Ultrafast Two-Dimensional Infrared Spectroscopy Resolves the Conformational Change of an Evans Auxiliary Induced by Mg(ClO₄)₂

Structure determination of reactive species is a key step in understanding reaction mechanisms. We demonstrate the application of polarization-dependent two-dimensional infrared spectroscopy (P2D-IR) as a powerful tool combining structure resolution with ultrafast time resolution. We apply this technique to investigate the substrate–catalyst complexes in a Lewis acid catalyzed Diels–Alder reaction. Using Mg­(ClO₄)₂ as a Lewis acid, we found that an additional complex besides the chelate typically postulated as reactive species forms. Experimental access to this new species leads to a deeper understanding of the observed selectivities for the Diels–Alder reaction catalyzed by Lewis acids. Our findings are supported by density functional computations at the M06/6-31+G­(d,p) level, including solvent corrections.