Inadequacy of Continuum Solvation for Polar Reactions: Predicting the Mechanism of Carbonyl–Olefin Metathesis

In carbonyl–olefin metathesis, two π-bonds undergo a cycloaddition-cycloreversion process to form valuable alkenes from simple precursors. Although this synthetic methodology has advanced significantly, further improvements would be greatly facilitated by a clear understanding of whether Lewis-acid-catalyzed carbonyl–olefin metathesis reactions occur via a stepwise or concerted pathway. Here we use 12C/13C kinetic isotope effects (KIEs), 1H/2H KIEs, and Hammett studies to show that prototypical iron(III)-catalyzed ring-closing carbonyl–olefin metathesis reactions of aryl ketones are stepwise. Despite this strong experimental evidence, typical computational models incorrectly predict a concerted mechanism. We trace this failure to the use of conventional implicit solvation models and demonstrate that when solvent molecules are explicitly represented, the correct stepwise mechanism is predicted. These results call into question prior computational proposals of concerted carbonyl-olefin metathesis, highlight the importance of explicit solvent representations for charged intermediates, and have broad implications for how all polar reactions are studied.