Reductive elimination chain process

On the basis of the results described, there seem to be at least three processes which are responsible for the molecular weight reduction in substituted silane high polymers upon irradiation in solution (i) Chain abridgement by silylene extrusion which occurs only at short wavelengths (ii) chain scission by silicon-silicon bond homolysis and (iii) chain scission by 1,1-photochemical reductive elimination. 

A third process proposed by Michl to be involved in photodegradation is chain cleavage by reductive elimination according to reaction 3 ... 

Green and Rooney81 proposed an alternative mechanism (Scheme 11.13b) that also accounted for Z-N catalysis. The mechanism resembles a metathesis-like pathway by starting with a-elimination to give a metal-carbene hydride followed by cycloaddition with the alkene monomer to form a metallacyclobutane. Reductive elimination finally yields a new metal alkyl with two more carbon atoms in the growing chain. The Green-Rooney mechanism, although plausible overall, requires an a-elimination, a process that is difficult to demonstrate. 

After the reductive elimination, the palladium(0) is oxidised by atmospheric oxygen to Pd(ll). DMSO is apparently essential for this process. Noteworthy, and decisive for the Takasago muscone process, is its high (E)/(Z) selectivity, which can also be found in open-chain and macrocyclic systems. [206]... 

RX - [Ni(I), RX ], is superseded by more examples of the four types of processes concerted, radical, radical-chain, and two-step ionic. Under the mantle of reductive elimination, perhaps not quite correctly, /3 elimination of hydrogen is included since this process seems able to take place under very similar conditions to those under which a concerted reaction occurs. It is interesting to see the use of Hoffmann-type calculations in rationalizing elimination reaction pathways. Mention should be made also of the mechanism of processes involving two metal centers. 

The hydrosilylation of methylenecyclopropanes is proposed to proceed via oxidative addition to the olefin, followed by rhodium migration across the strained cyclopropane ring, and eventual reductive elimination to give the silyl-substituted olefins. The process is compatible with aromatic and aliphatic substitution on the olefin and often requires heating. Additionally, cyclopropyl-substituted methylenecyclopropanes may be selectively silylated to give alkenes containing one, two, or three /3-silylated olefin chains. 

Moving to processes in which the oxidation state changes by 2, one can find examples of all four mechanisms which have been cited for oxidative addition and reductive elimination, namely, 5n2, radical, radical chain, and concerted. 

In their seminal paper, Barton and McCombie describe a method to deoxygenate secondary alcohols through a radical chain mechanism. This process was presented as an alternative to the standard conditions of derivatization of an alcohol to a tosylate/mesylate followed by reduction. Such polar processes are problematic with hindered carbon centers and can lead to rearrangements and/or eliminations if carbocationic intermediates are involved. 

---