Pschorr reaction radical cyclizations

Although the bimolecular additions of aryl radicals to aromatic rings are not useful due to a lack of selectivity, the corresponding cyclizations are useful for constructing biaryl bonds. Equation (5) provides a generalized example of the Pschorr reaction.172 This cyclization of a diazonium salt is one of the classical methods for biaryl bond formation, and there are a variety of related reactions.138... 

Among the initiation methods available, photochemistry has already been used successfully for Pschorr reactions [122]. A new study, from which one example is shown in Scheme 24, revealed a dramatic influence of the substituents on the 2-bromobenzophenone 66. Methoxy groups in the radical ring led to a highly electrophilic cr-aryl radical which proved unreactive for hydrogen abstraction and cyclization. Substituents in the non-radical ring that stabilize the cyclohexadienyl... 

A related process that involves diazonium salts is the Pschorr reaction, hich also couples aryl diazo-nium compounds to other aromatic rings. This diazonium salt coupling can he done under acidic conditions, but addition of copper powder usually promotes the radical process. Aryl amines generate aryl diazonium salts upon treatment with nitrous acid.l An example is the reaction of 233 to give an aryl diazonium salt, which cyclized in the presence of copper to give thaliporphine (234) in 43% yield. Kupchan called this transformation an improved Pschorr reaction. 68... 

Ru Many examples of radical arylations by photoredox catalysis have been recently reported [62-64]. For instance, when ArN +X salts are reduced, a C—N bond cleavage occurs to give Nj, an Ar radical, and the anion X . An intramolecular photo-Pschorr reaction is exemplified by the conversion of ArN +X (89) into phenanthrene derivatives 92, in quantitative yields, photocatalyzed by [Ru(bpy)3](BF )j (Scheme 9.12). In this way, the excited state of this ruthenium complex was able to reduce the salt 89 to the corresponding aryl radical 90. This in turn cyclized, thereby... 

As shown in other sections of this chapter, overall attention has shifted from diazonium salts as aryl radical sources to bromo- or iodobenzenes. One of the few recent attempts to improve the classical Pschorr cyclization using diazonium ions as starting materials led to the discovery of new catalysts [119]. Results from a first samarium-mediated Pschorr type show the variety of products that can be expected from intramolecular biaryl syntheses under reductive conditions (Scheme 22). Depending on the substitution pattern of the target aromatic core and the reaction conditions, either the spirocycle 60, the biphenyl 61, or the dearomatized biphenyl 62 were formed as major product from 63 [120]. 

The photochemical reaction between the diazonium salt 1 and [Ru(bpy)3] + produces good yields of the Pschorr-type product 2, and the process is catalytic here, the reaction hrst gives a phenyl radical. This intermediate cycHzes, and then back electron transfer from the cyclized radical to Ru(lll) regenerates the catalyst complex (Scheme 4). In contrast, direct photocyclization of 1 in acetonitrile gives poor yields of the phenanthrene. The intermediate in this process now becomes the phenyl cation, and the... 

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