Homogeneous catalysis

1. A + B C Catalyst Homogeneous catalysis Heterogeneous Homogeneous A catalyst lower the activation barrier for a transformation, by introducing a new reaction pathway. It does not change the thermodynamics!!

2. Important catalyst properties * Activity: A reasonable rate of reaction is needed Turnover frequency (N) N = /[Q] Large turnover frequency – efficient catalyst * Selectivity: Byproducts should be minimized * Lifetime: It is costly to replace the catalyst frequently * Cost: The acceptable cost depends upon the catalyst lifetime and product value lifetime and product value

3. Transition metal organometallic compounds Metal-carbon bond 18 en rule Organic compounds – Octet rule Organometallic – 18 electron rule * 18 valance electron – inert gas configuration Oxidation state method Neutral atom method

6. 16 electron complexes Group 9 and 10 9+1+2+2+2 = 16 Exception to 16/18 electron rule V(CO)6 17 electrons W(CH3) 12 electrons

7. Catalytic steps (a) Ligand coordination and dissociation Facile coordination of the reactant and facile loss of products. Coordinatively unsaturated - 16-electron complexes (b) Oxidative addition Non-bonding electron pair in the metal Coordinatively unsaturated Oxidation of metal by two units – Mn to Mn+2

8. Oxidative addition…

9. (c) Insertion or migration Migration of alkyl and hydride ligands

10. (d) Nucleophilic attack

11. (e) Reductive elimination Involves decrease in the oxidation and coordination number

12. Wilkinson’s Catalyst Tris(triphenylphosphine)rhodium(I) chloride square planar 16 electron speciesd8 configuration

13. Geoffrey Wilkinson • Born July 14, 1921, Todmorden, Yorkshire, England • Received Ph.D from Cal Berkeley studying with Glenn Seaborg • First published compound in 1965 in Journal of the Chemical Society - Chemical Communications • Nobel Prize in Chemistry 1973 (shared with Ernst Otto Fischer) for their pioneering work, performed independently, on the chemistry of the organometallic, so called sandwich compounds.

14. Synthesis commercially available – Sigma Aldrich, Acros, etc.

15. (1) Oxidative addition 16 en Rh+1 18 en Rh3+ (2) Ligand Dissociation

16. (3) Ligand Association (4) Insertion

17. (5) Ligand association

18. (6) Reductive elimination

19. WC IN A C T I O N

20. Highly sensitive to the nature of the phosphine ligand Analogous complexes with alkylphosphine ligands are inactive Chiral phosphine ligands have been developed to synthesize optically active products.

21. Applications * Laboratory scale organic synthesis * Production of fine chemicals * Synthesis of L-DOPA Used for the treatment of Parkinson’s diseases Synthetic route was developed by Knowles and co-workers at Monsanto Dr. William S. Knowles received Nobel prize in chemistry 2001 along with other two scientists.

22. This reaction, developed by Knowles, Vineyard, and Sabacky, was used at Monsanto as a commercial route to the Parkinson's drug L-DOPA.

23. Metal Carbonyl Complexes CO as a ligand strong s donor, strong π-acceptor strong trans effect small steric effect CO is an inert molecule that becomes activated by complexation to metals Homoleptic Heteroleptic

24. Group Formula Valence en Structure 6 Cr(CO)6 6 +12 = 18 7 Mn2(CO)10 7+10+1 = 18 8 Fe(CO)5 8+10 = 18 9 Co2(CO)8 9+8+1= 18 10 Ni(CO)4 10+8 = 18

26. M O C  orbital serves as a very weak donor to a metal atom

28. Metal should be in low oxidation state ie en rich

29. Synthesis Direct combination 30oC and 1 atm CO Reductive carbonylation AlCl3 and Benzene 250oC, 350 atm CO

30. Substituted carbonyls Oxidative Addition Concomitant oxidation of the metal and addition of new ligand(s) Fe(CO)5 + Br2 Fe(CO)4Br2 + CO Fe0 Fe2+

31. Reductive Carbonylation Metal halide and CO

32. Properties Reduction to form metal carbonylates Substitution

34. Oxidation Oxidative cleavage of M-M bond Protonation …more negative the anion, higher its Bronsted basicity … use in the synthesis of different organometallic

35. Application Hydroformylation