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(ClO(4))(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.