Palladium-catalyzed aerobic alcohol oxidation supported by a new type of α-diimine ligands

Article abstract of DOI:10.1139/V08-075

A series of new ionic sulfonated α-diimine ligands were prepared and employed as supporting ligands for the palladium-catalyzed aerobic alcohol oxidation. The electronic properties, rigidity, and steric hindrance of ligands have remarkable influences upon the catalytic activity of the palladium complexes. The best catalytic result could be achieved using ligand 9 with an electron-withdrawing sulfonyl substituent together with bulky groups (iso-propyl in 2,6-positions). This diimine/palladium-catalyzed system is highly efficient for the oxidation of various benzylic alcohols, and showed moderate yields for the secondary aliphatic alcohols and cyclic alcohols.

Full text of DOI:10.1139/V08-075

782
Palladium-catalyzed aerobic alcohol oxidation
supported by a new type of ␣-diimine ligands
Jin Zhou, Xiaoyan Li, and Hongjian Sun
Abstract: A series of new ionic sulfonated α-diimine ligands were prepared and employed as supporting ligands for
the palladium-catalyzed aerobic alcohol oxidation. The electronic properties, rigidity, and steric hindrance of ligands
have remarkable influences upon the catalytic activity of the palladium complexes. The best catalytic result could be
achieved using ligand 9 with an electron-withdrawing sulfonyl substituent together with bulky groups (iso-propyl in
2,6-positions). This diimine/palladium-catalyzed system is highly efficient for the oxidation of various benzylic alco-
hols, and showed moderate yields for the secondary aliphatic alcohols and cyclic alcohols.
Key words: α-diimine, palladium, aerobic oxidation, alcohol.
Résumé : On a préparé une série de nouveaux ligands ioniques α-diimines sulfonées et on les a utilisés comme li-
gands de support pour des réactions d’oxydation aérobiques d’alcools catalysées par le palladium. Les propriétés élec-
troniques, la rigidité et l’empêchement stérique des ligands influencent profondément l’activité catalytique des
complexes du palladium. Le meilleur résultat catalytique a été obtenu en utilisant le ligand 9 qui porte un substituant
électroaffinitaire sulfonyle avec des substituants volumineux, isopropyles, dans les positions 2 et 6. Ce système de dii-
mine catalysée par le palladium est très efficace pour l’oxydation de divers alcools benzyliques et donne des rende-
ments modérés avec les alcools aliphatiques ou cycliques secondaires.
Mots-clés : α-diimine, palladium, oxydation aérobique, alcool.
[Traduit par la Rédaction] Zhou et al. 790
Introduction
tems and could give a norm for the performance of new
catalysts.
The oxidation of alcohols to the corresponding aldehydes
or ketones is one of the most important functional group
transformations in organic synthesis. Recently, the use of
molecular oxygen as terminal oxidant has received great at-
tention for both economic and environmental benefits, and
many highly efficient systems have been developed for cata-
lytic aerobic alcohol oxidation using Fe (1), Ru (2–5), Co
(6), Cu (7, 8), Mn (9), Ni (10), and Pd (11,12) catalysts or
even a transitional-metal-free catalyst system(13). Homoge-
neous palladium catalysts are particularly effective for the
oxidation of alcohols (14–24). Of these reports, the first sig-
nificant breakthrough was reported by Peterson and Larock,
who employed the Pd(OAc)2/DMSO catalyst system for oxi-
dation of a wide range of benzylic and allylic alcohols with
molecular oxygen as the sole oxidant (25). Furthermore, the
reported Uemura Pd(OAc)2/pyridine system (26, 27),
Sheldon’s Pd(OAc)2/phananthroline aqueous system (28),
Sigman’s Pd(OAc)2/NEt3 (29), and Pd(OAc)2/N-heterocyclic
carbene catalyst systems (30) are remarkable catalyst sys-
The mechanism of palladium-catalyzed aerobic oxidation
has been comparatively studied (31–33). Recently, Stahl
and co-workers reported a mechanistic study of Pd(OAc)2/DMSO
catalyst system (34) and indicated that oxidation of palla-
dium(0) might be the turnover-limiting step. In contrast to
the Pd(OAc)2/DMSO catalyst system, Stahl’s mechanistic
studies of Pd(OAc)2/pyridine catalyst system indicated a pal-
ladium(II) resting state with turnover-limiting oxidation of
the alcohol substrate, which is similar to the other catalyst
systems that employ nitrogen ligands (35, 36). An oxidase-
style mechanism was proposed for the aerobic oxidation of
alcohols (37, 38). The oxidation proceeds in two independ-
ent, sequential stages. Palladium(II) is the real oxidant for
the oxidation of alcohol. The role of molecular oxygen is to
oxidize the palladium(0) species to palladium(II). The lig-
ands facilitate the activation of dioxygen by increasing the
electron density on Pd(0). Experimental evidence, support-
ing this hypothesis, has been presented by Stahl and co-
workers while examining olefin-substitution reactions with
bathocuproine–Pd(0) complexes (39, 40).
