Synthesis reactions in high -temperature water.

Abstract

Acid- and base-catalyzed reactions are prevalent in the chemical industry. Environmental concerns attributed to the use of acid/base catalysts has prompted research into more environmentally benign methods to carry out such reactions. We have explored high-temperature water (HTW) as a reaction medium that can be used without any added acid or base. We have tested high-temperature water for carbon-carbon bond forming reactions by examining the benzil-benzilic acid rearrangement and crossed aldol condensations. Additionally, we have conducted the broadest study to date of pH effects on acid and base-catalyzed reactions (primarily hydrolysis) in HTW. We investigate claims by previous researchers that acid catalysis occurs exclusively via hydronium ions from HTW in these systems. We demonstrated the feasibility of two different crossed aldol condensations in pure liquid water at temperatures of 250, 300, and 350&deg;C. We synthesized benzalacetone from benzaldehyde and acetone, and chalcone from benzaldehyde and acetophenone. We provide evidence that these reactions can be acid and base catalyzed in HTW. A reaction network with reversible formation of the unsaturated ketone, its degradation, and a path for benzaldehyde disproportion provided the basis for a quantitative reaction model. We also found that the crossed aldol condensations performed at decreasing water:organic loadings promoted higher yields. Evidence of the on-water effect was found at low water loadings with a mixer speed of 1500 rpm. Additionally the reaction was found to occur neat at 250&deg;C despite the lack of any added acid/base catalyst. The rearrangement of benzil is base (and not acid) catalyzed under conventional conditions (water-dioxane mixture around 100&deg;C). We examined this reaction in HTW between 300-380&deg;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. We conclude from our pH studies 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. We have also elucidated the kinetics in neutral water for both the rearrangement of benzil to benzilic acid and for the subsequent reactions of benzilic acid. The rearrangement is rapid, and the benzilic acid formed can react via two parallel pathways. The set of reaction pathways is consistent with the experimental data obtained from the reactions of benzil, diphenylacetic acid, and benzhydrol, individually, in HTW. In all of the tested reaction systems we found that specific acid/base-catalysis did not appear to be the primary mechanism in HTW at near neutral conditions. These systems included ester (methyl benzoate) and ether (methyl t-butyl ether) hydrolyses, reaction systems in which H₃O<super>+</super> or OH<super> -</super> were postulated as the catalytic agent by previous researchers.