Palladium-Schiff base-triphenylphosphine catalyzed oxidation of alcohols

Article abstract of DOI:10.1002/aoc.1683

Novel palladium(II)-N-(2-pyridyl)-N'-(5-R-salicylidene) hydrazine triphenylphosphine complexes were synthesized and characterized by UV, IR, ¹HNMR and³¹P NMR spectral analysis, C, H, N analysis andmagnetic susceptibilitymeasurements. The complexes were effective in the catalytic oxidation of primary and secondary alcohols in presence of N-methyl-morpholine-Noxide as oxidant. The oxidation reactions were carried out in dichloromethane. A mechanistic study of the above reactions has been proposed. Copyright

Full text of DOI:10.1002/aoc.1683

Full Paper
Received: 28 December 2009
Revised: 31 March 2010
Accepted: 26 April 2010
Published online in Wiley Interscience: 1 June 2010
(
www.interscience.com) DOI 10.1002/aoc.1683
Palladium–Schiff base–triphenylphosphine
catalyzed oxidation of alcohols
∗
Dileep R. and Badekai Ramachandra Bhat
ꢀ
Novel palladium(II)-N-(2-pyridyl)-N -(5-R-salicylidene) hydrazine triphenylphosphine complexes were synthesized and
1
31
characterized by UV, IR, H NMR and P NMR spectral analysis, C, H, N analysis and magnetic susceptibility measurements. The
complexes were effective in the catalytic oxidation of primary and secondary alcohols in presence of N-methyl-morpholine-N-
oxide as oxidant. The oxidation reactions were carried out in dichloromethane. A mechanistic study of the above reactions has
been proposed. Copyright ꢀc 2010 John Wiley & Sons, Ltd.
Keywords: Pd complexes; schiff base; catalytic oxidation; NMO
−
Introduction
altered in complexes. The band in the region 1315–1330 cm 1
,
which was assigned to phenolic ν(C–O) in the free ligand, was
shifted to higher wavenumber in the complexes, suggesting
the coordination of phenolic oxygen to metal ion. The N–H
The selective oxidation of alcohols to carbonyl compounds is
an important transformation in synthetic organic chemistry,
as it is essential for the preparation of many key synthetic
−1
stretching frequency occurred around 3100 cm in ligands and
intermediates.[
1–4]
Traditionally such transformations have been
−1
was unaltered in complexes. The pyridine vibrations at 610 cm
performedwithinorganicoxidants,e.g. chromium(VI)compounds
in stoichiometric quantities
such as product separation from the catalyst and catalyst recovery
remainproblematic.Overthelastfewyears,manytransitionmetals
−1
(
in-plane ring deformation) and 490 cm
(out of plane ring
deformation) in the free ligand were found to be altered in the
complexes, indicating the participation of pyridine ring in the
coordination. The band at 550 cm in the complex was assigned
to ν(Pd–O). Bands at 1440, 1090 and 690 cm were assigned
[
5,6]
[7]
and in organic solvents. Issues
−1
[
8]
[9]
[10]
[11]
such as ruthenium, manganese, tungsten,
rhenium,
−1
1
iron[ and vanadium have been used as catalysts for alcohol
12]
[13]
[15]
for the peaks due to triphenylphosphine.
H NMR spectra
oxidation.
of the complexes exhibited a multiplet around 6.9–7.9 ppm
which was assigned to the protons of phenyl groups present
in Schiff base ligand and triphenylphosphine. A peak observed
at 8.5 ppm in the complexes was assigned to azomethine
proton (–CH N–). The absence of a resonance at 10.3 ppm
The Schiff base transition metal complexes are attractive oxi-
dation catalysts because of their cheap, easy synthesis and their
chemical and thermal stability. Considerable attention has been
paid to the preparation of transition metal complexes of Schiff
bases because they are considered to constitute new kinds of
potential antibacterial and anticancer reagents. Palladium com-
pounds are well known as very important reagents and catalysts
in many organic reactions because of their stability and easy
handling. Palladium(II) complexes with Schiff base ligands were
found to be efficient homogeneous catalysts under mild reaction
conditions.[ Herein, we present our attempts to synthesize a
series of palladium triphenylphosphine complexes (PdL1–PdL5)
due to phenolic hydrogen indicated the deprotonation of the
[16]
Schiff base.
A broad singlet at 3.8 ppm was observed, which
[17]
13
corresponded to the N–H proton. In the C NMR spectra of all
the complexes, azomethine carbon resonances were observed in
the 151.12–154.49 ppm range. The resonances for C–N, C–O and
C–P were observed in the regions 147.32–149.99, 162.44–164.93
14]
[18]
13
and 142.96–144.48 ppm, respectively.
of complexes revealed the presence of six different carbons
119.30, 129.46, 143.85, 143.98, 149.82 and 163.98 ppm). The three
quaternary carbons arising from triphenylphosphine aromatic
The C NMR spectra
ꢁ
containingN-(2-pyridyl)-N -(salicylidene)hydrazinewithitsderiva-
(
tives (Scheme 1) and their application as catalysts for the oxidation
of alcohols to carbonyl compounds in dichloromethane using
N-methyl-morpholine-N-oxide (NMO) as oxidant.
[18] 31
units were in same magnetic environments.
P NMR spectra
exhibited a singlet at 23.3–23.8 ppm, suggesting the presence of
one coordinated triphenylphosphine in the complexes. In order
to obtain further structural information, the magnetic moments
of the complexes were measured. The magnetic susceptibility
Results and Discussion
Synthesis and Characterization of Palladium Complexes
The electronic spectra of the complexes showed many bands
in the region 250–490 nm. The bands appearing in the region
∗
Correspondence to: Badekai Ramachandra Bhat, Department of Chemistry,
NationalInstituteofTechnologyKarnataka,Surathkal,Srinivasanagar-575025,
India. E-mail: chandpoorna@yahoo.com
2
50–350 nm were assigned to intraligand transitions. A less
intense band in the range 390–490 nm corresponded to the d
d forbidden transition. The IR spectra of the ligands exhibited
→
−
1
a strong band around 1610–1620 cm , which was assigned to
Department of Chemistry, National Institute of Technology Karnataka,
Surathkal, Srinivasanagar-575025, India
ν(C N) vibration. As a result of coordination, this band was
Appl. Organometal. Chem. 2010, 24, 663–666
Copyright ꢀc 2010 John Wiley & Sons, Ltd.

