Optically Controlled Recovery and Recycling of Homogeneous Organocatalysts Enabled by Photoswitches

Abstract

We address a critical challenge of recovering and recycling homogeneous organocatalysts by designing photoswitchable catalyst structures that display a reversible solubility change in response to light. Initially insoluble catalysts are UV-switched to a soluble isomeric state, which catalyzes the reaction, then back-isomerizes to the insoluble state upon completion of the reaction to be filtered and recycled. The molecular design principles that allow for the drastic solubility change over 10 times between the isomeric states, 87 % recovery by the lightinduced precipitation, and multiple rounds of catalyst recycling are revealed. This proof of concept will open up opportunities to develop highly recyclable homogeneous catalysts that are important for the synthesis of critical compounds in various industries, which is anticipated to significantly reduce environmental impact and costs.

Traffic Lights for Catalysis: Stimuli‐Responsive Molecular and Extended Catalytic Systems

“…[1,50,51,[67][68][69] This particular approach relies on rational design of a photoresponsive unit which undergoes, for instance, E-to-Z isomerization (e.g., azobenzene or hydrazone derivatives). [1,50,51,69] In this case, one photoisomer blocks the catalytically active site (Scheme 1) while the other provides open access for the substrate to reach this site. Thus, the photoswitchable moiety behaves similar to a gate, allowing or preventing catalyst-substrate interactions.…”

“…[69] Azobenzene-based photoswitches have also been employed for the design of biphasic organocatalysts which promote solution-phase reactions and can be precipitated from solution upon photoisomerization to facilitate separation of catalysts from the product mixture. [1] For instance, Han and co-workers developed a series of azobenzene-based organocatalysts bearing catalytically active tertiary amine groups along with amide groups to facilitate hydrogen bonding interactions. Notably, the E-isomers of these catalysts exhibited planar structures with significant π-π stacking interactions, while photoisomerization to the Zisomers caused a loss of crystalline packing resulting in a phase transition.…”

“…[51] This example illustrates that coupling of hydrazone derivatives with organocatalysts could be used for tuning the reaction rate through selective blocking of the active sites, i.e., preventing hydrogen bonding between the substrate and catalyst. [51] Although the majority of studies in the area of stimuliresponsive photocatalysts utilize azobenzene or hydrazone derivatives as described above, [1,50,51,69] other classes of relatively less explored photoswitches offer other avenues for switchable catalysis. [52,[73][74][75] For example, some overcrowded alkenes (i.e., tetrasubstituted alkenes with bulky substituents such as 9-(6-bromo-3,5,8-trimethylthiochroman-4-ylidene)-N,N-dimethyl-9H-fluoren-2-amine) exhibit photoswitchable behavior similar to E-to-Z isomerism.…”

“…[82] Thus, this approach demonstrates the successful preparation of a Lewis photoacid with switchable catalytic activity. [82] Based on the mentioned studies above, it is clear that the light-induced alteration of geometric parameters of photoswitch-catalyst pairs has the potential to modulate reaction rates, [1,51,69,79] yield, [1,50,52,79] and enantiomeric ratios for chiral products. [73] Rational design of photoswitchable catalysts in this area has not only resulted in "on/off" catalysts, but also in systems for which the possible outcomes are no longer binary.…”

Ampeln für die Katalyse: Stimulus‐reaktive molekulare und erweiterte katalytische Systeme

“…B. Azobenzol-, Hydrazon-oder Rotaxanderivate), zur selektiven Blockierung oder Freilegung katalytisch aktiver Stellen eingesetzt, wobei die Anregungswellenlänge als "Regler" für die katalytische Aktivität dient. [1,50,51,[67][68][69] Dieser besondere Ansatz beruht auf dem rationalen Entwurd einer photoempfindlichen Einheit, die beispielsweise eine E-zu-Z-Isomerisierung durchläuft (z. B. Azobenzol-oder Hydrazonderivate).…”