Photochemical flow microreactor

Another characteristic feature of microreactors derived from their much greater surface-to-volume ratios is that they make phase-boundary reactions such as gas-liquid, liquid-liquid, or solid-liquid reactions more efficient. This feature of flow microreactors is also advantageous for photochemical [66-75] and electrochemical [76-86] reactions, which have received significant attention from the viewpoint of environmentally benign syntheses. 

Another interesting photochemical transformation conducted in a microreactor was examined by Sugimoto et ol. [7]. A continuous flow microreactor is used to... 

Photochemical intermolecular and intramolecular Pauson-Khand reactions of the allq ne cobalt complexes [RC=CH Co(CO)3 2] with alkenes using a flow microreactor were reported. The reaction of [PhC=CH Co(CO)3 2] with norbornene in a batch reactor led to the product in 32% yield, while the flow reactor resulted in 88% yield. " ... 

In order to produce oxime 199 in greater quantities, the authors subsequently evaluated the use of DMF as the reaction solvent due to the increased solubility of the nitrite precursor 277 (36 mM). In conjunction with two serially connected microreactors, each containing 16 microchannels [1,000 pm (wide) x 500 pm (deep) x 1.0 m (length)] and eight black lights, the photochemical synthesis was performed continuously for 20h at a flow rate of 250 pi min 1 (residence time = 32 min). After an off-line aqueous extraction and silica gel column, 3.1 g of the oxime 199 was obtained equating to an isolated yield of 60% and successfully demonstrating the ability to use photochemical synthesis for the scalable preparation of pharmaceutically relevant compounds. 

T. Fukuyama, Y. Hino, N. Kamata, I. Ryu, Quick execution of [2 -F 2] type photochemical cycloaddition reactions by a continuous flow system using a glass-made microreactor, Chem. Lett. 2004, 33, 1430. 

Jensen and coworkers [25] demonstrated the photochemical synthesis of benzo-pinacol (Scheme 6.9) within a sUicon/quartz microreactor (channel dimensions 500 pm wide x 250 pm deep), which enabled UV light to pass through the quartz layer and irradiate the reaction mixture contained within the microchannel network. Employing on-line UV analysis, they were able to assess the reaction progress rapidly and with ease. Preliminary investigations conducted at flow rates of >10.0 pi min showed no benzopinacol formation. Only recovery of the unreacted starting materials benzophenone and 2-propanol was observed. By reducing the flow rate to... 

Photochemistry can potentially provide an environmental-friendly and green approach to chemical synthesis however, the ability to scale-up such photochemical processes is marred with problems, which are mainly associated with the power of light sources. The fact that a large number of microreactors are manufactured in glass, quartz, or transparent polymers is ideal for conducting photochemical processes, as the path length of such reactors is small meaning that it is very easy to irradiate the reaction mixture within the channel. Compared to other examples of chemical synthesis in flow reactors, the number of photochemical transformations performed under flow conditions has until recently been very limited. Early examples included benzopinacol formation [1], synthesis of cycloaddition products [2], and photosensitized diastereodifferentiation [3]. 

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