Herein, we give attention to the famous diimine ligands
family, which has been applied in many catalytic processes,
such as the notable olefin polymerization (41, 42) and cross-
coupling reactions (43, 44). We have synthesized a series of
new α-diimine ligands and employed these ligands for the
aerobic oxidation of alcohols. The catalytic efficiency of al-
cohol oxidation could be optimized by the rigidity, steric,
and electronic properties of the ligands, which could be ad-
Received 22 January 2008. Accepted 12 April 2008.
Published on the NRC Research Press Web site at
canjchem.nrc.ca on 2 July 2008.
J. Zhou, X. Li, and H. Sun.1 School of Chemistry and
Chemical Engineering, Shandong University, Shanda Nanlu
27, 250100 Jinan, Shandong Province, P.R. China.
1Corresponding author (email: hjsun@sdu.edu.cn).
Can. J. Chem. 86: 782–790 (2008)
doi:10.1139/V08-075
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Zhou et al.
783
justed through variation of the backbone and the substituents
of the N-phenyl rings.
DMSO-d6) δ: 168.28, 148.63, 143.33, 125.72, 123.78, 17.86,
16.19. Anal. calcd. for C20H22N2Na2O6S2: C, 48.38; H, 4.47;
N, 5.64. Found: C, 48.24; H, 4.23; N, 5.35.
Experimental section
DABMe–ArEt(4-SO3Na) (Ligand 5)
1
Yield: 52%. H NMR (400 MHz, CD3OD) δ: 7.62 (s, 4
Materials and methods
H), 2.42 (q, 8 H), 2.05 (s, 6 H), 1.17 (t, 12 H). 13C NMR
(100 MHz, CD3OD) δ: 163.15, 150.08, 140.96, 131.64,
126.14, 49.0, 23.78. Anal. calcd. for C24H30N2Na2O6S2: C,
52.16; H, 5.47; N, 5.07. Found: C, 51.93; H, 5.60; N, 5.17.
All solvents and reagents were used after standard purifi-
cation. Sodium salts of alkyl substituted sulfanilic acid (45,
46), DABH–AriPr, DABMe–ArMe, DABMe–AriPr, BIAN–AriPr
(ligands 10–13) (47, 48), and BIAN–Ar(3,5 − CF ,) (ligand 14)
3
(49) as well as BmimCl (1-nbutyl-3-methyl-imidazolium
chloride), BmimBF4 (1-nbutyl-3-methyl-imidazolium tetra-
fluoroborate), and BmimPF6 (1-nbutyl-3-methyl-imidazo-
lium hexafluorophosphate) (50) were prepared according to
the literature methods.
DABMe–AriPr(4-SO3Na) (Ligand 6)
1
Yield: 67%. H NMR (400 MHz, CD3OD) δ: 7.79 (s, 4
H), 2.15 (s, 6 H), 2.84 (m, 4 H), 1.34 (d, 12 H), 1.28 (d, 12
H). 13C NMR (100 MHz, CD3OD) δ: 163.15, 150.08,
140.96, 131.64, 126.14, 49.0, 23.78. Anal. calcd. for
C28H38N2Na2O6S2: C, 55.25; H, 6.29; N, 4.60. Found: C,
55.12; H, 6.50; N, 4.50.
