Catalytic decomposition and chemical reduction of diaryliodonium salts. Reactions involving ligand transfer

Radical reactions of arenediazonium ions: An easy entry into the chemistry of the aryl radical

“…Ferric ion, for example, is by far less effective.197 Titanium(IV) ion is also capable of transferring ligands to radicals. 198 Use in the Sandmeyer reaction (i) of copper salts in both valence states or (ii) of a suitable reductant other than the cuprous salt in combination with a Cu(II) salt not only satisfies the mechanistic formalism but also improves the efficiency of the halo-dediazoniation. 89 In fact, although the conventional procedure23 gives in general satisfactory yields of ArX, it is spoiled by numerous competing reactions.62 Formation of biaryls or of azo compounds has more chance to compete with the halo-dediazoniation when the amount of Cu(II) is low, as is the case when the latter is generated in situ and is not present from the beginning of the reaction.…”

“…The analogous transfer of the ligand methanol, for a synthesis of aryl ethers, is less favored by about a factor of 10, with respect to water.221 But the theoretically possible homolytic methoxy-dediazoniation is overwhelmed by the much higher rate of abstraction of hydrogen from the methyl of the solvent methanol221 (eq 55). It is also worthwhile to point out that the (II) 196,198 Therefore, they must be avoided as counterions of the diazonium salt in synthetic procedures of the Cohen type.220 This last feature of the homolytic mechanism is at variance with the situation of the heterolytic mechanism, where the decomposition of the diazonium salt in boiling water solution always affords major amounts of phenol, even in the presence of considerable amounts of halide ions.17-20…”

Copper-catalyzed O-alkenylation of phosphonates

“…Importantly, the bulky mesityl ligand allowed the selective transfer of the alkenyl group in all cases. In sharp contrast, no alkenylation product was ob- served when alkenyliodonium salts bearing aliphatic substituents were used likely due to a faster decomposition of the salt [36,37].…”

Homolytic Reduction of Onium Salts

“…The latter may involve an initial SET process generating of free phenyl radicals by decomposition of the transient diphenyliodine radical followed by competitive H-abstraction, ligand-radical transfer with metal salts and dimerization. [78] Thus, the reactivity of iodonium salts toward chromium dichloride was applied by Ochiai to reactions of diaryl-, alkenyl(aryl)-and alkynyl(aryl)iodonium tetrafluoroborates with aldehydes. For instance, treatment of (o-allyloxyphenyl)mesityliodonium salt with three equivalents of CrCl 2 in the presence of five equivalents of benzaldehyde afforded the substituted dihydrobenzofuran in 47% yield (Scheme 19).…”