Epoxides are 3-membered cyclic ethers. The synthesis of an expoxide involves an alkene, oxidizing agent and a catalyst. In this experiement, the reagents used to form epoxide on triene 41 are methyl rhenium trioxide (MTO), urea-hydrogen peroxide (UHP), and N-methylimidazole (50 mol %).
MCPBA is the reagent most commonly used for alkene epoxidation and Payne oxidation is a convenient and inexpensive alternative. In Payne. CH3CN or CCl3CN is the oxidant precursor, but in this experiment CH3CN reslults in mixture of unidentifiable products and CCl3CN leads to complete decomposition of molecule 41. As a result, this reaction used Re-catalyzed epoxidation (Re stands for rhenium). Methyl rhenium trioxide acts as the catalyst which better activates UHP, the oxidant of the double bond. UHP is an unstable combination of urea and hydrogen peroxide in equal amounts. It decomposes and becomes a source of water-free hydrogen peroxide. N-methylimidazole is a Lewis base that donates electrons (lone pairs on nitrogen atoms), and Lewis bases are ligands that accelerate the Re-catalyzed epoxidation. Methanol is the solvent because UHP is soluble in alcohol.
In this reaction, the actual epoxidation happens in two steps. First, the oxygen anion attackes the carbon in the alkene, and the electrons shift down the molecule. Then the electrons go back up and eventually attacks the attacked oxygen and releases the rest of the peroxide complex.