1H NMR and 13C NMR were recorded on a Bruker 300 or
400 MHz. GC–MS was carried out on a Shimadzu QP-
5050A. GC was Zheda Zhida 9790. Elemental analyses were
carried out on an Elementar Vario EL III.
BIAN–ArMe(4-SO3Na) (Ligand 7)
1
Yield: 50%. H NMR (300 MHz, DMSO-d6) δ: 8.16 (d, 2
Synthesis of new diimine ligands
H), 7.61 (t, 2 H), 7.49 (s, 4 H), 6.73 (d, 2 H,), 2.06 (s, 12 H).
13C NMR (75.5 MHz, DMSO-d6) δ: 160.51, 149.22, 144.10,
140.19, 131.29, 130.24, 129.32, 126.26, 123.61, 122.34,
119.48, 18.45. Anal. calcd. for C28H22N2Na2O6S2: C, 56.75;
H, 3.74; N, 4.73. Found: C, 56.37; H, 4.00; N, 4.56
DABH–ArMe(4-SO3Na) (Ligand 1)
Sodium salt of 3,5-dimethyl sulfanilic acid (2.23 g,
10.0 mmol) and 40% aq. glyoxal (0.80 g, 5.50 mmol) were
dissolved in 30 ml of anhyd. methanol. Five drops of formic
acid were added into this solution. The mixture was stirred
at 40 °C for 48 h. After cooling to room temperature, the de-
posit was isolated and washed by 5 ml of cold methanol.
The solid was recrystallized from MeOH and dried over an
aspirator for 10 h. Ligand 1 was isolated as a yellow powder.
BIAN–ArEt(4-SO3Na) (Ligand 8)
1
Yield: 51%. H NMR (400 MHz, CD3OD) δ: 1.13 (t, 12
H), 8.07 (d, 2 H), 7.77 (s, 4 H), 7.47 (t, 2 H), 6.81 (d, 2 H),
2.57 (q, 4 H), 2.50 (q, 4 H). 13C NMR (100 MHz, CD3OD)
δ: 162.16, 150.39, 142.21, 141.89, 132.07, 130.90, 129.53,
129.04, 125.50, 125.21, 124.28, 25.55, 13.76. Anal. calcd.
for C32H30N2Na2O6S2: C, 59.25; H, 4.66; N, 4.32. Found: C,
59.10; H, 4.79; N, 4.10.
1
(1.54g, 66%). H NMR (300 MHz, DMSO-d6) δ: 8.14 (s, 2
H), 7.34 (s, 4 H), 2.11 (s, 12 H). 13C NMR (75.5 MHz,
DMSO-d6) δ: 164.19, 150.05, 136.06, 125.60, 119.49, 49.07.
Anal. calcd. for C18H18N2Na2O6S2: C, 46.15; H, 3.87; N,
5.98. Found: C, 46.35; H, 3.99; N, 5.60.
BIAN–AriPr(4-SO3Na) (Ligand 9)
DABH–ArMe(4-SO3Na) (Ligand 2–9)
1
Yield: 73 %. H NMR (400 MHz, CD3OD) δ: 7.85 (s, 4
Ligands 2–9 were synthesized according to the method
given above for ligand 1. All the ligands were analyzed by
1H NMR and elemental analysis. Melting points could not
be obtained because the ligands were sodium salt of
sulfonic.
H), 7.49 (t, 2 H), 6.80 (d, 2 H), 6.08 (d, 2 H), 3.00 (m, 4 H),
1.26 (d, 12 H), 1.04 (d, 12 H). 13C NMR (100 MHz,
CD3OD) δ: 160.77, 147.79, 140.95, 140.55, 135.08, 130.90,
129.61, 127.94, 127.42, 123.16, 121.10, 28.28, 21.40, 21.33.
