Efficient [2 + 2] photocycloadditions under equimolar conditions by employing a continuous UV-flow reactor

• [2 + 2] alkene–enone cycloadditions were performed in a UV flow reactor.
• In flow, reactions are accelerated by more than a factor 100 compared to batch.
• Excess of alkene component was decreased to equimolar ratios.
• High reaction yields are obtained with various substrates.
• Almost all criteria for a Click reaction are fulfilled in flow.

The [2 + 2] photocycloaddition between maleimide and various alkenes was optimized in a simple custom-made UV flow reactor. While complete maleimide conversion is only achieved with excesses of at least 10 eq. of alkene and reaction times of 12–24 h in batch, complete reactions with yields in the range >98% could be obtained in 5 min in the flow reactor under strictly equimolar conditions. Functional alkenes carrying allyl, alcohol, amine and ether moieties were successfully conjugated in good yields under optimized reaction conditions. As is demonstrated, the reaction gains in flow most characteristics of a high-efficient reaction, making the process highly valuable for upscaled synthesis of maleimide conjugates.

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