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Catalytic Olefin Isomerization

Catalytic Olefin Isomerization. RuHCl(PPh 3 ) 3 will hydrogenate olefins in the presence of H 2 , but it also isomerizes a -olefins to internal olefins through reactions of the Ru-H bond. Catalytic Olefin Isomerization - Product Distribution. 4-centred planar transition state.

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Catalytic Olefin Isomerization

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  1. Catalytic Olefin Isomerization • RuHCl(PPh3)3 will hydrogenate olefins in the presence of H2, but it also isomerizes a-olefins to internal olefins through reactions of the Ru-H bond.

  2. Catalytic Olefin Isomerization - Product Distribution 4-centred planar transition state

  3. Enantiomers and their Properties • That 2-butanol and its mirror • image cannot be superimposed • shows that these are two • different molecules • these stereoisomers are enantiomers • Chiral molecules exist as enantiomers • due in most cases to the presence of • an asymmetric carbon • While the influence of chirality on biological activity can be pronounced, the physical properties of enantiomers are identical except for optical rotation.

  4. Chiral Compounds with Differing Biological Effects

  5. Diastereomers and their Properties • Stereoisomers that are not mirror images of each other are called diastereomers. • They may chiral molecules (2,3-pentanediol, below) but need not be, as seen for cis- and trans-2-butene. • (2R, 3R) (2S, 3R) • (2R, 3S) (2S, 3S) • Diastereomers have different melting points, boiling points, refractive indices, heats of formation and other physical properties. • Reaction of a racemic mixture with a single enantiomer generates isolable diastereomers.

  6. Production/Isolation of Chiral Compounds • Optical Purity: • where [a] is the specific rotation of the mixture and [a]o is that of the pure enantiomer • Enantiomeric Excess (ee): • Methods of producing/isolating asymmetric compounds: • Kinetic resolution and/or selective crystallization of racemates • Fermentation • Asymmetric transformations of prochiral compounds • enzyme catalyzed functionalizations • chemical hydrogenation, epoxidation, etc.

  7. Catalytic Asymmetric Hydrogenation • A leading example is the synthesis of L-dopa, an optically active drug generated from non-chiral starting materials for the treatment of Parkinson’s disease. Phosphine ligand of rhodium catalyst precursor

  8. Catalyst Precursors for Selective Hydrogenation • Horner and Knowles at Monsanto (1968) prepared an asymmetric phosphine which, when used in the place of PPh3 in Wilkinson’s catalyst, generated enantioselectivity in the hydrogenation of prochiral olefins. • Refinements in ligand structure • (steric bulk and basicity) • led to steady improvements • in enantiomeric excess. • Best results were observed • for bidentate phosphines.

  9. Catalyst Precursors for Selective Hydrogenation

  10. Catalyst Precursors for Selective Hydrogenation • Ruthenium-base systems have a broad • range of utility as asymmetric catalysts. • a,b-unsaturated carboxylic acids are • hydrogenated in high yield and ee • (S-Naproxen, below) as well as • allylic alcohols. • Note that the BINAP ligand is an • example of a chiral, bidentate phosphine • by virtue of it having atropisomeric forms (isomers that can be separated only because rotation about a single bond is prevented).

  11. Catalyst Precursors for Selective Hydrogenation • Organometallic compounds of the Schrock/Osborn-type have proven to be more selective hydrogenation catalysts than the Wilkinson derivatives: • This catalyst precursor is readily activated by H2 to generate a Rh(I) complex that is coordinated with solvent.

  12. Substrate Coordination in Asymmetric Hydrogenations • Achieving high enantiomeric excesses seems to require a • substrate that is capable of • bidentate coordination. • This secondary • coordination generates • diastereomeric adducts • with rigid • phosphine/ • substrate • arrangements. • Hydroxy, carbonyl, and • amino, groups in an • a-position to the double bond • are suitable.

  13. Hydrogenation of Dehydroamino Acid Derivatives

  14. Hydrogenation Mechanism - Achiral Phosphines • Mechanism of the [Rh(DIPHOS)]+ catalyzed hydrogenation of • methyl-(Z)-a-acetamidocinnamate (MAC).

  15. Reaction Coordinate of an Enantioselective Synthesis • To achieve high enantiomeric • excess, the diastereomeric • transition states of the rate • determining steps must be • substantially different in energy. • The theoretical ee is a strong • function of D(DG‡) as shown to • the left.

  16. Enantioselective Hydrogenation Mechanism k1’’ k-1’’ k1’ k-1’

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