Anal. calcd. for C36H38N2Na2O6S2: C, 61.35; H, 5.43; N,
3.97. Found: C, 61.04; H, 5.55; N, 4.16.
DABH–ArEt(4-SO3Na) (Ligand 2)
1
Yield: 56.6%. H NMR (CD3OD, 400 MHz) δ: 8.14 (s, 2
H), 7.62 (s, 4 H), 2.55 (m, 8 H), 1.16 (t, 12 H). 13C NMR
(100 MHz, CD3OD) δ: 163.15, 150.08, 140.96, 131.64,
126.14, 49.0, 23.78. Anal. calcd. for C22H26N2Na2O6S2: C,
50.37; H, 5.00; N, 5.34. Found: C, 50.05; H, 4.79; N, 5.25.
Typical experimental procedure
Pd(OAc)2 (0.05 mmol) and ligand (0.05 mmol) were dis-
solved in DMA (5 ml) in a 25 mL two-necked flask. The
flask, equipped with a magnetic stirring bar, was purged
with oxygen gas. Oxygen gas was introduced into the flask
from an O2 balloon under atmospheric pressure, and the
mixture was stirred at 40 °C for 10 min or stirred for 30 min
at room temperature. Then alcohol (1 mmol) and K2CO3
(1 equiv.) was added, and the reaction mixture was stirred
for appropriate time at 100 °C under oxygen. The oxidation
product was analyzed by GC–MS. The reaction yields were
determined by GC analysis using n-heptane as an internal
DABH–AriPr(4-SO3Na) (Ligand 3)
1
Yield: 55%. H NMR (300 MHz, DMSO-d6) δ: 8.17 (s, 2
H), 7.44 (s, 4 H), 2.86 (m, 4 H), 1.15(d, 24 H). 13C NMR
(75.5 MHz, DMSO-d6) δ: 163.91, 148.34, 145.22, 135.63,
120.89, 49.06, 28.06, 23.75. Anal. calcd. for
C26H34N2Na2O6S2: C, 53.78; H, 5.90; N, 4.82. Found: C,
53.68; H, 6.12; N, 4.70.
standard. Recoveries were 100
2% with this procedure.
DABMe–ArMe(4-SO3Na) (Ligand 4)
The purified products could be obtained by flash chromatog-
raphy on a short silica-gel column eluting with a mixture of
ethyl acetate and petroleum ether.
1
Yield: 76%. H NMR (300 MHz, DMSO-d6) δ: 7.35 (s, 4
H), 1.98 (s, 6 H,), 1.96 (s, 12 H). 13C NMR (75.5 MHz,
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Can. J. Chem. Vol. 86, 2008
Scheme 1. Preparation of sulfonated α-diimine ligands 1–9.
General procedure for catalyst-recycling trials
Pd(OAc)2 (0.05 mmol) and ligand 9 (0.05 mmol) were
dissolved in DMA (5 ml) in a 25 mL two-necked flask under
oxygen gas. Oxygen gas was introduced into the flask from
an O2 balloon under atmospheric pressure, and the mixture
was stirred at 40 °C for 10 min. Then, benzyl alcohol
(1 mmol) and K2CO3 (1 equiv.) was added, and the reaction
mixture was stirred for 5 h at 100 °C under oxygen. An
aliquot of 0.02 mL of reaction mixture was taken out for the
GC analysis. The volatiles of reaction mixture were removed
under vacuum. The residual was washed with n-pentane for
three times, and was applied to the next reaction cycle.
Results and discussions
Synthesis of new ␣-diimine ligands
At first, 2,6-dialkylaniline was sulfonated with concen-
trated sulphuric acid at 200 °C, followed by neutralization
with NaOH or NaOCH3 to yield the sodium salts. The new
α-diimine ligands were prepared by condensation of the
α-dione (2,3-butanedione, acenaphthenequinone, or 40%
aqueous glyoxal) and the corresponding anilines in anhyd.
methanol with a catalytic amount of formic acid (Scheme 1).
A small amount of the sulfonated anilines was found in the
crude product. This impurity could be removed by
recrystallization from methanol or methanol/diethyl ether.
These new sulfonated ligands are all easily diffluent and sta-
ble in polar organic solvents, such as DMF, DMA, and
DMSO, as well as in water, but slowly hydrolyze in the lat-
ter when heated.
