Epoxidation of α-pinenemediated by cobalt(III)
catalysts
DEPARTMENT OF CHEMISTRYGAUHATI UNIVERSITY
GUWAHATI INDIA
Birinchi K Das
RRB2 Conference, York (2006)RRB2 Conference, York (2006)RRB2 Conference, York (2006)
Green Chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. It also refers to the discovery of new chemistry and/or technology leading to prevention and/or reduction of environmental, health and safety impact at source.
– JH Clark
Atom Economy
C C
H
H
H
HC C
H
H
HH
H
H
H2, Ni
+
% Yield = Actual yield of product
Theoretical yield of productX 100
% Atom economy = Molecular weight of atoms utilized
Molecular weight of all reactants usedX 100
Some ways of practicing Green Chemistry
• Catalysis
• Use of good solvents or no solvents
• Use of non-hazardous substances
• Production of chemicals from renewable resources
• Others
Catalysis for Green Chemistry• important tool for implementing green
chemistry
• increasing selectivity
• decreasing processing and separation agents
• heterogeneous catalysts to make processes greener
• catalysts can significantly lower the energy demands of many manufacturing processes
PRODUCING CHEMICALS FROM RENEWABLE RESOURCES
Advantages
• Conservation of fossil resources
• CO2 neutrality
• Non-toxicity of raw materials
• Biodegradable substances
Disadvantages
• Expensive raw materials
• New technolgy requirements
• Business logistics and infrastructure require development
α-pinene
Pinene is a bicyclic monoterpene.
α-pinene is one of the two structural isomers found in nature, the other one being β-pinene.
Both forms are important constituents of pine resin, they also occur in other plants.
They are mostly obtained from turpentine, which is a clear liquid produced as a by-product of pulping of pine wood. Turpentine contains 58%-65% α-pinene and about 30% of β-pinene. α and β pinenes are fractionated by distillation.
α and β pinenes are key components in the synthesis of flavour and fragrance chemicals used in tooth paste, detergents, shampoos…
Selective oxidation of pinene gives important pharmaceutical and flavouring materials.
OXIDATION CHEMISTRY
‘..one of the most essential and one of the most polluting chemical technologies...’
– Anastas & Warner
Oxidation of hydrocarbon compounds
Use of air as the oxidant rather than peroxides
Catalyst stability
Heterogeneous catalysts
Metal leaching
Selectivity to products
Coordination Chemistry at the heart of Transition Metal Catalysed Oxidation
COBALT CATALYSTS FOR OXIDATION
Haber-Weiss cycle
ROOH + Co(II) RO· + Co(III) + HO–
ROOH + Co(III) RO2· + Co(II) + H+
? ROOH + Co(III) RO· + Co(IV) + HO–
ROOH + Co(IV) RO2· + Co(III) + H+
Feasibility of the first set of twin reactions is determined by the reduction potential for the Co3+/Co2+ couple.
Suitable reduction potential for the Co4+/Co3+ couple may also permit the use of Co(III) compounds as catalysts.
More on redox activity of cobalt ions
Reversibility of electrochemical processes means retention of molecular structure during and after electron transfer. It is essential for redox catalysts that are expected to have a long lifetime.
The Co3+/Co2+ redox couple has a suitable reduction potential of ~ +1.8 V to activate triplet O2 which is a powerful oxidant from a thermodynamic point of view, but is kinetically inert due to the necessity of overcoming the kinetic barrier involved in the triplet to singlet conversion.
3Σg1∆g
Metal ions activate O2 by accommodating dioxygen, at least momentarily, as either O2
-
(superoxo) or O22- (peroxo) ligands.
O22- has no unpaired electrons.
O2- has one unpaired electron. But in its formation,
triplet oxygen, which has two unpaired electrons, has already reacted to the presence of the metal ion in its proximity!
