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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
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.
enantiomers and their properties
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.
diastereomers and their properties
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.
production isolation of chiral compounds
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.
catalytic asymmetric hydrogenation
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

catalyst precursors for selective hydrogenation
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.
catalyst precursors for selective hydrogenation10
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).
catalyst precursors for selective hydrogenation11
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.
substrate coordination in asymmetric hydrogenations
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.
hydrogenation mechanism achiral phosphines
Hydrogenation Mechanism - Achiral Phosphines
  • Mechanism of the [Rh(DIPHOS)]+ catalyzed hydrogenation of
  • methyl-(Z)-a-acetamidocinnamate (MAC).
reaction coordinate of an enantioselective synthesis
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.
enantioselective hydrogenation mechanism
Enantioselective Hydrogenation Mechanism

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