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In organometallic chemistry, sigma-bond metathesis is a chemical reaction wherein a metal-ligand sigma bond undergoes metathesis (exchange of parts) with the sigma bond in some reagent. The reaction is illustrated by the exchange of lutetium (III) methyl complex with a hydrocarbon (R-H): [ 1 ]
The mechanism of this activation is debated. One possibility is the oxidative addition of a sigma coordinated C-H bond followed by the reductive removal of the proton. Another is a sigma-bond metathesis involving the formation of the M-C bond and a H-Cl or H-O bond. Regardless it is this step that kinetically imparts the chemoselectivity to the ...
Sigma-bond metathesis is a synthetic method for forming new carbon-carbon sigma bonds. Sigma-bond metathesis is typically used with early transition-metal complexes that are in their highest oxidation state. [19] Using transition-metals that are in their highest oxidation state prevents other reactions from occurring, such as oxidative addition.
The metal center engages the Si-H entity via a 3-center, 2-electron bond. It is widely assumed that these sigma complexes are intermediates in the oxidative addition of hydrosilanes to give metal silyl hydrides. This transformation is invoked in hydrosilylation catalysis. Evidence for sigma-silane complexes is provided by proton NMR spectroscopy.
Similar to sigma bond metathesis is the 1,2 addition reaction, where a four-membered transition state is also formed. However, a polarized double or triple metal-ligand bond is required in order to favor the formation of the desired product . [1]
Sigmatropic rearrangements are concisely described by an order term [i,j], which is defined as the migration of a σ-bond adjacent to one or more π systems to a new position (i−1) and (j−1) atoms removed from the original location of the σ-bond. [3]
Mechanism for hydrogenation is shown below, where the active catalyst is generated by sigma-bond metathesis, followed by olefin insertion and another sigma-bond metathesis to regenerate the catalyst. This is also the mechanism for other hydrofunctionalization.
The metal ion-catalyzed σ-bond rearrangement is a collection of chemical reactions that occur with highly strained organic compounds are treated with metal ions like Ag +, Rh(I), or Pd(II) based reagents. [1] [2+2] ring openings are sometimes observed: These rearrangements proceed via oxidative addition of strained rings.