Elucidating the Mechanism of Reversible Oxiranations via Magnetization Transfer Spectroscopy

Abstract

The reversible [2 + 1] cycloadditions between an N,N'-diamidocarbene (DAC) and eight aldehydes were examined using NMR spectroscopy. Variable temperature magnetization transfer experiments revealed higher exchange rates and lower activation barriers when electron-deficient aldehydes were employed. Likewise, competitive equilibrium studies indicated a thermodynamic preference for electron-deficient aryl and sterically unhindered alkyl aldehydes compared to more electron-rich or bulkier substrates. Collectively, these and other data collected were consistent with the oxiranation process proceeding in an asynchronous manner.