Photo enhancement reveals ( E , Z ) and ( Z , Z ) 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, ( E , Z ) and ( Z , Z ) imidazolidinone iminium ions, i.e. ( Z )-CC configurations, have been elusive so far. Herein we describe how in situ photoisomerization enables the observation and assignment of high energetic ( Z )-configured intermediates below the detection limit of NMR spectroscopy for ( E , Z ) and ( Z , Z ) iminium perchlorate complexes derived from MacMillan's 1st generation catalyst and cinnamaldehyde. Traces of ( E , Z ) 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 ( E , Z ) and ( Z , Z ) isomers for several hours via ion pair aggregation. Thus, at least ( E , Z ) should be considered for future investigations in asymmetric iminium ion catalysis.