Final Report: Photochemical Alternatives for Pollution PreventionEPA Grant Number: R825330
Title: Photochemical Alternatives for Pollution Prevention
Investigators: Kraus, George , Tanko, James
Institution: Iowa State University , Virginia Polytechnic Institute and State University
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1996 through September 30, 1999
Project Amount: $400,000
RFA: Technology for a Sustainable Environment (1996) RFA Text | Recipients Lists
Research Category: Sustainability , Pollution Prevention/Sustainable Development
Objective:The objectives were to: (1) extend the photochemically mediated acylation and alkylation reactions; (2) use photochemistry to produce acyl radicals that will decarbonylate to alkyl radicals; (3) evaluate supercritical solvents for our photochemically mediated additions of aldehydes to quinones (with Dr. James Tanko, VPI); and (4) understand the factors that influence the scale-up of the reaction.
Significant progress was made on all four of these objectives. A major goal of Objective 1 was to develop photochemically based annulation procedures. Dr. Kraus's group has created seven different photochemically based annulation strategies. They have successfully synthesized anthrones that are precursors to dyes using an annulation onto benzoquinone initiated by the photochemical generation of acyl radicals derived from ortho alkenyl benzaldehydes. The yields of anthrones range from 53?78 percent.
A second annulation example uses photochemical acylation to a quinone followed by intramolecular radical addition. Using this chemistry, anthrones related to the natural product MS-444 are generated in approximately 70 percent overall yield. The same sequence also works well with furan-2-carboxaldehyde.
An effective route to dihydroxy flavonoids based on the photochemically mediated reaction of 3-substituted cinnamaldehydes with quinones also has been developed. The sequence involves an addition to the quinone followed by intramolecular addition of the resulting phenol and oxidation. The yields range from 55?87 percent. There are plans to extend this science to trihydroxy flavonoids. Several trihydroxy flavonoids such as apigenin are important as dietary supplements.
The photochemical chemistry also is successful when alpha alkoxy groups are present in the aldehyde. The product of the reaction, an alkoxyacetophenone, is a key intermediate of a synthesis of halenaquinone.
It was demonstrated that the acyl radicals react regioselectively with the acyl quinones. This result is in contrast to earlier published work that proposed a meta relationship for the acyl groups. The structure for one product was determined by x-ray diffraction.
Recently, 2-phenyl-3-methoxypropenal was synthesized and irradiated in the presence of benzoquinone and benzophenone. Preliminary results indicate that the isoflavone structure has been formed. Isoflavones constitute a large group of natural products, many of which exhibit useful biological activity.
Objective 2 involved the study of the decarbonylation of acyl radicals. It was intended to provide a useful route to alkyl radicals. The reaction does occur; however, the yield of the reaction shows significant dependence on solvent. As part of an effort to find alternative pathways for decarbonylation, they examined the persulfate-catalyzed reaction. Surprisingly, no decarbonylation occurred, but this does represent a convenient, inexpensive, and benign alternative to the photochemical generation of acyl radicals.
Objective 3 was designed to evaluate supercritical solvents for our photochemically mediated additions of aldehydes to quinones. To determine whether the benzophenone-mediated additions of aldehydes to benzoquinone is a free-radical process, in addition to the photochemical route, Dr. Tanko's group carried out several reactions in benzene between benzaldehyde and benzoquinone utilizing free-radical initiators, such as t-butyl peroxide and benzoyl peroxide. In addition to probing the mechanism of the reaction, they were hoping to decrease reaction times significantly. However, the yield never exceeded 1 percent with (60-99 percent unreacted benzoquinone). Consequently, this reaction does not appear to be a chain process, and the mechanism of this reation likely involves hydrogen abstraction from the aldehyde by the excited state quinone, followed by in-cage radical-radical coupling.
Dr. Tanko's group built the 6 ml high-pressure reactor with two sapphire windows that allowed them to check solubility of chemicals in SC-CO2 by visual examination. They have observed that most of 1,4-benzoquinone at pressures of 1200 psi and 2500 psi is soluble in this solvent. Benzophenone and benzaldehyde were fully soluble in SC-CO2 at these pressures. They have performed several experiments with benzaldehyde and benzoquinone in benzene with alcohols as cosolvents. From these experiments, they learned that only when t-buthanol was used as a cosolvent for benzene it was possible to obtain similar product yields (56 percent in benzene and 46 percent in benzene + 5 percent t-buthanol). Methanol, ethanol, or isopropanol gave no more than 19 percent yield. Finally, the reactions were performed in SC-CO2 using 17 ml high-pressure reactor and 150 Watt Xenon lamp with Pyrex glass filter.
Objective 4 involved optimization of reaction conditions. The initial concern was to improve the quantum yield of the reaction. Because the products of the reaction of benzaldehyde with benzoquinone are benzophenones, they absorb light in competition with benzophenone. Unfortunately, the benzophenone products undergo a reversible intramolecular photoreaction, thus lowering the quantum yield. Ways to render the products (2,5-dihydroxybenzophenones) insoluble by metal ion complexation were explored. This would not only ensure that the products do not lower the quantum yield, but also would facilitate product recovery. The use of potassium carbonate has led to precipitates and improved yields.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
|Other project views:||All 9 publications||3 publications in selected types||All 2 journal articles|
||Kraus GA, Melekhov A. A direct route to acylhydroquinones from α-keto acids and α-carboxamido acids. Tetrahedron Letters 1998;39(23):3957-3960.||
||Kraus GA, Khwang K, Lu Y. Photoalkylation of quinones with ethers. Journal of Photochemistry and Photobiology A: Chemistry 1999;129(1-2):49-50.||