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Chapter 3
Aerobic Oxidation of Benzyl Alcohols
Application of solid supported----------- Chapter 3
115
Application of solid-supported palladium catalyst for oxidation reaction
3.1 Introduction:
Oxidation of organic compounds is important and widely used reaction in organic
chemistry [Tsuji (2000)]. Among various oxidation reactions, oxidation of alcohols (Figure
1) is a fundamental organic transformation in synthetic organic chemistry [Hudlicky
(1990); Larock (1999); Smith and March (2001)] since carbonyls have huge industrial
applications as solvents, perfumes, intermediates in the manufacture of dyes and in
pharmaceuticals [Pillai and Sahle-Demessie (2003)].
R OHR
H
O1o alcohols
Oxidising agent
R1 OH
R2
R1
R2
O
Oxidising agent
2o alcohols ketone
aldehyde
Figure 1. General Scheme for the oxidation of alcohols
Traditionally, stoichiometric oxidants having transition metals notably chromium based
reagents (such as PCC, PDC etc.) [Lee and Spitzer (1970), Cainelli and Cardillo (1984);
Muzart (1992)], permanganates [Regen and Koteel (1977); Menger and Lel (1981)],
ruthenium (VIII) oxide [Berkowitz and Rylander (1958); Griffith (1992)], DMSO-coupled
reagents [Mancuso and Swern (1981)], Dess-Martin periodinane [Dess and Martin (1983)],
tert-butylhydroperoxide [Krohn et al.(1996)] and TPAP/NMO system [Ley et al. (1994)]
are used to affect the oxidation of alcohols which are often toxic and demonstrate poor
atom efficiency and their use causes significant environmental issues which render them
impractical. For this reason, there has been a strong move toward the development of
catalytic reactions using benign oxidizing reagents.
3.2 Different metal catalyzed protocols using green oxidants:
From economical and environmental viewpoint, there is a definite need for catalytic
oxidation reactions that use dioxygen (O2) or hydrogen peroxide as oxidants. The use of
oxygen has great benefits from both economic and green chemistry viewpoints in oxidation
reactions because oxygen is relatively cheap and produce water as the only by product. In
this context, significant progress has been made in the field of transition metal catalyzed
oxidations [Chen et al. (2010); Ji et al. (2007); Li et al. (2007); Kockritz et al. (2006)]
Application of solid supported----------- Chapter 3
116
particularly utilizing molecular oxygen as green oxidant. The catalysts including
heterogenized complexes, mixed oxides and nanoparticles, containing transition metals
such as vanadium [Jiang and Ragauskas (2007)], cobalt [Sharma et al. (2003)], copper
[Marko et al. (1996)], ruthenium [Yamaguchi and Mizuno (2002)], palladium [Jensen et al.
(2003)], platinum [Wang et al. (2007)], iron [Martin and Suarez (2002)] etc. have been
screened for oxidation reactions. Among them, palladium based catalysts have very
interesting and promising catalytic activity and different types of palladium based
homogeneous and heterogeneous catalysts in the form of metal complexes or nanoparticles
have been developed for oxidation reactions which are discussed in details as below:
3.2.1 Homogeneous palladium-catalyzed aerobic oxidation of alcohols:
Oxidation of primary and secondary allylic and benzylic alcohol to aldehyde and ketone
has been achieved [Peterson and Larock (1998)] using catalytic Pd(OAc)2 in dimethyl
sulfoxide (DMSO) with oxygen gas as the sole oxidant. Nishimura et al. have described an
improved system of simple combination of commercially available reagents,
Pd(OAc)2/pyridine/MS3Ao for aerobic oxidation of benzylic and aliphatic alcohols using
O2 [Nishimura et al. (1999)]. This apparoach could also be employed using catalytic
amount of a novel perfluoroalkylated-pyridine as a ligand in a fluorous biphase system
(FBS) composed of toluene and perfluorodecalin [Nishimura et al. (2000)]. The fluorous
phase containing the active palladium species reused several times without significant loss
of catalytic activity. Further Schultz et al. have developed a convenient aerobic alcohol
oxidation using Pd(OAc)2/TEA system at room temperature and found that triethylamine
(TEA) is best additive or base for oxidation than pyridine. The initial mechanistic studies
provided evidence that the active catalyst may be a palladium complex of TEA (Scheme 1)
[Schultz et al. (2002)].
R1 OH
R2
R1
R2
O
Pd(OAc)2, O2
Ligand / Toluene
80 oC
R2 = H, CH3
R1 = alkyl, aryl
Scheme 1. Palladium-catalyzed aerobic oxidation of alcohols
Brink et al. have demonstrated that water-soluble complexes of palladium (II) with
phenanthroline derivatives are stable and recyclable catalysts for the selective aerobic
Application of solid supported----------- Chapter 3
117
oxidation of a wide range of alcohols in a biphasic liquid-liquid system and the active
catalyst found to be a dihydroxy-bridged palladium dimer (Scheme 2) [Brink et al. (2002)].