Oxidation of olefins
TWO PATHWAYS
Epoxidation
Allylic oxidation
CH
CH2H2C
Allylic position(saturated)
Allyl groups have weaker C-H bonds (360 kJ/mol) compared to alkenes and alkanes (bond energies ~400 kJ/mol). This is why an allyl radical is more stable than an alkyl radical.
Oxidation of α-pinene
Uncatalysed autoxidation at 100°C in the dark leads to the formation of 9% verbenone, 16% verbenol, 13% α-pinene epoxide, 8% trans-pinocarveol, 2% trans-carveol and
1% myrtenal [1956].
Reaction scheme of α-pinene conversion
OH O
OH O
OHOH
O
OH OH
OH
O
H+
H+
α-pinene
verbenol verbenone
α-pinene oxide isopinocamphenol isopinocamphenone
trans carveol trans sobrerol
1,2-pinanediol campholenic aldehyde
H2O/ Lewis acid
Rearrangement of α-pinene epoxide is known to produce over 100 products under various reaction conditions, particularly in presence of acid sites in the catalysts.
Selectivity thus appears to be highly important in
catalytic oxidation of α-pinene.
Common Oxidants:
Air, O2, TBHP, NaOCl, H2O2 or peroxy acids
Catalysts reportedBoth homogeneous and heterogeneous
• Zeolite encapsulated ruthenium and cobalt schiff basecomplexes (allylic oxidation); higher Ru activity. Air
• Ti-HMS catalyst (Campholenic aldehyde). TBHP
• (Zn-Al) Layered double hydroxide-hosted chiral sulfonato-salen manganese(III) complex catalysts (stereoselectiveepoxidation). Air or O2
• Mn(II) complex on montmorillonite clay. NaOCl
• Co(OAc)2/bromide. O2
• Polystyrene supported Co(II) acetylacetonate complex (epoxidation). O2/sacrificial aldehyde
• CoCl2, CoBr2, Co(OAc)2 and Co(NO3)2. Air
Cobalt(III) oxidation catalysts• Co(II) compounds are important as (even
industrial) catalysts. They are still good catalysts if environmental acceptability is not an issue.
• Under ordinary circumstances Co(II) is very stable.
• Once oxidized in presence of suitable ligands, Co(III) also may remain stable because ‘cobalticcomplexes are kinetically inert’.
• Thermal activation may be necessary in case of cobalt(III)-based oxidation catalysts.
B. K. Das and J.H. Clark, Chem. Commun., 2000, 605
Investigation on the applicability of Co(III) systems for α-pinene (an olefin) oxidation
Cubane-like cobalt(III) complexes
[Co4(µ3-O)4(µ-O2CR)4L4]L = pyridine or its derivatives such as 4-Mepy, 4-CNpy
Co(III) Cubane Cluster
. Pink SolutionNaO2CRCo(NO3)2 LMeOH
6H2O + +
Olive Green ComplexesH2O2 (30%, v/v)
[Co4(µ3-O)4(µ-O2CMe)4(NC5H5)4]
Cyclic voltammogram
E½ = +0.73 V
In MeCN
HOMOGENEOUS CATALYSISOxidation of α-pinene by compressed air under homogeneous conditions at atmospheric pressure has been investigated using [Co4
III(µ3- O)4(µ-O2CC6H5)4(4-CNpy)4] as the catalyst.
• Both epoxidation and allylic oxidation products are found to form
• Effects of reaction temperature and catalyst concentration are studied
O
OH O+ +Compressed Air
Catalyst I, ∆
α-pinene α-pinene oxide Verbenol Verbenone
Effect of Reaction Temperature on α-pinene Epoxidation
60 80 1000
5
10
15
20
25
30
35
Yiel
d (%
)
Reaction temperature (0C)
α-pinene oxide verbenol verbenone
Reaction Conditions:α-pinene = 3.97mL (25mmol)
1,4 dioxane = 40mL
Amount of catalyst = 25mg
Oxidant, O2 = 15mL/min
T = 60, 80 & 100ºC
Reaction time = 24h
Effect of reaction temperature on α-pineneoxidation by [Co4
III(µ3- O)4(µ-O2CC6H5)4(4-CNpy)4]
15.4615.194.7631.4366.84100
7.927.95.079.7330.6280
02.191.283.56.9760
Other ProductsVerbenoneVerbenolα-Pinene
oxide
Composition of product yield (%)Conversion
(%)
Reaction temperature (°C)
Effect of Catalyst Concentration• The catalyst concentration was varied between 0.01mol% to 0.5mol%.