Dileep R. and B. R. Bhat
P
R
NH
O
Pd
N
EtOH, NaOEt
N
PdCl (PPh )
2
3 2
N
N
OH
R
N
H
Scheme 1. Synthesis of palladium complexes (L1: R = H, L2: R = Cl, L3: R = Br, L4: R = NO2, L5: R = OCH3).
100
Table 1. Optimization of reaction conditions for oxidizing benzyl
alcohol to benzaldehyde
a
80
Entry Amount of PdL1 (mmol) Amount of NMO (mmol) Yield (%)
1
2
3
4
5
6
7
8
9
1
1
0
1.0
1.0
1.0
1.0
1.0
1.0
0
1.5
29.9
89.1
89.1
89.3
89.2
23.1
48.2
89.1
89.2
89.1
0.01
0.02
0.03
0.04
0.05
0.02
0.02
0.02
0.02
0.02
60
4
2
0
0
0
0.5
1.0
1.5
2.0
0
1
a
0
50
100
Time (min)
150
200
250
1
mmol benzyl alcohol, 20 ml CH2Cl2, 150 min, reflux.
Figure 1. Effect of time on conversion of benzyl alcohol to benzaldehyde
1 mmol benzyl alcohol, 0.02 mmol PdL1, 1 mmol oxidant, 20 ml CH2Cl2).
(
measurements showed that the complexes are diamagnetic in
nature and support square planar geometry.
experiments were carried out in an air atmosphere since there is
no change in conversion if reaction is carried out under nitrogen.
This indicates that air is not involved in oxidation process and
palladium complexes are air-stable. A plausible mechanism for the
oxidation of alcohols by palladium complexes in the CH2Cl2 –NMO
system is illustrated in Fig. 2.[
Catalytic Oxidation of Alcohols
The optimization of the reaction conditions was studied by taking
benzyl alcohol as substrate with PdL1 in the CH2Cl2 –NMO system
(Table 1). The benzaldehyde formed was quantified by GC. In order
19,20]
to study the effect of time on the activity, the product analysis was
done at regular intervals of time under similar reaction conditions
(
1
Fig. 1). It was observed that the total reaction time was only
50 min at reflux. This implies that the Pd (II)-complexes–CH2Cl2-
Conclusions
NMO catalytic system showed good efficiency (Table 1). Therefore
thiscatalyticsystemwasstudiedindetail(Table 2).Inordertostudy
theeffectof theconcentrationofcatalystwithrespecttosubstrate,
the reaction was carried out at different substrate to catalyst ratios.
Aminimumamountof12 mg (0.02 mmol)ofcatalystwassufficient
for the effective transformation of benzyl alcohol to benzaldehyde
Insummary,fromreadilyavailablestartingmaterials,aseriesofpal-
ladium complexes containing N-(2-pyridyl)-N -(5-R-salicylidene)
ꢁ
hydrazine and triphenylphosphine have been prepared and char-
acterized. Oxidation of primary and secondary alcohols in the
presence of the above synthesized complexes gave a yield of the
corresponding carbonyl compounds. Further applications of the
present catalytic system are ongoing and will be reported in due
course.
(Table 1, entry 3). The reaction was also studied in the absence of
catalyst. The yield was insignificant in this case (Table 1, entry 1).
This observation reveals the catalytic role of palladium complexes.
The reaction was studied at various substrate-to-oxidant ratios.
A minimum quantity of 1 mmol of the oxidant was sufficient for
the effective oxidation of benzyl alcohol to benzaldehyde (Table 1,
entry 9). All the alcohols were oxidized in good to excellent
yields without the necessity of any additives. All the synthesized
palladium complexes were found to catalyze the oxidation of
alcohols to corresponding carbonyl compounds in 74–95% yield.
Benzylic primary and secondary alcohols oxidized smoothly to
give aldehydes and ketones respectively. Aliphatic alcohols were
also oxidized to corresponding carbonyl compounds. All the
Experimental
Materials and Methods
All the chemicals used were of analytical grade. Solvents
[
21]
were purified and dried according to standard procedures.
Anhydrous PdCl2 was purchased from Merck and was used
without further purification. [PdCl2 (PPh3)2] was prepared by
the reaction of anhydrous PdCl2 (CDH) and triphenylphosphine in
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Copyright ꢀc 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 663–666