Table 1. Optimization of solvents.
Solvent
Yieldf
a
PhCH3
H2Ob
99
25g, 30h
45i, 52h
H2O/DMSOc
DMF
74
50
99
30
0
Dioxane
DMA
DMAd
ILse
Influence of solvents on aerobic benzyl alcohol
oxidation
Note: Reaction conditions: 5 mol% Pd(OAc)2 and
ligand 9, K2CO3 (1 equiv.), benzyl alcohol (1
mmol), solvent (5 ml), 1 atm O2 (1 atm = 101.325
kPa), and 100 °C for 5 h.
Initially, the goal of this work was to develop a new
Pd/ionic α-diimine catalytic system with the introduction of
sulfonic group for aerobic oxidation in ionic liquid medium
where immobilization of the catalysts was expected. That
catalytic system could be profitable in the recycling of the
catalyst and in the separation of products. Unfortunately, the
combination of Pd/α-diimine showed no catalytic activity
for oxidation of aerobic alcohols in ionic liquids, such as
BmimCl, BmimBF4, and BmimPF6. Therefore, our attention
was transferred to the catalytic activity of this Pd(OAc)2/α-
diimine system in other solvents.
aPd black occurred.
b25 mL, 10 atm O2, NaOAc, and 10 h.
c25 mL H2O/DMSO (1:1), 10 atm O2, NaOAc,
and 10 h.
dNo ligand.
eIonic liquid, including BmimCl, BmimBF4, and
BmimPF6, and 10 atm O2.
fGC yield, average of two runs.
g1 mol% Pd(OAc)2 and ligand 9.
h5 mol% Pd(OAc)2 and ligand 9.
i0.5mol% Pd(OAc)2 and ligand 9.
Six solvents were examined, excluding the ionic liquid,
for the aerobic oxidation of benzyl alcohol (Table 1). The
use of toluene as the solvent led to complete conversion of
the starting alcohol and afforded the desired aldehyde in
99% yield, but a great deal of Pd-black was produced. High
pressure of O2 (10 atm, 1 atm = 101.325 kPa) was employed
for the reactions in aqueous phase or ionic liquid to increase
the solubility of O2. Using low loading of catalyst, catalytic
reactions in water or DMSO/H2O mixed solvent showed
higher TON value compared with other solvents, but the
conversions were low or moderate because of the Pd-colloid
produced (Table 1, entries 2 and 3). Increasing the loading
of catalyst of 5 mol% did not markedly improve the conver-
sions. The employment of DMA (N,N-dimethyl acetamide)
gave excellent yield without any Pd-black (Table 1, entry 4),
while DMF and dioxane displayed moderate results (Ta-
ble 1, entries 5 and 6). In conclusion, DMA appears to be
the most effective solvent under given reaction conditions.
Electronic and steric effects of diimine ligands
Benzyl alcohol was selected as a model substrate to inves-
tigate the electronic and steric effects of ligands on
diimine/palladium(II)-catalyzed aerobic oxidation of alcohol
(Scheme 2).
Table 2 showed us the catalytic activities of various lig-
ands for the aerobic oxidation. We recognized that no palla-
dium black was produced in most of the experiments except
when ligands 2 and 3 were used. The use of ligands in-
creased the electron density on Pd(0) and facilitated the acti-
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Zhou et al.
785
Table 2. Influence of various ligands for aerobic oxidation of benzyl alcohol.
Scheme 2. Palladium-catalyzed oxidation of benzyl alcohol.
ligands. It was shown that in runs employing α-diimine lig-
ands with a strongly electron-withdrawing sulfonic group
showed much higher yields were obtained than in those em-
ploying normal α-diimine ligands (ligands 10–15) in equal
reaction times. This electron-deficiency facilitates the oxida-
tion of alcohol because the overall rate of oxidation in-
creased. Faster oxidation was also achieved by using ligand
14 with the strongly electron-withdrawing trifluoromethyl
groups (Table 2, entry 14). A similar observation that
electron-withdrawing substituents on the ligand increase the
reaction rate in alcohol oxidation was reported by Sheldon
vation of dioxygen. Then, the palladium(0) species could be
smoothly re-oxidized to palladium(II) without evident aggre-
gation when supported by the diimine ligands. An interest-
ing electronic effect seems to arise in the series of diimine
© 2008 NRC Canada

786
Can. J. Chem. Vol. 86, 2008
Table 2 (concluded).