• The reaction with the lowest amount of catalyst shows the highestconversion of 81.4% with a turnover frequency (TOF) of 105.
• High selectivity of 62-68% for α-pinene oxide has been observed with0.01mol% of catalyst at 100°C.
Product Yield (selectivity) (%)
11.3313.984.8333.074863.2150mg,
0.5mol%
15.4615.194.7631.4328866.825mg,
0.08mol%
11.8916.974.5348.0 (68)252081.43mg,
0.01mol%
Other products
(%)
Verbenone(%)
Verbenol(%)
α-pineneoxide (%)
TONConversion (%)
Amount of catalyst
0 5 10 15 20 250
20
40
60
80
100 Amount of Catalyst = 0.01mol%Yi
eld
(%)
Time (h)
α-pinene pineneoxide verbenol verbenone
Effect of catalyst concentration on air oxidation of α-pinene
0 5 10 15 20 250
20
40
60
80
100 α -pinene α -pinene oxide verbenol verbenone
Amount of catalyst = 0.08mol%Yi
eld
(%)
Time (h)
Effect of catalyst concentration on air oxidation of α-pinene
0 5 10 15 20 250
20
40
60
80
100 Amount of catalyst = 0.5mol%
Yiel
d (%
)
Time (h)
α -pinene α -pinene oxide verbenol verbenone
Effect of catalyst concentration on air oxidation of α-pinene
HETEROGENEOUS CATALYSIS
Immobilization of known homogeneous catalysts on porous supports provides a way towards eco-friendliness of chemical synthesis
Work-up procedures become simpler
Efficiency sometimes rises
Strategies for Immobilisation
Ion exchange
Physisorption
Covalent binding
Encapsulation
Incorporation into support framework
Immobilisation of Co(III) cubanes onHexagonal Mesoporous Silica
SiO
OO
O SiO
OO O
CNEtOH/H2O, RT
CNHMS
COOHHMS
H2SO4(aq.)
Co(III) Cubane Cluster
∆, H2O (MeCN)Catalyst A
n-DDA
Immobilisation via ligand exchange
Immobilisation of Co(III) cubanes onHexagonal Mesoporous Silica
TEOS CTESn-dodecylamine
EtOH + H2OHMS-(CH2)2-CN
H2SO4 (aq)
HMS-(CH2)2-COOHCo(OAc)2 + 4-CNpy
H2OCATALYST B
In-situ immobilisation
Scanning electron micrograph of the supported reagent Co4O4(O2CCH3)4(4-CNpy)4 on HMS
CATALYST B
Scanning electron micrograph of the supported reagent Co4O4(O2CCH3)4(4-CNpy)4 on HMS
CATALYST B
Scanning electron micrographs of the supported reagent
Co4O4(O2CCH3)4(py)4 on HMS
N2 Adsorption Data and Metal Loading of Cobalt(III) Cubane Complexes Supported on HMS
1.08
1.23
1.7
0.86
1.07
0.99
AAS Cobalt Loading
mM/g
870.60622Co4O4(O2CCH3)4(4-CNpy)4(in situ prepared)
250.28376Co4O4(O2CCH3)4(4-tBupy)4
400.71542Co4O4(O2CCH3)4(4-NH2py)4
360.51426Co4O4(O2CCH3)4(py)4
850.53714Co4O4(O2CPh)4(4-CNpy)4
910.57620Co4O4(O2CCH3)4(4-CNpy)4
% of Pores between
3.2-6.0 nm
Pore Volume (ml/g)
BET SurfaceArea, m2/g
ImmobilisedComplex
Ps/Po (Adsorption) = 0.9814
