Homolysis, aryl halides

Some aryl halide derivatives of pyrazoline can undergo photocyclization. Thus, 5-(2-halogenphenyl)-l,3-diphenyl-A2-pyrazoline 120 proceeds by a simple bond homolysis from the Si state to give 2-phenylpyrazolo[l,5-/Iphenathridine 121 [171] (Scheme 2.33). 

Intermolecular photoreaction of an aryl halide with another aromatic compound may lead to the formation of biaryls. In this section several examples of such reactions will be discussed. In some cases, information concerning the reaction mechanism is available but the depth to which mechanisms have been investigated varies greatly. In many cases aryl radicals formed by homolysis of the carbon-halogen bond are the reactive species. Such radicals may also be produced via electron transfer, followed by departure of halide anion. In some cases aryl cations have been proposed as intermediates. Intermolecular bond formation may also be preceded by charge transfer within an exciplex or by formation of radical ion pairs. 

In the presence of hydrogen sources and electron donors, the reaction mechanisms for photodechlorination of aryl halides may follow two general mechanisms, i.e., homolysis and electron transfer processes. In the homolysis process, C-Cl bond photolytic cleavage is the primary reaction, forming an aryl radical, which abstracts a hydrogen from the hydrogen source, and gives the product ArH. 

Irradiation of both aryl and vinyl halides (bromides are commonly used) induces homolysis of the carbon-halogen bond and produces aryl radicals from aryl halides and (apparently) both vinyl radicals and vinyl cations from vinyl halides. 

Homolysis of photoexcited aryl iodides and bromides has been exploited as a means of producing aryl radicals in some cases, these reactions have synthetic potential for the formation of biaryls, including the use of intramolecular arylation in order to construct polycyclic systems. Reductive dehalogenation has been suggested as a synthetic route to specifically deuterated aromatic compounds by photolysis of aryl halides in solvents containing abstractable deuterium atoms. 

Aryl and Alkyl Halides. Although the factors governing photodissociation of alkyl halides are not well known, there exists clear experimental evidence for electron exchange between the radical fragments formed by C-X homolysis. For example, the ultraviolet photolysis of 1-octyliodide shows photorearrangement to the 2-halogenated isomer, eq. 58 (181) ... 

Simple aromatic halides display two principal superimposing absorption bands, n,n and a,a, tailing over 250 nm. Intersystem crossing (ISC) to T, can be fast and efficient due to increased spin orbit coupling attributed to the heavy-atom effect (Section 4.8) of halogen atoms (Br, y 42- 49,i3i6 subsequent C—X bond homolysis may afford aryl radicals and... 

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