Note: Reaction conditions: Pd(OAc)2 (0.05 mmol), ligand (0.05 mmol), benzyl alcohol
(1.0 mmol), K2CO3 (1 equiv.), DMA (5 ml), O2 1atm, 100 °C, and 5 h.
aGC yield with n-heptane as an internal standard, average of two runs.
bPd black pruduced.
and co-workers in the studies of a range of 4,4-disubstituted-
2,2-bipyridines as ligands for the palladium-catalyzed aero-
bic oxidation of 2-hexanol (51). Variation of the electron
density on the diimine ligand changes the redox properties
of the catalyst. Therefore, this variation should provide evi-
dence for whether an oxidation step or a reduction step is
rate-limiting. The re-oxidation step of Pd(0) to Pd(II) should
be facilitated by electron-donating substituents. On the other
hand, if the reduction step is rate-limiting, then the reaction
should be facilitated by electron-withdrawing substituents in
the ligands. The electronic effect of diimine ligands indi-
cated that the reduction step is slow.
Scheme 3. Oxidation of the other alcohol.
backbone of ligand effectively prevent aggregation of
palladium(0) species and facilitates the re-oxidation of palla-
dium(0).
Everntually, the optimized catalytic combination for the
aerobic oxidation of alcohol was found to be Pd(OAc)2/
ligand 9/K2CO3/DMA/O2/100 °C.
Comparing catalytic results of BIAN/diimine ligands 7–9
(Table 2, entries 7–9), it was shown that bulky groups on 2
and 6 positions of the N-phenyl ring facilitated the oxidation
of benzyl alcohol. The same trend is observed in a series of
DABMe/diimine ligands 4–6 (Table 2, entries 4–6). A similar
conclusion is reported by Sheldon and co-workers (52) for
the Pd/phenanthroline system. It was reported that the
methyl groups in 2 and 9 positions of the phenanthroline lig-
ands clearly increase the TOF value by raising the cleavage
rate of dimeric palladium catalyst species. Generally, the
BIAN/diimine ligands (ligands 7–9) showed higher rates of
oxidation as compared with the DAB/diimine ligands (lig-
ands 1–6). A more stable coordination and a longer lifetime
of the active palladium species in the catalytic cycle (53)
was achieved by using a rigid ligand backbone (BIAN). In a
word, the bulky group on arene of ligands and the more rigid
Oxidation of various alcohols
The scope of this optimized procedure for the oxidation of
the other alcohols (Scheme 3) was listed in Table 3. The pri-
mary benzylic alcohols were converted to the corresponding
aldehydes in 87%–99% yields (Table 3, entries 1–8 and 13).
An electronic effect seems to exist for these benzylic sys-
tems. Comparing benzyl alcohol (Table 3, entry 1) with the
benzylic alcohols with electron-donating substituents (Ta-
ble 3, entry 2), we found that the reaction times for the
electronic-rich arenes were significantly shorter. It was also
shown that the benzylic alcohols with electron-withdrawing
groups (Table 3, entries 3 and 4) gave lower yields in the
same reaction times with benzyl alcohol and required longer
reaction times for complete conversion. It is noteworthy that
the primary benzylic alcohols with electron-withdrawing
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Zhou et al.
