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General characteristics of oxidative addition

General characteristics of oxidative addition. The metal behaves as both a Lewis acid and a Lewis base. The oxidation state of the metal center increases by two: if the metal does not have an accessible oxidation state, which is two units higher than the initial OS, then OA cannot occur.

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General characteristics of oxidative addition

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  1. General characteristics of oxidative addition • The metal behaves as both a Lewis acid and a Lewis base. • The oxidation state of the metal center increases by two: if the metal does not have an accessible oxidation state, which is two units higher than the initial OS, then OA cannot occur. • Increasing the electron density of the metal center will facilitate the OA process. • Non-polarized substrates undergo a concerted OA, while polarized substrates go through a non-concerted mechanism. • It is especially common for the following d8 or d10 metal centers: Ru(0), Os(0), Rh(I), Ir(I), Pd(0), Pt(0), Pt(II).

  2. Concerted mechanism • Nonpolar reagents: H2, C-H, Si-H bonds • Aryl halides If a concerted OA mechanism is operating, the incoming ligands will end up in mutually cis positions, even though isomerization may lead to a more stable product containing the incoming ligands in mutally trans positions.

  3. C-H activation of hydrocarbons • “The C-H bond is the un-functional group.” • C-C and C-H bonds are strong and localized: no empty orbitals of low energy or filled orbitals of high energy. • Activity of metal complexes • Selectivity • C-H bond has to react faster with the metal center than another functional group. • Terminally functionalized alkanes are the most desirable. • C-H bond next to a functional group is usually “activated”. • Thermodynamics • C-H bond: 90 - 100 kcal/mol • pKa = 45 - 60

  4. C-H oxidative addition Intramolecular • Typical for electron-rich, low valent complexes of late TMs. • The reactive species (LnMx) is generated in situ by thermal or photochemical decomposition of a suitable precursor. • Remarkable selectivity among C-H bonds: aryl (strongest C-H bond) > 1° > 2° >> 3°. Less substituted groups are favored both TD and kinetically. It can lead to selectivity of activation of the alkane vs. the derivatized product (product usually has weaker C-H bonds than the parent alkane). Intermolecular

  5. Nucleophilic displacement (SN2) • Methyl, allyl, acyl, and benzyl halides • The ligands have a strong effect on the nucleophilicity of the metal center. • Reactivity of R-X: • X: I > Br > Cl • R: Me > Et > iPr > tBu • If chiral RX are used then inversion of configuration is observed:

  6. Radical mechanism • Indications of a radical mechanism • Alkyl halides reactivity order: PhCH2 > tBu > iPr > Et > Me • Other products than alkyl complexes are formed in the reaction • Reactions are sensitive to O2 and traces of paramagnetic impurities • If chiral halides are used, loss of optical activity is observed. • Non-chain mechanism • Chain mechanism

  7. Reductive elimination RE is favored by low electron density at the metal: oxidation, loss of ligand.

  8. Mechanistic considerations Ligands to eliminate must be in mutually cis positions.

  9. Catalytic cycles: OA/RE 2010 Nobel prize • R and R’ are sp2-hybridized • X = I > OTf > Br >> Cl (not common, but very desirable) • M = Sn, B, Zr, Zn (others not so useful) • Catalyst = Pd (sometimes Ni) Richard F. Heck Akira Suzuki Ei-ichi Negishi

  10. Observations about Pd(0)/Pd(II) catalysis • Pd(0)/Pd(II) catalytic cycles are very numerous and have many applications to organic synthesis. Most of them are based on the elementary steps shown on the previous slide and add another step, which usually occurs after the oxidative addition step. • Usually the species observed in solution at the beginning of a reaction is a precursor to the catalytically active species, not the catalyst. • A catalyst is a very reactive species and is rarely observed by conventional experiments. • Pd(II) square planar species are stable as 16 e species. In order for a precursor to the 16 e species to undergo oxidative addition, it should have a lower electron count than 16 e. • Reductive elimination can only occur if the two ligands are in mutually cis positions, so an isomerization has to take place after the transmetalation step.

  11. Circle the correct answer and explain your choice: More reactive in palladium-catalyzed coupling reactions: Faster reductive elimination:

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