organometallic catalysts presenter : saber askari advisor : dr.mirzaaghayan may 2012

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  • Slide 1
  • Organometallic Catalysts Presenter : Saber Askari Advisor : Dr.Mirzaaghayan May 2012
  • Slide 2
  • Contents : The basis for catalysis Catalytic Cycle History Mechanistic Concept Homogeneous Catalysis Wilkinsons Catalyst Asymmetric hydrogenation Hydroformylation Monsanto Acetic acid Process CATIVA Process Wacker Process Heterogeneous Catalysis Ziegler-Natta Catalyst
  • Slide 3
  • A Catalyst is a substance which speed up the rate of a reaction without itself being consumed. A catalyst lowers the activation energy for a chemical reaction The catalyzed reaction goes by a multistep mechanism in which the metal stabilizes intermediates that are stable only when bound to metal. The basis for catalysis
  • Slide 4
  • Importance of catalysis Many major industrial chemicals are prepared with the aid of catalysts Many fine chemicals are also made with the aid of catalysts Reduce cost of production Lead to better selectivity and less waste
  • Slide 5
  • Catalytic Cycle The catalytically active species must have a vacant coordination site to allow the substrate to coordinate Late transition metals are privileged catalysts (from 16e species easily) In general, the total electron count alternates between 16 and 18 One of the catalytic steps in the cycle is rate-determining The establishment of a reaction mechanism is always a difficult task. It is even harder to definitively establish a catalytic cycle as all the reactions are going on in parallel!
  • Slide 6
  • History
  • Slide 7
  • Fundamental Reaction
  • Slide 8
  • Mechanistic Concept
  • Slide 9
  • Homogeneous Catalysis Wilkinson`s Catalyst : Olefin Hydrogenation Hydroformylation Monsanto Acetic acid Process Wacker Process Heterogeneous Catalysis Ziegler-Natta Catalysts
  • Slide 10
  • Homogeneous Catalysis Homogenous catalysts are used when selectivity is critical and product-catalyst separation problems can be solved.
  • Slide 11
  • Advantages : Relatively high specificity Relatively low reaction temperatures far more easily studied from chemical & mechanistic aspects far more active Generally far more selective for a single product Disadvantages : o far more difficult for achieving product/catalyst separations
  • Slide 12
  • Catalytic steps in homogeneous reactions Most catalytic process can be built up from a small number of different types of step Association / dissociation of a ligand requires labile complexes Insertion and elimination reactions Nucleophilic attack on a coordinated ligand Oxidation and reduction of a metal center Oxidative addition / reductive elimination
  • Slide 13
  • Wilkinsons Catalyst: RhCl(PPh3)3 was the first highly active homogeneous hydrogenation catalyst and was discovered by Geoffrey Wilkinson (Nobel prize winner for Ferrocene) in 1964. Wilkinsons Catalyst is a Rh(I) complex, Rh(PPh3)3Cl containing three phosphine ligands and one chlorine. As a result of the olefin insertion (hydrogen migration) we obtain a Rh (III), 16e-, five coordinate species. A solvent occupies the sixth coordination site to take it to a 18e- species. Reductive elimination occurs to give the hydrogenated product and the catalytically active species.
  • Slide 14
  • Bennett, IC, 1977, 16, 665
  • Slide 15
  • Olefin Hydrogenation using Wilkinsons Catalyst The complex RhCl(PPh3)3 (also known as Wilkinsons catalyst) became the first highly active homogeneous hydrogenation catalyst that compared in rates with heterogeneous counterparts. Wilkinson, J. Chem. Soc. (A) 1966, 1711
  • Slide 16
  • Hydrogenation mechanism Steps: (1) H2 addition, (2) alkene addition, (3) migratory insertion, (4) reductive elimination of the alkane, regeneration of the catalyst Halpern, Chem. Com. 1973, 629; J. Mol. Cat. 1976, 2, 65; Inorg. Chim. Acta. 1981, 50, 11
  • Slide 17
  • Wilkinsons catalyst selectivity The rate of hydrogenation depends on : (a) presence of a functional group in the vicinity of the C=C bond (b) degree of substitution of the C=C fragment
  • Slide 18
  • Hydrogenation is stereoselective: Wilkinsons catalyst selectivity Rh preferentially binds to the least sterically hindered face of the olefin:
  • Slide 19
  • Cis-disubstituted C=C react faster than trans-disubstituted C=C: Wilkinsons catalyst selectivity Schneider, JOC 1973, 38, 951
  • Slide 20
  • Cationic catalysts Cationic catalysts are the most active homogeneous hydrogenation catalysts developed so far:
  • Slide 21
  • Slide 22
  • Halperns mechanism of hydrogenation for cationic Rh catalysts with bidentate phosphines Steps: (1)alkene addition, (2)(2) H2 addition, (3) migratory insertion, (4) reductive elimination of the alkane, regeneration of the catalyst. Halpern, Science 1982, 217, 401.