Adsorption isotherms for (a) HMS-(CH2) 2COOH and HMS-supported cobalt(III) catalysts prepared by immobilizing
(b) Co4O4(O2CCH3)4(4-CNpy)4 and (c) Co4O4(O2CPh)4(4-CNpy)4
Type IV isotherms
2500 2000 1500 1000 500
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
0
ν(CN) = ~ 2243 cm-1
Tran
smitt
ance
(%)
Wavenumber (cm-1)
HMS-COOH Co4O4(O2CCH3)4(4-CNpy)4 Immobilised catalyst(in-situ)
2500 2000 1500 1000 5000.4
0.5
0.6
0.7
0.8
0.9
1.0
0
Tran
smitt
ance
(%)
Wavenumber (cm-1)
in-situ ligand exchanged
Immobilised Co4O4(O2CCH3)4(4-CNpy)4 prepared in-situImmobilised Co4O4(O2CCH3)4(4-CNpy)4 prepared via ligand exchange
Effect of temperature on α-pinene epoxidation
Catalyst B:Immobilised Co4O4(O2CCH3)4(4-CNpy)4 prepared in-situ
60 80 1000
10
20
30
40
50
Yiel
d (%
)
Reaction Temperature (0C)
α-pinene oxide verbenol verbenone
Reaction Conditions:α-pinene = 3.97mL (25mmol)
1,4 dioxane = 40mL
Amount of catalyst = 0.09mol%
Oxidant, O2 = 15mL/min
T = 60, 80 & 100ºC
Reaction time = 24h
0 5 10 15 20 250
10
20
30
40
50
60
70
80
90
100
0
0
Amount of Catalyst = 0.005 mol%
Yiel
d (%
)
Time (h)
α-pinene α-pinene oxide verbenol verbenone
T = 100ºC
0 5 10 15 20 250
10
20
30
40
50
60
70
80
90
100
0
Yiel
d (%
)
Time (h)
α-pinene α-pinene oxide verbenol verbenone
Amount of Catalyst = 0.02 mol%
T = 100ºC
0 5 10 15 20 250
20
40
60
80
100
0
Yiel
d (%
)
TIme (h)
α-pinene α-pinene oxide verbenol verbenone
Amount of Catalyst = 0.09 mol%
T = 100ºC
Homogeneous and Heterogeneous α-pinene epoxidation
% GC Yield Catalyst Amount of catalyst
(Co)
TOF α-pinene 2,3-
epoxypinane verbenol verbenone
Co/benz/4-CNpy 3 mg (0.01mol%)
105
18.87
48.01
4.53
16.97
Co/benz/4-CNpy 25 mg (0.08mol%)
12
33.16
31.43
4.76
15.19
Co/benz/4-CNpy 150 mg (0.5mol%)
2
36.79
33.07
4.83
13.98
Catalyst B 5 mg (0.005mmol)
157
24.22
24.21
8.51
16.33
Catalyst B 25 mg (0.02mmol)
38
27
41.08
8.05
14.36
Catalyst B 100 mg (0.09mmol)
9
18.54
50.07
2.9
16.94
Reaction condition: α-pinene(25mmol), 1,4-dioxan(40mL), reaction time(24h), temperature(100ºC),
ACKNOWLEDGEMENTS
Rajesh Chakrabarty
Green Chemistry Network
Department of Science & Technology Government of India
Ironwood Tree
Mesua ferrea
Nahar (Assamese)
Nageswar (Sanskrit)
The sweetly fragrant flowers do not smell of diesel in spite of the fact that this plant is gaining importance as a source of biodiesel.
Nahar comes into full bloom around the middle of April when people celebrate a festival called Bihu in Assam. Boys and girls dance, make merry and sometimes they marry one another.
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