Palladium–Schiff base–triphenylphosphine catalyzed oxidation of alcohols
R1
(II)
Table 2. Oxidation of alcohols catalyzed by Pd complexesa
Pd
O
Time
R2
O
Entry
1
Alcohols
Product
(min) Yield (%)b TONc
N+
O-
OH
O
150
150
89 (87)
445
H
2
3
OH
O
87 (65)
435
H
(III)
(III)
H
3
C
R1
O N+
O
Pd
Pd
O
3
H C
O
OH
O
150
180
89 (75)
88 (30)
445
440
R2
H
H
1/2 O2
O
N
CH
3
O
CH3
4
5
6
OH
O
H
Cl
Cl
Cl
Cl
R2 R1
H O
2
OH
O
150
150
89 (45)
95 (88)
445
475
H
HO
H
NO
2
NO
2
Figure 2. Proposedmechanismforpalladiumcomplexcatalyzedoxidation
of alcohols by NMO.
OH
CH
O
3
CH3
frequency range 400–4000 cm 1. The C, H and N contents were
determined by Thermoflash EA1112 series elemental analyzer.
−
7
8
9
OH
O
120
120
90
93 (84)
88 (76)
82 (70)
85 (82)
465
440
410
425
1
13
H NMR and C NMR spectra were recorded in Bruker AV
00 instrument using TMS and H3PO4 as internal standards
4
OH
O
31
respectively. P NMR spectra were recorded in Bruker AV 400
instrument using H3PO4 as internal standard.
H
OH
O
General Procedure for the Synthesis of Pd (II) Complexes
Complexes PdL1–PdL5 were prepared by stirring a mixture of
1
0
1
OH
OH
O
150
[PdCl (PPh ) ] in 3 ml of 0.1 M sodium acetate and the respective
2
3 2
OH
H
ligands in 15 ml alcohol in a 1 : 1 ratio for 5 h. The red solid was
filtered off, washed with ethanol and dried in vacuo.
1
OH
O
O
120
81 (42)
405
3
H C
H
3
C
H
Data for the Complexes
1
1
1
2
3
4
H
3
C
OH
H
3
C
120
120
120
81 (36)
75 (66)
78 (60)
405
375
390
PdL1
H
−
1
Yield: 79%. IR (KBr, cm ): 3105 (s), 623, 479 (m), 1592 (s), 1345
(
H
3
C
OH
H C
O
1
3
w), 550 (m), 1446, 1095, 697. H NMR (CDCl3, δ ppm): 3.8 (s,
H
1
H, N–H), 6.6–7.3 (m, 15H, Ar–H), 7.4–7.7 (m, 8H, Ar–H), δ 8.5
1
3
(d, 1H, –CH N–). C NMR (ppm): 163.98 (C–O), 154.51 (C N),
143.98 (C–P), 161.46, 147.57, 137.41, 114.05, 106.97 (carbon atoms
in pyridine), 117.55, 119.84, 133.73, 136.92 (carbon atoms in
OH
O
H
3
C
3
H C
H
15
H
3
C
H
120
74 (45)
370
31
OH
salicylaldehyde). P NMR (H3PO4, δ ppm): 23.4. CHN found (calcd)
3
H C
O
for C30H25N3OPPd: C: 61.84 (62.02), H: 4.21 (4.34), N: 7.12 (7.23), Pd:
18.15 (18.32); UV–vis: λmax (nm) intraligand interactions: 229, 272,
346, d → d forbidden transition: 441.
a
1
mmol alcohol, 1 mmol NMO, 0.02 mmol PdL1, 20 ml CH2Cl2, reflux.
GC yields, isolated yields are given in parentheses, confirmed by
b
derivative.
c
TON = moles of product/moles of the catalyst.
PdL2
−
1
Yield: 75%. IR (KBr, cm ): 3110 (s), 620, 478 (m), 1597 (s), 1330
(
1
w), 545 (m), 1435, 1101, 690. H NMR (CDCl3, δ ppm): 3.8 (s, 1H,
tetrahydrofuran (Merck) under reflux for 5 h.[22] The Schiff bases
were prepared in 70–80% yield by condensation reactions of 2-
hydrazinopyridine (Aldrich) with the corresponding 5-substituted
N–H), 6.6–7.3 (m, 15 H, Ar–H), 7.4–7.8 (m, 8 H, Ar–H), 8.5 (d, 1 H,
1
3
–CH N–); C NMR (ppm): 160.65 (C–O), 154.05 (C N), 144.66
(C–P), 161.61, 147.17, 137.01, 114.55, 105.97 (carbon atoms in
pyridine), 110.15, 118.99, 121.07, 133.51, 135.61 (carbon atoms in
[
23]
salicylaldehyde (Loba) in methanolic media.
3
1
Electronic spectra were measured on a GBC Cintra 101 UV–vis
double beam spectrophotometer in CH3OH solution of the
complexes in the 200–800 nm range. FT-IR spectra were recorded
onaThermoNicoletAvatarFT-IRspectrometerasKBrpowderinthe
salicylaldehyde). P NMR (H3PO4, δ ppm): 22.3. CHN found (calcd)
for C30H24ClN3OPPd: C: 58.25 (58.55), H: 3.68 (3.93), N: 6.46 (6.83),
Pd: 17.19 (17.29); UV–vis: λmax intraligand interactions: 233, 271,
346, d → d forbidden transition: 447.
Appl. Organometal. Chem. 2010, 24, 663–666
Copyright ꢀc 2010 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/aoc

