title: Structure analysis of substrate catalyst complexes in mixtures with ultrafast two-dimensional infrared spectroscopy.
abstract: The understanding of reaction mechanisms requires structure elucidation of short-lived intermediates, even in the presence of other, similar structures. Here we show that polarization dependent two-dimensional infrared spectroscopy is a powerful method to determine the structure of molecules that participate in fast equilibria, in a regime where standard techniques such as nuclear magnetic resonance spectroscopy are beyond their limits. Using catalyst-substrate complexes in a Lewis acid catalyzed enantioselective Diels-Alder reaction as an example we present two methods that allow the resolution of molecular structure in mixtures even when the spectroscopic signals partially overlap. The structures of N-crotonyloxazolidin-2-one, a reactant carrying the Evans auxiliary, and its complex with the Lewis acid SnCl(4) were determined in a mixture as used under the typical reaction conditions. In addition to the chelate that mainly forms, three additional substrate-catalyst complexes were detected and could be tentatively assigned. Observation of minor complex conformers suggests a rationale for the observed diastereoselectivity of the reaction using SnCl(4) as compared to other Lewis acids. Knowledge about additional species may lead to a better understanding of the different selectivities for various Lewis acids and allow reaction optimization.
authors.affiliation: Institute of Biophysics, Johann Wolfgang Goethe-University, Max-von-Laue-Str. 1, 60438 Frankfurt, Germany.
authors.forename: Andreas T
authors.initials: AT
authors.lastname: Messmer
authors.forename: Katharina M
authors.initials: KM
authors.lastname: Lippert
authors.forename: Peter R
authors.initials: PR
authors.lastname: Schreiner
authors.forename: Jens
authors.initials: J
authors.lastname: Bredenbeck
mesh_terms: D002384:Catalysis; D061565:Cycloaddition Reaction; D058116:Lewis Acids / Q000737:chemistry*; D023303:Oxazolidinones / Q000737:chemistry*; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared; D013237:Stereoisomerism; D013379:Substrate Specificity
chemical_list: D058116:Lewis Acids; D023303:Oxazolidinones