| Abstract
| - The epoxidation of β-methylstyrene catalyzed by the Shi fructose-derived ketone is studied usingexperimental kinetic isotope effects and DFT calculations. The observation of a large β olefinic 13C isotopeeffect and small α carbon isotope effect is indicative of an asynchronous transition state with more advancedformation of the C−O bond to the β olefinic carbon. By varying the catalyst conformation and alkeneorientation, diverse transition structure geometries were located calculationally, and the lowest-energystructure leads to an accurate prediction of the isotope effects. Given this support for the accuracy of thecalculations employed, the nature of enantioselectivity in this and related epoxidations is considered. Thelowest-energy transition structures are generally those in which the differential formation of the incipientC−O bonds, the “asynchronicity,” resembles that of an unhindered model, and the imposition of greater orless asynchronicity leads to higher barriers. In reactions of cis-disubstituted and terminal alkenes usingShi's oxazolidinone catalyst, the asynchronicity of the epoxidation transition state leads to increased stericinteraction with the oxazolidinone when a π-conjugating substituent is distal to the oxazolidinone butdecreased steric interaction when the π-conjugating substituent is proximal to the oxazolidinone. Overall,the asynchronicity of the transition state must be considered carefully to understand the enantioselectivity.
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