Photo enhancement reveals (,) and (,) configurations as additional intermediates in iminium ion catalysis.

Imidazolidinone-based α,β-unsaturated iminium ions are the reactive species within countless synthetic protocols in asymmetric organocatalysis. However, (,) and (,) imidazolidinone iminium ions, ()-CC configurations, have been elusive so far. Herein we describe how photoisomerization enables the observation and assignment of high energetic ()-configured intermediates below the detection limit of NMR spectroscopy for (,) and (,) iminium perchlorate complexes derived from MacMillan's 1st generation catalyst and cinnamaldehyde. Traces of (,) could even be detected under synthetic conditions at 25 °C in MeCN. Using back isomerization studies and diffusion ordered spectroscopy, conditions were found to stabilize the (,) and (,) isomers for several hours ion pair aggregation. Thus, at least (,) should be considered for future investigations in asymmetric iminium ion catalysis.