Epoxidation of α-pinene mediated by cobalt(III) …...Epoxidation of α-pinenemediated by...

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)


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


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 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)



T = 100ºC

0 5 10 15 20 250

20

40

60

80

100

0

Yiel

d (%

)

TIme (h)



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


12

33.16

31.43

4.76

15.19


2

36.79

33.07

4.83

13.98

Catalyst B 5 mg (0.005mmol)

157

24.22

24.21

8.51

16.33


38

27

41.08

8.05

14.36


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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