Sigma-bond metathesis Textbook H: Chapter 6.5.1 – 6.5.2 Textbook A: Chapter 3.3.7

Sigma-bond metathesis It avoids the TD barriers of the C-H activation /substitution step. It is found for early TM with d0 configuration.

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.

g-H elimination g-hydride elimination as the reverse of s-bond metathesis

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.

Electrophilic activation of C-H bonds Shilov, 1972 Alkane activation step Pd2+, Pt2+ and/or Pt4+, Hg2+, Tl3+.

1,2-Addition M=N bonds M=C bonds Legzdins, 2003

a-H elimination a-hydride elimination as the reverse of 1,2-addition

Migratory insertions

Migratory insertions Two possible mechanisms: Labeling studies show that Me migrates. experimental results

If CO moves

If Me moves

Migratory insertion examples

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

b-H elimination: retro-migratory insertion Modes of blocking b-H elimination No b-hydrogens The alkyl is oriented so that the beta position cannot access the metal center (steric bulk or rigidity). The alkyl would give an unstable alkene as the product.