R1 OH
R2
R1
R2
O
R2 = H, CH3
R1 = alkyl, arylair, NaOAc, H2O
PhenS*Pd(OAc)2
Scheme 2. PhenS*Pd(OAc)2-catalyzed aerobic oxidation of alcohols
Buffin et al. have described the application of an aqueous palladium catalyst that is
stabilized by a water-soluble biquinoline-based ligand for the aerobic oxidation of primary
and secondary alcohols in water and air as oxidant. The catalyst system was recycled
(Scheme 3) [Buffin et al. (2005)].
R1 OH
R2
R1
R2
O
Pd(II) biquinoline catalyst
R2 = H, CH3
R1 = alkyl, arylO2, NaOAc, H2O
Scheme 3. Palladium(II)biquinoline catalyzed aerobic oxidation of alcohols
Steinhoff et al. have studied the effect of molecular sieves on the Pd(OAc)2/pyridine and
Pd(OAc)2/DMSO catalystic systems for aerobic oxidation of alcohols. It has been found
that molecular sieves enhance the rate of the Pd(OAc)2/pyridine-catalyzed oxidation due to
ability of molecular sieves to serve as a bronsted base, but no rate enhancement was
observed for the Pd(OAc)2/DMSO catalyzed reaction [Steinhoff et al. (2006)]. It has been
observed that both catalytic systems exhibit improved catalytic stability in the presence of
molecular sieves. The performances of palladium complexes with phenanthroline ligands
have been studied for alcohol oxidation in water as well as in water/polar co-solvent
mixtures [Arends et al. (2006)]. The Pd(OAc)2-neocuproine catalyst has been found
efficient in water or polar co-solvents for oxidation of alcohols and tolerates a wide variety
of functional groups in the alcohol (Scheme 4).
R1 OH
R2
R1
R2
O
Pd (II) neocuproine, air
DMSO/ H2O
80 oC
R2 = alkyl, aryl
R1 = aryl, alkyl
Scheme 4. Palladium-neocuproine catalyzed aerobic oxidation of alcohols
Application of solid supported----------- Chapter 3
118
Zhou et al. have investigated the use of palladium-diamine catalyst in the aerobic oxidation
of alcohols (Scheme 5). The electronic properties, rigidity and steric hindrance of ligands
have remarkable influence upon the catalytic activity of the palladium complexes [Zhou et
al. (2008)]. It has been found that the catalyst showed high efficiency for the oxidation of
benzylic alcohols than secondary aliphatic and cyclic alcohols.
R1 OH
R2
R1
R2
O
Pd (OAc)2 / diamine, O2
K2CO3, DMA
100 oC
R2 = alkyl, aryl
R1 = aryl, alkyl
Scheme 5. Palladium-diamine catalyzed aerobic oxidation of alcohols
Water-soluble palladium nanoparticles stabilized by the functionalized-poly(ethylene
glycol) [Feng et al. (2010)] as a protective ligand has been demonstrated for aerobic
oxidation of alcohols (Scheme 6). UV/Vis and XPS study proved the electronic
interactions between the bidentate nitrogen ligand and palladium atoms. It has been found
that both the size and surface properties of palladium nanoparticles are important in
affecting catalytic performance and the catalyst can be recycled at least four times without
any loss of catalytic activity.
R1 OH
R2
R1
R2
O
R2 = H, CH3
R1 = alkyl, arylO2, K2CO3, 80 oC
Pd nanoparticles
Scheme 6. Aerobic oxidation of alcohols using water soluble palladium nanoparticles
Palladium pincer complexes have been used as catalysts for aerobic oxidation of secondary
benzyl alcohols in PEG-400 and showed good catalytic activity (Scheme 7) [Urgoitia et al.
(2011)].
R1 OH
R2
R1
R2
O
Pd pincer complex, O2
NaOAc,PEG-400
120 oC
R2 = alkyl, aryl
R1 = aryl
Scheme 7. Oxidation of secondary benzyl alcohols using palladium pincer complex
Although homogeneous catalyst (typically a soluble metal complex) acquire the advantage
of having all catalytic sites accessible to all reagents. However, their use in industrial
Application of solid supported----------- Chapter 3
119
applications is limited by the difficulties encountered in catalyst separation from the final
products. To overcome the separation problems encountered in homogeneous catalysis,
efforts have been made to develop heterogeneous catalysts which can be easily separated
from the reaction mixture and reused several times.
3.2.2 Heterogeneous palladium-catalyzed aerobic oxidation of alcohols:
Kakiuchi et al. have described the aerobic oxidation of primary and secondary alcohols
into the corresponding aldehydes and ketones with atmospheric pressure of air under mild
conditions in toluene using a heterogeneous Pd(II)-hydrotalcite catalyst (Scheme 8)
[Kakiuchi et al. (2001)].
R1 OH
R2
R1
R2
O
Pd(II)hydrotalcite, O2
Pyridine/ Toluene
R2 = H, alkyl, aryl
R1 = aryl, alkyl
Scheme 8. Aerobic oxidation of alcohols using palladium hydrotalcite catalyst
Mori et al. have described the formation of palladium hydroxyapatite (PdHAP-0) as a
heterogeneous catalyst by grafting of monomeric PdCl2 species on the surface of
stoichiometric hydroxyapatite and its application in aerobic alcohol oxidation (Scheme 9)
[Mori et al. (2004)].
