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.

Von ultraschnellen Strukturbestimmungen bis zum Steuern von Reaktionen: mehrdimensionale gemischte IR/nicht‐IR‐Schwingungsspektroskopie

“…[22,31] Die Polarisationsabhängigkeit der Kreuzsignale lässt Rückschlüsse auf den Winkel zwischen den Übergangsdipolmomenten der wechselwirkenden Schwingungen zu, was zur Strukturbestimmung genutzt werden kann. [32,33] Auch die anharmonische Kopplung,w elche aus den beobachteten anharmonischen Verschiebungen bestimmt wird, kann verwendet werden, um die molekulare Struktur abzuleiten. [34,4] Doch direkte anharmonische Kopplung ist nicht die einzige Mçglichkeit wie Kreuzsignale entstehen kçnnen.…”

“…Die Beispiele dieses Abschnitts zeigen, dass die Analyse von transienten Strukturen mittels T2D-IR bisher nur auf sehr grundlegendem Niveau betrieben wurde.H ingegen konnte man mit 2D-IR-Spektroskopie in Gleichgewichtssystemen bereits detaillierte Strukturanalysen durchführen. [31][32][33][34]83] T2D-IR-Spektroskopie kann solch detaillierte Information auch für transiente Spezies liefern, bei denen die hohe Zeitauflçsung von Femtosekundenspektroskopie ihren vollen Nutzen entfalten kann.…”

Structure analysis of substrate catalyst complexes in mixtures with ultrafast two-dimensional infrared spectroscopy

“…All computations were performed by employing the program suite Gaussian 09, 25 using density functional theory (DFT) with Truhlar's hybrid meta-exchange-correlation functional M06 26 in conjunction with a 6-31+G(d,p) basis set. 1 Computations on Sn-containing species were performed using a 6-31+G(d,p) basis set with Stuttgart-Dresden effective core potentials (SDD) on the Sn atom. 27 Computations that use a DGDZVP basis set are reported in the ESI.…”

“…In a recent study we showed that polarization-dependent two-dimensional infrared (P2D-IR) spectroscopy is capable of resolving the conformation of small molecules in solution and their complexes with a catalyst. 1 Both, complex formation and dissociation are fast compared to the timescale of NMR. With its inherent (sub)picosecond time resolution and the high sensitivity of infrared spectroscopy, P2D-IR is a powerful technique to investigate the structure of short-lived intermediates.…”

From Ultrafast Structure Determination to Steering Reactions: Mixed IR/Non‐IR Multidimensional Vibrational Spectroscopies

“…Cross‐peaks can thus be used to disentangle and assign vibrational spectra of mixtures 22. 31 The polarization dependence of cross‐peaks gives the angle between the transition dipole moments of the interacting vibrations, which can be used to determine molecular structure 32. 33 The anharmonic coupling, which can be deduced from the observed anharmonic shifts, can also be used to infer molecular structure 4.…”