Dileep R. and B. R. Bhat
PdL3
(30 m length and 0.25 mm diameter) and a flame ionization detec-
tor (FID). The initial column temperature was increased from 60 to
Yield: 72%. IR (KBr, cm 1): 3106 (s), 630, 475 (m), 1585 (s), 1340
−
◦
◦
◦
1
4
50 C at the rate of 10 C/min and then to 220 C at the rate of
0 C/min Nitrogen gas was used as the carrier gas. The tempera-
1
(w), 552 (m), 1439, 1093, 698. H NMR (CDCl3, δ ppm): 3.8 (s, 1H,
◦
N–H), 6.7–7.3 (m, 15 H, Ar–H), 7.8–7.9 (m, 8 H, Ar–H), 8.5 (d, 1 H,
tures of the injection port and FID were kept constant at 150 and
1
3
–
CH N–); C NMR (ppm): 160.55 (C–O), 153.46 (C N), 144.48
C–P), 161.61, 147.17, 137.01, 114.55, 105.97 (carbon atoms in
pyridine), 111.34, 119.31, 121.77, 135.61, 139.63 (carbon atoms in
◦
2
50 C, respectively, during product analysis. The retention times
(
for different compounds were determined by injecting commer-
cially available compounds under identical gas chromatography
conditions. The oxidation products are commercially available,
and were identified by GC co-injection with authentic samples.
The products were isolated and were further confirmed by the
derivative test.
3
1
salicylaldehyde). P NMR (H3PO4, δ ppm): 22.5. CHN found (calcd)
for C30H24BrN3OPPd: C: 54.48 (54.61), H: 3.45 (3.67), N: 6.19 (6.37),
Pd: 16.05 (16.13); UV–vis: λmax intraligand interactions: 231, 277,
3
46, d → d forbidden transition: 443.
PdL4
Yield: 72%. IR (KBr, cm ): 3099 (s), 619, 481 (m), 1595 (s), 1351
Acknowledgments
−
1
The authors are thankful to the Technical Education Quality
Improvement Programme, NITK, for financial support. The authors
also thank Indian Institute of Science, Bangalore, for NMR analysis.
1
(w), 549 (m), 1429, 1087, 695. H NMR (CDCl3, δ ppm): 3.8 (s, 1H,
N–H), 6.6–7.4 (m, 15 H, Ar–H), 7.4–7.8 (m, 8 H, Ar–H), 8.6 (d,
1
3
1
1
H, –CH N–); C NMR (ppm): 166.18 (C–O), 153.39 (C N),
44.42 C–P), 161.61, 147.17, 137.01, 114.55, 105.97 (carbon atoms
in pyridine), 119.03, 119.41, 129.81, 131.69, 140.73 (carbon atoms
in salicylaldehyde). P NMR (H3PO4, δ ppm): 22.8. CHN found
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3
1
(
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3
–
(
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[
3
1
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[
[
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[
5% diphenyl and 95% dimethyl siloxane Restek capillary column
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Copyright ꢀc 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 663–666