Liquid‐Liquid Slug‐Flow‐Accelerated [2+2] Photocycloaddition of Cinnamates

Abstract

[2+2] photocycloaddition of cinnamates, using a previously described cone reactor and a bis‐thiourea catalyst, is greatly accelerated by employing liquid–liquid slug flow. In most cases, a four‐fold acceleration in reaction time was observed with equivalent or superior yields. This approach has enabled significant improvement in the reaction throughput and expansion of the substrate scope to include challenging substrates. It has also enabled, to our knowledge, the first reported direct photodimerization in solution of electron‐rich cinnamamide substrates.

Photon Transport and Hydrodynamics in Gas‐Liquid Flows Part 1: Characterization of Taylor Flow in a Photo Microreactor

“…As can be seen in Figure 4, the variation found in our actinometric measurements is similar to the values obtained in the Paternò-Büchi photoreactions by Terao et al [4b] and Nakano et al [7] However, even if an exponential increase of conversion with the gas content is present also in the work of Telmesani et al, [6] the acceleration of the reaction observed by them is significantly higher. This could be due to the different physical properties of their solvent (ethyl acetate vs. toluene and hexane), smaller tube diameter (0.8 mm vs. 1 mm) and different light absorption properties of the employed reaction mixture.…”

“…To calculate the photon flux per liquid volume and the optical pathlength from the chemical actinometric measurements we need to determine how long the liquid was Nakano et al [7] and Telmesani et al [6] illuminated when passing through the microreactor. This was realized using residence time distribution (RTD) experiments.…”

“…Moreover, a lower productivity is expected in two-phase flow as a fraction of the reactor is occupied by the inert phase. However, higher productivities in two-phase flow compared to single-phase were achieved by using a very low fraction of the inert phase (Nakano et al [7] ) or by using highly concentrated solutions (Telmesani et al [6] ). Starting from our model, we can propose an additional optimization strategy which involves a rational design of the Taylor flow characteristics (i. e. bubble and slug lengths, film thickness) by finding the optimal liquid volume in the highly irradiated region of the film and in the darker region of the bulk liquid.…”

“…[7] Moreover, water was used as the inert phase by Telmesani et al who investigated the photocycloaddition of cinnamates in an ethyl acetate-water Taylor flow. [6] They achieved a conversion of 43 % in two-phase flow at a water fraction of 0.9 in comparison to 6 % when the water was absent. The addition of water allowed a reduction of the reaction time below 2 hours compared to around 8 hours which was typical for the singlephase reaction.…”

“…[5] While these strategies are limited by the properties of the photoreactor and pump, adding an inert phase can be a more accessible option for implementing difficult photochemical processes. [6] Figure 1 illustrates the factors reported to cause this improvement: (i) the presence of a thin liquid film which receives a high photon flux, (ii) light scattering at the interfaces, and (iii) mixing in the liquid slug in two-phase flow. [4a] Nakano et al investigated the [2 + 2] photocycloaddition of carbonyl compounds with olefins, Paternò-Büchi-type photoreactions, in the presence of nitrogen bubbles.…”