R1 OH
R2
R1
R2
O
PdHAP-0, O2
TFT, 90 oC
R2 = H, alkyl, aryl
R1 = aryl, alkyl
Scheme 9. Aerobic oxidation of alcohols using palladium hydroxyapatite catalyst
Kwon et al. have prepared a recyclable aluminium hydroxide-supported palladium catalyst
by a one-pot synthesis through nanoparticle generation and gelation which have catalytic
activity for aerobic alcohol oxidation (Scheme 10) [Kwon et al. (2005)]. A silica-based
Pd(II) interphase catalyst having high thermal stability has also been applied for the
oxidation of alcohols [Karimi et al. (2005)].
R1 OH
R2
R1
R2
O
Pd catalyst, O2
Toluene or TFT
80-100 oCR2 = H, CH3, alkyl, aryl
R1 = alkyl, aryl
Scheme 10. Oxidation of alcohols using supported palladium catalyst with O2
Application of solid supported----------- Chapter 3
120
Alumina supported palladium (Pd/Al2O3) catalyst has been prepared by the adsorption
method [Wu et al. (2005)] which results in mononuclear or oligonuclear palladium species
stabilized on Al2O3 surface after calcination and found to be efficient in the solvent-free
selective oxidation of alcohols (Scheme 11). The catalyst was recycled and palladium
species transformed to small Pd nanoparticles during the course of oxidation.
R1 OH
R2
R1
R2
O
R2 = H, CH3
R1 = alkyl, aryl
150 oC
Pd/ Al2O3, O2
Scheme 11. Aerobic oxidation of alcohols using Pd/Al2O3
The biphasic aerobic oxidation of alcohols by using palladium nanoparticles in a
poly(ethylene glycol) (PEG) matrix as the catalyst and supercritical carbon dioxide
(scCO2) as the substrate and product phase has been established (Scheme 12) [Hou et al.
(2005)]. It was observed that the PEG matrix effectively stabilizes and immobilizes the
active particles, whereas the solubility and mass-transfer properties of scCO2 allow
continuous processing at mild conditions.
R1 OH
R2
R1
R2
O
Pd nanoparticles, O2
R2 = H, CH3
R1 = alkyl, aryl
scCO2 / PEG
Scheme12. Aerobic oxidation using PEG stabilized palladium nanoparticles
An efficient and recoverable palladium-based catalyst for the aerobic oxidation of alcohols
has been developed [Karimi et al. (2006)]. It has been demonstrated that the combination
of an organic ligand and ordered mesoporous channels resulted in an interesting synergistic
effect that led to enhanced activity, the prevention of the agglomeration of the Pd
nanoparticles and the generation of a durable catalyst. Choudhary et al. have prepared a
series of silica-supported palladium catalysts bearing N-N, N-S and N-O chelating ligands
for the aerobic oxidation of alcohols [Choudhary et al. (2006)] and found that N-N ligands
were highly selective for oxidation without forming over-oxidation products (Scheme 13).
R1 OH
R2
R1
R2
O
Palladium catalyst, O2
K2CO3,Toluene,
80-90 oCR2 = H, alkyl, aryl
R1 = alkyl, aryl
Scheme 13. Aerobic oxidation of alcohols using palladium catalyst
Application of solid supported----------- Chapter 3
121
An amphiphilic polystyrene-poly(ethylene glycol) (PS-PEG) resin-dispersion of palladium
nanoparticles [Uozumi and Nakao (2003); Uozumi et al. (2007)] has been prepared by
Uozumi et al. and the catalytic oxidation of alcohols has been achieved in water under
atmospheric pressure of molecular oxygen (Scheme 14). The catalyst was recycled up to
four runs and catalyst leaching into the aqueous phase under the reaction conditions was
not observed.
R1 OH
R2
R1
R2
O
ARP-Pd, O2
K2CO3, H2O,reflux
R2 = H, alkyl, aryl
R1 = aryl, alkyl
Scheme 14. Aerobic oxidation of alcohols using ARP-Pd
Chen et al. have prepared SiO2-Al2O3-supported Pd nanoparticles by adsorption of PdCl42-
ions onto the support followed by calcination and reduction with either hexanol or H2. The
supported Pd nanoparticles catalyzed the aerobic oxidation of benzyl alcohols under
solvent free conditions [Chen et al. (2008)]. The Pd/SiO2-Al2O3 catalyst with appropriate
Si/Al ratios showed significantly higher benzyl alcohol conversions than both the Pd/SiO2
and the Pd/Al2O3. Hara et al. have synthesized a NiZn-Pd nanocomposite catalyst
containing highly stable divalent Pd species by simple intercalation of the anionic Pd(II)
hydroxyl complex. The intercalated anionic palladium hydroxide complex rigidly fixed by
the strong electrostatic interactions between the NiZn host and anionic Pd(II) species could
act as an efficient heterogeneous catalyst for the oxidation of alcohols under an air
atmosphere (Scheme 15) [Hara et al. (2009)].
