Toward coherent anti-Stokes Raman scattering-Raman optical activity for reaction monitoring in organocatalysis: Chiral vibrational signatures of the two atropisomers (R)- and (S)-MOM-BINOL in solution

Abstract

Coherent anti-Stokes Raman scattering-Raman optical activity (CARS-ROA) is a coherent version of spontaneous ROA spectroscopy, the latter being a well-established chirally sensitive vibrational spectroscopic technique. A central major advantage of heterodyne-detected CARS-ROA over ROA is the drastic reduction of acquisition times from several hours to approximately 1 min due to the high ratio of the chiral Raman signal relative to the achiral CARS background. Here, we present the first demonstration of heterodyne-detected CARS-ROA of chiral molecules dissolved in an achiral solvent by using an experimental setup built from a conventional broadband CARS spectrometer. Specifically, the two atropisomers (R/S) of 2,2′-Bis(methoxymethoxy)-1,1′-binaphthyl, a methoxymethyl (MOM) bis-protected derivative of 1,1′-Bi-2-naphthol (BINOL), dissolved in the achiral organic solvent dichloromethane (DCM) were characterized by employing ~1 ps, 1 mW of pump pulses and ~60 fs, 10 mW of Stokes pulses within approximately 4-min acquisition time. MOM-BINOL is an important precursor for the synthesis of a great variety of many chiral auxiliaries including binaphthyl phosphoric acids, which are an important class of chiral organocatalysts used in asymmetric reactions. Overall, this work is a first step toward kinetic reaction monitoring in organocatalysis by coherent ROA providing the necessary short acquisition times.