title: Two-dimensional infrared spectroscopy reveals the structure of an Evans auxiliary derivative and its SnCl4 Lewis acid complex.
abstract: Determining the structure of reactive intermediates is the key to understanding reaction mechanisms. To access these structures, a method combining structural sensitivity and high time resolution is required. Here ultrafast polarization-dependent two-dimensional infrared (P2D-IR) spectroscopy is shown to be an excellent complement to commonly used methods such as one-dimensional IR and multidimensional NMR spectroscopy for investigating intermediates. P2D-IR spectroscopy allows structure determination by measuring the angles between vibrational transition dipole moments. The high time resolution makes P2D-IR spectroscopy an attractive method for structure determination in the presence of fast exchange and for short-lived intermediates. The ubiquity of vibrations in molecules ensures broad applicability of the method, particularly in cases in which NMR spectroscopy is challenging due to a low density of active nuclei. Here we illustrate the strengths of P2D-IR by determining the conformation of a Diels-Alder dienophile that carries the Evans auxiliary and its conformational change induced by the complexation with the Lewis acid SnCl(4), which is a catalyst for stereoselective Diels-Alder reactions. We show that P2D-IR in combination with DFT computations can discriminate between the various conformers of the free dienophile N-crotonyloxazolidinone that have been debated before, proving antiperiplanar orientation of the carbonyl groups and s-cis conformation of the crotonyl moiety. P2D-IR unequivocally identifies the coordination and conformation in the catalyst-substrate complex with SnCl(4), even in the presence of exchange that is fast on the NMR time scale. It resolves a chelate with the carbonyl orientation flipped to synperiplanar and s-cis crotonyl configuration as the main species. This work sets the stage for future studies of other catalyst-substrate complexes and intermediates using a combination of P2D-IR spectroscopy and DFT computations.
authors.affiliation: Institute of Biophysics, Johann Wolfgang von 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: Sabrina
authors.initials: S
authors.lastname: Steinwand
authors.forename: Eliza-Beth W
authors.initials: EB
authors.lastname: Lerch
authors.forename: Kira
authors.initials: K
authors.lastname: Hof
authors.forename: David
authors.initials: D
authors.lastname: Ley
authors.forename: Dennis
authors.initials: D
authors.lastname: Gerbig
authors.forename: Heike
authors.initials: H
authors.lastname: Hausmann
authors.forename: Peter R
authors.initials: PR
authors.lastname: Schreiner
authors.forename: Jens
authors.initials: J
authors.lastname: Bredenbeck
mesh_terms: D058116:Lewis Acids / Q000737:chemistry*; D015394:Molecular Structure; D023303:Oxazolidinones / Q000737:chemistry*; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared; D013237:Stereoisomerism; D017971:Tin Compounds / Q000737:chemistry*
chemical_list: D058116:Lewis Acids; D023303:Oxazolidinones; D017971:Tin Compounds; C041694:stannic chloride