Probing the Mechanisms of Aldehyde Decarbonylation Using Radical Clock Substrate Analogues.

[摘要] Global warming has led to increasing research into environmentally-friendly energy sources.One of the greatest sources of greenhouse gases is consumption of fossil fuels to power modern transportation.In this context, the recent discovery of a soluble, stable enzyme that generates alkanes, cyanobacterial aldehyde deformylating oxygenase (cADO), has garnered considerable interest for its potential use in biofuel production. Biological alkane formation in biology is chemically challenging and the mechanisms of the enzymes that catalyzes alkane formation poorly understood. The mechanistic characterization of aldehyde decarbonylation through the use of radical clock substrate analogues is the subject of this dissertation.A β-cyclopropyl aldehyde was used as a ;;radical clock” to probe the mechanism of cADO.Reaction with the enzyme yielded only ring-opened product, providing evidence of homolytic scission of the aldehyde formyl group.The minimum lifetime of the intermediate cyclopropylcarbinyl radical formed was calculated to be ≥10 ns.The compound also acted as a mechanism-based inhibitor of cADO, and was found to form a covalent adduct after deformylation.The subsequent use of an α-oxiranyl aldehyde as a slow radical clock, allowed the lifetime of the radical formed after C-C bond homolysis to be more accurately estimated to be between 10 and 100 μs.Using isotopically labeled α-oxiranyl aldehydes also revealed the stereorandom nature of electron-proton transfer reaction that constitutes the final step in formation of the alkane product.A reaction that mimics the aldehyde decarbonylation catalyzed by the insect enzyme, CYP4G1, was uncovered through studies of an α-cyclopropyl aldehyde.This molecule was found to undergo nonenzymatic decarbonylation in the presence of O2 and Fe2+ salts.The reaction produced CO2 as a byproduct, and exhibited retention of the carbonyl hydrogen in the alkane product.The simplicity of this model system allowed computational simulations to be performed, which identified an energetically feasible mechanism to explain the experimental findings.The simulations indicated that control over the electrophilicity of the carbonyl carbon was key in directing the aldehyde towards decarbonylation.

[发布机构] University of Michigan

[效力级别] Aldehyde Decarbonylation

[关键词] Radical Clock;Aldehyde Decarbonylation;Biofuel;Biological Chemistry;Science;Biological Chemistry