R1 OH
R2
R1
R2
O
Pd/SiO2-Al2O3, O2
R2 = H, CH3
R1 = alkyl, arylTFT
orPd/ NiZn, air
Scheme 15. Pd/SiO2-Al2O3 or Pd/ NiZn catalyzed aerobic oxidation of alcohols
Palladium catalysts supported on TUD-1 mesoporous molecular sieves functionalized with
various organosilanes (APS, ATMS, HMDS and MPTMS) have been prepared using a
post-synthesis grafting method combined with a metal adsorption-reduction procedure. The
catalytic activity has been explored in the solvent-free selective oxidation of benzyl alcohol
using molecular oxygen (Scheme 16). Among all the catalysts, 1Pd/1.2APS-TUD
Application of solid supported----------- Chapter 3
122
(containing 1 wt% Pd and 1.2 mmol of APS/g TUD) showed high catalytic activity for
benzyl alcohol conversions [Chen et al. (2010)].
OH
R2
R1
R2
O
R1
O2, 160 oC
R1=H, Me,NO2,BrR2= H,Me
1Pd/1.2APS-TUD
Scheme 16. 1Pd/1.2APS-TUD catalyzed aerobic oxidation of benzyl alcohols
Palladium-charcoal (Pd/C) has been used as a heterogeneous catalyst along with NaBH4 in
aqueous ethanol or methanol and either K2CO3 or KOH as base at room temperature under
molecular oxygen or air for oxidation of alcohols (Scheme 17). The catalyst was recycled
upto ten times without the loss of activity [An et al. (2010)].
OH
R2
R1
R2
O
R1
K2CO3, EtOH-H2O
R1=H,OMe,Me,NO2,FR2= H,Me,Ph
Pd/ C, NaBH4, O2
Scheme 17. Pd/C catalyzed aerobic oxidation of alcohols
Karimi et al. have demonstrated the efficiency of ionic liquid based PMO material in the
production and stabilization of active palladium species for aerobic oxidation of alcohols
(Scheme 18). The catalyst was easily recovered and reused even after ten runs [Karimi et
al. (2011)]. The hot filtration test, atomic spectroscopy and the results of the effect of
poisons have confirmed that the Pd@PMO-IL catalyst operated through a heterogeneous
pathway.
R1 OH
R2
R1
R2
O
Pd@PMO-IL, O2
K2CO3,TFT, 95 oC
R2 = H, alkyl, aryl
R1 = alkyl, aryl
Scheme 18. Pd@PMO-IL catalyzed aerobic oxidation of alcohols
Nanocrystalline magnesium oxide-stabilized palladium(0) [NAP-Mg-Pd(0)] as an efficient
catalytic system has been employed for the selective oxidation of alcohols using
atmospheric oxygen as a oxidant at room temperature (Scheme 19). This catalyst was
recovered and reused for several cycles without any significant loss of catalytic activity
[Layek et al. (2011)].
Application of solid supported----------- Chapter 3
123
R1 OH
R2
R1
R2
O
NAP-Mg-Pd(0), O2
K2CO3,Toluene
R2 = H, alkyl, aryl
R1 = alkyl, aryl
Scheme 19. NAP-Mg-Pd(0) catalyzed aerobic oxidation of alcohols
The supported palladium catalyst based on ligand functionalized amorphous and ordered
mesoporous silica (SBA-15) has been prepared and applied for the oxidation of alcohols
[Karimi et al. (2009)]. Recently Zang et al. have prepared mesoporous SBA-15 supported
catalyst PdLn@SBA-15 via click route in which the click-triazole acted as a stable linker
and good chelator [Zang et al. (2012)]. The obtained solid catalyst demonstrated a
promising catalytic activity for the aerobic oxidation of benzyl alcohols (Scheme 20). The
catalyst was recovered by simple filtration and reused upto seven cycles with a consistent
catalytic activity.
OH
R2
R1
R2
O
R1
K2CO3, Toluene
80-100 oC
R1=H,OMe,Me,NO2,FR2= H,Me,Ph, Et
PdLn$SBA -15, O2
Scheme 20. Aerobic oxidation of benzylic alcohols using PdLn$SBA-15
In view of the above literature precedents, it would be apparent that a majority of the
prevalent oxidation approaches employ additives and basic conditions. Although there
have been noteworthy efforts to devise more benign oxidation conditions with various
improvements, but these have also been found to be constrained by the use of complicated
ligands which are not commercially available or require multiple steps for their synthesis.
On the other hand, various metal free approaches mainly employ oxidants such as TEMPO
or hypervalent iodine based reagents. But the utility of TEMPO mediated system is limited
due to its expensiveness and lengthy work up procedures [Jiang and Ragauskas (2005);
Mannam et al. (2007); He et al. (2009)]. In this context, it would be highly desirable to
develop a mild methodology for oxidation of benzyl alcohols using green oxidants.
