine (1.1 equiv) was added slowly. The resulting solu-
tion was stirred overnight at 1008C under argon. The reaction mixture
was washed with water and extracted with dichloromethane. The solvent
was removed and the product was oxidized with an excess of H2O2 in tol-
uene before purification by short column chromatography or crystalliza-
tion.
(O=PPh3)3Cl]+ the P
ꢀ
1.526 ꢃ, whereas in the complex [Pd
ꢀ
O distances become longer (1.562–1.578 ꢃ) and the O Pd
distances are in the range of 2.062–2.118 ꢃ. The stability of
[PdACHTUNGTRENNUNG
(O=PPh3)3Cl]+ was estimated on the basis of the dissoci-
ation enthalpy of the oxide ligand from Equation (1):
½PdðO¼PPh3Þ3Clꢁþ ¼ ½PdðO¼PPh3Þ2Clꢁþ þ O¼PPh3
ð1Þ
The BP86-computed OPPh3 dissociation enthalpy of [Pd-
ACHTUNGTRENNUNG
Acknowledgements
This work was supported by the State of Mecklenburg-Vorpommern and
the BMBF (Bundesministerium fꢀr Bildung und Forschung).
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Conclusion
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system for the benign oxidation of primary and secondary
alcohols with O2 as the sole reoxidant. By applying stable
and inexpensive TPO ligands such as L3, the active palladi-
um species are stabilized and a variety of alcohols including
demanding aliphatic substrates give the desired aldehyde
and ketones in good yields.
Chem. Eur. J. 2013, 19, 15979 – 15984
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Received: July 1, 2013
Published online: October 14, 2013
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