HHydrogenationydrogenation

Sumeet chavanSumeet chavan

Msc-II, Sem-IVMsc-II, Sem-IVPaper- IIPaper- II

ContentContent- Introduction.- Introduction.

- Wilkinson catalyst .- Wilkinson catalyst .

-Mechanism of hydrogenation.-Mechanism of hydrogenation.

IntroductionIntroduction

▪ The addition of hydrogen to an The addition of hydrogen to an alkene to form an alkane is alkene to form an alkane is favoured thermodynamically. favoured thermodynamically. however the reaction rates is however the reaction rates is negligible at ordinary conditions negligible at ordinary conditions in the absence of the catalyst.in the absence of the catalyst.

▪ Efficient homogenous and Efficient homogenous and heterogenous catalyst are known heterogenous catalyst are known for the hydrogenation of alkenes. for the hydrogenation of alkenes.

Wilkinson catalystWilkinson catalyst

Chlorotris (triphenylphosphine) rhodium(I)Chlorotris (triphenylphosphine) rhodium(I)

Rh

Ph3P PPh3

PPh3Cl

Basic properties and Basic properties and SynthesisSynthesis

▪ The compound is a square planar, The compound is a square planar, 16-electron complex. It is usually 16-electron complex. It is usually obtained in the form of a red-violet obtained in the form of a red-violet crystalline solid from the reaction of crystalline solid from the reaction of rhodium(III) chloride with excess rhodium(III) chloride with excess triphenylphosphine.triphenylphosphine.

Rh

Ph3P PPh3

PPh3Cl

+EtOH

78 oC

+ Ph3PORhCl3 3 H2O + >4 PPh3

Step-1 Oxidative addition.Step-1 Oxidative addition.

The given reaction involves the The given reaction involves the oxidative addition of H2 to the 16e- oxidative addition of H2 to the 16e- complex complex RhCl(PPh3)3 to form a 18e- dihydro complex.

RhPh3P Cl

Ph3P PPh3

+ H2 RhPh3P

Cl

Ph3P PPh3

H

H

RhRh3+3+RhRh+1+1

Step-2 Ligand dissociation.

The dissociation of a phosphine ligand from dihydro complex results in the formation of the co-ordinatively unsaturated complex.

RhPh3P

Cl

Ph3P PPh3

H

HRh

Ph3PCl

Ph3PH

H + PPh3

Step- 3 Alkene coordination.

The co-ordinatively unsaturated complex forms the alkene complex.

RhPh3P

Cl

Ph3PH

HCH2

CH2Rh

Ph3PCl

Ph3PH

H

CH2

CH2+

Step-4 migratory insertion.Step-4 migratory insertion.

Hydrogen atom transfers from the Hydrogen atom transfers from the Rh atom in alkene complex to the Rh atom in alkene complex to the co-ordinated alkene yield a co-ordinated alkene yield a transient 16e- alkyl complex.transient 16e- alkyl complex.

RhPh3P

Cl

Ph3PH

H

CH2

CH2

RhPh3P

Cl

Ph3PCH2

H

CH2H

Step-5 Ligand association.Step-5 Ligand association.

The phosphine ligand is then The phosphine ligand is then added to the 16e- complex.added to the 16e- complex.

+ PPh3RhPh3P

Cl

Ph3PCH2

H

CH2H

RhPh3P

Cl

Ph3PCH2

H

CH2H

PPh3

Step-6 Reductive elimination.Step-6 Reductive elimination.

Hydrogen get migrated to carbon Hydrogen get migrated to carbon which results in reductive elimination which results in reductive elimination of the alkane and regeneration of the of the alkane and regeneration of the catalyst takes place.catalyst takes place.

RhPh3P Cl

Ph3P PPh3RhPh3P

Cl

Ph3PCH2

H

CH2H

PPh3

+ CH3 CH3

(note: regeneration of the catalyst)

- The rates of hydrogenations - The rates of hydrogenations decrease with increase in the alkyl decrease with increase in the alkyl group substitution on double bond group substitution on double bond mirroring their relative binding mirroring their relative binding affinities to the metal center. It is also affinities to the metal center. It is also partly due to steric factors. Therefore, partly due to steric factors. Therefore, for a successful hydrogenation, for a successful hydrogenation, oxidative addition of dihydrogen prior oxidative addition of dihydrogen prior to the binding of olefin is crucial.to the binding of olefin is crucial.

SELECTIVE HYDROGENATION SELECTIVE HYDROGENATION BY WILKINSON'S CATALYSTBY WILKINSON'S CATALYST

▪ Less substituted and sterically Less substituted and sterically less hindered double bonds are less hindered double bonds are selectively hydrogenated.selectively hydrogenated.

- Exocyclic double bonds are - Exocyclic double bonds are selectively hydrogenated over selectively hydrogenated over endocyclic double bonds.endocyclic double bonds.

- Cis alkenes are reduced rapidly - Cis alkenes are reduced rapidly than trans alkenes.than trans alkenes.

- Isolated double bonds are rapidly hydrogenated over conjugated dienes.

References -References -

▪ R.C.Mehrotra and A.Singh, R.C.Mehrotra and A.Singh, Organometallic Chemistry-A Organometallic Chemistry-A Unified Approach.Unified Approach.

▪ D.F.Shriver, P.W.Atkins and C.H. D.F.Shriver, P.W.Atkins and C.H. Langford, Inorganic Chemistry.Langford, Inorganic Chemistry.