Hence, we have investigated the scope of our previously synthesized heterogeneous SS-
Pd(0) catalyst in the oxidation of benzyl alcohols to the corresponding aldehydes and
ketones using molecular oxygen as the oxidant. Moreover, the catalyst system was
recycled up to five runs without significant loss of activity.
Application of solid supported----------- Chapter 3
124
3.3 Results and discussion:
Initially concentration based studies of solid supported palladium catalyst were performed
to optimize the concentration of palladium required to give best performance of the
catalyst for aerobic oxidation of benzyl alcohols. In this regard, solid supported palladium
catalysts (SS-Pd) were prepared following our previously described procedure with
different concentrations of Pd(OAc)2 on solid surface (5 mg/g and 10 mg/g) and analyzed
by scanning electron micrographs (SEM) (Figure 2 and 3). With the change of
concentration, no significant particle size changes were observed but 10 mg Pd(OAc)2 on 1
g solid surface gave the best result with good recyclability. Through these studies, we
realized that 10 mg/g concentration of catalyst occupied sufficient space of the solid
surface which showed highest interaction with the substrate required for heterogeneous
catalyst to give high performance of the catalyst. With decrease of concentration (5mg/g)
huge space of the solid surface remained unoccupied by the catalyst and decrease the
optimal interaction with substrate, thereby, decreasing the rate of reaction.
SS-Pd
5um
a b
Figure 2. (a) SS-Pd (10 mg Pd(OAc)2/g solid surface) (b) SEM of SS-Pd (10 mg
Pd(OAc)2/g solid surface)
Figure 3. (a) SS-Pd (5 mg Pd(OAc)2/g solid surface) (b) SEM of SS-Pd (5 mg Pd(OAc)2/g
solid surface)
Application of solid supported----------- Chapter 3
125
The solid surface of the catalyst (SS-Pd) behaves as an interface where liquid and gaseous
molecules such as alcohols and oxygen interacted with each other to produce
corresponding carbonyls (Figure 4).
Figure 4. Performance of reaction on solid surface of catalyst which act as interface
Using the standard SS-Pd catalyst (10 mg/g), further optimization studies were performed
for oxidation of 2-ethoxybenzylalcohol 1a as the model substrate (Table 1). The progress
of reaction was monitored by TLC and GC-MS analysis. From standard SS-Pd catalyst (10
mg/g), different loadings of SS-Pd containing 1-6 mol% palladium with respect to
substrate were investigated for the aerobic oxidation of 2-ethoxybenzyl alcohol. Highest
turnover number (TON, 2266) and turnover frequencies (TOF, 453 h-1
) were calculated
when SS-Pd (3 mol % Pd) was used for the oxidation reaction of 1a (Table 1, entry 2).
However, only 68% yield of 2a was observed by GC-MS analysis. SS-Pd with 5 mol% Pd
in toluene afforded 80% yield of 2a and TON/TOF was calculated as 1600/320 h-1
(Table
1, entry 3). No further improvement of yield was observed by enhancing the catalyst
loading up to 6 mol% Pd (Table 1, entry 4). Different solvents were investigated to see
their effect on the product yield. Dioxane and THF were not found to be good solvents
under the reaction condition. Moderate yield of product was obtained in benzene and
acetonitrile. Among different solvents, toluene gave the best result under standard reaction
condition. Comparative studies of different catalytic systems were also performed. The
reaction with Pd/C and Pd2C6H10Cl2 gave poor yield of product while moderate yields
Application of solid supported----------- Chapter 3
126
were obtained in presence of Pd2(dba)3 and Pd[P(C6H5)3]4. Pd(OAc)2 was also found to be
good but the best result was obtained using SS-Pd under the standard reaction condition.
Hence SS-Pd (5 mol% Pd) in the presence of toluene was found to be optimum under the
reaction conditions.
Table1. Effect of catalysts and solvents for the oxidation of 2-ethoxybenzylalcohol
OH OSolvent, 110 0C
1a 2a
OC2H5 OC2H55h
Catalyst,O2
Entry Castalyst (mol % Pd) Yield (%)aTON TOF h-1
2
3
4 SS-Pd (6)
SS-Pd (3)
SS-Pd (1)
68
11
2266 453
1100 220
Solvent
Toluene
Toluene
2
69
80
Pd/C
Pd(OAc)2
6
5
Toluene
Toluene
Toluene
1333 267
1380 276
(5)
(5)
40 8
aDetermined by means of GC,based on the 2-ethoxy benzyl alcohol
TON = % conversion x mmole of substrate /mmole of catalyst, TOF = TON / time
1
SS-Pd (5)
i. Toluene 80 1600 320ii. THF 9 180 36iii.Dioxane 3 60 12iii. C6H6 55 1100 220iv. CH3CN 23 460 92
7 Pd2C6H10Cl2(10) Toluene 1
8 Pd2(dba)3(10)
9 Pd[P(C6H5)3]4(5)
Toluene
Toluene
20
39
20 4
400 80
780 156
Further investigations were performed to enhance the scope of oxidation reactions in
different substituted primary and secondary benzyl alcohols (Table 2).
