Dual wavelength asymmetric photochemical synthesis with circularly polarized light† †Electronic supplementary information (ESI) available: Full detailed methods used for the entire study; further discussion of the work not central to the main message of the paper; full derivation of the kinetics models used to predict the dual wavelength enantioselectivity; computational details and energy breakdown; more complete mechanism for the reaction. See DOI: 10.1039/c4sc03897e

Asymmetric photochemical synthesis using circularly polarized (CP) light is theoretically attractive as a means of absolute asymmetric synthesis and postulated as an explanation for homochirality on Earth. Using an asymmetric photochemical synthesis of a dihydrohelicene as an example, we demonstrate the principle that two wavelengths of CP light can be used to control separate reactions. In doing so, a photostationary state (PSS) is set up in such a way that the enantiomeric induction intrinsic to each step can combine additively, significantly increasing the asymmetric induction possible in these reactions. Moreover, we show that the effects of this dual wavelength approach can be accurately determined by kinetic modelling of the PSS. Finally, by coupling a PSS to a thermal reaction to trap the photoproduct, we demonstrate that higher enantioselectivity can be achieved than that obtainable with single wavelength irradiation, without compromising the yield of the final product.