Selective Oxidant Simplifies Making Phenol

Phthaloyl peroxide promises to lower the cost and streamline manufacture of agricultural chemicals, pharmaceuticals and other compounds, say researchers at the University of Texas at Austin, Austin, Texas. The material acts as a selective oxidant, transforming arenes to phenol under mild conditions, they report. It does not require use of acids or catalysts to work, and can oxidize a variety of compounds, they add.

"There are no special conditions. You just combine the reagents, mix them, and go. It's very simple and straightforward," notes Dionicion Siegel, an assistant professor of chemistry in the College of Natural Sciences at the university. The process involves a so-called reverse-rebound mechanism. He likens it to a crab tearing food apart (Figure 1).

CONCEPTUALIZED REACTION

Figure 1. The crab, representing phthaloyl peroxide, grabs a molecule, forming a carbon-oxygen bond while tearing away hydrogen. Source: Mingjun Quan, University of Texas, Austin.

The researchers were studying the reactivity of phthaloyl peroxide and decided to see if it could transform aromatic hydrocarbons into phenols by selectively oxidizing aromatic C-H bonds. Previous attempts using other materials typically have resulted in over-oxidation and required the use of expensive metals. "This is a chemical transformation that is underdeveloped and at the same time pivotal in the production of many chemicals important to life as we know it," says Siegel.The researchers optimized reaction conditions using trifluoroethanol or hexafluoroisopropanol as solvent, increasing the reaction yields to 78% and 97%, respectively. They then looked at a wide variety of arenes. For simple and polycyclic ones, yields were moderate to excellent (46%–96%) at 23–50°C. More details on the process appear in a recent Nature article.Using the material does pose hazards. Current work focuses on fully understanding the safe use of phthaloyl peroxide on large scales, and should take three to six months to complete."We are currently examining the potential of the material to detonate under a variety of forcing conditions. We are also simultaneously developing an in situ preparation that would remove a step and avoid isolating the peroxide," says Siegel. In addition, the group plans to develop a protocol that enables recycling of phthalic acid, allowing only consumption of sodium percabonate or hydrogen peroxide for the oxidation of arenes.Siegel and his colleagues also hope to expand the scope of chemicals that can be transformed by developing more reactive agents. He is working with chemical supply companies to make phthaloyl peroxide or its precursors available to other researchers."It hasn't even come out yet, but there are a lot of people that are already picking up the technique and working on it," says Siegel.Siegel says commercialization could happen in two to three years, "if everything goes without a hitch," but still needs to find a collaborator interested in carrying out the reaction on the pilot-plant scale.