Application of solid supported----------- Chapter 3
127
Table 2. Oxidation of primary and secondary benzyl alcohols to aldehydes and ketones
OH
R2
R1
R2
O
R1
SS-Pd, O2
Toulene, 110 0C
1a-1m 2a-2m
3-5 h
Entry Product Yield (%)a
1
2b1b
90
Substrate
2
1d
1e
3
4
5
6
1f
1g
1h
70
2c
7
8
9
10
80
83
85
2d
2e
2f
1i
1j
11
2g
96
12
1k
1l
1m
2h
2i
2j
2k
2l
2m
90
90
90
93
90
74
aIsolated Yield
Time (h)
5
3
3
3
3
3
5
5
5
5
5
513
5
2a
78CHO
OC2H5
1a
CH2OH
OC2H5
OH O
1c
CH2OHO2N CHOO2N
CH2OH
MeO
CHO
MeO
CH2OHBr CHOBr
CH2OHCl CHOCl
CH2OH
Br
CHO
Br
OH O
OHMeO
OMe
OMeO
OMe
OHOMe
MeO
OOMe
MeO
OHMeO
OMeO
C6H5
OHCl
C6H5
OCl
OHBr
OBr
TON /TOF-h
1963/393
1400/467
1800/600
1600/533
1660/553
1700/567
1800/360
1860/372
1800/360
1800/360
1480/296
1800/360
1560/312
Application of solid supported----------- Chapter 3
128
Both the primary and secondary benzyl alcohols on oxidation gave satisfactory to good
yields of the corresponding aldehydes and ketones. Secondary benzyl alcohol was also
found to be reactive toward oxidation under the same reaction condition and the product 2b
was obtained in excellent yield (Table 2, entry 2). Primary benzyl alcohols with electron
donating and withdrawing groups afforded products 2c and 2d in good yields (Table 2,
entries 3 and 4). Primary benzyl alcohols bearing halogen groups were also successfully
oxidized to benzaldehydes 2e-g with satisfactory yields (Table 2, entries 5-7). Secondary
benzyl alcohols with electron donating groups afforded products 2h-k in good yields
(Table 2, entries 8-11) and halogen substituted secondary benzyl alcohols also gave
products 2l and 2m in satisfactory yields (Table 2, entries 12 and 13).
Evaluation of recyclability was done on the same test substrate 1a. After completion of
reaction, the product was extracted with ethyl acetate and the recovered catalyst was
washed with acetone, dried and used for further reactions. The catalytic activity of SS-Pd
remained almost the same upto five runs as indicated by the yields of the corresponding
aldehyde 2a. Gradual decrease in yield was encountered after six to seven runs of SS-Pd
catalyst. As shown in Table 3, on recycling, from the first to the seventh run the TONs
drop from 1600 to 1000 and total turnover number over the seven cycles was 10400.
Table 3. Recyclability experiments of 2a using SS-Pd
1st2nd 3rd
80 79
4th Run
79 78
5th
Yield (%)a
aDetermined by means of GCbIsolated Yield
TTON = total turnover number; procedure followed as described for 2a
6th 7th
74b50b
TON
TTON over seven cycles
1600 1600 1580 1580 1560
10400
1480 1000
80
3.4 Conclusion:
Aerobic oxidation of benzyl alcohols using SS-Pd has been developed. Different primary
and secondary benzyl alcohols afforded corresponding products in good yields. Due to the
stability of SS-Pd catalyst in moisture and air, it is very easy to handle under reaction
conditions. This methodology will find interest in both academic as well as industry due to
Application of solid supported----------- Chapter 3
129
its atom-economy and cost-effective process by using heterogeneous nano and
microparticles as a ligand free catalyst.
3.5 Experimental section:
3.5.1 General procedure:
Reagents of high quality were purchased from Sigma Aldrich. Silica gel (60-120 mesh
size) for column chromatography was procured from SD Fine-Chem. Ltd. Commercial
reagents and solvents were of analytical grade and were purified by standard procedures
prior to use. Thin layer chromatography was performed using precoated silica gel plates 60
F254 (Merck) in UV light detector. 1H and
13C NMR spectra were recorded using a Bruker
Avance 300 spectrometer operating at 300 MHz (1H) and 75 MHz (
13C). Spectra were
recorded at 25 °C in CDCl3 [residual CHCl3 (δH 7.26 ppm) or CDCl3 (δC 77.00 ppm) as
international standard] with TMS as internal standard. Chemical shifts were recorded in δ
(ppm) relative to the TMS and CDCl3 signal, coupling constants (J) are given in Hz and
multiplicities of signals are reported as follows: s, singlet; d, doublet; t, triplet; m,
multiplet; br, broad singlet.
