Hydroformylation of Alkenes and Mechanistic Investigation

CONTENTS

vIntroduction (concept and importance)

vCatalysts for Hydroformylation

vCobalt Catalysts for Hydroformylation-

vRhodium – Phosphine Catalysts

vWater-soluble rhodium catalysts

vReaction parameters and n/iso ratio obtained with different catalysts

vFactors Affecting the n/iso Ratio of Hydroformylation Products

vBidendate Phosphine Rh Catalyst

vEnantioselective Hydroformylation

vConclusion

vBibliography

  What is Hydroformylation of Alkenes?

• Reaction was first discovered by Otto Roelen in 1938, often called Oxo Process.
• Hydroformylation means addition of formaldehyde to the alkene.
• First commercially important homogeneous catalytic reaction.

Why hydroformylation is industrially important?

• Ready availability of 1-alkene from the petrochemical industry.

The large increase in production of plastics, which require plasticizing agents (diester of phthalic acid), derived from hydroformylation.

Industrially useful compounds produced by hydroformylation (long carbon chain alcohols  are used  for making synthetic detergents).

C3 to C15 aldehydes are produced by the oxo process and the are subsequently converted into amines, carboxylic acids and most importantly to primary alcohols.

CH3CH=CH2  +  CO  +  H2  →   CH3CH2CH2CHO 

∆H = -150 kJ/mol; ∆G = -42 kJ/mol

CH3CH=CH2  +  H2  →    CH3CH2CH3 

∆H = -126 kJ/mol; ∆G = -88 kJ/mol

Catalysts for Hydroformylation:

1. The Co2(CO)8 catalysed process,
2. The Co2(CO)8/PR3 catalysed process,
3. The HRh(CO)(PPh3)3 catalysed process,
4. The biphasic HRh(CO)(PR3)3 process (R = m- C6H4SO3Na)
5. Aqueous phase Rhodium Catalyst: TPPTS (Triphenylphosphinetrisulfonate)
6. New generation of Rhodium Catalyst: bidentate phosphine ligands. 

COBALT CATALYSTS FOR HYDROFORMYLATION: 

—Oldest homogeneous catalysis process still in use

—Under H2/CO pressure, cobalt salts produce HCo(CO)4 as the active catalytic species.

—Most widely accepted mechanism for the catalytic cycle for cobalt based catalyst Co2(CO)8 was proposed by Heck and Breslow in 1961.

—Kinetic studies support a general rate expression as given below:

d[aldehyde]/dt = k[alkene][Co][pH2][pCO]-1

—Total H2/CO (ratio= 1:1) pressures of  200-300 bar and 110-160°C

—n:iso ratio = 3:1 (not good; should be high)

Catalytic cycle using HCo(CO)4:

The general relative reactivity of alkenes  for hydroformylation is as follows:

RHODIUM – PHOSPHINE CATALYSTS

—In 1965, Osborn, Wilkinson and others reported that Rh(I) catalysts with PPh3.

—HRh(CO)(PPh3)3 and Rh(acac)(CO)2 are two commonly used catalyst

—Kinetic studies on the rhodium catalyst

      Rate α [propylene][Rh][pH2]

—High temperature is required for separation of the long chain aldehyde products, the catalyst decomposes at that temperature.

—Application has been limited to C3 and C4 alkenes.

—This problem was solved by using a water soluble phosphine along with the catalyst and also by resorting to biphasic catalysis.

WATER-SOLUBLE RHODIUM CATALYSTS:

—Water soluble catalyst are made using sulfonated PR3 

  ligands (3,3′,3″-phosphanetriyltris(benzenesulfonic acid) 

  trisodium salt; TPPTS)

—Runs at mild conditions (at 18 bar and 85- 90°C)

—Easily separated because water-soluble catalysts remain

  in aqueous phase and aldehyde is separated into

  organic phase with higher regioselective ratio between 

  linear and branch.

