Free-radical reactions Pschorr reaction

Another free-radical arylation method consists of the photolysis of aryl iodides in an aromatic solvent. Yields are generally higher than in 14-17 or 14-21. The aryl iodide may contain OH or COOH groups. The mechanism is similar to that of 14-17. The aryl radicals are generated by the photolytic cleavage ArI AR + T. The reaction has been applied to intramolecular arylation (analogous to the Pschorr reaction). A similar reaction is photolysis of an arylthallium bis(trifluoroacetate) (12-21) in an aromatic solvent. Here too, an unsymmetrical biaryl is produced in good yields. ... 

Reduction of phenyldiazonium chloride in acetonitrile containing a high concentration of an aromatic substrate, which can act as a free-radical trap, leads to phenylation of the substrate in 14 - 33% yields together with 50 - 50% of benzene formed by phenyl radical attack on the acetonitrile [132], Intramolecular capture of the phenyl radical, in an electrochemical equivalent of the Pschorr reaction, is much more successful and phenanthrene derivatives can be prepared in 90 - 96% yield [133],... 

The Pschorr reaction is valuable in the synthesis of carbolines. The 3-aminopyridine (165) gives the a-carboline (166 78%) under free-radical conditions, but the aniline... 

The reactions are performed by slow addition (10-15 h) of benzene solution of tri-K-butyltin hydride (1.3 eq.) and AIBN (0.7 eq.) to the boiling benzene solution of the corresponding Motherwell s precursor. Tri- -butyltin hydride in the presence of radical initiator such as AIBN is well known reagent for the generation of aryl radicals from aryl iodides and bromides [5]. Side-reactions in this approach are the formation of direct free-radical addition and dehalogenation products. The formation of the latter side-product is minimized by slow addition of tin hydride. However, the formation of direct free-radical addition, the Pschorr-type product is a serious side-reaction. For example, compound 423 at 75% conversion gives 34% of biaryl 424 as zp o-product (Motherwell reaction pathway), and 39% of biaryl 425, as the product of direct free-radical arylation, analogously to the Pschorr reaction pathway [1], Scheme 2. 

Methodology similar to Pschorr and Motherwell reactions, was employed in the synthesis of phenanthrene-type structures, where an intramolecular free-radical arylation was accomplished by the reaction of bromo-c/s-stilbene 428 with tri-n-butyl tin hydride, giving 429 in 85% yield [6], respectively. Scheme 4. 

---