Synthesis and Photochemistry of New Carbene Precursors
Creators:Cassara, Christopher M.
Advisor:Platz, Matthew S.
MetadataShow full item record
Publisher:The Ohio State University
Series/Report no.:The Ohio State University. Department of Chemistry Honors Theses;2005
Carbenes are neutral divalent reactive intermediates containing a carbon atom surrounded by only six valence electrons. Because of this electron deficiency, carbenes are very short-lived intermediates and react with a variety of functional groups. One of the most commonly used applications of carbenes is in cyclopropane synthesis. This research has focused on the synthesis of new, novel carbene precursors and the study of their photochemistry. The purpose of this research is twofold: 1) to trap the carbene with pyridine and characterize the UV spectra of the carbene ylide intermediate and 2) to determine the lifetimes and reaction rates of carbenes with various reagents. The lifetime of the carbene and the rate of its reaction with alkenes will be used to form a better understanding of the relationship between carbene structure and reactivity. Laser Flash Photolysis (LFP) techniques were used to generate the carbene, which subsequently reacted with pyridine to form an ylide. This reaction was necessary because we were not able to detect the carbene directly. The carbenes being studied cannot be directly detected because they do not exhibit a UV chromophore at or above 300 nm. Trapping the carbene with pyridine yields a species with a strong UV absorbtion at 480 nm, which is easily detected. The lifetimes of bromo-substituted carbenes are much shorter than their fluoro-substituted or chloro-substituted analogs. Fluorine is able to stabilize the lone pair of electrons on the carbon much better than the bromine. Fluorine can also stabilize carbenes by donation of a lone pair of electrons into an empty orbital of the carbene. It has also been observed that as the other substituent on the carbene precursor becomes more electron-withdrawing, the carbon-bromine bond undergoes homolysis to form a bromine radical when irradiated with 308 nm light. To overcome this complication the bromine has been replaced with a hydrogen atom. A carbon-hydrogen bond does not undergo homolytic bond cleavage to form a hydrogen radical when irradiated with 308 nm light, and, therefore, the desired carbene can be formed and react with pyridine to form the intended ylide.
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