Dual-catalysis approach

Du J, Skubi KL, Schultz DM, Yoon TP (2014) A dual-catalysis approach to enantioselective [2 +2] photocycloadditions using visible light. Science 344 392-396... 

Scheme 40.24 Dual-catalysis approach to the asymmetric Steglich rearrangement...

More recently, Jacobsen s group [67] developed a similar approach under dual catalysis combining a chiral thiourea and a weak Brpnsted acid for the enantioselective synthesis of A-unsubstituted carboline derivatives. Alternatively, Hiemstra and co-workers [68] considered the use of A-sulfenyliminium ions as intermediates in the asymmetric Pictet-Spengler reaction in the presence of chiral phosphoric acids. Due to the easy acid-mediated removal of the sulfenyl group, this methodology is also powerful for accessing A-unsubstituted products. 

Allylic amines also proved to be suitable substrates for the dual catalysis/anion-binding approach developed by Seidel et al. however, the selectivities were slightly lower than that obtained with either primary aryl alkyl amines or propargylic amines [128]. This decrease in selectivily was attributed to the higher nucleophilic-... 

In 2011, Bode et al. reported a unique dual-catalysis redox approach to the KR of secondary amines based on the catalytic generation of an acyl azolium species [132]. Indeed, these species, which can undergo rapid acyl transfer in the presence of various nucleophiles such as water, alcohols, or thiols, are absolutely inert in the presence of amines except if an additive such as imidazole or l-hydroxy-7-azabenzotriazole (HOAt) is added to the reaction mixture [133]. 

As far as we know, this is the first molecular probe that includes two different types of reporter units activated upon on a specific stimulus. The other option to achieve dual detection would be to use two separate probes. However, in this case there could be a problem of competitive catalysis (circumstances in which the Km of the two substrate is not identical). In our probe, 6-aminoquinoline and 4-nitrophenol, detected by fluorescence and absorbance spectroscopy, respectively, were used as reporter units. Due to the synthetic flexibility of our approach, other reporter molecules with different types of functional groups, like amine or hydroxyl, can be linked to our molecular probe. The two assays must be orthogonal to each other, in order to prevent disturbances in the detection measurement. Another advantage of the probe is the aqueous solubility... 

With the dual approach of Sections 5.3.1 and 5.3.2 we would like to share our experience in particular with readers working in the field of transition metal catalysis, but so far inexperienced in using ionic liquids. Our section aims to provide some guidelines and is intended to encourage these scientists to test and develop ionic liquids as parts of a tool box for their future research. 

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