Reactions of organometallic complexes

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Reactions of organometallic complexes

• Chapters 3.7 3.10, 5.1 5.2.2, 6.1 6.2

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Outline

• Sigma bond metathesis
• OA of C-C bonds
• Oxidative coupling and reductive decoupling
• Ligand substitution reactions
• Dissociative mechanism
• Associative mechanism
• Slippage mechanism
• Rearrangement of coordinatively unsaturated
  species
• Migratory insertion
• Double carbonylation

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Sigma-bond metathesis
Watson, 1983

• It avoids the TD barriers of the C-H activation
  /substitution step.
• It is found for early TM with d0 configuration or
  high oxidation-state late TMs.

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Why position b cannot be C
The reaction is best described as a nucleophilic
substitution of H at either C or Si in the
coordination sphere of Ln.
The transition state is a pentacoordinated
anionic CH5- or SiH5- which is energetically
highly unfavorable for C and much more favorable
for Si.
The energy barrier for C at the b position is
lowered with electronegative substituents (F),
known to stabilize a hypervalent species, but not
vinyl or phenyl.
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s bond metathesis with high-valent, late TMs

• Midterm 2005 Recently, Hartwig et al. (J. Am.
  Chem. Soc. 2005, 127, 14263-14278) published
  mechanistic studies on the functionalization of
  arenes by diboron reagents catalyzed by iridium
  complexes

For each step shown in the catalytic cycle
indicate the mechanism (type of reaction).
Where you can envision more than one
possibility, write down all of them (at least
two) discussing arguments that support your
proposal or that are against it (at least one of
each). If you consider that some intermediates
are not shown, draw those intermediates.
Indicate formal oxidation state and electron
count for each iridium complex.
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Oxidative addition of C-C bonds
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Oxidative coupling and reductive decoupling
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Ligand substitution reactions
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Ligand substitution reactions overview

• Studied systematically for the reactions of
  phosphines with metal carbonyls (Basolo)
• Classification
• D Dissociative (comparable to the SN1 limiting
  case)
• A Associative (comparable to the SN2 limiting
  case)
• I Interchange / Intermediate
• Ia (comparable to typical SN2 reactions)
• Id (comparable to typical SN1 reactions)
• Notes
• "Labile" and "Inert" are kinetic terms
• "Stable" and "Unstable" are thermodynamic terms

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Dissociative displacements

• Observed for 18e carbonyls
• Rates TBP d8 gt Td d10 gt Oh d6
• Y-intermediate is favored by L being a good
  p-donor T-intermediate is favored by a high
  trans-effect L.
• Rates for TM 3rd row lt 2nd row gt 1st row
• Dissociation is accelerated for bulky ligands.
• Weakly bound solvent molecules are often useful
  ligands synthetically.

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Associative substitutions

• Often adopted by 16e complexes
• Found for 18e complexes that have a ligand which
  can rearrange (slip)

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Rearrangements of coordinatively unsaturated
species

• When a ligand dissociates, one of the remaining
  ligands rearranges to fill the vacant site
  created the reverse of the slippage process.
• Analogous to neighboring group participation in
  organic chemistry

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Migratory insertions
Two possible mechanisms
Labeling studies show that Me migrates.
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CO insertion
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Double carbonylation
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