  • Slide 23
  • Asymmetric hydrogenation A variety of bidentate chiral diphosphines have been synthesized and used to make amino acids by hydrogenation of enamides: Burk, Acc. Chem. Res 2000, 33, 363.
  • Slide 24
  • Synthesis of derivative of L-dihydroxyphenylalanine
  • Slide 25
  • Catalysts similar to Wilkinsons but using chiral phosphine ligands have been used for the asymmetric hydrogenation of small molecules. Important in the fine chemicals /pharmaceutical industry Noles and Nyori received the 2001 chemistry Nobel prize for the development of asymmetric hydrogenation catalysis Chiral hydrogenation catalysts
  • Slide 26
  • Knowles, JACS 1975, 97, 2567. Intermediates in Noyoris transfer hydrogenation
  • Slide 27
  • Lanthanide Hydrogenation Catalysts Tobin Marks reported the extraordinary activity of (Cp*2LuH)2 for the hydrogenation of alkenes and alkynes. The monometallic complex catalyzes the hydrogenation of 1- hexene with a TOF = 120,000 hr-1 at 1 atm H2, 25C!! This is one of the most active hydrogenation catalysts known.
  • Slide 28
  • Catalytically active species With bidentate ligands, olefin coordination can precede oxidative addition of H2 (S = methanol, ethanol, acetone). Halpern, JACS 1977, 99, 8055
  • Slide 29
  • Hydroformylation Hydroformylation was discovered by Otto Roelen in 1938. The reaction of an alkene with carbon monoxide and hydrogen, catalyzed by cobalt or rhodium salts to form an aldehyde is called hydroformylation.
  • Slide 30
  • Heck, JACS,1961,83,4023
  • Slide 31
  • Cobalt Phosphine modified catalyst
  • Slide 32
  • Cobalt Phosphine catalyst Mechanism
  • Slide 33
  • Monsanto Acetic acid Process 1960 basf 1966 monsanto
  • Slide 34
  • Slide 35
  • CATIVA Process
  • Slide 36
  • Slide 37
  • Wacker Process This is one of the earliest industrial processes developed in Germany for the conversion of ethylene into acetaldehyde. Wacker process is more complex than the other catalytic processes described above.
  • Slide 38
  • Slide 39
  • Heterogeneous Catalysis Heterogeneous catalysts dominate chemical and petrochemical industry: ~ 95% of all chemical processes use heterogenous catalysts.
  • Slide 40
  • Ziegler-Natta Catalysis for the Polymerization of olefins Polymers are large molecules with molecular weights in the range of 104 to 106. These consist of small building units known as monomers For example polyethylene is made up of ethylene monomers In all of these cases a single monomer is repeated several times in the polymer chain. The number of repeating units determines the molecular weight of the polymer.
  • Slide 41
  • Giulio Natta (1903-1979), an Italian chemist, extended the method to other olefins like propylene and developed variations of the Ziegler catalyst based on his findings on the mechanism of the polymerization reaction. The German chemist Karl Ziegler (1898-1973) discovered in 1953 that when TiCl3(s) and AlEt3 are combined together they produced an extremely active heterogeneous catalyst for the polymerization of ethylene at atmospheric pressure. The Ziegler-Natta catalyst family includes halides of titanium, chromium, vanadium, and zirconium, typically activated by alkyl aluminum compounds Ziegler and Natta received the Nobel Prize in Chemistry for their work in 1963.
  • Slide 42
  • There are typically three parts to most polymerizations:
  • Slide 43
  • Slide 44
  • Thanks for your attention