Radical-site initiated cleavage

Note The term a-cleavage for this widespread radical-site initiated process with charge retention can be misleading, because the bond cleaved is not directly attached to the radical site, but to the next neighboring atom. 

The alpha bond to the radical cation site can be broken by a radical-site-initiated reaction, that is by a transfer of the unpaired electron to form a new bond to an adjacent atom (a atom) with concomitant cleavage of another bond of this atom. The new bond compensates energetically for the cleaved bond. In this case we speak about a radical-site-initiated a fragmentation ... 

For the radical site initiation (a cleavage), the tendency for electron pairing drives the formation of a new bond to an adjacent atom to the radical site and the cleavage of a bond to an a atom. 

Inasmuch as this chapter covers the cyclization of hydroxy-biradicals, it is important to remember that excited-state hydrogen abstraction forms a biradical with its two radical sites initially very close to each other. Unless some of the connecting bonds rotate very rapidly, disproportionation back to ketone would be aU that happens. One would think that rather simple conformational changes are all that is required for cyclization to take place. In order to predict cyclization quantum efficiency, the challenge is to understand the kinetics of those bond rotations as well as other conformational changes that lead to competing disproportionations and cleavage (only 1,4-biradicals). 

There are four distinct processes initiated by y-hydrogen abstraction in excited carbonyl compounds Norrish type II photoelimination, Yang photo-cyclization (cyclobutanol formation), Yang photoenolization (o-xylylenol formation), and (3-cleavage of radicals from carbons adjacent to the radical sites of the 1,4-biradicals. Some of these require unique structures and generate distinct products. 

Thus, the charge and the radical sites induce different cleavage reactions. The indication i and a involve all types of reactions initiated respectively at a charge or a radical site. 

Another a cleavage initiated by the radical site leads to the benzoylium ion ... 

The added 1-alkenes reach the metal sites aided by the SCF and adsorb onto the active sites as alkyl radicals to initiate carbon chain growth the resulting chains are indistinguishable from other carbon chains formed directly from synthesis gas. These new alkyl radicals consume additional methylene units to initiate new carbon chain propagation processes. Thus the selectivity for methane, which is formed mainly from methylene hydrogenation, decreases. CO adsorption and cleavage of CO to carbide on the metal site, as well as hydrogenation of carbide to methylene species, are both accelerated. This is attributed to increased consumption of the adsorbed methylene species. Experimentally, the CO conversion increased with addition of 1-alkene. This acceleration may contribute to the suppressed CO2 selectivity as well in the alkene-added reaction, as CO2 is the byproduct from CO in the water-gas shift reaction. 

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