Distinguishing Organomagnesium Species in the Grignard Addition to Ketones with X-Ray Spectroscopy

The addition of Grignard reagents to ketones is a well-established textbook reaction. However, a comprehensive understanding of its mechanism has only recently begun to emerge. X-ray spectroscopy, because of its high selectivity and sensitivity, is the ideal tool for distinguishing between an ensemble of competing pathways. With this aim in mind, we investigated the concerted mechanism of the addition of methylmagnesium chloride (CH₃MgCl) to acetone in tetrahydrofuran by simulating the X-ray spectra of different molecules in solution. We used electronic structure methods to calculate the X-ray absorption spectra at the Mg K- and L₁-edges and the X-ray photoelectron spectra at the Mg K-edge for different organomagnesium species, which coexist in solution due to the Schlenk equilibrium. The simulated spectra show that individual species can be distinguished throughout the different stages of the reaction. Each species has a distinct spectral feature which can be used as a fingerprint in solution. The absorption and photoelectron spectra consistently show a blue shift as the reaction progressed from reagents to products.