Heterolytic radicals fragmentation

Figure 4.17. Ion-pair formation in a heterolytic radical fragmentation.

Triplet sensitization of sulfonium salts proceeds exclusively by the homolytic pathway, and that the only arene escape product is benzene, not biphenyl or acetanilide. However, it is difficult to differentiate between the homolytic or heterolytic pathways for the cage reaction, formation of the isomeric halobiaryls. Our recent studies on photoinduced electron transfer reactions between naphthalene and sulfonium salts, have shown that no meta- rearrangement product product is obtained from the reaction of phenyl radical with diphenylsulfinyl radical cation. Similarly, it is expected that the 2- and 4-halobiaryl should be the preferred products from the homolytic fragments, the arene radical-haloarene radical cation pair. The heterolytic pathway generates the arene cation-haloarene pair, which should react less selectively and form the 3-halobiaryl, in addition to the other two isomers. The increased selectivity of 2-halobiaryl over 3-halobiaryl formation from photolysis of the diaryliodonium salts versus the bromonium or chloronium salts, suggests that homolytic cleavage is more favored for iodonium salts than bromonium or chloronium salts. This is also consistent with the observation that more of the escape aryl fragment is radical derived for diaryliodonium salts than for the other diarylhalonium salts. 

Attempts at 4-exo nucleophilic cyclization failed, presumably because of a heterolytic fragmentation of the intermediate radical cation (Scheme 32) [139], not unlike that proposed (Scheme 15) for the decomposition of a mannose-derived alkene radical cation. 

The kinetics of the reaction of 2 -deoxyuridin-L-yl radicals (11) with thiols, with superoxide release from the peroxyl radical (13) generated, have been reported. Radical (11) is produced by photolysis of precursor (10). When the radical is produced in the presence of thiols, (12) is formed. Second-order kinetics were found for the reactions with thiols. Peroxyl radical (13) is formed in the presence of oxygen. This undergoes heterolytic fragmentation to the superoxide anion O2 and cation (14), which ultimately leads to 2-deoxyribonolactone (15). 

The gaseous dichlorocarbene radical cation reacted with alkyl halides via a fast electrophilic addition to form a covalently bonded intermediate (CI2C—X—R)+ in a Fourier transform ion cyclotron resonance mass spectrometer. This intermediate fragments either homolytically or heterolytically to produce net halogen atom or halogen ion transfer product. Addition of carbonyls to the carbene ion is followed by homolytic cleavage of the C-O bond to yield a new carbene radical cation. 

Acyclic alkoxycarbenes can fragment by homolytic (radical) or heterolytic (ionic) pathways. For example, the allyloxymethoxycarbene (74) fragments in benzene at 110 °C to a radical pair that recombines (Scheme 7.38). The radical pair can... 

In heterolytic cleavage, a pair of electrons move together toward the charged site as shown by the conventional curved arrow the fragments are again an even-electron cation and a radical, but here the final charge site is on the alkyl product. 

Water Elimination, Heterolytic P-Fragmentation and Formation of Radical Cations 118... 

A dissociative (D) process, in which the rate-determining step is the breaking of the E-Y bond. This rupture may be homolytic (the electron pair constituting the E-Y bond is equally shared by the two fragments, which will be free radicals) or heterolytic, i.e. the bonding pair remains on one of the two atoms. 

A mechanism was proposed in which the perferryl iron-oxeme, resulting from heterolytic cleavage of the 0-0 bond of the iron-peroxy intermediate, abstracts an electron from the 0=0 double bond of the carbonyl group of the aldehyde. The reduced perferryl attacks the 1-carbon of the aldehyde to form a thiyl-iron-hemiacetal diradical. The latter intermediate can fragment to form an alkyl radical and thiyl-iron-formyl radical. The alkyl radical then abstracts the formyl hydrogen to produce the hydrocarbon and C02 (Reed et al 1995). 

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