Stepwise bond cleavage

B. Stepwise Bond Cleavage of Two C—O Bonds Via the T State V. Conclusions... 

Polychloroethylenes follow a stepwise reductive cleavage mechanism involving c -Cl + e -Cl sequences (Figure 3.11), as expected from the resemblance between chlorine-carbon bonds in vinylic and aromatic positions. 

The reductive cleavage of iodobenzene and 3-methyliodobenzene was studied by cyclic voltammetry in both DMF and acetonitrile at 21 and 56 °C at different scan rates and has shown that there is a transition between stepwise and concerted mechanisms at lower scan rates. 1-Iodonaphthalene undergoes a stepwise reductive cleavage with mixed kinetic control by electron transfer and follow-up bond breaking, whatever the scan rate. ... 

Organic reactions that involve the cleavage and/or formation of two or more bonds may proceed by either a stepwise mechanism, in which bond cleavage and... 

The description of the borderline between stepwise and concerted nucleophilic substitution remains murky in cases where there is no significant stabilization of the transition state for the concerted reaction through the coupling of bond cleavage and formation. The reason is that there are no simple experimental protocols to detect the point at which the energy well for the carbocation intermediate of the stepwise reaction in the upper right hand corner of Figure 2.3 is transformed into... 

The electrochemical measurements were carried out in the presence of one equivalent of a weak acid (acetanilide) to ensure protonation of the electrogenerated terf-butoxy anion. This was necessary to avoid the interference of the father-son reaction between t-BuO and the perbenzoate, leading to the corresponding ester. The initial one-electron reduction proceeds with 0—0 bond cleavage leading to the formation of t-BuO and ArCOO according to a stepwise (equations 71, 72) or concerted (equation 73) mechanism. At the working potentials, f-BuO is reduced (equation 74) to the anion t-BuO (E° = —0.23 V)and thus the overall process is a two-electron reduction. 

Oxidative cyclization is another type of oxidative addition without bond cleavage. Two molecules of ethylene undergo transition metal-catalysed addition. The intermolecular reaction is initiated by 7i-complexation of the two double bonds, followed by cyclization to form the metallacyclopentane 12. This is called oxidative cyclization. The oxidative cyclization of the a,co-diene 13 affords the metallacyclopentane 14, which undergoes further transformations. Similarly, the oxidative cyclization of the a,co-enyne 15 affords the metallacyclopentene 16. Formation of the five-membered ring 18 occurs stepwise (12, 14 and 16 likewise) and can be understood by the formation of the metallacyclopropene or metallacyclopropane 17. Then the insertion of alkyne or alkene to the three-membered ring 17 produces the metallacyclopentadiene or metallacyclopentane 18. 

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