787
Table 3. Oxidation of the other alcohols.
group at ortho position (Table 3, entries 9 and 10) showed
much lower reaction activities. The secondary benzylic sub-
strates generally gave very clean reactions with high yields
(Table 3, entries 11, 12, and 14). About 3% yield of 3-
formylbenzyl alcohol as halfway product of oxidation was
determined by GC and GC–MS for the oxidation of 1,3-
benzenedimethanol (Table 3, entry 18) with 94% yield of
iso-phthalaldehyde. It is noticable that the oxidation of
hydrobenzoin (Table 3, entry 19) gave 87% yield with no
benzoin produced and a trace of benzaldehyde formed by
oxidative C=C cleavage, possibly of an intermediate ben-
zoin. As expected, benzaldehyde was also found in the oxi-
dation of benzoin by GC–MS (Table 3, entry 20).
sponding aldehydes or ketones in moderate yields. The cata-
lytic system showed no activities for the oxidation of pri-
mary aliphatic alcohols (Table 3, entries 21 and 22) because
of the decreased electron density on the primary aliphatic al-
cohols.
So far, our catalytic system is limited in scope to benzylic
or secondary aliphatic alcohols, needing higher reaction tem-
perature (100 °C) and longer reaction times (3–12 h for
benzylic alcohols) compared with other excellent catalytic
systems for aerobic alcohol oxidation, such as the
Pd/pyridine and Pd/NEt3 systems. But one certain advantage
is their modularity because the electronic and steric proper-
ties could be modulated independently to optimize the cata-
lytic activities. We believe that further investigation of the
diimine ligands is worthwhile.
The secondary aliphatic alcohols and cyclic alcohol (Ta-
ble 3, entries 15–17) could be transformed into the corre-
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788
Can. J. Chem. Vol. 86, 2008
Table 3 (concluded).
aYield determined by GC.
bRun 2.
cRun 3.
d10 mol% catalyst used.
e3% yield of 3-formylbenzyl alcohol.
fTrace benzaldehyde was observed.
It is confirmed that the Pd(OAc)2/ligand 9 catalytic sys-
tem could be recycled three times, while allowing for ex-
tended reaction times: 8 h for second cycle and 20 h for
third cycle were required after the first run.
obic alcohol oxidation mimics the palladium–oxidase path-
way (38). This process is composed of two independent
steps: substrate oxidation and catalyst oxidation. The trend
of the activities of the Pd (OAc)2/dimine system for alcohol
oxidation is in contrast with that for the diimine/palladium-
catalyzed Suzuki cross-coupling reaction reported by Nolan
and co-workers (44). The observation that electron-
Mechanistic discussion
Generally, the mechanism of the palladium-catalyzed aer-
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Zhou et al.
789
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withdrawing substituents on the ligand increase the reaction
rate in alcohol oxidation is a strong indication that the re-
duction step (substrate oxidation) is slow and that under
these conditions re-oxidation of (diimine)Pd(0) is facile
(51), which is similar to other palladium catalyst systems
with nitrogen-containing ligands (21–31). A bulky sub-
stituent on the arene group in the ligand effectively prevents
aggregation of palladium(0) species and facilitates the re-
oxidation of palladium(0), still allowing an approach of the
substrate alcohol to the catalytic species. An anionic base,
such as K2CO3, can significantly enhance the rate of alcohol
oxidation. Complete conversion of benzyl alcohol could be
achieved using only 0.1 equiv. of the base. More mechanis-
tic investigation is clearly desired.
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Conclusions
A series of new ionic sulfonated α-diimine ligands were
prepared and employed as supporting ligands for the palla-
dium-catalyzed aerobic oxidation of alcohols. The electronic
properties, rigidity, and steric hindrance of ligands have re-
markable influence upon the catalytic activity of the palla-
dium catalysts. The best catalytic results could be achieved
by using ligand 9 with an electron-withdrawing sulfonyl
substituent together with bulky groups (iso-propyl in 2,6-
positions). This diimine/palladium system is highly efficient
for the catalytic oxidation of various benzylic alcohols but
shows only moderate yields for the secondary aliphatic alco-
hols and cyclic alcohols. The observation that electron-
withdrawing substituents on the ligand increase the reaction
rates of alcohol oxidation strongly indicates that the reduc-
tion of Pd(II) is a slow step.
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Acknowledgements
This work was supported by Natural Science Foundation
of China No. 20372042, the Doctoral Program of Ministry
of Education No. 20050422010 and 20050422011,
Shandong Scientific Plan 032090105, and the Science Foun-
dation of Shandong Y2006B18.
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