Phenyl radicals hydrogen atom transfer

The photo- RNl reaction of enolate anions with aryl halides has been shown to be useful for both inter- and intra-molecular annelation reactions, but has now been found to suffer one major drawback in its general applicability. When a hydrogen atom is present at the /8-carbon of the enolate anion, hydrogen atom transfer to the transient phenyl radical is the preferred reaction pathway. 

In both light and high energy irradiation induced reactions, phenyl-thiol, 2-mercaptomesitylene, and their sulfides inhibit nonchain processes,201202 e.g., the light-induced conversion of benzophenone in 2-propanol to benzopinacol and acetone.99 217 The reaction converts compounds into radicals by removal or addition of hydrogen atoms. The sulfur compounds in rapid hydrogen transfer processes convert the free radicals to stable molecules and may do so repeatedly.46 46... 

As to the next step, namely, the reaction of aryl radicals with nucleophiles, we should take into account the fact that air molecular orbital, which initially accommodates the incoming electron, is available in the aryl halide. The electron is subsequently transferred in-tramolecularly from the it to the o molecular orbital of the carbon-halogen bond. Aryl radicals effectively scavenge H atoms. Therefore, an abstraction of a hydrogen atom from the solvent may occur. However, in the case of nucleophiles that can act as effective traps of aryl radicals, the addition of a nucleophile to the phenyl radical takes place. At this point, let us focus on the step of addition of the nucleophile (Y ) to the intermediate radical (Ph). When a new a bond begins to form between the sp3 carbon-centered radical (H5C6) and... 

Since the Barton reaction and the Hofmann-Lofifler-Freytag reaction generate very reactive oxygen-centered and nitrogen-centered radicals respectively, the next 1,5- and 1,6-hydrogen atom abstraction reaction readily happens. However, 1,5-H shift does not proceed effectively by carbon-centered radicals, because there is not so much energy difference between the C-H bond before and after 1,5-H shift. So the reactions are quite limited. Eq. 6.21 shows iodine transfer from reactive 1-iodoheptyl phenyl sulfone (40) to a mixture of 5-iodoheptyl phenyl sulfone (41a) and 6-iodoheptyl phenyl sulfone (41b) initiated by benzoyl peroxide, through 1,5-H shift by an sp3 carbon-centered radical [56-58]. 

---