3.5.2 Procedure of aerobic oxidation:
O 1-Phenyl ethanone (2b): A mixture of 1-phenyl ethanol 1b (100 mg, 0.818
mmol), SS-Pd (918 mg, 0.04 mmol Pd) and 3 ml of toluene was purged with molecular
oxygen and stirred at 110 0C for 5h. The progress of reaction was monitored by TLC. After
completion of reaction, the reaction was cooled, diluted with ethyl acetate and filtered
through cotton bed. The combined organic layer was evaporated under reduced pressure
and crude residue was purified by silica gel (mesh 60-120) column chromatography
(hexane:EtOAc :: 95:5) afforded 1-Phenyl ethanone 2b as colourless liquid (94 mg, 96%);
1H NMR (300 MHz, CDCl3-d1) δ 2.59 (s, 3H), 7.44-7.55 (m, 3H), 7.93-7.96 (m, 2H);
13C
NMR (75 MHz, CDCl3-d1) δ 26.49, 128.22 (2C), 129.48 (2C), 133.01, 137.06, 198.07.
CHO
OC2H5
2-Ethoxy benzaldehyde (2a): Prepared as described for 2b; starting from 2-
ethoxybenzyl alcohol 1a (100 mg, 0.66 mmol) gave, after purification with silica gel
column chromatography (hexane:EtOAc :: 95:5) 2a as colourless liquid (81 mg, 78%); 1H
NMR (300 MHz, CDCl3-d1) δ 1.27-1.35 (m, 3H), 4.10-4.21 (m, 2H), 6.97-7.05 (m, 2H),
Application of solid supported----------- Chapter 3
130
7.52-7.57 (m, 1H), 7.85 (d, J = 7.6 Hz, 1H), 10.53 (s, 1H); 13
C NMR (75 MHz, CDCl3-d1)
δ 35.02, 64.92, 113.16, 116.67, 121.28, 125.66, 129.06, 136.64, 190.77.
CHOO2N 4-Nitro benzaldehyde (2c): Prepared as described for 2b; starting
from 4-nitrobenzyl alcohol 1c (100 mg, 0.66 mmol) gave, after purification with silica gel
column chromatography (hexane:EtOAc :: 97:3) 2c as a brown solid (70 mg, 70%), mp
106-107 oC;
1H NMR (300 MHz, CDCl3-d1) δ 8.09 (brs, 2H), 8.41 (brs, 2H), 10.17 (s, 1H);
13C NMR (75 MHz, CDCl3-d1) δ 124.29 (2C), 130.71 (2C), 140.04, 151.12, 190.23.
CHO
MeO 3-Methoxy benzaldehyde (2d): Prepared as described for 2b; starting
from 3-methoxy benzylalcohol 1d (100 mg, 0.72 mmol) gave, after purification with silica
gel column chromatography (hexane:EtOAc :: 95:5) 2d as colourless liquid (89 mg, 90%);
1H NMR (300 MHz, CDCl3-d1) δ 3.90 (s, 3H), 7.20-7.21 (m, 1H), 7.42-7.49 (m, 3H),
10.01 (s, 1H); 13
C NMR (75 MHz, CDCl3-d1) δ 55.48, 112.02, 121.55, 123.57, 130.03,
137.81, 160.16, 192.13.
CHOBr4-Bromo benzaldehyde (2e): Prepared as described for 2b; starting
from 4-bromobenzyl alcohol 1e (100 mg, 0.54 mmol) gave, after purification with silica
gel column chromatography (hexane:EtOAc :: 96:4) 2e as white solid (80 mg, 80%), mp
56-57 oC;
1H NMR (300 MHz, CDCl3-d1) δ 7.61-7.71 (m, 2H), 7.75-7.78 (m, 2H), 9.99 (s,
1H); 13
C NMR (75 MHz, CDCl3-d1) δ 129.77, 130.95 (2C), 132.43 (2C), 135.06, 191.04.
CHOCl4-Chloro benzaldehyde (2f): Prepared as described for 2b; starting
from 4-chlorobenzyl alcohol 1f (100 mg, 0.70 mmol) gave, after purification with silica gel
column chromatography (hexane:EtOAc :: 96:4) 2f as white solid (82 mg, 83%), mp 46-47
oC;
1H NMR (300 MHz, CDCl3-d1) δ 7.43-7.46 (m, 2H), 7.45-7.77 (m, 2H), 9.91 (s, 1H);
13C NMR (75 MHz, CDCl3-d1) δ 129.45 (2C), 130.89 (2C), 134.71, 140.96, 190.84.
CHO
Br
2-Bromo benzaldehyde (2g): Prepared as described for 2b; starting from 2-
bromobenzyl alcohol 1g (100 mg, 0.54 mmol) gave, after purification with silica gel
column chromatography (hexane:EtOAc :: 95:5) 2g as colourless liquid (85 mg, 85%); 1H
NMR (300 MHz, CDCl3-d1) δ 7.40-7.45 (m, 2H), 7.60-7.63 (m, 1H), 7.86-7.89 (m, 1H),
Application of solid supported----------- Chapter 3
131
10.31 (s, 1H); 13
C NMR (75 MHz, CDCl3-d1) δ 126.88, 127.73, 129.68, 132.82, 133.37,
135.12, 191.48.