Reaction parameters and n/iso ratio 
obtained with different catalysts

Catalyst (active form)	Reaction parameters	(n/iso) ratio maximum
Co2(CO)8       [HCo(CO)4]              Co2(CO)8/PR3       R = n-Bu and other similar groups       HRh(CO)(PPh3)3	Pressure 200-300 bar       Temp. 110-160oC      Cat. Concentration* 0.1-1.0       Pressure 50-100 bar       Temp. 160-200oC       Cat. Concentration* 0.6      Pressure 15-25 bar	3:1 7:1 16:1

HRh(CO)(PR3)3       R = mC6H4SO3Na

Temp. 80-120oC     Cat. Concentration* 0.01-  0.05         Pressure 15-25 bar         Temp. 80-120oC       Cat. Concentration* 0.01-0.05

19:1

      ^*percentage of metal/olefin

FACTOR AFFECTING the n/iso RATIO OF HYDROFORMYLATION PRODUCTS:

—Recent research in hydroformylation reactions has been to improve the  n/iso ratio of aldehyde products.

—The first major development was the discovery of a chelating biphosphine BISBI developed by Eastman Kodak.

—Rh with BISBI as ligand gave an n/iso ratio 96:4 under mild condition.

—when PPh3 groups in BISBI were replaced with dibenzophosphole units, it resulted is an n/iso ratio of 99.4:0.6

—use of phosphites as ligands instead of phosphines also led to a higher n/iso ratio

—BIDENDATE PHOSPHINE Rh CATALYSTS:

—Over the past 20 years, research was focused on bidentate ligands because of remarkably increased regioselectivity between n/iso ratio of  aldehydes.

— High regioselectivity is the related to the stereochemistry of complex combined with the electronic and steric factors of bidendate PR3.

—In Rh-catalyzed hydroformylation, the n:iso ratio increases with the bite angle = (preferred P–M–P angle) of a chelate phosphine, probably because these ligands facilitate the RE step in the mechanism.

—The Rh complex (9.27) of the wide bite angle ligand, BISBI, has proved particularly useful.

OTHER ASPECTS OF HYDROFORMYLATION: The overall effectiveness of other metals are compared with Co and Rh.

                             Rh >   Co > Ir > Ru > Os > Mn  >   Fe  >  Cr, Mo, W, Ni, Re

Rel. Reactivity: 104-103    1      10-1  10-2 10-3   10-4      10-6        < 10-6  

ENANTIOSELECTIVE HYDROFORMYLATION:

•     Relatively recent development in hydroformylation reactions.
• chiral aldehyde will be formed only when the addition of H2/CO to the alkene occurs in the Markownikoff manner
• Rhodium based chiral catalysts such as HRh(CO)2(R,S)-BINAPHOS have been developed which give high n/iso ratio as well as good enantiomeric excess.

CONCLUSION

•     Through the catalyzed hydroformylation reaction, olefins are converted into aldehydes; mechanism and corresponding energy calculation were demonstrated.
•     The different type of phosphine ligands and cobalt- and rhodium-based catalysts were introduced; bidendate phosphine Rh catalyst showed the highest  ratios of linear to branched aldehyde even at ambient conditions.
•     Enantio- and regio-selectivity can be increased if specifically designed ligands on Rh catalysts are used.

BIBLIOGRAPHY

• B D Gupta and A J Elias, Basic Organometallic Chemistry : Concepts, Synthesis and Application, Second Edition, Universities Press( India) Private Limited,2010, 2013, pp 245-252.
• Ajai Kumar, Organometallic and Bioinorganic Chemistry, First Edition, Aaryush Education, 2014, pp 7-9 to 7-11.
• Robert H. Crabtree, The Organometallic Chemistry of the Transition Metals, Sixth Edition, John Wiley & Sons, 2014, pp 242-245.
• "Organometallic Chemistry", Spessard and Miessler
• Chem. Rev. 2012, 112, 5675 6 – 5732
• L. H. Slaugh and R. D. Mullineaux11 , J. Organometal. Chem., 1968, 13, 46

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