The benzil-benzilic acid rearrangement in high-temperature water

The rearrangement of benzil is base ( and not acid) catalyzed under conventional conditions ( water - dioxane mixture around 100 degrees C). We examined this reaction in high-temperature water (HTW) between 300 - 380 degrees C with the intent of studying a reaction that proceeds solely by base catalysis in this more environmentally benign medium. The rearrangement proceeds in neutral HTW without addition of base, but the yield of rearrangement products is nearly insensitive to pH at near-neutral conditions. Adding larger amounts of base, however, leads to much higher yields and 100% selectivity to rearrangement products. Likewise, adding larger amounts of acid leads to comparable yields, but less than 100% selectivity. The selectivity to rearrangement products generally increased with pH at near-neutral and basic conditions whereas the selectivity to benzil decomposition products ( a competing thermal pathway) exhibited a maximum at near-neutral conditions. We conclude that the benzil rearrangement is catalyzed by acid, base, and water in HTW. The dominant mechanism shifts as the pH changes. This system shows that mechanisms that are unimportant at conventional reaction conditions can become dominant in HTW. It also demonstrates the ability to use pH to direct the selectivity of a reaction in HTW.