Asymmetric Brønsted catalysis is one of the most utilized synthetic strategies to perform functionalization of double bonds. In the past decade, various potent catalysts have been developed and applied effectively in many synthesis. However, the mechanistic understanding of the transformation using chiral Brønsted acid is rather difficult due to experimental limitation. Recently a new methodology has been introduced facilitating the mechanistic insight of catalyzed transfer hydrogenation of imines using chiral phosphoric acid and Hantzsch ester as hydride donor. The method employs the combination photo-induced isomerization and Brønsted acid catalysis. The illumination transforms the E-imine into the Z-imine, which was postulated to be the major reactive species. In the case of slow isomerization process and competing E/Z-isomers, the increasing amount of Z-imine would cause the increase in ee and rate. Unexpectedly, we observed a significant increase of rate but not in ee when comparing to the reaction in the dark. This would mean that the enantiomeric outcome of the reaction was generated through the competition of two pathways containing only Z-imines. In the photoisomerization experiment, three different wavelengths (278, 365 and 455 nm) were employed in the reaction. In all cases, the reaction was accelerated to different degree depending on the wavelength. However, the UV absorption spectra showed that the maxima of the investigated imines are rather found in the range from 320 nm to 385 nm. This suggests that almost no excitation of imines can take place when 455 nm light is applied. Thus, the observed acceleration might be due to the change of mechanism. Indeed, with the combination of NMR spectroscopy and TD-DFT calculation new possible pathways, which proceeds via single electron transfer and hydrogen atom transfer, were revealed.

Wavelength dependent rate acceleration in hybrid “photoisomerization-Brønsted acid catalysis”

254th National Meeting (2017)

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