Transient radical species bonds

In summary, alkenes are reactive compounds and are removed rapidly from the atmosphere by a variety of processes. Reaction with OH radicals, ozone, and NO3 radicals all play important roles. These reactions proceed via addition to the unsaturated bond giving an adduct which decomposes and/or reacts with 02 leading to the generation of a variety of transient radical species which react to form the first generation closed-shell products (principally carbonyl compounds). 

Referring to a reaction intermediate or free radical that has a lifetime longer than that of a transient species, typically on the time-scale of at least several minutes in dilute solution in inert solvents. Persistence is therefore a kinetic property related to reactivity. The stability of an intermediate or free radical is a thermodynamic property, often expressed in terms of the appropriate bond strengths. See Transient Chemical Species D. Griller and K. U. Ingold (1976) Acc. Chem. Res. 9, 13. 

With the second charge separation initiated by hv-2, again a neutral Yz radical is formed which abstracts a hydrogen atom from the hydroxide forming a transient 0x0 species (not shown), ending with two hydroxides at both terminal catalytic centers in the S2-state. Upon hydrogen atom abstraction in the S2->S3 transition, the hydrogen-bonded 0x0 species is formed at the lower terminal Mn-site. 

The mechanism for the transformation of 5 to 4 was not addressed. However, it seems plausible that samarium diiodide accomplishes a reduction of the carbon-chlorine bond to give a transient, resonance-stabilized carbon radical which then adds to a Smni-activated ketone carbonyl or combines with a ketyl radical. Although some intramolecular samarium(n)-promoted Barbier reactions do appear to proceed through the intermediacy of an organo-samarium intermediate (i.e. a Smm carbanion),10 ibis probable that a -elimination pathway would lead to a rapid destruction of intermediate 5 if such a species were formed in this reaction. Nevertheless, the facile transformation of intermediate 5 to 4, attended by the formation of the strained four-membered ring of paeoniflorigenin, constitutes a very elegant example of an intramolecular samarium-mediated Barbier reaction. 

Indeed, the (200-fs) laser excitation of the EDA complexes of various benz-pinacols with methyl viologen (MV2+) confirms the formation of all the transient species in equation (59). A careful kinetic analysis of the decay rates of pinacol cation radical and reduced methyl viologen leads to the conclusion that the ultrafast C—C bond cleavage (kc c = 1010 to 1011 s- ) of the various pinacol cation radicals competes effectively with the back electron transfer in the reactive ion pair. 

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