O 4-Methyl acetophenone (2h): Prepared as described for 2b; starting from
4-methyl phenyl ethanol 1h (100 mg, 0.72 mmol) gave, after purification with silica gel
column chromatography (hexane:EtOAc :: 95:5) 2h as colourless liquid (88 mg, 90%); 1H
NMR (300 MHz, CDCl3-d1) δ 2.43 (s, 3H), 2.60 (s, 3H), 7.27-7.29 (m, 2H), 7.87-7.89 (m,
2H); 13
C NMR (75 MHz, CDCl3-d1) δ 21.61, 26.49, 128.44, 129.23, 134.78, 143.85,
197.81.
OMeO
OMe 2,4-Di-methoxy acetophenone (2i): Prepared as described for 2b;
starting from (2,4-di-methoxy phenyl) ethanol 1i (100 mg, 0.54 mmol) gave, after
purification with silica gel column chromatography (hexane:EtOAc :: 95:5) 2i as white
solid (92 mg, 93%), mp 46-48 oC;
1H NMR (300 MHz, CDCl3-d1) δ 2.53 (s, 3H), 3.80 (s,
3H), 3.84 (s, 3H), 6.34-6.48 (m, 2H), 7.73-7.79 (m, 1H); 13
C NMR (75 MHz, CDCl3-d1) δ
31.57, 55.20, 55.25, 98.00, 104.95, 120.84, 132.74, 160.90, 164.36, 197.37.
OOMe
MeO
2,5 Di-methoxy acetophenone (2j): Prepared as described for 2b; starting
from (2,5 di-methoxy phenyl) ethanol 1j (100 mg, 0.54 mmol) gave, after purification with
silica gel column chromatography (hexane:EtOAc :: 95:5) 2j as colourless liquid (88 mg,
90%); 1H NMR (300 MHz, CDCl3-d1) δ 2.64 (s, 3H), 3.80 (s, 3H), 3.90 (s, 3H), 6.90-7.06
(m, 2H), 7.28-7.30 (m, 1H); 13
C NMR (75 MHz, CDCl3-d1) δ 31.75, 55.79, 56.02, 113.20,
113.82, 120.31, 128.42, 153.41, 153.50, 199.34.
O
MeO
4-Methoxy acetophenone (2k): Prepared as described for 2b; starting
from (4-methoxy phenyl)ethanol 1k (100 mg, 0.64 mmol) gave, after purification with
silica gel column chromatography (hexane:EtOAc :: 95:5) 2k as white solid (87 mg, 90%),
mp 37-39 oC;
1H NMR (300 MHz, CDCl3-d1) δ 2.54 (s, 3H), 3.85 (s, 3H), 6.91 (d, J =
8.72, 2H), 7.92 (d, J = 8.70 Hz, 2H); 13
C NMR (75 MHz, CDCl3-d1) δ 26.65, 55.75, 113.96
(2C), 130.57, 130.89 (2C), 163.77, 197.12.
Application of solid supported----------- Chapter 3
132
C6H5
OCl
4-Chlorobenzophenone (2l): Prepared as described for 2b; starting
from (4-chlorophenyl)-phenylethanol 1l (100 mg, 0.44 mmol) gave, after purification with
silica gel column chromatography (hexane:EtOAc :: 95:5) 2l as white solid (73 mg, 74%),
mp 76-78 oC;
1H NMR (300 MHz, CDCl3-d1) δ 7.46-7.53 (m, 3H), 7.60-7.64 (m, 2H),
7.76-7.80 (m, 4H); 13
C NMR (75 MHz, CDCl3-d1) δ 128.38 (2C), 129.61 (2C), 129.90
(2C), 131.44 (2C), 132.62, 135.86, 137.23, 138.87, 195.46.
OBr
4-Bromo acetophenone (2m): Prepared as described for 2b; starting
from (4-bromophenyl) ethanol 1m (100 mg, 0.48 mmol) gave, after purification with silica
gel column chromatography (hexane:EtOAc :: 95:5) 2m as white solid (87 mg, 90%), mp
47-49 oC;
1H NMR (300 MHz, CDCl3-d1) δ 2.60 (s, 3H), 7.60-7.63 (m, 2H), 7.82-7.84 (m,
2H); 13
C NMR (75 MHz, CDCl3-d1) δ 26.52, 128.28, 129.82 (2C), 131.87 (2C), 135.81,
197.00.
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Application of solid supported----------- Chapter 3
138
NMR spectra of some compounds
1H NMR (in CDCl3) spectrum of 4-Bromo benzaldehyde
13
C NMR (in CDCl3) spectrum of 4-Bromo benzaldehyde
Application of solid supported----------- Chapter 3
139
1H NMR (in CDCl3) spectrum of 4-methoxy acetophenone
13
C NMR (in CDCl3) spectrum of 4-methoxy acetophenone
Application of solid supported----------- Chapter 3
140
1H NMR (in CDCl3) spectrum of 4-chlorobenzophenone
13
C NMR (in CDCl3) spectrum of 4-chlorobenzophenone