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Jacobsen asymmetric epoxidation of olefins

Jacobsen asymmetric epoxidation of olefins. R = aryl, alkenyl, alkynyl R’ = bulky group. For the Jacobsen epoxidation, a “lock-and-key” mechanism operates: the transition state complex with the lowest energy is the one leading to the major product.

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Jacobsen asymmetric epoxidation of olefins

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  1. Jacobsen asymmetric epoxidation of olefins R = aryl, alkenyl, alkynyl R’ = bulky group For the Jacobsen epoxidation, a “lock-and-key” mechanism operates: the transition state complex with the lowest energy is the one leading to the major product. Metal complexes of porphyrins, salens, phthlocyanins catalyze the reaction with O2 or O–donors (H2O2, ROOH, PhIO, NaOCl, RCOOOH, py-O, etc). Typically, the O-donor generates a high-valent metal-oxo complex and the electrophilic oxo- atom is transferred to the hydrocarbon substrate. Although porphyrin complexes are not so readily available as salen complexes, they can be used in combination with O2. Reference: Katsuki, T.Synlett. 2003, 3, 281

  2. Mechanistic considerations The observed selectivity is based on a side-on approach of the olefin. cis-disubstituted olefins are better substrates than trans-disubstituted olefins. Trisubstituted olefins are very good substrates. The olefin approaches such that its bulkier substituent (L) is away from the 3’-substituent to minimize repulsion. The substituents on the benzylic carbons in the 3 or 3’ positions are directed away from the incoming olefin and strong repulsion cannot be expected. In order to improve enantioselectivities, a binaphtyl unit as the chiral element was used. Trans-olefins are not good substrates because the desired orientation of the incoming olefin is destabilized by the interaction of the downward substituent (S) with the salen ligand. Deeply-folded Mn(salen)s are expected to be the catalyst suitable for trans-olefins. Reference: Katsuki, T.Adv. Synth. Catal. 2002, 344, 131

  3. Sharpless asymmetric dihydroxylation of olefins

  4. Ligands used for AD

  5. Oligomerization and Polymerization of Olefins Textbook H: Chapter 22.1 – 22.4, 22.9.1.1 Textbook A: Chapter 15.1, 15.3

  6. Outline • Oligomerization • SHOP: Shell Higher Olefin Process • Polymerization • Polymers: definitions, structural/property relationships • Historical aspects of Ziegler/Natta polymerization • Heterogenous catalysts • Metallocene catalysts: co-catalysts, mechanism • Living polymerization • Late transition metal catalysts

  7. Ni-catalyzed oligomerization C6 – C18 olefins: commercial value The rest: isomerization and metathesis SHOP: commercialized in 1977; in 1993 global annual production capacity was 106 tons.

  8. Polymers: Definitions • Monomer: Any substance that can be converted to polymers. • Polymer: Macromolecule built up by linking together large numbers of smaller molecules. • Copolymer: Macromolecule built up by linking together two different monomers. Naturally occurring polymers: Synthetic polymers: • Polymer structure greatly affects polymer properties.

  9. Synthesis of polymers • Condensation reactions: all molecules are involved in the steady growth of species of higher molecular weight. • Addition reactions: reaction of the initiating species with monomers; a limited number of growing polymer molecules exists in excess of monomers. • Radical (and living radical) polymerization • Anionic polymerization • Cationic polymerization • Coordination polymerization

  10. Polymer molecular weight • Many important mechanical properties of a polymer depend on and vary with molecular weight: melting point, Tm (crystalline part); glass transition temp., Tg; crystallinity; strength; modulus (the relation between stress and deformation); viscosity; morphology of the polymer particles. Weight fraction • Gel Permeation Chromatography (GPC) is a tool for polymer molecular weight determination. • Molecular weight distribution gives information about the distribution of different molecular weight chains within a polymer sample. Mn: number average molecular weight; Mw: weight average molecular weight

  11. Ziegler/Natta polymerization: introduction • Karl Ziegler: German chemist, Nobel prize 1963 • 1953/54: oligomerization of ethylene by trialkylaluminum compounds (high pressure and temperature). • In the presence of trace transition metals, it was found that the reaction took place at a much lower temperature and pressure. • Polymer had a linear, unbranched structure with high molecular weight, i.e. HDPE • Giulio Natta: Italian chemist, Nobel prize 1963 • Learned of Ziegler’s research, and applied findings to other a-olefins such as propylene and styrene. • Resulting polypropylene was made up of two fractions: amorphous (atactic) and crystalline (tactic).Polypropylene is not produced in radical initiated reactions. http://www.nobel.se/chemistry/laureates/1963/

  12. Metallocene catalysts • 1955: Natta reported that Cp2TiCl2 activated by AlEt3 could polymerize ethylene with low activities. • 1981: Sinn and Kaminsky discovered a high-activity catalyst: • The key to reactivity was the co-catalyst generated from the adventitious water, “methylalumoxane(s)” or MAO:

  13. Role of MAO • Ziegler/Natta catalysts are water sensitive; excess MAO serves to dry the solvent and monomers. • MAO can alkylate metal halides. • MAO can abstract alkyl groups from a complex and generate cations. • In this case, MAO is transformed into a weakly-coordinating anion. Reference: Eilertsen, J. L. et al. Inorg. Chem. 2005, 44, 4843

  14. Activation of metallocenes and olefin insertion • The active metallocene catalyst is a cationic alkyl. • The alkyl resides in one of the equatorial sites and the olefin binds to the other.

  15. a-Olefin insertion • a-olefins can insert from two positions: 1,2-insertion 2,1-insertion • 1,2-addition is the major mode of insertion; 2,1-insertion usually leads to chain termination.

  16. Chain termination • b-Hydrogen elimination • b-Alkyl elimination • Chain transfer to co-catalyst

  17. Living polymerization: A special case • Initiator and intermediates are stable under reaction conditions. • There is no chain termination. • ki ≥ kp This means that the rate of initiation is greater than rate of propagation and that all the metal centers are initiated before propagation takes place. • Polymers with narrow molecular weight distributions are obtained.

  18. Ti-based heterogeneous Ziegler/Natta catalysts Cossée-Arlman mechanism First generation (Solid solution) • Different crystalline modifications of Ti(III) chloride, (TiCl3)a,b,g,d, and Al(C2H5)2Cl Second generation (Donor modified) • TiCl3/AlR3/Lewis Base (e.g. ethers, esters, ketones, amines and phosphines) • Certain Lewis bases increase the stereospecificity of polymerization and increase the activity of the catalyst. Third generation (Supported) • TiCl4 + Al/MgCl2/Lewis Base/AlR3 • Increase in catalyst surface area greatly increases polymerization activity.

  19. Late transition metal catalysts

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